This entry is missingOverview, add relevant content to make the entry more complete, and it can also be upgraded quickly. Hurry upeditCome on!
Either of gas phase and solid phase in non catalytic reactionHeterogeneous reaction process, including gas-solid catalytic reaction process and gas-solid non catalytic reaction process.
Chinese name
Non catalytic reaction
Meaning
Gas phase composition under the action of solid catalyst
Classification
Gas-solid catalytic reaction process and gas-solid non catalytic reaction
The reaction process of gaseous components under the action of solid catalysts is the most widely used and largest reaction process in the chemical industry.According to statistics, about 90% of the catalytic reaction process is gas-solid catalytic reaction process.The earliest industrial gas-solid catalytic reaction process was built in 1832sulfur dioxideIn solidPlatinum catalystOveroxidationSulfur trioxideReaction process.At present, many important reaction processes in industry, such asCO conversionAnd ammonia synthesis, naphthalene oxidation to phthalic anhydride and benzene oxidation to maleic anhydride in organic chemical industrycatalytic cracking And catalytic reforming, etc.
Non catalytic reaction process
Announce
edit
Reactions between gas phase reactants and solid phase reactants, such as roasting of pyrite and sphalerite, gasification of coal;The reaction in which solid reactants are decomposed by heating to produce gaseous products, such as the calcination of limestone.The reaction of gas phase reactants forming solid products can also be classified into this category, but it is rare in the chemical industry.Generally speaking, the gas-solid non catalytic reaction process is also composed of the transfer of gas-phase reactants to the solid phase, the transfer within the solid phase and the surface reaction in series. However, since solid particles directly participate in the reaction, the surface reaction andtransmitIn addition to the reaction process, the influence of particle characteristics change in the reaction process must also be considered.According to the characteristics of particle behavior changes, a variety of gas-solid non catalytic reaction models have been proposed, among which the shrinking core model and shrinking particle model are widely used.
Basic steps
Announce
edit
The gas-solid catalytic reaction process usually includes the following steps: the reactant diffuses from the main gas phase toSolid catalystOuter surface of particles (1);The reactant diffuses to the internal surface (2) of the solid catalyst particles through the micropores in the particles;The reactant is adsorbed by the surface active center of the catalyst (3);React on the surface active center (4);The reaction product is desorbed from the surface active center (5);The reaction product diffuses tocatalyzerThe outer surface of the particle (6);The reaction product diffuses from the external surface of the catalyst particle and returns to the main flow body (7).Steps (1) and (7) are collectively referred to as external diffusion processes, and steps (2) and (6) are collectively referred to as internal diffusion processes, both of which belong to mass transfer processes.Steps (3), (4) and (5) are collectively referred to as surface reaction;Steps (2) to (6) can be regarded as internal processes of catalyst.If the resistance of one step is much greater than that of other steps, this step is a control step.
In exothermic reaction, the heat released on the active center of the catalyst surface must be transferred to the external surface of the catalyst particles first, and then to the main gas phase;In endothermic reaction, the heat is transferred in the opposite direction, and the heat required for reaction is provided by the main body of air flow.Therefore, in addition to mass transfer, heat transfer generally coexists with surface reaction.Mass transfer and heat transfer outside catalyst particles are collectively referred to as external transfer processes (see mass transfer and heat transfer outside reaction phase);What happens in catalyst particles is called internal transfer process (see mass transfer and heat transfer in reaction phase).
Catalytic reaction kinetics
Announce
edit
The mechanism of gas-solid catalytic reaction and the relationship between the reaction rate and the partial pressure and temperature of components, as well as the properties of catalyst and other factors are often expressed in the form of reaction kinetics equations.
The kinetic equation of gas-solid catalytic reaction can be derived according to the assumed reaction mechanism and confirmed by experimental verification.There are two kinds of kinetic equations of gas-solid catalytic reaction commonly used in engineering, namely, hyperbolic equation derived from uniform surface adsorption theory and heterogeneous surface adsorption theoryPower function equation(See reaction kinetics).The basic assumption of the hyperbolic equation is relatively simple. Although the form of the kinetic equation derived is complex, the physical meaning of the model parameters is clear. This equation is mainly applicable to the active surface coverage of medium sized catalysts (this is the case for most industrial gas-solid catalytic reaction processes).The basic assumptions of power function equations are relatively complex, but the form of dynamic equations is relatively simple.Power function equations are sometimes used as empirical formulas to correlate experimental data.When the activity of the catalyst decays rapidly, the deactivation rate equation of the catalyst must also be established.
Impact of transmission process
Announce
edit
Mass transfer (external diffusion and internal diffusion) makes the reactant concentration on the catalyst surface lower than that of the gas phase main body, and the product concentration higher than that of the gas phase main body.When the mass transfer effect is very serious, the concentration of reactants on the outer surface isIrreversible reactionThe process is controlled by external diffusion.The heat transfer makes the surface temperature of the catalyst higher than (reaction exothermic) or lower than (reaction endothermic) the temperature of the main gas phase.When the heat transfer effect is serious, the temperature difference can reach hundreds of degrees or even thousands of degrees.
The difference between the catalyst surface concentration and temperature caused by the transfer process and the main gas phase will affect the reaction rate, selectivity (see chemometrics) and catalyst life.In industrial gas-solid catalytic reaction process, the influence of internal diffusion cannot be ignored.In order to improve the selectivity of the reaction, appropriate catalyst particle size and active component distribution can be selected.For example, the "eggshell" catalyst with active components concentrated on the surface layer of the catalyst is suitable for reactions with series side reactions.Study on the process of gas-solid catalytic reaction in the laboratoryIntrinsic dynamicsIn order to eliminate the influence of the transfer process, the gas velocity is often increased and the catalyst particle size is reduced.
Shrinking kernel model
Announce
edit
It is applicable to the case that the ash layer formed by reaction products or reaction residues covers the unreacted solid core.As the reaction proceeds, the unreacted core gradually shrinks, the ash layer gradually thickens, and finally the particles completely become ash particles.The process is usually controlled by the diffusion of gaseous components through the gradually thickened ash layer, but it may also be controlled by the reaction on the surface of the solid core, or the influence of both cannot be ignored.
Shrinkage model
Announce
edit
It is applicable to the situation where solid particles gradually shrink and disappear with the reaction.As the reaction proceeds, the reaction products will either escape in gaseous state or fall off in solid state.The process can be controlled by the external diffusion of gaseous components, or by the reaction on the solid surface.In chemical production, gas-solid reaction usually occurs inFixed bed reactor、Fluidized bed reactororMoving bed reactorIn.[1]
