*Article* **The Effects of Using Steam to Preheat the Beds of an Adsorption Chiller with Desalination Function**

**Karol Sztekler \*, Wojciech Kalawa, Lukasz Mika, Lukasz Lis, Ewelina Radomska and Wojciech Nowak**

Department of Thermal and Fluid Flow Machines, Faculty of Energy and Fuels, AGH University of Science and Technology, Mickiewicza 30 St., 30-059 Krakow, Poland<EMAIL_ADDRESS>(W.K<EMAIL_ADDRESS>(L.M<EMAIL_ADDRESS>(L.L<EMAIL_ADDRESS>(E.R<EMAIL_ADDRESS>(W.N.) **\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** Adsorption chillers are a promising alternative to traditional compressor-based devices. Adsorption chillers can be supplied with low- or medium-temperature waste heat or heat from renewable energy sources. In addition, they can be used for water desalination purposes. Thus, the adsorption chillers are unique devices that meet essential problems of the modern world: a need to limit the negative impact of humankind on the natural environment and growing problems with access to drinking water. However, adsorption chillers also have disadvantages, including ineffective operation and large size. Therefore, in this paper, the influence of steam utilization on the operation of an adsorption chiller with water desalination function was investigated experimentally, which has not been done before. The research was carried out on the adsorption chiller, working on a silica gel–water pair, installed in the AGH UST Center of Energy. The chiller was modified to preheat the sorbent with the use of steam. The results show that the use of steam instead of water for preheating the bed leads to higher temperatures in the heat exchanger and the bed. As a result, heat transfer from the heating medium to the bed is more intense, and a significant shortening of the desorption process is observed. In the case of using steam for preheating, the desorption time was about 30 s, while for water, it was 300 s. Thanks to this solution, it is possible to reduce the size of the device and increase its efficiency. The proposed solution opens a new course of research on adsorption chillers and broadens the horizon of their applications, as steam is a by-product of many industrial processes.

**Keywords:** adsorption chiller; desalination; preheating; silica gel; steam

#### **1. Introduction**

Cooling processes are used in many industries, including food, pharmaceutical, heavy and energy industries. The International Institute of Refrigeration (IIR) estimated that the total number of refrigeration, air-conditioning and heat pump systems worldwide in 2019 was around 5 billion [1], with electrically powered compressors being the most abundant. As a result, electricity consumption in the refrigeration industry accounts for around 20% of total electricity consumption in the world [1]. Moreover, the dynamically progressing economic development, population growth, observed climate changes and the increasing desire to increase thermal comfort among people mean that by 2050 the global demand for electricity in the refrigeration industry will double [1].

It should be noted that more than 65% of electricity in the world is generated by burning fossil fuels [2], which adversely affects the natural environment due, among others, to greenhouse gas emissions. Additionally, the operation of compressor refrigeration equipment is based on the appropriate thermodynamic transformations of refrigerants such as hydro-chlorofluorocarbons (HCFC) and chlorofluorocarbons (CFC). These refrigerants are characterized by a high ozone depletion potential (ODP) and a high global warming potential (GWP) [3]; therefore, various legal regulations are introduced regarding the possibility of using these refrigerants [4].

**Citation:** Sztekler, K.; Kalawa, W.; Mika, L.; Lis, L.; Radomska, E.; Nowak, W. The Effects of Using Steam to Preheat the Beds of an Adsorption Chiller with Desalination Function. *Energies* **2021**, *14*, 6454. https://doi.org/10.3390/en14206454

Academic Editor: Ron Zevenhoven

Received: 24 August 2021 Accepted: 29 September 2021 Published: 9 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Another challenge faced by more and more people around the world is the growing problem of a lack of access to drinking water. It is estimated that currently around 3.6 billion people live in water-poor areas for at least one month a year, and this number could rise to 5.7 billion by 2050 [5]. Therefore, in order to meet the demand for drinking water, desalination of sea or ocean water seems inevitable. However, conventional methods of desalination of water, such as multi-effect distillation (MED) or reverse osmosis (RO), are energy-intensive processes. To obtain 1 m3 of desalinated water using MED and RO technologies, 14.2–21.6 kWh and 1.5–6.0 kWh of energy is required, respectively [6].

As a result, alternative and environmentally friendly technologies are being sought in both the refrigeration and desalination sectors. One of them is the use of adsorption chillers with the function of water desalination. Table 1 presents a comparison of water desalination using adsorption chillers and other technologies. Comparisons are made for solar energy coupled desalination technologies. As can be seen, water desalination using adsorption chillers is competitive with conventional water desalination technologies listed in Table 1.

**Table 1.** Comparison of selected water desalination technologies coupled with solar energy. Based on [6].

1 Calculated with the assumption that the latent heat of water vaporization is 2390 kJ/kg [7] and the thermal efficiency of the solar still is 60% [8].

Adsorption chillers, unlike compressor chillers, are supplied with low-temperature heat, the minimum required temperature of which is only 55–80 ◦C [9]. Therefore, to power adsorption chillers, low-temperature waste heat from industrial processes [10] or heat obtained from solar energy [11] can be used. Moreover, the adsorption devices do not use harmful HCFCs and HFCs, but are based on environmentally friendly sorbent-sorbate pairs, such as silica gel–water [12], zeolites–water [13], activated carbon–ethanol [14]. Additional advantages of adsorption chillers include low noise level, lack of system vibrations and no moving parts [15]. On the other hand, one of the main disadvantages of adsorption refrigeration equipment is discontinuity of operation and low coefficient of performance (COP), usually not exceeding a value of 0.6 [16], as well as a low specific cooling power (SCP) [17].

The solution to the first problem, i.e., the discontinuity in the operation of adsorption chillers, is the use of two- [11], three- [18], four- [19] or even six-bed chillers [20]. On the other hand, efforts to improve the COP and SCP include many different methods, some of which only consist in changing the operating conditions of the device, and some require interference with the construction of the chiller.

