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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 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, Engineering

Services, Saudi Aramco.

Chapter : Process For additional information on this subject, contact

File Reference: CHE10405 R.A. Al-Husseini on 874-2792

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Dehydration And Hydrate Inhibition

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Engineering Encyclopedia Process

Dehydration and Hydrate Inhibition

Saudi Aramco DeskTop Standards

Contents Pages

PRINCIPLES OF DEHYDRATION ....................................................................... 1

Background .................................................................................................. 1

Water Content of Hydrocarbons ................................................................... 2

Water Content Measurement for Natural Gas .............................................. 3

Hydrate Formation ....................................................................................... 3

Hydrate Inhibition ........................................................................................ 4

WATER REMOVAL PROCESSES ........................................................................ 6

Liquid/Solid Desiccants................................................................................ 6

Glycol Dehydration ...................................................................................... 6

Background....................................................................................... 6

Process/Design Variables.................................................................. 9

Solid Desiccant Dehydration ...................................................................... 12

Background..................................................................................... 12

Adsorption Calculations ............................................................................. 17

Regeneration Calculations .......................................................................... 20

Dehydrating Liquids ................................................................................... 21

Process Variables ............................................................................ 22

Other Dehydration Processes .......................................................... 26

OPTIMIZING AND TROUBLESHOOTING DEHYDRATOR ........................... 28

Operations .................................................................................................. 28

Glycol Maintenance ................................................................................... 28

Methanol ......................................................................................... 28

Oxidation ........................................................................................ 28

Thermal Decomposition.................................................................. 28

pH Control ...................................................................................... 29

Salt Contamination.......................................................................... 29

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Saudi Aramco DeskTop Standards

Hydrocarbons.................................................................................. 29

Sludge ............................................................................................. 29

Foaming .......................................................................................... 30

Analysis and Control of Glycol .................................................................. 30

Glycol Loss Prevention .............................................................................. 31

Glycol Filtration ......................................................................................... 32

Optimizing Adsorption-Type Dehydrators ................................................. 32

Desiccant Performance ................................................................... 32

Equipment Items ............................................................................. 34

WORK AID 1: SOLUBILITY OF WATER IN LIQUID HYDROCARBONS .. 35

WORK AID 2A:WATER CONTENT OF HYDROCARBON GAS .................... 36

WORK AID 2B: EFFECTIVE WATER CONTENT FOR CO2............................ 37

WORK AID 2C: EFFECTIVE WATER CONTENT FOR H2S ............................ 38

WORK AID 3: PHYSICAL PROPERTIES OF CHEMICAL INHIBITORS ..... 39

WORK AID 4: USEFUL EQUATIONS FOR DEHYDRATION

CALCULATIONS...................................................................... 40

WORK AID 5: TYPICAL DESICCANT PROPERTIES.................................... 43

GLOSSARY .......................................................................................................... 46

APPENDIX A - SAUDI ARAMCO SOLID DESICCANT

DEHYDRATION UNITS ............................................................ 48

APPENDIX B - REPRESENTATIVE VENDORS OF SOLID

DESICCANT EQUIPMENT ....................................................... 49

APPENDIX C - VENDOR INFORMATION........................................................ 50

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Saudi Aramco DeskTop Standards 1

PRINCIPLES OF DEHYDRATION

Background

Liquid water and/or water vapor are removed from natural gas to:

• Prevent formation of hydrates in transmission lines.

• Meet a water dew point requirement of a sales gas contract.

• Prevent corrosion.

Many sweetening agents used in gas treating utilize an aqueous solution. Therefore,

dehydration usually follows gas treating. Techniques for dehydrating natural gas include:

• Absorption using liquid desiccants.• Adsorption using solid desiccants.

• Dehydration by refrigeration.

Through absorption, the water in a gas stream is dissolved in a relatively pure liquid solvent

stream. The reverse process, in which the water in the solvent is transferred into the gas

phase, is known as stripping. The term regeneration is also used to describe stripping (or

purification) because the solvent is usually recovered for reuse in the absorption step.

Absorption and stripping are frequently used in gas processing and most gas sweetening

operations, as well as in glycol dehydration.

The second major process by which water vapor is removed from a gas stream is called

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.

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

stream.

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Water Content of Hydrocarbons

Work Aid 1, based on experimental data, shows the solubility of water in sweet hydrocarbon

liquids. In sour hydrocarbon liquids, water solubility can be substantially higher. For sour

liquids an equation of state may be used to estimate water solubility.

The water content of a gas depends on pressure, temperature, and composition. The effect of

composition increases with pressure. For lean, sweet natural gases containing small amounts

of "heavy ends," pressure-temperature correlations are suitable for many applications. Work

Aid 2A is an example of one such correlation that has been widely used for many years in the

design of natural gas dehydrators. When the gas contains more than about 5% CO2 and/or

H2S, correction for the acid gas components should be made, particularly above 700 psia.

Below 40% acid gas components, one method of estimating the water content is to use

Equation 1 (Work Aid 4) and Work Aids 2A, 2B, and 2C.

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Water Content Measurement for Natural Gas

Specifications are given in GPA Publication 2140 in the GPA Technical Standards Book.

These include the valve freeze method, the Bureau of Mines Dew Point Tester, and the cobalt

bromide method. Cobalt bromide color change occurs at about 25-30 ppm. Absolute water

content can be determined by titration with Karl Fischer reagents.

There are several commercial instruments available for monitoring water content based on

other principles. Measuring water content less than 10 wppm, or determinations at less than -

40°F can be very difficult.

Hydrate Formation

A hydrate is a physical combination of water and other small molecules to produce a solid

that has an "ice-like" appearance, but possesses a different structure than ice.

There are two crystalline structures for gas hydrates. Limiting hydrate numbers (ratio of

water molecules to molecules of included gaseous component) are calculated using the size of

the gas molecules and the size of the holes in the H2O lattice. Smaller molecules (CH4,

C2H6, H2S) form a body centered cubic (Structure I) with a limiting hydrate number of 5 3/4

for CH4 and 7 2/3 for C2H6. Larger molecules (C3H8, i-C4H10) form a diamond lattice

(Structure II) with a limiting hydrate number of 17. Mixed gases will form Structure II.

The conditions that promote hydrate formation are:

Primary Considerations

• Gas must be at or below its water dew point with "free" water present.

• Low temperature.

• High pressure.

Secondary Considerations

• High velocities.

• Pressure pulsations.

• Introduction of a small crystal of the hydrate.

• Physical site for crystal formation such as a pipe elbow, an orifice, thermowell, or line

scale.

