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Encyclopedia Of Bioprocess Technology Fermentation, Biocatalysis аnd Bioseparation

Categories: Chemistry
Volume: Volumes 1-5
発行年: 1999
言語: english
File: RAR, 25.93 MB

Most frequently terms


Plasma Instabilities and Nonlinear Effects

発行年: 1975
言語: english
File: PDF, 5.21 MB

FlickingerM.C., EncyclopediaOfBioprocessTechnologyFermentation,Biocatalysis, AndBioseparationVol.1-5(JohnWileyA&Sons,NY,1999)/38223_a1.pdf


Westvaco Corporation
Charleston, South Carolina


Activated carbon
Adsorption isotherms
Adsorption of radionuclides
BET surface area
Emissions, motor vehicles
Food, beverage, and cooking oil
Pore volume
Powdered carbon regeneration
Solvent recovery


Physical and Chemical Properties
Manufacture and Processing

Thermal Activation Processes
Chemical Activation Processes
Forms of Activated Carbon Products
Shipping and Storage

Economic Aspects
Analytical Test Procedures and Standards
Health and Safety
Liquid-Phase Applications

Potable Water Treatment
Groundwater Remediation
Industrial and Municipal Wastewater Treatment
Sweetener Decolorization
Chemical Processing
Food, Beverage, and Cooking Oil
Miscellaneous Uses

Gas-Phase Applications
Solvent Recovery
Gasoline Emission Control

Adsorption of Radionuclides
Control by Chemical Reaction
Protection against Atmospheric Contaminants
Process Stream Separations
Gas Storage

Additional Reading


Activated carbon is a predominantly amorphous solid that
has an extraordinarily large internal surface area and pore
volume. These unique characteristics are responsible for
its adsorptive properties, which are exploited in many dif-
ferent liquid- and gas-phase applications. Activated carbon
is an exceptionally versatile adsorbent because the size
and distribution of the pores within the carbon matrix can
be controlled to meet the needs of current and emerging
markets (1). Engineering requirements of specific appli-
cations are satisfied by producing activated carbons in the
form of powders, granules, and shaped products. Through
choice of precursor, method of activation, and control of
processing conditions, the adsorptive properties of prod-
ucts are tailored for applications as diverse as the purifi-
cation of potable water and the control of emissions from
bioproduct recovery processes.

In 1900, two very significant processes in the develop-
ment and manufacture of activated carbon products were
patented (2). The first commercial products were produced
in Europe under these patents: Eponite, from wood in
1909, and Norit, from peat in 1911. Activated carbon was
first produced in the United States in 1913 by Westvaco
Corp. under the name Filtchar, using a by-product of the
papermaking process (3). Further milestones in develop-
ment were reached as a result of World War I. In response
to the need for protective gas masks, a hard, granular ac-
tivated carbon was produced from coconut shell in 1915.
Following the war, large-scale commercial use of activated
carbon was extended to refining of beet sugar and corn
syrup and to purification of municipal water supplies (4).
The termination of the supply of coconut char from the
Philippines and India during World War II forced the do-
mestic development of granular activated carbon products
from coal in 1940 (5). More recent innovations in the man-
ufacture and use of activated carbon products have been
driven by the need to recycle resources and to prevent en-
vironmental pollution.


The structure of activated carbon is best described as a
twisted network of defective carbon layer planes, cross-
linked by aliphatic bridging groups (6). X-ray diffraction


patterns of activated carbon reveal that it is nongraphitic,
remaining amorphous because the randomly cross-linked
network inhibits reordering of the structure even when
heated to 3,000 �C (7). This property of activated carbon
contributes to its most unique feature, namely, the highly
developed and accessible internal pore structure. The sur-
face area, dimensions, and distribution of the pores depend
on the precursor and on the conditions of carbonization and
activation. Pore sizes are classified (8) by the International
Union of Pure and Applied Chemistry (IUPAC) as micro-
pores (pore width �2 nm), mesopores (pore width 2–50
nm), and macropores (pore width �50 nm).

The surface area of activated carbon is usually deter-
mined by application of the Brunauer-Emmett-Teller
(BET) model of physical adsorption (9,10) using nitrogen
as the adsorptive (8). Typical commercial products have
specific surface areas in the range 500–2,000 m2/g, but val-
ues as high as 3,500–5,000 m2/g have been reported for
some activated carbons (11,12). In general, however, the
effective surface area of a microporous activated carbon is
far smaller because the adsorption of nitrogen in micro-
pores does not occur according to the process assumed in
the BET model, which results in unrealistically high val-
ues for surface area (10,13). Adsorption isotherms are usu-
ally determined for the appropriate adsorptives to assess
the effective surface area of a product in a specific appli-
cation. Adsorption capacity and rate of adsorption depend
on the internal surface area and distribution of pore size
and shape but are also influenced by the surface chemistry
of the activated carbon (14). The macroporosity of the car-
bon is important for the transfer of adsorbate molecules to
adsorption sites within the particle.

Functional groups are formed during activation by in-
teraction of free radicals on the carbon surface with atoms
such as oxygen and nitrogen, both from within the precur-
sor and from the atmosphere (15). The functional groups
render the surface of activated carbon chemically reactive
and influence its adsorptive properties (6). Activated car-
bon is generally considered to exhibit a low affinity for wa-
ter, which is an important property with respect to the ad-
sorption of gases in the presence of moisture (16). However,
the functional groups on the carbon surface can interact
with water, rendering the carbon surface more hydrophilic
(15). Surface oxidation, which is an inherent feature of ac-
tivated carbon production, results in hydroxyl, carbonyl,
and carboxylic groups that impart an amphoteric charac-
ter to the carbon, so that it can be either acidic or basic.
The electrokinetic properties of an activated carbon prod-
uct are, therefore, important with respect to its use as a
catalyst support (17). As well as influencing the adsorption
of many molecules, surface oxide groups contribute to the
reactivity of activated carbons toward certain solvents in
solvent recovery applications (18).

In addition to surface area, pore size distribution, and
surface chemistry, other important properties of commer-
cial activated carbon products include pore volume, parti-
cle size distribution, apparent or bulk density, particle den-
sity, abrasion resistance, hardness, and ash content. The
range of these and other properties is illustrated in Table
1 together with specific values for selected commercial

grades of powdered, granular, and shaped activated carbon
products used in liquid- or gas-phase applications (19).


Commercial activated carbon products are produced from
organic materials that are rich in carbon, particularly coal,
lignite, wood, nut shells, peat, pitches, and cokes. The
choice of precursor is largely dependent on its availability,
cost, and purity, but the manufacturing process and in-
tended application of the product are also important con-
siderations. Manufacturing processes fall into two catego-
ries, thermal activation and chemical activation. The
effective porosity of activated carbon produced by thermal
activation is the result of gasification of the carbon at rela-
tively high temperatures (20), but the porosity of chemi-
cally activated products is generally created by chemical
dehydration reactions occurring at significantly lower tem-
peratures (1,21).

Thermal Activation Processes

Thermal activation occurs in two stages: thermal decom-
position or carbonization of the precursor and controlled
gasification or activation of the crude char. During carbon-
ization, elements such as hydrogen and oxygen are elimi-
nated from the precursor to produce a carbon skeleton pos-
sessing a latent pore structure. During gasification, the
char is exposed to an oxidizing atmosphere that greatly
increases the pore volume and surface area of the product
through elimination of volatile pyrolysis products and from
carbon burn-off. Carbonization and activation of the char
are generally carried out in direct-fired rotary kilns or
multiple-hearth furnaces, but fluidized-bed reactors have
also been used (22). Materials of construction, notably steel
and refractories, are designed to withstand the high-
temperature conditions (i.e., �1,000 �C) inherent in acti-
vation processes. The thermal activation process is illus-
trated in Figure 1 for the production of activated carbon
from bituminous coal (23,24).

Bituminous coal is pulverized and passed to a briquette
press. Binders may be added at this stage before compres-
sion of the coal into briquettes. The briquetted coal is then
crushed and passed through a screen, from which the on-
size material passes to an oxidizing kiln. Here, the coking
properties of the coal particles are destroyed by oxidation
at moderate temperatures in air. The oxidized coal is then
devolatilized in a second rotary kiln at higher tempera-
tures under steam. To comply with environmental pollu-
tion regulations, the kiln off-gases containing dust and vol-
atile matter pass through an incinerator before discharge
to the atmosphere.

The devolatilized coal particles are transported to a
direct-fired multihearth furnace where they are activated
by holding the temperature of the furnace at about 1,000
�C. Product quality is maintained by controlling coal feed
rate and bed temperature. As before, dust particles in the
furnace off-gas are combusted in an afterburner before dis-
charge of the gas to the atmosphere. Finally, the granular
product is screened to provide the desired particle size. A


Table 1. Properties of Selected U.S. Activated Carbon Products

Manufacturer Gas-phase carbons Liquid-phase carbons


Product grade
Product form
Typical range






WV-A 1100



SA 3




Particle size, U.S. mesha,b �4 12 � 30 3.8 mm 10 � 25 8 � 30 60% �325 65–85% �325
Apparent density, g/cm3 0.2–0.6 �0.48 0.43 0.27 0.52 0.46 0.34–0.37
Particle density, g/cm3 0.4–0.9 0.80 0.50 0.80
Hardness number 50–100 �90 99
Abrasion number �75
Ash, wt % 1–20 �8 6 �10 6 3–5
BET surface area, N2 m2/g 500–2,500 1,050–1,150 1,100–1,200 1750 900–1,000 750 1,400–1,800
Total pore volume, cm3/g 0.5–2.5 0.8 0.9 1.2 0.85 2.2–2.5
CCl4 activity, wt% 35–125 �60
Butane working capacity,

g/100 cm3 4–14 �11.0
Iodine number 500–1,200 �1,050 �900 800 �1,000
Decolorizing index

Westvaco 15–25 �20
Molasses number

Calgon 50–250 �200
Norit 300–1,500 440

Heat Capacity at 100 �C,
J/(g K)c 0.84–1.3 1.05 1.05

Thermal conductivity,
W/(m•K) 0.05–0.10

Note: Specific values shown are those cited in manufacturers’ product literature (19). Typical ranges shown are based on values reported in the open literature.
aUnless otherwise noted.
bApproximate mm corresponding to cited meshes are (mesh:mm 4:4.76; 8:2.38; 10:2; 12:1.68; 25:0.72; 30:0.59; 325:0.04.
cTo convert J to cal, divide by 4.184.







