Specialized Dehydration Processes

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Specialized Dehydration Processes

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE20605

For additional information on this subject, contact R.A. Al-Husseini on 874-2792

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Process Specialized Dehydration Process

CONTENTS

PAGES

OPERATION OF AND EQUIPMENT THAT IS USED FOR SPECIALIZED DEHYDRATION PROCESSES......................................................... 1 CALCIUM CHLORIDE (CACL2) .............................................................................. 1 Equipment.................................................................................................... 3 Operation .................................................................................................... 4 PRESSURE SWING ADSORPTION (PSA) ................................................................. 5 LOW-TEMPERATURE DEHYDRATION SYSTEMS ..................................................... 6 Low-Temperature Separation With Hydrate Formation ............................. 7 Low-Temperature Separation Without Hydrate Formation (LTS) ............ 11 Refrigeration-Aided Low-Temperature Separation................................... 11 AIR DRYERS ....................................................................................................... 13 Process Flow ............................................................................................. 13 Uses of Dry Air.......................................................................................... 14 Equipment.................................................................................................. 15 Pressure Swing Adsorption ....................................................................... 16 Instrument Air Dryers................................................................................ 17 Instrument Air Filters ................................................................................ 18 ADVANTAGES, DISADVANTAGES, AND APPLICATIONS OF SPECIAL DEHYDRATION PROCESSES................................................................ 19 CALCIUM CHLORIDE ........................................................................................... 19 Advantages............................................................................................... 19 LOW-TEMPERATURE ........................................................................................... 19 Low-Temperature Separation with Hydrate Formation............................ 19 CALCIUM CHLORIDE ........................................................................................... 20 Disadvantages/Limitations ........................................................................ 20 Disadvantages/Limitations ........................................................................ 20

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CALCIUM CHLORIDE ........................................................................................... 21 Applications............................................................................................... 21 Applications............................................................................................... 21 Low-Temperature Separation Without Hydrate Formation ...................... 21 Refrigeration-Aided Low-Temperature Separation................................... 21 PRESSURE SWING ADSORPTION (PSA) ............................................................... 21 ADVANTAGES, DISADVANTAGES, AND APPLICATIONS OF GLYCOL AND REGENERATIVE SOLID DESICCANT SYSTEMS ..................... 22 GLYCOL (TEG) DEHYDRATION PROCESSES ....................................................... 22 Advantages ................................................................................................ 22 Disadvantages ........................................................................................... 23 Applications............................................................................................... 23 REGENERATIVE SOLID DESICCANT PROCESSES .................................................. 23 Advantages ................................................................................................ 23 Disadvantages ........................................................................................... 24 Applications............................................................................................... 24 COMPARISON OF DEHYDRATION PROCESSES ...................................................... 25 Dew Point .................................................................................................. 25 Capacity .................................................................................................... 25 Cost............................................................................................................ 26 Maintenance .............................................................................................. 26 Materials ................................................................................................... 26 CALCIUM CHLORIDE DEHYDRATORS .................................................................. 27 LOW-TEMPERATURE SEPARATION UNITS ........................................................... 27 GAS PLANT AIR SYSTEMS ................................................................................... 27 GLOSSARY ............................................................................................................... 28

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OPERATION OF AND EQUIPMENT THAT IS USED FOR SPECIALIZED DEHYDRATION PROCESSES This section covers the operation of and equipment that is used for the following specialized dehydration processes: • • •

Calcium chloride (CaCl2) Low-Temperature Dehydration Systems Pressure swing adsorption (PSA)

Calcium Chloride (CaCl2) Nonregenerable solid desiccant dehydrators are used to dehydrate natural gas that is produced by small, remote fields and for offshore retrofits with severe space and weight limitations. They are also used to dehydrate instrument air. Although many other brines have been experimented with, most nonregenerable dehydrators use calcium chloride for their solid desiccants. CaCl2 exists in anhydrous form and in four levels of hydration. Figure 1 shows how the water content (lb H2O/MMSCF) of natural gas in equilibrium with solid CaCl2•4H2O varies with gas pressure and temperature. Similar graphs are available for the other hydrates: CaCl2•H2O, CaCl2•2H2O, and CaCl2•6H2O.

