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Usually, Steam is used for CDU's stripping. I wonder that can I use fuel gas that comes out of the top of the column for stripping? (& Recirculating fuel gas). I think that by using fuel gas as stripping medium, we can save money & there will be less corrosion at the top of the column.
Can you tell me the advantages & disadvantages about this idea?
 
Answers
07/11/2018 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Steam acts an 3rd not-miscible phase, lowering the partial pressure of the hydrocarbon vapor phase and increasing the evaporation of hydrocarbon liquid at the same temperature and total pressure (vacuum).
Fuel gas is completely miscible with the hydrocarbon vapor and thus DOES NOT lower its partial pressure and DOES NOT increase the evaporation of hydrocarbon liquid.
There is no free lunch in thermodynamics.
04/10/2018 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Fuel gas can be used for stripping but most units could not handle it without significant capital modifications. Steam essentially all condenses in the overhead system so has minimal impact on the gas yield from the unit. Fuel gas used for stripping steam, however, will not condense and will all go out the tower overhead drum. To get the amount of fuel gas equivalent to recommended stripping steam rates (10lb/bbl of atmos tower bottoms) would required around 20 MMSCFD of fuel gas for a 100 mbpd crude unit. A crude unit of this size would only produce maybe 1 MMSDFD of gas normally so going to fuel gas for stripping would increase the gas make from the crude unit by at least 20x. Also that volume of non condensibles would increase DP in the overhead system which could have a negative impact on yields.
22/08/2018 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
In principle it is possible to use fuel gas as a stripping medium instead of steam with some modifications in existing units. However using fuel gas as stipping medium might reduce the yield of naphtha, as increased light ends in the overhead vapor will change the dew point temperature.
Stripping steam dilutes the hydrocarbosns on each tray, reducing thus there partial pressure, the reduced partial pressure helps to evaporate more hydrocarbons, similar effect can be achieved with fuel gas.
The disadvantage with using fuel gas will be change in some properties of the products, particularly the flash point of Kero, diesel. Some part of fuel gas will be absorbed in Kero/diesel side strippers and this will significantly reduces the flash point. Another thing is pressure at top of the column is lower than bottom hence need a compressor (with cooler and knock out drum) to recycle the fuel gas from overhead to bottom of column.