Dehydration FAQs
DEHYDRATION
Frequently Asked Questions
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Dehydration FAQs
1) Define gas dehydration?
The process of removing water from a gas stream is called gas dehydration.
2) Why Dehydrate Natural?




Meet a pipeline spec
Avoid hydrates
Avoid corrosion
Dehydration is also done so that a dry gas can be used for fuel in the processing
plant.
 Typical fuel gas consumers in a process plant are building heaters, fire-tube in
heaters, instrumentation, purging during a start-up operation, etc.
3) What is Dew Point?
The dew point is the temperature at which water will condense from gas stream as it is
cooled.
4) Which value of Dew Point corresponds to 7 lbs of water/MMSCF at 1000 psia?
0°C or 32F correspond approximately for 7lb/MMSCF.
5) What are hydrates?
Hydrates are loosely-linked, crystal-like chemical compounds of hydrocarbon and water,
resembling dirty ice.
6) How can we dissolve the Hydrates plug?
Methanol or glycol can be injected to dissolve the hydrate plug,the line pressure can be
lowered to lower the hydrate formation temperature or temperature can be increased to
melt the plug.
7) What is Dew Point Depression?
The dew point depression is the difference between the gas dew point before and after
dehydration.
8) Which are the most commonly used liquid desiccants?
Most commonly used liquid desiccants in the conventional dehydration unit are
a.
b.
c.
d.
Ethylene glycol.
Diethylene glycol.
Triethylene glycol.
Tetra ethylene glycol.
9) What is formula and molecular structure of TEG?
Molecular Structure
Molecular formula
C6H14O4
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Dehydration FAQs
10) What is glycol?
Any of a class of organic compounds belonging to the alcohol family; in the molecule
of a glycol, two hydroxyl (OH) groups are attached to different carbon atoms.
11) What is the concentration of lean glycol is maintained?
99.5%
12) Name of some commonly used methods to increase glycol purity?
 Increase the temperature of reboiler. (400°F for TEG)
 Lowering the pressure on still with a vacuum pump.
 Use of stripping gas.
13) Glycol circulation depends upon?
 Glycol purity
 Number of trays in contactor.
14) What is “Turndown”?
The minimum flow of gas required to make the bubble caps or valves work efficiently in
contactor is typically taken as 25% of design flow rate. This is called “turndown”.
15) What is purpose of Reflux condenser/coil in dehydration system?
 To control glycol loses
 To pre heat rich glycol before entering reboiler.
16) What is the retention time of glycol in Flash Drum?
The standard three phase separator can either a horizontal or vertical, sized for 15 to 30
minutes retention time for liquids.
17) Which kinds of pumps are used in dehydration for glycol?
Normally pumps having low flow rates and high differential pressure are used for glycol.
Usually, an electric motor-driven plunger pump is used.
On small units, and where electricity is not available, glycol powered pumps are
sometimes used. A glycol powered pump uses high pressure rich glycol with a small
amount of entrained gas to pump lean glycol.
18) What is the formula and molecular weight of TEG?
 HO(C2H4O)3H
 Molecular wt. 150.17
19) What is Boiling point and flash point of glycol?
 Boiling point
287.3°C
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Dehydration FAQs
 Flash point (open cup) 165.5°C
20) What is Flash Point?
The flash point of a flammable liquid is the lowest temperature at which it can form an
ignitable mixture in air. At this temperature the vapor may cease to burn when the source
of ignition is removed.
21) .Define desiccant?
Desiccant is a substance (liquid or solid) which has an ability to remove moisture
from a gas stream.
22) .What is inhibition?
Inhibition is the process of adding some chemical in gas stream to prevent hydration
formation.
23) . What are the methods to dehydrate natural gas commercially?
Nature is commercially dehydrated in one of three ways:
 Absorption
 Adsorption
 Condensation
24) . What is Adsorption?
Adsorption is the process in which molecules from the gas are held on the surface of a
solid by surface force.
25) Write some solid desiccants?
There are number of commercially available solid desiccants used to remove water
vapors from natural gas. Some of these are:




Activated alumina.
Molecular sieves.
Silica gel
Calcium chloride.
26) What are the disadvantages of solid desiccants?
Since this system requires multi towers, switching valves, etc., this type of dehydration is
typically more expensive than liquid desiccant system. These are used only
27) What should be the temperatures of glycol and gas at absorber inlet?
Inlet glycol temperature should be 5-10°F greater than inlet gas temperature.
28) Why glycol is kept hotter than inlet gas?
If the glycol is colder than the gas, it may cause liquid hydrocarbon to condense, and
resulting condensate could contaminate the glycol and cause foaming.
