# Lessons Learned

from Natural Gas STAR Partners



# Replacing Glycol Dehydrators With Desiccant Dehydrators

Executive Summary

There are approximately 30,000 high-pressure, onshore 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 maintena nce costs. In a desiccant 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 demonstrat e that replacing a glycol dehydrator processing 1 million cubic feet per day (MMcfd) of gas with a desiccant dehydrator can save up to $9,232 per year in fuel gas, vented gas, and operation and maintenance (O&M) costs and reduce methane emissions by 444 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.



## Technology Background

Produced natural gas is normally saturated with water. If not removed, the water can condense and/or freeze in gathering, transmission, and distribution piping causing plugging, pressure surges, an d 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 glycol (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 fo r drying gas: desiccant dehydrators. These dehydrators use moisture- absorbing salts to remove water from the gas without emitting large quantities of methane, VOCs, or HAPs.

Method for Reducing Natural

Gas Losses a Methane

Emissions Savings (Mcf/year) b Volume of

Natural Gas

Savings (Mcf/year) c Value of Natural Gas Savings ($/year) Implementation Cost ($) d Other

Costs

($/ year) e Payback (Months)

$3 per

Mcf $5 per

Mcf $7 per

Mcf $3 per

Mcf $5 per

Mcf $7 per

Mcf

Replacing a Glycol Dehydrator with a

Desiccant Dehydrator 444 1,063 $3,189 $5,315 $7,441 $15,787 $(1,791) 38 27

General Assumptions:

a Based on a 1 MMcfd dehydrator operating at 450 psig and 47°F.

b Difference between methane vented from t he glycol and desiccant dehydrators.

c Sum of net gas emissions reductions and fuel gas savings.

d Installed cost of desiccant dehydrator minus surp lus equipment value for the replaced glycol dehydrator.

e Difference between glycol and desiccant dehydrators O&M costs. 21 

## Economic and Environmental Benefits

1

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

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 59oF and pressures above 250 psig. Lithium chloride, which is more expensive, has a wider operating range: up to 70oF and above 100 psig. Appendix A provides equilibrium moisture contents of natural gas dehydrated by commercially available calcium and lithium

chloride salts.

Process Description

A desiccant dehydrator is a ve ry 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 dehydrator 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 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 moisture 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 dehydrat or, most installations use a 

## Exhibit 1: Schematic of Single Vessel Desiccant Dehydrator

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 minimum temperature expected in the pipeline. If the gas is not dried appropriately, 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 temperature 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 common 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 given moisture content. In this example, an inlet gas at 47oF 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 calcium and lithium chloride are shown, although lithium chloride is rarely used
because of its cost.

2

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Exhibit 2: Desiccant Performance Curves at Maximum Pipeline Moisture Content Requirement (7 lb. of water/MMscf)

Refilling Desiccants and Draining Brine

As the desiccant tablets absorb moisture from the gas, the depth of the desiccant tablets in the drying bed slowlydecreases. Some manufacturers place a “window” (sightglass) on the vessel (see Exhibit 1) at the minimum

desiccant level. When the to p 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 another dehydration vessel, shutting valves to isolate the “empty” vessel, venting gas pressure to the atmosphere, opening the top filler hatch, and pour ing desiccant pellets into the vessel. This requires the operator to dump one or more 30 to 50 pound bags of salt in to 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 dehydrators typically accumulate from 10 to 50 gallons of brine a week). Draining to an evaporation pond

is best done afte r the vessel is depressurized, while draining to a produced water tank can be done before the vessel is depressurized—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 pneumatic “duplex-type” pump. 

## 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 desiccant 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 occu r during desiccant vessel depressurizing for salt refilling, typically one vesselvolume 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 dehydrators with desiccant dehydrators using the following five steps.

Step 1: Identify appropriate locations.

Desiccant dehydrators are an economic choice under certain operating conditions. Their applicability is determined primarily by gas throughput and produced gas temperature and pressure. 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.

