Review and Evaluation of Alternative Chemical Disposal Technologies (1996) / Chapter Skim
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5 MEDIATED ELECTROCHEMICAL OXIDATION SILVER II
5 MEDIATED ELECTROCHEMICAL OXIDATION SILVER II
Pages 72-101
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From page 72... ...
At the start of operation, the composition of the anolyte is approximately ~ molar in nitric acid, 0.5 molar in silver nitrate, and 0.02 to 0.03 molar in agent. The catholyte is 4 molar nitric acid.
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From page 73... ...
The anticipated overall anode reactions for VX and HD are as follows: HNO3 + 2H+ + 2e = HNO2 + H2O Nitrous acid will partially decompose to NO gas, nitric acid, and water. In the laboratory tests observed by the AltTech Panel, the gas leaving the cathode compartment had the characteristic red-brown color of NO2, which can form by oxidation of NO in the gas phase when O2 is present.
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From page 74... ...
Source: AEA Technology. The reaction products are treated in subsequent steps outside the cell to reoxidize HNO2 to HNO3 and to neutralize the acids to their corresponding sodium salts.
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From page 75... ...
However, agent destruction is likely to proceed in several steps, some of which may produce volatile organic intermediates that will enter the gas 75 Clean ~ Offgas phase and require further treatment. In laboratory tests, for example, the TPC identified varying levels of alkyl nitrates in the anolyte offgas, which was mainly CO2.
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From page 76... ...
· agent receipt and supply · anolyte feed circuit · catholyte feed circuit · electrochemical cell · anolyte offgas condenser · NOx reformer system | Anolyte Circuit Anolyte Bulk Vessel [~ [~1 S101 V103 _ _ ~ _ ~ P1 02 · Catholyte silver nitrate recovery circuit combined offgas treatment circuit silver management system utilities infrastructure Figure 5-4 is a process flow diagram. Each of the key system components is discussed below.
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From page 77... ...
For HD processing, a hydrocyclone is added to remove some of the silver chloride precipitate. The anolyte vessel is fed from batch tanks of silver nitrate and nitric acid, a head tank of water, the catholyte silver nitrate recovery circuit, and an agent-slurry tank.
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From page 78... ...
The NOx reformer reconstitutes nitric acid from the products of the cathode reaction. The catholyte silver nitrate recovery circuit captures silver that has migrated across the cell membrane from the anolyte.
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From page 79... ...
The Porton Down facility also includes anolyte and catholyte feed circuits, an anolyte offgas condenser, an NOX reformer system, and a modified version of the combined offgas treatment circuit, which culminates in a sodium hydroxide scrubber. The silver management system will be tested at Dounreay on the effluent generated at Porton Down.
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From page 80... ...
Transfer Pump Boiler V301 P308 NOX Reformer Hydrogen Peroxide T ran sfer Pump FIGURE 5-5 Anolyte offgas condenser, NOx reformer, silver nitrate recovery circuit, and combined offgas treatment circuit. Source: AEA Technology.
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From page 81... ...
The agent-transfer system that the Army has proposed 81 for use in the neutralization process (see Chapter 7) is equally well suited to Silver II
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From page 82... ...
~,7 at' 'a V406 l M Sodium Hydroxide Batch Tank Stream Number V403 P403 r V402 l ~ CWR -, CWS E402 P402 V403 Nitric Acid Distillation Column B P403 Nitric Acid Transfer Pump E404 Nitric Acid Reboiler The loss of silver nitrate has two causes: the transport of Ag(0 from the anode to the cathode compartment, which occurs with any organic feed material, and the precipitation of silver chloride, which happens when a feed material contains chlorine, as does HD. The TPC reports that transport of Ago)
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From page 83... ...
The catholyte silver nitrate recovery circuit, which is discussed below, recovers the silver from the catholyte by crystallizing silver nitrate
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From page 84... ...
The makeup silver nitrate is added to the anolyte feed circuit through a manifold in the top of the anolyte vessel and mixes into the bulk anolyte as the solution circulates. Silver concentration must be monitored during a campaign, and feedback systems must be designed to automate the addition of proper quantities of silver nitrate to the anolyte circuit.
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From page 85... ...
The silver nitrate solution is filtered off for reuse as anolyte feed. Any remaining silver chloride solids are recycled to repeat the treatment with sodium hydroxide for conversion to silver oxide.
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From page 86... ...
Energy Requirements The Silver IT process consumes a great deal of electrical energy for cell operation and for auxiliary heating, refrigeration, and pumping. The theoretical energy for EVALUATION OF ALTERNATIVE TECHNOLOGIES > Water Feed A> N~ _ ~ 1 ~h V701 V701 Silver Chloride Heated Reactor Vessel P701 Steam Trap ~ r Condensate S701 Silver Chloride, Silver Oxide Solids Separation a 2-volt cell is about 9,400 kW.h per metric ton of HD and 16,440 kW.h per metric ton of VX.
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From page 87... ...
MEDIATED ELECTROCHEMICAL OXIDATION _ Pr (~ S701 ~ r, P702 V702 S702 11 ~ V702 Silver Nitrate Reactor P702 Silver Nitrate Transfer Pump Cell requirement DC power supply losses Refrigeration Steam Compressor for plant air Instrumentation and control Blast air coolers 87 ~ Silver Nitrate \ \ Neutral Salt Solution to / Hold-Up Tank it ~ S702 Waste Solids Separator P701 Neutral Salt Solution Transfer Pump 360 kW 360kW 2kW 622 kW lOkW lOkW 360 kW > Based on the TPC's estimates that a single 360-kW module, operated 24 hours per day, could destroy 137.6 metric tons of mustard or 74.1 metric tons of VX in 245 days, the total electric energy consumption is 72,600 kW.h per metric ton of HD destroyed and 134,900 kW.h per metric ton of VX destroyed. The silver management system, which requires additional electric power of 507 kW, is expected to operate for about 6 hours following completion of each
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From page 88... ...