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1,000 Note:

800 600 400 300

Curves for 120°F, 130°F, and 140°F are fictitious in that CaCl2•4H2O does not exist at temperatures above 113.5°F. However, these extrapolated values at elevated temperatures are useful for predicting dehydrator performance.

Humidity, lb H

200

100 80 60 40 30

140

20 120 10 8

110

6 4 3 2

1 10

Source:

130 113.5

90

100

70

80

T=60°F

20

40

60

100

200 400 600 1,000 Pressure, psia

2,000

5,000

Dow Chemical in Manning, Francis S. and others, Oilfield Processing of Petroleum, Volume One: Natural Gas; PennWell Books, Tulsa: © 1991; p. 192, Figure 9-24.

FIGURE 1: WATER CONTENT OF NATURAL GAS IN EQUILIBRIUM WITH CaCl2•4H2O

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Equipment Except for its liquid level controls, calcium chloride dehydrators have no moving parts. Figure 2 shows the three sections of a calcium chloride dehydrator: separation section, tray section, and the solid CaCl2 bed section.

Heater Source:

Gas Dehydration and Hydrate Inhibition; Version 1; Operations Division; June 1992; p. 100, Figure 41.

Exxon Production Research Company, Production

FIGURE 2: CALCIUM CHLORIDE DEHYDRATOR

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Separation Section - As in other types of dehydration systems, the inlet separator removes free water and other contaminants from the inlet gas. After separation, the inlet gas enters the tray section above it. Tray Section - Three to five trays contact concentrated CaCl2 with the process gas. The brine flows downward and the process gas flows upward. As the brine flows downward, the wet gas increasingly dilutes it and as the gas flows upward, it is progressively dehydrated. The trays are specially designed so that the gas velocity is used to recirculate the brine on each tray. Because very small amounts of brine are formed from large gas volumes, liquid recirculation is required to obtain good liquid-vapor contact or high tray efficiency. CaCl2 Bed Section - The top section contains pellets (3/8 in. to 3/4 in.) of CaCl2. The wet gas rising up from the tray section contacts the pellets. The anhydrous CaCl2 absorbs water in the process gas and turns into a brine. The brine flows down into the tray section. As the process gas flows up through the bed it contacts increasingly drier CaCl2. As the CaCl2 at the bottom of the bed changes to liquid, the rest of the bed settles and takes its place. Beds with at least 2 ft of CaCl2 pellets provide satisfactory gas dehydration. In addition to the three sections, Figure 2 shows a heater at the base of the dehydrator. The heater operates only when the ambient temperature falls below the freezing point of the brine. Operation The inlet gas enters the bottom of the dehydrator, rises through the tray section and the bed section, then returns down the sides of the dehydrator and exits through the outlet. Both the brine and the solid CaCl2 absorb water from the process gas. Normally, the pressure drop across the entire unit (trays and bed section) is less than 8 psi.

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At typical inlet gas temperatures (80°F to 100°F), the brine that drips from the CaCl2 bed section contains about 1 lb of water per lb of CaCl2. In the tray section, this brine absorbs enough water from the wet gas to dilute it to about 20 wt% to 25 wt% CaCl2. With five trays, a CaCl2 dehydrator removes about 3.5 lb of water for each pound of CaCl2 consumed. Maximum dew point depressions of 60°F to 70°F are achievable. For example, a well producing 1 MMSCFD of gas at 1000 psig and 80°F contains about 34 lb H2O/MMSCF. Reducing this amount of water to a pipeline specification of 7 lb H2O/MMSCF, requires the removal of approximately 27 lb H2O/MMSCF. If efficiently used, 7.7 lb of CaCl2 would be consumed in removing this water. At this rate, a 350-lb drum would last 45 days. Minimum unit capacity is two drums, or 700 lb. These units are manufactured in capacities up to 10 to 15 MMSCFD and in working pressures up to 3000 psig for wellhead use. Packed nonregenerable CaCl2 dehydrators are also used in instrument gas-dehydration services. Dryers are available from a number of suppliers. Pressure Swing Adsorption (PSA) Instead of using high temperatures to regenerate solid desiccants, dehydrators that use the pressure swing adsorption cycle use changes in pressure to regenerate solid desiccants. As pressure decreases, the adsorption capacity of solid desiccants decreases. Therefore, pressure swing adsorption dehydrators cycle adsorber towers between high and low pressures. The only changes in temperature are caused by the heat of adsorption and desorption. The desiccant adsorbs water at high pressure. The desiccant regenerates at low pressure. Pressure swing adsorption is commonly used to regenerate molecular sieves in dehydrators designed to recover high-purity hydrogen from demethanizer off-gases. Saudi Aramco uses PSA units to dry instrument air (covered later).