Five Steps for Evaluating a Desiccant Dehydrator:



##  Step 1: Identify a ppropriate locations.



##  Step 2: Determine dehydrator capacity.



##  Step 3: Estimate the capital and operating costs.



##  Step 4: Estimate savings.



##  Step 5: Conduct economic analysis.



## 3

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

If the inlet temperature of the gas is too high, desiccants can form hydrates that precipitate 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 typically 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 produced 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.



## Exhibit 3: Optimum Operating Conditions for Dehydration Technologies

Low Pressure (<100 psig) High Pressure (>100 psig)

Low Temperature (<70 ° F) Desiccant/Glycol a Desiccant

High Temperature (>70 ° F) Glycol Glycol/Desiccant b

a 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.

b 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 natu ral gas water vapor content graph (example shown in Appendix B), a moisture content table, or a sizing program su ch as the Hanover Company’s Quick Size program, found at <www.hanover-co.com/home/ products/index.html>, 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 47oF 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.

In order to meet a pipeline moisture specification of 7 pounds per MMcf, calcium

chloride desiccant must remove 14 pounds of water 

## Vendor’s Rule-of-Thumb

One pound of desiccant removes three pounds of moisture from the gas. 

## Nelson Price Indexes

In order to account for in flation in equipment and operating & maintenance costs, Nelson-Farrar Quarterly Cost Indexes (avail able 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-Fa rrar index number, and, finally multiply by the appropriate costs in the Lessons

Learned.

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 will be dissolved per day. Exhibit 4 summarizes this calculation.



## Exhibit 4: Determine the Daily Consumption of Desiccant

Where:

D

F

I

O

B = Daily consumption of desiccant (lb/day) = Gas flow rate (MMcf/day) = Inlet water content (lb/MMcf) = Outlet water content (lb/MMcf) = Desiccant-to-water ratio (lb desiccant/lb water)

Given:

F

I

O

B = 1 MMcf/day of production gas at 47 ° F and 450 psig

= 21 lb/MMcf = 7 lb/MMcf (pipeline moisture requirement) = 1 lb desiccant/3 lb water (vendor rule-of-thumb)

Calculate:

D = F * (I-O) * B = 1 * (21-7) * 1/3 = 4.7 lb desiccant/day



## The next step is to size the vessel. Vendors supply desiccant dehydrator vessels in standard sizes, usually specified by outside diameter and maximum gas throughput at various operat ing 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.

4

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Exhibit 5: Determine the Size of the Desiccant Dehydrator

Where:

ID

D

H

T

B = Inside diameter of the desiccant vessel (in) = Daily desiccant consumption (lb/day) = Working salt bed height (in) = Time between refilling (days) = Bulk density (lb/ft 3 )

Given:

D

H

T

B = 4.7 lb/day (Exhibit 4) = 5 in (vendor rule-of-thumb) = 7 days (operator’s choice)

= 55 lb/ft 3 (vendor’s data)

Calculate:

ID = 12 *

= 12 *

= 16.2 in

Select standard vessel size from Exhibit 6:



##  Select next larger size than ID = 20 in BPH

TD

  12 4

555 1274.74

 

P



## Partners can select the desi ccant vessel size from the vendor’s table or calculate the size using the equations in Exhibit 5. For the 1 MMcfd dehydrator 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 thickness). To use Exhibit 6, follow the 450-psig column down to the throughput 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.

Step 3: Estimate the capital and operating costs.

Capital costs for single ve ssel desiccant dehydrators suitable for gas production rates from 0.1 to 5 MMcf per day (including the initial fill of desiccant) range between $3,500 and $21,000. 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 dehy drator is $8,048. For a twovessel dehydrator, the cost would be $16,096.

