Temperature Management of Centrifugal Contactor for Caustic-side Solvent Extraction of Cesium from Tank Waste PDF Download

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Languages : en
Pages : 12

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A 32-stage centrifugal contactor underwent proof-of-concept testing for its potential use in caustic-side solvent extraction for removal of cesium from radioactive waste now stored in underground tanks at the Savannah River Site. This contactor met the performance goals. Maintaining this performance, however, depends on adequate temperature management. Subsequent calculations and measurements determined the effect of several sources of heat on the temperature of the liquid inside the contactor stages: heats of solvation and dilution, heat generated by the rotor in the mixing zone, and heat from the motor used to spin the rotor. The results indicated that over 90% of the heat comes from the motors, generating 12 W of thermal energy per stage. It was concluded that satisfactory temperature management of the contactor should be achievable by cooling the extraction section to remove 12 W of heat per stage.

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Languages : en
Pages : 44

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A test program has been conducted in which the use of pilot-scale centrifugal solvent extraction contactors for cesium removal from an alkaline waste stream has been successfully demonstrated. The program was designed specifically to evaluate the use of centrifugal contactors having 5-cm-diam rotors for the removal of cesium from alkaline high-level waste (HLW) that was generated and is being stored at the U.S. Department of Energy's Savannah River Site (SRS). The removal of cesium from this waste is highly desirable because it will reduce the volume of waste that must be treated and disposed of as HLW. The parameters applied in the test effort are those that have been established for the Caustic-Side Solvent Extraction (CSSX) process, a multistage extraction operation that has been designed by researchers at Oak Ridge National Laboratory (ORNL) and Argonne National Laboratory (ANL). In the CSSX process, cesium is extracted by calix(4)arene-bis-(fert-octylbenzo-crown-6), commonly referred to as BOBCalixC6. The extract is scrubbed with dilute (0.05 M) nitric acid, both to remove coextracted elements (primarily potassium and sodium) and to adjust the pH of the extract to facilitate recovery of the cesium. The scrubbed solvent is contacted with 0.001 M HNO3, which results in the stripping of the cesium from the solvent into the aqueous acid. The CSSX process flow rates have been established so to produce a cesium concentration in the strip effluent that is 12 to 15 times the concentration in the waste stream that enters the extraction section of the cascade. Results from initial hydraulic testing of a commercially available 5-cm contactor under CSSX conditions indicated that the mixing of feed solutions within the unit (which is critical to efficient solute transfer) was limited by a feature of the contactor that was designed to increase throughput and improve separation performance. In the design, phase separation is improved by reducing turbulence within the contactor. Subsequent to the initial hydraulic test: cesium transfer tests were performed using contactors arranged in both single-stage and multistage arrangements. Results of these tests confirmed that phase mixing within the contactor was inadequate. In an effort to improve mixing within the contactor and thereby increase mass transfer efficiency, two minor modifications were made to a single contactor unit. One modification was the replacement of the bottom plate from the vendor-supplied contactor housing, which was equipped with curved (impeller-type) vanes, with a bottom assembly that had straight radial vanes. The latter configuration is the standard used in all existing ANL, ORNL, and SRS contactor designs. The second modification involved enlargement of the opening in the bottom of the rotor through which dispersion from the contactor mixing zone enters the rotor for separation. By increasing the rotor opening sufficiently, the rotor loses pumping efficiency to such an extent that accumulation of a hydrostatic head in the annular mixing zone is required for solution to be pumped through the contactor to the organic and aqueous discharge ports. By causing a volume of liquid to accumulate in the mixing zone, it is expected that phase mixing will be improved. Following modification of a contactor, hydraulic testing was repeated to determine flow parameters to be applied in mass transfer testing using the modified device. As expected, test results indicated that the maximum throughputs that could be achieved using the modified contactor under extraction and stripping conditions were lower than those obtained using the ''as-received'' unit. However, phase separation performance within the reduced operating envelope was excellent. Most importantly, cesium transfer stage efficiencies were significantly improved over those obtained using the unmodified device and resulted in attainment of the target CSSX process decontamination factor of 40,000 when extrapolated to the baseline CSSX contactor cascade.

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Languages : en
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The objectives of this report are: to demonstrate complete CSSX process flowsheet (proof of concept)--decontamination factor[ge] 40,000, and concentration factor[approx]15; Scientific and technical issues evaluated--stage efficiency, temperature control, hydraulic performance, long time (multi-day) operation, short-term shutdown, effect of solids, and recovery from Cs moving through strip section.

