Page 419 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Two examples are given:
1. The key components for the main separation are identified as benzene and toluene. The
composition of the top product is specified to be 99.6 mole% benzene, and the recovery of
toluene in the bottoms product is 0.98.
2. The top composition is specified to be 99.6 mol% benzene, and that of the recovery of benzene
in the bottoms product is 0.01.

The first specification violates the material balance, whereas the second specification does not. Looking
at the first specification, if 98% of the toluene in the feed is recovered in the bottoms product, then 2% or
0.7 kmol/h must leave with the top product. Even if the recovery of benzene in the top product were
100%, this would yield a top composition of 106.3 kmol/h benzene and 0.7 kmol/h toluene. This
corresponds to a mole fraction of 0.993. Therefore, the desired mole fraction of 0.996 can never be
reached. Thus, by specifying the recovery of toluene in the bottoms product, the specification for the
benzene purity is automatically violated.

The second specification shows that both specifications can be achieved without violating the material
balance. The top product contains 99% of the feed benzene (105.2 kmol/h) and 0.4 kmol/h toluene, which

gives a top composition of 99.6 mol% benzene. The bottoms product contains 1.0% of the feed benzene
(1.1 kmol/h) and 34.6 kmol/h of toluene.

When giving the top and bottom specifications for a distillation column, make sure that the

specifications do not violate the material balance.

If problems continue to exist, one way to ensure that the simulation will run is to specify the top reflux
rate and the boil-up rate (reboiler duty). Although this strategy will not guarantee the desired purities, it
will allow a base case to be established. With subsequent manipulation of the reflux and boil-up rates, the
desired purities can be obtained.

Absorbers and Strippers. Usually these units are simulated using the rigorous distillation module given
above. The main differences in simulating these types of equipment are that condensers and reboilers are
not normally used. In addition, there are two feeds to the unit; one feed enters at the top, and the other at
the bottom.

Liquid-Liquid Extractors. A rigorous tray-by-tray module is used to simulate this multistaged
equipment. It is imperative that the thermodynamic model for this unit be capable of predicting the
presence of two liquid phases, each with appropriate liquid-phase activity coefficients.

13.2.6 Selection of Output Display Options

Several options will be available to display the results of a simulation. Often, a report file can be
generated and customized to include a wide variety of stream and equipment information. In addition, a

simulation flowsheet (not a PFD), T-Q diagrams for heat exchangers, vapor and liquid flows, temperature
and composition profiles (tray-by-tray) for multistaged equipment, scheduling charts for batch operations,
environmental parameters for exit streams, and a wide variety of phase diagrams for streams can be
generated. The user manual should be consulted for the specific options available for the simulator you