When mixing reactive powder systems, precise thermal management is decisive for controlling reaction kinetics, product quality and process safety. Depending on the reactivity of the system, the batch size and the desired temperature profile, various technical heating and cooling options are available.

Double-jacket systems (indirect heating and cooling)

The most common method is the use of double-walled mixing vessels through whose jacket a heat transfer medium circulates.

Typical media are:

• water for moderate temperature ranges
• thermal oil for high process temperatures
• brine or glycol-water mixtures for cooling below 0 °C
• steam for rapid heating, for example during drying or sterilisation steps

This makes it possible both to remove exothermic heat of reaction reliably and to preheat powder components in a targeted way, for instance to control viscosity or to activate solid-state reactions.

Temperature-controlled mixing tools and internal heat transfer surfaces

Because heat transfer through the vessel wall is limited at large volumes, mixing shafts and mixing tools can be of hollow design with a temperature-control medium flowing through them.

The following are also possible:

• internal heat exchanger tubes or plates
• temperature-controlled base areas to prevent condensation

The advantage of this solution lies in direct heat transfer within the core of the product. This improves thermal homogeneity, shortens process times and permits large quantities of heat to be removed or introduced at a high rate of intermixing.

Direct electrical heating

For extremely high temperatures, electrical heating jackets or heating mats are used. Low-current trace heating is also employed where silos or pipework have to be kept warm.

This solution is comparatively simple and inexpensive and is suitable for applications with a limited heat demand.

Gas purging and gas temperature control

Temperature-controlled gas streams are passed through the mixing chamber and transfer heat directly to the moving bulk material. Preheated or cooled inert gases such as nitrogen serve two purposes at once: they establish the desired temperature level and keep the oxygen content low, which is relevant to safety with reactive or explosible dusts. Heat transfer is lower than with surface-based methods, but it acts directly on the particle surface and can be regulated quickly. Gas temperature control is therefore frequently used to supplement jacket or tool temperature control, for example to remove reaction peaks or to discharge moisture at the same time.

Vacuum-assisted evaporative cooling

Where moisture or solvent is present in the system, the boiling point of the liquid phase can be lowered by applying a vacuum.

Effect:

Evaporation of the liquid phase withdraws heat from the material being mixed (enthalpy of evaporation). This produces efficient, gentle cooling that is suitable for temperature-sensitive products. This form of evaporative cooling can be used at the same time for drying and for setting the residual moisture content.

Cryogenic cooling

For strongly exothermic processes or very low process temperatures, cryogenic media such as liquid nitrogen (LN₂) or CO₂ are introduced directly into the mixing chamber.

Properties:

• very rapid shock cooling through the enthalpy of evaporation
• the possibility of keeping the system inert at the same time and thereby increasing explosion safety with reactive dusts

This method is particularly suitable for grinding-mixing processes and for systems with a high heat release potential.

Radiant heating

Less commonly, heat is introduced by means of infrared emitters directed at the surface of the moving bulk material. The energy acts without contact and without a thermal transfer medium, but reaches only the uppermost layer of particles; sufficient circulation is therefore a prerequisite. The method is used above all with thin layers, with sensitive surface reactions and where a heat transfer circuit is to be avoided.

Important selection criteria for process design

The choice of the appropriate heating and cooling option – often a combination of several methods – depends in particular on:

• the enthalpy of reaction and activation energy of the system
• the required heating or cooling rate
• the specific heat capacity and thermal conductivity of the bulk material
• the temperature sensitivity of the raw materials and the desired temperature window
• safety requirements (e.g. ATEX, avoidance of thermal runaway)
• batch size, geometry and the available heat transfer surfaces
• economic and energy efficiency considerations