Reactors for determination of kinetics

P.C. Borman et. al. A novel reactor for determination of kinetics for solid catalyzed gas reactions, AIChE Journal, (1994) 40, 862-869. Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1994 AIChE. All rights reserved. 

Fig. 3.1.3-1 Laboratory reactors for determining the kinetics of heterogeneously catalyzed reactions [Luft 1987a, Forni 1997, Cavalli 1997] ...

Additionally, online monitoring methods have been developed to adapt off-line characterization methods into in situ (i.e., in-reactor) probes for determination of kinetics and monomer conversion with optical methods such as mass spectroscopy (MS), ESR, FTIR, near IR, and Raman spectroscopy. However, frequently, due to high turbidity and viscosity of the polymer reaction milieu, the optical surfaces are easily fouled, leading to frequent sensor failure. Furthermore, data acquired with these probes are model dependent the empirical and inferential calibration schemes used can be expensive and time consuming to develop and can drift and become unreliable as reactor conditions change and as sensors become fouled. Another limiting feature of these methods is that they usually measure only one characteristic of the reaction, such as monomer conversion and are not directly sensitive to polymer molecular mass and intrinsic viscosity. More detailed discussion of these techniques can be found in Chapters 6-10 of this book. 

Zauner and Jones (2000a) describe an experimental set-up for determination of precipitation kinetics, as shown in Figure 6.19. Briefly, the jacket glass reactor (1) (300 ml, d = 65 mm) is equipped with a polyethylene draft tube and four baffles. The contents are stirred using a three-blade marine-type propeller (5) with motor (Haake), which pumps the suspension upwards in the annulus and downwards inside the draft tube. Measured power inputs ranged from 3.3 X 10- to 1.686 W/kg. 

Micro reactors show, under certain conditions, low axial flow dispersion reactions with unstable intermediates can be carried out in a fast, stepwise manner on millisecond time-scales. Today s micro mixers mix on a millisecond scale and below [40]. Hence in micro reactors reactions can be carried out in the manner of a quench-flow analysis, used for determination of fast kinetics [93]. 

Stirred suspensions of droplets have proven to be a popular approach for studying the kinetics of liquid-liquid reactions [54-57]. The basic principle is that one liquid phase takes the form of droplets in the other phase when two immiscible liquids are dispersed. The droplet size can be controlled by changing the agitator speed. For droplets with a diameter < 0.15 cm the inside of the drop is essentially stagnant [54], so that mass transfer to the inside surface of the droplet occurs only by diffusion. In many cases, this technique can lack the necessary control over both the interfacial area and the transport step for determination of fundamental interfacial processes [3], but is still of some value as it reproduces conditions in industrial reactors. 

To obtain experimental data suitable for the determination of kinetic parameters, two different operation modes of reactors should be considered, the differential and the integral mode. In the differential mode rates are calculated from small conversions (<10%) within a fixed time span dt. For reactions with two reactants, it is suitable to use excess of one reactant to suppress undesired side reactions. 

The evaluation of catalyst effectiveness requires a knowledge of the intrinsic chemical reaction rates at various reaction conditions and compositions. These data have to be used for catalyst improvement and for the design and operation of many reactors. The determination of the real reaction rates presents many problems because of the speed, complexity and high exo- or endothermicity of the reactions involved. The measured conversion rate may not represent the true reaction kinetics due to interface and intraparticle heat and mass transfer resistances and nonuniformities in the temperature and concentration profiles in the fluid and catalyst phases in the experimental reactor. Therefore, for the interpretation of experimental data the experiments should preferably be done under reaction conditions, where transport effects can be either eliminated or easily taken into account. In particular, the concentration and temperature distributions in the experimental reactor should preferably be described by plug flow or ideal mixing models. 

The most important methods for the determination of kinetics of catalyzed reactions are described here. We emphasize the problems and pitfalls in obtaining reliable reaction rates. The many diagnostic tests are briefly discussed and some warnings are given to limitations of commonly used laboratory reactors. Finally, it is worth noting that reaction rates can be expressed per unit mass of catalyst, per unit catalytic surface, per unit external particle area or per unit volume of the reactor, fluid or catalyst. For chemical reactor design it is best to express reaction rates in terms of unit catalyst volume. 

Conversion and coke formation during catalytic ethene oligomerization catalyzed by HZSM-5 have been investigated in the TEOM and in a conventional gravimetric microbalance under similar conditions (2). The results show that the TEOM is a powerful tool for determination of the kinetics of deactivation of catalysts, with a design that makes determination of the true space velocity (or space time) easy. The TEOM combines the advantages of the conventional microbalance with those of a fixed-bed reactor, and the same criteria can be used to check for plug flow and differential operation. 

