Dehydration

GENERAL DESCRIPTION

Before starting on the processes used in the treatment of natural gas for the removal of water

vapour, look at the following diagrams that show definitions of 'Absorption' and 'Adsorption'. 

ABSORPTION : 

Uses liquid desiccants to take in gases. These liquids are called 'Absorbents' and are used not

only for water removal but also to remove other unwanted gases from the natural gas, such as: 

Hydrogen Sulphide (H2S) and Carbon Dioxide (CO2). These processes use different absorbents

to the Glycol used in dehydration. These processes are discussed in other booklets.

Absorption then is:

'The ability of some liquids to 'Take in' or 'Absorb' gases' 

ADSORPTION : 

Uses solid desiccants. These are called 'Adsorbents' and are generally in the form of pellets or

granules. Adsorption processes are used for the removal of any unwanted impurities mainly from

gases or liquids such as: 

Water Vapour (H2O), Hydrocarbon Compounds and other undesirable substances. 

Adsorption then is:

'The ability of the molecules of some solids, to hold on their surface, molecules of other

substances - Gases, Liquids or Solids'. 

e.g. A cigarette filter will adsorb nicotine and tar; A car oil filter will adsorb solid particles from the

circulating oil; Blotting paper will adsorb ink or other liquid etc. 

See Figures: 21 & 22 

EXAMPLE OF ABSORPTION 

Figure: 21 

EXAMPLE OF ADSORPTION 

Figure: 22 

Most produced natural gas contains water. Some of this water is called 'FREE' water, (liquid

phase) and may be removed by passing the gas through a separator or scrubber. After

scrubbing, the gas will still contain Water Vapour. This is the water we are concerned about in

this discussion. 

The term 'DEHYDRATION' is a process of 'WATER REMOVAL' from a substance or

the Drying of a substance. 

The process of water vapour removal from the natural gas stream is carried out by a process

of 'ABSORPTION'(using a LIQUID desiccant), or 'ADSORPTION' (using a SOLID desiccant). 

In many systems we use the 'Absorption' processes - with a liquid desiccant called 'GLYCOL'.

(Generally Tri-Ethylene Glycol (TEG). The process is carried out in towers

called 'ABSORBERS' or 'CONTACTORS'. 

In some processes, the water vapour is removed from natural gas by 'Adsorption' using a solid

desiccant called 'Activated Alumina' (Aluminium Oxide), or 'Molecular Sieve' in a 'Drying'

process. 

Other processes using the Adsorbent principle are: - Instrument air systems generally use 'Silica

Gel' adsorbent; Activated Charcoal is used to remove hydrocarbons and other impurities from a

process stream etc.

GENERAL DEHYDRATION OF NATURAL GAS

All of the medium, low and very low pressure gases from the gosps and degassing areas are

compressed to the same pressure and added to, the high pressure, first stage gosp gas. 

The wet HP gas then enters the absorber tower bottom and flows upwards through contacting

devices (Raschig rings .. etc, or Bubble-caps). The glycol (called LEAN glycol), enters the tower

top and flows down across the contacting devices which give intimate contact between the rising

gas and down-flowing glycol . 

Dry gas leaves the top of the tower and goes for further processing while the wet glycol, (now

called RICH glycol), leaves the tower bottom and passes to the glycol 'REGENERATOR' or

'RECONCENTRATOR' where, by DISTILLATION, the water is vaporised out and passed to the

atmosphere as steam. 

The rich glycol, is then re-circulated around the dehydration unit. 

(See Figures: 23 & 24) 

Figure: 23 -Operation of a Bubble - Cap Tower 

Figure: 24 - Operation of Bubble-caps 

The up-flowing gas has to pass through the RISER and into the BUBBLE-CAPS. The cap turns

the gas flow through 180° forcing it into the liquid on the tray. The gas bubbles through the glycol

and gives up water vapour to the liquid and, after the top tray, the gas passes through a demister

screen that coalesces glycol mist into droplets which fall back down the tower. 

The trays are fitted with 'DOWNCOMERS', (Weirs), that maintain a liquid level on the tray and

carry the glycol down to the tray below and so on down the tower. the down flowing glycol

becomes 'Richer' (absorbs more water) as it flows across each tray to the bottom of the tower. 

The rich absorbent is then piped to the REGENERATION unit. 

The dry gas leaving the tower top, goes to a knock-out drum to separate entrained glycol should

any be carried over with the gas. This glycol, if any, is returned to the glycol system. 

