Emission Control Technology
By Gene Anguil, CEO and Founder of Anguil Environmental
Systems, Inc.
This handbook was originally written by Gene Anguil as a chapter
in the Odor and VOC Control Handbook by Harold J. Rafson
(Editor). It has recently been updated for publication on our
website to reflect technology advances and terminology changes.
Before reading the handbook it is important to understand that
many distinctly different industries have very similar pollution
control challenges as well as solutions. It is our goal at Anguil
to utilize our experiences in every industry to help customers
apply the proper technology on their specific application. Always
feel free to contact an Anguil Representative near you
to discuss your unique destruction requirements, efficiency demands
and process parameters.
Chapter 8. Emission Control Technology
8.4 Thermal Oxidation
8.4.1 INTRODUCTION
The objective of this section is to present various VOC thermal
oxidation technologies, their strength and weaknesses and the
elements required to determine what is the preferred technology for
a given application.
In 1970 the environmental authorities started to identify
various industries which had to bring their emissions within
boundary limits. At the beginning of 1997, there are over 6000
systems operating throughout the world controlling VOC emissions.
The majority of the equipment is in the United States although
systems have proliferated in Canada, Europe, Taiwan, Korea and
Japan. The major industries that have employed thermal oxidation
technology are shown in Table 8.4.1. Since the early 1990's the
baking industry, the chemical process industry, pharmaceutical
industry, fabric coating, rubber extrusion, electronic components,
soil remediation, metal coating, wood working, formaldehyde and
sterilizers have become the specific industries being targeted by
the environmental authorities.
TABLE 8.4.1 MAJOR INDUSTRIAL SOURCES
VOCs Emitted From Stationary Sources
|
INDUSTRY
|
VOCS, TYPICAL SOLVENTS (S)
AND
OTHER OFF GASES
|
|
Acetate Finish Coating
|
Silicate Solution
|
|
Alcohol Synthesis
|
C1, C2, C3, C6 Hydrocarbons
|
|
Automobile Coating
|
Ketones, Xylene, Toluene, Phenols
|
|
Bakery Ovens
|
Ethanol
|
|
Can Coating
|
Ketones, Alcohols, Aromatic Hydrocarbons, Ethers
|
|
Chemical Processing
|
|
|
Coffee Roasting
|
Heavy Oils from coffee beans
|
|
Coil Coating
|
Phosphates, Solvesso, Cyclohexanol, Alcohols, Carbitols,
Hydrocarbons
|
|
Electronic Components
|
Butyl Acetate, Xylene, MEK, Cellosolve
|
|
Fiberglass Coating
|
Teflon Emulsion, Fiberglass, Synthetics, Styrene
|
|
Flexographic Printing
|
Flexographic Ink Derivatives, Alcohols, Glycol
|
|
Formaldehyde
|
Formaldehyde, Methanol, CO
|
|
Lithographic Print/Paint
|
Butyl Cellosolve, Ciacetone Alcohol, Solvesso, Cellosolve
Acetate, Xylene, MIBK
|
|
Metal Coating
|
Alcohols, Cellosolve Acetate, Phthalates, Solvesso
|
|
Paper Coating
|
High-Boiling Organics, Latexes
|
|
Pharmaceuticals
|
Isoproponol, Toluene, Hydrocarbons
|
|
Phthalic Anhydride Mfg.
|
Organic Acids
|
|
Resin Plant
|
Formaldehyde, Phenols, Phthalic Anhydride
|
|
Rubber Processing
|
|
|
Soil Remediation
|
Benzene, Toluene, Ethylene, Xylene
|
|
Sterilizers
|
Ethylene Oxide
|
|
Vinyl Surgical Glove
|
Polyvinyl Chloride, Cioctyl Phathalate
|
|
Wire Enameling
|
Cellosolve Acetate
|
The air pollution control solutions that are available are
normally dictated by the volume of air that is to be processed. The
volume of airflow, measured in cubic feet per minute, is designated
as ACFM for Actual Cubic Feet per minute of ACFM where "S" stands
for standard cubic feet per minute, at 70 degrees Fahrenheit, sea
level and one atmosphere.
Thermal oxidizer sizes range from 100 SCFM up to 100,000 SCFM.
Each industry has operations that dictate the exhaust flow that
must be processed. Table 8.4.2 shows various industries and typical
airflows from their operations.
TABLE 8.4.2 INDUSTRIAL PROCESS VOLUMES
|
INDUSTRY
|
SCFM
|
|
Soil Remediation
|
100-5,000
|
|
Vent Treatment
|
100-1,000
|
|
Automobile Coating
|
10,000-100,000
|
|
Semiconductor
|
20,000-50,000
|
|
Flexographic Printing
|
5,000-15,000
|
|
Rotogravure Printing
|
10,000-30,000
|
Most thermal oxidizers are pre-engineered, that is the equipment
is designed on the principle that in order for the equipment to be
competitive in the marketplace, then a series of products of
fundamentally standard designs are tailored to the application by
changing some of the parameters as dictated by the requirements.
Since thermal oxidation equipment has a burner, the designs require
controls for safety and operation. Controls are termed user
friendly with control logic being relay or computer based.
Particular hydrocarbons affect health in several ways. Some
affect the respiratory system, while others are air toxins. The EPA
issued new rules and regulations in 1990 regarding VOC's and Air
Toxins. Subsequent controls or regulations have been issued which
have identified that air toxins are particularly dangerous to
industrial workers. One of the major air toxins is benzene, and
benzene is one of the principal components of gasoline. The
remediation of benzene from underground storage tanks and leaky
underground petroleum bases is a significant source of air toxins.