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Languages : en
Pages : 21

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A caustic-side solvent extraction (CSSX) process to remove cesium from Savannah River Site (SRS) high-level waste was tested in a minicontactor (2-cm centrifugal contactor). In the first phase of this effort, the minicontactor stage efficiency was improved from 60% to greater than 80% to meet the SRS process requirements using a 32-stage CSSX flowsheet. Then, the CSSX flowsheet was demonstrated in a 32-stage unit, first without solvent recycle, then with it. In both cases, the key process goals were achieved: (1) the cesium was removed from the waste with decontamination factors greater than 40,000 and (2) the recovered cesium was concentrated by a factor of 15 in dilute nitric acid. Oak Ridge National Laboratory (ORNL) analysis of the recycled solvent showed no evidence of impurity buildup.

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Languages : en
Pages : 31

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Tests have been conducted to determine if satisfactory mass transfer performance is achieved using a fully pumping 5-cm centrifugal contactor under conditions present in the Caustic-Side Solvent Extraction (CSSX) process. Tests utilized a commercially available contactor that had been modified by installation of a rotor housing bottom that had straight radial vanes on the process side. As received from the vendor, the housing bottom was equipped with curved (impeller-type) vanes that were intended to promote phase separation by minimizing mixing of influent solutions. Stage efficiencies exceeding 85% were obtained under conditions, present in the extraction section of the CSSX flowsheet. Under CSSX stripping conditions the stage efficiency exceeded 90%. In both cases, the efficiencies obtained exceed the minimum requirement for acceptable transfer of cesium in the CSSX process.

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Languages : en
Pages : 5

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This project seeks a fundamental understanding and major improvement in cesium separation from high-level waste by cesium-selective calixcrown extractants. Systems of particular interest involve novel solvent-extraction systems containing specific members of the calix[4]arene-crown-6 family, alcohol solvating agents, and alkylamines. Questions being addressed bear upon cesium binding strength, extraction selectivity, cesium stripping, and extractant solubility. Enhanced properties in this regard will specifically benefit applied projects funded by the USDOE Office of Environmental Management to clean up sites such as the Savannah River Site (SRS), Hanford, and the Idaho National Environmental and Engineering Laboratory. The most direct beneficiary will be the SRS Salt Processing Project, which has recently identified the Caustic-Side Solvent Extraction (CSSX) process employing a calixcrown as its preferred technology for cesium removal from SRS high-level tank waste.

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Languages : en
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The Caustic-Side Solvent Extraction (CSSX) process has been selected for the separation of cesium from Savannah River Site high-level waste. The solvent composition used in the CSSX process was recently optimized so that the solvent is no longer supersaturated with respect to the calixarene crown ether extractant. Hydraulic performance and mass transfer efficiency testing of a single stage of 5.5-cm ORNL-designed centrifugal contactor has been performed for the CSSX process with the optimized solvent. Maximum throughputs of the 5.5-cm centrifugal contactor, as a function of contactor rotor speed, have been measured for the extraction, scrub, strip, and wash sections of the CSSX flowsheet at the baseline organic/aqueous flow ratios (O/A) of the process, as well as at O/A's 20% higher and 20% lower than the baseline. Maximum throughputs are comparable to the design throughput of the contactor, as well as with throughputs obtained previously in a 5-cm centrifugal contactor with the non-optimized CSSX solvent formulation. The 20% variation in O/A had minimal effect on contactor throughput. Additionally, mass transfer efficiencies have been determined for the extraction and strip sections of the flowsheet. Efficiencies were lower than the process goal of greater than or equal to 80%, ranging from 72 to 75% for the extraction section and from 36 to 60% in the strip section. Increasing the mixing intensity and/or the solution level in the mixing zone of the centrifugal contactor (residence time) could potentially increase efficiencies. Several methods are available to accomplish this including (1) increasing the size of the opening in the bottom of the rotor, resulting in a contactor which is partially pumping instead of fully pumping, (2) decreasing the number of vanes in the contactor, (3) increasing the vane height, or (4) adding vanes on the rotor and baffles on the housing of the contactor. The low efficiency results obtained stress the importance of proper design of a centrifugal contactor for use in the CSSX process. A prototype of any centrifugal contactors designed for future pilot-scale or full-scale processing should be thoroughly tested prior to implementation.

