GAS PROCESSING Jaouhar Ktari 1 What are the various typical specs for gas dehydration per service? There are four methods that are used for gas dehydration: they vary in efficiency and cost. The methods used for gas dehydration are: Absorption Adsorption Membrane processes Refrigeration Jaouhar Ktari 2 Jaouhar Ktari 3 Water Dew-point The efficiency of the dehydration is measured on the water contents in the dry gas. The dew-point temperature for the water in the gas is often a more useful parameter than the total water contents. The dew-point temperature must be below the minimum pipeline temperature, to avoid liquid in the gas pipeline. Jaouhar Ktari 4 A dew-point temperature of 6 to 11 °C (10 to 20 °F) below the desired dew-point may be used to insure against non-ideal situations. The water Dew-point may differ from the gas Dew-point. The total gas dew-point may be influenced by other hydrocarbons in the gas. This can result in condensation of hydro-carbons in the gas pipeline. This is also undesirable but much less so than water condensation. Water content in natural gas Based on typical gas composition : separate corrections for actual compositions acid gas content. Takes into account non-idealities. Take care if gas is specified as wet or dry basis. ( dry basis does not include the amount of water in the MMSCF). Jaouhar Ktari 5 Wet Basis XH2O (NH2O/MH2O)/(NHC+NH2O)= YH2O/MH2O Dry Basis XH2O Jaouhar Ktari (NH2O/MH2O)/(NHC)= (YH2O/MH2O )/(1-yH2O) 6 Jaouhar Ktari 7 Water content on field Equal fugacities for each component in each phase. Between gas/water phases: yi= xi*Ki where Ki = Фi,l/Фi,v For a gas in contact with pure water: Y H2O = (PH2OVap)/P (xH2O =1) Formation of the water phase will control the water content in the gas phase : • Increasing water in the feed increases the amount of free water, not the concentration of water in the gas. • Can decrease the gas water content by adding compounds that are water soluble. Jaouhar Ktari 8 What are hydrates? Hydrates are a possibility in oil/gas exploration, production, transportation, or processing, which involves water and molecules smaller than n-pentane. When small (< 9 Å) nonpolar molecules contact water at ambient temperatures(typically < 100°F) and moderate pressures (typically > 180 psia), a water crystal form may appear a clathrate hydrate. 1 Å = 10^(-10) m Jaouhar Ktari 9 What are the types of hydrates (Explain Type I and Type II hydrates)? Type I Type II Type H Type I Type I hydrates consist of 46 water molecules. It is made from two types of cages: Dodecahedron, a 12-sided polyhedron where each face is a regular pentagon. Tetrakaidecahedron, a 14-sided polyhedron with 12 pentagonal faces and two hexagonal faces. The dodecahedral cages are smaller than the tetrakaidecahedral cages. Jaouhar Ktari 10 Type II The Type II hydrate consists of 136 molecules of water. The structure of the Type II hydrates is significantly more complicated than that of the Type I. The Type II hydrates are also constructed from two types of cages. Dodecahedron, a 12-sided polyhedron where each face is a regular pentagon. Hexakaidecahedron, a 16-sided polyhedron with 12 pentagonal faces and four hexagonal faces. The dodecahedral cages are smaller than the hexakaidecahedron cages. Jaouhar Ktari 11 What are the thermodynamic hydrate inhibitors? A thermodynamic hydrate inhibitor (THI) is a chemical substance used to balance the reactions between hydrocarbons and water that result in the formation of crystalline compounds. Thermodynamic hydrate inhibitors do this by shifting the crystalline equilibrium curve in a direction that lowers the reaction temperature to a level that blocks compound formation. Crystalline build-up in pipelines is reduced when a thermodynamic hydrate inhibitor is used as an additive. Two main thermodynamic hydrate inhibitors are commonly used: Methanol and monoethylene glycol (MEG) A thermodynamic inhibitor is sensitive to changes in subsystem cooling. Therefore, the rate that an inhibitor is injected into a system must correlate to temperature changes within the system. In the case of methanol, ideal operating conditions are about 0.5°C (33°F). A methanol injection of 40% by volume in an aqueous phase is required to prevent hydrate crystalline formation. Jaouhar Ktari 12 What are the LDHI (Low Dosage Hydrate Inhibitor)? There are two types : Kinetic Hydrate Inhibitors (KHI) Anti-Agglomerants (AA) KHI Interfere with hydrate crystal growth or nucleation by embedding themselves into the lattice structure, delaying significant growth for longer than the fluids residence time. AA Prevent the agglomeration of hydrate crystals into large masses by dispersing water droplets within the condensate or oil phase. Jaouhar Ktari 13 The main Glycols used for Gas Dehydration Glycol is a common name for diols: with the two alcohols these substances have a high affinity for water. In dehydration 1,2-ethandiol also known as