Lessons
Learned
NaturalGas
EPA POLLUTION PREVENTER
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From Natural Gas STAR Partners
REPLACING GLYCOL DEHYDRATORS WITH DESICCANT
DEHYDRATORS
Executive Summary
There are approximately 30,000 high-pressure, on-shore gas wells producing 4 trillion cubic feet (Tcf) of natural
gas annually in the United States. About 700 of these wells have conventional glycol dehydrators, emitting an
estimated 1 billion cubic feet (Bcf) of methane per year to the atmosphere. Glycol dehydrators vent methane,
volatile organic compounds (VOCs), and hazardous air pollutants (HAPs) to the atmosphere from the glycol
regenerator and also bleed natural gas from pneumatic control devices. This process wastes gas, costs money,
and contributes to local air quality problems as well as global climate change.
Natural Gas STAR partners have found that replacing glycol dehydrators with desiccant dehydrators reduces
methane, VOC, and HAP emissions by 99 percent and also reduces operating and maintenance costs. In a des-
iccant dehydrator, wet gas passes through a drying bed of desiccant tablets. The tablets pull moisture from the
gas and gradually dissolve in the process. Since the unit is fully enclosed, gas emissions occur only when the
vessel is opened, such as when new desiccant tablets are added.
Economic analyses demonstrate that replacing a glycol dehydrator processing 1 million cubic feet per day
(MMcfd) of gas with a desiccant dehydrator can save up to $4,403 per year in fuel gas, vented gas, and opera-
tion and maintenance (O&M) costs and reduce methane emissions by 564 thousand cubic feet (Mcf) per year.
This Lessons Learned study describes how partners can identify areas where desiccant dehydrators can be
implemented and determine their economic and environmental benefits.
Method for
Reducing Gas
Loss1
Replacing a
Glycol Dehydrator
with a Desiccant
Dehydrator
Annual Methane
Emission Savings
(Mcf)2
564
Annual Gas
Savings
(Mcf)3
1,063
Value of Gas
Saved
($)4
3,189
Capital and
Installation
Cost ($)5
12,750
O&M
Cost
($)6
(1,214)
Payback
(Years)
2.9
Based on a 1 MMcfd dehydrator operating at 450 psig and 47°F.
Difference between methane vented from the glycol and desiccant dehydrators.
Sum of net gas emissions reduction and fuel gas savings.
Based on $3 per Mcf price of gas.
Installed cost of desiccant dehydrator minus surplus equipment value for the replaced glycol dehydrator.
Difference between glycol and desiccant dehydrators O&M costs.
This is one of a series of Lessons Learned Summaries developed by EPA in cooperation with the natural gas industry on superior
applications of Natural Gas STAR Program Best Management Practices (BMPs) and Partner Reported Opportunities (PROs).
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Technology
Background
Produced natural gas is normally saturated with water. If not removed, the
water can condense and/or freeze in gathering, transmission, and distribu-
tion piping causing plugging, pressure surges, and corrosion. To avoid these
problems, the produced gas is typically sent through a dehydrator where it
contacts a dewatering agent such as triethylene glycol (TEG), diethylene gly-
col (DEG), or propylene carbonate. In the most common process, glycol
dehydration, the TEG absorbs water from the gas along with methane,
VOCs, and HAPs. The absorbed water and hydrocarbons are then boiled off
in a reboiler/regenerator and vented to the atmosphere. (See EPA's Lessons
Learned: Optimize Glycol Circulation and Install Flash Tank Separators in
Glycol Dehydrators.)
Natural Gas STAR partners have reported success using an alternative
method for drying gas: desiccant dehydrators. These dehydrators use mois-
ture-absorbing salts to remove water from the gas without emitting large
quantities of methane, VOCs, or HAPs.
Desiccants
Deliquescent salts, such as calcium, potassium and lithium chlorides, have
been used by the oil and gas industries to dehydrate petroleum products for
more than 70 years. These salts naturally attract and absorb moisture
(hygroscopic), gradually dissolving to form a brine solution. The amount of
moisture that can be removed from hydrocarbon gas depends on the type
of desiccant as well as the temperature and pressure of the gas. Calcium
chloride, the most common and least expensive desiccant, can achieve
pipeline-quality moisture contents at temperatures below 59°F and pressures
above 250 psig. Lithium chloride, which is more expensive, has a wider
operating range: up to 70°F and above 100 psig. Appendix A provides equi-
librium moisture contents of natural gas dehydrated by commercially avail-
able calcium and lithium chloride salts.
Process Description
A desiccant dehydrator is a very simple device; it has no moving parts and
needs no external power supply; therefore, it is ideal for remote sites.
As shown in Exhibit 1, wet natural gas enters near the bottom of the dehydra-
tor vessel, below the desiccant support grid. The support grid and ceramic
ball pre-bed prevent the desiccant tablets from dropping down into the brine
sump (claim area). The wet gas flows upward through the drying bed. When
the gas comes into contact with the surface of the tablets, the desiccant salts
remove water vapor from the gas (hydrate). As the desiccant continues to
remove water vapor from the gas, droplets of brine form and drip down
through the drying bed to the brine collection sump (claim area) at the bottom
of the vessel. This brine formation process gradually dissolves the desiccant.