All of these primary and secondary considerations should be minimized when forced to

operate near a possible hydrate region.

Conditions for hydrate formation can be calculated using Hyprotech's HYSIM, SimSci's

PROCESS or PRO/II simulation programs. These calculations are based on the gas

composition and vapor-solid equilibrium constants.

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Hydrate Inhibition

The formation of hydrates can be prevented by dehydrating to prevent a free water phase, or

by inhibiting hydrate formation in the free water phase. Dehydration is usually preferable, but

inhibition can often be satisfactory.

Inhibition utilizes injection of one of the glycols or methanol to lower the hydrate formation

temperature at a given pressure. Both glycol and methanol can be recovered. For continuous

injection in non-cryogenic conditions, one of the glycols usually offers an economic

advantage. At cryogenic conditions, methanol usually is preferred because glycol's higher

viscosity makes effective separation very difficult.

Ethylene, diethylene, and triethylene glycols have been used for glycol injection. The most

popular has been ethylene glycol because of its lower cost, lower viscosity, and lower solubility in liquid hydrocarbons. Physical properties of these glycols are tabulated in Work

Aid 3.

The inhibitor must be present at the very point where the wet gas is cooled to its hydrate

temperature. Therefore, the inhibitor is sprayed upon the face of the feed gas chiller tube

sheet where free water is present. Injection must provide good distribution to every tube in

chillers and heat exchangers operating below the gas hydrate temperature.

Glycol and its absorbed water are separated from the gas stream, possibly along with

hydrocarbons. The glycol-water solution and liquid hydrocarbons can emulsify when

agitated, or when let down together from a high pressure to a lower pressure. Carefulseparator design will allow nearly complete recovery of the glycol for regeneration and

recycle.

The regenerator in a glycol injection system should be operated to produce a regenerated

glycol solution that will have a freezing point below the minimum temperature encountered in

the system. Figure 1 shows the freezing point of various concentrations of glycol water

solutions. To use this plot, locate the glycol concentration, read up to the glycol type, and

then read across to find the freezing point temperature. Glycol concentrations less than

70-75 wt% are typically used.

The minimum inhibitor concentration in the free water phase may be approximated byHammerschmidt's equation (Equation 2 in Work Aid 4). The quantity of inhibitor required to

prevent hydrate formation can also be calculated using Hyprotech's HYSIM, SimSci's

PROCESS or PRO/II simulation programs.

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The amount of inhibitor to be injected not only must be sufficient to prevent freezing of the

inhibitor water phase, but also must be sufficient to allow for some vaporization and the

solubility of the inhibitor in any liquid hydrocarbon. The vapor pressure of methanol is high

enough that significant quantities will vaporize. The total injection rate required may be

about three times that needed to maintain the water concentration desired. The amount of

methanol that will vaporize can be estimated using the above computer programs. No

allowance for glycol vaporization is necessary.

Inhibitors can cause problems in downstream process units. In these cases efficient inhibitor

separation should be provided.

FREEZING POINTS OF AQUEOUS GLYCOL SOLUTIONS

With permission from Gas Processors Suppliers Association. Source: GPSA Engineering

Data Book.

FIGURE 1

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WATER REMOVAL PROCESSES

Liquid/Solid Desiccants

In those situations where inhibition is not feasible or practical, dehydration must be used.

Both liquid and solid desiccants may be used, but economics favor liquid desiccant

dehydration when it will meet the required dehydration specification.

Liquid desiccant dehydration equipment is simple to operate and maintain. It can easily be

automated for unattended operation; for example, glycol dehydration at a remote production

well. Liquid desiccants can be used for sour gases, but additional precautions in the design

are needed due to the solubility of the acid gases in the desiccant solution.

Solid desiccants are normally used for extremely low dew point specifications as required to

recover liquid hydrocarbons.

Glycol Dehydration

Background

The more common liquids in use for dehydrating natural gas are diethylene glycol (DEG),

triethylene glycol (TEG), and tetraethylene glycol (TREG). In general, glycols are used for

applications where dew point depressions of the order of 60°F to 120°F are required.

DEG was the first glycol to be used commercially in natural gas dehydration and can providereasonable dew point control. With the exception of TEG, DEG is the best liquid available.

However, with normal field equipment, DEG can be concentrated to only 95% purity,

whereas TEG concentrations can reach 98 to 98.5% without special equipment. Although

both glycols perform sufficient dehydration in many situations, TEG is used more commonly

because it requires lower circulation rates for a comparable dew point depression than DEG

does and can reach lower dew points. It is not advisable to use triethylene glycol for

dehydration at low temperatures (approximately 50°F), due to its high viscosity. TREG is

primarily used when dehydration conditions fall between those encountered in normal TEG

operations, and those in which gas stripping or vacuum distillation becomes necessary. The

properties of these glycols are compared in detail in Work Aid 3.

A process flow diagram of a glycol dehydration unit is shown in Figure 2. Good practice

dictates installing an inlet gas scrubber, even if the dehydrator is near a production separator.

The inlet gas scrubber will prevent accidental dumping of large quantities of water,

hydrocarbons, and/or salt water into the glycol contractor. Even small quantities of these

materials can result in excessive glycol losses due to foaming, reduced efficiency, and

increased maintenance.

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PROCESS FLOW DIAGRAM FOR GLYCOL DEHYDRATION UNIT


Data Book.

FIGURE 2

In the glycol dehydration process, regenerated glycol is pumped to the top tray of the

contactor (absorber). The glycol flows down through the contactor countercurrent to the gas

flow. Water rich glycol is removed from the bottom of the contactor, passes through the

condenser coil, flashes off gas in a flash drum, and flows through the glycol-glycol heat

exchanger to the regenerator. In the regenerator, absorbed water is removed from the glycol

at atmospheric pressure by heating. The regenerated glycol is cooled in the glycol heat

exchangers and is recirculated to the contactor by the glycol pump.

TEG will absorb about 1 SCF of natural gas per gal at 1000 psig absorber pressure. There

will be more absorption if aromatic hydrocarbons are present. A three to five minute

residence time in the flash drum is required for degassing. Excessive hydrocarbons in the

glycol may cause high glycol losses and foaming. The overhead vent from the glycol

regenerator may contain hydrocarbons and should be piped to a safe location.

The separation of TEG and water in the regenerator is accomplished easily with only internal

reflux. The separation of DEG and water is more difficult due to DEG's higher vapor

pressure.

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EQUILIBRIUM WATER DEW POINTS FOR GASES IN CONTACT WITH

VARIOUS CONCENTRATIONS OF TEG


Data Book.

