Cascading flash separators

EXAMPLE 3.1 PROPANE-BUTANE CASCADED FLASH SEPARATION... 

Edmister applied the group method to complex separators where cascades are coupled to condensers, reboilers, and/or other cascades. Some of the possible combinations, as shown in Fig. 12.24, are fractionators (distillation columns), reboiled strippers, reboiled absorbers, and refluxed inert gas strippers. In Fig. 12.24, five separation zones are delineated (1) partial condensation, (2) absorption cascade, (3) feed stage flash, (4) stripping cascade, and (5) partial reboiling. 

One difficulty with the cascade shown in Figure 3-1 is that the intermediate product streams, L, L2, V4, and V5, are of intermediate concentration and need further separation. Of course, each of these streams could be fed to another flash cascade, but then the intermediate products from those cascades would have to be sent to additional cascades, and so forth. A much cleverer solution is to use the intermediate product streams as additional feeds within the same cascade. 

Catalyst mixing vessel 2 Distillation column 3 Water separator 4 Reactor cascade 5 Flash column 6 Crystallizer 7 Decomposer 8 and 9 Vacuum columns... 

Flash vaporization and multistage distillation calculation methods have been shown to be adaptable to membrane separations, which indicates the degree of separation that can be achieved. Furthermore, the use of multistage or cascade operations enhances the sharpness of separation and can be used to separate components with low relative selectivity. 

Techniques used in steady-state flash vaporization calculations and multistage distillation calculations can be utilized to show that membrane separations are enhanced by the use of cascade or multistage operations. This is of importance particularly in the use of membrane materials showing low selectivity between the components to be separated. 

---