Glycol-Type Gas Dehydration Unit

A natural gas stream can be dehydrated by contacting the gas with glycol. This process (see Figure 1) is normally carried out at an elevated pressure in a vessel called a contactor or absorber. After absorbing the water, the glycol is reconcentrated by boiling off the water at atmospheric pressure in a regenerator. A pump is used to recirculate the glycol to the contactor.

Fig 1: Gas Dehydration Unit

Inlet Scrubber: An inlet scrubber is required, either integral with the contactor or as a separate vessel upstream, to remove free liquids from the gas stream going to the contactor. The mist extractor in this vessel removes larger droplets entrained in the gas.

Contactor: The contactor vessels may be categorized as to the manner in which the absorption process is accomplished. One type uses trays equipped with bubble caps, valves, other devices, to maximize gas-to glycol contact. The action of the gas flowing upward through the glycol layer on each tray creates a froth above the tray, where most of the absorption takes place. The other type of contactor is referred to as a packed tower. It is filled with packing, which has a large surface area per unit volume. Glycol flowing downward wets the entire packing surface. Absorption takes place as the gas flows upward through the packing, contacting the wetted surface. In either type of vessel, a mist extractor removes entrained glycol droplets from the dehydrated gas stream before it leaves the top of the contactor. On larger units, an optional residue gas scrubber may be justified. Rich (wet) glycol is directed from the bottom of the contactor to the regeneration system.

Gas/Glycol Heat Exchanger: Absorption is improved with lower temperature glycol. A gas/glycol heat exchanger is required which uses dehydrated gas to cool the lean (dry) glycol before it enters the top of the contactor.

Regeneration System: The regeneration system consists of several pieces of equipment. If glycol-gas powered pumps are installed, energy from the high pressure rich glycol along with a small amount of gas is used to pump the lean glycol. If an optional reflux coil in the still column is provided, the rich glycol flows through it before entering the glycol/glycol heat exchanger. The glycol/glycol heat exchanger serves two purposes: 1) to cool the lean glycol to a temperature as recommended by the pump manufacturer, and 2) to conserve energy by reducing the heat duty in the reboiler.

Gas-Condensate-Glycol Separator:  A frequently used option in regeneration systems is a gas-condensate glycol separator, and should be included when the inlet gas contains condensate. It may be located upstream or downstream of the glycol/glycol heat exchanger and usually operates at a pressure of 25-75 psig. It removes condensate from the glycol prior to the reboiler, which minimizes coking and foaming problems. The separator also captures flash gas that is liberated from the glycol and exhaust gas from the glycol-gas powered pumps, so that the gas may be used as fuel. Glycol is regulated from the separator to the reboiler by means of a level controller and dump valve. Condensate removal may be controlled automatically or manually.

Reboiler.:Rich glycol enters the reboiler through the stili column. It is then heated to 350-400°F, which causes the water that was absorbed in the contactor to vaporize. The reboiler is usually heated by combustion of natural gas, but may utilize other fuels, steam, hot oil or other heat sources. The regenerated lean glycol gravity feeds from the reboiler, through the glycol/ glycol heat exchanger, and into the pump suction for recirculation back to the contactor. Either electric, gas powered, or glycol-gas powered pumps may be used.

Still Column: Water and glycol vapors from the reboiler enter the bottom of the still column, which is mounted on top of the reboiler. The bottom section contains packing, while the top section of the still column may contain a reflux coil or external fins. Reboiier vapors are cooled and partially condensed to provide reflux, which improves the separation between glycol and water. The remaining water vapor leaves the top of the stili column and vents into the atmosphere.

Filters and Strainers: Regeneration systems contain various types of filters and strainers. A particle filter or fine mesh strainer is required to protect the pump. To reduce foaming, an activated carbon filter may be installed to remove heavy hydrocarbons from the glycol.

Some important factors to be considered on GDU Unit:

Firetube Heat Flux: The average heat flux shall be no higher than 10,000 BTU/hr.-sq. It. of exposed area.

Example: 8%‘‘ O.D. Sch. 20, 0.25” wall fire tube having 25.0 square feet of surface, 51.85 sq. in. cross sectional area and rated at 250,000 BTU/hr. heat duty.

