Corrosion and Chemical Agent Destruction - ACS Symposium Series

Chapter 21

Corrosion and Chemical Agent Destruction Research on Supercritical Water Oxidation of Hazardous Military Wastes Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 10, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch021

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Kevin W. Downey, Richard H. Snow, D. A. Hazlebeck , and Adele J. Roberts 1

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General Atomics, 3550 General Atomics Court, San Diego, CA 92121 IIΤ Research Institute, 10 West 35th Street, Chicago, IL 60616 2

Supercritical water oxidation (SCWO) is a developing technology for the destruction of hazardous and nonhazardous wastes. General Atomics and its subcontractors are currently conducting two comprehensive research and demonstration programs geared toward the destruction of Department of Defense (DoD) wastes utilizing SCWO technology. Wastes of primary interest include chemical agents and solid propellants. Corrosion testing performed in support of these programs identified platinum and titanium as candidate reactor construction materials. Kinetics testing emphasized chemical agent destruction. Kinetics results for chemical agents show destruction and removal efficiencies for GB, VX, and mustard agents to be in excess of 99.9999%, limited only by detection capability.

Supercritical water oxidation (SCWO), also called hydrothermal oxidation (HTO), is a developing technology for the destruction of hazardous and nonhazardous wastes. SCWO destroys combustible materials using an oxidant in water at temperatures and pressures above the critical point of water, 374°C and 218 atm. General Atomics (GA) and its subcontractors are currently conducting two comprehensive research and demonstration programs geared toward the destruction of Department of Defense (DoD) hazardous wastes. The first program, performed for the Advanced Research Projects Agency (ARPA), involves the destruction of chemical agents, solid propellants, and other DoD hazardous wastes. The second program, performed for the U.S. Air Force, involves the destruction of Hazard Class 1.1 propellant removed from rocket motor casings. Both the ARPA and Air Force programs required significant research efforts to provide sufficient data for pilot plant design. Numerous research areas

0097-6156/95/0608-0313$12.00/0 © 1995 A m e r i c a n Chemical Society

In Innovations in Supercritical Fluids; Hutchenson, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 10, 2015 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch021

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INNOVATIONS IN SUPERCRITICAL FLUIDS

were investigated, including corrosion, kinetics, hydrolysis, catalysis, solids handling, and chemical agent destruction verification/kinetics (1-4). However, only the corrosion and chemical agent destruction data, generated under the ARPA program, are discussed herein. Corrosion testing investigated the corrosion resistance of various high nickel alloys, reactive metals, refractory metals, noble metals, and ceramics at temperatures of 350 to 550°C under the highly acidic conditions (pH=0) corresponding to SCWO of 5 wt% solutions of the chemical agents GB, VX, and mustard. Materials capable of withstanding these extremely aggressive conditions were identified. Chemical agent testing was performed at the facilities ofÏÏTResearch Institute (IITRI), a facility approved by the U.S. Army for the handling of chemical surety materials. Separate test programs were conducted for the chemical agents, GB, VX, and mustard. Corrosion Testing. Under supercritical conditions, the products that result from the oxidation of chemical agents are highly corrosive to a wide variety of materials. For example, the oxidation of GB agent yields hydrofluoric and phosphoric acids; the oxidation of VX agent yields sulfuric and phosphoric acids; and the oxidation of mustard agent yields sulfuric and hydrochloric acids. At desired pilot plant agent throughput rates, acid concentrations of several percent can be present, resulting in solutions with a pH of ~0. In order to perform basic corrosion research using materials coupons, one must have a testrigcapable of withstanding the corrosive environment for sufficient time to compile coupon data adequate for analysis and pilot plant design. Therefore, a partial solution to the corrosion problem was required during the test rig design and assembly stage because typical high strength materials such as Hastelloy C276 are grossly inadequate for even relatively short-term testing of highly acidic feeds. A literature search showed platinum to be a good materials candidate for many of the feeds in question, but the available data did not cover the supercritical regime (5-6). A platinum-lined reactor was designed and fabricated for testing candidate materials coupons at temperatures of 350,450, and 550°C at a pressure of approximately 4000 psig. Numerous coupons could be suspended in an isothermal section of the reactor for extended periods of time. Exposure times ranged from 5 to 100 nr. The control system was designed to allow unattended overnight operation with automatic shutdown if temperature, pressure, or flow conditions outside of prescribed limits occurred. The corrosion test rig is shown in Fig. 1. Table I shows the materials coupons studied during the test program. Most coupons were 3/8" square and approximately 65 mils thick. Table II shows the range of corrosive media tested. These test solutions corresponded to the composition expected following complete oxidation of 5 wt% chemical agent feeds. The H2O2 was present to provide oxygen, upon thermal decomposition, in the approximate concentration expected during later full-scale operations.


