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Dehydration with glycol
All raw natural gas is fully saturated with water vapor when produced from an underground reservoir. Because most of the water vapor has to be removed from all natural gas before it can be commercially marketed, all natural gas is subjected to a dehydration process.
Water vapor contents
The water vapor content of natural gas at equilibrium saturation is shown in Fig. 1, which is based on the well-known McKetta and Wehe chart and expanded to 400°F on the basis of data of Olds, Sage, and Lacy. As can be seen, the water content increases with increasing temperature and decreasing pressure.
- For gas sales in colder areas of North America, the specification limit for water content in the sales gas is 4 lbm/MMscf.
- For gas sales in warmer, southern areas, the limit for water content is generally 7 lbm/MMscf in sales gas.
When natural gas is a feedstock to a turboexpander plant for high natural gas liquids (NGL) recovery, virtually all the water must be removed before chilling the gas to very low temperatures.
There are four glycols that are used in removing water vapor from natural gas or in depressing the hydrate formation temperature. Table 1 lists these glycols and shows some of the properties of the pure material. Ethylene glycol (EG) is not used in a conventional glycol dehydrator, as described in Process Description. The main use of EG in the dehydration of natural gas is in depressing the hydrate temperature in refrigeration units. Of the other three glycols, triethylene glycol (TEG) is the most commonly used glycol for dehydration of natural gas because of the advantages relative to diethylene glycol (DEG):
- TEG is more easily regenerated to a higher degree of purity
- vapor losses are lower
- operating costs are lower
Tetraethylene glycol would have to be regenerated at higher temperatures than TEG to reach the required purity for application in a glycol dehydration unit. Thus, of the four glycols, TEG is the best suited for dehydration of natural gas. In glycol dehydration, TEG is usually referred to only as “glycol.” Unless otherwise specified, that convention is used in the rest of this page.
Fig. 2 is a schematic drawing of the typical process equipment for glycol dehydration. While the overall process equipment is similar for all glycol dehydration units, there can be considerable variation among installations.
The gas flows through a separator to remove condensed liquids or any solids that might be in the gas. Some absorbers incorporate the separator in a bottom section of the vessel, in which case the gas then flows upward through a chimney tray into the glycol absorber portion of the vessel. The glycol contactor or absorber can contain:
- random packing
- structured packing
If it is a trayed vessel, it will contain several bubble-cap trays. Lean glycol is pumped into the upper portion of the contactor, above the top tray but below the mist eliminator. The trays are flooded with glycol that flows down from tray to tray in downcomer sections. The gas rises through the bubble caps and is dispersed as bubbles through the glycol on the trays. This provides the intimate contact between the gas and the glycol. The glycol is highly hygroscopic, and most of the water vapor in the gas is absorbed by the glycol. The rich glycol, containing the absorbed water, is withdrawn from the contactor near the bottom of the vessel above the chimney tray through a liquid level control valve and passes to the regeneration section. The treated gas leaves the contactor at the top through a mist eliminator and usually meets the specified water content.
The rich glycol can be routed through a heat exchange coil in the top of the reboiler column called the still. The heat exchange generates some reflux for the separation of the water from the glycol in the top of the still and also heats the rich glycol somewhat. In some installations, the rich solution passes to a flash tank operating at about 15 to 50 psig, which allows absorbed hydrocarbon gas to separate from the glycol. The glycol then flows into the still through a filter and a heat exchanger, exchanging heat with the regenerated glycol. It drops through a packed section in the still into the glycol reboiler vessel, where it is heated to the necessary high regeneration temperature at near atmospheric pressure. At the high temperature, the glycol loses its ability to hold water; the water is vaporized and leaves through the top of the still. The regenerated glycol flows to the surge tank, from which it is routed through the lean/rich heat exchanger to the glycol pump. The pump boosts the pressure of the lean glycol to the contactor pressure. Prior to entering the contactor, it exchanges heat with the dry gas leaving the contactor or some other heat exchange medium.
