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Commercial Biofilter Applied to an Optic
Lens Manufacturer to Abate VOCs
by
Scot Standefer & Ray Willingham - PPC Biofilter,
Longview, TX
Rod Dahlstrom - OccuHealth, Inc., Mansfield, MA
ABSTRACT
Volatile Organic Compound (VOC) emissions are generated from a proprietary
coating process of optic lenses at a major optic lens production facility
in central Massachusetts. This facility required air pollution control (APC)
to significantly reduce VOC emissions and assure compliance with Massachusetts
Department of Environmental Protection (MADEP) facility wide emission limitations.
Air pollution control was also required to maintain Clean Air Act "minor
source" designation. VOC abatement alternatives were reviewed and biofiltration
technology was selected to achieve these goals. This resulted in the installation
of a Biofilter to reduce solvent emissions from a combined 4,500 cubic feet
per minute (cfm) (7650 m3/hr) exhaust.
The following paper provides information on the first Biofilter installation
in the optic lens industry, including economic issues, system design and performance
data.
INTRODUCTION
A leading optic lens manufacturer, producing polycarbonate lenses for corrective
eye wear generates VOCs from coating processes. The lenses are produced in
stock form for the correction of near and far sightedness, astigmatism, etc.
and delivered to retail outlets throughout the world. The attraction of polycarbonate
is it's light weight, particularly for heavy correction. To increase durability
and prevent the lenses from scratching, they are coated in the production
process. The main lens production facility is located in central Massachusetts.
This production facility emits VOCs as a result of coating
the lenses and solvent vaporization in the lens curing ovens. Coating the
lens is a crucial step in the production process and the coating mixture is
proprietary and highly protected. Consequently, the exact VOC emissions can
not be itemized in this paper. However, the general classifications of the
VOCs are alcohols and ketones. The production facility releases 4,500 cfm
(7650 m3/hr) of solvent laden air resulting in a peak emissions rate of 3.4
pounds per hour (1.55 kg/hr) VOCs. While this is a relatively small emissions
rate, the goal was to assure that Hazardous Air Pollutant (HAP) emissions
are less than the major source limitation of 10 tons per year(tpy) (9000 kg/yr)
for a single HAP and that total VOC emissions are less than MADEP facility
wide emission limitations. It was concluded that 90% removal of total VOCs
would reach this goal.
VOC ABATEMENT ECONOMICS
While many of the solvents in the coating process are water soluble, scrubbing
the solvents would result in a substantial amount of water consumption and
discharge. The facility is restricted in both water consumption and discharge
by the local water municipality, which eliminated scrubbing as an option.
An activated carbon system posed problems because of the ketones in the exhaust
and potentially high regeneration and replacement costs.
Thermal oxidation was also considered, however, the low fuel
value (~120 ppm) of the VOCs in the gas stream and the high cost of natural
gas ($0.55-$0.70/therm) would result in excessive operating costs to treat
this exhaust. Operating costs for thermal oxidation were projected to be approximately
$7.00/hr. This particular air district is also restricted on NOx emissions
which, when coupled with CO emissions, added additional complications to implementing
thermal oxidation as a control option.
The temperature and chemical composition of this exhaust make
this an ideal candidate for biofiltration. The relatively low concentration
of highly soluble, biologically active compounds results in a biofilter with
relatively low gas residence time. This resulted in a biofiltration system
with a relatively low capital cost. Since the bacteria metabolize the VOCs
to CO2 and H2O through natural respiration processes, the only power consumers
in the system are a fan and pump, resulting in an operating cost of only $0.55/hour.
This results in a savings of $56,000/year when compared to a thermal oxidizer.
Unlike the thermal oxidizer, the biofilter produces neither NOX or CO emissions.
The biofilter also produces 1/10th the CO2 produced by a thermal oxidizer.
When all these factors were considered, it was apparent that the biofilter
was not only the most economical alternative, but the most environmentally
friendly alternative as well.
COMMERCIAL BIOFILTRATION SYSTEM
In March of 1997, PPC Biofilter was contracted to build a commercial Biofiltration
system to treat 4,500 cfm (7650 m3/hr) containing 3.4 lb/hr (1.55 kg/hr) of
coating solvents. The system was designed to achieve a minimum removal efficiency
of 90%. PPC Biofilter did not conduct a pilot unit test prior to commercial
design.
The site was limited in available space for pollution abatement
equipment and existing construction limited the ability to build on site.
