Upgrading Natural Gas via Membrane Separation Processes Page: 5 of 51
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Consequently, a membrane process that could economically reduce the concentrations of
these "acid" gases to pipeline specifications will require polymer membranes that exhibit
both high H2S/CH4 and CO2/CH4 selectivities. However, neither the cellulose acetate
membranes nor the newly developed polyimide membranes exhibit a sufficiently high
H2S/CH4 selectivity for this application. Recent studies have shown that certain
polyurethane block polymers are very promising membrane materials for the removal of
H2S from natural gas. Thus, the H2S/CH4 selectivity of a poly(ether urethane urea)
membrane synthesized at Syracuse University and designated hereafter as PU4 is about 4
times higher than that of cellulose acetate membranes [at 95F (35"C) and 147 psia (10.14
bars)] [13]. Even more impressive is the fact that the permeability of the PU4 membranes
to H2S is 93 times higher than that of cellulose acetate membranes under the same
conditions. However, the CO2/CH4 selectivity of PU4 membranes is relatively low.
The above results suggest that it may be possible to develop membraneprocesses
for the upgrading of low-quality natural gas by utilizing both highly C02-selective
membranes, such as the new polyimide membranes mentioned above, and highly H2S-
selective membranes, such as the newly-developed PU4 membranes. Comprehensive
membrane process simulations and economic evaluations have been completed in order
to identify the process configurations that will most effectively combine the two types of
membranes mentioned above. It is also important to determine if the pipeline
specifications for H2S and CO2 can be met by means of a simpler membrane process
utilizing only the highly H2S-selective PU4 membranes, at least for some ranges of acid
gas concentrations in natural gas.
Both types of membranes considered in this study are much more permeable to
H2S and CO2 than to CH4. Consequently, in the membrane processes under consideration
the two acid gases will concentrate in the permeate (low-pressure) stream, whereas the
CH4 will concentrate in the retentate (high-pressure) stream, i.e., in the fraction of natural
gas (the "feed") not permeating through the membranes. Hence, the retentate is the
desired product, i.e., the upgraded natural gas. These are very favorable process
conditions because the retentate will be obtained at almost wellhead pressure. The main
pressure loss in the retentate stream will be caused by the pressure drop inside the
membrane modules, which is generally very small compared to the pressure drop across
the membranes.
II. MEMBRANE PROCESS DESIGN
1. Operating Conditions and Economic Parameters
Computer simulations and -economic assessments of membrane separation
processes for the upgrading of low-quality natural gas were performed for natural gas
"feeds" containing up to 40 mole-% CO2 as well as up to 10 mole-% H2S, the balance
being CH4. It is assumed that in all cases the concentrations of CO2 and H2S in the
retentate stream (the desired product) will meet pipeline specifications. It should be noted
that the pipeline specifications for H2S are much more stringent than those for
CO2 (< 4 ppm H2S, < 2 mole-% C02).
The separation performance of eight different membrane process configurations
without or with recycle, and utilizing either C02- or H2S-selective membranes or both,
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S.A.Stern; Rice, P.A. & Hao, J. Upgrading Natural Gas via Membrane Separation Processes, report, March 1, 2000; United States. (https://digital.library.unt.edu/ark:/67531/metadc786004/m1/5/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.