Flexibility of Liquid Redox Processing in Refinery Sulfur Management

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Courtesy of Merichem Company

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With over 25 years of experience, sulfur recovery via iron-based, liquid redox processing is a commercially proven approach to refinery sulfur management. The inherent characteristics of liquid redox processing, such as 100% turndown in respect to H2S concentration, flowrate and sulfur loading and single stage removal efficiencies in excess of 99.9%, makes the process attractive as a stand alone device for applications of less than 15 LTPD of sulfur or in conjunction with a Claus unit at higher capacities resulting in an overall system having a removal efficiency of 99.9+% and 100% turndown capability.

Another attractive inherent feature of iron-based, liquid redox processing is its ability to process any type of gas stream such as fuel gas, amine acid gas, and sour water stripper gas. Consequently, for refineries with Claus plants, the liquid redox process can be employed as a tail gas treatment unit, with or without a hydrogenation/hydrolysis unit, while also directly processing the sour water stripper gas relieving the Claus unit of this burden. All of this can be accomplished without recycling gas back to the Claus unit, thus increasing the capacity of the Claus unit.

Treating Acid Gas Streams

As illustrated in Figure 1, Autocirculation type LO-CAT units are used when treating acid gas streams and streams, which can be mixed with air without creating a safety problem. In this type of unit, the absorber and oxidizer are contained in one vessel and separated by baffles. Due to the large differences in aerated densities between the liquids in the absorber and the oxidizer large circulation rates are achieved between the various compartments of the vessel without having to employ pumps. The acid gas enters the absorber section of the vessel (centerwell) where it is contacted with oxidized LO-CAT solution and where the H2S is converted to elemental sulfur. The partially reduced solution then circulates to the oxidizer section where it is contacted with air, which reoxidizes the iron. The exhaust air from the oxidizer and the sweet acid gas from the absorber are combined and exhausted to the atmosphere.

In the conical portion of the vessel, the sulfur will settle into a slurry of approximately 15 wt%. A small stream is withdrawn from the cone and sent to a vacuum belt filter where the sulfur is further concentrated to approximately 65 wt% sulfur. Some units stop at this stage and sell the sulfur cake as a fertilizer. Drier sulfur cake can be formed by employing pressure filters.

Removal efficiencies of greater than 99.99 % and turndowns of 100% can easily be achieved with an Autocirculation unit.

Treating sour water stripper gas streams

Contrary to Claus units in which sour water stripper gas (SWS) can create operational problems, processing SWS gas in a LO-CAT unit is actually beneficial. As previous described, all reactions in a liquid redox process occur in the liquid phase and are very rapid; however, the mass transfer of the H2S from the gas phase to the liquid phase is relatively slow, and is thus the rate limiting step in the process. One method of increasing the mass transfer rate is to increase the alkalinity or pH of the circulating liquid solution. This is usually accomplished by adding caustic in the form of NaOH or KOH to the system, which creates an operating expense; however, due to the ammonia, which is usually contained in the SWS gas, no caustic addition is required when processing SWS gas, thus the operating cost is reduced.

Gas from sour water strippers, which contain reflux overhead systems, can be processed in a LO-CAT unit as illustrated in Figure 2; however, for non-reflux sour water strippers, the SWS gas will require cooling to reduce the temperature to approximately 140 – 150°F (60 – 65°C), otherwise a water balance problem will be created in the LO-CAT system. The condensed water from this cooling step would be recycled back to the sour water stripper.  

As shown in Figure 2, the SWS gas is contacted with LO-CAT solution in a venturi absorber, which serves two purposes. First, the venturi is an excellent, non-plugging absorber for this service and second, the venturi will produce a draft, which reduces the pressure requirement of the sour water stripper. The oxidizer and sulfur removal is the same as previously described.

The effluent gas from the Oxidizer will contain ammonia and water vapor and a good deal of dilution air. Depending on the amount of ammonia present and local regulations, this stream may need to be process through a catalytic reduction unit to remove the ammonia or combusted in a Claus incinerator if one is available. Again removal efficiencies of 99.99% and turndowns of 100% can easily be achieved with this processing scheme.

Removing the SWS gas from the Claus unit and processing it in a liquid redox system will not only free up capacity in the Claus unit but it will also eliminate the operational problems associated with processing this gas stream.

For more information about the LO-CAT Process click here.

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