DEHYDRATION UNITS

  1. Dehydrator Units

    1. Glycol Dehydrator Design: The dehydration of natural gas is defined as the removal of the water that is associated with the natural gas in the vapor form. It has long been recognized that the dehydration of natural gases is necessary to ensure efficient operation of gas transmission lines. The removal of the water vapor prevents the formation of gas hydrates and reduces corrosion in the pipelines. It also improves the efficiency of the pipelines by reducing liquid accumulations at low spots in the lines.
      One of the most popular methods of dehydration of natural gas now in use is an absorption process employing diethylene or Triethylene glycol as the desiccant.
      In recent years tri-ethylene glycol has emerged as the most popular chemical to be used. Triethylene glycol has a high affinity for the water vapor that is to be removed from the natural gas stream, and also has other desirable properties such as non-corrosiveness, is easy to regenerate, and chemical losses are generally quite low

Description of the Process: For the following description of the process and flow through a typical glycol dehydration unit refers to the schematic flow diagram as shown in Figure No. 1.
The wet inlet gas stream first enters the unit through a separate vertical inlet gas scrubber. In this scrubber, any liquid accumulations in the gas stream are removed. The inlet scrubber is normally provided with a tangential inlet diverter which affects a circular flow of the well fluids around the wall of the vessel for centrifugal separation. The wet inlet gas stream first enters the unit through a separate vertical inlet gas scrubber In this scrubber any liquid accumulations in the gas stream are removed. The inlet scrubber is normally provided with a tangential inlet diverter which affects a circular flow of the well fluids around the wall of the vessel for centrifugal separation. The wet gas then passes out of the top of the scrubber through a high capacity, high efficiency, stainless steel wire mesh mist eliminator which allows for virtually no liquid carry over. The separated well fluids drain into a quiet settling chamber in the bottom of the vessel and are discharged through a diaphragm operated motor valve operated by liquid level control. The vertical inlet gas scrubbers may be equipped for either a two-phase (oil-gas) operation or three phase (oil-gas-water) operation. If there is any liquid water in the inlet gas stream it is most desirable to use a three-phase gas scrubber to remove this liquid water before the gas enters the glycol-gas contactor.
The wet gas leaves the top of the inlet scrubber and passes to the vertical glycol-gas contactor. The gas enters the bottom of this vessel and flows upward through the contact medium countercurrent to the glycol flow. The contact medium in the glycol-gas contactor may be valve type trays or bubble cap trays. In smaller capacity units dumped packing may be used in the place of trays. The operation is the same in that the liquid glycol flows down through the packing and the gas vapor flows up through the packing contacting the glycol. In trayed columns, the
gas contacts the glycol on each tray as it passes through the vessel and the glycol absorbs the water vapor from the gas stream. Above the top tray in the contractor is an open space for entrainment settling where most of the entrained glycol particles in the gas stream will settle out. Any
glycol not settling out will be removed by a high-efficiency mist eliminator in the top of the contactor vessel.
The dry gas then leaves the contactor column at the top. The dry gas flows downward from the top of the glycol-gas contactor through an external glycol-gas heat exchanger attached to the side of the contactor vessel This heat exchanger is usually fabricated in the form of a concentric pipe ex-
changer. The incoming dry glycol from the surge tank is cooled in this heat exchanger before it enters the contactor for a maximum contacting efficiency The dry gas then leaves the unit at the bottom of the glycol-gas heat exchanger The dry concentrated glycol is picked up from the surge tank by the glycol pump and is pumped at the contactor operating pressure through the external glycol-gas heat exchanger and into the top of the contactor column. The dry glycol enters the contactor on the top of the tray. The dry glycol flows downward through the contactor vessel by passing across each tray and spilling over the weir box on the tray and then passing down through a downcomer to the next tray. By this countercurrent flow of gas and glycol, the driest incoming glycol on the top is in contact with the driest outgoing gas for maximum dehydration of the gas stream. The bottom tray downcomer is fitted with a seal as illustrated to hold a liquid seal on the trays.
The wet glycol which has now absorbed the water vapor from the gas stream leaves the bottom of the glycol-gas contactor column and passes through a high-pressure glycol filter. The high-pressure glycol filter will remove any foreign solid particles that may have been picked from the gas stream in the contactor before the glycol enters the power side of the glycol pump. This is generally considered to be the ideal location for primary filtration of the glycol stream. Filtration at this point will protect the glycol pump which is the most critical part of the unit because it has the only moving parts in the entire system.
