Minimum MEG / Methanol rates in Gas-Gas Exchangers (H/C DPCU)

[Careful1]

What's the minimum MEG rate required (per tube?) in a Gas-Gas exchanger in a hydrocarbon dewpoint control unit?

The theoretical MEG rate I have calculated for hydrate-inhibition-only is very low (0.2 gpm for a 15 MMscfd plant) and I cannot believe that this would adequately wet the tubes (still to size the exchanger and no. of tubes, but 0.2 is very low). Presumably there must be a rule of thumb about the minimum gpm per tube?

Thread

http://www.eng-tips.com/viewthread.cfm?qid=70137

had some notes on minimum gas rate and I can confirm one reliable source I know recommended 3 m/s minimum gas velocity, and that this was supported by HTFS and HTRI (reportedly).

Follow-up question: if you cannot get the gas velocity up (e.g. pressure drop restrictions or at turndown) and/or MEG is not suitable for any reason, are these issues the same with methanol or does its presence in the vapour phase get over the wetting/distribution problems?

Thx in advance

 

[maddocks]

You typically require a minimum of about 0.25 USGPM per nozzle - the EG should be introduced at every change in direction. You may need 2 or 3 nozzles in the gas.gas exchanger and 1 or 2 in the chiller depending on the configuration. You can certainly use methanol but you will likely experience some significant methanol losses as it is partially soluble in the hydrocarbon liquids. As long as you design for and inject EG at 80 wt%, it doesn't really matter if it dilutes to 70 or 75 wt% or even less. If you have minimal water coming into the facility, you may not require EG at all, but I would certainly put the connections on the exchangers so you can quickly get some EG or MEOH into the system. Alternately, you could install a TEG dehydrator in fron of the DPCU.