INTRODUCTION Produced natural gas contains water vapor. The gas is usually saturated under reservoir conditions of temperature and pressure. As it flows up the tubing, some of this water vapor may condense as "free water." The remainder will remain as water vapor. The process of removing water vapor from a gas stream is called "gas dehydration. The amount of water vapor contained in a gas stream can be expressed in terms of concentration, mg/m3 in metric units (in the U.S., the normal units are pounds of water vapor per million standard cubic feet of gas, lbs/MMSCF), or in terms of the "dew point" of the gas. The dew point is the temperature at which water will condense from the gas stream as it is cooled. The higher the concentration of water vapor in a given gas stream the higher its dew point. Thus, gas dehydration is also called "dew point depression," the lowering of the dew point of a gas by lowering the concentration of water vapor in the gas. Gas is dehydrated to prevent hydrate formation, to prevent corrosion, or to meet a sales gas contract. Hydrates are loosely-linked, crystal-like chemical compounds of hydrocarbon and water resembling dirty ice. If hydrates form, they can accumulate in valves and fittings, blocking or restricting gas flow. In order for hydrates to form, water must be present in liquid form, and the gas must be cooled to below its hydrate formation temperature, which is a function of gas composition and pressure. Thus, a gas stream which is dehydrated so that its dew point is lower than the temperature to which it will be cooled will have no water condensing from the gas, and hydrates will not form. Water condensing in a gas line which is cooled below its dew point can cause corrosion, especially if the gas contains carbon dioxide (CO2) or hydrogen sulfide (H2S). Also, water in a gas line reduces the line capacity, increases the pressure drop in the line, and can produce undesirable or damaging slugging in the pipeline. For these reasons most gas sales contracts specify a maximum amount of allowable water vapor in the gas. For the Southern U.S. the limit is normally 112 mg/m3 (7 lbs/MMSCF), for the Northern U.S. 64 mg/m3 (4 lbs/MMSCF), and for Canada 32 to 64 mg/m3 (2 to 4 lbs/MMSCF). These limits are low enough to prevent water from dropping out in the line at normal transmission pressures and the lowest anticipated gas line temperature. These values correspond to dew points of approximately 0C for 112 mg/m3 (32F for 7 lbs/MMSCF), -7C for 64 mg/m3 (20F for 4 lbs/MMSCF) and -18C for 32 mg/m3 (0F for 2 lbs/MMSCF) in a 6900 kPa (1,000 psi) gas line. Gas can be dehydrated by cooling and separating the condensed liquids, by using specially designed low temperature separation processes, by using solid desiccants, or by using liquid desiccants. Cooling the gas stream and removing the free water with a separator is the simplest method of dehydration. However, this method is limited by the hydrate formation temperature unless some other hydrate preventative method has been taken. Low temperature separation methods can be used when the produced well stream is at a higher pressure than the pipeline delivery pressure and there is adequate pressure differential to make use of the Joule-Thompson cooling effect caused by a pressure drop. This process is sometimes called a Low Temperature Exchange or LTX Unit. In an LTX Unit shown in Figure1 (Schematic of a low temperature exchange unit), the temperature upstream of the choke must be controlled so that it is above the hydrate temperature when it enters the choke body.

Figure 1

Hydrates are formed through the temperature reduction across the choke and blown into the separator where they are melted in the liquid section of the separator by heat exchange with the inlet gas stream. If the inlet gas stream is 13,800 to 20,700 kPa (2,000 to 3,000 psi), temperatures downstream of the choke on the order of -18 to -29C (0 to -20F) are possible. This corresponds to the dew point of the gas leaving the cold temperature separator ("residue gas"). There are a number of commercially available solid desiccants used to remove water vapor from natural gas. Some of these are activated alumina, molecular sieves, silica gel and calcium chloride. With the exception of calcium chloride, all of the desiccants can be regenerated and reused. A solid desiccant system normally consists of multiple towers containing the desiccant, with the wet gas flowing through one or more of the units while one unit is being regenerated. Since this system requires multiple towers, switching valves, etc., this type of dehydration is typically more expensive than liquid desiccant systems. They are capable, however, of reducing the water dew point to extremely low levels and are commonly used for dehydrating gas prior to further processing in cryogenic plants. Calcium chloride desiccants undergo several chemical reactions with water vapor, and the solid desiccant gradually becomes a brine solution. This brine solution is then discarded and the vessel is refilled with fresh solid calcium chloride. These units are not widely used, but are sometimes found in low volume fuel gas systems where a heat source or electricity is not available. The most commonly used method of gas dehydration is the liquid desiccant unit, typically using triethylene glycol (TEG) as the desiccant. Under certain conditions diethylene glycol and tetraethylene glycol can also be used. In these systems the glycol contacts the gas and absorbs water vapor from the gas in a "contact tower" or "absorber" and is then circulated to a "regenerator" or "reconcentrator" where the absorbed water is boiled off as steam. This presentation discusses conventional TEG dehydration and presents a method to size and design the glycol dehydration equipment. Its purpose is to provide basic information for the project engineer to specify and evaluate vendors' proposals and troubleshoot glycol units. THEORY Water Content of Natural Gas It can normally be assumed that hydrocarbon gas produced from wells is saturated with water vapor at bottom hole conditions. As the gas is produced and the flowing pressure and temperature change, the water content of the gas changes. Often water condenses out of the gas and is recovered in a separator. The amount of water vapor a natural gas can contain depends upon the pressure, temperature, and the gas composition. Figure1 (Water content of natural gas.

Figure 1

(Courtesy of GPSA)) is a suitable correlation for lean, sweet natural gases containing 70 percent or greater methane. As an example, assume it is desired to determine the water content for a natural gas with a molecular weight of 26 that is in equilibrium with 3 percent brine at 20,700 kPa at 66C (3000 psia at 150F). From Figure1 (Water content of natural gas. (Courtesy of GPSA)) at a temperature of 150F and pressure of 3,000 psia there is 104 lb of water per MMSCF of wet gas. The correction for salinity is 0.93 and for molecular weight is 0.98. Therefore, the total water content is 104 x 0.93 x 0.98 = 94.8 lb/MMSCF or 1518 mg/m3. From an examination of Figure1 (Water content of natural gas. (Courtesy of GPSA)), it is evident that the higher the temperature and the lower the pressure, the more water vapor a gas is capable of holding. If coded to 40C (100F), a water-saturated gas that was initially at 90C and 7000 kPA (200F and 1000 psi) will have a lower capacity for water vapor, and free water will condense in the line. When gases contain more than about 5 percent CO2 and/or H2S, corrections should be made for the acid gas components. Figures 2a

Figures 2a

(Effective water content for carbon dioxide gas) and Figures 2b (Effective water content for hydrogen sulfide gas) can be used to estimate the amount of water content of acid gases.

Figures 2b

As an example, assume the gas from the previous paragraph contains 15 percent H2S. The water content of the hydrocarbon gas is 94.8 lb/MMSCF. From Figure2b (Effective water content for hydrogen sulphide gas), the water content of H2S is 400 lb/MMSCF. The effective water content of the stream is approximately equal to (0.85)(94.8) + (0.15)(400) or 141 lb/MMSCF or 2260 mg/m3. With high concentrations of acid gas (above 40 percent), additional experimental data should be obtained to verify the calculated water contents. Figures 2a (Effective water content for carbon dioxide gas) and 3b are not meant to show water contents of pure H2S and pure CO2. Figure2c (Water content of CO2) shows the water content of pure CO2.

Figure 2c

It can be seen that unlike natural gas, pure CO2 goes through a minimum at 4800 to 6200 kPa (700 to 900 psi), and then its ability to hold water vapor increases with pressure. For this reason, dehydration associated with gas produced from CO2 floods normally occurs at 4800 to 6200 kPa (700 to 900 psi).HYDRATE FORMATION A hydrate in the gas system is a physical combination of water and other small molecules to produce a solid, which resembles ice but has a different structure than ice. Hydrates resemble snow in appearance and can form at temperatures well above 0C (32F). If a gas stream contains free water, and is at a temperature and pressure at or below its hydrate point, hydrate formation is possible. Other conditions which promote hydrate formation when the above conditions are met are: physical site for crystal formations such as rust, elbows, thermowell, line scale, orifice, etc.; pressure pulsations; high velocities; or introduction of a small crystal of the hydrate. Removing the hydrate once it is formed is often difficult. Since the line can become completely plugged, gas flow may be stopped and delivery interrupted until the hydrate can be removed. Methanol or glycol can be injected to dissolve the hydrate plug, the line pressure can be lowered to lower the hydrate formation temperature and thus "melt" the plug, or the temperature can be increased. In lowering the line pressure, extreme care must be exercised as gas can be trapped between two plugs. One plug can blow out when it begins to melt, causing large impact forces and a discharge of gas. Hydrates can also cause physical damage to equipment. Slugs of hydrates moving through a pipeline at normal gas velocities can produce high impact forces on valves, orifice plates, strainers, and other equipment that impede their movement. Hydrate slugs leaving lines and flowing into separators, scrubbers, compressors, and other equipment can likewise cause serious damage. Figure3 (Approximate hydrate temperature formation.) can be used to predict approximate temperatures and pressures where hydrate formation is possible.

