Chemical
Reactor is the core equipment of chemical production, the form of reactor has a very important impact on chemical production, can directly affect the production safety and product quality. According to the characteristics of the reactor form, it can be mainly divided into kettle reactor, tube reactor, tower reactor, bed reactor, micro reactor and so on.
Kettle reactor The kettle reactor is also known as reactor and pot reactor. It is the simplest and most widely used of all types of reactors, and is widely used in petroleum, chemical, rubber, pesticide, dye, pharmaceutical and other fields. It can be used to carry out homogeneous reactions or non-homogeneous reactions mainly in liquid phase, such as liquid-liquid phase, liquid-solid phase, gas-liquid phase, gas-liquid-solid phase, etc.
Kettle reactors are characterised by a wide range of temperatures and pressures, adaptability, flexibility of operation, easy control of temperature and concentration during continuous operation and homogeneous product quality. These reactors are usually most commonly used under relatively mild operating conditions, such as atmospheric pressure, low temperature and below the boiling point of the material. When the reaction conditions are more severe (e.g. high temperature, high pressure, strong corrosion, etc.), special kettle reactors can also be used for production.
The main structure of the kettle reactor consists of the kettle body, stirring device, transmission device, shaft sealing device and heat exchange device.
Kettle reactors can be divided according to their mode of operation into
(1) Intermittent kettle, also known as intermittent kettle reactor, is characterised by flexible operation and can be adapted to different operating conditions and product varieties, and is particularly suitable for the production of small batches, multiple varieties and long reaction times. The disadvantage of intermittent kettles is that they require auxiliary processes such as charging and discharging, and the quality of the product is not easily stabilised. However, some reaction processes, such as fermentation and polymerisation reactions, are still difficult to achieve continuous production, so intermittent kettles are still used for production.
(2) Continuous kettle, also known as continuous kettle reactor, consists of several reactors in series. Compared with intermittent kettles, continuous kettles can save time for charging and discharging, produce continuously and have a more stable product quality. The disadvantage of the continuous kettle is that the stirring effect can easily cause the material to be mixed back, which affects the conversion rate of the product.
(3) Semi-continuous kettle, also known as semi-continuous kettle reactor, refers to a reactor in which one or more raw materials are added at once and another or more raw materials are added continuously, and its characteristics are between intermittent and continuous kettle. The reactor can be divided into vertical vessel centre stirring, eccentric stirring, tilting stirring, horizontal vessel stirring and other types according to the different stirring methods, among which the vertical vessel centre stirring reactor is the most commonly used.
Tubular reactors The tubular reactor is usually large in length and diameter and tubular in shape. The tube reactor is characterised by low re-mixing, large specific surface area and high volumetric efficiency (capacity per unit volume), which makes it particularly suitable for processes requiring high conversion rates or where there are tandem side reactions. However, for slow reactions, longer pipelines are required, resulting in a larger pressure drop in the reactor, which affects the reaction effect. In addition, the tubular reactor can also achieve process condition control in sections, creating suitable temperature gradients, pressure gradients and concentration gradients. As a result, tubular reactors are characterised by high conversion rates and high selectivity.
In continuous operation, the relatively large length and diameter of the tubular reactor can be approximated as an ideal displacement flow reactor. It is suitable for both liquid and gas phase reactions. Due to the high pressure, the tube reactor is suitable for pressurised reactions. Compared to the kettle reactor, the tube reactor has a large heat exchange area and a high cooling capacity, so the tube reactor can be used for strongly exothermic reaction processes.
Tube reactors can be divided according to their structure into
(1) Horizontal tube reactor Horizontal tube reactor is made of seamless steel tube connected with U-shaped tube or flange, which is characterised by simple manufacture, easy maintenance and can withstand high pressure
(2) Standpipe reactors Standpipe reactors are widely used in industrial production and are currently used in reactions such as liquid-phase ammonification, liquid-phase hydrogenation and liquid-phase oxidation. It includes single-programmed standpipe reactors and multi-programmed standpipe reactors.
