Page 169 - Gas Purification 5E

Alkanolamines for Hvdrogen Surfide and Carbon Dioxide Removal 157

tact can be accomplished by the use of a single countercurrent packed bed or by the use of an
efficient cocurrent contactor. Cocurrent contactors used for LPG treating usually consist of
an eductor or in-line mixer followed by a phase separator.
Figure 2-96 depicts a typical LPG treating system. In this case the LPG feed is the con-
densed product of a refinery debutanizer column. The LPG is pumped on level control from the
debutanizer column overhead accumulator and flows to a countercurrent liquid-liquid contactor
which contains three beds of random packing. The LPG enters the bottom of the column, while
filtemi lean amine is fed to the top of the column at a controlled, constant flowrate.
The LPG feed is distributed evenly and formed into droplets by injection into the continu-
ous amine phase at the bottom of the column. The lean amine is distributed across the top of
the packing where it joins the continuous amine phase. The density difference between the
two phases causes the dispersed LPG to flow upward through the continuous amine phase.
The LPG/amine interface is maintained above the top bed and the amine distributor by an
interface level controller which controls the rate of discharge of rich amine from the bottom
of the contactor.
The treated LPG leaves the contactor and flows to a gravity settler or coalescer where
entrained droplets of amine are removed from the LPG. Figure 2-96 shows a gravity settler
which also includes a LPG water wash system. Washing the treated LPG with water
improves the recovery of entrained amine and also removes dissolved amine from the LPG.
As shown in Figure 2-96, makeup wash water is added on flow control; while the water-
amine purge stream is discharged on level control. A portion of the water-amine stream from
the settler is recycled to the LPG stream entering the water wash mixer. Usually the wash
water flow rate is about 25% of the LPG flow. The combined water-LPG stream flows
through a static mixer or other mixing device and then to the gravity settler. If required, the
LPG from the settler may be further processed in other treating units, such as an extractive
Merox unit to remove mercaptans. The operating pressure of the entire LPG treating system
is controlled by a pressure controller located downstream of the final LPG treating unit.
An alternate design, depicted in Figure 2-97, is often used when only one theoretical con-
tact stage is required. In this application, LPG and amine are contacted in either an eductor
or a static mixer and the intimate mixture of the two liquid phases is sent to a settling tank
where the two phases are separated. The rich amine is discharged from the settler on level
control and lean amine is added on flow control. A relatively large flow of rich amine is
recycled through the eductor to assure an adequate surface area for transfer of C02 from the
LPG to the amine. A single stage contactor system is commonly used for C02 removal from
natural gas plant liquids (NGL) when hydrogen sulfide is not present and very high efficien-
cy carbon dioxide removal is not required (Bacon, 1972; Perry, 1977, B; Honerkamp, 1975).

Design Data

Liquid-Liquid Equilib~uium Data for Hfi and CO,

The development of rigorous design methods for LPG treaters has been hampered by the
lack of liquid-liquid equilibrium (LLE) data for amines and sour hydrocarbon liquids. Early
designs were based on empirical rules of thumb and field experience with existing units
operating under similar conditions.

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