GAS CONDITIONING & PROCESSING (Gas Dehydration)
Contents
- Water in Natural Gas
- Dehydration
- Glycol
- Basic Process
- Absorption
- Distillation
- Process flow and Components
- Inlet Separator
- Contactor Column
- Heat Exchanger
- Flash Vessel
- Filters
- Reboiler
- Surge Tank
- How to care for Glycol
BASIC PRINCIPLES
In this session we will discuss about the need for dehydration then discuss the basic principles of absorption, distillation and other process involved in the glycol dehydration system.
- Water in Natural Gas
Produced natural gas contains water vapor. The gas is usually saturated under reservoir conditions of temperature and pressure. As it flows up to the plant some of this water vapor my condense as “free water”. The remainder will remain as water vapor. The process of removing water vapor from a gas stream is called “gas dehydration” .
The amount of water vapor contained in a gas stream can be expressed in terms of concentration or in terms of “water dew point” of the gas. The water dew point is the temperature at which water will condense from the gas stream as it is cooled. The higher the concentration of water vapor in a given stream the higher the dew point. Thus gas dehydration is also called dew point depression; the lowering of the dew point of a gas by lowering the concentration of water vapor in the gas.
Natural gas causes three major problems in transmission lines: corrosion, hydrate formation and slug formation. Corrosion causes pitting and damage in pipelines. Hydrate deposit on pipeline interiors and restrict the flow of gas especially in the valves and fittings. Hydrates are loosely linked , crystal-like compounds of hydrocarbon and water resembling dirty ice. Hence to prevent hydrate formation during the cooling process ( refrigeration process ) the gas should be dehydrated before it. Slugs damage equipment and cause pressure drop in the transmitting lines.
Natural gas when produced is usually saturated with water vapour at that pressure and temperature. As long as water in vapour form pose no problems, but as soon as it condenses and forms free water the above problems may start.
The following graph illustrates the amount of water that natural gas carries at different pressure and temperature.
Water content of Natural gas
- Dehydration
The process of removing water from a substance is called dehydration. Gas can be dehydrated by cooling and separating the condensed liquids, by using specially designed low temperature separation process, by using solid desiccants, or by using liquid desiccant. Cooling the gas and removing the free water with a separator is the simplest method of dehydration. However, this method is limited by the hydrate formation temperature unless some other hydrate preventive method has been taken. Although there are several methods for removing water from gas, the most commonly used dehydration method utilizes a liquid desiccant known as glycol
- Glycol
Liquid desiccants which are commonly used in the conventional dehydration unit are diethylene glycol, triethylene glycol, and tetra ethylene glycol. Ethylene glycol has been used in some applications, but its vapor pressure is too high for use in conventional dehydration units without excessive vapor losses. The more commonly used glycols for dehydration are diethylene glycol, triethylene glycol, and tetra ethylene glycol. Diethylene glycol is the cheaper of the three glycols; however it has a higher vapor pressure and cannot be regenerated to as high a purity as the other glycols. It is more commonly used for injection systems, and is only sometimes used for limited dew point depression in conventional dehydration units. In most facilities DEG is being used.
Chemical name. Formula. B.P Thermal decomposition.
Ethylene Glycol OHCH2CH2OH 197.6C 165C
CH2CH2OH
|
Di Ethylene Glycol O 246C 165C
|
CH2CH2OH
CH2-O-CH2CH20H
Tri Ethylene Glycol | 278C 206C
CH2-O-CH2CH2OH
The one change which can have the greatest effect on dew point depression is the glycol concentration, normally stated as a percent of purity. The higher the purity of the glycol the drier it is possible to get the gas, other conditions being equal.
Another factor that influences the water vapor content of a natural gas is pressure. The higher the pressure the less water vapor a gas can contain. In most instances the operating pressure of a dehydration unit is fixed due to pipeline delivery requirements, however if a unit is designed for a given pressure and is then operated at a considerably lower pressure the unit may not be able to deliver the designed dew point depression without making other operating adjustments. At lower than design pressures the contact tower may have too small a diameter to handle the design flow rate of natural gas.Another method of increasing the glycol purity is by lowering the pressure on the still with a vacuum pump.
Absorption
Much like a sponge, glycol is used to absorb water from natural gas. By mixing together the wet gas and glycol, water is absorbed by the glycol, thereby removing it from the natural gas.
Distillation
In distillation, water is separated and removed from glycol by heating (boiling ). Glycol does not begin to boil until approximately 245C. Water boils at 100C. Distillation of water from glycol involves heating the glycol-water mixture to a temperature between 100C and 245C, allowing water to separate as vapor.
- Basic Process
There are two basic purposes of a glycol dehydration unit. The primary purpose is to dehydrate natural gas. The other purpose is to remove water from wet glycol so that it can be used over and over again in the dehydration process.
The process, while it may seem somewhat complicated, is actually quite simple. The wet gas that is, natural gas with water in it has water removed from it in a dehydration process. Glycol literally soaks up the water, leaving dry gas. The wet glycol then goes through a process of distillation where the water is removed by boiling. The lean glycol is then sent back to function again in the dehydration of gas. In this manner, glycol is recycled.