(3) Coil reactor The coil reactor makes the tube reactor into a horizontal coil shape, which is compact and space-saving, but is not conducive to maintenance and pipe cleaning.
(4) U-shaped tube reactor U-shaped tube reactor is equipped with porous baffle or stirring device to strengthen the heat and mass transfer process. u-shaped tube reactor has a large pipe diameter and long material residence time, which can be applied to chemical reactions with slow reaction rate. For example, U-tube reactors with perforated baffles have been widely used for the polymerisation of caprolactam. U-tube reactors with stirring are suitable for non-homogeneous reactions or liquid-solid phase suspension reactions, such as continuous nitration of toluene and continuous sulfonation of anthraquinone.
(5) Multi-tube parallel reactor As the structure of the tube reactor is more flexible, in industrial production to meet the production needs of different processes, often use a multi-tube parallel structure of the tube reactor. For example, the gas phase hydrogen chloride and acetylene are reacted in a multi-tube parallel reactor with a solid phase catalyst to prepare vinyl chloride, and the gas phase nitrogen and hydrogen mixture is synthesised in a multi-tube parallel reactor with a solid phase catalyst to synthesise ammonia.
Tower reactors In addition to being widely used for processes such as distillation, absorption and extraction, tower reactors can also be used as reactors for chemical reactions such as hydrogenation, sulphonation and halogenation. Common tower reactors are mainly divided into the following categories.
(1)
Packed Tower reactor Packed tower reactor is mainly used for gas-liquid phase involved in the chemical reaction, is the tower of the packing as a gas-liquid two-phase mass transfer equipment. Liquid from the top of the tower through the liquid distributor sprayed onto the packing, and along the surface of the packing downstream, in the packing surface to form a liquid film. The gas is fed from the bottom of the tower, through the gas distributor (small diameter tower can also not be set up gas distributor) after the distribution, and the formation of liquid flow in the opposite direction, continuous through the packing layer of the gap. On the packing surface, the two phases of gas-liquid close contact, mass transfer. Packed towers are continuous contact gas-liquid mass transfer equipment, the composition of the two phases along the tower height continuous change. In general, the gas phase is continuous and the liquid phase is dispersed.
The simple structure and low pressure drop of packed tower reactors make them suitable for reactions in which corrosive materials are involved or generated. However, in packed tower reactors, the contact time between the phases of material is short, which enables large product conversions to be obtained for fast and instantaneous reaction processes, but is not suitable for slower chemical reactions. In addition, the tower reactor heat exchange effect is poor, for the reaction with large heat release, can only take to increase the amount of liquid spray to reduce the temperature inside the reactor.
(2)
Bubble Tower reactor bubble tower reactor is filled with liquid inside the tower, the gas from the bottom of the reactor continuous entry, dispersion into bubbles, along the liquid rise, and liquid phase contact for reaction at the same time, stirring the liquid in the tower to increase the rate of mass transfer. This type of reactor is suitable for medium and slow reactions involving the liquid phase and reactions with large heat release. Such as various organic compounds involved in the oxidation reaction.
The bubble tower reactor is simple, inexpensive, easy to use and maintain, and the contact area between the gas and liquid phases involved in the reaction is large and well mixed. However, the pressure drop consumed during the bubbling is large, and the material in the tower is seriously mixed back, so it is difficult to obtain a high liquid-phase conversion rate in a single continuous reactor.
(3) Plate tower reactor The plate tower reactor is the liquid lateral flow through the tower plate after the overflow weir overflow into the drop tube, the liquid in the drop tube release entrained gas, from the bottom of the drop tube gap flow to the next layer of the tower plate. The gas below the tower plate passes through the gas phase channels on the tower plate, such as sieve holes, float valves, etc., into the liquid layer bubble on the tower plate, gas-liquid contact for mass transfer. The gas phase leaves the liquid layer and runs to the upper plate for multi-stage contact mass transfer.
