Problem
In September 1998 the EPA promulgated a ruling in 40 CFR,
imposing strict new standards to reduce emissions of toxic air
pollutants from the manufacture of pharmaceutical products,
including prescription and over-the-counter drugs. The agency's
rule was intended to reduce emissions of a number of air toxics and
hazardous air pollutants (HAPs), including methylene chloride,
methanol, toluene and HCI. It was estimated at the time that the
ruling would reduce air toxins annually by approximately 24,000
tons or 65 percent from contemporaneous levels. The affected
pharmaceutical manufacturing processes included chemical synthesis
(drawing a drug's active ingredient) and chemical formulation
(producing a drug in its final form).
Action
One of the approximately 100 facilities affected by the ruling
was a pharmaceutical plant in upstate New York. Determined to stay
below acceptable MACT levels, the company set out to establish
compliance needs and subsequent direction by contracting with a
consultant to formulate a compliance plan. The result was a
specification package that required oxidation and caustic scrubber
technologies.
Solution
Specifically, the design included a primary and backup system,
each consisting of two 35,000-SCFM regenerative thermal oxidizers
(RTOs) and two 35,000-SCFM caustic scrubber systems. The RTOs were
to process the emissions, including methylene chloride, acetone,
ethanol, isopropyl, alcohol, methanol and mineral spirits, and were
designed to achieve over 99-percent destruction efficiency. The
scrubbers were designed to process treated gases and were to
achieve 99.5-percent reduction of the HCI derived from the
oxidation process. After the design phase, the two technologies
were selected in order to oxidize the VOC/HAP compounds, and to
remove the resultant HCI emissions from the outlet of the oxidizer.
Upon receipt of the specification package, the engineering staff at
Anguil Environmental Systems Inc. began to design the system within
the boundaries of the specifications.
The specification package included the following requirements
and parameters:
- Process producing emissions - multiple (~60) sources
including:
- Conservation vents
- Reactor vents
RTO materials of construction
- RTO refractory selection: The RTO included the installation of
high-purity ceramic fiber insulation, because of its resistance to
HCI attack.
- Purification chamber outer skin: The purification chamber was
constructed of 0.25inch A36 Steel. It was internally blasted and
coated with a vinyl ester corrosion-resistant coating. This coating
resists any HCI (vapors or condensed acid) that could potentially
reach the RTO shell behind the insulation. The coatings were
pigmented so that the first coating was light gray and the second
coat was dark gray. This allowed a visual inspection to identify
that the coating was thoroughly applied during fabrication, and
provides an easy means of checking for coating degradation after
operation.
- Ceramic media support grid: The ceramic media support grid was
constructed from Hastelloy C276 in order to support the ceramic
media. This "cold face" has the potential too see condensed acid
gas. Hastelloy C276 provides high strength and resists both
chloride stress corrosion cracking and chloride pitting and crevice
corrosion.
- Inlet plenum: The inlet plenum is the duct located underneath
the RTO that connects to the three inlet butterfly diverter valves.
The air flowing through this duct to the RTO contains VOCs and HAPs
but does not contain acid gases. However, as a precaution against
corrosion, this ductwork was constructed of AL-6XN, a corrosion
resistant alloy.
- Outlet plenum: The outlet plenum is the duct located underneath
the RTO that connects to the three outlet butterfly diverter
valves. The air flowing through this duct to the RTO contains acid
gases. This ductwork was constructed of Hastelloy C276.
- Bed/plenum/hopper: The plenum beneath each of the three ceramic
media support grids was constructed of Hastelloy C276.
- Butterfly valves (bed inlet, outlet and purge): The three inlet
diverter valves process air containing VOCs and HAPs. The three
outlet diverter valves contain air containing acid gas. All sic
valves were constructed of Hastelloy C276. The three purge valves
were also constructed of Hastelloy C276.
- Transition to acid gas scrubber quench: The transition from the
RTO outlet plenum to the acid gas scrubber quench contains acid
gases. This ductwork was also constructed of Hastelloy C276.
Floor sweeps
- Waste stream flow rate: 6,500-35,000 SCFM
- Waste stream temperature: 50 to 100ºF
- VOC/HAP breakdown
The company possessed a long history treating halogenated
compounds, including oxidation and acid gas scrubbing equipment.
The two RTO/scrubber systems supplied here were actually installed
after a smaller oxidizer/scrubber was operational within the same
facility.
General operational description
Designing the oxidizer first, engineers specified that each RTO
would process up to 35,000 SCFM of VOC/HAP-laden air, providing
99.5 percent destruction efficiency.
The oxidizer consisted of three reinforced,
insulated, steel chambers filled with high-temperature, structured
ceramic energy recovery media. Each oxidizer would utilize two
burners to maintain its oxidation temperature set-point, and
provide even temperature distribution within the combustion chamber
for maximum VOC/HAP destruction. Located below each of the energy
recovery chambers would be inlet and outlet diverter valves and the
associated air duct plenum passages. These would allow the process
airflow to be diverted into and out of each of the heat recovery
chambers. One duct would act as an inlet to the energy recovery
chamber, the other as an outlet from the chambers to the acid gas
scrubber. A third, smaller duct would direct heated purge air to
each chamber. A purge valve for each chamber would control the flow
of purge air into the chamber. The directional mode and purging
would be controlled by a PLC, which would change the direction of
airflow at regular intervals to optimize system efficiency. The
typical flow directions within the RTO would be adjusted every 90
seconds.
