Gas Dehydration - Chapter 5 - Part 3

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Gas Dehydration - Chapter 5 - Part 3

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Fundamentals of Oil and Gas Processing Book
Basics of Gas Field Processing Book
Prediction and Inhibition of Gas Hydrates Book
Basics of Corrosion in Oil and Gas Industry Book

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5.14 Glycol System Cleaning
Chemicals are frequently needed to clean the glycol system. If chemical cleaning is done properly, it can be quite beneficial to plant operations. While if done poorly, it can be quite costly and create long-lasting problems.
The most effective type of cleaner is a very heavy-duty alkaline solution. To provide optimum cleaning, the concentration, temperature, and pumping rate of the solution must be carefully controlled and an experienced, reputable vendor employed. A cascading technique can be used to save on the cost of cleaning chemicals.
Cleaning Techniques to Avoid
Steam cleaning is not effective and can be damaging and dangerous. It tends to harden the deposits in the system, making them almost impossible to remove.
The use of cold or hot water, with or without high detergent soaps, will do little good in cleaning the system. High-detergent soaps can create a serious problem by leaving trace quantities of soap after the cleaning job. Soap traces left in the system can make glycol foam for a long time.
Acid cleaning is good for removing inorganic deposits. Since most deposits in the glycol system are organic, acid cleaning is not very effective. It can easily create additional problems in the glycol system after the cleaning job.
5.15 Eliminating Operating Problems
Most operating problems are caused by mechanical failure. It is important to keep equipment in good working order. Following operating and maintenance suggestions helps provide a trouble-free operation.

5.15.1 Inlet Scrubber/Microfiber Filter Separator
The cleaner the inlet gas entering the absorber, the fewer operating problems there will be.
Potential problems if there was not an inlet scrubber of filter separator are:
Liquid water carryover, results in one or more of the following:
Dilutes the glycol
Lowers the absorber efficiency
Requires a greater glycol circulation rate
Increases the vapor-liquid load on the still column
Floods the still column
Vastly increases the reboiler heat load and fuel gas requirements
These problems also cause:
High glycol losses
Wet sales gases
If the water contained salt and solids, they would be deposited in the reboiler to foul the heating surfaces and possibly cause them to burn out.
The scrubber also prevents salt or other solids from entering the glycol system, where they could be deposited in the reconcentrator to:
Foul the heating surfaces
Burn out as hot spots

If liquid hydrocarbons were present:
They would pass onto the still column and reboiler
Lighter fractions would pass overhead as vapor and could create a fire hazard
Heavy fractions would collect on the glycol surface in the storage tank and could overflow the system.
Flashing of the hydrocarbon vapor can flood the still column and vastly increase the reboiler heat load and result in glycol losses.
Well corrosion control program should be planned and coordinated to prevent glycol contamination.
Scrubber or filter separator may be an integral part of the absorber or preferably a separate vessel. Vessel should be large enough to remove all solids and free liquids to keep these impurities from getting into the glycol system. Vessel should be regularly inspected to prevent any malfunction. Liquid dump line should be protected from freezing during cold weather.
Separator should be located close to the absorber so the gas does not condense more liquids before it enters the absorber.
Sometimes an efficient mist extractor, which removes all contaminants over one micron is needed between the inlet separator and the glycol plant to clean the incoming gas; this is particularly useful when paraffin and other impurities are present in a fine vapor form
When gas is compressed prior to dehydration, a coalescing type of scrubber (microfiber filter separator) placed ahead of the absorber insures removal of compressor oil in vapor form. Compressor oil and distillate can coat the tower packing either in the absorber or still column and decrease its effectiveness.

5.15.2 Absorber
This vessel contains valve or bubble cap trays or packing to give good gas-liquid contact.
Cleanliness is very important to prevent high sales gas dew points caused by foaming and/or poor gas-liquid contact. Plugged trays or packing could also increase glycol losses.
Unit startup considerations are as follows:
The pressure on the absorber should be slowly brought up to the operating range and then the glycol should be circulated to get a liquid level on all trays.
Next, the gas rate going to the absorber should be slowly increased until the operating level is reached.
If the gas enters the absorber before the trays are sealed with liquid, it will pass through the downcomers and bubble caps. When this condition occurs and the glycol is pumped into the absorber, the liquid has difficulty in sealing the downcomers, and the liquid will be carried out with the gas stream instead of flowing to the bottom of the absorber.
Gas flow rate should be increased slowly when changing from a low to a high flow rate.
Rapid surges of gas through the absorber may cause; Sufficient pressure drop through the trays to break the liquid seals, and/or Glycol to be lifted off the trays, which will flood the mist extractor and increase glycol losses
Unit shutdown considerations are as follows:
First, the fuel to the reboiler should be shut down.
Then the circulating pump should be run until the reboiler temperature is lowered to approximately 2000F (94 0C).
This precaution will prevent glycol decomposition caused by overheating.
The unit can then be shut down by slowly reducing the gas flow to prevent any unnecessary shocks on the absorber and piping.
The unit should be depressurized slowly to prevent a loss of glycol.
The dehydrator should always be depressurized from the downstream (gas outlet) side of the absorber.

A dehydrator installed on the discharge side of a compressor should be equipped with a check valve in the inlet line, located as close as possible to the absorber. Experience has shown that some glycol is sucked back into this line when a compressor backfires or is shut down.
Internal absorber damage to the trays and mesh pad may also occur with a compressor failure.
The installation of the check valve usually eliminates this problem.

All compressors taking gas from or feeding gas to a dehydrator should have pulsation dampeners. The absence of this safety device may cause fatigue failure of instruments, trays, coils, mesh pads and other parts of the dehydrator.
The glycol dump valve and level controller should be set for throttling action to give an even flow of glycol to the regenerator. This will prevent slugs, which could flood the stripper and cause excessive glycol losses.
The absorber must be vertical to insure the proper flow of glycol in the vessel and adequate contact of the glycol and gas. Inspection ports at the trays can be very useful when inspecting or cleaning the vessel.
If dry gas from a glycol unit is used for gas lift, care must be used in both sizing and operating the unit because of the unsteady gas rate required in this service. Control valves or backpressure valves are used to prevent a sudden overloading of the absorber which can break the downcomer seals in a tray type of vessel and cause excessive loss of glycol in the sales gas.
Absorbers sometimes need to be insulated when excessive condensation of light hydrocarbons collect on the vessel walls. This often occurs when dehydrating rich, warm gases in cold climates.
These very light hydrocarbons can cause tray flooding in the absorber and excessive glycol losses from the regenerator.
The mist extractor should receive special attention because glycol entrainment and well-crawling are difficult to effectively control.
The type and thickness of the mesh pad should be carefully studied to minimize glycol losses.
Care should also be taken after installation to avoid mesh pad damage. The maximum pressure drop through the contractor to avoid damage to the mesh pad is approximately 15 psi.

5.15.3 Glycol-Gas Heat Exchanger
Most units are supplied with a glycol-gas heat exchanger that uses the gas leaving the absorber to cool the lean glycol entering the absorber. This exchanger may be a coil in the top of the absorber or an external one. A water-cooled exchanger may be used when heating of the gas must be avoided. This exchanger may accumulate deposits, such as salt, solids, coke or gum which foul the heat exchanger surface, reduce the heat transfer rate, and increase the lean glycol temperature. All of the above increase glycol losses and make dehydration difficult. The vessel should be inspected regularly and cleaned when needed.

