Lessons
Learned
NaturalGas
EPA POLLUTION PREVENTER
SBl
N
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CD
From Natural Gas STAR Partners
OPTIMIZE GLYCOL CIRCULATION AND INSTALL FLASH TANK
SEPARATORS IN GLYCOL DEHYDRATORS
Executive Summary
There are approximately 38,000 glycol dehydration systems in the natural gas production sector emitting an esti-
mated 22 Bcf of methane per year into the atmosphere. Most dehydration systems use triethylene glycol (TEG)
as the absorbent fluid to remove water from natural gas. As TEG absorbs water, it also absorbs methane, other
volatile organic compounds (VOCs), and hazardous air pollutants (HAPs). As TEG is regenerated through heating
in a reboiler, absorbed methane, VOCs, and HAPs are vented to the atmosphere with the water, wasting gas and
money.
The amount of methane absorbed and vented is directly proportional to the TEG circulation rate. Many wells pro-
duce gas far below the original design capacity but continue to circulate TEG at rates two or three times higher
than necessary, resulting in little improvement in gas moisture quality but much higher methane emissions and
fuel use. Reducing circulation rates reduces methane emissions at negligible cost.
Installing flash tank separators on glycol dehydrators further reduces methane, VOC, and HAP emissions and
saves even more money. Recovered gas can be recycled to the compressor suction and/or used as a fuel for the
TEG reboiler and compressor engine. Economic analyses show flash tank separators installed on dehydration
units payback costs in 4 to 17 months.
Method for
Reducing Gas
Loss
Reducing TEG
circulation rates
Flash Tank
Separators
TEG
Circulation
Rates (gal/hr)
50% to 200 %
over-circulation1
150
450
Value of Gas Saved ($/yr)2
Energy-
exchange
Electric
Pump
390 to 39,400/yr1
2,1303
21,2953
71 03
8,7623
Cost of
Reducing Gas
Loss
Negligible
$5,000-$5,600
$7,000-$14,000
Payback
(months)
Immediate
6-17
5-8
1 Optimal circulation rates ranged from 30 to 750 gal TEG/hr.
2 At $3.00/Mcf.
3 Includes recovered natural gas liquids sales revenue.
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
Many producers use triethylene glycol (TEG) in dehydrators to remove water
from the natural gas stream and to meet pipeline quality standards. In a typi-
cal TEG system, shown in Exhibit 1, "lean" (dry) TEG is pumped to the gas
contactor. In the contactor, the TEG absorbs water, methane, VOCs, and
HAPs (including benzene, toluene, ethylbenzene and xylenes (BTEX)), from
the wet production gas. The "rich" (wet) TEG leaves the contactor saturated
with gas at sales pipeline pressure, typically between 250 and 800 psig. The
gas entrained in the rich glycol, plus additional wet gas bypassing the con-
tactor, expands through the energy-exchange driver for the TEG circulation
pump. The TEG is then circulated through a reboiler where the absorbed
water, methane, and VOCs are boiled off and vented to the atmosphere. The
lean TEG is then sent through an energy-exchange pump back to the gas
contactor, and the cycle repeats.
I
Exhibit 1: TEG system without Flash Tank Separator
To Atmosphere
(Methane/other
vapors and water)
Fuel Gas
Energy
Exchange
Pump
Lean TEG
Source: Exxon U.S.A.
Because the system described above is primarily designed to remove water
from the gas stream, significant methane emissions can also result.
Fortunately there are several steps that operators can take that will minimize
gas loss:
1) Reduce the TEG circulation rate.
