Each year, new federal and state environmental regulations require higher destruction
and improved capture of volatile organic compounds (VOCs). The coating industry
and VOC control equipment manufacturers must develop technologies to meet these
regulations while providing improvements in both capital investment and operating
costs. A variety of auxiliary pollution control approaches are currently available.
It is important to understand the features, benefits and potential drawbacks
of the different types.
Solvent recovery. Adsorption is defined as the concentration of a substance (adsorbate) into the surface of a solid (adsorbent). The basic principles of adsorption are applied to air emission control when a solventladen air stream passes through a bed of high-surface-area solids material (typically carbon,
alumina, silica gel or molecular sieve) and is “captured” or adsorbed
into the surface of the material. Once the micropores of the adsorbent material
are filled to capacity, the process stream is then directed to another bed of
adsorbent while the original bed is “desorbed.”
Desorption, also referred to as regeneration, is a secondary process whereby
solvent is removed from the adsorbent by passing high-temperature steam or other
gases through it. The highly concentrated solvent air stream then goes through
a condenser and a separation or distillation column where the solvent is separated
or recovered from the air stream condensate.
Characteristics:
- High capital investment
- Moderate operating cost
- 95-98% removal/destruction efficiency
- Maintenance intensive (steam, condensing and distillation systems, activated
carbon material)
- High maintenance cost (replacement or regeneration of carbon material)
- Additional refining necessary if using multiple solvents
- Wastewater quality must be monitored for disposal/treatment purposes
- Potential of solvent to be reused or sold
|
| Schematic of new type of RTO |
Thermal oxidation. Thermal oxidizers are typically used to convert hydrocarbons
into carbon dioxide (CO2) and water (H2O) through oxidation. This process is
carried out by raising the temperature of the process exhaust to break the hydrogencarbon bonds, which allows new bonds to form, creating CO2 and H2O. When this occurs, an exothermic reaction takes place, and heat is released.
Oxidizers typically are designed with a 0.5 sec or greater total residence time.
Residence time is critical, not for the oxidation reaction, but for proper mixing.
Many designs fail to complete the mixing in the time available. In this instance,
to meet a performance specification, more fuel must be burned than would be
needed if mixing were complete.
Nitrogen oxides (NOx), products of combustion, increase as temperature and fuel
inputs increase. NOx output should be considered because it is a primary component
in the production of ozone.
Regenerative thermal oxidizer. Regenerative systems (see Figure 1) are thermal
oxidizers that operate at high temperatures, up to 1,500-1,800F. These systems
use a ceramic stoneware or other heat exchange media. In most designs, the media
is mounted in vertical columns. The process air stream is passed through a column
of media as it enters the oxidizer. The air stream is heated further to oxidation
temperatures by a burner in the central combustion chamber. The air stream then
exits the oxidizer through a second media column. The second column “stores”
energy from the hot air stream. By constantly cycling the air stream between
the columns, the incoming air stream is heated by the media, which in the previous
cycle was absorbing heat from the air stream exiting the central chamber. As
this column loses heat to the incoming air stream, it cycles and becomes the
receptor of heat, repeating the cycle. The theory is quite simple and has proven
to be successful in many applications. Regenerative thermal oxidizers can be
designed with one, two, three or more columns.
|
| 1. Schematic of regenerative thermal oxidizer. |
Characteristics:
- Moderate capital/installation costs
- High thermal efficiencies
- Low operating cost
- Moderate to high destruction efficiencies (95-99%)
- Higher radiation losses due to system surface area
- Best for low concentrations of VOCs in the process exhaust with high-volume
exhaust flows
- Heavy weight requires additional installation support
Recuperative thermal oxidizers. There are a number of recuperative thermal
oxidizer designs that can be used to destroy VOCs. The diagram of the thermal
oxidizer shown in Figure 2 is a simple design that passes air through an airtoair
heat exchanger to preheat it before entering the burner chamber. In the burner
chamber, the process exhaust air is heated to a sufficiently high temperature
and held at this temperature with some degree of turbulence to ensure VOC destruction.
