Aromatic emission from TEG unit 1

[27may2002]

To all members:

Hi, I am going to be involved in trouble shooting of a typical TEG dehyd unit using stripping gas process to increase lean glycol concentration. We are satisfied with TEG concentration but the trouble is releasing aromatic hydrocarbons from the top of the regenerator, which present an environmental problem.
I heard about DRIZO process, but I am thinking about condensing BTEX in a partial
Condenser and releasing vent gas including nocondenseabale natural gas in condenser condition. (Shortcut and cheaper). Does anyone have any comment or experience on this?
Idea?
Apart from DRIZO, what is other commercial method to prevent aromatic emissions from glycol unit?
Any suggestion or inputs is highly appreciated.

Regards,

 

[bchoate]

bchoate
Although, NG dehydration is not a process I have worked with a lot, there are some things I can contribute. The TEG dehydration process, unassisted, can regenerate TEG to an assay of 99%. This will provide gas with a dew point of -20 C. Enhancing the process depends on what dew point NG has to be achieved. There are three major means of improving the TEG assay after regeneration.
a) secondary water extraction which is partial condensation of the distillate vapor phase as in a 'cold finger' or 'Trimdryer' process. 99.4% TEG can be achieved.
b) dry gas stripping of lean TEG - which can achieve 99.95% TEG. Additional water is desorbed by countercurrent contact of lean TEG with dry gas. There is a cost associated with the amount of gas needed to produce higher TEG assays. The downside is that the gas that desorbs water also desorbs BTEX and increased aromatics in the vent are a result.
c) DRIZO process - solvent stripping instead of gas stripping. A liquid solvent is vaporized for stripping duty. In some cases the BTEX in the NG can be used as the stripping solvent.
The question posed in your post concerned partial condensation of BTEX from the vent gases. Depending on the gas flow rate, the aromatic concentration is probably too low for partial condensation to work. What permitted BTEX emissions are allowed and what gas dew point is necessary to reduce emissions to this level? Another important issue is the CO2 in the NG feed. This is vented in the gas stripped TEG process. The DRIZO process also removes CO2. There are three variations of the DRIZO process giving -60 C, -80 C, and -100 C dewpoints.
Previous posts have asked about a spec for purchased TEG. Certainly, TEG with an assay >/= 99% is desirable. Since TEG is produced from ethylene oxide, BTEX impurities are not a real problem. Water content is an issue. TEG producers face the same TEG drying issues NG drying does.
Bill Choate

 

[MGlycol]

Emission from still vent of natural gas dehydration units have been identified as significant air pollution problem.
Benzene, toluene, ethylbenzene and xylenes (BTEX) are emitted from the still vent during the glycol regeneration process.
Incineration can destroy glycol regeneration vent stream and prevent their release, but dedicated incineration systems are expensive.
Regenerator vent feed to a fuel system or flare might also be possible, expect that glycol regenerator design pressures are often so low as to preclude this option.
However, immediate condensation and recovery is the least complex and potentially most economical.
Three cooling media (refrigerants, cooling water, and air) are most commonly used in condensers.
Refrigerants and cooling water are often unavailable on site, expect at a high cost. Air Condensers are one of the most commonly used options due to the relatively simple design and competitive capital cost. However, the thermodynamic limitations require the condenser operating temperature to be higher than the ambient dry-bulb air temperature.
The Radiant Corporation, under contract to GRI (Gas Research Institute) sponsored the development of the R-BTEX process (patent #5,209,762) to reduce the BTEX emissions.
In the R-BTEX process the still vent vapours first pass though an air-cooled heat exchanger in which most of the water and some of the hydrocarbons are condensed. The partially condensed stream then goes to a water-cooled heat exchanger where most of the remaining hydrocarbon vapours are condensed. The condensed, three-phase stream flow to a separator, where it is partitioned into noncondensable gas, hydrocarbon liquids, and water. The small amount of noncondensable gas can be recompressed, burned (used as fuel gas in the reboiler or sent to flare) or vented, depending on local regulations. The hydrocarbon layer is decanted and stored for sale.
The condensed water is returned to the cooling system as make-up water, and the excess water is blow down as needed to maintain the water balance. The water is then passed through a packed tower where it is cooled by evaporation and hydrocarbons are stripped by air. Utility requirements for the R-BTEX process are minimal (water may be supplied at process start-up or can be recovered initially by running only the air-cooled exchanger).
Test data on this system shows a BTEX removal efficiency in the 98+ per cent range.
For more information contact:
Gas Research Institute
8600 West Bryn Mawr Avenue Chicago, Illinois 60631
312/399-8100