Figure 3

The presence of acid gases, especially H2S, will increase the temperature at which hydrates will form. There have been instances where hydrates have formed in sweet gas at temperatures as much as 6C (10F) above or below the hydrate temperatures predicted by the charts. For this reason a safety factor should be used when designing a system until some experience with the particular stream in question is available. DEW POINT DEPRESSION The dew point depression is the difference between the gas dew point before and after dehydration. For example, a sweet natural gas saturated at 6900 kPa and 38C (1,000 psia and 100F) is dehydrated to 112 mg/m 3 (7 lbs of water per MMSCF). This 7 lbs at 1000 psia corresponds to a temperature of 1C (34F) (see Figure1 ) (Water content of natural gas.) and consequently a dew point depression of 37C (38C -1C) or 66F (100F -34F).

Figure 1

The actual operation of a glycol dehydration unit must fall within the design range of the unit in order for it to perform satisfactorily. The dew point depression depends upon a number of factors, such as: inlet gas temperatures, type of glycol used, concentration and temperature of the lean glycol, circulation rate of the glycol, number of trays in the contactor, gas inlet pressure, and the quality or condition of the glycol. The inlet gas temperature has a large effect on the amount of water vapor a gas can contain at a given pressure. A gas saturated with water vapor at 6900 kPa abs (1000 psia) will contain approximately 1660 mg/m3 (104 lbs/MMSCF) at 49C (120F) as compared to 288 mg/m3 at 16C (18 lbs/MMSCF at 60F). This is a difference of almost 600 percent. Six times more water vapor will have to be removed from a gas with an inlet temperature of 49C (120F) as compared to one with an inlet temperature of 16C (60F). If an outlet dew point temperature of 1C (34F) is required, the dew point depression is 48C for 49C (86F for 120F) inlet and only 15C for 16C (26F for 60F) inlet gas temperature.GENERAL PROCESS DESCRIPTION The glycol dehydration unit consists of an inlet separator, glycol contactor, glycol reboiler, glycol pumps, filters, heat exchangers, and often a three-phase (glycol, condensate, gas) separator. (See Figure4 ) (Schematic flow diagram for a typical glycol dehydration unit).

Figure 4

The wet gas goes through an inlet scrubber to remove any free liquids before going to the glycol contactor. The gas enters the bottom of the glycol contactor and flows up through the vessel exiting at the top of the tower. Dry or "lean" glycol enters the top of the tower and flows down through the vessel, exiting at the bottom of the contactor. The glycol and gas are intimately mixed on the trays as they flow counter-current to each other. Normally the contactor contains trays and the glycol and gas are mixed on each tray. Packing is sometimes used in lieu of trays. As the glycol flows down the column it picks up water from the gas and becomes wet or "rich" glycol. The wet glycol leaves the tower, goes through a reflux exchanger section in the reboiler still column, a heat exchanger and then to the glycol/condensate separator known as a "skimmer" in Figure4 (Schematic flow diagram for a typical glycol dehydration unit). The glycol leaving the separator goes through filters to remove solids and dissolved hydrocarbon traces and then to the reboiler where the water vapors are driven off to atmosphere. The hot regenerated lean glycol goes to a storage tank and from there to the glycol/glycol heat exchangers where it is cooled before being pumped to contactor pressure. The lean glycol goes through a glycol/gas exchanger where it is cooled to within 3 to 6C (5 to 10F) of the contactor temperature. It then enters the top of the contactor and the process is repeated. As the wet inlet gas enters the contactor, the water vapor is removed as the gas flows up the contactor and is mixed with the glycol. The gas becomes drier as it goes from tray to tray up the contactor. The dry gas leaving the contactor goes to the glycol/gas heat exchanger where it cools the inlet glycol. The gas and glycol approach equilibrium as they are mixed on the trays; therefore, the higher the purity of the glycol, the drier the gas leaving the tray. Approximately four actual trays are normally assumed to be required to equal one theoretical tray where the gas and glycol are in equilibrium.GLYCOL PROPERTIES Liquid desiccants which are commonly used in the conventional dehydration unit are diethylene glycol, triethylene glycol, and tetraethylene glycol. Ethylene glycol has been used in some applications, but its vapor pressure is too high for use in conventional dehydration units without excessive vapor losses. The more commonly used glycols for dehydration are diethylene glycol, triethylene glycol, and tetraethylene glycol. Diethylene glycol is the cheapest of the three glycols, but it has a higher vapor pressure and cannot be regenerated to as high purity as the other glycols. It is more commonly used for injection systems and is only sometimes used for limited dew point depression in conventional dehydration units. Triethylene glycol is the most commonly used glycol for conventional dehydration units because of its higher decomposition temperature allowing regeneration to a higher purity than diethylene glycol. This higher purity of the reconcentrated glycol allows corresponding higher dew point depressions of the gas stream. Triethylene glycol is more expensive than diethylene but cheaper than tetraethylene. Tetraethylene glycol has recently become commercially available. It has a higher decomposition temperature than the others and can be regenerated to higher temperatures and purities, allowing a slightly higher dew point depression than triethylene glycol, using the same regeneration equipment. Another advantage it has over triethylene glycol is its lower vapor equilibrium, resulting in lower glycol losses. This is particularly true at elevated gas contact temperatures (above 49C (120F)). The major disadvantages are its higher cost and higher viscosity, which becomes a factor at low gas contact temperatures. Physical properties of the different glycols, together with viscosities, specific gravities, boiling points and condensation temperatures, vapor pressures, specific heats, toxicological properties and thermal conductivities are included in a later section. Material safety data sheets for these glycols can be obtained from your local chemical supplier. A triethylene glycol dehydration unit should operate between a maximum of about 49C (120F) to a minimum of about 16C (60F). At temperatures higher than 49C (120F) the glycol vaporization loss becomes excessive when the glycol enters the contactor and flashes on the first tray. At temperatures below about 16C (60F) the glycol viscosity increases leading to low efficiency of the gas-glycol contact and an increased tendency of the glycol to foam. Tetraethylene glycol can be used at higher gas contact temperatures. Another factor that influences the water vapor content of a natural gas is pressure. The higher the pressure the less water vapor a gas can contain. In most instances the operating pressure of a dehydration unit is fixed due to pipeline delivery requirements, but if a unit is designed for a given pressure and is then operated at a considerably lower pressure, the unit may not be able to deliver the designed dew point depression without making other operating adjustments. At lower than design pressures the contact tower may have too small a diameter to handle the design flowrate of natural gas. Design Parameters The effects of glycol concentration, number of trays, and glycol circulation rate are all interrelated. In this presentation, equilibrium data for water vapor over TEG-H20 solutions is computed at various temperatures and pressures from an equation of state and a liquid-phase activity model. The Peng-Robinson-Stryjek-Vera (PRSV) equation of state is used for the vapor phase, to calculate fugacities. Liquid fugacities are based on an empirical correlation presented by Parrish and Won for pure water fugacity, modified by liquid activity coefficients. The activity coefficients are based on the Parrish-3rd order Margules equation up to 100C (212F), and on the Edwards-Won hyperboliccorrelation at higher temperatures. The equilibrium relationship below is used, in conjunction with mass balances, to perform stagewise contactor and reconcentrator design and performance calculations. Equation 1

Glycol Concentration The one change which can have the greatest effect on dew point depression is the glycol concentration, normally stated as a percent of purity. The higher the purity of the glycol the drier it is possible to get the gas, other conditions being equal. Figure5 (Equilibrium water dew points with various concentrations of TEG) shows the equilibrium water dew point with various concentrations of triethylene glycol.