Brine collected in the claim area can be periodically drained to either a brine
(or produced water) storage tank, or (where permitted) to an evaporation
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Exhibit 1: Schematic of Single Vessel Desiccant Dehydrator
Sight Window
Coupling(s)
1 on PFD's
2 on OSD's
"met map- ED
Pressure Gauge
- Filler Hatch
(Quick Opening
Threaded Closure)
Lifting Lug
^> Outlet
Drying Bed
Pre-Bed
Support Grid
Coupling for (optional)
Pilot-operated Diaphragm
Dump System
Claim Area
Drain Coupling
Source: Van Air
pond. Produced water and brine may be deep-well injected near the site, or
periodically picked up for disposal offsite.
With a drying bed of sufficient depth, the gas reaches equilibrium moisture
content with the desiccant before it reaches the top of the drying bed.
Excess salt, above the minimum depth needed to achieve equilibrium mois-
ture content, is referred to as the "working salt bed." This working inventory
is refilled periodically. To avoid halting gas production or bypassing wet gas
to a sales line when refilling the desiccant dehydrator, most installations use
a minimum of two vessels: one in drying service while the other is being
refilled with salt.
Operating Requirements
To protect their pipelines, producers dry gas to a dew point below the mini-
mum temperature expected in the pipeline. If the gas is not dried appropri-
ately, water and other free liquids can precipitate as the gas cools which can
lead to pipeline blockage or corrosion. To avoid this, producers normally
dehydrate gas to a pipeline moisture specification between 4 and 7 pounds
of water per MMcf of gas. Desiccant performance curves show the temper-
ature and pressure combinations that will result in gas meeting pipeline
moisture standards. Exhibit 2, derived from the moisture content table in
Appendix A, shows the gas temperature and pressure combinations that
would result in 7 pounds of water per MMcf of gas for two of the most com-
mon desiccants. The shaded region above the saturation line in Exhibit 2
represents a "safe operating region" for calcium chloride dehydrators where
the gas will be at or below pipeline moisture specification. Operators use
these curves to determine the minimum gas pressure required to ensure a
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given moisture content. In this example, an inlet gas at 47°F passing through
a calcium chloride desiccant dehydrator must be pressurized to at least 450
psig to meet the 7 pounds of water per MMcf standard. Curves for both cal-
cium and lithium chloride are shown, although lithium chloride is rarely used
because of its cost.
Exhibit 2: Desiccant Performance Curves at Maximum Pipeline
Moisture Content Requirement (7 Ib. of water/MMscf)
Pressure (Psig)
« 1 1
Chloride
— — -Lithium
_..•"
35 37 39 41 43 45 47 49 51 53 55 57 59
Source: Air & Vacuum Process, Inc. Temperature (°F)
Refilling Desiccants and Draining Brine
As the desiccant tablets absorb moisture from the gas, the depth of the
desiccant tablets in the drying bed slowly decreases. Some manufacturers
place a "window" (sight-glass) on the vessel (see Exhibit 1) at the minimum
desiccant level. When the top of the desiccant reaches the sight-glass, the
operator needs to refill the desiccant up to the maximum level. Refilling the
working bed is a manual operation that involves switching gas flow to anoth-
er dehydration vessel, shutting valves to isolate the "empty" vessel, venting
gas pressure to the atmosphere, opening the top filler hatch, and pouring
desiccant pellets into the vessel. This requires the operator to dump one or
more 30 to 50 pound bags of salt into the vessel, depending on dehydrator
design. Because this procedure needs to be performed more frequently the
higher the gas throughput, desiccant dehydrators are usually used when the
volume of gas to be dried is 5 MMcfd or less.
The brine in the claim area is sometimes drained manually (desiccant dehy-
drators typically accumulate from 10 to 50 gallons of brine a week). Draining
to an evaporation pond is best done after the vessel is depressured, while
draining to a produced water tank can be done before the vessel is depres-
sured—taking advantage of the gas pressure to push the brine into the tank.
On rare occasions brine may be pumped into a tank truck using a pneu-
matic "duplex-type" pump.
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Economic and
Environmental
Benefits
Using desiccant dehydrators as alternatives to glycol dehydrators can yield
significant economic and environmental benefits, including:
* Reduced capital cost—The capital costs of desiccant dehydrators
are low compared to the capital costs of glycol dehydrators. A desic-
cant dehydrator does not use a circulation pump, pneumatic controls,
a gas heater, or a fired reboiler/regenerator.
* Reduced operation and maintenance cost—Glycol dehydrators
burn a significant amount of produced gas for fuel in a gas heater and
glycol regenerator. If the brine drain valve is automatic, the only O&M
cost for a desiccant dehydrator is for refilling the desiccant bed.
* Minimal methane, VOC, and HAP emissions—Glycol dehydrators
continuously vent gas to the atmosphere from pneumatic devices and
the TEG regenerator vent. The only gas emissions from desiccant
dehydrators occur during desiccant vessel depressuring for salt refill-
ing, typically one vessel-volume per week. Brine is produced in small
quantities and absorbs little hydrocarbon.
Decision
Process
Partners can evaluate potential
locations and economics for
replacing existing glycol dehydra-
tors with desiccant dehydrators
using the following five steps.
