| 4 Suspension mixing equipment Suspension mixing equipment generally includes several parts such as agitator, tank and baffle. There are many factors affecting solid-liquid suspension, mainly the following: 4.1 Stirrer For complete off-bottom suspension, only one layer of impeller is used. For uniform suspension, a multi-layer impeller must be used, but the critical speed is still determined by the lowest impeller. Some high-efficiency axial flow impellers are well suited for solid-liquid suspension operations, and these impellers feature variable vane width and dip angle. A typical solid-liquid mixing impeller is shown below. 4.2 Ratio of Paddle Diameter to Groove Diameter When using a turbine or paddle impeller, if the viscosity does not change much, the ratio of the pitch diameter to the groove diameter is generally between 0.35 and 0.5. 4.3 Slot bottom shape The flat bottom groove and the tapered groove are prone to particle accumulation, and the dish groove has a large power consumption, and the curved bottom groove can avoid the above difficulties. 4.4 Off-bottom height of the impeller The impeller is too close to the bottom, and the accumulation of particles at the bottom of the trough can cause obstacles to start the impeller. The impeller is too far from the bottom and the suspension of the particles at the bottom of the tank will be weakened. A suitable height is about 0.25 times the groove diameter. 4.5 Baffles and Guide Tubes To avoid the formation of liquid turns, it is common to install baffles and sometimes to install guide tubes. 5 Selection of Suspension Mixing Equipment The choice of suspension mixing equipment mainly depends on the needs of the process, including the following aspects: 5.1 Process issues (1) Batch, semi-batch or continuous process? (2) What phase will occur during the process? (3) Is there a chemical reaction between the solid and liquid? (4) What are the physical properties of the liquid-solid phase? (5) How much suspension is needed? (6) What is the minimum speed required to reach this suspension state? (7) What happens if the agitation speed is reduced or the agitation is interrupted? (8) What is the change in suspension when the agitation speed increases? (9) How does the geometry of the container affect the process? (10) What is the most suitable equipment material for this process? 5.2 Problems with the tank and the agitator include the design of the shape of the groove bottom, the size and diameter of the groove, the baffle and other accessories. Including the shape, number and direction of the paddle; the position of the paddle; the speed and power of the paddle; the diameter and length of the paddle; motor and sealing system. 6 Application of suspension mixing equipment The application of suspension mixing equipment is mainly applied in the following aspects: 6.1 Solid Dispersion The action of the agitator allows the particles or agglomerates to be dispersed and suspended in the liquid to form a uniform suspension or slurry. It is applied to prepare a solid reactant slurry and a catalyst slurry, and then to the next reactor; or simply disperse the solid into particles suspended in the liquid. 6.2 Dissolution and Filtration Dissolution is the unit operation that transfers the liquid-solid mass. The solid particles are absorbed by the liquid and become smaller and eventually disappear. Filtration is a unit operation in which the liquid is soluble and analyzed. Some resins and plastics swell when they are precipitated due to absorption of liquid. In many systems, the density and viscosity of the dissolved and filtered liquid changes. In this process, the purpose of the agitation is to obtain the desired dissolution or filtration rate. 6.3 Crystallization and Precipitation Before the seeding, the particles in the solution are free particles, which are crystallized or precipitated to form particles. During operation, the diameter and quantity of the particles increase synchronously, and at the same time, the density and viscosity of the slurry are also changes happened. The purpose of this process is to control the rate of nucleation and particle growth to minimize particle breakage and wear. The average particle size and particle size distribution are an important indicator. Controlling the concentration of the liquid phase to avoid excessive local concentrations also requires control. 6.4 Absorption, desorption and ion exchange are also processes of mass exchange. 6.5 Catalytic Particle Reaction This operation absorbs the reactants onto the surface of the catalyst and removes the product from the surface of the catalyst. The uniform suspension of the catalyst in the liquid is the key to the operation. In addition, the agitator reduces the boundary layer for mass transfer and enhances the mass exchange of liquid and solid. 6.6 Polymerization At the beginning of the reaction, the agitator is required to stably disperse the monomer droplets. As the reaction proceeds, the resulting polymer becomes very viscous, and the agitator controls the contact of the monomer with the catalyst and, in turn, the particle size and particle size distribution of the polymer. In the polymerization, the purpose of the agitation is to maintain uniform dispersion of the monomer and the polymer.
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