See Figure: 25 

Figure: 25 - Simplified Dehydration Unit 

The presence of water vapour in natural gas can lead to many problems. The dehydration of

natural gas is therefore carried out for the following reasons:

Water vapour reduces the ability of gas to flow in the flowlines and process systems.

Water vapour causes corrosion in lines and equipment.

At low temperature, water vapour & hydrocarbons form hydrates complicated molecules

of hydrocarbon liquid and water, causing blockage of lines and equipment.

Natural gas may contain from 10 to 300 pounds of water vapour per million cubic feet of gas

produced - (10 to 300 lb. water/mmcf), depending on the temperature and pressure of the natural

gas; the warmer the gas, the more water vapour it will contain. 

Tri-Ethylene Glycol (TEG) dehydration systems are the most common means used for the

process. When lean TEG is brought into contact with wet natural gas, it absorbs the water vapour

from the gas stream. 

The picture below shows two absorber towers using Tri-ethylene Glycol as absorbent for the

dehydration of Natural Gas. 

The towers each have sixteen bubble-cap trays as contacting devices to give intimate contact

between the rising gas and down-flowing glycol. 

Absorber Towers 

The pictures on the next page view a Glycol Regeneration Unit from each side:

PRINCIPLES AND OPERATION OF GLYCOL DEHYDRATION UNIT

DESCRIPTION OF PROCESS AND EQUIPMENT

In the contactor, the up-flowing gas gives up water vapour to the glycol flowing down from the top

tray. At the tower top, the dry gas passes through mist extractor elements and then leaves the

contactor top to go on to other processes. 

The mist extractor coalesces fine particles of liquid into large droplets which fall back into the

glycol passing down the tower. In this way, glycol carry-over with the gas stream is minimised.

See Figure: 26. 

Referring to Figure: 48, the wet natural gas enters the bottom of the glycol contactor tower and

rises through the column where it is brought into contact with the lean glycol flowing downwards

across bubble cap trays. 

Figure: 26 - Demister Pad Operation 

The absorption of water vapour during the process, gradually dilutes (weakens) the glycol. The

rich (dilute) solution collects in the bottom of the glycol contactor tower from where it is

discharged to the glycol regeneration unit by way of a level control system. 

At the regeneration unit, the wet glycol flows first to the glycol flash tank where a pressure drop

takes place causing the dissolved gases to leave the glycol as it passes into the flash tank. (This

is similar to opening a bottle of Pepsi for example. As the cap is removed, the gas bubbles out of

the liquid). 

Flash tank pressure is controlled by a PCV in the gas line. (The released gas may be piped to a

flare or fuel system or may be passed into the still column). The rich glycol leaves the bottom of

the flash tank under level control and passes through a 'Reflux coil' placed in the top of the still

column. (This will be explained later). 

After the reflux coil, the glycol is filtered and then passed through a double-pipe heat exchanger

to be heated by the regenerated 'lean' glycol leaving the unit reboiler. (This in turn, cools the

regenerated (lean) glycol). 

The rich glycol now enters the still column at the top tray (below the reflux coil), and flows down

across the bubble-cap trays.

THE REFLUX COIL

The cool, rich glycol as it passes through the reflux coil, picks up heat from the hot, rising gases

passing up the tower from the reboiler. These hot vapours consist of water vapour (steam),

entrained gases and glycol vapour. 

The exchange of heat between the liquid in the reflux coil and the rising hot vapours causes the

glycol vapour to condense and drop back down the still column. 

Some water vapour will also condense but, as it drops back, it is re-vaporised on the top trays of

the column. These liquids dropping down from the reflux coil form the internal reflux in the tower

thus controlling the tower top temperature and therefore the final separation process. Above the

reflux coil, the uncondensed vapour consisting of water vapour and entrained gases pass from

the tower top to atmospheric vent stack. 

Improper operation of the still column - excess vapour flow, fouling of the reflux coil, low flow rate

of rich glycol through the coil .. etc, will result in glycol vapour remaining uncondensed and

escaping to atmosphere causing glycol losses. 

As the glycol flows down the tower across the contacting devices, the absorbed water is stripped

out by hot rising vapours from the reboiler. 

The glycol, as it collects in the still column bottom section, is now partially regenerated and is

referred to as 'Semi-lean' glycol. It is then passed into the reboiler for final water removal. 

Generally, the reboiler is of the 'Fire-tube' type and contains a 'weir' which ensures that the fire

tube is completely immersed in glycol. 

(In systems that have separate reboiler and still column, the weir also maintains the level in the

still column bottom). 

In the reboiler, the glycol flows over the weir and enters a stripping section containing Raschig

rings or other contacting devices. 