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Dehydration FAQs
29) Why TEG is used commonly instead of other liquid desiccants?
Triethylene is used because of its higher decomposition temperature allowing
regeneration to a higher purity than diethylene glycol. It is used in temperature limitation
of maximum 120°F to a minimum of about 60°F.(absorber inlet temperature)
Ethylene glycol and diethylene glucol are cheaper but they have higher vapor pressure
and cannot be regenerated to as high as the Triethylene and teraethylene glycols.
30) At which conditions tetra ethylene is used for dehydration?
Due to its higher viscosity, Tetra ethylene is use when absorber inlet glycol temperature
is operating above 49°C (120°F).
31)
What is auto ignition temperature of triethylene and diethylene glycol?
Auto ignition temperature of TEG is 700°F and of DEG is 425°F
32)
.Why does the temperature of gas at absorber outlet increase?
Absorption is exothermic reaction so the temperature rises due to the heat of absorption.
33)
.What should be the characteristics of liquid desiccant in dehydration absorption
system.
Liquid desiccant used should be hygroscopic, non-corrosive, non-volatile, easy
regeneration at high temperature, insoluble in liquid hydrocarbons, and un-reactive with
hydrocarbons, CO2 and sulfur compounds.
34)
.What is Reflux?
The portion of the overhead liquid product from a distillation column or fractionator that
is returned to the upper part of the column.
35)
. What is Reflux Ratio?
The ratio between the total product of a column and reflux is called Reflux Ratio.
36)
What should be the reflux ratio for glycol reboiler?
The reflux ratio employed in TEG system typically ranges from 0.1 to 0.2 moles/mole.
This is equivalent to condensing 10-20% of total over head vapor stream.
37)
What is vapor pressure?
Vapor pressure, also known as vapour pressure, is the pressure of a vapor in equilibrium
with its non-vapor phases. All liquids and solids have a tendency to evaporate to a
gaseous form, and all gases have a tendency to condense back into their original form
(either liquid or solid). At any given temperature, for a particular substance, there is a
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Dehydration FAQs
pressure at which the gas of that substance is in dynamic equilibrium with its liquid or
solid forms. This is the vapor pressure of that substance at that temperature
38)
Relation between vapor pressures and normal boiling points of liquids?
The higher the vapor pressures of a liquid at a given temperature, the lower the normal
boiling point of the liquid.
39) . How many gallons of glycol required to remove one pond (1 lb.) water?
2 to 5 gallon glycol required for one pound.
40) . What is the hazardous decomposition product of TEG?
Carbon dioxide and carbon monoxide form when heated to decomposition.
41) .On what factors the amount of water removed in glycol absorber depends upon?
 Glycol concentration
 Glycol circulation rate
 Number of contacts in absorber.
42) . Describe parameters to design an absorber?
The design of absorber is based on two parameters:
 Gas rate which determines the contractor diameter
 Number of contacts, which determines the contactor height.
43) .Which heat sources can be used in glycol reboiler?
 Direct Fire in fire tube
 Steam
 Hot oil
 Electric heater.
44) .What is Heat Flux?
Heat Flux
The rate at which heat is transferred is represented by the symbol Q . A common unit
for heat Q transfer rate is Btu/hr. Sometimes it is important to determine the heat transfer
rate per unit area, or heat flux, which has the symbol Q”. Units for heat flux are Btu/hrft2. The heat flux can be Q”
determined by dividing the heat transfer rate by the area through which the heat i
s being transferred.
Q” =
Q/A
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Dehydration FAQs
Where
Q” = heat flux (Btu/hr-ft2)
Q = heat transfer rate (Btu/hr)
A = area (ft2)
45) .What should be the maximum heat flux in for Direct Fired, and why?
Maximum heat flux for direct fired is 19 kW/m2 (6000 Btu/hr-ft2). High heat flux
rate can lead to some degradation of glycol in contact with the heat transfer surface.
Operating Problems & Troubleshooting
46) . What is the one common symptom of problems in glycol dehydration?
One common symptom of many problems is excess glycol loss.
47) .What are the common operating problems in glycol dehydration?
 Foaming
 Degradation
 Salts plugging the regenerator still column
 Lean glycol at absorber inlet is too hot
 Loss of glycol from top tray and bottom chimney tray of absorber.
 Leakages from different points.
48) .What is thermal decomposition in glycol?
Excessive heat will decompose the glycol and form corrosive compounds.
49) At which conditions thermal decomposition of glycol occurs?
 A reboiler temperature above the glycol decomposition level.
 A high heat flux rate. (Usually caused by trying to operate the unit above
design capacity.)