The simplest methods of improving the COP and SCP of adsorption chillers, which do not require modification of the chillers' structure, are the selection of the appropriate time of the adsorption/desorption cycles [21], or the use of a heat source with a higher temperature [18]. However, it should be kept in mind that the heat supplying adsorption chillers is usually waste heat with a given temperature, which usually cannot be changed. Therefore, another solution to improve COP may be, for example, the use of mass recovery, which requires the connection of sorbent beds [22]. As demonstrated by Ng et al. [22], the use of mass recovery can improve the COP of the chiller by up to 20%. Another possibility is to use heat recovery, which takes heat from the bed during the adsorption process and transfers it to another bed, which is in the desorption or preheating phase [23]. Such action can improve the COP of the chiller by about 40% [23].

The heat exchange between the sorbent and the bed heating/cooling medium has a significant impact on the course of the desorption/adsorption process and, therefore, on the effectiveness of adsorption chillers [24,25]. The intensification of heat transfer

in beds can be implemented in several ways: Firstly, by using a heat exchanger of an appropriate design [26], as well as being made of an appropriate material, which should be characterized by a high thermal conductivity coefficient, low heat capacity and low density [27]. Secondly, the intensification of heat exchange can take place by reducing the thermal resistance between the sorbent layer and the heat exchanger, which can be achieved through the use of various types of glues [25]. The heat transfer can also be intensified in the bed itself by adding various materials with high thermal conductivity to it, such as graphite flakes [28], copper, aluminum or carbon nanotubes [29,30].

Therefore, based on the above paragraph, it can be concluded that much attention is paid to the intensification of heat transfer within the bed itself or between the bed and the heat exchanger. However, the rate of heat flow from the heat transfer medium (the medium heating the bed during desorption) to the bed is also dependent on the thermal resistance between the medium and the heat exchanger as well as on the temperature difference between the medium and the bed [24]. Therefore, it seems justified to look for the possibility of intensifying heat transfer on the side of the heat transfer medium.

The effectiveness of desorption depends on the inlet temperature of the heating medium, and the abovementioned effectiveness augments with increasing temperature of the heating medium [31]. This observation was also reported by Ghilen et al. [32], who concluded that desorption is much faster for high temperatures of the heating medium compared to low temperatures. As a result, when the heating water inlet temperature increased from 55 ◦C to 95 ◦C, the chiller COP increased by up to 80%. A similar result was obtained experimentally by Sekret and Turski [33], as well as Sztekler et al. [18]. Sekret and Turski [33] found that the chiller COP increased by 17% when the heating water inlet temperature increased from 80 ◦C to 95 ◦C, while Sztekler et al. [18] reported a COP improvement of 190% when the heating water inlet temperature increased from 57 ◦C to 85 ◦C.

It is worth noticing that in most cases of adsorption chillers, the medium that heats or preheats the bed is water and, in some cases, also thermal oil [34,35]. Ng et al. [36] suggested that adsorption chillers can be driven by low-temperature steam, but they did not provide any information about the effects of using steam for bed regeneration on the operating parameters, e.g., temperatures, pressures and COP, of the chiller. In turn, Shahzad et al. [37] investigated the performance of the adsorption chiller integrated with two ejectors driven by steam at a pressure of 2 bar. However, to the best of the authors' knowledge, the literature lacks research concerning the effect of using steam to directly preheat the bed of the adsorption chiller, without using additional devices such as ejectors. Steam can have a much higher temperature than water, which, as shown in the above paragraph, is expected to improve the performance of the adsorption chiller. Such an innovative solution has not been investigated before.

This article presents the results of experimental research on the use of steam for the preheating of the beds of a three-bed adsorption chiller with water desalination function. Such a solution has never been used in the technology of adsorption chillers and is undoubtedly a novelty that will contribute to the development of this direction of research. As a result of using high-temperature steam, the heat exchange between the medium and the bed takes place faster, thanks to which the desorption process can take place in a much shorter time. Additionally, steam is generated in many technological processes and, therefore, the use of steam for preheating adsorption chiller beds can significantly expand the scope of their applications, as well as develop a new direction of research on adsorption chillers.

#### **2. Methodology**

#### *2.1. Test Stand*

The test stand, the basis of which is an adsorption chiller with a water desalination function, is located at the AGH UST Center of Energy. The tested chiller is a three-bed chiller in which silica gel and water are used as a sorbent–sorbate pair. A detailed description of the test stand and the general principle of operation of the tested chiller can be found in the papers [18,21]. Nevertheless, for the needs of this experiment, the chiller worked in a two-bed configuration, and the test stand was modified so that it was possible to investigate the effect of bed preheating with steam on the parameters of the chiller operation. The diagram and photos of the test stand with applied modifications are presented in Figures 1 and 2, respectively.

**Figure 1.** Simplified diagram of the test stand. 1—condenser; 2—distillate tank; 3—adsorption bed; 4—brine tank; 5—evaporator; 6—degasification tank; 7—steam generator; 8—manifold; 9—hot water tank; 10—electric heater; T1—chilled water inlet temperature; T2—chilled water outlet temperature; T3—cooling water inlet (to the condenser) temperature; T4—cooling water outlet (from the condenser) temperature; T5—heating water/steam inlet temperature; T6—cooling water inlet (to the bed) temperature; T7—heating water/steam outlet temperature; T8—temperature inside the heat exchanger; T9—temperature of the free space of the bed; T10—temperature of water vapor leaving the bed and flowing to the condenser; F1—chilled water flow rate; F2—cooling water flow rate; F3—heating water/steam flow rate; P1—steam inlet pressure; P2—steam outlet pressure; P3—pressure inside the bed; P4—pressure inside the condenser.

The main components of the chiller are three adsorption beds (3), an evaporator (5) and a condenser (1). Each bed is equipped with a finned Tichelmann lamella heat exchanger with a total length of 880 mm and a total width of 450 mm. The construction of the evaporator and condenser is cylinder-shaped with a length and diameter of 1500 and 350 mm (evaporator) and 1450 and 220 mm (condenser), respectively. In addition, the system is equipped, among other things, with a distillate tank (2), a brine tank (4), a degasification tank (6) and an expansion valve not shown in Figure 1. Additionally, a steam generator (7) and manifold (8) were installed at the test stand to enable investigating the use of steam for preheating the bed. In addition to the main components, the investigated unit is equipped

with three circulating pumps (not shown in Figure 1), one for heating, one for cooling and one for chilled water circuits. However, these pumps are not an integral part of the adsorption chiller in contrast to, for example, a compressor in the case of compressor chillers. Theoretically, it is even possible to completely exclude the above-mentioned pumps thanks to the gravity flow of water. Fluids in various forms and under different conditions flow through the system in seven main circuits, as marked in Figure 1.

