Lessons Learned 

from Natural Gas STAR Partners 

Optimize Glycol Circulation And Install 

Flash Tank Separators In Glycol 

Dehydrators 

Executive Summary 

There are approximately 36,000 glycol dehydration

systems in the natural gas production sector emitting an 

estimated15 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

produce 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 11 months. 

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 typical 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, ethyl benzene and

xylenes (BTEX)), from the wet production gas. The 

“rich” (wet) TEG leaves the contactor saturated with gas at 

Method for Reducing 

Natural Gas Losses 

Volume of 

Natural Gas 

Savings (Mcf) 

Value of Natural Gas Savings 

($/year) Implementation 

Cost ($) 

Payback (Months) 

$3 per 

Mcf 

$5 per 

Mcf 

$7 per 

Mcf 

$3 per 

Mcf 

$5 per 

Mcf 

$7 per 

Mcf 

Reducing TEG circulation 

rates 1 

370 to 37,000 

per year 

$2,750 – 

$118,000 

$1,960 – 

$196,000 

$2,700 – 

$275,000 Negligible Immediate Immediate Immediate 

Energy Exchange 

pump and a 150 gal/hr 

TEG circulation rate 

3,573 $10,719 $17,865 $25,011 $6,500 – $7,600 7 – 9 4 – 6 3 – 4 

Energy Exchange 

pump and a 450 gal/hr 

TEG circulation rate 

10,717 $32,151 $53,585 $75,019 $9,500 – $18,800 3 – 7 2 – 5 1 – 3 

Electric pump and a 

150 gal/hr TEG 

circulation rate 

1,191 $3,573 $5,955 $8,337 $6,500 – $7,600 22 – 26 13 – 16 9 – 11 

Electric pump and a 

450 gal/hr TEG 

circulation rate 

3,553 $10,659 $17,765 $24,871 $9,500 – $18,800 11 – 22 6 – 13 5 – 9 

1

 50% to 200% TEG over-circulation rate. Optimal circulation rates ranged from 30 to 750 gal TEG/hr. 

Flash Tank Separators 

Economic and Environmental Benefits 

1 
 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

Exhibit 1: TEG system without Flash 

 Tank Separator 

sales pipeline pressure, typically between 250 and 800 

psig. The gas entrained in the rich glycol, plus additional 

wet gas bypassing the contactor, 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. 

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 

proportional to the amount of TEG circulated through the

system. The higher the circulation rate, the more methane

is vented from the regenerator. Over-circulation results in

more methane emissions without significant and necessary

reduction in gas moisture content. Natural Gas STAR 

partners have found that dehydrator systems often 

recirculate TEG at rates two or more time 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 atmosphere.

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 captures 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, remaining methane, and VOCs. 

These gases are normally vented to the atmosphere and

the lean TEG is circulated back to the gas contactor.

Exhibit 2 shows a TEG dehydrator with a flash tank 

separator. 

Exhibit 2: Dehydrator Schematic—with Flash Tank 

Separator 

2 
 

 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

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. 

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 volume of gas absorbed in the

rich TEG leaving the contactor, two more volumes 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 energyexchange 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 

information on this practice, see EPA’s Lessons Learned: 

Replacing Gas-Assisted Glycol Pumps with Electric Pumps. 

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 replacement 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-condensate gas, primarily methane. A condenser

recovers natural gas liquids (NGLs), and HAPs more

efficiently than flash tanks 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 compressors (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 Points 

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 optimal 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 

The average methane content of natural gas varies by natural gas 

industry sector. The Natural Gas STAR Program assumes the 

following methane content of natural gas when estimating 

methane savings for Partner Reported Opportunities. 

Production 79 % 

Processing 87 % 

Transmission and Distribution 94 % 

Methane Content of Natural Gas 

NESHAP Regulation 

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

operators need to install equipment to either reduce

HAPs from dehydrator vents by 95 percent using 

closed-vent control systems or making process 

modifications, or combust HAPs below 20 ppmv. These 

standards are also triggered if total benzene emissions

exceed 1 ton/year. 

3 
 

 

 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

Five Steps for Evaluating TEG Circulation Rate Optimization and 

Flash Tank Separator Installation: 

1. Optimize circulation rate. 

2. Identify dehydration units without flash tanks. 

3. Estimate capital and installation costs. 

4. Estimate value of gas saved. 

5. Conduct economic analysis 

absorbed, and if the unit has an energy exchange pump,

two more cubic feet of gas will be necessary 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 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 circulation 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 typically ranges from 10 to 30 percent 

above the minimum circulation rate. The formulas used to 

determine the minimum and optimum TEG circulation

rates are shown in Exhibit 3. 