Function of the inlet separator
The first and foremost piece of equipment that the gas flows through is the inlet separator. This vessel can be either of the following:
- a separate, detached vessel
- on smaller units, built in to the bottom of the contactor
Its function is to separate any condensed liquid from the gas before the gas enters the contactor. If the gas does not contain condensate (liquid hydrocarbon), the vessel is a two-phase separator. If the gas is a rich gas, with some condensate as well as liquid water forming at the inlet conditions of pressure and temperature in the separator, then a three-phase separator is installed. It is absolutely essential that none of the following enters the absorber section:
The separator is usually equipped with a mist eliminator section in the top of the vessel. As the gas moves through the mist eliminator section, small droplets that might be in the gas will coalesce on the fine wire mesh and form larger droplets that drop down through the gas into the liquid section below.
The inlet separator is equipped with liquid level controls, allowing the accumulated liquids to exit the vessel through a level control valve. If for some reason the liquid level in the vessel should rise above a certain limit, a high-level alarm or shutdown automatically occurs.
Function of the contractor or absorber
The contactor is the vessel in which the mass transfer of the water occurs from the gas to the glycol. Most of the water vapor is extracted from the gas phase into the liquid glycol phase. For this to occur, it is necessary to create a large surface area between the gas and the liquid glycol. This is accomplished with specific internal equipment configurations, such as through the installation of:
- structured packing
- random packing
The most common trays used in this application are bubble cap trays, as illustrated in Fig. 3. The gas flows from below each tray through the bubble cap and forms small bubbles of gas in the glycol liquid that flows across and on top of each tray. After flowing across one tray, the glycol flows down to the next tray below through a downcomer, which ensures that the gas cannot bypass any tray. The gas bubbles provide the large surface area needed to effect the transfer of the water from the gas to the glycol. Because of the short contacting time on each tray, equilibrium in mass transfer is not reached—several trays are needed in a contactor to bring about the necessary dehydration of the gas. In the design of dehydrators, the calculations make use of theoretical equilibrium stages for determining how many times the gas and glycol must be contacted. Because of the dynamic conditions, equilibrium in mass transfer is not reached—approximately four actual trays are used for each theoretical equilibrium stage.
- In practice, about 6 to 10 trays are installed in a contactor, usually spaced 24 in. apart.
- In more recent designs, 12 to 14 trays are installed in glycol absorbers to minimize glycol circulation.
Fig. 3—Illustration of bubble cap and bubble cap tray in contractor (after Engineering Data Book of Gas Processors Suppliers Association and Gas Processors Association).
Structured packing consists of arrangements of corrugated steel internals, over which the glycol flows downwards as a thin film. Elements of structured packing are illustrated in Fig. 4. The gas flows upward through the structured packing and is in intimate contact with the large surface area of the glycol that flows downward as a film on the packing. This creates a very efficient model for mass transfer to occur. The design for the height of the packing required is related to the number of theoretical stages required. The suppliers of structured packing have developed the relationship of packing height per theoretical equilibrium stage. When using structured packing, it is essential that the glycol be evenly distributed across the top of the packing. To ensure that the large surface area is provided by the downward flowing glycol, it is also essential that the steel be thoroughly clean, so that all of the steel is wetted by the glycol.
Similarly, random packing of various types can also be used in glycol contactors to create the surface area for mass transfer. Various types of random packing are illustrated in Fig. 5. Again, the total height of the packing in the vessel is related to the number of theoretical stages used in the design and the height of packing per theoretical stage. The suppliers of the packing have correlations for packing height per theoretical stage.
Fig. 5—Illustration of types of random packing (after Engineering Data Book of Gas Processors Suppliers Association and Gas Processors Association).
Function of the reboiler
The rich glycol leaving the absorber must be regenerated to a high purity so that it can be recirculated to the absorber to continue its dehydration function. The regeneration is accomplished in the reboiler and the still column above the reboiler.
The rich glycol is preheated through heat exchange with the regenerated glycol and enters the top of the still at atmospheric pressure. By heating the glycol in the still and reboiler to near its boiling point, the glycol releases virtually all of the absorbed water and any other compounds and is then cooled for reuse. The heat is usually supplied through a fire tube in the reboiler in which natural gas is burned. The fire tube is always submerged in the glycol by having the glycol flowing from the reboiler over a weir or pipe, which is higher than the top of the fire tube.
