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.

Glycol Dehydration - Glycol Gas Heat Exchanger Leak

The hot glycol from the reboiler is cooled by heat exchange with the wet glycol from the contactor. This heat transfer typically takes place in a double-pipe or plate-type exchanger. On one of the double-pipe heat exchangers, I noticed that the reboiled glycol was being cooled to a rather low temperature. I suspected that this could be an indication of a leaking feed-effluent exchanger. That is, cooler (120°F) wet glycol might be leaking into warmer (165°F) dry glycol. To verify my suspicions, I blocked in the dry glycol at the reboiler and at the suction to the pump. The appearance of a steady stream of liquid at an intervening bleeder confirmed that the feed-effluent exchanger was leaking, hi effect, wet glycol was bypassing the reboiler and flowing straight back to the contactor tower.

After fixing the leak, this reboiler and the units that had suffered from an inefficient pump and a faulty temperature controller were put back on-line. The treated natural gas was checked and found to meet pipeline moisture specifications.

Glycol Dehydration

The gas exiting the top of the contactor in Figure 6-1 can be assumed to be in equilibrium with the reboiled—i.e., dry—glycol. The higher the glycol reboiler temperature, the dryer the glycol. The dryer the glycol, the dryer the treated natural gas. For most of the year in El Gringo, critical control of the glycol reboiler temperature gas was not vital. Relatively cool ambient temperatures maintained the top temperature of the contactor towers below 110°F. But now, in mid-July, this temperature was peaking at 122°F every afternoon. I checked my gas purification data book1 and calculated that, for the 1,020 psig operating perssure of the contactors, it should be possible to meet the required moisture specification. My calculations were based on a reboiler temperature at 375°F. For triethylene glycol, which is the work horse of the gas drying industry, the maximum recommended reboiler temperature to prevent thermal degradation of the glycol is 400°F. The six El Graingo dehydrator reboilers were all set to hold 375°F. But by checking the actual reboiler temperatures with a calibrated thermometer, I determined that one of the reboilers was actually operating at 350°F as opposed to 375°F. This reduced temperature was sufficient to greatly increase the water concentration of the “dry” glycol, so that the moisture content of gas treated with this glycol stream was doubled.

A simple recalibration of the reboiler temperature controller rectified this problem. Incidentally, operating a triethylene glycol reboiler at 375°F-400°F does not necessarily result in a noticeable increase in glycol degradation. The trick is to keep the glycol filters in good repair. Dirty glycol fouls the reboiler heat-transfer tube. This in turn causes hot spots on the heat-transfer surface, which accelerates thermal decomposition.