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Absorption

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.

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Absorption

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  1. 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_... Aerosol & Particulate Research Lab

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

  3. Equipment Spray tower Countercurrent packed tower Clean gas out Clean gas out Mist Eliminator Spray nozzle Liquid Spray Packing Dirty gas in Dirty gas in Redistributor Liquid outlet Limitations of a spray tower? Why redistributor? Mycock et al., 1995 Aerosol & Particulate Research Lab

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

  5. Design considerations: What are known? What are we looking for? Aerosol & Particulate Research Lab

  6. Gas out Liquid in Gas in Liquid out (for a dilute system) Lm: molar liquid flow rate Gm: molar gas flow rate x: mole fraction of solute in pure liquid y: mole fraction of solute in inert gas Slope of Operating Line = Lm/Gm Mass Balance In = Out Aerosol & Particulate Research Lab

  7. Dirty air Clean air Dirty water Clean water Aerosol & Particulate Research Lab

  8. Generally, actual liquid flow rates are specified at 25 to 100% greater than the required minimum. How much is X2 if fresh water is used? If a fraction of water is recycled, what’s X2? Ex. Pure water is used to remove SO2 gas in gas flowing at 84.9 m3/min (= 3538 mole/min). 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. Aerosol & Particulate Research Lab

  9. Problems with high gas flow • 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 Ex. 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. Aerosol & Particulate Research Lab

  10. 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) Aerosol & Particulate Research Lab

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

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

  13. Mole balance on the solute in the gas only a: packing area per volume (tower height) HTU Resistivity of the equipment NTU Difficulty in separating the pollutants

  14. Mass balance x1, y1 Equilibrium x1, y1* xZ, yZ xZ, yZ* Alternative solution: HTU NTU Aerosol & Particulate Research Lab

  15. 0.04% CO2 Pure amine Lm = 0.46 gmole/s Ex. 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. 1.27% CO2 Gm = 2.31 gmole/s C* = 7.3% CO2 in amine Assumptions for dilute/soluble systems? Aerosol & Particulate Research Lab

  16. Absorption of concentrated vapor Mole balance on the controlled volume x1, y1 x1, y1* Gas flux Liquid flux xZ, yZ xZ, yZ* Aerosol & Particulate Research Lab

  17. Mole balance on the gas in a differential tower volume Aerosol & Particulate Research Lab

  18. HTU (ft) HTU HTU HTU HTU L' L' For a given packing material and pollutant, HTU does not change much. Aerosol & Particulate Research Lab

  19. Quick Reflection Aerosol & Particulate Research Lab

  20. According Henry’s Law, current Voltage difference two resistance in series Aerosol & Particulate Research Lab

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