Improving sulphur recovery units
A new alternative to the Claus unit tail gas treatment has been found in the wet gas scrubber. This can maintain or increase acid gas feed rates to the front end of the sulphur recovery unit, while reducing SO2 emissions
Steven F Meyer and Cristina Kulczycki, MECS
Ed Juno, Sinclair Oil Corporation
Nick Watts, CEntry Constructors and Engineers
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Many refiners operating sulphur recovery units (SRUs) must consider installing a tail gas treatment unit (TGTU) in order to reduce sulphur emissions. For example, in the US, Refinery MACT II requires that SO2 emissions from SRUs (in refineries) be kept below 250ppm, corrected to 0% oxygen (on a dry basis). This usually equates to an overall sulphur recovery/removal rate of 99.8% or higher.
Regenerable amine-type TGTUs are most often utilised to achieve 99.8% sulphur recovery. In an amine-type unit, the Claus tail gas sulphur compounds, including SO2, CS2, COS and elemental sulphur, are converted to H2S with a hydrogenation/hydrolysis reactor. The H2S is then absorbed in a selective amine solvent, concentrated and recycled to the front end of the SRU. This approach typically increases the overall sulphur-recovery efficiency of the Claus unit to 99.8% or higher. However, the fresh acid gas feed rate to the SRU is reduced by the size of the recycle stream, which reduces the SRU fresh acid gas capacity by approximately 5%. The residual H2S not recycled to the SRU is incinerated to convert it to SO2 prior to emitting it to the atmosphere.
Until recently, there were few viable alternatives to this approach to SRU tail gas treatment that could also provide an overall sulphur recovery/removal of 99.8%. However, recently a wet gas scrubber was shown to be a viable, alternate approach for SRU tail gas treatment. When using a wet gas scrubber, the sulphur compounds present in the tail gas from the Claus unit are first oxidised to SO2. The SO2 is then captured by contacting the gas stream with an alkaline reagent such as sodium hydroxide in the wet gas scrubber, producing a concentrated brine waste stream. Residual SO2 not captured in the scrubber is discharged to the atmosphere. With this approach, there is no H2S recycle to the SRU, and the overall sulphur recovery/removal efficiency is increased to 99.95%. In addition, equipment costs are less than for conventional amine absorption-type systems. Finally, wet gas scrubbing systems can operate over a wide range of feed conditions, as is often found in refineries.
Sinclair Oil Corporation operates two refineries in the state of Wyoming (US). The Sinclair refinery is located just outside of the city of Rawlins in Sinclair, Wyoming, and is rated at 72 000bpd. The other refinery is located just outside of Casper and is rated at 22 500bpd. The Sinclair refinery operates two three-stage SRUs with a combined capacity of 47.7ltpd, while the Casper refinery operates one three-stage SRU with a total capacity of 18ltpd.
Throughout 2003, Sinclair investigated several options that would bring the SRUs at both sites into compliance with the newly promulgated Refinery MACT II standards. In early 2004, it decided that its preferred approach was to treat the tail gas from the SRUs at its Sinclair and Casper refineries with incineration/wet gas scrubbing technology. The decision was based on an economic review of the competing technologies as well as the fact that Sinclair had successfully operated a wet gas scrubber on the SRU at its Tulsa refinery for several years.
After evaluating several different wet gas scrubbing technologies, Sinclair settled upon the proprietary DynaWave (supplied by MECS Inc). The technology was selected for its ability to handle a wide range of upstream operational scenarios, while still reducing SO2 in the offgas to levels required by MACT II, as well as its ability to reduce chemical oxygen demand (COD) in the scrubber effluent.
Sinclair worked with CEntry Constructors & Engineers to develop the project scope, process design basis and engineering specifications for the units. CEntry had assisted with the previous technology studies and was selected as the EPC firm for the project. As the Sinclair refinery has two SRUs, a main process requirement was that the DynaWave performance would not be compromised when only one of the SRUs was in operation. On the other hand, the Casper refinery only has one SRU. Therefore, the Casper refinery wanted to have the capability to send a large percentage of the feed stream that would normally be sent to the front end of the SRU directly to the TGTU should the SRU need to be taken offline for maintenance. With this arrangement, the refinery could continue to operate even during periods when the SRU was offline. As the Casper refinery required the ability to operate a portion of the refinery when the SRU was down, the tail gas systems at both the Casper and the Sinclair refineries were designed to handle similar amounts of sulphur throughput.
The final scope for the Sinclair and Casper refineries each consisted of a single tail gas treatment train with a forced draft thermal incinerator, a waste heat boiler and a single DynaWave scrubbing system. The Sinclair refinery also included a single ID fan downstream of the TGTU waste heat boiler to avoid imposing the TGTU’s pressure drop on the two SRUs.
Under CEntry’s direction, MECS upgraded the standard DynaWave system engineering specifications in order to provide a final design that met Sinclair’s refinery engineering standards. The result is a design that provides the redundancy and safety features required in a refinery environment. These systems were placed on order in the summer of 2004 and were operational by April 2005.
Oxidation of sulphur compounds in the tail gas
The first major piece of equipment in each TGTU system is the forced draft thermal incinerator, also known as a thermal oxidiser supplied by John Zink. The incinerator combusts the tail gas stream from the SRU with sufficient oxygen, at the proper temperature, for a given residence time. Based on the tail gas composition from the SRUs and allowing for 10ppm of H2S in the incinerator tail gas, the incinerators at the Sinclair and Casper refineries were designed to be operated at 1250–1350°F respectively, with a minimum residence time of 1.0 second. Normal operation requires fuel gas to maintain the incineration temperature. The Sinclair refinery incinerator is rated at 10 MMBTU/hr, while Casper’s incinerator is rated at 4 MMBTU/hr.
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