Since there is a large difference between the boiling point of triethylene glycol (546°F) and water (212°F), the still column can be relatively short (10 to 12 ft of packing). The glycol liquid in the reboiler is heated to 340°F to 400°F to provide the heat necessary for the still column to operate. Higher temperatures would vaporize more water, but may degrade the glycol.
If a very lean glycol is required, it may be necessary to use stripping gas. A small amount of wet natural gas can be taken from the fuel stream or contactor inlet stream and injected into the reboiler. The stripping gas can be taken from the fuel stream or the contactor inlet stream and injected into the reboiler. The “leaness” of the gas depends on the purity of the wet glycol and the number of stages below the reconcentrator. The stripping gas is saturated with water at the inlet temperature and pressure conditions, but adsorbs water at the reboiler conditions of atmospheric pressure and high temperatures. The gas will adsorb the waler from the glycol by lowering the partial pressure of the water vapor in the reboiler. Stripping gas exits in the still column with the water vapor. If necessary.
I he gas can be recovered by condensing the water and routing the gas to a vapor recovery compressor.
The lean glycol flows from the reboiler to a surge tank which could be constructed as an integral part of the reboiler as in Figure 8-6. The surge tank must be large enough to allow for thermal expansion of the glycol and to allow for reasonable time between additions of glycol. A well designed and operated unit will have glycol losses to the dry gas from the contactor and the water vapor from the still of between 0.01 and 0,05 gal/MMscf of gas processed.
The lean glycol from the atmospheric surge tank is then pumped back to the contactor to complete the cycle. Depending upon the pump design, the lean glycol must be cooled by the heat exchangers to less than 200UF to 25()CF before reaching the pumps. There are many variations to the basic glycol process described above. For higher “wet” gas flow rates greater than 500 MMscfd, the “cold finger” condenser process as shown in Figure 8-7 is often attractive. A cold finger condenser tube bundle with cold rich gas from the contactor is inserted either into the vapor space at the reboiler or into a separate separator. This creates a “cold finger” in the vapor space. The hydrocarbon liquid and vapor phases along with the
glycol/water phase are separated in a three-phase separator. The lean glycol from the bottom of the condenser is cooled, pumped, cooled again, and fed to the contactor.