Glycol Dehydration - Glycol Gas Heat Exchanger Leak

The hot glycol from the reboiler is cooled by heat exchange with the wet glycol from the contactor. This heat transfer typically takes place in a double-pipe or plate-type exchanger. On one of the double-pipe heat exchangers, I noticed that the reboiled glycol was being cooled to a rather low temperature. I suspected that this could be an indication of a leaking feed-effluent exchanger. That is, cooler (120°F) wet glycol might be leaking into warmer (165°F) dry glycol. To verify my suspicions, I blocked in the dry glycol at the reboiler and at the suction to the pump. The appearance of a steady stream of liquid at an intervening bleeder confirmed that the feed-effluent exchanger was leaking, hi effect, wet glycol was bypassing the reboiler and flowing straight back to the contactor tower.

After fixing the leak, this reboiler and the units that had suffered from an inefficient pump and a faulty temperature controller were put back on-line. The treated natural gas was checked and found to meet pipeline moisture specifications.

Glycol Dehydration

The gas exiting the top of the contactor in Figure 6-1 can be assumed to be in equilibrium with the reboiled—i.e., dry—glycol. The higher the glycol reboiler temperature, the dryer the glycol. The dryer the glycol, the dryer the treated natural gas. For most of the year in El Gringo, critical control of the glycol reboiler temperature gas was not vital. Relatively cool ambient temperatures maintained the top temperature of the contactor towers below 110°F. But now, in mid-July, this temperature was peaking at 122°F every afternoon. I checked my gas purification data book1 and calculated that, for the 1,020 psig operating perssure of the contactors, it should be possible to meet the required moisture specification. My calculations were based on a reboiler temperature at 375°F. For triethylene glycol, which is the work horse of the gas drying industry, the maximum recommended reboiler temperature to prevent thermal degradation of the glycol is 400°F. The six El Graingo dehydrator reboilers were all set to hold 375°F. But by checking the actual reboiler temperatures with a calibrated thermometer, I determined that one of the reboilers was actually operating at 350°F as opposed to 375°F. This reduced temperature was sufficient to greatly increase the water concentration of the “dry” glycol, so that the moisture content of gas treated with this glycol stream was doubled.

A simple recalibration of the reboiler temperature controller rectified this problem. Incidentally, operating a triethylene glycol reboiler at 375°F-400°F does not necessarily result in a noticeable increase in glycol degradation. The trick is to keep the glycol filters in good repair. Dirty glycol fouls the reboiler heat-transfer tube. This in turn causes hot spots on the heat-transfer surface, which accelerates thermal decomposition.

GLYCOL REGENERATION PROCESS AND EQUIPMENT

THE GLYCOL REBOILER

The glycol regeneration process is very important to maintain the correct concentration of the lean glycol. Refer to Figures: 50 & 52 for the equipment used in the Glycol Regeneration Process.


The glycol reboiler is the main piece of equipment that plays this role in the regeneration process. The reboiler supplies heat to separate the glycol and water by a simple distillation process.


The system consists of a ' U ' shaped, combustion chamber with gas burners, set into the shell of the reboiler and includes an outlet stack for the waste combustion gases.


The shell also contains a ' Weir ' that maintains the level of glycol above the fire-tube in order to prevent overheating of the tube and subsequent damage and/or glycol decomposition by excess heat.



Figure: 32 - Fire-tube Reboiler


The temperature of the reboiler should be in the range of 375 to 390 °F. This temperature will usually give good distillation of the rich glycol and evaporate all water out of it.


The glycol should never be heated above 400 °F as it begins to decompose above that temperature.


Note: When making adjustments to reboiler temperature, never increase the temperature setting by more than five degrees at a time.


Too great an increase will cause the control system to open the fuel gas valve too wide, giving a large burner flame which in turn will cause flame impingement on the inside of the fire-tube. This will lead to ' Hot-spots ' and cause damage to the fire-tube and breakdown of the glycol into corrosive organic acids.


If coke , salts or tar deposits form on the fire tube, the heat transfer into the glycol is reduced, the control system will increase the fuel to maintain the glycol temperature and tube failure can result. Localised overheating, especially where salt deposits accumulate, will decompose the glycol.


Salt deposits can be detected by shutting off the burner on the glycol reboiler system at night and looking down the fire-box. A bright red glow will be visible at the hot spots on the fire tube walls where salt deposits have collected. An analysis of the glycol will determine the degree of the contamination.


It is highly recommended that, during a plant start-up, make sure the reboiler is up to the desired operating temperature before flowing gas through the contactor .


Some fires have been caused by leaks in the gas lines near the fire-box. The best precaution is to have valves and regulators in the gas line at a suitable distance from the firebox.


Another very effective measure is the addition of a flame arrestor around the fire-box. If the flame arrestor is properly designed, even severe gas leaks in the immediate vicinity of the fire-box will not ignite.