Average Heat Flux = (Firetube Rating (BTU/hr))/ (Sq. Ft. of Firetube Surface) = 250000/25.0 =10,000 BTU/hr-sq. ft

Stack Height: The height of the stack shall be no less than required to provide draft sufficient to overcome the pressure drop in the firetube, flame arrestor, stack, returns. turbulators, dampeners, and stack flame arrestor if provided. The operating site elevation shall be considered in the draft calculations.

Process Considerations:

Inlet Gas Temperature: One of the key design and operating variables of a glycol-type gas dehydration unit is the temperature of the entering wet gas. For operation, this temperature should be maintained between 60°F and 120°F. At lower gas temperatures, glycol on the contactor trays will become very viscous, resulting in reduced tray efficiency, increased pressure drop, and glycol carryover. Higher Temperatures will increase the amount of water vapor to be removed, as well as require very pure lean glycol to meet the dehydration specification. Glycol vaporization losses will also increase at higher gas temperatures.

Gas/Glycol Heat Exchanger: It is important that the glycol entering the contactor be cooled to a 10°F to 30°F above the temperature of the gas stream. This is necessary because the equilibrium conditions between the glycol and the water vapor in the gas are affected by temperature. At higher temperatures, more water vapor will remain in the gas stream. A cooler glycol temperature will decrease the glycol vaporization losses but hydrocarbons may condense in the contactor.

Glycol Reboiler Heat Flux/Temperature:Glycol degradation should be minimized by designing the glycol reboiler firetube with an average heat flux of no higher than 10,000 BTU/hr/ft2. The normal range of heat flux is 6,000 - 10,000 BTU/hr/ftZ. Burner flame pattern and flame length should also be designed to avoid hot spots on the firetube. Bulk temperature for triethylene glycol should not exceed 400°F. The maximum tube wail temperature should not exceed 430°F

Circulation Rates: Typical glycol type gas dehydration units have glycol circulation rates from 2.0 to 3.0 gallons of glycol per pound of water removed- Varies depends on Unit Spec & gas quality.

Glycol Losses: For a properly designed gas dehydration unit during normal operation, the glycol losses should not exceed 0.1 gallon of glycol per million standard cubic feet of gas dehydrated.

GDU- CORROSION CONTROL:

Variables affecting Corrosion Potential: Stream compositions, operating pressure and temperature conditions, and design/fabrication details such as metallurgy, stress, welding procedures and heat treatment all have a part in the corrosion potential of a system. Since carbon steel is the major material of construction for typical glycol-type gas dehydration units, corrosive environments require special considerations.

Stream Compositions. Of primary concern is the presence of acid gases (carbon dioxide- CO, and/or hydrogen sulfide-H,S) and/or oxygen-02 in the flow streams.

Carbon dioxide partial pressures in the gas phase below 3 psia typically do not require corrosion control. Between 3 and 30 psia, some form of corrosion control may be required, such as pH control or inhibitor injection.

Corrosion resistant metals may also be needed. For carbon dioxide (CO,) partial pressures above 30 psia, design/operational corrosion control measures will be required. Hydrogen sulfide (H2S) and oxygen (O2) are corrosive at very low concentrations. In addition to corrosion, hydrogen sulfide (H,S) can lead to sulfide stress cracking (SSC)

INSTALLATION, START-UP, OPERATION AND MAINTENANCE

Installation:

All equipment must be installed on an adequate foundation. The equipment should be as level as possible for the most efficient operation. All items shipped loose should be installed on the unit. This may include the stack, still column, piping between the regenerator and contactor, and the vent line from the still column. Normally the still column vapors are vented directly to the atmosphere.

Vent piping should be kept to a minimum. It should be remembered that these vapors contain combustible hydrocarbons, corrosive components, and water which may condense and freeze. Therefore, consideration must be given to the location  assembled, all screwed and bolted connections should be checked for tightness.

 Start-up:

The unit should be inspected before start-up to make certain that all valves are closed and all regulators are backed off.

