Page 301 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 303

Table 11.5 Typical conservative values of h (%) for industrial columns.
0
Process Service Enrichment section Stripping section
Amine absorber DEA or MDEA e 50
Regenerator column for amine Sour gas e DEA e 10
absorption system
Sour gas e MEA or MDEA e 15
Sour water stripper WatereH 2 S e 25e35
Solvents Hexane/Heptane 75 80
Light hydrocarbon separation Demethanizer/Deethanizer 100 100
Ethylene fractionator 95 95
Depropanizer 90 75
Propyleneepropane 95 90
fractionator
Debutaniser/naphtha 90 75
stabiliser
Aromatic separation Benzene column 70 70
Toluene column 65 60
Xylene column 80 80
C8eC9 splitter 80 70
Others Ethanol e water 60 60
Isopropanol e water 70 70

transfer surface during complete vaporisation can lead to overheating of the tubes. Therefore total
reboil if at all resorted to must be for a very short period.
At minimum reflux condition the operating lines of the rectification and the stripping section meet
the feed line (q-line or f-line) on the equilibrium curve as shown in the McCabeeThiele construction in
Fig. 11.11. Accordingly, the column requires infinite number of trays to effect the required separation.
It is important to determine the minimum reflux ratio (R min ) since it forms a basis for establishing
the design R. This is done by using the ratio (R/R min ) as an independent variable for optimising the
design of a distillation system. Typically this ratio lies between 1.1 and 1.5.
Analytical expression for R min in case of a binary distillation with saturated liquid feed and the
relative volatility being a is given by

x D ½1 þða 1Þ x F Š a x F
. (11.7)
R min ¼
a x F x F ½1 þða 1Þ x F Š
This is derived from the geometrical properties of the McCabeeThiele diagram.