FIGURE 3

To obtain the high glycol concentrations required for high dew point depressions, stripping

gas or vacuum distillation must be used in the reboiler portion of the regeneration unit. The

amount of stripping gas required to reconcentrate the glycol to a high purity ranges from 2 to

10 ft3 per gallon of glycol circulated. If stripping gas is used, a recovery system may be

justified.

The dew point depression obtainable with triethylene glycol can be estimated from Figure 3

based on the contact temperature and the concentration of the reconcentrated glycol that is

used. Figure 3 shows the equilibrium water dew point at different temperatures for gases in

contact with various concentrations of glycol. To use this plot, locate the contact temperature,

read up to the glycol concentration, and then read across to find the equilibrium water dew

point. In practice it is seldom economical for actual gas dew points to approach equilibrium

dew points closer than 20°F.

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Process/Design Variables

Several process and design variables have an important effect on the successful operation of a

glycol dehydration system.

Gas Temperature

Plant performance is especially sensitive to the temperature of the incoming gas. At constant

pressure, the water content of the inlet gas increases as this temperature is raised. Glycol

vaporization losses are also increased at the the higher temperature. Furthermore, problems can

result from too low a temperature (below 50°F) because glycol becomes very viscous.

Lean Glycol Temperature

The temperature of lean glycol entering the absorber has a significant effect on the gas dew

point depression, and should be held to a minimum to achieve the best operation. However, it

should be kept at least 10°F above the inlet gas temperature to minimize hydrocarboncondensation in the absorber and subsequent foaming.

Glycol Reboiler Temperature

The reboiler temperature controls the concentration of the water in the glycol. With a

constant pressure, the glycol concentration increases with higher reboiler temperatures. The

reboiler temperature should never be allowed to remain at or above the glycol degradation

temperatures (see Work Aid 3) for any period of time. When higher glycol concentrations are

required, stripping gas can be added to the reboiler.

Regenerator Top Temperature

The temperature in the top of the regenerator is also important. A high temperature can

increase glycol losses due to excessive vaporization. The recommended temperature in the

top of the column is about 225°F. If the temperature in the top of the column drops too low,

too much water can be condensed and washed back into the regenerator to flood the column

and fill the reboiler with excessive liquids.

Contactor Pressure

At constant temperature, the water content of the inlet gas decreases with increasing pressure.

Therefore, less glycol circulation is required at higher pressures. However, if carbon dioxide

is present, at a certain point a higher pressure will actually increase the water content. If not

otherwise fixed, optimum dehydration pressure is typically in the range of 700 to 1100 psig.

Reboiler Pressure

Reducing the pressure in the reboiler at a constant temperature results in higher glycol purity.

This pressure reduction lowers the water partial pressure in the vapor, increasing the driving

force under which water leaves the glycol solution.

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Glycol Concentration

The water content of the dehydrated gas depends primarily on the lean glycol concentration.

The dry gas leaves the contactor approaching equilibrium with the lean glycol. The leaner the

glycol flowing to the absorber, the more efficient the dehydration. Figure 3 shows the effectof glycol concentration on gas dew point.

Glycol Circulation Rate

When the number of absorber trays and glycol concentration are fixed, the dew point

depression of a saturated gas is a function of the glycol circulation rate. Whereas the glycol

concentration mainly affects the dew point of dry gas, the glycol rate controls the total amount

of water that can be removed. A typical glycol circulation rate is about three gallons of glycol

per pound of water removed (seven maximum). The minimum circulation rate to assure good

glycol-gas contacting is about two gallons of glycol for each pound of water removed.

A greater dew point depression is easier to achieve by increasing the glycol concentrationrather than by increasing the glycol circulation rate (see Figure 4). To use this plot, locate the

glycol circulation rate, read up to the glycol concentration, and then read across to find the

dew point depression. An excessive circulation rate, especially above the design capacity,

overloads the reboiler and prevents good glycol regeneration. It also prevents adequate

glycol-gas contacting in the absorber, increases pump maintenance problems, and can

increase glycol losses.

EFFECT OF TEG CIRCULATION RATE AND CONCENTRATION ON DEW POINT

DEPRESSION

From non-proprietary information from Exxon Production Research Company, Production

Operations Division, "Dehydration and Hydrate Inhibition," July, 1986, from Surface

Facilities School, Vol. II.

FIGURE 4

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Solid Desiccant Dehydration

Background

Since solid desiccant units cost more than glycol units, their use is usually limited to

applications such as very sour gases, very low water dew point requirements, simultaneous

control of water and hydrocarbon dew points, and special cases such as oxygen containing

gases, etc. In cryogenic plants, solid desiccant dehydration usually is preferred over methanol

injection to prevent hydrate and ice formation. Solid desiccants are also often used for the

drying and sweetening of NGL liquids. Appendix A is a listing of Saudi Aramco's solid

desiccant dehydration units.

Desiccants in common commercial use fall into one of three categories:

• Alumina - Regenerable aluminum oxide base desiccant.

• Silica Gel - Regenerable silicon oxide adsorbent.

• Molecular Sieves - Regenerable solid desiccants composed of crystalline metal

aluminosilicates (zeolites).

Each desiccant category offers advantages in different services. The best choice is not

routine. A listing of representative vendors of solid desiccants is presented in Appendix B.

Activated alumina has a strong affinity for water and high internal adsorption area due to the

presence of pores or very fine capillaries. Alumina condenses and holds the water in the

pores by surface adsorption and capillary attraction. Activated alumina desiccant can be used

for drying liquids which do not contain unsaturates such as olefins or diolefins. It is less

costly than molecular sieve desiccant but its capacity for absorbing water also tends to be

lower, particularly when attempting to reach very low water levels, e.g. 5 wppm in the

product.

Silica gel has a higher equilibrium adsorption capacity (see Figure 6) than alumina because its

available surface is greater. Due to silica gel's higher price per pound, alumina is generally

the economic choice. Silica gel is not used where free water can be present, because free

water destroys silica gel. Free water over long-term operation, either as droplets or slugs, willalso damage molecular sieve and activated alumina by mechanical attrition and should be

avoided.

Molecular sieves have a high water equilibrium capacity at low relative humidities (see Figure

6). Molecular sieves also have the feature of uniform pore size, which allows them to exclude

molecules based on size. Because different pore size molecular sieves are produced, selection

of proper type of sieve can alleviate the problem of undesirable coadsorption. Molecular

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sieves have a higher design adsorption capacity than the other regenerable desiccants, but this

is often offset by their considerably higher price per pound.

