Research On Optimization Theory And Application Of Reactive Distillation With Side Reactors

The preparation process of benzyl chloride by photochlorination of toluene is a typical consecutive reaction taking intermediate product as end product. The boiling point of benzyl chloride is 69°C higher than that of toluene. It is very essential and feasible for improving percent conversion of toluene and selectivity of benzyl chloride using reactive distillation. It was already studied that benzyl chloride was prepared by reactive distillation through tray column, but the production capacity of existed full scale plant was only about 1000t/a limited to not enough reactive space. The first aim of this text is to seek a kind of new-type reactive distillation device which is suitable for this system to improve the production capacity of the single set of device by a wide margin so as to realize the upsizing of the device. The second aim is to carry on the relevant theoretical research of this new-type reactive distillation device and set up new method of process simulation and optimum design. Thereby, the upsizing problem of reactive distillation device is solved from theory and engineering application.The structure of reactive distillation equipment is various. Packing structure and tray structure are ever changing too. Towards reaction process of preparation of benzyl chloride, the key of realizing the upsizing of the device is to offer enough photocatalysis space. There are no more requests for the structure of tray. And reactive distillation with side reactors in this paper quite fulfills the request.The reaction which should be carried out inside column takes place outside column with side reactors. Both reaction and distillation don't take place in the same place exactly. But mass transfer exists between column and side reactors. So, it can be regard as a kind of expansion mode of traditional reactive distillation device. Such a device will become more and more unified with traditional device when there are many reactors. Differed from traditional reactive towers, little information is available on the study of simulation and optimization design on reactive distillation with side reactors.The mathematical model, simulation and optimum design method for process of photochlorination of toluene within a column with side reactors were set up. The model comprises mass and heat balance of rectifying tower and reactors. And equations of gas-liquid equilibrium and reaction kinetics are also involved. Since rectifying tower and reactors are relatively independent, the coupling of rectification and reaction is embodied by quantity, composition and state exchanged among fluids on the basis of independent calculation for simulation and optimum design. The model was solved through Newton-Raphson Method.The extent of reaction is decided by quantity of chlorine entered reactors towards the process of photochlorination of toluene within a column with side reactors. However, it can be called as a reactive distillation process with independent amount of reaction because the quantity of chlorine entered reactors can be controlled independently. It was proposed that the matching of reactive potency to separating power was the key of optimization of reactive distillation process. The method and procedure of optimum design for the reactive distillation process with independent amount of reaction were set up. The sum of equipment investment of the whole set of device and operating cost could be minimum by seeking a maximum amount of reaction to match a certain separating power at assured column trays and boiling up rate. Considering little yield of benzal chloride in reactive distillation process, the model of rectifying section was simplified to a constant molar flow distillation model including only toluene and benzyl chloride. The yield of benzal chloride could be calculated through reaction kinetics. Thus, the calculation amount could be reduced greatly. Calculation of optimum design was carried out with LINGO 8.0. The structure parameter and operational parameter, such as number of trays and reactors, feed rate and distribution of chlorine, could be obtained simultaneously. And the distribution of flow rate, composition and temperature in both gas and liquid phase could be obtained too. And then, the actual result was obtained by strict calculation in HYSYS with received result from simplified model. The essence of this method was to carry out optimum design for the reactive distillation process with amount of reaction as variables. And then, thereactor or loading amount of catalyst was designed from the calculated amount of reaction to match the rectifying tower. A new way to design a reactive distillation device was opened up.The production cost of benzyl chloride showed a monotone decreasing tendency along with the increase of number of reactors when boiling up rate was 15kmol/h regarding 5000t/a process units for benzyl chloride as the design goal. But the cost dropped slowly when the number of rectors exceeded 3. The optimum result was that, tray number of separation area was 12 including reboiler, number of reactors was 3 and 2 trays between two rectors were appropriate.The coupling device made of glass of reactive distillation with 3 200ml side reactors was set up. And the tray number of separation area was 8 including reboiler and there were 2 trays between two reactors. According to the characteristic of the system of chlorination of toluene, the coupling operation of reactive distillation was taken place in the upper reactive distillation area and the purification of benzyl chloride was taken place in the lower separation section. The influence of the change of top condensed flow rate of the tower and feed rate on reactive distillation tower was investigated. Under the operational condition of experiment, the conversion of toluene could be more than 99% and the selectivity of benzyl chloride could be above 96%. The distribution of concentration and temperature within the reactive distillation tower was measured.The engineering design, construct of device and trial run of commercial plant for preparation of benzyl chloride with annual output of 5000 ton were carried out. It is different from lab experiment that the coupling of different work condition was realized, column was operated at vacuum while reactors were operated at normal atmosphere. And good result was obtained. The tower diameter of commercial plant was 800mm. 8000mm high ceramic packing was packed into the separation section. The number of reactors was 3 and there were 2 trays between two reactors. Under the operational condition of experiment, the conversion of toluene was 99.7% and the selectivity of benzyl chloride was 98%.An additional application study on reactive distillation device with side reactorswas carried out. The lab experiment of preparation of chlorocyclohexane by chlorination of cyclohexane with reactive distillation was carried out. The yield of chlorocyclohexane could be above 90%. It was much higher than the literature value by 30%. The process of photochlorination of cyclohexane was preliminarily analyzed. The course of reaction was considered as sequential reactions including a small amount of parallel reactions. And that could serve as basis for further improvement of selectivity of chlorocyclohexane.