Glycol Dehydration – Process Description #2
On larger streams filter separators are used as inlet scrubbers to further reduce glycol contamination and thus increase the life of the glycol charge. Due to their cost, filter separators are not normally used on streams less than approximately 50 MMscfd. Often on these smaller units a section in the bottom of the contactor is used as a vertical inlet scrubber as shown in Figure 8-5.
Dry gas from the top of the gas/glycol contactor flows through an external gas/glycol heat exchanger. This cools the incoming dry glycol to increase its absorption capacity and decrease its tendency to flash in the contactor and be lost to the dry gas. In some systems, the gas passes over a glycol cooling coil inside the contactor instead of the external gas/glycol heat exchanger.
The glycol reconcentration system is shown in Figure 8-6. The rich or “wet” glycol from the base of the contactor passes through a reflux condenser to the glycol/glycol preheater where the rich glycol is heated by the hot lean glycol to approximately 170°F to 200°F. After heating, the glycol flows to a low pressure separator operating at 35 to 50 psig, where the entrained gas and any liquid hydrocarbons present are removed. The glycol/condensate separator is a standard three-phase vessel designed for at
least 15-30 minutes retention time and may be either horizontal or vertical. It is important to heat the glycol before flowing to this vessel to reduce its viscosity and encourage easier separation of condensate and gas.
The gas from the glycol/condensate separator can be used for fuel gas, In many small field gas packaged units this gas is routed directly to fire lubes in the reboiler, and provides the heat for reconcentrating the glycol. This separator is sometimes referred to as a gas/glycol separator or “pump gits” separator.
The wel glycol from the separator flows through a sock filter to remove solids and a charcoal filter to absorb small amounts of hydrocarbons that may build up in the circulating glycol. Sock filters are normally designed for the removal of 5-micron solids. On units larger than 10 gpm it is common to route only a sidestream of 10 to 50% of total glycol flow through the charcoal filter. The filters help minimize foaming and sludge build-up in the reconcentrator.
The glycol then flows through the glycol/glycol heat exchanger to the still column mounted on the reconcentrator, which operates at essentially atmospheric pressure. As the glycol falls through the packing in the still column, it is heated by the vapors being boiled off the liquids in the reboiler. The still works in the same manner as a condensate stabilizer. The falling liquid gets hotter and hotter. The gas flashing from this liquid is mostly water vapor with a small amount of glycol. Thus, as the liquid falls through the packing it becomes leaner and leaner in water. Before the vapors leave the still, they encounter the reflux condenser. The cold rich glycol from the contactor cools them, condensing the glycoi vapors and approximately 25 to 50% of the rising water vapor. The result is a reflux liquid stream, which reduces the glycol losses to atmosphere to almost zero. The water vapor exiting the top of the still contains a small amount of volatile hydrocarbons and is normally vented to atmosphere at a safe location. If necessary, the water vapor can be condensed in an aerial cooler and routed to the produced water treating system to eliminate any potential atmospheric hydrocarbon emission.