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Designing a Scrubber to Control Emergency Releases

Posted on: Tuesday, 19 July 2005, 03:01 CDT

Here are guidelines for determining the scope of the control system, sizing the enclosure, and specifying the scrubber design.

Scrubbers are commonly used for air pollution control, and are designed based on a known continuous emission rate from a process and well-understood chemistry. The design of scrubbers for emergency releases, however, is more complicated. Usually neither the composition of the emissions nor the emission rate are known and must be estimated. Furthermore, during an emergency release, the emission rate is not continuous, and may initially be quite large. This article explains how to design an emergency scrubber, and illustrates the stepby-step procedure for a hypothetical facility.

Define the scope of the problem

The first step in designing an emergency scrubber for a particular application is to decide what process areas or potential events will be controlled by the enclosure and control system. This decision should be based on previous experience with releases (if any) and a detailed analysis of potential scenarios to assess their probability.

At the example facility, the batch process involves mixing highly reactive chemicals in a series of steps. The process equipment includes stationary mixers, portable mixing vessels (pots and drums), interim storage vessels, and unloading equipment (which transfers the product into smaller containers for shipment). The risk of an emergency release exists due to the potential for exothermic polymerization at a critical step in the process, during a short period immediately after the addition of the reactive chemicals. Any deviation from the process protocol (for example, if the chemicals are introduced in incorrect quantities) can trigger polymerization.

The portable mixing vessels, storage drums, and unloading vessel will be controlled by the emergency scrubber.

The stationary mixer will not be controlled. An exotherm in the mixing pot is unlikely due to agitation, which improves heat transfer and prevents hot spots. In addition, the mixing pot is too large to fit inside the enclosure.

The batch unloading equipment will also be excluded, since during normal operation no exotherm is expected during the short time the vessel is on the unloader. In the event of an upset or power failure, the vessel can be removed manually from the unloader. In addition, the 20-ft-tall unloading mechanism is also too large to fit inside the enclosure.

Estimate the mass emission rate

Next, the chemicals of concern must be identified and their mass emission rates during a potential release quantified. The chemical composition of a gaseous release may be quite different than the solid or liquid substrate from which it was derived due to chemical reactions and reaction rates. Therefore, obtaining data that properly represent the potential chemical composition of the release gas is critical.

An accelerating rate calorimeter (ARC) can be used to study the exothermic polymerization reaction in a laboratory. A sample is placed into a small spherical bomb and then put into an oven and blanketed with nitrogen. The oven temperature is steadily increased to generate the exothermic reaction. The gaseous release is collected and analyzed in portions, for example by gas chromatographymass spectrometry (GC-MS) or ionic chromatography, to identify the types and quantities of chemicals present in the release gas on a mass-emitted-per-sample-mass basis.

Table 1. Immediately dangerous to life and health (IDLH) and short-term exposure limit (STEL) values for various chemicals.

Table 2. Comparison of control technologies for emergency release control.

These data can be used to calculate the total mass that could potentially be emitted during an actual release. In addition, the ARC data can provide a means to calculate mass emission rates that fluctuate until the reaction ceases. As a conservative approach, a constant rate equal to the maximum mass emission rate should be assumed for the estimated duration of the release. Estimating the duration of the release depends on the nature of the release, and whether it is relatively short (15 min to 1 h) or long (several hours).

Setting the outlet concentration

Unless a limit on outlet concentration has been specified, for instance by a particular regulation, it can be set by applying a safety factor to published exposure limits, such as immediately dangerous to life and health (IDLH) values or short-term exposure limits (STEL).

Table 1 lists IDLH data for the chemicals identified in the ARC testing - ammonia, dimethylamine and trimethylamine (1, 2). A typical safety factor of 10 is also shown.

Selecting the control system

Table 2 compares three common air-pollution control systems - carbon adsorber, aqueous scrubber and thermal oxidizer (which can include catalysis and may be equipped with heat recovery) - in terms of such variables as operating temperature, chemical application, special exclusions, waste treatment, cost and overall feasibility.

For emergency releases of the type expected from this mixing process - organics containing ammonia and amines - a scrubber appears to be the best choice for several reasons:

Table 3. Basic design features of a scrubber for the mixing process example.

Table 4. PTE design criteria and specifications for the example enclosure.

