Page 366 - Petroleum and Gas Field Processing

Solution

Let us calculate the value of S by applying formula (2). S is found to be 5;
the five schemes are shown in Figure 9. Now, in order to select the best
scheme, the total load of all sequences have to be calculated:

Total load of a sequence ¼ sum of individual loads of all
components undergoing separation
¼ sum of L i

Because L i ¼ D i n, where n is defined as the number of columns or separators
component i, goes through before final separation,

Total load of a sequence ¼ Sum of D i n ð3Þ

Calculations are carried out as shown in Figure 9. It can be concluded that
sequence 3 has less load (8D).

Example 3

This example is a direct industrial application for some of the rules
or heuristics presented in the course of separation of NGL constituents. It
deals with ethylene and propylene manufacture. Figure 10 describes a
sequence of distillation separation processes for a gas mixture produced by a
catalytic cracking plant of natural gas. The plant built by Sinclair produces
500 million lbs/year of ethylene. Ethylene and propylene are the valuable
products formed by catalytic cracking of the hydrocarbons in the natural gas.
The following observations are cited in accordance with the heuristics stated:

1. Difficult separation is last: Because of the close boiling points of
propane and propylene, separation between them is kept the very
last (splitter). The next most difficult separation is between
ethane and ethylene. Again, it is kept to a last position (splitter).
2. Favor overhead removal in plentiful quantities: In the first
distillation column, the demethanizer separates the volatile com-
ponents hydrogen and methane (18% and 15%, respectively).
3. Remove valuable products as distillates: Both ethylene and propyl-
ene are separated as top products. This ensures that the materials
do not experience discoloration and separate in pure form.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.