electrical equipment W Static electricity grounding system W Heating, cooling appliances for

chemical stability, freeze/boil pro- tection and comfort control

W Eyelface wash station W Explosion relief construction

Segregation features for separating incompatible chemicals and accumulat- ing hazard0.w chemicals in a safe area incMe-the following.

W Interior partitions and divided sumps

W Shelving -wood or metal W Separate detached lockers in a

centralized storage area W Site arrangement in compliance

with codes for spacing and set- back distances

Security features (from entry by unauthorized personnel) include:

Tamper-proof heavy-gauge steel construction

W Door with a three-point locking mechanism

W External light for area illumination against vandalism

W Outside electrical switches to ensure safety External hazard signs and safety alarms

Selecting a Locker The National Fire Protection Associ-

ation recognizes hazardous material storage lockers. As defined by NFPA, a locker is a relocatable prefabricated structure manufactured at a site other than the final location of the structure and transported assembled or in a ready to assemble package to the final site. Other major codes that apply to the lockers derive from the three Model Building Code Groups, and various state and local regulations, codes, and ordinances. There are differences between these codes. The UBC code states that “secondary containment is required, and shall be designed to retain the spill from the largest single container plus the design flow rate of the sprinkler system.. .for a period of 20 minutes.” This would far exceed any of the sump capacities available with pre- fabricated storage lockers. The solution to compliance would be a third type of containment (or overflow drainage area, as listed in the code) or elimina- tion of the water sprinkler as a fire pro- tection system.

The UBC code also says that floors containing hazardous materials must be made of non-combustible materials, which eliminates the use of plywood and fiberglass flooring in hazardous materials storage lockers. A steel grat- ing would comply, but this creates an undesirable sparking hazard for flam- mable and combustible liquids. Various rating and testing agencies also have standards that affect hazardous materi- als lockers.

When selecting a locker, or any other hazardous materials equipment, it is critical to deal with a reputable manufacturer. Look for a Factory Mutual or Underwriters Laboratory label (or state seal if applicable), and ask for documentation that the product meets the compliance standards i n your community. Many states and municipalities also have specific programs for approval or permitting of hazmat lockers. 0

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The National Environmental Journal January/February 1994 65

Tighter regulations mean that emissions that have gone untreated must now be controlled. Economic airborne emission control for the CPI presents several unique challenges. As with any add-on control system, the goal is to minimize the annualized total cost. The challenges result from the nature of the emission sources and the wildly fluctuating exhaust stream variables.

A few of the emission streams presenting con- trol challenges in the CPI include: 0 Emissions from multiple small sources, often

spread out over a wide area 0 Emissions from batch operations 0 Unpredictable fugitive emissions

For economic treatment, as many hydrocarbon emission sources as possible should be collected into as few streams as possible in the lowest air volume possible. For some sites, this means that an extensive duct system is necessary to collect and deliver the emissions to one location. Examples may be a product storage facility with multiple tanks or vessels, each venting small, irregular volumes when product is loaded, displacing a vapor phase: or a facility with many batch processes, each venting a variety of hydrocar- bons in unknown concentrations whenever the process is initi- ated or terminated. Collecting fugitive emissions is a more unique challenge and must be tackled on a case-by-case basis. Fugitive emissions should also be collected in the lowest air vol- ume possible.

I

E x ~ a u Once collected, emission streams from the CPI often exhibit

a range of process variables around which the control system must be designed. 0 Fluctuating air volumes 0 Multi-component emissions 0 Varying hydrocarbon blends 0 Varying hydrocarbon concentrations

Other variables affecting technology and feature selection include operating characteristics of the emitting process, the method of emission collection, and the number and magnitude of sources. How these details affect technology selection is examined more closely below.

Air emission control systems can either recover or destroy hydrocarbons. If reuse is possible, recovery is generally pre- ferred. In most cases, however, the only recovery value is as a fuel source; the recovered hydrocarbons are eventually burned. The most commonly applied form of destruction is thermal oxidation. In this process, hydrocarbons are converted at an elevated temperature to carbon dioxide and water vapor. Several types of oxidation processes are available: 0 Recuperative thermal oxidizer 0 Catalytic thermal oxidizer 0 Regenerative thermal oxidizer

Recuperative thermal oxidizers (recups) include a combus- tion chamber with a primary heat exchanger to recover waste heat from the hot incinerated exhaust air to preheat incoming air (see Figure 1). These oxidizers typically include a shell- and-tube type heat exchanger capable of up to 70 percent pri-

mary heat recovery. With an oxidation temperature of about 1 400°F, sufficient waste heat is available for secondary heat recovery. This heat is typically recovered for process heating or to generate steam or hot water. A recuperative oxidizer is therefore best suited for those applications where the excess heat can be used. This offsets the relatively high operating (fuel energy) cost for the unit.