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Molecular sieve dehydrators are commonly used ahead of NGL recovery plants where

extremely dry gas is required. Cryogenic NGL plants designed to recover ethane produce

very cold temperatures and require very dry feed gas to prevent formation of hydrates.

Dehydration to approximately 1 ppmw is possible with molecular sieves.

Two types of molecular sieves, Type 3A and Type 4A, are commonly used for drying

hydrocarbon liquids. Type 4A sieves are less costly than Type 3A sieves and are used for

distillates which do not contain unsaturates. When unsaturates are present in the feed, Type

3A are used to assure good regeneration.

WATER VAPOR ADSORPTION AT 60°F




FIGURE 6

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TYPICAL MOLECULAR SIEVE GAS DEHYDRATION VESSEL




FIGURE 8

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Adsorption Calculations

Adsorption calculations for a molecular sieve dehydrator are discussed below. The allowable

superficial vapor velocity through the bed is the first parameter that must be estimated using

Figure 9. To use this plot, locate the operating pressure, read up to the type sieve, then read

across to find the allowable superficial velocity. Once the allowable superficial velocity is

estimated, the bed diameter can be calculated for a design vapor rate. The design pressure

drop through the bed is calculated using Equation 4 in Work Aid 4 and should be about five

psi. A design pressure drop higher than eight psi is not recommended.

ALLOWABLE VELOCITY FOR MOLE SIEVE DEHYDRATOR


Data Book.

FIGURE 9

The next step is to choose a cycle time and calculate the pounds of sieve required. Eight to

twelve hour cycles are common. Cycles greater than 12 hours may be justified, especially if

the gas is not water saturated. Long cycles mean fewer regenerations and longer sieve life,

but larger beds and additional capital investment are required.

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During the adsorption cycle, the bed operates with three zones The top zone is called the

saturation zone. The molecular sieve in this zone is in equilibrium with the wet inlet gas. The

middle or mass transfer zone (MTZ) is where the water content of the gas is reduced from

saturation to < 1 ppm. Normally a system is designed so that there is a moisture analyzer to

indicate when the mass transfer zone is likely to break through the end of the bed. A guard

bed zone (typically one to two feet deep) is provided after this point to prevent actual

breakthrough before the system has a chance to change to the regenerated bed.

Unfortunately, both the water capacity and the rate at which the molecular sieves adsorb water

change as the molecular sieves age. The object of the design is to install enough sieve so that

three to five years into the life of the sieve, the mass transfer zone will be at the bottom of the

bed at the end of the adsorption cycle.

In the saturation zone, the molecular sieve is expected to hold approximately 13 pounds of water per 100 pounds of sieve. This capacity needs to be adjusted when the gas is not water

saturated or when the temperature is above 75°F. See Figures 10 and 11 for the correction

factors. To determine the pounds of molecular sieve required in the saturation zone, calculate

the amount of water to be removed during the cycle and divide by the sieve capacity (use

Equations 5 and 6 in Work Aid 4).

Even though the MTZ will contain some water, the saturation zone is calculated assuming it

will contain all the water to be removed. The length of the mass transfer zone can be

calculated using Equation 7 from Work Aid 4. The total bed height is the summation of the

saturation zone, mass transfer zone, and guard bed zone heights. Approximately six feet free

space above and below the bed is needed.

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MOLE SIEVE CAPACITY CORRECTION FOR UNSATURATED INLET GAS


Data Book.

FIGURE 10

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MOLE SIEVE CAPACITY CORRECTION FOR TEMPERATURE


Data Book.

FIGURE 11

Regeneration Calculations

Regeneration calculations for a molecular sieve dehydrator are discussed below. The first

step in the regeneration calculations is to calculate the total heat required to desorb the water

and heat the sieve and vessel. A 10% heat loss is assumed. For the entire regeneration cycle,

only about 1/2 of the heat put into the regeneration gas is utilized. This is because by the endof the cycle the gas is leaving the bed at about the same temperature at which it enters. The

heating time is usually 1/2 to 5/8 of the total regeneration time, which must include a cooling

period. For 8 hour adsorption cycles, the regeneration normally consists of 4-1/2 hours of

heating, 3 hours of cooling and 1/2 hour for standby and switching. For longer cycles, the

heating time can be lengthened as long as a minimum pressure drop of 0.1 psi/ft is

maintained. Figure 12 can be used to estimate the required minimum velocity to meet 0.10

psi /ft. To use this plot, locate the operating pressure, read up to the type sieve, then read

across to find the minimum superficial velocity.

The regeneration cycle frequently includes depressuring/repressuring to match the

regeneration gas pressure and/or to maximize the regeneration gas volume to meet thevelocity criterion. Some applications, termed pressure swing adsorption, regenerate the bed

only with depressurization and sweeping the bed with gas just above atmospheric pressure.

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MINIMUM REGENERATION VELOCITY FOR MOLE SIEVE DEHYDRATOR


Data Book.

FIGURE 12

Dehydrating Liquids

Typical liquid dehydration systems have filtering equipment ahead of the dehydration unit to

remove particulates that may clog the solid desiccant. There are usually two beds containing

the desiccant, although it is sometimes necessary to have additional beds to handle very large

streams. The desiccant beds are regenerated on an alternating basis. Downstream from the

desiccant beds, another filter must be used to capture small particles of desiccant which may

be attrited during the drying step.

The diameter of desiccant beds is usually determined by the volumetric flow rate. For

activated aluminas, a maximum of 30 gallons per minute per square foot of cross-sectionalarea is recommended. Bed depth determines the time required for the drying step. In liquid

dehydration, drying times are typically 12 to 120 hours before regeneration.

Liquids that cannot be dried with activated alumina are those exhibiting complete miscibility

with water, and highly acidic and caustic compounds. The latter types can cause

disintegration of the activated alumina in addition to being corrosive to equipment.

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Desiccant regeneration is performed in one of two basic ways. At times the only available

medium for heated regeneration is a slipstream of the process stream which must be

vaporized. An alternative scheme uses vapor or gas other than the vaporized process stream

to regenerate the desiccant.

The heat requirement for regeneration include the sensible heats for the desiccant and

equipment plus the heat required to desorb the water and hydrocarbons. The latter consists of

the sensible heats, the latent heats of vaporization, and the heats of wetting. These values

may be calculated individually and added together or may be estimated at 1,600 Btu/lb water

and 200 - 800 Btu/lb hydrocarbons. The amount of water for this calculation is assumed to be

the water content of the inlet stream over the drying period. The hydrocarbon liquids present

are estimated to be the amount contained in the total pore volume of 0.048 gallons for

hydrocarbon liquid per pound of adsorbent.

