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Gas Plant Training

Module 4

TEG Dehydration Systems

Gas Plant Training

Module IV: TEG Dehydration Systems

Effective Date: 05/07/2012 Module #4: Dehydration Systems

4-1

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

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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

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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.

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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

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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 ~195oC

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

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Contactor C301

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.

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Glycol Regenerator

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.

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Flash Separator V308

Flash Separator V308 (Figure 4-6) is a 2-phase separator maintained approximately half-

full 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

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Lean/Rich Exchanger E309

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.

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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.

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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

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Still O/H Condenser AR311

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

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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

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5. Which unit boils hydrocarbons from the glycol?

b. Regenerator

c. Still O/H Condenser

d. Flash Separator

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

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Answer Key

Multiple Choice:

1. d 6. b

2. c 7. c

3. c 8. b

4. a 9. c

5. c 10. d

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