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Category: Process Control
Question: What would be the best option for limiting or eliminating ethanol vapor emissions from storage tanks at ambient conditions?
Keywords: v1i1,ethanol,emissions,limiting,storage,tank,reflux,condenser,vent
Answer: BACKGROUNDOften times, ethanol plants are equipped with very old external storage tanks that contain my relief devices. Emissions from such tanks are being targeted to limit ethanol entering the atmosphere.ANSWERSeveral options exist:1. The easy and "process-smart" answer to tankage emissions is pressurized storage; for ethanol, conventional API tank design standards allow for 2.5 psig design and this is more than ample for containing the vapors within the tank at most ranges of ambient temperatures. However, this may be a major stumbling point in your industry. The existing tanks may be very old and without calcs to put any basis on. I would strongly recommend you upgrade (as soon as you have the economic or process justification) to stainless material and a 2.5 psig design. This will make life a lot better for everyone involved. However, this may not be an immediate option ?Ǫ?Ǫ2. Try to get your existing tanks rated for pressurized service. Of course you need conservation vents on them, for tank protection. Rating them may be easier said than done. You may have already looked at this and discovered the futility of dealing with old, workable tanks. But this is an option nonetheless3. Consider cooling the incoming ethanol with the coldest water you have available and store at that temperature. This lowers the storage vapor pressure and reduces the emissions. This doesn?ÇÖt stop the emissions; it just reduces them. This was probably the most obvious option that may have already been explored. The point here is that it is more cost effective to cool a liquid than to partially condense a saturated vapor. The disadvantage here is that you must conserve the cool liquid temperature if your ambient is appreciably higher. This calls for insulation and more maintenance.4. You might be able to ?Ç£control?Ç¥ the ethanol emissions by venting through a small, continuous water scrubber and concentrating the aqueous ethanol to a level where you can recycle it back to process. This may or may not be feasible or cost effective.5. The least attractive option, in my opinion, is putting an atmospheric vent condenser on the existing atmospheric tanks and cooling-condensing the emissions (like a ?Ç£kick-back?Ç¥ condenser) with a refrigerant or brine. This method requires relatively expensive refrigeration equipment and controls --- with additional maintenance on equipment. Economic evaluation of this method usually shows little or no return.Source: CERP Message Board, Mr. Art Montemayor


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