(**a**) (**b**)

(**c**)

**Figure 2.** Pictures of the test stand: (**a**) front view on the beds, evaporator and steam generator; (**b**) front view on the steam generator and manifold; (**c**) back view on the condenser, distillate tank, degasification tank and brine tank.

In order to test the use of steam for the preheating of the chiller beds, the electric steam generator MAXI 24 (marked with the number 7 in Figure 1) was installed on the test stand. Detailed technical parameters of the steam generator are summarized in Table 2. The device has modulated power, thanks to which it is possible to control the parameters of the steam supplied to the chiller. The steam generator was connected to the distribution manifold (8), which in turn was connected to the heating water circuit. Thanks to this, two variants of bed preheating are possible:

**Table 2.** Technical parameters of the steam generator MAXI 24.

*2.2. Measuring Devices and Uncertainty Analysis*

The following parameters were measured during the experiments:

The measuring sensors used are listed in Table 3.

**Table 3.** Technical parameters of measuring sensors.

1 The Pt-100 sensor was used to measure the temperature of steam for the preheating of bed. The remaining temperatures were measured with Pt-1000 sensors.

All values were measured in the steady state of the chiller operation, achieved after about one hour from the moment of its start-up, and then, 300 adsorption/desorption cycles were performed. The values were saved every 5 s on a personal computer thanks to the use of special software. The test results were exported to csv files, which enabled their further processing. In addition, the test stand is equipped with an appropriate control system, thanks to which it is possible to fully control the parameters of the chiller.

The total measurement uncertainties were calculated as a quadratic sum of the random and systematic uncertainties [38]:

$$
\mu(k) = \sqrt{\delta k\_{ran} + \delta k\_{sys}} \tag{1}
$$

where *k*, *u(k)*, *δkran* and *δksys* are the temperature or the pressure or the mass flow rate, the total measurement uncertainty, the random uncertainty and the systematic uncertainty, respectively. The systematic uncertainties are equal to the uncertainties of the measuring devices (Table 3), while the random uncertainties were calculated as follows:

$$
\delta k\_{\rm run} = \sqrt{\frac{1}{N \cdot (N-1)} \cdot \sum \left(k\_n - k\_{\rm avy}\right)^2} \tag{2}
$$

where *N*, *kn* and *kavg* are the number of measurements (*N* = 300), the value of n-th measurement and average from 300 measurements, respectively.

The coefficient of performance of the investigated chiller was calculated as [39]:

$$\text{COP} = \frac{\dot{m}\_{\text{c}} \cdot c\_{p,\text{c}} \cdot (T\_{i,\text{c}} - T\_{o,\text{c}})}{\dot{m}\_{\text{h}} \cdot c\_{p,\text{h}} \cdot (T\_{i,\text{h}} - T\_{o,\text{h}})} \,\text{}\tag{3}$$

where *m*, *cp* and *T* are the mass flow rate, (kg/s), the specific heat capacity, (J/kg/K) and the temperature, (◦C), respectively. The subscripts c, h, i and o correspond to the chilled water, heating medium, inlet and outlet, respectively. The uncertainty of the *COP* was calculated from the general formula for error propagation [38]:

$$
\mu(\text{COP}) = \frac{\sqrt{\sum \left(\frac{\partial \text{COP}}{\partial k\_i} \cdot u(k\_i)\right)^2}}{\text{COP}} \cdot 100\% \tag{4}
$$

where *ki* stands for quantities from Equation (3).

The calculated measurement uncertainty of temperatures, pressures, mass flow rates and COP is 0.2 ◦C, 0.1 kPa, 0.01 kg/s and 9.7%, respectively.

#### *2.3. Experimental Procedure*

During the experiments, the influence of the use of steam for preheating the adsorption chiller beds on its operating parameters was investigated. For this purpose, two variants of the chiller operation were investigated, during which the beds were preheated with hot water (variant 1) or steam (variant 2). The working parameters of the chiller in variants 1 and 2 are presented in Table 4.

As mentioned before, the investigated adsorption chiller has three beds. Nevertheless, for this study, the chiller operated in a two-bed mode. Thus, the adsorption and desorption processes occur in the beds periodically. Primarily, the refrigerant from the evaporator flows to the bed, and the adsorption, which is an exothermic process, begins. The cooling water flows through the bed and receives heat from the bed through the heat exchanger. When the adsorption is completed, the bed is firstly preheated with water (variant 1) or steam (variant 2) for 50 s (until its inlet and outlet temperatures are equalized). Next, the desorption starts. As desorption is an endothermic phenomenon, heat in the form of hot water is supplied to the bed through the heat exchanger. After finishing the desorption, the refrigerant (desorbed water vapor) flows to the condenser, while the heat recovery process starts in the bed. Then, the entire process (adsorption, preheating, desorption, heat recovery) repeats. During all processes (adsorption, preheating, desorption and heat recovery), supply water is continuously supplied to the evaporator, and to ensure the continuous cooling capacity of the adsorption chiller, two beds are being used and working alternately. That is, desorption takes place in one bed, while adsorption takes place in

the second bed. The preheating/desorption (adsorption)/heat recovery cycle time was 50/300/30 s in both variant 1 and 2.

**Table 4.** Chiller operating parameters in two variants of operation.

#### **3. Results and Discussion**

#### *3.1. Preheating with Steam and Water*

Figure 3 shows the temperatures of heating water (variant 1) and steam (variant 2) at the inlet to the bed, while the outlet temperatures of the media (heating water and steam) are shown in Figure 4. As the preheating process starts, the inlet and outlet temperatures of the media increase in the first 50 s due to the cooling water circuit closure and the heating water/steam circuit opening. When analyzing the preheating with steam, it can be noticed that after opening the valve supplying water to the bed (transition from high to low temperature and vice versa), there is a certain time shift related to the change in the medium. This shift time is approximately 5 s.

In the case of variant 1, the heating water temperature at the bed inlet reaches about 80 ◦C after 50 s and remains at this level throughout the desorption process (300 s). When steam is used to preheat the bed, it can be observed that its maximum temperature at the inlet to the bed is about 120 ◦C after about 50 s from the beginning of the process. Thereafter, a decrease in the temperature of the medium to about 80 ◦C is visible, caused by the closure of the steam flow and the opening of the flow of heating water.