The plate tower reactor has the characteristics of plate by plate operation, the more plates used, the smaller the axial re-mixing, so as to obtain a higher liquid phase conversion rate. In addition, heat transfer devices can be set up between the plates to move heat out and in.
(4) Spray tower reactor Spray tower reactor structure is relatively simple, the liquid after spraying, in the form of fine droplets dispersed in the gas, gas for the continuous phase, liquid for the dispersed phase, with a large contact area and small gas pressure drop and other advantages. It is suitable for instantaneous, interfacial and fast reaction processes, especially for reaction systems with sludge, precipitation and generation of solid products. However, the spray tower reactor has a small liquid holding capacity, a small mass transfer coefficient and serious re-mixing of the gas and liquid phases.
Bed reactor Bed reactors are mainly used for chemical reactions involving solid phase materials or solid catalysts. Common bed reactors can be divided into the following categories.
(1) Fixed-bed reactors Fixed-bed reactors, also known as filled-bed reactors, are filled with solid catalysts or solid reactants to achieve a multi-phase reaction process. The solids are usually in the form of granules and are built up into a bed of a certain height or thickness, which is stationary and through which the fluid reacts.
There are several basic forms of fixed bed reactors
① Axial adiabatic fixed bed reactor. The fluid flows through the bed from top to bottom along the axial direction, and the bed has no heat exchange with the outside world.
② radial adiabatic fixed bed reactor. Fluid along the radial flow through the bed, can use the centrifugal flow or centripetal flow, the bed and the outside world without heat exchange.
Radial reactor compared with the axial reactor, the fluid flow distance is shorter, large cross-sectional area, the fluid pressure drop is smaller. But the structure of the radial reactor is more complex. The above two forms are adiabatic reactors, suitable for the reaction thermal effect is not large, or the reaction system can withstand the temperature change caused by the thermal effect of the reaction under adiabatic conditions.
(3) Tube type fixed bed reactor. Composed of several reaction tubes in parallel. The catalyst is arranged within or between the tubes and the hot and cold carriers flow through the tubes or within the tubes for heating or cooling. The tubular fixed bed reactor is suitable for reactions with relatively large thermal effects.
④ Multi-stage fixed-bed reactor. According to the needs of different process conditions in the reaction process, the above basic forms of reactors are connected in series and combined into a reactor. For example: when the thermal effect of the reaction is large or when the temperature needs to be controlled in sections, multiple adiabatic reactors can be connected in series to form a multi-stage adiabatic fixed-bed reactor, with
Heat Exchangers or supplementary materials between reactors to adjust the system temperature so that the reaction can be carried out under the best process conditions.
The fixed bed reactor has a simple structure, the material is not easily mixed back, the catalyst mechanical loss is small and the fluid can be in effective contact with the catalyst. However, the fixed bed reactor has poor heat transfer and poor heat transfer, which can lead to thermal runaway and exceed the maximum allowable process temperature range when the reaction heat release is large, thus affecting production safety. In addition, the catalyst cannot be replaced during the operation of the fixed bed reactor, and the catalyst needs to be regenerated frequently for the reaction is generally not applicable, such cases usually replace the fixed bed reactor with fluidised bed reactor.
(2) Fluidised bed reactor A fluidised bed reactor is an apparatus in which gases are chemically reacted in a boiling bed of solid materials or catalysts. The gas at a certain flow rate will be stacked into a certain thickness (bed) of catalyst or solid material strongly stirred, so that it is like a boiling liquid and has some characteristics of liquid, such as the role of pressure on the vessel wall, can overflow, with viscosity, etc.. The top of the reactor has an enlarged section with a cyclone separator to recover the catalyst or solid material carried away by the gas. The bottom section is equipped with a raw material inlet pipe and a gas distributor, while the middle section is equipped with a cooling water pipe and a guide baffle to control the reaction temperature and improve the contact conditions between the gas and solid phases.
