Page 358 - Petroleum and Gas Field Processing

From the above data, it is evident that in order to separate a mixture of
NGL, say propane plus, as a bottom product, then there must be a
difference in boiling point between the top product (ethane) and the
bottom product. This difference is an indication of the degree of difficulty
of separation or the value of SF. As an equilibration separation process,
SF should be much greater than unity for ethane to concentrate in the top
and propane plus to concentrate in the bottom.
A difficult separation implies the following:
1. Higher number of distillation trays, which affects column size
2. Higher reflux ratio, which influences pump size and power
consumption
3. Additional reboiler heat duty, which influences reboiler size and
energy consumption

13.3.3 Distil ation Processes and Types of Fractionators
The separation of NGL may require different modes of distillation as well
as other methods of separation techniques. A summary-up of different
distillation processes and other separation methods is shown in Figures 6
and 7 respectively.
Fractionators of different types are commonly used in gas plants:

Type of fractionator Feed Top product Bottom product
Demethanizer C 1 /C 2 Methane Ethane
Deethanizer LPG Ethane Propane plus
Depropanizer Deethanizer Propane Butanes plus
bottoms
Debutanizer Depropanizer Butanes Natural gasoline
bottoms (iso þ n) (pentanes plus)
Deisobutanizer Debutanizer Isobutane Normal butane
top

Control of the following key operating variables will ensure efficient
results of fractionation operations [2]:
1. Top tower temperature, which sets the amount of the heavy
hydrocarbons in the top product. This is controlled by the reflux
ratio. Increasing the reflux rate will decrease this amount. The
reader should observe that reflux liquid is produced as a result of
overhead condensation of vapors. For columns using total
condensers, such as depropanizers and debutanizers, all vapors

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.