Water dewpoint depression
Water dewpoint is the temperature at any given pressure at which the natural gas is saturated with water. Because sales gas generally has a maximum water content specification in mass per unit volume (e.g., 4 lbm/MMscf), it is necessary to determine from Fig. 1 the dewpoint temperature at the contactor pressure. For example, if the treating pressure were 1,000 psia, then the dewpoint temperature for 4 lbm/MMscf would be about 18°F. If the treating pressure were 500 psia, the dewpoint temperature would be about 5°F.
The overall objective of dehydration is to remove a sufficient amount of water from the natural gas so that the specification for maximum allowable water content in the treated gas is met. If the gas has not been processed, then the gas entering the glycol contactor is fully saturated with water at the pressure and temperature of the separator ahead of the contactor. The dewpoint depression to achieve is the lowering of the dewpoint temperature from the separator temperature to a temperature at the separator pressure where the water content meets the specified limit for the dehydrated gas. The next example shows the difference in dewpoint depression and the amount of water to be removed per MMscf, if the gas is entering the glycol contactor at 90°F and at 1,000 psia or at 500 psia. The water content specification in this example is 4 lbm per MMscf for the treated gas.
Examples of water dewpoint depression for two operating pressures are shown at 90°F.
- Water content at 1,000 psia and 90°F = 45 lbm/MMscf (Fig. 1).
- Dewpoint temperature for 4 lbm/MMscf and 1,000 psia = 18°F (Fig. 1).
- Water to remove = 45 – 4 = 41 lbm/MMscf.
- Dewpoint depression = 90°F – 18°F = 72°F.
- Water content at 500 psia and 90°F = 78 lbm/MMscf.
- Dewpoint temperature for 4 lbm/MMscf and 500 psia = 5°F.
- Water to be removed = 78 – 4 = 74 lbm/MMscf.
- Dewpoint depression = 90°F – 5°F = 85°F.
Fig. 6 provides a correlation of the equilibrium water dewpoint when gas is in contact with triethylene glycol of various purities at different contacting temperatures.6 In actual operations, equilibrium is not achieved on any tray. To enhance the transfer rate of the water from the gas to the glycol, an approach temperature to equilibrium of 10 to 20°F is used in designing glycol dehydration units. For example, assuming a 10°F approach is realistic, to achieve the dewpoint temperature of 18°F at 1,000 psia contacting pressure, an equilibrium dewpoint of 8°F (18°F minus 10°F approach) is used in the design of the glycol dehydration unit. At a 90°F contacting temperature, the glycol purity must be about 98.8% to achieve this equilibrium dewpoint, according to Fig. 6. At a contacting pressure of 500 psia and 90°F on the other hand, the glycol purity must be about 99.3% to depress the water dewpoint to –5°F with an approach of 10°F. The lower the water dewpoint temperature requirement is for the treated gas, the higher the glycol purity has to be.
Fig. 6—Equilibrium water dewpoint vs. temperature at various TEG concentrations (after Engineering Data Book of Gas Processors Suppliers Association and Gas Processors Association).
The degree of water removal from natural gas by glycol, or the depression of the water dewpoint of the gas, depends on certain conditions:
- glycol purity
- glycol circulation rate (up to a certain limit)
- number of contacting stages (trays) or packing height
- amount of water in the inlet gas, which depends on the pressure and temperature of the gas
These parameters must be considered at the design stage of the contactor, in addition to the maximum rate, pressure, and temperature of the gas. The higher the glycol purity, the more effective is the dewpoint temperature depression. If the glycol purity is insufficient, increasing the glycol circulation rate will not necessarily achieve the desired dehydration of the gas.
Glycol purity enhancement methods
In some operating situations, a high glycol purity is required that cannot be achieved by the temperature in the glycol reboiler alone. There are several ways of enhancing the purity of the glycol beyond what is achieved in the reboiler. One such method is the application of a small amount of stripping gas in the regenerating section. Fig. 7 shows the effect of using stripping gas to enhance the purity of the lean glycol solution. Stripping gas is simply a small stream of natural gas that is flowed into the hot glycol. The flow of this gas is usually regulated manually with a small needle valve and is measured by means of a small rotameter.