(Extra stripping action may be provided by an injection of dry stripping gas into the reboiler

stripping section). 

On leaving the reboiler, the lean glycol passes through the glycol/glycol exchanger into the glycol

accumulator (or storage drum) from where the circulation pumps take suction and discharge the

glycol back to the contactor via the glycol cooler, to complete the circuit. 

(In some small field units, the still column may be a packed type and is usually an integral part of

the reboiler). 

(See Figures: 27 & 28). 

Figure: 27 -Complete

Glycol Unit -Absorption & Regeneration 

Figure: 28 - Package type Glycol Unit as used in Field Locations 

Figure: 29. Shows the 'Double-pipe, Glycol/glycol exchanger, in more detail. 

Figure: 29 - Double-pipe Glycol/Glycol Exchanger 

Figure: 30 - Types of Packing Used in Some Still Columns

ALTERNATIVE DEHYDRATION PROCESS

PRINCIPLES OF OPERATION

This dehydration process differs from other gas plants in two ways :

1. The glycol used to remove water from the wet gas is MONO-ETHYLENE glycol (MEG)

instead of TRI-ETHYLENE glycol (TEG) which is used in most other fields. (See Table of

Comparison)

2. The lean glycol (MEG) is injected into the wet gas via various points in the system - (i.e.

there is no contactor or absorption tower.

The injection of glycol will prevent the freezing of the water in the gas when a refrigerant (usually

propane) is used to cool the gas to below 0 °F for glycol recovery. The rich glycol is collected,

separated from gas condensate in a two section separator and then sent to a glycol regeneration

system similar to the systems used in other field plants. 

It is important to keep the Reflux Coil in the Still column at a temperature sufficient to condense

the MEG and allow only water vapour to pass to the vent stack to atmosphere. 

The lean glycol (after regeneration) is returned from the reboiler to the injection points. 

Figure: 31 - Glycol injection system and Propane Refrigeration Unit

MAIN OPERATING VARIABLES AND LIMITS

In order to understand the operating mechanism of the glycol dehydration process, it is

necessary to consider and understand the effect of the following four major variables: 

1. Temperature

2. Pressure

3. Glycol flow-rate

4. Glycol concentration

1. TEMPERATURE

Temperature of the incoming wet natural gas to the contactor is very important, and is the key

factor affecting the potential use of a glycol dehydrator. 

This can be seen clearly by looking at the following points :

The higher the gas temperature, the more water it will contain in vapour form.

If the temperature of the wet natural gas is around 140°F or above, the natural gas does

not want to give up the water vapour to the glycol. On the other hand, if the natural gas

temperature is 40°F or below, the glycol becomes viscous and does not want to pick-up

the water vapour. Therefore, dehydration will take place at temperatures between 50 to

130°F. The best results will be obtained between 80 and 110°F .

The temperature of the lean glycol entering the top tray of the contactor tower should be

10 to 15 °F above the temperature of the gas to be treated. If the glycol temperature is

too much higher than the gas temperature, the glycol will tend to foam and be carried out

of the contactor tower with the gas.

Conversely, if the glycol temperature is much lower than the gas temperature, liquid

hydrocarbons (condensate) will tend to form and fall to the bottom of the contactor tower causing

problems in the glycol regeneration system.

2. PRESSURE

At constant temperature, the lower the pressure, the higher the water content of the inlet gas.

Other than affecting the water content of the inlet gas stream, pressure has very little effect on

the mechanics of glycol dehydration.

3. GLYCOL CIRCULATION FLOW-RATE

Determining the proper glycol circulation rate is not an easy task due to several limitations and

considerations involved. There are many factors that must be considered but, for simplicity, over

a normal pressure range up to 1200 psi, about 3 to 5 gallons of glycol must be circulated for

every pound of water removed at a 55 °F dew point depression. 

This quantity of glycol is based on equilibrium conditions, plant design, glycol concentration and

other factors, and is calculated by the Plant Chemists.

4. GLYCOL CONCENTRATION

Since the main objective of natural gas dehydration is maximum dew point depression, relatively

high glycol concentrations are used. The usual practice is to introduce, at the top of the glycol

contactor tower, a solution of regenerated glycol with a concentration ranging from 97 to 99 %,

and to remove the solution from the base of the contactor tower at a glycol concentration of 80 to

90 %. 

In general, high glycol concentrations will give larger dew point depression (the larger the better),

if the glycol circulation flow-rate is proportional to the water content of the feed gas.

