Catalytic Oxidizers…What You Should Know.
Within the U.S., depending upon specific plant operations, geographic locations, etc., many companies concerned with controlling VOC emissions have been operating air pollution control systems for years. However, due to business expansion, plant modifications or recent changes in EPA guidelines, an increasing number of companies are now finding it necessary to install new emission control systems or upgrade existing equipment with more modern and efficient control technology. With well over twenty-five years of proven success, catalytic oxidation is a popular method for controlling volatile organic compound (VOC) emissions.
When a decision is made to purchase a catalytic oxidizer, it is advisable that the purchaser takes the same approach in selecting a pollution control system as they would when purchasing more sophisticated production equipment. That is, very specific oxidizer design details should be gathered and evaluated. Unfortunately, it can be difficult to understand the bona fide differences between oxidizer manufacturers. When evaluating a catalytic oxidizer for purchase, there is no real product testing that can be done and no sample production to be reviewed, so purchasing decisions are typically made by relying on the vendor’s credibility and on the assembled design data.
Which Components To Evaluate
The following is a comprehensive list of catalytic oxidizer design specifications, major components and related vendor services, along with a brief explanation as to why each item should be considered when evaluating a catalytic oxidizer system for purchase.
Maximum and minimum design airflow. The oxidizer should be sized to handle your maximum exhaust rate, but should include provisions for sufficient turndown to idle and to operate any one process independently. Typical volumetric turndown ratios range from 4:1 up to 6:1.
Fan type and design. An oxidizer supply fan should be of an industrial grade design with an impeller wheel suited to the air stream. While a belt drive model is preferred to keep costs down, a direct drive unit (with bearing temperature and vibration monitors) is recommended when the motor size exceeds 150 horsepower.
Catalyst type and design. There are various metal catalyst types available. Although precious-metal catalyst has proven to be the most versatile, base-metal catalyst has made advances in recent years. Precious-metal catalysts can be of either the monolith or bead type design. Both have advantages and disadvantages depending upon your specific application. If you work with a qualified vendor that offers both catalyst types, they can help you select which one is best suited to your application and explain why.
Catalyst manufacturer. Again, there are many catalyst manufacturers, so it would be wise to verify that your oxidizer vendor is offering a catalyst from a proven source that will allow the end user to purchase replacement catalyst directly from the manufacturer when necessary.
Catalyst warranty/VOC destruction efficiency. In many industries, a full three-year catalyst warranty has become very common. A minimum of 98+ percent VOC destruction efficiency can be achieved at low temperatures with minimal amounts of catalyst. A 99+ percent VOC destruction efficiency can usually be guaranteed by designing the unit with additional catalyst.
Catalyst life expectancy. Depending on the type and amount of catalyst used, life expectancies can range from three to four years for manganese dioxide catalyst and from seven to ten years or more for precious-metal catalysts. Keep in mind that the life expectancy of any catalyst is directly proportional to the amount (cubic feet) of catalyst originally installed. The more catalyst used, the longer the life expectancy.
Catalyst cleaning capabilities. If a monolith catalyst is used, it can be easily removed and cleaned by using a high-pressure air knife, a soap and water wash and/or an acid bath if necessary. All cleaning types are designed to extend the catalyst life. Bead and/or pellet type catalysts cannot realistically be cleaned.
Catalyst attrition rate. Since monolith catalyst is basically static, it has no attrition rate. Brittle by nature, bead and pelleted catalysts will have a small percentage of catalyst destroyed as the unit expands and contracts with temperature changes. If bead or pelleted catalyst is used, preventative maintenance is periodically required to replenish and repack the catalyst beds.
Catalyst testing/test cores. The oxidizer vendor should be offering a test-core program where samples can be removed from the oxidizer and returned to the manufacturer every six months to be tested for destruction efficiency and possible masking or poisoning agents. A report should be generated for the customer and kept for monitoring catalyst activity.
Catalyst inlet/outlet temperature requirements. Low catalyst temperature set point requirements have two obvious advantages: the lower the set point, the lower the operating costs; and a lower temperature set point allows the oxidizer to operate at higher solvent load levels without risking an over temperature shutdown situation.
Maximum oxidizer operating temperature capabilities. Maximum temperature capabilities are determined by the construction materials and the catalyst type. The higher the oxidizer temperature capabilities, the higher the solvent load that can be treated without an over temperature situation.
Primary heat exchanger design and construction materials. The primary heat exchanger built into the oxidizer usually accounts for the single highest cost of any component used. A high-quality heat exchanger is very important if you’re choosing a unit with long life expectancies. The heat exchangers typically found in higher quality catalytic oxidizers are manufactured of #304 stainless steel with continuously welded seams and rolled expansion joints.
Heat exchanger efficiency. Obviously, the higher the heat exchanger efficiency, the lower the operating costs. However, when efficiencies get above 70 to 75 percent, the cost of the heat exchanger itself can become very expensive. If you have an application where high heat exchanger efficiencies could be used, ask the vendor to do a payback analysis to determine what efficiency is best suited to your application.
Internal insulation and external surface temperatures. The amount and type of internal insulation will determine heat loss through the shell of an oxidizer. A sufficient amount of insulation should be used to minimize heat loss, thus reducing operating costs, and also to reduce outside skin temperatures for obvious safety reasons.
Construction materials. The type and thickness of materials used in manufacturing the oxidizer will have a major impact on the life expectancy. The internals of higher quality catalytic oxidizers are typically manufactured of 12 and 14-gauge
#304 stainless steel. The exteriors are manufactured of either 14-gauge carbon steel and painted with a suitable outdoor corrosion-resistant paint or aluminum cladding, which does not require paint. The internal components and the equipment skid are manufactured of heavyweight structural steel.
