Which reactor functions do mixing plants offer for crystallisation or solid-state reactions?
Mixing plants can support crystallisation and solid-state reactions effectively, but they are not automatically full-fledged reactors for every task. Their suitability depends on reaction kinetics, heat release, solids content, temperature and pressure range and the desired product properties.
In crystallisation, the mixing tool ensures temperature, concentration and suspension conditions that are as uniform as possible. This makes it possible to reduce local supersaturation peaks after the addition of solvent, antisolvent, reactant or seed crystals. Mixing intensity, cooling rate and dosing strategy influence nucleation, crystal growth and particle size distribution. Excessive shear, however, can cause crystal breakage and unwanted formation of fines.
Heatable or coolable jackets and temperature-controlled mixing tools permit defined temperature profiles for crystallisation, evaporation, drying and temperature-dependent solid-state reactions. Under vacuum, solvents can be removed at lower temperatures, products degassed or temperature-sensitive substances dried gently. In suitable cases, reaction, crystallisation, solvent separation and final drying can be combined in a single closed apparatus.
In solid-state reactions, mixing improves contact between solids, liquids and gases and reduces concentration and temperature gradients. Typical applications are neutralisations, precipitations, surface modifications, coatings and impregnations. For high-temperature processes such as calcining or sintering, by contrast, special thermal units are usually required.
Speed, tool geometry, fill level, residence time, temperature profile and point of addition influence agglomeration, de-agglomeration, bulk density and flowability. Temperature, pressure, torque, moisture and – depending on the process – pH value, conductivity or turbidity can be used for process monitoring. PAT is not a rigid equipment package here but an application-related approach to controlling critical process and quality parameters.
amixon® vacuum mixer-dryers and synthesis reactors: mixing, reacting, crystallising and drying in one apparatus
amixon® develops vacuum mixer-dryers and synthesis reactors that can combine mixing, temperature control, reacting, crystallising, evaporating and drying in a single closed apparatus. Particularly relevant for such tasks are the VMT vacuum mixer-dryer and reactor and the AMT conical mixer-dryer and reactor. Both machines are suitable for powdered, moist, pasty or suspended products.
In crystallisation and precipitation processes, the three-dimensional product movement supports low-gradient blending and homogeneous temperature control. This can reduce local differences in concentration and temperature and thus contribute to reproducible product quality. The mixing action is deliberately designed to protect the product – an important aspect where the crystal structure being formed is to be preserved and unwanted crystal breakage or formation of fines is to be limited.
Heat transfer takes place via heatable or coolable product-contact surfaces. Depending on the version, walls, base and mixing tools can be temperature-controlled. amixon® machines thus offer a very large heat transfer area. Temperature profiles for reactions, crystallisations, evaporations and drying processes can therefore be managed in a targeted way.
VMT and AMT are of gas-tight design and can be operated under vacuum. This makes it possible to remove solvents or moisture at reduced product temperatures, to degas products and to dry sensitive substances gently. The machines can also be used as batch reactors, with temperature and pressure kept under controlled management.
A particular advantage of process integration is that reaction, crystallisation, washing steps, solvent separation and final drying can – depending on the process – be carried out with fewer product transfers. This reduces interfaces, product losses and potential contamination risks. The conical vessel geometry of the AMT also supports good residual discharge and makes product changes easier.
For the development and design of demanding processes, pilot plant trials are also available at amixon®. Mixing quality, heat transfer, vacuum operation, crystallisation behaviour and product-protecting drying can be investigated there with original products, among other things. An amixon® high-temperature reactor in the pilot plant allows trials under vacuum or overpressure at product temperatures up to 350 °C and pressures up to 30 bar.