KW International Glycol Dehydration Units can dry up any gas stream. The KWI Standard Glycol Units are designed to reduce water content to 7 lbs or less at a pressures from 300 psig to 2160 psig and at gas temperature up to 130°F. KW International offers gas producers unmatched efficiency and trouble free operation in gas dehydration units.
All natural gas wellstreams contain water vapor as they leave the reservoir. In many instances, free water is produced along with the natural gas. Natural gas cools as it travels up the well bore to the surface as a result of pressure reduction and conduction of heat through the pipe to cooler formations. Therefore, since the ability of gas to hold water vapor decreases as the gas temperature decreases, natural gas is nearly always saturated with respect to water vapor when it reaches surface equipment. Additional cooling of the saturated gas will cause the formation of free water. Should the natural gas further cool into the hydrate range, hydrates will form and serious equipment damage and stoppage of flow will occur. Thus it is understandable why it is important to remove water vapor from natural gas. The process for removal of water vapor from natural gas is known as DEHYDRATION.
There are three principal reasons for dehydrating natural gas:
As stated before, two conditions must exist before hydrates can form - free water must be present in the gas stream, and the stream must be at or very near the hydrate temperature for the system pressure. By reducing the water content of natural gas with dehydration, the operators can be sure that no freewater with resulting hydrates will form in the pipeline until the gas in the line reaches its saturation temperature. A more detailed discussion on hydrates.
Freewater in the pipeline occupies volume and therefore reduces the line's gas carrying capacity. Any volume of water in the line means a loss in line capacity as the water will collect at low places in the line. Therefore, it is desirable to dehydrate the entering gas to a water vapor content that will prevent the formation of freewater in the pipeline.
The third reason for dehydrating natural gas is to eliminate or retard corrosion. Not only will water moisture alone cause corrosion in a pipeline, but also liquid water in the presence of any acid in the stream will form even more corrosive acid solutions. By removing water vapor by dehydration to keep the gas in an under-saturated condition and eliminating the condensing of freewater in the pipeline, corrosion can be reduced.
There are five basic methods for dehydrating or "drying" natural gas. These are:
The glycol dehydrator of today consists of - an absorber (or contactor, as you prefer) for contacting the gas with highly concentrated triethylene or tetraethylene glycol, a glycol regenerator for reconcentrating the used glycol, glycol pumps for pumping the reconcentrated glycol to the top of the absorber, heat exchangers for fuel conservation and satisfaction of process requirements, and filters to keep the glycol clean and free from hydrocarbons and sediment. Improvements in reconcentrator designs, using stripping gas to reach higher regenerated glycol concentrations, has greatly extended the range of this equipment.
The largest number of these units is on single-well -applications, but many large volume central plants have been installed in recent years. Practically all of these units utilize triethylene glycol. Tetraethylene glycol has been used on relatively few installations. Even though it is quite expensive, it offers some advantages over triethylene in certain special applications.
The actual operational conditions of a glycol dehydrator must fall within the range of limitations of the unit if it is to perform satisfactorily. The extent of water removal that can be obtained is determined by inlet gas temperature, type of liquid absorbent used, concentration of the absorbent, circulation rate, number of trays in the absorber, inlet pressure, and quantity of contaminants in the absorbent. Two terms used in natural gas dehydration should be explained before proceeding further; these are dew point temperature and dew point depression.
These two terms are quite important to the operator since they are used to describe the unit's performance. For under-saturated streams, dew point temperatures are physically determined with special instruments. From this information the actual water content of the gas stream at the point analyzed can be determined. Moisture analyzers such as the Bureau of Mines' unit, MECCO, DuPont, Panametrics, etc., have all been developed to quickly and accurately determine a wellstream's dew point temperature.
