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

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.

GLYCOL REGENERATION PROCESS AND EQUIPMENT

THE GLYCOL REBOILER

The glycol regeneration process is very important to maintain the correct concentration of the lean glycol. Refer to Figures: 50 & 52 for the equipment used in the Glycol Regeneration Process.


The glycol reboiler is the main piece of equipment that plays this role in the regeneration process. The reboiler supplies heat to separate the glycol and water by a simple distillation process.


The system consists of a ' U ' shaped, combustion chamber with gas burners, set into the shell of the reboiler and includes an outlet stack for the waste combustion gases.


The shell also contains a ' Weir ' that maintains the level of glycol above the fire-tube in order to prevent overheating of the tube and subsequent damage and/or glycol decomposition by excess heat.



Figure: 32 - Fire-tube Reboiler


The temperature of the reboiler should be in the range of 375 to 390 °F. This temperature will usually give good distillation of the rich glycol and evaporate all water out of it.


The glycol should never be heated above 400 °F as it begins to decompose above that temperature.


Note: When making adjustments to reboiler temperature, never increase the temperature setting by more than five degrees at a time.


Too great an increase will cause the control system to open the fuel gas valve too wide, giving a large burner flame which in turn will cause flame impingement on the inside of the fire-tube. This will lead to ' Hot-spots ' and cause damage to the fire-tube and breakdown of the glycol into corrosive organic acids.


If coke , salts or tar deposits form on the fire tube, the heat transfer into the glycol is reduced, the control system will increase the fuel to maintain the glycol temperature and tube failure can result. Localised overheating, especially where salt deposits accumulate, will decompose the glycol.


Salt deposits can be detected by shutting off the burner on the glycol reboiler system at night and looking down the fire-box. A bright red glow will be visible at the hot spots on the fire tube walls where salt deposits have collected. An analysis of the glycol will determine the degree of the contamination.


It is highly recommended that, during a plant start-up, make sure the reboiler is up to the desired operating temperature before flowing gas through the contactor .


Some fires have been caused by leaks in the gas lines near the fire-box. The best precaution is to have valves and regulators in the gas line at a suitable distance from the firebox.


Another very effective measure is the addition of a flame arrestor around the fire-box. If the flame arrestor is properly designed, even severe gas leaks in the immediate vicinity of the fire-box will not ignite.