What are chemical reactors? Types of chemical reactors

The chemical reaction is a process that leads to the transformation of the reagents. It is characterized by changes that result in one or more products different from the original ones. Chemical reactions are of a different nature. It depends on the type of reagents, the substance obtained, the conditions and time of synthesis, decomposition, displacement, isomerization, acid-alkaline, oxidation-reduction, organic processes, etc.

Chemical reactors are containers designed to carry out reactions in order to produce the final product. Their design depends on various factors and should provide the maximum yield in the most cost-effective way.

Kinds

There are three basic basic models of chemical reactors:

• Periodic action.
• Continuous with a stirrer (HPM).
• Reactor with a piston flow (PFR).

These basic models can be modified in accordance with the requirements of the chemical process.

Batch reactor

Chemical aggregates of this type are used in batch processes with small volumes of production, long reaction times or where better selectivity is achieved, as in some polymerization processes.

For this, for example, stainless steel containers are used, the contents of which are mixed with internal working blades, gas bubbles or with pumps. Temperature control is carried out using heat exchangers, refrigeration coolers or pumping through a heat exchanger.

Batch reactors are currently used in the chemical and food processing industry. Their automation and optimization create difficulties, since it is necessary to combine continuous and discrete processes.

Semi-periodic chemical reactors combine operation in continuous and periodic modes. The bioreactor, for example, is periodically loaded and constantly releases carbon dioxide, which must be continuously removed. Similarly, in the chlorination reaction, when one of the reactants is chlorine gas, if it is not introduced continuously, most of it evaporates.

To ensure a large volume of production, chemical reactors of continuous action or metal tanks with a stirrer or with a continuous flow are mainly used.

Continuous stirred tank reactor

Stainless steel tanks are supplied with liquid reagents. To ensure proper interaction, they are blended with working blades. Thus, in reactors of this type, the reactants are continuously fed into the first tank (vertical, steel), then they fall into the next, while carefully mixing in each tank. Although the composition of the mixture is uniform in each individual tank, the concentration in the system as a whole varies from capacity to capacity.

The average amount of time that a discrete amount of reagent spends in the reservoir (residence time) can be calculated by simply dividing the volume of the tank by the average volumetric flow rate through it. The expected percentage of completion of the reaction is calculated using chemical kinetics.

The containers are made of stainless steel or alloys, as well as with enamel coating.

Some Important Aspects of HPM

All calculations are performed taking into account the ideal mixing. The reaction proceeds at a rate associated with the final concentration. In a state of equilibrium, the flow velocity must be equal to the flow rate, otherwise the tank will overflow or empty.

It is often economical to work with several sequential or parallel HPMs. Stainless tanks, assembled in a cascade of five or six units, can behave like a reactor with a piston flow. This allows the first unit to work with a higher concentration of reagents and, consequently, a higher reaction rate. Also, several HPM stages can be placed in the vertical steel tank, rather than processes taking place in different capacities.

In the horizontal version, the multi-stage unit is partitioned by vertical partitions of various heights, through which the mixture enters the cascades.

When the reagents are poorly mixed or significantly different in density, a vertical multistage reactor (enamelled or stainless steel) is used in countercurrent mode. This is effective for carrying out reversible reactions.

A small pseudo-liquid layer is completely mixed. A large commercial fluidized bed reactor has an almost uniform temperature, but combines mixing and displaced flows and transition states between them.

The chemical reactor of ideal displacement

A PFR is a (stainless) reactor in which one or more liquid reagents are pumped through a pipe or pipes. They are also called tubular flow. It can have several pipes or tubes. Reagents are constantly supplied through one end, and the products come out from the other. Chemical processes proceed as the mixture passes.

In RPP, the reaction rate is gradient: at the inlet it is very high, but with a decrease in the concentration of reagents and an increase in the content of the output products, its rate slows down. Usually, a state of dynamic equilibrium is achieved.

Both horizontal and vertical orientation of the reactor are common.

When heat transfer is required, individual pipes are placed in a jacket or a shell-and-tube heat exchanger is used. In the latter case, chemicals can be found both in the casing and in the pipe.

Large-diameter metal tanks with nozzles or baths are similar to PFR and are widely used. In some configurations, an axial and radial flow is used, multiple shells with built-in heat exchangers, a horizontal or vertical position of the reactor, and so on.