PPC designed a Biofiltration system to be fabricated in their shop in Longview,
Texas, transported to the site and set with the assistance of a crane. The
Biofiltration system included a pre-treatment humidifier and Biofilter fabricated
in one 11' wide by 12' tall by 44' long (3.35 m x 3.66 m x 13.41 m) vessel.
The humidifier and Biofilter were separated by a common wall. The Biofilter
vessel was set on elevated, structural beams with the induced draft (ID) fan
(negative pressure) installed underneath the vessel and terminating into a
33' (10 m) discharge stack.
HUMIDIFIER - The Humidifier was designed with an 18"
(45.7 cm) diameter co-current quench duct and a counter current packed tower
using 100 cubic feet (2.8 m3) of 2" (5 cm) pall rings. A 5 HP (3.73 kW)
pump recirculates 200 g.p.m (45.4 m3/hr). of water through the quench duct
and over the packing to raise the relative humidity of the gas stream to 95%
+. The function of the humidifier is to prevent drying of the filter media
in the Biofilter. Saturating the gas stream prior to entering the Biofilter
eliminates wide fluctuations in the filter media moisture content and therefore
provides more consistent operation and removal efficiencies. A chevron blade
demister was installed on the humidifier exhaust to prevent water droplet
carryover on to the filter media.
Water in the humidifier evaporates into the exhaust gas increasing the humidity
of the process gas stream. This continuous evaporation can result in mineral
salts accumulating in the recirculation water creating scaling and equipment
fouling unless a blowdown is employed. The facility is limited both on water
consumption and discharge which limits the amount of humidifier blowdown.
To solve both of these competing objectives, the humidifier blow down was
reduced to a bare minimum.
BIOFILTER - The Biofilter represents a vessel with dimensions
of 40' long by 10' wide by 12' tall (12.2 m x 3.05 m x 3.66 m). 2,050 cubic
feet (58 m3) of proprietary filter media was installed to a height of 5' (1.5
m) on elevated air distribution grids providing plenums above and below the
filter bed for air distribution. The Biofilter provides a 27 second empty
bed gas residence time and operates at a velocity of 11.25 feet per minute
(fpm) (0.06 m/s).
The system was designed for down flow operation with the humidifier
exhausting above the filter media and the Biofilter exhausting out of the
bottom of the vessel. Down flow operation is beneficial for consistent operations.
Removal efficiency is a direct relationship to the bacteria populations in
the filter media. The bacteria populations are a direct function of media
moisture content. When the media dries out, the bacteria populations decline
as does the removal efficiency. Maintaining a stable moisture content provides
a stable biomass population and therefore stable removal efficiency. When
the VOCs contact the filter media, the biological oxidation begins to take
place. The biological oxidation process is an exothermic reaction which generates
heat and dehydrates the filter media. A high percentage (50-70%) of the overall
removal occurs within the initial 14"-18"(35-45 cm) of media contact.
Therefore, in an up-flow system, the media at the bottom of the filter is
experiencing excessive drying. The only way to replenish moisture content
is by adding water to the top of the media where it trickles down to areas
in the bottom of the filter which are dehydrated. Many times this results
in over saturating the media which leads to excessive biomass growth and associated
pressure drop problems. A down flow design allows water to be added to the
top of the filter media where most of the drying activity occurs and is needed
most.
Twenty seven fine mist nozzles were installed above the filter
media to provide complete and uniform water distribution to the filter media.
Moisture control is provided by a programmed logic controller (PLC) timer
and monitored with the assistance of a load cell placed under a floating grid
which supports the filter media. The control logic activates solenoid valves
to supply water to the spray nozzles.
Two filter media loading hatches are provided in the top of
the vessel and two media removal hatches are provided at grid level on the
side of the unit. One man way is installed in the Humidifier, and the Biofilter
has one man way above and below the filter media. The 15 HP (20 kW) fan is
installed underneath the vessel to save on footprint area. The fan is oversized
to accommodate fluctuating air flows between 2,500 cfm (4,250 m3/hr) and 5,000
cfm (8,500 m3/hr). A variable frequency drive was installed to save on electricity
consumption when the full capacity of the fan is not required. The fan discharges
into a 33' (10 m) high stack.
Temperature is an important factor in the Biofilter design.
