GENERAL PROCESS DESCRIPTION
The basic function of the triethylene glycol (TEG) dehydration unit is to remove water vapor from natural gas streams to an outlet water content meeting pipeline specifications, usually seven lb./MMSCF or less. QBJ has designed and built special TEG dehydration units which have met water specifications of less than 1 lb. / MMSCF.
Dehydration, or water vapor removal, is accomplished by depressing the water dew point (defined as the temperature at which vapor begins to condense into a liquid) from the inlet dew point temperature to the dew point temperature for the outlet water content required.
The most common dehydration method used for natural gas is the absorption of water vapor in the liquid desiccant, Triethylene Glycol. The wet gas is brought into intimate contact with lean dry glycol in the tray or structured packing section of an absorber tower where water vapor is absorbed in the glycol thus depressing the water dew point. The wet rich glycol is pressure reduced and then flows from the absorber to the regeneration system. The wet rich TEG is pre-heated (via heat exchange), entrained gas is separated, additionally heated (via heat exchange) and fractionated in the still column and reboiler by heating and boiling off the absorbed water vapor. The water dry lean glycol is cooled (via heat exchange) and pumped back to the absorber.
The QBJ regeneration systems can be equipped with proprietary baffling and stripping columns. These units produce lean TEG glycol concentrations from 99.1% up to 99.95% (wt.) concentration by the use of dry gas to strip more water out of the hot regenerated glycol. The proprietary QBJ “Enhancelator” produces TEG glycol concentrations of 99.5% (wt.) without the use of stripping gas.
With special design, material selection and fabrication requirements, the TEG dehydration process can be applied to CO2, H2S, oxygenated gases or any other application.
Dewpoint depression is dependent on TEG circulation rate (gallons per pound of water in the gas), lean TEG concentration, number of equilibrium stages (number of trays) in the absorber, contact temperature and pressure.
Dewpoint depression is especially sensitive to inlet gas temperature. TEG dehydration units yield a higher dew point depression with an increase in temperature and correspondingly a lower dew point depression with a decrease in inlet gas temperature. This performance change is primarily due to the change in gas-glycol contact efficiency in relation to temperature. Even though the dew point depression increases with an increase in inlet gas temperature, the outlet gas dew point will be higher. For the lowest obtainable outlet dew points it is desirable to have a low inlet gas temperature.
Normal operating temperatures range from 50 to 135 F. 50 F is considered to be the minimum operating temperature due to the high viscosity of glycol at lower temperatures. 135 F. is the upper practical temperature limit for TEG dehydrators because of the increased TEG vaporization losses at higher temperatures.
Pressure appears to have little effect on dewpoint depression in the dehydration process. Existing data indicates that the dewpoint depression is essentially constant over a range of 0 to at least 3000 PSIG. Pressure does however affect the water vapor capacity of the gas. At lower pressures the gas can absorb more water per unit volume.
Typically, increasing the number of trays, the glycol circulation rate or the lean TEG concentration will increase the dew point depression. Increasing glycol rates above 4 to 6 gallons per pound does not usually have an appreciable effect on dewpoint depression. Increasing TEG concentration or the number of trays is usually more effective than increasing glycol circulation rate in maximizing dewpoint suppression.
GENERAL EQUIPMENT DESCRIPTION
The dehydration unit consists of an absorber and regenerator. The mass transfer section of the absorber may be trays, structured packing or random packing. A standard design, unique to QBJ, is a large TEG disengagement space above the top tray and below the mist extractor. This space allows additional time for glycol - gas separation and minimizes TEG losses. The standard QBJ regenerator is a high concentration type concentrating the glycol to over 99.1% by the use of stripping gas or by concentrating the glycol to 99.5% without the use of stripping gas in the Enhancelator.
The lean, dry TEG entering the absorber should be cooled to within 10 degrees of the gas temperature to maximize the efficiency of the mass transfer media and to minimize TEG vaporization losses. The absorber is normally equipped with an external glycol/gas heat exchanger; however, some units are supplied with a fin fan air-cooled exchanger.
The water rich glycol is withdrawn from the bottom section of the absorber via a level controller and level control valve on units utilizing electric or gas fired pumps. Units using an energy exchange glycol / gas powered pump, do not require a level control system since the pump transfers the energy available from the wet glycol, at the absorber pressure, to an equivalent volume of dry glycol at reboiler pressure. The additional energy to overcome friction, line and system pressure losses is supplied by gas at the absorber pressure.
The absorber may be provided with an integral scrubber to remove free liquid from the incoming gas. The scrubber may be two phase for separation of gas and liquids, or three phase to separate gas, hydrocarbon liquid and water. The gas stream to the absorber must be free of liquids prior to entering the mass transfer section of the absorber. A filter separator or coalescer upstream of the absorber will fulfill this requirement.
Cold weather protection can be provided for the dehydration unit by the addition of an enclosure, extra insulation, heat trace or a catalytic heater.
TEG heat exchange is optionally provided by a three-section glycol/glycol heat exchanger. The heat exchanger is of a fixed tube sheet BEM construction, with bolted bonnets. The first section preheats the water rich TEG for optimum gas separation in the low-pressure three-phase separator. The second and third sections raise the rich TEG from separation temperature to approximately 300 F (for maximum heat conservation) prior to the inlet of the still column. The lean TEG from the surge tank provides the heat and is cooled by exchange for pump suction.
The QBJ standard regenerator consists of the reboiler with a firetube, burner control assembly, flame arrestor, stack with down draft diverter, still column with reflux condenser and integral surge accumulator with stripping column. The reflux condenser, integral to the QBJ still column, aids in the condensation of glycol vapors and reduces glycol losses. QBJ units are test fired in the shop to verify performance and reduce startup problems.
The stripping column uses a source of dry gas, usually taken from the dry gas leaving the absorber to strip additional water out of the regenerated glycol. The amount of stripping gas varies with the application and dew point depression requirements from 0.5 SCF/gal. of glycol circulated up to 6 SCF.
Fuel gas and instrument supply gas is usually supplied from the dry gas leaving the absorber but other sources may be used. A fuel gas scrubber with fuel gas safety valve is provided to prevent free liquids from entering the fuel gas system.
The regenerator temperature is controlled by the temperature controller and fuel gas control valve(s) along with a high temperature shutdown.
The glycol pump(s) are provided with valves and strainers. The electric driven positive displacement pump is provided with a suction stabilizer and pulsation bottle in the discharge line. A redundant valve system bypasses glycol back to pump suction for manual flow control of lean TEG. A bypass relief valve is provided on the discharge of all electric and gas powered pumps.
Standard units are provided with low pressure three phase separator (flash separator) to separate solution gas (and glycol powered pump energy gas) from the glycol and hydrocarbon condensate. The flash separator is installed on the rich glycol line between first pass of the glycol/glycol heat exchanger and the glycol sock filter. The flash gas is generally routed to the fuel gas system.
Additional Optional Features
- Enhancelator for 99.5% (wt.) without the use of stripping gas.
- Standby glycol pump
- Charcoal filter
- Flame failure shutdown
- Low temperature shutdown - glycol reboiler
- High temperature shutdown - stack
- High and Low level shutdown - glycol reboiler, surge tank and flash separator
- High and Low level shutdown – inlet separation / integral scrubber
- Cold weather enclosure
- PLC control and DCS interface
QBJ glycol dehydration units GUARANTEE outlet gas dewpoints and unit performance.