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Low-Temperature Dehydration Systems When wellhead pressures are high, low-temperature processes efficiently perform the following tasks: •

The dehydration of natural gas streams to pipeline specifications.

•

The recovery of additional hydrocarbon liquids (condensate) from gas streams.

•

The separation of hydrocarbon liquids and water from natural gas streams.

If the wellhead pressure exceeds that of the pipeline, then the gas can be passed through a choke or throttled in a constant-enthalpy Joule-Thomson expansion to provide cooling. Figure 3 can be used to estimate the magnitude of this Joule-Thomson cooling.

FIGURE 3:

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TEMPERATURE DROP ACCOMPANYING A GIVEN PRESSURE DROP

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This section covers the following classes of low-temperature processes: • • •

Systems that intentionally form and melt hydrates Systems that require the use of hydrate inhibition Systems that require additional cooling (refrigeration)

Low-Temperature Separation With Hydrate Formation Cooling wet gases condense the water vapor held by the gas stream. Once condensed, the water can be removed easily by a separator. Lowering the temperature of natural gas streams, however, can cause hydrates to form. Low-temperature dehydration systems, therefore, must either avoid the formation of hydrates or use the formation of hydrates to advantage. These systems take advantage of the three following properties of hydrates: • • •

Hydrates are less dense than water. Hydrates are more dense than condensate. The formation of hydrates extracts water from a gas stream.

Equipment - Low-temperature separation units that use hydrate formation use the following major components: • • • • • • •

Indirect heater (if the inlet gas requires heating) Low temperature (LTX) separator High pressure knockout (HPKO) Inlet gas/sales gas heat exchanger Choke Flash separator or stabilizer Piping and controls

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Operation - Figure 4 shows a typical LTX separation unit. The following section describes the process flow of this system.

Source:

NATCO in Manning, Francis S. and others, Oilfield Processing of Petroleum, Volume One: Natural Gas; PennWell Books, Tulsa: ©1991; p. 189, Figure 9-19.

FIGURE 4: LOW TEMPERATURE SEPARATION UNIT (LTX)

If the inlet gas requires heating for use as a heating fluid in the LTX separator, it is heated by the indirect heater. The inlet gas stream then flows through the hydrate melting coils in the bottom of the LTX separator. In addition to heating the hydrate melting coils, the gas stream provides enough heat to prevent the buildup of hydrates and the plugging of the choke located nearby.

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From the hydrate melting coils, the gas stream flows to the HPKO drum. This drum separates the free liquids from the process gas stream. The temperature drop from the wellbore at the system temperature condenses some of the water and the heavier hydrocarbons from the process gas stream. These liquids may be handled in one of the three following ways: •

The HPKO removes all liquids (water and condensate) from the process gas. The HPKO dumps all of the separated liquids into the lower liquid section of the LTX (as shown in Figure 4).

•

The HPKO removes all liquids (water and condensate) from the process gas. The HPKO dumps the water to disposal and feeds the condensate to the lower liquid section of the LTX separator. This requires that the temperature of the HPKO be warm enough (about 100°F) to inhibit the formation of hydrates in the liquid dump valves.

•

The HPKO removes only water from the process stream. The process gas and the condensate both pass through the choke.