Step 2: Identify dehydration units without flash tanks. 

Most new dehydration 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 proceeding to the next step, operators

first should identify dehydration units without flash tank 

separators. 

Exhibit 3: Calculating the Optimal 

TEG Circulation Rate 

For example a 20 MMcf/d dehydrator has a TEG circulation rate set at 280 

gal/hr, and the wet gas stream has 60 lb 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 (lb/MMcf) 

O = Outlet water content (lb/MMcf) (Rule-of-thumb is 4) 

G = Glycol-to-water ratio (gal TEG/lb water) (Rule-of-thumb 3) 

L(min) = minimum TEG circulation rate (gal/hr) 

W = Water Removal Rate (lb/hr) 

Calculate: L(min) = Minimum TEG Circulation Rate (gal/hr) 

L(min) = W*G 

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 150 gal 

TEG/hr. For example: 

L(opt) = Optimal circulation rateL(opt) = 140 gal TEG/hr * 1.15 = 160 gal 

TEG/hr 

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 separator, partners must choose a design and 

size that meets their needs. Selecting a flash tank depends 

on a number of factors including composition 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 threephase 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 two4 
Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

phase separator (natural gas, TEG) with a 5 to 10 minute 

retention time. Vertical vessels are best suited for twophase systems. 

Manufacturers sell a wide range of standard, “off-theshelf” flash tank separators, 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 settling 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 setting volume (gallons) 

SV = (L * T)/60 

Note: Add site-specific volume for accumulating NGLs for 

periodic pick-up 

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 $3,375 and 

$6,751, uninstalled, 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. 

instrumentation. Information provided by flash tank 

separator manufacturing companies suggests an average 

installation cost of $1,684, 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 performed with 

minimal downtime. To minimize installation costs, 

partners suggest 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 during

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) 

Diameter 

(feet) 

Height 

(feet) 

Capital 

Costs ($) 

Installation 

Costs ($) 

O&M 

Costs 

($) 

8.2 1.08 4 $3,375 $1,684-3,031 N/A 

13.5 1.33 4 $4,455 $1,684-3,031 N/A 

22.3 1.66 4 $5,806 $1,684-3,031 N/A 

33.6 2 4 $6,751 $1,684-3,031 N/A 

Note: Cost information provided by Sivalls, Incorporated, updated to 2006 

equipment/labor costs. 

(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 

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. 

5 
 

 

 

 

 

 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

Exhibit 5B: Typical Horizontal Three-Phase Separator Sizes and Costs 

Settling 

Volume 

(gallons) 

Diameter 

(feet) 

Height 

(feet) 

Capital 

Costs ($) 

Installation 

Costs ($) 

O&M 

Costs 

($) 

49 2 3 $4,050 $1,684-3,031 N/A 

65 2 5 $4,320 $1,684-3,031 N/A 

107 2.5 5 $4,590 $1,684-3,031 N/A 

158 3 5 $6,481 $1,684-3,031 N/A 

225 3 7.5 $6,751 $1,684-3,031 N/A 

Note: Cost information provided by Sivalls, Incorporated, updated to 2006 

equipment/labor costs. 

Nelson Price Indexes 

In order to account for inflation in equipment and 

operating & maintenance costs, Nelson-Farrar 

Quarterly Cost Indexes (available in the first issue of 

each quarter in the Oil and Gas Journal) are used to 

update costs in the Lessons Learned documents. 

The “Refinery Operation Index” is used to revise

operating costs while the “Machinery: Oilfield Itemized 

Refining Cost Index” is used to update equipment 

costs. 

To use these indexes in the future, simply look up the 

most current Nelson-Farrar index number, divide by 

the February 2006 Nelson-Farrar index number, and, 

finally multiply by the appropriate costs in the Lessons 

Learned. 

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 (ft3

/gallon TEG) (Rule-of-thumb 

is 2) 

H = Hours per year (8,760) 

P = Sales price of gas (Assume $7/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) 

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. 

      

1,000 

L Original L Optimal PHEA V    

Original 

Circulation Rate 

Optimal 

Circulation Rate 

Annual 

Methane Savings 

(Mcf) 

Annual 

Savings 

(@ $7/Mcf) 

45 30 394 $2,758 

90 30 1,577 $11,039 

225 150 1,971 $13,797 

450 150 7,884 $55,188 

675 450 5,913 $41,391 

1350 450 23,652 $165,564 

1125 750 9,855 $68,985 

2250 750 39,420 $275,940 

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 $1,272 and $127,064 per year, depending on

the amount of over circulation and the heating value

of natural gas. 