Monoethylen glycol (MEG) and the small polymers of MEG (diethylen glycol (DEG), triethylen glycol (TEG) and tetraethylen glycol (TREG)) are the most commonly used for absorbents. Higher polymers than TREG is usually not used for dehydration because they become too viscous compared to the smaller polymers. Jaouhar Ktari 14 What are the main components of TEG dehydration unit? Inlet cooler An inlet cooler may be used because dehydration is more efficient at low temperatures. An inlet cooler is used when the inlet gas temperature is higher than the desired temperature in the contactor. It is also a helpful tool in simulation if the temperature in the contactor needs to be optimized. Inlet scrubber The inlet scrubber removes free liquid and liquid droplets in the gas, both water and hydrocarbons. Removing liquid water in the scrubber decreases the amount of water that has to be removed in the contractor. Contactor The contactor is the absorption column where the gas is dried by the glycol. The lean glycol enters at the top of the contactor while the rich glycol is collected at the bottom of the contactor and sent to regeneration. The wet gas enters the contactor at the bottom, while the dry gas leaves at the top. Jaouhar Ktari 15 The glycol temperature into the contactor must be 3 to 11 °C (5 to 20 °F) higher than the gas entering the contactor to minimize hydrocarbon condensation into the glycol. At contactor temperatures below 10 °C (50 °F) TEG becomes too viscous, thus reducing the column efficiency. The contactor temperature may be as high as 66 °C (150 °F), but glycol vaporization loss is often deemed unacceptably high above 38 °C. Flash valve After the contactor column the pressure is reduced to the regeneration pressure by a flash valve. The pressure drop over this valve depends on the pressure in the contactor and the pressure loss in the pipes and equipment until the regeneration column. Flash separator It is a good idea to install a separator after the flash valve. Because of the decreased pressure hydrocarbons absorbed in the glycol will be released. With a flash separator the hydrocarbon rich gas, can be used as process gas in the plant. The pressure in the flash separator must be above the pressure in the system that the gas is vented too. Jaouhar Ktari 16 Filters Filters are only necessary if there is a problem with solid particles or liquid hydrocarbons in the glycol. Solid particles in the glycol accumulate, increasing the wear on the equipment and can create plugs in heat exchangers. Solid particles can easily be removed with sock filters, which can be made of cloth fabrics, paper or fiberglass. Heat exchangers The numbers of heat exchangers varies with the design of the process plant. Because of the large temperature difference between the contactor and regenerator column, rich glycol needs to be heated while lean glycol must be cooled. With proper design of heat exchangers between the rich and lean glycol most of the energy can be conserved. Regenerator The regenerator is a distillation column, where glycol and water is separated. The rich glycol is preheated in heat exchangers before it is feed to the regenerator column. The energy required to separate glycol and water is supplied by the reboiler at the regenerator column. Jaouhar Ktari 17 The pressure in the regeneration system is just above atmospheric pressure, this is to insure that no air can enter the system from the atmospheric vent. Stripping column Glycol purities up to 99.9 wt% can be achieved by using a stripping column after the regenerator. The stripping gas from the top of the stripping column is routed to the regenerator boiler, like when stripping without the stripping column. Cool stripping gas can be used in the stripping column, because the glycol needs to be cooled after the regenerator. If on the other hand stripping gas is added directly to the regenerator boiler it might be preferable to preheat the gas, to keep a uniform temperature in the boiler. Glycol storage tank This is an optional instalment that ensures a constant glycol flow to the contactor column. Because there will be a loss of glycol in the dehydration system, a storage tank can act a buffer to prevent insufficient glycol flow, and also be used to measure the glycol contents in the system. Jaouhar Ktari 18 Glycol circulation pump Because of the pressure difference between the regenerator and the contactor, the glycol pressure needs to be increased. This is done with the glycol regeneration pump. The glycol is cooled below 80 °C before pumping to protect the pump. Jaouhar Ktari 19 How does the TEG Dehydration unit work? The gas flows through a separator to remove condensed liquids or any solids that might be in the gas. Some absorbers incorporate the separator in a bottom section of the vessel, in which case the gas then flows upward through a chimney tray into the glycol absorber portion of the vessel. The glycol contactor