Mixing continuous reactors

The same consecutive reactions considered in Prob. 6.18 are now carried out in two perfectly mixed continuous reactors. Flow rates and densities are constant. The volumes of the two tanks (P) are the same and constant. The reactors operate at the same constant temperature. 

J1. An isothermal, first>oidei, liquid-phase, reversible reaction is carried out in a constant-volume, perfectly mixed continuous reactor. 

For the perfectly mixed continuous reactor, the CSTR, the ratio VT/ Fy only represents the mean residence time, /p,av however, it is still possible to compare the performance of the CSTR with the performance of the BR by letting the mean residence time fp av = t. Interestingly, when the reaction rate shows a positive dependence on reactants concentration, the BR is more effective than the CSTR. This is because the batch reactor experiences all the system compositions between initial and final values, whereas the CSTR operates at the final composition, where the reaction rate is smaller (under the above hypotheses). Finally, one can compare the two continuous reactors under steady-state conditions. The CSTR allows a more stable operation because of back-mixing, which however reduces the chemical performance, whereas the PFR is suitable for large heat transfer but suffers from larger friction losses. 

Steady, well-mixed continuous reactor (CSTR)... 

Fluidized bed reactors are used for the gas phase polymerization of olefins to produce LLDPE and HDPE (Unipol PE, Innovene and Spherilene processes [22]) as well as for the manufacture of polypropylene (Unipol PP process [22]). The residence time distribution of a fluidized bed reactor is close to that of a well mixed continuous reactor. Therefore, the use of a single fluidized bed reactor leads to a broad particle size distribution and to large variations in the catalyst productivity from particle to particle. The use of two fluidized bed reactors in series narrows the residence time distribution leading to a more uniform product. 

In this chapter the most important operation modes of reactors are considered. Models are developed by combining simple reaction kinetics for single-phase reactions with mass balances for five ideal model reactors the ideal batch reactor the semi-batch reactor the plug flow reactor the perfectly mixed continuous reactor and the cascade of perfectly mixed reactors. For isothermal conditions, conversions can be calculated on the basis of chemical kinetics only. 

In section 1.2 the concept of the CSTR was introduced, and its merits briefly described. When the mixing is strong and the reactions that take place in the reactor are not too rapid, the reactor may be considered as perfectly mixed. Because these assumptions are in some cases quite realistic, the concept of the perfectly mixed continuous reactor is useful. 

The calculation of the conversion of chemical reactions in perfectly mixed continuous reactors is simpler than for the cases of batch- and plug flow reactors. On the basis of data obtained for a given chemical reaction in a batch-reactor, the conversion in perfectly mixed continuous reactors can be calculated easily. 

In considering the chemical kinetics of complex reaction systems the differential selectivities are directly relevant, but for reactor design the use of integral selectivities is often more practical. Note that for perfectly mixed continuous reactors differential and integral selectivities are the same. 

For perfectly mixed continuous reactors we use the following mass balances... 

In this chapter the principle of selectivity was introduced. Selectivities could be calculated as a function of the degree of conversion for several types of reaction pairs, and for various reactor configurations. It was shown that, for the case of competitive reactions, the use of a well mixed continuous reactor results in a higher selectivity when the undesired reaction has the highest order. When the... 

For consecutive reactions batch and plug flow reactors will always give a higher selectvity than well mixed continuous reactors. For competitive-consecutive reactions the best selectivity can be found in semi-batch reactors. However, this type of reactions can in some cases be carried out with an acceptable selectivity in a continuous well mixed reactor, when sufficient excess of the "other reactant (that does not cause undesired reactions) is applied. 

In Oiapter 3 the importance of mixing in chemical reactors was indicated with the defmition of two ideal reactor types the plug flow reactor, in which no mixing takes place at all, and the perfectly mixed continuous reactor, with infinite mixing rates. The concept of mixing itself, however, was not analysed. 

In section 3.4 the selectivities of competitive and consecutive reactions were calculated for two reactor types the batch or plug flow reactor (no backmixing), and the perfectly mixed continuous reactor. The following general conclusions can be formulated in terms of backmixing ... 

Continuous reactors in the pharmaceutical and specialty chemical industries may not only be needed for high productivity as in other segments of the chemical industry, but additionally to solve specific reactor design problems caused by limitations in batch operation. These limitations include heat transfer, mass transfer, and mixing. Continuous reactors are also used to minimize the reacting volume of thermally potent and/or noxious reactions and to decrease the potential and exposure for catastrophic failure of a vessel. Chemical industry reactor standards such as packed bed, fluid bed, and trickle bed reactors And limited utility since this type of phase contacting can usually be achieved in a slurry reactor, where residence time distribution variations, which can lead to changes in product distributions, are eliminated. Continuous stirred tank reactor operation is used only... 

The wide spread of residence times inherent in completely mixed continuous reactors may be a disadvantage if complete conversion of the solids is desired [48]. For this reason sometimes stagewise contacting is arranged... 

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