After 300 s, the desorption process is completed and the cooling water flow is opened, as evidenced by the drop in its inlet and outlet temperatures. In about 320 s, a sharp rise in temperature is visible in the steam temperature curve. A possible reason for such an undesirable system behavior is improperly selected solenoid valves shutting off the steam circuit from the bed.

**Figure 3.** Temperature of the medium inlet (T5) to the adsorption chiller bed during desorption.

**Figure 4.** Temperature of the medium outlet (T7) from the adsorption chiller bed during desorption.

Figure 5 shows the change in pressure inside the bed as a function of time for preheating with water and steam. During steam preheating, the pressure in the bed increases much faster than in the case of water preheating. In addition, preheating the bed with steam leads to a greater maximum pressure inside the bed (about 12 kPa) than preheating with water (maximum pressure of about 8 kPa), which indicates that the desorption process is faster when the bed is preheated with steam.

Faster heat transfer between steam and silica gel than between water and silica gel is also confirmed by the temperature changes inside the heat exchanger, which is presented in Figure 6. The temperature rises and stabilizes faster for steam preheating. The use of steam to preheat the bed leads to higher temperatures inside the heat exchanger than in the case of the use of heating water. This is because the steam at the inlet to the bed has a higher temperature than heating water. Furthermore, the steam condenses in the heat exchanger, which additionally intensifies heat transfer. The high temperature of the steam enables faster heat transfer to the silica gel, which in turn leads to an intensification of the desorption process. It is also worth paying attention to the time after which

the heat exchanger temperature reaches its maximum. In the case of steam preheating, the maximum temperature of the heat exchanger is about 70 ◦C, reached after about 100 s from the beginning of the process, and remains constant until the end of the desorption cycle. On the other hand, for water preheating, the temperature in the heat exchanger increases throughout the desorption phase until it reaches its maximum (approximately 65 ◦C) after about 300 s.

**Figure 5.** Pressure inside the bed (P3) during desorption.

**Figure 6.** Temperature inside the heat exchanger (T8).

As already mentioned, the use of steam to preheat the bed leads to the intensification of heat transfer and acceleration of the desorption process. Therefore, in the case of preheating with steam, the desorption ends practically a few seconds after the preheating process. As a result, the desorption lasts for about 30 s for steam preheating, while in the classic cycle (water preheating), it lasts for about 300 s.

The temperature of the free space of the bed is presented in Figure 7. It can be noticed that the temperature of the free space in the bed preheated with steam increases and stabilizes faster and is higher than in the case of preheating with water. With the use of steam, the temperature of the free space in the bed reaches a constant value of 50 ◦C already

after 80 s from the beginning of the cycle. On the other hand, the temperature of the free space in the bed preheated with water increases gradually, reaching its maximum (about 48 ◦C) after 300 s.

**Figure 7.** Temperature of the free space of the bed (T9) during desorption.

Figure 8 shows the temperature of water vapor, i.e., the sorbate after desorption, leaving the bed and flowing to the condenser. It can be noticed that the sorbate temperature increases faster in the case of steam preheating than water preheating. Such a course of the curves proves that the desorption of water vapor from the silica gel surface is more intense and occurs earlier in the case of steam preheating compared to the preheating with water. Thus, the transfer of sorbate from the bed to the condenser takes place more quickly, resulting in an increased COP (Equation (3)). The COP for water and steam preheating is 0.49 and 0.66, respectively, which means an increase of 35.2%.

As shown in the Introduction, numerous researchers reported that the COP of adsorption chillers increases with an increasing temperature of the heating medium. Among the works cited in the Introduction, the greatest improvement (by 190%) was obtained by Sztekler et al. [18]. Thus, the improvement of the COP achieved in this work (by about 35%) seems to be modest. However, it should be noted that the COP of adsorption chillers depends not only on the temperature of the heat source but on many other factors, including the number of beds, sorbent–sorbate pair, the temperature of chilled water, the pressure in the condenser and evaporator, the time of adsorption/desorption cycles, the construction of the chiller and others. Therefore, the improvements of the COP reported by different researchers cannot be compared directly, since, as explained above, the COP depends on several factors. Thus, the reported values of the COP enhancement could be compared only if the tests were carried out on the same chillers and with the same operational parameters. Additionally, the reference temperature of the heating source is meaningful, e.g., Sztekler et al. [18] compared the COP for the heat source temperatures of 57 ◦C and 85 ◦C, while in this paper, the temperatures of the heat source are 80 ◦C and 120 ◦C. As reported by Ng et al. [40], the rate of water desorption from silica gel is temperaturedependent but this relationship is not linear, i.e., the enhancement of desorption rate is much higher when the heat source temperature is increased from 40 ◦C to 80 ◦C than from 80 ◦C to 120 ◦C.

**Figure 8.** Temperature of the water vapor leaving the bed and flowing to the condenser (T10).

Such observations are confirmed by the pressure distribution in the condenser, which is shown in Figure 9. For bed preheating with steam, the pressure change in the condenser is more dynamic than in the case of water. Regarding steam preheating, after the initial pressure increase (facilitating the formation of condensate in the condenser), the pressure drops quite quickly, which leads to the flow of larger amounts of water vapor from the bed and, therefore, enhances the effectiveness of water desalination.

**Figure 9.** Pressure inside the condenser (P4).

#### *3.2. Cost Calculation*

As presented in Section 3.1., using steam for bed preheating can be beneficial in the terms of improving heat transfer and reducing the desorption time. Therefore, if free waste steam is easily available, the proposed solution seems to be cost-effective (ignoring the costs of chiller rebuilding). However, if there is no access to waste steam, the steam must be generated. Thus, a simple cost analysis is done to estimate the cost-effectiveness of this solution.

