Page 342 - Distillation theory

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316 Synthesis of Separation Flowsheets

D

M 1

a) b)
D st M 2
M 1
st M 3
M 2
st M 4
F F

st B st B

D 2
c)
D 1
st M 1

st
st M 2
F

st B 1 st B 2

D 1 D 2
d)

st st
M 1 M 3
st st
M 2 M 4
F
st st B 2
Figure 8.33. Vacuum unit: (a,b) in one column, (c,d) in two columns.

A two-column flowsheet is applied for crude with a large content of gases that are
separated in the first column (at pressure 3–5 atm), which keeps sufficiently low
pressure in the main column (1,3 ÷ 2,0 atm).
The second unit usually consists of one column with side strippings (Fig. 8.33b)
and serves to obtain vacuum gas-oil and tar (fuel flowsheet) or lubricant fractions
and tar (lubricant flowsheet).
In some (Fig. 8.33a) cases, steam is not used at separation for strippings (“dry”
separation) and deeper vacuum is applied (10 ÷ 20 mm Hg).
To increase the number of theoretical trays, vacuum unit is sometimes made as a
two-columnone(Fig.8.33c,d),butaccordingtoenergyexpendituresonseparation