GLYCOL REGENERATION PROCESS AND EQUIPMENT

THE GLYCOL REBOILER

The glycol regeneration process is very important to maintain the correct concentration of the

lean glycol. Refer to Figures: 50 & 52 for the equipment used in the Glycol Regeneration

Process. 

The glycol reboiler is the main piece of equipment that plays this role in the regeneration

process. The reboiler supplies heat to separate the glycol and water by a simple distillation

process. 

The system consists of a ' U ' shaped, combustion chamber with gas burners, set into the shell of

the reboiler and includes an outlet stack for the waste combustion gases. 

The shell also contains a ' Weir ' that maintains the level of glycol above the fire-tube in order to

prevent overheating of the tube and subsequent damage and/or glycol decomposition by excess

heat. 

Figure: 32 - Fire-tube Reboiler 

The temperature of the reboiler should be in the range of 375 to 390 °F. This temperature will

usually give good distillation of the rich glycol and evaporate all water out of it. 

The glycol should never be heated above 400 °F as it begins to decompose above that

temperature. 

Note: When making adjustments to reboiler temperature, never increase the temperature setting

by more than five degrees at a time. 

Too great an increase will cause the control system to open the fuel gas valve too wide, giving a

large burner flame which in turn will cause flame impingement on the inside of the fire-tube. This

will lead to ' Hot-spots ' and cause damage to the fire-tube and breakdown of the glycol into

corrosive organic acids. 

If coke , salts or tar deposits form on the fire tube, the heat transfer into the glycol is reduced, the

control system will increase the fuel to maintain the glycol temperature and tube failure can

result. Localised overheating, especially where salt deposits accumulate, will decompose the

glycol. 

Salt deposits can be detected by shutting off the burner on the glycol reboiler system at night and

looking down the fire-box. A bright red glow will be visible at the hot spots on the fire tube walls

where salt deposits have collected. An analysis of the glycol will determine the degree of the

contamination. 

It is highly recommended that, during a plant start-up, make sure the reboiler is up to the desired

operating temperature before flowing gas through the contactor . 

Some fires have been caused by leaks in the gas lines near the fire-box. The best precaution is

to have valves and regulators in the gas line at a suitable distance from the firebox. 

Another very effective measure is the addition of a flame arrestor around the fire-box. If the flame

arrestor is properly designed, even severe gas leaks in the immediate vicinity of the fire-box will

not ignite.

OPERATING PROBLEMS AND GLYCOL CARE

Most operating and technical problems usually occur when the circulating glycol solution gets

dirty. In order to get a long, trouble-free life with the glycol system, it's necessary and very

important to recognise these problems and know how to prevent them. 

Some of the major problems are :

1. GLYCOL LOSS

2. FOAMING

3. THERMAL DECOMPOSITION OF GLYCOL

4. DEW POINT CONTROL

5. GLYCOL pH CONTROL

6. SALT CONTAMINATION

7. GLYCOL OXIDATION

8. SLUDGE FORMATION

1. GLYCOL LOSS

The physical loss of glycol is probably the most important operating problem in the dehydration

system. Most dehydration units are designed for a loss of less than 0.10 gallons of glycol per

million cubic feet of natural gas treated. However, if the system is not operated properly, the loss

might be much higher than this. 

The glycol contactor (the absorber) and glycol regenerator are the most common places in the

dehydration system where about 90% of glycol loss occurs. High gas velocity through the glycol

contactor will cause carryover of glycol into the pipeline and a poor mist eliminator (mist

extractor) in the top of the glycol contactor will pass some glycol even at normal gas velocity . 

The glycol losses occurring in the glycol regenerator are usually caused by excessive reboiler

temperature which causes vaporisation or thermal decomposition of glycol (TEG). Also,

excessive top temperature in the still column allows vaporised glycol to escape from the still

column to atmosphere with the water vapour.

2. FOAMING

Foaming of glycol is another problem frequently encountered. It can increase glycol loss and

reduce the plant capacity. Entrained glycol will carry over from the contactor (absorber) with the

sales gas. Also, foaming can cause poor contact between the gas and the glycol solution ;

therefore , the drying efficiency is decreased. The best cure for glycol foaming, is the proper care

of the glycol solution. The most important measures in the program are, effective gas cleaning

ahead of the glycol system and good filtration of the glycol solution. 

De-foaming agents such as Mono-ethanolamine (MEA) are widely used to control the problem.

However, it's very important to point out that, the use of these does not solve the basic problem,

and its only a temporary measure until the cause of the foaming can be determined and

eliminated. 

Some factors that can cause foaming are:

Low glycol solution concentration to the contactor.

