Isothermal reactor, description

Continuously operated, fixed bed reactors are frequently used for kinetic measurements. Here the reactor is usually a cylindrical tube filled with catalyst particles. Feed of a known composition passes though the catalyst bed at a measured, constant flow rate. The temperature of the reactor wall is usually kept constant to facilitate an isothermal reactor operation. The main advantage of this reactor type is the wealth of experience with their operation and description. If heat and mass transfer resistances cannot be eliminated, they can usually be evaluated more accurately for packed bed reactors than for other reactor types. The reactor may be operated either at very low conversions as a differential reactor or at higher conversions as an integral reactor. 

In this chapter, the dynamics of ideally stirred tank reactors will be analyzed. First, the assumptions, required to limit model complexity, will be discussed. Next, various types of reaction will be considered such as simple first-order reactions, equilibrium reactions, parallel reactions, etc. Subsequently, the analysis will be expanded to include non-isothermal reactors. Numerical examples of chemical reactors are given and the non-linear model descriptions are compared with the linearized model descriptions. 

The reaction is also influenced by the heat of reaction developing during the conversion of the reactants, which is a problem in tubular screening reactors. In microstructures, the heat transport through the walls of the channels is facilitated by their small dimensions, which allows the development of isothermal reaction conditions. Thus, by decoupling the heat and mass balance, an analytical description of the flow in the screening reactor is achievable. 

Again, the simple isothermal 1-D plug-flow reactor model provides a good basis for quantitative descriptions. This model allows to explore the potential of using series connections of several membrane reactor segments. The corresponding mass balance for a component i and a segment k can be formulated as follows ... 

Description Hydrocarbon feed is preheated and desulfurized (1). Process steam, generated from process condensate in the isothermal shift reactor (5) is added to give a steam ratio of about 2.7 reformer feed is further preheated (2). Reformer (3) operates with an exit temperature of850°C. 

Description The main reactor (1) converts essentially all the feed isothermally in the liquid phase at a thermodynamically-favorable low temperature using a continuously-injected soluble catalyst. The catalyst s high activity allows use of relatively low hydrogen partial pressure, which results in fewer side reactions, e.g., isomerization... 

The reactors for the basic propane and n-butane pyrolysis were of monolithic annular quartz construction (Type I reactor). The reaction space was kept virtually isothermal by a surrounding bath of Ottawa sand fluidized vigorously by a stream of nitrogen. Temperature profiles were measured by calibrated Pt-Rh couples in a central thermowell. A description of this type of reactor has been given elsewhere (6). 

An example of a mapping from the equipment representation to the thermodynamic state representation is shown in Fig. 5. It represents an isothermal vertical packed-bed catalytic reactor equipped with temperature and pressure sensors, an explosion vent, and a distributor plate. Notice that the equipment and sensors are not associated with the state representation. They are contained in the base representation and reside in the process description at the equipment level. As discussed earlier, flow, work, heat, and mass interactions are all modeled independently. This allows us to evaluate independently the effect of these processes. Independent evaluation assists in the identification, evaluation, and assessment of event pathways leading to hazardous states. 

Polymerization reactions require stringent operating conditions for continuous production of quality resins. In this paper the chain-growth polymerization of styrene initiated with n-butyllithium in the presence of a solvent is described. A perfectly mixed isothermal, constant volume reactor is employed. Coupled kinetic relationships descriptive of the initiator, monomer, polystyryl anion and polymer mass concentration are simulated. Trommsdorff effects (1) are incorporated. Controlled variables include number average molecular weight and production rate of total polymer. Manipulated variables are flow rate, input monomer concentration, and input initiator concentration. The... 

We shall recapitulate the governing equations in the next section and discuss the economic operation in the one following. The results on optimal control are essentially a reinterpretation of the optimal design for the tubular reactor. We shall not attempt a full derivation but hope that the qualitative description will be sufficiently convincing. The isothermal operation of a batch reactor is completely covered by the discussion in Chap. 5 of the integration of the rate equations at constant temperature. The simplest form of nonisothermal operation occurs when the reactor is insulated and the reaction follows an adiabatic path the behavior of the reactor is then entirely similar to that discussed in Chap. 8. 

The statistical description of multiphase flow is developed based on the Boltzmann theory of gases [37, 121, 93, 11, 94, 58, 61]. The fundamental variable is the particle distribution function with an appropriate choice of internal coordinates relevant for the particular problem in question. Most of the multiphase flow modeling work performed so far has focused on isothermal, non-reactive mono-disperse mixtures. However, in chemical reactor engineering the industrial interest lies in multiphase systems that include multiple particle t3q)es and reactive flow mixtures, with their associated effects of mixing, segregation and heat transfer. 

For non-isothermal or non-linear chemical reactions, the RTD no longer suffices to predict the reactor outlet concentrations. From a Lagrangian perspective, local interactions between fluid elements become important, and thus fluid elements cannot be treated as individual batch reactors. However, an accurate description of fluid-element interactions is strongly dependent on the underlying fluid flow field. For certain types of reactors, one approach for overcoming the lack of a detailed model for the flow field is to input empirical flow correlations into so-called zone models. In these models, the reactor volume is decomposed into a finite collection of well mixed (i.e., CSTR) zones connected at their boundaries by molar fluxes.4 (An example of a zone model for a stirred-tank reactor is shown in Fig. 1.5.) Within each zone, all fluid elements are assumed to be identical (i.e., have the same species concentrations). Physically, this assumption corresponds to assuming that the chemical reactions are slower than the local micromixing time.5... 

