Catalytic Oxidizers: What You Should Know

As the full effect of the Clean Air Act Amendments continues to impact the finishing industry, many different companies will be required to install air pollution control equipment. With more than 25 years of proven success, catalytic oxidation is a popular method for controlling 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 he 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 oxidizers. 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.

Operating

The oxidizer should be sized to handle your maximum exhaust rate but should have provisions for sufficient turndown to idle and to operate any one process independently. Typical volumetric ratios range from 4:1 to 6:1.

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 processes at any given time. As the airflow from the processes fluctuates, so should the air volume being treated by the oxidizer. A volume control system will save fuel and reduce operating costs. The AC drive will save electricity and further reduce operating costs.

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 also should be used for volumetric control. If it is an outdoor installation, electric damper actuators are preferred.

Maximum temperature capabilities are determined by the construction materials and the catalyst type. Low catalyst temperature set point requirements have two 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.

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 per minute allowed and brings the unit up to temperature slowly. Typically a 30-minute warm-up period is sufficient to extend the unit's life.

Look for a startup procedure that is fully automated and requires minimal operator interface. A startup procedure of pushing no more than one or two buttons can be easily achieved. The control panel should also be designed in a way that the operator can understand what's happening with the oxidizer with just a glance. Electrical troubleshooting can be made easier if the unit uses a programmable controller and a message display panel.

Catalysts

There are various types of catalysts available, however, the precious metal catalyst has proven to be the most versatile. Precious metal catalysts can be 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, it can help you select the one that is best suited to your application and explain why.

Depending on the type and amount of catalyst used, life expectancies can range from 3-4 years for manganese dioxide catalysts and from 7-10 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. In many industries, a full 3-year catalyst warranty has become very common. A minimum of 98%+ VOC destruction efficiency can be achieved at low temperatures with minimal amounts of catalyst. A 99%+ VOC destruction efficiency can usually be generated by designing the unit with additional catalyst.

If a monolith catalyst is used, it can be easily removed and cleaned by using a high-pressure air knife, 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 can't realistically be cleaned. The monolith catalyst has an advantage in that it has no attrition rate. All bead and pelleted catalysts are basically brittle by nature and will have a small percentage of catalyst destroyed as the unit expands and contracts with temperature changes. If bead or pelleted catalyst is used, preventive maintenance is periodically required to replenish and repack the catalyst beds.

The oxidizer vendor should be offering a test-core program where samples can be removed from the oxidizer and returned to the manufacturer every 6 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.

Heat recovery

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 No. 304 stainless steel with continuously welded seems and rolled expansion joints.

Obviously, the higher the heat exchanger efficiency, the lower the operating costs. However, when efficiencies get above 70-75%, the cost of the heat exchanger itself can become very expensive. If you have an application where high heat exchange efficiencies could be used, ask the vendor to do a payback analysis to determine what efficiency is best suited to your application.

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 safety reasons.

Construction

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 12- and 14-gage No. 304 stainless steel. The exteriors are either 14-gage 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.

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 seal tightly without the use of many bolts and be designed in a way that they are large enough for an average size man to enter.

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.
Catalytic oxidizers with airflows 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.

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 2-4 weeks depending upon the size of the job.

Burner/Fuel Train

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 a 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.

The oxidizer's gas pressure requirement could determine whether an expensive gas booster or even a new gas service is required. Check the pressure requirements against available pressure.

A continuous temperature recorder to document the catalyst inlet temperature, the catalyst output temperature and the stack temperature should be an integral part of the control panel. Many recorder types are available, but look for a design that offers a 30-day chart as this makes record keeping easier.

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.

Startup

Qualified vendors should perform a FID destruction test at the time of startup. 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.

Ask for at least two sets of operator and maintenance manuals, then keep one set in a safe place. You should also receive two full sets of drawings and cut sheets from all major components. The manuals should be shipped before or with the oxidizer so they are available for installation, startup and training.

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 startup takes more than 5-7 days, the service technician is troubleshooting factory problems that may not be specific to your project.

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, it must understand each major component and its function.