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TABLE I. MATERIALS INCLUDED IN CORROSION STUDIES Category Material


Nickel alloys Cobalt alloys Reactive metals & alloys Refractory metals Ceramics

C22, C276, 625, 825, HR-160 188 Ti Gr 7, TÎ-21S, Nb/55% Ti, Zr 704, Hf Mo Gr361,Nb, Ta A1 0 , AI2O3 (sapphire), A1N, SiC, Si N , ΖιΌ /9% Y 0 Pt, Pt/10% Rh, Pt/20% Ir PTFE 2

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Noble metals & alloys Other

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TABLE IL CORROSIVE CHEMICAL AGENT SIMULANT MEDIA Simulant Compo Temperature Pressure Time (hr) (psi) Test Simulant sition (wt%) (°Q 350,450, 550

4000

0, 5, 20, and 65 or 90

1.8%H S0 , 1.8% 7.0% H 0

350, 450, 550

4000

0, 5, 20, and 65 or 90

Mustard agent, 3.1%H S0 , 2.3% 5 wt% HC1, 7.0% H 0

350,450, 550

4000

0, 5, 20, and 65 or 90

GB agent, 5 wt%

0.7% HF, 3.5% H3PO4, 7.0% H 0

VX agent, 5 wt%

H3PO4,

2

2

4

2

2

2

4

2

2

2

A summary of the corrosion results is presented in Table III. All corrosion rates were determined from a combination of weight loss data and a visual inspection to confirm that weight loss calculations were reasonable. Additionally, microscopic inspections were performed for the most promising materials to look for signs of nonuniform corrosion. The data were compiled and presented in three broad classifications: (1) Good (200 mil/yr corrosion rate) - clearly unacceptable materials of construction. These relatively broad categories were selected based on perceived limits of corrosion acceptability. For example, corrosion rates >200 mil/yr were deemed too great for these materials to be of practical use, while materials with corrosion rates of intermediate levels may be acceptable for some of our applications. Because of economics, expensive materials such as platinum compounds can only be used under conditions where corrosion is quite low. The yearly corrosion rates used to categorize the materials coupons were calculated by extrapolating the rates observed during short-term testing to a yearly rate, assuming 24 hr/day, 365 day/yr exposure. Caution must be exercised in


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DOWNEY ET AL.

TABLE III. CORROSION RESULTS SUMMARY HF and H P0 (GB)


3

4

H S0 and H P0 (VX) 2

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HC1 and H S0 (mustard)

4

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Material

350°C

450°C

550°C

350°C

450°C

550°C

350°C

450°C

550°C

Pt Pt/Ir Pt/Rh Hf Ti Timet 21S Zr 704 Mo Nb Nb/Ti Ta A1 0 AIN Sapphire Si N SiC Zr0 C22 Hast. C276 Hayn. 188 HR-160 Inc. 825 Inc. 625

