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Gas-solid reactions in the mechanically generated fluidised bed
Individualists are called for

There is a wide range of options for gas-solid reactions in reaction process technology. This is particularly true for heterogeneous syntheses, as required in modern chemical and pharmaceutical production. For this reason, it is important that reactor solutions are customised and application-specific. Even small procedural details can influence the quality of the process and product.

Author Dr. Frank Spranzel Sales Department Mixing and Reacting Technologies, Lödige

There are probably as many different chemical gas-solid reactions as there are industrial gases. As a result, controlled performance of chemical reactions is no longer limited to homogeneous, single-phase systems. On the contrary, heterogeneous syntheses using catalysts dominate today where feasible.
The options for reactions with gaseous reactants in the chemical and pharmaceutical industry are always highly diversified: for instance, many reaction processes use carbon dioxide gas or dry ice. Carboxylation for producing carbonic acid is roughly based upon the reaction of organometallic compounds with carbon dioxide, known as the Grignard reaction. The reaction of carbon dioxide with phenolate, which produces salicylic acid derivatives (known as the Kolbe-Schmitt reaction – a reaction stage in the synthesis of aspirin), is conducted in like manner. Ammonia gas, a primary product of the chemical industry, also forms the basis for many syntheses, amongst other things in fertiliser making. Oxidation with pure oxygen or an oxygen-ozone mixture, for example for disinfection or bleaching, is another widespread reactor process, along with processes involving slightly boiling liquids such as ethylene oxide or propylene oxide and high vapour pressure (in the threshold range of the gas phase). Methyl chloride is often utilised as a methylation reagent in organic chemistry while dimethylamine is employed in many areas of industry to introduce dimethyl amino groups.
All these chemical processes take place in reactors. Yet which process can be optimally realised in which reactor? The answer to this question is, of course, critical for success. Small details generally determine which procedural solution makes the most sense for a particular task. Customised reactor solutions are the key here.
Process-related tasks
There are three types of system to choose from – especially when it comes to gases reacting with solids, as is the case with the above-mentioned processes – namely rotary kilns, solid bed reactors and fluidised bed reactors such as the Druvatherm reactor from Lödige. These three reactor types differ in their control of the respective thermodynamic and kinetic reaction processes. Rotary kilns make use of a solid material bed over which gas flows. In a solid bed reactor the gas flows through a solid material bed. In fluidised bed reactors the solid material bed is moved intensively during the reaction process.
A fluidised bed reactor is a special, complex type of mixer. When designing an optimal fluidised bed reactor for use in the chemical or pharmaceutical industry, the requirements of the mixing process in the reactor for the prod-uct in question must be precisely analysed. Specifically, the customer and the mechanical engineer must cooperate during the planning process to develop the most suitable reaction and machine technology.
Tailor-made reactor types
There are a multitude of technical options for meeting the application-specific requirements of gas-solid reactions. The broad portfolio of Druvatherm fluidised bed reactors from Lödige provides the perfect solution whatever the application – from simple solid material mixers through drying systems to a pressure vessel for reactions subject to overpressure. In practice, Lödige has already realised working overpres-sures of up to 40 bar in a reactor. The reactor sizes vary from small laboratory devices with a drum capacity of 5 l to production machinery with a volume of more than 50,000 l.
These machines are equally suited for batch operation and continuous production lines. The solid particles move in the reactors in a circulating fluidised bed which is generated mechanically in the drum. The energy neces-sary to do this is put in by means of a horizontal shaft fitted with mixing elements. Thermal energy is supplied and discharged via a heat transfer medium such as water, steam, thermal oil or molten salt. Electric heating permits temperatures up to +700 °C. High heat transfer coefficients are realised using a mixing drum equipped with a double jacket. The shaft can optionally be included in the heating or cooling circuit. Additional, rapidly rotating choppers in the reaction chamber introduce shearing strain into the product and open up further options. If a vacuum or carrier gas is used, the performance range of the Lödige fluidised bed reactor is enhanced with drying systems, known as shovel dryers. The material qualities which are selected, as well as the mixing elements and the sealing technology, are always individually adapted to the process.
Reactions in the fluidised bed reactor
If the above-mentioned mixing technology is implemented in the fluidised bed reactor, reactions can be performed in multiphase systems (solid-liquid-gas) with outstanding results. There is a homogeneous distribution of heat and concentration in the mechanically generated fluidised bed. This is important in that temperature gradients – and hence an inhomogeneous temperature distribution – would have a limiting influence on the reaction sub-steps that determine the rate (material transport, diffusion). There are two options for gas-solid reactions: with the first, the reaction gas flows through the fluidised bed whereas with the second, the reactor is filled with the required stoichiometric amount and the reaction conducted in a closed system under pres-sure and with the addition of heat. The latter method has a correspondingly positive effect on the reaction kinetics, of course.
Fluidised bed reactors thus offer a wide range of possibilities for variable liquid and gas dosage with reaction control. Parameters such as overpressure, temperature control by means of a double jacket and cooler or final drying in a vac-uum can be controlled very precisely. For this reason, certain reactors are ideal for what are known as one-pot reactions. A continuous gas flow through the solid fluidised bed of a continually operated reactor is also conceivable.
Conclusion
Gas-solid reactions, which are a daily occurrence in the chemical industry, pose complex procedural challenges. A broad range of suitable reactors are available to ensure optimal adaptation to application-specific requirements. Choosing the best tailor-made solution from this portfolio is decisive here. In the end, procedural details determine which machine type is most appropriate in each case.
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