GAS DEHYDRATION AND TEG REGENERATION

ByM. Sulochana

Why Dehydration RequiredNatural gases either from natural production or storage reservoirs contain water, which can form hydrates to block pipeline flow and control systems.

Natural gas in transit to market should be dehydrated to a controlled water content to avoid hydrate as well as to minimize the corrosion problems.

Basic Definitions

Natural Gas - Natural gas is a combustible mixture of hydrocarbon gases consisting essentially methane, other hydrocarbons and non hydrocarbon gases in gaseous state. It is one of the safest, cleanest and most useful of all energy sources.Dehydration Removal of water vapor from gas.Hydrated gas Natural gas which contains H2O.De-hydrated gas Natural gas after removal of H2O.

Basic Definitions

Dry gas: Natural gas is considered as DRY when it is almost pure methane, having most of the other commonly associated liquid hydrocarbons and water removed. Glycol - A hygroscopic liquid. Mono-ethylene Glycol (MEG) and Di-ethylene Glycol (DEG) are commonly used in hydrate inhibition service and Tri-ethylene Glycol (TEG) is most common in gas dehydration service. Free water - Liquid water which is not dissolved in any other substance.

Basic Definitions

Hydrate - A clathrate compound formed by a combination of methane, ethane, propane, iso-butane, H2S or CO2 and water at elevated pressure and low temperature. Gas hydrates are crystals of natural gas and water which can appear far above the temperature where ice is formed.

Basic Definitions

Saturated gas - A gas stream which contains the maximum amount of water vapor at a given temperature and pressure without condensing the water. Dew point The temperature at which vapour begins to condense into a liquid at a particular system pressure.

Major methods of dehydration

Direct cooling - The saturated vapor content of natural gas decreases with increased pressure or decreased temperature. Hot gases saturated with water may be partially dehydrated by direct cooling. Gases subjected to compression are normally "after cooled", and this cooling may remove water from the gas.Adsorption - Molecular sieves (zeolites), silica gel, and bauxite are the desiccants used in adsorption processe. Adsorption is used for cryogenic systems to reach very low moisture contents.

Major methods of dehydration

In some situations, such as remote gas wells, use of a salt desiccant, such as CaCl2 , may be economically feasible. The system can reduce the water content down to 20 ppmv. Typical salt capacities are 0.3 lb CaCl2 per lb H2O. Absorption - In absorption processes, the most frequently used desiccants are diethylene and triethylene glycols. Usually, the absorption/stripping cycle is used for removing large amounts of water.

Absorption Method

Glycols are extremely stable to thermal and chemical decomposition.Readily available at moderate cost, useful for continuous operation and are easy to regenerate. These properties make glycols as obvious choice as dehydrating agents.

TEG Properties

Triethylene Glycol

TEG has gained universal acceptance as the most cost effective choice.TEG is more easily regenerated to a concentration of 98-99.99% (wt) in an atmospheric stripper because of its high boiling point and decomposition temperature.Vaporization temperature losses are lower than MEG or DEG. Capital and operating cost are lower.

Triethylene Glycol

High absorption efficiency.A high affinity for water and a low affinity for hydrocarbons.A low volatility at the absorption temperature to reduce vaporization losses. A low potential for corrosion.A good thermal stability to prevent decomposition during regeneration.No operational problems when used in high concentrations.

UCR

UCR (Unit Circulation Rate) - Volumetric or mass flow rate of lean glycol per mass flow rate of water removed. Inlet Water Content lb/ MMscf Outlet Water Content lb/ MMscf Gas Flowrate MMscfd TEG Circulation rate = (I O) x 2-3 US Gallons/lb of water x (G) / (24 x 60)

Inlet Scrubber

An inlet scrubber is required, either integral with the contactor or as a separate vessel upstream, to remove free liquid and liquid droplets in the gas, both water and hydrocarbons. The mist extractor in this vessel removes larger droplets entrained in the gas.Removing liquid water in the scrubber decreases the amount of water that has to be removed in the contractor.

Inlet Scrubber

This decreases the size of the contactor and the glycol circulation needed to reach the required conditions for the outlet gas specification.Liquid hydrocarbons are also a problem in the contactor because they increase the TEG tendency to foam, thereby decreasing the contactors efficiency and increasing the TEG loss in the contractor and from the regeneration system. Another problem is that hydrocarbons can be accumulated in the glycol polluting it and thereby decreasing the dehydration efficiency.

