Kinetically controlled reactions laboratory scale

Use of medium-scale heat flow calorimeter for separate measurement of reaction heat removed via reaction vessel walls and via reflux condenser system, under fully realistic processing conditions, with data processing of the results is reported [2], More details are given elsewhere [3], A new computer controlled reaction calorimeter is described which has been developed for the laboratory study of all process aspects on 0.5-2 1 scale. It provides precise data on reaction kinetics, thermochemistry, and heat transfer. Its features are exemplified by a study of the (exothermic) nitration of benzaldehyde [4], A more recent review of reaction safety calorimetry gives some comment on possibly deceptive results. [5],... 

As an alternative to investigating the kinetics of a gas-liquid reaction on a laboratory scale, the mass transfer resistance may be minimised or eliminated so that the measured rate corresponds to the rate of the homogeneous liquid-phase reaction. This method of approach will be considered after first describing those reactors giving rise to controlled surface exposure times. 

For laboratory-scale modification, distinction has to be made between static and dynamic adsorption procedures. In a static procedure, the substrate is contacted with a known volume of gas at a well-defined pressure. The modifying gas may be stationary or circulating in a closed loop. Modification in a static gas adsorption apparatus allows the careful control of all reaction parameters. Temperature and pressure can be controlled and easily measured. Adsorption kinetics may be determined by following the pressure as a function of the reaction time. Figure 8.13 displays a volumetric adsorption apparatus, in which mercury is used, as a means to change the internal volume and for pressure measurement. 

Temperature control for laboratory reactors is typically easy because of high heat transfer area-reactor volume ratios, which do not require large driving forces (temperature differences) for heat transfer from the reactor to the jacket. Pilot- and full-scale reactors, however, often have a limited heat transfer capability. A process development engineer will usually have a choice of reactors when moving from the laboratory to the pilot plant. Kinetic and heat of reaction parameters obtained from the laboratory reactor, in conjunction with information on the heat transfer characteristics of each pilot plant vessel, can be used to select the proper pilot plant reactor. 

Assuming that the contribution made by the free oxygen was controlled by boundary layer diffusion [35] whereas those made by carbon dioxide and water vapour were controlled by the rates of surface reactions [57], the authors derived separate equations to calculate the components on the right-hand side of Equation (8.8). Based on laboratory examination results, the authors believed that when the steel was oxidized in dilute O2-N2 atmospheres, the oxidation rate followed a linear kinetics law until the scale thickness was 400-500 microns. Thereafter, the oxidation kinetics gradually changed from linear to parabolic. 

For a gas-liquid reaction which is gas-phase controlling, the chemical kinetics must be well understood. The importance of laboratory studies must therefore be emphasized. However, for successful scale-up, pilot plant studies are very critical because of the difficulties in reliably modeling gas behavior on a small scale (due to hydrodynamics) and its influence on reaction rates. 

Research and development determining chemical kinetic mechanisms and parameters from laboratory or pilot-plant reaction data exploring the effects of different operating conditions for optimization and control studies aiding in scale-up calculations. 

Laboratory Extractors. Pilot-Scale Testing, and Scale-Up. Several laboratory units arc useful in analysis, process control, and process studies. The AKUFVE contactor incorporates a separate mixer and centrifugal separator. It is an efficient instrument for rapid and accurate measurement of partition coefficients, as well as for obtaining reaction kinetic data. Miniature mixer-settler assemblies set up as continuous, bench-scale, multistage, countercurrent, liquid-liquid contactors are particularly useful Tor the preliminary laboratory work associated with flow-sheet development and optimization because these give a known number of theoretical stages. 

With miniaturized bioreactors, however, scale-up is a much simpler process. The procedure involves putting miniaturized bioreactors into series and/or parallel in order to produce larger output quantities. This approach keeps the reaction kinetics and hydrodynamics predictable for each component regardless of plant scale. Hence, the process is often referred to as numbering-up. In other words, the laboratory bioreactor is very similar to the industrial production, with the bioreactor quantity and controls being the most significant differences. This property keeps the start-up and development costs lower and more flexible (Ehrfeld et al., 2000 Watts and Wiles, 2007). 

---