Reaction autocatalytic volume

Example 4.10 Consider a reactor train consisting of a CSTR followed by a piston flow reactor. The total volume and flow rate are fixed. Can series combination offer a performance advantage compared with a single reactor if the reaction is autocatalytic The reaction is... 

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

The reaction was shown to undergo decay untill equilibrium is attained autocatalytical behaviour could not be identified. The volume change is a good measure for the extent of reaction. 

The general rule is that combinations of isothermal reactors provide intermediate levels of performance compared with single reactors that have the same total volume and flow rate. The second general rule is that a single, piston flow reactor will give higher conversion and better selectivity than a CSTR. Autocatalytic reactions provide the exception to both these statements. 

The CSTR and the PFR each have a volume of 0.1 m3. The rate at which A is fed to the first reactor is 150 kmoles/ksec. Note that this is an autocatalytic reaction in which a product acts as a catalyst for subsequent reaction. 

The encapsulation results in a chance collection of molecules that then form an autocatalytic cycle and a primitive metabolism but intrinsically only an isolated system of chemical reactions. There is no requirement for the reactions to reach equilibrium because they are no longer under standard conditions and the extent of reaction, f, will be composition limited (Section 8.2). Suddenly, a protocell looks promising but the encapsulation process poses lots of questions. How many molecules are required to form an organism How big does the micelle or liposome have to be How are molecules transported from outside to inside Can the system replicate Consider a simple spherical protocell of diameter 100 nm with an enclosed volume of a mere 125 fL. There is room within the cell for something like 5 billion molecules, assuming that they all have a density similar to that of water. This is a surprisingly small number and is a reasonable first guess for the number of molecules within a bacterium. 

For an autocatalytic reaction, Example 15-10 shows that a recycle PFR operating with an optimal value of R requires the smallest volume for the three reactor possibilities posed. (In the case of a PFR without recycle, the size disadvantage can be offset at the expense of maintaining a sufficient value of cBo (in the feed), but this introduces an alternative disadvantage.) A fourth possibility exists for an even smaller volume. This can be realized from Figure 15.8 (although not shown explicitly), if the favorable characteristics of both normal and abnormal kinetics are used to advantage. Since this involves a combination of reactor types, we defer consideration to Chapter 17. 

In the discussion following Example 15-10 about reactor volumes for an autocatalytic reaction, it is suggested that a further possibility exists for an even smaller volume than that obtained with a recycle PFR. 

Figure 17.6 Graphical illustration of basis for using CSTR + PFR series combination for minimum volume for autocatalytic reaction...

For the autocatalytic reaction described in Example 15-10 and the data given there, calculate the volume of a combined CSTR + PFR reactor arranged as in Figure 17.7. 

The enzymatic activity versus pH is thus generally a bell-shaped curve as in Fig. 6. An autocatalytic effect may appear when the reaction products have an acid-base effect (often the case). A simple example is the glucose oxidase reaction, shown in Fig. 7. Notably, the rate versus product (H+) curve indicates an autocatalytic effect on the alkaline branch, that is, for pH>pH (see Fig. 6). Systems presenting analogous properties have been studied by R. Caplan et al.27 and we also learn more about them from D. Thomas in this volume. 

For most reactions, the rate decreases as the reaction proceeds (important exceptions being a number of biological reactions which are autocatalytic). For a reaction with no volume change, the rate is represented by the slope of the curve of X (moles converted per unit volume) versus time (Fig. 1.10), which decreases steadily with increasing time. The maximum reaction rate occurs at zero time, and, if our sole concern were to obtain maximum output from the reactor and the shutdown time were zero, it appears that the best course would be to discharge the reactor after only a short reaction time tr, and refill with fresh reactants. It would then be necessary, of course, to separate a large amount of reactant from a small amount of product. However, if the shut-down time is appreciable and has a value ts then as we have seen in the example on ethyl acetate above, the average production rate per unit volume is ... 

Figure 4.6 presents kinetic curves of cyclohexadiene accumulation at an optimal temperature of 580 °C. It is shown that in the initial period cyclohexadiene accumulation as an intermediate product is intensified until its consumption and accumulation rates are equalized and cyclohexadiene concentration reaches the maximum. Further increase of conditional contact time reduces cyclohexadiene concentration. For example, at 580 °C and r = 0.7 h the cyclohexadiene yield is 17.5%, decreasing to 9.5% with r increased to 2.5 h. Under optimal conditions (T = 580 °C, cyclohexene volume rate 1.4ml/(mlh), cyclohexene 20% aqueous H202 = 1 3), yields were 17.1% for cyclohexadiene and 5.8% for benzene. The reaction selectivity approached 100%. The entire process was of a consecutive autocatalytic type. 

The hydrolysis of methyl acetate is an autocatalytic reaction and is first order with respect to both methyl acetate and acetic acid. The reaction is elementary, bimolecular and can be considered irreversible at constant volume for design purposes. The following data are given ... 

Experiments on RVX hydrolysis with an equimolar amount of water were performed as follows. A mixture of 74 pi of RVX and 5.6 pi of water was exposed at room temperature for 3.5 months without stirring. An ash-gray thiek uniform material formed after hydrolysis and completely dissolved in 5 ml acetonitrile. The solution was diluted 100 times with acetonitrile and analyzed by GC-EIMS (sample D). An aliquot of this sample was mixed with an equal volume of bis-trimethylsilyltrifluoroacetamide (BSTFA), and the mixture was heated at 70°C for 30 min and then analyzed by GC-EIMS (sample E). The autocatalytic hydrolysis of RVX was almost complete by the end of the experiment, since the RVX content in the sample was no more than 0.01%. Qualitatively, the reaction mixture in the latter case was much poorer than in the hydrolysis with... 

The autocatalytic nature of the reaction, described by Hinshelwood and Williamson [1], is in sharp contrast with the effect of water on the surface reaction at lower temperatures, which is poisoned by steam, and also with the inhibiting effect of water vapour on the second limit explosions. The autocatalysis has been studied in some detail by Chirkov [36], who used a reaction vessel of Durobax glass with diameter 5 cm and volume 200- 250 cm. For hydrogen oxygen ratios of about 2 1 at 550 torr initial pressure and 524 °C, he found the reaction rate w (torr sec ) to be given in terms of the initial pressure p and the amount of gases reacted x by... 

When confined in the closed cell and subjected to the isothermal storage test performed at a 7) of several tens of degrees C., however, 0.4 g of nitrocellulose, the volume of which is about 2 cm, behaves as a chemical of the AC type in such a manner as to start the autocatalytic reaction a long time after the start of the isothermal storage test in accordance with Eq. (59). This manner is the same as that shown in Fig. 6 in Section 3.1. 

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