Very rapid, ropid, and slow second-order reactions were studied in an isothermal turbulent flow reactor. The two aqueous reactant solutions were separately introduced through many alternote jets and the reaction took place in the resulting nonhomogeneous mixture. Very rapid reactions were diffusion controlled and were in agreement with earlier theory. All reactions followed second-order rote laws based on time average quantities. The apparent reaction velocity constant was controlled by the mixing for very rapid reactions, by the chemical kinetics for slow reactions, ond by both mechanisms for ropid reactions.When a homogeneous chemical reaction takes place in a turbulent fluid the local instantaneous rate of reaction can be assumed to be described by the normal laws of homogeneous chemical kinetics. However, the local time average rate of reaction, r,, the mean rate, may be greater than, equal to, or less than the homogeneous rate at the local time average concentration, the homogeneous mean rate. Although the above statements are true whether or not the system is isothermal, in this paper only isothermal situations are considered.For the purposes of this discussion a homogeneous solution or mixture is one in which the RMS concentration fluctuations of all species are zero. The mean rate obviously equals the homogeneous mean rate in homogeneous solutions and this is true even in nonhomogeneous solutions if the reaction is first order.In nonhomogeneous solutions the mean rate of a type I second-order reaction, A -+ A + product, is greater than the homogeneous mean rate, while the mean rate of a type I1 second-order reaction, A + B + product, may be greater or less than the homogeneous mean rate, depending upon how the reactants are introduced; greater if the reactants are premixed, less if they are not. (The above assertions are easily justified by examining the appropriate time average expressions and noting that premixing gives a positive correlation between the fluctuating reactant concentrations. while lack of premixing gives a negative correlation. ) The deviation from the homogeneous mean rate depends upon the rate of the reaction relative to the rate of mixing. Three arbitrary divisions are convenient: very rapid reactions, rapid reactions, and slow reactions. They correspond, respectively, to reaction rates much faster than the rate of mixing, reaction rates of the same order as the rate of mixing, and reaction rates much slower than the rate of mixing. The latter case is the simplest, and most reactor design studies have been concerned with this problem. It is under reasonably good control. [It is noted that in the flow methods of studying rapid chemical reactions in solution ( 1 4 ) , the reactions are slow in the above sense, since the experiments are designed to make the mixing much faster than the reaction.]Very rapid and rapid reactions present an entirely different problem, for here the detailed turbulent motion can have a profound effect on the rate of reaction, es- pecially with type I1 second-or...
A study has been made of the effect of fixed packing on the properties of a gas-fluidized bed, including minimum fluidization velocity, pressure drop, and bed expansion. Experiments using a range of glass beads as fluidizing solid with smooth uniform spheres as packing indicate that both packing size and the ratio of particle to packing diameter are the main variables in correlating the results. Other solids of varying density and shape have also been used. In addition to smooth spherical packing, rough spheres and variously shaped packings such as Raschig rings, Bert saddles, and a cylindrical, open-ended screen packing have received preliminary study. With the screen packing, which occupies only 5% of the column volume, i t has been possible to operate a fluidized bed at a much higher gas throughput without slugging than is possible with a conventional bed.A preliminary study has also been made of heat transfer rates, and the results indicate that the same factors are significant. The fluidization of particulate material is a technique which has many industrial applications. Good mixing and high rates of heat and mass transfer are generally associated with its use. However, the nature of conventional gas-solid fluidization imposes certain restrictions on the design of fluid-bed reactors. For example, there is a limit to the length-to-diameter ratio which can be used without serious slugging. Also, if high gas flow rates are desired, the reactor depth must include a Iarge freeboard to allow for fluctuations in bed height. Although many proposals have been made for expanding the useful range of gas-solid fluidization, very little quantitative information about their effect on the behavior of fluidized beds has been published. The data which are available, including an extensive study by Massimilla into the use of screen baffles (9, 10, 11) and a paper by Gabor, et al. on heat transfer rates in fluidized beds containing fixed spherical packing ( 1 2 ) , indicate that the changes resulting from the insertion of these devices can be very marked. Reactor internals affect not only the overall quality of the fluidization but also the expansion and the flow pattens of both gas and solid. As a result, heat and mass transfer rates can deviate significantly from those occurring in conventional beds.The work described in this paper is an introductory investigation into the effect of fixed packing on the properties of gas-fluidized beds. Most of the experimental work was performed with beds of packed spheres, but a limited number of experiments were done using packings of different shapes, in particular a cylindrical screen packing which was felt to be particularly suitable for use in fluidized beds. Results are reported for unit pressure drop and gas velocity at incipient fluidization and for bed expansion. A limited number of results showing the effect of packing on heat transfer rates are also given. APPARATUS AND PROCEDUREThe equipment used was of conventional design. The column consisted of a 3-in. I.D. plastic tube mou...
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