Page 340 - Distillation theory

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314 Synthesis of Separation Flowsheets

D/F, %
52
p
1
51
50
p
49 2
p
48 3
47
46 N
2 3 4 5 6 7 8
Figure 8.31. The yield of light oil products (<360 C) D/F as
◦
function of a pressure p in crude oil distillation column and
number of trays N in the first section above feed cross-section
(p 1 , 0.8 bar; p 2 , 1.6 bar; p 3 , 2.3 bar, West Siberian petroleum).

Because petroleum is a mixture with a wide interval of bubble temperatures
of components and the required purity of products is not very great, reflux and
vapor numbers in the sections are not large. Therefore, the heat brought in is
used up mostly not for creation of vapor reflux, but for evaporation of those
products that are withdrawn above the feed cross-section. Therefore, the split
(direct, indirect, intermediate) has but a weak influence over energy expenditures,
but it is very important to exclude multiple evaporation and condensation like in
multicolumn sequences of simple columns. Columns with side strippings exclude
multiple evaporation and condensation.
Replacement of multicolumn sequences with multisection columns also leads
to a decrease in capital expenditures because, due to the wide interval of bubble
temperatures, there can be but a small number of trays in each section, and it is
profitable to unite these sections into one shell.
Among multisection distillation complexes, only columns with side strippings
bring practically the whole heat into the feeding and bring live steam into the bot-
tom. Application of pumparounds decreases energy expenditures and recuperates
withdrawn heat for heating of petroleum before separation.
At the same time, the application of pumparounds decreases vapor flow rate in
the top sections of the column (i.e., to decrease the diameter and capital expendi-
tures).
Therefore, column with side strippings, with live steam into the bottom, and
with pumparounds is the best distillation complex for petroleum refining. An
optimum way of designing such column is discussed in Section 7.5.2.
8.7.4. Main Succession of Petroleum Refining

Large numbers of products are obtained from petroleum: liquefied gases (pro-
pane + butane), benzine and its fractions, kerosene and jet fuel, diesel oil − light
one (winter) and heavy one (summer), mazut, gas-oil, lubricant fractions, and tar.
Only part of these products is obtained in each unit. Nevertheless, it is not
possible to obtain all the products in one column with side strippings because of