From the HPKO, the process gas flows through the tube side of the inlet gas/sales gas heat exchanger. This exchange cools the process gas to the coldest temperature that will not cause the formation of hydrates on the upstream side of the choke. This low temperature ensures the lowest separation temperature in the LTX separator and the maximum recovery of hydrocarbon liquids. To maintain the lowest safe temperature in the heat exchanger, a threeway valve controls the flow of the sales gas through the heat exchanger. The inlet gas/sales gas heat exchanger also warms the sales gas before it enters the sales pipeline. Warming the sales gas prevents the cooling of the gas in the sales pipeline to a point below its hydrate formation temperature. From the inlet gas/sales gas heat exchanger, the process gas flows through the choke. The choke expands the gas from the wellbore pressure to the salesline pressure. The choke can be used to regulate either the flow rate of the process gas or the pressure in the low temperature separator. The expansion of the process gas in the choke condenses most of the water and some of the hydrocarbon gas. This expansion also causes hydrates to form in the choke, but sonic flow breaks them up and carries them into the LTX separator. This flow of broken up hydrates sounds like pellets entering the LTX vessel. The liquid, vapor, and solid stream that leaves the choke tangentially enters a cylindrical spinner box. A tangential entry absorbs inlet momentum, directs the process stream onto the hydrate melting coils, and helps to separate the vapor from the liquid.

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The hydrate melting coils in the bottom of the LTX separator heat the condensate to 65°F to 75°F and heat the water to a temperature 15°F to 20°F hotter than the condensate. Therefore, the condensate remains the top liquid layer while the hydrates sink through the condensate. The hydrates float on the layer of liquid water, but beneath the condensate, at the bottom of the LTX separator. As the liquids flow through the LTX separator toward the liquid outlets, the hydrates decompose (melt). Figure 5 shows the temperatures and layers of liquids in a horizontal LTX separator.

*

Source:

Mapes in Manning, Francis S. and others, Oilfield Processing of Petroleum, Volume One: PennWell Books, Tulsa: ©1991; p. 190, Figure 9-20a.

Natural Gas;

FIGURE 5: HORIZONTAL LTX SEPARATOR

If the choke pressure drop is barely adequate, then the HPKO is generally placed ahead of the inlet gas/sales gas heat exchanger. On the other hand, the placement of the HPKO downstream from the inlet gas/sales gas heat exchanger allows the removal of additional liquids, feeds the liquid-free gas to the choke, and generates the lowest expansion temperatures in the choke.

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Low-Temperature Separation Without Hydrate Formation (LTS) Low-temperature separation units without hydrate formation are similar to LTX separation units. However, in LTS systems the inlet gas/sales gas heat exchanger cannot cool the inlet gas below its hydrate-formation temperature. Therefore, the available pressure drop from the inlet gas pressure to the sales gas pressure controls the resulting temperature in the lowtemperature separator (LTS). When this Joule-Thomson cooling cannot achieve the required gas dehydration, then the inlet gas requires additional cooling before it enters the choke. Because this additional cooling lowers the temperature of the gas stream below its hydrate-formation temperature, a system that requires additional cooling also requires the use of hydrate inhibition (covered in ChE 206.02). Refrigeration-Aided Low-Temperature Separation External refrigeration is required when the combined cooling of the gas-to-gas exchanger and any Joule-Thomson expansion is insufficient to achieve the desired water and hydrocarbon dew points. Figure 6 shows an LTS separation unit that uses mechanical refrigeration to supply the required additional cooling.

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Source:

NATCO in Manning, Francis S. and others, Oilfield Processing of Petroleum, Volume One: Natural Gas; PennWell Books, Tulsa: ©1991; p. 191, Figure 9-22.

FIGURE 6: SIMPLIFIED FLOW IN A REFRIGERATION-AIDED LOW-TEMPERATURE SEPARATION UNIT

In Figure 6, the system injects glycol into the inlet gas stream ahead of the inlet gas/sales gas heat exchanger. The chiller then supplies the required additional cooling to condense the water and heavier hydrocarbons in the process stream. In this example, the process gas enters the chiller at about 50°F and leaves it at about -20°F. This temperature and the other temperatures shown in Figure 6 are typical, but operating temperatures vary with different applications. After the chiller, the cold separator separates the process fluid into dry gas, hydrocarbon liquids, and water (mixed with glycol). The system then pipes the dry gas to the sales line, the glycol-water solution to the glycol reboiler, and the condensate to the stabilizer. The glycol reboiler reconcentrates the glycol. The stabilizer separates the condensate into overhead gas and natural gas liquids.