Carbon tetrachlorethylene, Hydrogen Cyanide, and Ketones are all
air toxins being controlled today or being regulated, since they
are carcinogenic and may create an immediate problem or
delayed.
8.4.2 CHARACTERIZING THE PROCESS STREAM
In determining the most appropriate technology to controlling a
process stream, it is necessary to characterize the air stream. The
first step in the characterization is to establish what organics
and other contaminants need to be destroyed or controlled.
Standard forms called Application Data Sheets, see Table 8.4.3,
are available whereby a series of questions are asked to determine
what are the possible technologies that can control the VOCs being
generated. The sizing of the equipment required is dependent on the
air flow and the organic loading.
In general for a given flow and concentration, preferred
oxidation technologies exist. In general, low flow, low
concentration streams are best handled by a catalytic recuperative
oxidizer. When the concentration of the stream is between 15%-20%
LEL then both a catalytic recuperative or thermal recuperative is
the best technology. For process streams between 20%-25% LEL then
thermal recuperative is the preferred solution.
RTO/RCO and Rotor/Concentrator systems are typically higher in
capital costs, but the operating cost savings on high volume, low
concentration streams make these technologies attractive in this
region. Subsequently the capital and operating cost of the
equipment and the cost of the installation of that equipment is
related to the sizing. Since oxidation equipment requires fuel and
electricity, the operating costs will also depend on the hours or
operation per day, per week, and per year. In addition, the costs
will depend on the quantity of organics that need to be processed,
in pounds per hour (lb/hr); or in parts per millions by volume
(ppmv). A critical consideration is whether the organic loading is
steady state or if it has peaks or spikes.
The volume of the process stream that is to be treated must be
determined. Since most companies have not needed to buy pollution
control equipment in the past, their approach to eliminating
pollution has been the more exhaust, the better. This is
particularly true in the low margin industries. The more
sophisticated industries, say the chemical industry, have normally
been concerned about the air exhausted from their plant and have
paid attention to the exhaust volume by process control. The
industries that have not paid attention to exhaust volume are not
sufficiently aware that when buying pollution control equipment the
cost of that equipment is going to depend on two elements:
(1) The air volume that must be processed
(2) The pounds per hour of organics that must be processed.
Process flow reduction should be targeted and must be in such a
way that it will not impact upon the operation of the process or
plant. The normal first response by a plant engineer is that air
flow reduction can not be done. However, subsequent investigation
has resulted in that most of the time flow reduction can be
done.
ORGANIC LOADING
The determination of the concentration of the organics is often
difficult. However, in an operation where solvents or other
organics are purchased, the number of pounds or tons of the
organics that has been bought in the past year is known. Since this
information must also be supplied to the environmental authorities.
Hence, the first step that needs to be determined is done through
the purchasing records. Given the tons of organics purchased in the
previous years, and the number of operating hours per year, the
ratio of those two numbers yields the pounds per hour, a
conservative initial estimate of the average organic loading. This
methodology assumes that everything that is purchased, will be
emitted up the stacks.
CONTAMINANTS
It is necessary to determine if there are compounds in the
exhaust stream which can be deleterious to catalyst, thereby
precluding the use of this technology. As an example, when
automotive catalytic mufflers and converters were introduced a
number of years ago the automobile industry required the
petrochemical industry to eliminate lead from gasoline since lead
degraded and reduced the effectiveness of the catalyst and the
destruction of the gasoline. One set of industrial compounds that
can harm catalysts are halogens, a family of compounds which
include chlorine, bromine, iodine, and fluorine. Bromine, while not
prevalent in industry, is present in chemical plants. Freons are
fluorine compounds. Silicone is another compound which is
deleterious to catalyst. It is used as slip agent, or a lubricant
in many industrial processes. Phosphorous, heavy metals (zinc,
lead), sulfur compounds and any particulate can result in
shortening the life of the catalyst. It is necessary to estimate
the volume or the amount of each of those contaminants, to assess
the viability of catalytic technologies for the application.
OPERATING COSTS
Since all oxidation equipment has to operate at an elevated
temperature, it is necessary to know the type of auxiliary energy
of fuel available. All oxidizers can operate on electric heating,
natural gas or LPG, whereas thermal oxidizer can operate with
sulfur based fuels such as number two or number six fuel oil.
Electric heat is only suitable for small air flows.
Since a continuing cost of air pollution control is the
operating cost, all thermal oxidizers should have some form of heat
recovery. Available heat recovery efficiencies vary depending on
the type of thermal oxidation technology selected. The specific
heat recovery efficiency selected for an application depending on
the organic concentration. Table 8.4.4 shows a series of different
economic analyses for a particular air stream.
