Chemical Engineering November 2010 - 7

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Comparing Physical Solvents for Acid Gas Removal
PROCESS INSIGHT
Physical solvents such as DEPG, NMP, Methanol, and Propylene Carbonate
are often used to treat sour gas. These physical solvents differ from chemical
solvents such as ethanolamines and hot potassium carbonate in a number of
ways. The regeneration of chemical solvents is achieved by the application
of heat whereas physical solvents can often be stripped of impurities by
simply reducing the pressure. Physical solvents tend to be favored over
chemical solvents when the concentration of acid gases or other impurities
is very high and the operating pressure is high. Unlike chemical solvents,
physical solvents are non-corrosive, requiring only carbon steel construction.
A physical solvent's capacity for absorbing acid gases increases signifi cantly
as the temperature decreases, resulting in reduced circulation rate and
associated operating costs.
Typical Physical Solvent Process
PC (Propylene Carbonate)
The Fluor Solvent process uses JEFFSOL®
PC and is by Fluor
Daniel, Inc. The light hydrocarbons in natural gas and hydrogen in synthesis
gas are less soluble in PC than in the other solvents. PC cannot be used for
selective H2
S treating because it is unstable at the high temperature required
to completely strip H2S from the rich solvent. The FLUOR Solvent process
is generally limited to treating feed gases containing less than 20 ppmv;
however, improved stripping with medium pressure fl ash gas in a vacuum
stripper allows treatment to 4 ppmv for gases containing up to 200 ppmv H2
S.
The operating temperature for PC is limited to a minimum of 0°F (-18°C) and
a maximum of 149°F (65°C).
Gas Solubilities in Physical Solvents
All of these physical solvents are more selective for acid gas than
for the main constituent of the gas. Relative solubilities of some selected
gases in solvents relative to carbon dioxide are presented in the following
table.
The solubility of hydrocarbons in physical solvents increases with
the molecular weight of the hydrocarbon. Since heavy hydrocarbons tend
to accumulate in the solvent, physical solvent processes are generally not
economical for the treatment of hydrocarbon streams that contain a substantial
amount of pentane-plus unless a stripping column with a reboiler is used.
Gas Component
DEPG (Dimethyl Ether of Polyethylene Glycol)
DEPG is a mixture of dimethyl ethers of polyethylene glycol.
Solvents containing DEPG are marketed by several companies including
Coastal Chemical Company (as Coastal AGR®
UOP (Selexol). DEPG can be used for selective H2
confi gured to yield both a rich H2
), Dow (Selexol™), and
S removal and can be
S feed to the Claus unit as well as bulk CO2
removal. DEPG is suitable for operation at temperatures up to 347°F (175°C).
The minimum operating temperature is usually 0°F (-18°C).
MeOH (Methanol)
The most common Methanol processes for acid gas removal are
process (by Prosernat). The
the Rectisol process (by Lurgi AG) and Ifpexol®
main application for the Rectisol process is purifi cation of synthesis gases
derived from the gasifi cation of heavy oil and coal rather than natural gas
treating applications. The two-stage Ifpexol process can be used for natural
gas applications. Methanol has a relatively high vapor pressure at normal
process conditions, so deep refrigeration or special recovery methods
are required to prevent high solvent losses. The process usually operates
between -40°F and -80°F (-40°C and -62°C).
NMP (N-Methyl-2-Pyrrolidone)
The Purisol Process uses NMP®
and is marketed by Lurgi AG.
The fl ow schemes used for this solvent are similar to those for DEPG. The
process can be operated either at ambient temperature or with refrigeration
down to about 5°F (-15°C). The Purisol process is particularly well suited
to the purifi cation of high-pressure, high CO2
synthesis gas for gas turbine
integrated gasifi cation combined cycle (IGCC) systems because of the high
selectivity for H2
S.
Bryan Research & Engineering, Inc.
P.O. Box 4747 * Bryan, Texas USA * 77805
979-776-5220 * www.bre.com * sales@bre.com
Circle 8 on p. 58 or go to adlinks.che.com/29256-08
H2
Methane
Ethane
CO2
Propane
n-Butane
COS
S
H2
n-Hexane
Methyl Mercaptan
DEPG
at 25°C
0.013
0.066
0.42
1.0
1.01
2.37
2.30
8.82
11.0
22.4
PC
at 25°C
0.0078
0.038
0.17
1.0
0.51
1.75
1.88
3.29
13.5
27.2
NMP
at 25°C
0.0064
0.072
0.38
1.0
1.07
3.48
2.72
10.2
42.7
34.0
MeOH
at -25°C
0.0054
0.051
0.42
1.0
2.35
-
3.92
7.06
-
-
Choosing the Best Alternative
A detailed analysis must be performed to determine the most
economical choice of solvent based on the product requirements. Feed gas
composition, minor components present, and limitations of the individual
physical solvent processes are all important factors in the selection process.
Engineers can easily investigate the available alternatives using a verifi ed
process simulator such as ProMax®
which has been verifi ed with plant
operating data.
For additional information about this topic, view the technical
article " A Comparison of Physical Solvents for Acid Gas Removal " at
http://www.bre.com/tabid/147/Default.aspx. For more information about
ProMax, contact Bryan Research & Engineering or visit www.bre.com.
http://www.bre.com/tabid/147/Default.aspx http://www.bre.com http://www.bre.com http://adlinks.che.com/29256-08
Chemical Engineering November 2010

Table of Contents for the Digital Edition of Chemical Engineering November 2010