How It Works
The concentrated or "lean" triethylene glycol is continuously pumped to the top tray of the contactor tower where it flows across the tray and is intimately mixed with gas from the tray below, passing out through the slots in the bubble caps. The glycol spills over the edge of the down comer pipes and passes from the top tray to the next tray below where the process is repeated. This process is repeated for each tray until the glycol reaches the bottom of the tower (or the hat tray if an integral scrubber is furnished as shown here). The wet or "rich" glycol passes out of the tower, through a strainer, and then to the power side of the glycol pump. In the pumps shown, the rich glycol helps furnish the motive power for pumping the dry glycol to the contractor. The additional power required by the pump is furnished by gas from the contactor. The wet glycol with the expanded gas passes through a glycol-glycol heat exchanger, where it is preheated (by the hot, dry reconcentrated glycol) before entering the reboiler to be concentrated.
The reboiler drives off the water vapor through the still column to the atmosphere and the re-concentrated glycol flows over a weir into the storage compartment. The hot re-concentrated glycol passes out of the storage compartment through the glycol-glycol exchanger, then through a filter and to the glycol pump, where it is pumped through the gas-glycol exchanger to the top tray of the contactor tower. The glycol-glycol exchanger serves to cool the hot lean stream to protect the pump as well as to pre-heat the cool rich glycol for heat conservation as mentioned before. The gas-glycol heat exchanger further cools the lean glycol to improve dehydration efficiency and minimize glycol losses from the contactor.
It is imperative that the gas entering the glycol contactor be free of entrained liquid and solid particles if the desired dew points are to be achieved. This means that the gas must already have passed through an adequately sized separator just prior to entering the contactor. KW International separators or coalescer scrubbers are recommended for installation ahead of glycol contactors.
The alternate solution is to place an integral scrubber in the bottom section of the contactor tower. The contactor shown here has an integral scrubber section. This offers a savings in capital investment, simplified erection, foundations, and hook-up. An integral scrubber is included in the KW International standard dehydrators. The wet gas enters the scrubber section in the bottom of the contactor releasing its entrained liquid. The gas then passes upward through a wire mesh mist extractor, where the fine liquid particles are coalesced and removed; next through the chimney tray and then on through the contact trays. Liquids removed from the gas stream in the integral scrubber section are dumped by the liquid level controller to a distillate and/or water disposal system.
Standard Absorber Accessories
Standard Reconcentrator Accessories
Methods for Drying Natural Gas
There are five basic methods for dehydrating or "drying" natural gas:
Cooling the stream is perhaps the simplest method of removing water vapor from natural gas; however, the process is limited by the hydrate forming temperature for any given system pressure. For example, a 0.6 specific gravity natural gas at 1000 psig has an average hydrate formation temperature of 64°F. This would limit the cooling to approximately 70°F because of the variation
Considerable water vapor can be removed from the gas by cooling to 70°F. Assuming the gas ahead of the cooling system is at 100°F, the initial water content at 1000 psig is 61 lbs/mmscf. The water content at 70°F is approximately 24 lbs/mmscf. By cooling from 100°F to 70°F the water content is reduced 37 lbs/mmscf or approximately 4.4 gallons/mmscf. This method is often used when handling gas originating from extremely hot reservoirs such as those in volcanic regions or very deep formations. Air cooling and/or water cooling are commonly used as the preliminary steps to reduce the subsequent requirements for more complicated equipment for further removal of water vapor.
Compression and Cooling
Compression followed by cooling is a variation of the first process. This process takes advantage of the effect which pressure has upon the saturation water content of natural gas. Gas acts like a sponge in that the harder it is "squeezed", the less water it can hold. Therefore, reduction of water content can be obtained by compressing the gas to a higher pressure and then cooling it. The compression -process causes the gas to heat up, therefore cooling is required to bring it back or near the hydrate temperature. The liquid water can then be removed with a separator. This method of water vapor removal is limited by the hydrate formation temperature just as the simple cooling method is. Any further reduction of the gas temperature will require additional dehydration or hydrate protection of some type.
Low Temperature Separation
The low temperature separation method can be used either where adequate pressure differential exists between the wellstream flowing pressure and the pipeline delivery pressure or where cooling by mechanical refrigeration can be used. This former method makes use of the Joule-Thompson or auto-refrigeration effect that results from taking an appreciable pressure drop across a choke on the inlet to the low temperature separator. Normally this method requires an initial wellstream pressure of 1500 psig or higher with an available differential of at least 1000 psig. When the wellhead pressure and available differential declines with reservoir age, the amount of dehydration and liquid recovery begins to reduce until it becomes necessary to use an alternate method or to supplement the cooling with a mechanical refrigeration unit.