The recup can handle a wide variety of solvent blends, as well as the fluctuating solvent concentrations often seen in the CPI. For high inlet temperatures, the oxidizer heat recovery must be limited. Excessive heat recovery could result in prema- ture hydrocarbon ignition in the preheater section. In cases where this is possible, the preheater must be partially or totally bypassed to limit thermal efficiency. The recup will efficiently destroy halogenated compounds, but more exotic metallurgy is required, making it a less economical alternative. The pressure drop through the unit is steady, requiring no automatic volume control. If the emitting process is variable volume, some type of automatic control for steady burner operation is required.

Catalytic oxidizers operate on the same principal as a recu- perative oxidizer, but use a catalyst to aid in the oxidation process. This can reduce the operating cost of the unit when compared against a- recuperative thermal oxidizer. The catalyst - either a noble or base metal type - can lower the necessary preheat temperature to below 600°F. This allows the oxidizer to be built from less exotic steels as well, resulting in lower fabrica- tion costs.

The major disadvantages of catalytic units are higher mainte- nance costs due to the need to monitor catalyst in order to ensure performance. Heavy metals and halogens are known to deactivate a catalyst, as do certain organic silicones, although some newer catalysts are available to be applied on halo- genated streams. In general, the user needs to be very knowl- edgeable about the various organic and inorganic compounds in an exhaust stream to consider using a catalytic oxidizer. The catalytic thermal oxidizer is not recommended for air streams containing changing hydrocarbon blends and fluctuating con- centrations, and is a poor technology for many of the exhaust streams found in the CPI.

Regenerative Thermal Oxidizers (RTOs) consist of a purifi- cation chamber located above three energy recovery cham-

The National Environmental Jouvnal January/February 1994 67

ferred for storage to the heat exchange media. The olern air then passes through the exhaust and is discharged through a stack to atmosphere. The temperature of the air as it leaves the unit is close to the temperature of the polluted air entering the RTO. At least one chamber is always on inlet and outlet mode at the same time to allow the RTO to continuously process a pol- luted air stream efficiently. ~

The RTO is equipped with a purge system that allows the evacuation of solvent-laden air trapped below the heat exchange media. The automatic purge cycle forces this polluted air into the purification chamber where the hydrocarbons are destroyed. This fea- ture ensures a continuous high destruc- tion efficiency. RTOs are typically designed for heat recovery of up to 95 percent and hydrocarbons destruction efficiency up to 99 percent. The advan- tages of an RTO include very high ther- mal and destruction efficiency, low NOx emission and less susceptibility to the type of hydrocarbons and lower operat- ing cost. Disadvantages include large size, more expensive installation, higher capital cost, and more moving parts for maintenance.

Many features of the RTO make it an idea technology for the CPI. In some cases design modifications are neces- sary for the unit to safely handle the wide range of exhaust stream charac- teristics. The high thermal efficiency of the RTO means fuel costs are low for very dilute streams. Unfortunately, if the exhaust stream feeding the unit can at times emit very high hydrocarbon concentrations, the thermal efficiency of the unit becomes a negative. As the concentration increases, the excess energy available from the hydrocarbon

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bers (see Figure 2). The energy recovery chambers are filled with ceramic heat exchange media. The hydrocarbon-laden air enters the inlet header and is directed to one of the energy recovery chamhers through an in!e? c ~ n t r o ! va!\ve. The air passes through the heat exchange media, absorbing heat from the media. It then enters the purification chamber at a temperature very close to the oxidation temperature. The oxi- dation process is completed in the purification chamber. A gas burner maintains a preset incineration temperature. If the incoming polluted gas contains sufficient concentration of sol- vents, the energy in the solvents provides the necessary heat to raise the temperature to the combustion set-point. In this case, the burners will go to pilot.

The purified air leaves the unit through the heat exchange media of an adjacent chamber. The heat in the hot air is trans-

heat of combustion must beexhausted to atmosphere. In this situation, some form of heat rejection must be built in.

When directed into the RTO through the ceramic-filled tower (its usus! path), the process air enters the combustion chamher at over 1300°F. During periods of high hydrocarbon concentra- tions, the heat rejection system routes process air at inlet condi- tions directly into the combustion chamber of the unit. By bypassing the ceramic-filled tower and flowing directly to the combustion chamber, the inlet air acts as a cooling media for the excess combustion energy, maintaining control over the combustion chamber temperature. Of course, the excess com- bustion energy must still be exhausted to the atmosphere. The stack temperature gradually increases during periods of high hydrocarbon concentrations, indicating heat rejection.

