Removing most of the water vapor from the gas is required by most gas sales contracts, because it prevents hydrates from forming when the gas is cooled in the transmission and distribution systems and prevents water vapor from condensing and creating a corrosion problem. Dehydration also increases line capacity marginally. Most sales contracts in the southern United States call for reducing the water content in the gas to less than 7 Ib/MMscf. In colder climates, sales requirements of 3 to 5 Ib/MMscf are common. The following methods can be used for drying the gas:
1. Cool to the hydrate formation level and separate the water that forms. This can only be done where high water contents (±30Ib/MMscfd) are acceptable.
2. Use a Low-temperature Exchange (LTX) unit designed to melt the hydrates as they are formed. Figure 2-15 shows the process. LTX units require inlet pressures greater than 2,500 psi to work effectively. Although they were common in the past, they are not normally used because of their tendency to freeze and their inability to operate at lower inlet pressure as the well FTP declines.
3. Contact the gas with a solid bed of CaCl2. The CaCl2 will reduce the moisture to low levels, but it cannot be regenerated and is very corrosive.
4. Use a solid desiccant, such as activated alumina, silica gel or molecular sieve, which can be regenerated. These are relatively expensive units, but they can get the moisture content to very low levels. Therefore, they tend to be used on the inlets to low temperature gas processing plants, but are not common in production facilities.
5. Use a liquid desiccant, such as methanol or ethylene glycol, which cannot be regenerated. These are relatively inexpensive. Extensive use is made of methanol to lower the hydrate temperature of gas well flowlines to keep hydrates from freezing the choke.
6. Use a glycol liquid desiccant, which can be regenerated. This is the most common type of gas dehydration system and is the one shown on the example process flowsheet.
Figure 2-16 shows how a typical bubble-cap glycol contact tower works. Wet gas enters the base of the tower and flows upward through the bubble caps. Dry glycol enters the top of the tower, and because of the downcomer wier on the edge of each tray, flows across the tray and down to the next. There are typically six to eight trays in most applications. The bubble caps assure that the upward flowing gas is dispersed into small bubbles to maximize its contact area with the glycol. Before entering the contactor the dry glycol is cooled by the outlet gas to minimize vapor losses when it enters the tower. The wet glycol leaves from the base of the tower and flows to the reconcentrator (reboiler) by way of heat exchangers, a gas separator, and filters, as shown in Figure 2-17. In the reboiler the glycol is heated to a sufficiently high temperature to drive off the water as steam. The dry glycol is then pumped back to the contact tower. Most glycol dehydrators use triethylene glycol, which can be heated to 340°F to 400°F in the reconcentrator and work with gas temperatures up to 120°F. Tetraethylene glycol is more expensive but it can handle hotter gas without high losses and can be heated in the reconcentrator to 400°F to 430°F.