Specialized Dehydration Processes

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready 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, EngineeringServices, Saudi Aramco.

Chapter : Process For additional information on this subject, contactFile Reference: CHE20605 R.A. Al-Husseini on 874-2792

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Specialized Dehydration Processes

Engineering Encyclopedia Process

Specialized Dehydration Process

Saudi Aramco DeskTop Standards

CONTENTS PAGES

OPERATION OF AND EQUIPMENT THAT IS USED FORSPECIALIZED 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 OFSPECIAL DEHYDRATION PROCESSES................................................................19

CALCIUM CHLORIDE ...........................................................................................19

Advantages...............................................................................................19

LOW-TEMPERATURE ...........................................................................................19

Low-Temperature Separation with Hydrate Formation............................19


Disadvantages/Limitations ........................................................................ 20

Disadvantages/Limitations ........................................................................ 20



Saudi Aramco DeskTop Standards


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 OFGLYCOL 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



Saudi Aramco DeskTop Standards 1

OPERATION OF AND EQUIPMENT THAT IS USED FOR SPECIALIZEDDEHYDRATION PROCESSES

This section covers the operation of and equipment that is used for the following specializeddehydration 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 producedby 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 beenexperimented with, most nonregenerable dehydrators use calcium chloride for their soliddesiccants. 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 withsolid CaCl2•4H2O varies with gas pressure and temperature. Similar graphs are available forthe other hydrates: CaCl2•H2O, CaCl2•2H2O, and CaCl2•6H2O.




1,000

600

800

400

300

200

1008060

40

30

20

108

6

4

3

2

110 20 40 60 100 200 400 600 1,000 2,000 5,000

Hum

idity

, lb

H

Pressure, psia

120 130

113.5110

10090

8070

T=60°F

Note: 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.

140

Source: 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




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, traysection, and the solid CaCl2 bed section.

Heater

Source: Gas Dehydration and Hydrate Inhibition; Version 1; Exxon Production Research Company, ProductionOperations Division; June 1992; p. 100, Figure 41.

FIGURE 2: CALCIUM CHLORIDE DEHYDRATOR




Separation Section - As in other types of dehydration systems, the inlet separator removesfree water and other contaminants from the inlet gas. After separation, the inlet gas enters thetray section above it.

Tray Section - Three to five trays contact concentrated CaCl2 with the process gas. The brineflows downward and the process gas flows upward. As the brine flows downward, the wetgas increasingly dilutes it and as the gas flows upward, it is progressively dehydrated. Thetrays are specially designed so that the gas velocity is used to recirculate the brine on eachtray. Because very small amounts of brine are formed from large gas volumes, liquidrecirculation 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 wetgas rising up from the tray section contacts the pellets. The anhydrous CaCl2 absorbs water inthe process gas and turns into a brine. The brine flows down into the tray section. As theprocess 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 itsplace. 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. Theheater 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 bedsection, then returns down the sides of the dehydrator and exits through the outlet. Both thebrine and the solid CaCl2 absorb water from the process gas. Normally, the pressure dropacross the entire unit (trays and bed section) is less than 8 psi.




At typical inlet gas temperatures (80°F to 100°F), the brine that drips from the CaCl2 bedsection contains about 1 lb of water per lb of CaCl2. In the tray section, this brine absorbsenough 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 wellproducing 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 theremoval of approximately 27 lb H2O/MMSCF. If efficiently used, 7.7 lb of CaCl2 would beconsumed in removing this water. At this rate, a 350-lb drum would last 45 days. Minimumunit capacity is two drums, or 700 lb. These units are manufactured in capacities up to 10 to15 MMSCFD and in working pressures up to 3000 psig for wellhead use.

Packed nonregenerable CaCl2 dehydrators are also used in instrument gas-dehydrationservices. Dryers are available from a number of suppliers.

Pressure Swing Adsorption (PSA)

Instead of using high temperatures to regenerate solid desiccants, dehydrators that use thepressure swing adsorption cycle use changes in pressure to regenerate solid desiccants. Aspressure decreases, the adsorption capacity of solid desiccants decreases. Therefore, pressureswing adsorption dehydrators cycle adsorber towers between high and low pressures. Theonly changes in temperature are caused by the heat of adsorption and desorption. Thedesiccant adsorbs water at high pressure. The desiccant regenerates at low pressure.

Pressure swing adsorption is commonly used to regenerate molecular sieves in dehydratorsdesigned to recover high-purity hydrogen from demethanizer off-gases. Saudi Aramco usesPSA units to dry instrument air (covered later).