There are two basic types of low temperature separation units using the Joule-Thompson effect one with hydrate inhibitor injection and one without. The unit with hydrate inhibitor injection takes full advantage of available auto-refrigeration. The injection of hydrate inhibitor into the wellstream ahead of the heat exchanger allows maximum cooling of the wellstream before pressure reduction. This produces the lowest possible temperature in the low temperature separator. The cold gas from the low temperature separator is heat-exchanged with the inlet well stream. If the inhibitor is a solution of ethylene glycol or diethylene glycol, it can be separated and recovered in a reconcentrator for re-use. If the inhibitor is an alcohol such as methanol, generally no attempt is made to recover it. The methanol is then dumped with the water phase from the low temperature separator.
The unit without hydrate inhibitor is very similar in design; however, the amount of wellstream cooling ahead of the choke must be controlled so the wellstream is still a few degrees above the hydrate temperature as it enters the choke body. Hydrates are actually produced and blown into the separator. The inlet wellstream is used to melt the hydrates by conducting it through a pipe coil in the separator near the region where the hydrates are collected. The wellstream is then cooled through heat-exchange with the cold gas from the separator.
The low temperature separation system using mechanical cooling is identical to the above system with hydrate inhibition except that refrigeration and a chiller are put in place of the expansion choke. The reduced separation temperature lowers the water content of the gas stream through condensation. The lower temperature also usually results in an increase in liquid hydrocarbon recovery, which will often amortize the equipment investments.
Any of the above low temperature separation systems can be supplied by KW International. However, each application requires that the equipment be sized and designed specifically for its conditions. All details for a low temperature separation system should be forwarded to the KW International Houston office for the best type of unit and the equipment required.
There are several solid or "dry" desiccants used to remove water vapor from natural gas. The more common are activated alumina, silica gel, molecular sieves, and calcium chloride. Except for the calcium chloride, all of these desiccants can be regenerated and re-used many times. Also except for the calcium chloride units, solid desiccant gas dehydrators are multi-bed units which have the wet gas flowing through one or more beds to remove the water vapor while the other bed or beds are being regenerated and readied for placement on the wet gas stream when the bed or beds on line reach near-saturation. This alternate usage of towers or beds is the normal manner in which the solid desiccants are used. The requiring of multiple pressure vessels, switching valves, associated piping, and equipment make dry desiccant dehydrators the most expensive of all types of dehydrators. However, they are capable of reducing the water content of a natural gas to an extremely low level. When the silica gel type desiccant is used, the unit can be designed to also extract marketable liquid hydrocarbons. These latter units are referred to as the "Short Cycle" hydrocarbon units.
As in the case of low temperature separation units, each dry bed dehydrator is specifically designed for its own application. Based on your information and requirements, KW International can furnish your dry desiccant dehydrator needs.
The calcium chloride desiccant is deliquescent; i.e., it undergoes several successive chemical reactions with water vapor, gradually transforming from a solid to a brine solution. It is, therefore, used in a batch type system. The brine solution cannot be regenerated and is discarded. In most cases, only one specially designed tower is used and periodically taken off the line for recharging. Some installations use two towers so that one is always on the line while the other is being recharged. These units are infrequently used today because of the associated corrosion problems of calcium chloride brines and the fact that the water content of the outlet gas increases rapidly as the charge in the tower is spent.