Ο 0

Ο

ο

0

0 0 0

Δ Δ Δ

0

0 0 0 0 0

0

0 0 0 Δ

Ο 0

ο

0 0

0 0 0 Δ

0

ο

0

0

0

Δ

ο

0 0

Δ Δ

Δ Δ

0 0 0

Δ Δ Ν/Α

Ν/Α

0

0

Ν/Α Δ Ν/Α Δ

2

3

3

4

2

Ο Δ 0 N/A -

Δ 0

0 0 0 Δ Δ 0 0 0 0 Δ Δ Δ 0 Δ 0 0 0 0 0 0 ο Ο ο 0 0 0 0 Δ 0 0 0 0 0 Δ 0 0 0 Δ 0 0 0 0 0 0 0 0 0 0 0 0 0 Δ Δ 0 Ο 0 0 Δ 0 Δ 0 0 Δ 0 Δ 0 0 Δ Δ 0 0 0 0 0 0 0 0 Δ 0 0 0 Ο Δ Good (200 mil/yr corrosion rate) Not available 0 0

ο

0 Δ Δ Δ Δ

0 0 Δ

0 0 Δ Δ

0 0 Δ

0 0 0 0 0 0

Δ Ο

0 0 0 0 0 0 0 0 0 0 0 0

0 Δ

0 0 0 0 0 0 0 0 0 Δ

0 0 Δ

ο

0 Δ

0 0 Δ

0 0 0 0 0 0

using these data since extrapolation from short-term data presents some risk. It is recommended that longer-term corrosion testing be performed. For the nonchloride-containing feeds tested (GB and VX simulants containing HF, H P0 , and/or H S0 ) platinum (and platinum alloyed with either iridium or rhodium) was found to be highly resistant to attack. It was clearly the best of all materials tested. Microscopic examination of platinum alloy coupons did not reveal any evidence of nonuniform modes of corrosion. High nickel alloys and ceramics showed poor resistance. Some refractory metals, e.g., titanium, showed reasonable resistance for some conditions, but not as good as platinum, especially over the entire range of test conditions and media. During the 350°C testing of mustard agent simulant in the platinum-lined reactor, it became clear that a platinum-containing compound or complex was produced which was soluble in the subcritical fluid. High platinum losses 3

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occurred in both the platinum coupons and the reactor liner. Upon further investigation, the platinum loss was found to be confined to temperatures below the supercritical transition region (at 350°C in our testing). At both 450 and 550°C., platinum was much more resistant to chloride attack. Because of the high rate of platinum attack at the transition temperature with chloride feeds, the platinum-lined reactor was unsuitable for long-term testing of these feeds. A titanium-lined reactor was therefore constructed and used to perform mustard agent simulant testing. Titanium exhibited poor resistance to GB and VX simulants, especially with regard to fluoride attack. But, based on preliminary results, titanium appeared to exhibit reasonably good resistance to chloride/sulfur attack. Further testing confirmed the preliminary results, and titanium was found to be acceptable for chloride-containing feeds. Microscopic examination and microhardness testing of the titanium coupons did not reveal any evidence of nonuniform modes of corrosion or oxygen embrittlement. As with the GB and VX simulant feeds, high nickel alloys and ceramics showed very poor corrosion resistance when exposed to chloridecontaining feeds. Chemical Agent Testing. The primary focus of the ARPA SCWO program, from both a research and design standpoint, was the processing of the nerve agents GB and VX, and the blister agent mustard. The safety requirements necessary for processing chemical agents dictated much of the pilot plant design, and the extremely corrosive conditions accompanying SCWO of chemical agents dictated the corrosion test conditions. To ensure a comprehensive, workable design, it was imperative that laboratoryscale SCWO testing of actual chemical agents be conducted. (None had been performed prior to this program.) Testing was performed at ÏÏTRI in discreet phases to provide sufficient time for changeover of equipment and agent monitors, validation of analytical techniques (7-9), and agent deliveries from the Army. Agent testing was performed in a bench-scale test rig specifically designed for testing at IITRI. The rig was made as compact as possible to allow insertion into an IITRI agent test hood. The hood provided a ventilated, protected workspace for agent testing. The major features of the test rig included: (1) storage reservoirs for oxidant (H2O2) and water mixtures, (2) a high pressure pump, (3) an 8.5-ft long, 3/4" O.D., heated reactor lined with either platinum (for GB and VX testing) or titanium (for mustard testing), (4) slip-on heaters in three independently controlled temperature zones, (5) cooled reactor end fittings with O-ring seals, (6) pressure monitors at both ends of the reactor, (7) numerous external thermocouples positioned axially along the entire reactor length, (8) a pressure letdown valve to control the system operating pressure, (9) liquid effluent collectors, (10) a sealable, reinforced stainless steel enclosure capable of containing a worst-case system depressurization, and (11) a data acquisition and control system. Figure 2 shows a photograph of the rig inserted in the IITRI agent test hood. The agent feed pump, supplied by IITRI, is positioned to the far right




