Heterogeneous reactors isothermal

An isothermal batch reactor produces a fairly narrow MWD except possibly at high conversions. Conditions will vary from the start to the end of a batch reaction, and the MWD will broaden. The dynamic chain length of a free-radical polymer will because of changes in monomer concentration and initiation rate, although usually not by much when measured by the cumulative In and Iw of the finished polymer. When the catalyst is heterogeneous, an isothermal batch reactor will produce... 

The work is organized in two parts in the first part kinetics is presented focusing on the reaction rates, the influence of different variables and the determination of specific rate parameters for different reactions both homogeneous and heterogeneous. This section is complemented with the classical kinetic theory and in particular with many examples and exercises. The second part introduces students to the distinction between ideal and non-ideal reactors and presents the basic equations of batch and continuous ideal reactors, as well as specih c isothermal and non-isothermal systems. The main emphasis however is on both qualitative and quantitative interpretation by comparing and combining reactors with and without diffusion and mass transfer effects, complemented with several examples and exercises. Finally, non-ideal and multiphase systems are presented, as well as specific topics of biomass thermal processes and heterogeneous reactor analyses. The work closes with a unique section on the application of theory in laboratory practice with kinetic and reactor experiments. 

The previous description of the processes considered in a fixed-bed reactor leads to a heterogeneous non-isothermal 2D model (symmetry with respect to the angular coordinate is assumed). The unsteady state mass and energy balances for a single reactant species with dimensional concentration (subscript i is omitted for convenience) and appropriate boundary conditions are given in Table 3.1. The independent variables are normalized as follows (see Figure 3.2) ... 

The yield of a gas-solid heterogeneous reaction depends not on the total time that molecules spend in the reactor but on the time that they spend on the catalyst surface. The contact time distribution provides a standardized measure of times spent in the absorbed state. A functional definition is provided by the following equation applicable to a first-order, heterogeneous reaction in an isothermal reactor ... 

Restelli and Coull [AIChE J., 72 (292), 1966] have studied the transmethylation reaction of dimethylamine in a differential flow reactor using montmorillonite as a catalyst. They measured initial reaction rates under isothermal conditions for this heterogeneous catalytic process. Steady-state operating data were recorded. 

Vocabulary of Terms Used in Reactor Design. There are several terms that will be used extensively throughout the remainder of this text that deserve definition or comment. The concepts involved include steady-state and transient operation, heterogeneous and homogeneous reaction systems, adiabatic and isothermal operation, mean residence time, contacting and holding time, and space time and space velocity. Each of these concepts will be discussed in turn. 

In general, if heterogeneous catalytic reactions are to be conducted isothermally, the reactor design must provide for heat flow to or from the particles of catalyst so as to keep the thermal gradients small. Otherwise, temperatures within the catalyst bed will be non-uniform. The differential reactor and the various forms of the gradientless reactors are advantageous in this regard. 

In a recent survey [19] it was noted that a realistic model for catalytic oxidation reactions must include equations describing the evolution of at least two concentrations of surface substances and account for the slow variation in the properties of the catalyst surface (e.g. oxidation-reduction). For the synchronization of the dynamic behaviour for various surface domains, it is necessary to take into consideration changes in the concentrations of gas-phase substances and the temperature of the catalyst surface. It is evident that, in the hierarchy of modelling levels, such models must be constructed and tested immediately after kinetic models. On the one hand, the appearance of such models is associated with the experimental data on self-oscillations in reactors with noticeable concentration variations of the initial substances and products (e.g. ref. 74) on the other hand, there was a gap between the comprehensively examined non-isothermal models with simple kinetics and those for the complex heterogeneous catalytic reactions... 

The differential equation can be transferred into its dimensionless form by dividing Eq. 8.19 by the feed concentration cfeed and the solid flow rate Vsoiid and multiplying by the section length Isec. Assuming a heterogeneously catalyzed reversible reaction as well as linear adsorption isotherms, the following dimensionless differential equations describe the behavior of the TMB reactor ... 

Tubular reactors are commonly used in laboratory, pilot plant, and commercial-scale operations. Because of their versatility, they are used for heterogeneous reactions as well as homogeneous reactions. They can be run with cocurrent or counter-current flow patterns. They can be run in isothermal or adiabatic modes and can be used alone, in series, or in parallel. Tubular reactors can be empty, packed with inert materials for mixing, or packed with catalyst for improved reactions. It is often the process that will dictate the design of the reactor, as discussed in this entry. 

A tank reactor and separator (Fig. 12-6) are used to study the heterogeneous reaction between pure liquid A (phase 1) and reactant B dissolved in phase 2 (also liquid). The solvent in phase 2, reactant B, and the products of reactior are all insoluble in liquid A. No reaction occurs in the separator. The reactor operates isothermally at 25°C, and at this temperature A has a limited solubility in phase 2, the value being 2.7 x 10 g mole/liter. Phase 2 is dispersed aj bubbles in continuous phase 1, which is recycled. There is excellent stirring in the reactor, but the fluid motion within the bubbles of phase 2 is insufficien to prevent some mass-transfer resistance. From independent measurements it is estimated that at average conditions the reaction resistance within the bubbles is 75% of the total resistance (mass-transfer plus reaction resistance) (n) Derive a relationship between the concentration of reactant B entering the reactor in phase 2 and the concentration leaving the separator. 

Equations (13-16a) are of the same form as the equations for homogeneous plug-flow reactors [Eq. (4-5)] they are the constant-density version because the system is isothermal, and because for gaseous reactions no allowance has been made for a change in number of moles. Equation (12-2) is the analog of Eq. (3-13) for heterogeneous reactions and is applicable for variable-density conditions. The application of these equations to reactor design is the same as discussed, for example, in Example 4-6. 

H. Redlingshofer, O. Krocher, W. Bock, K. Huthmacher, G. Emig, Catalytic wall reactor as a tool for isothermal investigations in the heterogeneously catalyzed oxidation of propene to acrolein, Ind. Eng. Chem. Res. 41 (2002) 1445. 

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