Fig. 7—Effect of stripping gas on TEG concentration (after Engineering Data Book of Gas Processors Suppliers Association and Gas Processors Association).
There are a couple of ways of flowing this gas into the hot glycol. One way is simply to flow the gas into the glycol below the overflow line from the reboiler to the surge drum or directly into the glycol in the reboiler vessel through a perforated pipe below the fire tube. The other way is to install a small packed column between the reboiler and the surge drum and admit the gas at the base of this column. By contacting the hot glycol with natural gas, an additional small amount of water is “stripped” from the glycol into the gas, increasing the purity of the lean glycol. If a packed column is used as a contacting means between the glycol and the stripping gas, the stripping efficiency is considerably improved, as seen in Fig. 7.
The stripping gas that is added to the glycol in the regeneration section is emitted into the atmosphere with the released water vapor, unless the vapors from the still columns are routed to a heater or an incinerator or are captured by a compressor and recompressed.
Components of the glycol circulating system
The glycol circulating pump
The circulation of glycol is done with a reciprocating pump. The pump is driven by:
- an electric motor
- natural gas pressure
- the high-pressure, rich glycol returning from the contactor
Electric-motor-driven pumps are usually employed in central dehydration facilities where electric power is available. In field installations, a natural gas powered pump or a glycol powered pump can be used. In the latter case, the high pressure, rich glycol, with the assistance of a small amount of high pressure gas, flowing out of the contactor, is used to provide the power needed to stroke the pump. The required pump rate in field dehydration units is usually small. Therefore, a single plunger pump is normally used. When the gas rate is large, duplex or triplex pumps are used.
The main problems with glycol pumps are leaks through the packing around the plunger, as well as sticking check valves. If the packing gland nut is tightened too much, the rod may get scored. Usually, a small pan is placed under the pump or the plunger portion of the pump to contain the leaked glycol.
Because glycol must be relatively cool when entering the absorber and is heated to near its boiling temperature for regeneration, the liquid is continuously subjected to heating and cooling. To minimize the use of energy in the regeneration of the glycol at high temperature, several heat-exchange opportunities are built into most glycol circulating systems. Heat exchange usually occurs in certain locations:
- coil in the top of the reboiler still
- coil in the surge tank or separate heat exchanger between rich and lean glycol
- pipe-in-pipe heat exchanger ahead of the glycol absorber or a coil in the top of the absorber
In some cases, an additional heat exchanger is necessary to exchange heat with air, to cool the lean glycol ahead of the contactor. It is necessary to limit the temperature of the lean glycol to only a few degrees above the temperature of the gas to increase the absorption of the water by glycol. Too high a glycol temperature reduces the transfer of water from the gas to the glycol, and the water dewpoint may not be met. This is frequently the problem in summer operations, in which the gas is dehydrated after compression. On hot days, the glycol, as well as the gas, might be above normal temperatures. Usually, by exchanging heat with the dried gas through a double pipe exchanger or through a coil in the top of the contactor in small units, the temperature of the glycol is adjusted to a few degrees above the temperature of the gas leaving the contactor.
It is very important to maintain the glycol in as clean a condition as possible. For this reason, filters are always incorporated in a glycol circulating system. These filters are usually particulate filters and carbon filters.
The particulate filters are intended to remove solids down to a 5-μm diameter. Solids can occur from corrosion in the glycol system. Carbon filters are designed to remove dissolved impurities, such as compressor oil or condensate from the glycol solution. Particulate filters are usually installed on the rich glycol side and are in operation all the time. Carbon filters may be bypassed most of the time, if there is no dissolved hydrocarbon in the glycol. Impurities in the glycol solution might promote foaming in the contactor or still.
Because the glycol that is being circulated might not always flow evenly at the same rate throughout the system, a vessel, the surge drum, is required that can handle any surges in the circulation rate. The reboiler always contains a liquid level above the fire tube. Glycol levels in the absorber or flash tank are essentially constant but might fluctuate slightly. Thus, there is a need for a vessel that can absorb slight temporary differences in circulation flow between the various vessels, as well as the thermal expansion of the glycol upon startup.