5.15.4 Lean Glycol Storage Tank or Accumulator
Normally this vessel contains a glycol heat exchanger coil which cools the lean glycol coming from the reboiler and preheats the rich glycol going to the stripper
The lean glycol is also cooled by radiation from the shell of the storage tank.
This accumulator should normally be insulated. Water cooling can also be used to help control the lean glycol temperature.
On conventional regenerators without stripping gas:
Accumulator must be vented to prevent trapping gas
Vapors, trapped in the storage tank, could cause the pump to vapor lock
A connection is usually provided in the top of the storage tank for venting
Vent line should be piped away from the process equipment and should not be connected to the stripper vent because this could cause steam to dilute the concentrated glycol

Some units are equipped to provide a dry gas blanket (no oxygen or air) in the storage tank. Blanket gas is normally piped to the regular vent connection on top of the storage tank. If blanket gas is used, it is commonly taken from the fuel gas line. Only a very slight flow of gas is required to prevent steam generated in the reboiler from contaminating the regenerated glycol.
The vessel should be inspected occasionally to see that sludge deposits and heavy hydrocarbons are not collecting in the bottom of the vessel. The heat exchanger coil should be kept clean so proper heat transfer can be made. This also prevents corrosion. If the heat exchanger develops a leak, the water rich glycol could dilute the lean glycol.
Glycol level in the storage tank should be checked and a level in the gauge glass should always be maintained. Glycol should be added as the level is pumped down. Records of the amount of glycol added should be maintained. Make certain the storage tank is not overfilled.

5.15.5 Stripper or Still Column
The stripper, or still column, is generally a packed column located on top of the reboiler to separate the water and glycol by fractional distillation. Packing is usually a ceramic saddle but 304 stainless steel pall rings can be used. A standard stripper usually has a finned atmospheric condenser in the top to cool the steam vapors and recover the entrained glycol.
Atmospheric condenser depends upon air circulation to cool the hot vapors. Increased glycol losses can occur on extremely hot days when insufficient cooling in the condenser causes poor condensation. High glycol losses can also occur on extremely cold, windy days when excessive condensation (water and glycol) overloads the reboiler. Excess liquids percolate out the stripper vent.
If stripping gas is used, an internal reflux coil is normally provided to cool the vapors to prevent excessive glycol losses. This is due to a larger mass of vapor leaving the stripper which will carry glycol. Adequate reflux is provided by passing the cool, rich glycol from the absorber through the condenser coil in the stripper. If properly adjusted, it can provide uniform condensation throughout the year.

Manual/automatic valve in the piping is furnished to bypass the reflux coil.
Under normal circumstances this valve will be closed and the total flow will be through the reflux coil. In cold weather operation, with extreme low ambient temperatures, this could produce too much reflux and the regenerator could become overloaded. Therefore, a portion or all of the rich glycol solution should bypass the reflux coil. This is accomplished by opening the manual/automatic valve until the reboiler can hold the temperature. This lowers the amount of reflux produced by the coil and reduces the load on the reboiler.
Sometimes a leak can develop in the cool glycol reflux coil in the top of the stripper. When this happens, excess glycol can flood the tower packing in the still column, upset the distillation operation, and increase glycol losses.
Broken, powdered packing can cause solution foaming in the stripper and increase glycol losses.
Packing is usually broken by excessive bed movement which is caused when hydrocarbons flash in the reboiler. Careless handling when installing the packing can also cause powdering.
As particles break down, the pressure drop through the stripper increases. This restricts the flow of vapor and liquid and causes the glycol to percolate out the top of the stripper.
Dirty packing, caused by sludge deposits of salt or tarry hydrocarbons, will also cause solution foaming in the stripper and increase glycol losses. Packing should be cleaned or replaced when plugging or powdering occurs. The same size tower packing should be used for replacement.
The standard size of the ceramic saddle or a stainless steel pall ring is one inch.
A large carryover of liquid hydrocarbons into the glycol system can be very troublesome and dangerous. The hydrocarbons will flash in the reboiler, flood the stripper, and increase glycol losses. Heavy hydrocarbon vapors and/or liquids could also spill over the reboiler and create a serious fire hazard. Therefore, the vapors leaving the stripper vent should be piped away from the process equipment as a safety measure. The vent line should be properly sloped all the way from the stripper to the point of discharge to prevent condensed liquids from plugging the line.

Still Emissions
Vapor from the still column can contain some hydrocarbon gases that flashed from the glycol, stripping gas and aromatics. Glycol preferentially absorbs aromatics and napthene components over paraffinic components in the inlet gas.
Aromatics:
Include benzine, ethylene, toluene, and xylene (commonly called BETX)
Condense with water vapor
Could lead to “soluble” oil in the produced water discharge
Treatment consists of condensing the water vapor and BETX exiting the still column and then compressing the non-condensables (hydrocarbon gases) (Figure 5-46)

Image
Fig. 5-46. Process flow diagram of treatment of still emissions.

5.15.6 Reboiler
The reboiler supplies heat to separate the glycol and water by simple distillation. Large plant locations may use hot oil or steam in the reboiler. Remote field locations are generally equipped with a direct-fired heaters, with the following characteristics:
Use a portion of the gas for fuel
Heating element usually has a U-Tube shape and contains one or more burners
Conservatively designed to insure long tube life and prevent glycol decomposition caused by overheating (Forming a sludge covering parts of flame “u-tube” due to glycol decomposition).
Reboiler should be equipped with a high-temperature safety overriding controller to shut down the fuel supply gas system in case of malfunction of the primary temperature controller.
The firebox heat flux (a measure of the heat transfer rate in Btu/hr/ft2) should be high enough to provide adequate heating capacity but low enough to prevent glycol decomposition. Excessive heat flux, a result of too much heat in a small area, will thermally decompose the glycol.
Flame should be correctly adjusted to give a long, rolling, and slightly yellow-tipped flame.
Nozzles are available that distribute the flame more evenly along the tube:
Decreases the heat flux of the area nearest the nozzle without actually lowering the total heat energy transferred
Avoids direct and hard impingement of the flame against the firetube
A continuous spark ignition system, or a spark igniter to relight the pilot if it goes out, is also useful.
Orifices on the air-gas mixers and pilots should be cleaned regularly to prevent burner failures.
The following temperatures in the reboiler should not be exceeded:

Type of Glycol Thermal Decomposition Temperatures
Ethylene 3290F (1650C)
Diethylene 3280F (1640C)
Triethylene 4040F (2070C)
Image
Table.5-6. Thermal decomposition of Glycols.

Excessive discoloration and very slow degradation will result when the reboiler bulk temperature is maintained about 100F (50C) in excess of the above listed temperatures. If coke, tarry products, and/or salt deposit on the firetube, the heat transfer rate is reduced and a tube failure can result.
Localized overheating, especially where salt accumulates, will decompose the glycol.
An analysis of the glycol determines the amounts and types of these contaminants.
Salt deposits can also be detected by shutting off the burner on the reboiler at night and looking down the firebox. A bright red-glowing light will be visible at spots on the tubes where salt deposits have collected. These deposits can cause a rapid firetube burnout, particularly if the plant inlet separator is inadequate and a slug of salt water enters the absorber.

Coke and tarry products present in the circulating glycol can be removed by good filtration.
More elaborate equipment is needed to remove the salt. Contaminates, which have already deposited on the firetube and other equipment, can only be removed by using chemicals.
The heating process must be thermostatically controlled and fully automatic.
The reboiler temperature should be occasionally verified with a test thermometer to make sure true readings are being recorded.
If water and/or hydrocarbons enter the reboiler from the absorber, it may be impossible to raise the temperature until this problem is corrected. Standard orifices furnished for reboiler burners are sized for 1000–1100 Btu/scf of gas. If the rating of the fuel gas is less than this, it may be necessary to install a larger orifice or drill out the existing orifice to the next higher size.

