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Large storage tanks may require mixing to achieve homogeneity of
contents for several
reasons: prior to sampling for mass balance purposes, for blending in
reagents, for suspending
settled solids for removal, or for use as a feed tank to a process.
At Oak Ridge National
Laboratory, mixed waste evaporator concentrates are stored in
∼190-m3 (50 000-gal) horizontal
tanks, about 3.7 m (12 ft) in diameter and 18 m (60 ft) in length.
This tank configuration has
the advantage of permitting transport by truck and therefore
fabrication in the shop rather
than in the field. A survey of the literature revealed no
information on mixing large storage
tanks with length-to-diameter ratios greater than 2. Jet mixing
experiments were carried out
in two model tanks: a 0.87-m3 (230-gal) Plexiglas tank
that was ∼1/6 linear scale of the
actual
waste tanks and a 95-m3 (25 000-gal) tank that was about
2/3 linear scale of the actual
waste
tanks. Mixing times were measured by the use of a sodium chloride
tracer and several
conductivity probes distributed throughout the tanks. Several jet
sizes and configurations were
tested. In the 0.87-m3 tank, jet diameters of 0.016,
0.022, and 0.041 m (0.62, 0.87, and 1.61 in.)
were used. In the 95-m3 tank, jet diameters of 0.035
and 0.049 m (1.38 and 1.93 in.) were used.
One-directional and two-directional jets were tested in both
tanks. Mixing times for each tank
were correlated with the jet Reynolds number and for the two tank sizes
using the recirculation
time for the developed jet.
Millions of gallons of radioactive liquid and sludge wastes must be retrieved fi-om underground storage tanks at the U.S. Department of Energy sites to be staged and transfemed to treatment facilities and processed into final waste forms. Retrieval operations involve mixing solid and liquid wastes to create slurries that can be transported via underground pipelines to specified locations for treatment or disposal. A major concern during the transfer operations is plugging of the transfer lines. Blocked transfer lines could significantly escalate the remediation costs both in terms of pipeline replacement costs and costs of maintenance of inactive facilities and operating personnel. Technologies Evaluated The three main factors that contribute to transfer line plugging are (1) settling of solid particles during transfer, (2) crystallization of the waste, and (3) gelation of the waste. Depending on the mechanism, plugging or its onset may result in a change in the particle size distribution or the density of the slurry. In addition, the onset of pipeline plugging may also lead to an increase in the pressure drop in the transfer lines. Therefore, process monitoring, including measuring the slurry density and particle size distribution and measuring the pressure drop in the transfer lines, represents several significant on-line, real-time methods for measuring the quantity of waste transfemed and monitoring for process control and early detection/prevention of pipeline plugging. To reduce the likelihood of pipeline blockage during waste-transfer operations, the Accelerated Site Technology Deployment (ASTD) project with funding from Project W-320 (Waste-Retrieval Sluicing System) and Hanford Tanks Initiative (H'H), evaluated three on-line slurry-monitoring devices for use at the Hanford and Oak Ridge National Laboratory (ORNL) sites and for potential use at other U.S. Department of Energy (DOE) sites. These instruments include: (1) the Lasentec M600 Particle Size Analyzer(a) developed by Laser Sensor Technology, Inc., Redmond, Washington, (2) the Red Valve Pressure Sensor manufactured by Red Valve Company, Inc., Pittsburgh, Pennsylvani~and (3) an ultrasonic densimeter developed at the Pacific Northwest National Laboratory (PNNL).o) These slurry monitors present numerous benefits over the baseline instrumentation plan that calls for grab sampling of the waste followed by off-line analysis. Benefits include: (a) The LasentecM600 is an in-line analyzerfor measuring chord-len=@distributionof suspendedsolid particles. Chord lengthand particlesize are not exactlyequivalentterms, but there is a direct correlationbetweenthe two. For the purposesof the testingperformed,the LasentecM600 was used to evaluatethe particle size distributionof the suspendedsolid particlesin the slurries. As such,the instrumentwill be referredto as a particle size analyzer elsewherein this report.
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