4 replies to this topic

[omarzohairy]

Dear all,

If I have a Glycol dehydration unit of MAWP 1440 PSI and a 2 stages reciprocating gas compressor

where :

First stage suction is 200 psi and discharge 700 psi.

Second stage suction is 700 psi and discharge 1800 psi.

 

Can I locate the glycol dehydration unit in between the stages and adding the necessary scrubber to make sure no Glycol carry over enters the second stage of the compressor.

 

The Glycol unit will be added after the gas is discharged from 1st stage then cooled then passes through a scrubber to remove the free liquids then enters the contactor tower. Then it passes again through a scrubber before entering the second stage of the compressor.

 

And what are the possible problems that the process may face 

Edited by omarzohairy, 11 March 2018 - 02:41 AM.

[Art Montemayor]

Omar:

 

The basic response to your query is:

• Yes, you certainly can make good use of the interstage pressure and use the TEG unit at that point in the process.  I’ve never done this, but I’ve done a lot of unit processing in between reciprocating compressors in the past - such as adsorption dryers, scrubbers, etc.
• Depending on the required purity of the compressed gas, it is wise to use generous and conservative mechanical separation before the TEG contactor and a conservatively sized water scrubber to ensure that any entrained TEG is not swept into the last stage of compression.

I don’t know if you - as a student - thought of this process scheme, but it’s a smart process design decision.  What this shows is that you are taking advantage of the interstage pressure to dry the gas under the maximum pressure available to you that is under the MAWP (Maximum Allowable Working Pressure) of the TEG absorber and thereby dry the gas when it has the lowest available amount of water moisture within the design limitations of the TEG absorber.  Of course, you would be better off if you could dry the gas at the higher 1,800 psig - but you don’t have the absorber MAWP that allows it.  But you reap another advantage by drying in the interstage: you don’t introduce water moisture into the last compression stage.  This helps the cylinder lubrication and the wear and tear of the valves, piston, and cylinder.  It’s a good application.

Some problems you may encounter in this application are:

 

• Be mindful that you are dealing with a positive displacement device that has inherent pulsation characteristics in its operation.  What this means is that this compressed gas pulsation requires that you design your separator, absorber, and water scrubber appropriately with respect to capacity and operation in order to avoid any liquid entrainment into the absorber or into the last stage of compression.
• The pulsation encountered depends on the size, type, and rpms of your compressor and the size of the piping and pulsation vessels in your unit.  It is best to take a conservative design attitude in applications such as these.
• If you have an oil-lubricated reciprocating compressor, you don’t want to introduce lube oil into your circulating TEG.  This would create a serious fouling problem around the TEG reboiler tubes that would gradually cause an eventual shutdown.  Similarly, you can’t tolerate any TEG droplets entering your last compression stage.
• The effects on the purity of the gas is one thing, but the dangers of introducing any incompressible liquids into a positive displacement compressor are very serious hazards.  This is the kind of engineering experience and knowledge that isn’t normally taught in university training for engineers and you would be wise to bear this in mind and even bring it up with your instructor(s) or teachers.

I hope this information helps you out.  In my opinion you show wise curiosity in challenging and probing this idea and application while still in the academic phase of your career.  Keep up the questions and curiosity.

 

 

[omarzohairy]

Dear Sir,

 

Thank you for your answer and support.

[benabed]

Hi,

 

I personally  worked in a crude oil treatment plant where the produced gas is dehydrated in a TEG contactor prior to being sent to wells as lift gas. The TEG contactor is located between two stages of a centrifugal compressor with suction and discharge KODs for each stage.  The pressures are the same as you mentioned and the whole system is working perfectly.  For your case, and as Art pointed out, you should think about the impact of the pulsations of  your reciprocating compressor on the operation of the contactor.

[omarzohairy]

Thank you for your answer and support sir.