Options for Small-Scale Sulfur Recovery
- Tom Royan (Tartan Engineering Corp. Ltd.) | Edward Wichert (Gascan Resources Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1997
- Document Type
- Journal Paper
- 267 - 272
- 1997. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.1.4 Gas Processing, 4.3.4 Scale, 4.1.5 Processing Equipment, 4.6 Natural Gas, 4.1.2 Separation and Treating, 5.7.2 Recovery Factors, 5.8.3 Coal Seam Gas, 5.2.1 Phase Behavior and PVT Measurements, 6.5.3 Waste Management
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With the issuance by the Alberta Energy Resources Conservation Board and Alberta Environment of their report entitled Sulphur Recovery Guidelines for Sour Gas Plants in Alberta in August of 1988, the requirement in Alberta to recover sulfur was broadened to a sulfur content of 1 tonne/D or greater in the inlet gas to a new sour-gas treating plant. This paper reviews the processes in use for recovering sulfur from sour-natural-gas streams that have a total sulfur content of 5 tonne/D or less. These processes are the modified Claus process, the recycle Selectox process, and the reduction/oxidation processes LO-CAT and SulFerox. While the modified Claus process is used in large sulfur-recovery plants, the other processes may be more economical for sulfur recovery on a small scale. A description of the sour-gas treating and sulfur-recovery processes is given, and a comparison of estimated capital and operating costs for typical sour-gas streams is provided.
All of the above processes are in operation in North America. Operating experiences with these processes in Alberta are discussed. The quality of the end-product sulfur varies among these processes, and the options for sulfur disposal are reviewed.
Approximately 40% of the natural gas produced to date in Alberta was sour; i.e., it contained some hydrogen sulfide (H2S). Most of the H2S that was extracted from the sour natural gas was treated in modified Claus plants to recover elemental sulfur. When the quantity of sulfur as H2S was less than 10 tonne/D entering a sour-gas plant (before 1989), the usual method of disposal of the sulfur was the flaring of the H2S and the CO2 through plant flare stacks by adding sweet gas to the acid-gas stream. After 1988 in Alberta sulfur had to be recovered if the gas contained 1 tonne/D or more in the sour gas of new plants.1 Between 1 and 5 tonne/D, the sulfur-recovery efficiency requirement is 70%.
Since 1989, several plants have been built in Alberta for the recovery of less than 5 tonne/D of sulfur. Various technologies have been applied to suit the particular situations. Capital and operating costs, as well as simplicity of operation, are the most common process-selection criteria for such small-scale sulfur-recovery facilities.
Nature of Sour Natural Gas
When raw natural gas produced from an underground reservoir contains more than the limit of H2S for sales gas, it has to be treated for the removal of most of the H2S. This is called gas sweetening. All raw sour natural gases also contain CO2. If a regenerative solvent is used for gas sweetening, the CO2 is extracted along with the H2S. These two gases are referred to as acid gases. When dealing with a sour gas that contains only a small amount of H2S (such as less than 1%), the content of CO2 is usually considerably greater than the H2S content. Upon regeneration of the sweetening solution, the resulting acid-gas stream may consist mainly of CO2. Such acid-gas streams are difficult to handle in a modified Claus plant. For this reason, other processes can be more economical on a small scale to recover the sulfur from such acid-gas streams that are lean in H2S.
The Sweetening Process
In most sulfur-recovery processes, the H2S and CO2 are first extracted from the sour natural gas by means of a sweetening solvent. There are various chemical and physical solvents available for this purpose.2 Fig. 1 illustrates the equipment used in such a process. The various options for the subsequent treatment of the acid gas stream for the recovery of 1 to 5 tonne of sulfur from the H2S form the basis for the remainder of this paper.
Modified Claus Process.
The modified Claus process consists of the following basic process equipment: inlet scrubber for acid gas; air blower; reaction furnace; heat-recovery boiler; condenser; one, two, or three sets of reheaters; catalytic converters and sulfur condensers/separators; and incinerator and stack.
The basic equipment is usually arranged in one of two modes, namely straight through or split flow.3 Fig. 2 illustrates the equipment for the split-flow process, which is the mode of operation used for lean-H2S acid-gas-feed streams to the sulfur plant, namely gases with an H2S content less than about 35% in the acid gas.
The chemical conversion of H2S to elemental sulfur occurs in two exothermic reaction steps. First, one-third of the H2S is burned with air to form sulfur dioxide (SO2) and water vapor, as in
This reaction occurs in the combustion zone of the reaction furnace. As soon as the above reaction takes place, a second reaction can occur between the remaining two-thirds of the H2S and the formed SO2, as in
The second reaction is an equilibrium reaction, which means that it does not go 100% to completion. Combining these two reactions results in the overall reaction of
In the split-flow mode, only about 35 to 45% of the total acid-gas stream is flowed through the reaction furnace, with the balance bypassing the combustion stage.
Below about 20% H2S in the acid-gas feed to a modified Claus plant, it becomes more difficult to operate the plant, as the flame in the reaction furnace becomes unstable. In such cases, the acid gas and air can be preheated before mixing in the front-end burner.4 The degree of preheating depends on the acid-gas composition. A flame temperature of about 1,000°C would provide for stable combustion in the reaction furnace.
Acid-Gas Enrichment. When the acid-gas stream contains less than 10% H2S, it may be too difficult to operate the facilities and achieve the necessary recovery efficiency in a split-flow modified Claus plant. In such a situation, the acid-gas stream can be treated in an additional sweetening step with a specialty solvent that preferentially absorbs the H2S and leaves most of the CO2 behind.5 Fig. 3 illustrates this process. Upon solvent regeneration, the acid-gas feed to the Claus plant is sufficiently enriched in H2S content so that a split-flow plant can achieve the necessary operating stability. The exit gas stream is disposed into the atmosphere through an incinerator and stack.
Selectox and Recycle-Selectox Processes.
The Selectox and recycle-Selectox processes were developed in the late 1970's as an adaptation of the Beavon Sulfur Removal/Selectox Tail Gas Cleanup process (BSR/Selectox).
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