TABLE 8.4.4 OPERATING COST COMPARISONS
|
Operational Costs
($/hr)
|
|
Process Stream
10,000 SCFM
Percent LEL
|
Catalytic Recuperative
Oxidizer
|
Thermal Recuperative
Oxidizer
|
Regenerative Catalytic
Oxidizer
|
Regenerative Thermal
Oxidizer
|
Rotor Concentrator with
Thermal Oxidizer
|
|
1% LEL
|
$11.00/hr
|
$28.37/hr
|
$2.50/hr
|
$5.65/hr
|
$0.30/hr
|
|
10% LEL
|
$3.70/hr
|
$16.72/hr
|
$1.65/hr*
|
$1.60/hr*
|
N/A
|
|
|
1. Assumes a 10,000 SCFM process stream with an inlet
temperature of 70oF.
|
|
2. Assumes a process mixture of 16,720 BTU/LB with a mixture LEL
of 1.10%.
|
|
3. Assumes an electrical cost of $0.06 per KWH.
|
|
4. Assumes a natural gas cost of $4.50 per MMBTU.
|
|
5. The Catalytic Recuperative Oxidizer assumes a 65% efficient
heat exchanger.
|
|
6. The Thermal Recuperative Oxidizer assumes a 65% efficient
heat exchanger.
|
|
7. The Regenerative Catalytic Oxidizer assumes a 95% efficient
heat exchanger.
|
|
8. The Regenerative Thermal Oxidizer assumes a 95% efficient
heat exchanger.
|
|
9. The rotor concentrator wheel assumes a 6:1 concentration
ratio.
|
|
|
* Excess heat has been produced at this loading and a hot side
heat exchanger bypass is required.
|
A process stream of 10,000 SCFM with an inlet temperature of
70°F was utilized in the analyses shown in Table 8.4.4. Toluene was
chosen as the contaminant in this example. Toluene has a calorific
content of 16,720 BTU/LB and a LEL of 1.10% by volume. For each of
the technologies, an economic analysis was performed at a toluene
loading of 1% LEL and at 10% LEL. The results of this analysis are
reported as operational costs in $/HR. The operational cost is the
sum of the fuel usage as well as the electricity needed to run the
system fans. All the technologies shown have been assumed to be
running on natural gas at a cost of $4.50/MMBTU. The electricity
price has been taken at $0.06/KWH.
From Table 8.4.4 one can see the operating cost advantages of
one technology over another at two loading conditions. A catalytic
recuperative oxidizer, with a 65% effective heat exchanger, is more
economical to operate at the 1% to 10% LEL range than a thermal
recuperative oxidizer with a 65% effective heat exchanger. The
thermal recuperative oxidizer will only begin to show a significant
reduction in operational costs around the 15% to 25% LEL range.
Both of the regenerative oxidizers utilize a 95% effective heat
exchanger. These technologies are ideal for high air flow, low
loading process streams. As shown on Table 8.4.4 both of the
regenerative oxidizers begin to show high temperature bypass at the
10% LEL loading. This means that the loading point at which the
oxidizers would require zero supplemental fuel has been exceeded.
Excess heat is now being produced in the oxidizer . This heat must
be able to escape from the oxidizer by way of a high temperature
bypass. The table also shows that the rotor concentrator wheel is
another technology suited for high airflow, low loading process
streams. The rotor concentrator will concentrate the process stream
from anywhere between a 6:1 to a 13:1 concentration ratio. Thus,
the airflow will be reduced 6 times, while the contaminant loading
will be increased 6 times. The rotor concentrator technology is not
suited for process streams at a 10% LEL loading.
DESTRUCTION EFFICIENCY
Typically throughout the United States 95% destruction of VOC is
required, and when toxic chemicals are present the destruction
efficiency is raised to 99%. Characterization of the VOC laden
stream assists in selecting the appropriate technology to achieve
the required destruction efficiency.
MULTIPLE PROCESS LINES
If multiple process lines exist, a means to combine the
airstream and to control the volume to the oxidizer must be
present. For example, several process exhausts feed into the
collection plenum, go through a roughing filter and are then drawn
through a system fan. The pressure drop across that filter is
monitored to determine when to clean the filters. The amount of
flow going into the system fan is controlled by a vortex damper or
alternatively, a variable frequency drive. The amount of air from
multiple process lines into this plenum can vary. The pressure in
the plenum is sensed by a pressure transducer and this signal is
used to vary the speed of the fan VFD or position the vortex damper
to control the amount of volume drawn from the plenum.
Consequently, the fan will slow down or speed up until the amount
of air drawn from the plenum is the same as the amount of air
entering. This pressure control loop is standard on all systems
that have multiple process lines. It is not required when a single
process line exists, but is beneficial to minimize air volume
variations drawn from the process. When a fixed speed fan is used,
variation in organic loading cause differing pressure drops across
the oxidizer. The differing pressure drops, result in varying flow
from the process unless a pressure control loop is unitized.
8.4.3 THERMAL OXIDATION TECHNOLOGIES
Thermal oxidation, by definition, converts a hydrocarbon, in the
presence of oxygen and heat, to carbon dioxide and water vapor. A
general equation showing this relationship is presented in equation
(1)
Cn H2m + (n + m/2) O2 → n CO2 +
H2O + Heat
The particular n and m subscripts of the equation are used to
define the number of carbon atoms and hydrogen atoms. The number of
oxygen atoms convert to n molecules of carbon dioxide and m
molecules of water vapor and heat which is given off in the
exothermic reactor.
In the section entitled Characterizing the Process Stream, it
was stated that the process air volume and the organic loading were
important variables in identifying the appropriate oxidation
technology. One measure of the organic loading or the organic
concentration of the process stream is the "lower explosive limit",
designated as the LEL or the "lower flammable limit", designated as
the LFL. The LEL of a stream is the lowest organic concentration in
a stream that would, in the presence of an ignition source, yield a
combustible mixture. At this concentration, the stream is said to
be at 100% LEL. As examples, if a steam has toluene or alcohol and
it is at 100% LEL, this means that the concentration within that
stream is explosive if a spark or ignition source is present.