Installation costs typically ra nge 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 woul d cost $6,036 to install. The two-vessel dehydrator would cost $12,072 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



## Exhibit 6: Cost and Maximum Throughput Capacity (Mcfd) of Desiccant Dehydrators

Outside

Diameter (inches) Cost ($) a,b 100 psig 200 psig 300 psig 350 psig 400 psig 450 psig 500 psig

10 3,529 95 177 260 301 342 383 424

12 4,674 132 247 362 419 476 533 590

16 7,262 214 400 587 680 773 866 959

20 8,048 311 620 909 1,054 1,199 1,344 1,489

24 11,014 481 900 1,319 1,528 1,738 1,948 2,158

30 15,911 760 1,422 2,085 2,416 2,747 3,078 3,409

36 21,092 1,196 2,230 3,270 3,789 4,308 4,827 5,346

a The capital cost is for pressure ratings up to 500 psig, including one vessel with vessel supports, valves, piping, all appurt enances and the initial fill of calcium chloride desiccant tablets.

b Dehydrator cost includes all appurtenances: vessel, support structure, valves, and piping. Source: Van Air, updated to 2006 equipment costs.



## 5

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

cost of calcium chloride can range from $0.80 to $1.49 per pound. Using $1.49 per pound for the cost of calcium chloride, the total cost for refilling 4.7 pounds per day (from Exhibit 4) is $2,556 per year. In the example given in Exhibit 4, very little brine is produced 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

CD

CB

I

O

F

P

D

S

BD

LC

LT

LR = Total operating cost ($/year) = Cost of desiccant ($/year) = Cost of brine disposal ($/year) = Inlet water content (lb/MMcf) = Outlet water content (lb/MMcf) = Gas flow rate (MMcf/day) = Price of the desiccant ($/lb) = Daily desiccant consumption (lb/day) = Density of CaCl 2 brine (lb/bbl) = Cost of brine disposal ($/bbl) = Labor cost ($) = Labor time for operator to refill with desiccant (hr) = Labor rate for operator ($/hr)

Given:

F

P

D

S

BD

LT

LR = 1 MMcf/day of production gas at 47 ° F and 450 psig
= $1.49/lb of calcium ch loride (vendor data) = 4.7 lb desiccant/day (Exhibit 4)

= 490 lb/bbl
= $1.40/bbl a

= 1 hr/week
= $40/hr

Calculate:

CD

CB

LC

TO = D*P*365 day/yr

= 4.7*1.49*365 = $2,556/yr

=

=

= $20/yr

= LT*LR*52 weeks/yr

= 1*40*52 = $2,080/yr

= CD+CB+LC = $2,556+$20+$2,080 = $4,656/yr

a GRI Atlas of Gas-Related Produced Water for 1990 , May 1995, updated to 2006 costs. S days yr BDDFOI /365]))[(( 



### 490

4.7] 1.4 365 [((21 7) 1)  

## Step 4: Determine economic replacement threshold.

Replacing a glycol dehydrator with a desiccant dehydrator significantly saves gas an d 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 determine the gas savings by determining the following five factors.

 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 demonstrates this calculation for the 1 MMcfd dehydrator example. In this example, an energy exchange pump without a flash tank separator is a ssumed. Using rules-of

thumb from EPA’s Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol Dehydrators , methane gas emissions of 69 Mcf per year is calculated.

 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 se parators, 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 controll ers—level controllers on the contactor and reboiler and temperature

controllers on the reboiler and gas
Determine the Net Gas Savings: 

## heater. It does not
have a flash tank Add Savings from eliminating: 

## separator. Pneumatic

 Gas vented from glycol dehydrator. 

## devices are assumed to

 Gas vented from pneumatic controllers. 

## be a typical mix of

 Gas burned as fuel in glycol reboiler. 

## high bleed and low

 Gas burned as fuel in a gas heater. 

## bleed (i.e., they bleed

Subtract: 

## in excess of 50 Mcf of

 Gas vented from desiccant dehydrator. 

