Sulfur Recovery from Refinery Process Emissions

11JJ SULFUR RECOVERY FROM REFINERY PROCESS EMISSIONS 

Stretford plants have been in operation for 30 years. There are hundreds of such plants worldwide, used in a variety of sulfur removal operations (Dalrymple 1989). In a Stretford process, the hydrogen sulfide in the feed gas stream is absorbed and oxidized to elemental sulfur in aqueous phase, using pentavalent vanadium which is subsequently reduced from a pentavalent form to a tetravalent form. Later in the process, the vanadium is re-oxidized back again, using anthraquinone disulfonic acid (ADA) as a catalyst, and the elemental sulfur is floated to the surface of the solution and removed. 

In addition to the reactions that produce elemental sulfur, competing reactions also occur that produce undesirable by-products such as sodium thiosulfate. This is detrimental, because the thiosulfate remains in solution, and its concentration can generally be reduced only by bleeding off a portion of the solution inventory. This solution purge waste stream is hazardous, largely because it also contains vanadium compounds. The key to reducing the metal content of the waste stream is to reduce the rate of thiosulfate formation. 

The undesirable formation of thiosulfate is aggravated by several factors, including  

The rate of thiosulfate formation also increased with increasing temperature and pH. Many Stretford plants employ a sulfur melter to separate the elemental sulfur from Stretford solution. This subjects the solution to 150°C temperatures, which enhance generation of thiosulfate. 

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The recovered sulfur industry exists primarily as a result of the necessity of removing sulfur values from hydrocarbon fuels before combustion so that sulfur emissions to atmosphere are reduced. In the case of sour gas, the principal source of recovered sulfur, the product that results from recovery of the sulfur is clean-burning, non-polluting methane. In the case of refineries handling high sulfur crude the product is low sulfur gasoline and oils. Thus every ton of sulfur recovered is a ton that is not added to the atmosphere. The recovery process itself however, is also the subject of optimization and recent developments in recovery efficiency have further ensured that the environmental impact in the immediate vicinity of these desulfurization facilities will be minimized. 

The Clean Air Act, passed in 1990 and scheduled to become effective in 1995, will force chemical companies to reduce their emissions, into the atmosphere, of many environmentally detrimental chemicals. These include sulfur which occurs as H2S in sour natural gas and refinery gas. Recognizing that sulfur recovery is a fast-growing business, you have recently formed your own engineering company, SULFREC, which specializes in sulfur recovery. A small chemical company has requested that SULFREC submit a bid to design a process for removal of sulfur from a 23-metric-ton-per-day gas stream (90 wt% H2S and 10 wt% CO2) using the modified Claus process described below. The principal reactions are ... 

The next four papers concentrate on another emission source, hydrogen sulfide from Claus units. The Claus process has been used for several years to remove hydrogen sulfide from petroleum refinery waste gases. However, current environmental concerns require a more efficient recovery of the sulfur values. Some of the new technology to improve Claus plant efficiencies is discussed in these chapters. 

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