Process Description
The Gas Dehydration Unit removes water vapor from produced gases destined for pipelines using tri-ethylene glycol (TEG) as the desiccating agent in the process. Removal of water vapor is essential to avoid the formation of slugs of water and hydrates (ice-like solids) in downstream operations. TEG remains completely in the liquid state at the normal operating conditions of the Gas Dehydration process. The so-called wet gas contacts lean, regenerated TEG in the Glycol Contactor T-601. The rich TEG is then regenerated in the Still Column T-611 by heating up the TEG with hot oil. The lean TEG is pumped back to Glycol Contactor. Dried gas produced from the Glycol Contactor contains a fraction of the water vapor concentration in the wet gas.
Glycol Contactor T-601
Wet gas from battery limit (e.g. a gas-oil separation plant) enters the bottom of Glycol Contactor T-601 and flows up into the bottommost tray of the column. The Glycol Contactor consists of 15 trays and the wet gas contacts TEG that falls by gravity through the trays from the top of the Contactor. As a result of this intimate contacting, TEG will absorb the water vapor from the feed gas into the liquid phase. The concentration of water vapor in the feed gas is normally 780 ppm and is reduced to 37 ppm by the TEG. The wet gas flow rate is 260.7 MMCF/D and the water removed in the Contactor is 384 LB/H. The lean TEG flow rate is 20 GPM (1,200 GPH). The treat ratio of TEG to water removed is 3.1 GAL/LB. Lean TEG contains 0.7 WT% water, while rich TEG contains 3.8 WT% water.
Cool, lean TEG enters the top of the Glycol Contactor T-601 and is distributed evenly on to the topmost tray. A water-rich TEG solution leaves the bottom of T-601. Because the rate of water absorption is low relative to the total gas flow rate, the temperatures of rich TEG and dried gas leaving T-601 are only slightly above the gas feed temperature. The rich TEG is then sent to the Still Condenser E-611 in the regeneration section. The dried gas is sent to battery limits under pressure control. A wire mesh pad is installed at the top of T-601 to coalesce any lean TEG that may be entrained in the gas leaving the top tray.
Any coalesced TEG falls back to the top tray. The residence time of rich TEG in the bottom of T-601 is normally 10 minutes
Gas/Glycol Exchanger E-601
Lean TEG pumped from the regeneration section is cooled to within 10 degrees of the wet gas feed temperature in Gas/Glycol Exchanger E-601 prior to entering the
top of T-601. The final temperature of the lean TEG is controlled by bypassing some of the flow of warm TEG around E-601.
Note that there is no exhaustion of heat to air or cooling water in the process because the cool product gas from T-601 is used to cool down the warm lean TEG.
Still Condenser E-611
Rich TEG from the bottom of T-601 is used to condense a portion of the water vapor generated from heating the rich TEG in Still Column T-611. The Still Condenser E611 is located in the top of T-611. Any condensed water produced by E-611 falls directly to the top tray of T-611 to provide reflux that will wash back any entrained TEG from the top tray. Normally, all the rich TEG is routed through E-611. If desired, a portion of the rich TEG can be bypassed around E-611 to control the rich TEG temperature entering the Flash Separator D-611. Controlling this temperature also changes the amount of reflux generated by E-611 for the Still Column.
Flash Separator D-611
Rich TEG from E-611 is routed to the Flash Separator D-611. The Flash Separator separates any entrained gases and hydrocarbon liquids from the rich TEG prior to entering the Still Column T-611. Flashed gases are routed to the flare system. To ensure pressure control of D-611 in case there are no flash gases in the rich TEG, fuel gas is added to the vapor space of D-611.
Hydrocarbon liquids will settle out and float on the rich TEG that accumulates in the main part of D-611. A baffle is provided in D-611 to allow any accumulated hydrocarbon liquids to spill over to the hydrocarbon sump. These hydrocarbons are recycled back to the production unit at battery limits.
The residence time of rich TEG in D-611 is normally 5 minutes. The residence time of hydrocarbon liquid in the sump is normally more than 60 minutes.
Glycol Filters F-611A/B
Rich TEG, free of dissolved gases and liquid hydrocarbons, is routed from the bottom of D-611 through the Glycol Filters F-611A/B and on to the Rich/Lean Exchanger E612 before entering the Still Column T-611. The first filter is a mechanical cartridge filter that traps solid particles in the rich TEG. The second is a charcoal filter designed to remove organic impurities from the rich TEG.
Rich/Lean Exchanger E-612
Rich/Lean Exchanger E-612 cools the hot, lean TEG drawn from Glycol Drum D-612 using rich TEG from the Glycol Filters F-611A/B. The rich TEG is heated up prior to entering the Still Column, T-611. E-612 significantly reduces the amount of heat required to regenerate the rich TEG in the Still Column.
Still Column T-611
Still Column T-611 consists of 10 trays and receives warm, rich TEG from E-612 on the fourth tray from the top of T-611. The top three trays of T-611 wash any entrained TEG back to the feed tray using reflux (essentially water) generated by Still Condenser E-611.
The base of T-611 is directly connected to the Still Reboiler E-613. Vapor from E-613 directly enters the bottommost tray of T-611 and heats up the rich TEG falling through the trays. As the rich TEG heats up it will release some of water it contains. Because the water is now in a solution that is mostly TEG (approximately 99 weight%), the temperature of the TEG must be significantly higher than the boiling point of water in order to vaporize it. TEG has a very high boiling point so that very little TEG is vaporized in T-611 and E-613.