TEG ContactorContactor is most important mass transfer equipment in TEG dehydration.The gas stream is dried in contactor using counter current TEG stream with temperature difference of around 3 - 6 deg C with that of gas stream.The typical contactor is provided with internals like Inlet Device, Chimney trays, trays/ packings and demister .The inlet device reduces the momentum of feed gas and also allows for considerable reductions of the vessel height and inlet nozzle size.

TEG ContactorLean TEG is fed to the top of an TEG contactor, where it is contacted with the wet natural gas stream. Lean TEG removes water from the natural gas by physical absorption and is carried out at the bottom of the column. Chimney trays are provided for proper distribution of the gas through the column.

TEG ContactorIn earlier days, bubble cap trays were commonly used in contactor. But in recent decade, due to proven performance of the structured packing is highly recommended due to, Its high specific gas capacity, Ease of operation Low glycol entrainment characteristicsBetter mass transfer efficiency

TEG ContactorAs the glycol flows down the surface of the structured packing, it absorbs water from the gas. Rich glycol is collected on the top of the chimney tray and is discharged under level control valve LV to the TEG reflux condenser.The dehydrated gas leaves of the column passing through vane pack or mist eliminator which is provided to minimize the TEG losses.

TEG ContactorThe required water dew-point of the dry gas dictates the lean TEG temperature and purity. The TEG temperature into the contactor must be 3 to 6C higher than the gas entering the contactor to minimize hydrocarbon condensation into the glycol.At contactor temperatures below 10 C TEG becomes too viscous, thus reducing the column efficiency. The contactor temperature may be as high as 66 C, but glycol vaporization loss is often deemed unacceptably high above 38 C.

TEG Contactor - Internals

Heat ExchangersBecause 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.Heating before the flash separator increases hydrocarbon recovery along with glycol loss.

TEG Reflux condenserTEG Reflux condenser is located at the top of the TEG Still Column and is used to effect the rectification of the vapour and minimize TEG losses.Rich TEG flowing through the TEG Reflux condenser is warmed by steam and rich TEG vapors, where the water vapour present in the top of the column are condensed on the shell side. The overhead temperature is adjusted manually based on the temperature transmitter, by allowing the rich TEG to bypass the TEG Reflux condenser by opening the by-pass globe valve.

TEG Reflux CondenserThe TEG Reflux Condenser coil normally operates at lower pressure but is designed for design pressure of contactor by considering failure of the TEG Contactor level control in the event of causing accidental gas blow-by and blocking of the Cold Lean/Rich Glycol Exchanger.Rich Glycol from the TEG Reflux Condenser is sent to the tube side of the Cold Lean/Rich Glycol Exchanger.

Cold Lean / Rich Glycol ExchangerRich Glycol is heated by countercurrent flow of lean TEG to approximately 60-70 C prior to entering the TEG Flash Drum.By using the energy of shell side lean TEG from hot lean/rich TEG exchanger which is sent to the TEG surge drum for storage.

Flash VesselIn flash vessel hydrocarbon vapors are removed and any liquid hydrocarbons are skimmed from the TEG.Due to the composition of the rich TEG, a vapor phase having a high hydrocarbon content will form when the pressure is lowered.This step is necessary as the contactor is typically operated at high pressure and the pressure must be reduced before the regeneration .

Flash vesselIf not separated off, these hydrocarbon components would flash in the regenerator and lead to an increased still column vapor load, a higher reboiler duty requirement, greater TEG losses and a loss of recoverable product.These components could also lead to coking of the reboiler heating elements, fouling, foaming and a higher BTEX level in the water condensed from the overhead. The TEG Flash Drum is sized for nominally 20 minutes liquid retention timewhile operating at 60% full.

Flash vesselThe condensate is collected at the HC Skim bucket provided in the flash vessel.LP fuel gas is provided as blanket gas to supplement the gas flow if necessary to maintain the vessel pressure.In the TEG Flash Vessel the dissolved hydrocarbons are separated from the TEG and sent to the TEG particulate filters.Separated gas is sent to LP flare header under back pressure regulator.