Gas production fields experience declining production, as pressure is drawn
off the reservoir. Wellhead glycol dehydrators and their TEG circulation rates
are designed for the initial, highest production rate, and therefore, become
over-sized as the well matures. It is common that the TEG circulation rate is
much higher than necessary to meet the sales gas specification for moisture
content. The methane emissions from a glycol dehydrator are directly pro-
portional to the amount of TEG circulated through the system. The higher
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NESHAP Regulations
On June 29, 2001 EPA finalized the National
Emission Standards for Hazardous Air
Pollutants(NESHAP) for Oil and Natural Gas
Production Facilities (40 CFR 63 Subpart
HH) and for Oil and Gas Transmission and
Storage Facilities (40 CFR 63 Subpart HHH).
These standards set a throughput floor of
3MMscf/day for production facilities and a
higher 10MMscf/day for transmission and
storage facilities. Above these floors opera-
tors need to install equipment to either
reduce HAPs from dehydrator vents by 95
percent using closed-vent control systems
or making process modifications, or com-
bust HAPs below 20 ppmv. These standards
are also triggered if total benzene emissions
exceed 1 ton/year.
the circulation rate, the more methane is vented from the regenerator. Over-
circulation results in more methane emissions without significant and neces-
sary reduction in gas moisture content. Natural Gas STAR partners have
found that dehydrator systems often recirculate TEG at rates two or more
times higher than necessary. Operators can reduce the TEG circulation rate
and subsequently reduce the methane emissions rate, without affecting
dehydration performance or adding any additional cost.
2) Install a Flash Tank Separator
Most production and processing sector dehydrators send the glycol/gas
mixture from the TEG circulation pump directly to the regenerator, where all
of the methane and VOCs entrained with the rich TEG vent to the atmos-
phere. One industry study found that flash tank separators were not used in
85 percent of dehydration units processing less than one MMscfd of gas, 60
percent of units processing one to five MMscfd of gas, and 30 to 35 percent
of units processing over five MMscfd of gas.
In a flash tank separator, gas and liquid are separated at either the fuel gas
system pressure or a compressor suction pressure of 40 to 100 psig. At this
lower pressure and without added heat, the gas is rich in methane and
lighter VOCs but water remains in solution with the TEG. The flash tank cap-
tures approximately 90 percent of the methane and 10 to 40 percent of the
VOCs entrained by the TEG, thereby reducing emissions. The wet TEG,
largely depleted of methane and light hydrocarbons, flows to the glycol
reboiler/regenerator where it is heated to boil off the absorbed water, remain-
ing methane, and VOCs. These gases are normally vented to the atmos-
phere and the lean TEG is circulated back to the gas contactor. Exhibit 2
shows a TEG dehydrator with a flash tank separator.
Note: Installing flash tank separators on large dehydrators may be required
to achieve compliance with Maximum Available Control Technology (MACT)
standards under the oil and gas industry NESHAPs. When these installations
are required by law, the partner should not include associated methane
emissions reductions in their Natural Gas STAR Annual Reports.
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Exhibit 2: Dehydrator Schematic - with Flash Tank Separator
To Atmosphere
(Methane/other
vapors and water)
Fuel Ga
Energy
Exchange
Pump
Lean TEG
Source: Exxon U.S.A.
3) Use of electric pumps in place of energy-exchange pumps
Remote gas fields do not have electrical power and instead use "energy-
exchange" pumps to power the lean TEG circulation pump. For every vol-
ume of gas absorbed in the rich TEG leaving the contactor, two more vol-
umes of gas must be added from wet feed gas to supply enough power in
the driver for the lean TEG pump. Therefore, using either a piston or gear
type "energy-exchange" pump triples the amount of gas entrained with the
TEG and vented to the atmosphere when there is no flash tank separator.
Installing an electric motor in place of an energy-exchange pump eliminates
this additional emissions source. Conventional piston type energy-exchange
pumps also often leak rich (wet) TEG into the lean (dry) TEG. Leakage of
only 0.5 percent can double the circulation rate necessary to maintain sales
gas moisture content, thus increasing potential emissions. For more informa-
tion on this practice, see EPA's Lessons Learned: Replacing Gas-Assisted
Glycol Pumps with Electric Pumps.