Five Steps for Evaluating A
Desiccant Dehydrator:
1. Identify appropriate locations
2. Determine dehydrator capacity
3. Estimate the capital and operating costs
4. Estimate savings
5. Conduct economic analysis
Step 1: Identify appropriate
locations. Desiccant dehydra-
tors are an economic choice
under certain operating condi-
tions. Their applicability is deter-
mined primarily by gas throughput and produced gas temperature and pres-
sure. Desiccant dehydrators work best when the volume to be dried is 5
MMcfd or less, and absorb moisture down to pipeline specifications when
the wellhead gas temperature is low and the pressure is high. If the inlet
temperature of the gas is too high, desiccants can form hydrates that pre-
cipitate from the solution and cause caking and brine drainage problems.
While it is possible to cool or compress the produced gas in order to use
desiccant dehydrators, these measures increase system complexity and typ-
ically are cost prohibitive.
In contrast, glycol dehydrators are a better choice for higher producing well
sites and work best for higher temperature gas at any pressure. If the pro-
duced gas temperature is too low for the TEG process, however, operators
will need to heat the gas prior to entering the dehydrator. Since heating the
gas requires more product to be burned as fuel, these situations are likely to
be good candidates for desiccant dehydrators. Exhibit 3 shows which gas
drying systems work best under various operating conditions.
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Exhibit 3: Optimum Operating Conditions for Dehydration Technologies
Low Temperature (<70°F)
High Temperature (>70°F)
Low Pressures oo psig)
Desiccant/Glycol1
Glycol
High Pressure (>1 oo psig)
Desiccant
Glycol/Desiccant2
1 The gas may need to be heated to use a glycol dehydrator, or the gas may need to be compressed
to use a desiccant dehydrator.
2 The gas may need to be cooled to use a desiccant dehydrator.
Step 2: Determine dehydrator capacity. The first step in estimating the
size of a desiccant dehydrator is to determine the inlet and outlet moisture
content of the gas. This is required to calculate the quantity of desiccant
needed, and from that the size of the vessel. Operators use a natural gas
water vapor content graph (example shown in Appendix B), a moisture con-
tent table, or a sizing program such as the Hanover Company's Quick Size
program, found at , to
estimate the water content in the gas stream. For this analysis, we will
assume the dehydrator is being designed to handle a 1 MMscf/day gas
stream at 47°F and 450 psig. For this scenario, using any of these methods
yields the same results—the natural gas stream contains 21 pounds of
water per MMcf.
Vendor's Rule-of-Thumb
In order to meet a pipeline mois-
ture specification of 7 pounds per
MMcf, calcium chloride desiccant
must remove 14 pounds of water
per MMcf of gas. For a 1 MMcfd
dehydrator, and using a vendor's
rule-of-thumb that 1 pound of
desiccant removes 3 pounds of water, 4.7 pounds of calcium chloride
dissolved per day. Exhibit 4 summarizes this calculation.
One pound of desiccant removes three
pounds of moisture from the gas.
be
The next step is to size the vessel. Vendors supply desiccant dehydrator
vessels in standard sizes, usually specified by outside diameter and maxi-
mum gas throughput at various operating pressures, as shown in Exhibit 6.
The bed dimensions are fixed to achieve equilibrium gas moisture content.
This includes a standard size working bed depth: 5 inches for this vendor.
Partners can select the desiccant vessel size from the vendor's table or
calculate the size using the equations in Exhibit 5. For the 1 MMcfd dehy-
drator example above, using Exhibit 5 gives a vessel with a 16.2 inch
inside diameter (about 17 inch outside diameter with a 3/8 inch wall thick-
ness). To use Exhibit 6, follow the 450-psig column down to the through-
put capacity equal to or greater than what is needed; in this example,
1,344 Mcfd (1.344 MMcfd). Following this row to the left yields an outside
diameter of 20 inches.
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Exhibit 4: Determine the Daily Consumption of Desiccant
Where:
D
F
1
0
B
Daily consumption of desiccant (Ib/day)
Gas flow rate (MMcf/day)
Inlet water content (Ib/MMcf)
Outlet water content (Ib/MMcf)
Desiccant-to-water ratio (Ib desiccant/lb water)
Given:
F
1
0
B
1 MMcf/day of production gas at 47°F and 450 psig
21 Ib/MMcf
7 Ib/MMcf (pipeline moisture requirement)
1 Ib desiccant/3 Ib water (vendor rule-of-thumb)
Calculate:
D
F * (I-O) * B
1 * (21 -7)* 1/3
4.7 Ib desiccant/day
Exhibit 5: Determine the Size of the Desiccant Dehydrator
Where:
ID = Inside diameter of the desiccant vessel (in)
D = Daily desiccant consumption (Ib/day)
H = Working salt bed height (in)
T = Time between refilling (days)
B = Bulk density (Ib/ft3)
Given:
D = 4.7 Ib/day (Exhibit 4)
H = 5 in (vendor rule-of-thumb)
T =7 days (operator's choice)
B =55 (Ib/ft3) (vendor's data)
Calculate:
4*D*T*12
ID = 12*A
4*4.7*7*12
= 12* Af
5*n*55
= 16.2 in
Select standard vessel size from Exhibit 6:
• Select next larger size than ID = 20 in
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Exhibit 6: Cost and Maximum Throughput Capacity (Mcfd)
of Desiccant Dehydrators
Outside
Diameter
(Inches)
10
12
16
20
24
30
36
Cost12
($)
2,850
3,775
5,865
6,500
8,895
12,850
17,034
100
Psig
95
132
214
311
481
760
1,196
200
Psig
177
247
400
620
900
1,422
2,230
300
Psig
260
362
587
909
1,319
2,085
3,270
350
Psig
301
419
680
1,054
1,528
2,416
3,789
400
Psig
342
476
773
1,199
1,738
2,747
4,308
450
Psig
383
533
866
1,344
1,948
3,078
4,827
500
Psig
424
590
959
1,489
2,158
3,409
5,346
1 The capital cost is for pressure ratings up to 500 psig, including one vessel with vessel supports,
valves, piping, all appurtenances and the initial fill of calcium chloride desiccant tablets.
