Foaming in recycle gas amine absorber

[Naiduat]

Recently, we have noticed foaming and high pressure drop in our recycle gas amine absorber column in our Diesel HDS unit. DEA is being used in the system. The column was isolated , water washed and put back in service. However, the foaming problem still persists. The antifoam chemical (non silicone based) dosage rate also was increased. But, the foaming continues. The pressure drop of the column gradually increases from 0.12 to 0.22 Kg/cm2 after introduction of minimum gas through the column and foam is being observed in the bottom of the column. Due to this, liquid is getting carried over to the suction of Recycle gas and causing tripping of the compressor.

15 days prior to the observed problem, a new antifoam chemical trial supplied by alternate vendor has been started. After addition of this antifoam chemical, it has been noticed that the amine colur changed to milky white. During the initial 15 days, the column pressure drop was observed to be normal. After 15 days, while the unit restarted after trip, foaming and resultant high pressure drop in the column was noticed.

1. What could be the possible reasons for foaming in the column?
2. Does anyone have similar experience? If so, what was done to suppress the foaming?

 

[NiravK]

One of the reasons for foaming might be the entrained heavy hydrocarbons alongwith sour gas which gradually built up inside the column.

I do not think that mere water flushing of the column will be useful. You need to 'de-grease' the tower and associated piping in order to ensure complete removal of oily compounds.

 

[dcasto]

Typically we acidize with 15% HCL inhibited. The solution can be heated to assist in removing waxes and oils. There is also some fine particles of iron sulphide that can cause foaming, these can also be removed with washing. CAUTION, the HCL will react with FeS and make H2S, have you PPE on and ready to handle it.

 

[quadtracker]

I have had some experience with foaming with MDEA, WE injected anti-foam at different rates but no success. I believe you can get too much anti-foam in the system. We found that good clean amine solved the problem. We filtered down to 10 or 20 micron and after about two weeks of operation, we never had any more problems. But when we were foaming before, we also would carry over liquid. We also kept our amine feed temp at about 125 F. Gas temp was kept lower 90 to 105 F. We also changed out the carbon filter elements a couple of times in those two weeks. At first We thought that the carbon filter were absorbing the anti-foam, but I think we bypassed the filter and found that the anti-foam still made no difference. You really have keep the amine free from contamination by solids and hydrocarbon saturation.

 

[Hookem]

Keep hydrocarbon liquids out of the system, including compressor discharge lubricating oil that comes in the form of a mist. If you already have a scrubber, evaluate the efficiency of the mesh pad, or of the vanes. Oftentimes, process conditions change (pressure, temperature, gravity, flowrates, etc.) that cause higher or lower velocities.

 

[epoisses]

Agree with quadtracker, there is such thing as overinjecting antifoam. My experience with similar systems is that filtering helps; small particles affect the surface tension; the smaller mesh size filter you use, the better. You may want to go down to even below 1 micron. Try in the lab what mesh size you need to get your color right (milky white is a bad sign!).
And I guess you will want to keep hydrocarbons out as well, they don't really help...
Good luck!

 

[pkpbip]

One thing to check is whether you have a high liquid level in the bottom of the column. If the liquid level gets above the gas inlet the column will flood and DP will increase. One problem that causes a high liquid level that might not show up in the level reading of the sight glass is if you have a hydrocarbon level on top of the amine layer. Since it has a lighter specific gravity than the amine layer it will read a smaller level than there really is if it the level reader is a DP gauge. This also might not show up in the sight glass if the hydrocarbon layer is in between the uppper and lower tap. The level shown in the sight glass will be less than the level in the vessel because the sight glass fluid will have a SG of 1.0 while the average SG of the fluid in the vessel will be less than 1.0. Just a thought worth checking.

 

[22082002]

Keep Amine temperature atleast 5-6°C above the recycle gas temperature. If hydrocarbons condense into amine, then it's a problem.
Never allow in HC-level build up in RGKOD, check physically the level is not there in the drum.
What's amine concentration, try 1-1.5% lower concentration and see if it improves.
Antifoam dosing beyond 6(could be specific for silica based AF solutions) ppm is waste and then it starts leading to aggreviate the problem.
Check Amine side-stream(slip stream) filtraion is also working fine.There should not be any metallic contamination(corrosion products from piping/columns' internals).

Update, if you have any further info...