Characteristics of some common organics are shown in Table 8.4.5
where they are divided into five general types, Alcohols,
aliphatics, aeromatics, esters, and keytones.
TABLE 8.4.5 ORGANIC COMPOUND CHARACTERISTICS
|
Gas/Vapor
|
Chemical Formula
|
LEL
|
Calorific Valve
(BTU/lbs)
|
|
(1) Alcohols
|
|
Methanol
|
CH3OH
|
3.4
|
9,033
|
|
Isopropanol
|
(CH3)2CHOH
|
1.1
|
|
|
N-Butyl Alcohol
|
CH3(CH2)2CH2OH
|
2.7
|
|
|
N-Propyl Alcohol
|
CH3CH2CH2OH
|
1.2
|
12,960
|
|
(2) Aliphatics
|
|
Naphtha
|
No chemical description
|
-
|
16,960
|
|
Methyl Cellosolve
|
CH3OCH2CH2OH
|
1.5
|
9,274
|
|
N-Hexane
|
CH3(CH2)4CH3
|
2.8
|
19,391
|
|
N-Heptane
|
CH3(CH2)5CH3
|
2.4
|
19,314
|
|
N-Pentane
|
CH3(CH2) 3CH3
|
-
|
19,493
|
|
Dimethyl Formamide
|
(CH3) 2NCHO
|
0.8
|
|
|
Cyclohexane
|
CH2(CH2) 2CH2
|
2.0
|
18,826
|
|
(3) Aeromatics
|
|
Benzene
|
C6H6
|
3.2
|
17,443
|
|
Toluene
|
C6H5CH3
|
1.7
|
17,599
|
|
Tetrahydrofuran
|
OCH2CH2CH2CH2
|
-
|
|
|
Xylene
|
|
|
17,725
|
|
(4) Esters
|
|
Isopropyl Acetate
|
(CH3)2CHOOCCH3
|
12.5
|
10,960
|
|
Iso Butyl Acetate
|
CH3COOCH2CH(CH3)
2
|
6.5
|
|
|
Ethyl Acetate
|
CH3COOC2H5
|
Gas
|
10,235
|
|
N-Butyl Acetate
|
CH3COOC4H9
|
3.0
|
12,460
|
|
Propyl Acetate
|
C3H7OOCCH3
|
10.0
|
10,525
|
|
(5) Keytones
|
|
Acetate
|
|
|
|
|
Methyl Ethyl Ketone
|
CH3COC2H5
|
1.0
|
13,729
|
|
Methyl Butyl Ketone
|
CH3CO(CH2)3CH3
|
2.2
|
|
|
MTBK
|
|
|
|
|
|
|
|
For organic benzene, it is seen that 12,000 PPM, is 100% LEL.
The National Fire Protection Association (NFPA) states that
equipment can operate, without LEL monitors or controls, if the LEL
is less than 25% LEL. For benzene then, 25% LEL is equal to 3,000
PPM. This upper boundary becomes a dictating factor in the
selection and design of the oxidation equipment. If the
concentration is higher than 25% LEL, the NFPA requirements state
that an LEL monitor is required. Using an LEL monitor, NFPA
guidelines allow operation up to 50% LEL (a 2:1 safety factor). In
summary, 100% LEL is explosive; if the stream is at 25%, a factor
safety of four exists.
In addition to the explosive aspects of the LEL, another issue
is the heat energy given off during oxidation. An estimate of the
exotherm is that there will be a 25 degrees F rise per 1% LEL in
the stream. Hence, if the process air enters the oxidizer at a
given temperature, and if the stream has a concentration of 2% LEL,
then a 50 degrees F rise in process stream temperature is expected
after oxidation. If the process stream were running at a 10% LEL,
then a 250 degrees F temperature rise would be predicted. A maximum
LEL of 25%, yields a 625 degrees F temperature rise of the process
stream.
The concentration that is required to yield 100% LEL varies with
the characteristics of the identified organics. For example,
ethanol has a 20,000 PPM by volume concentration whereas benzene
has a 12,000 ppmv.
Not only do different organics have different LELs but every time
a pound of a different organic is oxidized, a different amount of
heat will be given off. An example that demonstrates this energy
release is the catalytic muffler in an automobile. When mufflers
were initially installed, the high level of unburned gasoline which
went through the muffler caused excessive heating and subsequently
caused fires if the auto had been parked on leaves. The reason for
the excessive heat was that for each pound of gasoline being
oxidized 20,000 BTU's were being released.
Chlorinated organics are hydrocarbons that have one or many
chlorine atoms. Oxidation of chlorinated hydrocarbons yield CO2,
water vapor and hydrogen chloride (HCl) gas. Some typical
chlorinated organics are TCE and PCE. These organics have calorific
values as low as 5,000 BTU/lb.
CATALYST
The characteristics of oxidation catalyst are many and varied.
Fundamentally, if an airstream containing organics is heated and
passed across catalyst, the organics will be converted to carbon
dioxide and water vapor. However, the percent conversion happens at
different temperatures for different organics and for different
catalysts. (See Figure 8.4.4 and 8.4.5)
Consider the compound - Toluene. To destroy 25% of the Toluene
in an air stream, the chart is entered on the y axis at 25%, come
over to the toluene line, and down to the Fahrenheit line, and it
is seen that a temperature of about 300 degrees F is required.
