STIRRED FLOW REACTOR

A perfectly-stirred flow reactor is characterized by a homogeneous state of the reaction mixture throughout the entire volume of the reactor, i.e. the concentration of each constituent, the temperature and the pressure have the same value at each point of the reactor. Xhere are two consequences of this  

To calculate the second-order rate constant for the reaction between hydroxyl ion and ethyl acetate from measurements using a stirred-fiow reactor. 

In an experiment carried out by Stead, Page, and Denbigh (Discuss. Faraday Soc., 1947, No. 2, 263) solutions of barium hydroxide of hydroxyl ion concentration Xq and of ethyl acetate of concentration j/q were introduced into a reaction vessel of volume V = 602 ml at flow rates u and w respectively through capillary inlet tubes and upon entering the reaction vessel were thoroughly mixed. The filled reaction vessel overflowed at a steady rate, and after several hours running titration of the free alkali in the overflow solution showed that its composition, equal to that of the solution in the reaction vessel, had attained a constant value. We denote by x and y the steady-state concentrations of hydroxyl ion and ethyl acetate respectively. The results are recorded in table 1. 

We note that the value obtained for k is considerably lower than the value A = 6.761 mole min cited in problem 156. A single measurement by Stead, Page, and Denbigh (loc. cit.) using sodium hydroxide gave A = 6.61 mole min.  

The chief potential advantage of the stirred-flow reactor technique is in the study of complex reactions proceeding in several stages, since it avoids the use of cumbrous integrated rate equations, and facilitates the study by physical methods of transient reaction intermediates. 

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The interpretation of the above data on iodination has been questioned by Buss and Taylor217, and by Grovenstein et a/.218,219. The former workers studied the iodination of 2,4-dichlorophenol at about 25 °C using a stirred flow reactor, the advantages of which are that once a steady state has been reached there is no change in the concentration of the reactive species in the reactor with time and the rate of reaction is simply a product of extent of reaction multiplied by the reciprocal ol the contact time hence it is possible to use unbuffered solutions and low iodide ion concentrations. They found general catalysis by the base component of added phosphate buffers and the observed rate coefficients varied with [H+ ] according to... 

Houser and Lee [J. Phys. Chem., 71 (3422), 1967] have studied the pyrolysis of ethyl nitrate using a stirred flow reactor. They iiave proposed the following mechanism for the reaction. 

Young and Hammett [J. Am. Chem. Soc., 72 (286), 1950] have studied the alkaline bromi-nation of acetone in a stirred flow reactor (VR = 118 cm3). The stoichiometric equation for the bromination is usually considered to be ... 

Keairns and Manning AIChE J., 15 (660), 1969] have used the reaction between sodium thiosulfate and hydrogen peroxide in a well-stirred flow reactor to check a computer simulation of adiabatic CSTR operation. Data on their experimental conditions and the reaction parameters are listed below. The reaction may be considered second-order in sodium thiosulfate. 

The complete mixing in a stirred flow reactor has been discussed by Denbigh. In stirred flow, the composition becomes uniform throughout the entire volume of the reactor as a result of efficient stirring. 

In the steady-state approach to determining the rate law, solutions containing reactants are pumped separately at a constant flow rate into a vessel ( reactor ), the contents of which are vigorously stirred. After a while, produets and some reactants will flow from the reactor at the same total rate of inflow and a steady state will be attained, in which the reaction will take place in the reactor with a constant concentration of reactants, and therefore a constant rate. This is the basis of the stirred-flow reactor, or capacity-flow method. Although the method has been little used, it has the advantage of a simplified kinetic treatment even for complex systems. 

In chapters 2-5 two models of oscillatory reaction in closed vessels were considered one based on chemical feedback (autocatalysis), the other on thermal coupling under non-isothermal reaction conditions. To begin this chapter, we again return to non-isothermal systems, now in a well-stirred flow reactor (CSTR) such as that considered in chapter 6. 

Following Jorgensen and Aris (1983), we take a well-stirred flow reactor fed by a stream of reactant A. This reactant is successively converted to an... 

If one desires to make meaningful use of small changes in entropies of activation, for example of AA values as might be caused by small structural changes in a reactant, then one must insist on kinetic measurements of really high precision. For well-behaved reactions the necessary precision can be obtained even with conventional techniques. Special techniques such as the stirred flow-reactor, first applied by Denbigh et al. (1948), and later modified by Hammett ef al. (1950), are occasionally helpful. 