The most widely used method of drying natural gas is the liquid desiccant unit. Desiccants commonly used are: methanol, ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol. Methanol, ethylene glycol and diethylene glycol are normally used with injection systems as hydrate inhibitors. This is discussed under low temperature separation. Methanol and ethylene glycol are most commonly used only in an emergency or temporary system because they are not easily recovered for re-use. Diethylene glycol is commonly used in injection systems as it can be readily recovered, reconcentrated, and re-used. It is also used in the same manner as the other higher glycols, but it is not capable of producing as great a reduction in water content of the gas as the higher molecular weight glycols. There are three types of glycol used in dehydration systems. These are diethylene, triethylene, and tetraethylene glycol. Ethylene glycol has been used in some special applications but its vapor pressure is too high for use in conventional reconcentrators without experiencing very high losses. Consequently, it is not considered as one of the major absorbents. Diethylene glycol is used because its vapor pressure is lower than ethylene glycol and it is not as soluble in liquid hydrocarbons as triethylene glycol or tetraethylene glycol. Diethylene glycol is primarily used in glycol injection systems as a hydrate inhibitor, but it can also be used in the conventional gas dehydrators where a limited dew point depression is acceptable. Because of its lower decomposition temperature, diethylene glycol cannot be regenerated to as high percent reconcentration as triethylene glycol. Diethylene glycol also has a price advantage over triethylene and tetraethylene glycol.
Triethylene glycol is the predominant glycol used in dehydration and has largely supplanted diethylene glycol for this purpose. Because of its higher decomposition temperature and a much lower vapor pressure, triethylene glycol can be more readily reconcentrated to a higher purity with a resultant increase in dew point depression without incurring decomposition and high losses from the still. Recent improvements in reconcentrating equipment (making use of stripping gas) have resulted in achieving even higher purities with subsequent increases in dew point depressions.
Tetraethylene glycol has become commercially available for use as a liquid desiccant. It has an even higher decomposition temperature than triethylene glycol. It can usually provide a slight increase in dew point depression over that obtained by triethylene glycol, using the same equipment. The tetraethylene glycol must be reconcentrated at a higher reboiler temperature. It has another advantage over triethylene glycol in that there is a considerable reduction in glycol loss due to its lower vapor equilibrium at elevated contact temperatures due to a high temperature of the inlet gas. This is especially important where gas-glycol contact temperatures are above 120F. The major disadvantages are its high cost and its higher viscosity, which becomes a factor with low ambient air conditions and in cold climates. The recommended safe ranges of reconcentrator (reboiler) temperatures are as follows:
Diethylene Glycol 315F - 340F
Triethylene Glycol 340F - 400F
Tetraethylene Glycol 400F - 430F
Conditions Affecting Design and Operation of Gas Dehydrators
Equipment size and amount of water removed by a glycol dehydrator are affected by the following variables
These variables must be controlled if the desired water content reduction is to be achieved.
Inlet Gas Temperature
The inlet gas temperature has a very profound effect on the water content of the gas entering the contactor. If the gas temperature is increased, while still in contact with free water, additional water vapor will be absorbed by the gas.
If the inlet gas temperature is above the ambient temperature, another operation problem can be encountered. Contactors operating with rich gas at temperatures above the ambient can have condensation of the heavier hydrocarbon fractions on the wall of the contactor. These will accumulate in the system and contaminate the glycol unless provision is made for their removal.
When line heater are used to heat the gas stream to prevent hydrate formation ahead of the dehydrator during cold weather, the inlet gas temperature to the dehydrator should not be allowed to rise excessively; however, the inlet temperature should be maintained above 60oF. At gas inlet temperatures below 60oF, the glycol will be cooled sufficiently so that the increase in viscosity of the glycol will result in low efficiency in the gas-glycol contact, and increase the tendency of the glycol to foam. Foaming results in a significant reduction of dehydration of the gas stream and loss of glycol.
Inlet Gas Pressure
Normally, the inlet gas pressure will not fluctuate enough from the design conditions to be a critical factor. However, if the inlet gas pressure is very low, water content of the gas will be very high. In the pressure range of 125 psig to 250 psig, the quantity of water to be absorbed by the circulating glycol is quite large and consideration must be given to the heat of reaction or absorption. This can raise the glycol temperature on the contactor trays several degrees above the gas temperature. Units which are operated below the design pressure cannot produce the designed dew point depression without increasing the glycol circulation rate in terms of gallons of glycol circulated per pound of water vapor removed and/or the lean glycol concentration. In some cases there may be sufficient circulation and reboiler capacity available to allow increasing the circulation and/or lean glycol concentration to enable a given unit to reach the desired outlet dew point temperature. It will be necessary that the gas rate be reduced in order to keep the actual gas velocity in the proper range through the contactor. Each case will have to be reviewed by the operator in charge.