DOWNEY ET AL.

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S C3

ο

U Β



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of the test rig. The effluent collection and off-gas monitoring equipment were located in an adjacent hood. Corrosion testing, discussed previously, identified platinum as the optimum material of construction for SCWO processing of GB and VX agents. A thin (0.010") platinum liner was contained within a Hastelloy C276 pressurebearing tube. For processing of mustard agent, titanium was identified as the best material of construction, again as a thin liner within a Hastelloy C276 pressure bearing wall. A platinum-lined reactor was therefore used for the GB and VX test series. This reactor was removed and replaced with a titanium-lined reactor for the mustard agent test series. The testrigdata acquisition and control system consisted of an Apple Macintosh Ilci computer coupled with National Instruments' data acquisition hardware. The software utilized was National Instruments' Lab VIEW, a flexible, multipurpose, graphics-based data acquisition and control program. Inputs included 3 pressure and 29 temperature readouts. Outputs included pump actuation signals, back pressure control, and independent control of three reactor heating zones. Prior to the start of agent testing, a test program was completed using chemical agent simulants in order to verify SCWO test rig capabilities. Tests were performed at temperatures of 450 to 550°C and flow rates of up to 100 ml/min. All tests were performed at a pressure of approximately 4000 psi. Agent simulants used were diethylmethylphosphonate (DEMP, GB and VX simulant), diisopropylethylamine (DIEA, VX simulant), and thiodiglycol (TDG, mustard simulant). Target simulant feed concentrations were 1 wt%. GB Agent Testing. Following successful completion of the Preoperational Test for agent operations, an Army requirement, and the receipt of final Army approvals, GB agent testing commenced on May 10, 1993. All tests were performed at GB concentrations of approximately 1 wt% with 100% excess oxygen. Table IV presents the GB test matrix .

TABLE IV. GB AGENT TEST MATRIX Test Pressure Temperature Total Flow No. (psig) ÇQ Rate (ml/min)

2 3 4 5

4000 4000 4000 4000 4000

550 450 550 450 500

50.5 39.4 50.5 43.4 46.4

Residence Time (sec)

Test Duration (min)

16 29 16 26 20

42 15 54 71 58

Gas and liquid samples were collected and analyzed throughout the test series. No agent was detected in any liquid samples, signifying a Destruction and Removal Efficiency (DRE) in excess of 99.99999%. [Higher DREs may have



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been achieved, but a DRE of 99.99999% is the maximum that can be measured given an influent agent concentration of 10,000 ppm (1 wt%) and the ~1 ppb detection limit for GB in liquid samples.] Additionally, no agent above the allowable exposure limit (AEL) was found in the gaseous effluent samples as analyzed on-line by a Minicams analyzer. After the gaseous and liquid effluent samples were confirmed to be agent free, they were shipped to the Institute of Gas Technology (IGT) and the University of Texas Balcones Research Center (UTBRC), respectively, for further analysis. Gas samples were found to contain oxygen, nitrogen, argon, carbon dioxide, and trace amounts of methane. Liquid samples showed essentially quantitative conversion of the GB agent to complete oxidation products, i.e., HF and H3PO4. Small amounts (

Corrosion and Chemical Agent Destruction - ACS Symposium Series

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