During a unit startup, it is imperative the reboiler temperature be up to the desired operating level before flowing gas through the absorber.
The reboiler must be horizontal when erected. A nonhorizontal position can cause a firetube burnout. The reboiler should also be located close enough to the absorber to prevent excessive cooling of the lean glycol during cold weather. This will prevent hydrocarbon condensation and high glycol losses in the absorber.

5.15.7 Stripping Gas
Stripping gas is an optional item used to achieve very high glycol concentrations which cannot be obtained with normal regeneration. It will provide the maximum dew point depression and greater dehydration. Stripping gas is used to remove the residual water after the glycol has been reconcentrated in the regeneration equipment. It is used to provide intimate contact between the hot gas and the lean glycol after most of the water has been removed by distillation.
Lean glycol concentrations in the range of 99.5 - 99.9% and dew-point depressions of 1400F and above have been reported. There are several methods of introducing stripping gas into the system.
One method is to use a vertical tray or packed section in the downcomer between the reboiler and storage tank where the dry gas strips the additional water out of the regenerated glycol.
The glycol from the reboiler flows down through this section, contacts the stripping gas to remove the excess water, and goes into the storage tank.
Another method is to use glycol stripping gas sparger in the reboiler beneath the firetube.
As the glycol flows through the reboiler, gas is injected into this vessel and is heated by the glycol. Stripping gas contacts the glycol in the reboiler and removes some of the additional water.
Gas then passes out the stripper to the waste pit. The lean glycol flows from the reboiler down into the storage tank.
Stripper gas is normally taken from the reboiler fuel gas line (if dehydrated gas) at the fuel drip pot pressure. Air or oxygen should not be used. Stripping gas is usually controlled by a manual valve with a pressure gauge to indicate the flow rate through an orifice.
Stripping gas rate has the following characteristics:
Will vary according to the lean concentration desired and the method of glycol-gas contact
Usually between 2 and 10 scf/gallon of glycol circulated
Should not get high enough to flood the stripper and blow glycol out to the pit.
When stripping gas is used it is necessary to provide a more reflux in the still column to prevent excessive glycol losses. This is usually provided by using a cool glycol condenser coil in the stripper.

5.15.8 Glycol Circulating Pump
A circulating pump is used to move glycol through the system. It can be powered by electricity, gas, steam, or gas and glycol, depending upon the operating conditions and unit location.

Glycol-Gas Powered Pump
Powered by gas entrained in the wet glycol leaving the contactor. Where it utilizes the rich glycol under pressure in the absorber to furnish part of its required driving energy.
Does not require contactor glycol liquid level control, dump valve, or external power (electricity).
Gas consumption is relatively low (at 1000 psi operating pressure on the absorber, the volume of gas required is approximately 5.5 scf per gallon of lean glycol circulated)
Have few moving parts, which translates into less wear and simplified repairs.
Contact with hydrocarbon distillate, which may be entrained in glycol passing through the pump, swells o-ring seals in the pumps causing premature pump failure.
Generally used on small isolated systems
Temperatures above 200 0F damage o-ring seals.
Controls are serviceable, dependable, and, if adjusted properly, should give a long, trouble free operation, and inexpensive.

Electric Driven Positive Displacement Piston/Plunger Pump
Usually used in large installations
Require a small glycol leak in the piston rod packing for lubrication.
Resilient to hydrocarbon distillate, grit, and debris that would damage the glycol-gas powered pumps.

Pump rate should be commensurate with the gas volume being processed. Proportioning adjustments allow increased gas-glycol contact time in the absorber.
When the pump check valves become worn or clogged, the pump will operate normally except no fluid will go to the absorber. Even a pressure gauge will indicate a pumping cycle. The only evidence of this type of failure is little or no dew-point depression. One sure way to check the volume flowing is to close the valve on the absorber outlet and calculate the flow by measuring the rise in the gauge glass (if one is available) versus the amount pumped normally
One of the most common sources of glycol loss occurs at the pump packing gland. If the pump leaks over one quarts (0.25 gallon) of glycol per day, the packing needs to be replaced. An adjustment will not recover the seal. Packing should be installed hand-tight and then backed off one complete turn. If the packing gets too tight, the pistons can score and require replacement.
Glycol circulation rate of 2 to 3 gallons/lb of water to be removed is sufficient to provide adequate dehydration. An excessive rate can overload the reboiler and reduce the dehydration efficiency.
The rate should be checked regularly by timing the pump to make sure it is running at the proper speed.
Proper pump maintenance will reduce the operating costs. When the pump is not working the whole system must be shut down because the gas cannot be dried effectively without a good continuous flow of glycol in the absorber. Pumps should be lubricated regularly.
If there is insufficient glycol circulation:
Check the pump suction strainer for plugging and/or open the bleeder valve to eliminate air lock.
Glycol strainers should be regularly cleaned to avoid pump wear and other problems.

The maximum operating temperature of the pump is limited by the moving O-ring seals and nylon D slides. A maximum temperature of 2000F (940C) is recommended.
Packing life will be extended considerably if the temperature is held to a maximum of 1500F (660C). Therefore, sufficient heat exchange is necessary to keep the dry, lean glycol below these temperatures when it goes through the pump.
The pump is usually the most overworked and overused piece of equipment in the glycol process system.
The glycol system usually contains a second spare pump to avoid shutdowns when the primary pump fails. It is not uncommon for operators to use the second pump to send more glycol to the absorber to avoid wet sales gas problems. This procedure just increases operating problems. All of the other process variables should first be checked before a second pump is used.
A pressure gauge is furnished on the discharge side of the pump. Pressure gauge can be used to see that the pump is working by watching the gauge “kick” as the pump piston strokes. The sensing element in the pressure gauge is a bourdon tube. The flexing or movement of this tube indicates the pressure. A bourdon tube will fatigue or fail if subjected to continuous fluctuations in pressure on the pump discharge. Pressure should be kept off the gauge except when testing the unit or to determine glycol loss from the gauge failure.

5.15.9 Flash Tank or Glycol-Gas Separator
The flash tank, or glycol-gas separator, is an optional piece of equipment used to recover the off-gas from the glycol-powered pump and the gaseous hydrocarbons from the rich glycol. The recovered gas can be used as fuel to the reboiler and/or stripping gas. Any excess gas is usually discharged through a back pressure valve. The flash tank will keep volatile hydrocarbons out of the reboiler. The separator usually works best in a temperature range of 1300F to 1700F (550C to 770C). A two-phase separator, with at least a five minute retention time, can be used to remove the gas. If liquid hydrocarbons are present in the rich glycol, a three-phase separator should be used to remove these liquids before they get in the stripper and reboiler. A liquid retention time of 20 to 45 minutes, depending on the type of hydrocarbons, API gravity, and the amount of foam, should be provided in the vessel. Vessels should be located ahead of or behind the preheat coil in the storage tank, depending on the type of hydrocarbons present.

5.15.10 Gas Blanket
A gas blanket prevents air from contacting glycol in the reboiler and storage tanks. A small amount of low-pressure gas is bled into the storage tank. Gas is piped from the storage tank to the bottom of the stripper and it passes on overhead with the water vapor. Elimination of air helps prevent glycol decomposition by slow oxidation. The gas blanket equalizes the pressure between the reboiler and storage tank. The gas blanket also prevents the liquid seal from breaking down between these two vessels.

