Differences of Behavior Between Filtering Hydrocyclones With Bradley and Rietema Geometry

Transcription

1 Differences of Behavior Between Filtering Hydrocyclones With Bradley and Rietema Geometry Edu A.Barbosa, Luiz G. M.Vieira, Celso A. K. Almeida, João J. R. Damasceno, Marcos A. S. Barrozo. Federal University of Uberlândia, Faculty of Chemistry Engineering -FEQUI, Block K, Campus Santa Mônica, Uberlândia-MG - Brazil. ZIP: Fax: masbarrozo@ufu.br Keywords: Separation; Rietema; Bradley; Hydrocyclones; Filtering Hydrocyclones, Filtration Abstract Hydrocyclones are extensively devices used by the industry in processes involving separation solid-liquid. Those equipments are divided in several families [1], between them can be mentioned Bradley, Rietema, Krebs, CBV/DEMCO, and other, which differ to each other in the geometric relationships between the dimensions of the hydrocyclone and the diameter of the cylindrical part, called Dc. The Table 1 displays the geometric relationships of the families Bradley and Rietema. The filtering hydrocyclone maintains the same geometric relationships of the conventional family, however it differs for having the conical area constituted of a filtering medium. Previous studies, [2] and [3], they showed that the hydrocyclones of Bradley and of Rietema they suffer interesting alterations in their performances when they operate with a filtering middle, giving them characteristics quite different from a conventional similar equipment. 1 - Introduction The hydrocyclone promotes the separation solid-liquid or liquid-liquid by the density difference between the fluid and the material to be separate. In that equipment, due to the formation of a centrifugal field caused by the tangent feeding, the material of larger density is thrown against the wall of the hydrocyclone and dragged to the underflow while the one of smaller density proceeds for the overflow, forming a descending vortex and other ascendancy. The concentrations of these two streams depend on several factors, among them the diameter of the underflow, the pressure drop in that the equipment is operated and, mainly, of the family of hydrocyclone employed. Fig. 1 displays the path of the suspension and the outline of your geometric dimensions. The filtering hydrocyclone, object of this study, possesses another stream constituted of filtrate originating from of the filtration in the conical area, which can come limpid or it darkens, depending on the operational conditions what the equipment is submitted. Like this, it is interesting to study the principal variables associated to the hydrocyclones, such as: the volumetric feed flowrates, underflow-to-throughput ratios, efficiencies, Euler numbers and reduced cut sizes. 2 - Materials Table 1 Dimension of Rietema and Bradley hydrocyclones D i /D C D o /D C L/D C l/d C Angle Rietema Bradley 1/7 1/5 1/ The used particulate material was pyrochlore (65% Nb 2 O 5 ), which presents appropriate density (ρ s =4.03g/cm 3 ) and size distribution for the hydrocyclone studies, that can analyzed by gamma ray techniques. The size distribution of this material can be visualized in Fig. 2, where the model RRB (Eq.1) was the best adjustment.

2 D C D 0 Di Feeding Overflow L1 Cylindrical section L Glass Filtering section Angle Underflow (Du) Filtrate Figure 1 Trajectory of suspension for the conventional hydrocyclone and filtering device scheme X 0.60 o Exp. Data --- RRB Model D' : micron n: 0.73 R: d Stk (microns) Figure 2 Size distribution of the pyrochlore particles. The filtering hydrocyclones had diameter of the cylindrical section (D C ) of 3 cm and underflow diameters of 0.3, 0.4 and 0.5 cm. Three filtering cones were built of sinter bronze for Rietema's hydrocyclone, called R1, R2, R3. Besides, were built two cones of sinter bronze and one of polypropylene in according to the Bradley geometry, called of B1, B2 and P, respectively. The volumetric flowrates and the concentrations of the principal streams were obtained by gravimetric techniques and the size distributions by gamma ray attenuation. The experimental apparatus was composed of a well-stirred tank, a 5-HP centrifugal pump, an agitator, PVC tubes, valves and the filtering hydrocyclone, in according to the Fig. 3. Figure 3. Experimental apparatus

3 3 - Methodology The experimental results were treated in according to the Massarani methodology [4]. Statistical models: The particle size distribution is represented by the model of Rosin-Rammler- Bennet [5], shown in Eq.1, where X is the fraction mass of smaller particles than the certain Stokes diameter (d Stk ), n and d' are parameters of the model. n d Stk X = 1 exp (1) D' Efficiency total or global (E T ): It relates the flowrate of solids collected in the underflow (W SU ) and the solids from the feed (W S ), in agreement with Eq.2. WSU E T = (2) WS Reduced total efficiency (E T '): When the hydrocyclone operates without the action of the centrifugal field, it can work as a flow divisor ("T" effect)[1]. In this case, the amount of solids removed on the concentrate it is proportional to the underflow-to-throughput ratio, described by Eq. 3. Because this, the underflow-to-throughput ratio must be discounted in the evaluation of the total efficiency, as displays the Eq.4: Q U ( 1 CVU ) R L = (3) Q( 1 CV ) and: ' ( ET R L ) ET = (4) ( 1 R L ) Granulometric Efficiency (G): Also known as efficiency by size, it supplies the value corresponding to the size of the particle that is collected with efficiency of 50%. The calculation is made in agreement with Eq.5 and it evaluates the cut size (d 50 ) when G assumes 0.5. dx U G = ET (5) dx Reduced granulometric efficiency (G'): Here, also the "T" effect is discounted. When G' assumes 0.5, the reduced cut size (d 50 ) is obtained, in according to Eq. 6. ' ( G R L ) G = (6) ( 1 R L ) Euler Number (Eu): It is the relationship between the pressure drop and the volumetric feed flowrate, as displays the Eq.7. In this expression, V C is the velocity of the suspension in the cylindrical area and it can be calculate by the Eq.8. P Eu = (7) 2 VC ρ 2 4Q VC = (8) 2 πdc Design Equation: The Eq.9 is known as the design equation, where the parameter K depends on the employed geometry [5]. ' d50 DC µ DC K Q S ( ρ ρ) 0.5 f ( R ) g( C ) = (9) L V

