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.
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| Many
catalytic oxidizers can be completely assembled before being shipped, minimizing
installation time and cost.
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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.
