Fuel Gas Efficiency BMP - Efficient Use of Fuel Gas in Glycol Dehydrators (Module 9)

May 2008

Disclaimer

This publication was prepared for the Canadian Association of Petroleum Producers, the Gas Processing Association Canada, the Alberta Department of Energy, the Alberta Energy Resources and Conservation Board, Small Explorers and Producers Association of Canada and Natural Resources Canada by CETAC-West. While it is believed that the information contained herein is reliable under the conditions and subject to the limitations set out, CETAC-West and the funding organizations do not guarantee its accuracy. The use of this report or any information contained will be at the user’s sole risk, regardless of any fault or negligence of CETAC-West or the sponsors. Acknowledgements This Fuel Gas Efficiency Best Management Practice Series was developed by CETAC WEST with contributions from:

• Accurata Inc. • Clearstone Engineering Ltd. • RCL Environmental • REM Technology Inc. • Sensor Environmental Services Ltd. • Sirius Products Inc. • Sulphur Experts Inc. • Amine Experts Inc. • Tartan Engineering

CETAC-WEST is a private sector, not-for-profit corporation with a mandate to encourage advancements in environmental and economic performance in Western Canada. The corporation has formed linkages between technology producers, industry experts, and industry associates to facilitate this process. Since 2000, CETAC-WEST has sponsored a highly successful eco-efficiency program aimed at reducing energy consumption in the Upstream Oil and Gas Industry. Head Office # 420, 715 - 5th Ave SW Calgary, Alberta Canada T2P2X6 Tel: (403) 777-9595 Fax: (403) 777-9599 [email protected]

Table of Contents

1. Applicability and Objectives .......................................... 1 2. Basic Improvement Strategies....................................... 2 2.1 Technology and Equipment 2.2 Efficiency Assessment 2.3 Improving Efficiency 3. Inspection, Monitoring and Record Keeping................ 3 4. Efficiency Assessment and Adjustments..................... 4 4.1 Types of Dehydrators and Uses 4.2 Fuel Gas Consumption 4.3 Operational Optimization 5. Technical Checks and Evaluation ............................... 10 5.1 Performance Evaluation 5.2 Performance Specification and Assessment Process 6. Appendices

Appendix A Glycol Dehydration Turndown Considerations Appendix B Design Considerations for Optimization Appendix C Example Calculations for Optimization Appendix D Water Content of Hydrocarbon Gas Appendix E Water Removal vs. TEG Circulation Rate Appendix F References

Tables Table 1 Operating Targets for Glycol Dehydrator

Components Table 2 Glycol Dehydrator Fuel Gas Use Table 3 Turndown Liquid Rate Reduction

Table 4 Case Study: Fuel Gas Savings from Using a Flash Tank Separator Table 5 Case Study: Potential Savings from Using an Electric Pump vs. Pneumatic Pump

Figures

Figure 1 Glycol Dehydration Schematic Figure 2 Typical Fuel Gas Usage for Glycol Dehydrators Figure 3 TEG Operating Graph

Background The issue of fuel gas consumption is increasingly important to the oil and gas industry. The development of this Best Management Practice (BMP) Module is sponsored by the Canadian Association of Petroleum Producers (CAPP), the Gas Processing Association Canada (GPAC), the Alberta Department of Energy, Small Explorers and Producers Association of Canada (SEPAC) Natural Resources Canada (NRC) and the Energy Resources and Conservation Board (ERCB) to promote the efficient use of fuel gas in glycol dehydrators used in the upstream oil and gas sector. It is part of a series of 17 modules addressing fuel gas efficiency in a range of devices.

This BMP Module:

• identifies the typical impediments to achieving high levels of operating efficiency with respect to fuel gas consumption;

• presents strategies for achieving cost effective improvements through inspection, maintenance, operating practices and the replacement of underperforming components; and

• identifies technical considerations and limitations. The aim is to provide practical guidance to operators for achieving fuel gas efficient operation while recognizing the specific requirements of individual glycol dehydrators and their service requirements.