Silver nitrate is not included as an input stream in the mass balance on the assumption that there is no significant net loss of silver. The mass balances are presented to the nearest tenth of a ton.
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From page 89... ...
(For further discussion, see Scale-Up Requirements below.) 89 Under normal operating conditions, the submitted design for Silver II anticipates that the following process residuals will be produced: · End-of-pipe gaseous emissions from the combined offgas treatment circuit will be a mixture primarily of carbon dioxide, oxygen, and nitrogen.
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From page 90... ...
Of the offgas with hydrogen peroxide, followed by carbon filtering, will remove both agent and volatile organics from the offgas. The TPC reports that the aqueous effluent from the silver management system is slightly acidic (pH 6~.
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From page 91... ...
_E303 NOX Reformer Condenser 20°C ~ 470 kW ·1 11 0°C ~ 80°C ·( / ) ·E202 Catholyte Bulk Vessel \ C JCoolerTrain 2 20°C ~~ 130 kW 70°C 11 0°C ~ - 80°C ·( / ~ ·E201 Anolyte Bulk Vessel 20°C\~60 kW CoolerTrain 2 70°C 11 0°C 't 80°C .( / ~ · E102 Catholyte Bulk Vessel ~ C ~CoolerTrain 1 20°C ~30 kW 70°C 110°C ;_ 80°C - ~ / ~ · E101 Anolyte Bulk Vessel \- < ~Cooler Train 1 20°C = 60 kW ~70°C Q 1 1 0°C I 80°C ~~_ E40 1 ~30 kW 70°C Silver Management Unit Neutralization Reactor Cooler 110°C _~_ 80°C _t ~ ~ ~ _` / ~ · E403 Silver Management unit 20°C \~60 kW effluentwillbeontheorderof50~g/m3 (about 50 parts per trillion)
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From page 92... ...
The aqueous residuals from the silver management system and the NOX reformer are retained in a holding tank for analysis. After that, disposal may be by one of three routes: (1)
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From page 93... ...
MEDIATED ELECTROCHEMICAL OXIDATION TABLE 5-3 Mass Balance for VX Destruction (all figures in metric tons) Inputs Agent Nitric Acid Hydrogen Sodium Peroxide Hydroxide Oxygen Total VX 2.0 HNO3 0 7 H2O 0.3 H2o2 1.9 H2O 3.6 NaOH 1.8 H2O 0.1 O2 4.9 N2 0.5 Total Input 2.0 1.0 5.5 1.9 5.4 15.8 Neutral Salt Outputs Offgas Waste Acid Solution Total CO2 3.8 02 0.1 N2 0.5 NOx 0.004 HNO3 1.1 H20 3 9 NaNO3 Na2SO4 Na3PO4 H20 0.6 1.1 1.2 3.6 Total Output 4.4 5.0 6.5 15.9 PROCESS INSTRUMENTATION AND CONTROL The heart of the proposed system of process instrumentation and control is a computer-based system for 93 supervisory control and data acquisition (SCADA)
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From page 94... ...
One is the agent destruction system, which is composed of the electrochemical cell and its supporting circuits; the offgas treatment circuits; and all supporting unit operations, processes, and plumbing. The other is the silver management system, which operates separately at the end of a campaign.
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From page 95... ...
Minor process fluctuations under normal operating conditions might vary the temperature between 87°C and 93°C. During the course of a campaign, some process conditions will change substantially, particularly in the anolyte circuit, but the rate of change is slow under normal operating conditions.
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From page 96... ...
A large loading of silver chloride precipitate during an HD campaign can cause many problems, including malfunction of the electrochemical cells, inadequate heat transfer in the heat exchangers, and pump malfunctions. The pilot demonstration is critical not only to determining the effectiveness of the hydrocyclone in removing the very fine precipitate expected but also to assessing the effect of suspended particles on cell operation.
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From page 97... ...
MATERIALS OF CONSTRUCTION Systems and Materials In the design submitted for Silver IT, the core agentdestruction process is carried out in aqueous concentrated nitric acid at close to atmospheric pressure and at temperatures below 90°C. Temperatures at points in the secondary circuits where nitric acid solutions are distilled will reach the boiling point of the still bottoms: (The boiling point of concentrated nitric acid is 105°C; additional salts in the still bottoms may further elevate the boiling point.)
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From page 98... ...
Neither normal nor emergency procedures will cause significant thermal stress on the materials of construction. Failure Definition The TPC assembled a multidisciplinary team for two days in September 1995 to conduct a first-phase hazard and operability study for the design of a Silver IT facility for chemical agent destruction.
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From page 99... ...
process has been limited to the electrochemical cell. However, the pilot testing under way at Porton Down will combine the electrochemical cell with the auxiliary fluid systems (anolyte and catholyte feed circuits, anolyte offgas condenser, NOX reformer, and a modified version of the combined offgas treatment circuit)
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From page 100... ...
of a mixture of nitric, sulfuric, and phosphoric acids, plus silver nitrate, silver chloride, and various impurities, is neutralized with sodium hydroxide. This reaction has a high heat release and is prone to spattering, but the operation is well within the current state of practice.
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From page 101... ...
Agent destruction could be completed in a shorter time by adding modules. As noted in the section on Utility Requirements, the electrical power requirement correlates with the number of modules.
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