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Air Dryers Saudi Aramco gas plants use air for many purposes and produce the three following classes of air: • • •

Instrument air Plant air Process air

Process Flow Figure 7 shows the flow of air through a Saudi Aramco gas plant air system. The air enters the compressors and coolers from the atmosphere. From the compressors, the air enters the compressed air receivers. After the receivers, the system diverts instrument air from the process and plant air for further processing. The system then cools, prefilters, dries, and afterfilters the instrument air. Air Process and plant air

Air compressor and coolers

Air compressor and receivers

Auxiliary after- cooler

Instrument air prefilters

Instrument air dryers

Instrument air afterfilters

Instrument air

FIGURE 7: PROCESS FLOW OF SAUDI ARAMCO GAS PLANT AIR SYSTEM

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Uses of Dry Air Plant Air and Process Air - Gas plants use plant air to run air-operated equipment and for many jobs in different areas of the gas plant. Dry air is preferred for all uses. Wet plant air can cause corrosion in piping and damage equipment and tools. Also, a small amount of instrument air is used to keep the flame scanners of fired heaters and boilers clean. Process air is also used to regenerate Merox catalysts. Instrument Air - Gas plants use instrument air to operate many of their instruments. To keep operating, gas plants require that instrument air be available at all times. Each type of air must be clean, but instrument air must be very clean and very dry. Dirty air can cause equipment damage. Moisture in instrument air damages instruments and can cause erratic instrument readings.

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Equipment Figure 8 shows the flow of air through the filters and the dryers. The prefilters remove particles too small to be removed by the intake filters. The afterfilters remove any desiccant picked up by the air in the dryer.

Vent to atmosphere

Drain

FIGURE 8:

SAUDI ARAMCO GAS PLANT AIR SYSTEM PROCESS FLOW THROUGH AIR DRYER

As with solid desiccant dehydrators that dry natural gas, the drying of air requires at least two drying vessels. While one vessel dries the instrument air, dry instrument air regenerates the other vessel. These Saudi Aramco systems use activated alumina and silica gel for their solid desiccants.

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Pressure Swing Adsorption Saudi Aramco also uses air dryers that use pressure swing adsorption to regenerate their solid desiccants. Saudi Aramco uses these heatless dryers to dry instrument air in new facilities. Figure 9 shows a pressure swing adsorption (heatless) regenerative air dryer.

Source: Van Air Systems, Inc.; Regenerative Compressed Air dryers; Lake City, PA; p. 6.

FIGURE 9:

PRESSURE SWING ADSORPTION (HEATLESS) REGENERATIVE AIR DRYER

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The operation of this dryer is similar to that of other solid desiccant dehydrators. While one tower dries the air, the other tower is regenerating. Valves at the top and bottom of the dehydrator direct inlet and product air through the towers. Wet, inlet air flows up thorough one tower while a portion of the dry, product air flows down through the other. In Figure 9, the pressure of the inlet air ranges from 60 psig to 150 psig and the dry air used for regeneration is at atmospheric pressure. At the higher pressure, the desiccant adsorbs water from the inlet air. At the lower pressure, the dry air strips water from the desiccant and regenerates it. As with other solid desiccant systems, either timers or moisture analyzers can be used to control cycle times. The dehydrator in Figure 9 uses activated alumina for its desiccant. The following specifications on instrument air dehydration are covered in Saudi Aramco Engineering Standard SAES-J-901. Instrument Air Dryers Dryers shall be supplied to deliver dry air to the air system at a maximum dew point of -4°F at system pressure (75 to 125 psig). The maximum air inlet temperature, inlet flow rate, and inlet pressure range shall be clearly stated to vendors for correct sizing of desiccant chambers. Dryers shall be heatless regeneration, desiccant type. Refrigeration type dryers shall not be used. Dryers shall be automatic cycle type using two desiccant chambers. An inline continuous moisture indicator shall be provided in the dryer discharge. One chamber regenerates while the other chamber adsorbs moisture from the air. Regeneration shall consume less than 20% of the total air capacity. This consumption shall be included in the system air requirements. The desiccant shall be a type that does not disintegrate upon contact with water. Activated alumina is preferred. Pneumatic cycle timers and switching valves are to be used only in areas where electronic power is not available.

