Module 4 TEG Dehydration Systems
Table of Contents Introduction...2 Objectives...2 Glycol Dehydration...3 Dehydration System Process Control...4 Contactor C301...6 Glycol Regenerator...7 Flash Separator V308...8 Lean/Rich Exchanger E309...9 Gas/Glycol Exchanger E302...9 Glycol Filters F306/307... 10 Glycol Pumps P305A/B... 11 Still O/H Condenser AR311... 12 Module References... 12 Module 4 Exercises... 13 Written Exercises ~ Self-Assessed... 13 Answer Key... 15 4-1
Introduction In this module, you will learn about TEG (triethylene glycol) gas dehydration systems. You will understand the principles behind natural gas dehydration using glycol as well as the process for regenerating the water-saturated glycol. Objectives After completing the Dehydration Systems module, you will be able to: Describe process flows for: Gas stream Rich glycol stream Lean glycol stream Fuel gas State normal operating parameters for: Contactor Regenerator Flash Separator Glycol pumps Explain why glycol temperature control is critical, and why temperature differs for the Contactor and Regenerator processes 4-2
Glycol Dehydration The main functions of a glycol dehydration system are to: 1. Absorb water from the gas stream using lean glycol 2. Regenerate water-rich glycol to a lean state Dehydrating the gas is critical in order to prevent hydrates from forming in downstream pipelines or equipment. The absorption of water into lean glycol is favored by high pressure and low temperature. The glycol regeneration process (whereby the absorbed water is boiled from the glycol) takes place at low pressure and high temperature. As shown in Figure 4-1, the central component used for glycol regeneration is a small distillation column (stripping tower) connected to a reboiler and surge drum. The solution is heated at the bottom by the reboiler and cooled at the tower top by rich glycol from the Gas/Glycol Contactor. Steam and gases are drawn from the stripping tower top, while the lean glycol drains to the reboiler tank and surge drum. Figure 4-1: Basic glycol regeneration system Cool rich glycol from the Contactor flows (under level control) to the preheat coil in the stripping tower top. The glycol experiences a significant pressure drop across the level control valve, which lowers the boiling point of the absorbed water. 4-3
The pre-heat coil has two purposes: 1. The rich glycol picks up some free heat on its way to being regenerated, and 2. The rich glycol cools vapors rising through the stripping tower, thereby condensing any residual glycol vapors entrained with the O/H water vapor. From the pre-heat coil, the rich glycol flows through the lean/rich exchanger, which in the example above is a coil submerged in the lean glycol accumulator other systems use external plate-type heat exchangers (or a combination of both types). The lean/rich exchanger uses inflowing rich glycol to cool the outflowing lean glycol (recall that water absorption works best at low temperatures). At the same time, the rich glycol picks up more free heat on its way to regeneration. (Use of a lean/rich exchanger reduces reboiler heat requirements by ~50%.) From the lean/rich exchanger, the rich glycol enters the glycol flash drum (also termed a flash tank or flash separator ), where any dissolved hydrocarbon gases boil off due to the combined effects of: 1. The pressure drop across the LCV 2. The temperature increase from preheating Flash Drum O/H gas is recycled (or flared), while the rich glycol flows through filters to the distillation column (stripping tower), where it mixes with hot, rising vapors generated in the reboiler. Reboiler heat vaporizes water in the rich glycol returning it to a lean state. Following regeneration, the lean glycol is approximately 70% glycol and 30% water. Dehydration System Process Control The Dehydration System shown in Figure 4-2 is comprised of: Contactor C301 Gas/Glycol Exchanger E302 TEG Level Chamber Flash Separator V308 Particulate Filter F306 Carbon Filter F307 Lean/Rich Glycol Exchanger E309 Glycol Regenerator H303 Glycol Pumps P305A/B 4-4
Figure 4-2: Dehydration System process flows and instrumentation The main process controllers are: TIC030301 ON/OFF Fuel gas valve TV030301 controller; maintains Reboiler H303 temperature at ~195 o C LC030801 Maintains Flash Separator V308 level setpoint by controlling rich glycol flow through LV030801 PCV030601 Maintains Flash Separator V308 pressure at 414 kpa by controlling O/H gas flow to the Recycle Compressor PCV030803 Relieves excess gas to Flare if V308 pressure rises above 448 kpa LC030101 Removes liquid water from Contactor C301 s gas inlet chamber through LV030101 to the Produced (Sour) Water Tank LC030102 Maintains a minimum level in Contactor C301 s glycol collection chamber by controlling rich glycol flow through LV030802 4-5
