Chemical reactor
A chemical reactor is a process engineering apparatus in which (generally more valuable) products are manufactured from raw materials or intermediate products by means of chemical reactions . Often, however, full conversion is not achieved in the reactions or by-products are formed. For this reason, a reactor is usually followed by separation devices (e.g. rectification or extraction columns ) in which the product is purified to the required purity. Separated components of the reaction mixture - unconverted reactants and solvents - can be returned to the reactor in so-called recycling streams. A chemical reactor is therefore only one - albeit central - component of a chemical production plant .
View of an enamelled stirred kettle
Sketch of a stirred tank
Designs
Chemical reactors come in two main forms,
- the stirred tank and
- the flow tube , a flow reactor.
Operating modes
Essentially, a distinction can be made between a continuous and a discontinuous mode of operation. In the continuous operating mode, the reactor is flowed through evenly, the state variables ( temperature , pressure , concentrations ) change - apart from start-up and shutdown processes and malfunctions - not in time, but only locally. For example, the product concentration increases from the inlet to the outlet of a tubular reactor, but in the steady state it is constant over time at every location (by definition). Due to the enthalpy of reaction of the reaction, the temperature can have a maximum or minimum between inlet and outlet, while the pressure usually decreases in the direction of flow due to the pressure loss . Length and flow rate must be coordinated with one another in order to achieve the optimal residence time of the reactants. This in turn determines the conversion of the reaction (and possibly of undesired secondary reactions ).
In the case of discontinuous operation, a distinction can be made between record or batch operation and fed-batch operation . While all reactants are initially introduced in batch mode and removed again after a certain dwell time, in fed batch mode only some of the reactants are initially introduced while the remaining reactants (continuously or discontinuously) are added. This is used, for example, to keep the reactant concentration and thus the reaction rate low in highly exothermic reactions in order to always be able to dissipate sufficient heat and to prevent the reactor from " running away " . Both batch and fed-batch reactors are emptied, cleaned and refilled as required after the reaction is complete, which means a significantly higher manual effort compared to the mostly highly automated continuous processes. The advantages are the higher flexibility in the dwell time and the possibility to produce frequently changing products.
In principle, continuous processes and thus continuous reactors are used in plants for large production quantities (e.g. in refineries and in the production of basic chemicals ), while batches are used for smaller product quantities (e.g. in the production of active pharmaceutical ingredients ). A more recent development are so-called micro- and millireactors , which enable continuous operation even with lower product flows.
interpretation
The design of a chemical reactor is the task of chemical reaction engineering . The following influencing variables are required for the device-related and business calculations in order to optimize the target variable .
- For the overall reaction and essential sub-processes to proceed , the reaction mechanism , kinetics and heat balance should be known.
- For the reactor size, values for the material transport, such as flow type, mixing, and also material data for viscosity and the heat coefficient are necessary.
- The heat balance and the reaction process provide information on the cooling or heating requirement.
- Special hazards arising from the substances involved ( corrosiveness , reactivity of intermediate or by-products, possible thermal runaway ) and from the reaction conditions (extreme temperatures, pressure vessel) must be known.
- Finally, it must be clarified how the starting materials are added and the products discharged (purification, recycling, subsequent reactions).
For this design, idealized reactors are initially considered. This is understood to be models of the desired reaction vessel which, due to simplifications, enable mathematical modeling .
In a first step, mainly software-based models are used for the design of chemical reactors ( process simulation ). In a second step, the mathematical models are optimized by building the reactors on a laboratory scale, possibly also on a pilot plant scale. Finally, the plant is built to production size by scale-up .
Special designs
- In the manufacture of polymers , the design is influenced by the often extremely high viscosities of the reaction mixtures.
- Catalytic processes require the necessary large contact surfaces to be created with solid catalytic materials.
- Due to the temperature sensitivity of biological substances, special designs are necessary for biotechnological processes .
Various reactors in chemical production
In chemical production, a distinction is made between different reactors. The best known in chemistry is the stirred tank (also called stirred tank). Chemical reactions are carried out in the liquid phase in stirred tanks. The response time is relatively long. The quantities processed vary depending on the size of the stirred tank and the residence time. Stirrer tanks are large steel tanks equipped with a stirrer and heating jacket. Baffles protrude into the container through nozzles . They prevent the entire contents of the container from rotating with the stirrer and mix the reactants. Additional nozzles in the cover ensure that the reactants are fed in and are used to insert measuring instruments. At the bottom of the stirred tank there is the drainage nozzle, which is opened and closed with a valve disk. Batch reactions are carried out in the stirred tank, which take place under ambient conditions or at moderately increased pressure and temperature up to approx. 250 ° C.
Depending on the operating requirements, the stirred tanks are made of unalloyed or alloyed steel or they are clad, enamelled (glass-coated) or rubberized. For particularly aggressive media (e.g. HF and HCl ), linings made of perfluoroalkoxylalkane (PFA) can be used, which, depending on the medium and design, are resistant up to a temperature of 250 ° C.
There are also high-pressure, high-temperature, tubular, loop and fluidized bed reactors.
literature
- Klaus Hertwig, Lothar Martens: Chemical process engineering: calculation, design and operation of chemical reactors , Oldenbourg, Munich 2007, ISBN 978-3-486-57798-3 .