McKenzie Dehydration of Natural Gas

Dehydration of Natural Gas

Natural Gas usually contains significant quantities of water vapor. Changes in temperature and pressure condense this vapor altering the physical state from gas to liquid to solid. This water must be removed in order to protect the system from corrosion and hydrate formation.

In 1810, an English scientist by the name of John Dalton stated that the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the components. This statement, now known as Dalton's Law of Partial Pressures, allows us to compute the maximum volume of water vapor that natural gas can hold for a given temperature and pressure.

The wet inlet gas temperature and supply pressures are the most important factors in the accurate design of a gas dehydration system. Without this basic information the sizing of an adequate dehydrator is impossible.

As an example, one MMSCF (million standard cubic feet) of natural gas saturated @ 80 degree F. and 600 PSIG (pound per square inch gauge) will hold 49 pounds of water. At the same pressure (600 PSIG) one MMSCF @ 120 degree F will hold 155 pounds of water.

Common allowable water content of transmission gas ranges from 4 to 7 pounds per MMSCF. Based upon the above examples, we would have two very different dehydration problems as a result of temperature alone.

There are many other important pieces of design information required to accurately size a dehydration system. These include pressures, flow rates and volumes.

All gasses have the capacity to hold water in a vapor state. This water vapor must be removed from the gas stream in order to prevent the formation of solid ice-like crystals called hydrates. Hydrates can block pipelines, valves and other process equipment. The dehydration of natural gas must begin at the source of the gas in order to protect the transmission system. 

The source of the gas moved through the transmission lines may be producing wells or developed storage pools. Pipeline drips installed near well heads and at strategic locations along gathering and trunk lines will eliminate most of the free water lifted from the wells in the gas stream. Multi stage separators can also be deployed to insure the reduction of free water that may be present.

Water vapor moved through the system must be reduced to acceptable industry levels. Typically, the allowable water content in gas transmission lines ranges from 4 lb. to 7 lb. per MMSCF. There are basically three methods employed to reduce this water content. These are: 

1.  Joule-Thomson Expansion
2.  Solid Desiccant Dehydration
3.  Liquid Desiccant Dehydration 

Joule-Thomson Expansion utilizes temperature drop to remove condensed water to yield dehydrated natural gas. The principal is the same as the removal of humidity from outside air as a result of air conditioning in your house. In some cases glycol may be injected into the gas stream ahead of the heat exchanger to achieve lower temperatures before expansion into a low temperature separator. 

Solid desiccant dehydration, also known as solid bed, employs the principal of adsorption to remove water vapor. Adsorbents used include silica gel (most commonly used), molecular sieve (common in NGV dryers), activated alumina and activated carbon. The wet gas enters into an inlet separator to insure removal of contaminants and free water. The gas stream is then directed into an adsorption tower where the water is adsorbed by the desiccant. When the adsorption tower approaches maximum loading, the gas stream is automatically switched to another tower allowing the first tower to be regenerated. 

Heating a portion of the mainstream gas flow and passing it through the desiccant bed regenerates the loaded adsorbent bed. The regeneration gas is typically heated in an indirect heater. This undersaturated regeneration gas is passed through the bed removing water and liquid hydrocarbons. 

The regeneration gas exits the top of the tower and is cooled most commonly with an air-cooled heat exchanger. Condensed water and hydrocarbons are separated and the gas is recycled back into the wet gas inlet for processing. The third method of dehydration is via liquid desiccant and is most common in the Northeast United States. This method removes water from the gas stream by counter current contact in a tray type contactor tower with tri-ethylene glycol (TEG). Natural gas enters the unit at the bottom of the adsorber tower and rises through the tower were it intimately contacted with the TEG solution flowing downward across bubble trays. Through this contact, the gas gives up its water vapor to the TEG. 

The water laden TEG is circulated in a closed system, where the water is boiled from the TEG. The regenerated TEG then is recirculated to the contacting tower.

Dehydration

Dehydration

Natural gas often comes out of the ground mixed with water vapor. This "wet gas" can be separated using two primary methods:

Glycol dehydration – Wet gas moves through an inlet pipe into a tank called a "contactor." A rounded cap over the inlet pipe forces the gas to flow down into a pool of glycol solution at the bottom of the tank. Glycol has a strong affinity for water, so the water molecules from the wet gas bond to the glycol molecules in the solution. The vapor-free natural gas is collected from the top of the contactor.

Because water boils at 212F and glycol doesn't boil until 400F, simple heating is all that is required to vaporize the captured water so the glycol solution can be reused.

Solid-desiccant dehydration - This method is typically more effective than glycol dehydration, but requires higher volumes of natural gas moving under high pressure. The wet gas is pumped downward through a tower filled with a solid desiccant (drying agent). The desiccant attracts and binds the water molecules so that only dry gas flows out the bottom of the tower.

When the desiccant has captured all the water it can, operators flush the tower with heated gas that re-vaporizes the water molecules, thereby "reactivating" the desiccant.