However, the EPA does not require 25% destruction, but 95-99%
destruction. In order to destroy 95% of the Toluene, enter the
graph at 95% destruction, come over to the Toluene line, down
vertically and it is seen that 500-550 degree range is required in
order to destroy the Toluene. Some organics require higher
temperatures to be destroyed than others, catalytically. Alcohols,
isopropyl alcohol and ethanol, can be destroyed relatively simply
whereas the Acetates, particularly the ethyl acetates and propyl
acetates, may require temperatures in the 750 degrees F range in
order to achieve adequate destruction. Depending on the process
stream, either a single organic may be present as found in the
chemical industry, or in printing operations, a multiplicity of
organics exists. Having a multiplicity of organics imposes the
requirement of focusing on the ability to destroy the most
difficult organic
Some organics can not be effectively destroyed by catalyst. For
example: heptane and hexane, can be destroyed at temperatures of
600-700 degrees. Whereas propane, ethane and methane require
temperatures beyond a reasonable temperature range. Since methane
is not a smog producing organic, a guarantee to destroy 95% of the
organics means that the methane is not considered and is removed
from that stream in the computation process. However, if propane is
the auxiliary fuel, that is if the burners are being driven by LPG
or by propane directly, it means that there will be contribution to
the VOC at the end of the stack.
Catalyst samples are shown in Figure 8.4.6. Some catalyst is
deposited on a ceramic substrate. These ceramics are extruded in a
malleable state and then fired in ovens. The process consists of
starting with a ceramic and depositing an aluminum oxide coating.
The aluminum oxide makes the ceramic, which is fairly smooth, have
a number of bumps. On those bumps a noble metal catalyst, such as
platinum, palladium or rubidium, is deposited. The active sited,
wherever the noble metal is deposited, is where the conversion will
actually take place.
An alternate to the ceramic substrate is a metallic substrate.
In this process, the aluminum oxide is deposited on the metallic
substrate to give the wavy contour. The precious metal is then
deposited onto the aluminum oxide. Both forms of catalyst are
called monoliths.
An alternate form of catalyst is pellets. The pellets are
available in various diameters or extruded forms. The pellets can
have an aluminum oxide coating with a noble metal deposited as the
catalyst. The beads are placed in a tray or bed and have a depth of
anywhere from 6 to 10 inches. The larger the bead (1/4 inch versus
1/8 inch) the less the pressure drop through the catalyst bed.
However, the larger the bead, the less surface area is present in
the same volume which translates to less destruction efficiency.
Higher pressure drop translates into higher horsepower required for
the oxidation system. The noble metal monoliths have a relatively
low pressure drop and are typically more expensive than the pellets
for the same application.
BASE METAL CATALYST
An alternate to a noble metal catalyst is a base metal catalyst.
A base metal catalyst can be deposited on a monolithic substrate or
is available as a pellet. These pellets are normally extruded and
hence are 100% catalyst rather than deposition on a substrate. A
benefit of base metal extruded catalyst is that if any poisons are
present in the process stream, a deposition of the poisons on the
surface of the catalyst occurs. Depending on the type of
contaminant, it can frequently be washed with water. When it is
washed, abraded or atritted, the outer surface is removed and
subsequently new catalyst is exposed. Hence, the catalyst can be
regenerated. Noble metal catalyst can also be regenerated but the
process is more expensive and often doesn't work effectively.
A noble metal catalyst, depending on the operation, will
typically last around 30,000 hours. As a rule of thumb, a single
shift operation of 40 hours a week, 50 weeks a year results in a
total of 2,000 hours per year. Hence, the catalyst might have a 15
year life. It may degrade, it may need some washing, it may need
some other operations on it, but it should last that long. If a
plant operates three shifts a week, or 6,000 hours per year, then
the expected catalyst life would be about five years. From a cost
factor, a typical rule of thumb, catalyst might be 10%-15% of the
overall capital cost of the equipment.
There are seven fundamental oxidizer technologies that achieve
the oxidation of organics in alternate ways. These technologies
are:
A catalytic recuperative oxidizer consists of several main
elements, as shown in Figure 8.4.2, and classified as:
- System Fan
- Heat Exchanger
- Reactor
- Catalyst
- Exhaust Stack
Figure 8.4.3, shows a typical P & ID for such a system. The
process stream is ducted to the oxidizer and enters a system fan.
The system fan is driven by a motor and the process stream is
forced under positive pressure through a heat exchanger. The heat
exchanger shown is a cross flow heat exchanger of the shell and
tube type. Plate type heat exchangers can be used in the
application. Normally, the process stream enters through the tube
side of the heat exchanger due to the ability of cleaning the
tubes. As the air enters in and goes through the heat exchanger, it
is heated and will then exit into the section called the reactor.
As it comes into the reactor, the process stream will be further
heated by a burner, controlled by a thermocouple measuring the
temperature of the air and a temperature controller regulating the
burner firing to bring the process stream up to the catalyzing
temperature of 300-700 degrees F. The catalyzing temperature
depends on the organic, the requirement for the destruction of the
organic and the type and volume of catalyst. At the catalyzing
temperature, the process stream will pass through a series of beds,
having catalyst in them. As the air containing organics comes
across the catalyst, the organic is converted to CO2 and water
vapor and an exothermic reaction occurs. This exotherm will raise
the temperature of the stream exiting the catalyst bed. Hence the
catalyst outlet temperature will be higher than the temperature
going into the catalyst bed. The process stream is then directed
though the shell side of the heat exchanger where it preheats the
incoming air and is then exhausted to the atmosphere.
