What should I do if the residence time of the microchannel reactor is not enough?

The microchannel reactor can significantly improve the mass and heat transfer effects of materials, shorten the reaction time, increase the product yield, and is safe, energy-saving, and environmentally friendly. Therefore, it has received extensive attention in the chemical, pharmaceutical, dye and pesticide industries. There are many success stories. However, in the process of experimental development and exploration on the micro-reactor pilot device, the experimenters often encounter such a problem: when the best experimental conditions of the batch reactor are applied to the microchannel reactor, the results are often not ideal. , Sometimes the yield of the batch reactor is not reached. As soon as the product was tested, it was found that there were still a lot of raw materials, so it was determined that the residence time was not enough, causing the raw materials to not react completely.

1. Increase the reaction temperature

We know that the optimal reaction conditions of the batch reactor are matched with the reactor. The industrial reactor has a large volume and a large amount of materials processed at one time, so the mass transfer effect is not good, and the effective number of collisions between molecules per unit time is small, so the reaction rate is relatively slow. It can be seen that the reaction rate constant is directly proportional to the reaction temperature. This is because increasing the reaction temperature can increase the average kinetic energy of the molecules. The number of molecules that transform from the ground state to the transition state per unit volume increases, and the ratio becomes larger, and more molecules can cross the barrier, which is commonly referred to as activation energy. Therefore, the reaction is more likely to occur, which is manifested in the macroscopic view that the reaction rate is accelerated. It can be seen that the reaction temperature plays a crucial and decisive role in the reaction rate.

  Microreactor: However, in many reaction processes, we cannot arbitrarily increase the reaction temperature. On the one hand, excessive temperature may cause side reactions such as coking, and on the other hand, it is mainly restricted by the poor heat transfer effect of traditional reactors. Chemical reactions are often accompanied by severe thermal effects. For exothermic reactions, the faster the reaction rate, the greater the exotherm. However, the limited heat exchange area of industrial reactors cannot meet the heat exchange requirements of the reaction. In order to avoid flying temperature, Effective control of temperature is achieved by lowering the reaction temperature and sacrificing the reaction rate, and then the purpose of full reaction is achieved by extending the reaction time. This is why many low-temperature organometallic reactions are carried out at -20°C or even -40°C, and why most nitration reactions are carried out by dripping raw materials.

 However, these problems are no longer the shackles of the microchannel reactor. There are many successful cases of increasing the reaction temperature of the microchannel reactor, and it can even be said to be one of the effective ways to shorten the reaction time. Therefore, when an incomplete reaction of the raw materials occurs in the experiment, first consider: Is the reaction temperature set too low? Generally speaking, the reaction temperature of the microchannel reactor can be increased by about 10-20°C than the temperature of the batch reactor.

2. Increase the concentration of reactants

  In addition to the average kinetic energy of the molecules, the reaction rate is also related to the number of reactant molecules per unit volume, as shown in Figure 4. That is, theoretically, the higher the reactant concentration, the more conducive to the reaction. However, in batch reactors, sometimes a relatively low concentration of reactants is selected, which is mainly considered from the following aspects:

(1) Control the reaction rate. As mentioned earlier, in batch reactions, sometimes the heat exchange cannot keep up or the stirring is not uniform, which may easily cause local overheating, which may cause coking or other side reactions, which will affect the product selectivity and yield. At this time, it is a wise choice to use a lower reactant concentration, but it also has to pay the price of long reaction time accordingly.

(2) Avoid secondary reactions. In addition to coking, a higher concentration of reactants can easily lead to secondary reactions, such as overnitrification, peroxidation, etc. In order to avoid the formation of non-target products, it is often necessary to adjust the concentration of reactants.

(3) Use solvent to exchange heat. In addition to dispersing the reactants, some solvents also have good heat absorption capacity, so they are good heat exchange media. For example, concentrated sulfuric acid often acts as a catalyst and heat exchange medium at the same time in the nitration reaction. At this time, the amount of solvent will also be increased, thereby reducing the concentration of reactants.

  Therefore, in the process of conducting the experiment, we must first clarify the reason why the reactant chooses this concentration. If it is restricted by the heat exchange capacity of the reactor, you can boldly try to increase the concentration of the reactants in the slightest reaction, and satisfactory results will often be obtained. A nitrification reaction recently done by the editor. The concentration of nitric acid in the reactor is 16-20%. The same acid concentration is used in the micro-reactor, and the product yield is only 60-70%. The gas chromatography analysis results show that there are still many raw materials. No reaction was complete. When the acid concentration was increased to 35-40%, the product yield immediately reached over 95%. It can be seen that in the previous experiment, the reaction rate was limited by the low concentration of nitric acid. If you choose a lower concentration to avoid the occurrence of secondary reactions, you must carefully increase the concentration of reactants. However, due to the short residence time of the micro-reactions, sometimes the secondary reactions are terminated before the reaction occurs. In this case, the residence time can also be shortened by increasing the concentration of reactants, so as to achieve the purpose of increasing the product yield.

3. Reduce traffic

  If the method of increasing the reaction temperature and the concentration of reactants cannot effectively shorten the residence time of the reaction and make the reaction complete, then you can try to reduce the flow rate of the materials at this time while other conditions remain unchanged. The standard configuration of the microchannel reactor pilot equipment is 10 reaction plates, so the volume of the reactor is determined. When the liquid holding capacity remains the same, reducing the flow rate of the material will correspondingly extend the residence time of the reaction. However, it should be noted here that the flow rate of the material cannot be reduced to a particularly low level, otherwise the material cannot completely fill the microchannel, and the mixing and mass transfer effects of the microchannel will be lost.

4. Increase coil or reaction film

  Through the three methods introduced above, most of the problem of insufficient residence time in the reaction process should be effectively solved. However, for individual special reactions, if the above three methods are all unsuccessful, you can only consider increasing the liquid holding capacity of the reactor, that is, increasing the volume of the reactor. The usual measure is to first connect a section of coil to the outlet of the microreactor, so that the reaction system continues to react in the coil, and then analyze and test the material at the coil outlet. If the analysis results show that the extension of the residence time is indeed helpful for the conversion of raw materials and can make the reaction more complete, then you can customize the microreactor with more reaction plates according to the actual required residence time, or directly combine the two One or more micro-reactors are used in series.

Discuss

There are many influencing factors in the chemical reaction process, including temperature, pressure, flow rate, raw material concentration, material ratio, feeding sequence, residence time, etc., and the various factors are interrelated and mutually restricted. Therefore, in the face of a given result in the experiment, such as the incomplete reaction of the raw materials, one should not only consider its direct influence factors, such as whether the residence time is insufficient, but should be grasped from a macro perspective, in order to understand the context of the entire reaction and the various influences. After the relationship between the factors, it can be solved by increasing the reaction temperature, increasing the concentration of the reactant, reducing the material flow, or increasing the coil or reaction plate.