or absorber can contain: Trays Random packing Structured packing If it is a trayed vessel, it will contain several bubble-cap trays. Lean glycol is pumped into the upper portion of the contactor, above the top tray but below the mist eliminator. The trays are flooded with glycol that flows down from tray to tray in down comer sections. The gas rises through the bubble caps and is dispersed as bubbles through the glycol on the trays. This provides the intimate contact between the gas and the glycol. The glycol is highly hygroscopic, and most of the water vapor in the gas is absorbed by the glycol. The rich glycol, containing the absorbed water, is withdrawn from the contactor near the bottom of the vessel above the chimney tray through a liquid level control valve and passes to the regeneration section. The treated gas leaves the contactor at the top through a mist eliminator and usually meets the specified water content. Jaouhar Ktari 20 The rich glycol can be routed through a heat exchange coil in the top of the reboiler column called the still. The heat exchange generates some reflux for the separation of the water from the glycol in the top of the still and also heats the rich glycol somewhat. In some installations, the rich solution passes to a flash tank operating at about 15 to 50 psig, which allows absorbed hydrocarbon gas to separate from the glycol. The glycol then flows into the still through a filter and a heat exchanger, exchanging heat with the regenerated glycol. It drops through a packed section in the still into the glycol reboiler vessel, where it is heated to the necessary high regeneration temperature at near atmospheric pressure. At the high temperature, the glycol loses its ability to hold water: the water is vaporized and leaves through the top of the still. The regenerated glycol flows to the surge tank, from which it is routed through the lean/rich heat exchanger to the glycol pump. The pump boosts the pressure of the lean glycol to the contactor pressure. Prior to entering the contactor, it exchanges heat with the dry gas leaving the contactor or some other heat exchange medium. Jaouhar Ktari 21 Jaouhar Ktari 22 What is the typical temperature difference at dehydration column inlet between gas and glycol? Who is hotter? TEG to gas contactor is limited from 10°F to 15°F above the inlet gas temperature. If hotter, some TEG will vaporize with gas. If colder, gas condensation of the hydrocarbons may cause foam and glycol loss. The glycol temperature into the contactor must higher than the gas. What is the regeneration temperature for TEG/DEG/MEG? TEG: (204.4°C) or (400°F) DEG: (177°C) or ( 350,6°F) MEG: (163°C) or (325,4°F) Jaouhar Ktari 23 What is the freezing temperature of TEG/DEG/MEG? TEG: (-7°C) or (19°F) DEG: (-10,5°C) or (13,1°F) MEG: (-12,9°C) or (8,78°F) Jaouhar Ktari 24 What is DRIZO Process? DRIZO regenerates the glycol by solvent stripping instead of the conventional gas stripping. Solvent stripping allows to obtain much higher glycol purities than gas stripping (up to 99.998 wt% instead of the typical 99.95 wt%) and consequently allows to get much larger water dew point depressions up to 100°C (180°F) and even higher in some cases. The solvent required by the DRIZO process is usually obtained from the BTEX present in the natural gas itself and in most cases, the process will even produce some liquid hydrocarbons. Glycol solvent stripping by condensates (instead of conventional gas stripping) allows: Higher glycol purities down to < 1 ppmv H2O in treated gas Reduced BTEX / CO2 emissions Possible recovery of liquid aromatics DRIZO solvent is not a solvent, it is a hydrocarbon recovered from feed gas Jaouhar Ktari 25 The Drizo regeneration system utilizes a recoverable solvent as the stripping medium. The patent operates with iso-octant solvent, but the typical composition of the stripping medium is about 60 wt% aromatic hydrocarbons, 30 wt% naphthenes and 10 wt% paraffins. The threephase solvent water separator is crucial for this method. Jaouhar Ktari 26 Jaouhar Ktari 27 What is COLDFINGER Process? TEG regeneration by Coldfinger technology has been recognized as one of the promising processes for dehydrating natural gas. The Coldfinger regeneration system employs a cooling coil (the coldfinger) in the vapor space of the surge tank. The cooling that takes place there causes condensation of a high amount of vapors. The condensate is a water-rich TEG mixture, which is led to a further separation process . Jaouhar Ktari 28 The principle of the Cold Finger instrument is an inverted pipeline. The cooled metal finger replicates the inner wall of a pipeline. The heated and moved oil flows around it. When the finger’s temperature falls below the Wax Appearance Temperature (WAT), wax starts to deposit on its surface. We use the cold finger test to calculate the efficiency of wax inhibitors. As the piece of equipment replicates a pipeline