The design temperature for mesophilic bacteria is 55 F to 105 F (13 C - 40
C) . With metabolic activity doubling approximately every 12 F (7 C), a higher
operating temperature can increase the system performance. However, elevated
temperatures can also affect the physical properties of the VOCs, inhibiting
mass transfer from gas to biofilm. Operating at too low of a temperature for
some compounds can affect the removal performance of the Biofilter. This exhaust
has a dry bulb temperature of 110 F (43 C), which is above the biological
design parameter. However, it is a relatively dry exhaust (6% relative humidity)
and after adiabatic cooling in the humidifier, the Biofilter operating temperatures
fluctuate between 60 and 75 F (15-24 C). This geographic location also experiences
some very cold winter months subjecting the Biofilter to ambient temperatures
of 15-25 F (-9 - -4 C). A saturated 70 F (21 C) exhaust contacting external
vessel temperatures of 20 F (-7 C) can create condensation in the Biofilter
which can result in over saturating the filter media, eventually leading to
air flow and pressure drop problems. Consequently, the entire Biofilter vessel
is covered with insulation and then covered with aluminum lagging. An immersion
heater is provided in the sump of the humidifier to prevent freezing during
shut down periods.
The system is controlled with an Allen Bradley PLC with real
time display and data logging using a PC operating with Intellution MMI software.
Start and Stop function are provided for the fan and pump. A flow switch monitors
the humidifier recirculation with the control system providing an alarm and
aborting power to the fan and opening the bypass damper in the event of a
pump failure which could dry out the filter media. A thermocouple monitors
the inlet temperature to the Biofilter with the control system providing an
alarm and aborting power to the fan and opening a bypass damper in the event
the operating temperature exceeds 105 F (40 C) for 30 minutes or immediately
if the temperature exceeds 115 F (46 C). Differential pressure transmitters
measure the pressure drop over the humidifier and the filter media with the
control system providing alarms if operations are above normal conditions.
All pertinent information is data logged including alarm history.
SYSTEM PERFORMANCE
The system was placed into operation on September 4, 1997. Removal Efficiency
tests are performed by measuring the concentration in the inlet duct to the
humidifier and the concentration in the exhaust stack. The Biofilter is tested
on a periodic basis with the use of Drager tubes to insure performance. These
test results have a relatively low accuracy but consistently show removal
in excess of 96% of the target VOCs.
Independent third party test results have been conducted using
EPA method 25A. Removal efficiency of target hydrocarbons using FID are reported
at 91%. The FID has a poor response to oxygenated compounds thereby understating
the inlet concentration (numerator) and understating the overall removal efficiency.
Independent GC/FID tests show removal efficiencies on the alcohols of 93%,
and 82% on the ketones. A removal efficiency of 97% was recorded for the major
HAP constituent in this off gas stream.
CONCLUSIONS
Biofiltration provided a cost effective, economic approach to pollution abatement
with significantly lower cost per ton treated than alternative Air Pollution
Control (APC) technologies.
Biofiltration provides many advantages to other APC technologies. Aside from
the direct cost to the customer, there is an intermediate national cost in
natural resources including, but not limited to, the expense of drilling for
natural gas, generating electricity (coal), and the associated pollution that
these activities generate. Biofiltration avoids these cost with a natural
solution. Pressure drop through the system is lower than catalytic or regenerative
thermal oxidation which saves on electricity consumption. Oxidation occurs
at ambient temperatures with the use of naturally occurring bacteria eliminating
the need for natural gas consumption in incineration. Because no supplementary
fuel is required, the biofilter produces substantially less CO2 reducing greenhouse
gas emissions from the APC equipment. Biofiltration is a destructive technology,
oxidizing the compounds to CO2 and H2O eliminating the regeneration cost of
activated carbon and the water treatment cost of scrubbing.. The Biofilter
does not create secondary pollutants such as NOx compounds or CO.
Biofiltration was selected to achieve both high VOC removals
(90%+) and reap additional economic benefits from the technology.
Unfortunately, many U.S. industries coming under new Federal MACT guidelines
may not be so fortunate. Some industries will be required to meet very high
(98%+) percentage removals which may not be attainable on a consistent basis
with Biofiltration. So the question becomes, are we better off at 90% removal
with Biofiltration or 98% with Regenerative Thermal Oxidation (RTO): understanding
that the RTO has 2-3 times the electrical requirements, requires natural gas
and produces secondary NOx and CO emissions as well as much higher levels
of CO2.
For more information, contact Scot Standefer, president of PPC Biofilter,
3000 E. Marshall Avenue, Longview, TX 75601; phone (903) 758-3395; fax (903)
758-6487.
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