4 The function f(r L ) represents the influence of underflow-to-throughput ratio on the separation process, like shows the Eq.10. The function g(cv) relates the volumetric concentration of the solids in the volumetric feed flowrate and it can be described by the. Eq f ( R L ) = (10) 1 + AR L g ( CV ) = exp( 4.5C V ) (11) 4 - Results and discussion The Table 2 displays the values of the experimental filtering medium resistance (R m ). The conventional hydrocyclones (without filtering cone) were called BC and RC, referring to Bradley and Rietema geometry, respectively. Table 2 Filtering medium resistance to the hydrocyclones Cone of Bradley s hydrocyclone Cone of Rietema s hydrocyclone B1 B2 P BC R1 R2 R3 RC R m (cm -1 ) 10.63x x x x x x10 9 The underflow-to-throughput ratio in both geometry suffered significant changes by the presence of the filtering medium, as display Fig. 4. Du: 5 mm Du: 4 mm Du: 3 mm R L (%) RCR1R2R3 P B2 B1BC RCR1R2R3 P B2 B1BC RCR1R2R3 P B2 B1BC Figure 4. Medium underflow-to-throughput ratio for all the hydrocyclones. Both geometry suffered changes in its underflow-to-throughput ratios because the presence of the filtration. The more pronounced changes were verified to the Rietema hydrocyclones that supplied more diluted underflow streams. Depending on the geometry, the filtering hydrocyclones presented a distinct behavior in relation to its respectively conventional device, in agreement with the Fig.5. While the volumetric feed flowrates decreased for the Rietema s filtering hydrocyclones, there was an increment in this same variable for the Bradley s filtering hydrocyclones.

5 Feed Volumetric Flowrate (cm 3 s -1 ) kgf/cm kgf/cm kgf/cm kgf/cm R1 R3 B1 BC R1 R3 B1 BC Hydrocyclones R1 R3 B1 BC R1 R3 B1 P Figure 5. Volumetric feed flowrates of the hydrocyclones at different pressure drops (D u : 0.5 cm) As well as the changes for the volumetric feed flowrates, the filtering hydrocyclones presented new values for the Euler numbers, like can be seen in the Fig.6. Within the limits of the experimental region investigated in this work, the Rietema s filtering hydrocyclones show increments for the Euler number in relation to the conventional device, while the Bradley s filtering hydrocyclones suffered reductions for this same variable Eu RC R1 R2 R3 P B2 B1 BC Hydrocyclones Figure 6. Euler numbers for the hydrocyclones. The changes on the underflow-to-throughput ratios affected the total efficiency in both geometry that were employed in this study, like is shown on the Fig.7. There were increases in the total efficiency for the Rietema s filtering hydrocyclones. This characteristic behavior for the total efficiency occurred due to the increase of the values of the underflow-to-throughput ratio, that were responsible by the larger drag of solids to the underflow stream. For the Bradley s filtering hydrocyclones happened the opposite, because the reduction on the underflow-to-throughput ratio that caused decreases on their total efficiencies.

6 Du: 5 mm Du: 4 mm Du: 3 mm E T (%) RC R1 R2 R3 P B2 B1 BC RC R1 R2 R3 P B2 B1 BC RC R1 R2 R3 P B2 B1 BC Hydrocyclones Figure 7. Total efficiencies for the hydrocyclones. 5 - Conclusions The experimental results obtained with the filtering hydrocyclones in the same operation conditions that their conventional devices, allow to concluded that: The performance of the hydrocyclones of the families Bradley and Rietema is significantly influenced by the presence of the filtering medium. The underflow-to-throughput ratios of the both filtering devices suffered changes in relation to their conventional device, increasing for Rietema s Filtering hydrocyclone and decreasing for the Bradley s filtering hydrocyclone. While the Rietema s filtering hydrocyclones supplied smaller volumetric feed flowrate in relation to the conventional device, the Bradley s filtering hydrocyclones presented increments for this same variable. While the Euler numbers for the Rietema s filtering hydrocyclones suffered increments in the same operational conditions when it was compared to the conventional device, for the Bradley s filtering hydrocyclones there were reduction for them. In both geometry, the total efficiencies were influenced by the underflow-to-throughput ratio. For the Rietemas s filtering hydrocyclones the underflow-to-throughput ratio collaborated on the increment of the total efficiency. The total efficiencies for the Bradley s filtering hydrocyclones were reduced due to the reduction the solids on the underflow stream. The filtration and the geometry were simultaneously important for understanding the separation process in filtering hydrocyclone. 6 - References [1] Svarovsky, L. Hydrocyclone. Holt, Rinehart & Winston, Eastbourne, UK p [2] Souza, F. J., Analysis of the influence of the filtering media in the behavior of the filtering hydrocyclone of Bradley. Thesis of master s degree, Federal University of Uberlândia, MG, Brazil, [3] Vieira, L. G. M., Study of the performance of Rietema's filtering hydrocyclones, Thesis of master s degree, Federal University of Uberlândia, MG, Brazil, 2001 [4] Massarani, G., Fluid dynamic in particulates systems, Federal University of Rio de Janeiro, [5] Perry, R. H. & Green, D. Perry s Chemical Engineering Handbook, 6 th edition, McGraw Hill, New York