Figure 5

Since the gas leaving the top tray or the top section of the tower is in contact with the leanest (highest purity) glycol, the chart shows the ideal dew point depression for a gas in equilibrium with the glycol. However, true equilibrium can never be reached, and in practice it is common to approach these equilibrium dew points within about 11C (20F). That is, to obtain a 0C (32F) dew point with 49C (120F) gas in the contactor it is necessary to have an equilibrium dew point of approximately 7C (12F) and a 99.5 percent concentration of lean glycol is required. There are several ways to increase the glycol purity. The simplest method is to increase the temperature of the reboiler, thus driving off more water and obtaining a higher purity glycol. The Figure6 (Glycol purity versus reboiler temperature at different levels of vacuum) shows the glycol concentration as a relationship to the reboiler temperature.

Figure 6

A temperature of 204C (400F) in the reboiler is about the highest recommended temperature for TEG as thermal decomposition begins to occur at higher temperatures. Many operators like to limit reboiler temperature to 200C (390F). Another method of increasing the glycol purity is by lowering the pressure on the still with a vacuum pump. Figure6 (Glycol purity versus reboiler temperature at different levels of vacuum) also shows the effect of different levels of vacuum on the glycol purity. One other method of increasing the glycol purity is through the use of stripping gas. The wet gas entering the contactor is saturated with water at contactor operating pressure and temperature. Under the conditions of near atmospheric pressure and high temperatures in the reboiler, this gas can absorb large quantities of water. In a stripping gas system the hot glycol leaving the reboiler is contacted with wet gas from the contactor in a counter-current packed tower below the reboiler. Dehydrated gas could also be used from the contactor. The gas bubbles up through the glycol, "stripping" the glycol of additional water and exiting through the still on top of the reboiler. The longer the stripping gas contactor the greater the number of equilibrium stages "below the reboiler." Often the gas is injected through a sparger in the base of the reboiler, and just bubbles through the glycol. The still provides approximately one or two equilibrium stages above the reboiler. Figure7 Effect of different stripping gas flow rates on the concentration of triethylene glycol) shows the effect of different stripping gas flow rates on the concentration of triethylene glycol at various temperatures,

Figure 7

assuming a sparger is used in the reboiler. At a reboiler temperature of 204C (400F) and no stripping gas the glycol purity is 99.05 percent. The glycol purity can be increased to 99.5 percent with the reboiler temperature remaining constant by adding 33.7 m3/m3 (4.5 SCF/gal) of stripping gas to the glycol. Stripping gas can be used more efficiently with a packed column located between the reboiler and the storage tank, as shown in Figure8a (Effect of stripping gas rate and adding stages below the reboiler on the percent purity of lean TEG for 90.0 percent wet TEG),

Figure 8a

Figure8b (Effect of stripping gas rate and adding stages below the reboiler on the percent purity of lean TEG for 95.0 percent wet TEG),

Figure 8b

and Figure8c (Effect of stripping gas rate and adding stages below the reboiler on the percent purity of lean TEG for 97.5 percent wet TEG).

Figure 8c

Using the same conditions shown above, but with one stage below the reboiler, the glycol concentration can be increased from 99.05 percent to 99.87 percent with 33.7 m 3/m3 (4.5 SCF/gal) of stripping gas. Glycol Circulation Rate Glycol circulation rates from 3.4 to 8.6 l of glycol per kg (2 to 5 gal of glycol per lb) of water to be removed are normally used. The circulation rates required depend upon the glycol purity and the number of trays in the glycol contactor. The higher the circulation rate the greater the dew point depression, other factors being equal. However, the higher the circulation rate the greater the reboiler duty. A higher circulation rate can also result in an increase in the glycol temperature entering the contactor. This will cause an increase in the glycol overhead losses and will increase the glycol pump maintenance. Although circulation rates lower than 3.4 l/kg (2 gal/lb) of water removed are sometimes used, a minimum circulation rate of about 3.4 l/kg (2 gal/lb) is needed for adequate glycol flow across the trays. Figure9 (Effect of circulation rate and number of trays on the dew point depression) shows the effect of circulation rate on dew point depression for a specified glycol concentration.

Figure 9

It can be seen that the number of trays has a greater effect on dew point depression than glycol circulation rate. Figure10 (Effect of circulation rate and glycol purity on the dew point depression) shows that increasing the glycol purity has a greater effect on dew point depression than increasing the circulation rate.

Figure 10

Number of Trays Tray efficiencies in glycol contactors normally range from 25 to 35 percent. Due to the low liquid flow rates, care must be used in the design of the trays to maintain a liquid seal on the tray. The more trays there are in a contactor the closer the glycol and gas approach equilibrium and the greater the dew point depression. Most glycol contactors are designed for one to three theoretical trays or four to twelve actual trays. See Equipment Description subject for further descriptions of tray types and contactor design. Combining Parameters Each dehydration requirement involves a balance and trade off between glycol concentration, circulation rate, and number of trays or height of contactor if packing is used. Figure11a : (Water removed versus recirculation rate for various glycol puritie) ,

Figure 11a

Figure11b (Water removed versus recirculation rate for various glycol purities),

Figure 11b

Figure11c (Water removed versus recirculation rate for various glycol purities),

Figure 11c

Figure11d (Water removed versus recirculation rate for various glycol purities),

Figure 11d

qnd Figure11e (Water removed versus recirculation rate for various glycol purities) can be used to select one of these parameters if the others are fixed.

Figure 11e

. In using these figures, W is the water content of the gas at contactor inlet conditions and W is the desired change in water content. Example: 0.675 S.G. gas at 6900 kPa (1000 psig), 43C (110F), desire to dehydrate to 112 mg/m3 (7 lb/MMSCF) From Figure13 (Water content of natural gas. Courtesy of GPSA):

Figure 13

EQUIPMENT DESCRIPTION Contactor/Separator Before entering a glycol contactor the gas should go through a separator to remove any free liquid. This can be either a horizontal or vertical separator or a filter/separator. On units with capacities greater than 1.4 MMm3 (50 MMSCFD) and units with high condensate rates or gas coolers before the dehydration system, a filter/separator is often used to decrease the possibility of contaminating the glycol. On smaller units, the separator is usually included as a part of the contactor in the bottom section as shown in Figure1 (Schematic flow diagram of a typical glycol contact tower).

Figure 1

In the case shown, the gas leaving the separator section flows up through a hat or chimney tray into the contactor section. The contactor is a tower using trays or packing to affect an intimate contact between the glycol and gas. The wet gas entering at the bottom of the contactor flows upward, contacting the downward flowing glycol. The gas gets leaner and leaner in water vapor as it rises, and the glycol gets richer and richer in water as it falls. In this manner a series of counter-current equilibria are set up with the leanest glycol contacting the driest gas. The glycol is forced to flow across each tray by a series of downcomers and spill over weirs. The distance between trays is set so that the gas can disengage from the glycol prior to reaching the underside of the tray above. Because there is a pressure drop as the gas flows upward from one tray to another, a seal weir is needed to create a liquid seal at the base of the downcomer. Without this weir, some of the gas will blow up the downcomer and not contact the glycol correctly. The downcomer must be sized so that the pressure drop for the designed glycol flowrate does not exceed the head available. Otherwise glycol will build on the top trays. This condition is called "flooding." Typically, to provide a margin of safety the downcomer is designed for 75 percent or less of flooding. A good rule of thumb is to size the downcomer so that the glycol velocity does not exceed 0.25 ft/sec. The trays are orifice type devices, which disperse the gas uniformly on the trays and throughout the liquid on the tray. Trays are commonly spaced 610 mm (24 in) apart vertically. The most commonly used tray in a glycol contactor is the bubble cap tray, although valve trays are also used. An example of a bubble cap tray is shown in Figure2 (Typical bubble cap tray.

