This paper reviews changes made to the Rimbey Sulphur Plant to handle an acid gas feed that can vary from 60% H2S to 15% H2S due to various inlet gas streams from different gas fields. The main areas covered by this paper include the theoretical calculations, plant modifications and operating test results. In addition, this paper outlines modification required to handle reduced acid gas flowrates.
The Rimbey gas plant was built in 1961 to process wet sour gas from the Westerose South and Homeglen-Rimbey fields. Initial design capacity was 326 MMSCFD. The original plant had two sulphur trains each rated at 122 tons/day. In 1964, an amine unit addition and also a third sulphur unit, with a capacity of 122 tons/day, increased the plant capacity to 420 MMSCFD. Figure 1 shows the original gas analysis from the two fields and the resulting acid gas feed analysis to the sulphur plant. As with all oil and gas reservoirs, there is a limited volume of gas that can be recovered and over time, production rates from the original fields declined. To maximize plant production and maintain the utilization of existing equipment, additional amounts of outside, or non unit, gas was processed by the lant. Provided that the non-unit gas analysis was similar to the original gas analysis, no problems would occur. Indeed, some outside gas with a substantially different analysis could be handled if the flowrate of this gas were low.
As increasing volumes of outside gas were brought into the plant for processing, the resulting acid gas feed to the sulphur plant could shift from the original H2S: CO2 ratio of 62.3: 37.7 to 40: 60 with the possibility of some outside gas combinations resulting in a H2S: CO2 ratio as low as 15: 85.
The original plant was designed as a straight-through Claus process (Figure 2), consisting of two parallel units. Each unit consisted of an acid gas knock-out drum, a reaction furnace and waste heat recovery boiler, and two stages ofconversion with a common three ass condenser. Reheat to eachconverter was originally provided using a hot-gas bypass from the reaction furnace waste heat boiler. This was later changed to indirect steam reheaters before each converter on the "B" train.
The straight through Claus process is best suited to recovering sulphur from an acid gas feed having more than 50% H2S. At this level, the exothermic reaction of hydrogen sulphide and oxygen results in furnace temperature in the range of 1095 ° C - 1260 ° C. This high temperature ensures flame stability and maintains the secondary reaction of 2H2S + SO2 → S to the extent that some 60% to 65% of the total sulphur in the feed gas is recovered after the waste heat boiler and condenser. The remaining two stages of conversion are sufficient to raise the overall sulphur recovery to about 95% - 96%.