The temperature necessary for regeneration is that needed to assure the desired effluent water

content in the next drying step. For activated aluminas, temperatures of approximately 400°F

at the exit end of the bed during regeneration result in effluent water levels down to about 10

ppmw. Increasing the temperatures to about 550°F results in effluent water contents of 1

ppmw or below. The extent of removal of hydrocarbons depends primarily on the boiling

points of the hydrocarbons. The regeneration temperature should be that which gives as

complete removal of the hydrocarbons as possible within the economics of the system.

Drying of hydrocarbon liquids (NGL, liquid propane and butane) is done at Saudi Aramco's

Shedgum, Uthmaniyah, Ju'aymah, and Yanbu facilities (see Appendix A).

Process Variables

Several process variables can have a major effect on solid desiccant bed sizing and operating

efficiency.

Quality of Inlet Gas

The most important variable in sizing a desiccant bed is the relative saturation of the inlet gas.

This variable is the driving force that affects the transfer of water to the adsorbent.

The performance of a desiccant bed designed to remove water is adversely affected by gas

containing quantities of carbon dioxide, heavy hydrocarbons (even in vapor phase), andsulfur-bearing compounds. The greater the molecular weight of a compound, the greater its

adsorption potential.

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Temperature

The following are important temperature-related factors:

• Molecular sieves and most other adsorbents have significantly higher adsorptivecapacity at low temperatures.

• Temperature of the regeneration gas that mixes with the incoming wet gas ahead of the

dehydrators is also important. If the temperatures of these two gas streams differ more

than 15 to 20°F, liquid water and hydrocarbons may condense as the hotter gas stream

cools. Condensed liquids that strike the bed can shorten the desiccant's life.

• The temperature of the hot gas entering and leaving a desiccant tower during the heating

cycle can significantly affect plant efficiency and the desiccant life. To assure good

desorption of the water and contaminants, a high regeneration gas temperature is

needed. The maximum hot gas temperature needed depends on the type of contaminants and the "holding power" or affinity of the desiccant for the contaminants.

• The desiccant bed temperature reached during the cooling cycle is important. If wet gas

is used to cool the desiccant, the cooling cycle should be terminated when the desiccant

bed reaches a temperature of about 125°F. Additional cooling may cause water to be

adsorbed from the wet gas stream and presaturate or preload the desiccant bed, before

the next adsorption cycle begins. If dry gas is used for cooling, the desiccant should be

cooled within 10 to 20°F of the incoming gas temperature during the adsorption cycle.

This will keep the desiccant's adsorption efficiency high.

• The temperature of the regeneration gas going through the regeneration gas scrubber or

water knockout should be held low enough to condense and remove as much of the

water and hydrocarbons from the gas as possible.

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Pressure

Generally, the adsorption capacity of a desiccant bed decreases as the pressure is lowered. At

constant temperature, the water content of the inlet gas increases as the separation pressure is

lowered. If the dehydrators are operated well below design pressure, the desiccant will haveto work harder to remove the additional water and to maintain the desired effluent dew point.

With the same volume of incoming gas, the increased gas velocity occurring at the lower

pressures could affect the effluent moisture content and damage the desiccant. At pressures

above 1300 to 1400 psia, the co-adsorption effects of hydrocarbons are very significant.

Cycle Time

Most adsorbers operate on a fixed drying cycle time, and that time is frequently set for the

worst conditions. However, the adsorbent capacity is not a fixed value; it 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

typically be achieved. As the desiccant ages, the cycle time can be shortened to saveregeneration fuel costs and improve the desiccant life.

Gas Velocity

As the gas velocity during the drying cycle decreases, both lower effluent moisture contents

and longer drying cycle times to the breakthrough point are usually obtained. However, low

linear velocities require towers with large cross-sectional areas to handle a given gas flow. In

selecting the linear flow rate, therefore, a compromise must be made between the tower

diameter and the maximum utilization of the desiccant. At lower velocities, the gas may

channel through the desiccant bed without being properly dehydrated. A lower adsorption

efficiency and desiccant damage may occur at higher velocities.

The regeneration gas velocity is important, especially when effluent moisture contents below

0.1 ppm are needed. A minimum heating gas velocity of 10 ft/min may be required to

achieve this superdehydration. At lower velocities, the hot gas may channel through the

desiccant bed, tending to leave excess water in the bed after regeneration and resulting in poor

dehydration.

Sources of Regeneration Gas

The source of gas for heating and cooling desiccant beds depends on plant requirements and,

possibly, on the availability of a suitable gas stream. When low effluent moisture contents (in

the range of 0.1 ppm) are required, the regeneration stream should be dry. Plant tail gas cannormally be used for this purpose. If only moderate drying is required, a portion of the wet

feed gas can be used.

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Direction of Gas Flow

The flow direction influences effluent purity, regeneration gas requirements, and desiccant

life. The direction of flow during the drying cycle is normally downward. This direction

permits higher velocities without lifting or fluidizing the desiccant bed. Fluidization can

severely damage the desiccant.

The direction of heating gas flow is generally countercurrent (opposite) to the direction of the

adsorption flow. This flow permits better reactivation of the lower portion of the desiccant

bed, which must perform the superdehydration during the drying cycle, especially in

cryogenic plants.

Bed support and screens on top of the bed consist of three to five layers in graduated sizes.

Since the flow is both directions through the bed, both ends must be protected.

Desiccant Selection No desiccant is suitable for all applications. In some cases, the choice is determined primarily

by economics. Sometimes process conditions control the choice. Many times desiccants are

interchangeable, and the equipment designed for one desiccant can often operate effectively

with another.

The desirable characteristics of a solid desiccant are listed below:

• 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.

• High adsorptive capacity retained after repeated regeneration, allowing smaller initial

charge and longer service before replacement.

• Low resistance to gas flow, to minimize gas pressure drop through the unit.

• 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, avoiding the need for special alloys and costly

measures to protect the operator's safety.

• Low cost, to reduce initial and replacement costs.

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Desiccant Selection (Cont' d)

Certain physical characteristics of the more common solid desiccants are given in Work Aid 5

and Appendix C.

Molecular sieves have the highest adsorptive capacity of all the desiccants, when the feed gas

is at very high temperatures or at low relative saturation. If the feed gas entering the

dehydrators is saturated with water vapor, however, silica gel or alumina may be a better

selection. For water-saturated gases, these desiccants can adsorb twice as much water as

molecule sieves and at lower first cost. Silica gel has another advantage in that it can be

regenerated to a lower water content than molecular sieves and at much lower temperatures

(400°F for silica gel versus 500 to 600°F for sieves).

