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Multiphase Flows

Experiments on Cake Development in Crossflow Filtration for High Level Waste

[+] Author and Article Information
Mark R. Duignan2

Savannah River Nuclear Solutions, Savannah River National Laboratory, Aiken, SC 29802mark.duignan@srnl.doe.gov

Charles A. Nash

Savannah River Nuclear Solutions, Savannah River National Laboratory, Aiken, SC 29802charles.nash@srnl.doe.gov

Nuclear Waste Definitions: An HM Waste refers to a liquid waste that came from the H-Canyon (at SRS) Modified Purex process, and a Purex Waste resulted from the Plutonium URanium EXtraction process.

2

Corresponding author.

J. Fluids Eng 134(8), 081302 (Jul 27, 2012) (9 pages) doi:10.1115/1.4007055 History: Received September 26, 2011; Revised June 13, 2012; Published July 27, 2012; Online July 27, 2012

Crossflow filtration is a key process step in many operating and planned waste treatment facilities to separate undissolved solids from supernatant slurries. This separation technology generally has the advantage of self-cleaning through the action of wall shear stress created by the flow of waste slurry through the filter tubes. However, the ability of filter wall self-cleaning depends on the slurry being filtered. Many of the alkaline radioactive wastes are extremely challenging to filtration, e.g., those containing compounds of aluminum and iron having particles whose particle size and morphology reduce cake permeability. Low filter flux can be a bottleneck in waste processing facilities such as the Salt Waste Processing Facility at the Savannah River Site and the Waste Treatment Plant at the Hanford Site. To date, increased rates are generally realized by either increasing the crossflow filter axial flow rate, limited by pump capacity, or by increasing filter surface area limited by space and increasing the required pump load. The Savannah River National Laboratory (SRNL) set up both dead-end and crossflow filter tests to better understand filter performance based on filter media structure, flow conditions, and filter cleaning. Using nonradioactive simulated wastes, both chemically and physically similar to the actual radioactive wastes, the authors performed several tests to demonstrate increases in filter performance. With the proper use of filter flow conditions, filter flow rates can be increased over rates currently realized today. This paper describes the selection of a challenging simulated waste and crossflow filter tests to demonstrate how performance can be improved by varied filter operation methods. Those methods were a slow startup to better develop the filter cake and scouring the filter wall. The results showed that for salt waste and metal oxide hydroxide sludges, the process of backpulsing is not necessary to maintain a good filter flux, and the process of periodically scouring the filter improves filter performance. The results also imply that initial filter operation is important to develop a filter cake that minimized pressure drop, the presence of a filter cake can lead to improved solids separation, and a well-developed cake with periodic scouring may allow a good filter flux to be maintained for long periods of time.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Typical crossflow filter arrangement

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Figure 2

Schematic of cake on filter surface with most of the smallest particle in the cake at the (a) bottom and (b) top

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Figure 3

Schematic of the crossflow ultrafiltration test facility

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Figure 4

Three filter tubes studied, left to right: 0.0095-m PALL, 0.0095-m MOTT, and 0.0127-m MOTT

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Figure 5

Three filter tubes in filter housings

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Figure 6

Close-up of one of the three tubes

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Figure 7

Particle size distribution of sludge simulant

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Figure 8

Sludge Batch 6 was used for the test (note: 10 dyn/cm2 = 1 Pa)

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Figure 9

Range of yield stress for wastes stored in SRS tanks

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Figure 12

Comparison of the filterability of a Purex-type waste (Tank 8F) to an HM-type waste (SB6) in a dead-end filter normalized by the fastest filter flux

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Figure 10

Long-term 12-day filtration test done at the conditions listed in the legend and with the slurry at 5 wt. % solids loading with a constant temperature of 25 °C ± 2 °C

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Figure 11

Filter performance at a single set of flow conditions at 5 wt. % solids loading

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