A Breath of Fresh Air for Your Oxidizer System
As the full effects of the Federal Clean Air Act (CAA) Amendments continue to impact the manufacturing industry, many processors will be required to install air pollution control equipment. With more than 25 years of proven success, catalytic oxidation continues to be a popular method for controlling volatile organic compound (VOC) emissions.
According to the Clean Air Act, any organic compound that contains the element carbon (excluding carbon monoxide, carbon dioxide, carbonic acid and methane) is a VOC and, at various emission levels, must be controlled. VOCs consist of a range of organic compounds such as aliphatic and aromatic hydrocarbons, alcohols, ketones, esters, ethers, formaldehyde and many others.
Emissions containing VOCs from stationary sources contribute to the air quality problems that adversely affect public health and welfare. Uncontrolled VOC emissions combine with NOX and, in the presence of ultraviolet radiation, produce ground-level ozone. Certain VOCs now are also classified as air toxics or hazardous air pollutants (HAPs) that pose special health risks.
How Does a Catalytic Oxidizer Work?A catalytic oxidizer destroys harmful VOCs and HAPs at temperatures ranging from 500 to 650oF (260 to 343oC). A precious metal industrial-grade catalyst is used to promote this oxidation when the process exhaust fumes are passed through the heated catalyst chamber. An energy-efficient catalytic oxidizer will utilize a high efficiency, air-to-air heat exchanger to preheat the incoming exhaust fumes, thus further reducing the unit's operating costs.
Some of the advantages that a catalytic oxidizer has over other technologies include: low operating costs, low capital equipment costs, high VOC/HAP destruction efficiency, long life expectancy due to low operating temperatures, quick startup/warmup times and minimal maintenance requirements. In addition, catalytic units are preassembled and tested at the factory to minimize site installation costs.
In many areas, the local air quality regulators will not allow a manufacturing facility to operate certain types of machinery if the oxidizer is not operating properly.
When a decision is made to purchase a catalytic oxidizer, the purchaser should take the same approach in selecting a pollution control system as when purchasing more sophisticated production equipment. That is, very specific oxidizer design details should be gathered and evaluated. 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 typically are made by relying on the vendor's credibility and the assembled design data.
The following is a comprehensive list of catalytic oxidizer design specifications, major components and related vendor services, along with a brief explanation 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 the maximum exhaust rate but should include provisions for sufficient turndown to idle and operate any one process independently. Typical volumetric turndown ratios range from 4:1 up to 6:1.
Catalyst Type and Design. Various metal catalyst types are available. Precious-metal catalysts can be of either the monolith or bead-type design. Both have advantages and disadvantages depending upon the specific application. If you work with a qualified vendor that offers both catalyst types, the vendor can help you select which one is best suited to an application and explain why.
Catalyst Manufacturer. Again, there are many catalyst manufacturers, so it would be wise to verify that the oxidizer vendor is offering a catalyst from a proven source that will allow the end user to purchase replacement catalysts directly from the manufacturer when necessary.
Catalyst Warranty/VOC Destruction Efficiency. In many industries, a full three-year catalyst warranty has become common. A minimum of 98-plus percent VOC destruction efficiency can be achieved at low temperatures with minimal amounts of catalyst. A 99-plus percent VOC destruction efficiency usually can 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 catalysts and from seven to 10 years or more for precious-metal catalysts. 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 removed and cleaned by using a high-pressure air knife, soap and water wash, or an acid bath.
Catalyst Attrition Rate. The monolith catalyst has an advantage as there is no attrition rate. Bead and pelleted catalysts are 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, preventative maintenance is required periodically to replenish and repack the catalyst beds.
Catalyst Testing. The oxidizer vendor should offer a test-cores 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 setpoint requirements have two obvious advantages: The lower the setpoint, the lower the operating costs; and a lower temperature setpoint allows the oxidizer to operate at higher solvent load levels without risking an overtemperature shutdown situation.
Maximum Oxidizer Operating Temperature Capabilities. Maximum temperature capabilities are determined by the construction materials and catalyst type. The higher the oxidizer temperature capabilities, the higher the solvent load that can be treated without an overtemperature 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 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 percent to 75 percent, the cost of the heat exchanger itself can become expensive. In an application where high heat exchanger efficiencies could be used, ask the vendor to perform a payback analysis to determine what efficiency is best. Internal Insulation and External Surface Temperatures. The amount and type of internal insulation will determine heat loss through the oxidizer shell. A sufficient amount of insulation should be used to minimize heat loss, thus reducing operating costs.
Construction Materials. The type and thickness of materials used to manufacture the oxidizer will impact life expectancy. The internals of higher quality catalytic oxidizers typically are manufactured of 304 stainless steel. The exteriors are manufactured of either carbon steel and painted with a suitable outdoor corrosion-resistant paint or finished with 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 and local code. The burner (maximum BTU/hr) should be sized in such a way that it could maintain the oxidizer's setpoint temperature at a full exhaust rate with no solvents present in the airstream.
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 prepipe, prewire 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, catalyst outlet and exhaust stack.
Startup Time from Cold Start. If the burner is sized properly, the unit can heat to temperature within minutes from a cold start. However, to increase the unit's life expectancy, it is important to control the rate of thermal expansion. A built-in temperature ramp controller will restrict the change in temperature allowed per minute and brings the unit up to temperature slowly. Typically, a 30-min warmup period is sufficient to extend the unit's life.
Airflow Volumetric Control. Depending on 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. 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 placed strategically 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 would 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 Startup/Operator Interface. Look for a startup procedure that is automated and requires minimal operator interface. Also, the control panel should be designed in a way that the operator can understand at a glance what's happening with the oxidizer. Electrical troubleshooting can be made easier if the unit utilizes a programmable controller and message display panel.
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 look for a design that offers a 30-day chart as this simplifies record-keeping.
Number of Pieces for Site Assembly. Catalytic oxidizers with airflows of up to 30,000 scfm can be manufactured and tested completely in the factory and shipped to the job site in one major piece. Manufacturing quality can be controlled better in the plant, and this approach 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 two to four weeks depending upon the size of the job.
Operator/Maintenance Manuals. Ask for at least two sets of operator and maintenance manuals and keep one set in a safe place. Also, get two full sets of drawings and cut sheets for all major components. The manuals should be shipped before or with the oxidizer so they are available for installation, startup and training.
Startup and Training Included. It is important that a customer utilizes factory-trained service technicians to start up the equipment and provide operator training. Make sure the vendors include the costs of startup service in your proposal. If an oxidizer startup takes more than five to seven days, the service technician may be troubleshooting factory problems that are not specific to your project.
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 any equipment, ductwork and spare parts that may be shipped separately.
Cost of Turnkey Installation. If seeking a full turnkey installation, ask the vendors to offer a firm turnkey price for the entire project. Special efforts will be required on your part to understand the offering and to be sure that all aspects of the job actually have 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 sometimes surface. One can often judge a vendor on how its staff solved these problems and how they reacted to different situations as they arose.
If a company intends to purchase a catalytic oxidizer that will provide many years of trouble-free service, they must understand each major component and its function.