Contactor C301 Gas Plant Training The Glycol Contactor (Figure 4-3) is a steel column with upper and lower chambers. Wet gas from the Inlet Compressor enters the lower chamber, which functions as a cyclone separator. Liquid water separates from the gas stream, collects in the chamber bottom, and is level controlled to the Produced Water Tank. The gas exits the lower chamber though a demister screen, which minimizes liquid carryover to the upper chamber. Figure 4-3: Glycol Contactor C301 The upper chamber contains 8 evenly spaced bubble cap trays. Lean glycol is injected above the top tray. On each tray, the glycol forms a layer before overflowing to the tray below. Gas rises through the tray caps and bubbles through successive layers of glycol. Process conditions vary with height - As the gas rises up the column, its water content decreases; as the glycol drains to successively lower trays, its water content increases. The process continues until water absorption is complete. Dry gas exits the upper chamber through a second demister, which traps and coalesces glycol mists. The dry gas continues to the Sales outlet (or compressor), while the rich glycol is level-controlled from the upper chamber bottom to the Glycol Regenerator. 4-6
Glycol Regenerator Gas Plant Training Glycol regeneration returns rich glycol to a lean state so that it can be reused in the Contactor. The main components of the Glycol Regenerator are the distillation (still) column (Figure 4-4), Reboiler H303 Figure 4-5), and the accumulator tank. Figure 4-4: Regenerator still column Figure 4-5: Reboiler H303 (Top) & Accumulator Glycol exiting the Contactor is relatively cool and contains impurities picked up from the gas stream, so it requires both preheating and filtering prior to regeneration. Contactor pressure drives the rich glycol through a heat exchange coil located in the still column top. Here the rich glycol picks up heat from reboiler vapors before flowing to the Flash Separator, where dissolved hydrocarbons are boiled off. The glycol is then filtered before picking up more heat in the lean/rich exchanger. Finally the glycol is sprayed into the still column (above the lower packed section), where the regeneration process is completed by direct heating and contact with reboiler steam. The fuel gas fired reboiler normally operates at ~200 C. Rising steam heats down flowing rich glycol, which strips away water vapor and returns the glycol to a lean state. While the steam rises, the glycol drips down through the column packing into the reboiler. Rich glycol flowing through the still column s heat exchange coil cools the column top, which condenses any glycol vapor entrained with the stripping steam. Excess lean glycol from the reboiler overflows into the accumulator tank, which provides both surge and storage capacity for the system. It also contains a heat exchange coil for warming fuel gas supplied to the Reboiler burner. 4-7
Flash Separator V308 Gas Plant Training Flash Separator V308 (Figure 4-6) is a 2-phase separator maintained approximately halffull of glycol solution to aid the flashing process. A demister pad on the O/H outlet minimizes glycol carryover with the flash gas.. Figure 4-6: Flash Separator V308 When warm rich glycol solution flows into V308, the sudden pressure drop (from ~4000 kpa to 414 kpa) causes dissolved hydrocarbons to boil off. H/C vapors from the flash tank s overhead connection normally flow to the Recycle Compressor. The glycol is removed from the tank s bottom under level control. Some hydrocarbons will not boil at flash tank pressure, and will instead remain in liquid form. Due to differences in density, liquid hydrocarbons form a layer on top of the rich glycol. Since glycol is drawn from the tank s bottom, the hydrocarbons accumulate over time, forming an ever thickening layer. For this reason, the flash tank has a connection for periodically skimming liquid hydrocarbons to the Hydrocarbon Drain system 4-8