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Instrument Air Filters Instrument air shall be filtered before entering the air dryer. The filters shall be of the coalescing type, capable of removing entrained droplets of oil or water, and dust or other foreign matter down to a particle size of a one micrometer absolute. Filters are to be fitted with automatic drains. All desiccant type dryers shall be provided with an afterfilter capable of removing 100% of all particles one micrometer absolute or larger to prevent desiccant dust entering the downstream system. The pressure drop caused by the drying and filtering equipment when it is clean shall not exceed 5 psi at maximum flow rates.

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ADVANTAGES, DISADVANTAGES, DEHYDRATION PROCESSES

AND

APPLICATIONS

OF

SPECIAL

This section covers the advantages, disadvantages, and applications of the following specialized dehydration processes: • • •

Calcium chloride (CaCl2) Low-Temperature Dehydration Systems Pressure swing adsorption (PSA)

Calcium Chloride Advantages Calcium chloride dehydrators have the following advantages: • • • • • •

Can operate unattended until they require fresh desiccant Contain few moving parts Do not require fuel or heat to operate Have low capital costs Present no fire hazard Very compact

Low-Temperature Low-Temperature Separation with Hydrate Formation Advantages •

Because of minimal energy consumption, these units have low operating costs.

•

These units experience minimal corrosion, especially when they do not use hydrate inhibitors.

•

With adequate wellhead pressure, these units can achieve pipeline specification for water and hydrocarbon dew points.

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Calcium Chloride Disadvantages/Limitations Calcium chloride dehydrators have the following disadvantages or limitations: •

CaCl2 solutions are highly corrosive in the presence of air.

•

Even with five trays, 1 lb of CaCl2 removes only 3.5 lb of H2O.

•

The CaCl2-H2O equilibrium limits the dew point depression they can achieve.

•

They are subject to bridging, which normally causes channeling.

Bridging is the joining or fusing (freezing) of adjacent CaCl2 pellets in the desiccant bed. Intermittent or cyclic operation of a calcium chloride dehydrator is the most common cause of bridging. The following situations can cause bridging: •

A slight temperature drop freezes the brine that drips off the CaCl2 pellets. This fuses the pellets.

•

The dehydrator is removed from service, left idle, and returned to service. While the dehydrator is idle, a slight temperature drop causes the CaCl2 pellets to fuse together.

•

Wet gas or free water contacts the bed, forms more brine, and intensifies an existing bridging condition.

Low Temperature Disadvantages/Limitations Low temperature systems have the following disadvantages or limitations: •

The addition of a hydrate inhibition system increases both its capital and operating costs.

•

The addition of mechanical refrigeration increases both its capital and operating costs.

•

They require adequate wellhead pressure, otherwise they are unable to function properly.

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Calcium Chloride Applications Generally, calcium chloride dehydrators are used to dry small volumes of natural gas that are gathered from wells located in remote areas. CaCl2 dehydrators are particularly useful for wells located in areas in which the terrain, climate, or other conditions make servicing expensive. CaCl2 dehydrators are also useful for offshore retrofits with severe space and weight limitations. Low Temperature Applications Low-temperature separation systems are generally selected for wells producing sweet gas with wellhead pressures considerably higher than pipeline pressure. Low-Temperature Separation Without Hydrate Formation These systems cannot cool the process gas below its hydrate-formation temperature. Therefore, these systems require a greater pressure drop from the well to the separator or the aid of mechanical refrigeration. The addition of a hydrate inhibition system increases both its capital and operating costs. Refrigeration-Aided Low-Temperature Separation Mechanical refrigeration can make up for an inadequate pressure drop from the well to the separator. However, the addition of mechanical refrigeration increases both its capital and operating costs. Pressure Swing Adsorption (PSA) Because PSA systems do not require heat, they are more easily installed than systems that use heat to regenerate solid desiccants. However, the venting and low-pressure purging of the regeneration gas produces greater losses. Pressure swing adsorption is used to regenerate molecular sieves in dehydrators that are designed to recover high-purity hydrogen from demethanizer offgases. These systems generally produce very high purities of hydrogen, but they recover only about 70% of the hydrogen in the gas stream. However, the remaining gas can be used for fuel gas.