Effect of Regeneration Gas on Outlet Gas Quality

Regeneration gas desorbs molecular sieve beds in the reverse order of the adsorption bond.

For example, adsorbed methane and ethane would be desorbed first, then propanes andheavier 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.

Other Dehydration Processes

Two other general categories of dehydrators are briefly discussed below. The first of these is

the nonregenerable dehydrator, of which the calcium chloride brine unit will be described.

The second is refrigeration, in which dehydration is not necessarily the prime purpose; but the

availability of dehydration as a side benefit further justifies its installation.

Nonregenerable Dehydrator

Calcium chloride is used as a consumable desiccant. Solid calcium chloride combines with

water vapor to form a brine solution. The drying tower must periodically be recharged with

fresh calcium chloride and the brine frequently removed from the bottom of the tower (see

Figure 13). A tray section in the tower is often used in conjunction with a dry bed of calcium

chloride to take advantage of the brine's capability to combine with additional water vapor.

Up to 3.5 lb H2O/lb CaCl2 can be absorbed with the addition of trays versus 1.1 lb H2O/lb

CaCl2 for a dry bed type. This type of installation is efficient for remote, small-capacity gas

wells without heat or fuel. It is reported that units of this type can lower the dew point to 7°F

with a bed depth as low as 2 feet and a gas temperature of 127°F.

Refrigeration

Whether cooling is obtained by gas expansion or mechanical refrigeration, its net effect is the

condensation of water from saturated inlet gas when the gas is cooled. The removal of water

from the system achieves dehydration. Of course, the water condensation and water removal

steps require an environment resistant to hydrate formation, either by use of inhibitors or by

spot heating.

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CALCIUM CHLORIDE (NONREGENERABLE) DEHYDRATOR




FIGURE 13

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OPTIMIZING AND TROUBLESHOOTING DEHYDRATOR

Operations

Glycol Maintenance

Operating and corrosion problems usually occur when the circulating glycol gets dirty.

Therefore, to achieve a long, trouble-free life from the glycol, it is necessary to recognize

these problems and know how to prevent them. Some of the major areas are discussed below:

Methanol

Methanol in the feed gas to a glycol dehydrator will be absorbed by the glycol. This results in

the following problems:

• Methanol will add additional heat duty on the reboiler and additional vapor load on the

regenerator. High methanol injection rates and slug carryover can cause flooding.

• Aqueous methanol causes rust in carbon steel, so corrosion can occur in the regenerator

and reboiler vapor space.

Most of the methanol absorbed in the rich glycol solution can be removed by flashing in the

regenerator. Activated carbon filters are used to adsorb methanol from the lean glycol

solution to avoid these problems.

Oxidation

Oxygen enters the system with the incoming gas, through unblanketed storage tanks and

sumps, or through the pump packing glands. Sometimes glycol will oxidize in the presence

of oxygen and form corrosive acids. To prevent oxidation, bulk storage tanks should have a

gas blanket to keep air out of the system. Oxidation inhibitors can also be used to minimize

corrosion.

Thermal Decomposition

Excessive heat, a result of one of the following conditions, will decompose glycol and form

corrosive products:

• High reboiler temperature above the glycol decomposition level.

• High heat-flux.

• Localized overheating, caused by deposits of salt or tarry products on the reboiler fired

tubes or by poor flame direction on the fired tubes.

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pH Control

New glycol has a neutral pH of approximately seven. As it is used, however, the pH always

decreases and the glycol becomes acidic and corrosive, unless pH neutralizers or buffers are

used. The equipment corrosion rate increases rapidly with a decrease in the glycol pH. Acids

created by glycol oxidation, thermal decomposition products, or acid gases picked up from

the gas stream are the most troublesome of corrosive contaminants. A low pH accelerates the

decomposition of glycol. Ideally, the glycol pH should be held at a level of 7.0 to 7.5. A

value above 8.0 to 8.5 tends to make glycol foam and emulsify.

Borax, ethanolamines (usually triethanolamine), or other alkaline neutralizers can be used to

control the pH. These neutralizers should be added with great care -- slowly and continuously

-- for best results. An overdose of neutralizer will usually precipitate a suspension of black

sludge in the glycol. The sludge could settle and restrict glycol circulation. Frequent filter-

element changes should be made while pH neutralizers are added.

Salt Contamination

Salt deposits accelerate equipment corrosion, reduce heat transfer in the reboiler tubes, and

alter specific gravity readings when a hydrometer is used to measure glycol-water

concentrations. This troublesome contaminant cannot be removed with normal regeneration.

Therefore, an efficient scrubber upstream of the glycol plant should be used to prevent salt

carryover with the incoming gas. In areas where large quantities of brine are produced, some

salt contamination will occur. The removal of salt from the glycol solution is then necessary.

Salt contaminated glycol may be reclaimed by several methods. Scraped-surface heat

exchangers in conjunction with centrifuges are used in cases of extreme contamination. Other

reclamation methods are vacuum distillation or ion exchange.

Hydrocarbons

Liquid hydrocarbons, a result of carryover with the incoming gas or condensation in the

absorber, increase glycol foaming, degradation, and losses. They must be removed with a

glycol-gas separator, hydrocarbon liquid skimmer, or activated carbon beds.

Sludge

An accumulation of solid particles and tarry hydrocarbons very often forms in the glycol.

This sludge is suspended in the circulating glycol; over a period of time, the accumulation becomes large enough to settle out. This action results in the formation of black, sticky,

abrasive gum that can cause trouble in pumps, valves, and other equipment, usually when the

glycol pH is low. The gummy substance becomes hard and brittle when deposited on the

absorber trays, stripper packing, and other places in the circulating system. Good solution

filtration prevents a buildup of sludge.

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Foaming

Foaming can increase glycol losses and reduce plant capacity. Entrained glycol will be

carried over the top of the absorber with the sales gas when stable foam builds up on the trays.

Foaming also causes poor contacting between the gas and glycol, decreasing the drying

efficiency.

Some foam promoters are:

• Hydrocarbon liquids.

• Field corrosion inhibitors.

• Salt.

• Finely divided suspended solids.

Excessive turbulence and high liquid-to-vapor contacting velocities usually cause the glycolto foam. This condition can be caused by mechanical or chemical problems.

The best way to prevent foaming is proper care of the glycol. This involves effective gas

cleaning ahead of the glycol system and good filtration of the circulating solution. The use of

defoamers does not solve the basic problem, and serves only as a temporary control until the

conditions generating foam can be identified and removed.