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Economic and
Environmental
Benefits
Optimizing glycol circulation and installing flash tank separators provide
several environmental and economic benefits:
* Reducing glycol circulation to the optimum rate saves glycol replace-
ment costs as well as fuel consumption in the reboiler.
* Reducing VOC and HAP (BTEX) emissions improves ground level
air quality. BTEX emission reductions can be significant for large
dehydrators.
* Using flash tank separators on dehydration units with a condenser on
the reboiler vent improves the efficiency of the condenser by removing
most of the non-condensable gas, primarily methane. A condenser
recovers natural gas liquids (NGLs), and HAPs more efficiently than flash
tank separators alone.
* Using the gas recovered in the flash tank for fuel gas reduces operating
costs.
* Piping recovered flash tank gas to the suction of an upstream
compressor (a common design practice in new installations) reduces
production costs.
* Piping a dehydrator's regenerator vent to a vapor recovery unit allows
flash tank gas to be used as a stripping gas in the glycol reboiler.
Decision
Process
Operators can estimate the costs and the benefits of optimizing the TEG
circulation rate and installing a flash tank separator by following these five
steps:
Step 1: Optimize Circulation Rate. Operators can easily calculate the opti-
mal circulation rate by following a few simple calculations. First obtain the
current circulation rate by reading the flow controller, which measures gallons
of TEG circulated. For each gallon of TEG circulated, one standard cubic
foot of methane is absorbed,
and if the unit has an energy-
exchange pump, two more
cubic feet of gas will be nec-
essary to drive the pump. All
of this gas is vented to the
atmosphere when there is no
flash tank separator.
Next, determine the minimum
circulation rate necessary to
dewater the gas stream. The
Five Steps for Evaluating TEG
Circulation Rate Optimization and
Flash Tank Separator Installation:
1.
2.
Optimize circulation rate.
Identify dehydration units without flash
tanks.
3. Estimate capital and installation costs.
4. Estimate value of gas saved.
5. Conduct economic analysis.
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minimum TEG circulation rate at a particular site is a function of the gas flow
rate, the water content of incoming gas, and the desired water content of
outgoing gas. The water removal rate is a function of the gas flow rate and
the amount of water to be removed from the gas stream. The TEG-to-water
ratio (how many gallons of TEG are required to absorb 1 pound of water)
varies between 2 and 5 gallons of TEG per pound of water; the industry
accepted rule-of-thumb is 3 gallons of TEG per pound of water removed.
The greater the water removal rate or the higher the TEG-to-water ratio, the
higher the TEG circulation rate must be. Some STAR partners report lower
TEG-to-water ratios than the norm (i.e., <3 gallons TEG per pound of water),
which lowers their optimal TEG circulation rates.
Problems can arise if the TEG circulation rate is too low; therefore a certain
amount of over-circulation is desired. For instance, an overly restricted circu-
lation rate can cause problems with tray hydraulics, contactor performance,
and fouling of glycol-to-glycol heat exchangers. Therefore, operators should
include a margin of safety, or "comfort zone," when calculating reductions in
circulation rates. An optimal circulation rate for each dehydration unit typical-
ly ranges from 10 to 30 percent above the minimum circulation rate. The for-
mulas used to determine the minimum and optimum TEG circulation rates
are shown in Exhibit 3.