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ADVANTAGES, DISADVANTAGES, AND APPLICATIONS OF GLYCOL AND REGENERATIVE SOLID DESICCANT SYSTEMS This section covers the advantages, disadvantages, and applications of the following dehydration processes: • •

Glycol Regenerative solid desiccant

Glycol (TEG) Dehydration Processes Advantages Glycol dehydration systems using TEG have the following advantages: •

Can dehydrate natural gas to 0.5 lb H2O/MMSCF (0.25 lb H2O/MMSCF in special applications).

•

Dehydrates natural gas continuously. process).

•

Easily automated for unattended operation in remote locations.

•

Operate effectively in the presence of materials that would foul solid desiccant dehydrators.

•

Lower installed costs than solid desiccant dehydrators for smaller plants (Solid desiccant plants cost about 50% more for 10-MMSCFD applications and about 33% more for 50-MMSCFD applications.)

•

Lower pressure drop: 5 psi to 10 psi for glycol dehydrators versus 10 psi to 50 psi for solid desiccant dehydrators.

•

Lower utility costs: glycol units require less regeneration heat per pound of water removed than solid desiccant dehydrators.

•

Simple to operate and maintain.

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Disadvantages Glycol dehydration systems using TEG have the following disadvantages and limitations: •

Dehydrating gases to dew points below -25°F requires the use of a stripping gas and column.

•

Glycol is susceptible to contamination.

•

When contaminated or decomposed, glycol is corrosive.

•

Glycol is susceptible to foaming.

Applications TEG dehydrators are by far the most common system for dehydrating natural gas. TEG dehydrators are used unless the conditions listed under the other dehydration systems are present. Regenerative Solid Desiccant Processes Advantages Regenerative solid desiccant dehydrators have the following advantages: •

Achieve dew points as low as -150°F [1 ppm(vol)].

•

Small changes in gas pressure, temperature, or flow rate affect them less than other dehydrators.

•

They are less susceptible to corrosion and do not foam.

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Disadvantages Solid desiccant dehydrators have the following disadvantages or limitations: •

High regeneration heat requirements and high utility costs.

•

High space and weight requirements.

•

Higher capital costs.

•

Higher pressure drops.

•

Solid desiccants are susceptible to crushing and other mechanical breakage.

•

Solid desiccants are susceptible to desiccant poisoning by heavy hydrocarbons, H2S, CO2, and other contaminants.

Applications Solid desiccant dehydrators are generally used for the following applications: •

The dehydration of natural gas to dew points low enough for cryogenic processing (below -30°F).

•

The dehydration of natural gas to pipeline specifications (4 to 7 lb H2O/MMSCF or water dew points of 10°F to 30°F) when TEG is not effective. For example, desiccant dehydrators are used to dehydrate sour gas and aboard floating production platforms, where wave action disturbs glycol flow on the contactor trays.

•

To recover hydrocarbon liquids from lean (0.5 GPM C3+ or less) natural gas (often with refrigeration).

•

To simultaneously remove water and hydrocarbons to meet both water and hydrocarbon dew points.

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Comparison of Dehydration Processes This section compares the dehydration processes on the basis of the following factors: • • • • •

Dew Point Capacity Cost Maintenance Materials

Dew Point All of the dehydration processes discussed in this module can meet natural gas pipeline specifications (7 lb H2O/MMSCF). With the use of a stripping gas, TEG systems can dehydrate natural gas streams to 0.5 lb H2O/MMSCF. Solid dessicants that use molecular sieves can dehydrate natural gas streams to 1 ppm(vol) of water. Capacity Solid desiccant dehydrators most efficiently dehydrate the largest capacities of natural gas. Calcium chloride dehydrators efficiently dehydrate natural gas gathered from small wells in remote areas.

