What are chemical reactors? Types of chemical reactors

2024 Author: Howard Calhoun | [email protected]. Last modified: 2024-01-02 13:48

A chemical reaction is a process that leads to the transformation of reactants. It is characterized by changes that result in one or more products that are different from the original. 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-base, redox, 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 maximum output in the most cost effective way.

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There are three basic basic models of chemical reactors:

• Periodic.
• Continuous Stirred (CPM).
• Plunger Flow Reactor (PFR).

These basic models can be modified to meet the requirements of the chemical process.

Batch reactor

Chemical units of this type are used in batch processes with low production volumes, 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 using pumps. Temperature control is carried out using heat exchange jackets, irrigation coolers or pumping through a heat exchanger.

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

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

To ensure large production volumes, continuous chemical reactors or metal tanks with agitator or continuous flow are mainly used.

Continuous stirred reactor

Liquid reagents are fed into stainless steel tanks. To ensure proper interaction, they are mixed by the working blades. Thus, inIn reactors of this type, the reactants are continuously fed into the first tank (vertical, steel), then they enter the subsequent ones, while being thoroughly mixed in each tank. Although the composition of the mixture is homogeneous in each individual tank, in the system as a whole the concentration varies from tank to tank.

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

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

Some important aspects of NPM

All calculations are based on perfect mixing. The reaction proceeds at a rate related to the final concentration. At equilibrium, the flow rate must be equal to the flow rate, otherwise the tank will overflow or empty.

It is often cost effective to work with multiple serial or parallel HPMs. Stainless steel tanks assembled in a cascade of five or six units can behave like a plug flow reactor. This allows the first unit to operate at a higher reactant concentration and therefore a faster reaction rate. Also, several stages of HPM can be placed in a vertical steel tank, instead of processes taking place in different containers.

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

When the reactants do not mix well or differ significantly in density, a vertical multi-stage reactor (lined or stainless steel) is used in countercurrent mode. This is effective for carrying out reversible reactions.

The small pseudo-liquid layer is fully mixed. A large commercial fluidized bed reactor has a substantially uniform temperature, but a mix of miscible and displaced streams and transition states between them.

Plug-flow chemical reactor

RPP is a reactor (stainless) in which one or more liquid reactants are pumped through a pipe or pipes. They are also called tubular flow. It may have several pipes or tubes. Reagents constantly enter through one end and products exit from the other. Chemical processes occur as the mixture passes through.

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

Both horizontal and vertical reactor orientations are common.

When heat transfer is required, individual tubes are jacketed or a shell and tube heat exchanger is used. In the latter case, the chemicals may beboth in shell and tube.

Metal containers of large diameter with nozzles or baths are similar to RPP and are widely used. Some configurations use axial and radial flow, multiple shells with built-in heat exchangers, horizontal or vertical reactor position, and so on.

The reagent vessel can be filled with catalytic or inert solids to improve interfacial contact in heterogeneous reactions.

It is important in the RPP that the calculations do not take into account vertical or horizontal mixing - this is what is meant by the term "plug flow". Reagents can be introduced into the reactor not only through the inlet. Thus, it is possible to achieve a higher efficiency of the RPP or reduce its size and cost. The performance of RPP is usually higher than that of HPP of the same volume. With equal values of volume and time in piston reactors, the reaction will have a higher percentage of completion than in mixing units.

Dynamic Balance

For most chemical processes, it is impossible to achieve 100 percent completion. Their speed decreases with the growth of this indicator until the moment when the system reaches dynamic equilibrium (when the total reaction or change in composition does not occur). The equilibrium point for most systems is below 100% process completion. For this reason, it is necessary to carry out a separation process, such as distillation, to separate the remaining reactants or by-products fromtarget. These reagents can sometimes be reused at the start of a process such as the Haber process.

Application of PFA

Piston flow reactors are used to carry out the chemical transformation of compounds as they move through a tube-like system for large scale, rapid, homogeneous or heterogeneous reactions, continuous production, and high heat generating processes.

An ideal RPP has a fixed residence time, i.e. any liquid (piston) entering at time t will leave it at time t + τ, where τ is the residence time in the installation.

Chemical reactors of this type have high performance over long periods of time, as well as excellent heat transfer. The disadvantages of RPPs are the difficulty in controlling the process temperature, which can lead to unwanted temperature fluctuations, and their higher cost.

Catalytic reactors

Although these types of units are often implemented as RPP, they require more complex maintenance. The rate of a catalytic reaction is proportional to the amount of catalyst in contact with the chemicals. In the case of a solid catalyst and liquid reactants, the rate of processes is proportional to the available area, the input of chemicals and the withdrawal of products and depends on the presence of turbulent mixing.

A catalytic reaction is in fact often multi-step. Not onlythe initial reactants interact with the catalyst. Some intermediate products also 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.

Applying new technologies

RPP are used for biomass conversion. High-pressure reactors are used in the experiments. The pressure in them can reach 35 MPa. The use of several sizes allows the residence time to be varied from 0.5 to 600 s. To achieve temperatures above 300 °C, electrically heated reactors are used. Biomass is supplied by HPLC pumps.

RPP aerosol nanoparticles

There is considerable interest in the synthesis and application of nanosized particles for various purposes, including high-alloy alloys and thick-film conductors for the electronics industry. Other applications include magnetic susceptibility measurements, far infrared transmission, and nuclear magnetic resonance. For these systems it is necessary to produce particles of a controlled size. Their diameter is usually 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. Application examples for nanoparticles include SnO₂ for sensorscarbon monoxide, TiO₂ 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, to a lesser extent, palladium, magnesium and bismuth. All these materials are synthesized in aerosol reactors. In medicine, nanoparticles are used to prevent and treat wound infections, in artificial bone implants, and for brain imaging.}

Production example

To obtain aluminum particles, an argon flow saturated with metal vapor is cooled in a RPP with a diameter of 18 mm and a length of 0.5 m from a temperature of 1600 °C at a rate of 1000 °C/s. As the gas passes through the reactor, aluminum particles nucleate and grow. The flow rate is 2 dm3/min and the pressure is 1 atm (1013 Pa). As it 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.