Page 343 - Distillation theory

P1: JPJ/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c08 CB644-Petlyuk-v1 June 11, 2004 20:20

8.7 Petroleum Mixtures 317

D/F, %
28
27
26
25
24
23
22
21
20 <360°C, %
0 5 10 15 20
Figure 8.34. The yield of oil distillates D/F in vacuum column as
function of fraction below 360 C in reduced crude (West Siberian
◦
petroleum).
it is more profitable to increase the number of trays in a one-column flowsheet by
means of applying highly efficient contact devices with small hydraulic resistance
(of regular packs).

8.7.5. Modernization of Units for Petroleum Refining

Existing crude units can be reconstructed for a stronger recovery of the most
valuable products – light oil products (benzine, kerosene, diesel fuel) and lubricant
fractions. The above-listed methods of improving separability have to be used in
this reconstruction:

1. Part of diesel fuel has to be recovered not in an atmospheric unit but in a
vacuum one. This creates a double effect: (1) in the feeding of a vacuum
column the fraction of light components increases, which, due to their strip-
ping influence, increases recovery of lubricant fractions or gas-oil; and (2)
the volatilities of components grow under vacuum, which separates diesel
fuel from heavier products more sharply and deeply. Figure 8.34 shows
the calculation dependence of yield of lubricant fractions in “dry” vacuum
column on the fraction in its feeding of components with bubble tempera-
tures below 360 C. However, increase of vapor flow rate in vacuum column
◦
requires application of more productive contact devices.
2. If a crude unit contains only a two-column atmospheric unit, then in the
main column a moderate vacuum has to be kept (≈0,8 atm) with the help
of vacuum-creating equipment joined with c reflux drum.

Increase of the fraction of vapor phase in the feeding at moderate vacuum
increases sharpness of separation in the sections of the main column above feeding
because of sharp increase of reflux number (in a few times). Besides that, sharpness
of separation grows because of increase of relative volatilities of components at
lower pressure. This increases output of light oil products (< 360 C) at ensuring
◦
their quality (Fig. 8.31) and/or to decrease the flow rate of steam.