Molecular Sieve Sweetening of Propane And Butane
Authors: Kowalchuk Arthur 
Published: December 2007

Molecular sieves have been used successfully in sour-gas processing plants in Alberta to sweeten propane and butane products even when plants in Alberta to sweeten propane and butane products even when H2S and COS are present in the feed streams. A system of handling regeneration gas has been devised that minimizes the attention required of the operator and that eliminates pollution problems.

Location and Size of Plants
Gulf Oil Canada operates seven major gas plants in Alberta, Canada. Three of the plants located at Pincher Creek, Nevis, and Rimbey are sour-gas Pincher Creek, Nevis, and Rimbey are sour-gas plants that yield propane and butane products in plants that yield propane and butane products in addition to sales gas, sulfur, and condensate. Rimbey processes 423 MMcf/D, recovering 8,000 B/D of propane and 5,500 B/D of butanes; Nevis processes propane and 5,500 B/D of butanes; Nevis processes 76 MMcf/D, recovering 1,235 B/D of propane and 788 B/D of butanes; and Pincher Creek processes 100 MMcf /D, recovering 1,050 B/D of propane and 955 B/D of butanes.
Gulf Canada's experience with molecular sieve sweetening of propane and butane has been gained over a total of 11 years' operation at the three plants. (Molecular sieve is a granular dry desiccant material manufactured from zeolites to trap particular molecules such as H2S, COS, and mercaptan sulfins, and allow other molecules such as paraffin hydrocarbons to pass through the bed.)
Our decision to replace the regenerative caustic sweetening system with the molecular sieve system (Fig. 1) was prompted by common problems related to vapors from regeneration of the caustic system, as well as by pollution, operating, and maintenance problems resulting from disposal of the spent caustic. problems resulting from disposal of the spent caustic. The Rimbey plant, where process gas contains about 1 1/2 percent hydrogen sulfide, was the first to be changed from the regenerative caustic system to the mol sieve system. This was in 1964 when deep cut facilities were installed to increase propane recovery. In 1968, after 4 years of successful operation at Rimbey, a molecular sieve was installed at the Nevis plant, and in 1970 one was installed at the Pincher Creek plant.

Sampling and Analysis Method
At the Nevis plant, for design purposes, considerable time and money were spent on stream sampling to achieve repeatability of results of tests for carbonyl sulfide (COS), methyl mercaptan, and ethyl mercaptan. At that time there were no local laboratory facilities capable of this analysis, so samples had to be shipped about 2,000 miles to Gulf Canada's Research and Development Center near Toronto, and to the mol sieve manufacturer's laboratory in New York State. Analyses from the two laboratories showed variations in sulfur levels; these we subsequently attributed to reaction with the sample container during the shipping periods, which were sometimes as long as 3 weeks. By sending the containers by air, thus minimizing the reaction time, we were able to improve repeatability.
The Gulf Canada Sheridan Park Laboratory used a microcoulometric detector for the mercaptans and COS, and also developed a portable absorption-potentiometric system for on-site analysis of sulfur components. First, the sampling apparatus was connected to the inlet of the mol sieve to determine the level of sulfur compounds entering the bed. Next, the unit was connected to the outlet of the mol sieve to determine the level of remaining sulfur compounds in the product that was leaving.
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