TEG FiltersFrom the flash vessel the rich glycol flows through a full flow particle filter and an activated carbon filter often in slipstream service, to remove solids, dissolved hydrocarbons and degradation products.Solid particles in the glycol accumulate, increasing the wear on the equipment and can create plugs in heat exchangers.Liquid hydrocarbons like condensate and BTEX can be removed from the glycol by activated carbon filters.

Hot Lean / Rich TEG ExchangerRich TEG from the Activated carbon filter is sent to the tube side of Hot Lean/Rich TEG Exchanger, where it is further heated approximately 165 -170 deg C prior to entering the TEG still column.The TEG Flash Drum level control valve located downstream of the Hot lean/rich TEG exchanger is required in order to maintain back pressure on the exchanger and minimize vapor generation within the tubes.

TEG Still ColumnThe TEG Still Column is a vertically mounted atmospheric distillation (fractionation) column on top of the TEG Reboiler.The TEG Still Column is designed with one theoretical stage above the feed and one theoretical stage below.The Still Column consists of a random packing. The TEG flows downward across the packing and is partly regenerated in the TEG Still Column through contact with rising vapours generated by the TEG Reboiler.

TEG Still ColumnVapour rising through the TEG Still Column is partially condensed by the TEG Reflux Condenser.The partly regenerated TEG leaving the bottom of the TEG Still Column enters the front end of the TEG Reboiler for regeneration to 98.6 wt%.

TEG ReboilerThe reboiler may be directly fired or indirectly heated by electricity, hot oil or steam.The pressure in the regeneration system is just above atmospheric pressure.The operating conditions for the regenerator influence the purity of glycol. At 204 C TEG yields a lean glycol concentration of 98.6 wt%.

TEG ReboilerWhen TEG decomposes it becomes MEG and DEG, therefore it will not influence the dehydration process, only give a slightly larger glycol loss because MEG and DEG are more volatile than TEG.By adding stripping gas to the regenerator boiler the TEG purity can be increased up to 99.9 wt%.

TEG Stripping ColumnAs stated earlier, applications requiring high dew point depressions will virtually always utilize stripping gas in the regenerator.Low dew points simply cannot be achieved using the maximum 98.8 wt % TEG obtainable with a 204 deg C reboiler at atmospheric pressure.Increasing reboiler temperature is not an option due to the thermal degradation temperature of 206 deg C for TEG.

TEG Stripping ColumnThe use of Stripping gas column is provided to reduce the water content of the regenerated glycol by counter current contact with hot fuel gas flowing upward through the stripping gas column.The stripping gas also reduces the partial pressure of the vapour in the TEG reboiler. Hence increasing the regeneration capability of the TEG reboiler.These low dew points will need up to 99.9 wt % glycol in the contactor. Glycol purities up to 99.9 wt% can be achieved by using a stripping column.

TEG Stripping ColumnThe stripping gas is usually nitrogen, dry gas or flash gas from the flash separator. A gas-stripping column consists of randomly packed column with pall ring.A small stripping gas rate of 1 scf/gal circulated has a pronounced difference. With this stripping gas rate, the dry gas will contain about half the water of the same process without stripping gas.

TEG Stripping ColumnIncreasing the stripping gas rate beyond 2 to 3 scf/gal will have little impact on dew point depression.Regenerated Lean TEG flows from the bottom of the TEG Stripping Column by gravity into the shell side of the Hot Lean / Rich TEG Exchanger where it is cooled and then into the shell side of the Cold Lean / Rich TEG Exchanger where it is further cooled.

TEG Surge DrumLean TEG from Cold Lean/Rich TEG exchanger is sent to the TEG surge vessel. 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.The TEG Surge Drum provides storage capacity for the lean TEG Pumps and will be used to dump TEG inventory of the system.

TEG Circulation PumpsThe pressure difference between the regenerator and the contactor, the Lean TEG pressure needs to be increased.Glycol circulation pump is used to discharge the lean glycol at contactor operating pressure.The lean glycol is pumped to the contactor by normally a positive displacement pumps.

Lean TEG CoolerThe lean glycol from circulation pumps then flows to the lean glycol cooler, which is often an air or water-cooled even may be gas/glycol exchanger.where it is cooled to 3-6 deg C above the feed gas temp of TEG contactor by using an cooler.A temperature differential control valve is provided at cooling water return line of Lean TEG cooler to ensure the system temperature.

