催化燃烧的应用原理

2019-11-17 10:54
催化燃烧基本原理
 
催化燃烧是借助催化剂在低温下(200~400℃)下,实现对有机物的完全氧化,因 此,能耗少,操作简便,安全,净化效率高,在有机废气特别是回收价值不大的有机废气净化方面,比如化工,喷漆、缘材料、漆包线、涂料生产等行业应用较广,已有不少定型设备可供选用。
 
一、催化原理及装置组成
 
[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
 
 其反应速度较慢。当加入催化剂K后,反应从一条很容易进行的途径实现:  A+B+2K→[AK]+[BK]→[CK]+K→C+2K
 
 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。
 
 ①废气预处理  为了避免催化剂床层的堵塞和催化剂中毒,废气在进入床层之前必须进行预处理,以除去废气中的粉尘、液滴及催化剂的毒物。
 
 ②预热装置  预热装置包括废气预热装置和催化剂燃烧器预热装置。因为催化剂都有一个催化活性温度,对催化燃烧来说称催化剂起燃温度,必须使废气和床层的温度达到起燃温度才能进行催化燃烧,因此,必须设置预热装置。但对于排出的废气本身温度就较高的场合,如漆包线、缘材料、烤漆等烘干排气,温度可达300℃以上,则不必设置预热装置。   
 
预热装置加热后的热气可采用换热器和床层内布管的方式。预热器的热源可采用烟道气或电加热,目前采用电加热较多。当催化反应开始后,可尽量以回收的反应热来预热废气。在反应热较大的场合,还应设置废热回收装置,以节约能源。
 
预热废气的热源温度一般都超过催化剂的活性温度。为保护催化剂,加热装置应与催化燃烧装置保持一定距离,这样还能使废气温度分布均匀。
 
从需要预热这一点出发,催化燃烧法比较适用于连续排气的净化,若间歇排气,不仅每次预热需要耗能,反应热也无法回收利用,会造成很大的能源浪费,在设计和选择时应注意这一点。
 
催化燃烧装置  一般采用固定床催化反应器。反应器的设计按规范进行,应便于操作,维修方便,便于装卸催化剂。
 
在进行催化燃烧的工艺设计时,应根据具体情况,对于处理气量较大的场合,设计成分建式流程,即预热器、反应器立装设,其间用管道连接。对于处理气量小的场合,可采用催化焚烧炉,把预热与反应组合在一起,但要注意预热段与反应段间的距离。
 
在有机物废气的催化燃烧中,所要处理的有机物废气在高温下与空气混合易引起爆炸,安全问题十分重要。因而,一方面必须控制有机物与空气的混合比,使之在爆炸下限;另一方面,催化燃烧系统应设监测报警装置和有防爆措施。
 
二、催化燃烧用催化剂
 
由于有机物催化燃烧的催化剂分为贵金属(以铂、钯为主)和贱金属催化剂。贵金属为活性组分的催化剂分为全金属催化剂和以氧化铝为载体的催化剂。全金属催化剂是以镍或镍铬合金为载体,将载体做成带、片、丸、丝等形状,采用化学镀或电镀的方法,将铂、钯等贵金属沉积其上,然后做成便于装卸的催化剂构件。由氧化铝作载体的贵金属催化剂,一般是以陶瓷结构作为支架,在陶瓷结构上涂覆一层仅有0.13mm的α-氧化铝薄层,而活性组分铂、钯就以微晶状态沉积或分散在多孔的氧化铝薄层中。
 
但由于贵金属催化剂价格昂贵,资源少,多年来人们特别注重新型的、价格较为便宜的催化剂的开发研究,我国是世界上稀土资源比较多的,我国的科技工作者研究开发了不少稀土催化剂,有些性能也较好。
 
[if !supportLists]三、[endif]催化剂中毒与老化  
 
在催化剂使用过程中,由于体系中存在少量的杂质,可使催化剂的活性和选择性减小或者消失,这种现象叫催化剂中毒。这些能使催化剂中毒的物质称之为催化剂毒物,这些毒物在反应过程中或强吸附在活性中心上,或与活性中心起化学作用而变为别的物质,使活性中心失活。
 
 毒物通常是反应原料中带来的杂质,或者是催化剂本身的某些杂质,另外,反应产物或副产物本身也可能对催化剂毒化,一般所指的是硫化物如H2S、硫氧化碳、RSH等及含氧化合物如H2O、CO2、O2以及含磷、砷、卤素化合物、重金属化合物等。
 
毒物不单单是对催化剂来说的,而且还针对这个催化剂所催化的反应,也就是说,对某一催化剂,只有联系到它所催化的反应时,才能清楚什么物质是毒物。即使同一种催化剂,一种物质可能毒化某一反应而不影响另一反应。      按毒物与催化剂表面作用的程度可分为暂时性中毒和性中毒。暂时性中毒亦称可逆中毒,催化剂表面所吸附的毒物可用解吸的办法驱逐,使催化剂恢复活性,然而这种可再生性一般也不能使催化剂恢复到中毒前的水平。性中毒称不可逆中毒,这时,毒物与催化剂活性中心生成了结合力很强的物质,不能用一般方法将它去除或根本无法去除。
 
催化剂的老化主要是由于热稳定性与机械稳定性决定的,例如低熔点活性组分的流失或升华,会大大降低催化剂的活性。催化剂的工作温度对催化剂的老化影响很大,温度选择和控制不好,会使催化剂半熔或烧结,从而导致催化剂表面积的下降而降低活性。另外,内部杂质向表面的迁移,冷热应力交替所造成的机械性粉末被气流带走。所有这些,都会加速催化剂的老化,而其中比较主要的是温度的影响,工作温度越高,老化速度越快。因此,在催化剂的活性温度范围内选择合适的反应温度将有助于延长催化剂的寿命。但是,过低的反应温度也是不可取的,会降低反应速率。
 
为了提高催化剂的热稳定性,常常选择合适的耐高温的载体来提高活性组分的分散度,可防止其颗粒变大而烧结,例如以纯铜作催化剂时,在200℃即失去活性,但如果采用共沉积法将Cu载于Cr2O3载体上,就能在较高的温度下保持其活性。
 
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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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