Thermal Oxidizers for Natural Gas Savings

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Natural Gas Disciplines and Case Study

It’s in the news every day; the natural gas production industry is experiencing record growth.  The gas reserves in the United States are thought to hold enough Natural Gas for hundreds of years and the gas and oil industry has improved extraction techniques that make gas production more affordable and less impactful on the landscape.  This is good news for the country and our environment.  

The natural gas industry is broken up into three main disciplines:

1. Exploration and Production – this is the effort to locate appropriate gas and oil well drilling locations and extract the resource from the ground.  The well is ultimately connected to a pipeline system for transport and directed to gas processing plants.
2. Midstream – The midstream operator accepts the products from the pipeline for treatment.  In different regions of the country and different regions within particular shale plays there exist different varieties of gas and oils.  For instance, in the Eagle Ford shale, areas to the North are considered wet gas.  It’s this wet gas that holds a bonus product for producer, condensates.  Condensates are liquid hydrocarbons that can be separated and sold at even higher costs than the natural gas.  In some cases condensates are valued at six times that of natural gas (considered on a dollar per btu basis)  The processor separates the liquids from the gas, treats the gas by removing harmful impurities, and pushes the gas product into a downstream pipeline for distribution to end users.
3. Downstream – The downstream handlers take the gas from the processing facilities and ultimately sell the product.  Most of the gas is sold directly to customers, while some gas is also stored in geological formations such as; salt domes, spent aquifers and oil wells, and certain porous earthen structures.  Downstream processors are also liquefying the natural gas for export to other countries.  And lastly, downstream processors can break the natural gas down into its rudimentary components - in a process called fractionation - that allows the sale of certain hydrocarbons for use in chemical processing facilities. 

All of these areas of the natural gas circuit have EPA Clean Air Act Guidelines that are strictly enforced.  This paper will focus in on the midstream operations of one of the country’s largest integrated operators.

The Midstream Process

Midstream gas processing plants are complicated production facilities.  These plants process large quantities of natural gas and hydrocarbons.  Safety comes first at every site.  And high quality standards drive every decision.  Natural gas has to be processed to very demanding industry standards.  

The process involves cleaning natural gas of harmful impurities and water.  When natural gas is extracted from the ground it contains high amounts of CO2, water, and corrosive sulfur compounds.  A Common processing operations include glycol dehydration and amine stripping.  An amine stripper is a counter current stripping operation where natural gas is directed into the bottom of a tall tower and an amine liquid solution is poured into the top of the tower.  As the gas contacts the liquid, the CO2, water, and H2S impurities are adsorbed from that gas and held in the liquid.  The natural gas is now processed and ready to be directed to the downstream users.  The amine system uses a re-boiler that boils off the CO2, water, other VOC’s, and H2S so the amine solution can be re-used.  The vent gas from the re-boiler is the subject to EPA guidelines; 40 CFR, Part 63, Subpart HH.  Under this rule, greater than 95 percent of the pollutants must be destroyed prior to release to atmosphere.  

This may sound easy, but there are a lot of design considerations that must be understood in order to provide a long lasting – highly reliable abatement device.

The Problem

The typical off gas from an amine treatment system is rich in CO2, contains substantial amounts of water, and varying concentrations of hydrogen sulfide (H2S).  These three components combine to form a corrosive gas that has a tendency to condense and form a corrosive liquid.  The gas also contains a variety of hydrocarbons such as; methane, ethane, propane, butane, benzene and others, all in varying concentrations. To further complicate the abatement plan, the gas contains no oxygen.  Oxygen is a necessary ingredient when “oxidizing” VOC’s.  

Given the complications that the vent gas presents, many operators have opted to use flares as the abatement device.  However, flares have several drawbacks that prevent efficient operation and under recently proposed EPA changes to the rules, flares will surely be discontinued in many future plant designs.  (See, for example, 40 CFR, Part 63.11b for flare design criteria that will apply to flare installations in the future).As a replacement flares, thermal VOC abatement can be broken down into two common technologies: Catalytic Oxidation and Thermal Oxidation.  

1. Catalytic Oxidation uses a catalyst to initiate conversion of hydrocarbons to CO2 and water vapor at low temperatures.  However, given the presence of sulfur compounds (sulfur is a catalyst poison), Catalytic Oxidation is not a good long term choice.  
2. Thermal Oxidation is a process by which the vent gas is raised to 1,400 F, held in a reaction chamber where the hydrocarbons are oxidized to CO2 and water vapor.  Thermal Oxidation provides very high destruction efficiency and is considered a safer method than flaring.  

