Introduction

Considerable effort has been put forth by gas producers to evaluate and permit production sites for compliance with environmental requirements. Complexity of site assessment and difficulty understanding regulations has made this a major undertaking by most companies. Based on results of these evaluations, many production sites require emission control now, and many others may require retrofit control in the future.

Field proven control methods are available to significantly reduce emission levels at gas production sites.  While technology exists to achieve virtually any level of emission reduction, a balance between efficiency and economics must be considered. This balance can best be accomplished through a thorough design evaluation of each application and then apply a proven technology to satisfy requirements.

The information to follow concerns a single aspect of natural gas production, liquids storage. The vapors generated as a result of this storage will require control under regulations imposed by the Clean Air Act legislation. Final emission levels requiring control will be implemented under state authority. Control of these condensate vapor emissions provide some unique qualifying factors.

Application Description

The majority of gas production site emissions are generated from two sources; liquids storage tank vapors and glycol dehydration unit still column vents. Both sources have emissions containing Hazardous Air Pollutants (HAP's) and Volatile Organic Compounds (VOC's). The characteristics of each source are unique, as are the control solutions. Quantities of emissions generated is site specific and dependent on both the production equipment used and characteristics of the gas being processed. Each state government will institute regulations that require control of these vapors to reduce the amount of VOC's and {L~p~s released to the atmosphere.

From a well head, natural gas flows to a separator which separates free liquids from the gas. The gas phase normally flows to a dehydration unit, while liquids are accumulated in the separator dump tank. At periodic intervals, the dump tank containing liquids is relieved and the liquids flow to large, low pressure storage vessels. The liquids consist of primarily hydrocarbons with smaller amounts of other compounds.

Quantities of condensate vapors generated are a combination of events and conditions. As liquids are relieved from the high pressure separator dump tank to the low pressure storage tanks, a significant amount of vapors are produced which is termed 'flashing losses". The volume of liquids transported to the tanks also displaces an equal volume of vapor in the tanks contributing to the total volume. This vapor displacement is known as "working losses". The third loss consideration is termed 11breathing loses" which is a function of the ambient temperature and barometric pressure. The characteristic composition of the liquids, storage volume, and other aspects also influence the total volume of vapors generated at any given site.

The variety of operating parameters used in the industry combined with the many other influences on condensate storage makes accurate prediction of vapor flow from storage tanks virtually impossible. The fluctuations in flow are also significant, adding to the complications of control.

Process Considerations

As stated, gas production separators remove free liquids (condensate) from the gas. These liquids are most often sent to storage tanks, on location, where they remain until pumped out and transported for marketing. Vapors from these tanks are high in "VOC's" and can contain "HAP's". Volume of vapor generated will depend on:

• Well production rates
• Separator dump pressure
• Characteristics of gas produced
• Liquid storage volume
• Ambient temperature
• And more

Production of liquids, and likewise vapors from those liquids, will be at the highest rate during the initial phase of well production. The amount of liquids produced will decline considerably sometime within the first year. Liquids production continues to fall off in the years to follow, but at a much slower rate of decline. This reduction in liquids produced, naturally reduces the volume of vapor released and hence, changes the control equipment requirements.

Management of effective control for ever changing and declining vapor volume provides a special challenge. The control equipment capacity requirements after one-year of production may be 20% of initial design. A single control system to meet requirements at the beginning of production would be increasingly oversized from then on. For the same control equipment needed during initial production to remain on site can isolate an otherwise useful investment and negatively impact production economics.

Combustion/Destruction Efficiency

Although vapors from natural gas liquids are certainly of value to recover, it is often not economical, and sometimes impossible to accomplish. The most common and proven technology then becomes combustion, or the process of burning.

Combustion by open pipe methods, or flaring, will not consistently accomplish complete combustion or destruction efficiency. Only under controlled conditions, or very isolated circumstances, could this be accomplished. It is important to understand that there is a difference between mere combustion and destruction efficiency.