5.15.11 Reclaimer
The reclaimer purifies the glycol for further use by vacuum distillation. Clean glycol is driven off and all the dirty sludge is left in the vessel and then washed to the sewer. It is normally used only in very large glycol systems.

5.16 Improving Glycol Filtration
Filters will extend the life of pumps, will prevent an accumulation of solids in the absorberand in the regeneration equipment
Solids that settle out on metal surfaces will frequently set up cell corrosion. Filters remove the solids to also eliminate fouling, foaming, and plugging. Filters should be designed to remove all solid particles 5 microns and larger. They should be able to operate up to a differential pressure of 20–25 psi without loss of seal or channeling of flow. Normal differential pressure is 3-6 psi.
An internal relief valve with a setting of about 25 psi and differential pressure gauges are very helpful. New elements should be installed before the relief valve opens. When this differential increases to 15-20 psi, the element should be replaced. Filters are not usually placed in the rich glycol line, but the lean glycol can also be filtered to help keep the glycol clean. Frequent filter changes may be needed during unit startup or when neutralizers are added, to control the glycol pH.
It is important to know when and how to change elements to keep air out the glycol system. Valves and gauges should be inspected occasionally for corrosion and scale buildup. To determine the proper use of filter elements, cut them to the core and inspect them. If they are dirty throughout, the filter is being used properly. If the element is clean on the inside, an element with a different micron size may be needed. It is also a good practice to occasionally scrape some sludge from a dirty element and have it analyzed. This will help establish the types of contaminants present. A record of the number of elements replaced will establish the amount of contaminants present.

Use of Carbon Purification
Activated carbon can effectively eliminate most foaming problems by removing the hydrocarbons, well-treating chemicals, compressor oils, and other troublesome impurities from the glycol.
Two ways glycol purification can be achieved are as follows:
a- One method is to use two carbon towers installed in series but piped so they can be taken off-stream or interchanged without difficulty. In large systems about 2% of the total glycol flow should pass through the carbon towers. In small systems 100% of the total glycol flow should pass through the carbon towers. Each carbon bed should be sized to handle 2 gallons of glycol per square foot of cross-sectional area per minute. Towers should be designed to permit back flushing with water to remove the dust after the carbon is loaded. To achieve this, a retainer screen, with a smaller mesh size than the carbon should be installed above the carbon bed between the liquid inlet distributor and the outlet water drain nozzle to hold the carbon to the vessel. The liquid distributor is needed to avoid glycol channeling through the carbon. The inlet water nozzle for back-flushing should be placed below the screen in the bottom of the tower. The appearance of the glycol can generally be used to determine when the carbon needs to be regenerated or replaced. The pressure drop across the carbon bed can also be used. The pressure drop normally across the carbon bed is only 1 or 2 psi. When the pressure drop reaches 10 to 15 psi, the carbon is usually completely plugged with impurities. Steam cleaning can sometimes be used to regenerate the carbon by removing the impurities. However, this can be hazardous and offers only limited success.
b- Another method of purification is to use activated carbon in elements, such as Peco-Char.
Either purification system should be placed downstream from the solids filter. This will increase the carbon adsorptive efficiency and life.


Adsorption
5.17 Overview of Adsorption Processes
The two types of adsorption are physical adsorption and chemisorption. In physical adsorption, the bonding between the adsorbed species and the solid phase is called van der Waals forces, the attractive and repulsive forces that hold liquids and solids together and give them their structure. In chemisorption, a much stronger chemical bonding occurs between the surface and the adsorbed molecules. This chapter considers only physical adsorption, and all references to adsorption mean physical adsorption.
Adsorption is a physical phenomenon that occurs when molecules of a gas are brought into contact with a solid surface and some of them condense on the surface.
Physical adsorption is an equilibrium process like vapor−liquid equilibria. Thus, for a given vapor-phase concentration (partial pressure, and temperature), an equilibrium concentration exists on the adsorbent surface that is the maximum concentration of the condensed component (adsorbate) on the surface.
Dehydration of a gas (or a liquid hydrocarbon) with a dry desiccant is an adsorption process in which water molecules are preferentially held by the desiccant and removed from the gas stream.
Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas. Water forms a thin film that is held to the desiccant surface by forces of attraction, not by chemical reaction.
Desiccant is a solid, granulated dehydrating medium with a large effective surface area (large number of small pores) per unit weight.
Figure 5.47 shows the equilibrium conditions for water on a commercial molecular sieve. Such curves are called isotherms. The figure is based upon a water−air mixture but is applicable to natural gas systems. The important parameter is the partial pressure of water.
The achievement of equilibrium on a small surface displays the following pattern:
Some passing molecules will condense on the surface (physical as opposed to chemical absorption).
After some finite time the molecule may acquire sufficient energy to leave and be replaced by another.
After sufficient time, a state of equilibrium will be reached wherein the number of molecules leaving the surface will equal the number arriving.

Because adsorbate concentrations are usually low, generally only a few layers of molecules will build up on the surface. Thus, adsorption processes use solids with extremely high surface-to-volume ratios. Typical desiccants (zeolite) might have as much as 4 million square feet of surface area per pound. In the case of molecular sieves, the adsorbent consists of extremely fine zeolite particles held together by a binder. Therefore, adsorbing species travel through the macropores of the binder into the micropores of the zeolite. Adsorbents such as silica gel and alumina are formed in larger particles and require no binder.
Pore openings that lead to the inside of commercial adsorbents are of molecular size.
Molecular sieves have an extremely narrow pore distribution, whereas silica gel and alumina have wide distributions. However, a molecular sieve binder, which is usually about 20% of the weight of the total adsorbent, has large pores capable of adsorbing heavier components.
Two steps are involved in adsorbing a trace gas component. The first step is to have the component contact the surface and the second step is to have it travel through the pathways inside the adsorbent. Because this process is a two-step process and the second step is relatively slow, solid adsorbents take longer to come to equilibrium with the gas phase than in absorption processes.
In addition to concentration (i.e., partial pressure for gases), two properties of the adsorbate dictate its concentration on the absorbent surface: polarity and size. Unless the adsorbent is nonpolar, which is not the case for those used in gas plants, polar molecules, like water, will be more strongly adsorbed than weakly polar or nonpolar compounds. Thus, methane is displaced by the weakly polar acid gases that are displaced by the strongly polar water.
How size affects adsorption depends upon the pore size of the adsorbent. An adsorbate too large to fit into the pores adsorbs only on the outer surface of adsorbent, which is a trivial amount of surface area compared with the pore area.
If the pores are sufficiently large to hold different adsorbates, the less volatile, which usually correlates with size, adsorbates will displace the more volatile ones. Therefore, ethane is displaced by propane.
In commercial practice, adsorption is carried out in a vertical, fixed bed of adsorbent, with the feed gas flowing down through the bed. As noted above, the process is not instantaneous, which leads to the formation of a mass transfer zone (MTZ) in the bed. Figure 5.48 shows the three zones in an adsorbent bed:
Image
Fig. 5.47 Water loading on UOP Adsorbent 4A-DG MOLSIV Pellets. Activation conditions for the adsorbent were 662°F (350°C) and less than 10 microns Hg.