environment, the results we obtain are transferable to how our products would work in field conditions. Our wax inhibitors are crude oil specific. They work via a lock and key mechanism whereby a specific paraffin inhibitor combines with a specific crude oil to prevent wax formation. The test involves the immersion of a series of tubes in molten crude oil. The temperature of the tubes is set colder than the oil, such that waxes from the oil can deposit on the cool surfaces. The wax deposits are then weighed. With the addition of an effective paraffin inhibitor, the weight of the deposits should decrease and is presented as a percentage of wax inhibition. Jaouhar Ktari 29 Jaouhar Ktari 30 How to simulate a complete glycol dehydration unit using HYSYS? The dehydration unit of a plant that processes natural gas uses triethylene glycol (TEG) as an absorbent to remove water from the gas to prevent blockages in pipes due to the formation of hydrates. Although TEG is recyclable, it is usually lost in the system due to vaporization and carryover, which results in economic issues. Therefore, it is necessary to optimize the dehydration process to achieve the allowable water concentration in the gas, to minimize the use of energy, and to minimize the loss of TEG. We use ASPEN HYSYS to construct and simulate the dehydration process. The chosen affecting parameters to the process were: - Lean glycol circulation rate. - The temperature of the reboiler - The number of trays in the contactor column. Jaouhar Ktari 31 Whereas, the response parameters included: - The amount of glycol that was lost. - The reboiler duty. - The concentration of water in the dry gas. - The temperature at which the hydrate formed. Jaouhar Ktari 32 How does KIMRAY Glycol Pumps work? The pump does two things at the same time: Moves wet (or rich) glycol from the contactor to the reboiler Moves dry (or lean) glycol from the reboiler to the contactor The pump uses energy from the wet glycol and a small quantity of gas at contactor pressure to stroke the piston inside the pump. High-pressure wet glycol from the contactor enters the top of the pump. It is then diverted to the end of the piston. This causes the piston to stroke. As the piston strokes, three additional processes occur: the pump sends high-pressure dry glycol on the inside of the pump cylinder out the top of one end of the pump to the contactor the pump pulls low-pressure dry glycol from the reboiler into the cylinder through the back of the pump the pump sends low-pressure wet glycol from the other end of the pump to the reboiler Jaouhar Ktari 33 As the piston reaches the end of its stroke, a series of check valves and d-slides switch to redirect the high-pressure wet glycol to the other end of the piston. This continual back-and-forth movement of the piston is what pumps the glycol in and out of the pump to the different components of the dehydration system. The glycol pump does not require a power source outside of the dehydration system. Kimray’s Energy Exchange Glycol Pump is a durable, low-maintenance solution for recirculation of glycol in your dehydration system Jaouhar Ktari 34 Troubles related KIMRAY Pumps Symptoms - The pump will not operate - The pump will start and run until the glycol returns from the absorber. The pump then stops or slows appreciably and will not run at its rated speed. - The pump operates until the system temperature is normal then the pump speeds up and cavities. - The pump lopes and pumps on one side only. - Pumps stops and leaks excessive gas from wet glycol discharge. - Erratic pump speed. Pump changes speed every few minutes. - Broken pilot piston. Jaouhar Ktari 35 Causes - One or more of the flow lines to the pump are completely blocked or the system pressure is too low for standard pumps (below 300 psig). - The wet glycol discharge line to the reboiler is restricted. A pressure gauge installed on the line will show the restriction immediately. - The suction line is too small and increase in temperature and pumping rate cavities the pump. - Traps in the wet glycol power piping sends alternate slugs of glycol and gas to the pump. - Insufficient glycol to the main piston D-slides port. Elevate the control valve end of the pump to correct. Jaouhar Ktari 36 How to protect KIMRAY Pumps? A new pump or new dehydrator should be put into operation by first bringing the glycol circulation and operating temperature to an equilibrium conditions by using 300 to 400 psig absorber pressure. This can be done with or without gas flow. The maximum operating temperature of the pump is limited by the moving “O” ring seals. A maximum of 200 °C is recommended. Packing life will be extended considerably at 150°C. If a pump has been deactivated for several months, the check valves should be removed and inspected before attempting to operate the pump. The pump start up should be similar to that of a new pump by first bringing the system to