Lower Rich Glycol Temperature Causes Glycol Flash Tank Operational Problem

One of major problem in glycol dehydration plant is hydrocarbon carry over. Plant observation conclude, glycol flash tank is one of equipments which hydrocarbon carry over tends to occur. One factor which participate in hydrocarbon carry over on glycol flash tank is temperature.


Since lower temperature of rich glycol has higher viscosity, rich glycol solution tends to form emulsion.
Many ways can be taken to observe whether hydrocarbon carry over take place in glycol flash tank or not.

1. Take sample from glycol flash tank, observe while taking sample. If glycol solution comes out from the tank and forms foam or emulsion, we can say "WE HAVE PROBLEMS HERE". Since vapour hydrocarbon is trapped (carry over) in to glycol solution.
2. If glycol particulate or glycol charcoal filter is installed, observe it's pressure drop. Pressure differential will tell many. If pressure drop is fluctuating, it seems "WE HAVE PROBLEM HERE". Trapped vapour in pressure differential tube line can causes wrong reading of instrumentation. 
3. Another ways to make sure is open vent on top of glycol particulate/charcoal filter (if vent valve available). If large amount of vapour comes out while venting is opened, it seems the problems is confirmed. Hydrocarbon carry over in rich glycol solution will caused vapour trapped in glycol particulate or glycol charcoal filter. Since there is no on line vent on glycol filter, it will accumulate on top of filter. 

If hydrocarbon carry over take place, several methods can be applied to avoid

1. Operating at higher temperature, since hydrocarbon vapour will be flashed into top of glycol flash tank and reduce hydrocarbon carry over.
2. Operate at lower pressure also result same effect, increase flash effect and reduce hydrocarbon carry over
3. Check glycol flash tank design. Glycol flash tank may not be design for three phase separator.

Other problem may be found here

Glycol Dehydration -Stripping Column Overhead Temperature

A higher temperature in the top of the still column can increase glycol losses due to excessive vaporization. The boiling point of water is 212°F and the boiling point of TEG is 546°R The recommended temperature in the top of the still column is approximately 225°F. When the temperature exceeds 250°F the glycol vaporization losses may become substantial. The still top temperature can be lowered by increasing the amount of glycol flowing through the reflux coil.

If the temperature in the top of the still column gets too low, too much water can be condensed and increase the reboiler heat load. Too much cool glycoi circulation in the reflux coil can sometimes lower the still top temperature below 220°F. Thus, most reflux coils have a bypass to allow manual or automatic control of the stripping still temperature.

Stripping gas will have the effect of requiring reduced top still temperature to produce the same reflux rate.

Glycol Dehydration - Feed Gas Temperature

At constant pressure, the water content of the inlet gas increases as the inlet gas temperature increases. For example, at 1,000 psia and 80°F gas holds about 34 Ib/MMscf, while at 1,000 psia and 120°F it will hold about 104 Ib/MMscf. At the higher temperature, the glycol will have to remove over three times as much water to meet a pipeline specification of 7 lb/MMscf.

An increase in gas temperature may result in an increase in the required diameter of the contact tower. As was shown in separator sizing , an increase in temperature increases the actual gas velocity, which in turn increases the diameter of the vessel.

inlet gas temperatures above 120°F result in high triethylene glycol losses. At higher gas temperatures tetraethylene glycol can be used, but it is more common to cool the gas below 120°F before entering the contactor. The more the gas is cooled, while staying above the hydrate formation temperature, the smaller the glycol unit required.

The minimum inlet gas temperature is normally above the hydrate formation temperature and should always be above 50°F. Below 50°F glycol becomes too viscous. Below 60°F to 70°F glycol can form a stable emulsion with liquid hydrocarbons in the gas and cause foaming in the contactor.

There is an economic trade-off between the heat exchanger system used to cool the gas and the size of the glycol unit. A larger cooler provides for a smaller glycol unit, and vice versa. Typically, triethylene glycol. units are designed to operate with inlet gas temperatures between 80°F and 110°F.

Glycol Dehydration - Feed Gas Temperature

We invariably cool the compressor discharge prior to dehydration. Unfortunately, natural gas will be reheated—sometimes by 10°F — in a typical gas field dehydration contactor. This occurs because of two factors:
• The circulating glycol may be 70° hotter than the contactor gas inlet temperature.
• The heat of condensation or absorption of the water vapor contained in the wet natural gas must be dissipated into the dried natural gas.

If the glycol contactor is properly designed (see chapter 6) this temperature rise will not effect dehydration efficiency. However, transmission temperatures will increase.

Glycol Dehydration - Tower Flooding

The field supervisor’s first indication of a flooded contactor tower is usually a report of excessive glycol loss. A check of a lowpoint bleeder on the gas pipeline downstream of the tower will show glycol. After refilling the glycol reboiler, the level in the reboiler gauge glass noticeably decreases after a few hours. This is a further indication of flooding. Of course, a dehydration system loosing glycol this fast cannot dry natural gas on a continuous basis.

One simple explanation of such glycol losses is a leaking dry gas to dry glycol heat exchanger (Figure 6-2). Note that the glycol pressure in this heat exchanger will be slightly higher than the gas pressure. To check for leakage, shut off and block in the glycol pump, block in the dry glycol at the contactor tower, and open an intervening bleeder between the pump and the tower. If gas does not blow out of the bleeder, the exchanger is not leaking.