1. The equilibrium zone, where the adsorbate on the adsorbent is in equilibrium with the adsorbate in the inlet gas phase and no additional adsorption occurs
2. The mass transfer zone (MTZ), the volume where mass transfer and adsorption take place
3. The active zone, where no adsorption has yet taken place.
In the mass transfer zone (MTZ), the concentration drops from the inlet value, yin, to the outlet value, yout, in a smooth S-shaped curve. If the mass transfer rate were infinite, the MTZ would have zero thickness. The MTZ is usually assumed to form quickly in the adsorption bed and to have a constant length as it moves through the bed, unless particle size or shape is changed.
The length of the MTZ is usually 0.5 to 6 ft (0.2 to 1.8 m), and the gas is in the zone for 0.5 to 2 seconds. To maximize bed capacity, the MTZ needs to be as small as possible because the zone nominally holds only 50% of the adsorbate held by a comparable length of adsorbent at equilibrium. Both tall, slender beds, which reduce the percentage of the bed in the MTZ, and smaller particles make more of the bed effective. However, smaller particle size, deeper beds, and increased gas velocity will increase pressure drop.
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Fig. 5.48 Vapor-phase concentration profile of an adsorbate in the three zones of an adsorption bed.

MTZ lengths can be obtained experimentally for various materials and systems and used in graphical correlations for design purposes.
As the flow of gas continues, the MTZs move downward through the bed and water displaces all of the previously adsorbed gas until, finally, the entire bed is saturated with water vapor. When the bed is completely saturated with water vapor, the outlet gas is just as wet as the inlet gas.
Towers must be switched from the adsorption cycle to the regeneration cycle (heating and cooling) before the desiccant bed has become completely saturated.
Thus, higher velocities increase the MTZ thickness.
MTZ is a function of the following factors:
Adsorbent, adsorbent particle size, fluid velocity, fluid properties, adsorbate concentration in the entering fluid, adsorbate concentration in the adsorbent if it is not fully reactivated, temperature, pressure, and past history of the system.
5.18 Properties of Industrial Adsorbents for Dehydration
Three types of commercial adsorbents are in common use in gas processing plants:
• Silica gel, which is made of pure SiO2
• Activated alumina, which is made of Al2O3
• Molecular sieves, which are made of alkali aluminosilicates and can be altered to affect adsorption characteristics.
Table 5.7 lists the more important properties of three adsorbents compiled primarily from commercial literature. The properties are representative and vary between manufacturers.
Silica Gel Activated Alumina Molecular Sieve 4A
Shape Spherical Spherical Pellets (extruded
cylinders) and beads
Bulk density lb/ft3 (kg/m3) 49 (785) 48 (769) 40 −45 (640 − 720)
Particle size 4 − 8 mesh
5 −2 mm 7−14 mesh, 1/8-inch,
3/16-inch, 1/4-inch
diameter (3-mm,
5-mm, 6-mm) 1/16-inch,1/8-inch,1/4-inch diameter cylinders
(1.6-mm, 3.2-mm, 6-mm)
Packed bed % voids 35 35 35
Specific heat
Btu/lb-°F (kJ/kg-K) 0.25 (1.05) 0.24 (1.00) 0.24 (1.00)
Surface area m2/g 650 − 750 325 – 360 600 – 800
Pore volume cm3/g 0.36 0.5 0.28
Regeneration
temperature, °F (°C) 375 (190) 320 to 430 (160 to 220) 400 to 600 (200 to 315)
Average pore diameter (A0) 22 NA 3,4,5,10
Minimum dew point temperature of effluent, °F (°C) −80 (−60) −100 (−75) −150 (−100)

Average minimum
moisture content of
effluent gas, ppmv 5 – 10 10 – 20
lb/MMscf ~ ppmv / 21.4 0.1
Image
Table 5.7. Representative Properties of Commercial Silica Gels, Activated Alumina, and Molecular Sieve 4A
5.19 Solid Bed Adsorption Process
Although this discussion uses molecular sieve as the example of an adsorbent to remove water, with the exception of regeneration temperatures, the basic process is the same for all gas adsorption processes. Figure 5.49 shows a schematic of a two-bed adsorber system. One bed, adsorber #1 dries gas while the other bed, adsorber #2, goes through a regeneration cycle. The wet feed goes through an inlet separator that will catch any entrained liquids before the gas enters the top of the active bed. Flow is top-down to avoid bed fluidization. The dried gas then goes through a dust filter that will catch fines before the gas exits the unit. This filter must be kept working properly, especially if the gas goes on to a cryogenic section with plate-fin heat exchangers, as dust can collect in the exchangers and reduce heat transfer and dramatically increase pressure drop.
5.20 Principles of Operation
5.20.1 Introduction
The adsorption process is a batch process, with multiple desiccant beds used in cyclic operation to dry the gas on a continuous basis. The number and arrangement of the desiccant beds may vary from two towers, adsorbing alternatively (Figure 5-49), to many towers.
Three separate functions or cycles must alternatively be performed in each dehydrator tower:
Adsorbing or gas-drying cycle
Heating or regeneration cycle
Cooling cycle (prepares the regenerated bed for another adsorbing or gas-drying cycle)

Image
Fig. 5.49 Schematic of a two-bed adsorption unit. Valves are set to have absorber #1 in drying cycle and absorber #2 in regeneration cycle.
5.20.2 Drying Cycle
Several automatically operated switching valves and a controller route the inlet gas and
regeneration gas to the right tower at the proper time.
As the wet gas flows downward through the tower on the adsorption cycle, each of the adsorbable components is adsorbed at a different rate.
The water vapor is immediately adsorbed in the top layers of the desiccant bed.
Some of the light hydrocarbon gases and heavier hydrocarbons moving down through the bed are also adsorbed.
Heavier hydrocarbons will displace the lighter ones in the desiccant bed as the adsorbing cycle proceeds.
As the upper layers of desiccant become saturated with water, water in the wet gas stream begins displacing the previously adsorbed hydrocarbons in the lower layers.
For each component in the inlet gas stream, there will be a section of bed depth, from top to bottom, where the desiccant is saturated with that component and where the desiccant below is just starting to adsorb it.

5.20.4 Regeneration Cycle
One regeneration-gas supply scheme consists of taking a portion (5 to 15%) of the entering wet gas stream across a pressure-reducing valve that forces a portion of the upstream gas through the regeneration system. (Sales gas is sometimes used instead of a slip stream. The sales gas stream has the advantage of being free of heavier hydrocarbons that can cause coking.) The regeneration gas is heated to about 600°F (315°C) to both heat the bed and remove adsorbed water from the adsorbent. If COS formation is a problem, it can be mitigated by lowering regeneration temperatures to 400 °F (200°C) or lower, provided sufficient time for regeneration is available, or by switching to 3A. Regeneration gas enters at the bottom of the bed countercurrent to flow during adsorption to ensure that the lower part of the bed is the driest and that any contaminants trapped in the upper section of the bed stay out of the lower section.
Initially, the hot regeneration gas must heat up the tower and the desiccant.
The water begins vaporizing when the effluent hot gas temperature reaches between 240 0F and 250 0F. The bed continues to heat up slowly as the water is being desorbed or driven out of the desiccant. After all the water has been removed, heating is maintained to drive off any heavier hydrocarbons and contaminants that would not vaporize at lower temperatures.
The desiccant bed will be properly regenerated when the outlet gas (peak-out) temperature has reached between 350 0F and 550 0F.
After the heating cycle, the desiccant bed is cooled by flowing unheated regeneration gas until the desiccant is sufficiently cooled. Gas flow during this step can be concurrent or countercurrent.
All of the regeneration gas used in the heating and cooling cycles is passed through a heat exchanger (normally an aerial cooler) where it is cooled to condense the water removed from the regenerated desiccant bed.
This water is separated in the regeneration gas separator, and the gas is mixed with the incoming wet gas stream. This entire procedure is continuous and automatic.
The regeneration gas velocity is important, especially when effluent moisture contents below 1 ppm are needed (lb/MMscf ~ ppmv / 21.4). Figure 5.58 or for conservative operation velocities should not be less than 10 ft./sec., to prevent hot gases from channeling through the bed, leaving excess water in the bed after regeneration which results in poor dehydration.
Image
Table 5.8 lists design parameters that are guidelines for typical molecular sieve dehydrators

