A Hybrid Gas Cleaning Process for Production of Ultraclean Syngas Page: 4 of 16
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As an alternative to the high cost of catalyst supports, several low cost and moderate surface area
materials were considered as support media for the active ingredient, sodium carbonate. Such
materials include (1) pyrolyzed rice hulls, (2) diatomaceous earth, and (3) sepiolite minerals. A
bench-scale system is being used at SRI to determine the achievable residual HCl vapor level,
the rate of chloride uptake as a function of time, and the maximum achievable chloride capacity
of the sorbents.
Bulk Sulfur Removal. For the second stage of the syngas cleanup train, RTI is working with
MEDAL (MEmbrane DuPont Air Liquide)a joint venture between DuPont and Air Liquide
(and a wholly owned subsidiary of Air Liquide) to develop and commercialize gas separation
membrane technologies-and with North Carolina State University (NCSU) and University of
Texas (UT) at Austin to develop polymer membrane technology for bulk removal of H2S from
syngas. These membranes are being specifically engineered to enhance the "solubility
selectivity" of the membrane for removal of the acid gas components (H2S, C02, NH3, and H20)
from syngas.
Commercial polymer membrane systems (e.g., air separation, H2 recovery from NH3 purge gas,
and CO2 removal from natural gas) are based on stiff chain rigid glassy materials, which exhibit
high diffusivity selectivity. In these membranes, small penetrants permeate through the
membrane faster than larger penetrants (Stern 1994; Freeman and Pinnau 1997). If this type of
polymer were used for syngas conditioning, H2 would easily pass through the membrane and
collect in large concentrations on the low-pressure side of the membrane (permeate). However,
recompression of this H2-rich stream to feed pressure is economically prohibitive.
However, diffusivity selectivity is only one of the components affecting overall selectivity. The
other component is solubility selectivity. The important factors affecting solubility selectivity are
the relative condensability of the penetrants in the polymer matrix and the affinity of the
penetrants for the polymer matrix. In solubility selective membranes, specific components from
gas mixtures are separated based on their chemical properties and not their size, unlike in
conventional diffusivity selective membranes such as cellulose acetate. If suitable polymeric
membrane materials are developed to selectively permeate H2S, CO2, and other acid or polar
gases (such as NH3 and H20) from mixtures with light gases (such as H2, CO, or N2), this
technology can become an attractive option for a host of applications requiring the removal of
reduced sulfur species (such as H2S, COS and CS2) from syngas. Thus, the impurities in syngas
could be removed and the desirable syngas components, namely H2 and CO, could be maintained
at high-pressure conditions as a non-permeate stream.
Sulfur Polishing. Removal of H2S using a membrane is limited by the finite selectivity of the
membrane for H2S over H2. Any additional sulfur removal from syngas required for fuel cell or
chemical production applications will necessitate a sulfur polishing step. The current commer-
cially available technologies used for polishing sulfur removal to sub-ppm levels are activated
carbons, molecular sieve absorbents, disposable ZnO pellets, and newly available regenerable
ZnO and zinc titanate pellets. However, because activated carbon and molecular sieve
absorbents must be used at ambient temperature, applying them in the polishing sulfur removal
process is problematic. Additional disadvantages include low activity, high cost, low sulfur
capacity (typically <1 wt%), inability to remove low-boiling sulfur compounds such as COS, and
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Merkel, Timothy C.; Turk, Brian S.; Gupta, Raghubir P.; Cicero, Daniel C. & Jain, Suresh C. A Hybrid Gas Cleaning Process for Production of Ultraclean Syngas, article, September 20, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc788144/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.