EFFICIENT USE OF FUEL GAS IN THE UPSTREAM OIL AND GAS INDUSTRY

MODULE 9 of 17: Glycol Dehydrators

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Efficient Use of Fuel Gas in Glycol Dehydrators Rev Date 27/05/2008 Module 9 of 17 Page 1 of 25

1. Applicability and Objectives This module provides guidance for operating staff to identify reasons for lower than achievable operational efficiency of glycol dehydrators. The determination of operational efficiency is generally not readily apparent but is achieved by insuring that each fuel using component is operating optimally. The majority of well-site glycol dehydrators use gas operated equipment due to the absence of electrical power. In these cases, dry sales gas is used as a motive force for the glycol circulation pumps and as fuel for the glycol regenerator. Many compressor site dehydrators use electric glycol pumps and dry sales gas for burner fuel. In both cases they are usually prefabricated skid mounted systems. These systems are typically robust standardized designs that operate with minimal operator intervention. It is not uncommon to find field dehydrators that have been in service for decades with few equipment changes. Dehydrators are typically designed for an initial high gas production rate. As production rates typically decline over time resulting in dehydrators that are operated with circulation rates that are higher than needed or have oversized components. An efficient dehydrator is one that operates with the least amount of fuel while producing adequately dry gas. Dry gas is defined as natural gas with a dew point lower than -10°C at operating pressures. Due to the significant differences in service requirements and lack of installed measurement devices on glycol dehydrators, a single fuel use objective is not feasible. Thus, operating targets and best management practices for individual components are used. The operating targets for the two most commonly used types of glycol (Tri-ethylene glycol TEG, and Di-ethylene glycol DEG) are presented in Table 1.

Table 1 Operating Targets for Glycol Dehydrator Components

Component Target Values

TEG Circulation Rate 25L/kgH2O or 3 usg/lbH2O DEG Circulation Rate 25L/kgH2O or 3 usg/lbH2O TEG Reboiler Temperature 200oC DEG Reboiler Temperature 160oC


2. Basic Improvement Strategy The most significant elements of long-term operating efficiency are the application of best available technology, implementation of operating and maintenance systems and management commitment. Efficient operation of glycol dehydrators requires:

• understanding what the dehydrator is currently required to do,

• periodic checks and adjustments to maintain efficient operation,

• periodic assessment of current operations to identify the benefits of equipment changes, and;

• maintenance of records.

2.1 Technology and Equipment The first step in moving toward higher levels of fuel gas efficiency is to understand what the water removal requirements are and to ensure that the equipment installed is capable of meeting these requirements. Knowledge of the equipment will also help to identify what changes may be required to achieve higher levels of fuel gas efficiency. Section 4 of this module provides guidance for the assessment of various technologies used in glycol dehydrators.

2.2 Efficiency Assessment The fuel gas efficiency of a glycol dehydrator cannot be tested directly. An efficiency assessment that incorporates an evaluation of each component is the most effective way of calculating efficiency for a glycol dehydrator. Section 4 of this module describes methods to assess individual component performance.

2.3 Improving Efficiency Decisions to carry out adjustments and/or replace components should be made on a case by case basis having consideration for health, safety, environmental and economic considerations. Where adjustments to existing systems are practical, these should be carried out at the time of testing. Section 5 of this module provides a logic diagram and evaluation method for improving glycol dehydrator performance.


3. Inspection, Monitoring and Record-Keeping Operators should have a record program to support the company’s glycol dehydrator testing and improvement program. Proper record keeping should assist in ensuring that operations are maintained at peak efficiency rather than simply trouble-free operation. The information collected in record keeping will assist in establishing the required checking frequency and demonstrate improvements in fuel gas efficiency. In addition to the routine operating information, consideration should be given to recording and retaining the following efficiency related information:

• glycol pump specifications,

• ideal circulation rate for the current water removal demand,

• pump rate to meet the ideal circulation rate,

• actual pump rate,

• temperature and firing frequency of the glycol reboiler, and

• fuel gas usage. Gathering and compiling this data can be useful for:

• determining if fuel gas metering is required in accordance with ERCB Directive 017,

• compliance with Alberta Environment emissions reduction guidelines,

• identifying benzene emissions as required in the ERCB Directive 039 Dehydrator Engineering and Operations Sheets (DEOS), and

• operational efficiency assessments and performance improvements. Completing a performance evaluation with input from operators and engineers can have significant benefits. Performance evaluation should follow the suggestions presented in section 5.2.


4. Efficiency Assessment and Adjustments

4.1 Types of Dehydrators and Uses While most dehydrators are of a standard skid mounted package design, there are usually small differences in the detailed design of each unit. Every dehydrator contains: a contactor where the glycol and gas mix, glycol pumps to circulate glycol, regenerator to remove water from the glycol and an accumulator to store glycol for later use and provide some surge capacity.