Feed rate 10 to 1500 MMscfd (0.3 to 42 MMSm3/d)
Superficial velocity Approximately 30 to 35 ft/min (9 to 11 m/min)
Pressure drop Approximately 5 psi (35 kPa), not to exceed 10 psi (69 kPa)
Cycle time Four to 24 hours; 8 or a multiple thereof is common
Temperatures and pressures of adsorption
Temperatures and pressures of regeneration Temperatures: 50 to 115°F (10 to 45°C) Pressures: to 1500 psig (100 barg),
Temperatures: 400 to 600°F (200 to 315°C) Pressures: Adsorption pressure or lower.
Table. 5.8. Typical Operating Conditions for Molecular Sieve Dehydration Units
5.21 Adsorption System Performance
Advantages Disadvantages
- Can achieve very low dew points (-150 0F “less than 1 ppm”)
- High contact temperatures are possible
- Adaptable to large rate and load changes - High initial cost
- Batch process
- Experiences high-pressure drop through the bed
- Desiccant is sensitive to poisoning with liquids or other impurities in the gas
Image
Table.5.9. Adsorption system performance.

5.22 Effect of Process Variables
For the commonly used 4A molecular sieve, the Engineering Data Book suggests that the design water content of a molecular sieve when at equilibrium with saturated gas at 75°F (24°C) will be 13 lb H2O/100 lb sieve compared with a new molecular sieve, which holds about 20 lb H2O/100 lb sieve. Two main factors affect this number: water content of entering gas and adsorption temperature.

5.22.1 Quality of Inlet Gas
Performance of dry bed dehydrator is affected by moisture content and components in the natural gas stream. The relative saturation of the inlet gas determines the size of a given desiccant bed, and affects the transfer of water to the adsorbent.
If the gas comes to the dehydration unit fully saturated, which is often the case, cooling the gas and removing the condensed water before the gas enters the bed lowers water loading.
Compounds in produced natural gas adversely affect performance of the dry bed dehydrator.
Components of concern are carbon dioxide, heavy hydrocarbons, and sulfur-bearing compounds
The greater the molecular weight of a compound, the greater its adsorption potential.
Trace amounts of oxygen affect bed life and performance in a variety of ways. At the normal regeneration gas temperature of 600°F for molecular sieves, 2 moles of oxygen react with methane to form 2 moles of water and 1 mole of CO2. As this reaction is exothermic, higher amounts of oxygen in the gas can lead to temperatures above the design temperature of the molecular sieve vessel. When oxygen is present, the temperature of the beds during regeneration must be monitored for safety reasons. Oxygen undergoes partial oxidation reactions with heavier hydrocarbons, which are adsorbed in the binder, to form alcohols and carbocylic acids that ultimately turn to water and CO2. If H2S is present, it undergoes oxidation to elemental sulfur, sulfur dioxide, and water. Oxygen concentrations greater than 20 ppmv also generate olefins that become coke in the bed. These reactions reduce molecular sieve capacity by forming solid deposits and by causing incomplete removal of water during regeneration because the partial pressure of water is higher (see Figure 5.47).
To avoid the above reactions as well as COS formation, regeneration temperatures are lowered to the 300 to 375°F range. However, this range increases the required regeneration time and the amount of regeneration gas used, which increases recompression cost.
As in all processes, ensuring that the beds are protected from entrained water and hydrocarbons is important. Even trace amounts of entrained water load the bed quickly and increase the regeneration heat load.

5.22.2 Temperature
Operation is very sensitive to the temperature of the incoming gas.
Adsorption efficiency decreases as the temperature increases.
Molecular sieves and most other adsorbents have significantly higher adsorptive capacity at low temperatures. Cooling the gas to lower temperature in order to improve adsorption is limited due to condensation and/or hydrate formation.
Temperature of the regeneration gas that commingles with the incoming wet gas ahead of the dehydrators is important. The temperature must remain within 10 0F to 15 0F, otherwise liquid water and hydrocarbons will condense as the hotter gas stream cools. Condensed liquids that strike the bed can shorten the solid desiccant’s life.
Temperature of the hot gas entering and leaving a desiccant tower during the heating cycle affects plant efficiency and the desiccant life. High regeneration gas temperature assures good removal (desorption) of water and contaminants from the bed. 450 0F to 600 0F is usually used as a regeneration temperature.
Desiccant bed temperature reached during the cooling cycle is important.
If wet gas is used to cool the desiccant: Terminate the cooling cycle when the bed reaches 125 0F. Additional cooling may cause water to be adsorbed from the wet gas stream and preload (presaturate) the bed before the next adsorption cycle begins. If dry gas is used to cool the desiccant: Terminate the cooling cycle within 10 0F to 20 0F of the incoming gas temperature. It maximizes adsorption capacity of the bed.
The temperature of the regeneration gas going through the regeneration gas scrubber should be held low enough to condense and remove the water and hydrocarbons without causing hydrate problems.

5.22.3 Pressure
The adsorption capacity of a dry bed unit decreases as pressure is lowered and with usage.
Operating dry bed dehydrators well below the design pressure requires the desiccant to work harder to remove the additional water, and maintain the desired effluent dew point. With the same volume of incoming gas, the increased gas velocity occurring at the lower pressure could affect the effluent moisture content, and damage the desiccant.
At pressure above 1300 to 1400 psia, the co-adsorption effects of hydrocarbons are very significant.

5.22.4 Cycle Time
In principle, beds can be run until the first sign of breakthrough. This practice maximizes cycle time, which extends bed life because temperature cycling is a major source of bed degeneration, and minimizes regeneration costs. However, most plants operate on a set time cycle to ensure no adsorbate breakthrough. Adsorbent capacity is not a fixed value and declines with usage. For the first few months of operation, a new desiccant normally has a high capacity for water removal. If a moisture analyzer is used on the effluent gas, a much longer drying cycle can be achieved. As the desiccant ages, the cycle time can be shortened to save regeneration fuel costs and improve the desiccant life. Common cycle times are as follows: 8 hours on stream - 5 to 6 hours heating - 2 to 3 hours cooling

5.22.5 Gas Velocities
As the gas velocity during the drying cycle decreases, the ability of the desiccant to dehydrate the gas increases. On the surface, it would seem desirable to operate at minimum flow rates to utilize the desiccant fully.
Low linear velocities:
Require towers with large cross-sectional areas to handle a given gas flow
Allow wet gas to channel through the desiccant bed and thus not be properly dehydrated.
Compromise must be made between the tower diameter and the maximum utilization of the desiccant.
High linear velocities:
Lower adsorption efficiency
May cause desiccant damage
Higher inlet compression discharge pressures to maintain the same refrigeration requirements and outlet pressure.
Increased mechanical load on the adsorbent, which leads to particle breakdown and causes further increases in pressure drop.

