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Avoid trouble with slurries

By Thomas R. Blackwood, Healthsite Associates

ChemicalProcessing.com

Keywords: slurries, "powder and solid mix", "powder handling", "fluid handling" "slurry handling", "Thomas R. Blackwood" and "Healthsite Associates"

Handling slurries (a mix of solids and liquids), should be based on experience and experiments, not theory. Much of the knowledge obtained from pneumatic conveyors and fluidization systems can be used in understanding slurries.

In almost all solids processing operations there comes a time when the engineer must deal with a slurry (a mixture of solids and liquids). This can be in a reactor, crystallizer, centrifuge feed tank, or a pipeline. Often slurries can be treated as just another dense liquid, especially when residence time isn’t a concern or horsepower issues have been addressed. What sets a slurry apart from normal liquids is a longer list of physical attributes, including: particle size, distribution, and particle shapes, with the resulting shear, settling, and drag of the particles. Most of the major problems encountered with slurries are due to a poor understanding of these factors, or from poorly obtained slurry experimental data. However, much of the knowledge obtained from pneumatic conveyors and fluidization systems can be used in understanding slurries.

Some of the properties of the liquid become more complicated once it becomes part of a slurry. Viscosity is especially difficult to determine, due to the range of solids concentrations or changes in particle size distribution that can be encountered in normal plant operations. Just as in pneumatic conveyors, the solids can build up in concentration or size along a pipeline or in a tank. Experimental work obtained under laboratory conditions won’t be of much use unless it is obtained over a wide range of conditions and with scaleable tests. Even the type of viscometer used during the test influences measurements.

As an example, obtaining viscosity data requires consideration for particles bridging the gaps in testing equipment. Using viscosity instruments that test a wide range of disk or cone geometries and sizes can eliminate this problem. Particle size and particle attrition can be the primary concern for handling slurries. This may be a lesser problem than in pneumatic conveyors that use gas because of the lower velocities, but it can alter the product or lessen equipment life due to the higher shear rate of the liquid. Attrition products, due to their high surface energy, also may cause excessive build-up on pipe walls, requiring frequent cleaning of the pipes and vessels. Another complication in handling slurries is the reactivity between the solids and the liquid.

While many of the experiences and design rules of solids suspended in air can be applied to slurries, reactivity can lead to scale formation, secondary products of reaction, and agglomeration. This problem is noticeable in a crystallizer. New particles form from a reaction or because of nucleation. Unfortunately, there may be nuclei formed from attrition of larger particles. Newly formed particles tend to be extremely surface-reactive and the source of operational problems, such as scale. Excessive nuclei hinder growth or can cause products of a different morphology to be formed. The principles of good slurry pipeline design, suspension, and handling have been well defined since the early 1950s.

However there have been significant refinements to detailed hydraulic design. In most cases the strategy is to make a settling slurry into a non-settling slurry so that it can be treated as a normal, homogeneous fluid. Settling slurries are more complex, and rely on determination of the settling rate, at least for pipeline design. Several design methods are described in References 1-5 and in some proprietary publications such as Ref. 6. Some authors suggest that there are slurries that can’t be effectively pumped or suspended because they can’t be classified as settling or non-settling slurries [7]. For these, you must rely entirely on previous experience. This article focuses on the basic principles of suspension for solids in vessels and pipes, with an emphasis on practical rather than theoretical considerations.

Vessel considerations

Maintaining a uniform suspension in a crystallizer or a slurry tank is impossible, no matter what the theory will tell you. Although colloids and very fine particles can be suspended with appropriate agitation, only a portion of the large particles will be suspended. Agitators should be selected to provide enough suspension to keep most of the solids off the bottom of the vessel. This is very important when the solids are cohesive. In addition to agitator selection, you must consider tank design, baffle selection, and slurry takeoff points. Many solids can tolerate incomplete suspension with solids accumulating on the bottom of the vessel, but this isn’t good design practice. This is especially true with bottom drain tanks where the outlet can become plugged. Although the solids can be kept off the bottom of the vessel by maintaining a minimum agitator speed, they can accumulate in the bottom drain and nozzles of the vessel. Any design should assume that and provide for back-flushing of the nozzle or dilution of the slurry by having a side port where liquid from the top of the tank can be injected into the nozzle. Specialized valves can minimize the buildup in the nozzle, but aren’t foolproof.

The most common method for estimating this minimum suspension condition is Zwietering’s formula for the minimum agitator speed, N [8]:
N = S dp^0.2 µ₁^0.1 (g Δρ)^0.45 B ^0.13 / ρ₁^0.55 D^0.85      (1)
where:
 S = Dimensionless parameter from agitator type, and agitator/tank diameter ratio;
 dp = Particle diameter, m;