Analysis and Control of Glycol

Analysis of glycol is essential to good plant operation. Meaningful analytical information

helps pinpoint high glycol losses, foaming, corrosion, and other operating problems.

Analyses enable the operator to evaluate plant performance and make operating changes to

obtain maximum drying efficiency.

A glycol sample should first be visually inspected to identify some of the contaminants:

• A finely divided black precipitate may indicate the presence of iron corrosion products.

• A black, viscous solution may contain heavy hydrocarbons.

• The characteristic odor of decomposed glycol (a sweet aromatic odor) usually indicates

thermal degradation.

• A two-phase liquid sample usually indicates the glycol is heavily contaminated with

hydrocarbons.

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Analysis and Control of Glycol (Cont' d)

The visual inspections should next be supported by chemical analysis. Samples of the lean

and rich glycol should be taken and routine tests performed: salt analysis, solids content, pH,

iron content, foam test, and titration procedure (to determine the amount of neutralizer necessary to raise the pH to a safe level). These analyses usually provide sufficient

information to determine the condition of the glycol.

Glycol Loss Prevention

Glycol losses can be defined as liquid carryover from the contactor (normally 0.10 gal/ MSCF

with a standard mist eliminator) plus vaporization from the contactor and regenerator, and

spillage. Glycol losses, exclusive of spillage, range from 0.05 gal/MSCF for high pressure,

low temperature gases to as much as 0.30 gal/MSCF for low pressure, high temperature gases.

There are several ways to reduce glycol losses.

• A certain amount of glycol always vaporizes in the sales gas stream. Adequate cooling

of the lean glycol before it enters the absorber minimizes these losses.

• Normally, most of the glycol entrainment is removed by a mist eliminator in the top of

the absorber. Excessive gas velocities and glycol foaming in the absorber sharply

increase the glycol carryover. A downstream gas scrubber can pay for itself quickly and

save much money by trapping the carryover and recovering the excess glycol. This gas

scrubber also helps prevent problems downstream of the glycol plant.

• Vaporization losses in the stripper can be held to a minimum with good glycolcondensation and control of the tower top temperature. Glycol entrainment, or

mechanical carryover, can be reduced with proper maintenance of the stripper and

reboiler.

• Mechanical leaks can be reduced by keeping the pump, valves, and other fittings in

good order. The glycol from these leaks should be collected and reprocessed.

• Excessive entrainment losses may be the result of foaming in the absorber and/or

regenerator. Defoamers are sometimes used.

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Glycol Filtration

Filters extend the life of the glycol pumps, and prevent an accumulation of solids in the

absorber and regeneration equipment. Solids that settle on metal surfaces frequently set up

cell corrosion. Filters also remove the solids that contribute to fouling, foaming, and

plugging. Sock-type filters are preferred, although fine screens and cartridge filters are also

suitable. The filters should be designed to remove all solid particles over 5 microns in size.

They should be able to operate up to pressure drops of 20 to 25 psi. For best results, filters

should be 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 plant start-up, or when

neutralizers are added to control the glycol pH.

Activated carbon filters can eliminate most problems caused by hydrocarbons, well-treating

chemicals, compressor oils, and other troublesome impurities in the glycol. Use of carbon

filters increases glycol efficiency and life. The carbon filter should be placed on the leanstream since it has the lowest hydrocarbon load (a large portion of entrained hydrocarbons are

flashed off in the regenerator) and it should be placed downstream of the solids filter. The

carbon filter is generally sized for 1 to 5 gpm per ft2 of carbon bed cross-sectional area with a

bed depth of 3 to 10 feet. A slipstream of lean glycol equal to 10% of the circulation rate is

typically fed to the carbon filter. A full stream filter would require such a large vessel that it

would not be economical or practical to use.

Optimizing Adsorption-Type Dehydrators

Desiccant Performance

Operating data should be monitored to try to prevent permanent damage to the desiccant.

Performance tests are frequently scheduled on a routine basis, ranging from monthly during

early operations, to six months or longer. The size of the unit and the quantity of the

desiccant also affect the frequency of performance tests.

Desiccants decline in adsorptive capacity at different rates under varying operating

conditions. Markedly different capacity-decline rates may be experienced for the same

desiccant under similar conditions of gas flow, temperature, pressure, water removal

requirements, cycle times, and regeneration temperatures. Desiccant aging is a function of

many factors, including the number of cycles experienced and exposure to any harmful

contaminants present in the inlet stream. Many of these contaminants are not completely

removed during normal reactivation. Contaminants may be the cause of 90% of

unsatisfactory solid desiccant operations. Therefore, the single most important variable

affecting the decline rate of desiccant capacity is the chemical composition of the gas or liquid

to be dried. Feed stream composition should always include the contaminants.

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The capacity of a new desiccant will decline slowly during the first few months in service

because of cyclic heating, cooling, and wetting. Desiccant capacity usually stabilizes at about

55 to 70% of the initial capacity. To get maximum use out of the desiccant, a moisture

analyzer can be used to optimize the drying cycle time. That time can be shortened as the

desiccant ages. Both inlet and outlet moisture-analyzer probes should be used. Moisture

analyzers for very low water contents require care to prevent damage to the probes. Sample

probes and temperature probes must be installed to reach the center of the gas phase.

Proper conditioning of the inlet gas is important. Compressor oils, corrosion inhibitors,

glycols, amines, and other high-boiling contaminants present in the feed gas cause a further

decline in desiccant capacity, because normal reactivation temperatures will not vaporize the

heavy materials. The residual contaminants slowly build up on the desiccant's surface,

reducing the area available for adsorption. Many corrosion inhibitors chemically attack

certain desiccants, permanently destroying their usefulness. A layer of less expensivedesiccant can be installed on the top of the bed to catch these contaminants.

Although gases rich in heavier hydrocarbons may be dried satisfactorily with molecular

sieves, the use of this same rich gas in a 550 to 600°F regeneration service aggravates coking

problems. Lean dry gas is always preferable for regeneration, if it is available.

Methanol in the inlet gas is a major contributor to the coking of molecular sieves where

regeneration is carried out at temperatures above 550°F. Polymerization of methanol during

regeneration may produce dimethyl ether and other intermediates that will cause coking of the

beds.

Monitoring bed differential pressure is important. An increase in differential pressure can

indicate desiccant problems such as excessive coking or the formation of fines. The

differential pressure along with the bed run length should also be recorded when doing a

performance test on a desiccant bed.