Exhibit 3: Calculating the Optimal TEG Circulation Rate
A 20 MMcf/d dehydrator has a TEG circulation rate set at 280 gal/hr, and the wet gas
stream has 60 Ib water/MMcf. A comfort zone of 15 percent over the minimum rate is
desired. The optimal TEG circulation rate can be calculated as follows:
Given:
F = gas flow rate (MMcf/d)
I = inlet water content (Ib/MMcf)
0 = outlet water content (Ib/MMcf) (Rule-of-thumb is 4)
G = glycol-to-water ratio (gal TEG/lb water) (Rule-of-thumb is 3)
L(min) = minimum TEG circulation rate (gal/hr)
W = Water Removal Rate (Ib/hr)
Calculate: L(min) = Minimum TEG Circulation Rate (gal/hr)
L(min) = W*G
w-
24hr/day
W = 20*(60'4) = 46.66 Ib water/hr
24hr/day
G = 3
L(min) = 46.66*3 = 140 gal TEG/hr
This is the minimum circulation rate. Adding 15 percent over L(min) for the comfort
zone yields an optimal circulation rate of 160 gal TEG/hr. For example:
L(opt) = Optimal circulation rateL(opt) = 140 gal TEG/hr * 1.15 = 160 gal TEG/hr
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Natural Gas STAR partners and other indus-
try experts have identified five common
reasons operators of glycol dehydrators
over-circulate TEG:
• Gas-powered energy-exchange pumps
can contaminate lean glycol, making the
glycol less effective at absorbing water
from the wet gas stream. To compensate,
operators over-circulate to attain the
same dew point depression as would be
attained by non-contaminated glycol
circulating at a lower rate.
• Circulation rates are set to match the
plant design capacity, rather than actual
throughput.
• Higher rates ensure adequate dehydration
at fluctuating gas throughput rates.
• Dehydration units are in remote locations
making frequent adjustments
inconvenient.
• Dehydrators are operated by independent
contractors that have little incentive to
optimize the circulation rate and reduce
methane losses.
Step 2: Identify dehydration units without flash tanks. Most new dehy-
dration units include flash tank separators as standard equipment.
Approximately two-thirds of operating units, however, do not have flash tank
separators; these are mainly smaller, older, or more remote units. Before pro-
ceeding to the next step, operators first should identify dehydration units
without flash tank separators.
Step 3: Estimate capital and installation costs. For the purposes of this
analysis, the cost of optimizing the glycol circulation rate is assumed to be
very small (1/2 hour at $25/hour).
Before estimating the costs of purchasing and installing a flash tank separa-
tor, partners must choose a design and size that meets their needs.
Selecting a flash tank depends on a number of factors including composi-
tion of the gas stream (i.e., recovery rate of gas liquids), construction code
requirements, cost, and ease of implementation. Flash tank separators are
manufactured in two designs—vertical and horizontal. In general, operations
that have significant volumes of NGLs in their gas stream should use a
three-phase horizontal separator (natural gas, TEG, NGLs) with a retention
time of 10 to 30 minutes. Operations that do not have marketable amounts
of NGLs can use a two-phase separator (natural gas, TEG) with a 5 to
10 minute retention time. Vertical vessels are best suited for two-phase
systems.
Manufacturers sell a wide range of standard, "off-the-shelf" flash tank sepa-
rators, which are specified based on settling time and volume. To determine
the appropriate size of a flash tank separator, partners should calculate the
settling volume required for each system.
Exhibit 4 presents the basic calculation for determining the necessary set-
tling volume for a flash tank separator based on the TEG circulation rate.
Additional volume might be necessary if operators also settle out NGLs in
the flash tank separator for periodic pickup by a tank truck. For example, if
the TEG circulation rate indicates a settling volume of 75 gallons, and 35
gallons of NGLs will be accumulated, the settling volume should be
increased by 35 gallons.
Exhibit 4: Sizing the Flash Tank
Given: L = TEG circulation rate in gal/hr
T = retention time in minutes
Calculate: SV = liquid settling volume (gallons)
SV = (L * T) -r 60
Note: Add site-specific volume for accumulating NGLs for periodic pick-up.
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The total cost of a flash tank separator depends on: (A) capital costs and (B)
installation and operating costs.
(A) Capital Costs
Costs of flash tank separators can range between $2,500 and $5,000, unin-
stalled, depending on flash tank design and size. If the required size exceeds
the largest standard flash tank available, operators can either have a custom
tank built, install multiple flash tanks in parallel, or install a separate NGL
accumulation tank.