There are three principal uses for glycol dehydrators:

• reduce pipeline corrosion,

• control hydrate formation, and

• meet pipeline water content specifications. The two most common glycols used in Canada are:

• triethylene glycol (TEG),

• diethylene glycol (DEG), Triethylene glycol (TEG) is the most common type of glycol used. It has the advantage over diethylene glycol (DEG) of a higher boiling point that reduces losses during regeneration. Its low vapour pressure also helps to reduce losses from vaporization into the gas stream. The higher molecular weight of TEG increases its solubility with hydrocarbons and hence it has a higher affinity for benzene. The use of TEG is preferable in processes where the absorber contact temperature is relatively high and the required dew point is relatively low. All types of glycols are regenerated and used as part of a continuous dehydration cycle. TEG units are used throughout the industry at gas wells, gathering stations, and processing facilities. Glycol flows in a continuous process from contactor to reboiler and back to the contactor (See Figure 1 below for details).


Dry Gas

Wet Gas

Flash GasWater Vapor (with Benzene)

Free Liquid

GlycolContactor Reboiler

Still

Surge Drum

Exhaust gasLeanGlycol

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Glycol

Figure 1

Glycol Dehydration Schematic

4.2 Fuel Gas Consumption Fuel gas is used:

• in the reboiler burner and as makeup fuel in flares or incinerators,

• as motive gas in instruments, controllers and glycol circulation pumps,

• as stripping gas when needed to increase the purity of lean glycol. Table 2 identifies the typical ranges of fuel gas use rates per litre of TEG circulated in glycol dehydrators.

Table 2 Glycol Dehydrator Fuel Gas Use

Areas of Fuel Gas Use Fuel Gas Use m3/l TEG

Stripping Gas 0-0.050* Glycol Pumping (800psi) 0.015- 0.033

Glycol Reboiler 0.003-0.008 * Not always required

Stripping Gas


As seen in the glycol dehydrator fuel gas use table (Table 2 above), the largest fuel gas usage can be from stripping gas, however in many cases stripping gas is not required. The largest fuel gas consumer in the glycol dehydration system is typically the glycol pump. Figure 2 identifies the fuel gas usage for a range of water concentrations on a typical glycol dehydrator operating with a Kimray pump and no stripping gas.

Fuel Gas used in Reboiler

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Figure 2 Typical Fuel Gas Usage for Glycol Dehydrators

Figure 2 assumes a temperature into the still of 150˚C, a reboiler temperature of 200˚C, a circulation rate of 25L TEG/kg H2O and a fuel gas heating value of 40 GJ/e3m3. The red horizontal line on the graph indicates the fuel gas usage of 500 m3/day, over which metering is required as per ERCB directive 017. See Appendix C for an example on fuel gas usage calculations. Stripping gas is dry fuel gas introduced in the stripping section of the reboiler specifically to regenerate high concentration glycol for improved dehydration. Stripping gas is only used if the glycol concentration must be higher than 98.5%. If gas stripping is NOT used the required temperature for 99% TEG is approximately 204˚C. This temperature is borderline with regard to TEG degradation; therefore, stripping gas provides an effective but fuel gas intensive method to keep glycol strength at the 99%+ required to meet spec in some plants. Reduction or elimination of stripping gas by use of design changes can translate directly into savings in fuel gas and should be a major focus for optimization.


In most cases, the greatest potential for fuel gas reduction is that used by circulating. The majority of glycol dehydrators use a Kimray pump in which fuel gas is used as supplemental motive force and requires significant amounts of fuel gas as identified in Figure 2. Depending on the availability of electricity, electric pumps can provides a significant reduction in fuel gas usage. Refer to Appendix B for a discussion and example of fuel gas savings as a result of replacing pneumatics with electric pumps. The reboiler maintains the glycol at the required temperature to boil off water (without exceeding the thermal degradation temperature of the glycol) and other components picked up in the TEG. Reboiler load is largely determined by the glycol circulation rate and the quantity of water to be removed.

