Absoiption column

While ethyl chloride is one of the least toxic of all chlorinated hydrocarbons, CE is a toxic pollutant. The off-gas from the reactor is scrubbed with water in two absoiption columns. The first column is intended to recover the majority of unreacted ethanol, hydrogen chloride, and CE. The second scrubber purifies the product fiom traces of unreacted materials and acts as a back-up column in case the first scrubber is out of operation. Each scrubber contains two sieve plates and has an overall column efficiency of 65% (i.e., NTP = 1.3). Following the scrubber, ethyl chloride is finished and sold. The aqueous streams leaving the scrubbers are mixed and recycled to the reactor. A fraction of the CE recycled to the reactor is reduced to ethyl chloride. This side reaction will be called the reduction reaction. The rate of CE depletion in the reactor due to this reaction can be approximated by the following pseudo first order expression ... 

The required column diameter and pressure drop in a packed absoiption column can be estimated quickly with the use of Fig. 15-7 or obtained quite accurately with one of several appropriate compute simulation programs. Both approaches will be used in the solution of this problem. 

The total mass of the system is m, n is the total mass flux (mass flow per unit area) relative to the stem boundaiy at any point, and S is the cross-sectional area nonnal to flow at that same location. The summations extend over all the mass entry and exit locations in the system. The mass flux at any poim is equal to pv, where p is the mass density and o is the velocity relative to the boundary at that point. Equation (2.2-1) can be applied equally well to a countercuirent gas absoiption column or to a lake with input and output streams such as rain l, evaporation, streams flowing to or from the lake, deposition of sediment on the lake bottom, or dissolution of minerals fiom the sides and bottom of the lake. The steady-state version of Eq. (2.2-1) ( 0) is of use in chemical process analysis because it permits calculation of various flow rates once some have been specified. 

Effective countercurrent contacting of two phases and good heat transfer are often difficult to combine. There are examples of gas/liquid absoiption columns with internal cooling (such as in nitric acid and sodium carbonate processes). In other cases one might opt for cross flow instead, and use a cascade of gasAiquid contactors. 

The spreadsheet below shows some of the data. You will need to use all the data. Column A contains [mesitylene] and column B contains [I2]tot. Column C lists the measured absorbance. Guess a value of the molar absoiptivity of the complex, e, in cell A7. Then compute the concentration of the complex (= Ale) in column D. The equilibrium constant in column E is given by E2 = [complex]/ ([I2J [mesitylene]) = (D2)/((B2-D2) A2). 

To determine the required size of an absorption or stripping nrtl, it is necessary to know not only the equilibrium soluhility of the solute in the solvent and the material balance atound the column bas also the rate at which solute is transferred from one phase to the other within the tower. This rale directly affects the volume of packing needed in a packed tower, the degree of dispersion requited in a spray contactor, and (somewhat less directly) the number of trays required in a nay tower. The last effect occurs as a result of the influence of mass transfer rms on tray efficiency which is discussed in a later section. Because of its direct effect ou packed tower design and the importance of this type of contactor in absoiption. this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theoty is given in Chapter 2. 

A very approximate estimate or the overall efficiency of babble-cap columns used for absoiption can be obtained from Fig. 6.4-13. This simple cormlation was proposed by O Connell.1 1... 

The raw gas consists of an inert gas G, and of the absorptives. On the way up through the column, the absorptives are removed from the inert gas and, in turn, the amount of the gas phase decreases. Analogously, the amount of the liquid phase is increased during the absorption process. Therefore, the ratio of liquid to gas L/G changes and, in turn, the operating line becomes curved. This problem can be solved by using a concentration where the amounts of absoiptives are referred to the amount of pure inert gas and pure solvent, respectively ... 

The processes of absoiption and distillation are similar in a high extent. In principle, the same processes are effected within the column. A countercurrent flow of gas and liquid is superimposed by a mass transfer between the phases. Both phases are saturated, i.e., the gas is at its dew point and the hquid at its boiling point. ... 

Spray contactors ate paiticularly important for the absorption of impurities from large volumes of flue gas where low pressure drop is of key importance. They are used where materials in die liquid phase (e.g., particles of limestone) or in the gas phase (e.g., droplets of tar) may cause plugging of packing or trays. Other important applications of spray contactors (which are outside the scope of this discussion) include particulate removal and hot gas quenching. When used for absoiption, spn devices are not applicable to difficult separations and generally are limited to about four transfer units even with countercurrent spray column designs. The low efficiency of spray columns is believed to be due to entrainment of droplets in the gas and backmixing of the gas induced by the sprays. 

Gas- and liqirid-tide mass transfer coeflicients in packed absoiption ccdumns exhibit complex dependencies on the gas and liquid rates aixl the column packing, transfler area will be a liuiction of the hydrodynamics and packing. Correlations on eqieiimenial data, are usually developed in terms of the height of a transfer and liquid hmghts of transfer units are defined 1 die reiatioiiships. with countercurrent flows In addition, the interfiKial for such situations, based unit (HTU) concqjt. Gas... 

The Kiemser equation is useful in the pieliminaiy design of plate columns fw fdiydcal absoiption processes, such as the ddiydiation of natural gas witii glycol solutions (see Chapter 11) and the absoiption of CO2 and H2S in nonreactive solvents (see Chapter 14). 

The effect of hindered amine promoters on the rate of CO2 absorption into a potassium carbonate solution has been evaluated using a single sphere absorber (Sartori and Savage, 1983) and a pilot scale packed column (Weinberg et al. 1983). The single sphere absorber tests showed that unpromoted potassium carbonate provides a mass transfer enhancement in the range of 10-30 times that of simple physical absoiption. The addition of a conventional amine activator (DEA) further increases the mass transfer rate by a factor of about 3, and the substitution of a hindered amine for the DEA gives an additional increase by a factor of about 2. 

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