Solid - Liquid
Solids suspension is a flow-controlled process. For solids to be suspended in a fluid we need to impart an adequate fluid velocity to the solid particles. This velocity must be greater than the terminal settling velocity of the particles, otherwise, solids suspension will not occur.
Axial flow impellers are best suited for solids suspension applications because of their flow patterns, volumetric flow rates and fluid velocities.
The data required: solids particle density, particle size, percent solids composition, fluid density and viscosity. The data is used to
calculate the settling velocity of the solid particles in the fluid. As the particles become larger and/or more dense, the settling velocity increases. As the fluid density increases, the settling velocity decreases.
Degree of solids suspension
Solids suspension applications are usually categorized as:
Off-Bottom Suspension - All particles are moving with a vertical velocity at some time, though not necessarily simultaneously. While larger particles may be moving upward, but only at small distances from the bottom, smaller or lighter particles may be suspended to the upper surface of the fluid. Off-Bottom Suspension is also called the "just-suspended" condition.
Uniform Suspension - All particles are suspended uniformly throughout the vessel. Complete Uniformity is a relative term and is defined as the condition for which a further increase in mixer speed or power will not appreciably affect the solids concentration profile throughout the vessel.
Off-Bottom Suspension is adequate for most solids suspension applications.
Axial flow impellers are commonly used for solids suspension operations. They have higher pumping capacity per unit of applied torque than other practical impellers. Applied mixer torque translates into fluid flow velocity, and flow velocity suspends solids. Such impellers work well when positioned relatively far off tank bottoms. This aids mixed design by minimizing agitator shaft overhangs in vessels that are usually quite large, and require long shaft overhang at best.
An impeller position up to at least one impeller diameter (1 D) off bottom is effective.
Tank size and shape -This is much more important in solids suspension than in blending.
As the liquid level to tank diamter ratio (Z/T) increases, thetanks get taller and the solids must be suspended at higher levels in the vessel, requiring more power. At Z/T > 1.25, dual impellers are normally required if uniform suspension is required.
Dish bottom tanks or tanks with fillets are optimum for solids suspension because the bottom shape aids in directing the flow. Steep cone bottoms are shapes to be avoided as solids settle in the cone and can be very difficult to suspend.
Anti-swirl baffles are an absolute necessity for developing the vertically upward flow streams required for development of best uniformity of solids suspended in a mixing vessel Always use such baffles, unless there is some reason why they can't be tolerated; such a reason might be extreme build-up of solids in the mix in the vicinity of any internal fitting in the tank.
Baffles should be standard—as used for all-liquid operations—for the majority of installations. 4 baffles, 90° spaced, each baffle 1/12th tank diameter in width, always set out from tank wall (to maximum 30% of baffle width). Extend downward in tank as far as possible, for best mixing action at bottom, but terminate above bottom to avoid build-up of solids at that point.
Higher solids content mixes—but ones where baffles can still be tolerated—may benefit from use of slightly narrower than standard baffles.
Where the solids have any floating tendency—or in any operation where wetting-out of solids is a requirement —the baffles should be terminated well short of the top of the mix. This will promote surface vortexing, for better draw-down. An upper impeller may also be desirable for such operations.
Properties of solids suspension
A solids suspension process can be judged to be either free settling or hindered
Determining settling by examining a representative sample in a glass beaker. The solids are suspended by stirring, then the settling process is observed. If the solids fall to the bottom of the beaker after a 'few seconds, it is a free settling system. If the
settling process is gradual and the particle sizes segregate during settling, it is a hindered settling system.
The bulk density of a solid is the weight of a specific volume of the solid (typically pounds per cubic foot). It has little value in sizing agitation jobs because it does not define the porosity of the particles or the voids between them.
When low bulk density solids (containing air in pores and voids) are added to a batch and wetted out, the air is dispersed in the batch, sometimes completely enveloping the impeller. This can be so serious that the impeller will stop pumping. Shutting off the mixer and allowing the air to escape from the batch is called "burping" and restores the pumping action of the impeller.
Light solids that dissolve to form high viscosity solutions (like gums and pectins) often wet out with a viscous layer on the outside of the agglomerates while remaining dry on the inside. These are called "fish eyes" and are difficult to dissolve further as the solution must penetrate through the thick viscous layers. Higher shear impellers may be needed to eliminate fish eyes.
Packed bed effect is a measure of the degree of difficulty of starting up an impeller in settled solids. Slurries having a wide variation in particle size usually pack in very
tightly because the smaller particles filter down through the porous bed and fill up the voids. When starting a mixer in settled solids, provision must be made to protect the mixer from mechanical failure.
As the viscosity of the liquid increases, more power is required to produce the same fluid velocity. On the other hand, the settling velocity of solids decreases as viscosity increases. These effects largely offset each other for free settling solids