High differential temperature between wet gas inlet and lean glycol inlet to the contactor.

High glycol pH - (Note: Basic glycol solution of pH > 9 tends to foam and emulsify)

Hydrocarbon liquids (condensate)

Finely divided suspended solids

Salt contamination

Field corrosion inhibitors

3. THERMAL DECOMPOSITION OF GLYCOL

It has been established that the glycol reboiler temperature is limited by the Tri-ethylene Glycol

decomposition temperature , and glycol vaporisation losses. Laboratory data indicates that glycol

(TEG) is thermally stable up to about 400°F. Excessive heat as a result of one or more of the

following conditions will decompose the Tri-Ethylene glycol (TEG) and form corrosive compounds

.

A high reboiler temperature above the glycol decomposition level.

Localised overheating, caused by deposits of salt or tarry compounds on the reboiler

fire tube or by flame impingement on the fire tube

4. DEW POINT CONTROL

'Dew Point' is the temperature at which the water vapour first starts to condense to liquid. In

industry, the dew point is used to indicate the water vapour content in the gas stream. For the

dew point to have meaning as a descriptive term , the pressure at which it is determined must be

stated . 

When the dew point depression of the treated gas is too low, there can be several causes such

as; Low glycol circulation rate; Low lean glycol concentration - i.e poor regeneration of the rich

glycol solution; Foaming (leads to poor contact between the wet gas and the lean glycol

solution); Blocked or dirty contacting devices in the absorber tower; High gas velocity in the

contactor .... etc.

Check the glycol circulation rate.

Check the glycol reboiler temperature and make sure its on the right setting. If

temperature setting is normal , verify the reboiler temperature with a test thermometer

and make sure that the temperature control system is working properly.

As a conclusion, the dew point depression indicates the extent to which the moisture content of a

gas is lowered. For example, a 50° dew point depression below a saturation temperature of 80

°F at 600 psia, would indicate that the natural gas, after dehydration, would have to be cooled, to

30 °F before any condensation of water vapour would occur. From the water vapour content

curves, it is seen that the concentration of water vapour would be decreased from 51.00 lb /

mmcf to 9.4 lb / mmcf, representing the removal of 41.6 lb / mmcf or 5 gallons of water per one

million cubic feet of gas. 

(The greater the dew point depression, the more water vapour removed).

5. GLYCOL pH CONTROL

The pH of a glycol solution is the measure of its acidity or alkalinity, and is measured on a scale

of 0 - 14. A pH of less than 7 is an acid solution , 7 is neutral and, greater than 7 is an alkaline

solution. 

The corrosion rate of equipment increases rapidly with a decrease in the glycol pH. The

formation of organic acids, resulting from the oxidation of glycol, thermal decomposition products

or acid gases picked up from the gas stream, are the most troublesome corrosive compounds.

Therefore, the glycol pH should be checked periodically and kept on the basic side by

neutralising the acidic compounds with borax, Ethanol-amines or other suitable alkaline

chemicals to maintain the pH at 7.5 to 8.0. A glycol solution that is too alkaline - i.e. pH greater

than 9.00, tends to foam and emulsify .

6. SALT CONTAMINATION

Salt deposits accelerate equipment corrosion, reduce heat transfer in the glycol reboiler and

change the specific gravity readings when a hydrometer is used to determine glycol

concentration. These troublesome compounds cannot be removed by normal regeneration

processes. Salts should be prevented by the use of effective filters or an efficient scrubber.

7. GLYCOL OXIDATION

Oxygen can enter the glycol system via the vapour space of an un-blanketed storage tank or

through the glycol make-up pump packing glands ... etc. The glycol will oxidise readily in the

presence of oxygen (air) and form corrosive organic acids 

Precautions should be taken to prevent glycol oxidation. It is highly recommended, that process

vessels that can draw in air as the liquid level is lowered, should contain a gas blanket to keep

oxygen (air) out of the system. Oxidation inhibitors, such as Hydrazine can be used to prevent

the formation of corrosive, organic acids.

8. SLUDGE FORMATION

Accumulation of solid particles and tarry hydrocarbons very often forms in the glycol solution.

This sludge is suspended in the circulating glycol and, over a period of time, the accumulation

becomes large enough to settle out. 

This action results in the formation of a black, sticky and abrasive gum which can cause erosion

of the equipment. It usually occurs when the glycol pH is low and becomes very hard and brittle

when deposited on the absorber trays, still column parts and other areas in the circulating

system. Good, effective filtration will prevent the build-up of sludge in the glycol system.