- PROCESS FLOW AND COMPONENTS
OVER VIEW OF THE SYSTEM
A typical glycol dehydration system sketch is shown below. It consists of the following components: inlet scrubber, contactor column, heat exchangers, flash tank, filters, re boiler, and surge tank
Inlet Separator
Before entering a glycol contactor the gas should go through a separator to remove any free liquid . This could be either a horizontal or vertical separator or a filter- separator. On large units with high condensate rate or gas coolers before the dehydration system a filter-separator is often used to decrease the possibility of contaminating the glycol. On small units the separator is usually included as part of the contactor in the bottom section as shown in the previous overview .In the case shown , the gas leaves the separator section flows up through a chimney tray into the contactor section
Contactor Column
In this vertical pressure vessel, water is removed from gas. Inside the contactor column there are several trays . Most common trays are bubble cap trays . Valve trays are also in use. In smaller units packing are used in place of trays. The purpose of this arrangement is to make the gas in thorough contact with glycol. The most efficient one is bubble cap trays.
Each tray has a number of evenly arranged openings, covered with bubble caps. After passing through the separator the wet gas enters the contactor through an inlet near the bottom. The gas, traveling upward in the contactor column, is forced through the openings below the caps and bubbles through the glycol.
During the bubbling process, the gas gives up water vapor to the glycol. As gas passes upward through each succeeding tray it becomes dry.
Before leaving the contactor, the dry gas passes through a mist pad to remove any glycol that may be in vapor form. As the glycol particles collect and become heavier in the mist extractor, they drop back into the top tray and rejoin the glycol stream.
Dry glycol enters the contactor tower at an inlet near the top and flows across the top tray,
then downward and across other trays.
A level of glycol is maintained on a tray by means of a dam known as weir. This level is above the slots in the bubble caps so the gas is forced to bubble through the glycol.
The glycol flows over the weir through an opening known as downcomer and into the tray below. Maintaining the level of glycol on the next tray above the bottom of the down comer prevents gas from bypassing the bubble cap tray.
As the glycol spills downward through each succeeding tray, it becomes saturated with the water it has absorbed from the gas and collects in the bottom of the contactor. The rich glycol from the contactor then passes to the regeneration system.
HEAT EXCHANGER
In the regeneration system the glycol passes through a reflex column at the top of the re-boiler still and enters to the first heat exchanger. Top of the still column temperature to be maintained so that water vapor should not condense out and at the same time glycol vapor should condense out. This can be done by adjusting the reflex by-pass valve.
Then the rich glycol enters the first heat exchanger and get heated up with the hot lean glycol. Glycol should be sufficiently warm in order to flash out all the dissolved hydrocarbon and the lighter liquid hydrocarbons in the flash vessel.
FLASH VESSEL
From the heat exchanger the glycol enters the flash vessel. The purpose of this vessel is to remove the gas and condensate hydrocarbons that were picked up by the glycol on its path through the contactor.
Heat from the heat exchanger helps to separate hydrocarbons from the wet glycol. The hydrocarbon condensate is separated from the glycol and any remaining gas vapors leave from the top.
SOCK FILTER
The glycol leaving the flash tank is normally filtered through a sock type filter to remove 5 microns and larger solids. A differential pressure indicator is installed to measure the pressure drop across the filter. When the pressure drop increases the filter element to be replaced. Manufacture’s recommends to be followed regarding pressure drop requirement before replacing filters. Sock type filters consists of a quick opening closure and a number of filter elements. The filter elements are replaced when they become clogged and dirty.
CHARCOAL FILTER
The charcoal filter is similar to sock filter with charcoal instead of sock elements. For small glycol units the charcoal filter is normally full flow, where as for larger units a small side stream of 25 to 30% of total glycol is filtered. The charcoal filter removes hydrocarbons . A differential pressure indicator is installed to measure the pressure drop across the bed . When pressure drop reaches to the alarm point the charcoal needs to be replaced.
HEAT EXCHANGER
This one is the second heat exchanger in the system. In this exchanger , rich glycol is getting heated up before reaching the re-boiler. Hence the heat requirement in the re-boiler to boil off the water will be reduced.
Re boiler and Still Column
From the heat exchanger the glycol flows to the re boiler. Here the glycol is heated to vaporize the water before entering the still column (“stripper”) on the re boiler. The still column removes the water vapor from the glycol.
RE BOILER
Inside the still column is a section, known as distillation column is filled with ceramic, stainless steel or carbon packing known as saddles or raschig rings. The glycol spreads out uniformly over the packing and drips down through the lower portion of the packed column. The water vapor rises to the top of the column.
From the packed column the wet glycol drops downward into the bottom of the re boiler. A source of heat circulated through a tube in the lower section of the re boiler maintains the temperature of the glycol solution at approximately 165 C (in the case of DEG) which is just below the boiling and decomposition point of Diethylene Glycol. Many installations use a gas fired heater. The temperature of the glycol in the re boiler is critical and must be controlled within the range.
The remaining water boils out of the glycol solution and moves upward through the still column as vapor. Some hot glycol vapors also are mixed with the water vapor. As this mixture passes upward through the still column, it comes in contact with a cooler part of the column and the glycol vapors are condensed and drop back down into the re-boiler. The water leaves the top of the still column as vapor.
The glycol level in the re boiler is maintained above the heating tube by the location of the overflow tube. The dried, purified glycol spills into the overflow tube and flows into the surge tank.
Surge Tank
The surge tank is a holding tank which stores hot regenerated glycol from the re boiler before it is sent to the contactor with the pump.There are chances of heavier hydrocarbon liquid accumulation and form a layer at the top of the glycol. Skim lines are provided to skim off the these hydrocarbon liquids from the surge tank.