Low-Temperature Dehydration Systems

When wellhead pressures are high, low-temperature processes efficiently perform thefollowing 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 achoke 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: TEMPERATURE DROP ACCOMPANYING A GIVENPRESSURE DROP




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, thewater 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, musteither 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 followingmajor 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




Operation - Figure 4 shows a typical LTX separation unit. The following section describesthe 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 bythe indirect heater. The inlet gas stream then flows through the hydrate melting coils in thebottom of the LTX separator. In addition to heating the hydrate melting coils, the gas streamprovides enough heat to prevent the buildup of hydrates and the plugging of the choke locatednearby.




From the hydrate melting coils, the gas stream flows to the HPKO drum. This drum separatesthe free liquids from the process gas stream. The temperature drop from the wellbore at thesystem temperature condenses some of the water and the heavier hydrocarbons from theprocess 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. TheHPKO 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. TheHPKO dumps the water to disposal and feeds the condensate to the lower liquidsection of the LTX separator. This requires that the temperature of the HPKO bewarm enough (about 100°F) to inhibit the formation of hydrates in the liquid dumpvalves.

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

From the HPKO, the process gas flows through the tube side of the inlet gas/sales gas heatexchanger. This exchange cools the process gas to the coldest temperature that will not causethe formation of hydrates on the upstream side of the choke. This low temperature ensuresthe lowest separation temperature in the LTX separator and the maximum recovery ofhydrocarbon liquids. To maintain the lowest safe temperature in the heat exchanger, a three-way 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 salespipeline. Warming the sales gas prevents the cooling of the gas in the sales pipeline to a pointbelow its hydrate formation temperature.

From the inlet gas/sales gas heat exchanger, the process gas flows through the choke. Thechoke expands the gas from the wellbore pressure to the salesline pressure. The choke canbe used to regulate either the flow rate of the process gas or the pressure in the lowtemperature separator. The expansion of the process gas in the choke condenses most of thewater and some of the hydrocarbon gas. This expansion also causes hydrates to form in thechoke, but sonic flow breaks them up and carries them into the LTX separator. This flow ofbroken up hydrates sounds like pellets entering the LTX vessel.

The liquid, vapor, and solid stream that leaves the choke tangentially enters a cylindricalspinner box. A tangential entry absorbs inlet momentum, directs the process stream onto thehydrate melting coils, and helps to separate the vapor from the liquid.




The hydrate melting coils in the bottom of the LTX separator heat the condensate to 65°F to75°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 ofthe 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: Natural Gas;PennWell Books, Tulsa: ©1991; p. 190, Figure 9-20a.

FIGURE 5: HORIZONTAL LTX SEPARATOR

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




Low-Temperature Separation Without Hydrate Formation (LTS)

Low-temperature separation units without hydrate formation are similar to LTX separationunits. However, in LTS systems the inlet gas/sales gas heat exchanger cannot cool the inletgas below its hydrate-formation temperature. Therefore, the available pressure drop from theinlet gas pressure to the sales gas pressure controls the resulting temperature in the low-temperature separator (LTS).

When this Joule-Thomson cooling cannot achieve the required gas dehydration, then the inletgas requires additional cooling before it enters the choke. Because this additional coolinglowers the temperature of the gas stream below its hydrate-formation temperature, a systemthat requires additional cooling also requires the use of hydrate inhibition (covered inChE 206.02).

Refrigeration-Aided Low-Temperature Separation

External refrigeration is required when the combined cooling of the gas-to-gas exchanger andany Joule-Thomson expansion is insufficient to achieve the desired water and hydrocarbondew points. Figure 6 shows an LTS separation unit that uses mechanical refrigeration tosupply the required additional cooling.




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-TEMPERATURESEPARATION UNIT

In Figure 6, the system injects glycol into the inlet gas stream ahead of the inlet gas/sales gasheat exchanger. The chiller then supplies the required additional cooling to condense thewater and heavier hydrocarbons in the process stream. In this example, the process gas entersthe chiller at about 50°F and leaves it at about -20°F. This temperature and the othertemperatures shown in Figure 6 are typical, but operating temperatures vary with differentapplications.

After the chiller, the cold separator separates the process fluid into dry gas, hydrocarbonliquids, and water (mixed with glycol). The system then pipes the dry gas to the sales line, theglycol-water solution to the glycol reboiler, and the condensate to the stabilizer. The glycolreboiler reconcentrates the glycol. The stabilizer separates the condensate into overhead gasand natural gas liquids.




Air Dryers

Saudi Aramco gas plants use air for many purposes and produce the three following classes ofair:

• 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 entersthe compressors and coolers from the atmosphere. From the compressors, the air enters thecompressed air receivers. After the receivers, the system diverts instrument air from theprocess and plant air for further processing. The system then cools, prefilters, dries, andafterfilters the instrument air.

