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156 Gas Purification

Fortunately, mercaptans do not appear to react with any of the amines to form nonregener-
able compounds. Rahman et al. (1989) looked for possible reactions between methyl mer-
captan and MEA, DEA, DGA, DIPA, and MDEA and found none.

Process Description

Amine-based liquefied petroleum gas (LPG) and gas treating processes are similar in that
both involve contacting a low density hydrocarbon phase (liquid or gas) with a heavier,
immiscible liquid phase (aqueous ahnolamine solution). Carbon dioxide, hydrogen sulfide,
and carbonyl sulfide in the hydrocarbon phase are transferred to the aqueous phase where
they react with the amine. Spent amine solution is removed from the contactor, regenerated,
and recycled. In gas treating units, the gas is usually (but not always) the continuous phase
because of the large volumes of gas involved and the limited gas flow capabilities of flooded
columns. In LPG treaters, however, the volumetric flowrate of the hydrocarbon phase is rela-
tively low, although usually larger than that of the amine, and designs with either phase con-
tinuous may be considered.
According to Tse and Santos (1993), the choice of amine as the continuous phase and
LPG as the dispersed phase offers two advantages:

1. The total surface area for mass transfer is maximized when the dispersed phase has the
larger flow rate.
2. Droplet residence times and mass transfer rates are increased when LPG is dispersed into
the higher viscosity continuous amine phase.

Using amine as the continuous phase also increases the LPG treater amine residence time
and provides more time for the operator to intervene if the interface level controller fails.
This minimizes the chance of hydrocarbon breakthrough causing a major sulfur plant upset.
The advantage of using LPG as the continuous phase is that the LPG Amine Treater
capacity (gpm of LPG/fP of contactor area) is substantially higher when LPG is the continu-
ous phase. Therefore, substantial capacity increases can often be achieved by a revamp
which makes LPG the continuous phase. However, when LPG is the continuous phase, the
height of packing equivalent to a theoretical stage is greater because the mass transfer area is
less and the droplet residence time is decreased as previously noted. The vast majority of
LPG treaters have amine as the continuous phase. And, unless otherwise noted, this is
assumed in the following discussion.
LPG contactors are typically countercurrent columns containing 1.5 or 2.0411. stainless
steel or ceramic random packing. As the LPG flows upward through the bed, the droplets
coalesce and mass transfer efficiency decreases. The maximum effective bed height is gener-
ally thought to be about 12 ft (Strigle, 1994), and bed heights in the range of 8 to 12 ft are
typically used. When more than 12 ft of packing are required, it is necessary to use multiple
beds and to collect and redisperse the LPG entering each bed. Specially designed
disperserlsupport plates are used beneath each bed to collect and redisperse the LPG and pro-
vide support for the packing.
Most LPG treating applications require no more than three beds of packing. In some cases,
e.g., removal of carbon dioxide alone, a single bed of packing is adequate. Single stage con-