Assuming the initial water temperature *T0*, heating water temperature *Tw* and steam temperature *Ts* as 20 ◦C, 80 ◦C and 120 ◦C, respectively, their specific enthalpies are *i*0 = 83.9 kJ/kg, *iw* = 335 kJ/kg and *is* = 2705.9 kJ/kg. Therefore, the energy necessary to increase the temperature of 1 kg of water from 20 ◦C (initial condition) to 80 ◦C and to generate 1 kg of steam (120 ◦C) is *Ew* = 251.1 kJ/kg and *Es* = 2622 kJ/kg, respectively. The cost of generating steam and preparing heating water can be calculated from Equation (5):

$$\mathbb{C} = \mathfrak{c}\_{field} \dots \dots \to \mathbb{Z}/\mathbb{Q}\_{\prime} \tag{5}$$

where *C*, *Q* and *cfuel* are the cost of generating the steam (or preparing the heating water), the calorific value of fuel and the unit price of fuel, respectively. The assumed values of fuel unit price, calorific value and the results of calculation are summarized in Table 5.

**Table 5.** Cost calculation of generating steam and heating water from different fuels. Based on data from [41].

Based on Table 5, it can be concluded that the cost of generating 1 kg of steam is approximately 10 times higher than the cost of preparing 1 kg of heating water with predetermined parameters. Nevertheless, if steam being a by-product of, e.g., the industrial process is available, then the cost of steam is zero.

#### *3.3. Challenges and Design Guidelines*

As explained in the Introduction and shown in the Results and Discussion, higher temperatures of the medium used for bed preheating, achieved by using steam instead of water, lead to faster heat transfer and shorten the desorption time from 300 s to 30 s. As a result, the COP of the adsorption chiller is improved by 35.2%, and more adsorption/desorption cycles can be completed in a given time. Therefore, the size of the chiller can be reduced maintaining the same cooling capacity. Nevertheless, using steam instead of water for bed preheating proposed in this paper involves a completely different approach to the desorption process and the material requirements of the device. Under given operating conditions of the chiller (Table 4), the average velocity of the steam is about 7.3 times higher than the average velocity of the water during the preheating. As a result, during the supply of steam to the bed, a very strong mechanical reaction of the system, especially of the pipelines supplying the steam to the bed, takes place, manifested by strong vibrations of the system.

The heat transfer rate between the heating medium and bed is directly proportional to the overall heat transfer coefficient, heat transfer surface area and temperature difference between the medium and the bed. Therefore, based on the results presented in this paper, the following general guidelines for designing steam-preheated adsorption chillers can be outlined:

In addition to the design guidelines listed above, the results presented in this work may also be a basis for creating and validating a mathematical model of the desorption in the adsorption chillers, which may be a topic of the next paper. Furthermore, although the research was carried out for one adsorption chiller on a laboratory scale, it is expected that the presented results translate into other adsorption chillers, including the industrial-scale ones.

#### **4. Conclusions**

This article analyzes the possibilities of bed preheating with steam at a pressure of 3 bar and the maximum temperature of 120 ◦C.

During the experimental tests, a significant effect of the use of steam for preheating on the reduction in the desorption time was observed. The maximum temperature of the inlet steam to the bed was 120 ◦C, and a dynamic equalization of the outlet temperature to the inlet temperature was observed. The temperatures equalization lasted up to 30 s, which proves a very intense heat exchange process between the bed and the heating medium.

The heat exchanger temperature reaches its maximum after about 80 s from the start of the cycle (a few seconds after the end of steam feeding to the heat exchanger) and remains constant until the end of the desorption cycle, while the temperature in the case of water preheating slowly increases to reach its maximum at the end of desorption.

In the case of steam preheating, the pressure in the bed reaches a maximum value of 12 kPa, where in the case of heating water, it is 8 kPa. A higher pressure facilitates the desorption process, which is much more dynamic in the case of steam preheating.

When analyzing the use of steam, it should be noted that when using steam for preheating, the desorption process is shortened to about 30 s compared to 300 s for heating water. Reducing the desorption time to 30 s causes the device, using the same amount of sorbent, to increase its cooling capacity, because at the same time the sorption/desorption cycle can run several times in relation to the preheating the bed using hot water. Thus, the shortening of the desorption time can also lead to the reduction in the dimensions of the device. This is because the same cooling capacity can be obtained from a smaller amount of sorbent. Additionally, the COP of the chiller increased by 35.2% when steam is used instead of water for bed preheating.

If steam is used as a heat source for bed preheating, special preparation of the entire steam supply system to the device is required, and structural changes in the adsorption chiller are necessary. Nevertheless, using steam to preheat the beds of the adsorption chiller shows the potential of increasing its efficiency by reducing the desorption time.

**Author Contributions:** Conceptualization, K.S. and W.K.; methodology, K.S., L.M., L.L. and W.K.; formal analysis, K.S.; investigation, W.K. and L.L.; data curation, K.S., W.K. and E.R.; writing—original draft preparation, K.S., L.M., E.R. and L.L.; writing—review and editing, K.S. and E.R.; supervision L.M. and W.N.; project administration, K.S. and W.N.; funding acquisition, L.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The article was funded by the Faculty of Energy and Fuels AGH UST number <IP_ADDRESS>6 (research subsidy).

**Data Availability Statement:** The data presented in this paper are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

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**Protein Fibre Surface Modification**

## Jolon Dyer1,2,3,4 and Anita Grosvenor1

*1AgResearch Lincoln Research Centre, Christchurch, 2Lincoln University, Canterbury, 3Biomolecular Interaction Centre, University of Canterbury, Christchurch, 4Riddet Institute at Massey University, Palmerston North, New Zealand*

#### **1. Introduction**

Many natural fibres, including wool, cashmere and silk, are protein-based materials; the dry weight of wool is almost entirely derived from proteins (Maclaren & Milligan 1981). As such, they possess an inherent structural and chemical heterogeneity not found in synthetic polymers. Although typically less heterogeneous than biological fibres, the rapidly emerging range of commercially available protein-based biomaterials also contain a wide range of functionality derived from their constituent primary and secondary protein structure.

The response of fibres to processes such as dyeing and finishing treatments correlates directly to their structural and chemical properties, and this is particularly true for surface treatments. Due to its barrier function in the fibre, modification of the surface has a profound impact on processing and performance. Keratinous fibres such as wool and cashmere have an outer lipid layer which results in a hydrophobic surface. Recently a range of innovative and novel fibre surface technologies has been developed, many of which involve altering surface properties by the removal of the lipid layer, which exposes a proteinaceous surface with a variety of reactive chemical moieties. Treatments that can be covalently bound to fibre surface components, rather than simply physically applied to the surface, offer the potential for superior durability.

