Problem
Emission control and the operating costs associated with meeting
environmental regulations were nothing new for the Coated Products
Division of Brady Corp. The company has been manufacturing coated
films for nearly 60 years, demonstrating over that time a
commitment to pollution prevention and emission reduction programs.
But these considerations were magnified when the Milwaukee coating
facility chose to implement a new, energy-efficient emission
control system. The division maintains 200 different coating
formulas on three main continuously operating process lines. Two of
the company's three coaters are operated as so-called "white rooms"
to allow the manufacturing of exceptionally clean products.
Applying adhesives, topcoats, cast films and other coatings onto a
range of substrates requires solvent-based coatings with mixtures
of chemicals. Some of the many solvents used in the process are:
toluene, MEK, MIBK, heptane, hexane, ethyl acetate, IPA,
nitroethane, nMP, cyclohexanone, and 1,3-dioxolane. The dynamic
process stream poses many challenges to the company, particularly
in that it eliminates the option of solvent recovery.
Action
The capital cost of emission control equipment can be negligible
compared to the operating costs if careful consideration is not
given to proper equipment selection. With natural gas prices
continuing to rise, the company has focused on getting the most
efficiency from its incineration equipment. Since the early 1990s,
the facility has spent millions of dollars on air pollution control
equipment to meet a variety of EPA regulations imposed on coating
companies. Including thermal recuperative and regenerative thermal
oxidizers (RTOs), as well as concentrator systems, the company has
purchased a total of 12 units with a 13th on order for the
Milwaukee location alone. The oxidizers have been used to treat
everything from coating emissions to low-point floor sweeps located
throughout the facility. As natural gas prices started rising in
the late 1990s, and with associated costs for emission control
equipment steadily increasing, the company looked for ways to
reduce their yearly operating and maintenance costs.
The operating and maintenance costs were overwhelming. When one of
the old electric RTOs would fail, it generally took over a week to
replace the cold face support grid and electric heating elements,
and then bring the unit back up to temperature. It occurred so
frequently that the maintenance department constructed a special
tent so the repairs could be done in the rain or snow. The thermal
recuperative oxidizers on site had so many problems with internal
heat exchanger failure that a roller system was installed just to
move the large duct transition, allowing access to weld the tube
sheet without bringing in a crane.
Solution
The decision was made to begin replacing the oldest and least
efficient oxidizers; the type of systems would be determined by the
maintenance team. The first phase of what plant personnel started
referring to as its "efficient emission control plan" would replace
one of the thermal recuperative oxidizers with a 35,000 SCFM RTO
from Anguil Environmental Systems Inc.
The new system tested out at a destruction efficiency rate of
99.2 percent, and was equipped with a hot-gas bypass that allowed
it to process VOCs at rates up to 850 lbs/hour. This high-capacity
VOC processing allowed some of the other less efficient oxidizers
to shift their load over to the new RTO through a unique common
manifold collection system. With the concentration of hydrocarbons
in the process air stream, the heat energy content of the VOCs was
self-sustained and the oxidation process required no additional
fuel for destruction.
RTO technology utilizes ceramic media in two or more beds as a
high-efficiency heat exchanger. Process gas with VOC contaminants
enters the RTO through an inlet manifold. A flow diverter valve
diverts the gas into an energy recovery chamber, which preheats the
process stream. The process gas and contaminants are progressively
heated by the ceramic bed as they move toward the combustion
chamber.
The VOCs are then oxidized, releasing energy that is transferred
to the second ceramic bed, thereby reducing any auxiliary fuel
requirement. Heat is transferred from the gas to the ceramic bed so
that the outlet gas temperature is only slightly higher than the
inlet temperature. A flow diverter valve switches, alternating the
ceramic beds so each is in inlet and outlet modes over time.
If the process gas contains sufficient VOCs, the energy released
from their combustion promotes self-sustained operations. For
example, at 95-percent thermal energy recovery, the outlet
temperature may be only 77°F higher than the inlet process gas
temperature.
The maintenance team investigated several types of RTO systems,
including a new rotary valve system. The rotary valves seemed to be
a viable option, but they were a close-tolerance proprietary item
that could only come from the specific vendor. The rotary valve
location underneath the RTO also presented major maintenance
concerns. The company went with Anguil's poppet valve design,
believing the maintenance levels were more satisfactory.
The company also was pleased that the RTO manufacturer was
willing to share its complete computer operating program, something
other vendors were not willing to do. Some of the other items on
the system included:
- A stairway for access to platforms rather than the usual
vertical ladders - a feature especially appreciated in Wisconsin
winters
- Replaceable valve seats on the poppet valves and large access
doors.
- Heavier gauge access doors with fewer bolts to be removed.
- Block-off plates after the system fan as required for confined
space entry.
The second stage of the plan would prove to be a little more
challenging, but even more effective in reducing the company's
operating costs. The EPA's requirement of a permanent total
enclosure, or PTE, required coaters to create a negative pressure
in all areas of the facility that process any volume of solvents.
Due to the layout of the coaters, this became a large volume of
exhaust air with very low VOC levels.
Two of the old electric RTOs were treating this high-volume,
low-concentration stream from pump rooms, wash-up areas,
compounding areas and floor sweeps located throughout the facility.
Large volumes of natural gas were consumed to burn a very small
amount of pollutants. In addition, the unit could not be turned off
during plant shutdowns because of time-consuming reheat procedure,
which could take up to four days.
After evaluating the solvent vapors and various concentrations,
an Anguil Model 350 (35,000 SCFM) rotor concentrator and Model 50
(5,000 SCFM) RTO were selected to handle this portion of the
process. By absorbing and concentrating the VOCs they were able to
achieve a 10-to-one concentration ratio, requiring an oxidizer only
a tenth the size to handle the concentrated process stream. The
energy contained in the concentrated stream entering the RTO proved
sufficient to allow self-sustaining operation, requiring little to
no auxiliary fuel.
The third but not final stage of the company's plan is still in
motion. They have placed an order for another 35,000 SCFM RTO to
replace the last thermal recuperative system on site. When this
system has been installed, heat from the RTO will be used to
preheat the facility's ovens, further reducing energy consumption.
The system will have enough capacity to eliminate the final thermal
recuperative unit and another aging electric RTO.
In addition to replacing old oxidation technologies at the
facility, careful consideration has been given to all the oxidizers
as a single system. The company has implemented a dual collection
and distribution manifold that allows operators to divert process
streams from one oxidizer to another for maintenance or equipment
shutdowns.
The impact of these efforts has exceeded expectations for
reliability and efficiency. Gas usage on the company's three
coating lines have continued to drop at a steady rate. At a time
when gas prices continue to trend high, coupled with increases in
production, the reduction in energy consumption drops straight to
the bottom line.
The company is continuing to investigate energy reduction
strategies, and is currently investigating the option of placing
secondary heat exchangers on all of its oxidizers. The process
would return waste heat to preheat the air streams on all of its
other coating lines.