Figure 2

(Courtesy of Smith Industries, Inc.)). In order to assure good contact between the gas and the liquid flowing across the tray, a minimum upward flow of gas is necessary. The minimum flow of gas required to make the bubble caps or valves work efficiently is typically taken as 25 percent of design flowrate. This is called "turndown." At lower flowrates, liquid could fall from tray to tray through the valves or bubble caps, thereby bypassing most of the gas, or gas will not be sheared into small enough bubbles to maximize the contact area between the gas bubble and the liquid. Bubble caps, by virtue of their design, are less sensitive to turndown than valves. There are instances where bubble cap contactors have successfully dehydrated gas at flowrates of 10 percent of design. If turndown is a problem, individual caps or valves can be blanked off, forcing the gas flow to be shared by those remaining. For this reason, and also to service the caps or valves, a 203 mm (8 in) hand opening or larger manway is often provided in the space above each tray. The diameter of the tray tower determines the gas velocity. When the gas velocity is above a certain point, significant quantities of glycol may be transported upward from one tray to the next. To avoid this problem, the tower should be sized for m (micron) drops from the gas stream.(removal of 120 to 150 Packing of various types, sizes and shapes can be randomly packed in the tower in place of using trays. An example of some of the types of random packing used is shown in Figure3a

Figure 3a

and Figure3b Various types of packing).

Figure 3b

Random packing provides a surface area upon which contact can be made between the gas and glycol by creating a tortuous path for the gas to rise and glycol to fall. At low gas rates the glycol and gas may channel, largely bypassing each other. Thus, randomly packed towers should not be expected to adequately dehydrate the gas at flowrates less than 25 percent of design. At high gas velocities, glycol may build above the packing, flooding the contactor. The diameter of the tower is set to assure that gas velocities are kept below those required for flooding. The height of packing is given by the manufacturer in terms of the number of feet required for an equilibrium tray. Structured packing is also sometimes used. Structured packing is made up of folded perforated plates welded together as shown in Figure4a Example of structured packing.

Figure 4a

(Courtesy of Koch Engineering Co.) ) and Figure4b Example of structured packing.

Figure 4b

(Courtesy of Koch Engineering Co.)) . Due to the defined nature of the gas and glycol paths through the structured packing, structured packing is more efficient than either random packing or trays at creating contact between the gas and glycol. Thus, with structured packing smaller contactor diameters and shorter contactors are possible. Good lean glycol inlet distribution is essential for structured packing to perform as desired. Glycol contactors commonly use from one to three equilibrium trays (four to twelve actual trays) with the higher number of trays required for higher dew point depressions. Glycol/Gas Exchanger The glycol/gas exchanger is used to cool the lean glycol before it contacts the gas in the glycol contactor. If the lean glycol's temperature is more than 5.5 to 11C (10 to 15F) higher than that of the gas in the contactor, some glycol will vaporize on the top tray and be lost with the dry gas. If the glycol is colder than the gas, it may cause liquid hydrocarbons to condense, and the resulting condensate could contaminate the glycol and cause foaming. The most commonly used glycol/gas exchanger consists of two concentric pipes with one pipe inside the other. The glycol flows counter current to the gas in the annulus space. The gas leaves the contactor in the inner pipe. This is an economical type exchanger and requires very little additional space. If more efficient heat transfer is required, a shell and tube exchanger or a multi-tube exchanger can be used. This exchanger, as well as all the other exchangers in the system, can be sized using the procedures contained in the presentation on Heat Transfer. For a first approximation, an overall heat transfer coefficient ("U" value) of 57 to 113 W/m2K (10 to 20 Btu/hr-ft2-F) can be used for the two concentric pipe type exchangers. Glycol Regeneration System The glycol regeneration system consists of several different pieces of equipment designed to regenerate a wet ("rich") glycol by removing hydrocarbons and water vapor. A typical glycol regeneration system is shown in Figure5 Schematic flow diagram of a typical glycol regeneration system). A brief description of the various pieces of equipment follows:

Figure 5

Reflux Condenser The rich glycol enters the system and goes through a coil in the top of the glycol reboiler (or "glycol regenerator") where it exchanges heat with the water vapor leaving the column and acts as a reflux condenser. The rich glycol is heated in the reflux condenser. The reflux condenser controls glycol losses to the atmosphere by condensing some of the water vapor in the reboiler still overhead. The falling water helps to condense glycol vapors entrained in the vapor coming off the still. Typically, the reflux condenser is designed with enough surface area to condense 50 percent of the water vapor being removed in the still. The temperature of the water vapor reaching the reflux condenser is approximately 150 to 160C (300 to 320F) as the vapors are cooled with the incoming glycol. The temperature of the vapors leaving the condenser are usually at the boiling point of water, that is, 100C (212F) if there is no stripping gas and the regenerator is operated at atmospheric pressure. A typical overall heat transfer coefficient for the reflux condenser is 400 W/m2K (70 Btu/hr-ft2-F). Glycol/Glycol Heat Exchanger The rich glycol flows to a glycol/glycol heat exchanger where it is heated by the lean glycol. The glycol/glycol exchanger can be a double pipe exchanger, plate fin exchanger, or shell and tube type exchanger. The heat exchanger cools the lean glycol and heats the rich glycol, thus reducing the size of the reboiler and cooling the lean glycol prior to going to the glycol pumps. An approximate overall heat transfer coefficient for glycol-glycol exchangers is 170 to 284 W/m2K (30 to 50 Btu/hr-ft2-F). Often there are two glycol-glycol exchangers, one upstream and one downstream of the glycol/condensate separator as shown in Figure5 Schematic flow diagram of a typical glycol regeneration system). It is best to heat the rich glycol to between 66 to 93C (150 and 200F) before routing it to the glycol/condensate separator. This reduces the viscosity of the glycol, which is the continuous phase, making it easier to separate condensate. It also helps liberate dissolved gases from the glycol before the glycol enters the still. The temperature of glycol to the still is typically on the order of 135 to 150C (275 to 300F). Higher temperatures would save on reconcentrator duty, but would add to heat exchanger cost. Glycol/Condensate Separator The glycol condensate separator (sometimes called a "glycol flash tank" or "pump gas separator") is a three-phase separator used to separate hydrocarbon liquids, glycol/water mixture, and vapors. This is a standard three-phase separator and can be either a horizontal or vertical separator, sized for 15 to 30 minutes retention time for the liquids. The procedures in the presentation on Three Phase Separators can be used to size this vessel. The separator operates at a low pressure, approximately 200 to 350 kPa (30 to 50 psig). The gas leaving the separator can be used for fuel or recompressed for sales. The condensate flows to the oil stream and the glycol/water mixture goes to the regenerator. Since the glycol will absorb some of the heavier hydrocarbons from the gas, a glycol condensate separator should be included when the gas stream contains heavier hydrocarbon components. For a gas stream of methane, ethane and CO2 with very little or no heavier components a two-phase separator can be used to separate only the gas which is absorbed by the glycol at the high pressures in the contactor or the gas which is used to drive glycol-powered pumps. Some small units do not have a separator at all. In these cases entrained gas is vented with the still vapors. This is not recommended as it wastes gas, which could be used for fuel and could lead to pollution. In Figure5 Schematic flow diagram of a typical glycol regeneration system) the level control valve is located downstream of the filters and heat exchangers to avoid flashing of the glycol upstream of this equipment. Often the level control valve is located at the glycol condensate separator for convenience and because the amount of flash gas is small. Glycol Filters The glycol leaving the separator is normally filtered through a sock type filter to remove 5 micron and larger solids. A differential pressure indicator is installed to measure the pressure drop across the filter. When the pressure drop increases to about 70 kPa (10 psi), the filter elements need to be replaced. Manufacturer's recommendations should be followed regarding pressure drop requirements before replacing filters. Sock type filters consist of a pressure vessel with a quick opening closure and a number of filter elements. The filter elements are replaced when they become clogged and dirty. The charcoal filter is a similar type of filter with charcoal instead of sock elements. For small glycol units, less than 2.3 m3/hr (10 gpm), the charcoal filter is normally full flow, whereas for larger units a small side stream of 10 to 25 percent of total glycol is filtered. The charcoal filter removes hydrocarbon and chemical impurities. A differential pressure indicator is installed to measure the pressure drop across the bed. When the pressure drop reaches about 70 kPa (10 psi), the charcoal needs to be replaced. Glycol Pumps Glycol pumps normally have low flow rates and high differential pressures. Usually, an electric motor-driven plunger pump is used. The presentation on Reciprocating Pumps provides information for sizing and selecting this type of pump, pulsation dampers, and associated equipment. On smaller units, and where electricity is not readily available, glycol powered pumps are sometimes used. A glycol powered pump uses high pressure rich glycol with a small amount of entrained gas to pump the lean glycol. Figure6 Schematic flow diagram of a glycol powered pump.