2 Dehydrator cost includes all appurtenances: vessel, support structure, valves, and piping.
Source: Van Air
Step 3: Estimate the capital and operating costs. Capital costs for single
vessel desiccant dehydrators suitable for gas production rates from 0.1 to 5
MMcf per day (including the initial fill of desiccant) range between $3,000
and $17,000. After determining the necessary vessel size (Step 2), partners
can use Exhibit 6 to determine the capital costs of a desiccant dehydrator.
For the example given in Step 2, the capital cost of a 20-inch, single vessel
desiccant dehydrator is $6,500. For a two-vessel dehydrator, the cost would
be $13,000.
Installation costs typically range from 50 to 75 percent of the equipment
cost. Using an installation factor of 75 percent of the equipment cost, the
single vessel desiccant dehydrator described above would cost $4,875 to
install. The two-vessel dehydrator would cost $9,750 to install.
The operating cost of using a desiccant dehydrator includes the costs of
desiccant replacement and brine disposal. Because the desiccant tablets
dissolve as they remove moisture from the gas, the working salt bed will
need to be replenished periodically. The resulting brine also requires removal
and treatment or disposal.
Exhibit 7 shows the operating cost calculations for the 1 MMcfd dehydrator
example. Depending on the vendor, the cost of calcium chloride can range
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from $0.65 to $1.20 per pound. Using $1.20 per pound for fhe cosf of calci-
um chloride, fhe fofal cosf for refilling 4.7 pounds per day (from Exhibit 4) is
$2,059 per year. In the example given in Exhibit 4, very little brine is pro-
duced removing moisture from gas to achieve the desired pipeline moisture
specification (i.e., 7 pounds per MMcf): 4.7 pounds per day of salt plus the
14 pounds of water per day removed from the gas, or 18.7 pounds of brine
per day—a little over 2 gallons per day.
Exhibit 7: Determine the Operating Cost of a Desiccant Dehydrator
Where:
TO = Total operating cost ($/year)
CD = Cost of desiccant ($/year)
CB = Cost of brine disposal ($/year)
I = Inlet water content (Ib/MMcf)
0 = Outlet water content (Ib/MMcf)
F = Gas flow rate (MMcf/day)
P = Price of the desiccant ($/lb)
D = Daily desiccant consumption (Ib/day)
S = Density of CaCI2 brine (Ib/bbl)
BD = Cost of brine disposal ($/bbl)
LC = Labor cost ($)
LT = Labor time for operator to refill with desiccant (hr)
LR = Labor rate for operator ($/hr)
Given:
F = 1 MMcf/day of production gas at 47°F and 450 psig
P = $1.20/lb of calcium chloride (vendor data)
D = 4.7 Ib desiccant/day (Exhibit 4)
S = 490 Ib/bbl
BD =$1.00/bbl1
LT = 1 hr/week
LR = $30/hr
Calculate:
CD = D*P*365 days/yr
= 4.7*1.2*365
= $2,059/yr
CB = [((I-0)*F)+D]*BD*365 days/yr
S
= [((21-7)*1)+4.7]*1.0*365
490
= $14/yr
LC =LT*LR*52 weeks/yr
= 1*30*52
= $1,560/yr
TO = CD+CB+LC
= $2,059+$14+$1,560
= $3,633/yr
1 GRI Atlas of Gas-Related Produced Water for 1990, May 1995.
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Step 4: Estimate savings. Replacing a glycol dehydrator with a desiccant
dehydrator significantly saves gas and reduces operation and maintenance
costs.
Determining Net Gas Savings
The amount of gas saved can be
determined by comparing the
gas emissions and usage for the
existing glycol dehydrator to the
gas vented from a desiccant
dehydrator. Partners can deter-
mine the gas savings by deter-
mining the following five factors.
Determine the Net Gas Savings:
Add Savings from eliminating:
• Gas vented from glycol dehydrator.
• Gas vented from pneumatic controllers.
• Gas burned as fuel in glycol reboiler.
• Gas burned as fuel in a gas heater.
Subtract:
• Gas vented from desiccant dehydrator.
Estimate the gas vented from glycol dehydrator—The amount of
gas vented from the glycol regenerator/reboiler is equal to the gas
entrained in the TEG. To determine this, partners will need to know the
gas flow rate, the inlet and outlet water content, the glycol-to-water
ratio, the percent over-circulation, and the methane entrainment rate.
Exhibit 8: Gas Vented from the Glycol Dehydrator
Where:
GV
F
W
R
OC
G
Amount of gas vented annually (Mcf/yr)
Gas flow rate (MMcf/day)
Inlet-outlet water content (Ib/MMcf)
Glycol-to-water ratio (gal/lb)1
Percent over-circulation
Methane entrainment rate (ft3/gal)1
Given:
F
W
R
G
OC
1 MMcfd of gas at 47°F and 450 psig
21 -7 = 14lbwater/MMcf(Exhibit4)
3 gal/lb (rule-of-thumb)1
3 ftVgal for energy exchange pumps (rule-of-thumb)1
150%
Calculate:
rw
(F*W*R*OOG*365days/yr)
1,OOOcf/Mcf
(1*14*3*1.5*3*365)
1,000
= 69 Mcf/yr
1 From EPA's
Dehydrators.
Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol
10
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Exhibit 8 demonstrates this calculation for the 1 MMcfd dehydrator
example. In this example, an energy exchange pump without a flash
tank separator is assumed. Using rules-of thumb from EPA's Lessons
Learned: Optimize Giycoi Circulation and Install Flash Tank Separators in
Glycol Dehydrators, methane gas emissions of 69 Mcf per year is calcu-
lated.
* Estimate the gas vented from pneumatic controllers—Pneumatic
controllers are commonly used to monitor and regulate gas and liquid
flows, temperature, and pressure in glycol dehydrator units.
Specifically, the controllers regulate gas and liquid flows in dehydrators
and separators, temperature in dehydrator regenerators, and pressure
in flash tanks (when in use). In this example, the glycol dehydrator unit
with a gas heater is assumed to have four bleeding pneumatic con-
trollers—level controllers on the contactor and reboiler and tempera-
ture controllers on the reboiler and gas heater. It does not have a flash
tank separator. It also is assumed that all the pneumatic devices are
high bleed devices (i.e., they bleed in excess of 50 Mcf of gas per year
during operation). Based on the GRI/EPA study, Methane Emissions
From the Natural Gas Industry, Volume 12-Pnuematic Devices, the
annual emission factor for an average high bleed pneumatic device is
estimated to be 126 Mcf per year. Therefore, the four pneumatic
devices will contribute 504 Mcf of the methane emissions annually.
Exhibit 9 summarizes this example.
Exhibit 9: Gas Vented from Pneumatic Controllers
Where:
GB
EF
PD
Gas bleed (Mcf/yr)
Emission factor (Mcf natural gas bleed/pneumatic device per year)1
Number of pneumatic devices
Given:
EF
PD
126Mcf/device/yr
4 pneumatic devices/glycol dehydrators
Calculate:
GB
EF*PD
126*4
504 Mcf/yr
1 GRI/EPA study, Methane Emissions from the Natural Gas Industry, Volume 12.
Estimate the gas burned for fuel in glycol reboiler—The glycol
dehydrator uses natural gas in the reboiler/regenerator to boil-off water
from the rich glycol. Assuming that the heat duty of the reboiler is
1,124 Btu per gallon of TEG, the gas used by the reboiler is 17 Mcf
per year. Exhibit 10 summarizes this calculation.
11
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Exhibit 10: Gas Burned for Fuel in Glycol Reboiler
Where:
FGR = Fuel gas for reboiler (Mcf/yr)
F = Gas flow rate (MMcfd)
W = Inlet-outlet water content (Ib/MMcf)
Qr = Heat duty of reboiler (Btu/gal TEG)1
Hv = Heating value of natural gas (Btu/scf)2
R = Glycol-to-water ratio (gal TEG/lb water)'
Given:
F = 1 MMcfd
W = 21 -7 = 14lbwater/MMcf
Qr =1,124 Btu/gal TEG
Hv =1,027 Btu/scf
R = 3 gal TEG/lb water removed
Calculate:
FGR
(F*W*Qr*R*365days/yr)
Hv*1,000cf/Mcf
_ (1*14*1,124*3*365)
1,027*1,000
= 17 Mcf/yr
1 Based on calculation in Engineering Data Book, Volume II, 11th edition, Gas Processors Supply
Association, 1998, Section 20-Dehydration.
2Energy Information Administration (EIA), Monthly Engineering Review, Table A4.
3From EPA's Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol
Dehydrators.
* Estimate the gas burned for fuel in a gas heater—TEG does not
perform well on low temperature gas. As a result, the gas is typically
heated prior to entering the dehydrator unit. Natural gas is used to fuel
the gas heater. The amount of fuel gas used to heat 1 MMcfd of pro-
duced gas from 47°F to (assumed) 90°F is 483 Mcf per year. Exhibit
11 shows this calculation.
* Estimate the gas loss from desiccant dehydrator—The gas loss
from a desiccant dehydrator is determined by calculating the amount
of gas vented from the vessel every time it is depressurized for the
refilling process. To determine the volume of gas vented, partners will
need to determine the volume of the dehydrator vessel and what per-
centage of this volume is occupied by gas. The 20-inch OD vessel in
Exhibit 6 would have an approximately 19.25-inch ID (assuming a 3/8
inch wall thickness). The vessel has an overall length of 76.75 inches
with 45 percent of its volume filled with gas. Using Bolye's Law, the
amount of gas vented to the atmosphere during depressurizing of the
vessel is 10 Mcf per year. Exhibit 12 summarizes this calculation.