5.22.6 Source of Regeneration Gas
Source of regeneration gas depends on plant requirements and the availability of a suitable gas stream. Regeneration gas should be dry when low effluent moisture contents (in the range of 0.1 ppm) are required. Plant tail gate gas can normally be used.
If only moderate drying is required, a portion of the wet feed gas can be used.
Figure 5-52 is an equilibrium diagram showing lines of constant water loading. For example:
A molecular sieve bed at 100 0F in equilibrium with a gas having a -80 0F water dew point will contain about 4 wt.% water. Equilibrium curves for a given adsorbate-adsorbent can be used to estimate the regeneration conditions necessary to provide the required outlet conditions. For example, If the regeneration gas is taken from inlet gas with a dew point of 40 0F and is heated to 450 0F, the mol sieve will contain 3 wt.% water after regeneration.
If the gas to be treated is at 100 0F, the intersection of the 3 wt.% line with an adsorbent temperature of 100 0F gives the minimum attainable dew point at -95 0F.
If this dew point is not satisfactory, either the regeneration gas must be heated to above 450 0F or a gas of a higher dew point (e.g., residue gas) must be used for regeneration gas.

Image
Fig. 5-52 Equilibrium diagram showing lines of constant water loading for a type 4a molecular sieve.

5.22.7 Direction of Gas Flow
Direction of flow during the drying cycle is downward, which:
Permits higher velocities without lifting or fluidizing the desiccant bed (fluidization can severely damage the desiccant)
Direction of flow during the heating cycle is counter-current to the direction of the adsorption flow.
It permits better reactivation of the lower portion of the desiccant bed, which must perform the super-dehydration during the drying cycle, especially in cryogenic plants.
If flow is co-current, all water and/or other contaminants must move through the entire bed, thus causing additional desiccant contamination and requiring longer regeneration times.
Direction of flow during the cooling cycle:
When dry gas is used, the flow direction is counter current to the adsorption flow, thus simplifying piping and valve configuration.
When wet gas is used, the flow direction is in the same direction as the adsorption flow so that the water adsorbed during the cooling cycle as the desiccant cools will preload on the inlet end of the bed.
If counter current flow is used in cooling with wet gas, water is deposited on the exit end of the bed. When the next adsorption cycle begins, the wet gas is immediately dried, but as the dry gas continues to move down through the bed, it picks up some of the water deposited during the cooling cycle and sometimes puts too much moisture in the effluent stream.
If wet gas is used, the additional water load, deposited during the cooling cycle, should be included when the amount of desiccant needed for dehydration is calculated.

5.22.8 Desiccant Selection
Desiccant selection is based upon:
Economics
Process conditions
Desiccants are usually interchangeable. Equipment designed for one desiccant can often operate effectively with another. No desiccant product will remain effective with massive liquid carryovers.
All desiccants are damaged by heavy impurities carried into the bed with gases. These include:
Crude oil and condensate
Glycols and amines
Most corrosion inhibitors
Well treating fluids
All desiccants exhibit a decrease in capacity (design loading) with an increase in temperature.
Molecular sieves are less affected and Aluminas are most affected.

Aluminas and molecular sieves act as a catalyst with H2S to form COS, which deposits sulfur on the desiccant bed during regeneration. (Carbonyl sulfide (COS) is formed in the following reaction: H2S + CO2 ↔ H2O + COS. Its concentration in feed gas is normally extremely low.)
Alumina gels, activated aluminas, and molecular sieves are all chemically attacked by strong mineral acids and thus decrease their adsorptive capacity.
Table 5-7 provides certain physical characteristics of the more common solid desiccants.

5.22.8.1 Molecular Sieves
Molecular sieves are a class of aluminosilicates. They produce the lowest water dewpoints, and can be used to simultaneously sweeten and dry gases and liquids. Their equilibrium water capacity is much less dependent on adsorption temperature and relative humidity. They are usually more expensive.
Molecular sieves offer the highest adsorptive capacity of all desiccants when the feed gas is at very high temperatures or at low relative saturation. It is the only desiccants capable of dehydrating gas to less than 1 ppm of water content are required for cryogenic temperatures (lb/MMscf ~ ppmv / 21.4) (dew points down to -150 0F). Therefore, for gas going into cryogenic processing, the only adsorbent that can obtain the required dehydration is a molecular sieve. Of these, 4A is the most common, but the smaller pore 3A is sometimes used. It has the advantage of being a poorer catalyst for generation of COS if both H2S and CO2 are present because a portion of the more active sodium cations in 4A has been replaced with potassium. (Carbonyl sulfide (COS) is formed in the following reaction: H2S + CO2 ↔ H2O + COS. The equilibrium constant for the reaction is of the order of magnitude of 10−6 at adsorption temperatures but increases to 10−4 at regeneration temperatures.)
If both oxygen and H2S are present 3A reduces the production of elemental sulfur that can block adsorbent pores. However, plant operators usually have little incentive to use 3A for dehydrating gas going to hydrocarbon recovery.

5.22.8.2 Silica Gel
Silica Gel and Alumina are generally offer a lower first cost.
Silica Gel is essentially pure silicon dioxide, SiO2. It is used for gas and liquid dehydration and hydrocarbon (iC5+) recovery from natural gas. When used for hydrocarbon removal, the units are often called HRUs (Hydrocarbon Recovery Units) or SCUs (Short Cycle Units). When used for dehydration, silica gel will give outlet dewpoints of approximately –60°F.
Silica gel can be regenerated to a lower water content than molecular sieves and at much lower temperatures (400 0F for gels versus 500 0 to 600 0F for sieves).
It shatters in the presence of free water or light hydrocarbon liquids. The problem is minimized by using a 4 to 6 inch buffer bed of mullite ball (or equivalent) to protect the silica gel from direct contact. Silica gels are used mostly where a high concentration of water (>1 mol%) vapor is present in the feed, and low levels of water in the dehydrated gas are not needed. They are relatively noncatalytic compounds.

5.22.8.3 Aluminas
Alumina is a hydrated form of alumina oxide (Al2O3). It is used for gas and liquid dehydration and will give outlet dewpoints of about –90°F. Less heat is required to regenerate alumina and silica gel than for molecular sieve, and the regeneration temperature is lower. Molecular sieves give lower outlet water dewpoints. Aluminas are very polar and strongly attract water and acid gases. They are used for moderate levels of water in the feed when low levels of water in the product are not required. They have the highest mechanical strength of the adsorbents considered here.

5.22.8.4 Desirable Characteristics of Solid Desiccants
High adsorptive capacity (lb/lb), which reduces contactor size.
Easy regeneration, for simplicity and economics of operation.
High rate of adsorption, which allows higher gas velocities and thereby reduces contactor size.
Low resistance to gas flow, to minimize gas pressure drop through the unit.
High adsorptive capacity retained after repeated regeneration, allowing smaller initial charge and longer service before replacement.
High mechanical strength, to resist crushing and dust formation.
Inert chemicals, to prevent chemical reactions during adsorption and regeneration.
Volume unchanged when product is wet, which would otherwise necessitate costly allowance for expansion.
Noncorrosive and nontoxic properties, eliminating the necessity for special alloys and costly measures to protect the operator’s safety.
Low cost, to reduce initial and replacement costs.

5.22.9 Effect of Regeneration Gas on Outlet Gas Quality
Regeneration gas desorbs molecular sieve beds chromatographically in the reserve order of the adsorption bead. For example:
Adsorbed methane and ethane would be desorbed first, then propanes and heavier hydrocarbons, then carbon dioxide, followed by any hydrogen sulfide that might have been in the inlet gas, and last of all, the water. The effect of the concentration of these impurities in the regeneration gas stream may be significant when regeneration gas is 10 to 15% of the net inlet gas.
In the regeneration circuit, the bulk of the water and some heavy hydrocarbons are removed from the system:
They may render the sales gas off specification for a short period.
The peak of ethane could cause the sales gas to exceed its heating value.
Concentrations of 3 to 4 ppm of H2S can be concentrated up to 20 times that amount, and thus render the composite stream far off spec.
Figure 5-49 shows the cooled regeneration gas stream is recombined with the main gas inlet to be processed. This recycle essentially eliminates the problem of making the sales gas off-specification, but it adds cost to the extent that the main gas processing capacity must be increased appropriately.
If the sales gas limits are no problem, or if there is other downstream processing, the cooled, scrubbed regeneration gas may be admitted directly to the dried outlet gas without this recycle.