(B) Installation and Operating Costs
Installation costs depend on location, terrain, foundation, weather protection
(vessel fabrication codes are based on the amount of hydrogen sulfide in the
gas), NGL accumulation and pickup capability, and automation and instru-
mentation. Information provided by flash tank separator manufacturing com-
panies suggests an average installation cost of $1,200, including delivery,
assembly and labor costs. This cost could increase by as much as 80
percent, depending on site-specific factors.
Flash tank separators installed at existing dehydration units are prefabricated,
and include tubing, valves, and associated equipment. Installation can be per-
formed with minimal downtime. To minimize installation costs, partners sug-
gest installing a flash tank separator when a dehydration unit is being repaired
or during other system overhauls.
Flash tanks are designed as simple pressure vessels, with few operating parts.
Therefore, operating and maintenance (O&M) costs are negligible. Partners
have found that flash tank separator maintenance can be accomplished dur-
ing routine O&M practices for the dehydration unit.
Capital and installation costs for a range of flash tank types and standard
sizes are provided in Exhibits 5A and 5B.
Exhibit 5A: Vertical Separator Sizes and Costs
Settling
Volume
(gallons)1
8.2
13.5
22.3
33.6
Diameter
(feet)
1.08
1.33
1.66
2
Height
(feet)
4
4
4
4
Capital
Costs ($)
2,500
3,300
4,300
5,000
Installation
Costs ($)
1,200-2,160
1,200-2,160
1,200-2,160
1,200-2,160
O&M Costs
($)
Negligible
Negligible
Negligible
Negligible
Note: Cost information provided by Sivalls, Incorporated.
1 Settling Volume = half of total volume (not including NGL accumulation requirements).
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Exhibit 5B: Typical Horizontal Three-Phase Separator Sizes and Costs
Settling
Volume
(gallons) 1
49
65
107
158
225
Diameter
(feet)
2
2
2.5
3
3
Length
(feet)
3
5
5
5
7.5
Capital
Costs ($)
3,000
3,200
3,400
4,800
5,000
Installation
Costs ($)
1,200-2,160
1,200-2,160
1,200-2,160
1,200-2,160
1,200-2,160
O&M Costs
($)
Negligible
Negligible
Negligible
Negligible
Negligible
Note: Cost information provided by Sivalls, Incorporated.
1 Settling Volume = half of total volume (not including NGL accumulation requirements).
Step 4: Estimate value of gas saved. Gas savings can be achieved by
optimizing the circulation rate alone, installing a flash tank separator, and
in certain circumstances, doing both. Exhibit 6 shows how to determine
the amount of gas savings from optimizing the TEG circulation rate with
no flash tank separator. Additional savings from reducing TEG circulation
rates include:
* Lower fuel requirements for the regenerator. Reducing the load on a
regenerator with a heat duty of 1,340 Btu/gal of TEG circulated could
save between $545 and $54,456 per year, depending on the amount of
overcirculation and the heating value of the natural gas.
* Reducied frequency of glycol replacement. Industry experts estimate
that 0.5 percent of TEG volume is lost per hour. Annual savings could
range from $393 (if circulation rates are reduced from 45 to 30 gallons
per hour) to $39,300 (if rates are reduced from 3,000 to 750 gallons per
hour).
Installing a flash tank allows partners to recover most of the gas entrained in
the TEG. The amount of gas saved from installing a flash tank is a function
of the type of TEG circulation pump, the dehydrator's glycol circulation rate
and the pressure in the flash tank separator. Typically, about 90 percent of
the methane can be recovered from TEG using a flash tank separator.
The type of circulating pump used in the dehydrator has the largest effect on
gas recovery. As a rule-of-thumb, each gallon of TEG leaving the contactor
has one cubic foot of methane dissolved in it. Energy-exchange pumps
require additional high-pressure gas in conjunction with that in the rich TEG
flow to supply the energy necessary to pump the lean TEG back to the con-
tactor. As a result, they increase the amount of methane entrained to three
cubic feet per gallon of TEG circulated.