12
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Exhibit 11: Amount of Fuel Gas Used to Heat the Gas
Where:
FGH
Hv
Cv
D
AT
F
E
Fuel gas used in heater (Mcf/yr)
Heating value of natural gas (Btu/cf)
Specific heat of natural gas (Btu/lb°
Density of natural gas (Ib/cf)
(T2 - T,) change in temperature (F°)
Flow rate (MMcf/d)
Efficiency
F)
Given:
Hv
Cv
D
AT
F
E
1,027 Btu/cf
0.441 Btu/lb°F
0.0502 Ib/cf
43 F° (90 - 47) P
1 MMcf/d
70%
Calculate:
FRH
=
(F*D*Cv*AT*365days/yr*1,OOOMcf/MMcf )
(Hv*E)
(1*0.0502*0.441*43*365*1,000)
(1,027*0.7)
483 Mcf/yr
* Estimate the total gas savings—The total gas savings is the total
avoided emissions and gas use of the glycol dehydrator minus the gas
lost from venting of the desiccant dehydrator when replacing the desic-
cant. In this example, total gas savings are 1,063 Mcf per year. Using a
gas price of $3.00 per Mcf, the gas value saved is $3,189 per year.
Natural gas contains 90 percent methane. Therefore, the total methane
emission savings is 90 percent of the difference between the gas emit-
ted by the glycol dehydrator and its pneumatic controllers (Exhibits 8
and 9 respectively), and the desiccant dehydrator (Exhibit 12); in this
case, 507 Mcf per year. Exhibit 13 summarizes this example.
13
-------
Exhibit 12: Gas Lost from the Desiccant Dehydrator
Where:
GLD
H
D
P,
P2
n
%G
T
Gas loss from desiccant dehydrator (scf/yr)
Height of the dehydrator vessel (ft)
Inside diameter of the vessel (ft)
Atmospheric pressure (psia)
Pressure of the gas (psig)
Pi
Percent of packed vessel volume that is gas
Time between refilling (days)
Given:
H
D
P,
P2
n
%G
T
76.75 in (6.40 ft)1
19.25 in (1.6 ft)
14.7 psia
450 psig + 14. 7 (464. 7 psig)
3.14
45% (vendor's rule-of-thumb)1
7 days
Calculate:
rci n
=
1 Based on
(H*D2*n*P2*%G*365days/yr)
(4*P1*T*1,OOOcf/Mcf)
(6.4*1.62*3.14*464.7*0.45*365)
(4*14.7*7*1,000)
10 Mcf/yr
product data provided by Van Air.
Exhibit 13: Total Gas Savings
Calculate:
TGS = Total Gas Savings (Mcf/yr)
= Exhibit 8 + Exhibit 9 + Exhibit 1 0 + Exhibit 1 1 - Exhibit 1 2
= 69 + 504 + 17 + 483-10
= 1,063 Mcf/yr
Savings = 1 ,063 Mcf/yr * $3/Mcf
= $3,189/yr
Methane Emissions Reduction
TMER = Total methane emissions reduction
TMER = 90% * (Exhibit 8+ Exhibit 9 - Exhibit 12)
= 0.9* (69 + 504 -10)
= 507 Mcf/yr
14
-------
Determining Operations and Maintenance Savings
Other savings include the difference between the operating and mainte-
nance cost (labor cost) of a desiccant dehydrator and a glycol dehydrator.
The operation cost of a desiccant dehydrator includes the refill cost of the
desiccant, disposal of the brine, and labor costs. Since a desiccant dehydra-
tor has no moving parts and does not require power to operate, maintenance
costs are negligible. The refill and brine disposal costs previously calculated in
Exhibit 7 are $2,059 and $14 per year, respectively. Labor costs assume one
hour per week for the operator to refill the desiccant dehydrator. At $30 per
hour, this would cost about $1,560 per year.
Operating cost for a glycol dehydrator includes topping-up the glycol sump to
maintain glycol levels. Maintenance and labor include inspecting and cleaning
the mechanical systems, periodically repairing the circulation pump and pneu-
matic controls, and annually cleaning the fire-tubes of the reboiler and gas
heater. Glycol costs $4.50 per gallon, and a typical make-up rate is 0.1 gal-
lons per MMcf of gas processed. For this example, this works out to about
37 gallons of glycol per year, or $167 per year. Labor costs assume operators
spend an average of two hours per week maintaining and repairing the unit.
At $30 per hour this amounts to about $3,120 per year. Spare parts are esti-
mated at half the labor cost, or $1,560 per year. Based on this, total opera-
tion, maintenance, and labor costs for our example glycol dehydrator system
is $4,847 per year.
Step 5: Conduct economic analysis. The final step is to compare the
implementation and annual operating and maintenance costs of each option
and the value of gas saved or used/lost by each unit. Exhibit 14 provides a
comparison of the implementation and operating and maintenance costs of
a desiccant dehydrator and a glycol dehydrator (dehydrating 1 MMcfd
natural gas at 450 psig pressure and 47°F temperature). Exhibit 15 com-
pares the amount and the value of gas used and lost by each system.
Exhibit 16 shows the savings a Natural Gas STAR partner could expect over
a 5-year period by replacing an existing glycol dehydrator of 1 MMcfd at 450
psig and 47°F gas with a desiccant dehydrator.
