ABSORPTION Operations and Equipments

Gas Absorption and Scrubber Operations 

is a unit operation in which a gas mixture having solute gas and insoluble gas when exposed to the liquid solvent the soluble gas is dissolved in the absorbent. The absorption may be a purely physical phenomenon for example the removal of SO2 from the flue gases by alkaline solution. As the global warming is caused by the carbon dioxide there are studies going on the absorption of the harmful gas from the atmosphere. Gas separating membranes operate at a high pressure and a high pressure differential. Separation of mixture by addition of new phase into the mixture can explained by absorption.  Free scrubber design sheet

 For absorption of NH3 from a dilute NH3 – air mixture in water, gas side resistance is relatively more important than the liquid side resistance. For absorption of benzene from a coal gas in wash oil, the major resistance to mass transfer comes from the gas side. Rate of absorption of a sparingly soluble gas in a liquid can be increased by increasing the liquid side mass transfer coefficient. The equilibrium relation for distribution of a solute between a gas and liquid phase is given by y = mx (at a particular temperature). If ky and kx are individual gas and liquid phase mass transfer coefficients, respectively, the overall gas phase mass transfer coefficient is given by the relation 1 / Ky = 1 / ky + m / kx At a particular temperature the equilibrium relation for distribution of a solute between a gas and a liquid phase is given by p = HC. If kg and kL are, respectively, the gas and liquid phase mass transfer coefficients, the overall liquid phase mass transfer coefficient is given by the relation1 / KL = 1 / kL + 1 / Hkg At a particular temperature the equilibrium relation for distribution of a solute between a gas and a liquid phase is given by y =2x (x and y are in mole-fractions).

 If the individual gas and liquid phase mass transfer resistances are equally important for absorption of the solute in the liquid phase, the overall gas phase mass transfer coefficient, Ky is 50 % of ky A stage is a device in which mass transfer between two immiscible phases occurs in which two immiscible phases are brought into intimate contact at the exit of which both phases are in equilibrium Multistage contacting is preferred over single stage contacting in CPI because solvent requirement is less in multistage contacting and equipment size is smaller in multistage contacting Kremser equation may be used to determine the number of stages in a staged column when the operating and equilibrium lines are straight. A good solvent for gas absorption should have low vapor pressure and low viscosity. The solvent widely employed for natural gas dehydration is ethylene glycol. For sweetening of sour gases, solvents that are widely employed are mono-and diethanolamines aqueous NaOH solution.

For a given gas and liquid flow rates and a specified gas phase concentration change, the number of plates in a plate column is N. If the gas and liquid flow rates are doubled, the number of plates required for the same gas-phase concentration change will be N. Mole fraction, y, is related to the mole ratio, Y, by the relation y = Y / 1 + Y.

Absorption factor is defined as L/mG The number of plates in a plate column will be larger for L/mG greater than one .The expression for HtOG is (G /KGaPtyBM). Height of transfer unit (HTU) gives an idea about the ease of separation Height of transfer unit and height equivalent to a theoretical plate (HETP) are equal when L/mG is equal to one. 

HTU is equal to HETP when the liquid phase resistance is negligible. For a certain gas absorption process (dilute gas), HtG = 0.3 m, HtL = 0.24 m and L/mG = 1.2. Then HTU is equal to 0.5m. Number of transfer unit gives us an idea about the difficulty of separation. Number of gas phase transfer unit is given as overall change in gas phase concentration/average driving force. Absorption accompanied by heat evolution results in larger number of plates (than that required for isothermal absorption) for the same degree of separation .Lewis number, Le is given by Sc /Pr.