Thermal oxidizers clean process air streams by burning or oxidizing them at
high temperatures. Typical VOC reduction is 99%. Carbon monoxide (CO) can be
either created by the partial oxidation of VOCs or destroyed by the complete
oxidation of CO to CO2 in a thermal oxidizer, depending upon operating temperatures.
CO production tends to rise with increasing temperature until it reaches a maximum
at about 1,200F. Above this temperature, the CO content tends to decrease rapidly.
At 1,400F, CO emissions from thermal oxidizers are relatively low.
Characteristics:
- Moderate to high capital/installation costs
- High operating cost at low solvent loading
- Destruction efficiencies can be very high (greater than 99%)
- Need high operating temperatures (1,380-1,500F) to get low CO levels
- NOx formation, especially at operating temperature above 1500F
- High exhaust gas temperatures (secondary energy recovery is possible
and high-temperature exhaust stack construction is required)
- Quality materials of construction necessary for longevity
Catalytic oxidation. Catalytic oxidizers (see Figure 3) are an alternative
to thermal oxidizers for oxidizing gaseous, combustible contaminants into carbon
dioxide and water. Their successful operation is limited to a more restricted
range of applications than thermal oxidizers. But, where applicable, catalytic
units offer the potential of significantly lower fuel consumption and operating
costs plus reduced CO and NOx emissions. The basic elements of the catalytic
unit are a preheat/mixing section, designed to achieve a uniformly preheated
and distributed waste stream flow, and the catalyst bed or catalyst matrix,
where the majority of the oxidation reactions take place.
|
| 2. Schematic of recuperative thermal oxidizer |
The oxidation of most hydrocarbons and carbon monoxide with catalysts occurs
rapidly in the range of 300-900F. With thermal oxidizers, the oxidation reaction
requires a high temperature, 1,200-1,600F, to break the carbon, hydrogen and
oxygen bonds.
Besides reduced energy consumption, NOx emissions from catalytic units are very
low because of the lower oxidation temperatures as well as the lower burner
firing rates. In addition, the oxidizing nature of the catalyst results in very
low CO emissions. However, there are tradeoffs involved to gain these advantages.
Catalytic oxidizers can be subject to masking agents or poisons that inhibit
the effectiveness of the catalyst.
Catalytic systems are limited to applications in which the waste stream has
negligible particulate loading and/or “poisons” that can reduce the
effectiveness of the catalyst. These poisons are primarily silicon and phosphorus,
which coat the catalyst; halogens, such as chlorine, which directly attack the
active metals converting them to an inactive form; and sulfur, which inhibits
the activity of some catalysts. The oxidation activity of the catalyst can also
be reduced by the loss of active components through attrition, deposition of
unreacted VOCs (coking) onto the catalyst surfaces or sintering of the catalyst
(collapse of the catalyst structure caused by high temperatures).
Regenerative catalytic oxidizers. This oxidizer is similar in design to the
regenerative thermal oxidizer. The addition of catalyst to either the media
or the top of the media column allows lower operating temperatures. Depending
on component design, this unit could be operated as a thermal oxidizer after
catalyst degradation.
Characteristics:
Moderate to high capital/installation costs
High thermal efficiencies
Low operating costs
Very low levels of CO emission
Very little or no NOx formation
Best for low-VOC, high air flow applications
New developments. This section discusses some recent developments in catalytic
and regenerative thermal oxidation technology.
Catalytic oxidizers. Noble metal catalysts require inlet temperatures above
600F to achieve more than 98% VOC conversion. Given that the maximum exhaust
temperature is 200300F, the exhaust gases from these applications need to be
heated to the desired operating temperatures to generate significant supplemental
fuel use. A catalyst that functions at a lower temperature would lower the operating
cost by reducing the need to heat the process system.
