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Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization 1 Introduction and Background The United States began the destruction of its stockpile of chemical weapons long before the passage of the Chemical Weapons Convention (CWC), which came into force April 29, 1997. The U.S. Army, as the executive agent for the U.S. Department of Defense for stockpile destruction, is using a combination of incinerators at two sites (Johnston Island and Tooele, Utah) where the chemical agents are present in assembled munitions, such as rockets and artillery projectiles. The Army plans to use incinerators at some additional stockpile sites with assembled munitions, as well. In response to public concerns associated with incineration, the Army is currently proceeding with development of alternative technologies for two sites (Aberdeen, Maryland, and Newport, Indiana) where chemical agents are stored in bulk and are more accessible than those contained in weapons and where the destruction of explosives and propellants is not required. The technology chosen was chemical neutralization, which detoxifies the chemical agent but does not completely destroy its potential precursors. This decision was reached with input from the National Research Council (NRC) Panel on Alternative Chemical Disposal Technologies (AltTech Panel) (NRC, 1996a). A number of technologies have been considered for the safe and environmentally sound disposal of the neutralization products, including biodegradation, incineration, wet air oxidation followed by biodegradation, and supercritical water oxidation (SCWO)(NRC, 1994a). SELECTION OF NEUTRALIZATION FOLLOWED BY SUPERCRITICAL WATER OXIDATION FOR STOCKPILE DISPOSAL AT NEWPORT, INDIANA The U.S. Army plans to destroy VX nerve agent stored in bulk at its Newport, Indiana, facility with a disposal technology based on chemical neutralization. This destruction process results in the production of a solution, called “hydrolysate,” 1 that retains some undesirable characteristics. The hydrolysate produced by the neutralization reaction is greatly reduced in toxicity compared to the agent but requires further treatment to meet the requirements for safe and environmentally acceptable disposal. Further treatment is also required to destroy remaining constituents that contain carbon-phosphorus bonds to meet the requirements of the CWC. The Army has tested biological oxidation to destroy the organic compounds that remain in the solution. However, research to date, under the auspices of the Program Manager for Chemical Demilitarization, has shown that biodegradation does not effect adequate destruction. As an alternative, the Army has selected SCWO, an approach that was recommended by the NRC Committee on Review and Evaluation of the Army Chemical Stockpile Disposal Program (Stockpile Committee) in its 1994 report (NRC, 1994a). VX STORAGE AT NEWPORT, INDIANA The Newport Chemical Activity was the U.S. production facility for VX nerve agent. The agent is no longer produced there or at any other U.S. site, but 1,689 ton containers holding a total of 1,269 tons of VX are stored on the site. VX is one of the most toxic chemical agents. As little as 10 mg of VX absorbed through the skin or lungs will 1 Chemical neutralization of other agents (e.g., blister agents such as mustard and nerve agents such as GB) also produces hydrolysates. However, the composition and potentially viable treatment alternatives for each hydrolysate varies based on the type of agent. For the purposes of this document, “hydrolysate” and the discussion of treatment process effectiveness refer only to hydrolysate produced by the neutralization of VX with aqueous sodium hydroxide.
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Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization kill or incapacitate a 70 kg individual by massive disruption of the central nervous system (U.S. Army, 1996a). Moreover, VX exhibits a characteristic persistence if released into the environment because it has a low vapor pressure and evaporates slowly under normal atmospheric conditions. It is slowly deactivated by moist air but can persist for a long time in an arid climate. The VX agent stored at Newport is of reasonably pure chemical composition. The major impurities are derived from a dialkylcarbodiimide, which was added to the agent to prevent hydrolysis during transfer operations and storage. The carbodiimide additive reacts with water to form a dialkyl urea: Based on a 1996 survey of the VX stored at Newport, the VX stored in a typical container is about 94 percent pure, with the remainder consisting of a large number of minor impurities. The largest percentage of impurities are diisopropylcarbodiimide (1.74 percent), diethyl dimethylpyrophosphonate (0.99 percent), and 2-(diisopropylamino)ethane thiol (0.89 percent) (U.S. Army, 1996b). NEUTRALIZATION OF VX The Army has evaluated several ways to neutralize (detoxify) VX by chemically destroying the phosphorussulfur bond associated with the neurotoxicity of the VX molecule. Chemical hydrolysis of this bond by neutral or alkaline aqueous solutions appears to be the most attractive approach based on the general simplicity of the process. The ease of process control was a significant factor in the selection of hydrolysis by aqueous caustic solution as the best candidate for further development. Other neutralization process options studied by the Army include using nonaqueous bases, such as methanolic KOH and monoethanolamine (U.S. Army, 1996c; NRC, 1993; Yang, 1995). In the hydrolysis of VX, the primary reaction is cleavage of the phosphorus-sulfur bond to form the sodium salt of ethyl methylphosphonic acid (EMPA) and a thiol: An undesirable side reaction is the cleavage of a phosphorus-oxygen bond in VX to form ethyl alcohol and a salt of EA-2192: Because EA-2192 retains a phosphorus-sulfur bond, its toxicity is only slightly reduced from the toxicity of VX. Fortunately, extending the reaction time for the caustic hydrolysis of VX also causes hydrolysis of EA-2192 to the relatively innocuous methylphosphonic acid (MPA). With a reaction time of six hours at 90°C, the overall efficiency for destruction of VX exceeds 99.9999 percent, and the toxicity of the resulting aqueous solution is 40,000-fold less than the toxicity of VX (NRC, 1996a). The alkaline hydrolysis of VX has been tested on a substantial bench scale in the Army's research and development facility at Edgewood, Maryland. Many of the tests