The surface modification of proteinaceous fibres has a long history. These include plasma applications, which expose and generate functional groups on the protein surface, etching into the surface of the cuticle scales, to improve properties such as surface wettability, dyeability, shrink-resistance and felting-resistance. Chemical approaches utilised include ozone treatments, which cause oxidation of the surface and altered ionic balance, leading to a more plastic and reactive fibre surface and shrinkage control; chlorination, which improves sorption characteristics and reduces shrinkage; hydrogen peroxide treatment; and acid anhydride acetylation of silk and wool, which improves the textile response to dyeing, shrink resist, and setting treatments. Enzymatic treatments have been utilised to decuticulate the surface and improve properties such as shrink resistance. More recent developments include reaction of functional chemical agents or branched molecules to exposed reactive groups on the fibre surface, enabling the attachment of covalently-bound smart treatments, or the amplification of reactive groups for increased functionality.

This chapter outlines developments in the area of targeted surface modification of proteinbased fibres and textiles, including summarising applications and future directions. It is not

Protein Fibre Surface Modification 113

Yuen 2007). As an alternative to ablation, with the use of carbon- and hydrogen-rich plasma

The physical changes (roughening) caused by ablative plasma treatments result in altered yarn strength, frictional and spinning properties and decreased felting behaviours (Höcker 2002). A detrimental effect is a firm or harsh handle (Rakowski 1997). The chemical changes (surface oxidation and the loss of surface-bound fatty acids) result in improved dyeing and resistance to shrinkage (Rakowski 1997). Shrink-resistance is obtained at levels comparable to those obtained from other treatments (like chlorination, reduction and resin application) by the addition of resin, causing surface smoothing. Dyeing depth, speed and bath exhaustion are also improved (Höcker 2002). Atmospheric pressure plasma treatment is also seen to improve the dyeability of mohair (Demir 2010). An unusual application of wool plasma treatment is the deposition of antibacterial polymers for woollen wound dressings

Due to the wide variety of techniques utilised to treat protein-based fibres with plasma, it can be difficult to compare the effects reported by different groups. Relating this problem to dyeing, Naebe *et al*. showed that the effect of plasma on dyeing depended on the dye used – with hydrophobic dyes unaffected by plasma treatment, and hydrophilic dyes greatly affected (Naebe *et al.* 2010). One general feature of plasma-based approaches to modifying the surface of protein materials is the high level of surface oxidation observed after

Hydrophilicity can be a desirable attribute for textiles, allowing improved dyeing, and enhancing comfort and wear properties. The natural hydrophobic surface of wool makes it practically impossible to print wool fabrics without prior treatment. Plasma treatment improves fabric wettability, and also reduces felting (Kan & Yuen 2007). This can be accomplished by the removal of surface lipids (oxygen plasma) or by the deposition of hydrophilic monomers, such as the plasma-assisted deposition of acrylic acid (Kutlu *et al.* 2010). Aside from dye properties and processing, the wool dyeing rate is primarily determined by fibre morphology and by the state of adsorbed water in the fibre (Ristić *et al.* 2010). These properties may be modified by a variety of surface treatments. When the outer lipid layer is even partially removed from wool, the wettability and dyeing rate are reduced. Plasma treatments remove this lipid layer and generate functional groups (such as thiols) that are more reactive to certain dyes. Corona discharge has been reported to incorporate oxygen atoms into the fibre and enhances wettability, which increases the acid-dye intensity of printed fabrics (Ryu *et al.* 1991). Combining plasma treatments with chitosan results in increases in colour intensity and dyeability over those observed with either treatment alone

UV/ozone (UVO) treatments also achieve textile improvement effects able to compete with aqueous treatments. UV radiation at certain frequencies generates atomic oxygen and ozone from molecular oxygen (at 184.9 nm) and atomic oxygen from ozone (at 253.7 nm). Organic hydrocarbons may also be excited at 253.7 nm. Textiles placed close (within 5 mm) of these UVO sources are cleaned by the reaction of UV-excited surface molecules with atomic

This surface treatment results in more wettable wool, which improves dyeing and printing properties, even at low temperature (Xin *et al.* 2002). The wool also yellows, although this can be lessened when the process is combined with peroxide pad bleaching (Shao *et al.*

oxygen. The volatile reaction products desorb from the surface (Shao *et al.* 1997).

gases, polymers may be deposited on the surface (Kan & Yuen 2007).

using fluorinated post-discharge plasmas (Canal *et al.* 2009).

treatment (Meade *et al.* 2008b).

(Ristić *et al.* 2010).

**2.2 UV/Ozone**

intended to cover every aspect of fibre surface modification, but rather focus on key types of surface modification that exploit the unique properties of proteinaceous fibres.

#### **2. Physical modification**

The increasing pressure for environmentally friendly processing approaches has motivated the textile industry to exchange aqueous treatments involving potential chemical pollutants and effluents with high adsorbable organic halide (AOX) contents for physical 'dry' processes, such as plasma and UV/ozone treatment. With respect to dyeing, the nature of physical modification to protein fibre surfaces and where this occurs in production are important and must be taken into consideration.

#### **2.1 Plasma**

Plasma treatments, which utilise a gaseous electrical discharge, are reported to be surface specific for protein fibres (Höcker 2002) and offer significant potential in terms of being simple, clean, solvent-free and relatively inexpensive. These treatments can be used to modify surfaces by the deposition of polymers or can 'clean' the surface by surface etching. Plasma treatments are increasingly replacing wet (chemical) textile treatments to achieve outcomes such as shrink-resistance and improved dyeability. Plasma and corona treatments oxidise the surface of protein-based textiles, generate chemically active radicals, induce functionalisation, and etch the surface (Höcker 2002; Ceria *et al.* 2010). Those utilising certain gases may result in the deposition of atoms from those gases, such as fluorine (Höcker 2002). The effects of plasma treatments are restricted to the wool surface, and are therefore unlikely to result in changes to the bulk properties resulting from damage to the interior of the fibres.

