Page 305 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

11.4 Fractionator 307

Light key and heavy key components
All components may not be present in the top as well as the bottom product of multicomponent
distillation columns. An example of this is splitting of the debutaniser feed to produce LPG as the top
product. LPG is primarily a mixture of C 3 and C 4 hydrocarbons. The lightest component C 2 H 6 does not
condense in overhead condenser and is bled out from the reflux drum vapour space. Some small amount
of C 3 H 8 is lost along with C 2 H 6 leaving. This is inevitable but should be kept to a minimum. The feed
composition is shown in Table 11.6 with the components arranged in decreasing order of volatility.

Table 11.6 Splitting of a multicomponent feed.
Adjacent key case Split key case

Presence in Presence in
% mole in distillate Presence in distillate Presence in
Component feed stream bottom stream stream bottom stream
0.8 X X
C 2 H 6
6.61 X X
C 3 H 8
8.67 X X
iC 4 H 10
14.77 X(LK) X X(LK) X
nC 4 H 10
11.61 X X (HK) X X
iC 5 H 12
10.06 X X X (HK)
nC 5 H 12
C 6 20.84 X X
C 7 14.4 X X
C 8 8.77 X X
C 9 3.47 X X
Total 100

LK, Light key; HK, Heavy key.

The distillate liquid, i.e., LPG is a high valued product. It should accommodate as much of C 3 and
C 4 components as possible to have higher yield. This requires focussing on the split between the liquid
distillate (LPG) and the column bottom streams.
Maximum yield of LPG can be achieved with the ideal split having all of nC 4 H 10 and lighter
components in distillate and all of iC 5 H 12 and heavier components in the bottom product. In reality,
nC 4 H 10 and iC 5 H 12 will be partially separated and will appear in the top as well as the bottom product.
These components nC 4 H 10 and iC 5 H 12 are therefore termed as the light key (designated as LK) and the
heavy key (HK) components based on which the separation target is defined. The light and the heavy
key components are in decreasing order of volatility. The other components are termed non-keys. Non-
key components more volatile than the light key are termed light non-keys (i-C 4 H 10 and lighter) and
those less volatile than the heavy key are heavy non-keys (e.g., C 7 and heavier).
In the example case, the light and the heavy key components appear consecutively in the volatility
sequence list. This is a case of adjacent key components. When the light and the heavy keys are