Multiple emission sources, such as process vents or small

68 The National Environmental Journal January/February 1994

batch operations ducted together, can usually be treated with an RTO. Its flexibility when oxidizing most any hydrocarbon mix or concentration means even irregular emissions are not a major problem. Additionally, the RTO can handle a very wide range of air volumes, but if a widely varying process dictates that the unit be capable of turndown lower than 25 to 50 percent of design, the RTO must be modified. Generally, recycle capabilities are installed to ensure that even with inlet flow as little as 2 to 4 per- cent of design, the unit will operate steady state with continued high destruction efficiency.

I

For large air streams with relatively low hydrocarbon concen- tration, rotary adsorbers can be used to concentrate the emis- sions into smaller air streams that can be handled much more economically (see Figure 3).

The hydrocarbon-laden air passes through the rotary adsorp- tion unit where the hydrocarbons are adsorbed onto zeolite or carbon media. The purified air is exhausted to the atmosphere. The hydrocarbons adsorbed on the media are then removed by desorption with higher-temperature, low-volume air. The des- orbed air containing a high concentration is delivered to an oxi- dation device.

The rotary concentration system is designed to continuously adsorb hydrocarbons from an air stream onto zeolite or carbon and discharge purified air. This is achieved through the use of a moving adsorbing wheel, a section of which is simultaneously desorbed. This design eliminates the need for dual running and stand-by adsorption beds. Solvents are adsorbed onto the adsorbent material and the purified air exits through the center of the cylinder. A portion of the rotating cylinder is simultane- ously desorbed by passing hot air through a section of the cylin- der. This desorption section is sealed off from the remaining adsorption section of the rotor by accurate seals, so that very high efficiencies can be obtained in the system. Capture effi- ciencies for this type of system may be as high as 99 percent.

Rotary concentration systems can be designed using zeolite or activated carbon as adsorption media. In certain applications, a granular activated carbon (GAC) prefilter is used upstream of the rotor. The GAC prefilter efficiently removes the higher boiling hydrocarbons before the honeycomb rotor. If the process emits hydrocarbons in widely varying concentrations, the GAC will also dampen the concentrations so that what reaches the rotor is much more consistent. The consistent concentration will improve removal efficiency in the rotor, enabling this system to be applied to a much wider range of CPI applications. The advantages of rotary concentration systems include low energy consumption, low operating cost, lack of NOx, low pressure drop across the system, good reliability, and ease of operation and maintenance. As with any adsorption system, particulate filtration is required upstream of the adsorber to prevent blinding of the media.

Hybrid systems using rotary concentration adsorbers to con- centrate solvents in large dilute air streams and oxidizers to destroy the concentrated hydrocarbons are common (see Fig- ure 4). The operating cost benefits of using a concentrator in con- junction with a small oxidizer make this a very attractive option.

A chemical manufacturer in the northwest has an onsite prod- uct storage facility. As product is loaded into the various tanks and vessels, hydrocarbons trapped in the vapor space are vent-

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The National Environmental Journal January/February 1994 69

VOC Concentrator/Oxidation System Exhaust Exhaust

95-99% Clean Fresh Air 9599% Clean 4 10% of A

Process Exhaust

Process Exhaust Secondary Heat

Recovery System

ed to the atmosphere. The hydrocarbon-laden air is exhausted in small, irregular volumes and contains a wide range of hydro- carbon concentrations. Because of high total tonnage of emis- sions, the manufacturer was forced to control the emissions.

All the sources were collected in an extensive duct system routed throughout the facility. The total air volume fluctuated with- in a wide range over the course of routine facility operations. At any given time, the hydrocarbon mix within the stream was vir- tually unknown and contained concentrations ranging from

almost zero to several thousand ppmv. The total air stream was ducted to a

regenerative thermal oxidizer. To ensure consistently high destruction as well as smooth operation despite the fluctuating inlet conditions, several features were designed into the unit. A heat rejection system was incorpo- rated to control combustion tempera- ture even with extremely high hydrocarbon concentrations. A vari- able frequency drive was included with the exhaust fan to automatically com- pensate for air volume changes. For extremely low air volumes, an exhaust recycle feature interfaced with a fresh air inlet to provide stable operation at ultra-low volumes. The resulting sys- tem oxidizes more than 98 percent of

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the incoming hydrocarbons to carbon dioxide and water vapor. Operation is very smooth and steady state, virtually operating without operator intervention. No problems related to the wide range of inlet conditions have been experienced.

In choosing the right technology, it is important to begin with sampling to determine the types and concentration of hydrocar- bons/toxics to be expected. A careful review of current and future regulations, along with local site considerations, should then be

0 used to select the appropriate option.

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70 The National Environmental \ournal January/February 1994