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