Gas dehydration is a process of extracting moisture out of natural gas and gaseous mixtures. It often precedes either a pipeline transportation of gas or a low temperature based gas processing.
Gas dehydration Process
Dehydration of natural gas is a necessary part of gas operations. Pipeline companies usually require the water content of the gas to be below 7 LBS/MMSCF and corrosion in our own pipelines can be reduced by dehydrating the gas. In many systems, compressed dehydrated gas is mixed with wet produced gas effectively eliminating the benefits of dehydration. Except at times when freezing may be a problem substantial fuel savings can be realized by not dehydrating field gathering lines.
The two factors that have the biggest effect on fuel consumption of gas fired glycol dehydrators are the glycol regeneration temperature and the glycol circulation rate.
Glycol Regeneration Temperature
Depending on the type of glycol used it is important for the reboiler temperature not to exceed the decomposition temperature of the glycol.
Decomposition temperatures are as follows:
Triethylene glycol is preferred in most systems. Maintaining a reboiler temperature between 350ºF and 400ºF will prevent thermal degradation of the glycol while allowing re-concentration to about 98.8%.
The recommended temperature in the top of the column is 225ºF. Below 220ºF water may start condensing and falling back into the reboiler. Above 250ºF glycol vaporization losses increase.
Glycol Circulation Rate
The amount of glycol circulated will have a direct effect on both the amount of water removed from the gas and the amount of energy used in the reboiler. Standard glycol dehydrator design is usually based on a minimum circulation of two gallons per pound of water removed, a maximum circulation of seven gallons per pound of water removed with an average of three gallons per pound of water removed. If the downstream equipment/pipeline requires that the water content of the gas be less than 7 LBS/MMSCF then the circulation rate should be adjusted to dehydrate to as near to 7 LBS/MMSCF as possible allowing for upsets in the system. It should be noted that the reboiler energy consumption is directly proportional to the amount of glycol circulated, if you are circulating twice the glycol needed fuel consumption will double.
To insure optimum operation the operator should keep accurate records of the following:
- Gas rate MMSCF/DAY
- Reboiler temperature ºF
- Still column temperature ºF
- Glycol circulation rate GAL/HR
- Glycol temperature to contactor ºF
- Maintain reboiler temperature between 350ºF and 400ºF.
- Still column temperature should be maintained at 225ºF.
- Fuel gas pressure should not exceed 10 PSIG.
- Adjust burner for 80 – 90% burn time with a small pilot during idle times.
- Adjust burner using stack gas analyzer. Adjust for 3 – 4% oxygen in stack gas.
- If necessary modify air intake opening to give 3 – 4% oxygen reading in stack gas.
- Combustibles in the stack gas must not exceed 0.1%.
- Optimum stack gas temperature is 500 – 550ºF.
- Increase turbulence in fire-tube by installing an in-line mixer.
- Insulate reboiler to reduce heat loss.
- Reboiler fuel consumption is directly proportional to glycol circulation rate.
- Keep accurate records.
- Install pump-gas separator to use previously vented gas as fuel.
- Dehydrate to as close as practical to maximum allowable water content.