Lean/Rich Exchanger E309 Gas Plant Training The lean/rich glycol exchanger (Figure 4-7) heats rich glycol before it is sprayed into the still column, and cools lean glycol before it is sprayed into the Contactor. Figure 4-7: Lean/Rich Glycol Exchanger E309 Lean glycol entering the Contactor must be cooled, and rich glycol entering the Still column must be warmed, as only cool glycol can absorb water, and only warm glycol can release water. Hot lean glycol from the Reboiler flows though Lean/Rich Glycol Exchanger E309, where it gives up heat that is no longer useful. The cool rich glycol flowing to the still column requires that heat and readily absorbs it. Gas/Glycol Exchanger E302 From the Contactor s O/H outlet, the dry gas flows through in-line Gas/Glycol Exchanger E302 (Figure 4-8). E302 is designed to cool the incoming lean glycol to about 5.5 C above the temperature of the incoming wet gas. It has manual block and bypass valves to regulate the degree of cooling. If the glycol is too cool, heavier hydrocarbons in the gas stream can condense. Hydrocarbon condensate in the Contactor causes foaming, which decreases tower capacity and increases glycol losses. 4-9
Figure 4-8: Gas/ Glycol Exchanger E302 Glycol Filters F306/307 Rich glycol from the Flash Separator passes through cartridge filter F306 (Figure 4-9), which removes dirt and debris. The filter elements must be replaced as they become saturated with particulates. If the filter elements are not replaced, they will eventually become completely plugged and will either stop the glycol flow or collapse and allow the glycol to bypass the elements. Glycol flowing through new filter elements will usually show a pressure drop of 20 to 40 kpa. As the elements pick up particles from the glycol stream, they become clogged and the pressure drop increases. When the pressure drop reaches 100 to 140 kpa, the elements should be replaced. Charcoal Filter F307 (Figure 4-10) primarily removes heavy hydrocarbons. 4-10
Figure 4-9: Particulate Filter F306 Figure 4-10: Charcoal Filters F307 Glycol Pumps P305A/B Pumps P305A and P305B (Figure 4-11) are high pressure (5099 kpa) low capacity (30.3 L/min) positive displacement triplex pumps driven by 5 hp electric motors. One pump normally operates while the other is on standby. The online pump transfers lean glycol solution from the Reboiler s accumulator tank through the heat exchange coil in the Contactor bottom and then through the Gas/Glycol Exchanger to spray nozzles located above the bubble cap trays in the Contactor s upper chamber. Figure 4-11: Glycol Pump P305B 4-11
Still O/H Condenser AR311 Gas Plant Training Steam from the still column s O/H outlet is collapsed back to liquid in Vapor Condenser AR311 (Figure 4-12). The condenser fan is driven by a 5 HP 1800 RPM motor. Manual louvers can be used to adjust air flow across the condenser tube bundle. Both condensed liquids and NCGs (non-condensable gases) flow to the VRU (Vapor Recovery Unit) liquid knock-out drum.. Figure 4-12: Vapor Condenser AR311 Module References Glossary 4-12
Module 4 Exercises Written Exercises ~ Self-Assessed The following questions apply to normal conditions unless otherwise stated. From the given choices, select the best answer to each question. Although other choices may apply under different conditions, do not consider these choices as being the best answers. Put your answers on a separate piece of paper. Do not write in the manual. Multiple Choice 1. The purpose of the dehydration system is to a. remove water from the gas stream b. remove hydrocarbons from the rich glycol c. remove water from the rich glycol d. All of the above 2. The absorption of water into glycol takes place in the a. Flash Separator b. Reboiler c. Contactor d. Still O/H Condenser 3. The glycol Regeneration process takes place at a. high pressure, low temperature b. low pressure, low temperature c. low pressure, high temperature d. high pressure, high temperature 4. Rich glycol leaving the Contactor is first warmed in the a. distillation stripping column preheat coil b. Lean/Rich Glycol Exchanger c. Flash Separator d. Reboiler 4-13
5. Which unit boils hydrocarbons from the glycol? b. Regenerator c. Still O/H Condenser d. Flash Separator Gas Plant Training 6. Rich glycol is returned to a lean state in the a. Flash Separator b. Reboiler c. Contactor d. Still O/H Condenser 7. Which unit is responsible for cooling lean glycol just before it enters the Contactor? a. Glycol Filters b. Lean/Rich Glycol Exchanger c. Gas/Glycol Exchanger d. Accumulator 8. Glycol pumps P305A/B transfer a. rich glycol from the Contactor to the Regenerator b. lean glycol from the Regenerator to the Contactor 9. Steam from the Reboiler s distillation column is collapsed back to vapour in the a. Accumulator b. Flash Separator c. Still O/H Condenser d. Contactor 10. Water from the Contactor bottom transfers to the a. Flash Separator b. VRU Knockout Drum c. Regenerator d. Produced Water Tank 4-14
Answer Key Multiple Choice: 1. d 6. b 2. c 7. c 3. c 8. b 4. a 9. c 5. c 10. d 4-15