Figure 6

(Courtesy of Kimray Inc.) ) shows a cutaway of the glycol/gas powered pump. Referring to Figure6 Schematic flow diagram of a glycol powered pump. (Courtesy of Kimray Inc.) ), with the Pilot "D" slide in the left position, high pressure wet glycol flows through the right hand speed control valve to the right side of the main piston. This causes both pistons to move to the left. At this point the Pump "D" slide places wet glycol discharge pressure on the left side of the Pilot "D" slide, allowing the Pilot "D" slide to stay in its left position. As the pistons move to the left, dry glycol is pumped from the right piston to the glycol contactor at high pressure, and wet glycol is discharged from the left piston to the regenerator system. Dry glycol is also sucked into the left piston from the glycol surge tank. When the pump piston moves completely to the left, the Pump "D" slide places the high pressure wet glycol on the left side of the Pilot "D" slide and the low pressure wet glycol on the right side of the Pilot "D" slide. This causes the Pilot "D" slide to switch to the right as shown in Figure6 Schematic flow diagram of a glycol powered pump. (Courtesy of Kimray Inc.) ). The Pilot "D" slide now places high pressure wet glycol to the left side of the left pump piston, causing the pump piston to move to the right. Wet glycol is now discharged to the regenerator system from in front of the right piston, and dry glycol is sucked in behind the right piston. Dry glycol is discharged to the contactor from in front of the left piston. This process then proceeds as a reciprocating motion as first the pump slide and then the pilot slide change positions. Gas is entrained with the wet glycol from the contact tower to provide the energy to make the system overcome friction losses. If a glycol powered pump is used, a level control is not installed on the glycol outlet of the contactor. If the level falls in the contactor, more gas flows to the pump, speeding it up and allowing it to pump drier glycol into the contactor. The pump speed is controlled by the throttling valves. Gas consumption rates are shown in the following table. CIRCULATION RATE -Gallons/Hour

*Pump Speed -Strokes/Minute. Count one stroke for each discharge of pump.

Model Number 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

1715 PV 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

4015 PV 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

9015 PV 27 31.5 36 40.5 45 49.5 54 58.5 63 67.5 72 76.5 81 85.5 90

21015 PV 66 79 92 105 118 131 144 157 171 184 197 210

45015 PV 166 200 233 266 300 333 366 400 433 466

* It is not recommended to attempt to run pumps at speeds less or greater than those indicated in the above table

GAS CONSUMPTION

Operating Pressure -psig. 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Cu. Ft./Gallon @ 14.4 & 60 oF 1.7 2.3 2.8 3.4 3.9 4.5 5.0 5.6 6.1 6.7 7.2 7.9 8.3

Model Number Dimensions, inches

"PV" Series "SC" Series A B C D E F G H J K L M N P

1715 PV 815 SC 5 1/4 5 11/16 5 3/4 3 7/16 1 1/2 3 1/2 7 1/4 10 7/8 10 3/16 9 5/8 15 2 1/8 1 3/4 3

4015 PV 2015 SC 5 1/4 5 11/16 5 3/4 3 7/16 1 1/2 3 1/2 7 1/4 10 7/8 10 3/16 9 5/8 15 2 1/8 1 3/4 3

9015 PV 5015 SC 6 1/4 8 1/8 6 3/8 5 1 3/4 4 1/4 8 3/4 13 1/4 13 7/8 11 3/4 20 2 1/2 2 3

21015 PV 10015 SC 7 3/8 10 1/8 1/8 7 5 3/8 2 1/4 5 3/4 9 1/4 14 3/4 16 5/8 13 24 3 3/16 2 1/2 4

45015 PV 20015 SC 10 3/4 14 1/8 9 6 5/8 2 5/8 6 1/2 11 3/8 19 21 1/8 16 3/8 34 3 3/4 3 1/2 6

Model Number Max. Cap. Size of Pipe Connections Mounting Bolts Approx. Weight

G.P.M. G.P.H.

1715 PV .67 40 1/2" N.P.T. 3/8" Dia. 66 Lbs.

4015 PV .67 40 1/2" N.P.T. 3/8" Dia. 66 Lbs.

9015 PV 1/5 90 3/4" N.P.T. 1/2" Dia. 119 Lbs.

21015 PV 3.5 210 1" N.P.T. 1/2" Dia. 215 Lbs.

45015 PV 7.5 450 1 1/2" N.P.T. 1/2" Dia. 500 Lbs.

No matter which type of pump is chosen, reciprocating or glycol powered, the lean glycol should be cooled to about 60C (140F) before it reaches the pump suction to promote longer seal and packing life and to conserve heat energy. The glycol should be cooled to 11C (20F) below the maximum allowable temperature recommended by the pump manufacturer. The glycol pumps circulate the lean glycol from the storage tank at atmospheric pressure to the glycol contactor at higher pressure. Glycol circulating rates can be determined by using a meter on the lean glycol line to the contactor or by counting pump strokes on a glycol powered pump. Glycol pumps are one of the high maintenance items in the glycol regeneration system. Often one extra pump is installed in the system so that the system can be operated when one pump is out of service for maintenance. Glycol Reboiler The glycol reboiler heats the rich glycol and boils off the water vapor through the still column to atmosphere. Normally the glycol reboiler operates at atmospheric pressure and between 190 and 204C (375 and 400F). The purity of the glycol for various temperatures and pressures is discussed before under Design Parameters section. A temperature controller in the glycol reboiler is set to maintain the desired temperature and thus the purity. Most onshore glycol reboilers use a fire tube with a natural gas burner to heat the glycol. The fire tube in the reboiler is normally sized for a 1.89 to 2.52 W/sq cm (6,000 to 8,000 Btu/hr-ft2) flux rate. Steam coils or heat medium coils can also be used to heat the reboiler. Electric heating elements can also be used to provide all or part of the heat for the reboiler. Sometimes the electric heating elements help load a gas engine or turbine generator and the waste heat from the generator is used to heat a circulating heat medium to provide the remainder of the heat required. The presentation on Heat Transfer Systems discusses the design of circulating heat media systems, and the presentation on Waste Heat Recovery discusses the recovery of heat from engines and turbines. Many operators prefer not to use fire tubes on offshore platforms as a leak in the fire tube could cause the glycol to burn. Others feel that the safety devices recommended by API RP 14C, Recommended Practice for Analysis, Design, Installation and Testing of Basic Surface Safety Systems for Offshore Production Platforms, provide sufficient protection. The vapors leaving the reboiler go through a still column, with the reflux condenser in the top of the section used to condense part of the vapors to lower TEG losses. Because of the large difference in the boiling points between glycol 286C (546 F) and water 100C (212 F), the still can be relatively short. Normally the still has ten to twelve feet of random packing although it is possible to use structured packing or trays on larger diameter stills. The design of distillation columns is beyond the scope of this presentation. Still Overhead The vapors leaving the still are normally vented to atmosphere in a pipe routed away from the regenerator skid and any potential source of ignition. Sometimes a steam trap or simple tee in the line is used to capture any liquids which are condensed in the vent pipe. These liquids may contain hydrocarbons and impurities such as benzene. This should be collected for disposal or pumped back to the oil system and should not be allowed to just run out on the ground. Because the still overhead vapors contain small amounts of hydrocarbons and may contain H2S in sour gas applications, it may be necessary from an air pollution standpoint to recover or incinerate the vapors. Recovery can be done by cooling the stream to condense the water and routing it to a two-phase separator. The liquid from the separator is pumped to the water treating equipment for separation of hydrocarbon liquids and disposal. The gas is routed to a vapor recovery unit for compression and eventual sales or routed to an incineration point. This method of handling still overhead vapors is becoming more prevalent due to more stringent air pollution regulations. Glycol Piping The glycol regeneration system normally operates at low pressure and, due to the small volumes circulated, usually requires piping of less than 50 mm (2 in) in diameter. Hot glycol is hard to contain, and the piping for this system is often specified as socket welded with a minimum of screwed fittings to minimize leaks. Some companies specify the low pressure glycol piping as a 300 ANSI rating rather than a 150 ANSI rating to increase the number of bolts at flange connections. Since glycol pumps are normally plunger type there is some vibration associated with the glycol piping. Because of the corrosive nature of vapors and liquids contacting the reflux condenser, often the pipe coils in the condenser are made of stainless steel. DESIGN PROCEDURE General The design procedure presented in this section can be used for initial sizing determinations, to evaluate vendor quotations, and to review existing systems. In the design, all functions and requirements should be considered, including the operating variables of design flow rates and properties. For this reason, there is no substitute for operating experience and good engineering evaluation of the glycol dehydration system. The following information is required to design a glycol dehydration system. Inlet gas flow rate Gas composition, or Gas Gravity and Percent Content of H2S & CO2 Inlet Gas Temperature and Pressure Inlet Gas Water Content Required Gas Outlet Water Content or Dew Point Outlet Gas Line Pressure Water Content Inlet/Outlet Unless other information is available, assume the inlet gas is saturated with water vapor at the inlet pressure and temperature conditions. The maximum anticipated inlet temperature should be used when calculating the water content of the inlet gas. The water content can be obtained from Figure1 (Water content of natural gas.