Gas processors are now focused on efficient operations of their plants.  The old school mentality of the gas processor did not consider natural gas fuel efficiency because they had an unlimited supply of the fuel.  Now the modern gas processing facility understands that the gas they use in their operations is less gas they can sell downstream.  Plus, recent modifications to EPA standards consider Greenhouse Gas (GHG) emissions in their inventory of emissions.  Methane, the primary component in natural gas, is a GHG with a Global Warming Potential 21 times that of carbon dioxide, according to the EPA.  

• Midstream processors have multiple issues to think about when designing their plants:
• Reliable VOC abatement systems
• Long lasting designs that consider the corrosive nature of the vent gas
• Safe designs that are capable of operating in hazardous areas
• Fuel efficient to help lower processing costs, provide more product for sale, and limit GHG emissions

The Solution

The first step to developing a suitable action plan is choosing the right partner.  For a successful project, the processor should work with a company that has a long history in the petro-industries, offers a complete product line, has the knowledge to integrate a safe system, and a support staff with a proven track record of success.  

Next, a thorough review of the expected vent gas must be had.  As noted earlier, different natural gas regions have different varieties of gas, and thus different vent gases can be expected.  Once the vent gas has been defined the final choice of abatement technology can be considered.  

Thermal Oxidation can be segmented into three primary technologies for the natural gas processor:

• Regenerative Thermal Oxidation – uses ceramic media and switching valves
• Recuperative Thermal Oxidation – uses traditional shell and tube heat exchanger
• Non-recuperative Thermal Oxidation – commonly called an afterburner, this system does not employ heat recovery.  These systems easily meet EPA requirements, but consume substantial fuel.

We consider all three systems appropriate technology, but the final choice is usually based on the expected amount of VOC’s contained in the vent gas.  If the vent gas contains low amounts of VOC (usually less than 10% of the LEL) then the Regenerative Thermal Oxidizer (RTO) is an appropriate device.  However, if the vent gas contains higher LEL or upset conditions that may exceed 25 percent LEL, then the Recuperative Thermal Oxidizer is a better choice.  Certain circumstances may point to the use of the non-recuperative Thermal Oxidizer, but considering the high fuel use, these systems are being employed less.

It’s important to partner with a company that offers all technologies.  The right equipment provider can work with the gas processor to identify all considerations in equipment design and finally offer the right solution, based on the gas processors unique needs; not the equipment providers product line.  

Case Study: Recuperative Thermal Oxidizer

A large midstream natural gas processing facility was in urgent need of a VOC abatement device for their newly expanded gas processing plant. The operator was expanding their operations at an unusually fast pace. In the process of bringing on new gas treating capacity, they found themselves up against the limits of their current operating permit. Any new sources would put them over their limits as defined under EPA guidelines; 40 CFR, Part 63, Subpart HH – National Emission Standards for Hazardous Air Pollutants from Oil and Natural Gas Production Facilities.

The process included treatment of four separate amine trains. The exhausts from each were combined together and ducted to one QUADRANT SR-6,000 Thermal Oxidizer. From the amine re-boiler, the vent gases were piped into a knock-out vessel where excess water and liquids are removed. The resultant gas was piped to an inlet manifold located at the QUADRANT SR-6,000 Thermal Oxidizer skid. The manifold and control dampers were constructed of stainless steel and incorporated a hot air recirculation system designed to raise the inlet temperature high enough to prevent the gas from condensing and forming additional liquids and more importantly, add the necessary amount of oxygen to allow the vent gas to sufficiently oxidize and destroy the pollutants.

After the vent gas has been heated with sufficient air (and oxygen) the gas is moved through the QUADRANT SR-6,000 Thermal Oxidizer by use of a main booster fan. The booster fan will push the gas through the shell and tube heat exchanger. One of the primary differences that the QUADRANT SR-6,000 Thermal Oxidizer offers any user is our unique FLOATING TUBE Shell and Tube Heat Exchanger. This system is unlike anything in the industry and is custom designed to provide high thermal efficiency and long life. Unlike most shell and tube heat exchangers used in industry, the FLOATING TUBE System does not use any expansion joints. The result is long life without worry to expansion joint failure. The FLOATING TUBE Heat Exchanger raised the vent gas temperature to over 1,100 F before the gas was directed to the combustion chamber.