Fluidised bed reactors have been widely used in the petroleum, chemical, metallurgical and nuclear industries.
Fluidised bed reactors have the following advantages
① Continuous input and output of solid materials can be achieved.
②The movement of fluid gives the bed good heat transfer performance, uniform temperature inside the bed and easy control, especially suitable for strong exothermic reactions.
③Easy for continuous regeneration and recycling of catalyst, suitable for reaction process with high catalyst deactivation rate.
(3) Moving bed reactors Moving bed reactors are suitable for reactions involving solid particles or solid catalysts, similar to fixed bed reactors, with the difference that solid materials or solid catalysts are continuously added from the top of the reactor and the liquid or gas phase passes through the solid bed in order to carry out the reaction. As the reaction proceeds, the solid material gradually moves down and is finally discharged from the bottom.
Compared to fixed bed reactors and fluidised bed reactors, the main advantage of a moving bed reactor is that the residence time of the solids and fluid can be controlled and the liquid phase is less likely to re-mix; the disadvantage is that it is more difficult to control the uniform downward movement of the solid particles.
Microreactors Microreactors, also known as microchannel reactors, are devices manufactured using microfabrication techniques and are suitable for chemical reaction research. Microreactors are more complex in structure and are a microreaction system that combines heat transfer, mixing, separation and control.
In scientific research, microreactors are mainly used for catalyst evaluation and kinetic studies, but also for thermal analysis of reactions, which has a high sensitivity. With the increasing development of microreactor technology, according to its own performance characteristics, it can also be used for the following chemical reaction processes.
(1) Exothermic reactions. For chemical reactions with high reaction heat, conventional reactors generally use the way of adding materials drop by drop, when adding materials instantaneously, due to the local concentration is too high, it is easy to occur local violent reaction, generating a large amount of reaction heat and generating a certain amount of by-products. Micro reactor can export heat in time, can realize the precise control of reaction temperature, eliminate the disadvantage of large local heat effect, and improve the conversion rate of products.
(2) Chemical reactions in which the reactants or products are unstable. Some chemical reactions, due to the instability of the reactants or products, are prone to decomposition when left in the reactor for a long time, thus affecting the yield of the product. The micro reactor is a continuous flow reaction system, and the residence time of the reactants or products can be precisely controlled, thus avoiding material decomposition due to long residence time and ensuring product yield and safe production.
(3) Chemical reactions requiring precise proportioning of reactants. For reaction systems requiring precise ratios, the micro reactor can achieve uniform mixing in a short time, avoiding local excesses and reducing the amount of by-products generated.
(4) Dangerous chemical reactions and high temperature and pressure reactions. For some dangerous chemical reactions, the production process is prone to runaway, resulting in a sharp rise in temperature and pressure, which can easily cause flushing and even explosion. Micro reactor can quickly move out of the reaction heat, and can withstand higher pressure, using micro reactor for such reactions is safer.
As microreactor technology has matured, microreactor systems have been widely used in scientific research. However, microreactors are small in size, low in production capacity, require a large equipment base for industrial production, have cumbersome monitoring and control processes, have small internal reactor channels that are easily clogged and difficult to clean, and are costly to industrialise. All of these factors restrict the use of microreactors on a large scale in industrial production.
In addition to the reactor forms described above, there are also trickling bed reactors, cyclone reactors, circulating flow reactors and biofilm reactors. Chemical production involves a wide range of complex processes, and reactors are the core equipment in chemical production. Choosing the right reactor not only improves production efficiency and product conversion rate, but also makes the chemical production process more stable, and is extremely important for chemical production process safety. Therefore, according to the material and process characteristics, to meet the process conditions and meet the actual production needs as a criterion, scientific and reasonable selection of reactor form is essential.
Source: Chemical 707
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