Condensate Vessel Pressure Drop

[B62plant]

I have gas at 1500 psig dropping to 850 psig across a JT valve.
The resultant condensate from the letdown flows into a 2 phase separator through a Schoepentoeter baffle with liquid flowing out to further processing and gas coming out through a demister.
I am seeing at certain times a DP across the separator of 40 psi (usually DP <2 psi) and I cannot understand why as no other process conditions change.
I thought at first it was hydrate formation but I ran HYSIS simulation and crystal formation is not possible at these conditions (water content <0.2 lbs/mmscf).
I am at a loss as to why I experience this DP and it does not cause me any problems, no other process changes.
Only thing I notice is it happens at high ambient temperatures in afternoon (I am in Middle East so quite hot).
Instrumentation is fine, it's a genuine DP.

Anyone ever experienced this ?

 

[LittleInch]

Can you sketch it out?

Vertical column or horizontal separator?

Levels between inlet and outlet and liquid levels?

Anything odd about the condensate ( does it foam, wax or create "gunge")

any additives or injection of chemicals into the feed stream?

Clearly a blockage somewhere so will need a change in composition causing an issue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[B62plant]

Horizontal
Both on same level
Condensate is fine, may possibly have very low content of lube oil from compressor injection
No additives

 

[LittleInch]

assume you're talking about the TS vessel on the RHS.

Sounds like the de-mister pad is getting blocked but what with isn't too clear as we have not a lot to go on here.

Are you sure you're not getting some sort of hydrate chystal blockage in the inlet device or the de-mister pad?

What sort of temps, flows, etc are we talking about here. high ambient could be increasing temperature beyond what was envisaged.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[B62plant]

Yes vessel in question is labelled LTS. The JT valve is the one upstream on it's inlet.
25 F
850 psig
90 mmscfd
0.2 lbs/mmscf water content
HYSIS tells me hydrate formation not possible until water content is >4lbs/mmscf

I would add that this high DP does not cause me any problems !!
There is no plant loss of pressure or flow up or downstream of this system.
Obviously I'm concerned that it may cause a problem at some point because I don't know it's cause
I just see a rise in DP across the vessel in times of high ambient temp of 40 psi.
The sun goes down the DP goes down !!!

 

[georgeverghese]

You havent said anything about the process upstream of this LTS unit - how does this gas get to 0.2 lb/mmscf water content? Very likely there is some malfunction upstream. The fact that this high dp is seen at high ambient tells me there is an air cooler upstream that knocks out most of the water at night, leaving the dehydration unit less loaded.

 

[B62plant]

There is a nitrogen removal (membrane based) system upstream which removes most of moisture

 

[georgeverghese]

Lube oil vapor, resulting from thermal degradation of lube oil at the upstream compressor, may be fouling up the RO membranes ?

 

[B62plant]

Even if true that would not explain anything as the up and down steam pressures both cycle. I have discovered (by fitting local PI) that the pressure drop is across a much wider section of the process and extends to the downstream cooler exchangers though

 

[don1980]

When troubleshooting any problem, the simplest explanation is the most likely explanation (ref: Occam's Razor). In this case the simple, and most obvious, explanation is a hydrate. It's a big mistake to ignore an obvious explanation based on lab results (moisture concentration) and HYSIS results. If you haven't done so already, go inject some alcohol into the system and see if this clears the dP. Definitively rule out the simple explanations before moving on to consider complicated ones.

 

[georgeverghese]

Could we see a sketch or PFD of the upstream section of this plant, starting from the oil lubricated compressors up till where your earlier diagram starts? And also post us this time vs. pressure profile trend?
Frankly, I cant imagine why oil lubricated compressors were selected for this process scheme with these delicate gas membranes - could only be CAPEX - should have been oil free compressors.

 

[B62plant]

There is a MeOH injection system and the problem persists with or without injection on. The upstream is simply a feed from TEG dehydration plant. CAPEX was indeed the reason for compressor choice.

 

[georgeverghese]

So your scheme is oil lubricated compressors - RO membranes - TEG - LTS ?

If so, thats an ambitious water dewpoint spec you've got for the TEG unit to get down to 0.2 lb/mmscf - how does this happen - DRIZO or gas stripping in the regen unit ?

MeOH should be injected at up to a total corresponding to approx 3x the actual feed water composition in the gas feed at these 2 locations - the gas-gas HX ; the JT valve through special injector distributors / nozzles.

 

[B62plant]

Yes, that is the set up

I have a moisture analyser at TEG unit outlet so I know it's 0.2 lbs/mmscf and most of the water is taken off by the nitrogen membranes and removed via the KO drums before it even gets to the TEG contactor column (yes I have gas stripping in regen unit)

I have injection at those points but I'm not sure what quills are fitted.

I'm beginning to think I will not get to the bottom of this until I open these units up at next annual shutdown next year

 

[georgeverghese]

If you want to be sure of the water content in the gas exit from the TEG unit, run a manual Karl Fischer titration. Else have your inline moisture analyser calibrated with certified calibration gas.

 

[B62plant]

KF on rich and lean TEG shows 8.6 & 0.2. Moisture meter is brand new, only fitted last month and was calibrated then.

 

[georgeverghese]

Presume you mean 100-0.2 = 99.8% by wt lean TEG from the KF titration. If so, and if I take zero credit for water removal at the gas membranes, that would correspond, roughly, to a 65degC water dewpoint depression at the TEG contactor, which corresponds to -15degC water dewpoint in the exit gas, assuming feedgas to the TEG unit is at approx 50degC daytime. At -15degC water dewpoint at 1500psig, this translates to 0.19lb of water/mmscf, which matches what you say.
With these rough results, it doesnt seem like there is a water content / dehydration issue. The freeze point of pure TEG of -7degC is awfully close to your LTS operating temp of -4degC (25degF) though, so it may be TEG vapor / mist carryover into the LTS ??

 

[B62plant]

I think you may have something here, thanks. Is it possible I have TEG carryover and it is freezing in the separator ??
I'm going to try and increase the temperature by slipstreaming a little gas past the coolers and see what happens.

 

[georgeverghese]

Yes, that idea crossed my mind - gong up to -2degC or so at the TIC may do it, I think.
On the other hand, it may be that the lean TEG feed to the column top may also be a little too hot - is that an air cooled lean TEG cooler ? That may also explain the daytime / night time oscillations in dp. Typical design criteria is for no more than a 5degC dt between lean TEG feed and wet feed gas. So if daytime feedgas is at 50degC, operate at 55degC max on lean TEG exit from the air cooler.

 

[B62plant]

No, it's a gas/teg exchanger. I'm going to check the DT (gas/teg) as you suggest