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Exhibit 6: Calculating the Total Annual Savings from Optimizing TEG
Circulation in Dehydrators with no Flash Tank Separator
Given:
A = TEG absorption rate (ft3/gallon TEG) (Rule-of-thumb is 1)
E = Energy-exchange Pump gas, if applicable (ftVgallon TEG) (Rule-of-thumb is 2)
H = Hours per year (8,760)
P = Sales price of gas (Assume $3/Mcf)
L(original) = TEG circulation rate (gal/hour) before adjustment
L(optimal) = TEG circulation rate (gal/hour) after adjustment)
V = Value of Gas Saved ($/year)
V =
(L(original) - L(optimal)) * (A + E) * H * P
1,000
Applying this formula shows that minor reductions in circulation rates can yield
substantial savings as shown in the following examples. Note that savings should be
reduced by 2/3 where lean glycol is pumped using an electric motor instead of an
energy-exchange pump.
Original
Circulation Rate
45
90
225
450
675
1350
1125
2250
Optimal
Circulation Rate
30
30
150
150
450
450
750
750
Annual Methane
Savings (Mcf)
394
1,577
1,971
7,884
5,913
23,652
9,855
39,420
Annual Savings
(@ $3/Mcf)
$1,182
$4,731
$5,913
$23,652
$17,739
$70,956
$29,565
$118,260
Exhibit 7 shows how to calculate the amount of methane vented in the
absence of a flash tank separator, as well as the value of the gas that could
be saved by using a flash tank separator. This example assumes that TEG
circulation rates are optimized.
10
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Exhibit 7: Amount of Gas Vented without a Flash Tank and
Potential Savings
Assume a dehydration unit with an energy-exchange pump circulates 150 gallons of
TEG per hour, with a recovery rate of 90 percent, and a gas price of $3 per Mcf.
Given: L = TEG circulation rate (gal/hr)
G = Methane entrainment rate (rule-of-thumb is 3 cubic ft/gal for energy-
exchange pumps; 1 cubic ft/gal for electric pumps)
Calculate: V = amount of gas vented annually (Mcf/yr)
V = (L * G ) * 8,760 (hours per year) -=-1000 cf/Mcf
V = 150 gal/hr * 3 scf/gal * 8,760 hrs/yr -=-1000 cf/Mcf
V = 3,942 Mcf/yr
Savings = 3,942 Mcf X 0.9 X $3/Mcf = $10,643 per year
Exhibit 8 compares the potential savings using a flash tank separator,
calculated for energy-exchange and electric pumps at different circulation
rates. As the exhibit shows, smaller dehydration units, and units with electric
circulation pumps, have a lower economic potential for paying out the cost
of a flash tank separator.
Exhibit 8: Potential Savings of using a Flash Tank Separator
TEG Circulation
Rates (gal/hr)
30
150
300
450
Energy-exchange Pump
Mcf/y
710
3,548
7,096
10,643
$/yr
2,129
10,643
21,287
31,930
Electric Pump
Mcf/y
237
1,183
2,365
3,548
$/yr
710
3,548
7,096
10,643
It is important to note that additional revenue can be generated from the sale
of natural gas liquids (NGLs). When treating rich production gas, NGLs often
condense and are separated out in the flash tank separator. The quantity
varies based on temperature, pressures in the contactor and the flash tank,
produced gas composition, and gas entrainment in the TEG. This is a very
site-specific evaluation, beyond the scope of this study.
11
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Step 5: Conduct economic analysis. As demonstrated in Step 4, the opti-
mization of glycol circulation to a lower rate will always save money.
Therefore partners should always take this action first, regardless of whether
or not they decide to install a flash tank separator. The remainder of this
analysis focuses on flash tank separators, and assumes that the glycol cir-
culation rate has already been optimized.