Oxidizer kiln

Devolatilizing kiln








Hot gas

furnace Screen



Figure 1. Thermal activation of bituminous coal.

typical yield of activated carbon is about 30–35% by weight
based on the raw coal.

The process for the thermal activation of other carbo-
naceous materials is modified according to the precursor.
For example, the production of activated carbon from co-
conut shell does not require the stages involving briquet-
ting, oxidation, and devolatilization. To obtain a high-
activity product, however, it is important that the coconut

shell is charred slowly prior to activation of the char. In
some processes, the precursor or product is acid-washed to
obtain a final product with a low ash content (23,25).

Chemical Activation Processes

In contrast to the thermal activation of coal, chemical ac-
tivation is generally carried out commercially in a single


Activation kiln Drying kiln








Concentrated acid







Figure 2. Chemical activation of wood.

kiln. The precursor, usually wood, is impregnated with a
chemical activation agent, typically phosphoric acid, and
the blend is heated to a temperature of 450–700 �C (26).
Chemical activation agents reduce the formation of tar and
other by-products, thereby increasing carbon yield. The
chemical activation process is illustrated in Figure 2 for
the production of granular activated carbon from wood

Sawdust is impregnated with concentrated phosphoric
acid and fed to a rotary kiln, where it is dried, carbonized,
and activated at a moderate temperature. To comply with
environmental pollution regulations, the kiln off-gases are
treated before discharge to the atmosphere. The char is
washed with water to remove the acid from the carbon, and
the carbon is separated from the slurry. The filtrate is then
passed to an acid-recovery unit. Some manufacturing
plants do not recycle all the acid but use a part of it to
manufacture fertilizer in an allied plant. If necessary, the
pH of the activated carbon is adjusted, and the product is
dried. The dry product is screened and classified into the
size range required for specific granular carbon applica-
tions. Carbon yields as high as 50% by weight of the wood
precursor have been reported (26).

Novel Manufacturing Processes. Different chemical ac-
tivation processes have been used to produce carbons with
enhanced adsorption characteristics. Activated carbons of
exceptionally high surface area (�3,000 m2/g) have been
produced by the chemical activation of carbonaceous ma-
terials with potassium hydroxide (28,29). Activated car-
bons are also produced commercially in the form of cloths
(30), fibers (31), and foams (32) generally by chemical ac-
tivation of the precursor with a Lewis acid such as alu-
minum chloride, ferric chloride, or zinc chloride.

Forms of Activated Carbon Products

To meet the engineering requirements of specific applica-
tions, activated carbons are produced and classified as

granular, powdered, or shaped products. Granular acti-
vated carbons are produced directly from granular precur-
sors, such as sawdust and crushed and sized coconut char
or coal. The granular product is screened and sized for spe-
cific applications. Powdered activated carbons are obtained
by grinding granular products. Shaped activated carbon
products are generally produced as cylindrical pellets by
extrusion of the precursor with a suitable binder before
activation of the precursor.

Shipping and Storage

Activated carbon products are shipped in bags, drums, and
boxes in weights ranging from about 10 to 35 kg. Contain-
ers can be lined or covered with plastic and should be
stored in a protected area both to prevent weather damage
and to minimize contact with organic vapors that could
reduce the adsorption performance of the product. Bulk
quantities of activated carbon products are shipped in
metal bins and bulk bags, typically 1–2 m3 in volume, and
in railcars and tank trucks. Bulk carbon shipments are
generally transferred by pneumatic conveyors and stored
in tanks. However, in applications such as water treatment
where water adsorption does not impact product perfor-
mance, bulk carbon may be transferred and stored as a
slurry in water.


Activated carbon producers furnish product bulletins that
list specifications, usually expressed as a maximum or
minimum value, and typical properties for each grade pro-
duced. Standards helpful in setting purchasing specifica-
tions for granular and powdered activated carbon products
have been published (33,34).


Excluding eastern European countries and China, where
production figures have not been published, the world pro-


Table 2. World Production Capacity, Estimated 1990

Country Capacity (103 t)

United States 146
Western Europe 108
Japan 72
Pacific Rim, other 49

Total 375

Note: Excluding eastern Europe and China.

duction capacity of activated carbon was estimated to be
375,000 metric tons in 1990 (35). The price of most prod-
ucts was 0.70 to 5.50 $/kg, but some specialty carbons were
more expensive (36, pp. 731.2001P, 731.2001Q). Forty per-
cent of the production capacity was in the United States,
30% in western Europe, 20% in Japan, and 10% in other
Pacific Rim countries (Table 2).

Production capacity was almost equally split between
powdered and nonpowdered activated carbon products.
Powdered activated carbon, a less expensive form used in
liquid-phase applications, is generally used once and then
disposed of. In some cases, however, granular and shaped
products are regenerated and reused (35). In 1990 produc-
tion capacity for granular and shaped products was split,
with about two-thirds for liquid-phase and one-third for
gas-phase applications (37).

Over the last decade, production capacity in the United
States remained essentially unchanged, but minor fluctu-
ations occurred in response to changes in environmental
regulations (36pp.731.2000S–731.2000Y). A similar reac-
tion was noted worldwide (35). The current demand for
activated carbon is estimated at 93% of production capac-
ity. The near-term growth in demand is projected to be ap-
proximately 5.5%/year (37pp.4,5).

In 1970 the U.S. Congress enacted the Clean Air Act,
the Clean Water Act, and the Safe Drinking Water Act.
Because activated carbon can often be used to help meet
Environmental Protection Agency (EPA) regulations, the
U.S. activated carbon industry reacted by increasing its
production capacity. A proposed amendment to the Safe
Drinking Water Act in 1979 required the use of granular
activated carbon systems, but the amendment was not en-
acted. In response to the projected increase in demand for
activated carbon, production capacity remained high until
the late 1980s, but when the anticipated need did not ma-
terialize, some production facilities were shut down. Cur-
rently, because of stricter EPA regulations implementing
all three acts in 1990, the industry will increase production
capacity by 25% during the next several years (35,38).

The estimated production capacity of activated carbon
in the United States is shown in Table 3 for seven manu-
facturers (35pp.54–65). The principal producers are Cal-
gon Carbon (37%), American-Norit (26%), Westvaco (19%),
and Atochem (10%). Several other companies purchase ac-
tivated carbon for resale but do not manufacture products.

Western Europe has seven manufacturers of activated
carbon. The two largest, Norit and Chemviron (a subsidi-
ary of Calgon), account for 70% of western European pro-
duction capacity, and Ceca accounts for 13% (35p.13),
Japan is the third-largest producer of activated carbon,

having 18 manufacturers, but 4 companies share over
50% of the total Japanese capacity (35p.25–32). Six Pacific
Rim countries account for the balance of the world produc-
tion capacity of activated carbon, 90% of which is in the
Philippines and Sri Lanka (35p.13). As is the case with
other businesses, regional markets for activated carbon
products have become international, leading to consolida-
tion of manufacturers. Calgon, Norit, Ceca, and Sutcliffe-
Speakman are examples of multinational companies.

Activated carbon is a recyclable material that can be
regenerated. Thus the economics, especially the market
growth of activated carbon, particularly granular and
shaped products, is affected by regeneration and industry
regeneration capacity. The decision to regenerate an acti-
vated carbon product is dependent on the cost, size of the
carbon system, type of adsorbate, and the environmental
issue involved. Large carbon systems, such as those used
in potable and wastewater treatment, generally require a
high-temperature treatment, which is typically carried out
in rotary or multihearth furnaces. During regeneration,
carbon losses of 1 to 15% typically occur from the treat-
ment and movement of the carbon (39). However, material
loss is compensated for by the addition of new carbon to
the adsorber system. In general, regeneration of spent car-
bon is considerably less expensive than the purchase of
new activated carbon. For example, fluidized-bed furnace
regeneration of activated carbon used in a 94,600-m3/day
water treatment system cost only 35% of new material
(40). For this system, regeneration using either infrared or
multihearth furnaces was estimated to be more expensive
but still significantly less so than the cost of new carbon.

Because powdered activated carbon is generally used in
relatively small quantities, the spent carbon has often
been disposed of in landfills. However, landfill disposal is
becoming more restrictive environmentally and more
costly. Thus large consumers of powdered carbon find that
regeneration is an attractive alternative. Examples of re-
generation systems for powdered activated carbon include
the Zimpro/Passavant wet air oxidation process (41), the
multihearth furnace as used in the DuPont PACT process
(41pp.389–447,42), and the Shirco infrared furnace

Other types of regenerators designed for specific ad-
sorption systems may use solvents and chemicals to re-
move susceptible adsorbates (44), steam or heated inert
gas to recover volatile organic solvents (45), and biological
systems in which organics adsorbed on the activated car-
bon during water treatment are continuously degraded


Source references for frequently used test procedures for
determining properties of activated carbon are shown in
Table 4. A primary source is the Annual Book of American
Society for Testing and Materials (ASTM) Standards (54).
Other useful sources of standards and test procedures in-
clude manufacturers of activated carbon products, the
American Water Works Association (AWWA) (33,34), and
the U.S. Department of Defense (54).