4.3 Operational Optimization Circulation Rate Of all operating variables affecting fuel gas use, the circulation rate has the greatest impact. Over-circulation results in more fuel gas use without significant reduction in gas moisture content. Dehydrator systems often re-circulate TEG at rates two or more times higher than necessary. Laboratory studies and simulations have found that for most plants, a circulation of 25 litres of TEG per kilogram of water is the optimum ratio. This ratio should be calculated for an individual dehydrator using the performance optimization procedure (See section 5.2). Depending on the pressure of the gas, a rate of 25 litres of TEG per kilogram should result in a dewpoint of -6 to -40˚C. Circulation rate is the main parameter that affects both energy usage and benzene absorption into the TEG. The operator’s goal should be to keep the circulation rate as low as possible, while still maintaining the needed water content specification in the treated gas. Over circulation of glycol solutions are often seen in plants because it is thought to be a “safety blanket”. Increasing circulation rate beyond the optimal level however, provides only minimal increases in water removal. Over circulating glycol results in increased:

• fuel gas consumption,

• glycol losses,

• hydrocarbon and aromatic absorption leading to emission increases,

• risk of foaming,

• filter changes,

• corrosion and equipment failure.


Fluctuating Inlet Gas Volumes Often times gas volumes rise and fall with little or no notice to the operator. If the volume of gas to the absorber increases, so does the volume of water. To properly maintain the TEG/water ratio, the glycol circulation rate must be increased. Failure to do so can result in treated gas with a water content higher than regulated amount (64 mg/m3). In cases where raw gas volume is subject to fluctuations, it is recommended to determine the maximum expected gas volume and then set the system to run at the conditions designed to treat this. When the gas volumes are not at the maximum over-circulation will result. The operator then has the option to temporarily turning down the glycol rate, or to leave it in anticipation of the next increase in gas volume. Control over glycol circulation rate in this case requires frequent outlet gas water content monitoring. In order to have ideal fuel gas efficiency, a control system including an online gas flow meter connected to the glycol circulation pumps is desired. As gas flow either increases or decreases, the glycol flow rate will match. Reboiler Temperature Optimized reboiler temperatures are essential for achieving specification dryness in the product gas. Reboiler temperature deviations, either above or below the optimum temperature range, can make the dehydrator operation inefficient. Higher than optimum temperatures over 200˚C may result in adequate dehydration of the gas stream, but can lead to glycol losses, degradation of glycol and excess consumption of fuel gas. Lower than optimum temperatures may result in reduced water removal efficiency. Maintaining a reboiler temperature of 200˚C will provide a TEG purity of 98.6%. The TEG Operating Graph (See Figure 3) has been designed to provide a 200˚C reboiler temperature. If a higher TEG strength is needed, stripping can be considered or design changes made. Exchangers and Coolers: A common problem in the summer is the glycol systems inability to cool the lean glycol to acceptable absorption temperatures (usually below 49˚C). TEG quickly loses water holding capacity at high temperatures resulting in off spec gas. In this case the operator may be tempted to increase glycol circulation rate in an attempt to compensate for the high temperature. This “quantity over quality” approach rarely works. If the lean glycol cannot be cooled adequately, the coolers should be examined for fouling or replaced if undersized.


4.4 Operational Adjustments Logic Diagram

Make changes to circulation rateslowly to match the optimum ratio

and dry gas dewpoint. Adjust reboilerduty according to the glycol

circulation rate in the TEG operatinggraph pg 11.

Does the current TEG/waterratio match the optimum TEG/waterratio as identified in the performance

optimization?

No

Yes

The optimum TEG/water ratio and themaximum inlet water flow have beenspecified in performance optimization

section 5.1

Based on inlet gaswater content and flow, can theglycol circulation rate be safely

reduced?

Ensure pump isset to the

operating pressurerequired

Adjust circulation ratesand reboiler duty

according to the TEGoperating graph usinginlet water flow, andthe optimum TEG/

water ratio(See pg 11)

No

Yes

Glycol dehydration systemis optimized


5. Technical Checks and Evaluation

5.1 Performance Optimization The performance evaluation team, ideally the operating staff and technical resources, should perform a complete review of dehydrator performance including:

• analysis of the suitability of the glycol dehydrator in the overall process and the applicability of alternative technologies,

• analysis of current operational requirements and efficiency,

• calculation of operational requirements and resulting efficiency, and

• analysis of available modifications and design changes to improve efficiency.