## gas per year during operation). Based on

6

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

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 (lb/MMcf) = Glycol-to-water ratio (gal/lb) a

= Percent over-circulation = Methane entrainment rate (ft 3 /gal) a

Given:

F

W

R

G

OC = 1 MMcfd of gas at 47 ° F and 450 psig = 21 - 7 = 14 lb water/MMcf (Exhibit 4) = 3 gal/lb (rule-of-thumb) a

= 3 ft 3 /gal for energy exchange pumps (rule-of-thumb) a

= 150%

Calculate:

GV =

=

= 69 Mcf/yr

a From EPA’s Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol Dehydrators

. 

### Mcfcf days yr R OC G WF

/1,000 )/365 ( 



### 1,000 365)33 1.5 (1 14





## the GRI/EPA study, Methane Emissions From the Natural Gas Industry, Volume 12–Pneumatic Devices

, the annual emission factor for an average 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) a = Number of pneumatic devices

Given:

EF

PD = 126 Mcf/device/yr = 4 pneumatic devices/glycol dehydrators

Calculate:

GB = EF * PD

= 126 * 4 = 504 Mcf/yr

a 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.



## Exhibit 10: Gas Burned for Fuel in Glycol Reboiler

Where:

FGR

F

W

Qr

Hv

R = Fuel gas for reboiler (Mcf/yr) = Gas flow rate (MMcfd) = Inlet-outlet water content (lb/MMcf) = Heat duty of re boiler (Btu/gal TEG) a = Heating value of natural gas (Btu/scf) b = Glycol-to-water ratio (gal TEG/lb water) c

Given:

F

W

Qr

Hv

R = 1 MMcfd
= 21 - 7 = 14 lb water/MMcf = 1,124 Btu/gal TEG = 1,027 Btu/scf = 3 gal TEG/lb water removed

Calculate:

FGR =

=

= 17 Mcf/yr

a Based on calculation in Engineering Data Book, Volume II , 11th edition, Gas Processors Supply Association, 1998, Sect ion 20-Dehydration.

b Energy Information Administration (EIA ), Monthly Engineering Review, Table A4.

c From EPA’s Lessons Learned: Optimize Glycol Circulation and Install Flash Tank Separators in Glycol Dehydrators

. 

### Mcfcf Hv yrdays RQrWF

/1,000 )/365 (

    



### 1,000

1,027 365 )31,12414(1

    



##  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 produced gas from 47oF to (assumed) 90oF 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 refillin g process. To determine the volume of gas vented, Partners will need to determine the volume of the dehydrator vessel and what percentage of this volume is occupied by gas.

The 20-inch OD vessel in Exhibit 6 would have an

7

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Exhibit 11: Amount of Fuel Gas

Used to Heat the Gas

Where:

FGH

Hv

Cv

D

DT

F

E = Fuel gas used in heater (Mcf/yr) = Heating value of natural gas (Btu/cf) = Specific heat of natural gas (Btu/lb ° F) = Density of natural gas (lb/cf) = (T 2 - T 1 ) change in temperature ( ° F) = Flow rate (MMcf/d) = Efficiency

Given:

Hv

Cv

D

DT

F

E = 1,027 Btu/cf
= 0.441 Btu/lb ° F
= 0.0502 lb/cf

= 43 ° F (90 - 47) ° F
= 1 MMcf/d

= 70%

Calculate:

FGH =

=

= 483 Mcf/yr 

### )( )/1,000/365 (

EHv MMcfMcf days yr DTCvDF

 



0.7)(1,027 365 1,000) 430.441 0.0502(1



 

## Exhibit 12: Gas Lost from the Desiccant Dehydrator

Where:

GLD

H

D

P 1 P 2 P

%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 vess el volume that is gas = Time between refilling (days)

Given:

H

D

P 1 P 2 P

%G

T = 76.75 in (6.40 ft) a = 19.25 in (1.6 ft) = 14.7 psia = 450 psig + 14.7 (464.7 psig)

= 3.14 = 45% (vendor’s rule-of-thumb) a = 7 days

Calculate:

GLD =

=

= 10 Mcf/yr

a Based on product data provided by Van Air. 