Table 3. Production Capacity in the United States, Estimated 1990

Company Location Capacity (103 t)

Acticarb Division, Royal Oak Enterprises Romeo, Fla. 6.8
American Norit Co. Marshall, Tex. 38.6
Barneby and Sutcliffe Columbus, Ohio 3.0
Calgon Carbon Corp. Catlettsburg, Ky. and Pittsburgh, Pa. 53.5
Ceca Division, Atochem NA Pryor, Okla. 15.0
Trans-Pacific Carbon Blue Lake, Calif. 2.3
Westvaco Corp. Covington, Va. 27.2

Total 146.4

Table 4. Source References for Activated Carbon Test Procedures and Standards

Title of procedure or standard Source

Standard Definitions of Terms Relating to Activated Carbon ASTM D2652
Apparent Density of Activated Carbon ASTM D2854
Particle Size Distribution of Granular Activated Carbon ASTM D2862
Total Ash Content of Activated Carbon ASTM D2866
Moisture in Activated Carbon ASTM D2867
Ignition Temperature of Granular Activated Carbon ASTM D3466
Carbon Tetrachloride Activity of Activated Carbon ASTM D3467
Ball-Pan Hardness of Activated Carbon ASTM D3802
Radioiodine Testing of Nuclear-Grade Gas-Phase Adsorbents ASTM D3803
pH of Activated Carbon ASTM D3838
Determination of Adsorptive Capacity of Carbon by Isotherm Technique ASTM D3860
Determining Operating Performance of Granular Activated Carbon ASTM D3922
Impregnated Activated Carbon Used to Remove Gaseous RadioIodines from Gas Streams ASTM D4069
Determination of Iodine Number of Activated Carbon ASTM D4607
Military Specification, Charcoal, Activated, Impregnated Ref. 47
Military Specification, Charcoal, Activated, Unimpregnated Ref. 47
AWWA Standard for Granular Activated Carbon Ref. 33
AWWA Standard for Powdered Activated Carbon Ref. 34
BET Surface Area by Nitrogen Adsorption Refs. 6, 8, 9, 48
Pore Volume by Nitrogen Adsorption or Mercury Penetration Refs. 10, 49–52
Particle Density Ref. 53


Activated carbon generally presents no particular health
hazard as defined by NIOSH (55). However, it is a nuisance
and mild irritant with respect to inhalation, skin contact,
eye exposure, and ingestion. On the other hand, special
consideration must be given to the handling of spent car-
bon that may contain a concentration of toxic compounds.

Activated carbon products used for decolorizing food
products in liquid form must meet the requirements of the
Food Chemical Codex as prepared by the Food and Nutri-
tion Board of the National Research Council (56).

According to the National Board of Fire Underwriters,
activated carbons normally used for water treatment pose
no dust explosion hazard and are not subject to sponta-
neous combustion when confined to bags, drums, or stor-
age bins (67). However, activated carbon burns when suf-
ficient heat is applied; the ignition point varies between
about 300 and 600 �C (58p.353).

Dust-tight electrical systems should be used in areas
where activated carbon is present, particularly powdered
products (58pp.84,85). When partially wet activated car-

bon comes into contact with unprotected metal, galvanic
currents can be set up; these result in metal corrosion (59).

Manufacturer material safety data sheets (MSDS) in-
dicate that the oxygen concentration in bulk storage bins
or other enclosed vessels can be reduced by wet activated
carbon to a level that will not support life. Therefore, self-
contained air packs should be used by personnel entering
enclosed vessels where activated carbon is present (60).


Activated carbons for use in liquid-phase applications dif-
fer from gas-phase carbons primarily in pore size distri-
bution. Liquid-phase carbons have significantly more pore
volume in the macropore range, which permits liquids to
diffuse more rapidly into the mesopores and micropores
(61). The larger pores also promote greater adsorption of
large molecules, either impurities or products, in many
liquid-phase applications. Specific-grade choice is based on
the isotherm (62,63) and, in some cases, bench or pilot-
scale evaluations of candidate carbons.






Volume treated


C1 C2 C3 C4

C0 C0 C0 C0

Contaminated influent stream




Effluent stream

Figure 4. Adsorption and breakthrough curve for fixed bed of
granular or shaped activated carbon.

Table 5. Liquid-Phase Activated Carbon Consumption
(103 t) in the United States in 1987.

Granular-shaped Powdered Total

Potable water 4.5 13.6 18.1

Industrial 6.4 6.6 13.0
Municipal 0.9 2.0 2.9

Sweetener decolorization 6.8 9.1 15.9
Chemical processing

and misc. 4.1 2.3 6.4
Food, beverage, and oils 0.9 3.9 4.8
Pharmaceuticals 2.0 2.3 4.3
Mining 1.6 2.5 4.1
Groundwater 0.9 2.3 3.2
Household uses 1.4 0.9 2.3
Dry cleaning 0.7 0.4 1.1
Electroplating 0.2 0.4 0.6

Total 30.4 46.3 76.7

tank Separator






Partially spent

Partially purified



Figure 3. Multistage countercurrent application of powdered ac-
tivated carbon.

Liquid-phase activated carbon can be applied either as
a powder, granular, or shaped form. The average size of
powdered carbon particles is 15–25 lm (62). Granular or
shaped carbon particle size is usually 0.3–3.0 mm. A sig-
nificant factor in choosing between powdered and nonpow-
dered carbon is the degree of purification required in the
adsorption application. Granular and shaped carbons are
usually used in continuous flow through deep beds to re-
move essentially all contaminants from the liquid being
treated. Granular and shaped carbon systems are pre-
ferred when a large carbon buffer is needed to withstand
significant variations in adsorption conditions, such as in
cases where large contaminant spikes may occur. A wider
range of impurity removal can be attained by batch appli-
cation of powdered carbon, and the powdered carbon dose
per batch can be controlled to achieve the degree of puri-
fication desired (61).

Batch-stirred vessels are most often used in treating
material with powdered activated carbon (64). The type of
carbon, contact time, and amount of carbon vary with the
desired degree of purification. The efficiency of activated
carbon may be improved by applying continuous, counter-
current carbon-liquid flow with multiple stages (Fig. 3).
Carbon is separated from the liquid at each stage by set-
tling or filtration. Filter aids such as diatomaceous earth
are sometimes used to improve filtration.

Granular and shaped carbons are generally used in con-
tinuous systems where the liquid to be treated is passed
through a fixed bed (41pp.8–19,64). Compounds are ad-
sorbed by the carbon bed in the adsorption zone (Fig. 4).
As carbon in the bed becomes saturated with adsorbates,
the adsorption zone moves in the direction of flow, and
breakthrough occurs when the leading edge of the adsorp-
tion zone reaches the end of the column. Normally at least
two columns in series are on line at any given time. When
the first column becomes saturated, it is removed from ser-
vice, and a column containing fresh carbon is added at the
discharge end of the series. An alternative approach is the
moving bed column (41pp.8–19). In this design the adsorp-

tion zone is contained within a single column by passing
liquid upward while continuously or intermittently with-
drawing spent carbon at the bottom and adding fresh car-
bon at the top.

The total activated carbon consumption for liquid-phase
applications in the United States in 1987 was estimated
to be about 76,700 t, which accounted for nearly 80% of the
total activated carbon use. The consumption by application
is summarized in Table 5 (36pp.731.2000V–731.2001L)

Potable Water Treatment

Treatment of drinking water accounts for about 24% of the
total activated carbon used in liquid-phase applications
(36pp.731.2000V–731.2001L). Rivers, lakes, and ground-
water from wells, the most common drinking water
sources, are often contaminated with bacteria, viruses,
natural vegetation decay products, halogenated materials,


and volatile organic compounds. Normal water disinfection
and filtration treatment steps remove or destroy the bulk
of these materials (65). However, treatment by activated
carbon is an important additional step in many plants to
remove toxic and other organic materials (66–68) for safety
and palatability.

Groundwater Remediation

Concern over contaminated groundwater sources in-
creased in the 1980s, and in 1984 an Office of Groundwater
Protection was created by the EPA (36pp.731.2000V–
731.2001L). This led to an increase in activated carbon con-
sumption in 1987 for groundwater treatment to about 4%
of the total liquid-phase usage, and further growth is ex-
pected in the 1990s. There are two ways to apply carbon
in groundwater cleanup. One is the conventional method
of applying powdered, granular, or shaped carbon to adsorb
contaminants directly from the water. The other method
utilizes air stripping to transfer the volatile compounds
from water to air. The compounds are then recovered by
passing the contaminated air through a bed of carbon

Industrial and Municipal Wastewater Treatment

Wastewater treatment consumes about 21% of the total
U.S. liquid-phase activated carbon (36pp.731.2000V–
731.2001L), and governmental regulations are expected to
increase demand over the next several years. Wastewater
may contain suspended solids, hazardous microorganisms,
and toxic organic and inorganic contaminants that must
be removed or destroyed before discharge to the environ-
ment. In tertiary treatment systems, powdered, granular,
or shaped carbon can be used to remove residual toxic and
other organic compounds after the primary filtration and
secondary biological treatment (71). Powdered carbon is
also used in the PACT process by direct addition of the
carbon to the secondary biological step (41pp.389–447).

Sweetener Decolorization

About 21% of the liquid-phase activated carbon is used for
purification of sugar (qv) and corn syrup (36pp.731.2000V–
731.2001L). White sucrose sugar is made from raw juice
squeezed from sugarcane or sugar beets. The clarified li-
quor is decolorized using activated carbon or ion exchange
resins (58pp.87–125,274–292). High-fructose corn sweet-
eners (HFCS) are produced by hydrolysis of corn starch
and are then treated with activated carbon to remove un-
desirable taste and odor compounds and to improve stor-
age life. The demand for HFCS rose sharply in the 1980s
primarily because of the switch by soft drink producers
away from sucrose (83).

Chemical Processing

Activated carbon consumption in a variety of chemical
processing applications is about 8% of the total
(36pp.731.2000V–731.2001L). The activated carbon re-
moves impurities to achieve high quality. For example,
organic contaminants are removed from solution in the
production of alum, soda ash, and potassium hydroxide

(58pp.87–125,274–292). Other applications include the
manufacture of dyestuffs, glycols, amines, organic acids,
urea, hydrochloric acid, and phosphoric acid (35pp.92–

Food, Beverage, and Cooking Oil

Approximately 6% of the liquid-phase activated carbon is
used in food, beverage, and cooking oil production
(36pp.731.2000V–731.2000L). Before being incorporated
into edible products, vegetable oils and animal fats are re-
fined to remove particulates, inorganics, and organic con-
taminants. Activated carbon is one of several agents used
in food purification processes. In the production of alcoholic
beverages, activated carbon removes haze-causing com-
pounds from beer, taste and odor from vodka, and fusel oil
from whiskey (58pp.87–125,274–292). The feed water for
soft drink production is often treated with carbon to cap-
ture undesirable taste and odor compounds and to remove
free chlorine remaining from disinfection treatment. Caf-
feine is removed from coffee beans by extraction with or-
ganic solvents, water, or supercritical carbon dioxide prior
to roasting. Activated carbon is used to remove the caffeine
from the recovered solvents (35pp.92–135).


