TEG Dehydrator

TEG Dehydrators

GENERAL PROCESS DESCRIPTION

The basic function of the triethylene glycol (TEG) dehydration unit is to remove water vapor from natural gas streams to an outlet water content meeting pipeline specifications, usually seven lb./MMSCF or less. QBJ has designed and built special TEG dehydration units which have met water specifications of less than 1 lb. / MMSCF.

Dehydration, or water vapor removal, is accomplished by depressing the water dew point (defined as the temperature at which vapor begins to condense into a liquid) from the inlet dew point temperature to the dew point temperature for the outlet water content required.

The most common dehydration method used for natural gas is the absorption of water vapor in the liquid desiccant, Triethylene Glycol. The wet gas is brought into intimate contact with lean dry glycol in the tray or structured packing section of an absorber tower where water vapor is absorbed in the glycol thus depressing the water dew point. The wet rich glycol is pressure reduced and then flows from the absorber to the regeneration system. The wet rich TEG is pre-heated (via heat exchange), entrained gas is separated, additionally heated (via heat exchange) and fractionated in the still column and reboiler by heating and boiling off the absorbed water vapor. The water dry lean glycol is cooled (via heat exchange) and pumped back to the absorber.

The QBJ regeneration systems can be equipped with proprietary baffling and stripping columns. These units produce lean TEG glycol concentrations from 99.1% up to 99.95% (wt.) concentration by the use of dry gas to strip more water out of the hot regenerated glycol. The proprietary QBJ “Enhancelator” produces TEG glycol concentrations of 99.5% (wt.) without the use of stripping gas.

With special design, material selection and fabrication requirements, the TEG dehydration process can be applied to CO2, H2S, oxygenated gases or any other application.

Dewpoint depression is dependent on TEG circulation rate (gallons per pound of water in the gas), lean TEG concentration, number of equilibrium stages (number of trays) in the absorber, contact temperature and pressure.

Dewpoint depression is especially sensitive to inlet gas temperature. TEG dehydration units yield a higher dew point depression with an increase in temperature and correspondingly a lower dew point depression with a decrease in inlet gas temperature. This performance change is primarily due to the change in gas-glycol contact efficiency in relation to temperature. Even though the dew point depression increases with an increase in inlet gas temperature, the outlet gas dew point will be higher. For the lowest obtainable outlet dew points it is desirable to have a low inlet gas temperature.

Normal operating temperatures range from 50 to 135 F. 50 F is considered to be the minimum operating temperature due to the high viscosity of glycol at lower temperatures. 135 F. is the upper practical temperature limit for TEG dehydrators because of the increased TEG vaporization losses at higher temperatures.

Pressure appears to have little effect on dewpoint depression in the dehydration process. Existing data indicates that the dewpoint depression is essentially constant over a range of 0 to at least 3000 PSIG. Pressure does however affect the water vapor capacity of the gas. At lower pressures the gas can absorb more water per unit volume.

Typically, increasing the number of trays, the glycol circulation rate or the lean TEG concentration will increase the dew point depression. Increasing glycol rates above 4 to 6 gallons per pound does not usually have an appreciable effect on dewpoint depression.  Increasing TEG concentration or the number of trays is usually more effective than increasing glycol circulation rate in maximizing dewpoint suppression.

GENERAL EQUIPMENT DESCRIPTION

The dehydration unit consists of an absorber and regenerator. The mass transfer section of the absorber may be trays, structured packing or random packing. A standard design, unique to QBJ, is a large TEG disengagement space above the top tray and below the mist extractor. This space allows additional time for glycol - gas separation and minimizes TEG losses. The standard QBJ regenerator is a high concentration type concentrating the glycol to over 99.1% by the use of stripping gas or by concentrating the glycol to 99.5% without the use of stripping gas in the Enhancelator.

The lean, dry TEG entering the absorber should be cooled to within 10 degrees of the gas temperature to maximize the efficiency of the mass transfer media and to minimize TEG vaporization losses. The absorber is normally equipped with an external glycol/gas heat exchanger; however, some units are supplied with a fin fan air-cooled exchanger.

The water rich glycol is withdrawn from the bottom section of the absorber via a level controller and level control valve on units utilizing electric or gas fired pumps. Units using an energy exchange glycol / gas powered pump, do not require a level control system since the pump transfers the energy available from the wet glycol, at the absorber pressure, to an equivalent volume of dry glycol at reboiler pressure. The additional energy to overcome friction, line and system pressure losses is supplied by gas at the absorber pressure.

