Kinetic data from continuous stirred-tank reactors

Kinetic Data from Continuous Stirred-Tank Reactors 

For fast or moderately fast liquid phase reactions, the stirred-tank reactor can be very useful for establishing kinetic data in the laboratory. When a steady state has been reached, the composition of the reaction mixture may be determined by a physical method using a flow cell attached to the reactor outlet, as in the case of a tubular reactor. The stirred-tank reactor, however, has a number of further advantages in comparison with a tubular reactor. With an appropriate ratio of 

There is one further point of comparison. Interpretation of results from a stirred-tank reactor depends on the assumption that the contents of the tank are well mixed. Interpretation of results from a tubular reactor rests on the assumption of plug flow unless the flow is laminar and is treated as such. Which of these two assumptions can be met most satisfactorily in practical experiments Unless the viscosity of the reaction mixture is high or the reaction extremely fast, a high speed stirrer is very effective in maintaining the contents of a stirred tank uniform. On the other hand, a tubular reactor may have to be very carefully designed if back-mixing is to be completely eliminated, and in most practical situations there is an element of uncertainty about whether the plug flow assumption is valid. 

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Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Experimental data that are most easily obtained are of (C, t), (p, t), (/ t), or (C, T, t). Values of the rate are obtainable directly from measurements on a continuous stirred tank reactor (CSTR), or they may be obtained from (C, t) data by numerical means, usually by first curve fitting and then differentiating. When other properties are measured to follow the course of reaction—say, conductivity—those measurements are best converted to concentrations before kinetic analysis is started. 

Although little valuable kinetic information is obtainable from published data on flow systems, an autoclave operated continuously under conditions approaching perfect mixing, i.e., a continuous stirred tank reactor (CSTR). can be a useful source of kinetic data (42). Recently van der Molen published rate data obtained in a CSTR (43). 

In the previous sections, the use of surfactants to increase the rate of desorption of hydrophobic organic contaminants was discussed. For the current study, several different surfactants were tested to determine whether the rate of TCE desorption from a peat soil could be increased. The effects of the surfactants on the rate of TCE desorption was tested using a continuous-flow stirred-tank reactor (CFSTR) methodology. The observed data were simulated using a distributed-rate kinetic desorption model. The parameters determined from the model simulation were then use to discern the effects of the surfactants on the rate of TCE desorption from the peat soil. The experimental methodology and the modeling procedure are now described in detail. 

Real kinetics data To date, almost all the kinetics data on reaction systems in liquid phase or multiphase with liquid as the continuous phase have been measured in traditional stirred tank reactors. From the results reported in this chapter, it is likely that significant deviations exist in the existing kinetics data. On the other hand, the LIS device cannot yet be considered as absolutely ideal for kinetics investigation, not least because its micromixing time, tM, is not zero. What then is the ideal equipment and conditions for obtaining real kinetics data ... 

The simplicity and general utility of the Madon-Boudart criterion make it one of the most important experimental tests to confirm that kinetic data are free from artifacts. It can be used for heterogeneous catalytic reactions carried out in batch, continuous stirred tank, and tubular plug flow reactors. 

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