Within the context of the textiles industry, plasma means the products of the interaction of an electromagnetic field with gas; namely, a partially ionised gas that contains ions, electrons and neutral particles (Kan & Yuen 2007). At gas pressures similar to atmospheric pressure and high voltage, corona discharge is generated. At gas pressures of 0.1-10 MPa at lower voltage, glow discharge is generated (Rakowski 1997; Kan & Yuen 2007). Atmospheric corona discharge plasmas lack uniformity due to their filamentous nature (Prat *et al.* 2000). Glow discharge is most commonly referred to when plasma textile processes are described. Many plasma treatments have employed low pressure (vacuum) systems to keep the plasma stable, called cold plasma or non-equilibrium plasma. Lowered pressure also assists with the penetration of plasma effects through the thickness of a textile (Poll *et al.* 2001). Atmospheric pressure plasma systems, however, have an industrial advantage for large-scale textile applications because of the expense, time and space involved in maintaining a vacuum (Sugiyama *et al.* 1998; Demir 2010). These require a source frequency of 1-20 kHz and a carrier gas of helium (Prat *et al.* 2000). Microwave-induced glow-discharge plasma can be kept stable at atmospheric pressure (Sugiyama *et al.* 1998). Several types of discharge may be used to generate plasma, including direct current discharge, radio-frequency discharge, and microwave.

In wool, plasma treatment oxidises and partially removes (ablates) the hydrophobic surface lipid layer (both the loosely adhering lipids and those covalently bound). The disulfide bonds in the surface protein layer of wool (epicuticle) are also oxidised (Höcker 2002). The surface is the only part of the fibre affected; this becomes rougher (surface area increases), while its protein contents are hardly affected (Höcker 2002). The free radicals that remain on the wool surface following ablative plasma treatment stimulate the formation of functional groups and of bonds between the fibre surface and customised surface coatings (Kan &

intended to cover every aspect of fibre surface modification, but rather focus on key types of

The increasing pressure for environmentally friendly processing approaches has motivated the textile industry to exchange aqueous treatments involving potential chemical pollutants and effluents with high adsorbable organic halide (AOX) contents for physical 'dry' processes, such as plasma and UV/ozone treatment. With respect to dyeing, the nature of physical modification to protein fibre surfaces and where this occurs in production are

Plasma treatments, which utilise a gaseous electrical discharge, are reported to be surface specific for protein fibres (Höcker 2002) and offer significant potential in terms of being simple, clean, solvent-free and relatively inexpensive. These treatments can be used to modify surfaces by the deposition of polymers or can 'clean' the surface by surface etching. Plasma treatments are increasingly replacing wet (chemical) textile treatments to achieve outcomes such as shrink-resistance and improved dyeability. Plasma and corona treatments oxidise the surface of protein-based textiles, generate chemically active radicals, induce functionalisation, and etch the surface (Höcker 2002; Ceria *et al.* 2010). Those utilising certain gases may result in the deposition of atoms from those gases, such as fluorine (Höcker 2002). The effects of plasma treatments are restricted to the wool surface, and are therefore unlikely to result in changes to

Within the context of the textiles industry, plasma means the products of the interaction of an electromagnetic field with gas; namely, a partially ionised gas that contains ions, electrons and neutral particles (Kan & Yuen 2007). At gas pressures similar to atmospheric pressure and high voltage, corona discharge is generated. At gas pressures of 0.1-10 MPa at lower voltage, glow discharge is generated (Rakowski 1997; Kan & Yuen 2007). Atmospheric corona discharge plasmas lack uniformity due to their filamentous nature (Prat *et al.* 2000). Glow discharge is most commonly referred to when plasma textile processes are described. Many plasma treatments have employed low pressure (vacuum) systems to keep the plasma stable, called cold plasma or non-equilibrium plasma. Lowered pressure also assists with the penetration of plasma effects through the thickness of a textile (Poll *et al.* 2001). Atmospheric pressure plasma systems, however, have an industrial advantage for large-scale textile applications because of the expense, time and space involved in maintaining a vacuum (Sugiyama *et al.* 1998; Demir 2010). These require a source frequency of 1-20 kHz and a carrier gas of helium (Prat *et al.* 2000). Microwave-induced glow-discharge plasma can be kept stable at atmospheric pressure (Sugiyama *et al.* 1998). Several types of discharge may be used to generate plasma, including

In wool, plasma treatment oxidises and partially removes (ablates) the hydrophobic surface lipid layer (both the loosely adhering lipids and those covalently bound). The disulfide bonds in the surface protein layer of wool (epicuticle) are also oxidised (Höcker 2002). The surface is the only part of the fibre affected; this becomes rougher (surface area increases), while its protein contents are hardly affected (Höcker 2002). The free radicals that remain on the wool surface following ablative plasma treatment stimulate the formation of functional groups and of bonds between the fibre surface and customised surface coatings (Kan &

surface modification that exploit the unique properties of proteinaceous fibres.

**2. Physical modification**

**2.1 Plasma**

important and must be taken into consideration.

the bulk properties resulting from damage to the interior of the fibres.

direct current discharge, radio-frequency discharge, and microwave.

Yuen 2007). As an alternative to ablation, with the use of carbon- and hydrogen-rich plasma gases, polymers may be deposited on the surface (Kan & Yuen 2007).

The physical changes (roughening) caused by ablative plasma treatments result in altered yarn strength, frictional and spinning properties and decreased felting behaviours (Höcker 2002). A detrimental effect is a firm or harsh handle (Rakowski 1997). The chemical changes (surface oxidation and the loss of surface-bound fatty acids) result in improved dyeing and resistance to shrinkage (Rakowski 1997). Shrink-resistance is obtained at levels comparable to those obtained from other treatments (like chlorination, reduction and resin application) by the addition of resin, causing surface smoothing. Dyeing depth, speed and bath exhaustion are also improved (Höcker 2002). Atmospheric pressure plasma treatment is also seen to improve the dyeability of mohair (Demir 2010). An unusual application of wool plasma treatment is the deposition of antibacterial polymers for woollen wound dressings using fluorinated post-discharge plasmas (Canal *et al.* 2009).

Due to the wide variety of techniques utilised to treat protein-based fibres with plasma, it can be difficult to compare the effects reported by different groups. Relating this problem to dyeing, Naebe *et al*. showed that the effect of plasma on dyeing depended on the dye used – with hydrophobic dyes unaffected by plasma treatment, and hydrophilic dyes greatly affected (Naebe *et al.* 2010). One general feature of plasma-based approaches to modifying the surface of protein materials is the high level of surface oxidation observed after treatment (Meade *et al.* 2008b).