### )/1,000 (4 )/365% (

1 2 2



### McfcfTP yrdays GPPDH

 



### 1,000 )714.7(4 365 )0.45 464 .7 3.141.6(6.4 2



###  



## 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 Boyle’s Law, the amount of gas vented to the atmosphere during depre ssurizing of the vessel is 10 Mcf per year. Exhibit 12 summarizes this

calculation.

 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 desiccant. In this example, total gas savings are 1,063 Mcf per year. Using a gas price of $7.00 perMcf, the gas value saved is $7,441 per year. Nonassociated natural gas contains 78.8 percent methane. Therefore, the total methane emission savings is 78.8 percent of the difference between the gas emitted 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. 

## Exhibit 13: Total Gas Savings

Calculate:

TGS

Savings = Total Gas Savings (Mcf/yr)

= Exhibit 8 + Exhibit 9 + Exhibit 10 + Exhibit 11 - Exhibit 12

= 69 + 504 + 17 + 483 - 10 = 1,063 Mcf/yr

= 1,063 Mcf/yr * $7/Mcf = $7,441/yr

Methane Emissions Reduction

TMER

TMER = Total methane emissions reduction = 78.8% * (Exhibit 8 + Exhibit 9 - Exhibit 12) = 0.788 * (69 + 504 - 10) = 444 Mcf/yr



## Determining Operations and Maintenance Savings

Other savings include the difference between the operating and maintenance cost (labor cost) of a desiccant dehydrator and a glycol dehydrator.

The operation cost of a desiccant dehydrator includes the

8

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

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

refill cost of the desiccant, disposal of the brine, and labor costs. Since a desiccant dehy drator 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 $2,556 and $20 per year, respectively. Labor costs assume one hour per week for the operator to refill the desiccant dehydrator. At $40 per hour, this would cost about $2,080 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 pneumatic controls, and annually cleaning the fire-tubes of the reboiler and gas heater. Glycol costs $5.57 per gallon, and a typical make-up rate is 0.1 gallons per MMcf of gas processed. For this example, this works out to about 37 gallons of glycol per year, or $206 per year. Labor costs assume operators spend an average of two hours per week maintaining and repairing the unit. At $40 per hour this amounts to about $4,160 per year. Spare parts are estimated at half the labor cost, or $2,080 per year. Based on this, total operation, maintenance, and labor costs for our example glycol dehydrator system is $6,446 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 47oF temperature). Exhibit 15 compares 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 47oF gas with a desiccant dehydrator. 

## 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 Desiccant ($/yr) Glycol ($/yr)

Implementation Costs

Capital Costs

Desiccant a (includes the initial fill) 16,097

Glycol 24,764

Other costs (installation and engineering) b 12,073 18,573

Total Implementation Costs: 28,169 43,337

Annual Operating and Maintenance Costs

Desiccant

Cost of desiccant refill c ($1.20/lb) 2,556

Cost of brine disposal c 20

Glycol

Labor cost d 2,080

Cost of glycol refill d ($5.57/gal) 206

Material and labor cost d 6,240

Total Annual Operation and Maintenance Costs: 4,656 6,446

a Based on two desiccant vessels used alternatively. See Exhibit 5.

b Installation costs assumed at 75% of the equipment cost.

c Values are from Exhibit 7.

d 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 Desiccant Glycol

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

Gas Use

Fuel (Exhibits 10 and 11)

— — 500 3,500

Gas Loss

Pneumatic devices (Exhibit 9)

— — 504 3,528

Vents (Exhibits 8 and 12) 10 70 69 483

Total:

Methane Emissions b : 10

8 70

— 1,073

452 7,511

—

a Gas price based on $7/Mcf.

b Values are from Exhibit 12 and Exhibit 13.



