TEG contactor approach to equilibrium

[LouNathanson]

In the discussion of glycol gas dehydration systems in the GPSA Databook, there is a graph of equilibrium dew point versus TEG concentration (fig 20-54 in the current version). The numbers here are generally in agreement with what I come up with in a Hysys simulation using very many theoretical stages. (Using Peng-Robinson, Hysys dew points are perhaps 15 F higher, presumably due to hydrate versus metat-stable dew point calculationis.) So far, so good.

However, the GPSA suggests that approaches to equilibrium of 10-20 F are the norm in the field. However, most contactors that I've seen have about 12 trays. Tray efficiency in this service is generally 25-30% -- per GPSA, per equipment vendors, and per what I've generally found by backcalculating from field data.

Given that tray efficiency, Hysys generally tells me that my gas dewpoint is about 50 F higher than obtainable with infinite stages. This is also what I generally find in the field.

I do find that my dew points measured in the field (using a FRESHLY CLEANED panametrics portable analyzer) agree with the water content that I calculate from a material balance using my sampled rich TEG concentration and circulation rate.

So my questions:

Does anyone know where GPSA comes up with the assertion that approaches to equilibrium of 10-20 F are typical?

What approaches to equilibrium are other folks finding in the field?

Thanks,
-Lou

 

[dcasto]

you must be behind on databook revisions, fiquire 20-54 is for DEG.

In the text on page 20-31 of the 12th edition, it discusses why there is a 10 to 20 degree approach are typical.

I don't know why your model is predicting 50 degrees higher. Look into what could cause a 50 degree difference.

Glycol concentration, is it 99.5% or 95%. 12 tray columns are not the norm. Most contactors have 6, 8, or 10 trays with 8 the most common, followed by 6 and 10 are rare.

Is the glycol pure water-TEG, not hardly.

In the GPSA they state that that the condensed water is a metastable liquid, in reality, the phase would be a hydrate, therefore, the fiquires predict dews lower that what can be achieved.

Are you sure your TEG concentrator is 390F +/-0.000 F 24/7 and that the stripping gas is exactly 2 SCF/gal and that it has exactly 1 stage?

SO, in the field, 10 F approach is tough and 20 is doable. At 30 degrees, that is like falling off a log backwards, easy. I'd have to agree with the GPSA Book for well maintained systems and realize junky field units will be 20 to 30 degrees off from the charts.

BTW, my simulator, winsim, hits the canned freebie/secret TEG design programs right on and the smith tables very closely.