The theory of operation for thermal recuperative oxidation is
called the three "Ts" of combustion:
- Time
- Turbulence
- Temperature
Time, as used in thermal oxidizers, refers to the retention time
or residence which is the length of time that an organic is at the
appropriate oxidation temperature. Typically, if a 95% destruction
efficiency is required, a residence time of a half a second is
adequate. That is, the organic if brought up to a temperature of
about 1400 degrees F, and maintained at 1400 degrees F for a
retention time of half a second. Both retention time and turbulence
must be achieved, where turbulence is a term which implies an
adequate level of mixing. If the turbulence occurs and if the 1400
degrees for a half a second is achieved, with adequate oxygen, then
95% of the organics will be destroyed. In some cases, particularly
in the chemical industry, oxygen must be added to a process stream.
For each organic, a specific temperature and a specific retention
time is required to achieve the desired level of destruction. Table
8.4.6 illustrates these points for several organics, some being
chlorinated.
TABLE 8.4.6 TEMPERATURES AND RESIDENCE TIMES
|
TEMPERATURES AND RESIDENCE
TIMES
|
|
0.5 SEC RETENTION
TIME
|
1.0 SEC RETENTION
TIME
|
|
95% DE
|
99% DE
|
95% DE
|
99% DE
|
|
Toluene
|
1351
|
1372
|
1317
|
1338
|
|
Benzene
|
1489
|
1640
|
1415
|
1640
|
|
Styrene
|
1445
|
1480
|
1391
|
1424
|
|
PCE
|
1843
|
1921
|
1726
|
1797
|
|
TCE
|
1592
|
1675
|
1972
|
1545
|
|
|
|
|
|
|
PCE - Perchloroethane
|
|
TCE - Trichloroethane
|
|
DE - Destruction Efficiency
|
Toluene: 95% destruction efficiencies achieved
with a half a second residence time if the temperature is 1351
degrees . At one second residence time, the temperature can be
reduced slightly to 1317 degrees . To achieve 99% destruction with
a half a second residence time, the operating temperature would
have to be 1372 degrees F. Whereas if one second residence time
existed, then 99% destruction efficiency could be achieved at 1338
degrees F. Increased temperature gives better destruction
efficiency but increases operating costs. Increased retention time
gives better destruction efficiency but increases capital cost. The
balance between increased capital cost and lowered operating cost
must be weighed.
Benzene: Benzene, which is carcinogenic,
requires a higher destruction temperature than toluene. Rather than
operating at 1351 degrees , operating temperatures of 1489 degrees
are required at one half a second residence time.
PCE: The chlorinated organics require higher
temperatures. Rather than operating at the 1400 degrees range, the
oxidizer must operate around 1800 degrees , a longer residence time
allows a decreased temperature to about 1700 degrees F; an increase
to a 99% destruction requires an increased temperature to about
1900 degrees F.
One of the concerns that exists in thermally processing
chlorinated organics is that the by-products of combustion that are
formed may be more harmful than the initial product that is to be
destroyed. A by-product of combustion is a compound formed, either
catalytically or thermally, when most of the organic compound is
converted to CO2 and H2O. For example, when 95% of Toluene is
destroyed, what happens to the other 5%? Does it remain as Toluene
or are new products formed? Frequently, "by products of combustion"
are formed and it is not possible to predict what those by products
are. They could be a mixture of the original compounds or they
could be more harmful. The concern that people have in processing
chlorinated organics thermally at these high temperatures is that
dioxins may be generated. Dioxins are in effect much more harmful
than the organic that will cause VOCs or smog in the air.
OXIDIZER FABRICATION
Thermal oxidizers must be built to provide the residence time
and temperatures to achieve the destruction of the organics.
Therefore, thermal oxidizers are comparatively larger than
catalytic oxidizers since their residence time is two to four times
greater. Thermal oxidizers used to be manufactured with carbon
steel for the outer shell and castable refractory or brick as the
thermal liner. A refractory is like a cement, which is put on the
inside of the rector shell to act a thermal insulation barrier. New
thermal oxidizers, offered in the market place today, are designed
and built using ceramic fiber insulation on the inside. This is a
rather light weight material, and has a relatively long life,
whereas the old refractory would tend to fail over a period of
years by attrition of expansion and contraction.
Figure 8.4.7 shows a forced draft thermal recuperative oxidizer,
where the system fan forces the process airstream through a heat
exchanger. As the process flow exits the heat exchanger the burner
fires and the process stream is brought up to temperature. The
reactor chamber, which is lined with a high temperative ceramic
fiber, is designed for the required retention time. A shell and
tube heat exchanger is normally used for thermal oxidizers because
of expansion concerns. The material selection for the shell and
tube heat exchanger needs to be a high grade stainless, adequate
for expected steady state and peak temperatures.
The thermal oxidizer illustrated in Fig. 8.4.7 is a forced draft
system, but induced draft systems also exist. The induced draft
system is slightly more expensive but is recommended when
particulate or organic oils are present. Particulates impact upon a
forced draft fan and will have a negative effect on the system
performance. If the process stream is clean a forced draft system
is appropriate.