Gas Flow Rate
Units are designed to operate efficiently at a specified range in gas rate. Below this range, there will be some loss of efficiency in terms of increased outlet dew point and reduced dew point depression. Above this range the unit will not only lose dehydration efficiency but will also experience excessive glycol losses. Also, at flow rates above the normal maximum, the reconcentrator will become overloaded, resulting in insufficient glycol reconcentration and the outlet gas dew point will again increase. The flow rate must be relatively constant. Rapid surges or changes in flow rate can cause a loss of seal in the contactor’s tray downcomers. This will cause not only loss of dehydration but also excessive glycol loss as the gas will lift the glycol out of the contactor. Once the seal on the downcomers is lost, the only way it can be re-established is to shut-in the contactor and put it back on line gradually.
Glycol Inlet Temperature to the Contactor
The temperature of the glycol entering the contactor has a significant effect on the gas dew point depression and should be held to within 10oF above the inlet gas temperature. Higher glycol losses and higher outlet gas dew point occur when the lean glycol enters the contactor at a temperature more than 10oF above the gas temperature.
Number of Trays in the Contactor
Most manufacturers use five contact trays (bubble cap, valve, or sieve type) in their standard units. Our standard units typically have six or eight trays in the contactor to achieve better balance between the contactor and regenerator capacities and greater flexibility in operation.
For a given glycol circulation rate, higher dew point depressions are obtained as additional trays (or equivalent length of packing) are added to the contactor. The increase in unit cost for additional trays for a given dehydration problem is not as much as that resulting from increased glycol reconcentration and/or circulating capacity which would alternately be required. High dew point depression units require seven or more trays. Rarely does the number of contactor trays exceed ten.
In contactors that are too small to allow installation of trays, a “packing” is used to provide the contact between the wet gas and the lean glycol. “Packing” is a metal, ceramic, or plastic object that is designed to furnish a large surface area. The glycol spreads over these surfaces in a thin film. Contact is affected as the gas is passed over the glycol wetted surfaces.
Glycol Concentration Entering the Contactor
The one single change that can be made in a glycol system, which will produce the largest effect on dew point depression, is the degree of glycol reconcentration (usually stated as percent of purity), which is obtainable by the reconcentrator. For example, assuming a contactor with 6 trays and a glycol rate of 3 gal. Per lb. of water vapor in the inlet gas, the maximum dew point depression obtainable with 98.5% triethylene glycol is 67oF. Changing the concentration to 99.1%, the dew point depression would be 75oF. If stripping gas is used and the concentration increased to 99.9%, the maximum dew point depression is 95oF.
In reboiler where the glycol is heated and then cooled for storage, the concentration to 99.1% can be achieved simply by raising the reconcentrator temperature to 400oF. In these units, it is necessary to use one of the stripping gas methods to achieve a concentration above 99.1%.
Glycol Circulation Rate
The dew point depression with a given number of trays and a specific glycol concentration can be increased by increasing the circulation rate. With 98.5% triethylene glycol, six trays, and 100oF contact temperature, the dew point depression can be increased from 61oF to 69oF by changing the circulation rate from 2 gal per lb. of water removed to a 4 gal per lb. of water removed. However, care must be exercised when increasing the glycol rate as the reconcentrator capacity can easily be exceeded. Secondly, too much circulation can cause the temperature of lean glycol entering the absorber to increase with a subsequent increase in overhead glycol loss. This occurs because the capacity of the glycol-gas heat exchanger has been exceeded. Higher circulation rates will also increase pump maintenance requirements. The circulation rates should also be as low as possible and still produce the desire dew point depression.
A circulation ratio of 2 gal per lb. of water vapor removed is considered to be the minimum required to assure adequate glycol flow across the trays.
