M. R. Poirier
Westinghouse Savannah River Company
Aiken, SC 29808

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This report has been reproduced directly from the best available copy.

Available for sale to the public, in paper, from:  U.S. Department of Commerce, National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161,  phone: (800) 553-6847,  fax: (703) 605-6900,  email:  orders@ntis.fedworld.gov   online ordering:  http://www.ntis.gov/support/ordering.htm

Available electronically at  http://www.osti.gov/bridge/

Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy, Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831-0062,  phone: (865 ) 576-8401,  fax: (865) 576-5728,  email:  reports@adonis.osti.gov

SRS High Level Waste (HLW) requested SRTC to determine the requirements for suspending insoluble solids in the 2H-evaporator pot. The evaporator pot is a cylindrical vessel with a conical bottom, an air lance, and the suction line to a transfer pump. The air lance and pump suction are located at the bottom of the pot, which will contain insoluble sodium aluminosilicate and sodium diuranate particles.

SRTC performed the analysis by reviewing available information on the properties of the solids and fluid in the evaporator pot and by reviewing the technical literature for information on suspending solid particles. The evaporator pot was modeled as a spouting bed. A spouting bed is a cylindrical tank with a flat or conical bottom. A stream of air is injected through the bottom of the vessel to suspend insoluble particles. Spouting beds operate in different flow regimes. At very low gas flow rates, the solid particles remain settled and the bed behaves as a packed bed. As the flow rate increases, the bed shows internal spouting, and then good spouting. To prevent plugging of the transfer pump suction, SRS HLW should operate the air lance at a high enough flow rate to have good spouting. Internal spouting or a packed bed could lead to plugging of the pump suction line.

SRTC makes the following recommendations to HLW for suspending solid particles in the 2H-evaporator:

• Determine the height of the bed of particles in the evaporator pot.
• Determine the minimum air flow rate for good spouting from the table below. The table shows the existing air lance with a maximum flow rate of 10 SCFM is unlikely to suspend the solid particles sufficiently to prevent plugging the transfer pump suction line.

Aluminosilicate forms in the SRS high level waste system by the reaction of aluminum from the Separations Canyons with silica from DWPF recycle. The high temperature of an evaporator accelerates the normally slow reaction. The 2H-evaporator pot contains approximately 3500 kg of solids that include aluminosilicate and slightly enriched uranium (108 kg). SRS High Level Waste proposes to remove the solids by dissolving them with nitric acid, neutralizing the acid solution, and transferring the solution to the tank farm. The transferred solution will contain insoluble solid particles. HLW is concerned that the particles formed during neutralization could plug the transfer pump in the evaporator. They requested SRTC to determine the requirements for suspending insoluble solids in the evaporator pot with an air lance.

The evaporator pot mixing is similar to a spouting bed. A spouting bed is a process for fluid-solid contacting which is similar to fluidization.³ The fluid enters the flat or conical base of a cylinder through a small orifice, entrains solids, carries the solids to the top of the bed, and the solids fall back into the annular core. Spouting beds are used with coarse particles and have a single air stream. For this reason the evaporator was modeled as a spouting bed rather than a fluidized bed.

Figure 2 shows the flow regimes that exist with spouting beds. At very low gas flow rates, the solid particles remain settled and the bed behaves as a packed bed (a). As the flow rate increases, the bed shows internal spouting (b), and then good spouting (c).³ To prevent plugging of the transfer pump suction, SRS HLW should operate the air lance at a high enough flow rate to have good spouting.

The author determined the air flow rate needed to achieve good spouting by reviewing available information on the properties of the solids and fluid in the evaporator pot and by reviewing the technical literature for information on minimum spouting velocity.

The following assumptions were made to perform the analysis:

• The evaporator pot can be modeled as a spouting bed. In a spouting bed, the air enters through the bottom of the vessel and is directed upward. In the evaporator, the air enters on the side of the pot and is directed downward. Because of the differences between the two vessels, the calculated minimum spouting velocity could contain a large amount of uncertainty.
• The diameter of the spouting bed is 8 feet.
• The height of the solid particles in the evaporator pot is between 1 inch and 50 inches.
• The diameter of the air lance is 1.5 inches.
• The sodium aluminosilicate particle density is 2.9 g/cc. Typical measurements of aluminosilicate density are 2.25 – 2.9 g/cc.¹ A particle density of 2.9 g/cc was chosen to be conservative. If the particle density is less than 2.9 g/cc, the minimum sparge rate will be less than calculated. Additional calculations were performed with a particle density of 3.93 g/cc to determine the requirements for suspending sodium diuranate.
• The fluid density is 1.0 g/cc. This density was selected to be conservative (water density is 1.0 g/cc at 20°C).² If the fluid density is greater than 1.0 g/cc, the minimum sparge rate will be less than calculated.
• The particle diameter is 0.1 – 4.0 mm. Scanning electron microscope pictures taken by SRTC were reviewed to determine the particle size. The size of particles in the SEM pictures varied between 0.1 mm and 4.0 mm. Larger particle sizes would lead to larger minimum sparge rates.
• The fluid viscosity is 1 cp. This viscosity was selected to be conservative (water viscosity is 1.0 cp. at 20°C).³

Mathur and Gishler developed a correlation to predict the minimum spouting velocity for uniform size particles in conical bottom vessels.^3,4 Equation [1] describes the correlation

where U_ms is the minimum spouting velocity, d_p is the particle diameter, D_c is the evaporator pot diameter, D_i is the spout diameter (i.e., air lance inner diameter), g is gravitational acceleration, H is the bed height, Dr is the density difference between the fluid and the solid particles, and r_l is the fluid density. In the case of non-uniform particle size, the correlation is generally accurate to within 25%.