The burners employed are modulating burners, that is if the
organic is preheated to an adequate temperature, the burners will
modulate down to zero so that there is no energy required for the
continued oxidation.
One of the benefits of the thermal recuperative oxidizer, is that
it is possible to process organics that may be a poison or be
detrimental to catalyst. In addition, if the organic concentration
is very high, for example the organic level is of the 20-25% LEL,
then thermal recuperative oxidation is the appropriate
technology.
For relatively large volume and low concentration streams the
problem with the thermal recuperative design is that the metallic
heat exchanger only recovers 70% of the heat and operating cost
increase dramatically. The level of heat exchanger efficiency is
limited by material cost to increase heat exchanger efficiency and
the fact that autoignition, where organics ignite due to
temperature alone, becomes a problem with the life of metallic heat
exchangers.
The Regenerative Thermal Oxidizer is a thermal oxidizer
consisting two or more ceramic heat transfer beds, which act as
heat exchangers and a Purification Chamber or Retention Chamber
where the organics are oxidized and converted to CO2 and H2O
vapor.
The operation of a Regenerative Thermal Oxidizer (Dual-Bed) is
shown in Figure 8.4.9 and 8.4.10 requires the initial preheating of
the ceramic heat transfer beds to a temperature of 1500 degrees F
during the start-up mode. This is accomplished by operation of fuel
fired burner located in the purification chamber. To equalize the
preheating of the ceramic heat transfer beds, the air is directed
into and out of the ceramic heat transfer beds by operation of two
(2) pneumatic diverter valves located under each ceramic heat
transfer bed. During initial start-up outside air is supplied to
the oxidizer through the make-up air damper tee located on the
inlet side of the process air fan. A Programmable Logic Controller
(PLC) monitors and controls the direction of the air flow.
After the ceramic heat transfer beds have reached an operating
temperature of 1500 degrees F the unit is ready for the process
airstream. As the process airstream enters the ceramic heat
transfer beds, the heated ceramic media preheats the process
airstream to its oxidation temperature. Oxidation of the airstream
occurs when the auto-ignition of the hydrocarbon is reached. At
this point the heat released by the oxidation of the process
hydrocarbons is partially absorbed by the inlet ceramic heat
transfer bed. The heated air passes through the retention chamber
and the heat is absorbed by the outlet ceramic heat transfer bed.
If the oxidizer is self-sustaining the net increase in temperature
(inlet to outlet) is 100 degrees F.
During the normal Mode of Operation of the system the process
air enters the RTO System Fan and passes through the Inlet Diverter
Valve where the process air is forced into the bottom of the left
ceramic heat transfer bed. As the process air rises through the
ceramic heat transfer bed, the temperature of the process stream
will rise. The top of the beds are controlled to a temperature of
1500 degrees F. The bottom of the beds will vary depending upon the
temperature of the air that is coming in. If it is assumed that the
process air is at ambient conditions or 70 degrees F, then as the
air enters the bottom of the bed, the bottom of the bed will
approach the inlet air temperature of 70 degrees F. The entering
air is heated and the media is cooled. As the air exists the
ceramic media it will approach 1500 degrees F. The process air then
enters the second bed at 1500 degrees F and now the ceramic media
recovers the heat from the air, and increases in temperature. At a
fixed time interval of four to five minutes, or based on
thermocouple control, the diverter valves switch and the process
air is directed to enter the bed on the right and exits the bed on
the left. Prior to valve switching the air heated the right bed and
now this bed is being cooled. The cooling starts at the bottom and
continues upward because the media is hot and the energy is
transferred. The process air then goes through the purification
chamber and exits through the second bed.
DESTRUCTION EFFICIENCY
When the valves are switched, whatever organics had not been
destroyed prior to the flow being reversed are then exhausted out
of the stack. In addition, the rapidity of switching or closure of
the valves is critical to minimize the bypass of unoxidized
organics. If the emissions vs. time were plotted, the graph would
reflect a very low exhaust concentration level, say a 1 PPM, but
whenever the diverter valve switches an organic pulse occurs in the
exhaust stream. Since the valves shift every four minutes these
pulses reduce the overall destruction efficiency of the organics.
Several methods of processing the pulse exist in order to achieve
higher destruction efficiencies.
OXIDIZER FABRICATION
The retention chamber and heat transfer chamber are fabricated
of reinforced carbon steel exterior and ceramic fiber lining. The
thickness of the ceramic fiber lining is based on the required
destruction temperature of the organics and the desired outside
shell temperature.
The ceramic heat exchange media can be of various types
including ceramic saddles, tye pacs, or structured packing. The
ceramic structured packing is a recent development in the industry
reflecting lower pressure drops for equivalent heat transfer.
A reinforced carbon steel structure is provided to support the
loads of the oxidizer chambers and the structured packing support
grid, and the wind and/or earthquake loads. The packing support
grid is fabricated of stainless steel and is designed to support
the structured packing.
DIVERTER VALVES
The oxidizer has on process air diverter valve located under
each energy recovery chamber. The process air diverter valves
control the process air flow into and out of the energy recovery
beds. The valves can be fabricated of carbon steel or stainless
steel depending on the application. The valves are operated by
pneumatic actuators and controlled by a PLC to maintain optimum
energy efficiency.
There are a number of different types of valves that are used. All
valves must be highly reliable and have the ability to seal
effectively. Many systems use heavy duty case butterfly valves,
whereas newer systems utilize popit type valves.