equilibrium. Jaouhar Ktari 37 What are the periodic laboratory analysis we can do on Glycol? Why? pH: Most inhibited glycols provide a pH between 8 and 10.5. The pH of the glycol is measured to determine if the glycol has broken down into corrosive acids. - Specific conductance: it will vary from a low range of 1500 µmhos to a high range of 4500 µmhos. The higher the percent of inhibited glycol, the higher the conductivity. Pure uninhibited glycol may have a very low specific conductance under 100 µmhos. - Total Iron: should be below 2 ppm. The level of total iron is tested to determine if corrosion of mild steel or iron pipe is a concern. - Copper: should be below 0.2 ppm. Coppers levels are also tested to determine if corrosion of cooper pipe is a concern. - Jaouhar Ktari 38 Molybdenum: Molybdenum or molybdate is also used as a basic mild steel corrosion inhibitor to provide extra protection to the system metals. Molybdate levels may vary from a very low level of 15 ppm to a very high level of 100 ppm. - Glycol Percent by Volume: the percent by volume should be in the range of 20% to 40% for best protection. Levels below 20% have an increased chance for microbiological growth causing the glycol to break down. - Freeze Point °F: Most freeze points are between 10°F and -10°F. A 40% solution of propylene glycol provides a -8°F freeze point whereas a 40% solution of ethylene glycol provides a -13°F freeze point. - Reserve Alkalinity: A reserve alkalinity of 10 to 12 is generally adequate. This test is designed to check the level of the buffering agent and metal passivators that are included in the inhibited glycol formulation. - Jaouhar Ktari 39 - Orthophosphate as PO4 ppm: Ranges are typically 1000 ppm to 5000 ppm. The inhibitor package that is included in the inhibited glycol is dipotassium phosphate. This test, like the reserve Alkalinity, gives the level of this inhibitor. - Sodium Nitrite: levels may vary from a low of 500 ppm and to a high of 1500 ppm. Sodium nitrite is a basic mild steel corrosion inhibitor that is used to provide extra corrosion inhibition. Jaouhar Ktari 40 What are the main reasons for Glycol foaming? Excessive turbulence and high liquid-to-vapor contacting velocities can also cause the glycol to foam. This condition may point to underlying mechanical or chemical issues. Other causes of foam that may be present in the process fluid include field corrosion inhibitors, salt, or finelydivided suspended solids. How to detect Glycol foaming? One of the most common ways glycol is lost is through foaming. Glycol foaming happens when entrained hydrocarbons from production enter the glycol fluid. As the entrained glycol is processed through the contactor tower (or absorber), it will carry over the top of the tower with the sales gas when stable foam builds up on the trays. Foaming also causes poor contact between the gas and the glycol, significantly reducing the drying of the gas. Jaouhar Ktari 41 How to prevent Glycol foaming? To dehydrate natural gas properly, your system needs clean glycol that is free from hydrocarbons and any other impurities. If you encounter foaming, your first step is to lower pressure. How to remedy Glycol foaming? We use an anti-foaming agent to temporarily reduce the foam and continue processing gas. A defoamer or an anti-foaming agent is a chemical additive that reduces and hinders the formation of foam in industrial process liquids. - Oil based defoamers - Water based defoamers - Poweder defoamers - Silicone based defoamers - EO/PO based defoamers - Alkyl polyacrylates Jaouhar Ktari 42 What type of corrosion you might have on Glycol Units? Why? Corrosion problems in glycol systems are most commonly caused by thermal degradation of the glycol. Degradation of glycols results in the formation of organic acids which depress the glycol pH and lead to corrosion. Apart from monitoring the pH a regular sample should be drawn for visual inspection. Dirty amine with high suspended solids content is an indication of filter and corrosion problems. The odour is also a good guide to the condition of the glycol, with an aromatic odour indicating glycol degradation. Another major problem with glycol systems is the build-up of salt in the glycol. Salt dissolves in glycol which makes it corrosive. The presence of salt is particularly damaging if stainless steels are used as materials of construction as chloride pitting and stress corrosion cracking can occur. Jaouhar Ktari 43 How to prevent corrosion on the Glycol units? The feed gas coming to the dehydration system is high in hydrogen sulfide and carbon dioxide. At the temperatures in the lean/rich exchanger, the presence of hydrogen sulfide and carbon dioxide can be expected to significantly increase the corrosion rates of mild steel. The recommendation in this case was to upgrade the metallurgy of the lean/rich exchanger. The minimum upgrade recommended was to 12 Chrome steel. Jaouhar Ktari 44
