Hydrocarbon Compression

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Choice of Glycol

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The commonly available glycols and their uses are:
1. Ethylene glycol—High vapor equilibrium with gas so tend to lose to gas phase in contactor. Use as hydrate inhibitor where it can be recovered from gas by separation at temperatures below 50° F.
2. Diethylene glycol—High vapor pressure leads to high losses in contactor. Low decomposition temperature requires low reconcentrator temperature (315°F to 340°F) and thus cannot get pure enough for most applications.
3. Triethylene glycol—Most common. Reconcentrate at 340°F to 400°F for high purity. At contactor temperatures in excess of 120°F tends to have high vapor losses to gas. Dew point depressions up to
150°F are possible with stripping gas.
4. Tetraethylene glycol—More expensive than triethylene but less losses at high gas contact temperatures. Reconcentrate at 400°F to 430°F.

Almost all field gas dehydration units use triethylene glycol for the reasons indicated. Normally when field personnel refer to “glycol” they mean triethylene glycol and we will use that convention in the remainder of this chapter.

Written by Jack

September 20th, 2009 at 5:39 pm

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Glycol Dehydration Process Part 3

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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.

 Glycol Dehydration Process Part 3

Written by Jack

September 20th, 2009 at 3:51 pm

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Glycol Dehydration Process Part 2

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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.

 Glycol Dehydration Process Part 2

 Glycol Dehydration Process Part 2

Written by Jack

September 20th, 2009 at 3:48 pm

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Glycol Dehydration Process Part 1

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Most glycol dehydration processes are continuous. That is, gas and glycol flow continuously through a vessel (the “contactor” or “absorber”) where they come in contact and the glycol absorbs the water. The glycol flows from the contactor to a “reboiler” (sometimes called “reconcentrator” or “regenerator”1) where the water is removed or “stripped” from the glycol and is then pumped back to the contactor to complete the cycle.

Figure 8-4 shows a typical trayed contactor in which the gas and liquid are in counter-current flow. The wet gas enters the bottom of the contactor and contacts the “richest” glycol (glycol containing water in solution) just before the glycol leaves the column. The gas encounters leaner and leaner glycol (that is, giycol containing less and less water in solution), as it rises through the contactor. At each successive tray the leaner glycol is able to absorb additional amounts of water vapor from the gas. The counter-current flow in the contactor makes it possible for the gas to transfer a significant amount of water to the glycol and still approach equilibrium with the leanest glycol concentration.

The contactor works in the same manner as a condensate stabilizer tower. As the glycol falls from tray to tray it becomes richer and richer in water. As the gas rises it becomes leaner and leaner in water vapor. Glycol contactors will typically have between 6 and 12 trays, depending upon the water dew point required. To obtain a 7 Ib/MMscf specification, 6 to 8 trays are common.

As with a condensate stabilizer, glycol contactors may have bubble cap trays as shown in Figure 8-4, or they may have valve trays, perforated trays, regular packing or structured packing. Contactors that are 12% in. and less in diameter usually use regular packing, while larger contactors usually use bubble cap trays to provide adequate contact at gas flow rates much lower than design. Structured packing is becoming more common for very large contactors.

It is possible to inject glycol in a gas line and have it absorb the water vapor in co-current flow. Such a process is not as efficient as countercurrent flow, since the best that can occur is that the gas reaches near equilibrium with the rich glycol as opposed to reaching near equilibrium with the lean glycol as occurs in counter-current flow. Partial co-current flow can be used to reduce the height of the glycol contactor by eliminating the need for some of the bottom trays.

The glycol will absorb heavy hydrocarbon liquids present in the gas stream. Thus, before the gas enters the contactor il should pass through a separate inlet gas scrubber to remove liquid and solid impurities that may carry over from upstream vessels or condense in lines leading from the vessels. The inlet scrubber should be located as close as possible to the contactor.

 Glycol Dehydration Process Part 1

Written by Jack

September 20th, 2009 at 3:45 pm

Glycol Dehydration

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By far the most common process for dehydrating natural gas is to contact the gas with a hygroscopic liquid such as one of the glycois. This is an absorption process, where the water vapor in the gas stream becomes dissolved in a relatively pure glycol liquid solvent stream. Glycol dehydration is relatively inexpensive, as the water can be easily “boiled” out of the glycol by the addition of heat. This step is called “regeneration” or “reconcentration” and enables the glycol to be recovered for reuse in absorbing additional water with minimal loss of glycol.

 

Written by Jack

September 20th, 2009 at 7:06 am

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