Pharmaceuticals account for 6% of the liquid-phase acti-
vated carbon consumption (36pp.731.2000V–731.2000L).
Many antibiotics, vitamins, and steroids are isolated from
fermentation broths by adsorption onto carbon followed by
solvent extraction and distillation (58pp.87–125,274–292).
Other uses in pharmaceutical production include removal
of chromogenic impurities, process water purification, and
removal of impurities from intravenous solutions prior to
packaging (35pp.92–135).


The mining industry accounts for only 4% of liquid-phase
activated carbon use, but this figure may grow as low-
grade ores become more common (36pp.731.2000V–
731.2000L). Gold, for example, is recovered on activated
carbon as a cyanide complex in the carbon-in-pulp extrac-
tion process (58pp.87–125, 274–292). Activated carbon
serves as a catalyst in the detoxification of cyanides con-
tained in wastewater from cyanide stripping operations
(46pp.8–19). Problems caused by excess flotation agent
concentrations in flotation baths are commonly cured by
adding powdered activated carbon (58pp.87–125,274–

Miscellaneous Uses

Several relatively low-volume activated carbon uses com-
prise the remaining 6% of liquid-phase carbon consump-
tion (36pp.731.2000V–731.2000L). Small carbon filters are
used in households for purification of tap water. Oils, dyes,
and other organics are adsorbed on activated carbon in dry
cleaning recovery and recycling systems. Electroplatingso-
lutions are treated with carbon to remove organics that can
produce imperfections when the thin metal layer is depos-
ited on the substrate (58pp.87–127,274–292). Medical ap-


Table 6. Gas-Phase Activated Carbon Uses in the United
States in 1987

Application Consumption (103 t)

Solvent recovery 4.5
Automotive/gasoline recovery 4.1
Industrial off-gas control 3.2
Catalysis 2.7
Pressure swing separation 1.1
Air conditioning 0.5
Gas mask 0.5
Cigarette filters 0.5
Nuclear 0.3

Total 17.4

plications include removal of toxins from the blood of pa-
tients with artificial kidneys (35pp.92–135), and oral
ingestion into the stomach to recover poisons or toxic ma-
terials (58pp.87–125,274–292;72). Activated carbon also is
used as a support for metal catalysts in low-volume pro-
duction of high-value specialty products such as pharma-
ceuticals, fragrance chemicals, and pesticides (73).


Gas-phase applications of activated carbon include sepa-
ration, gas storage, and catalysis. Although only 20% of
activated carbon production is used for gas-phase appli-
cations, these products are generally more expensive than
liquid-phase carbons and account for about 40% of the total
dollar value of shipments. Most of the activated carbon
used in gas-phase applications is granular or shaped. Ac-
tivated carbon use by application is shown in Table 6 (74).

Separation processes comprise the main gas-phase ap-
plications of activated carbon. These usually exploit the
differences in the adsorptive behavior of gases and vapors
on activated carbon on the basis of molecular weight and
size. For example, organic molecules with a molecular
weight greater than about 40 are readily removed from air
by activated carbon.

Solvent Recovery

Most of the activated carbon used in gas-phase applica-
tions is employed to prevent the release of volatile organic
compounds into the atmosphere. Much of this use has been
in response to environmental regulations, but recovery and
recycling of solvents from a range of industrial processes
such as printing, coating, and extrusion of fibers also pro-
vides substantial economic benefits.

The structure of activated carbons used for solvent re-
covery has been predominantly microporous. Micropores
provide the strong adsorption forces needed to capture
small vapor molecules such as acetone at low concentra-
tions in process air (75). In recent years, however, more
mesoporous carbons, specifically made for solvent recovery,
have become available and are giving good service, espe-
cially for the adsorption of heavier vapors such as cumene
and cyclohexanone that are difficult to remove from micro-
pores during regeneration (75). Regeneration of the carbon

is performed on a cyclic basis by purging it with steam or
heated nitrogen.

Gasoline Emission Control

A principal application of activated carbon is in the capture
of gasoline vapors that escape from vents in automotive
fuel systems (76). Under EPA regulations, all U.S. motor
vehicles produced since the early 1970s have been
equipped with evaporative emission control systems. Most
other auto-producing countries now have similar controls.
Fuel vapors vented when the fuel tank or carburetor are
heated are captured in a canister containing 0.5–2 L of
activated carbon. Regeneration of the carbon is then ac-
complished by using intake manifold vacuum to draw air
through the canister. The air carries desorbed vapor into
the engine where it is burned during normal operation.
Activated carbon systems have also been proposed for cap-
turing vapors emitted during vehicle refueling, and acti-
vated carbon is used at many gasoline terminals to capture
vapor displaced when tank trucks are filled (77). Typically,
the adsorption vessels contain around 15 m3 of activated
carbon and are regenerated by application of a vacuum.
The vapor that is pumped off is recovered in an absorber
by contact with liquid gasoline. Similar equipment is used
in the transfer of fuel from barges (78). The type of carbon
pore structure required for these applications is substan-
tially different from that used in solvent recovery. Because
the regeneration conditions are very mild, only the weaker
adsorption forces can be overcome, and therefore the most
effective pores are in the mesopore size range (79). A large
adsorption capacity in these pores is possible because va-
por concentrations are high, typically 10–60%.

Adsorption of Radionuclides

Other applications that depend on physical adsorption in-
clude the control of krypton and xenon radionuclides from
nuclear power plants (80). The gases are not captured en-
tirely, but their passage is delayed long enough to allow
radioactive decay of the short-lived species. Highly micro-
porous coconut-based activated carbon is used for this ser-

Control by Chemical Reaction

Pick-up of gases to prevent emissions can also depend on
the chemical properties of activated carbon or of impreg-
nants. Emergency protection against radioiodine emis-
sions from nuclear power reactors is provided by isotope
exchange over activated carbon impregnated with potas-
sium iodide (81). Oxidation reactions catalyzed by the car-
bon surface are the basis for several emission control strat-
egies. Sulfur dioxide can be removed from industrial
off-gases and power plant flue gas because it is oxidized to
sulfur trioxide, which reacts with water to form nonvolatile
sulfuric acid (82,83). Hydrogen sulfide can be removed
from such sources as Claus plant tail gas because it is con-
verted to sulfur in the presence of oxygen (84). Nitric oxide
can be removed from flue gas because it is oxidized to ni-
trogen dioxide. Ammonia is added and reacts catalytically
on the carbon surface with the nitrogen dioxide to form
nitrogen (85).


Protection against Atmospheric Contaminants

Activated carbon is widely used to filter breathing air to
protect against a variety of toxic or noxious vapors, includ-
ing war gases, industrial chemicals, solvents, and odorous
compounds. Activated carbons for this purpose are highly
microporous and thus maximize the adsorption forces that
hold adsorbate molecules on the surface. Although acti-
vated carbon can give protection against most organic
gases, it is especially effective against high molecular
weight vapors, including chemical warfare agents such as
mustard gas or the nerve agents that are toxic at parts per
million concentrations. The activated carbon is employed
in individual canisters or pads, as in gas masks, or in large
filters in forced air ventilation systems. In air-conditioning
systems, adsorption on activated carbon can be used to
control the buildup of odors or toxic gases such as radon in
recirculated air (86).

Inorganic vapors are usually not strongly adsorbed on
activated carbon by physical forces, but protection against
many toxic agents is achieved by using activated carbon
impregnated with specific reactants or decomposition cat-
alysts. For example, a combination of chromium and cop-
per impregnants is used against hydrogen cyanide, cyan-
ogen, and cyanogen chloride, whereas silver assists in the
removal of arsine. All of these are potential chemical war-
fare agents; the Whetlerite carbon, which was developed
in the early 1940s and is still used in military protective
filters, contains these impregnants (87). Recent work has
shown that chromium, which loses effectiveness with age
and is itself toxic, can be replaced with a combination of
molybdenum and triethylenediamine (88). Oxides of iron
and zinc on activated carbon have been used in cigarette
filters to absorb hydrogen cyanide and hydrogen sulfide
(89). Mercury vapor in air can be removed by activated
carbon impregnated with sulfur (90). Activated carbon im-
pregnated with sodium or potassium hydroxide has long
been used to control odors of hydrogen sulfide and organic
mercaptans in sewage treatment plants (91). Alkali-
impregnated carbon is also effective against sulfur dioxide,
hydrogen sulfide, and chlorine at low concentrations. Such
impregnated carbon is used extensively to protect sensitive
electronic equipment against corrosion by these gases in
industrial environments (92).

Process Stream Separations

Differences in adsorptivity between gases provides a
means for separating components in industrial process gas
streams. Activated carbon in fixed beds has been used to
separate aromatic compounds from lighter vapors in pe-
troleum refining process streams (93) and to recover gas-
oline components from natural and manufactured gas

Molecular sieve activated carbons are specially made
with restricted openings leading to micropores. These ad-
sorbents are finding increasing use in separations utilizing
pressure swing adsorption, in which adsorption is en-
hanced by operation at high pressure and desorption oc-
curs upon depressurization (96). Larger molecules are re-
stricted from entrance into the pores of these carbons and,
therefore, are not retained as strongly as smaller mole-

cules. The target product can be either the adsorbed or
unadsorbed gases. Examples include separation of oxygen
from air and recovery of methane from inorganic gases in
biogas production. Hydrogen can be removed from gases
produced in the catalytic cracking of gasoline, and carbon
monoxide can be separated from fuel gases. Use of pres-
sure swing techniques for gas separation is an area of
growing interest in engineering research.

The hypersorption process developed in the late 1940s
used a bed of activated carbon moving countercurrent to
gas flow to separate light hydrocarbons from each other
and from hydrogen in refinery operations. The application
is of interest because of its scale, treating up to 20,000
m3/h of gas, but the plants were shut down within a few
years, probably because of problems related to attrition of
the rapidly circulating activated carbon (97). It should be
noted, however, that in recent years moving-bed and fluid-
bed adsorption equipment using activated carbon has been
successfully employed for solvent recovery (98).

