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We have a sour fuel gas amine absorber where sour fuel gas from the refinery is treated with amine from the amine unit to strip out H2S. Sweet FG from the outlet of the absorber passes through a cooler then a filter coalescer to separate carryover amine from fuel gas. We are continuously getting water from the filter coalescer instead of amine. We have checked the cooler for tube leakage but no leak is observed, also the pressure of the fuel gas side is 0.5 - 1kg/cm2 higher than the cooling water. After checking the strength of a coalescer boot sample, almost 99% water is found. Is this a normal outcome or what are the probable causes of water formation instead of carryover amine?
 
Answers
12/09/2020 A: Nagarathinam S Murthy, Ashphil Consultancy, Chennai, nssvdvr@gmail.com
Your inputs are a bit scanty as there are several missing data: operating conditions of the amine absorber (both sour gas and lean amine), temperature drop across the sweet gas cooler, coalescer effectiveness, quantity of water being removed, etc. Nevertheless, with boiling points for most amines more than say 170 deg C, vaporization owing to heat transfer between sour gas and amine can result in such preferential evaporation of moisture from the amine. Entrainment is different and as such it appears from the information given it looks to be in control. Also, it appears that there is no foaming of amine mentioned, implying amine quality is under the normal operating window. However, check the amine strength (typically 35% wt for MDEA) and pH of lean amine (close to 10+) supplied to the absorber. It should be as per design needs.
11/09/2020 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
The fuel gas will be saturated with water at the conditions of the top of the amine absorber (given that the amine solution is mostly water on a molar basis). Therefore, when cooling the fuel gas afterwards, some of that moisture will condense. This is the water you are getting out of the coalescer.
11/09/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Amine is an aqueous solution. The fuel gas is in intimate contact with the aqueous amine, and will leave the absorber saturated with water vapour. Cooling the sweet fuel gas will condense some of the water vapour - this is what you are observing. The fact that you are condensing water rather than carrying over amine is a good thing!

You seem confident that the pressure balance across the cooler is such that cooling water cannot enter the fuel gas. If you would like to confirm that cooling water isn’t leaking into the fuel gas you could ask your chemical vendor to test the sample – the chemicals they add to the cooling water would be present in the coalescer if there was a leak.
11/09/2020 A: Jon Isley, Fluor, jon.isley@fluor.com
Your fuel gas is water saturated leaving the amine absorber and is condensing in the cooler. You can check and confirm this using a process VLE simulation.
11/09/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The gas leaving the contractor is saturated with water. When you cool the gas water drops out because the saturation water concentration drops with temperature.