OXIDIZER BAKE-OUT
If organic particulates in the process exhaust builds up on the
"cold" surfaces at the bottom of the oxidizer, the process must be
shutdown and a volatilization of these organics or a "bake out" is
required. When bake-out is activated, the flow diverter valves will
stay in one position until the exhaust air temperature from the
outlet bed reaches 850 degrees F. At this temperature, most organic
oils will volatalize, as in a self cleaning oven. When the first
outlet bed reaches 850 degrees F, the flow diverter valves will
switch and will stay in position until the outlet temperature of
the second bed reaches 850 degrees F.
SUPPLEMENTAL FUEL INJECTION
The supplemental fuel system is designed for natural gas
operation, but allows the oxidizer to be operated with Natural Gas,
Propane, or Butane for any condition requiring energy input to
initiate or sustain operation. Supplemental fuel may be injected
via the combustion burner in the Purification Chamber or by
injection of natural gas properly mixed with the process stream. As
the natural gas mixture rises through the ceramic media it reaches
autoignition temperature and the exothermic reaction takes place,
thereby providing heat to the process stream.
The benefit of natural gas injection into the process stream
rather than through the burner results in utilization of 1,000
BTU/cubic feet of natural gas versus 600 BTU/cubic feet of gas,
when a burner is employed.
OPERATING TEMPERATURE COMPARISONS
The oxidation temperature for catalytic or thermal oxidation is
shown in Table 8.4.7.
TABLE 8.4.7 OXIDATION TEMPERATURES
|
CATALYTIC VS.
THERMAL
Oxidation
Temperatures
99% Destruction
|
|
Catalytic
|
Thermal
|
|
Benzene
|
440
|
1460
|
|
Carbon TET
|
610
|
1430
|
|
MEK
|
600
|
1500
|
|
Cyanide
|
480
|
1800
|
Lower temperature, translates into lower fuel costs assuming the
same heat exchanger efficiency. It takes energy to take the
airstream from whatever temperature it enters, up to its operating
temperature. Even though a heat exchanger will help reduce net fuel
costs, the lower the operating temperature, the lower the fuel
costs.
Catalytic oxidizer design uses the term Gas Hourly Space Velocity
(GHSV) which relates the amount of air to be processed and the
volume of catalyst required. The GHSV is the inverse of the
residence time. As shown in Table 8.4.8, for Benzene destruction,
the residence time in a catalytic unit, is under a quarter of a
second.
TABLE 8.4.8 RESIDENCE TIME (SECONDS)
|
CATALYTIC VS.
THERMAL
Residence Time
(Seconds)
99% Destruction
|
|
Catalytic
|
Thermal
|
|
Benzene
|
.18
|
1.0
|
|
Carbon TET
|
.18
|
1.0
|
|
MEK
|
.18
|
1.0
|
Flares are an oxidation technology that continues to be used
both domestically and internationally. They are used in the
petroleum, petrochemical, and other industries that require the
disposal of waste gases of high concentration of both a continuous
or intermittent basis. As other thermal oxidation technologies, the
three T's of combustion of time, temperature and turbulence are
necessary to achieve adequate emission control.
Flares ideally burn waste gas completely and smokelessly. Two
types of flares are normally employed. The first is called the open
flare, where as the second is called the enclosed flare. The major
components of a flare consist of the burner, stack, water seal,
controls, pilot burner and ignition system. Flares required to
process variable air volumes and concentrations are equipped with
automatic pilot ignition systems, temperature sensors and air and
combustion controls.
Open flares have a flare tip with no restriction to flow. The
flare tip being the same diameter of the stack. Open flares are
effectively a burner in a tube. Combustion and mixing of air and
gas take place above the flare with the flame being fully combusted
outside of the stack.
Enclosed flares are composed of multiple gas burner heads placed
at ground level in a stack like enclosure that is usually
refractory or ceramic lined. Many flares are equipped with
automatic damper controls that regulate the supply of combustion
air depending on temperature which is monitored up stream of the
mixing, but inside the stack. This class of flare is becoming the
standard in the industry due to its ability to more effectively
control emissions.
Requirements on emissions includes carbon monoxide limits and
minimal residence time and temperature. Exhaust gas temperatures
may vary from 1000 to 2000 F.
The ceramic filter collects, volatilizes and destroys
particulate and condensable organics emitted from industrial
process streams, such as paint spray, lost foam casting,
condensable organics, tenter frames and cured rubber operations. It
may be used independently as a hot or cold filter media or coupled
with an oxidization module for total odor and VOC control.
The ceramic filter is designed based on exhaust airflow volume,
type of contaminant and desired collection efficiency. During
operation, the process stream, containing hot or cold particulate
laden air, condensable organics or VOCs, is drawn into the ceramic
filter. The process stream passes over a ceramic matrix selected on
particle size and collection efficiency considerations.
The ceramic matrix is periodically heated by a natural gas
burner, whereby any organics collected on the ceramic matrix are
volatilized. Any non-organics collected on the ceramic matrix are
converted to inorganic ash and retained in the unit drop out
chamber. If required, the volatilized organics can then be
processed through a catalyst bed. As is in the catalytic oxidizer
these volatiles would be converted to CO2 and H2O.
Copyright 1998 by McGraw-Hill Companies, Inc. All rights
reserved. Except as permitted under the United States Copyright Act
of 1976, no part of this publication may be reproduced or
distributed in any form or by any means, or stored in a database or
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