A catalyst formulation has been developed that reduces the lightoff temperature
for oxygenated hydrocarbons. Figure 4 compares the lightoff curves for this
catalyst with a commercially available noble metal catalyst. Reactors with the
new catalyst can operate at temperatures 100-200F less than reactors containing
conventional catalysts.
|
| 3. Schematic of catalytic oxidizer |
Regenerative thermal oxidizers. RTO systems have emerged as the leading technology
because of their high heat recovery, which produces an outstanding operating
cost advantage in comparison to other technologies while maintaining high flexibility
in the types of processes that can be treated. Typically these systems are very
large, requiring expensive installation work. Development efforts have focused
on a compact, modular and cost-effective RTO for the low flow VOC control market.
There are some distinguishing features of these RTOs (see Figure 4):
- Oxidizers are designed such that flow-directing poppet valves are incorporated
into the oxidizer shell to minimize the oxidizer footprint and fabrication costs.
This feature eliminates the transition ducts that have been associated with
traditional designs and further minimizes the field installation.
- Oxidizers are modular and designed to ship as completed assemblies with
width dimensions that will comply with commercial trucking equipment without
special shipping considerations or permits. These oxidizers are completely preassembled,
including all control panels, piping, conduit and electrical wiring. This preassembly
minimizes the installation effort and maximizes quality control over the finished
product.
- Oxidizers undergo simulated run conditions, control systems check out
and calibration prior to shipment to facilitate shorter startup times.
- Oxidizers feature stateoftheart controls, including modems to enable
the use of telemetry. Through the use of telemetry, product support and oxidizer
uptime is maximized while field service requirements are minimized.
- Oxidizers are designed with minimal entrained volume for a design destruction
efficiency goal of 98% without capture of the VOCs exhausted during the poppet
valve switch.
- All oxidizers are designed to accept optional VOC capture vessels when
destruction efficiency requirements exceed 98%. The design will permit oxidizers
to be upgraded in the field if the end user requires.
Choosing the right technology. Which of these new technologies is best for a
specific facility? The answer can be difficult or easy, depending on the approach
you take in the evaluation process. The best approach is to find vendors that
offer the best technology available based on your production requirements.
Find a supplier that offers multiple control methods and works with you to determine
which technology is best for your requirements. This way you can be sure you
are purchasing the best system for your needs. For the vendor to assist you
in making the correct selection, you first need to accumulate some critical
data. The minimum data required for proper technology selection are the following:
- Number of sources;
- Uptime and annual hours for each source;
- Flow rates from each source (scfm);
- Typical VOC loading from each source (lb/hr);
- Composition of the process stream (such as VOCs and catalyst poisons)
used in your facility;
- Utility costs;
- Facility layout, identifying sources, utility connections and special
limitations that could affect location of the oxidizer; and
- Regulatory requirements for your facility (make sure you comply with
federal, state and local regulations).
With the data, a request for quotation (RFQ) can be sent to a number of qualified
vendors. A vendor should have the appropriate technologies in its product mix
and the financial ability to stand behind what it offers.
Guidelines for choosing. When evaluating the options, include operating, installation
and capital costs. Capital and operating costs should be based on the actual
utility costs, uptimes and annual operating cycles of the facility. It might
be best to develop a chart listing flows and annual hours for each source.
|
| 4. Comparison of light-off curves. |
Request a spreadsheet showing the features for the proposed system to ensure
you are making an apples-to-apples comparison. Most reputable vendors should
be able to provide this as part of their evaluation.
Request delivery lead times for the system to ensure that the facility can meet
the regulatory timetable.
Request references from the vendor and contact them to determine the accuracy
of the vendor’s claims. The references should be similar in size and scope
to your facility.
Vendors can determine which technology will best fit your needs and regulations.
While a certain system may appear to be the ideal system for your needs, the
expertise of reputable vendors that offer a broad range of products should help
you decide between a low temperature catalytic oxidizer, a regenerative thermal
oxidizer or another system. Local regulatory requirements, product limitations,
uptime, utility costs or facility limitations may all play an important role
in determining which technology is best for your facility. Working with a reputable
vendor and supplying proper data in the RFQ can help you select the best technology
for your company.
Schematic of new type of RTO
- Schematic of regenerative thermal oxidizer
- Schematic of recuperative thermal oxidizer
- Schematic of catalytic oxidizer
- Comparison of light-off curves