The surge drum is usually located below the reboiler or at least at a level below the glycol in the reboiler. The glycol level in the surge drum is important because in some instances there is a heat exchange coil in the surge drum, as explained earlier. The level of glycol in the surge drum should be about at the two-thirds full level. The liquid level in the surge drum is an item that is usually checked by the operators. If the level is lower than normal, this might be the first indication of trouble, such as:
- high glycol losses with the treated gas
- losses with the vapors leaving the reboiler still
- holdup in one of the vessels
- leaks in the piping
A strainer should always be installed upstream of the suction of the glycol pump. The glycol strainer ensures that no solid particles enter into the glycol pump. The main problem with solids entering the pump is that they might lodge in the suction or discharge valves and prevent the pump from pumping at maximum efficiency.
Glycol flash tank
Whenever gas is in contact with a liquid at elevated pressures, such as natural gas and glycol in the contactor, some of the gas physically dissolves in the liquid. The greater the contacting pressure, the more gas dissolves in the liquid. Thus, some natural gas dissolves in the glycol in the absorber in addition to the water vapor. When the glycol reaches the flash tank, its temperature has been raised through the coil in the reboiler still, and the pressure in the flash tank is at a much lower level, generally between 15 to 50 psig, than the pressure in the contactor. In light of these changed conditions of pressure and temperature between the absorber and flash tank, most of the dissolved gases evolve from the glycol in the flash tank.
On larger systems, the glycol flash tank can be designed as a three-phase separator to help remove any condensate that becomes entrained in the glycol. This bulk separator increases the operating life of the downstream filters.
All the vessels through which glycol is circulated are interconnected with steel piping. Glycol is a substance that is prone to leak through threaded connections in piping, as well as through the packing on glycol pump plungers. For this reason, some operators prefer welded piping rather than threaded piping for the glycol system. There are, however, many threaded glycol systems that have provided leak-free service.
Because the pump rate is usually small, the piping in most field installations is of small diameter. It is important to check the piping for leaks and to repair them as soon as possible.
Instrumentation and controls
Most glycol dehydration units are sufficiently automated that they can operate unattended. The degree of automatic control of the equipment can vary considerably and depends largely on the specifications by the owner company. The discussion in this section highlights the main control points, which may be considered as the minimum control of a dehydration unit. The controls relate mainly to:
- gas flow
- glycol circulation
- lean glycol concentration
Pressure gauges should be installed:
- on all vessels, including the reboiler
- on the discharge side of the pump
Similarly, thermometers should be installed on all vessels, as well as ahead and after all heat-exchange equipment on both cold and hot lines.
Control of gas flow
Gas flow is usually controlled with a flow control valve upstream of the inlet separator. The operator can set the flow to a certain rate. If the set rate is not met, then the valve opens fully and allows the available gas flow to enter the separator and the contactor. Downstream of the contactor, there may be a meter, which meters the gas flow, or the meter may also be located upstream of the separator. At some point downstream of the contactor, there usually is a back-pressure valve. This valve ensures that the pressure in the contactor is steady without abrupt changes. The pressure is set above the downstream line pressure to ensure steady operation of the contactor.
Lean glycol circulation rate
To achieve the required water dewpoint depression, it is necessary to circulate a certain amount of lean glycol per pound of water to be removed from the gas. The rate of glycol circulation depends on several conditions, which are all interrelated. These conditions are:
- lean glycol purity, after regeneration, which depends on the reboiler temperature and whether or not stripping gas is used, with zero or one stage contacting for the stripping gas
- water content of the gas, which depends on gas temperature and pressure in the inlet separator
- number of actual trays (or equivalent packing height) in the contactor
- the design approach temperature in the contactor
In general, a circulation rate of 3 to 5 gal of lean glycol per pound of water to be removed from the gas is required. If the glycol purity is not sufficiently high, any larger circulation rate might not give the necessary dewpoint depression.