Reduced frequency of glycol replacement. Industry 

experts estimate that 0.5 percent of TEG volume is 

lost per hour. Annual savings could range from $551

(if circulation rates are reduced from 45 to 30 gallons 

per hour) to $55,146 (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 contactor. As a result, they 

increase the amount of methane entrained to three cubic 

6 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

feet per gallon of TEG circulated. 

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. 

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 $7 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 $7/Mcf = $24, 835 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) 

Energy-exchange Pump Electric Pump 

Mcf/yr $/yr Mcf/yr $/yr 

30 710 $4,970 237 $1,659 

150 3,548 $24,836 1,183 $8,281 

300 7,096 $49,672 2,365 $16,555 

450 10,643 $74,501 3,548 $24,836 

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 sitespecific evaluation, beyond the scope of this study. 

Step 5: Conduct economic analysis. 

As demonstrated in Step 4, the optimization 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

circulation 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 straightforward 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, 

Exhibit 9A: Economics of Installing a Flash Tank 

Separator on a Dehydrator with Energyexchange Pump 

TEG 

Circulation 

Rate 

(gal/hr) 

Capital and 

Installation 

Costs ($)1 

Gas 

Savings2 

$/yr 

Total 

Savings3 

$/yr 

Payback 

Period 

(months) 

IRR4 

30 $6,967 $4,970 $5,005 17 66% 

150 $7,507 $24,836 $25,013 4 333% 

300 $9,667 $49,672 $50,012 3 517% 

450 $18,7945 $74,501 $75,019 4 399% 

1 Horizontal flash tank, 80 percent contingency on installation, 30 minute 

settling time plus weekly volume of accumulated NGL, where recovered. 2 Gas valued at $7.00/Mcf 

3 Higher total savings include natural gas liquids recovery (if present) at 1 

percent of recovered gas, valued at $25/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. 

7 
 

 

 

 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

Exhibit 9B: Economics of Installing a Flash Tank 

Separator on a Dehydrator with Electric Pump 

TEG 

Circulation 

Rate 

(gal/hr) 

Capital and 

Installation 

Costs ($)1 

Gas 

Savings2 

$/yr 

Total 

Savings3 

$/yr 

Payback 

Period 

(months) 

IRR4 

30 $6,9675 $1,669 $1,670 51 6% 

150 $6,967 $8,281 $8,338 11 117% 

300 $6,967 $16,555 $16,572 6 237% 

450 $9,667 $24,836 $24,869 5 257% 

1 Horizontal flash tank, 80 percent contingency on installation, 30 minute 

settling time plus weekly volume of accumulated NGL, when recovered. 2 Gas valued at $7.00/Mcf 

3 Higher total savings include natural gas liquids recovery (if present) at 1 

percent of recovered gas, valued at $25/barrel. This NGL recovery rate is for 

these examples only, each site must individually evaluate this potential. 

4 IRR based on5 years. 

5 Cost for minimum standard tank size. 

installation of a flash tank separator at a dehydration 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. 

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 

Natural Gas STAR partners and other industry 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. 

holding capacity. The increased revenues 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 compressor, 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. Sitespecific fuel use would be required to evaluate the savings

from employing both the flash tank and optimizing 

circulation. 

When assessing options for installing flash tanks on glycol 

dehydrators, natural gas price may influence the decision 

making process. Exhibit 10 shows an economic analysis of

installing a flash tank separator on a glycol dehydrator 

with a 150 gal/hr glycol circulation rate and an energyexchange pump at different natural gas prices. 

Exhibit 10: Gas Price Impact on 

Economic Analysis 

$3/ 

Mcf $5/Mcf $7/Mcf $8/Mcf $10/Mcf 

Value of Gas 

Saved $10,644 $17,740 $24,836 $28,384 $35,480 

Payback Period 

(months) 9 6 4 4 3 

Internal Rate of 

Return (IRR) 142% 238% 333% 380% 475% 

NPV $33,515 $60,414 $87,314 $100,76 

3 $127,663 

Lessons Learned 

TEG circulation rates at glycol dehydrators are often two 

to three times higher than the level needed to remove 

8 
 

Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

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. 

References 

American Petroleum Institute. Specification for GlycolType 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.P. 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 International 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. 

9 
Optimize Glycol Circulation And Install Flash Tank Separators In 

Glycol Dehydrators 

(Cont’d) 

United States 

Environmental Protection Agency 

Air and Radiation (6202J) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

October 2006 

EPA provides the suggested methane emissions estimating methods contained in this document as a tool to develop basic methane emissions estimates only. As 

regulatory reporting demands a higher-level of accuracy, the methane emission estimating methods and terminology contained in this document may not conform to 

the Greenhouse Gas Reporting Rule, 40 CFR Part 98, Subpart W methods or those in other EPA regulations. 

10