## 9

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Exhibit 16: Economics of Replacin g a Glycol Dehydration System with a Two-Vessel Desiccant Dehydrator System

Types of Costs and Savings a Year 0 ($/yr) Year 1 ($/yr) Year 2 ($/yr) Year 3 ($/yr) Year 4 ($/yr) Year 5 ($/yr)

Capital costs (28,169)

Avoided O&M costs 6,446 6,446 6,446 6,446 6,446

O&M costs - Desiccant ($/yr) (4,656) (4,656) (4,656) (4,656) (4,656)

Value of gas saved 7,441 7,441 7,441 7,441 7,441

Surplus equipment value 12,382 b

Total ($) (15,787) 9,232 9,232 9,232 9,232 9,232

NPV (Net Present Value) c = $19,208 IRR (Internal Rate of Return) d = 51% Payback Period (months) = 21

a All cost values are obtained from Exhibits 14 and 15. The gas price is assumed to be $7/Mcf.

b Based on a 50% of the capital cost of glycol dehydrator.

c The NPV is calculated based on 10% discount over 5 years.

d The IRR is calculated based on 5 years.



## When considering replacing glycol dehydrators with desiccant dehydrators, natura l gas price may influence the decision making process. Exhibit 17 shows an economic analysis of installing a 1 MMcfd desiccant dehydrator that operates at 450 psig and 47o F at different natural gas prices.



## Exhibit 17: Gas Price Impact on Economic Analysis

$3/Mcf $5/Mcf $7/Mcf $8/Mcf $10/Mcf

Value of Gas

Saved $3,189 $5,315 $7,441 $8,504 $10,630

Payback Period (months) 39 27 21 19 16

Internal Rate of

Return (IRR) 17% 35% 51% 59% 74%

Net Present Value ( i =10%) $3,089 $11,148 $19,208 $23,237 $31,296



## Lessons Learned

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 maintenance costs for low flow rate gas at higher pressures and lower temperature 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 reductions attributable to replacing glycol dehydrators with desiccant dehydrators in Natural Gas STAR Program annual
reports.



## References

Acor, Lori G. and David Mirdadian. Benefits of Using Deliquescing Desiccants 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.

10

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Dow Chemical Company, product literature. Gas Dehydration with

PELADOW DG 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.

11

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Appendix A

Moisture Content of Natural Gas in Equilibrium with Desiccants (lb water/MMcf of natural gas)