Knowledge of the reaction kinetics is important for designing industrial ammonia synthesis reactors, for determining the optimal operating conditions, and for computer control of ammonia plants. This means predicting the technical dependence on operating variables of the rate of formation of ammonia in an integral catalyst volume element of a converter. 

Enzyme thermistors have also found applications in more research-related topics, such as the direct estimation of the intrinsic kinetics of immobilized bio-catalysts [64]. Here, the enzyme thermistor offered a rapid and direct method for the determination of kinetic constants (K , Km and Vm) for immobilized enzymes. For the system being investigated, saccharose and immobilized invertase, the results obtained with the enzyme thermistor and with an independent differential reactor system were in very good correlation, within a flow-rate range of 1 to 1.5 ml/min. 

First one component is fed into the reactor column in a pulsed maimer, then a second one after a certain interval. The reaction proceeds during the time interval when the second component overtakes the first one in the column. The use of this method as applied to the qualitative identification of sample components has been described elsewhere [41]. The application of the method for determining the kinetic characteristics was proposed by Berezkin [55]. The method was developed in earlier work [61 ]. 

Fig. 3.1.3-2 Types of difFerential circulating reactors with catalyst beds [Buzzi-Ferraris 1999, Perego 1999, Forni 1997] for determining the kinetics of heterogeneously catalyzed reactions, a) Spinning-basket principle a rotating basket containing catalyst pellets acts as a stirrer, b) Jet principle with internal recirculation [Luft 1973b, Luft 1978, Dreyer 1982]. c) External recirculation.

Once ku has been experimentally determined (see section 3.5.2), the curve of reactor operation (X vs t) can be obtained for a certain enzyme concentration (meat)-Eq. 5.69 also allows reactor design (determination of reactor volume), since meat is simply the ratio of enzyme load to reaction volume (Mcat/VR). Simulation of batch bioreactor operation under different scenarios of enzyme inactivation is presented in Fig. 5.16 for simple Michaelis-Menten kinetics (a = 14-K/Si b = -1 c = 0) with Si/K =10. Enzyme load in the reactor was calculated to obtain 90% conversion after 10 h of reaction under no inactivation. The strong impact of enzyme inactivation on bioreactor performance can be easily appreciated. 

In most cases, a continuous reactor is used and the reactions are carried out in gas phase. Under these conditions, volnme change may occur due to expansion or contraction of volume. Analogously, we can nse the integral or differential methods. For the determination of kinetic parameters, a differential reactor is used. 

The determination of kinetic parameters is an essential step in developing a catalytic process. Parameters determined in laboratory-scale steady-state reactors are necessary to formulate models for scale-up to pilot plant and process-scale reactors. Kinetic parameters also provide insight into the fundamental processes that occur during a catalytic reaction and form the basis for creating microkinetic models that describe the individual steps (e.g., adsorption, surface reaction, and desorption) of a complex reaction. 

Polythermal methods have in common that a suspension containing known amounts of solvent and solute in excess is heated and the temperature where last particles dissolve is detected. For detection, visual observation (e.g., under a microscope), turbidity measurements, particle-detecting inline probes (e.g., FBRM probe (Lasentec , Mettler Toledo GmbH)), or calorimetry may be used. Since it is a dynamic method, the results depend on dissolution kinetics of the particular system. In general, polythermal measurements are easier to automate since just a temperature has to be followed and no special analytical technique is required. The above-mentioned Crystall6 multiple reactor system can also be used to perform such kind of measurements. To detect both the dissolution process for derivation of saturation temperatures (clear points) and the formation of particles (cloud points) for determination of the metastable zone width, the... 

A rotating enzyme-immobilized reactor and a flat pH electrode were incorporated into a sealed cell for use under continuous-flow/stopped-flow (SF) operation for the rapid determination of penicillins G and V in tablets and injectables [50]. A co-immobilization in a rotating bioreactor and amperometric detector resulted in a sensitive system for determination of succinylcholine and acetylcholine in pharmaceutical preparations [51]. A tandem system incorporating two rotating bioreactors into a continuous-flow/SF sample/reagentprocessing setup was apphed for the determination of alkaline phosphatase activity in serum samples [52]. By functional combination of the SF and flow-injection analysis (FIA), an automated micro apparatus was constructed resulting in significant reduction of the injection volumes of enzyme and substrate [53]. SF/continuous flow methods were apphed to acquire kinetic information also [54, 55]. 

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