 Localized overheating, caused by deposits of salts on fire tube or by poor
flame direction on fire tubes.
50) What is the desire pH of glycol solution?
The desire pH of glycol solution is 7.0 – 7.5
51) .What are the causes of low pH of glycol?
Organic acids, resulting from oxidation of glycol, thermal decomposition products or acid
gases picked up from gas stream are common causes of low pH of glycol.
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Dehydration FAQs
52) .Which chemicals are used to maintain the pH of glycol?
Borax, ethanol amines or other alkaline chemicals are used to neutralizing the acidic
compounds in glycol.
53) .Describe oxidation of glycol?
The reaction of oxygen with glycol is called oxidation of glycol. Corrosive organic acids
produce from oxidation like Formic acid and some acetic acid.
54) .How can oxygen enters in glycol system?
Oxygen enters the system with incoming gas, through unblanketed storage tanks or
through the pump packing glands.
55) .How can we avoid oxidation?
To prevent oxidation, tanks shall have a gas blanket to keep air out of system. Oxidation
inhibitors, such as Hydrazine, can also be used to prevent corrosion.
56) .List down the foam producers in glycol system?
Some foam producers are:
 Hydrocarbon liquids.
 Corrosion inhibitors.
 Salt
 Suspended solids.
57) .Define “flooding”?
In glycol absorber, when pressure drop for glycol flow in down comer exceeds the head
available, then glycol build on the top trays. This condition is called “flooding”.
58) What is the purpose of Carbon filter?
Carbon filter removes the hydrocarbon and chemical impurities.
59) . Which portion of glycol is filtered though charcoal filters?
For small units( less than 10 gpm) charcoal filter is normally full flow where as for large
units a small side stream of 10 to 25 percent of total glycol is filtered?
60) .What is antifoam?
Chemical that can be effective either to prevent foam (anti-foam) or to destroy it once
it has formed (defoamer). Actually the distinction between these two terms is usually
ignored. Most foam-fighting chemicals can serve either role.
61) . What are the types of antifoam available for glycol service?
Three types of antifoam are available for glycol service:
 Polyglycol
 Silicon based
 Higher molecular weight alcohols.
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Dehydration FAQs
62) .What is procedure to select antifoam for system?
Selection for the right antifoam to treat a plant is often a trial and error process.
63) .Why we use sock filters?
Sock filter is used to remove 5 micron and larger solids.
64) . In typically glycol absorber, what is the distance between two trays?
Trays are commonly spaced 24 inch apart vertically.
65) . What will be the maximum velocity of glycol in down comer in absorber?
A good rule of thumb is to size the down comer so that the glycol velocity does not
exceed 0.25ft/sec to avoid the “flooding” in tower.
66) . What is the purpose of scrubber before glycol absorber?
These separators are used to remove any free liquid(from gas stream) and to decrease the
possibility of contaminating glycol.
67) Define purging?
Purging is the process to displace the unlike gas in a close vessel with an inert gas.
68) .Write down the types of Flame scanners?
Flame scanners are categorized by their working principle as described below
 Flame ionization detectors
 Ultraviolet emission detectors
 Infrared rays detectors
There some other kind of flame monitoring devices
69) What is Back fire?
A backfire is an explosion in the intake manifold, or air cleaner of an internal combustion
engine. Although an explosion in the engine's exhaust manifold or exhaust system is
often referred to as a backfire, it is actually an after fire.
70) .How can back fire occur in glycol reboiler?
If the explosive mixture is present in reboiler, a spark present in chimney could be
the cause of back fire.
71) .Which type of gas can be used as stripping gas in re-boiler?
Any inert gas or natural gas can be used.
72) .How does stripping gas work in reboiler?
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Dehydration FAQs
 It decreases the partial pressure of water vapors in reboiler. As the result
of which stripping of water from glycol solution increases.
 By rumbling the glycol solution.
 Natural gas is good absorbent of moisture at high temperature and low
pressure.
73) . What is procedure to discard the TEG waste?
This product has been evaluated for RCRA characteristics and does not meet the criteria
of a hazardous waste if discarded in its purchased form. Under RCRA, it responsibility of
the user of the product to determine at the time of disposal, whether the product meets the
RCRA criteria for hazardous waste or not.
74) . What is abbreviation of RCRA?
“The Resource Conservation and Recovery Act” directed by Environmental Protection
Agency (EPA). Under this act industrial wastes are discarded
75) .what is the primary air in combustion?
The air flow to make the air-fuel ratio for combustion is called primary air.
76) Define secondary air?
Air used to maintain the quality of flame is called secondary air in combustion.
