Stage calculations multistage processing

The simulation procedure is then as follows The starting value for the number of theoretical stages is taken from short-cut methods. Distribution coefficients are provided from external correlations. Then, concentrations for the individual (pseudo) components are calculated with algorithms known from multicomponent multistage processes (Figure 2.2-8). 

Although the most useful extraction process is with countercurrent flow in a multistage battery, other modes have some application. Calculations may be performed analytically or graphically. On flowsketches like those of Example 14.1 and elsewhere, a single box represents an extraction stage that may be made up of an individual mixer and separator. The performance of differential contactors such as packed or spray towers is commonly described as the height equivalent to a theoretical stage (HETS) in ft or m. 

In Chapter 1 it was shown that the number of independent variables for any problem is equal to the difference between the total number of variables and the number of linking equations and other relationships. Examples of the application of this formal procedure for determining the number of independent variables in separation process calculations are given by Gilliland and Reed (1942) and Kwauk (1956). For a multistage, multicomponent column, there will be a set of material and enthalpy balance equations and equilibrium relationships for each stage (the MESH equations) and for the reboiler and condenser, for each component. 

EXAMPLE 12.7-1. Material Balance for Countercurrent Stage Process Pure solvent isopropyl ether at the rate of, = 600 kg/h is being used to extract an aqueous solution of Lq = 200 kg/h containing 30 wt % acetic acid (/4) by countercurrent multistage extraction. The desired exit acetic acid concentration in the aqueous phase is 4%. Calculate the compositions and amounts of the ether extract and the aqueous raffinate L. Use equilibrium data from Appendix A.3. 

The calculation of multistage separation processes involves the solution of phase equilibrium relationships, mass balances, and energy balances. Energy balances require the computation of enthalpies of streams entering and leaving an equilibrium stage. The enthalpy is a function of state, defined in Section 1.1.2 as H = f/ + PV. It is a function of the stream composition, its temperature, and its pressure. 

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