I. Catalytic Principle and Device Composition

　　1. Definition of a catalyst

　　A catalyst is a substance that can increase the rate of a chemical reaction, control the direction of the reaction, and remain chemically unchanged before and after the reaction.

　　2. Mechanism of Catalytic Action

　　The mechanism of catalysis is a highly complex issue, and here we will only provide a brief introduction. During a chemical reaction, the addition of a catalyst does not alter the original chemical equilibrium; rather, it only changes the rate of the chemical reaction. Importantly, the catalyst's own properties remain unchanged before and after the reaction. So how exactly does a catalyst speed up the reaction? If the catalyst itself doesn't change during the reaction, does it actually participate in the reaction at all? In fact, the catalyst does indeed take part in the reaction. It is precisely because of its participation that the reaction follows a different pathway, lowering the activation energy and thus accelerating the reaction rate. For example, consider the reaction A + B → C, which proceeds via an intermediate active complex (AB), as follows: A + B → [AB] → C. This reaction is relatively slow. However, when a catalyst K is added, the reaction takes place through a much easier pathway: A + B + 2K → [AK] + [BK] → [CK] + K → ...

　　The intermediate C+2K no longer requires the transition from [AB] to C, thereby accelerating the reaction rate, while the catalyst itself remains unchanged in nature.

　　Organic Waste Gas Purification

　　3. Process Composition of Catalytic Combustion

　　Different emission scenarios and different exhaust gases require different process flows. However, regardless of the specific process flow adopted, it always consists of the following process units.

　　① Wastewater pretreatment

　　To prevent blockage of the catalyst bed and catalyst poisoning, the exhaust gas must be pre-treated before entering the bed to remove dust, liquid droplets, and catalyst poisons from the exhaust gas.

　　② Preheating device

　　The preheating device includes an exhaust gas preheater and a catalyst burner preheater. Since all catalysts have a specific catalytic activity temperature—referred to as the ignition temperature for catalytic combustion—it is essential to raise both the exhaust gas temperature and the bed temperature to this ignition level before catalytic combustion can take place. Therefore, a preheating device must be installed. However, in cases where the exhaust gas itself already has a relatively high temperature—for example, in drying exhaust gases from enameled wires, insulating materials, or baked coatings—the temperature can reach as high as...

　　Above 300℃, a preheating device is not necessary. The hot gas heated by the preheating device can be distributed through heat exchangers and piping arranged within the bed layer. The heat source for the preheater can be flue gas or electric heating; currently, electric heating is more commonly used. Once the catalytic reaction has started, the waste gas can be preheated as much as possible using the recovered reaction heat. In cases where the reaction heat is substantial, a waste heat recovery unit should also be installed to conserve energy. The temperature of the heat source used to preheat the waste gas generally exceeds the catalyst's active temperature. To protect the catalyst, the heating device should be kept at a certain distance from the catalytic combustion unit; this arrangement also helps ensure a uniform temperature distribution in the waste gas.

　　Starting from the fact that catalytic combustion requires preheating, this method is ***particularly suitable for the continuous purification of exhaust gases. If the exhaust gas is intermittent, not only will each preheating cycle consume energy, but the heat generated by the reaction also cannot be recovered and utilized, leading to significant energy waste. This point should be carefully considered during design and selection.

　　③ Catalytic combustion device

　　A fixed-bed catalytic reactor is typically used. The reactor design should comply with relevant standards and be easy to operate, maintain, and load/unload catalysts. When designing a catalytic combustion process, depending on the specific circumstances, for applications involving large gas flow rates, it is advisable to adopt a modular process layout—where the preheater and reactor are installed separately and connected via pipelines. For applications with smaller gas flow rates, a catalytic incinerator (see Figure 16-13) can be employed, integrating the preheating and reaction stages into a single unit; however, care must be taken to ensure an appropriate distance between the preheating section and the reaction section.