The material presented in thi.s chapter has been for isothermal ideal reactors. We will build on the concepts developed in this chapter when we discuss nonideal reactors in Chapters 13 and 14. A number of practical guidelines for the operation of chemical reactors have been pre.senied over the years, and tables and some of these descriptions are summarized and presented on the CD-ROM and web. The articles are li.sted in Table 4-7. 

A description of the apparatus, gas analysis and conditioning of the catalyst are available in [5]. Essentially, rate measurements are conducted in a conventional, isothermal, fixed-bed tubular reactor. The gas analysis measures the composition and total flow of noncondensable gases (H2, C02 and CO) from the reactor. The catalyst used is the commercially available BASF K3-110. 

Finally a fourth boundary condition shall be valid to support the worst case character of the procedure. The reaction order necessary for the formal kinetic description of a process has a severe influence on the pressure/time and respectively the tempera-ture/time-profiles to be expected. Industrial experience has shown that approximately 90% of all processes conducted in either batch or semibatch reactors can be described with a second order formal kinetic rate law. But it remains uncertain whether this statement, which is related to isothermal or isoperibolic operation with a rather limited overheating, remains valid if the reaction proceeds adiabatically and if side reactions contribute to the gross reaction rate at a much higher degree. In consequence, it shall be assumed for a credible worst case evaluation that the disturbed process follows a first order kinetics. Any reactions occurring in reality will almost certainly proceed at a much lower rate. 

Kinetic and hydrodynamic analyses, and methods for the calculation of the parameters of industrial reactors are sufficiently developed today [2-6]. Computer simulation is also popular because if we know the kinetic and hydrodynamic parameters of processes and the principles of reactor behaviour, it is not a problem to calculate process characteristics and final product performance. This principle is an adequate tool for the description of low and medium rate chemical transformations with uniform concentration fields and isothermic conditions which are easy to achieve. In this case, it is easy to calculate and control all the characteristics of a chemical process under real conditions. 

In this chapter, a brief description of the main bio-SNG facilities and projects in Europe is presented as well as the main process units (gasification, gas cleaning and methana-tion) integrated for bio-SNG production. Therefore, a case study for bio-SNG production is modeled by using the CHEMCAD 6.3.1.4168 software. Two process technologies, a fixed (adiabatic case) or fluidized (isothermal) bed methanation reactors are considered, while three different product gas compositions from real biomass gasification data are fed as input syngas for the modeled system. Einally, CH4 yield and chemical efficiency of the different cases are compared and discussed. 

The fixed-bed laboratory reactor is regarded as an ideal isothermal plug flow reactor. The reactor model consists of the continuity equations for (1) N2, CO, NO, O2, CO2, N2O and NO2 in the gas phase, (2) surface species adsorbed on the noble metal surface, (3a) surface species adsorbed on the ceria surface, (3b) species in the ceria sub-layer, (4) CO2 adsorbed on the y-AI2O3 support. A detailed description can be found in [28]. 

The primary information about chemical transformations is obtained by the measmement of the chemical composition, that is, concentration measurements. The chemical processes occurring are complex and do not consist only of chemical reactions but also of physical phenomena, such as mass and heat transport. The major goal of chemical kinetic studies is to extract intrinsic kinetic information related to complex chemical reactions. Therefore, the transport regime in the reactor has to be well defined and its mathematical description has to be reliable. A typical strategy in kinetic experiments is the minimization of the effects of mass and heat transport on the rate of change of the chemical composition. In accordance with this, a kinetic experiment ideally has to fulfill two main requirements isothermicity of the active zone and uniformity of the chemical composition, which can be accomplished by, for example, perfect mixing within the reaction zone. 

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 isothermal and optimal pathway (and any other pathway) can be compared descriptively by a plot of SO2 conversion versus temperature and the curves of equal rate (Figure 6.3.9). The shape of these contours is dear At a constant conversion, the rate first increases with temperature but then decreases as the influence of the equilibrium becomes strong. The optimal pathway to minimize the reactor size (dashed line) is the one on top of the mountains. ... 

While transport effects may be eliminated in laboratory reactors, and experiments have shown that certain reactions oscillate under what may be considered isothermal and gradientless conditions, the Langmuir-Hinshelwood mechanism by itself with conventional mass-action kinetics does not give a satisfactory description of them. A number of "extra" features have been added in modeling studies reported in the literature. Among them are an activation energy which depends on the concentration of adsorbed species in one or more of the reaction steps [37, 52 - 54], transition between active and inactive forms of an adsorbed component [7, 17, 55, 56], and periodic switching of the reaction mechanism [16, 18, 40, 57]. 

A gaseous mixture of propylene and oxygen was fed to a reactor that contained a solid catalyst Ag/Si02 in the form of a very fine powder. The catalyst was suspended in liquid dibutyl phthalate. The unconverted oxygen and propylene, as well as the product propylene oxide and by-products such as carbon dioxide and water, left the reactor as a gas. The catalyst and the liquid remained in the reactor. The reactor was isothermal. The patent contains the following description of the reactor ... 

Chapter 2 presents calorimeters for measuring accurately the rate of heat release during discontinuous and continuous reactions versus time under isothermal and nonisothermal conditions. In addition, the chapter contains a description of an apparatus that can be used to record online the rate of heat release within a stirred tank reactor during a reaction. 

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