Performance reviews should be conducted annually and incorporated into the development of the DEOS sheets and annual fuel gas reporting required by ERCB directive 039 and 017 respectively. Analysis of the glycol dehydrator can be conducted using field operating data and technical evaluation programs to assess the performance of the dehydrator. These programs include the GRI-GlyCalcTM and Rich-Lean methods, or other programs such as Prosim, Hysim1, or “in-house” commercial simulators. Results of these evaluations can be used in a sensitivity analysis whereby specific components of the system can be identified as the most appropriate place for implementing fuel gas reduction strategies. Circulation Rate Several variables must be taken into account before adjusting circulation rate. There may be other factors in the plant that require the circulation rate to be higher than predicted such as tray hydraulics etc. Reboiler duty is directly related to glycol circulation rate, meaning if the glycol circulation rate can be lowered, so can the reboiler duty. Reboiler duty is also related to the lean/rich exchanger efficiency; the warmer the rich glycol entering the regenerator tower, the less energy needed to warm it up further to its boiling point.


TEG Operating Graph

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Circulation Rate (16 L TEG/Kg H2O) Circulation Rate (25 L TEG/Kg H2O) Circulation Rate (33 L TEG/Kg H2O)Reboiler Duty (16) Reboiler Duty (25) Reboiler Duty (33)

16 L TEG/Kg H2O 25 L TEG/Kg H2O 33 L TEG/Kg H2O

1625

33

Figure 3 TEG Operating Graph

The TEG operating graph is to be used in the following manner:

• Determine the water content of the inlet gas from the “water content of hydrocarbon gas” graph (See Appendix D) (mg water/Sm3 of natural gas).

• Calculate the required TEG/water ratio (See Appendix E).

• Multiply the water content by the volume of gas processed per day (m3/day) – this number is the starting point on the left Y-axis.

• Follow the graph over to the plant’s required TEG/water ratio in litres TEG/kg water. Values for 16, 25 & 33 L TEG/Kg H2O are illustrated; however other ratios may be interpolated quite easily. Dropping straight down to the X-axis gives the necessary circulation rate for this ratio.

• Draw a straight line down to the lower set of curves, again from the plant’s required TEG/water ratio. Following across to the right Y-axis on the right side to obtain the optimum reboiler duty for that ratio.

The required TEG/water ratio calculation depends on desired water removal efficiency. Using the water content of hydrocarbon, determine the water content of the inlet gas (“Win”). The desired water content out is designated Wout (generally 64 mg/m3 in Canada). Removal efficiency is calculated by: Water Removal Efficiency = Win – Wout Win


From the “water removal vs. TEG circulation rate” graph (See Appendix E), the required circulation rate can be found by using the water removal efficiency and the lean TEG concentration. By operating with higher lean TEG concentration and lower circulation rates significant savings in fuel gas can result. See Appendix C for examples on circulation calculations and fuel gas savings. In the case where the plant conditions do not match the graph predictions:

• A review is needed to see if the plant can or should be optimized.

• Operational changes made to the system need to be gradual, and made one change at a time, and

• If it is not possible to make any further changes to the system, then the difference between current operating conditions and the graph predictions is the efficiency loss in the plant.

These parameters are considered “best operating practices” and are generally what Canadian dehydrators are designed to run at. Of course, not all plants will meet all these criteria. The graph may also be used to determine the effect of retrofits or modifications on the plant’s efficiency (See Appendix B). Reboiler Analysis of the operational efficiency should include an evaluation of circulation rate and reboiler performance. For TEG plants, the recommended purity is often >98.5%. Achieving this purity involves maintaining a reboiler temperature of 195-200˚C (383 - 392˚F) with regular glycol analysis to ensure minimal degradation product build up. Plants that run with a reboiler temperature lower than recommended will have more than 1.5% water in the lean glycol. In most of these cases no amount of over circulation will produce adequately dry gas. It is better, from an energy and practicality standpoint, to optimize reboiler temperature and reduce the circulation rate. Lean/Rich Exchanger All dehydrators have some form of lean/rich exchanger. This exchanger may be plate and frame or a simple tube through the accumulator. The purpose of the exchanger is to heat up the rich solution, preparing it for regeneration while at the same time cooling the lean glycol in preparation for absorption. The lean/rich exchanger is intended to lowers the needed duty of the lean glycol cooler and reboiler. The performance of the exchanger depends on the circulation rate of the glycol being high enough to keep the solution in turbulent flow. The flow regime (turbulent or laminar) is measured by calculating the Reynolds number.