Effective destruction of vapors is only achievable through Thermal Combustion methods. Thermal Combustion in this case should be understood to mean "burning at an elevated temperature". Design considerations for effective vapor combustion and destruction efficiency include; temperature, stoichiometric mixing ratio, retention time, optimization of combustion components, and more.

Control Design Considerations

It is of paramount importance that the producer knows exactly what they need to accomplish and identify any application restraints. These restraints might include:

• Flame visibility
• Smokeless operation
• Combustion/Destruction efficiency
• Turn-down
• Safety
• Reliability
• Control options
• And more

For optimum Thermal Combustion equipment design, it is extremely valuable to obtain as much information as possible with respect to the application. This would include:

• Vapor analysis
• Dump volume and frequency
• Separator dump pressure
• Flow profile at conditions
• Niaximum Instantaneous flow
• Liquid storage volume
• Storage tank pressure
• Temperature variation
• Other

Control Solutions

As mentioned, any post storage condensation that can be achieved is certainly beneficial for recovery of a marketable product. When all efforts of recovery have been exhausted however, Thermal Combustion is the best means of control. Through computer aided design, years of combustion experience and numerous site applications, Tornado™ Flare Systems, Inc., has developed economic systems designed to meet the exacting requirements of condensate vapor control. A typical Tornado™ system would include liquid knockout(above or below ground), detonation arrestor, and a properly sized enclosed ground flare.

Tornado™ Flare Systems, Inc., has provided vapor control equipment to gas producers for many years. It is significant to recognize, and accommodate through design, the variation in requirements for every production location. This includes conditions from initial production through depletion. Through research and numerous field installations, Tornado has addressed the many issues involved and offers solutions to optimize vapor control and minimize customer expenditures. This allows customers to always meet regulatory requirements as efficiently and economically as possible, under any given site conditions, for years to come.

To meet the challenge of declining production, Tornado™ Flare Systems, Inc., offers a modular design of their vapor control systems. This concept includes parallel detonation arrestor and control systems designed to be transportable for relocation. Multiple units can be installed at the beginning of production in combination to meet the vapor flow requirements and optimize control efficiency. As liquids production declines, this concept allows a single unit to be isolated and removed for relocation at another site. When liquids production stabilizes, a single system would remain for continuous vapor control at optimum efficiency.

This modular concept, coupled with the experienced technical assistance of Tornado™ Flare Systems, Inc., assures producers optimum vapor control during every stage of well production life.  Utilizing this concept can reduce capital equipment expenditures for control on subsequent condensate storage installations at new or existing production sites.

Installation Considerations

As mentioned, natural gas liquids are certainly of value to recover if economically feasible. In addition, free liquids can complicate the combustion equipment operation. For these reasons, special consideration should be given to layout of the vent line from liquids storage tanks. It is recommended that two important factors be considered for incorporation at every production installation.

A knockout tank (above or below ground) should be installed between storage tanks and detonation arrestor - As close to the combustion system as possible. Care should be taken to assure a continuous slope to the knockout which will prevent condensed liquid buildup in the vent line itself.

Summary

All states, in conjunction with the EPA, will eventually set limits for control of condensate storage tank emissions. Technology exists to effectively manage these emissions utilizing thermal combustion. Understanding of application requirements and process criteria is of paramount importance for every application to effectively accomplish control. Proper equipment engineering and design will assure optimum performance and continued reliability.

As with all critical equipment applications, caution must be taken to incorporate only field proven combustion control equipment from a qualified manufacturer. Conformance to manufacturing code requirements is essential for reliable performance and safety. Equipment must be built to withstand extreme demands of the process as well as environmental exposure without compromise of performance.

Tornado™ Flare Systems, Inc., are industry leaders in combustion and pollution control equipment. Tornado's expertise in thermal combustion is evidenced by the many operating installations and continuous customer satisfaction.

For application assistance with natural gas liquids storage emissions, or information on any other combustion requirements, please contact Advanced Rupture Disk Technology.