The absorber may be provided with an integral scrubber to remove free liquid from the incoming gas. The scrubber may be two phase for separation of gas and liquids, or three phase to separate gas, hydrocarbon liquid and water. The gas stream to the absorber must be free of liquids prior to entering the mass transfer section of the absorber. A filter separator or coalescer upstream of the absorber will fulfill this requirement.

Cold weather protection can be provided for the dehydration unit by the addition of an enclosure, extra insulation, heat trace or a catalytic heater.

TEG heat exchange is optionally provided by a three-section glycol/glycol heat exchanger. The heat exchanger is of a fixed tube sheet BEM construction, with bolted bonnets. The first section preheats the water rich TEG for optimum gas separation in the low-pressure three-phase separator. The second and third sections raise the rich TEG from separation temperature to approximately 300 F (for maximum heat conservation) prior to the inlet of the still column. The lean TEG from the surge tank provides the heat and is cooled by exchange for pump suction.

The QBJ standard regenerator consists of the reboiler with a firetube, burner control assembly, flame arrestor, stack with down draft diverter, still column with reflux condenser and integral surge accumulator with stripping column. The reflux condenser, integral to the QBJ still column, aids in the condensation of glycol vapors and reduces glycol losses.  QBJ units are test fired in the shop to verify performance and reduce startup problems.

The stripping column uses a source of dry gas, usually taken from the dry gas leaving the absorber to strip additional water out of the regenerated glycol. The amount of stripping gas varies with the application and dew point depression requirements from 0.5 SCF/gal. of glycol circulated up to 6 SCF.

Fuel gas and instrument supply gas is usually supplied from the dry gas leaving the absorber but other sources may be used. A fuel gas scrubber with fuel gas safety valve is provided to prevent free liquids from entering the fuel gas system.

The regenerator temperature is controlled by the temperature controller and fuel gas control valve(s) along with a high temperature shutdown.

The glycol pump(s) are provided with valves and strainers. The electric driven positive displacement pump is provided with a suction stabilizer and pulsation bottle in the discharge line. A redundant valve system bypasses glycol back to pump suction for manual flow control of lean TEG. A bypass relief valve is provided on the discharge of all electric and gas powered pumps.

Standard units are provided with low pressure three phase separator (flash separator) to separate solution gas (and glycol powered pump energy gas) from the glycol and hydrocarbon condensate. The flash separator is installed on the rich glycol line between first pass of the glycol/glycol heat exchanger and the glycol sock filter. The flash gas is generally routed to the fuel gas system.

Additional Optional Features

1. Enhancelator for 99.5% (wt.) without the use of stripping gas.
2. Standby glycol pump
3. Charcoal filter
4. Flame failure shutdown
5. Low temperature shutdown - glycol reboiler
6. High temperature shutdown - stack
7. High and Low level shutdown - glycol reboiler, surge tank and flash separator
8. High and Low level shutdown – inlet separation / integral scrubber
9. Cold weather enclosure
10. PLC control and DCS interface
     

QBJ glycol dehydration units GUARANTEE outlet gas dewpoints and unit performance.

Dehydration Unit

GLYCOL DEHYDRATION UNIT The DPS Delta glycol dehydration Unit removes water vapour from a gas stream to allow further treatment and transportation without risk of hydrate formation or corrosion in the presence of H2S or CO2 by using ethylene glycol as a liquid desiccant. PROCESS DESCRIPTION Wet gas containing HC droplets enters the integral scrubber section of the contactor tower where free liquid is removed. Saturated gas flows up through the mass transfer section of the tower mixing with the downward flowing lean glycol. Dry gas will exit the tower via a demister pad and the rich glycol goes to a coil within the still column mounted on the reboiler. The condensing vapours provide reflux for the still column. The warmed rich glycol flows via the lean/ rich glycol exchanger to the Flash Drum to remove entrained gas and separate HC liquid from the rich glycol. The rich glycol then passes through a solids filter to remove particulates and a carbon filter to remove traces of aromatic compounds. After filtering the rich glycol is heated by the lean glycol from the Surge Vessel. Lean glycol flows from an integral gas stripping column via the Surge Drum to the Lean/ Rich Glycol Exchanger before flowing to the Lean Glycol Pump which sends the glycol under high pressure to the Glycol Contactor via the Lean Glycol Cooler. Please click on the image below to view a larger version.

