[if !supportLists](1) [endif]催化剂定义 催化剂是一种能提高化学反应速率，控制反应方向，在反应前后本身的化学性质不发生改变的物质。
[if !supportLists](2) [endif](2)催化作用机理 催化作用的机理是一个很复杂的问题，这里仅做简介。在一个化学反应过程中，催化剂的加入并不能改变原有的化学平衡，所改变的仅是化学反应的速度，而在反应前后，催化剂本身的性质并不发生变化。那么，催化剂是怎样加速了反应速度呢了既然反应前后催化剂不发生变化，那么催化剂到底参加了反应没有实际上，催化剂本身参加了反应，正是由于它的参加，使反应改变了原有的途径，使反应的活化能降低，从而加速了反应速度。例如反应A+B→C是通过中间活性结合物(AB)过渡而成的，
[if !supportLists](3) [endif]即： A+B→[AB]→C
中间不再需要[AB]向C的过渡，从而加快了反应速度，而催化剂并未改变性质。 (4)催化燃烧的工艺组成 不同的排放场合和不同的废气，有不同的工艺流程。但不论采取哪种工艺流程，都由如下工艺单元组成。
Basic principles of catalytic combustion
Catalytic combustion is with the aid of catalyst at low temperature (200 ~ 400 ℃), the realization of the complete oxidation of organic matter, and because of this, less energy consumption, easy operation, safety, high purification efficiency, especially the recovery of little value in organic waste gases of organic waste gas purification, such as chemical industry, paint, insulation materials, enameled wire, coating production and other industries widely used, has quite a few stereotypes equipment to choose from.
1. Catalytic principle and device composition
[if! SupportLists](1) [endif] catalyst is defined as a kind of material that can improve the rate of chemical reaction, control the direction of reaction, and keep the chemical properties before and after the reaction unchanged.
The mechanism of catalytic action is a very complicated problem, which is only introduced here. In a chemical reaction process, the addition of catalyst does not change the original chemical equilibrium, but only the speed of the chemical reaction, and before and after the reaction, the nature of the catalyst itself does not change. Catalyst, then, is how to accelerate the reaction rate? Since reaction catalyst before and after the change, then the catalyst took part in the reaction at all, in fact, the catalyst itself took part in the reaction, it is because of its, the reaction changed the original way, reduce the activation energy, so as to accelerate the reaction rate. For example, reaction A+B→C is formed by the intermediate active binder (AB),
[if! SupportLists](3) [endif] : A+B→[AB]→C
Its reaction speed is slow. When catalyst K is added, the reaction proceeds from an easy path: A+B+2K→[AK]+[BK]→[CK]+K→C+2K
The transition [AB] to C is no longer needed, thus speeding up the reaction, and the catalyst does not change its nature. (4) the process of catalytic combustion consists of different discharge occasions and different waste gases, with different process flows. However, no matter which process is adopted, it is composed of the following process units.
(1) waste gas pretreatment in order to avoid the clogging of the catalyst bed and catalyst poisoning, waste gas before entering the bed must be pretreatment, in order to remove the dust in the exhaust gas, liquid drops and the poison of the catalyst.
The preheating device includes a waste gas preheating device and a catalyst burner preheating device. Because the catalyst has a catalytic activity temperature, for catalytic combustion called catalyst ignition temperature, must make the exhaust gas and bed temperature to reach the ignition temperature to carry out catalytic combustion, therefore, must set up a preheating device. But for the exhaust gas itself temperature is higher occasions, such as enameled wire, insulating materials, paint drying exhaust, the temperature can reach 300℃ above, then do not need to set up preheating device.
The heating heat of the preheater can be made by the heat exchanger and the tube inside the bed. The heat source of the preheater can be flue gas or electric heating. When the catalytic reaction is started, the recovered reaction heat can be used to preheat the waste gas as much as possible. In the case of large reaction heat, waste heat recovery device should be set up to save energy.
The heat source temperature of preheating waste gas generally exceeds the active temperature of catalyst. To protect the catalyst, the heating unit should be kept at a distance from the catalytic combustion unit so that the exhaust gas temperature is evenly distributed.
Starting from the need for preheating, catalytic combustion method is most suitable for continuous exhaust purification, if intermittent exhaust, not only requires energy for each preheating, reaction heat can not be recycled, will cause a lot of energy waste, in the design and selection should pay attention to this.
