(483f) Removal of Trace Hydrogen Sulfide Contamination from Natural Gas by Pressure Swing Sorption

Use of compressed natural gas (CNG) as fuel for motor vehicles continues to grow. A key requirement for this application is the removal of trace H2S contamination from the natural gas. Although adsorption is the most efficient method for this task, a PSA process is impossible to apply because the adsorbent saturated with H2S was not able to regenerate at ambient temperature, and heating and heat-exchanging facilities must be included in the sweetening equipment. However, the heating/cooling equipment leads to the increase in the fuel cost and is quite often unacceptable due to the limited space of the CNG fuel station. This work demonstrates that a pressure swing sorption (PSS) process operated at the ambient temperature can be used to remove the trace H2S impurity by using a porous solid coated with a solvent (e.g. tri-ethanolamine) as the sorbent. The liquid layer outside the solid grain afforded the sorbent better selectivity and may allow the regeneration of sorbent easier. It was shown that the sorbent regeneration is achieved by pressure reduction and back-purge with a part of the purified natural gas; furthermore, the dual functions of absorption and adsorption of H2S provide a higher capacity than the solvent and solid adsorbent alone. Thus, there is no need for using the conventional thermal swing adsorption (TSA) mode of purification process, which represents a major breakthrough in the natural gas purification technology. Multi-cycle operation of a dual column process proved the feasibility of the new sweetening method. The columns are made of stainless steel with inner diameter 10 mm and length 250 mm. In each column, 11 g silica gel with the TEA loading ratio of 26% was packed in 230 mm long. The feeding stream contains 170 ppm H2S and fed to the process at a rate of 400 cm3/min. A computer program controls the experiments via the LabCards, and the composition of gas streams was in-situ analyzed by a mass spectrograph. Tests were carried out for the determination of adsorption and purging condition as well as of the time allocation for each step of an operation cycle. The pressure swing sorption was continuously run for 160 cycles at ambient temperature. It is shown that the product purity drops down from 170 ppm to about 1 ppm after 5 cycles, and the recovery reaches stable in about 30 cycles. The applicability of the new technology has thus been proved. Keywords. Natural gas, Hydrogen sulfide, Removal, CNG vehicle, Pressure Swing Sorption, New sorbent 19 Figures 9 References