* The scrubber does not require heat-up to operating temperature prior to use, as the thermal oxidizer does. The thermal oxidizer would not be feasible for an emergency release application due to the high costs for fuel to maintain the unit at the required temperature.

* The scrubber is suitable for ammonia and amines, since these chemicals are water-soluble and can be neutralized in the scrubber. The carbon adsorber is not suitable for ammonia or amines because they are not adsorbed effectively.

* The scrubber is expected to have medium capital cost (relative to high capital cost for the thermal oxidizer and low capital cost for the non-regenerable carbon adsorber), but relatively low operating cost, except for waste disposal costs, which could be significant.

Scrubber features

Other design considerations for a scrubber include the scrubbing solution, liquid handling method, provisions for pH control, packing type and system pressure drop. Table 3 summarizes the features of a scrubber handling a stream containing ammonia and amines.

Enclosure design

An emergency-release control system includes an enclosure sized to accommodate the fixed equipment and/or portable containers that have the potential for gaseous release. The enclosure should have a door that permits access and that may be closed during the release to minimize exposure to emissions. The door should have several openings to allow air to enter, displace the gaseous release, and move it into the ducting leading to the scrubber.

To ensure proper capture of the released gases, the enclosure should be designed to meet the permanent total enclosure (PTE) criteria established by the U.S. Environmental Protection Agency (EPA). A PTE is defined as "a permanently installed enclosure that completely surrounds a source of emissions such that all emissions are captured and contained for discharge to a control device" (3). Therefore, if an enclosure meets the PTE criteria, 100% capture of gaseous releases will be achieved, and the overall control efficiency will be equal to the destruction efficiency of the control device.

Figure 1. Plan view of enclosure showing locations of the drums and the mixing pot.

Table 5. Required efficiency is based on the inlet concentration and the required outlet concentration.

Table 6. Specifications for the example scrubber (at 3,000 actual ft^sup 3^/min).

Table 4 lists the PTE criteria and the specifications for an enclosure that meets these requirements for this mixing example, with either four 55-gal (22.8 in. dia.) drums or one 200-gal (36 in. dia.) mixing pot. Figure 1 illustrates such an enclosure for two 55- gal drums and one mixing pot.

Figure 2. Typical scrubber with a recirculating pump. Photo courtesy of Misonix, Inc.

Scrubber design

The scrubber must be designed to handle a volumetric rate that is set by the enclosure design. In this example, a flowrate of 3,000 actual ft^sup 3^/min is required, which was calculated based on a velocity of 200 ft/min and an opening area of 15 ft^sup 2^. The maximum inlet concentration for the chemicals of concern can be calculated by dividing the mass emission rate of the toxic gases by the selected ventilation rate.

For the example scrubber, required outlet concentrations of the toxic gases equal to 10% of the IDLH values were assumed. (If an IDLH value is not available for a particular chemical, the STEL can be used instead.) The inlet concentration and the required outlet concentration then determine the required reduction efficiency (Table 5). Based on the required reductions, we see that ammonia and trimethylamine are the limiting chemicals.

Table 7. Effect of design parameter changes on scrubber size and performance.

The proposed scrubber for this mixing process (Table 6) is a packed tower that uses recirculating dilute sulfuric acid \solution. The solution should be approximately 25% sulfuric acid initially, which is sufficient to neutralize the expected release quantity of ammonia and amines.

To minimize the amount of solution needed, the scrubber should be designed to operate in batch mode with no blowdown. Many scrubbers have an integral sump at the bottom to store the recirculating scrubbing solution. Keep in mind that at the end of the release event, the liquid collected in the sump will have to be disposed of, most likely as a hazardous waste.

Figure 2 shows a typical scrubber as described here.

Sensitivity analysis

Since developing the design basis for the emergency scrubber involves making estimates and assumptions when data are lacking, it is important to consider the sensitivity of the overall design to small changes in various design parameters. Table 7 presents an overview of the types of changes in initial assumptions that could affect the design.

Vendors should be asked to quote on a variety of options to allow the cost impacts of design changes to be evaluated. Where cost impacts are relatively small, a more conservative design should be adopted. For example, increasing the scrubber height and fan capacity slightly would allow the future addition of another few feet of packing material to achieve good performance should emission concentrations be higher than expected. Providing for extra and/or more-concentrated scrubbing solution is also beneficial.