Reynolds number = R = w · ℓ / v, where w = flow speed (m/s) ℓ = tube diameter (m) v = kinetic viscosity As shown by the equation above, if the flow speed decreases so does the Reynolds number. If the Reynolds number drops below 2300, the flow is considered outside of the turbulent regime (it will drop into the transitional or laminar flow regime) and heat transfer efficiency is drastically reduced. In this case, the reboiler and lean amine cooler have to work much harder to heat and cool the amine resulting in a fuel gas increase. Additional improvements in fuel gas efficiency may be achieved by changes to the design in favour of fuel saving technologies. Please see Appendix B for a brief overview of design considerations and technologies available.


5.2 Performance Specification and Assessment Process

Can the dehydratormeet the required dry

gas specification?

Determine thewater content ofthe maximum

anticipated inletgas flow (refer to

Appendix D)

Calculate therequired lean TEG

strength andoptimal TEG to

water ratio(Industry standard:25LTEG/kgH20)

No

Yes

No

Yes

Determine a viable optionbetween stripping gas ormaking a design changes

(See Appendix B) toachieve the required lean

TEG purity.

Is the dehydrator themost suitable choice?

Reviewapplicability of theglycol dehydratorin plant operationand compare with

availabletechnologies

Existing glycol dehydratoris acceptable

Replace glycoldehydration unit withappropriate option

Considerimprovements tosystem via designchanges outlined

in Appendix B

Using the TEGoperating graphdetermine the

required circulationrate and reboiler

duty

Perform ananalysis of glycol

systemreplacement

options identifiedin Appendix B


Appendix A Glycol Dehydration Turndown Considerations

Dehydration facilities are generally designed using bubble cap trays, which can handle a higher liquid turn down (reduction in glycol flow from design) than other types of trays. The accepted industry standard is that bubble cap trays may be turned down to 10% of the design circulation rate. Many absorbers are also filled with packing instead of trays. Packed towers have the advantage of being able to handle higher gas flows than a trayed tower of the same diameter. Packed towers have greater limitation on turn-down than bubble caps however. General turndown guidelines for valve trays are as described in Table 3 below:

Table 3 Turndown Liquid Rate Reduction

Gas Capacity (% Design) Liquid Rate Reduced (% Design)

>80 25 40-50 50 <25 Typically No Liquid Reduction Possible

Care must be taken when working with high gas or liquid turndowns. Proper gas and liquid distributors are a must to prevent channelling.


Appendix B Design Considerations for Optimizing Glycol Dehydrators

Alternative Process Technologies One way of avoiding the use of stripping gas is by choosing suitable alternative technologies. A common example in Alberta is the use of the Drizo® process, where glycol is regenerated by solvent stripping instead of the conventional gas stripping. Solvent stripping obtains much higher glycol purities than gas stripping (up to 99.998 wt% instead of the typical 99.95 wt %) and consequently allows much larger water dew point depressions: up to 100 °C and even higher in some cases. The solvent required by the Drizo® process is usually obtained from the BTEX present in the natural gas itself and in most cases the process will even produce some liquid hydrocarbons. Another example is the Cold Finger Process, where cool rich glycol is used for heat exchange purposes, causing water vapour in the accumulator vessel to condense. High glycol purity may be achieved without the direct use of stripping gas. The circulation rate and heat requirements for the regeneration of glycol are also reduced which potentially decreases benzene absorption by the glycol from the gas stream.2 Flash Tank 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. Many dehydrators in use today send a 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. In a flash tank separator, gas and liquid are separated at either the fuel gas system pressure or a compressor suction pressure of 275kPa to 689kPa. 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 conserving fuel gas and reducing emissions. This gas may be collected as marketable natural gas liquids from the flash tank separator and serve as a potentially significant source of additional revenue. Figure 1 demonstrates a glycol dehydration system with a flash tank. 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 and benzene more efficiently than flash tank separators alone. In addition, the liberated gas can be used as fuel gas (i.e. reboiler burner) to reduce operating costs. Piping recovered flash tank gas to the suction of an upstream compressor (a common design practice in new installations) also reduces production costs.


Piping a dehydrator's regenerator vent to a vapor recovery unit also allows flash tank gas to be used as a stripping gas in the glycol reboiler. There are some operational concerns in that the recovered methane from flash gases are wet and can cause problems with pneumatic instruments and operations due to freezing. Recovered methane should be routed to the compressor suction or to fuel use. Specific operating measures can be taken to prevent operational problems and should be taken into consideration prior to installation of new units.