In operation, solvent-laden air (SLA) would enter the oxidizer
via an energy recovery chamber, where the high-temperature ceramic
heat transfer media would rapidly preheat it prior to its
introduction into the oxidation chamber. After the chemical
oxidation purification reaction occurs, the hot, clean, outgoing
gas would heat the exit energy recovery chamber.
The SLA flow direction would be switched at regular intervals to
maintain optimum heat recovery efficiency by the automatic diverter
valves on demand from the PLC control system. After serving as an
inlet, an energy recovery chamber would be purged for a cycle
before serving as an outlet. This ensured that all air that entered
a bed would be treated to the maximum extent possible. With
sufficient concentration of hydrocarbons would self-sustain the
oxidation process. The oxidizer can be operated in an off-line
bake-out mode to allow the removal of organic buildup on the heat
exchange media. The potential organic material consists of
methylcellulose and lactose. In the bake-out mode, the RTO/scrubber
trains are taken offline from the process. At a reduced airflow,
the outlet temperature is allowed to rise before the flow direction
is switched. This hot air vaporizes organic particulate collected
on the cold face of the heat exchange media. The flow direction is
switched and other cold faces are cleaned in succession.
Process air flow/redundancy
Two 35,000-SCFM systems were specified to provide redundancy
while processing flows from 6,500 SCFM to 35,000 SCFM. Each
RTO/scrubber train would be functionally equivalent and operate in
conjunction with or independent of each other. Each system could be
returned down 6:1 (5,850 SCFM). If the airflow to an individual
RTO/scrubber was reduced below this level, a pressure control loop
could open the fresh air damper to maintain the minimum system
airflow. Butterfly isolation dampers were included at the inlet and
outlet of each RTO/scrubber. The inlet isolation damper would be
used to isolate an RTO/scrubber train during startup and shutdown.
The outlet isolation damper would be activated when the
RTO/scrubber was not in use. Butterfly isolation valves were
included down stream of the scrubber; the manually operated valves
would be used to isolate the RTO/scrubber train for service. A
flanged blind was also included for installation downstream of the
scrubber for isolation during service.
Fan Location
The RTO processes corrosive HCI vapors as the chlorinated
hydrocarbons are oxidized. In a forced draft arrangement, the RTO
is under positive pressure. In that configuration, the corrosive
gases will tend to leak to atmosphere at the instrumentation
(thermocouple) penetrations and the corrosive gases will condense
at this interface corroding the outer shell. Therefore, an induced
draft arrangement is typically preferred for chlorinated RTO
applications. A fan was designed for the conditions at the scrubber
outlet, including a radial blade fan wheel constructed of a
corrosion resistant alloy, Hastelloy C276, and a carbon steel
housing lined with rubber. This type of fan wheel was not as
efficient as backward inclined or air foil designs, but it would
not be as sensitive to aerosol droplets and offered a lower tip
speed, reducing fan noise.
99.5-percent destruction efficiency design
A residence time of 2 seconds at 1650°F was proposed to achieve
an average destruction efficiency of 99.5 percent. Actual
compliance test data demonstrated destruction efficiency in excess
of 99.9 percent. In order to guarantee the high destruction
efficiencies this project required, additional steps were taken to
reduce the air not fully treated when the airflow changed
direction. For the 99.5-percent destruction guarantee, the system
was designed with three chambers. At any one time, one chamber
would act as an inlet and one as an outlet, while the third was
being purged. After serving as an inlet chamber, each chamber would
be purged with heated clean air during the next cycle. The purge
air would be heated to minimize the potential for HCI gas water
vapor condensation and the resultant corrosion potential, even
though high nickel and molybdenum alloys were used to resist
corrosive effects. It would then become and outlet chamber during
the next cycle. The three-chamber design also minimized any
inlet/outlet bypass during valve cycling.
Acid gas scrubber
The acid gas scrubber was designed to process the maximum
exhaust capacity of the RTO exhaust, including the purge air
containing HCI vapor, providing 99.5-percent HCI removal. The
horizontal quench was designed to coo the RTO exhaust to
approximately 150°F. The re-circulation pumps provided water into
the adiabatic quench through three separate spray headers. The air
temperature was reduced to 150°F at the quench outlet. The water
that was not evaporated flowed to the recycle sump. Approximately
50 percent of the HCI was scrubbed in the scrubber quench. The air
left the horizontal quench and entered the bottom of a
countercurrent packed tower scrubber. Water and caustic solution
was sprayed in the top of the tower. The remaining acid gases were
absorbed by the solution as the air passed up the column. The air
passed through a mist eliminator to remove entrained water before
exiting the scrubber column. A sodium hydroxide solution was added
to the re-circulating water to neutralize the adsorbed HCI and form
sodium chloride (salt water), the sodium hydroxide addition rate
controlled by pH analyzer. Salt water blow-down was controlled by a
conductivity analyzer and by adding makeup water, causing sump
overflow. The company worked closely with the customer and the
consultant throughout the bid process, suggesting options and
clarifying details on this major environmental project, and was
also commissioned to install the systems. A tight site location
necessitated the careful use of oversized cranes so as to ensure no
interference with or rupture of critical pant gas, air and nitrogen
lines. Heavy rains throughout much of the process further
complicated installation but ultimately the installation of all the
equipment and the tie-in of the ductwork, electric and controls,
without affecting process, were achieved. The result was a system,
which surpassed the customer's objective of 99.5 percent
destruction efficiency and ensured compliance with the EPA's
pharmaceutical MACT.