15
-------
Exhibit 14: Cost Comparison of Desiccant Dehydrator and
Glycol Dehydrator
1 MMcfd natural gas at operating 450 psig and 47°F
Type of Costs and Savings
Implementation Costs
Capital Costs
Desiccant1 (includes the initial fill)
Glycol
Other costs (installation and engineering)2
Total Implementation Costs:
Annual Operating and Maintenance Costs
Dessicant
Cost of desiccant refill3 ($1.20/lb)
Cost of brine disposal3
Labor cost4
Glycol
Cost of glycol refill4 ($4.50/gal)
Material and labor cost4
Total Annual Operation and Maintenance Costs:
Desiccant
($/yr)
13,000
9,750
22,750
2,059
14
1,560
3,633
Glycol
($/yr)
20,000
15,000
35,000
167
4,680
4,847
1 Based on two desiccant vessels used alternatively. See Exhibit 5.
2 Installation costs assumed at 75% of the equipment cost.
3 Values are from Exhibit 7.
4 See Step 4, Estimate Savings.
Exhibit 15: Gas Use/Loss and Value Comparison
1 MMcfd natural gas at operating 450 psig and 47°F
Type of Loss/Use
Gas Use
Fuel (Exhibits 10 and 11)
Gas Loss
Pneumatic devices (Exhibit 9)
Vents (Exhibits 8 and 12)
Total:
Methane Emissions2:
Desiccant
Mcf/yr
—
—
10
10
10
$/yr1
—
—
30
30
—
Glycol
Mcf/yr
500
504
69
1,073
507
$/yr1
1,500
1,512
207
3,219
—
1 Gas price based on $3/Mcf.
2 Values are from Exhibit 12 and Exhibit 13.
16
-------
Lessons
Learned
Exhibit 16: Economics of Replacing a Glycol Dehydration System with a
Two-Vessel Desiccant Dehydrator System
Types of Costs
and Savings1
Capital costs
Avoided O&M costs
O&M costs -
Desiccant ($/yr)
Value of gas saved
Surplus equipment
value
Total ($)
YearO
($/yr)
(22,750)
10,0002
(12,750)
Yearl
($/yr)
4,847
(3,633)
3,219
4,433
Year 2
($/yr)
4,847
(3,633)
3,219
4,433
Years
($/yr)
4,847
(3,633)
3,219
4,433
Year 4
($/yr)
4,847
(3,633)
3,219
4,433
Years
($/yr)
4,847
(3,633)
3,219
4,433
NPV (Net Present Value)3 = $3,1 37
IRR (Internal Rate of Return)4 = 21%
Payback Period (yr) = 2.9
1 All cost values are obtained from Exhibits 14 and 15. The gas price is assumed to be $3/Mcf.
2 Based on 50% of the capital cost of glycol dehydrator.
3 The NPV is calculated based on 10% discount over 5 years.
4 The IRR is calculated based on 5 years.
Desiccant dehydrators can cost-effectively reduce methane emissions for
gas dehydration. Partner experience offers the following lessons learned:
* Desiccant dehydrators can provide significant economic benefits, such
as increased operating efficiency and decreased capital and mainte-
nance costs for low flow rate gas at higher pressures and lower temper-
ature conditions.
* Make-up (replacement) cost of the desiccant is slightly higher than the
glycol because the desiccants dissolve in water and must be replaced
regularly, while the glycol is recirculated.
* Desiccant dehydrators are an effective method for eliminating methane,
VOC, and HAP emissions, resulting in both economic and environmental
benefits.
* Include methane emissions reduction attributable to replacing glycol
dehydrators with desiccant dehydrators in Natural Gas STAR Program
annual reports.
17
-------
Acor, Lori G. and David Mirdadian. Benefits of Using Deliquescing Desic-
cants for Gas Dehydration. Society of Petroleum Engineers (SPE82138),
2003.
Bowman, Bob. Benefits of Using Deliquescing Desiccants for Gas
Dehydration, Society of Petroleum Engineers (SPE 60170), 2000.
Dow Chemical Company, product literature. Gas Dehydration with PELAD-
OWDG Calcium Chloride,
Energy Information Administration. Monthly Energy Review, 2002, Table A4.
Eskrigge, Charles. Air and Vacuum Process Inc. (Van Air), personal contact.
Gas Processors Supply Association. Engineering Data Book, Volume II, 11th
edition, 1998, Section 20-Dehydration.
Gas Research Institute. Atlas of Gas-Related Produced Water for 1990.
(GRI-95/0016, May 1995).
Gas Research Institute. Methane Emissions From the Natural Gas Industry,
1996, Volume 12 (GRI-94/0257.29). June 1996.
Murray, Curt. Practical Methods of Drying Natural Gas. Pride of the Hills
MFG., Inc.
Murray, Curt. Pride of the Hills Mfg., Inc., personal contact.
Smith, Reid. BP, personal contact.
The Hanover Compressor Company, personal contact.
Tingley, Kevin. U.S. EPA Natural Gas STAR Program, personal contact.
U.S. Environmental Protection Agency. Lessons Learned: Optimize Glycol
Circulation and Install Flash Tank Separators in Glycol Dehydrators (EPA430-
B-03-013, May 2003).
U.S. Environmental Protection Agency. Lessons Learned: Replacing Gas-
Assisted Glycol Pumps with Electric Pumps (EPA430-B-03-014, May 2003).
Vavro, Matthew E. Minimizing Natural Gas Dehydration Costs with Proper
Selection of Dry Bed Desiccants and New Dryer Technology. Society of
Petroleum Engineers (SPE37348), 1996.
Zavadil, Duane. Williams Production, personal contact.