Once the gas has been pre-heated, it is directed into the combustion chamber where a natural gas burner adds additional energy to raise the gas temperature to 1,400 F Efficient oxidation has to consider the three T’s; time, temperature, and turbulence. In order to meet the high destruction efficiencies necessary in modern processing facilities, all three of these primary design considerations have to be maximized. The unique design of the QUADRANT Series Thermal Oxidizer meets this challenge by incorporating a unique combustion chamber that is specially designed to provide even temperature uniformity, turbulent mixing of the vent gas, and a specially designed residence chamber. All three T’s (time, temperature, and turbulence) are specifically engineered for maximum performance.

Once the vent gas has been heated to 1,400 F and directed into the combustion chamber, the VOC’s contained in the gas are oxidized. (Volatile Organic Compound or hydrocarbons are compounds that contain hydrogen and carbon and which are regulated by the EPA).

The process of oxidation can be summarized by the following reaction:

This process is exothermic, meaning heat is given off. The amount of heat is proportional to the amount of VOC. Therefore, the more VOC contained in the vent gas, the more heat is given off in the reaction. The more heat from the reaction means less auxiliary natural gas is demanded by the burner. As the VOC concentration rises, the amount of natural gas fuel needed is reduced. This is the basis for very efficient operation of the oxidizer with very low amounts of natural gas.

The vent gas is now cleaned of VOC’s and other pollutants. The hot gas is directed over the FLOATING TUBE heat exchanger, where the heat is transferred to the incoming gas, and the exhaust gas is cooled prior to release to atmosphere. A portion of this hot gas is recirculated and mixed with fresh air for use in the first step of the process.

The QUADRANT SR-6,000 Thermal Oxidizer provides +99.9% VOC removal, very low CO and NOx emissions, and low natural gas cost. Emissions are reduced to well below applicable EPA standards. The system is supplied from Catalytic Products International (CPI) as a fully assembled package. The unit is assembled, wired, piped, and tested in the factory. This is an important construction feature for midstream operators as their plants are located in very remote areas of the country. The installation plan is simple; unload from the trailer, assemble on the concrete pad, plug in electricity and natural gas, and turn on. Plug-and-Play is a feature that affords quick installation, commissioning, and operations.

The materials of construction are a very important design element. The system has to be designed with materials that are resistant to corrosion, heat, and thermal cycling. Appropriate grades of stainless steel are used throughout the design and consider corrosion and high temperature operation. The vessels are internally insulated with ceramic lining system that helps to keep shell temperature low. And the interface between the vessel shells and ceramic lining is coated to help prevent corrosion. This combination of material selections affords long term operation with the highest uptime reliability.

CPI was asked to supply this system in record time. As noted above, the operator was faced with new process equipment coming on line and no air equipment on site to satisfy EPA air permit needs. CPI worked with the operator to deliver the QUADRANT SR-6,000 Thermal Oxidizer in eight weeks. This is twice as fast as a normal project usually requires. CPI also worked with the customer to assist a expediting the site needs for the installation.

The project team at CPI helped source and supply various valves and piping auxiliaries necessary to complete the installation. When the equipment arrived on site, CPI project managers were there to assist the rigging crew with fast assembly of the system and proper re-termination of all the instrumentation. Since the system is fully assembled in the CPI factory and the system is fully wired, CPI production staff can deliver a system with 100 percent certainty of operation.

Once the process is connected, the electrical supply is plugged into the control panel, and the natural gas is piped to the combustion system, the QUADRANT SR-6,000 Thermal Oxidizer is ready to run.

Summary

Catalytic Products International delivered, installed, and commissioned a unique Recuperative Thermal Oxidizer system in 10 weeks time. Beating the plants online goals by two weeks and saving the customer more than $3,000 per week in potential costs for flare rental fees.

When compared to a traditional flare, the QUADRANT SR-6,000 Thermal Oxidizer provides the following benefits:

• $269,000 annual natural gas savings (at a gas sales cost of $6.5/therm)
• Less than 2 year ROI when compared to a traditional flare 3456 tons per year of CO2-e reductions
• 99.9 percent VOC and H2S removal efficiency

Catalytic Products International is a 42 year old supplier of custom built VOC, HAP, CO, NOx abatement solutions and efficient energy conservation systems. CPI maintains an engineering staff made up of industry professionals that bring years of experience and valued competencies while offering a full array of service products such as; engineering audits, layout and integration, startup, preventative maintenance plans, and emergency service.

Whether your application calls for quick response to a challenging application, as in the story with this case study, or longer-term design and integration of more traditional air pollution control equipment; CPI has the proven product line and expert applications staff to assist your needs.