Figure 1

(Courtesy of GPSA)), Figure2a,

Figure 2a

(Effective water content for carbon dioxide gas) and Figure2b (Effective water content for hydrogen sulphide gas), as explained earlier.

Figure 2b

The required water content of the dehydrated gas is fixed by a sales contract or is known. The water content is normally given in pounds per million standard cubic feet. Using Figure1 (Water content of natural gas. (Courtesy of GPSA)) and the gas sales pressure a dew point temperature can be read. The difference between the inlet temperature and the dew point of the dehydrated gas is the required dew point depression. Determine Process Variables The process variables such as glycol concentration, glycol circulation rate, and number of trays required in the contactor are all interdependent. Their selection requires trade-off studies. As a rule of thumb, glycol circulation rates of 3.4 to 6.8 l/kg (2 to 4 gal/lb) of water removed are normally used. For dew point depressions of 33 to 44C (60 to 80F), a 98.5 to 99 percent (177 to 193C (350 to 380F) regenerator temperature at atmospheric pressure) glycol purity, and one to one and a half theoretical trays in the contact tower are reasonable selections. Once a selection of circulation rate and number of trays is made, the glycol purity can be determined as shown in Section 3.6.1 and the regenerator heat duty calculated as described below. Different circulation rates, purity, and tray selections can then be analyzed to determine the effect on regenerator heat duty. For dew point depressions in excess of 39C (70F), either a higher purity glycol, more trays, or a combination of these will be required. Dew points of less than -29C (-20F) have been obtained with triethylene glycol by using a vacuum on the regenerator, stripping gas and three theoretical trays in the contactor. Estimate Size of Glycol Contactor The size of the glycol contactor is determined by the operating pressure, temperature, gas flow rate, and type of trays or packing used in the contactor. The minimum inside diameter of trayed contactors can be determined by the need to keep the upward velocity of the gas from carrying entrained glycol droplets to the tray above. This is equivalent to the droplet settling theory explained in the presentation on Two Phase Separators. The equation establishing minimum diameter was derived in the presentation on Two Phase Separators and is given by: Equation 1

The values of the drag coefficient and of the gas compressibility factor can be determined using the procedures described in the presentation on Two Phase Separators. Figure4 Chart for estimating contactor diameter.

Figure 4

(Courtesy of GPSA ) can be used for an initial estimate of inside diameter. Both random and structured packing are normally sized so that the pressure drop for gas flowing through the packing is one-half inch of water per foot of packing. The different manufacturers have empirical procedures to evaluate the diameter required for the various designs of packing, which they manufacture. Trayed contactors are normally sized for a 0.61 m (24 in) actual tray spacing, which is equivalent to 1.83 to 2.44 m (6 to 8 ft) height per theoretical tray, assuming 25 percent to 35 percent tray efficiency. Structured packing is sized for 1.14 to 1.22 m (45 to 48 in) of packing per theoretical tray. The height of random packing per theoretical tray is normally taken as the same as for bubble cap trays. Table1 shows a comparison of tower diameters and heights of the trayed/packing section for dehydration of 1.4 MMm 3 (50 MMSCFD) of gas at 6900 kPa (1000 psig) and 38C (100F), assuming two theoretical trays. Table1: Contactor Sizes for Dehydrating 50 MMSCFD at 1000 psig and 100F Tower Internals Tower Diameter, m (in) Tray/Packing Height, m (ft)

A. Tray

Bubble Cap 1.22 (48) 4.87 (16)

B. Structured Packing

B1-300 0.91 (36) 2.44 (8)

B1-100 0.76 (30) 2.44 (8)

Flexipac #1 1.07 (42) 1.83 (6)

Flexipac #2 0.76 (30) 2.44 (8)

C. Random Packing

2" Pall ring 1.22 (48) 4.87 (16)

The actual height of the tower must consider an allowance for a mist eliminator and glycol inlet above the top tray (approximately 0.61 to 1.22 m (2 to 4 ft)), and for a wet gas inlet below the bottom tray (approximately 0.61 to 1.22 m (2 to 4 ft)). If an integral scrubber is required in the base, additional height may be required. For more exact sizing contact the tray or packing vendor with the gas and glycol flow rates.

Estimate Size of Regenerator

Using the amount of water to be removed from the gas in pounds per hour and the glycol circulation rate in gallons per pound of water to be removed as selected above, the glycol circulation rate in pounds per hour can be calculated. The glycol concentration required determines the glycol regenerator temperature. The glycol regenerator duty is the sum of the sensible heat required to heat the glycol, the heat required to vaporize the water, the heat required for reflux, and any heat losses. The procedures in the presentation on Heat Transfer Systems can be used to determine the regenerator heat duty if the temperature of the wet glycol entering the still is known. This temperature depends on the glycol-glycol exchanger designs. A typical incoming temperature is 135 to 150C (275 to 300F). Table2 can be used for first approximation of regenerator heat duty. Table2: Approximate Heat Duty

Design Glycol Circulation Reboiler Heat Duty (q)

L Glycol/kg of Water Removed Gal Glycol/lb of Water Removed J/L Glycol Btu/GalGlycol

12.5 1.5 346,980 1245

16.7 2.0 297,100 1066

20.8 2.5 262,800 943

25.0 3.0 240,200 862

29.1 3.5 224,400 805

33.3 4.0 212,400 762

37.5 4.5 203,200 729

41.6 5.0 195,400 701

45.8 5.5 189,500 680

50.0 6.0 183,700 659

Size at 150 percent of above to allow for start-up, increased circulation, fouling.

Size fire tube for 1.89 to 2.52 W/sq cm (6,000 to 8,000 Btu/hr ft2)

