Multistage separation process

Figure 6-1 shows a multistage separation process. By removing molecules of the light components in the first separator they are not available to flash to gas from the liquid in the second separator, and the partial pressure of intermediate components in the second separator is higher than it would have been if the first separator did not exist. The second separator serves the same function of increasing the partial pressure of the intermediate components in the third separator and so forth. 

Smith and Brinkley developed a method for determining the distribution of components in multicomponent separation processes. Their method is based on the solution of the finite-difference equations that can be written for multistage separation processes, and can be used for extraction and absorption processes, as well as distillation. Only the equations for distillation will be given here. The derivation of the equations is given by Smith and Brinkley (1960) and Smith (1963). For any component i (suffix i omitted in the equation for clarity)... 

Equations 9.109 and 9.110 are valid for a boiling process in a closed system the gaseous phase develops from a liquid with initial concentration Cq, is the mass fraction of developed gas, and K is the mass distribution constant of the component of interest between gas and liquid. Equations 9.111 and 9.112 describe a multistage separation process in which n is the number of separation stages, AFg is the mass fraction of gas separated in each stage, saiAK is the mean mass distribution constant of the process. Equations 9.113 and 9.114 refer to a boiling process in an open system the gas is continuously removed from the system as the process advances. 

The description of any operation or design problem in a multistage separation process requires assigning numerical values to, or setting, a certain number of independent variables. The number of variables to be set depends on the process, and is usually determined easily by the method the authors have called the description rule (HI). Alternatively the number of variables to be set may be determined by writing all of the independent equations which define the process, then counting the number of variables and the number of equations. In order to solve the equations, a sufficient number of independent variables must be set so that the number of dependent variables remaining equals the number of equations. 

Relaxation solutions are conceptually the most simple methods of solution for any multistage separation process. They were first proposed by Rose et al. (R2) in 1958 and Duflin (Dl) in 1959. Both authors in... 

D.N. Hanson, J.H. Duffin and G.F Sommerville, Computation of Multistage Separation Processes, Reinhold, New York, 1962. 

We have no better starting point to offer. We will discuss some rearrangements of flow schemes which are possible with multistage separation processes, and the accompanying recompression and cooling of flash vapors, which serve to partially simulate the arrangement of a conventional fractionation process. Several aspects of thfese processes which influence their selectivity will be discussed. 

The process of separation is done in multiple stages. The crude xylenol fraction is rectified in a tall fractionation column into fractions of a narrow boiling temperature range and then further purified via crystallization and centrifuging. This multiple stage purification will be economically justified only if pure isomers have reasonable demand at an attractive price. Otherwise lower purity materials (say upto 90% purity) will be offered for sale. In any commercial plant demand in bulk volume and price will ultimately dictate the viability of such a multistage separation process. 

---