LTS Glycol injection plant turndown 1

[JonnyZ]

I'm looking for some guidance on designing MEG glycol injected gas-gas exchangers for gas turndown on an LTS, say 5 to 1.

Minimum tube gas velocities of 12-15ft/s are recommended in thread124-70137 and elsewhere to provide good dispersion through the tubes.

The turndown of these exchangers is poor as they are most efficiently designed with very long tube lengths and/or shells installed in series, which tends to limit velocities much above 15ft/s anyway due to pressure drop limits, which in turn means very poor turndown requiring multiple shells in parallel.

-Are multi-shell parallel-series configurations normal for turndown on LTS? If not how is turndown dealt with?

-Are these exchangers always single tube pass, are u tubes or tube inserts ever used to reduce exchanger length or do these promote hydrates?

-Does anybody have experience of these gas-gas exchangers being installed vertically to minimise turndown issues?

-Is there any hands-on experience out there regarding how strictly the 12 - 15ft/s needs to be adhered to?

A few questions there, but if you can help on any it would be most appreciated.

 

[maddocks]

We've certainly used multiple shell configurations to achieve better turndown, but it starts to get complicated with multiple injection systems, large bore block valves on the gas lines, lots of extra insulated gas piping and of course, the support of the exchangers. Almost all gas/gas exchanger tubes are single pass straight tubes with the NEN style of exchanger being the most dominant.

At reduced flows (say below the 5:1 ratio), you could start plugging tubes - alternately we used to use turbulators to increase gas velocity in the tubes without drastically altering heat transfer area - haven't seen them used for 15 years plus. As for a vertical gas/gas, technically it's a good idea, but the support structure, piping and the resulting monstrosity would put the space shuttle gantry to shame.

I've often ignored the 12-15 ft/sec as long as the facility has backup methanol injection and they can monitor the differential pressure on the gas/gas and chiller. As well, be sure and recheck the nozzles to make sure you have the correct nozzle size and distance from the tubesheet.

 

[JonnyZ]

Thanks maddocks, you’ve obviously got a lot of good experience in this area.

To follow up on some of your advice...

Tube plugging is certainly an option available for planned shutdowns . Not sure on its usefulness in this instance as the production profile is relatively rapid ascent and descent. Something to consider though.

You mentioned you have used turbulators, my concern was whether this promotes emulsions or foaming, hydrates etc. Are you aware of any problems with them? Do they help swirl the glycol around at lower velocity and stop it stratifying?

Agree with you on the vertical, good idea from a process perspective but perhaps a bit pricey to install. Are you aware of anybody ever trying it?

We do have methanol for back-up. Have you had much experience operating/commissioning those plants at velocities significantly less than the recommended values. How quick/effective is methanol for blockage clearance.

I’ve also noticed when designing these exchangers on HTFS software that the tube side heat transfer coefficient is extremely sensitive to glycol injection rate, it almost halves when injection is enabled. I guess you might notice this when operating as the plant may begin to warm when glycol is turned on.

 

[maddocks]

I'm not aware of the turbulators causing any foaming or emulsion problems - the good thing is - at turndown conditions, your LTS residence time is improved and the gas k-factor is dropping. I've heard of one owner/operator that simply used full length sections of carbon steel rebar in the tubes to reduce area and speed up velocity but I never followed up to check performance after this change.

As for the vertical mounting, I've never heard of anyone trying it although have seen a vertical sulphur condenser.

The methanol is very quick to break or dissolve a hydrate - usually within minutes if not seconds as long as you don't let the hydrate become a massive blockage. At that point, you'll have to shutdown, warm up, and sweep the system with warm gas and methanol to try and bust the hydrate.

Although we use HTFS software as well, I haven't noticed the decrease in Hi when you put the glycol in service - I may have to try that to see what is happening - must be the wall effects of viscous glycol on heat transfer from the wall to the gas.

 

[JonnyZ]

Re the observations on HTFS i also notice it predicts a whopping 3x increase in pressure drop with glycol. You may think thats overinjection - i've set the rate to give 2 to 3wt% average dilution of the glycol to ensure operations are maintained in the safe concentration region.

On a slightly different note, since you mentioned residence time and k-factors, is it necessary to heat the glycol-condensate mixture to effect liquid-liquid separation. I can't see it separating at LTS temperatures, but i've heard it does. I'm heating it in any case.

 

[maddocks]

I would guard against heating - the duty you add to your glycol condensate mixture is simply duty that you need to extract with the refrig compressor to achieve the desired process temperature. Depending on process temperature, you should have a minimum of 15 minutes of residence time. If you want to cut LTS size, run it as a 2 phase vessel and then warm the 2 phases of condensate and glycol and then throw them into a feed flash drum to allow them to separate.

I usually inject glycol at 80 wt% and dilute to 75 wt% or even 70 wt% if it's not too cold. The 3X pressure increase is a bit tough to believe....

 

[JonnyZ]

This particular design is as you say run as a two-phase separator followed by heating then liquid-liquid separation. Still i'm surprised that liquid-liquid separation occurs when done in a cold 3-phase LTS separator, similar mixtures exist in gas reception facilities for instance were glycol is injected into offshore pipelines and as far as i understand those mixtures of condensate and glycol need/prefer temperatures of atleast 30C 80F+ to separate in good time.

I agree the pressure drop is difficult to believe, if i could prove it was an error in the design software the design would be greatly simplified, however there is a clear relationship on HTFS as the glycol rate is increased from zero steadily up to the dilution mentioned above. Shame i haven't got HTRI to do cross check.

Thanks again for the top notch advice.