Slurry hold

It may be mixed with water to form a slurry for manual application (50/50-STB/water) or for use in a power-driven decontaminating app (40/60-STB/water). Antiset added to the slurry holds the ingredients in suspension and prevents settling out of calcium which would clog pipes and strainers... 

The CAER SBCR plant was overhauled and redesigned to incorporate automatic slurry level control and wax filtration systems. These design changes will allow a more constant inventory of the catalyst to be maintained in the reactor while reducing slurry hold-up in the catalyst/wax separation system. In addition, the wax filtration system was rearranged to accept a variety of filter elements. These additions were meant to enhance the stability of the reactor operation so that long-term tests can be conducted to study catalyst deactivation and attrition under real-world conditions. 

Originally, the overhead separator vessel was designed to enhance settling of the catalyst particles. Thus, slurry to be filtered was extracted near the top of the vessel where the catalyst concentration would be lower than that near the bottom. Unfortunately, this approach required a large hold-up volume of slurry outside the reactor (greater than the reactor volume itself). Decreasing the volume of the overhead vessel from 18 to 4 liters lowered slurry hold-up outside the reactor. 

Slurry Modes of Transportation, 237 Slurry Hold-up and Solid Concentration, 239 Solids and Fluid Mean Resistance Times, 243 Prediction of Hold-up Solids Concentration, 243... 

An experimental run consisted of rotating the drum at a given speed and maintaining predetermined flow rates of solids and water. Steady state conditions were assumed to have been achieved when the discharge slurry had the same flow rate and composition as those of the feed inlet slurry. Steady state was usually reached about 30 minutes after start-up. When steady state conditions were reached, the drum rotation was stopped simultaneously with the slurry feed. The slurry hold-up in the drum and slurry solids concentration were determined from the mass and volume of the drum contents. The drum slurry hold-up and its solids concentration were measured within 5% of the value. The percent slurry hold-up is defined as the percentage of total volume of slurry in the drum to the drum volume. The solids volumetric concentration, C, was determined from the slurry hold-up density, P ,. using... 

Effect of Drum Rotational Speed on Slurry Hold-up... 

Figures 11 and 12 depict the effect of the drum speed on the slurry hold-up and its solids volumetric concentration, respectively. The slurry feed rate is 20 g/s, and the solids particle size is d = 2.0 mm. For a given slurry feed solids concentration. Figure 11 displays that the drum slurry hold-up decreased with the drum speed. Also, at a given drum rotational speed, the drum slurry hold-up was higher for a slurry feed having a higher solids concentration. At low drum speeds, say 7 rpm, the drum slurry hold-up for a feed slurry solids concentration of Cp = 46.1 vol.% was twice that for a Cp = 7.6 vol.%.

The variation of the solids concentration in the drum slurry hold-up with the drum speed is shown in Figure 12. The solids concentration in the drum hold-up decreases with the drum speed. Within the drum speed range examined by Afacan et al., the solids concentration in the drum slurry hold-up was always much higher than that in the slurry feed [38]. This is simply due to the fact that the axial velocities of the solids and the water within the drum are not the same. The water velocity is higher than that of the solids. A similar observation was made by Davis [31], Gupta et al. [33], and Rogovin [37],... 

Figure 14 shows the effect of the drum speed on the slurry hold-up for a medium sand, d, = 0.5 mm, for different feed slurry concentrations. The slurry feed flow rate was kept constant at 20g/s, similar to those given in Figures 11 and 12 for... 

Figure 15 shows the influence of slurry feed rate on the slurry hold-up for drum speeds of 7 and 35 rpm. The feed solids concentration and particle size were kept... 

Figure 18 depicts the effect of the drum speed on the slurry hold-up for various particle sizes at a constant slurry feed rate and feed solids concentration. It is clear that the slurry hold-up is a strong function of solids particle size as well as drum... 

Experimental studies show that for medium particles (dj = 0.5 mm), the cohesive forces (agglomeration) are dominant and lead to higher slurry hold-up as compared with the coarse particles. To reduce these forces, the fluid surface tension, o, was reduced from 70 to 35 mN/m by the addition of Triton X-100 (a nonionic surfactant) to tap water at a concentration of 120 ppm. Figure 19 shows that the effect surface tension on the slurry hold-up was not significant for both particle sizes (dj = 0.5 and 2.0 mm). 

Figure 19. Effect of fluid surface tension on slurry hold-up.