Once the capital and installation costs and the value of gas saved have been
estimated, partners should conduct an economic analysis. One straightfor-
ward way to evaluate the economics is through a discounted cash flow
analysis, in which the first year costs for installing the flash tank separator
are compared against the discounted value of the saved gas (plus sales of
NGLs) over the economic life of the project.
Exhibits 9A and 9B present hypothetical results of this type of analysis. For
all but the smallest systems, installation of a flash tank separator at a dehy-
dration unit with an energy-exchange pump will pay-out in less than a year,
while a unit with an electric pump should pay-out in less than two-and-a-half
years.
Exhibit 9A: Economics of Installing a Flash Tank Separator on a
Dehydrator with Energy-exchange Pump
TEG
Circulation
Rate (gal/hr)
30
150
300
450
Capital and
Installation
Cost ($) 1
5,160
5,560
7,160
13,9205
Gas
Savings2
$/yr
2,129
10,643
21,287
31,930
Total
Savings3
$/yr
2,158
10,792
21,573
32,365
Payback
Period
(months)
29
6
4
5
Return on
Investment4
31%
193%
301%
232%
1 Horizontal flash tank, 80 percent contingency on installation, 30 minute settling time plus weekly volume of accumu-
lated NGL, where recovered.
2 Gas valued at $3.00/Mcf.
3 Higher total savings include natural gas liquids recovery (if present) at 1 percent of recovered gas, valued at
$21/barrel. This NGL recovery rate is for these examples only, each site must individually evaluate this potential.
4 IRR based on 5 years.
5 Cost for two parallel FTS (for custom size) as settling volume exceeds standard size FTS.
12
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Exhibit 9B: Economics of Installing a Flash Tank Separator on a
Dehydrator with Electric Pump
TEG
Circulation
Rate (gal/hr)
30
150
300
450
Capital and
Installation
Cost ($) 1
5,1605
5,1605
5,1605
7,160
Gas
Savings2
$/yr
710
3,548
7,096
10,643
Total
Savings3
$/yr
719
3,596
7,110
10,671
Payback
Period
(months)
No
17
9
8
Return on
Investment4
No
64%
136%
149%
1 Horizontal flash tank, 80 percent contingency on installation, 30 minute settling time plus weekly volume of accumu-
lated NGL, where recovered.
2 Gas valued at $3.00/Mcf.
3 Higher total savings include natural gas liquids recovery (if present) at 1 percent of recovered gas, valued at
$21/barrel. This NGL recovery rate is for these examples only, each site must individually evaluate this potential.
4 IRR based on 5 years.
5 Cost for minimum standard tank size.
These exhibits also illustrate the effect of NGLs in the analysis. Because
energy-exchange pumps entrain three times more natural gas with the rich
TEG than electric pumps, the TEG releases more NGLs in the flash tank
separator. As a result, a glycol dehydration system with an energy-exchange
pump requires a flash tank with a larger holding capacity. The increased rev-
enues from NGL sales justify the additional cost of the larger tanks. With an
electric pump, NGLs are not present in economic quantities in the TEG, thus
minimum sized standard tanks can be used for circulation rates between
30 and 300 gal/hr. However, when the 450 gal/hr tank is needed, a very
small amount of NGLs can be collected and sold to reduce the cost of the
flash tank.
The economics of both installing a flash tank separator and optimizing glycol
circulation rates depends entirely on whether the site has a beneficial use for
all the gas recovered in the flash tank. Partners have reported cases where
well-head dehydrator installations did not include an engine-driven compres-
sor, and the reboiler fuel gas consumption was well below the amount of
gas recovered in a flash tank. In this case, the excess recovered gas would
have to be vented from the flash tank. In this type of operation, optimizing
glycol circulation has an economic value in reducing the gas vented from the
flash tank. Site-specific fuel use would be required to evaluate the savings
from employing both the flash tank and optimizing circulation.