The useful life of most desiccants ranges from one to four years in normal service. A longer

life is possible if the feed gas is kept clean. The effectiveness of reactivation can also play a

major role in slowing the decline of a desiccant's adsorptive capacity and in prolonging its

useful life. Obviously, if all the water is not removed from the desiccant during each

regeneration, its usefulness will sharply decrease.

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WORK AID 1: SOLUBILITY OF WATER IN LIQUID HYDROCARBONS


Data Book.

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WORK AID 2B: EFFECTIVE WATER CONTENT FOR CO2

Use Photostat


Data Book.

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• Molecular sieve pressure drop.

∆PL = BµV + Cρ V2 (Eqn. 4)

where: ∆P = Pressure drop, psi.

L = Length of packed bed, ft.

V = Vapor velocity, ft/min.

µ = Vapor viscosity, cP.

ρ = Vapor density, lb/ft3.

Constants:

Desiccant Type B C

1/8 in. bead 0.0560 0.0000889

1/8 in. extrudate 0.0722 0.000124

1/16 in. bead 0.152 0.000136

1/16 in. extrudate 0.238 0.000210

• Molecular sieve requirement in adsorber saturation zone

SS =Wr

0.13 CSS CT (Eqn. 5)

where: SS = Amount of molecular sieve required in saturation zone, lb.

CSS = Saturation correction factor for sieve (see Figure 10).

CT = Temperature correction factor (see Figure 11).

• Length of molecular sieve packed bed saturation zone.

LS =SS ρ bd 4

3.14 D2(Eqn. 6)

where: ρ bd = Bulk density (see Work Aid 5).

LS = Length of packed bed saturation zone, ft.

D = Bed diameter, ft.

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• Length of molecular sieve packed bed mass transfer zone.

LMTZ

= (V/35)0.3 (Z) (Eqn. 7)

where: LMTZ =Length of packed bed mass transfer zone, ft.

V = Vapor velocity, ft/min.

Z = 1.70 for 1/8 in. sieve.

0.85 for 1/16 in. sieve.

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WORK AID 5: TYPICAL DESICCANT PROPERTIES

DESICCANT SHAPE

BULK DENSITY

lb/ft3 PARTICLE SIZE

APPROXIMATEMINIMUM

MOISTURE

CONTENT OF

EFFLUENT GAS

(wppm)

Alumina Gel Spherical 52 1/4 in. 5-10

Activated

Alumina

Granular 52 1/4 in.-8 mesh 0.1

ActivatedAlumina Spherical 47-48 1/4 in.-8 mesh 0.1

Silica Gel Spherical 50 4-8 mesh 5-10

Silica Gel Granular 45 3-8 mesh 5-10

Mole Sieve Spherical 42-45 4-8 mesh or

8-12 mesh

0.1

Mole Sieve Extruded

Cylinder

40-44 1/8 in. or 1/16 in. 0.1


Data Book.

BULK

DENSITY

(lb/ft3)

SPECIFIC

HEAT

(Btu/lb/°F)

NORMAL

SIZES USED

DESIGN

ADSORPTIVE

CAPACITY

(wt%)

Activated alumina 51 0.24 1/4 in.-8 mesh 7

Mobil SOR beads 48 0.25 4-8 mesh 7

Florite 50 0.24 4-8 mesh 4-5

Alumina gel (H-151) 52 0.24 1/8 in.-1/4 in. 7

Silica gel 45 0.22 4-8 mesh 7

Molecular sieves (4A) 45 0.25 1/8 in. 14

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GLOSSARY

absorption The assimilation of one material into another. In natural gas

dehydration, the use of an absorptive liquid to selectively remove

water vapor from a gas stream.

adsorption Adhesion of molecules of gases, liquids, or dissolved substances to

a solid surface, resulting in relatively high concentration of the

molecules at the place of contact.

alumina A regenerable aluminum oxide base desiccant.

calcium chloride A type of consumable desiccant.

DEG Diethylene glycol.

dehydration The act or process of removing equilibrium water from gases or

liquids.

desiccant A substance used in a dehydrator to remove water and moisture.

EG Ethylene glycol.

hydrate A solid material resulting from the combination of a hydrocarbon

with water under pressure.

molecular sieves Regenerable solid desiccants composed of crystalline metal

aluminosilicates (zeolites).

MSCF Million standard cubic feet.

MTZ Mass transfer zone for an adsorption bed.

NGL Natural gas liquids are those hydrocarbons liquefied at the surface

in field facilities or in gas processing plants. Natural gas liquids

include propane, butanes and natural gasoline.

regeneration A process by which a catalyst or a chemical reagent is returned

close to its original reactiveness.

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silica gel A regenerable silicon oxide adsorbent.

stripping Substantially complete removal of the more volatile components

from a mixture. It is usually accomplished by passing the hot bottoms from a flash drum or tower through a stripping vessel

through which steam or inert gas is passed, to sweep out the volatile

components.

TEG Triethylene glycol.

TREG Tetraethylene glycol.

water dew point The temperature at which water vapor starts to condense from a gas

mixture.

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APPENDIX B - REPRESENTATIVE VENDORS OF SOLID DESICCANT

EQUIPMENT

Activated Alumina

Alcoa-Aluminum Company of America

Pittsburgh, Pennsylvania 15219

Telephone: (800) 533-4511

Rhone-Poulenc Chimie Minérale Fine

18 Avenue d'Alsace

Cedex 29, 92097 Paris La Defense

Courbevoie, France

Telephone: (1) 47 68 1234

Telex: 610500F

Molecular Sieve

W. R. Grace & Co.

Davison Chemical Division

Dept. TR

P.O. Box 2117

Baltimore, Maryland 21203

Telephone: (301) 659-9000

Telex: 87834

UOP (Union Carbide/Allied Signal) Molecular Sieve Adsorbents

Dubai International Trade Centre, Floor 25

P.O. Box 9248

Dubai, United Arab Emirates

Telephone: (971) 4-376846

Silica Gel

W. R. Grace & Co.

Davison Chemical Division

Dept. TR P.O. Box 2117

Baltimore, Maryland 21203

Telephone: (301) 659-9000

Telex: 87834

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APPENDIX C - VENDOR INFORMATION

TYPICAL PROPERTIES OF RHONE-POULENC ALUMINAS

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ALCOA F-200 ACTIVATED ALUMINA FOR ADSORPTION APPLICATIONS

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ALCOA F-200 ACTIVATED ALUMINA FOR ADSORPTION APPLICATIONS

(CONT'D)

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BASIC TYPES OF UNION CARBIDE MOLECULAR SIEVES

Documents

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