Gas Storage

Adsorption forces acting on gas molecules held in micro-
pores significantly densify the adsorbed material. As a re-
sult, activated carbon has long been considered a medium
for lowering the pressure required to store weakly ad-
sorbed compressed gases (99). Recent work with modern
high-capacity carbons has been directed toward fueling
passenger cars with natural gas, but storage volume tar-
gets have not yet been attained (100). Natural gas storage
on activated carbon is now used commercially in portable
welding cylinders (101). These can be refilled easily at
about 2,000 kPa and hold as much gas as a conventional
cylinder pressurized to 6,000 kPa (59 atm).


Catalytic properties of the activated carbon surface are
useful in both inorganic and organic synthesis. For exam-
ple, the fumigant sulfuryl fluoride is made by reaction of
sulfur dioxide with hydrogen fluoride and fluorine over ac-
tivated carbon (102). Activated carbon also catalyzes the
addition of halogens across a carbon-carbon double bond
in the production of a variety of organic halides (73) and is
used in the production of phosgene from carbon monoxide
and chlorine (103,104).


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(1900); Ger. Pat. 136,792 (1901); U.S. Pat. 739,104 (1903).
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Inc., New York, 1963, pp. 1–14. A comprehensive account of
the development and use of activated carbon products to
about 1960.

5. R.V. Carrubba, J.E. Urbanic, N.J. Wagner, and R.H. Zan-
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Porosity, 2nd ed., Academic Press, London, 1982. An indis-
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11. H. Marsh, D. Crawford, T.M. O’Grady, and A. Wennerberg,
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Carbon, vol. 21, Marcel Dekker, New York, 1989, pp. 147–
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umes through 1991, is a primary source of excellent review
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16. D. Atkinson, A.I. McLeod, K.S.W. Sing, and A. Capon, Car-
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ference on Carbon, Penn State University, Pa., June 25–30,
1990, extended abstracts, pp. 94–95.

19. Product data bulletins from activated carbon manufacturers,
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20. T. Wigmans, Carbon 27, 13–22 (1989).

21. F. Derbyshire and M. Thwaites, Proc. 4th Australian Coal
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and H.R. Johnson (to Westvaco Corp.).

23. W. Gerhartz, Y.S. Yamamoto, and F. Thomas Campbell eds.,
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activation processes.

24. Product literature on Pittsburgh activated carbon, Pitts-
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Pittsburgh, Pa., ca 1960.

25. U.S. Pat. 4,014,817 (Mar. 29, 1977), B.C. Johnson, R.K.
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26. A. Cameron and J.D. MacDowall, in J.M. Haynes and P.
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27. R.C. Bansal, J.-B. Donnet, and F. Stoeckli, Active Carbon,
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activated carbon based on a comprehensive review of the lit-

28. U.S. Pat. 4,082,694 (Apr. 4, 1978), A.N. Wennerberg and T.M.
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29. T. Kasuh, D.A. Scott, and M. Mori, Proceedings of an Inter-
national Conference on Carbon, The University of Newcastle
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30. Product literature on activated carbon cloth, Charcoal Cloth
Ltd., U.K., 1985, and on C-tex products, Siebe Gorman & Co.,
Ltd., U.K., 1985.

31. Product literature on KYNOL activated carbon fibers and
cloths, GUN EI Chemical Industry Co., Ltd., Japan, 1987;
product literature on AD’ALL activated carbon fibers, Uni-
tika, Ltd., Japan, 1989.

32. Product literature on KURASHEET activated carbon foam
sheets, Kuraray Chemical Co., Ltd., Japan, 1987.

33. AWWA Standard for Granular Activated Carbon, ANSI/
AWWA B604, American Water Works Association, Denver,
Colo., 1991.

34. AWWA Standard for Powdered Activated Carbon, ANSI/
AWWA B600, American Water Works Association, Denver,
Colo., 1990.

35. The Economics of Activated Carbon, 3rd ed., Roskill Infor-
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36. J. Goin, V. von Schuller-Goetzburg, and Y. Sakuma, Chemi-
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37. Wertheim Schroder, Calgon Carbon Corp., Company Report
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38. S. Irving-Monshaw, Chem. Eng. 97(2), 43–46 (1990).

39. W.G.P. Schuliger, Waterworld News 4(1), 15–17 (1988).

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41. P.N. Cheremisinoff and F. Ellerbusch, Carbon Adsorption
Handbook, Ann Arbor Science Publishers, Ann Arbor, Mich.,
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carbon, ranging from theoretical to applied aspects.

42. Product literature on PACT systems, Zimpro/Passavant,
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43. W.E. Koffskey and B.W. Lykins, Jr., J. Am. Water Works As-
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44. A. Yehaskel, Activated Carbon—Manufacture and Regener-
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45. P.N. Cheremisinoff, Pollut. Eng. 17(3), 29–38 (1985).

46. R.G. Rice and C.M. Robson, Biological Activated Carbon—
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Arbor Science Publishers, Ann Arbor, Mich., 1982, 611 pp.

47. Department of Defense Military Specifications, MIL-C-
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Amendment 1, Mar. 5, 1986; and MIL-C-17605C(SH), Mar.
22, 1989.

48. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area, and
Porosity, 1st ed., Academic Press (London) London, 1967, pp.

49. H.M. Rootare, Advanced Experimental Techniques in Powder
Metallurgy, Plenum, New York, 1970, pp. 225–252. A com-
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50. G. Horvath and K. Kawazoe, J. Chem. Eng. Jpn. 16, 470–
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51. D. Dollimore and G.R. Heal, J. Appl. Chem. 14, 109–114

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53. C. Orr, Jr., Powder Technol. 3, 117–123 (1970).


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56. National Research Council, Assembly of Life Sciences, Di-
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57. American Society of Civil Engineers, American Water Works
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Water Treatment Plant Design, American Water Works As-
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58. J.W. Hassler, Purification with Activated Carbon, 3rd ed.,
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information given in Ref. 4 but with more emphasis on the
commercial uses of activated carbon.

59. U.S. Environmental Protection Agency, Process Design Man-
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Pa., 1971, pp. 3–68.

60. Material safety data sheets on activated carbon products,
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61. R.A. Hutchins, Chem. Eng. 87(2), 101–110 (1980). A particu-
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62. M. Suzuki, Adsorption Engineering, Kodansha Ltd., Tokyo
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63. T.F. Speth and R.J. Miltner, J. Am. Water Works Assoc. 82(2),
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64. F.L. Slejko ed., Adsorption Technology, Marcel Dekker, New
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65. American Water Works Association, Water Quality and
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66. W.J. Weber, Jr. and B.M. Van Vliet, in I.H. Suffet and M.J.
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volume treatise with many key references.

67. J.L. Oxenford and B.W. Lykins, Jr., J. Am. Water Works As-
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68. I.N. Najm and co-workers, J. Am. Water Works Assoc. 83(1),
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69. L.W. Canter and R.C. Knox, Ground Water Pollution Control,
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71. G. Culp, G. Wesner, R. Williams, and M.V. Hughes, Waste-
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72. M. Smisek and S. Cerny, Active Carbon—Manufacture,Prop-
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73. A.J. Bird, in A.B. Stiles ed., Catalyst Supports and Sup-
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97. C. Berg, Chem. Eng. Prog. 47, 585–590 (1951).
98. Gastak Solvent Recovery System, product literature, Kureha

Chemical Industry Co., Ltd., New York, 1990.
99. H. Briggs and W. Cooper, Proc. R. Soc. Edinburgh 41, 119–

127 (1920–1921).
100. J. Braslaw, J. Nasea, and A. Golovoy, Alternative Energy

Sources: Proceedings of the Miami Int. Conf. on Alternative
Energy Sources, 4th ed., Ann Arbor Science Publishers, Ann
Arbor, Mich., pp. 261–270, 1980.

101. U.S. Pat. 4,817,684 (Apr. 4, 1989), J.W. Turko and K.S. Czer-
winski (to Michigan Consolidated Gas Co.).

102. U.S. Pat. 4,102,987 (July 25, 1978), D.M. Cook and D.C. Gus-
tafson (to The Dow Chemical Company).

103. H. Jüntgen, Fuel 65, 1436–1446 (1986).
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551 (1986).


“Active Carbon” under “Carbon” in Encyclopedia of Chemical
Technology (ECT) 1st ed., Vol. 2, pp. 881–899, by J.W. Hassler,
Nuchar Active Carbon Division, West Virginia Pulp and Paper


“Activated Carbon” under “Carbon” in ECT 2nd ed., Vol. 4, pp.
149–158, by E.G. Doying, Union Carbide Corp., Carbon Prod-
ucts Division.

“Activated Carbon” under “Carbon (Carbon and Artificial Graph-
ite)” in ECT 3rd ed., Vol. 4, pp. 561–570, by R.W. Soffel, Union
Carbide Corp.


Bioprocessing Limited
Consett, County Durham, United Kingdom


CPG (controlled pore glass)
Rigid matrices


Choice of Matrices for Bioprocess Separations
Inorganic Matrices

Controlled Pore Glass
Zirconia and Titania

Controlled Pore Glass
Manufacturing Process
Physical Characteristics
Surface Modification

Future Trends
List of Suppliers


The design of a process for the large-scale purification of
biological molecules is one of the several complex tasks
that must be successfully completed to bring a product
to market. Most downstream processes include some chro-
matography steps as well as other conventional separation

methods. Chromatography is generally recognized as the
technique that enables the highest degree of purification
of biomolecules. For example, the U.S. Food and Drug
Administration (FDA) recommends that at least two
chromatography steps be used in the purification of
therapeutic-grade monoclonal antibodies.

Chromatography separations are sophisticated and can
be expensive processes, which explains the continuous
development of effective approaches by scientists and en-
gineers to improve the productivity of chromatography
technologies. Fluidized bed adsorption, various column
technologies, computerized optimization strategies, and
continuous and semicontinuous processes are some of the
recent approaches that have been proposed. However, one
of the most important aspects of chromatography technol-
ogy is the choice of matrix.

In this article, we examine the use of inorganic rigid
matrices in chromatography separation processes of bio-
logical molecules, with emphasis on the preparation, prop-
erties, and applications of controlled pore glass (CPG).


