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 an alkaline solution. As global warming is caused by carbon dioxide there are studies going on the absorption of harmful gas from the atmosphere. Gas-separating membranes operate at high pressure and a high-pressure differential. Separation of the mixture by the addition of a new phase into the mixture can be explained by absorption.
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 the absorption of benzene from a coal gas in wash oil, the major resistance to mass transfer comes from the gas side. The 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 the 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 the 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 the 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 sour gases, solvents that are widely employed are mono-and diethanolamines aqueous NaOH solution.
For a given gas and liquid flow rate 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. The 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). The height of the transfer unit (HTU) gives an idea about the ease of separation Height of the transfer unit and the 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. The number of transfer units gives us an idea of the difficulty of separation. The number of the gas phase transfer unit is given as overall change in gas phase concentration/average driving force. Absorption accompanied by heat evolution results in a larger number of plates (than that required for isothermal absorption) for the same degree of separation. Lewis number, Le is given by Sc /Pr.
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