Gas Processing

Hi all,
I'm working on a project where the operations folks say that the devices they have at the inlet to a pair of recip compressors are not effective. They have 2 parallel horizontal separators that seem to do basically nothing (no liquid ever drains from these vessels), and a vertical scrubber that I think is plugged. The latter has a vertical element, containing wire mesh and vanes, a liquid collection pan, and a diptube to drain coalesced liquid to the bottom of the vessel. The bottom should contain enough liquid to form a seal. Unfortunately, the bottom is dry, and attempts to fill it with water have resulted in liquid slugs to the compressors. Compressor maintenance has been a problem. Operations suspects that some burnt glycol got into the system (perhaps pretty long ago) and has been slowly sloughing off the pipes and equipment where it resides. Recently, gas production has been accompanied with some very fine solids (referred to as "doughy"). We could call it fine sand though some refer to it as clay-like. The gas contains very, very little liquid normally, though there are wellhead drying systems at each well. Upsets or misoperation could possibly generrate liquids.

Given this situation, I am looking for a solution to propose to them. What do you think of a high-quality reverse flow cleaning device (a la King Tool)? Will only disposable filters work? Are there other solutions that might work?

Thanks for your inputs,
Doug

Doug:

I had similar problems a long time ago with MEA mist getting into the suction of my CO2 compressors. I designed and installed an apparatus much as you see in the enclosed sketch. I designed it using the Brown-Souders equation which Milt Beychok and I have mentioned frequently in this Forum. The scrubber is basically a sparger device with a demisting-scrubbing section made of packing material. Mine worked very well and we never got any more MEA particles into our compressors. I positively know it worked because I analyzed the liquid water I employed in the bottom of the vessel and it increased in amine concentration with time; so I knew we were doing something positive.

I assume your problem is with glycols or a glycol solution. It would probably also behave analogous to MEA in its affinity to water and you probably could use a water level at the bottom to capture the glycol solute particles.

I would not use a "filter". I have found that I can't depend on filtering a liquid out of a gas stream. Perhaps a coalescer is more appropriate, but even that is not as simple or fool-proof as a simple suction scrubber.

I also applied the same principle - except that I circulated cool water around the packing section - in removing the odor-causing particles in fermentation CO2 prior to entering a compressor. This was applied after a Potassium Permanganate scrubber tower and again, the apparatus succeeded in capturing entrained permanganate and also acted as a redundent scrubber for the odor particles.

The compressor capacities I was using were 150 - 200 Acfm (@ 5 psig & 90 oF). If you incorporate a generous top disengagement space on top of the packing bed you should not require any further suction drums downstream before entering the compressor suction.

Perhaps this type of equipments would be simpler and easier to operate and maintain.

Art Montemayor

Thanks for your input, Art. Though I am an experienced engineer, I've never before worked in the gas processing field. I was told that burnt glycol is an especially nasty substance that can only be removed by filtering, and that there is no possibility of backblowing (cleaning) the filter elements. Thus I assumed that only a disposable type filter arrangement would work. The fine particulates are also deemed a substantial problem, but I must confess to not knowing their contribution to the current situation. I think that neighboring fields have had sand/clay problems and that they are anticipated but not yet problematic. In any case, I'll suggest a wet scrubber and see what happens.

I can size the diameter using the B-S equation, but what about the height. Any suggestions on sparger submergence and packing height? Also, am I correct in believing that the packing would be unwetted? If so, I'd guess that liquid carryover would be captured by the packing and that this transport mechanism would be sufficient to make the packing effective. Please comment on my understanding of this.

Thanks a second time,
Doug

Doug:

You are correct in giving careful attention to the scrubber height. This is the part of the application where you can easily reach a discomforting conclusion – the old, expected Trade-Off. You can arrive at inordinate scrubber heights by considering the following:

1. You desire good, generous submergence. This ensures that you obtain good contact time between the gas and the liquid bath. This adds height.

2. You want good packing height in order to accentuate the contact between the liquid film and the resulting gas. You also want the packing to act as a separation medium between the liquid phase and the gas phase. The more the packing height, the more the pressure drop – and the resulting height. Any liquid entrainment in the lower portion is incorporated to further “scrub” the ascending gas – hence the equipment name.

3. You are correct. The packing is not sprayed or purposely kept wetted. The lower portion will be wetted; but the top portion is intended to accentuate phase separation and act as a de-mister.

4. You also require a dry, disengagement space above the packing in order to protect the downstream compressor suction. This also adds to the overall height. I usually added 1.5 ft of disengagement height above the packing as a minimum – depending on the gas velocity.

In the actual applications I found myself halving the superficial gas velocity by doubling the diameter recommended by application of the Brown-Souders equation. This, in effect, allowed me to reduce the scrubbers overall height. However, once again, I inherited another trade-off: the scrubbing effect is lessened with reduced superficial velocity. I was reluctant to fabricate a 2-diameter vertical vessel and stuck with one diameter. I applied most of my sacrificial pressure drop across the sparger orifices in order to accentuate gas distribution in the submerged sparger. Somehow, the results always came out in agreement with our expectations. The particle removal was effected and the compressor was protected.

I wish I could be more specific, but I suspect my application’s capacity was different from yours so the specifics would change. However, I’m confident you understand the principles applied and can foresee the expected results and the parameters you have at your disposal in order to contemplate this application.

Doug,

You have mentioned that, in vertical separator; no liquid is collected at bottom of vertical separator. You have also mentioned that, if you put water in bottom of separator; liquid slug enters into compressor.

This clearly states that, in vertical separator there is insufficient space between feed point and liquid level. (If entering point is to low, which also results into no separation.) Gas momentum is so high that gas picking up bottom liquid. So please ensure that distance between feed point and high liquid level is 0.3D or 0.3 m min.

You have also mentioned problem of solid particles entering into gas stream.

Considering all above problems, I suggest use cyclone type inlet feed device, which removes big liquid and dust particles and also reduces inlet gas momentum.

Best alternative will be multi-tube cyclone type separator, which removes liquid and dust very effectively and all most maintenance free. No liquid wash or no liquid entrainment fear. Compact in size. Disadvantage is higher pressure drop and; at lower turndown, efficiency drops. Pressure drop may be equal to water wash scrubber (packed column).

If you have any query, please fell free.
Hope this may solve your problem.

All the best!

Manish Shah

hi.

I am a fresh chemical engineer and have gone through the mails in between you and other fellow,
I am also interseted to learn practically about Design i want to know how to design Scrubber.
please give me your expr. in this regards

Regards

Annee

Dear Mr. Art,
First of all, let me express my gratitude and sincere thanks for your invaluable guidance to young engineers. You are simply great.
I was going through some of the old posts.
With reference to the "Sparger device with demister-scrubber made of packing material" for removing the MEA mists from compressor suction, do you refer to Vane packing or distillation packing ? Also, can wire mesh be installed instead of packing for such applications ?

Thanks and Regards
Narendra Kumar