On the other hand, it is very difficult to make adjustments after the fact for changes in exhaust rate, since the scrubber's size would have to be increased. Thus, it is preferable to carefully select this parameter at the start of the design process in conjunction with the enclosure design.

Literature Cited

1. National Institute for Occupational Safety and Health, "Documentation lor Immediately Dangerous Io Life or Health Concentrations (IDLH): NIOSH Chemical Listing and Documentation of Revised IDLH Values (as of 3/1/95), NTlS Publication No. PB-94- 195047, www.cdc.gov/niosh/idlh/intridl4.html.

2. National Institute for Occupational Safety and Health, "OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project, extracted from 54 FR 2332 et. seq., - Trimethylamine," www.cdc.gov/niosh/pel88/7550.html.

3. U.S. Environmental Protection Agency, "Method 204, Permanent Total Enclosures," Federal Register, vol. 62. p. 32502 (40 CFR 51, Appendix M) (June 16. 1997).

JOSEPH W. HOWER

JEFFREY MICHAEL FORDE

SUNIL OJHA

ANNE MCQUEEN*

ENVIRON INTERNATIONAL CORP.

*Anne McQueen is now with Geomatrix Consultants, Inc.

JOSEPH W. HOWER is a principal at ENVIRON International Corp. (707 Wilshire Blvd., Suite 4950, Los Angeles, CA 90017; Phone: (213) 943-6319; Fax: (213) 943-6301; E-mail: jhower@environcorp.com). He has over 26 years of experience in air quality management, including regulatory compliance, permitting, litigation support, expert witness work, risk management and pollution control engineering. He currently leads ENVIRON's work in the area of emissions trading. Previously, he held a variety of environmental- and energy-related positions with Radian Corp. and Procter & Gamble. He served on the board of directors for the South Bay Business Environmental Coalition, and on the South Coast Air Quality Management District (SCAQMD) Advisory Council, and has taught air-quality courses. He is a director of the West Coast section of the Air & Waste Management Association. He has an MS in mechanical engineering from the Univ. of Southern California and a BS in mechanical engineering from the Univ. of California, Irvine, and is a Diplomate of the American Academy of Environmental Engineers (DEE) and a licensed professional engineer in five states.

JEFFREY MICHAEL FORDE is a senior associate engineer at ENVIRON International Corp. (2010 Main St., Suite 900, Irvine, CA 92614- 7215; Phone: (949) 798-3625; Fax: (949) 261-6202; E-mail: jforde@environcorp.com), where he has been involved in regulatory compliance projects, air quality permitting and health risk assessments. His work has also included environmental site assessments and compliance audits for facilities located in the U.S. and Mexico; health risk assessments of toxics for California Proposition 65 and the California AB2588 "hot Spots" program; and dispersion modeling. Prior to joining ENVIRON, he worked as a staff engineer consultant at ENTRIX, Inc. He earned a BS in environmental engineering from California Polytechnic State Univ., San Luis Obispo, and is a licensed professional engineer (mechanical) in California.

SUNIL OJHA is a senior associate at ENVIRON International Corp. (2010 Main St., Suite 900, Irvine, CA 92614-7215; Phone: (949) 798- 3625; Fax: (949) 261-6202; E-mail: sojha@environcorp.com). His experience in air pollution control and air sciences includes regulatory compliance and permitting, emission calculation, health risk assessment, air dispersion modeling, air-quality management planning, air-pollution control technologies and evaluation of ventilation systems. Prior to joining ENVIRON, he worked as assistant manager with Reliance Power Ltd. in India. He holds an MS in civil engineering from Univ. of Toledo and is a certified protection professional (CPP).

ANNE McQUEEN is a senior engineer with Geomatrix Consultants Inc. (Costa Mesa, CA), where she manages projects in air quality permitting, air pollution control, and litigation support related to air quality issues, and she also does compliance auditing and due- diligence work. She has 15 years of experience in air quality engineering, including working at Radian Corp. and Environ Corp. She has led projects involving the design, specification, feasibility analysis, permitting, and troubleshooting of air-pollution control equipment, as well as large source-test programs for air toxic compliance, equipment diagnostics, and plume evaluation and mitigation. She holds a PhD in chemical engineering from California Institute of Technology and a BS in chemical engineering from McGiII Univ. She is a licensed professional engineer in California, and is a certified permitting professional in two California districts.

Copyright American Institute of Chemical Engineers Jul 2005

Source: Chemical Engineering Progress

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