Table 4 Case Study: Fuel Gas Savings from Using a

Flash Tank Separator4

TEG Circulation Rate (LPM)

Fuel Gas Saved Using Flash Tank Separator on a Pneumatic Pump

(m3/yr) 2 21939

10 109633 20 219266 30 328869

Vapour Recovery Unit Glycol dehydration units use electric or gas assist pumps to re-circulate the lean (dry) glycol back to the gas contactor. Gas assist pumps are driven by expansion of the high-pressure gas entrained in the rich (wet) glycol. This methane gas is either vented to the atmosphere with water vapor that is boiled off in the glycol regenerator, or recovered for beneficial use with a flash tank separator. When flash tank separators and other vents are piped to a vapor recovery unit (VRU), more gas can be recovered and less methane, volatile organic compounds (VOC), and hazardous air pollutants (HAP) vented from the reboiler. The VRU boosts the recovered gas pressure enough to inject it into a fuel gas system, compressor suction, or gathering/sales line. For full benefit from this practice, the existing VRU should have sufficient capacity to capture the maximum production tank vapor load simultaneously with the glycol dehydrator vent load. Electric Pumps in Place of Energy-Exchange (Kimray) Pumps Remote gas fields may not have electrical power and therefore use pneumatic pumps to power the lean TEG circulation pump. In energy exchange pumps, every volume of gas absorbed in the rich TEG leaving the contactor, requires two volumes of gas from the wet feed gas to supply enough motive gas for the lean TEG pump. Therefore, using a pneumatic pump triples the amount of gas


entrained with the TEG and vented to the atmosphere. Gas-powered pneumatic pumps can also contaminate lean glycol, making the glycol less effective at absorbing water from the wet gas stream. Worn O-rings in gas-assisted glycol pumps can cause contamination of the lean TEG stream in the dehydrator, reducing system efficiency and requiring an increase in glycol circulation rate, compounding the fuel gas use. The design of electric pumps eliminates the potential for this contamination to occur and thereby increases the operational efficiency of the system. Replacing gas-assisted glycol pumps often results in lower annual maintenance costs. The floating piston O-rings in gas-assisted pumps must be replaced when they begin to leak, typically every 3 to 6 months. The need for this replacement is eliminated when electric pumps are employed. Installing an electric powered pump in place of a pneumatic pump eliminates a major source of fuel gas use. Typical fuel gas savings from the use of flash tank separators and different pumps is presented in Table 5.

Table 5 Case Study: Potential Savings from Using

an Electric Pump vs. Pneumatic Pump4

TEG Circulation Rate (LPM)

Fuel Gas Saved Using an

Electric Pump (m3/yr)

2 14616 10 73079 20 146188 30 219236

Replacement of Glycol Dehydrators Alternatives are available for hydrate control and dehydration. Selection of alternative processes should be based on an evaluation of capital, operating costs, and emissions reduction. Hydrate control, without dehydration, can be achieved through the use of physical separators, chemical inhibitors, or line heaters. Methanol or Glycol Injection. These chemical inhibitors mix with the water vapour and condensed water and effectively suppress the hydrate temperature of the gas. The selection of the specific chemical depends mainly on the operating (chemical) cost, because both are equally effective as inhibitors for wellhead dehydration applications. However, glycols injected at a well site could be easily


recovered if the gas is further processed in a glycol dehydrator at a downstream central location. There is the potential for chemical inhibitors to create excess liquid hold-up in the gathering system which may require routine pipeline pigging. Separator Package. A separator provides physical separation and removal of free liquids (including water) in the inlet gas stream. This substantially reduces the volume of chemical hydrate inhibitor injection required in the pipeline gathering system. Line Heater. The use of a line heater elevates the gas temperatures above the temperature at which hydrates form. This option can be used when chemical injection or separator packages are not viable options. Line heaters require the use of insulated pipelines to minimize heat input requirements. Solid Desiccant / Molecular Sieve Plants. Solid desiccants are selectively chosen for treating high pressure gases when it is economical to carry out both the water and hydrocarbon (HC) dewpoint controls in a single step process, or when lower water dew points must be achieved. The pressure loss is considerably lower compared to the conventional two step processes involving separate dehydration and HC dewpoint control units. A variety of solid desiccants are available in the market for specific applications. Some are good only for dehydrating the gas while others are capable of performing both dehydration and removal of heavy hydrocarbon components. However, in both cases, the desiccant beds must be routinely regenerated with heat to liberate contaminants. The physically adsorbed material is released as a gas and typically disposed of by flaring, incineration, or recovered as liquids if economically feasible. Desiccant systems have substantial capital and operating costs. Refer to Desiccant Dehydration (Module 10) for details Membrane Technology. This technology is used for the selective removal of contaminants, such as water from a natural gas stream. Use of the technology can eliminate certain emissions, relative to glycol dehydration. Membrane technology is not currently in use for production-scale gas dehydration.3