Air compressorand coolers

Air compressorand receivers

Auxiliary after- cooler

Instrument air prefilters

Instrument air dryers

Instrument air afterfilters

Air

Process and plant air

Instrumentair

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




Uses of Dry Air

Plant Air and Process Air - Gas plants use plant air to run air-operated equipment and formany jobs in different areas of the gas plant. Dry air is preferred for all uses. Wet plant aircan cause corrosion in piping and damage equipment and tools. Also, a small amount ofinstrument air is used to keep the flame scanners of fired heaters and boilers clean. Processair is also used to regenerate Merox catalysts.

Instrument Air - Gas plants use instrument air to operate many of their instruments. To keepoperating, 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 aircan cause equipment damage. Moisture in instrument air damages instruments and can causeerratic instrument readings.




Equipment

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

Drain

Vent toatmosphere

FIGURE 8: SAUDI ARAMCO GAS PLANT AIR SYSTEM PROCESS FLOW THROUGH AIRDRYER

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




Pressure Swing Adsorption

Saudi Aramco also uses air dryers that use pressure swing adsorption to regenerate their soliddesiccants. 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




The operation of this dryer is similar to that of other solid desiccant dehydrators. While onetower dries the air, the other tower is regenerating. Valves at the top and bottom of thedehydrator direct inlet and product air through the towers. Wet, inlet air flows up thoroughone 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 usedfor regeneration is at atmospheric pressure. At the higher pressure, the desiccant adsorbswater from the inlet air. At the lower pressure, the dry air strips water from the desiccant andregenerates it. As with other solid desiccant systems, either timers or moisture analyzers canbe used to control cycle times. The dehydrator in Figure 9 uses activated alumina for itsdesiccant.

The following specifications on instrument air dehydration are covered in Saudi AramcoEngineering 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 atsystem pressure (75 to 125 psig). The maximum air inlet temperature, inlet flow rate, andinlet 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 beused.

Dryers shall be automatic cycle type using two desiccant chambers. An inline continuousmoisture indicator shall be provided in the dryer discharge. One chamber regenerates whilethe 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. Activatedalumina is preferred.

Pneumatic cycle timers and switching valves are to be used only in areas where electronicpower is not available.




Instrument Air Filters

Instrument air shall be filtered before entering the air dryer. The filters shall be of thecoalescing type, capable of removing entrained droplets of oil or water, and dust or otherforeign matter down to a particle size of a one micrometer absolute. Filters are to be fittedwith automatic drains.

All desiccant type dryers shall be provided with an afterfilter capable of removing 100% of allparticles one micrometer absolute or larger to prevent desiccant dust entering the downstreamsystem.

The pressure drop caused by the drying and filtering equipment when it is clean shall notexceed 5 psi at maximum flow rates.




ADVANTAGES, DISADVANTAGES, AND APPLICATIONS OF SPECIALDEHYDRATION PROCESSES

This section covers the advantages, disadvantages, and applications of the followingspecialized 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 hydrateinhibitors.

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




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 ofbridging. The following situations can cause bridging:

• A slight temperature drop freezes the brine that drips off the CaCl2 pellets. Thisfuses the pellets.

• The dehydrator is removed from service, left idle, and returned to service. Whilethe dehydrator is idle, a slight temperature drop causes the CaCl2 pellets to fusetogether.

• Wet gas or free water contacts the bed, forms more brine, and intensifies anexisting 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 operatingcosts.

• The addition of mechanical refrigeration increases both its capital and operatingcosts.

• They require adequate wellhead pressure, otherwise they are unable to functionproperly.




Calcium Chloride

Applications

Generally, calcium chloride dehydrators are used to dry small volumes of natural gas that aregathered from wells located in remote areas. CaCl2 dehydrators are particularly useful forwells located in areas in which the terrain, climate, or other conditions make servicingexpensive. CaCl2 dehydrators are also useful for offshore retrofits with severe space andweight limitations.

Low Temperature

Applications

Low-temperature separation systems are generally selected for wells producing sweet gaswith 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 theaid of mechanical refrigeration. The addition of a hydrate inhibition system increases both itscapital and operating costs.

Refrigeration-Aided Low-Temperature Separation

Mechanical refrigeration can make up for an inadequate pressure drop from the well to theseparator. However, the addition of mechanical refrigeration increases both its capital andoperating costs.

Pressure Swing Adsorption (PSA)

Because PSA systems do not require heat, they are more easily installed than systems that useheat to regenerate solid desiccants. However, the venting and low-pressure purging of theregeneration gas produces greater losses.

Pressure swing adsorption is used to regenerate molecular sieves in dehydrators that aredesigned to recover high-purity hydrogen from demethanizer offgases. These systemsgenerally produce very high purities of hydrogen, but they recover only about 70% of thehydrogen in the gas stream. However, the remaining gas can be used for fuel gas.