The subject of matrices for process chromatography and
the requirements of an ideal matrix have been reviewed
by many authors (1–3). In process-scale purification, the
important characteristics of a matrix are chemical and
physical stability, rigidity of the matrix to withstand high
flow rates and differing hydrodynamic pressure, uniform
porosity, and high surface area. In addition, for most chro-
matography applications, surface modification of the ma-
trix will be necessary to facilitate ligand immobilization,
which requires the presence of suitable functional groups
on the matrix. Finally, the cost and commercial availability
of the matrix are also important considerations in selecting
a matrix.

The majority of matrices used in chromatography pro-
cesses can be divided into two categories: natural and syn-
thetic. Natural matrices include glass, silica, ceramic, alu-
mina, agarose, chitosan, and cellulose. Synthetic matrices
are prepared by polymerization of functional monomers.
The incorporation of monomers with suitable functional
groups can provide activation sites on these supports for
ligand attachments. There are also the composite matri-
ces, which are mixed networks constituted of at least two
components. Generally, one component acts as a rigid
skeleton, and the other component has active groups that
interact with the biological molecule to be separated.
Dextran-coated silica and polyacrylamide-agarose are ex-
amples of composite matrices that have been applied to the
separation of biological molecules.


A large selection of inorganic matrices are commercially
available for incorporating homogenous catalysts, and
some have been applied to chromatography separations.
These matrices include silica, alumina, glass, zeolite, clay,
celite, zerconia, titania, magnesium oxide, and carbon.
Only a limited number of these matrices with suitable pore


Table 2. Properties of Alumina Matrix (Selecto Scientific)

Nominal pore diameter

Surface area

Particle size range

6 120–150 60–120
10 100 60–120
15 75 60–120
30 80 40
50 50 40

100 8 40
180 6–8 20

Table 1. Properties of Matrex� Silica

Nominal pore

Surface area

Pore volume


Particle size

6 540 0.93 20–500
10 320 1.20 45–300
25 160 1.10 45–70
50 90 1.10 50

100 45 1.10 50–130
150 25 1.10 50

structure, particle size, and mechanical stability can be
used for chromatography separation of biomolecules. Sil-
ica, alumina, and glass are available with a large variety
of surface areas, pore structure, and particle sizes, and
some have been used extensively in both analytical and
preparative chromatography of biological molecules.

Controlled Pore Glass

Controlled pore glass (CPG) has excellent mechanical sta-
bility and can be manufactured with a wide range of pore
diameters, a high surface area, and a range of particle
sizes. The material is almost pure silica and has a very
narrow pore size distribution. Surface-modified CPG with
the trade name PROSEP manufactured by Bioprocessing
Limited has been extensively used in large-scale purifica-
tion of biomolecules. (See later sections on CPG for a more
detailed discussion of this material.)


Silica is extensively used in the analytical field (HPLC)
and to a much lesser extent for preparative chromatogra-
phy of biomolecules. Silica matrices for chromatography
applications are generally produced by a wet process, that
is, by acidification of sodium silicate in water (4,5), and are
available in particle sizes of up to 40 lm and pore sizes of
up to 30 nm. In most cases, these materials are unsuitable
for large-scale purification processes. The small particle
size causes excessive backpressure in preparative chro-
matography columns, and the pore size of 30 nm is not
large enough for the separation of most proteins, especially
in the field of affinity chromatography, where both the li-
gand and the ligate could be large proteins.

In our laboratories, we have used Matrex� silica (Ami-
con) for the large-scale purification of biomolecules. This
material is available in a range of pore diameters and par-
ticle sizes (Table 1) and is manufactured by first forming
a hydrosol from organosilicic acid. Hydrosol is then allowed
to set to a jelly-like mass called hydrogel. After setting, the
hydrogel is thoroughly washed to remove salts resulting
from the reaction. Temperature and pH of the washing me-
dium and the washing time influence the specific surface
area, pore volume, and purity of the hydrogel. The hydro-
gel is subsequently dried to give a structurally stable xero-
gel. This material is 99.5% silica, with impurities including
Na�, Ca2�, Fe3�, , and Cl� ranging from 600 to 602�SO4


Alumina-based materials have a much wider pH stability
than silica matrices and therefore are of great interest in
chemical separations and industrial-scale chromatogra-
phy. Commercially available alumina matrices are all al-
uminium oxides prepared by thermal dehydration of alu-
minum hydroxide (6,7). The origin of the starting material
together with other factors, such as the heating rate, dry-
ing time, and temperature, influence the properties of the
final product.

Alumina matrices can be obtained in a wide range of
particles and pore sizes (Table 2). The physical stability of
alumina particles in chromatography columns, even under
moderate flow rates and pressure, has been a major prob-
lem in our laboratories. Generally, the particles break up
and cause a substantial increase in the backpressure gen-
erated by the column. Furthermore, any risk of contami-
nation of the purified product with aluminum may not be
acceptable to the pharmaceutical and biotechnology indus-
tries, particularly when the reports relating to Alzheimer’s
disease to aluminum are considered (8).

Zirconia and Titania

Recently, there has been an increasing interest in zirconia-
and titania-based matrices in chromatography applica-
tions, mainly because of their chemical stability over a
relatively wide pH range and their physical characteris-
tics. Zirconia or titania supports with properties suitable
for preparative chromatography or HPLC are not commer-
cially available, although zirconia and titania powders can
be obtained commercially from a number of sources and
have been used by researchers to prepare matrices with a
small particle size (1–10 lm) and pore sizes of up to 100
nm, suitable for HPLC applications (9). More recently,
larger porous zirconia particles (30–150 lm) have been
prepared and used in packed and fluidized beds for protein
purification (10–13).

Zirconia and titania as base stable matrices not only
enable separation of biomolecules at high pH values, but
they can also be sanitized with alkaline media, which is
the standard sanitization procedure in downstream pro-
cessing of biological molecules. Various adsorbents such as
affinity, ion-exchange, and reversed phase can be prepared
from these matrices for protein purification by either poly-
mer coating or reaction with organosilanes (14–17).



Hydroxyapatite is a special crystalline form of calcium
phosphate that has been used in the separation of biologi-
cal molecules. The adsorbent consists of a mosaic of crys-
tals that result from a combination of phosphate and cal-
cium salts after a number of steps of wet alkaline
treatment and heat aging. The application of hydroxyapa-
tite to the field of protein separation was first introduced
by Tiselius et al. (18). The standard procedures for the
operation of hydroxyapatite columns were developed by
Bernardi and Gorbunoff (19,20) using a number of differ-
ent proteins in several solvent systems, Bernardi also pro-
posed a mechanism for protein adsorption to, and desorp-
tion from, hydroxyapatite.

Proteins bind reversibly to hydroxyapatite by a complex
ionic mechanism different from the principle of ion-
exchange chromatography (19,20). Both the amino and
carboxyl groups on the protein molecules act in the ad-
sorption mechanism. The adsorption of proteins to hy-
droxyapatite via the amino groups is the result of their
positive charges and the negative charge on the adsorbent
when the column is equilibrated in phosphate buffer. The
surface of hydroxyapatite crystals consists of sites with
both positive (calcium) and negative (phosphate) charges.
When the hydroxyapatite columns are equilibrated in
phosphate buffers, normally at pH 6.8, the surface of the
adsorbent can be regarded as negative because of partial
neutralization of the positive calcium loci by phosphate
ions. The carboxyl groups act in two ways. First, they are
repelled electrostatically from the negative charge of the
adsorbent; second, they bind specifically by complex for-
mation with the calcium sites.

The elution of basic proteins from hydroxyapatite is
generally carried out by specific displacement with Ca2�

and Mg2� ions that neutralize the negative charges of the
hydroxyapatite phosphate sites by complex formation.
Acidic proteins are eluted by displacement of their car-
boxyl groups from hydroxyapatite calcium sites by ions,
such as fluoride or phosphate, which form stronger com-

The difficulty with using hydroxyapatite in medium-
and large-scale purification of biological molecules is the
physical nature of the material. The crystals are highly
fragile, generating small particles resulting in column
clogging and precluding any long-term usage of the col-
umn. An improved form of hydroxyapatite crystals has re-
cently been prepared and is commercially available from a
number of sources, including Bio-Rad. The new ceramic
material from Bio-Rad is spherical and has been reported
to be stable both chemically and physically.


The method used for the preparation of CPG was originally
invented by Hood and Nordberg (21). They discovered that
by heat treating a certain sodium borosilicate glass com-
position, the glass separated into two intermingled and
continuous glassy phases. One phase, rich in boric oxide,
was soluble in acids; the other phase was high in silica and
stable toward acid solutions. This process was used by the

inventors to prepare nonporous silica particles by first re-
moving the boron phase with an acid solution to obtain
porous particles and then revitrifying this material by
heating to 900 �C or above to yield transparent homogen-
ous nonporous particles having a composition of greater
than 95% silica.

The porous glass material obtained by this method typ-
ically had an average pore diameter of about 4 to 5 nm. A
procedure for enlarging the pore size to approximately 20
nm was invented by Chapman and Elmer (22). This was
carried out by impregnating a porous glass body with an
aqueous solution of a weakly reactive fluorine-containing
compound, reacting the compound in situ with a mineral
acid to release hydrofluoric acid at a temperature sufficient
to dissolve a portion of the glass body and washing the body
to remove the soluble constituents. Ammonium bifluoride
and nitric acid at 95 �C were applied to a porous glass disc
to enlarge the pore size from 4 to 24 nm.

The process for making porous glass was modified by
Haller (23) to produce the material with a range of con-
trollable pore sizes. This was achieved by varying the tem-
perature and duration of the heat treatment to obtain po-
rous glass with pore sizes from 12 to 250 nm. Larger pore
sizes were also obtained by the addition of a sodium hy-
droxide leaching process.

Manufacturing Process

CPG can be commercially obtained from a number of
sources, and the manufacturing processes are all based on
the process invented by Haller (23). The composition of the
starting metal oxides mixture and the exact conditions
used for the heat treatment and leaching of the glass,
which influence the quality and physical properties of the
porous glass, are the propriety technology of the manufac-
turing companies. An outline of the general procedure used
for preparing CPG is shown in Figure 1.

