GAS ABSORPTION

GAS ABSORPTION1DefinitionGas absorption is an operation in which a gas mixture is contacted with a liquid for the purpose of preferentially dissolving one or more components of the gas mixture and to provide a solution of them in the liquid. (Treybal p. 275).

Gas absorption is an operation in which a soluble gas (vapor) is absorbed from its mixture with an inert gas by means of a liquid in which the solute gas is soluble.2Application of Gas Absorption TechnologyRemoval of CO2 and H2S from natural gas or synthesis gas by absorption in amines and alkaline salt solutions.Removal of NH3 from gas from by-product coke ovens by washing with water.Removal of benzene and toluene vapors from gas from by-product coke ovens with oil after washing with water.3In the above situation mass transfer occur from the gas stream to the liquid stream and the operation is called Gas Absorption.However, when the mass transfer is in the opposite direction (liquid stream to the gas stream) the operation is referred to as Desorption or Stripping. Here the solute gas is removed from the liquid steam by contacting it with an inert gas.Absorption and Desorption operations are ordinarily used only for solute recovery or solute removal. Separation of solute from each other to any important extent requires the fractionation techniques of distillation.

4Gas SolubilityIf a gas and relatively non volatile liquid are in equilibrium, the resulting concentration of the dissolved gas in the liquid is said to be the gas solubility at the prevailing temperature and pressure. At a fixed temperature the solubility concentration will increase with pressure

5If the equilibrium pressure of a given gas at a given liquid concentration is high as in curve A, the gas is said to be relatively insoluble in the liquid while if it is low the gas solubility is said to be relatively soluble in the liquid.

The solubility of any gas is influenced by the temperature in a manner described by Vant Hoffs law of mobile equilibrium: if the temperature of a system at equilibrium is raised that change will occur which will absorb heat. Solution of a gas usually evolves heat and solubility of a gas decreases with increasing temperature.6

7Multicomponent SystemsIf a mixture of gases is in contact with liquid under certain conditions the equilibrium solubilities of each of the gases will be independent of the others provided however that the equilibrium is described in terms of the partial pressure in the gas mixture.

If all but one of the components of the gas mixture are substantially insoluble, their concentration in the liquid will be so small that these cannot influence the solubility of the relatively soluble component.8Mass Transfer for a One Component SystemMaterial balance

9Ls = non volatile solvent (mol/area)Let G = total gas stream at any point in the tower, [mol/ (area of tower cross section) (time)]Made up of diffusing solute A of mole fraction y, partial pressure and mole ratio YAnd non-diffusing, essentially insoluble gas Gs = [mol/ (area) (time)]Such that

10Similarly for the liquid stream,Ltotal = [mol/ (area) (time)] containing x mole fraction of soluble gas or mole ratio X and essentially non-volatile solvent Ls = [mol/ (area) (time)]

Such that

11NoteSolvent gas and Solvent liquid are essentially unchanged in quantity as they pass through the absorption/ stripping tower. It is convenient to express the material balance in terms of these quantities.

Which is equation of a straight line passing through (X1, Y1)

The operating line with slope Ls/Gs

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The operating line is straight when plotted in terms of mole ratio units13

Each point on the operating line indicates the relationship between the liquid and gas concentration at any level in the tower.Each point on the equilibrium-solubility curve represents the gas concentration in equilibrium with the corresponding liquid at its local concentration and temperature. 14The operating line is straight when plotted in terms of mole-ratio units. In terms of mole fraction or partial pressures the operating line is curved.

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Where the total pressure Pt can be considered constant at any point throughout the tower.16Minimum Liquid-Gas Ratio for Absorber

17When operating line concedes with the equilibrium line, driving force for mass transfer is zero and1.Contact time is infinitely long2.Tower height is infinitely high.The maximum possible liquid concentration and the minimum possible liquid rate Ls occur when the operating line just touches the equilibrium line. At this point the driving force or the concentration gradient for mass transfer is zero and an infinitely tall absorption tower is necessary.18The driving force for mass transfer is or where is the equilibrium composition in the gas phase. y* is proportional to the vertical distance between the operating line and the equilibrium curve on the above diagram.

In general the liquid rate for the absorber should be between 1.1 and 1.5 times the minimum rate

when the liquid for the absorption is to be discarded and not regenerated.

19Concurrent Flow Absorber

20For concurrent flow in an absorber, the operating line has a negative slope Ls/Gs.There is no limit on the ratio of liquid to gas and an infinitely tall tower will give an exit liquid and gas equilibrium at (Xe, Ye) on the plot above.

A concurrent flow may be usedIf gas to be dissolved in the liquid is a pure substance where there is no advantage to countercurrent operation.If a rapid, irreversible chemical reaction with the dissolved solute occurs in the liquid where only the equivalent of one theoretical stage is required.21Rate of Absorption

Rate of absorption per unit volume of packed column is given by any of the following equations

a = interfacial area per unit volume of packed column or device22kXa, kYa, kya and kxa are based on a unit volume.

Kxa and kya are overall coefficients

kYa volumetric mass transfer coefficient, kg mol/m3h unit mole fraction for gas phase.23Overall Transfer Coefficients

The overall transfer coefficients may be obtained from kya and kxa using the following equations:

And 24where m is the local slope of the equilibrium curve.

are resistances to mass transfer in the gas film and liquid film respectively.25Solubility and Slope of Equilibrium Curve

When solubility is very high, slope of equilibrium curve m is very small.

When solubility is very low, m is very high.

26Design of Absorption Tower

1. Calculation of tower height

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Let S be the cross sectional area of the tower.

Then differential volume in height dz is Sdz

Assume change in water flow rate V is negligible, then the amount absorbed in dz is Vdyie282. Calculation of Number of Transfer Units (number of equilibrium stages or number of ideal stages)

From equation (16) the column height is given as:

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The integral represent the change in vapor concentration divided by the average driving force and is the number of transfer units (NTU) NOY.

30HOY is obtained from literature or from mass transfer correlations.

If operating line and equilibrium line (curve) are parallel and straight,

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For straight operating and equilibrium lines

For liquid phaseNOX NOY unless operating line and equilibrium line are straight and parallel.

33Assignment April 2014Example: A gas stream containing 3% A is passed through a packed column to remove 99% of the A by absorption in water. The absorber will operate at 25oC and I atm, and the gas and liquid rates are to be 20 mol/h.m2 and 100 mol/h.m2 respectively. Mass transfer coefficients and equilibrium data are given below:y* = 3.1x at 25oCkx= 60 mol/h.m3 unit mol fractionky = 15 mol/h.m3 unit mol fractioni). Find NOy, HOy and ZT, assuming isothermal operation and neglecting changes in gas and liquid flow rates. What percent of the total resistance is in the gas phase?Ii) Calculate ZT using NOx and HOx