Page 122 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 107
Only light keys components and lighter go to the distillate F lights and heavy keys and
heavier F heavies go to the bottom. F lights and F heavies are introduced as the sum of
respectively all lights light and heavy feed components.

F lights = F A + F B + F C + F D + ´´´´ F LK

F heavies = F HK + ´´´´ F X +F Y + F Z

F = F lights + F heavies
Now, Eq. 4 can be rewritten as
.
R R
V= F lights + F min
a 1
Based on the vapor flow of each column, summation of the vapor flows of all columns
gives the overall vapor flow. This must be calculated for each sequence to enable selec-
tion of the most promising sequence. The above approach is not limited to simple
distillation ± complex distillations can also be evaluated in the same manner.

4.2.2.4 Simplification of separation
The most obvious candidate is distillation, since this is one of the most utilized sepa-
ration techniques and has high capital and operational costs. A practical example
may be taken of a process with a reactor and distillation train. The boiling range of
the components to be separated are shown in Figure 4.16. Note that there are two
types of lights, each with different boiling points. In this particular example the feed
to the reactor system contained lights which are inert for the reaction, but which are
included in the separation scheme.
The distillation train for this mixture of seven components for separation is
shown as a five-column separation flowsheet in Figure 4.17. A heterogeneous azeo-
tropic distillation is included in the last column to remove water. Based on this
scheme, different steps are shown in the idea generation, as presented in Fig-
ures 4.18±4.20.

Fig. 4.16. Boiling points of inlet and outlet streams of distillation example.