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 - 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: handling and operational problems pH - salt - oxidation

5. GLYCOL pH CONTROL

The pH of a glycol solution is the measure of its acidity or alkalinity, and is measured on a scale of 0 - 14. A pH of less than 7 is an acid solution , 7 is neutral and, greater than 7 is an alkaline solution.

The corrosion rate of equipment increases rapidly with a decrease in the glycol pH. The formation of organic acids, resulting from the oxidation of glycol, thermal decomposition products or acid gases picked up from the gas stream, are the most troublesome corrosive compounds. Therefore, the glycol pH should be checked periodically and kept on the basic side by neutralising the acidic compounds with borax, Ethanol-amines or other suitable alkaline chemicals to maintain the pH at 7.5 to 8.0. A glycol solution that is too alkaline - i.e. pH greater than 9.00, tends to foam and emulsify .

6. SALT CONTAMINATION

Salt deposits accelerate equipment corrosion, reduce heat transfer in the glycol reboiler and change the specific gravity readings when a hydrometer is used to determine glycol concentration. These troublesome compounds cannot be removed by normal regeneration processes. Salts should be prevented by the use of effective filters or an efficient scrubber.

7. GLYCOL OXIDATION

Oxygen can enter the glycol system via the vapour space of an un-blanketed storage tank or through the glycol make-up pump packing glands ... etc. The glycol will oxidise readily in the presence of oxygen (air) and form corrosive organic acids

Precautions should be taken to prevent glycol oxidation. It is highly recommended, that process vessels that can draw in air as the liquid level is lowered, should contain a gas blanket to keep oxygen (air) out of the system. Oxidation inhibitors, such as Hydrazine can be used to prevent the formation of corrosive, organic acids.

8. SLUDGE FORMATION

Accumulation of solid particles and tarry hydrocarbons very often forms in the glycol solution. This sludge is suspended in the circulating glycol and, over a period of time, the accumulation becomes large enough to settle out.

This action results in the formation of a black, sticky and abrasive gum which can cause erosion of the equipment. It usually occurs when the glycol pH is low and becomes very hard and brittle when deposited on the absorber trays, still column parts and other areas in the circulating system. Good, effective filtration will prevent the build-up of sludge in the glycol system.