Usually there is an attempt to match the circulation rate to near the minimum required rate to achieve the necessary drying. Overcirculation has disadvantages:
- the heat load on the regenerator is increased, requiring more fuel gas consumption
- the lean glycol returning to the contactor is at a higher temperature because of less efficient heat transfer
- more hydrocarbons are absorbed, especially compounds such as benzene, toluene, ethyl benzene, and xylene (BETX), if these compounds are present in the gas
- additional acid gas is absorbed, if sour gas is being dehydrated
Because there is concern about the absorption of other compounds besides water, as well as for energy efficiency, the glycol circulation rate should be set to remove the required water only. In field installations, using gas driven pumps, the pumps are set to the required pump rate by a gas control valve. This is usually a manually operated needle valve. The manufacturer of the pump provides a chart for the pump that shows the pump rate in volumetric units per time vs. the number of strokes of the plunger per minute.
The temperature of the glycol in the reboiler determines largely the purity to which the glycol is regenerated. However, there is a limit on the temperature to which the glycol can be heated. This limit is a few degrees below the decomposition temperature, as shown in Table 1, because above this temperature, the glycol molecule breaks down. In light of this, the normal temperature in which TEG is heated in the reboiler is about 380 to 390°F. This temperature range results in a lean glycol purity of just under 99% on a mass basis, the other 1% being water.
Thus, it is very important to control the reboiler temperature to the range of 380 to 390°F or some other range that provides adequate regeneration of the rich glycol. In most glycol dehydration units, the heat for regeneration is supplied by burning a small amount of the gas in a fire tube in the reboiler vessel. The size of this vessel is determined by the maximum design rate of glycol circulation, and the size of the fire tube itself is designed for a limit on the heat flux from the fire through the steel tube to the glycol on the shell side of the fire tube. The larger the fire tube, the lower is the heat transfer rate per unit area. The flame should be burning along most of the tube, as opposed to an intense flame at the front of the burner. The fire tube should be designed for a heat transfer rate per square foot of fire tube no greater than 7,000 Btu/h.
A thermowell located in the shell of the reboiler and immersed in the glycol is equipped with a temperature regulator that controls the instrument gas supply to a control valve on the fuel gas supply line to the burner. By setting the regulator at the desired temperature, the gas flow to the burner is automatically controlled, resulting in a narrow operating temperature range for the reboiler. A pilot light ignites the gas to the main burner when the controller allows the gas to flow. The reboiler controls also include a high-temperature shutdown and a shutdown of the fuel supply in case of pilot-light failure.
Most glycol reboilers are equipped with a flame arrestor at the air inlet to the burner. The flame arrestor consists of a tightly wound metal sheet, with sufficient space between the wound metal to allow sufficient air through the arrestor into the burner. If an external source of flammable vapors is sucked in with the air through the flame arrestor, such vapors will not ignite outside of the flame arrestor, as the temperature of the gas is cooled below the ignition point, thus preventing a backflash or explosion.
Liquid level controls
The main liquid level of concern is the level of the condensed liquids in the inlet separator. This vessel can be a two-phase or a three-phase separator. It is very important that no condensed liquid flows with the gas into the contactor. If condensate or salt water gets into the contactor, the result could be foaming or deposits of salt occurring on the fire tube. Heavy hydrocarbons will eventually gum up the packing in the reboiler column or plug the filter. Flashing of hydrocarbons in the still could damage the packing in the still column. In light of this, most glycol units are equipped with high-level alarms and shutdowns, which activate when the liquid level is exceeded in the inlet separator.
The glycol level in the contactor is also important, as any increase in level above the gas inlet pipe can result in interruption of circulation of glycol because of insufficient glycol returns. Both liquid levels, in the inlet separator and the contactor, are controlled by conventional liquid level floats and outlet valves.
Where a flash tank is employed, it is again important to ensure that the level of the glycol be maintained at a set level. The liquid outlet valve must be the throttling type, as opposed to snap acting, to ensure a smooth and steady flow of rich glycol to the regenerator. The glycol level in the surge drum should be at about 2/3 to 3/4 full. In small units, the heat-exchanger coil is in the surge drum and has to be totally submersed to be effective in heat transfer.