18
-------
Appendix A
Moisture Content of Natural Gas in Equilibrium with Desiccants (Ib water/MMcf of natural gas)
Type Calcium Chloride Deliquescent Desiccant Tab lets
80'F
75F
70F
65F
EOF
58F
56F
54F
52F
50F
45F
40 F
35'F
80F
75F
70F
65F
60 F
58F
56F
54F
52F
5Q'F
45'F
40'F
35F
ID
PSIG
344
232
246
207
174
162
150
140
130
121
100
83
68
ID
PSIG
128
108
91
77
65
60
56
52
48
45
37
30
25
25
PSIG
219
186
157
132
111
103
96
69
63
77
64
53
44
25
PSIG
81
69
59
49
41
38
37
33
31
29
23.8
19.6
16.1
50
PSIG
134
113
96
81
68
63
59
55
51
47
39
32
27
50
PSIG
50
42
36
30
25
23.4
21.7
20.3
18.3
17.5
14.5
12
8.9
75
PSIG
98
83
70
59
50
46
43
40
37
35
29
24
19.6
I]
75
PSIG
36
31
26
21,9
18.4
17.1
15.9
14.8
13.8
12.8
10.7
8.7
7.2
100
PSIG
77
65
55
47
39
36
34
32
29
27
22.7
188
15.5
125
PSIG
64
54
46
39
33
31
29
26
24.5
22.6
19.9
15.6
13
150
PSIG
55
46
39
33
29
26
24.1
22.5
21
19.5
16.2
13.4
11.1
175
PSIG
48
41
43
29
24.5
22.8
21.2
19.8
18.4
17.1
14.3
11.6
9.8
200
PSIG
43
36
31
26
21.9
20,3
18.9
17.6
16.4
15.3
12.7
10.5
8.7
225
PSIG
39
33
27
23.5
19.6
18.4
17.1
16
14.9
13.9
11.5
9.6
7.9
250
PSIG
35
30
25
21.4
18.1
16.8
15.7
14.6
14.4
12.7
10.6
6.8
7,2
275
PSIG
33
28
23.4
19.8
16.8
15.6
14.5
13.5
12.6
11.7
9.6
8.1
6.7
300
PSIG
38
26
21.7
18.4
15.5
14.4
13.4
12.6
11.7
10.9
9.1
7.5
6.2
350
PSIG
27
22.5
19.1
16.2
13.7
12.9
11.8
11.1
10.3
9.6
8
6.7
5.5
400
PSIG
23.6
20.1
17.1
14.5
12.3
11.4
10.6
9.9
9.3
8.6
7.2
6
5
500
PSIG
19.7
16.8
14.3
12.1
10.3
9.6
8.9
8.3
7.8
7.2
6.1
5
4.2
750
PSIG
14.3
12.2
10.4
8.9
7.6
7
6.6
6.2
5.9
5.4
4.5
3.8
3.1
1000
PSIG
11.6
9.9
8.5
7.3
6.2
5.8
5,4
5.1
4.7
4.4
3.7
3.1
2.6
tpe Lithium Chloride Deliquescent Desiccant Tablets
100
PSIG
29
24.2
20.4
17.2
14.5
13.5
12.5
11.7
10.3
10.1
8.4
6.9
5.7
125
PSIG
23.7
20
17
14.3
12.1
11.2
10.5
3.7
3
8.4
7
5.8
4.8
150
PSIG
20.2
17.2
14.5
12.2
10.3
9.6
8.9
8.3
7.7
7.2
6
4.9
4.1
175
PSIG
17.8
15.1
12.7
10.8
9.1
8.4
7.6
7.3
6.8
6.4
5.3
4.4
3.6
200
PSIG
15.8
13.4
11.3
9.6
8.1
7.5
7
6.5
61
5.6
4.7
3.3
3.2
225
PSIG
14.3
12.1
10.3
8.7
7.4
6.8
6.3
5.8
5.5
5.1
4.3
3.6
2.9
250
PSIG
13
11.1
9.4
7.9
6.7
6.2
5.8
5.4
5
4.7
3.9
3.2
2.7
275
PSIG
12
10.2
8.7
7.3
6.2
5.7
5.4
5
4.7
4.4
3.6
3
2.5
300
PSIG
11.1
9.5
8
6.8
5.7
5.3
5
4.6
4.3
4
3.3
2.6
2.3
350
PSIG
9.8
8.3
7.1
6
5
4.7
4.4
4.1
3.8
3.5
2.9
2.4
2
400
PSIG
8.7
7.4
6.3
5.4
4.5
4.2
3.9
3.7
3.4
3.2
2.6
2.2
1.8
500
PSIG
7.3
6,2
5,3
4.5
3.8
3.5
3.3
3.1
2.9
2.7
2.2
1.8
1.5
750
PSIG
5.3
4.5
3.8
3.3
2.8
2.6
2.4
2.3
2.1
2
1.6
1.4
1.1
1000
PSIG
4.3
3.7
3.1
2.7
2.3
2.1
2
1.8
1.7
1.6
1.3
1.1
0.9
Source: Van Air
-------
Appendix B
1000*
100*
NATURAL GAS AT SATURATION i
SMITH INDUSTRIES INC.
HOUSTON, TEXAS
CHART 2
-10 0 10 20 30 40 50 60 70 80 90 100 110 120 130
TEMPERATURE, °F
1000*
Source: Smith Industries, Inc., Houston, Texas
20
-------
&EPA
United States
Environmental Protection Agency
Air and Radiation (6202J)
1200 Pennsylvania Ave., NW
Washington, DC 20460
EPA430-B-03-01
November 2003
------- |