In a Refinery that produces H2S the gas can be burned in a furnace to generate SO2 gas which in turn can be converted into weak Sulphuric Acid (spent acid regeneration).
The gas needs to be cleaned, to remove more than 200 mg/Nm3 of insoluble solids which would otherwise block the V2O5 catalyst. The gas is hot (e.g. 400 – 500 °C) and to avoid cooling it down, traditionally that filtration was done using an Electrostatic Precipitator, but that has been an expensive and not always effective solution.
V2O5 catalyst blockage / high pressure loss
Begg Cousland Envirotec has a partnership with Caldo Engineering (UK), for their Hot Gas Ceramic Filter system in this application. Caldo is a leading UK supplier of hot gas filtration equipment with over 50 successful installations and more than 10 years experience. Caldo Hot Gas Filters handle gases at temperatures up to 800 °C and in the presence of corrosive gases. Efficiency is very good achieving <10 mg/Nm3 exit. This system is very economical in comparison to ESPs and is proven in this duty.
The ceramic filter element is vacuum formed from alumino-silicate ceramic fibres as a 1000 mm long tube, flanged at one end and closed at the other. Its composition is stable under extreme conditions of temperature and/or chemical corrosion. The element can withstand a temperature of 900 °C, but if the housing is fabricated in mild steel the filter operating temperature is limited to 450 °C. Stainless steel construction allows higher temperatures. The elements can resist all chemical attack with the exception of HF.
The elements hang vertically in the filter vessel from the header plate, which separates the clean and dirty compartments. In use the hot gas is sucked through the filter medium from outside to inside, depositing the particles on the outer surface of the medium. At controllable intervals a sharp pulse of air is blown back down the inside of the filter element causing a momentary reversal of flow. This reversal causes the accumulated solids to be detached from the outer surface of the filter elements. The solids fall into the hopper section of the vessel from where they may be discharged.
The capacity of the filter is determined by the number of filter elements contained in the filter vessel. Caldo has proven standard designs for 16-, 36-,64-, 144- and 256-element filters. If the 256-element design is not large enough, 2 or more vessels may be installed in parallel.
Filtration can remove only solid pollutants – unwanted materials present as gases or vapours will pass right through even the most efficient filter. If, however, the unwanted material can be reacted with a solid then it can be removed along with the solid. This technique is known as dry scrubbing and the solid material is called the sorbent.
Reactions between gases or vapours and sorbents are often quite slow. Generally there is not much sorbent compared with the total amount of waste gas and a long residence time is needed to ensure that the molecules of pollutant actually come into contact with the sorbent particles. Using a filter forces the gas through a fixed bed of the sorbent particles which gives excellent contact.
Acid gases are a common gas phase pollutant and these can be very effectively removed by the addition of lime or sodium bicarbonate to the gas upstream of the ceramic filter. Both lime and sodium bicarbonate react well with HCl, HF and H2SO4 and may even remove a proportion of any NOx that is present.