Title

Membrane processes for gas separations: Part I. Removal of carbon dioxide and hydrogen sulfide from low-quality natural gas. Part II. Enrichment of krypton in air

Date of Award

1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

S. A. Stern

Second Advisor

P. A. Rice

Keywords

Membrane processes, Gas separations, Natural gas, Krypton, Carbon dioxide, Hydrogen sulfide

Subject Categories

Chemical Engineering | Chemistry

Abstract

The objective of this study was to determine the process design characteristics and economics of membrane separation processes for reducing the concentrations of H 2 S and CO 2 in low-quality natural gas containing substantial amounts of the two acid gases to pipeline specifications ([Special characters omitted.] 2 mole-% CO 2 and [Special characters omitted.] 4 ppm H 2 S). The new processes considered the simultaneous use of two different types of polymer membranes for the above application, namely, one with higher CO 2 /CH 4 selectivity and the other with higher H 2 S/CH 4 selectivity.

The performance and economics of membrane process configurations comprising one, two, and three permeation stages, with and without recycle streams, were examined and optimized via extensive computer simulations. Most computations assumed as a "base-case", the processing of a medium-size natural gas stream of 35 MMSCFD at 800 psia. The natural gas was taken to contain [Special characters omitted.] 10 mole-% H 2 S and [Special characters omitted.] 40 mole-% CO 2 . The most economical process configuration was two permeation stages in series, with H 2 S-selective membranes in the first stage and CO 2 -selective membranes in the second stage . The most economical process configurations for upgrading natural gas containing either only substantial amounts of H 2 S or of CO 2 were also determined. The sensitivity of the process economics to feed flow rate, feed pressure, membrane module cost, and wellhead cost of natural gas was studied. A comparison of the processing cost of membrane processes with that of conventional gas absorption processes utilizing diethanolamine as solvent was also investigated.

II . A membrane process for enrichment of Kr in air was studied experimentally as a technique of improving the accuracy of Kr analysis. "Asymmetric" silicone rubber membranes were found to be most suitable for this application. The study was investigated with a feed gas mixture containing 0.99 mole-% Kr, 20.70 mole-% O 2 , and 78.30 mole-% N 2 . The Kr concentration could be increased from 0.99 to 2.23 mole-% in a single membrane stage and further raised to 3.73 mole-% in two stages in cascade. Computer simulations of "cross-flow" model yielded results in general agreement with experimental data.

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