Chemical Injection SkidpH control storage tank and Anti-Foam chemical storage tank are used to maintain the design TEG concentration and to prevent TEG from some major problems such as oxidation, salt contamination, pH control, hydrocarbon contamination, sludge and foaming, with the help of pH control injection pump and Anti foam chemical injection pump.

Optimizing The DesignWhen optimizing the design of dehydration facilities, the impact of the following parameters should normally be considered: Number of trays in the contactor Glycol circulation rate through the contactor Temperature of the reboiler in the regenerator Amount of stripping gas used Operating pressure of the regenerator

Effects On No Of Equilibrium StagesIncreasing the number of trays allows the gas to approach equilibrium with the lean glycol at a lower TEG rate.Considering a typical TEG circulation rate of approximately 3 gal TEG/lb water removed. Three equilibrium-stage contactor is virtually at equilibrium with the inlet glycol. In a two stage contactor, a circulation rate of 5 to 6 gal TEG/lb water would be required to approach equilibrium.

Effects On No Of Equilibrium StagesSignificantly higher flow rates would still be required when only one ideal stage is used.

Effect Of Reboiler TemperatureHigher temperatures would result in higher residual moisture at the same circulation rate. The reboiler temperature influences the overhead water content by changing the purity of the lean glycol. Glycol purities of 98.0, 98.5, and 98.8 wt % are obtained at 182, 194, and 204 deg C respectively, at one atmosphere pressure.

Effect Of Reboiler Temperature

TEG LossesTEG losses from a properly designed and maintained dehydration unit are typically between 20 and 60 litres per million m3(st).The principal glycol loss points are: in the product gas from the glycol contactor in the flash gas from the glycol flash drum in the overheads from the glycol still column

TEG LossesThe methods described may be used to reduce glycol losses.Adequate cooling Demister mat Vaporisation losses Mechanical leaksCold weather

TEG LossesAdequate cooling Cooling to within 3 to 6 C of the inlet gas temperature of the lean TEG before it enters the contactor will minimize the losses.Demister mat Most entrainment associated with bubble cap trays is removed by a demister mat in the top of the contactor.

TEG LossesVaporization losses Vaporization losses in the regenerator still column can be reduced with satisfactory TEG condensation. The still column overhead temperature is a good indicator of whether TEG is being lost as a vapor. If the temperature is above 100 C at sea level, TEG must be present in the overhead vapors.

TEG LossesMechanical leaksMechanical leaks can be reduced by keeping the pump, valves and other fittings in good condition. A small drip of glycol can amount to significant daily losses. Cold weather Low ambient temperatures, especially when coupled with a strong wind, will cause more vapors in the still column to condense and return to the reboiler where they are revaporised..

TEG LossesThis overloads the column with liquids which may cause it to flood. If this occurs, liquids are percolated out of the still with the water vapor. This can be overcome by reducing the load on the system. It should be noted that due to the cold weather, the gas temperature is likely to be below design which will mean that the system is operating in a turndown mode.

Lessons Learnt Gas filter separator filter element riser height should be more than filter side high high liquid level. Hence the liquid cannot block filter section, it will results in efficiency reduction. Rotary gear pump used as TEG circulation pumps used has encountered some practical problems such the small particles which enters the gear section can damage the pump.

Lessons Learnt Notes should be clearly stated on P&IDs. (e.g. Whole Free Draining to be routed to Surge drum). As a result all the vessels are to be located above Surge Drum.Since TEG is a super-critical fluid, it does not vaporize above its critical pressure and so TEG Contactor PSV has to be designed for un-wetted wall fire case.

Lessons LearntSince the operating pressure of the Shell side fluid of the Cold Lean/Rich TEG exchanger will be higher than the surge drum design pressure. Cold Lean/Rich TEG exchanger is to be checked for tube rupture case during PSV sizing.Lean TEG from all the vessels (Surge Drum + Hot Lean/Rich TEG Exchanger + Cold Lean/Rich TEG Exchanger + TEG Reboiler + Stripping Column) should be considered during PSV sizing for Fire case since rich TEG in the reboiler will transform to lean TEG after water vapor is released.

Standards 20041010 SHELL DEP - Glycol-type gas dehydration systems. API 12GDU - Specification for Glycol-Type Gas Dehydration Units (1990) Fifth Edition.

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