The reagent container can be filled with catalytic or inert solid particles to improve interfacial contact in heterogeneous reactions.

An important factor in the RFP is that vertical or horizontal mixing is not taken into account in calculations, which is what is meant by the term "piston flow". Reagents can be introduced into the reactor not only into the inlet. Thus, it is possible to achieve a higher efficiency of the PFR or to reduce its size and cost. The performance of the PFR is usually higher than that of the HPM of the same volume. With equal volume and time values in the reciprocating reactors, the reaction will have a higher completion percentage than in the mixing units.

Dynamic balance

For most chemical processes, 100% completion is not possible. Their speed decreases with the growth of this indicator until the moment when the system reaches a dynamic equilibrium (when the total reaction or the composition change does not occur). The equilibrium point for most systems is located below 100% completion of the process. For this reason, a separation process, such as distillation, is necessary to separate the remaining reagents or by-products from the target. These reagents can sometimes be reused at the beginning of the process, for example, such as the Haber process.

Application of PPP

Piston flow reactors are used to conduct chemical transformation of compounds during their movement through a system resembling pipes for the purpose of large-scale, rapid, homogeneous or heterogeneous reactions, continuous production and processes with the release of large amounts of heat.

The ideal RPP has a fixed residence time, that is, any liquid (piston) arriving at time t leaves it at time t + τ, where τ is the residence time in the facility.

Chemical reactors of this type have high performance for long periods of time, as well as excellent heat transfer. The disadvantages of PPP is the difficulty of monitoring the process temperature, which can lead to undesirable temperature changes, as well as their higher cost.

Catalytic Reactors

Although aggregates of this type are often implemented in the form of a PPP, they require more complex maintenance. The rate of the catalytic reaction is proportional to the amount of the catalyst in contact with the chemicals. In the case of a solid catalyst and liquid reagents, the rate of the processes is proportional to the available area, the receipt of chemicals and the selection of products, and depends on the presence of turbulent mixing.

The catalytic reaction is in fact often a multi-stage reaction. Not only the initial reagents interact with the catalyst. Some intermediate products react with it.

The behavior of catalysts is also important in the kinetics of this process, especially in high-temperature petrochemical reactions, as they are deactivated by sintering, coking and similar processes.

Application of new technologies

PFRs are used for biomass conversion. In experiments, high-pressure reactors are used. The pressure in them can reach 35 MPa. The use of several sizes makes it possible to vary the residence time from 0.5 to 600 s. To reach temperatures above 300 ° C, reactors with electric heating are used. The supply of biomass is done using HPLC pumps.

RPP of aerosol nanoparticles

There is considerable interest in the synthesis and use of nanosized particles for various purposes, including high-alloy alloys and thick-film conductors for the electronics industry. Other applications include measurements of magnetic susceptibility, far infrared transmission, and nuclear magnetic resonance. For these systems, it is necessary to produce particles of controlled size. Their diameter, as a rule, is in the range of 10 to 500 nm.

Due to their size, shape and high specific surface area, these particles can be used to produce cosmetic pigments, membranes, catalysts, ceramics, catalytic and photocatalytic reactors. Examples of nanoparticle applications include SnO ₂ for carbon monoxide sensors, TiO ₂ for light guides, SiO ₂ for colloidal silicon dioxide and optical fibers, C for carbon fillers in tires, Fe for recording materials, Ni for batteries and, in smaller volumes, palladium, magnesium And bismuth. All these materials are synthesized in aerosol reactors. In medicine, nanoparticles are used for the prevention and treatment of wound infections, in artificial bone implants, as well as for visualization of the brain.

Production example

To obtain aluminum particles, the flow of argon saturated with metal vapors is cooled in a PFD of 18 mm in diameter and 0.5 m in length from 1600 ° C at a rate of 1000 ° C / s. As the gas passes through the reactor, the formation and growth of aluminum particles occurs. The flow rate is 2 dm ³ / min, and the pressure is 1 atm (1013 Pa). As the gas moves, the gas cools and becomes supersaturated, which leads to the nucleation of particles as a result of collisions and evaporation of molecules, repeated until the particle reaches a critical size. As they move through the supersaturated gas, the aluminum molecules condense on the particles, increasing their size.