Oxidizer Helps Control Highly Variable Hydrogen Sulfide Emissions
Lubrizol France SARL (Paris) reduced hydrogen sulfide (H2S) emissions to less than 10 ppmv, from 820,000 ppmv, after it installed a Lo-Cat II hydrogen sulfide oxidation system from USFilter/Gas Technology Products (Schaumburg, IL). Lubrizol's Le Harve, France, facility produces a lube oil additive in a batch-type reaction that uses elemental sulfur as a reactant and processes H2S as a byproduct.
Lubrizol chose the Lo-Cat unit because it is designed to handle variations in H2S concentration. During the facility's 8-hr batch reaction cycle, H2S concentration in reactor effluent gas ranges from zero to approximately 100%. This variation represents a large, repetitive turndown requirement from the Lo-Cat unit. The cyclic rate of H2S entering the locate means that the maximum instantaneous H2S rate is 10 mt/d, even though the average daily H2S loading is equivalent to only 3 mt/d.
Lubrizol also required an oxidation system that yields elemental sulfur pure enough to recycle as feedstock to the batch reactors. During each two-batch cycle, the Lo-Cat II unit produces and recycles approximately 3000 kg of sulfur. Poor quality sulfur produced by the unit would degrade the product significantly.
Lo-Cat's design features enable it to produce high-quality sulfur, says US Filter VP/GM Gary Nagl. "The use of the Lo-Cat sulfur has not resulted in an deterioration of the customer's product," Nagl says. "Because the byproducts of the oxidation system are reused successfully as raw materials, this installation is truly environmentally friendly."
The Lo-Cat II process is an isothermal method of forming elemental sulfur by transferring electrons from sulfide ions to oxygen. The reaction takes place in an aqueous solution containing iron ions. (While many metals can mediate the transfer, US Filter chose iron because it is inexpensive and non-toxic). The basic reactions are divided into an absorption and regeneration phase.
The above figure illustrates the process flow of the Lubrizol Lo-Cat II system. Sour gas from the batch reactors run through a compressor, then though an inlet knockout pot to remove any liquid droplets. The gas is then sparged into the regenerated Lo-Cat solution in the absorber sections of the autocirculation vessel.
Rotary lobe air blowers supply air that is sparged through the reduced Lo-Cat solution in the oxidizer sections of the autocirculation vessel. The "air lift" created by differences between the aerated densities of the absorbers and oxidizers and the nonaerated densities of the regions between the baffles and wiers circulates the iron catalyst throughout the vessel. Both the H2S oxidation to sulfur and regeneration of the iron catalyst occur in the same vessel.
A pump removes a small slipstream of the resulting solution to a sulfur settler vessel, where it is concentrated into 10 wt% sulfur slurry. Progressive cavity pumps then deliver the slurry to a vacuum belt filter, which produces a sulfur cake of approximately 60 wt%. The filter cake is water washed, and the filtrate (catalyst solution) and the wash water are pumped back to the autocirculation vessel.
The filter cake falls off the belt filter into a reslurry tank, where water is added to produce 10 wt % sulfur slurry. Another progressive cavity pump is used to direct the sulfur slurry from the reslurry tank into the melter, a patented, vertical shell and tube exchanger. The molten sulfur then flows to a separator, where the sulfur settles to the bottom and water rises to the top. The resulting elemental sulfur then recycles back to the batch reactors.
The Lo-Cat II has performed its dual processes effectively since its startup in June 1996, saving Lubrizol money by recycling elemental sulfur and improving the air quality of Le Harve.
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Edited by Alan S. Brown