Pressure and temperature indicators
All vessels are usually equipped with pressure gauges, as well as pressure relief valves. An operator checking the operation of the glycol unit can quickly see the pressure at which each vessel is operating. The same cannot, in many instances, be said about the temperature, especially in the glycol lines upstream and downstream of each heat exchanger. Ample installation of dial thermometers on the glycol lines is helpful but lacking in many cases. Thermometers are usually installed in the inlet separator and the reboiler. As a minimum, additional thermometers should be installed on the glycol line ahead of the contactor after the heat exchanger and ahead of the reboiler still. Ideally, dial thermometers are installed on all lines entering and leaving the heat exchange equipment.
Contractor design considerations
While the contactor provides the intimate contact between the gas and the glycol, it also must have a diameter sufficiently large that there is separation between the gas and liquid phases. The gas flows upward, and any liquid glycol droplets that might form must be able to fall down through the gas stream. The basic equation requires the determination of the maximum superficial gas velocity in the tower. The contactor towers are equipped with a mist extractor installed at the top of the tower. The suppliers of packing have the correlations for packing height per theoretical stage.
The allowable superficial gas velocity in the tower using bubble cap trays is determined by
The density of the gas at contactor condition of pressure and temperature is determined by
The internal diameter of the vessel is approximated by Eq. 3, on the basis of the allowable gas velocity in the vessel, as determined by Eq. 1.
Design engineers make use of these formulas, with proprietary adjustments, to determine the diameter for maximum gas flow through the absorber. The height of the vessel is determined by:
- the number of theoretical stages used in the design
- the spacing between the trays
- the relationship between the theoretical stages and the number of actual trays or packing height
Additionally, there is space provided below the bottom tray and above the top tray for disengaging between the gas and the glycol. A stainless steel mesh mist eliminator is installed near the top of the vessel. The contactors are also designed to very stringent codes, with respect to shell thickness requirement related to the maximum operating pressure rating. These items are stamped on a plate attached to each contactor.
Water dewpoint determination
It is always of interest to know that the dehydration unit is performing as required and that the exit gas meets the necessary dewpoint specification. If the gas has to meet sales gas requirements for water content, the dewpoint temperature that has to be met at the operating pressure of the contactor can be obtained from Fig. 1. This merely allows the operator to determine the appropriate dewpoint temperature. To determine the actual dewpoint temperature, a dewpoint tester instrument is usually used.
Before electronic instruments were commonplace, the water dewpoint of natural gas was usually determined with a U.S. Bureau of Mines dewpoint tester. Currently, there are several different types of electronic instruments on the market that determine the amount of water in the gas, as opposed to determining the condensation temperature of the water. Such instruments need very clean gas to function properly. A slipstream is taken of the gas to be tested and is usually filtered to remove any impurities.
One brand of electronic dewpoint testers makes use of a metal oxide layer that adsorbs water molecules. By measuring the electrical impedance across the adsorbed metal oxide surface, a reading of the amount of adsorbed water is obtained. The amount of adsorbed water is in balance with the small amount of water in the surrounding gas. Thus, a direct readout is obtained of the moisture content of the gas being tested.
Normal operation checklist
While glycol dehydration units are designed to operate unattended, periodic inspection of the equipment and its operation is necessary. All items listed next should be checked.
- Check the still column vent. Water vapors should be visible. There should be no pressure on the reboiler. Ensure that there is no ice buildup in winter.
- Check the lean glycol temperatures across all heat exchangers and in the reboiler.
- Check the pump operation, strokes per minute, and lubrication oil.
- Check the operation of the glycol filter for pressure drop. Change the filter if necessary.
- Check the glycol level in the surge drum. Add makeup glycol if necessary.
- Check the stripping gas rate. Adjust rate as required.
- Check the liquid levels in the inlet separator, contactor, and flash tank.
- Drain any fluid from the fuel gas scrubber.
- Check the operation of the burner in the fire tube. Check sight glass to ensure it is not broken and gasket is in good shape; clean if necessary.
- Check equipment for liquid leaks, and repair if required.
Glycol dehydrators usually operate trouble free. However, there are some problem areas that can occasionally occur.
One of the more serious problems is foaming. The cause of foaming is usually difficult to determine. However, if the solution is not continuously cleaned by filtration, certain materials can cause foaming. One of the more common causes of foaming is entrained hydrocarbon liquids. It is essential that the inlet separator provide good separation between condensed liquids and gas going to the contactor. Antifoam agent is usually a temporary solution, and the real problem must be identified and corrected.