Size Glycol/Hydrocarbon Separator The glycol/hydrocarbon separator is sized as a three-phase separator, either horizontal or vertical, using 15 to 30 minutes liquid retention time. The vessel should be sized using the principles outlined in the presentation on Three Phase Separators. On small units handling lean gas, this is sometimes sized as only a two phase separator, and no attempt is made to separate hydrocarbon liquids. Size Glycol/Glycol Heat Exchangers and Gas/Glycol Exchangers Size the glycol/glycol heat exchangers and the gas/glycol heat exchangers using the principles outlined in the presentation Heat Exchangers. Typical overall heat transfer coefficients ("U" values) are 0.006 to 0.012 W/cm2-C (10 to 12 Btu/hr-ft2-F) for gas/glycol exchangers and 170 to 284 W/m3-K (30 to 50 Btu/hr-ft2-F) for glycol/glycol exchangers. UNIT OPERATING PROBLEMS & TROUBLESHOOTING Operating and corrosion problems usually occur when the circulating glycol solution gets dirty. Therefore, to get a long, trouble free life with the glycol, it is necessary to recognize these problems and know how to prevent them. Some of the major problems are presented below. Oxidation Oxygen enters the system with the incoming gas, through unblanketed storage tanks and sumps or through the pump packing glands. The glycols will oxidize readily in the presence of oxygen and form corrosive organic acids. The primary acid formed is formic acid although some acetic acid is also found to be present in oxidized glycols.To prevent oxidation, tanks should have a gas blanket to keep air out of the system. Oxidation inhibitors, such as hydrazine, can also be used to prevent corrosion. Thermal Decomposition Excessive heat, a result of one of the following conditions, will decompose the glycol and form corrosive compounds: A reboiler temperature above the glycol decomposition level. A high heat flux rate, usually caused by trying to operate the unit above design capacity. Localized overheating, caused by deposits of salt or tarry compounds on the reboiler fire tubes or by poor flame direction on the fire tubes. Low pH pH is a measure of the acid content of the glycol (lower pH means more acid) and is an indication of the degradation taking place. The desired pH of the glycol solution is 7.0 to 7.5. If proper measures are not taken the pH will continue to decrease and equipment corrosion rates will increase. 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. The glycol pH should be checked periodically and kept in the desired range by neutralizing the acidic compounds with borax, ethanol amines or other alkaline chemicals. The glycol should always be diluted at least 50:50 with distilled water before its pH is determined to get accurate results. The addition of bases should be made with care as large amounts of some of these bases will result in precipitation of black matter. The addition of the base over a prolonged period will raise the pH to the desired level without fouling the system. The amount of neutralizer to be added and frequency of addition will vary from location to location depending mainly on operating conditions. The glycol manufacturer should be consulted on the recommended amount and type of neutralizer. Salt Contamination Salt is normally contained in produced water, and it may carry over with the gas and dissolve in the glycol. Salt deposits accelerate equipment corrosion, reduce heat transfer in the reboiler tubes and alter specific gravity readings when a hydrometer is used to determine glycol-water concentrations. This troublesome compound cannot be removed with normal regeneration. Therefore, salt carryover, either in slugs or fine mist, should be prevented with the use of an efficient scrubber upstream from the glycol contactor. The solubility of salt in glycol solutions decreases with increasing temperature, and, therefore, salt will accumulate on the reboiler fire tube and decrease the heat transfer efficiency. When the salt content in the glycol reaches 200 to 300 parts per million (ppm), it may start depositing on the fire tube. The solubility limits of salt in triethylene glycol is approached when the salt content reaches 600 to 700 ppm. Above this level, the salt deposition rate will quickly accelerate. The fire tube should then be regularly inspected before it fails. Hydrocarbons Liquid hydrocarbons, a result of carryover with the incoming gas or condensation in the tower, increase glycol foaming, degradation and losses. They should be removed in a glycol condensate separator. Sludge An 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 pumps, valves and other equipment. It usually occurs when the glycol pH is low and becomes very hard and brittle when deposited onto the absorber trays, stripper packing and other places in the circulating system. Good solution filtration will prevent a buildup of sludge. Foaming Foaming can increase glycol losses and reduce capacity. Entrained glycol will carry over the top of the absorber with the sales gas when stable foam builds up on the trays. Foaming also causes poor contact between the gas and the glycol, decreasing the drying efficiency. Some foam producers are: Hydrocarbon liquids Field corrosion inhibitors Salt Finely-divided suspended solids Excessive turbulence and high liquid to vapor contacting velocities will usually cause the glycol to foam. This condition can be caused by mechanical or chemical problems. The best cure for foaming problems is proper care of the glycol. The most important measures in the program are effective gas cleaning ahead of the glycol system and good filtration of the circulating solution. The use of defoaming chemicals does not solve the basic problem. They should only be considered as a temporary control until the foam promoters can be determined and removed. The success of a defoamer is usually dependent upon when and how it is added. Some defoamers, when added after the foam is generated, act as good inhibitors, but, when added prior to foam generation, act as good foam stabilizers, which makes the problem even worse. Most defoamers are inactivated within a few hours under high temperature and pressure conditions, and their effectiveness can be dissipated by the heat of the glycol solution. Therefore, defoamers should generally be added continuously, a drop at a time, for best results. The use of a chemical feed pump will help meter the defoamer accurately and give better dispersion into the glycol. Water-soluble defoamers are sometimes made more effective by diluting them before addition into the system. Defoamers with limited solubility should be added via the pump suction to insure good dispersion into the glycol. If foaming is not a really serious problem, the defoamer can be added in slugs of 3 to 4 ounces when needed. The addition of too much defoamer is usually worse than no defoamer at all. Excessive amounts sharply increase the foaming problem. The following procedure can be used to determine the best defoamer and the proper dosage to control the foaming problem. Always experiment with the defoamer chemicals using clean sample bottles before adding to the systems. Do not mix defoamers when running foam tests. If one defoamer does not do the required job, start with another glycol sample to run the bottle foam test. The samples for the foam test should be taken from the system at the point where most of the foaming occurs. Pour a measured amount (a graduated cylinder can be used) of glycol sample into a clean bottle. Add about 5 ppm of defoamer, put a cap on the bottle and then shake the sample several times. (Shake the sample bottles the same way each time the test is run for best results.) Make a visual inspection and study: Type of foam-bubble size and consistency Time required for the foam to reach a maximum height and record the foam height. Time required for the foam layer to settle back to the liquid level. Continue adding the same defoamer, in small increments, to determine the optimum concentration of defoamer. There is normally a point at which addition of more defoamer causes the foam to become uncontrollable. When this point is reached, the defoamer should be discarded. When the most effective defoamer has been selected, slowly meter the recommended dosage into the system at the point where most of the foaming occurs. The use of a continuous feed pump will usually help give better foam control. Some antifoam agents are: Dow Corning Antifoam "A" Emulsion Dow Corning Antifoam "B" Emulsion Union Carbide "SAG" 470 Glycol Losses The loss of glycol can be a very serious and costly operating problem. Losses can be incurred by vaporization, entrainment, poor glycol condensate separator design/operation and mechanical leaks. The total glycol loss from a properly designed and maintained dehydration unit should not exceed 13.4 l/MMm3 (0.1 gal/MMSCF) or about one pound per MMSCF of gas treated. It is not uncommon, however, to see glycol losses averaging 134 to 536 l/MMm3 (1 to 4 gal/MMSCF) or even higher. Without proper controls, several hundred dollars per day of excessive glycol can be used. Some average-sized units can waste over $100,000 per year in excessive glycol losses, downtime, and equipment wear. The following are some ways to reduce glycol losses: A certain amount of glycol will always be vaporized in the sales gas stream. Adequate cooling of the lean glycol before it enters the absorber will minimize these losses. Proper design, operation, and maintenance practices require that the lean glycol entering the contactor be no more than 2.8 to 5.6C (5 to 10F) hotter than the gas leaving the contactor. Often, an efficient separator, placed on the sales gas line downstream of the contactor, can pay for itself quickly and save a great deal of money by recovering the excess glycol. It may be necessary to cool the inlet gas prior to the contactor. Higher than normal vaporization losses can be expected as gas temperatures approach 49C (120F). It may be necessary to use tetraethylene glycol if a high gas temperature cannot be avoided. Nearly all the glycol entrainment is removed by a mist extractor in the top of the absorber. Excessive gas velocities and glycol foaming in the absorber will sharply increase the losses. Vaporization losses in the stripper can be minimized with good glycol condensation. This can be accomplished by increasing the reflux rate in the top of the reboiler still. An increase in reflux rate will increase reboiler heat duty; too high a reflux rate will increase energy costs or may reduce reboiler temperature. A properly designed and operated glycol condensate separator with adequate retention time will efficiently separate the glycol and hydrocarbon, preventing dumping of the glycol to the oil system. Mechanical leaks can be reduced by proper maintenance of the pump, valves and other fittings. Analysis and Control of the Glycol Solution Analysis of the glycol solution is essential to good operation. Meaningful analytical information helps pinpoint high glycol losses, foaming, corrosion and other operating problems. It enables the operator to evaluate the plant performance and make operating changes to get maximum drying efficiency. A glycol sample should first be visually inspected to identify some of the contaminants. For example: A finely divided black precipitate may indicate the presence of iron corrosion products. A black, viscous solution may contain heavy, tarry hydrocarbons. The characteristic odor of decomposed glycol (a sweet, aromatic smell) usually indicates thermal degradation. A two-phase liquid sample usually indicates the glycol solution is heavily contaminated with hydrocarbons. The visual conclusions should then be supported by chemical analysis. Some of the routine tests, which can be run are pH, salt analysis, hydrocarbon determination, solids content, foam test and water content. These analyses usually provide sufficient information to determine the condition of the glycol solution. The glycol test results can be used to help prevent and solve operating problems. The following are some general comments on glycol analysis: Glycol weight percent. This establishes the amount of glycol in the solution. The lean glycol percent should be as required for the design conditions, usually about 98 to 99 percent. If the spread between the lean and rich glycol content is too narrow (about 0.5 to 1.5 percent), the glycol circulation rate may be too high and there may not be sufficient contact time in the contactor. If the spread is too wide (over 4 to 5 percent) it may mean the glycol circulation rate is too low and should be increased. Types and amounts of glycol. When triethylene glycol is used, the amount of other glycols, such as monoethylene and diethylene, should be fairly small. If the other glycol percentages (besides triethylene) start increasing, it usually means the glycol in the system is degrading and decomposing. Water content. This determines the amount of water in the samples. Hydrocarbon content. This shows how much oil, paraffin or condensate is in the glycol. The hydrocarbon content in the rich glycol will sometimes be higher since some of the hydrocarbons have not been exposed to the high reboiler temperatures and boiled off. If the hydrocarbon content continues to increase, corrective steps should be taken to remove the hydrocarbons. Salt content. This shows how much salt or chloride is present in the glycol. In a typical glycol unit, salt cannot be removed from the glycol once it gets into the system. Therefore, when the salt content exceeds 500 to 2000 ppm, the glycol should probably be drained from the system and regenerated with the proper equipment to remove salt and other impurities. The glycol system should be thoroughly cleaned before new glycol is added. Corrective steps, as previously discussed, should also be taken to prevent salt carryover into the glycol system. Salt deposition in the regenerator could occur at concentrations as low as 200 to 300 ppm and cause hot spots on fire tube surfaces. Solids content. This determines the suspended solids content in the glycol. When the solids content reaches 400 to 500 ppm, the filtration technique should be checked. The filter elements should possibly be changed more frequently and/or a new type element should be used to remove the solids. pH. Chemicals should be added, as necessary, to maintain the pH above 7.0. Amount of neutralizer needed for pH adjustment. This determines the amount of neutralizer needed to safely control the pH. Iron content. This gives an indication of the amount of corrosion present in the glycol system. An iron content of 5 ppm is usually the maximum Figurefor a non-corrosive glycol system. An iron content of 10 to 15 ppm would indicate some corrosion products present in the glycol. The corrosion products, like iron sulfide, could be coming in with the inlet gas or they could be forming in the plant itself. The iron content should usually not exceed 100 ppm. Foaming system. This is a measure of the amount of glycol foam present in the system. NOMENCLATURE = drag coefficient