were carried out in highly instrumented 12-liter reactors, but several tests were conducted on a larger scale in 114-liter reactors. The latter experiments, which typically destroyed 24 to 30 kg of VX per test, produced large quantities of hydrolysate for tests of predisposal treatment and provided operating experience that should be valuable in future tests of pilot plants. All of the bench-scale tests were done using munitions-grade nerve agent similar to the agent stored in ton containers at the Newport facility. Procedures for decontaminating emptied VX storage containers were also tested. A key finding in the bench-scale tests was that vigorous mixing is required to ensure the complete destruction of VX, which has only limited solubility in the alkaline reaction mixture. The VX dissolves as the reaction progresses because the major hydrolysis products (EMPA, MPA, EA-2192, and thiol) are soluble in the strongly basic mixture. However, a significant complication for subsequent treatment prior to disposal is that several minor products are insoluble. These compounds form a small liquid layer (up to 10 weight percent) that floats atop the alkaline layer that contains the major reaction products. The water-insoluble products largely derive from the thiol and the carbodiimide stabilizer that were added to the agent. The composition of the VX hydrolysate that was used for SCWO treatability studies
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Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization TABLE 1-1 Composition of Hydrolysate from Neutralization of VX Component Concentration (mg/l) Ethylmethylphosphonic acid (EMPA) 152,673 Methylphosphonic acid (MPA) 13,348 Diisopropylaminoethanethiol (Thiol) 160,000 Bis (diisopropylaminoethyl) disulfide 13,000 Bis (diisopropylaminoethyl) sulfide 970 1,9-bis (diisopropylamino)-3,4,7-trithianonane 1,700 Total organic carbon (TOC) 140,000 Sulfate 96.9 Phosphate (as phosphorus) 2.19 Total sulfur (S) 38,400 Total phosphorus (P) 37,700 Arsenic (As) 0.125 Barium (Ba) 0.236 Calcium (Ca) 121 Chromium (Cr) 1.38 Copper (Cu) 1.53 Iron (Fe) 2.97 Lead (Pb) 0.50 Magnesium (Mg) 2.79 Mercury (Hg) 0.004 Selenium (Se) 2.0 Sodium (Na) 87,900 Titanium (Ti) 0.25 Zinc (Zn) 0.25 is presented in Table 1-1. 2 This composition is typical of the VX hydrolysate produced by neutralization using sodium hydroxide. Source: Adapted from General Atomics, 1997a. As Table 1-1 shows, the primary P-containing products are EMPA and MPA. Prior to release from the neutralization process step to subsequent treatment steps, both VX and EA-2192 3 must be nondetectable in the hydrolysate, with detection limits established at 20 ppb for VX and 5 ppm for EA-2192. Release to the treatment and disposal operations (the next step) will not be permitted unless concentrations of VX and other toxic constituents meet appropriate standards. The Army should evaluate and confirm that the standard for releasing hydrolysate from VX neutralization to subsequent treatment steps is adequate and verifiable. Although EMPA and MPA have low toxicity, destruction of these compounds is required for environmentally responsible disposal, as well as for meeting the requirements of the CWC, which designates these compounds as “Schedule 2 precursors ” because they could potentially be reconverted to nerve agents if they could be recovered from the hydrolysate. The thiol resulting from VX hydrolysis gives rise to several by-products through secondary reactions. These by-products include disulfides (RSSR') and an assortment of diisopropylaminoethyl compounds, listed in Table 1-1 . Minor products that derive from the stabilizer are not included in the analysis reported in the table. The thiol-derived products have foul odors, and, like MPA derivatives, they must be destroyed before disposal. The thiol itself is a Schedule 2 precursor and must be destroyed to comply with CWC treaty obligations. The Army has decided to destroy the mixture of organic compounds listed in Table 1-1 by oxidative processes that “mineralize” the materials, i.e., convert the elements in these compounds to oxidized forms, such as carbon dioxide, water, phosphate, and sulfate. Of the various oxidation technologies, SCWO was considered the most likely to adequately destroy the organic constituents in the hydrolysate. Because there is little operating experience with SCWO on an industrial scale, however, the Stockpile Committee was asked to evaluate whether SCWO is an effective and appropriate means of preparing VX hydrolysate for ultimate disposition. The NRC was not asked to conduct an in-depth analysis of the entire integrated VX bulk agent destruction and disposal process for the Newport Chemical Agent Disposal Facility (NECDF). As the facility design is being finalized (March 1999-April 2000), the NRC will probably be asked to assess all aspects of facility design, including monitoring, containment, process control, and redundancy, as well as the quantitative risk assessment (QRA). This report outlines the elements of the proposed neutralization/SCWO technology, 2 The numbers used in tables throughout this report are reproductions of the numbers reported in the original citation. Precision of the numbers should not be inferred by the number of significant figures reported. 3 EA-2192 is an intermediate form during the neutralization reaction which further reacts to form MPA. EA-2192 is a nerve toxin. Analysis for VX and EA-2192 is carried out on a homogenized sample (including both aqueous and organic layers) of the hydrolysate.
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Using Supercritical Water Oxidation to Treat Hydrolysate from VX Neutralization evaluates the results of ongoing SCWO tests, and makes recommendations concerning aspects of the technology that require further development. The scope of this evaluation did not include evaluations of other potential technologies or management options for the treatment of VX hydrolysate. For the evaluation of SCWO technology for treatment of VX hydrolysate, the Stockpile Committee reviewed documents on SCWO process fundamentals, previous applications, testing carried out specifically for the treatment of VX hydrolysate, and the planned process design for the Newport facility; met with SCWO academic researchers, process developers from Sandia National Laboratory, and vendors; visited a pilot-scale SCWO testing facility; and held discussions with the Army and its process design contractors.
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