In the previous section, the flow of a sand-water slurry in a horizontal drum with an open-end discharge was reviewed. Slurry hold-up was found to be a function of particle size, drum speed, feed flow rate, and composition. The experimental results also showed relatively low slurry hold-up and poor radial (transverse) mixing inside the drum. To overcome these problems for the case of dry solids, an end-constriction and lifters are normally used. The effects of such additions on the slurry hold-up and mean residence times of solids and fluid phases will be discussed in the next section. 

Ejfect of Drum Speed on Slurry Hold-up. It is instructive to discuss the effect of the end-constriction on the minimum slurry hold-up, before examining the effect of various parameters on the drum hold-up. In the case of a rotary drum with an end-constriction, material cannot flow out of the drum until it is filled to the height of the lip (R - R ). This means that a minimum slurry volume must be present in the drum before any material begins to flow over the discharge lip. The minimum slurry hold-up (percentage of the drum volume) can be calculated as ... 

Effect of Feed Flow Rate on Slurry Hold-up. Figure 25 depicts the effects of the drum speed, and slurry feed concentration on the slurry mean solids concentration in the drum for the same conditions given in Figure 24. The mean slurry solids concentration in the drum varied significantly with the drum speed for the slurry feed solids concentrations of 3.5% and 7.5%, whereas a small variation in the mean solids concentration occurred for a slurry feed solids concentration of 45.8%. For... 

Figure 24. Effect of drum speed on slurry hold-up in a drum without lifters.

Effect of Solids Feed Rate on Solids Hold-up. The results discussed so far indicate that the slurry hold-up is controlled mainly by the movement of solid particles in... 

Slurry Hold-up and Mean Solids Concentration. Figure 30 depicts the influence of the drum rotational speed on the slurry hold-up for slurry feed concentrations of 7.8%, 21.0%, and 45.9%. The slurry hold-up-drum speed relationship exhibited a minimum for all the feed solids concentrations examined. At low drum speeds, the slurry hold-up increased with the feed solids concentration. However, at high drum speeds the effect of the slurry feed solids concentration on the slurry holdup was less significant. The most important aspect of Figure 30 is that the lifters eliminated the abrupt changes in the slurry hold-up observed for the drum without lifters. 

Effect of Feed Rate on Slurry Hold-up. Figure 33 shows the effect of the slurry feed rate on the slurry hold-up at drum speeds of 7 and 35 rpm for Cp = 21.1%. The slurry hold-up increased with the slurry feed flow rate. This trend is similar to that observed with the drum without lifters (Figure 28). However, the presence of lifters tended to decrease the slurry hold-up at a drum speed of 7 rpm. At a drum speed of 35 rpm, the effect of the lifters on the slurry hold-up was not significant. 

Figure 33. Effect of slurry feed flow rate on slurry hold-up In a drum with lifters.

Flow of Solids and Slurries in Rotary Drums 239 Slurry Hold-up and Solid Concentration... 

Figure 35 shows the variation of the percent slurry hold-up and hold-up slurry concentration for the case of 80 pm silica sand at a slurry flow rate of 0.02 kg/s. The working liquid was water at 23°C. Figure 35a shows that the percent slurry hold-up in the drum decreased with the drum speed. For the cases of 10 to 30% feed solids concentration, the asymptotic hold-up value was about 20%, which was slightly higher than the minimum hold-up of 16.2%. However, for the case of a feed solids concentration of 40%, the asymptotic percent slurry hold-up was higher. 

Figure 36a is similar to Figure 35a, but for a slurry feed rate of 0.04 kg/s. The percent slurry hold-up behavior was similar to that for = 0.02 kg/s. The variation of the hold-up solids concentration ratio, C/Cp, is shown in Figure 36b. It is clear that for drum speeds > 2.62 s, all the curves for the various Cp values approached a limiting value of C/Cp = 1.0. This result is, once again, indicative that both the water and the solids move with the same axial velocity similar to a homogeneous slurry. The maximum deviation of C/Cp from unity occurs at low drum speeds, signifying a large relative velocity for the water and the solids. This case is that of a stratified slurry flow.

Figure 38. Effect of slurry mass flow rate on percent slurry hold-up with p, = 1 mPa s.

For all slurry feed solids concentrations and particle sizes, the slurry hold-up decreased as the drum speed was increased. 

For constant feed solids concentration and drum speed, the slurry hold-up increased as the slurry feed rate was increased. 

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