Theoretical stages absorption

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

In concentrated wstems the change in gas aud liquid flow rates within the tower and the heat effects accompanying the absorption of all the components must be considered. A trial-aud-error calculation from one theoretical stage to the next usually is required if accurate results are to be obtained, aud in such cases calculation procedures similar to those described in Sec. 13 normally are employed. A computer procedure for multicomponent adiabatic absorber design has been described by Feiutnch aud Treybal [Jnd. Eng. Chem. Process Des. Dev., 17, 505 (1978)]. Also see Holland, Fundamentals and Modeling of Separation Processes, Prentice Hall, Englewood Cliffs, N.J., 1975. 

The actual stage can be a mixing vessel, as in a mixer-settler used for solvent extraction applications, or a plate of a distillation or gas absorption column. In order to allow for non-ideal conditions in which the compositions of the two exit streams do not achieve full equilibrium, an actual number of stages can be related to the number of theoretical stages, via the use of a stage-efficiency factor. 

Sieve plates are used, similar to those used for distillation and absorption. The stage efficiency for sieve plates, expressed in terms the height of an equivalent theoretical stage (HETS), will, typically, range from 1 to 2.5 m. 

Figure 14-8 illustrates the graphical method for a three theoretical stage system. Note that in gas absorption the operating line is above the equilibrium curve, whereas in distillation this does not happen. In gas stripping, the operating line will be below the equilibrium curve. 

Table 10.9 presents the main results. The absorption unit C-l working under pressure ensures high recovery of 99.9% vinyl acetate and minimum 98% acetic acid with 20 theoretical stages. Furthermore, the components are sharply sepa-... 

Dodecane (NC ) is the solvent or lean oil used to recover propane and heavier components from a rich gas mixture by absorption. The initial conditions of both feed streams are fixed at 38°C and 2760 kPa. Under these conditions the lean oil is a subcooled liquid, and the rich gas is a superheated vapor. The column has ten theoretical stages and is maintained at 2760 kPa. 

A hydrocarbon gas stream contains some heavy components which must be removed by absorption. The absorber has six theoretical stages and operates at 2750 kPa. The overhead product should contain 0.01 kmol/h nCg. Using the modular column section method, calculate the required absorbent flow rate and the product streams flow rates and compositions. The feed streams are deflned below, along with the component relative volatilities which are referred to pentane and assumed constant. The A -valuc of pentane at the column pressure of 2750 kPa is given by... 

A column is to be designed with a number of trays equivalent to 8 theoretical stages for an absorption operation to remove at least 60% of the butane in a hydrocarbon stream. 

The initial solvent and feed concentrahons, the desired hnal concentrations, and equilibrium behavior determine the direction of mass transfer, the minimum solvent-to-feed ratio, and the minimum theoretical tray requirements. These theoretical trays (or stages) are analogous in many respects to theoretical plates of a distillation colnmn, and absorption (or stripping) columns discussed by Fair in another chapter. For any particnlar solvent-to-feed ratio, equilibrium relationships and the operating line determine theoretical stage reqnirements. 

Equation (13.8) can be used for multiple solute cases, since it can accommodate absorption factors Ai, Aj, A, . .., each giving a different value of E. The liquid/gas ratio does not change, since it is based on overall flows. Thus, Equation (13.9) can give different values of N (theoretical stages). It is here that the key component enters into the picture. The required stages for that component are calculated, and then the same value of N is substituted back in Equation (13.8) to determine the relative absorption effectiveness of the other components. 

This is the general equation for the number of theoretical stages needed in exchange columns. It is a form of the Kremser [K5, S5] equation, derived originally for gas absorption. 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

---