Design Of Absorber With Reaction

Agenda  Introduction.  Physical and chemical absorption.  Concentration profile for absorption with chemical reaction.  Application.  Example.

Introduction  In gas absorption a soluble vapor is absorbed from its mixture with an inert gas by mean of liquid in which the solute gas is more or less soluble.

Absorber gas equipment: Mainly use three type of tower: 1-Packed tower. 2-Try towers. 3-Spry tower. Usually carried out in packed towers where the gas stream enters the bottom of the column and pass upward through a wetted packed bed, the liquid enters the top of the column and distributed over the column packing.

 Packed towers are used extensively in the Chemical Industry for Mass Transfer operations using gas absorption, whereby a solute is transferred between a gas and a liquid phase.  The liquid and gas are contacted, based on the solubility of the gas,can be absorbed into the liquid.  Successful design demands satisfactory performance, with regards to both fluid dynamic and mass transfer considerations.

Physical absorption The gas component being absorbed is simply dissolved in the liquid absorbent. Example:  CO 2 dissolves into Water.  Propylene Carbonate.  Methanol.

Chemical Absorption There is a chemical reaction between the gas component being absorbed and a component in the liquid to form a compound. Example:  Hot Potassium Carbonate Solution.  Aqueous Alkanol amines.

Absorber With Chemical Reaction: Many industrial absorption processes are accompanied by chemical reaction. Reaction in the liquid of the absorbed component with a regent in the absorbing liquid is especially common.

Concentration profile for absorption with chemical reaction Figure 1 Where: U shown the plane of the interface between gas and liquid. R shown the reaction zone. S shown the outer boundary of liquid film.

The processes of diffusion and chemical reaction be represent by an extension of the film theory. The chemical reaction is irreversible where A is absorbed from mixture by a substance B. A combine with B according to equation A+B→AB As the gas approaches the liquid interface,it dissolves and reacts at once with B. The new product AB,thus formed, diffuses towards the main body of the liquid.

The component (A) diffuses through the gas film as a result of the driving force (P AG -P Ai ) and diffuses to the reaction zone as a result of the driving force C Ai in the liquid phase. The component (B) diffuses from the main body of the liquid to the reaction zone under a driving force q. The non – volatile product AB diffuses back to the main bulk of the liquid under a driving force (m-n).

The different between a physical absorption and one in which a chemical reaction occurs can shown in figure a and b: Where Figure (a) : shown Normal concentration profile. Figure (b) : shown Concentration profile by the chemical reaction.

For transfer in the gas phase N A =k G (P AG -P Ai ) For transfer in liquid phase N A = k L (C Ai -C AL ) Where: N A : is the overall rate of mass transfer. C : is the molar concentrations. P : is the partial pressure. k G : is gas film transfer coefficient. k L :is liquid film transfer coefficient.

 The effect of chemical reaction is to accelerate the removal of A from the interface, and supposing that it is now r times as great then we get: N A = r k L (C Ai -C AL )  As we can see in figure a the concentration film through the liquid film of thickness Z L is represent by straight line such that k L =D L /Z L  But in figure b component A is removed by chemical reaction,so that the concentration profile is curved.  The dotted line gives the concentration profile if, for the same rate of absorption,A were removed only by diffusion.

Thus N A = r k L (C Ai -C AL )= rD L /Z L (C Ai -C AL ) Factor r related to C Ai, D L, k L to the concentration of B in the bulk liquid C BL and to the second- order reaction rate constant k 2 for the absorption of CO 2 in alkaline solutions. r= (k 2 D L C BL ) 1/2 / k L

 A major application of absorption technology is the removal of CO 2 and H 2 S from natural gas or synthesis gas by absorption in solutions of amines or alkaline salts.  Another example is the washing of ammonia from a mixture of ammonia and air by means of liquid water. Application

 In the absorption of carbon dioxide by caustic soda,the carbon dioxide reacts directly with the caustic soda and the process of mass transfer is thus made much more complicated.  But when Carbon dioxide is absorbed in an ethanolamine solution there is direct chemical reaction between the amine and the gas.

Advantage 1) Enhancing the absorption rate. 2) Increasing carrying capacities for gas components.

Disadvantage Chemical reaction causes difficulty for the release of the gas components from liquid.

Example For removal of carbon dioxide from a gas mixture given that: Removal is frequently accomplished by scrubbing with aqueous solutions of PH 8 to 10 containing compound like monoethanol amine(NH2CH2CH2OH). CO 2 +H 2 O ↔ H 2 CO 3 K=10 3 H 2 CO 3 ↔ H + + HCO 3 - K= mol/liter HCO 3 ↔ H + + CO 3 2- K= mol/liter RNH 2 + H + ↔ RNH 3 + K= mol/liter RNH 2 + HCO 3 - ↔ RNH CO 3 2- K= mol/liter RNH 2 + HCO 3 - ↔ RNHCOO - + H 2 O K=50 mol/liter Where: RNH 2 :represents the monoethanol amine. K : are equilibrium constant.

Solution: First we need to know in which forms the carbon dioxide occur. Second approximate the kinetics involved in that reaction. The overall reaction: CO 2 + 2RNH 2 ↔ RNH RNHCOO - We expect the reaction to be instantaneous described by k/k o = 1+ D 2 c 2 / Ѵ D 1 c 1i Where 1 and 2 refer to carbon dioxide and amine at the boundary of interfacial reaction. We expect c 1i to be fixed and C 2 = Ĉ(1-2θ) Where Ĉ total amine concentration and θ is the fraction of the amine already combined with carbon dioxide, the factor of 2 is stoichiometric.

Thus k/ko = 1 + D2Ĉ2/2D1c1i (1-2θ) This prediction is verified for industrial absorption tower for which k/ko = liter/mole Ĉ2 (1-2θ) Where k used for design.