CPG is manufactured in a range of particle sizes, 40 to
200 lm, and different pore sizes, 7 to 300 nm (Table 3). The
manufacturing batch sizes are generally much smaller
than silica particles, possibly because of the heat treat-
ment process. The largest batch size commercially avail-
able is in the region of 50 L (20 kg).

Physical Characteristics

Most commercially available CPG granules are irregularly
shaped and highly porous (Figs. 2 and 3). CPG has a nar-
row pore size distribution, with 80 to 90% of the pores
showing a deviation less than �10% from the nominal
pore diameter (Fig. 4). CPG has a very high surface area
that varies according to the pore diameter and pore vol-
ume. The surface area increases with increasing pore vol-
ume and decreases with increasing pore diameter, as is
shown by the data in Table 3. CPG is a rigid insoluble ma-
trix, compatible with most reagents used in the purifica-
tion of biological molecules. Because CPG is essentially
pure silica, it generally exhibits a high degree of solubility
in alkali solutions. The solubility of CPG is a function of
temperature, time, and pH of the solution.

The surface of native CPG is slightly hydrophobic and
contains acidic hydroxyl groups that could denature bio-


Glass mix;
Na2O, B2O3 and SiO2

Homogenous glass melt
furnace at 1,200–1,500 °C,

continuous stirring

Chill the melt
to form glass discs

Heat treat in furnace
500–700 °C, 4–16 h,
two-phase separation

Crush the glass discs and
fractionate the particles

Dry in oven
at > 100 °C

Acid wash; 3 M HCl
Water wash

Alkali leach
to remove colloidal silica
0.5 M NaOH, 25–30 °C

Water wash

Acid leach to
remove silica poor phase
1–3 M HCl, 50–90 °C

Second alkali leach
to enlarge pores

0.5 M NaOH, 25–30 °C

Figure 1. Process for the preparation of porous glass.

Table 3. Properties of Controlled Pore Glass (CPG, Inc.)



Surface area (m2/g);
pore volume � 1.0 cm3/g

Surface area (m2/g);
pore volume � 0.9 cm3/g

7.5 340 240
12 210 150
17 150 100
24 110 75
35 75 50
50 50 35
70 36 25

100 25 18
140 18 13
200 13 9
300 9 7

logical molecules and cause strong, often irreversible bind-
ing of proteins. For most separation applications, surface
modification of CPG with organosilanes is necessary, al-
though native CPG has been used for the purification of a
number of biological molecules, such as factor VIII (24).
Surface modification also enhances the stability and du-
rability of CPG.

Surface Modification

Siliceous materials such as CPG can be reacted with the
organosilane compounds in a number of different ways to
obtain the desired bonding through hydrolyzable groups of
the organosilane. Depending on the type of organosilane,
the reaction can be performed in organic, aqueous, or va-

pour phase, resulting in matrices suitable for different ap-
plications. The majority of the work cited on this subject
relates to the surface modification of silica material (25),
which is not always applicable to CPG because of the dif-
ference in the surface characteristics of the two matrices.

Surface modification of CPG is carried out by treating
the material with an organosilane containing an organic
functional group at one end and a silylalkoxy group at the
other. Attachment of the silane compound to the matrix is
through the surface silanol or oxide groups to the silyl-
akoxy groups with polymerization between the adjacent
silanes. The result is an inorganic matrix with available
organic functional groups that can be used in the prepa-
ration of chromatography adsorbents. A large number of
different organosilanes are commercially available with
functional groups such as epoxy, amine, and sulfhydryl
(Table 4).

The first step in the surface modification of porous glass
is to clean the surface by washing the matrix in nitric acid
at elevated temperature. The matrix is then washed with
water and dried in the oven at temperatures not exceeding
180 �C. The dry matrix is reacted with the silane compound
dissolved in water or an organic solvent at temperatures
ranging from 40 �C to 120 �C, depending on the solvent.
After removing the excess silane by washing the matrix
with suitable solvents or water, the functional groups on
the matrix can be further modified to obtain various ad-
sorbents. The exact conditions used in the surface modifi-
cation of CPG will depend on the nature of the silane com-
pound and the application of the resulting adsorbent.

Extensive optimization of the surface modification pro-
cedure is usually required to obtain a suitable adsorbent
for a specific application. At Bioprocessing, special tech-


Figure 2. The irregularly shaped granules of CPG matrix.

Figure 3. Scanning electron micrograph of CPG matrix.







Pore diameter (µm)
0.001 0.01 0.1 1 10 100









Figure 4. Pore size distribution of CPG matrix as determined by
mercury porosimetery method. Average pore diameter was deter-
mined to be 0.1125 l.

Table 4. Surface Modification of CPG

Organosilane Reactant Activated CPG

3-Aminopropyltriethoxysilane — CPG-amine
3-Aminopropyltriethoxysilane Glutaraldehyde CPG-aldehyde
3-Aminopropyltriethoxysilane Succinic anhydride CPG-carboxyl
3-Glycidoxypropyltrimethoxysilane — CPG-epoxy
3-Glycidoxypropyltrimethoxysilane Acid solution CPG-diol
3-Glycidoxypropyltrimethoxysilane Sodium periodate CPG-aldehyde
Mercaptopropyltrimethoxysilane — CPG-sulfyhyryl

niques for preparing surface modified CPG (PROSEP�)
have been developed for applications in affinity and other
types of separation processes. PROSEP� has been de-
signed for effective immobilization of both large and small
ligands, resulting in affinity adsorbents with a high capac-
ity for the target molecules, minimum nonspecific binding
of contaminating proteins, and low ligand leakage (26).


CPG has been effective in the separation of a range of bio-
logical molecules, using a variety of chromatography tech-
niques that include adsorption, size exclusion, affinity, and
ion exchange. It has also been used extensively in the field
of enzyme immobilization for the modification of biological
molecules. The rigidity and incompressibility of the matrix
allows it to be used in large columns at very high flow rates
generating low backpressure (Fig. 5), which makes this
matrix especially useful for processing large volumes of
feedstock in short cycle times.

Application of native CPG in adsorption chromatogra-
phy has been generally limited because of its denaturing
surface characteristics and irreversible binding of pro-
teins. A number of proteins, such as factor VIII (24), have
been purified from plasma by first blocking the surface of
CPG with albumin. The use of CPG in ion exchange chro-
matography applications has also been very limited be-
cause of the instability of the matrix in sodium hydroxide
solution, which is the standard cleaning and sanitization

solution for ion exchange adsorbents. Furthermore, ion ex-
change adsorbents prepared with CPG as the matrix have
generally shown low functional capacity when compared to
the polymetric matrices. Nevertheless, some applications
of surface-modified CPG in ion exchange chromatography
have been reported (27–29).

The use of CPG in size-exclusion chromatography has
generally been for the separation of macromolecules such






Linear velocity (cm/h)
0 1000 2000 3000 4000



Figure 5. Backpressure generated from a column (11.3 � 10.0
cm) packed with PROSEP�.

as viruses, cell components, and polymers (30,31). Sepa-
ration of these species from low molecular weight contam-
inants has been achieved by taking advantage of the very
sharp exclusion limits and narrow pore size distribution of
CPG. Generally, surface-modified CPG has been used for
size-exclusion chromatography to eliminate the problem of
nonspecific and irreversible binding of biological molecules
to native CPG. The surface of the matrix is modified with
glycidoxypropyltrimethoxysilane, converting the solid
phase to diol-CPG (Table 4). This is a nonionic, hydrophilic
coating compatible with aqueous and most solvent systems

Surface-modified CPG has been widely applied in the
area of affinity chromatography for the purification of bio-
logical molecules. Biologically active molecules will often
bind to other molecules in a reversible and highly specific
manner. The formation of these stable, specific, and dis-
sociable complexes forms the basis of this powerful sepa-
ration technique.

Affinity adsorbents prepared using PROSEP� and other
surface-modified CPG have been used in many applica-
tions for the purification of biological molecules, such as
antibodies (33,34) and enzymes (35). PROSEP� adsorbents
have been used in packed-bed chromatography columns
and in fluidized or expanded bed mode (Fig. 6) for the direct
capture of biological molecules from feedstocks containing

particulate or colloidal solids (36,37). Table 5 shows the
application of PROSEP� adsorbents in the field of affinity


The progress of inorganic matrices such as silica and CPG
as adsorbents in the downstream processing of biological
molecules is hampered by their instability toward alkaline
solutions. Sodium hydroxide solution up to 0.5 M concen-
tration is the standard cleaning and sanitizing reagent for
chromatography adsorbents used in the separation pro-
cesses of biomolecules. Furthermore, certain chemistries
used in the surface modification of matrices and some pu-
rification protocols require alkaline conditions.

Alkaline-resistant inorganic matrices are under devel-
opment using different approaches. Development of zirco-
nia and titania matrices by sol-gel techniques have already
been mentioned. Surface coating of silica and CPG with an
inert metal oxide such as zirconium oxide and preparation
of glass–ceramic matrices may result in suitable products
that satisfy all the requirements of an ideal matrix for the
purification of biomolecules.

Methods for coating silica matrices with zirconium ox-
ide were studied by Meijers et al. (38) and applied to CPG
at Bioprocessing. Great improvement in the stability of the

Figure 6. Principle of fluidized-bed adsorption. From left to right:
PROSEP� adsorbent is packed into a column specially adopted for
fluidized-bed operation. The bed is gradually expanded by an up-
ward flow of solution through the column until full expansion is

Table 5. Application of Surface-Modified CPG (PROSEP�) in Separation Processes

Product Trade name Application

Protein A adsorbent PROSEP�-A High Capacity Purification of IgG from all species and subclasses except rat IgG
Protein G adsorbent PROSEP�-G Purification of IgG from all species and subclasses
Heparin adsorbent PROSEP�-Heparin Purification of antithrombin III and blood clothing factors
Lysine adsorbent PROSEP�-Lysine Purification of plasminogen and tPA
Metal-chelating adsorbent PROSEP�-Chelating A metal-binding adsorbent used in purification of metal-binding proteins

and metal removals from dilute solutions
Thiophilic adsorbent PROSEP�-Thiosorb Purification of antibodies
Immobilized antibodies — Purification of specific antigens and second antibodies

Note: IgG, immunoglobulin G; tPA, tissue-type plasminogen activator.


coated CPG to alkaline solutions were observed; however,
total stability has not yet been achieved.

