Glycol dehydration: handling and operational problems
Most operating and technical problems usually occur when the circulating glycol solution gets dirty. In order to get a long, trouble-free life with the glycol system, it's necessary and very important to recognise these problems and know how to prevent them.
Some of the major problems are :
- Glycol loss
- Foaming
- Thermal decomposition
- Dew point control
- Glycol pH control
- Salt contamination
- Glycol Oxydation
- Sludge formation
- Glycol loss
The physical loss of glycol is probably the most important operating problem in the dehydration system. Most dehydration units are designed for a loss of less than 0.10 gallons of glycol per million cubic feet of natural gas treated. However, if the system is not operated properly, the loss might be much higher than this.
The glycol contactor (the absorber) and glycol regenerator are the most common places in the dehydration system where about 90% of glycol loss occurs. High gas velocity through the glycol contactor will cause carryover of glycol into the pipeline and a poor mist eliminator (mist extractor) in the top of the glycol contactor will pass some glycol even at normal gas velocity .
The glycol losses occurring in the glycol regenerator are usually caused by excessive reboiler temperature which causes vaporisation or thermal decomposition of glycol (TEG). Also, excessive top temperature in the still column allows vaporised glycol to escape from the still column to atmosphere with the water vapour.
2. FOAMING
Foaming of glycol is another problem frequently encountered. It can increase glycol loss and reduce the plant capacity. Entrained glycol will carry over from the contactor (absorber) with the sales gas. Also, foaming can cause poor contact between the gas and the glycol solution ; therefore , the drying efficiency is decreased. The best cure for glycol foaming, is the proper care of the glycol solution. The most important measures in the program are, effective gas cleaning ahead of the glycol system and good filtration of the glycol solution.
De-foaming agents such as Mono-ethanolamine (MEA) are widely used to control the problem. However, it's very important to point out that, the use of these does not solve the basic problem, and its only a temporary measure until the cause of the foaming can be determined and eliminated.
Some factors that can cause foaming are:
- Low glycol solution concentration to the contactor.
- High differential temperature between wet gas inlet and lean glycol inlet to the contactor.
- High glycol pH - (Note: Basic glycol solution of pH > 9 tends to foam and emulsify)
- Hydrocarbon liquids (condensate)
- Finely divided suspended solids
- Salt contamination
- Field corrosion inhibitors
3. THERMAL DECOMPOSITION OF GLYCOL
It has been established that the glycol reboiler temperature is limited by the Tri-ethylene Glycol decomposition temperature , and glycol vaporisation losses. Laboratory data indicates that glycol (TEG) is thermally stable up to about 400°F. Excessive heat as a result of one or more of the following conditions will decompose the Tri-Ethylene glycol (TEG) and form corrosive compounds .
A high reboiler temperature above the glycol decomposition level.
Localised overheating, caused by deposits of salt or tarry compounds on the reboiler
fire tube or by flame impingement on the fire tube
4. DEW POINT CONTROL
'Dew Point' is the temperature at which the water vapour first starts to condense to liquid. In industry, the dew point is used to indicate the water vapour content in the gas stream. For the dew point to have meaning as a descriptive term , the pressure at which it is determined must be stated .
When the dew point depression of the treated gas is too low, there can be several causes such as; Low glycol circulation rate; Low lean glycol concentration - i.e poor regeneration of the rich glycol solution; Foaming (leads to poor contact between the wet gas and the lean glycol solution); Blocked or dirty contacting devices in the absorber tower; High gas velocity in the contactor .... etc.
Check the glycol circulation rate.
Check the glycol reboiler temperature and make sure its on the right setting. If temperature setting is normal , verify the reboiler temperature with a test thermometer and make sure that the temperature control system is working properly.
As a conclusion, the dew point depression indicates the extent to which the moisture content of a gas is lowered. For example, a 50° dew point depression below a saturation temperature of 80 °F at 600 psia, would indicate that the natural gas, after dehydration, would have to be cooled, to 30 °F before any condensation of water vapour would occur. From the water vapour content curves, it is seen that the concentration of water vapour would be decreased from 51.00 lb / mmcf to 9.4 lb / mmcf, representing the removal of 41.6 lb / mmcf or 5 gallons of water per one million cubic feet of gas.
(The greater the dew point depression, the more water vapour removed).