Corrosion is usually caused by degradation products in the glycol, which can be generated by too high a skin temperature of the fire tube in the reboiler.
Not meeting water dewpoint
There can be many reasons for not meeting the required water dewpoint depression. The first step is to check the water dewpoint temperature with a dewpoint tester. A high water dewpoint can be caused by:
- Gas inlet temperature higher than design.
- Gas inlet pressure lower than design, combined with normal or higher temperature.
- Insufficient glycol circulation owing to too low a pump rate or a low glycol level in the surge drum, check valves on suction or discharge of pump not holding, or suction strainer plugged.
- Insufficient glycol regeneration because of a too low reboiler temperature, high water in inlet separator carrying water into absorber, a leak in the rich/lean glycol exchanger, insufficient stripping gas, or fouled stripping column packing.
- Foaming in absorber: check liquid level in inlet separator, place charcoal filter in service, or temporarily cut back throughput, if necessary.
Upon the regeneration of the rich glycol to lean glycol, the water that was absorbed in the contactor is released in the regenerator, and in the past was vented into the atmosphere. Unfortunately, the glycol not only absorbs water in the contactor, it also physically absorbs hydrocarbons and acid gas. The absorptivity of paraffinic hydrocarbons, such as methane, ethane, etc. is not great. However, the aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX) are more easily absorbed. The problem with these vapors is that they are considered carcinogenic, and all contribute to atmospheric pollution.
Glycol dehydrator BTEX and VOC emission control
The quantity of BTEX and volatile organic compounds (VOC) emitted from natural gas processing facilities has become a major environmental concern over the past decade. With passage of the 1990 amendments to the Clean Air Act in the United States and similar regulations in the rest of the world, several regulatory programs were established to control BTEX and VOC emissions from the still vent of glycol dehydration systems.
Most environmental regulations that were promulgated have focused on two main areas that affected the oil and gas industry. The first area was the reduction of VOC and nitrous oxide (NOx) emissions, which were known to react together in the presence of sunlight and create what is commonly referred to as smog. The second area was a list of hazardous air pollutants (HAP) that were to be controlled, most of which were found to be carcinogenic compounds. Among the list of HAP were the four BTEX compounds, which are commonly found in natural gas streams.
To minimize emissions of these vapors to the atmosphere, several emission control processes have been developed by the gas industry. One such method is to recover condensable compounds and to use the remaining vapors as part of the fuel gas supply to the reboiler. Fig. 8 illustrates the process equipment. The vapors emitted from the glycol reboiler still column are cooled in a natural or forced draft air cooler to temperatures below 120°F. The condensed liquids are collected in a small two-phase separator and pumped back into the process system for recovery of saleable hydrocarbon liquids. Noncondensable gas from the two-phase separator is burned in the glycol reboiler firebox to reduce fuel gas consumption and to achieve an overall minimum destruction efficiency of 99.7%.
It is important that all emission control devices be engineered with proper controls for safe operation. Safety controls should be used as a minimum. A level safety high (LSH) should be installed on the two-phase separator to prevent condensed hydrocarbon liquids from going into the reboiler firebox, if the condensed liquid pump fails. An in-line flash arrestor should be installed in the noncondensable gas piping to prevent flame propagation from the reboiler firebox back into the BTEX system. A pressure safety valve (PSV) should be installed on the glycol reboiler or emission control device to protect the system from overpressure. Block and bleed valves should be installed in the noncondensable gas piping to protect the glycol reboiler during a high-temperature situation.
GTI has sponsored research in the area of emission controls from glycol units. A computer program has been developed that is designed in part to estimate the amount of BTEX absorbed in a conventional glycol unit. The program is called GLYCalc™ and is available from GTI at a nominal cost.
|d||=||internal diameter, inches|
|Q||=||gas flow rate, MMscf/d|
|T||=||absolute temperature, °R|
|V||=||superficial gas velocity, ft/sec|
|z||=||gas compressibility factor, dimensionless|
|γ||=||gas specific gravity (air = 1)|
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