All relief valves and critical shutdown devices should be operational. Admit supply gas to the system and open isolation valves under all pressure gauges.

The contactor should be purged with natural gas to eliminate air. It then should be brought up to line pressure and checked for leaks.

Maintain the contactor pressure, but do not flow gas at this point. The flash tanks and piping should also be purged to eliminate air.

Open the cocks on the glycol surge tank level gauge and the valve in the line between the surge tank and the glycol/glycol heat exchanger.

Fill the reboiler with glycol until the level comes about half way up in the surge tank gauge. Allow approximately 25% of the surge tank for thermal expansion of the glycol.

The glycol circulation, including the return to the reboiler from the contactor, should be fully established prior to ignition of the main burner.

Light the pilot light and main burner as recommended. Heat the glycol until it reaches 390°F and set the temperature controller. Continue heating the glycol until it reaches 400°F and set the high temperature shutdown. These temperatures are typical: however, some manufacturers and operators prefer somewhat different temperatures.

Operating conditions can also sometimes require different operating temperatures. It is highly recommended that the glycol never be heated above 400°F because it starts decomposing at 405°F.

The glycol level in the surge tank should be brought to normal after circulation has been established. All gauge cocks should be open and level controls set at this time.

Gas flow may now be started through the contactor. The flow rate should be increased slowly to prevent losing

liquid seals and damage to the trays.

The unit is now ready for final adjustments. This includes checking the reboiler temperature setting, circulation rate, burner adjustment, valve function, level controller function, and glycol level in the sure tank.

It is very important to make sure that steam is coming out of the vapor outlet of the still column. The circulation rate should be in accordance with the process design specification.

Operation:

Routine operation of gas dehydration units primarily involves periodic visits to determine if everything is operating properly.

As a minimum, the following items should be checked:

a. inlet gas temperature and flow rate

b. contactor pressure

c. reboiler temperature

d. pump operation

e. steam from still column

f. level of glycol in surge tank

g. burner flame pattern and firetube appearance.

It is necessary to periodically add glycol to the surge tank because a certain amount of glycol  loss is normal.

Other than that, the units are designed for unattended operation as long as everything is functioning properly.

If the unit is designed for manual dumping of distillate from the reboiler and/or the glycol flash separator, it will be necessary to check these levels during the periodic visits.

There are numerous operating problems that can be encountered with these units. Some of the most common will be discussed here.

Two factors which greatly affect the ability of a unit to dehydrate gas are gas pressure and temperature.

Small changes from design in these variables can have a large effect on the water content of the gas. Gas flow rate has a somewhat smaller effect on equipment performance.

Cold outside air temperatures can render a unit inoperable. It can freeze instruments and controls, and can cause hydrates to form in scrubbers. If a unit is located in an area where this is a problem, precautions should be taken. Examples are heating coils in scrubbers, heating jackets on liquid discharge lines, cold weather shrouds on glycol/glycol heat exchangers, and housing the entire regenerator

Proper operation of a unit depends on the cleanliness of the gas being processed. Many times, it is necessary to install a coalescing filter separator immediately ahead of the unit. This will remove compressor lube oil fog, small solids, distillate, salt, etc. These impurities can plug equipment, coat packing, render the glycol less effective, and coat the firetube which will cause it to burn out.

  

Plugging in the still column or vent line can cause pressure to build up in the reboiler and surge tank. This pressure should be checked periodically. Caution should be used when opening connections: for example, to add glycol.

  

There are ways of removing distillate once it gets into the regeneration system. The surge tank may have a skimmer valve on it by which the distillate can be manually drained. If the glycol flash separator is designed as a three phase vessel, distillate may also be removed from the system at this point.

  

Maintenance:

It is necessary to check the pH of the glycol periodically. It should be a neutral solution. Values that vary from neutral can lessen the ability of the glycol to absorb water, and may cause foaming or corrosion.

The elements in all filters (coalescing, charcoal, sock, regulators, etc.) need to be checked periodically and replaced as necessary.

Pumps require routine maintenance and overhauling.

Dehydration units may become plugged and packing may get a coating buildup. When this happens, it is necessary that the system be thoroughly cleaned.