The vaporized water leaving the TEG feed tray will then contact the reflux from E613 in the top three trays. Some of the water vapor will be condensed on these three trays but most of it will make it to E-613 where a portion of the vapor will be condensed. The balance of the water vapor will continue to the flare system in case it contains any gases or hydrocarbons that may have gotten through the Flash Separator D-611.
The hot, lean TEG from the bottom tray of the Still Column falls into Reboiler E-613.
Reboiler E-613
Reboiler E-613 is a horizontal vessel with a baffle that divides the volume into a main compartment for heating TEG and an overflow channel to route lean regenerated TEG to the Glycol Drum D-612. The main compartment of E-613 contains heating tubes throughout the lower section. Hot oil from battery limits is circulated through the tubes to heat the TEG to the desired temperature. The Reboiler operates at 400 DEG F. Although higher reboiler temperatures will improve the unit’s dehydration performance, operation above this temperature should be avoided as the TEG will chemically degrade.
The residence time of lean TEG in E-613 is normally 8 minutes. Note that the baffle in E-613 has holes at the base to allow liquid on the boiling side of E-613 to drain into Glycol Drum D-612 when the unit is shut down.
Glycol Drum D-612
The Glycol Drum D-612 stores hot regenerated TEG overflowing from the Reboiler E613. The level of TEG in D-612 will vary if the levels of TEG in other equipment are changing. However, over time the level in D-612 should remain steady because the normal losses of TEG are very small because of its very low volatility. A declining level for no apparent reason most likely indicates a significant loss of TEG is occurring somewhere in the unit.
Hot lean TEG is taken from the bottom of D-612 and routed through Rich/Lean Exchanger E-612. Cooled lean TEG from E-612 is pumped by the Glycol Circulation Pumps P-611A/B to the Gas/Glycol Exchanger E-601.
The residence time of lean TEG in D-612 is normally 12 minutes.
Lines are provided for adding fresh glycol and draining D-612 to storage.
Glycol Circulation Pumps P-611A/B
Glycol Circulation Pumps P-611A/B are electric motor-driven multistage centrifugal pumps capable of delivering high head. Normally one pump is in operation at a rate of 20 GPM. The pumps take suction at roughly atmospheric pressure because Still Column T-611 operates open to the flare system. The pumps must ultimately push TEG into Glycol Contactor T-601 which operates at 1,550 PSIG. The pumps deliver warm TEG to the Gas/Glycol Exchanger E-601 prior to entering T-601.
Instrumentation
Basic Controls: Gas/Glycol Exchanger E-601
The flow of the wet gas to the bottom of the Glycol Contactor T-601 is controlled by FIC-601. AI-601 indicates the water content of the wet gas. Pressure and temperature of the wet gas stream are indicated on PI-601 and TI-601, respectively.
The temperature of the dry gas leaving the top of T-601 is indicated on TI-603. The temperature of the gas outlet stream from Gas/Glycol Exchanger E-601 is indicated on TI-604. AI-602 indicates the water content of the dried gas. The flow of the dried gas to battery limits is controlled by PIC-602.
The lean TEG circulation flow to T-601 is controlled by FIC-602.
The pressure drop across the trays of T-601 is indicated on PDI-603.
The level of rich TEG in the bottom of T-601 is controlled by LIC-601.
Basic Controls: Still Condenser E-611
The temperature of the rich TEG going from Still Condenser E-611 to the Flash Separator D-611 is controlled by TIC-611 which adjusts the position of a three-way valve that bypasses rich TEG around E-611. Normally, this controller is in manual mode and the valve is lined up with 100% flow to E-611.
Basic Controls: Flash Separator D-611
The pressure of D-611 is controlled by PIC-611 which does split-range control of the fuel gas and flare valves:
- When PIC-611’s output is 0%, the fuel gas valve is wide open and the flare valve is closed
- When PIC-611’s output is 50%, both valves are closed
- When PIC-611’s output is 100%, the fuel gas valve is closed and the flare valve is fully open.
LIC-611 controls the level of rich TEG in D-611. LIC-612 controls the level of oil in the side sump of D-611.
Basic Controls: F-611A/B Glycol Filters
The pressure drop across F-611A/B Glycol Filters is indicated on PDI-612. PDI-612 will alarm in case of plugging that causes a high pressure drop across the filters.
Basic Controls: Rich/Lean Exchanger E-612
The temperature of rich TEG from Rich/Lean Exchanger E-612 is indicated on TI-613. The temperature of lean TEG from E-612 is indicated on TI-616.
Basic Controls: Still Column T-611
The temperature of the vapor leaving the top tray of Still Column T-611 is indicated on TI-612.
The pressure drop across the trays of T-611 is indicated on PDI-613.
Basic Controls: Reboiler E-613
The temperature of the lean TEG in Reboiler E-613 is controlled by TIC-614 which adjusts the setpoint of hot oil flow controller FIC-613.
The level of liquid in E-613 is indicated on LI-613. This level should normally be around 78 – 80%. The liquid in E-613 spills over a baffle at this level. Note that the baffle contains holes at the base to drain the liquid on the heating side of E-613 to Glycol Drum D-612 when the unit is shut down.
Basic Controls: Glycol Drum D-612
The level of lean TEG in Glycol Drum D-612 is indicated on LI-614. This level will fluctuate as other levels in the unit are changing, but should return to approximately the same indication when levels stabilize.
The temperature of lean TEG drawn from D-612 is indicated on TI-615.
Fresh glycol can be added to D-612 using HIC-611.
Glycol from D-612 can be sent to storage using HIC-612.
Basic Controls: Glycol Circulation Pumps P-611A/B
The motors of Glycol Circulation Pumps P-611A/B are operated by switches HS-611A/B, respectively. If a motor should fail, the switch of the affected switch will stay in the STOP state.