From the glycol filter, the wet glycol passes to the power side of the glycol pump where it furnishes power to pump the dry glycol into the contactor. From the pump the wet glycol flows through a coil in the heat exchanger-surge tank where it is preheated by exchanging heat with the hot lean glycol coming from the reboiler A low-pressure glycol filter may be installed between the heat exchange coil and the reboiler for added filtration of the glycol stream.
The warm wet glycol stream flows from the heat exchange coil to a low-pressure glycol flash separator which allows for the release of the entrained solution gas which is necessary to power the glycol pump. The gas separated in the flash separator leaves the top of the vessel and may be used to supplement the fuel gas required for the reboiler. Any excess gas is discharged through a back pressure valve. The flash separator is normally equipped with a liquid level control and a diaphragm operated motor valve which discharges the wet glycol stream to the inlet feed connection on the glycol stripping still. If the wet glycol stream absorbs any liquid hydrocarbons in the contactor, it is desirable to equip the flash separator for three-phase operation to additionally separate the glycol from the liquid hydrocarbons before it enters the reboiler. Any liquid hydrocarbons present in the reboiler will cause undue glycol losses from the stripping still vent. The Liquid hydrocarbons that are separated in the flash separator are discharged from the vessel by a second diaphragm operated motor valve and liquid level control. The warmed and filtered wet glycol stream enters the lower part of the stripping still column which is packed with ceramic saddles and is insulated. An atmospheric reflux condenser is integral with the stripping still at the top of the still column and will condense any glycol vapors reaching the head of the still, plus some water vapor to provide the adequate reflux required for the stripping column. This reflux condenser is also packed with ceramic saddles to assure that all the vapor to be vented will come in contact with the cool wall of the condenser.
This is to ensure that the last possible remaining traces of any glycol vapor will be condensed and not lost out the water vapor vent.
The wet glycol after entering the stripping still column will flow downward toward the reboiler contacting hot rising glycol vapors, water vapors, and stripping gas. The water vapor has a lower boiling point than glycol; therefore, any rising glycol vapors will be condensed in the stripping still and returned to the reboiler section In the reboiler the glycol must travel a substantially horizontal path along the firebox to reach the liquid overflow exit at the opposite end. Here in the reboiler, the glycol is heated to between 350°F to 400°F to remove enough water vapor to re-
concentrate it to 99.5% or more.
For extra dry glycol (99% plus) it may be necessary to add some stripping gas to the reboiler. A manual valve and pressure regulator are provided to take a small amount of gas from the fuel gas supply system and inject it into the reboiler through a perforated spreader along the bottom of the vessel. This stripping gas will “slightly rol1” the glycol in the reboiler to allow any pockets of water vapor to escape, which might otherwise remain in the glycol due to its normal high viscosity. This gas will also aid in sweeping the water vapor out of the reboiler and stripping still. The gas will lower the partial pressure of the water vapor in the reboiler and still column, allowing the glycol to be concentrated to a higher percentage. For extra dry glycol (99% plus) it may be necessary to add some stripping gas to the reboiler. A manual valve and pressure regulator are provided to take a small
amount of gas from the fuel gas supply system and inject it into the reboiler through a perforated spreader along the bottom of the vessel. This stripping gas will “slightly roll” the glycol in the reboiler to allow any pockets of water vapor to escape, which might otherwise remain in the glycol due to its normal high viscosity. This gas will also aid in sweeping the water vapor out of the reboiler and stripping still. The gas will lower the partial pressure of the water vapor in the reboiler and still column, allowing the glycol to be concentrated to a higher percentage.
Standard field glycol dehydration units are normally equipped with a direct natural gas-fired firebox in the reboiler utilizing a portion of the natural gas stream and vent gas from the flash separator for fuel. A temperature control in the reboiler operates a fuel gas motor valve to maintain the proper temperature in the glycol The reboiler is also equipped with a high-temperature safety overriding temperature controller to shut down the fuel gas system in case of failure of the primary control. Standard field units are also equipped with a fuel gas scrubber, necessary pressure regulators, and a safety relief valve. In plant type dehydration units the reboiler may be fitted with a hot oil heated coil or steam coil in place of the direct fired firebox. In some cases, such as on offshore platforms, reboilers are sometimes heated with excess heat from compressor turbine exhaust.
The reconcentrated glycol leaves the reboiler through an overflow pipe and passes into the shell side of the heat exchanger-surge tank. In the surge tank, the hot reconcentrated glycol is cooled by exchanging heat with the wet glycol stream passing through the coil. The surge tank also acts as a liquid accumulator for feed for the glycol pump. The reconcentrated glycol flows from the surge tank through a strainer and into the glycol pump. From the pump, it passes to the shell side of the glycol-gas heat exchanger, flows upward through the heat exchanger and returns to the glycol-gas contactor column at the feed point on the top of the tray.

 

Sivalls