1. Government Engineering College, Valsad
A presentation on:
Material Balance of Co-current flow of Absorber & Choice of Solvent
Semester- 5, Division-2
Department of Chemical Engineering

2. Mass Transfer Operation- 1 Material Balance of Co-current flow of Absorber & Choice of Solvent
Prepared by:
Patel Shashikant C. – ( )
Patel Siddharth S. - ( )
Patel Rohan ( )

3. Material Balance for Co-current flow Absorber
When gas and liquid flow concurrently, as shown in figure, the operating line has a negative slope –Ls / Gs .
Figure: Co-current Absorber

5. There is no limit on this ratio, but an infinitely tall tower would produce an exit liquid and gas in equilibrium, as at (Xe , Ye ).
Co-current flow may be used when an exceptionally tall tower is built in two sections as shown in the figure, with the second section operated in concurrent flow to save on the large diameter gas pipe connecting the two.
Figure: Co-current arrangement for very
tall tower.

6. It may also be used if the gas to be dissolved in the liquid is a pure substance, where there is no advantage to counter-current operation, or if a rapid, irreversible chemical reaction with the dissolved solute occurs in the liquid, where only the equivalent of one theoretical stage is required.

7. Choice of solvent for absorption
If the principal purpose of the absorption operations is to produce specific solution. as n the manufacture of hydrochloric acid, for example the solvent is specified by the nature of the product. if the principal purpose is to remove some constituents from the gas, some choice is frequently possible. Water is of course , the cheapest and most plentiful solvent , but the following properties are important considerations.

8. Gas solubility- the gas solubility should be high, thus increasing the rate of absorption and decreasing the quantity of solvent required. generally solvents of a chemical nature similar to that of the solute to be absorbed will provide good solubility. Thus hydrocarbon oils and not water, are used to remove benzene from coke oven gas. for cases where the solutions formed are ideal, the solubility of the gas is the same in terms of mole fractions for all solvents. but it is greater in terms of weight fractions for solvent of low molecular weight, and smaller weights of such solvents as measured in pounds ,need to be used. A chemical reaction must be reversible . For example hydrogen sulfide can be removed fro gas mixtures using ethanolamine solutions since the sulfide is readily absorbed at low temperature and easily stripped at high temperature. Caustic soda absorbs hydrogen sulfide excellently but will not release it in a operation.

9. volatility- the solvent should have a low vapour pressure since the gas leaving an absorption operation is ordinary saturated with the solvent and much may thereby be lost. if necessary ,a second ,less volatile liquids can be used to recover the evaporated portion of the first as in fig. this is sometimes done , for example in the case of hydrocarbon absorbers , where a relatively volatile solvent oil is used in the principal portions of the absorber because of the superior solubility characteristics and the volatilized solvent is recovered from the gas by a nonvolatile oil. similarly hydrogen sulfide can be absorbed by a water solutions of sodium phenolate. but the desulfurized gas is further washed with water to recover the evaporated phenol.

10. Figure: Tray Absorber with volatile solvent recovery section

11. Corrosiveness- the material of construction required for the equipment should not be unusual or expensive.
Cost- the solvent should be inexpensive, so that losses are not costly and should be readily available.
Viscosity-low viscosity is preferred for rapid absorption rates , improved flooding characteristics in absorption towers, low pressure drops on pumping , and good heat-transfer characteristics.

12. Miscellaneous- the solvent if possible should be non-toxic, non- flammable, and chemically stable and should have a low freezing point.