Appendix C Case Study Calculations for Glycol Dehydrator Optimization

A TEG dehydration system is processing gas coming straight from an amine absorber. The TEG contactor has 10 bubble cap trays (2.5 theoretical stages). Gas conditions: flow is 1675 e3m3/d: Temperature = 30˚C Pressure = 5000 kPa Benzene = 200 ppm A gas at 30˚C and 5000 kPa has a water content of 780 mg H2O/m3. 780 mg/m3 x 1675 e3m3/d = 1306 kg of water/day The plant has a treated gas specification of 64 mg H2O/m3 of gas. Water removal efficiency needed = (Win –Wout)/Win = (780 – 64)/780 = 91.8% From the “water removal vs. TEG circulation rate” graph (See Appendix E), this efficiency can be achieved with a 98.6% strength TEG circulating at 25 L TEG/kg H2O. No stripping gas is needed in this unit:

TEG Operating Graph

0100200300400500600700800900

100011001200130014001500

0 5 10 15 20 25 30 35


Kg

Wat

er/D

ay

0255075100125150175200225250275300325350375

Reb

oile

r D

uty

(kW

)



1625

33

To meet the dehydration requirements, the system must circulate the glycol at 23 LPM, and run with a reboiler duty of 112 kW.


Operation Optimization Case Study The above plant was found to be circulating at 30 LPM.

TEG Operating Graph

0100200300400500600700800900

100011001200130014001500

0 5 10 15 20 25 30 35


Kg

Wat

er/D

ay

0255075100125150175200225250275300325350375

Reb

oile

r D

uty

(kW

)



1625 33

Over circulation of the glycol results in a 31.6% increase in reboiler duty, from 112 kW to 147.5 kW. The plant reduces the glycol circulation rate down to 22 LPM and can then lower the reboiler duty correspondingly. Fuel Gas Savings Reboiler Savings: Fuel gas value: $5.25/GJ Fuel gas heating value: 40 GJ/e3m3

1 kW*s = 1 kJ 1 kW*h = 0.0036 GJ therefore, 147.5 kW*h = 0.531 GJ 0.531 GJ = $66.91/d and 318.6 m3/d of fuel gas

= $24,420.69/yr and 116,289 m3/yr of fuel gas After optimization: 112 kW*h = 0.4032 GJ 0.4032 GJ = $50.80/d and 241.9 m3/d of fuel gas

= 18,543.17/yr and 88,301 m3/yr of fuel gas Economic savings = $5,877.52/yr (24% reduction) Fuel gas volume saved = 28 e3m3/yr


Pumping Savings: Assuming an operating pressure of 800 psi, a Kimray pump will consume 4.5 cu.ft./gal pumped. Economic savings = $29,820.00/yr Fuel gas volume saved = 142 e3m3/yr This fuel gas is often sent with sales gas instead of being burned, further adding to plant revenues.


Appendix D Water Content of Hydrocarbon Gas


Appendix E


Appendix F References

1. Hyprotech Limited, 1994. Hysim™ User’s Guide, Version C2.50. 300

Hyprotech Centre, Calgary, Alberta. 2. Gas Conditions International Company. Coldfinger - An Exhauster of

Removing Trace Quantities of Water from Glycol Solutions used for Gas Dehydration. Gas Conditioners International Co., Houston, Texas.

3. Newbold, 1995. Solvent-Resistant High-Pressure Membrane Modules for

Natural Gas Dehydration. Presented at 1995 Gas Research Institute Glycol Dehydrator/Gas Processing Air Toxics Conference. Denver, Colorado, November 5-8, 1995.

4. United States Environmental Protection Agency, 2003. Optimize Glycol

Circulation and install Flash Tank Separators in Glycol Dehydrators. Presented in Lessons Learned Natural Gas Stars Program.

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Fuel Gas Efficiency BMP - Efficient Use of Fuel Gas in Glycol Dehydrators (Module 9)

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