Selectivity in experimental reactors

Abstract

showing that reactor-product distribution is generally distorted in proportion to a reactor Peclet number. G. I. Taylor's theory of axial mixing is used to relate this Peclet number to the physical characteristics of the reactor.Although the effects of axial mixing are concluded to be negligible in most commercial or large-scale equipment, they are serious in the case of experimental or pilot-scale apparatus.Charts are drawn to illustrate design problems for these reactors. PURPOSE AND SCOPEThe successful scale-up of a process requires accurate knowledge of yields and reaction rates in the range of operation of the scaled-up process. To this end, a method is given here for designing experimental flow reactors in which good selectivity can be maintained, that is, reactors in which the products formed are characteristic of the reaction chemistry, concentrations, etc., and not characteristic of the peculiarities of the experimental reactor.Excessive axial mixing, or backmixing, in an experimental reactor makes the interpretation of results on yields and by-products highly uncertain. It is especially detrimental with integrated processes wherein recycle rates, separation procedures, etc., are based on the composition of the effluent obtained in experiments. Unrealistic amounts of by-products from a faulty experimental reactor may lead to establishment of a false optimum in operating conditions for the integrated process; and even if this mishap is later discovered, most of the experiments may involve conditions near the false optimum, thus yielding little information for the operating conditions of ultimate interest. Despite these pitfalls flow reactors are necessary in the study of certain reactions which cannot be explored with accuracy in bombs or continuous wellstirred vessels because of physical problems associated with the reaction mixture.Although there have been past studies of backmixing in homogeneous reactors ( 4 , 5,6,7,8, 9, 10) on the course of a reaction was not attempted here. Instead an approximate analysis was used which is valid for all kinds of reactions when the effect of mixing on the overall performance is not excessive. Therefore, the methods developed are useful not for interpreting data from reactors in which there has been excessive mixing, but rather for selecting reactor dimensions and operating conditions which make backmixing effects tolerably small. The product from the reactor is then substantially the same as from a batch reactor. THE EFFECT OF AXIAL MIXINGAxial mixing in an unpacked tube is caused by variation of axial velocity across the radius. Fluid elements near the center move more rapidIy than the average; thus they penetrate into downstream material near the walls and eventually mix with it. The magnitude of this effect depends greatly on the shape of the velocity profile and the rate of mixing in the radial direction. Contrary to first instinct, radial mixing inhibits axial mixing; this interrelation formed the basis for Taylor's analysis, ( I ) later extended by Tichacek,...

Improvement of the yield of physiologically active oligosaccharides in continuous hydrolysis of chitosan using immobilized chitosanases

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Effect of Catalyst Particle Size on Selectivity in Butene Dehydrogenation

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Optimal operation of a tubular chemical reactor

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