A preferred approach for making alkaline-stable inor-
ganic matrices is the glass–ceramic route. Porous glass–
ceramic with a skeleton of CaTi4(PO4)6 crystals has been
prepared by controlled crystallization of glasses in the
CaO-TiO2-P2O5-Na2O system and subsequent acid leach-
ing of the resulting dense glass–ceramic (39). The medium
pore diameter has been reported to be controllable in the
range of 40 to 150 nm by changing the temperature of the
heat treatment. Such an approach is being examined at
Bioprocessing in collaboration with a glass–ceramic re-
search center for developing alkaline stable inorganic ma-
trices suitable for downstream processing of biological mol-


Bioprocessing Limited, Medomsley Road, Consett, Co
Durham, DH8 6SZ, UK

Amicon Products–Millipore Corporation, 80 Ashby
Road, Bedford, MA 01730-2271, USA

Selecto Scientific, 5933 Peachtree Industrial Blvd.,
Bldg. B, Norcross, GA 30092, USA

Bio-Rad Laboratories, 2200 Right Avenue, Richmond,
CA 94804, USA

CPG Inc., 3 Borinski Road, Lincoln Park, NJ 07035,


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Amersham Pharmacia Biotech
Uppsala, Sweden


Expanded bed
Downstream process
Fluidized bed
Protein adsorption
Protein purification




Equipment and Methodology
Method Development
Operation of an Expanded Bed

Industrial-Scale Applications
Purification of Recombinant Human Serum
Albumin from Whole Yeast Culture
Purification of Monoclonal Antibodies from Whole
Mammalian Cell Culture
Purification of Bovine Serum Albumin from Whole
Yeast Culture
Potential Industrial-Scale Applications

Additional Reading


Expanded-bed adsorption is a technique based on fluidiza-
tion for recovery of proteins directly from unclarified feed-
stocks such as fermentation broths, cell homogenates, and
other solutions containing suspended particulates. An ex-
panded bed is a chromatography bed with an increased
void volume between the adsorbent particles, as compared
to a packed chromatography bed. An expanded bed is
formed when high-density adsorbent particles are lifted by
an upward liquid flow through a column specially designed
for expanded bed adsorption. When the bed has expanded
and is stable, the particulate-containing feedstock is ap-
plied, and the protein of interest can bind to the adsorbent
particles while the particulates in the feedstock can pass

unhindered through the void of the bed. Expanded-bed ad-
sorption is a one-passage technique designed for large-
scale recovery processes. Traditionally, when conventional
packed-bed chromatography is used to recover proteins,
the feedstock has to be clarified by centrifugation or filtra-
tion prior to application or the particulates can block the
bed. Expanded-bed adsorption can handle particulate-
containing feedstocks; as a result, clarification, concentra-
tion, and initial purification are accomplished in one unit
operation. Reducing the number of unit operations in a
downstream processing scheme decreases the risk of prod-
uct loss, decreases the processing time, and lowers the
overall production costs. The basic principles of expanded-
bed adsorption and some applications are described in this


The biotechnology industry produces proteins for thera-
peutic and diagnostic use by using biological systems such
as bacteria, yeast, and mammalian cells. A common bot-
tleneck in the production of these proteins is the initial
purification, during which the proteins are recovered from
large volumes of cell- and debris-containing feedstocks,
such as fermentation broths and homogenates. The puri-
fication processes are usually based on packed-bed chro-
matography techniques in which the protein is bound to
an adsorbent packed in a column. A packed bed (fixed bed)
cannot handle particulates very well and would eventually
become totally blocked if an unclarified feedstock was
passed through it. Another concern with these types of
feedstocks is that because of the sometimes low production
level of the protein of interest, the feedstock volume is
large. A large volume takes a long time to pass through a
chromatography column, and a long processing time in-
creases the risk of proteolytic breakdown of the protein.
Unit operations to clarify and reduce the volume of these
feedstocks are therefore vital for a successful purification
process. It is not uncommon for a purification process to
consist of a sequence of unit operations just to achieve ini-
tial recovery. The techniques traditionally used for feed-
stock clarification are centrifugation and filtration (1).

Centrifugation is well suited to industrial processes be-
cause the throughput is high and it can be performed in a
contained environment (a requirement when handling re-
combinant host organisms). The efficiency of the centrifu-
gation process will depend on the properties of the feed-
stock, but it is usually difficult to remove all particulates
by centrifugation, and the resulting fluid may therefore not
be sufficiently free of particulates for direct application to
a packed chromatography bed (2). Whole cells are rela-
tively easy to remove by centrifugation but disrupted cells
are not, and cells (especially mammalian) may even be
damaged by the centrifugation process (3) because of shear
stress and centrifugal forces. New and improved centri-
fuges that are more gentle can be used for mammalian
cells (4), but often the clarification problem remains be-
cause at harvest, culture fluids usually contain relatively
large amounts of subcellular particulates, such as debris.
When the feedstock is secreting bacteria or a bacterial ho-


mogenate, for example, clarification is even more difficult
and centrifugation is not very efficient.

When filtration is used for clarification of feedstocks, it
is usually performed in cross-flow mode (5,6). The fluxes
that can be used will depend on the properties of the feed-
stock, but it is often difficult to reach an acceptable eco-
nomic limit (100 L/m2/h) (7) for a cross-flow filtration pro-
cess because fluxes are typically less than 50 L/m2/h.
Cross-flow filtration can easily be operated as a completely
contained system, and it yields a particulate-free solution.
The fouling of the filtration membranes is a major concern,
and the resulting decrease in flux per filter area often leads
to long processing times. In addition, the cost of mem-
branes must be taken into account in a process where
membrane fouling is severe because the membranes have
to be replaced frequently. Because of the drawbacks asso-
ciated with both the centrifugation and the filtration tech-
niques, they are often used in combination in a purification
process to achieve the best clarification of the feedstock.

The volume of the feedstock is not significantly reduced
by either centrifugation or filtration, which is why product
concentration does not usually increase until the protein
has been adsorbed to a chromatography adsorbent in a
packed-bed column. Because the feedstock volume is not
reduced before application to the packed bed, processing
time will be long. Thus, for a number of reasons, the best
option for successful initial recovery of a protein from a
large-volume cell- or debris-containing feedstock is direct
adsorption of the protein.

A variety of methods for direct recovery have been tried.
One such method is to use a batch procedure in which an
adsorbent is added directly to the unclarified feedstock in
a stirred tank (8,9). The advantage of using a batch pro-
cedure is that the feedstock does not have to be clarified.
The disadvantage is that adsorption of protein to the chro-
matography adsorbent is inefficient, because the stirred
tank acts as one single theoretical plate in a separation
process (10). Several attempts, however, have been made
to improve batch adsorption, for example, continuous-
batch adsorption processes (11).

Another alternative for direct recovery is fluidized bed
adsorption. This technique uses high-density adsorbent
particles in an upflow fluidized bed column (12,13). As with
a batch procedure, a fluidized bed behaves like a column
with a low number of theoretical plates. The adsorbent
particles and the feedstock are constantly “back-mixed,”
which usually results in poor adsorption of the protein by
the adsorbent particles, making it necessary to recirculate
the feedstock through the column, leading to long process-
ing times. A number of solutions to improve the efficiency
of fluidized beds have been suggested, such as columns fit-
ted with perforated horizontal plates (14). This creates a
column with a number of smaller, each completely back-
mixed, fluidized beds. The number of fluidized beds de-
pends on the number of plates inserted. The end result is
a column with more than one theoretical plate and conse-
quently a more efficient process. Another way to improve
the efficiency of a fluidized bed is to use magnetically sta-
bilized fluidized beds (15). The adsorbent particles are
magnetically susceptible and are fluidized while being sta-
bilized by a magnetic field. Stabilizing the particles in this

way prevents back-mixing and gives the column several
theoretical plates. There are, however, drawbacks that
have prevented this technique from being used at indus-
trial scale, such as the heat generated by the magnetic

The concept of expanded-bed adsorption (or expanded-
bed chromatography) was introduced after another at-
tempt to stabilize and prevent back-mixing in a fluidized
bed. An expanded bed is a chromatography bed in which
the bed voidage is larger than in a settled or packed bed.
Furthermore, it displays no or very low back-mixing of ad-
sorbent and liquid. An expanded bed can be also be de-
scribed as a stable fluidized bed because the liquid flow
shows a constant velocity profile over the whole cross-
section of the bed (i.e., plug flow) (Worth mentioning is that
the terms fluidized bed and expanded bed are used differ-
ently in the literature, and sometimes authors do not differ
between the expressions.) The early expanded bed exper-
iments were performed using Sepharose� Fast Flow
(Amersham Pharmacia Biotech, Uppsala, Sweden) as the
adsorbent and a sintered glass filter as the liquid flow dis-
tributor at the bottom of the column (16). The results
showed that if the bed expansion was limited to approxi-
mately twice the settled bed height, the liquid flow through
the column was close to plug flow and the protein adsorp-
tion efficiency was similar to that of a packed bed. These
promising results showed that in an expanded bed it is
possible to combine the fluidized bed’s ability to handle
unclarified feedstocks with a packed bed’s characteristic of
efficient protein adsorption. The adsorbent used in these
early experiments is intended for use in packed beds. The
density of the particles is relatively low, which meant that
the flow through the expanded bed column also had to be
low, otherwise the bed would expand to an extent that ad-
sorbent would start to pack at the top of the column. For
this technique to be attractive at industrial scale, the flow
velocities must be significantly increased. The sintered
glass filter that was used as a flow distributor is not ap-
propriate for large-scale operations for a number of rea-
sons: particulates may get trapped in the filter, the filter
is difficult to clean, and it is difficult to produce large ho-
mogenous sinters. Hence, it soon became apparent that for
expanded-bed adsorption to be successful at industrial
scale, new types of adsorbent particles and new column
designs were needed.

Many different types of adsorbents have been tested:
agarose (17,18), zirconia, silica (19), glass (20,21), hydro-
philized perfluorocarbon (22), and composites such as kie-
selguhr/agarose (23), cellulose/titanium dioxide (24), and
dextran/silica (25). These