Conditions Affecting Glycol Losses
Contactor towers operating at gas rates in excess of manufacturer's recommended maximum rates will show higher glycol losses than towers operating below the maximum recommended ratings. Where dehydrated gas from a glycol unit is used for a gas lift system, care must be used in both sizing and operating the glycol unit because of the unsteady gas rate that exists in this type service. It is recommended that a backpressure valve be installed on the gas outlet from the contactor operating on a gas lift system. If this is not done, then a valve downstream of the contactor should be pinched to help even out the flow through the unit and to minimize overloading of the contactor.
Momentary overloading of the glycol contactor can break the down comer seals in a tray type tower and cause excessive loss of glycol. Contactor tower operating at temperatures above 100°F will show greater glycol losses than those operating below 100°F. Contactor towers operating at pressure below 800 psi will show greater glycol losses than those operating above 800 psi. Running the glycol pumps at maximum operating speed at all times will cause greater glycol losses when the gas flow rate is reduced than when the pump speed is reduced in proportion to the gas flow rate. Abnormally high losses will be encountered when the pumps are operated at maximum rates and the gas flow is reduced to a very low rate. Operation at very low gas inlet temperatures (60°F or less) can be expected to cause excessive glycol losses due to foaming of the glycol.
Pump Gas Separator
The pump used in glycol type gas dehydrators, which takes its driving force from the contactor, is very often referred to as a "Glycol Powered" pump. This is incorrect. In actuality, the driving force to the pump is provided in the form of gas from the contactor.
With present Manufacturers' Standard glycol type gas dehydrators, the gas used by the pump is vented to the atmosphere and is actually wasted. The addition of a separator on the rich glycol and gas stream from the pump exhaust can and will result in considerable savings in operating cost by making this otherwise wasted gas available for use as fuel. This separator is called a pump gas separator.
The pump gas separator must be properly designed and controlled and in the correct sequence in the flow through the unit. Failure on any one of these criteria, particularly the improper location in the process stream will adversely effect the operation of the pump gas separator and of the complete dehydration unit.
In this system the pump gas separator is located downstream of the glycol/glycol heat exchanger so that the water-rich glycol will be heated before the separation of rich glycol and gas is performed. If the water-rich glycol from the absorber is not heated before entering the separator, it has a strong tendency to foam. The heating ahead of the separator reduces the possibility and probability of foaming in the separator. On some units the glycol/glycol heat exchanger is split so the water-rich glycol and gas stream flows through only part of the heat exchanger before it flows to the separator.
This usually applies only to larger more sophisticated dehydration units when a lower or controlled operating temperature on the separator is desirable. In other dehydrators such as KW International's Standard Unit, the water-rich glycol and gas stream flows through the entire heat exchanger before going to the separator. In order to minimize the heat loss from the rich glycol due to radiation cooling, KW International offers insulation for the glycol section of the separator. To continue the process flow through the system; the rich glycol from the separator goes to the regenerator and the gas from the pump gas separator goes to fuel and/or recompression. In the event that recompression is not used, as is the case with most smaller dehydrators, the excess gas which is not used as fuel is vented. In the future as gas becomes more valuable, even the smaller dehydration units will be equipped with compressors to salvage this excess gas.
Control of the operating pressure of the pump gas separator is of prime importance. This pressure must always be high enough to force the rich glycol out and into the regenerator. At the same time, it must not be so high that it will exert sufficient backpressure against the pump exhaust to hinder the operation of the pump. Consequently, the pump gas separator must have pressure relief to prevent pressure build-up when there is gas in excess of fuel requirements and pressure make-up if the fuel requirement should be greater than the volume of gas being exhausted from the pump.
A secondary value of the pump gas separator is the reduction of upsets in the dehydrator by removing excess distillate from the rich glycol flowing into the regenerator. This requires that the separator be designed for 3 phase (Glycol/Gas/Distillate) separation. All KW International pump gas separators are constructed for 3 phase operation and can be furnished with or without controls for automatic operation of the third (Distillate) phase.
However, the primary purpose of the pump gas separator (Glycol/Gas/Distillate separator) is to recover for use as fuel, gas that would without this separator be wasted.