A chemical reaction is a process that leads to the conversion of reagents. 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 tanks designed to carry out reactions to produce the final product. Their design depends on various factors and should provide maximum output in the most cost-effective way.
Kinds
There are three main basic models of chemical reactors:
- Periodic action.
- Continuous with agitator (NRM).
- Piston flow reactor (RPP).
These basic models can be modified to meet the requirements of a 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 purpose, for example, stainless steel containers are used, the contents of which are mixed with internal working blades, gas bubbles or with the help of pumps. Temperature control is carried out using heat-exchange shirts, irrigation refrigerators or pumping through a heat exchanger.
Batch reactors are currently used in the chemical and food processing industries. Their automation and optimization create difficulties, since it is necessary to combine continuous and discrete processes.
Semi-batch chemical reactors combine continuous and batch operation. A bioreactor, for example, is periodically charged and constantly emits carbon dioxide, which must be continuously removed. Similarly, in the chlorination reaction, when chlorine gas is one of the reacting substances, if it is not introduced continuously, then most of it will volatilize.
To ensure large volumes of production, chemical reactors of continuous operation or metal tanks with a stirrer or with a continuous stream are mainly used.
Continuous stirred reactor
In stainless steel tanks, liquid reagents are fed. To ensure proper interaction, they are mixed with working blades. Thus, in reactors of this type, the reacting substances are continuously fed into the first tank (vertical, steel), then they enter the subsequent ones, while thoroughly mixing in each tank. Although the composition of the mixture is uniform in each individual tank, in the system as a whole, the concentration varies from tank to tank.
The average amount of time that a discrete amount of reagent spends in the 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 NRM
All calculations are carried out taking into account perfect mixing. The reaction proceeds at a rate related to the final concentration. In 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 NRMs. Corrosion-proof tanks assembled in a cascade of five or six units can behave like a piston-flow reactor. This allows the first unit to work with a higher concentration of reagents and, therefore, a higher reaction rate. Also in the vertical steel tank can be placed several steps of the HPM, instead of the processes taking place in different containers.
In a horizontal version, a multi-stage unit is partitioned by vertical partitions of various heights through which the mixture enters in cascades.
When the reactants mix poorly or vary significantly in density, a vertical multi-stage reactor (enameled or stainless steel) is used in countercurrent mode. It is effective for reversible reactions.
A small pseudo-fluid layer is completely mixed. A large commercial fluidized bed reactor has an almost uniform temperature, but combines miscible and displaced flows and transition states between them.
Ideal Displacement 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 flowing. It may have several pipes or tubes. Reagents constantly flow through one end, and products exit 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 output products, its speed 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 present in the casing as well as in the pipe.
Large-capacity metal containers with nozzles or bathtubs are similar to RPPs and are widely used. Some configurations use axial and radial flow, multiple shells with integrated heat exchangers, horizontal or vertical position of the reactor, and so on.
The container with the reagent can be filled with catalytic or inert solid particles to improve interfacial contact in heterogeneous reactions.
Of great importance in the RPF is that the calculations do not take into account vertical or horizontal mixing - this is what is meant by the term "piston flow". Reagents can be introduced into the reactor not only in the inlet. Thus, it is possible to achieve higher efficiency of the RPP or reduce its size and cost. RPP productivity is usually higher than HPM of the same volume. With equal values โโof volume and time in reciprocating reactors, the reaction will have a higher percentage of completion than in mixing units.
Dynamic balance
For most chemical processes, it is not possible 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 composition change does not occur). The equilibrium point for most systems is located below 100% of the completion of the process. For this reason, it is necessary to carry out a separation process, such as distillation, in order to separate the remaining reagents or by-products from the target. These reagents can sometimes be reused at the beginning of a process, for example, such as the Haber process.
The use of RPP
Piston flow reactors are used to carry out the chemical conversion of compounds during their movement through a pipe-like system, for large-scale, fast, homogeneous or heterogeneous reactions, continuous production and in processes with the release of a large amount of heat.
An ideal RPF has a fixed residence time, i.e., any liquid (piston) entering at time t leaves it at time t + ฯ, where ฯ is the time spent 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 the RPP is the difficulty of controlling the process temperature, which can lead to undesirable temperature differences, as well as their higher cost.
Catalytic reactors
Although units of this type are often sold as RPPs, they require more complex maintenance. The catalytic reaction rate is proportional to the amount of catalyst in contact with chemicals. In the case of a solid catalyst and liquid reagents, the speed of the processes is proportional to the available area, the flow of chemicals and the selection of products and depends on the presence of turbulent mixing.
The catalytic reaction is often often a multi-stage reaction . Not only the initial reagents interact with the catalyst. Some intermediate products also react with it.
The behavior of the catalysts is also important in the kinetics of this process, especially in high-temperature petrochemical reactions, since they are deactivated by sintering, coking, and similar processes.
Application of new technologies
RPPs 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 allows you to vary the residence time from 0.5 to 600 s. To reach temperatures above 300 ยฐ C, electrically heated reactors are used. Biomass is supplied using HPLC pumps.
RPP 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 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, as a rule, are in the range from 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 optical fibers, 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 for the prevention and treatment of wound infections, in artificial bone implants, as well as for imaging the brain.
Production example
To obtain aluminum particles, an argon stream saturated with metal vapor is cooled in an RPP with a diameter of 18 mm and a length of 0.5 m from a temperature of 1600 ยฐ C at a speed of 1000 ยฐ C / s. As the gas passes through the reactor, nucleation and growth of aluminum particles occurs. The flow rate is 2 dm 3 / 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, repeating until the particle reaches a critical size. As you move through a supersaturated gas, aluminum molecules condense on the particles, increasing their size.