= internal diameter, mm (in)

= droplet diameter, m (micron)

= pure water fugacity, kPa (psia)

= operating pressure, kPa (psia)

= gas flow rate, std m3/hr (MMSCFD)

= operating temperature, K (R)

= gas compressibility factor

= density of gas at operating conditions of temperature and pressure, kg/m3 (lb/ft3)

= density of liquid kg/m3 (lb/ft3)

= specific gravity of gas relative to air

= water, kg/m3 (lb/MMSCF)

= inlet water contact/outwater contact, mg/std m3 (lb/MMSCF)

= percent water removed

= mole fraction of water in water-TEG phase

= mole fraction of water in gas phase

= fugacity coefficient of water in vapor phase

= activity coefficient of water in liquid phase

DESIGN EXAMPLE Example Glycol Dehydration Given: Gas, 116,800 std m3/hr (98 MMSCFD) at 0.675 SG, Sat w/Water at 6895 kPa (1000 psig), 38C (100F) Dehydrate to 111 mg/std m3 (7 lb/MMSCF) Use Triethylene Glycol No stripping gas, 98.8 percent TEG Compressibility factor = 0.865 CD (contactor) = 0.852 Problem: Refer to Figure1 (Schematic flow diagram for a typical glycol dehydration unit).

Figure 1

Calculate contactor diameter. Determine glycol circulation rate and estimate reboiler duty. Calculate duties of heat exchangers. Note: Much of the available industry data for Glycol Properties and Design Procedures is available only in customary units. For this reason, it will often be simpler to convert conditions given in metric units to customary units, then solve design calculations using the methods for customary units. The following conversion factors are obtained from The Society of Petroleum Engineers "SPE Metric Standard," (1982). Table1: Conversion factors from SI to oilfield units. SI Units Oilfield Units

1 std m3/hr 8.389 x 10-4 MMSCFD

1 kPa 0.145 psi

1 mg/std m3 6.305 x 10-2 lb/MMSCF

1 mm 3.937 x 10-2 in

1 m3/hr 4.403 gpm

1 W 3.414 Btu/hr

1 kg/hr 2.205 lb/hr

T (C) x 9/5 +32 T (F)

To convert from SI units to oilfield units, multiply the metric value by the number in the right hand column, (except for the temperature, follow the formula explicitly).

Table2: Conversion factors from oilfield to SI units. Oilfield Units SI Units

1 MMSCFD 1192 std m3/hr

(60F, 14.696 psia) (15C, 100 kPa(A))

1 psi 6.895 kPa

1 lb/MMSCF 15.86 mg/std m3

1 in 25.4 mm

1 gpm 0.2271 m3/hr

1 Btu/hr 0.2929 W

1 lb/hr 0.4536 kg/hr

5[T(F)-32]/9 T(C)

To convert from oilfield units to SI units, multiply the customary value by the number in the right hand column (except for temperature, follow the formula explicitly)

Step 1. Calculate Contactor Diameter.

Use 1829 mm (72 in) I.D. Contactor Step 2. Determine Glycol Circulation Rate and Reboiler Duty.

Using the Figure2 :

Figure 2

Using n = 2 (8 actual trays) and Figure3 with 98.8 percent lean glycol:

Figure 3

Estimate Reboiler Duty

Use a 200 kW (700 MBtu/hr) Reboiler Step 3. Calculate Duties of Heat Exchangers. Rich TEG from Contactor T1 = 38C (100F) (Given) Rich TEG from Reflux Coil T2 = 54C (130F) Rich TEG to Separator T3 = 93C (200F) (Assume for good design) Rich TEG to Still T4 = 150C (300F) (Assume for good design) Lean TEG from Reboiler T5 = 190C (375F) Lean TEG to Pumps (Max) T7 = 93C (200F) (Given by manufacturer) Lean TEG to Contactor T8 = 43C (5C above contactor temperature) (110F (10F above contactor temperature)) Glycol/Glycol Preheater (Rich side, Duty) (See Figure1 (Schematic flow diagram for a typical glycol dehydration unit) Rich TEG T2 = 54C (130F) (Assume 14C (30F) increase in reflux coil)

T3 = 93C (200F)

Lean Glycol Composition

Rich Glycol Composition

Rich Glycol Flow Rate (wrich) Using Figure4 , one can obtain the specific heat of the glycol solution:

Figure 4

Rich Glycol Heat Duty (qrich)

Glycol/Glycol Exchanger

Rich T3 = 200 T4 = 300

Lean T5 = 375 T6 = ?

Rich Glycol Heat Duty

Lean Glycol Flow Rate (wlean)

Calculation of T6

Glycol/Glycol Pre-Heater -Lean Side Temperature Lean T6 = 132C (270F) T7 = ? Assume T7 = 80C (175F)

Gas/Glycol Exchanger Duty Lean T7 = 92C (197F) T8 = 43C (110F)

Summary of Exchangers

Glycol/Glycol Exchanger Rich T3 = 93C (200F), T4 = 150C (300F) Lean T5 = 190C (375F), T6 = 132C (270F) Duty q = 83,800 W (286 MBtu/hr) Glycol/Glycol Preheater Rich T2 = 54C (130F), T3 = 93C (200F) Lean T6 = 132C (270F), T7 = 92C (197F) Duty q = 53,600 W (183 MBtu/hr) Gas/Glycol Exchanger Lean T7 = 92C (197F), T8 = 43C (110F) Duty q = 60,000 W (205 MBtu/hr) PHYSICAL PROPERTIES OF GLYCOL Physical properties of glycol are presented in the following figures. Figure1 Vapor Pressures of Glycols vs. Temperature (Courtesy of Union Carbide Corp )

Figure 1

Figure2 Viscosities of Aqueous Ethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 2

Figure3 Viscosities of Aqueous Diethylene Glycol Solutions (Courtesy of Union Carbide Corp.)

Figure 3

Figure4 Viscosities of Aqueous Triethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 4

Figure5 Viscosities of Aqueous Tetraethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 5

Figure6 Heat of Vaporization of Glycols vs. Temperature (Courtesy of Union Carbide Corp )

Figure 6

Figure7 Specific Heats of Aqueous Tetraethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 7

Figure8 Specific Heats of Aqueous Diethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 8

Figure9 Thermal Conductivities of Aqueous Diethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 9

Figure10 Specific Heats of Aqueous Ethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 10

Figure11 Thermal Conductivities of Aqueous Ethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 11

Figure12 Specific Heats of Aqueous Triethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 12

Figure13 Thermal Conductivities of Aqueous Triethylene Glycol Solutions (Courtesy of Union Carbide Corp )

Figure 13

Figure14

Figure 14

Thermal Tetraethylene Glycol Solutions (Courtesy of Union Carbide Corp )