77) What should be the O2 percentage in emission of reboiler fire tube chimney?
Less than 5% of flue gas volume.
78) .Why should we maintain excess O2 in emission of a Furness.
Some excess O2 is maintained in emission to ensure the complete combustion in
farness.
79) What is quantity of NOX and SOX in emission?
80) Define NOx?
Nitrogen oxides, or NOx, are the generic term for a group of highly reactive gases, all of
which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are
colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along
with particles in the air can often be seen as a reddish-brown layer over many urban
areas.
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Dehydration FAQs
Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion
process. The primary manmade sources of NOx are motor vehicles, electric utilities, and
other industrial, commercial, and residential sources that burn fuels. NOx can also be
formed naturally.
81) Write the name of some NOx?
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
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Nitric oxide (NO), nitrogen(II) oxide
Nitrogen dioxide (NO2), nitrogen(IV) oxide
Nitrous oxide (N2O)
Dinitrogen trioxide (N2O3), nitrogen(II, IV) oxide
Dinitrogen tetroxide (N2O4), nitrogen(IV) oxide
Dinitrogen pentoxide (N2O5), nitrogen(V) oxide
82) .Define SOx?
83) . What is BTEX?
The aromatic hydrocarbons (benzene, toluene, ethylenebenzene, xylene) are called
BTEX.
84) .Which corrosive material can be can be formed?
Glycol can react with sulfur compounds. The resultant materials tends to polymerize
and form “gunk” which is very corrosive. Also when the partial pressure of CO2 is
high, CO2 absorbs in solution and rich TEG pH can be less than 5.
85) .Define viscosity. What is viscosity of TEG?
Viscosity is an internal property of a fluid that offers resistance to flow. It is commonly
perceived as “thickness”. Units are g/(cm sec) or poise. Centipoise (cp) is 0.01 poise.
Viscosity of TEG is 47.8 mPa.s at 20°C.
86) .What is the minimum space recommended between two trays in absorber?
610 mm (24 inch).
87) .What is the effect of acid gas concentration on the water content of gas?
With the increase of acid gas concentration, water content also increases in gas stream
keeping the other parameter constant.
88) .What is DRIZO technology?
This is a technology to enhance the stripping in glycol reboiler.in this method
avolitile hydrocarbon is fed into the reboiler. At reboiler temperature it vaporizes and
acts as stripping gas. This chemical is more effective than stripping gas as 99.99wt%
concentration with TEG can be achieved. Trade name of this chemical is DRIZO.
DRIZO circulates in close system.
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Dehydration FAQs
89) .Is co-current flow in an absorber possible?
Yes. Co-current contacting is applicable only for cases required less than 1.0
theoretical contact.
90) .What are the advantages of co-current flow in absorber?
According to Baker and Roger (1989) these advantages are:
 Simple operation
 Minimum pressure drop
 Small space required
 No flooding problem
 Low capital investment
91) .What is HETP?
HETP stands for Height Equivalent to a Theoretical Plate. For packed contact towers,
equilibrium stages are converted to packing height using HETP.
92) .Describe the lean glycol analysis?
Hydrocarbon %
Salt content ppm
Solid content ppm
pH
Iron content ppm
Foam, time to break seconds
1.0
200-300
100
7.0-7.5
10-15 (100 ppm max)
3.5
93) .Define glycol reclaiming?
The removal of salts directly from glycol solution by vacuum distillation in a reclaimer or by ion exchange using strong acid/base ion exchanger is called glycol
reclaiming.
94) .what are the reasons for replacing glycol in system?
There are only four reasons for replacing glycol:
 Inability to maintain proper dew point depression
 Fire tube failure due to fouling
 Foaming can not be controlled by an antifoam
 Excessive corrosion that cannot be controlled by corrosion inhibitor.
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Dehydration FAQs
95) .How much quantity of stripping is used to purify the glycol?
The amount of stripping gas quantity to be used is determined by correlation of glycol
purity at reboiler temperature, number of equilibrium stages, reboiler pressure and
stripping gas flow.
Glycol concentration can be increased from 99.05% to 99.87% with 4.5scf/gal of
stripping gas.
96) .How much efficient trays of glycol absorber work?
Trays efficiencies in glycol contactors normally range from 25 to 35%.
97) .Which sizes of particles are filtered in sock filters?
5 micron and larger solids.
98) .What is purpose of “thief hatch”?
In normal operation it prevents air to enter the system, but in case of vacuum in
system it works as vacuum breaker.
99) .What is IUPAC name of TEG?
2,2-(1,2-ethanedioxy)ethanol.
100)
.What is Auto ignition temperature of TEG?
371°C
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