Access doors to the inside of the unit. A catalytic oxidizer should not require much maintenance, but when the need arises, there should be enough access doors so that every major component can be reached inside the unit. The access doors should be designed in a way that they are large enough for an average size man to enter and also seal tight without the use of many bolts.
Burner manufacture and design. The burner/fuel train assembly must be designed to comply with all necessary NFPA regulations as well as those that may be set forth by the customer’s insurance carrier. The burner (maximum btu/hr) should be sized in such a way that it could maintain the oxidizer’s set-point temperature at a full exhaust rate with no solvents present in the air stream.
Gas pressure requirements. The oxidizer’s gas pressure requirements could determine whether an expensive gas booster or even a new gas service is required. Check the pressure requirements against available pressure.
Fuel train installation. The oxidizer manufacturer should have the ability to pre-pipe, pre-wire and install the fuel train at the factory. This can save both time and installation costs on the job site.
Thermocouple number and locations. Thermocouples are used to control and monitor temperatures at strategic points within the oxidizer. If you are required to record temperatures with a chart recorder, make sure the vendor installs dual element thermocouples at the catalyst inlet, the catalyst outlet and the exhaust stack.
Start-up time from cold start. If the burner is properly sized, the unit can heat to temperature within minutes from a cold start. However, to increase the unit’s life expectancy, it is very important to control the thermal expansion rate. Most quality oxidizers will have a built-in temperature ramp controller that restricts the change in temperature allowed per minute and brings the unit up to temperature slowly. Typically a 30-minute warm-up period is sufficient to extend the unit’s life.
Airflow volumetric control. Depending upon the size (volume) of the oxidizer and the number of processes connected to it, the airflow volume should be controlled either by means of a volumetric control damper or with an alternating current (AC) variable-speed drive connected to the supply fan motor. Either volume control system allows the oxidizer a method of turndown to treat only the air volume as required by the process at any given time. As the airflow from the processes fluctuate, so should the air volume being treated by the oxidizer. A volume-control system will save fuel and reduce operating costs. The AC drive unit will save electricity and further reduce operating costs.
Airflow dampers/actuators provided. Manual balancing dampers (one-time setup) should be strategically placed at various locations throughout the system. Control dampers with
Automated actuators should be installed to allow the oxidizer to purge and idle. Bypass dampers allow the process to purge to atmosphere as necessary. If an AC drive is not used for volume control, a damper with an automated actuator would also be used for volumetric control. If it is an outdoor installation, electric actuators are preferred even though they can be more costly.
Supply and control voltage requirements. The oxidizer’s fan motors will operate at a different voltage than the control circuits. The equipment manufacturer should install a transformer to alter the control voltage inside the control cabinet. This will eliminate the need and additional cost of separate power supply wiring in the oxidizer cabinet.
Method of operation/ operator interface/start-up. The control panel should be designed in a way that the operator can understand what’s happening with the oxidizer at a glance. A PLC based control system with an electronic operator interface panel is easier to operate and can help with troubleshooting. Where appropriate, a relay-based system offers a lower cost alternative. Ideally, the oxidizer should offer a fully automated start-up procedure that requires minimal operator interface. A start-up procedure as simple as pushing one or two buttons can be easily achieved.
Temperature recorder. A continuous temperature recorder to document the catalyst inlet temperature, catalyst output temperature and stack temperature should be an integral part of the control panel. Many recorder types are available, but a design that offers 30-day recording media (paper or electronic) will help simplify record keeping.
Number of pieces for site assembly. Catalytic oxidizers with air flows of up to 30,000 SCFM can be completely manufactured in the factory, tested, and shipped to the job site in one major piece. This has some advantages as the manufacturing quality can be controlled better in the plant. It also minimizes site installation time and costs.
Installation time. Review the vendor’s projected schedule for installation and try to plan your production schedule accordingly. A typical catalytic oxidizer installation should take no more than one to three weeks depending upon the size of the job.
Operator/maintenance manuals. Ask for at least two sets of operator and maintenance manuals with drawings, then keep one set in a safe place. You should also receive cut sheets for all major components. The manuals should be shipped before or with the oxidizer so they are available for installation, start-up and training.
Start-up and training included. It is important that a customer utilizes factory trained service technicians to start-up the equipment and to provide operator training. Make sure the vendors include the costs of start-up service in your proposal. Typically, if an oxidizer start-up takes more than five to seven days, the service technician is troubleshooting factory problems that may not be specific to your project.
F.I.D. testing at start-up. Qualified vendors can include the cost of performing an F.I.D. destruction test at the time of start-up. The test will provide immediate results so the customer knows that the vendor met the VOC destruction requirements before the installation is even considered complete.
Spare parts package. A limited spare parts package should be purchased with the equipment. Some parts should be considered consumables and others should be stocked for emergency situations. Have the vendors include spare parts with their proposal.
Cost of freight. The vendor can include the cost of all freight to the job site. This should include all shipments including the equipment, ductwork and spare parts that may be shipped separately.
Cost of turnkey installation. If you are seeking a full turnkey installation, ask the vendors to offer a firm turnkey price for the entire project. Special effort will be required on your part to understand the offering and to be sure that all aspects of the job have actually been included. If you receive a firm turnkey price, hold the vendor to it.
Check references. Ask for vendor references and call the contacts listed. Ask about problems that may have occurred at the job site. Installing an oxidizer can be a complicated project and problems can sometimes surface. You can often judge a vendor on how they solved these problems and how they reacted to different situations as they arose.
It can be difficult to make a fair comparison between one catalytic oxidizer manufacturer’s proposal and another’s. However, if a company intends to purchase a catalytic oxidizer that will provide many years of trouble-free service, one must understand each major component and its function.