Overprediction of water removal in glycol dehydrators could result in
costly freezing or hydrate formation

  • Data: from Surface Production Operations Vol. 2 (Ken Arnold and Maurice Stewart)
  • HYSYS-PR – Results from Aspentech HYSYS v7 using the Peng Robinson property package with
    default interaction parameters
  • GLYCalc – Results from GRI GLYCalc version 4.0
  • HYSYS-Glycol – Results from Aspentech HYSYS v7 using the Glycol property package with default
    interaction parameters

Three consistent trends could be observed for a range of lean glycol contents (from 98-99.5%), and
equilibrium trays (1 or 2):
  • HYSYS-PR tended to underpredict water removal, meaning that dry gas water content would be
    overpredicted.
  • GLYCalc significantly overpredicted water removal, meaning that dry gas water content would be
    underpredicted. We found that calculated dry gas water contents from GLYCalc were typically
    about half of what should be expected.
  • HYSYS-Glycol consistently gave the best match to water removal (and dry gas water content).
    Process Ecology recommends (and uses) the HYSYS-Glycol package for predicting BTEX
    emissions and water content in TEG dehydration units.

The impact of overpredicting water removal can be very serious.
  • For design, the glycol circulation requirements may be underestimated, resulting in a design
    which does not adequately dehydrate the gas
  • For rating and regulatory requirements, a target circulation rate may be proposed which again
    does not adequately dehydrate the gas.

In either case, serious operational issues could occur, namely freezing or hydrate formation.
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Most dehydration units use triethylene glycol (TEG) to remove water from natural gas.  Various simulation
tools such as Aspen HYSYS and GRI GLYCalc are used to model these glycol dehydration units - for
design, rating and regulatory requirements. Our experience, however, has shown that prediction of dry
gas water content can vary significantly between process simulators.

Of particular concern is the case where the simulator underpredicts the dry gas water content; in this
case there is a risk that operator actions taken in response to the simulator predicted values could result
in costly freezing or hydrate formation.

The following figure compares the predictions of water removal from a typical natural gas stream for a
contactor with two equilibrium trays (8 real trays) at 1000 psia and 100 F, with 99.0% lean TEG. Water
removal is of course related to dry gas water content; the higher the water removal, the lower the dry gas
water content.
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Another concern with inaccurately calculating the dry gas water
content is the fact that operators may not trust results from the
simulator, and run the circulation rate at a higher than necessary
rate in order to compensate. This would result in unnecessarily
higher emissions and higher energy usage.

Engineers and operators need to be very careful when interpreting
results from glycol dehydration simulations, and ensure that
these results are accurate.

Written by James Holoboff, Process Ecology