Page 284 - Petroleum and Gas Field Processing

and CO 2 as follows:
2ðRNH 2 Þþ H 2 S ,ðRNH 3 Þ S ð4Þ
2
ðRNH 3 Þ S þ H 2 S , 2ðRNH 3 ÞHS ð5Þ
2
2ðRNH 2 Þþ CO 2 , RNHCOONH 3 R ð6Þ
These reactions are reversible and are forward in the absorber (at low
temperature) and backward in the stripper (at high temperature).
Monoethanolamine will react with H 2 SandCO 2 nonselectively.
Unfortunately, it will react irreversibly with carbonyl sulfide and carbon
disulfide, which can result in a loss of solution and a buildup of solid
products in the circulating solution. These products could also be a source
of corrosion.
The MEA process is usually using a solution of 15–20% MEA
(wt%) in water. Loading is about 0.3–0.4 mol of acid removed per mole of
MEA. The circulation rate is between 2 and 3 mol of MEA per mole of
H 2 S, as expected from reactions (4) and (5). However, commercial plants
use a ratio of 3 to avoid excessive corrosion. Solution strength and loading
above this limits result in excessive corrosion. MEA forms foam easily due
to the presence of contaminants in the liquid phases; this foam results in
carryover from the absorber. These contaminants could be condensed
hydrocarbons, degradation products, iron sulfide, as well as corrosion
products and excess inhibitors. Solids can be removed by using a filter;
hydrocarbons could be flashed; degradation products are removed using a
reclaimer. The number of trays used in absorbers in commercial units is
between 20 and 25 trays. However, the theoretical number of trays
calculated from published equilibrium data is about three to four. If we
assume an efficiency of 35% for each tray, then the actual number of trays
is 12. It has been reported that the first 10 trays pick up all of the H 2 S and
at least another 10 trays are of not much value. Thus, it is suggested to use
15 trays. It is recommended that MEA be used if the feed does not contain
COS or CS 2 , which form stable products and deplete the amine. If the feed
has these compounds, a reclaimer must be used, where a strong base like
NaOH is used to regenerate and liberate the amine. This base has to be
neutralized later.

Diethanolamine Process
Diethanolamine (DEA) is replacing MEA and becoming the most widely
used gas sweetening solvent. It is a secondary amine with lower reactivity
and corrosivity than MEA. Moreover, it reacts with COS and CS 2 and the
product can be regenerated. It has a lower vapor pressure (lower losses)

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