13
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Lessons
Learned
TEG circulation rates at glycol dehydrators are often two to three times higher
than the level needed to remove water from natural gas. Most production
dehydrators do not have flash tanks, which can be an effective method for
recovering valuable methane from TEG that would otherwise be vented to the
atmosphere. Natural Gas STAR partners offer the following lessons learned:
* To keep the circulation rates near optimum, educate field O&M personnel
or contractors on the method for calculating and adjusting circulation
rates, including estimates of a "comfort zone." Incorporate circulation rate
adjustment into regular O&M practices.
* Operators should not reduce the quantity of glycol in the system, rather
than the circulation rate; this will not achieve the desired savings.
Reducing the quantity of glycol can cause problems with tray hydraulics,
contactor performance, and fouling of glycol-to-glycol heat exchangers.
* Identify all operating dehydrators without flash tank separators and
collect the necessary information to evaluate the economics of flash tank
installation.
* Where industrial power (440 volt or higher) is available, replacing an
energy-exchange pump with an electric motor-driven pump can reduce
the gas entrained with the TEG by as much as two thirds, significantly
reducing methane emissions. Where only 220-volt service is available, a
hybrid pump that combines gas-energy exchange with electric power to
reduce methane absorption can also reduce methane absorbed by the
TEG and lower emissions (see EPA's Lessons Learned: Replacing Gas-
Assisted Glycol Pumps with Electric Pumps).
* Route recovered methane to the compressor suction or to fuel use.
Partners have reported that recovered methane sometimes contains too
much water to be used for pneumatic instrument systems.
* Collect marketable natural gas liquids from the flash tank separator as a
potentially significant source of additional revenue.
* Over time, the seals on gas-powered energy-exchange pumps can leak,
contaminating the lean glycol and reducing dehydration effectiveness.
Operators should not compensate for the contaminated glycol by
increasing the TEG circulation rate. Instead, the energy-exchange pump
should be evaluated for repair or replacement.
* Record reduction at each dehydrator and report them with your Natural
Gas STAR Annual Report. Note: methane savings obtained by installing
technologies required by the NESHAP regulations should not be reported
to the Natural Gas STAR voluntary methane reduction program.
14
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American Petroleum Institute. Specification for Glycol-Type Gas Dehydration
Units (Spec 12GDU). July 1993.
Garrett, Richard G. Rotor-Tech, Inc. Personal contact.
Gas Research Institute Environmental Technology and Information Center
(ETIC). Personal contact.
GRI and U.S. EPA. Methane Emissions from Gas-Assisted Glycol Pumps.
January 1996.
Griffin, Rod. Sivalls, Incorporated. Personal contact.
Henderson, Carolyn. U.S. EPA Natural Gas STAR Program. Personal
contact.
Moreau, Roland. Exxon-Mobil Co. USA. Personal contact.
Robinson, R.N. Chemical Engineering Reference Manual, Fourth Edition.
1987.
Reuter, Curtis. Radian International LLC. Personal contact.
Rueter, C; Gagnon, P; Gamez, J.R GRI Technology Enhances Dehydrator
Performance. American Oil and Gas Reporter. March 1996.
Rueter, C.O.; Murff, M.C.; Beitler, C.M. Glycol Dehydration Operations,
Environmental Regulations, and Waste Stream Survey. Radian Intern^
LLC. June 1996.
Tannehill, C.C; Echterhoff, L.; Leppin, D. Production Variables Dictate Glycol
Dehydration Costs. American Oil and Gas Reporter. March 1994.
Tingley, Kevin. U.S. EPA Natural Gas STAR Program. Personal contact.
15
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&EPA
United States
Environmental Protection Agency
Air and Radiation (6202J)
1200 Pennsylvania Ave., NW
Washington, DC 20460
EPA430-B-03-013
December 2003
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