Smith and Reddy developed a correlation to predict the minimum spouting velocity for non-uniform size particles in conical bottom vessels.⁵ Equation [2] describes the correlation

where d_p-avg is the average particle size.

Uemaki et. al developed a correlation to predict the minimum spouting velocity for non-uniform size particles in conical bottom vessels.^3,8 Equation [3] describes the correlation

Lamont investigated air agitation with Pachuca tanks, which are similar to spouting beds and have conical bottoms. ⁶ He found a minimum spout velocity of 0.33 cm/sec was needed for moderate agitation.

The work developing correlations was performed with small diameter vessels. Fane and Mitchell developed a correction to apply the Mathur-Gishler correlation to larger diameter tanks.⁷ The correction is described by equations [4] and [5]

where D_c-ref is the reference column diameter (1.0 m).

The author used these correlations to predict the required air flow rate to produce good spouting of the solid particles in the 2H-evaporator pot.

Table 1 shows the input parameters used to calculate the minimum spouting velocity.

Tables 2 and 3 show the results of the calculations. Since the Mathur-Gishler correlation was developed for uniform size particles, the maximum particle size (0.4 cm) was chosen for that calculation. The Smith-Reddy correlation was developed for non-uniform size particles. Both the maximum and geometric average particle size were used with that correlation. The calculation made using the average particle size is probably more appropriate. The Uemaki correlation was developed for non-uniform size particles. Both the maximum and geometric average particle size were used with that correlation. The calculation made using the average particle size is probably more appropriate. Lamont’s criteria for suspending solid particles was developed in the 1930s and is given as a superficial velocity with no effect from particle size, particle density, spout diameter, or column diameter. Its accuracy is questionable.

Tables 2 and 3 also show the average minimum spouting velocity calculated by all six correlations and the average minimum spouting velocity calculated with the Mathur-Gishler correlation, the Smith-Reddy correlation using average particle diameter, and the Uemaki correlation using average particle diameter (i.e., average-likely).

Finally, the Fane correction for large diameter columns was applied to the Mathur-Gishler correlation and the average of the Mathur-Gishler correlation, the Smith-Reddy correlation using average particle diameter, and the Uemaki correlation using average particle diameter. These results are also shown in Tables 2 and 3. The tables show the minimum spouting flow rates for various depths of particles.

The calculations show the minimum required air sparge rate is probably greater than 10 SCFM which could explain HLW’s past experience in plugging the 2H-evaporator transfer pump suction line. Once the height of the solid bed in the evaporator is known, HLW can use Tables 2 and 3 to determine the required air sparging rate.

If the air sparging rate is less than the minimum flow rate for good spouting, the bed may behave as an internally spouting bed. This situation may be acceptable to prevent plugging. To better define the transition between a packed bed, internal spouting, and good spouting, experimental data are needed.

SRTC makes the following recommendations to CSTE for transporting solid particles from the 2H-evaporator to the Tank Farm:

• Determine the height of the bed of particles in the evaporator pot.
• Determine the minimum air flow rate for good spouting from the table below. The table shows the existing air lance with a maximum flow rate of 10 SCFM is unlikely to suspend the solid particles sufficiently enough to prevent plugging the transfer pump suction line.

References

1. David Lide, Ed., CRC Handbook of Chemistry and Physics, 71^st Ed., Boca Raton: CRC Press, 1990, pp. 4-160 – 4-177.
2. Robert Perry and Cecil Chilton, Eds., Chemical Engineers’ Handbook, 5^th Ed., New York: McGraw-Hill, 1973, p. 3-71.
3. T. Ishikura, H. Shinohara, and K. Fumatsu, "Minimum Spouting Conditions for Particle Mixtures", in N. P. Cheremisinoff, Ed., Encyclopedia of Fluid Mechanics: Volume 4: Solids and Gas-Solids Flows, Houston: Gulf Publishing, 1986.
4. K. B. Mathur and P. E. Gishler, "A Technique for Contacting Gases with Coarse Solid Particles", A.I.Ch.E. J., vol. 1, no. 2, pp. 157-164, 1955.
5. J. W. Smith and K. V. S. Reddy, Spouting of Mixed Particle-Size Beds", Can. J. Chem. Eng., vol. 42, no. 5, pp. 206-210, 1964.
6. A. G. W. Lamont, "Air Agitation and Pachuca Tanks", Can. J. Chem. Eng., August 1958, pp. 153-160.
7. A. G. Fane and R. A. Mitchell, "Minimum Spouting Velocity of Scaled-Up Beds", Can. J. Chem. Eng., vol. 62, pp. 437-439, 1984.
8. O. Uemaki, R. Yamada, and M. Kugo, "Particle Segregation in a Spouted Bed of Binary Mixtures of Particles", Can. J. Chem. Eng., vol. 61, pp. 303-307, 1983.