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Languages : en
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The Caustic-Side Solvent Extraction (CSSX) process has been selected for the separation of cesium from Savannah River Site high-level waste. The solvent composition used in the CSSX process was recently optimized so that the solvent is no longer supersaturated with respect to the calixarene crown ether extractant. Hydraulic performance and mass transfer efficiency testing of a single stage of 5.5-cm ORNL-designed centrifugal contactor has been performed for the CSSX process with the optimized solvent. Maximum throughputs of the 5.5-cm centrifugal contactor, as a function of contactor rotor speed, have been measured for the extraction, scrub, strip, and wash sections of the CSSX flowsheet at the baseline organic/aqueous flow ratios (O/A) of the process, as well as at O/A's 20% higher and 20% lower than the baseline. Maximum throughputs are comparable to the design throughput of the contactor, as well as with throughputs obtained previously in a 5-cm centrifugal contactor with the non-optimized CSSX solvent formulation. The 20% variation in O/A had minimal effect on contactor throughput. Additionally, mass transfer efficiencies have been determined for the extraction and strip sections of the flowsheet. Efficiencies were lower than the process goal of greater than or equal to 80%, ranging from 72 to 75% for the extraction section and from 36 to 60% in the strip section. Increasing the mixing intensity and/or the solution level in the mixing zone of the centrifugal contactor (residence time) could potentially increase efficiencies. Several methods are available to accomplish this including (1) increasing the size of the opening in the bottom of the rotor, resulting in a contactor which is partially pumping instead of fully pumping, (2) decreasing the number of vanes in the contactor, (3) increasing the vane height, or (4) adding vanes on the rotor and baffles on the housing of the contactor. The low efficiency results obtained stress the importance of proper design of a centrifugal contactor for use in the CSSX process. A prototype of any centrifugal contactors designed for future pilot-scale or full-scale processing should be thoroughly tested prior to implementation.

Author: National Research Council
Publisher: National Academies Press
ISBN: 030917158X
Category : Science
Languages : en
Pages : 154

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The Second World War introduced the world to nuclear weapons and their consequences. Behind the scene of these nuclear weapons and an aspect of their consequences is radioactive waste. Radioactive waste has varying degrees of harmfulness and poses a problem when it comes to storage and disposal. Radioactive waste is usually kept below ground in varying containers, which depend on how radioactive the waste it. High-level radioactive waste (HLW) can be stored in underground carbon-steel tanks. However, radioactive waste must also be further immobilized to ensure our safety. There are several sites in the United States where high-level radioactive waste (HLW) are stored; including the Savannah River Site (SRS), established in 1950 to produce plutonium and tritium isotopes for defense purposes. In order to further immobilize the radioactive waste at this site an in-tank precipitation (ITP) process is utilized. Through this method, the sludge portion of the tank wastes is being removed and immobilized in borosilicate glass for eventual disposal in a geological repository. As a result, a highly alkaline salt, present in both liquid and solid forms, is produced. The salt contains cesium, strontium, actinides such as plutonium and neptunium, and other radionuclides. But is this the best method? The National Research Council (NRC) has empanelled a committee, at the request of the U.S. Department of Energy (DOE), to provide an independent technical review of alternatives to the discontinued in-tank precipitation (ITP) process for treating the HLW stored in tanks at the SRS. Alternatives for High-Level Waste Salt Processing at the Savannah RIver Site summarizes the finding of the committee which sought to answer 4 questions including: "Was an appropriately comprehensive set of cesium partitioning alternatives identified and are there other alternatives that should be explored?" and "Are there significant barriers to the implementation of any of the preferred alternatives, taking into account their state of development and their ability to be integrated into the existing SRS HLW system?"

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Languages : en
Pages : 16

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The Modular Caustic Side Solvent Extraction (CSSX) Unit (MCU) facility at the Savannah River Site (SRS) is actively pursuing the transition from the current BOBCalixC6 based solvent to the Next Generation Solvent (NGS)-MCU solvent to increase the cesium decontamination factor. To support this integration of NGS into the MCU facility the Savannah River National Laboratory (SRNL) performed testing of a blend of the NGS (MaxCalix based solvent) with the current solvent (BOBCalixC6 based solvent) for the removal of cesium (Cs) from the liquid salt waste stream. This testing utilized a blend of BOBCalixC6 based solvent and the NGS with the new extractant, MaxCalix, as well as a new suppressor, tris(3,7dimethyloctyl) guanidine. Single stage tests were conducted using the full size V-05 and V-10 liquid-to-liquid centrifugal contactors installed at SRNL. These tests were designed to determine the mass transfer and hydraulic characteristics with the NGS solvent blended with the projected heel of the BOBCalixC6 based solvent that will exist in MCU at time of transition. The test program evaluated the amount of organic carryover and the droplet size of the organic carryover phases using several analytical methods. The results indicate that hydraulically, the NGS solvent performed hydraulically similar to the current solvent which was expected. For the organic carryover 93% of the solvent is predicted to be recovered from the stripping operation and 96% from the extraction operation. As for the mass transfer, the NGS solvent significantly improved the cesium DF by at least an order of magnitude when extrapolating the One-stage results to actual Seven-stage extraction operation with a stage efficiency of 95%.