How to understand reducing the volume first and then lowering the temperature?
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It’s easy to understand why the amount is increased first and then the temperature; it’s not quite clear how reducing the temperature first and then the amount can prevent overheating
Reduce temperature first, then reduce the volume: Consider a processing capacity of 160 tons with an outlet temperature of 326 degrees Celsius; if the volume is suddenly reduced to 100 tons, and if the flow rate of the main fuel gas remains unchanged while less oil is used, the outlet temperature of the furnace will naturally rise. It is only necessary to dynamically control the outlet temperature of the reactor within the specified range. With this rough explanation, you should be able to understand it.
First, reduce the temperature, that is, lower the degree of reaction. When the temperature drops, the reaction rate slows down, and the heat released during the reaction also decreases, making the reaction easier to control. If the amount of reactant is reduced first, the space velocity will drop, the degree of reaction will increase, and the reaction will intensify. This isn’t a problem in the case of purification reactions, but it has a significant impact in cases of modification or cracking. In fact, as long as the reduction in amount is small, there’s no major issue. Reducing the temperature first and then reducing the amount is just a principle; doing the opposite necessarily doesn’t lead to problems either. The key is to find the right balance
If the flow rate suddenly drops to 100 tons, and if the opening degree of the main fuel gas remains unchanged, there will be less oil available; with the fuel gas amount unchanged, the outlet temperature of the furnace will naturally rise sharply.
It’s easy to understand: if the flow rate is reduced first while the temperature remains unchanged, for example, if originally 50 tons per hour of material passed through the catalyst, now that amount becomes 30 tons! Catalysts will inevitably cause excessive cracking of the oil products; once the temperature rises and becomes difficult to control, it will be too late to lower it again
Reducing the feed rate increases the residence time, which leads to an increase in side reactions at the same reaction temperature; in severe cases, this can result in a spike in temperature. Therefore, it is necessary to lower the temperature before reducing the feed rate.