Liquid/Gas Coalescer

Pall Solutions

A Pall SepraSol™ Liquid/Gas coalescer in the gas feed line will remove virtually all of the entrained liquids in the feed gas. This eliminates the problem of hydrocarbons and amines mixing with the glycol, minimizing any problems with foaming.

Pall disposable filters are recommended on the recirculating glycol stream to remove iron oxide particulates. Since the glycol is a circulating system, the solids concentration will gradually increase unless removed by a filter. The high solids result in equipment fouling and stabilize foaming. The circulating glycol should contain less than 1ppm by weight of suspended solids and should be filtered to an efficiency of 10 µm absolute. This circulating glycol system often requires that coarser filters be installed at first to clean the system of solids which have been accumulating over a period of time. Progressively finer filters are introduced to the system until a filter efficiency of 10 µm absolute is obtained.

Figure 1. Glycol Dehydration System

Table 1. Filter Recommendations

Filter Location	Recommended Pall Assembly	Purpose of Filtration	Benefits of Filtration
1	Pall SepraSol Liquid/Gas Coalescer: CC3LG7A CC3LGO2-H13 CS604LGH13	Removes water, hydrocarbon, carried over amines and other liquids and solids from gas feed	Reduces foaming problems Increases absorber efficiency Prevents exchanger and reboiler fouling
2	Pall Profile® II or Ultipleat® High Flow cartridge: 10 µm	Removes scale, solid particles	Reduces foaming problems Reduces glycol losses Increases energy efficiency Increases absorber efficiency Prevents exchanger and reboiler fouling
3	Pall SepraSol Liquid/Gas Coalescer: CC3LG7A CC3LGO2-H13 CS604LGH13	Removes water, carried over glycol, and liquid hydrocarbon	Protects downstream processes Reduces downstream equipment maintenance costs

If glycol losses are significant due to operating at higher than design capacity, Pall’s SepraSol Liquid/Gas coalescers can be installed downstream of the contactor overheads to recover glycol and protect downstream equipment like compressors, desiccant beds and heat transfer equipment.

Glycol Dehydration process

Introduction The use of Glycol to dehydrate gas streams is an established method that has proven its functionality and versatility time and again. There are 3 common types of Glycol used for Gas Dehydration: Mono-Ethylene Glycol (MEG) Di-Ethylene Glycol (DEG) Tri-Ethylene Glycol (TEG) The type of Glycol used and the package design depends on several factors, and the end-users specific requirements and objectives for the gas stream being processed. Each package is typically designed in close consultation with the client to ensure the best overall design is achieved. Design Basis The design of TEG and MEG Dehydration Systems is unique for every requirement, and the overall package design will vary to meet the specified moisture content of the gas at the process conditions. Each system is typically designed and built as a complete turn-key package with particular emphasis given to the following issues: Discharge gas moisture content High gas dehydration capacity Minimum glycol losses Minimum power consumption Optimum plant efficiency & design integrity Compliance with HSE requirements Environmentally conscientious design Process Description In a typical TEG package, water saturated gas enters near the bottom of the Contactor Tower and flows upwards through the internal trays/packing (1).  Lean Glycol enters the Contactor Tower near the top and cascades down through the Contactor internals (9), making contact with the up-flowing gas stream.  The counter-current flow path of the Glycol and the high contact surface area adsorbs water into the Glycol from the gas stream. Dehydrated gas flows out of the top of the Contactor, while the Rich Glycol flows out of the bottom of the Contactor and to the Glycol Regeneration Package. The TEG Regeneration process typically involves passing the Rich Glycol through the still column to gain some heat (2) before entering the Flash Drum (3).  The Glycol is then passed through Particle Filters to remove particulates and Activated Carbon Filters to remove any dissolved hydrocarbon and/or chemical compounds (4).  The Rich Glycol is heated in a cross exchanger to preheat the feed (5) to the Still Column where the Glycol present in the water vapour leaving the Reboiler is recovered (6). Depending on the application, it may be necessary to increase the Lean Glycol concentration by using stripping gas (7), or running the Reboiler/Still Column under a slight vacuum.  Lean TEG (typically >99wt%) is then cooled and pumped back to the top of the Contactor Tower (8) to repeat the process.

Glycol dehydration: handling and operational problems

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 :

• Glycol loss
• Foaming
• Thermal decomposition
• Dew point control
• Glycol pH control
• Salt contamination
• Glycol Oxydation
• 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).