5.22.10 Pressure Drop Considerations
To achieve acceptable dehydration and extend the life of the desiccant, the pressure drop through the dehydration tower should not exceed 8 psi.
The combination of feed rate, pressure drop, and adsorbent crush strength dictates the adsorption bed geometry. As noted in the above discussion regarding minimizing MTZ thickness, the bed diameter should be kept small. This feature also reduces the wall thickness of the high-pressure vessels and increases the superficial velocity, which improves mass transfer in the gas phase. However, it does not affect intra-particle mass transfer, which is the slower of the two processes.
Trent (2004) presents data that show a change in the L/D from 0.8 to 2.7 in the bed increases the useful adsorption capacity from 8.7 to 10.0 wt% in useful water capacity for an equal amount of gas dried. However, the pressure drop increases from 0.4 to 4.3 psi.
Pressure drop through the tower can be estimated from either desiccant pressure drop curves furnished by the manufacturer (Figure 5-53), or Equation 5-17 which will be followerd.

Image
Fig. 5-53. Typical pressure-drop curve for silica gel type desiccants, 0.15-inch diameter beads.

5.22.11 Equipment
The proper selection of equipment is essential to good operations.

5.22.11.1 Inlet Gas Cleaning Equipment
All hydrocarbon liquids, free water, glycol, amine, or lube oil carry over must be cleaned from the inlet gas to ensure the best dry desiccant dehydrator operation. In all cases, the dry bed unit should have a scrubber (or a filter separator) between it and a primary well fluid separator. A microfiber filter separator (or its equivalent) should always be installed upstream of the inlet scrubber if a carryover of glycols, amines, or compressor lube oils is possible. Liquid level controls need to be checked frequently as well as the liquid dump line to ensure their operability.

5.22.11.2 Adsorber Tower
General considerations
An adsorber is a cylindrical tower filled with a solid desiccant. The depth of desiccant will vary from a few feet to 30 feet or more.
On top of the bed, a hold-down screen is provided, again covered with a layer of ceramic balls. In some cases, a layer of less expensive desiccant can be installed on the top of the bed to catch contaminants such as free water, glycol, hydrocarbons, amines, etc. This may extend the bed life. Good inlet separation of entrained contaminants is absolutely essential for long desiccant life.
Vessel diameter may be as much as 10 to 15 feet or more.
Bed height to diameter (L/D) ratio of 2.5–4.0 is desirable.
Lower ratios (1:1) are sometimes used, which could result in poor gas dehydration caused by:
Non-uniform flow
Channeling
Inadequate contact time between the wet gas and the desiccant
Three problems that frequently cause poor operation are:
Insufficient gas distribution
Inadequate insulation
Improper bed supports

1. Insufficient gas distribution
Poor gas distribution at the inlet and outlet of the desiccant beds has caused many costly problems, resulting in:
Channeling
Desiccant damage
The inlet gas distributor should be provided with adequate baffling before the gas enters the desiccant bed. Neither gas to be dehydrated nor the regeneration gas should impinge directly on the bed.
Channeling, high localized velocities and swirling can cause:
Desiccant attrition
High-pressure drop through the desiccant bed as attrition fines lodge between the regular particles
Screen-wrapped slotted pipe, with gas at low velocities exiting radially into the vessel is recommended. A 4- to 6-inch layer of large diameter (2 inch) support balls can be placed on top of the desiccant bed to improves gas distribution and prevents desiccant damage from swirling.
Swirling can destroy several feet of castable refractory lining by turning the powdered desiccant into a sandblasting agent which results in high heat losses and poor desiccant regeneration.

2- Inadequate Insulation
Internal or external insulation can be used.
Internal insulation, reduces the total regeneration gas requirements and costs.
Provision must be made for expansion and contraction so that there will be no cracking or weld failures. The lining is normally made from a castable refractory lining. Liner cracks permit some of the wet gas to bypass the desiccant bed where, a small amount of wet, bypass gas can cause freeze up in cryogenic plants. Ledges installed every few feet along the vessel wall can help eliminate liner cracks.

3- Improper Bed Supports
Two common bed supports include:
Horizontal screen supported by I-beams and a welding ring
Vessel whose bottom head is filled with graduated support balls

Screens are usually made of stainless steel or monel that have openings at least 10 meshes smaller than the smallest desiccant particle.
Screens should be securely fastened in the vessel. Provisions should be made for expansion and contraction as the adsorbers heat and cool. Annular space between the vessel wall and the edge of the bed support screen must be sealed to prevent the loss of desiccant:
Asbestos rope packing, forced in this space, is used.
A support ring around the edges of the screen is beneficial.

Support balls on the screens are helpful. 2 to 3 inches of ½-inch balls are gently placed on the screen and a 2 or 3 inch smooth layer of 1/4-inch balls is gently placed on top of the ½-inch balls. Bottom bed support typically includes three to five layers of inert ceramic balls in graduated sizes (smallest on top).
These layers prevent desiccant dust or whole particles from plugging the screen openings and forcing a high-pressure drop across the desiccant beds.
When calculating the regeneration needs of the system, it is important to include the heat requirements for the support balls.
If the bottom head of the vessel is filled with graduated support balls, a gas distributor may be required between the balls and the lower portion of the desiccant bed when upflow heating or cooling is used. This is important on large-diameter vessels to prevent channeling and poor reactivation of the desiccant. Many adsorbers have a void area in the bottom, below the bed supports, to collect contaminants, dust, and fines. A blowdown nozzle can be provided to discharge these materials. A moisture sample probe should be located in the adsorbers in cryogenic plants several feet from the outlet end of the bed and extending to the center.
This probe, used in conjunction with the outlet gas moisture probe, offers valuable flexibility in studying and solving dehydrator problems, particularly for determining if gas is being channeled down the walls of the vessel. It permits capacity tests for optimizing drying cycle times.
Since solid desiccants can produce dust, 1µm filters are frequently installed at the outlet of the dehydration unit to protect downstream equipment.

Pressurization
For best performance and maintenance of desiccant quality, adsorbers should:
Never be pressurized faster than 50 psi/min
Never be depressurized faster than 10 psi/min
Downflow pressure drop should not exceed 1 psi/ft.
Upflow pressure drop should not be less than 1/4 psi/ft.

Even with the best designs, some desiccant dust is swept out of the beds at design gas-flow rates. Certain amounts can be tolerated in many field dehydration systems.
It is not acceptable in turbo expander plant designs that involve extensive downstream heat exchange and processing. The problem is particularly significant where plate-fin or core-type heat exchangers are used. In many instances, this problem can be solved with microfiber filters (cleaning to 1 micron) with a differential pressure across them of 15 psi.

Image
Fig. 5-54. Molecular sieve gas dehydration tower

5.22.11.3 Regeneration Gas Exchangers, Heaters, and Coolers
A gas exchanger is usually designed with the following assumptions:
All of the water will be liberated from the bed in 1 hour at 250 0F.
Regeneration gas can be cooled to within 10 0F of the sales gas temperature.
A regenerative gas heater is sized to provide:
Heat to desorb the water
Heat for the desiccant of between 500 0 and 550 0F
Heat the contactor shell
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