For slurries

Slurry Viscosity. Viscosities of magnesium hydroxide slurries are determined by the Brookfield Viscometer in which viscosity is measured using various combinations of spindles and spindle speeds, or other common methods of viscometry. Viscosity decreases with increasing rate of shear. Fluids, such as magnesium hydroxide slurry, that exhibit this type of rheological behavior are termed pseudoplastic. The viscosities obtained can be correlated with product or process parameters. Details of viscosity deterrnination for slurries are well covered in the Hterature (85,86). 

FIG. 10-53 Pneumatically actuated diaphragm pump for slurry service. (Coutiesy of Dotr-Olvier Inc.)... 

Slurries of free-filtering sohds that are difficult to suspend are sometimes filtered on a top-feed drum filter or filter-diyer. An example application is in the production of table salt. An alternative for slurries of extremely coarse, dense solids is the internal drum filter. In the chemical-process industiy both top-feed and internal drums (which are described briefly by Emmett in Schweitzer, op. cit., p. 4-41) have largely been displaced by the horizontal vacuum filter (q.v). 

Equipment Operation Spray nozzles suffer from caking on the outside and clogging on the inside. When the nozzle is below the bed surface, fast capture of the liquid drops by bed particles, as well as scouring of the nozzle by particles, prevents caking. Blockages inside the nozzle are also common, particularly for slurries. The nozzle design shoiild be as simple as possible and provision for in situ cleaning or easy removal is essential. 

Tubular reactors have been the main tools to study continuous flow processes for vapor or gas-phase reactions. These are also used for reaction in tv o flowing phases over a solid catalyst. When the catalyst is in a fixed bed, the contact between the liquid on the outside surface of the particulate is uncertain. For slurry-type solid catalyst the residence time of the catalyst or the quantity in the reactor volume can be undefined. 

Applicability/Limitations Fluidized beds require frequent attention for maintenance and cleaning purposes. This treatment is ideal for slurries and sludges but not for bulky or viscous wastes. The waste particles should be of a certain size and be homogeneous. Wastes must have a low sodium content and a low heavy metal content. Some refractory wastes may not be fully destroyed since these units operate at low combustion temperatures (750 to 1000°C). 

This family of filters consist of a vertical pressure vessel with a horizontal filter plate at the bottom. The filtrate from this equipment flows out a nozzle on the bottom of the filter. These devises are usually used for slurries where large amounts of solids are being collected. Variations of this equipment include equipment with removable lower heads for easy cake removal, ability to pressure or vacuum filter, ability to wash the filter cake, an agitator to break-up and rewash the filter cake, and heating or cooling jackets for the whole vessel. The Nutsche filter is the industrial version of the well known laboratory scale Buchner Funnel with the exception that it is designed to operate under either on vacuum or pressure. 

Similarly equation 36 for slurries with nonspherical particles is... 

Lyachshenko number, dimensionless left hand side, dimensionless particle mass, kg pressure, N/m or force, N mass feed rate, kg/s or volumetric flowrate in mVhr drag or resistance force, N physical properties correction factor for slurries Reynolds number, dimensionless right hand side hydraulic radius, m... 

Figure 2.5 Flindered drag coejficients for slurries after Foscolo etai, 19H3)...

The settling rate and settling time ean be estimated using e.g. the Riehardson and Zaki equation (2.42). For slurries of irregular partieles, however, the assumptions in the eorrelation are exeeeded and the settling rate then beeomes more diffieult to ealeulate. Consequently, the Jar Test (see Chapter 2) is frequently used to determine R and /r in praetiee. 

In principle, filter bed permeabilities can be calculated using the Carman-Kozeny equation 2.53. For slurries containing irregular particles, however, cake filtrabilities together with filter medium resistance are determined using the Leaf Test (Figure 4.13). In this technique, a sample of suspended slurry is drawn through a sample test filter leaf at a fixed pressure drop and the transient volumetric flowrate of clear filtrate collected determined. 

More elongated devices are used for slurry thickening whilst those of more squat aspect ratio devices are suitable for product classification by size, as in fines-destruction circuits. The pressure drop in a hydrocyclone varies with the feed rate raised to a power between 2 and 3.3. The cut size is a weak function of pressure drop, varying with for dilute feeds. Large pressure drops are... 

These special throttling valves use an elastomer diaphragm to restrict or stop flow (Figure 15-7). They are suitable for slurry service and make an excellent valve for sand drains. Unfortunately, they do not provide a reliable, positive shut-off and should be installed in series with a ball or other on/off valve if positive shut-off is required. 

Figure 2-50. Representative plot of pressure drop for slurry flow. By permission, Turian, R. M. and Yuan, T. F., Flow of Slurries in Pipelines, AI.Ch.E. Journal, vol. 23, 1977, p. 232-243.

The Doppler meter may be used wherever small particulate solids, bubbles or droplets are dispersed in the fluid and are moving at essentially the same velocity as the fluid stream which is to be metered. A continuous ultrasonic wave is transmitted, again at an acute angle to the wall of the duct, and the shift in frequency between the transmitted and scattered waves is measured. This method of measurement of flowrate is frequently used for slurries and dispersions which present considerable difficulties when other methods are used. 

Boger et al. [50] analyzed the performance of MLRs with internal density-driven circulation (IMLR). They found the gas-hquid mass transfer superior and the overall mass transfer performance comparable with those for slurry reactors. 

Bubble columns and mechanically stirred reactors are the most common reactor types for slurry systems in laboratories, but they have many disadvantages from an industrialization perspective. Mechanically stirred reactors usually used for laboratorial studies are difficult to scale-up. In order to achieve good mixing and mass transfer between the gas and slurry phases, bubble column must be operated at a high space velocity, which leads to a relative low one-through conversion of the syngas. 

Mass transfer-limited processes favour slurry reactors over monoliths as far as the overall process rates are concerned. Moreover, slurry reactors are more versatile and less sensitive to gas flow rates. However, the productivity per unit volume is not necessarily higher for slurry reactors because of the low concentration of catalyst in such reactors. There also is no simple answer to the selectivity problem, and again, each process should be compared in detail for both reactors. 

Figure 11-4. Components for additives for slurry transport oleic acid, linoleic acid.

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