Hydrophilicity can be a desirable attribute for textiles, allowing improved dyeing, and enhancing comfort and wear properties. The natural hydrophobic surface of wool makes it practically impossible to print wool fabrics without prior treatment. Plasma treatment improves fabric wettability, and also reduces felting (Kan & Yuen 2007). This can be accomplished by the removal of surface lipids (oxygen plasma) or by the deposition of hydrophilic monomers, such as the plasma-assisted deposition of acrylic acid (Kutlu *et al.* 2010). Aside from dye properties and processing, the wool dyeing rate is primarily determined by fibre morphology and by the state of adsorbed water in the fibre (Ristić *et al.* 2010). These properties may be modified by a variety of surface treatments. When the outer lipid layer is even partially removed from wool, the wettability and dyeing rate are reduced. Plasma treatments remove this lipid layer and generate functional groups (such as thiols) that are more reactive to certain dyes. Corona discharge has been reported to incorporate oxygen atoms into the fibre and enhances wettability, which increases the acid-dye intensity of printed fabrics (Ryu *et al.* 1991). Combining plasma treatments with chitosan results in increases in colour intensity and dyeability over those observed with either treatment alone (Ristić *et al.* 2010).

#### **2.2 UV/Ozone**

UV/ozone (UVO) treatments also achieve textile improvement effects able to compete with aqueous treatments. UV radiation at certain frequencies generates atomic oxygen and ozone from molecular oxygen (at 184.9 nm) and atomic oxygen from ozone (at 253.7 nm). Organic hydrocarbons may also be excited at 253.7 nm. Textiles placed close (within 5 mm) of these UVO sources are cleaned by the reaction of UV-excited surface molecules with atomic oxygen. The volatile reaction products desorb from the surface (Shao *et al.* 1997).

This surface treatment results in more wettable wool, which improves dyeing and printing properties, even at low temperature (Xin *et al.* 2002). The wool also yellows, although this can be lessened when the process is combined with peroxide pad bleaching (Shao *et al.*

Protein Fibre Surface Modification 115

Keratin fibres such as wool, human hair and cashmere are covered in an outer lipid layer which is covalently bound to the surface to form a hydrophobic barrier. The major component of the surface wool lipids is 18-methyleicosanoic acid (18-MEA). 18-MEA is attached to the underlying protein mainly via covalent thioester bonding. A range of treatments have been reported to cleave the thioester bonds to form thiols on the epicuticle surface (Meade *et al.* 2008b). The generation of reactive surface sulfhydryl groups, with the sulfur able to act as a strong nucleophile, make these thiols attractive potential sites for subsequent covalent attachment of novel surface modifications (Meade *et al.* 2008a). Most research and development in this area has been performed with wool, but the principles

Wool fibres are comprised of a core of cortical cells surrounded by an outer sheath of overlapping cuticle cells. Each cuticle cell is enclosed within a resistant membrane termed the epicuticle (Höcker 2000). The epicuticle of wool covers the cuticle, and is comprised of both proteins and lipids (fatty acids); the hydrophobicity of the wool surface is largely attributable to this external lipid layer. The fatty acid component of the epicuticle accounts for approximately quarter of the epicuticle mass, with the surface-bound fatty acids forming a hydrophobic surface layer (Meade *et al.* 2008b). The branched chain fatty acid 18 methyleicosanoic acid (18-MEA) has been identified as the major lipid component of the wool surface, comprising approximately 65-70% of the surface lipid content (Negri *et al.* 1991; Ward *et al.* 1993). This 18-MEA is covalently bound to surface proteins via thioester linkages to cysteine, with the epicuticle estimated to have a content of 35% half-cystine (Negri *et al.* 1993; Evans & Lanczki 1997). The identity of the proteins that the surface lipids are bound to, forming proteolipids, is not yet well understood (Dauvermann-Gotsche *et al.* 1999). Thioesters are a relatively reactive group that can be cleaved relatively readily

If this outer lipid layer is removed in a controlled manner, it is possible to expose the underlying proteinaceous surface so that there are a variety of reactive functional groups (including hydroxyl, carboxyl and amine moieties) available for potential covalent attachment of surface treatments. There are various alkaline reagents that have been used to release covalently bound surface lipids from wool, yielding a hydrophilic and anionic

The use of alcoholic or aqueous alkali conditions cleaves the thioester bonds to form a substituted lipid and a thiol at the epicuticle surface (Negri *et al.* 1991; 1993; Dauvermann-Gotsche *et al.* 2000) The chemical mechanism for nucleophilic substitution of the thioester

Alcoholic alkali treatments use sodium butoxide, potassium tert butoxide, potassium hydroxide and hydroxylamine in water or in an anhydrous solvent such as tert butanol, dehydrated butanol, or ethanol (Leeder & Rippon 1983; Brack *et al.* 1996; Taki 1996; Meade *et al.* 2008b). These treatments are applied to control felting shrinkage, to improve polymer application, dyeing and printing, shrink resistance, and electrical conductivity (Leeder & Rippon 1983; Leeder *et al.* 1985). Alcoholic alkali treatment is less damaging than chlorination, as the effects are limited very much to the surface. The cortex is assumed to be unaffected; only the cuticle is removed, or de-scaled (Taki 1996). Lipid material is removed and a polar surface is generated. Covalently bound lipids are removed from the surface, revealing the polar surface of proteins beneath (Leeder & Rippon 1985; Brack *et al.* 1996; Meade *et al.* 2008b). As potassium t-butoxide is not found to be as good at removing the

surface with an increased frictional coefficient (Dauvermann-Gotsche *et al.* 2000).

bond is shown in Figure 1 [modified from Meade *et al*. 2008b].

have potential for application in other mammalian fibres used in textiles.

through nucleophilic substitution reactions.

**3.5 Delipidation**

2001). This is due to oxidation of the surface layers: Cysteic acid is detected, as disulfide bonds are broken. Carboxylic acid and carbonyl groups are detected at higher levels than after chlorination (Shao *et al*. 1997). The lipids at the surface are modified or volatised. UVO treatment results in a colour yield and dyeability comparable to that obtained after chlorination, permitting printing (Shao *et al*. 1997; 2001).