Stirred-flow reactors have been studied and used by chemical engineers for many years, but their application to chemical research is more recent, first by Denbigh (1944), and then by Hammett (1960). Stirred-flow reactors have recently been used by soil chemists to study soil chemical reaction... 

The analysis of data from a stirred-flow reactor is based on a mass balance equation similar to Eq. (3.2),... 

When one studies kinetics of soil chemical processes, where solid surfaces are involved, the analysis of data using a stirred-flow reactor is different from that presented above. The main difference is the presence of one reactant, i.e., soil, clay mineral, or some other solid surface, whose mass is constant throughout the experiment. Thus, a steady state is established together with an equilibrium state when the net reaction rate is zero. Therefore, the analysis of data is not based on steady state conditions. However, continuous short-incremental measurements can be carried out, which enables analysis of non-steady state conditions. 

A practical problem in using the stirred-flow reactor involves choosing proper experimental conditions such that a set of C(t) values are obtained that are significantly smaller than those obtained without a sorbent in the chamber (blank sample) and significantly different from those of an instantaneous reaction. With these concerns in mind, Seyfried et al. (1988) presented an empirical expression for the sorption or adsorption process which is given below... 

Tien (1987) studied the kinetics of heavy metal sorption-desorption on sludge using the stirred-flow reactor method of Carski and Sparks (1985). Sorption-desorption reactions were rapid with an equilibrium reached in 30 min. The sorption-desorption reactions were reversible. The sorption rate coefficients were of the order Hg > Pb > Cd > Cu > Zn > Co > Ni, while the desorption rate coefficients were of the order Cd > Cu > Hg >... 

Continuous isothermal perfectly stirred flow reactor (the reaction mixture is at equilibrium with the heat transfer medium). 

Continuous adiabatic perfectly stirred flow reactor. 

Recently, Barton and coworkers investigated the mechanism of the 1,2-silyl migration in a related system through a combination of experiment and theory40. Pyrolysis of 12 at 600 °C cleanly produced a mixture of 12 and methylenedisilacyclopentene 13 (25%) (equation 12). A kinetic study of this reaction was conducted over the temperature range of 520-600 °C in a stirred flow reactor. The Arrehnius parameters for the first order formation of 13 were logA = 12.5 s-1 and Ea = 54 kcalmol-1. In the pyrolysis of a related all-carbon system 14, decomposition occurred at 550 °C but no isomerization to the methylene cyclopentene 15 was observed up to 700 °C (equation 13). 

Example 12.12 Adiabatic stirred flow reactor Consider the following reaction ... 

The reaction occurs in an adiabatic stirred flow reactor with feed flow rate F, transient compositions cA, and cB and reaction rate JT, and total mass of reacting mixtures M. For small perturbations around the stationary state(s), the following expansions are used ... 

Stirred-flow reactor. The composition of the reacting volume, V, at temperature, T, is the same everywhere and at all times. Ff is the molar flow rate of species into the reactor while F, is the molar flow rate of species i out of the reactor. 

The equations provided above describe the operation of stirred-flow reactors whether the reaction occurs at constant volume or not. In these types of reactors, the fluid is generally a liquid. If a large amount of solvent is used, that is, dilute solutions of reactants/products, then changes in volume can be neglected. However, if the solution is concentrated or pure reactants are used (sometimes the case for polymerization reactions), then the volume will change with the extent of reaction. 

In a stirred-flow reactor of volume V = 0.602 L, the following data have been obtained at 298 K [Denbigh et al., Disc. Faraday Soc., 2 (1977) 263] ... 

Very similar dependences of the rate on the reactant concentrations were obtained by Longwell and Weiss [421], Hottel et al. [384], Williams et al. [422] and Dryer and Glassman [455] using stirred flow reactors. Hottel et al. [384] found... 

Ford and Endow studied the photolysis of NO2 at low pressures in a stirred flow reactor and postulated the following mechanism to explain their results. 

Kistiakowsky and Volpi, using a stirred flow reactor, added N atoms (produced in an electric discharge) to nitrous oxide at 553 °K and estimated ki63 10 I.moIe sec for the reaction... 

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