# Type Calcium Chloride Deliquescent Desiccant Tablets

10

PSIG 25

PSIG 50

PSIG 75

PSIG 100

PSIG 125

PSIG 150

PSIG 175

PSIG 200

PSIG 225

PSIG 250

PSIG 275

PSIG 300

PSIG 350

PSIG 400

PSIG 500

PSIG 750

PSIG 1000

PSIG

80oF 344 219 13 98 77 64 55 48 43 39 35 33 30 27 23.6 19.7 14.3 11.6

75oF 292 186 113 83 65 54 46 41 36 33 30 28 26 22.5 20.1 16.8 12.2 9.9

70oF 246 157 96 70 55 46 39 43 31 27 25 23.4 21.7 19.1 17.1 14.3 10.4 8.5

65oF 207 132 81 59 47 39 33 29 26 23.5 21.4 19.8 18.4 16.2 14.5 12.1 8.9 7.3

60oF 174 111 68 50 39 33 29 24.5 21.9 19.8 18.1 16.8 15.5 13.7 12.3 10.3 7.6 6.2

58oF 162 103 63 46 36 31 26 22.8 20.3 18.4 16.8 15.6 14.4 12.9 11.4 9.6 7 5.8

56oF 150 96 59 43 34 29 24.1 21.2 18.9 17.1 15.7 14.5 13.4 11.8 10.6 8.9 6.6 5.4

54oF 140 89 55 40 32 26 22.5 19.8 17.6 16 14.6 13.5 12.6 11.1 9.9 8.3 6.2 5.1

52oF 130 83 51 37 29 24.5 21 18.4 16.4 14.9 14.4 12.6 11.7 10.3 9.3 7.8 5.8 4.7

50oF 121 77 47 35 27 22.8 19.5 17.1 15.3 13.9 12.7 11.7 10.9 9.6 8.6 7.2 5.4 4.4

45oF 100 64 39 29 22.7 18.9 16.2 14.3 12.7 11.5 10.6 9.8 9.1 8 7.2 6.1 4.5 3.7

40oF 83 53 32 24 18.8 15.6 13.4 11.8 10.5 9.6 8.8 8.1 7.5 6.7 6 5 3.8 3.1

35oF 68 44 27 19.6 15.5 13 11.1 9.8 8.7 7.9 7.2 6.7 6.2 5.5 5 4.2 3.1 2.6



# Type Lithium Chloride Deliquescent Desiccant Tablets

10

PSIG 25

PSIG 50

PSIG 75

PSIG 100

PSIG 125

PSIG 150

PSIG 175

PSIG 200

PSIG 225

PSIG 250

PSIG 275

PSIG 300

PSIG 350

PSIG 400

PSIG 500

PSIG 750

PSIG 1000

PSIG

80oF 128 81 50 36 29 23.7 20.2 17.8 15.8 14.3 13 12 11.1 9.8 8.7 7.3 5.3 4.3

75oF 108 69 42 31 24.2 20 17.2 15.1 13.4 12.1 11.1 10.2 9.5 8.3 7.4 6.2 4.5 3.7

70oF 91 59 36 26 20.4 17 14.5 12.7 11.3 10.3 9.4 8.7 8 7.1 6.3 5.3 3.8 3.1

65oF 77 49 30 21.9 17.2 14.3 12.2 10.8 9.6 8.7 7.9 7.3 6.8 6 5.4 4.5 3.3 2.7

60oF 65 41 25 18.4 14.5 12.1 10.3 9.1 8.1 7.4 6.7 6.2 5.7 5 4.5 3.8 2.8 2.3

58oF 60 38 23.4 17.1 13.5 11.2 9.6 8.4 7.5 6.8 6.2 5.7 5.3 4.7 4.2 3.5 2.6 2.1

56oF 56 37 21.7 15.9 12.5 10.5 8.9 7.8 7 6.3 5.8 5.4 5 4.4 3.9 3.3 2.4 2

54oF 52 33 20.3 14.8 11.7 9.7 8.3 7.3 6.5 5.9 5.4 5 4.6 4.1 3.7 3.1 2.3 1.8

52oF 48 31 18.9 13.8 10.9 9 7.7 6.8 6.1 5.5 5 4.7 4.3 3.8 3.4 2.9 2.1 1.7

50oF 45 29 17.5 12.8 10.1 8.4 7.2 6.4 5.8 5.1 4.7 4.4 4 3.5 3.2 2.7 2 1.6

45oF 37 23.8 14.5 10.7 8.4 7 6. 5.3 4.7 4.3 3.9 3.6 3.3 2.9 2.6 2.2 1.6 1.3

40oF 30 19.6 12 8.7 6.8 5.8 4.9 4.4 3.8 3.6 3.2 3 2.8 2.4 2.2 1.8 1.4 1.1

35oF 25 16.1 9.9 7.2 5.7 4.8 4.1 3.6 3.2 2.9 2.7 2.5 2.3 2 1.8 1.5 1.1 0.9



## Source: Van Air

12

# Replacing Glycol Dehydrators With Desiccant Dehydrators
(Cont’d)

Appendix B

Water Vapor Content of Natural Gas at Saturation

Source: Smith Industries, Inc., Houston, Texas

13

# Replacing Glycol Dehydrators With Desiccant 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 em issions estimating methods contained in this do cument as a tool to develop basic methane e missions estimates only. As regulatory reporting demands a higher-level of accuracy, the methane emission estimating methods and terminology contained in t his document may not conform to the Greenhouse Gas Reporting Rule, 40 CFR Part 98, Subpart W methods or those in other EPA regulations.



## 14