Fixed bed catalytic reactor is usually used in catalytic combustion units. Reactor design according to the specification, should be easy to operate, maintenance, easy to load and unload catalyst.
In the process design of catalytic combustion, according to the specific situation, for the occasion of large gas volume, the design of component construction process, that is, preheater, reactor independent installation, which is connected with pipeline. In the case of small gas volume, a catalytic incinerator can be used to combine the preheating with the reaction, but the distance between the preheating section and the reaction section should be noted.
In the catalytic combustion of organic waste gas, the organic waste gas to be treated is likely to explode when mixed with air at high temperature. Therefore, on the one hand, the mixing ratio of organic matter and air must be controlled to make it at the lower explosive limit. On the other hand, the catalytic combustion system should be equipped with monitoring and alarm device and explosion protection measures.
Catalyst for catalytic combustion
Catalysts for catalytic combustion of organic compounds are divided into noble metals (mainly platinum and palladium) and base metals. The catalysts with noble metals as active components can be divided into all-metal catalysts and catalysts with alumina as carrier. All-metal catalyst is based on nickel or ni-cr alloy as the carrier, the carrier is made into strips, plates, pellets, wires and other shapes, electroless plating or electroplating is adopted to deposit platinum, palladium and other precious metals on it, and then it is made into catalyst components for easy loading and unloading. The precious metal catalyst with alumina as the carrier is usually supported by ceramic structure with 0. The active components, platinum and palladium, are deposited as microcrystals or dispersed in the porous alumina.
However, due to the high price of precious metal catalysts and few resources, over the years people have paid special attention to the development and research of new and cheaper catalysts. China is the country with the most rare earth resources in the world.
Three, [endif] catalyst poisoning and aging
In the process of catalyst use, because there is a small amount of impurities in the system, the activity and selectivity of the catalyst can reduce or disappear, this phenomenon is called catalyst poisoning. These substances that can make the catalyst toxic are called catalyst poisons. These poisons may adsorb on the active center strongly during the reaction, or they may react with the active center chemically and change into other substances, so that the active center is inactivated.
Poison is usually reactions caused by impurities in raw materials, or some impurities of the catalyst itself, in addition, the reaction product or by-product itself may also be the catalyst poisoning, generally refers to sulfide such as H2S, sulfur, carbon oxide, RSH and oxygen compounds such as H2O, CO2, O2 and phosphorus, arsenic, halogen compound, heavy metal compounds, etc.
The poison is not only for the catalyst, but also for the reaction that the catalyst catalyzes, that is to say, for a particular catalyst, only in relation to the reaction that it catalyzes can it be clear what is the poison. Even with the same catalyst, one substance may poison one reaction without affecting the other. According to the degree of surface action of poison and catalyst, it can be divided into temporary poisoning and permanent poisoning. Temporary poisoning also known as reversible poisoning, the surface of the catalyst adsorbed poison can be desorbed method expulsion, make the catalyst to restore activity, however, this kind of renewable generally can not make the catalyst to restore to the level before poisoning. Permanent poisoning is called irreversible poisoning, in which the poison and the active center of the catalyst form a substance with a strong binding force, which cannot be removed by ordinary means or cannot be removed at all.
Aging of catalysts is mainly determined by thermal and mechanical stability. For example, the loss or sublimation of active components at low melting point will greatly reduce the activity of catalysts. The operating temperature of the catalyst has a great influence on the aging of the catalyst, and poor temperature selection and control will lead to semi-melting or sintering of the catalyst, which will lead to the decrease of the surface area of the catalyst and reduce the activity. In addition, the migration of internal impurities to the surface and the mechanical powder caused by the alternations of cold and hot stress are carried away by airflow. All these will accelerate the aging of the catalyst, and the most important is the influence of temperature, the higher the working temperature, the faster the aging speed. Therefore, choosing the appropriate reaction temperature within the active temperature range of the catalyst will help to extend the life of the catalyst. However, too low reaction temperature is not desirable, will reduce the reaction rate.
In order to improve the thermal stability of the catalyst, often choose the appropriate carrier of high temperature to improve the dispersion of active components, can prevent the particles become larger and sintering, pure copper as catalyst, for example, in 200 ℃ or is inactive, but if the codeposition method is adopted to Cu on Cr2O3 carrier, can keep its activity at higher temperatures.
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