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Absorption Definition Equipment Packing materials Design considerations: Mass balance High gas flow Mass flow Concentrated systems HTU and NTU Reading: Chap. 13 img.alibaba.com/photo/50678451/Ceramic_Metal_...


Definition Transfer of a gaseous component (absorbate) from the gas phase to a liquid (absorbent) phase through a gas-liquid interface. Q: What are the key parameters that affect the effectiveness? Q: How can we improve absorption efficiency? Mass transfer rate: gas phase controlled absorption liquid phase controlled absorption Q: Does it matter if it’s gas phase or liquid phase controlled?


Equipment Mist Eliminator Liquid Spray Packing Liquid outlet Dirty gas in Spray nozzle Clean gas out Countercurrent packed tower Spray tower Mycock et al., 1995 Redistributor Q: Limitations of a spray tower? Q: Why redistributor? Clean gas out Dirty gas in


Berl Saddle Intalox Saddle Raschig Ring Lessing Ring Pall Ring Tellerette Three-bed cross flow packed tower Liquid spray Dry Cell Packing Mycock et al., 1995 Q: Criteria for good packing materials?


Design considerations: What are known? What are we looking for?


Mass Balance In = Out


Clean water Dirty water Clean air Dirty air


Generally, actual liquid flow rates are specified at 25 to 100% greater than the required minimum. G = 84.9 m3/min (= 3538 mole/min). Pure water is used to remove SO2 gas. The inlet gas contains 3% SO2 by volume. Henry’s law constant is 42.7 (mole fraction of SO2 in air/mole fraction of SO2 in water). Determine the minimum water flow rate (in kg/min) to achieve 90% removal efficiency. Q: How much is X2 if fresh water is used? What if a fraction of water is recycled?


Channeling: the gas or liquid flow is much greater at some points than at others Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the void space between packing Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow Problems with high gas flow Gas flow rate is 3538 mole/min and the minimum liquid flow rate is 2448 kg/min to remove SO2 gas. The operating liquid rate is 50% more than the minimum. The packing material selected is 2” ceramic Intalox Saddles. Find the tower diameter and pressure drop based on 75% of flooding velocity for the gas velocity. Properties of air:: molecular weight: 29 g/mole; density: 1.17×10-3 g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp.


L: mass flow rate of liquid G: mass flow rate of gas G’: mass flux of gas per cross sectional area of column F: Packing factor : specific gravity of the scrubbing liquid mL: liquid viscosity (in cP; 0.8 for water) (dimensionless)

Mass Transfer: 

Mass Transfer Two-Film Theory (microscopic view) Cussler, “Diffusion”, Cambridge U. Press, 1991. pG CI pI CL J: flux k: mass transfer coefficient (gas phase flux) (liquid phase flux) (overall flux)


Macroscopic analysis of a packed tower Mole balance on the solute over the differential volume of tower (equivalent concentration to the bulk gas pressure) (equivalent pressure to the bulk concentration in liquid) (overall liquid phase MT coefficient) (overall gas phase MT coefficient) 1 2 L’m: molar flux of liquid G’m: molar flux of gas


Mole balance on the solute in the gas only NTU? HTU? a: packing area per volume (tower height)


Mass balance Equilibrium x1, y1* x1, y1 xZ, yZ* xZ, yZ Alternative solution: Assumptions for dilute/soluble systems?


Pure amine Lm = 0.46 gmole/s 0.04% CO2 1.27% CO2 Gm = 2.31 gmole/s C* = 7.3% CO2 in amine Q: A Packed tower using organic amine at 14 oC to absorb CO2. The entering gas contains 1.27% CO2 and is in equilibrium with a solution of amine containing 7.3% mole CO2. The gas leaves containing 0.04% CO2. The amine, flowing counter-currently, enters pure. Gas flow rate is 2.31 gmole/s and liquid flow rate is 0.46 gmole/s. The tower’s cross-sectional area is 0.84 m2. KOGa = 9.34×10-6 s-1atm-1cm-3. The pressure is 1 atm. Determine the tower height that can achieve this goal.


Absorption of concentrated vapor Mole balance on the controlled volume Gas flux Liquid flux x1, y1 x1, y1* xZ, yZ* xZ, yZ


Mole balance on the gas in a differential tower volume


HTU (ft) HTU For a given packing material and pollutant, HTU does not change much.

Quick Reflection: 

Quick Reflection

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