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Cost Figure 10 summarizes and compares the relative operating costs of dehydration processes. DEHYDRATION PROCESS

CAPITAL COSTS

OPERATING COSTS

Calcium Chloride

Low

High

Glycol

Moderate

Moderate

Low Temperature

Moderate

Solid Regenerative Desiccant

High

Low (Moderate to High if Refrigeration is Used) High

FIGURE 10: COMPARISON OF CAPITAL AND OPERATING COSTS OF DEHYDRATION PROCESSES

Maintenance None of the dehydration processes discussed in this module require a lot of maintenance. Calcium chloride dehydrators periodically need to be reloaded with solid desiccant. Because of the high cost of replacing the contaminated desiccant, solid desiccant dehydrators require more monitoring than the other dehydration processes. Also, for efficient operation, the drying cycles of solid desiccant dehydrators that do not use moisture analyzers for control require periodic adjustment. Materials Only nonregenerative dehydrators need a constant supply of materials. Glycol dehydrators require periodic additions of glycol to replenish losses. Solid desiccants need large investments in desiccant when they are started, but do not require replenishment until the desiccant bed can no longer economically dehydrate natural gas. Except for calcium chloride dehydrators and solid desiccant dehydrators that use pressure swing adsorption, all of the dehydration processes require continuous supplies of fuel or heat. Solid desiccant dehydration systems need additional piping and instrumentation to control the adsorption/regeneration sequence.

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Process Specialized Dehydration Process

Calcium Chloride Dehydrators Calcium chloride dehydrators have separation, tray, and solid CaCl2 bed sections. The separation section uses an inlet separator to remove free water and other contaminants. The tray section uses three to five trays to contact the process gas and the CaCl2 brine (20 wt % to 25 wt % CaCl2). The design of the trays maximizes contact between the brine and the process gas. The bed contains solid CaCl2 pellets that adsorb water from the process gas. The water and solid bed form brine that flows down into the tray section. Calcium chloride dehydrators are simple and can operate unattended. They dry natural gas to pipeline specifications and have low capital costs. For these reasons, calcium chloride dehydrators are used to dehydrate small volumes of natural gas produced by wells located in remote areas. They are also useful for offshore retrofits with severe space limitations. Low-Temperature Separation Units Low-temperature separation units dehydrate natural gas streams by expanding the natural gas from well head pressure to pipeline pressure. The Joule-Thomson expansion of the gas cools the gas and condenses the water in it. It may also cause the formation of hydrates. A separator removes the hydrate or the free water from the process stream. Low-temperature separation units have low operating costs. With adequate wellhead pressure, they can dry gases to pipeline specifications. However, the addition of hydrate inhibition or mechanical refrigeration systems increases the capital and operating costs of these systems. Therefore, low-temperature separation units work best for high-pressure wells that produce sweet gas. Gas Plant Air Systems Plants typically produce two classes of air: instrument and plant/process air. Instrument air must be very clean and very dry. Plant air is used to run air operated equipment, process air is used in chemical processes, and instrument air is used to operate instruments. Air enters the compressors and coolers from the atmosphere. From the compressors, the air enters the air receivers. After the receivers, part of the air goes to the instrument air for further filtering and drying. The remainder of the air is available for use as plant or process. Saudi Aramco gas plants use two-tower solid desiccant dehydration systems to dry air. The system uses both activated alumina and silica gel. In new facilities, Saudi Aramco use pressure-swing adsorption dehydrators to dry instrument air.

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Engineering Encyclopedia

Process Specialized Dehydration Process

GLOSSARY HPKO

High pressure knockout.

inlet gas

Feed gas to a processing plant.

instrument air

Dry, compressed air used to operate pneumatic instruments and instrumentation.

LTS

Low temperature separation unit that does not use hydrate formation.

LTX

Low temperature separation unit that uses hydrate formation.

LTX separator

Low temperature separator used in low-temperature separation units.

Merox

A caustic treating process developed and licensed by UOP for mercaptan extraction and sweetening of light and middle boiling range distillates.

plant and processor air

Compressed air used in plants to run air-operated equipment and other uses.

process gas

See inlet gas.

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