ADVANTAGES, DISADVANTAGES, AND APPLICATIONS OF GLYCOL ANDREGENERATIVE SOLID DESICCANT SYSTEMS

This section covers the advantages, disadvantages, and applications of the followingdehydration 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 inspecial applications).

• Dehydrates natural gas continuously. (Solid desiccant dehydration is a batchprocess).

• Easily automated for unattended operation in remote locations.

• Operate effectively in the presence of materials that would foul solid desiccantdehydrators.

• Lower installed costs than solid desiccant dehydrators for smaller plants (Soliddesiccant plants cost about 50% more for 10-MMSCFD applications and about33% more for 50-MMSCFD applications.)

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

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

• Simple to operate and maintain.




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 gasand 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. TEGdehydrators are used unless the conditions listed under the other dehydration systems arepresent.

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 otherdehydrators.

• They are less susceptible to corrosion and do not foam.




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/MMSCFor 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 floatingproduction platforms, where wave action disturbs glycol flow on the contactortrays.

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

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




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 pipelinespecifications (7 lb H2O/MMSCF). With the use of a stripping gas, TEG systems candehydrate natural gas streams to 0.5 lb H2O/MMSCF. Solid dessicants that use molecularsieves 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 inremote areas.




Cost

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

DEHYDRATIONPROCESS CAPITAL COSTS OPERATING COSTS

Calcium Chloride Low High

Glycol Moderate Moderate

Low Temperature ModerateLow

(Moderate to High ifRefrigeration is Used)

Solid RegenerativeDesiccant

High High

FIGURE 10: COMPARISON OF CAPITAL AND OPERATING COSTS OF DEHYDRATIONPROCESSES

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. Becauseof the high cost of replacing the contaminated desiccant, solid desiccant dehydrators requiremore monitoring than the other dehydration processes. Also, for efficient operation, thedrying cycles of solid desiccant dehydrators that do not use moisture analyzers for controlrequire periodic adjustment.

Materials

Only nonregenerative dehydrators need a constant supply of materials. Glycol dehydratorsrequire periodic additions of glycol to replenish losses. Solid desiccants need largeinvestments in desiccant when they are started, but do not require replenishment until thedesiccant bed can no longer economically dehydrate natural gas. Except for calcium chloridedehydrators and solid desiccant dehydrators that use pressure swing adsorption, all of thedehydration processes require continuous supplies of fuel or heat. Solid desiccantdehydration systems need additional piping and instrumentation to control theadsorption/regeneration sequence.




Calcium Chloride Dehydrators

Calcium chloride dehydrators have separation, tray, and solid CaCl2 bed sections. Theseparation section uses an inlet separator to remove free water and other contaminants. Thetray section uses three to five trays to contact the process gas and the CaCl2 brine (20 wt % to25 wt % CaCl2). The design of the trays maximizes contact between the brine and theprocess 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 topipeline specifications and have low capital costs. For these reasons, calcium chloridedehydrators are used to dehydrate small volumes of natural gas produced by wells located inremote 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 gasfrom well head pressure to pipeline pressure. The Joule-Thomson expansion of the gas coolsthe gas and condenses the water in it. It may also cause the formation of hydrates. Aseparator removes the hydrate or the free water from the process stream.

Low-temperature separation units have low operating costs. With adequate wellheadpressure, they can dry gases to pipeline specifications. However, the addition of hydrateinhibition or mechanical refrigeration systems increases the capital and operating costs ofthese systems. Therefore, low-temperature separation units work best for high-pressure wellsthat produce sweet gas.

Gas Plant Air Systems

Plants typically produce two classes of air: instrument and plant/process air. Instrument airmust be very clean and very dry. Plant air is used to run air operated equipment, process air isused in chemical processes, and instrument air is used to operate instruments.

Air enters the compressors and coolers from the atmosphere. From the compressors, the airenters the air receivers. After the receivers, part of the air goes to the instrument air forfurther 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. Thesystem uses both activated alumina and silica gel. In new facilities, Saudi Aramco usepressure-swing adsorption dehydrators to dry instrument air.




GLOSSARY

HPKO High pressure knockout.

inlet gas Feed gas to a processing plant.

instrument air Dry, compressed air used to operate pneumatic instrumentsand instrumentation.

LTS Low temperature separation unit that does not use hydrateformation.

LTX Low temperature separation unit that uses hydrate formation.

LTX separator Low temperature separator used in low-temperature separationunits.

Merox A caustic treating process developed and licensed by UOP formercaptan extraction and sweetening of light and middleboiling range distillates.

plant and processorair

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

process gas See inlet gas.

Date post:	25-Dec-2015
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Specialized Dehydration Processes

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