Chemical reactors Kinetics

Next we need the reaction rates, and chemical reaction is the only aspect that differentiates a multiphase reactor from a separation unit. As with all chemical reactors, kinetics are the most difficult quantities to obtain in describing a reactor accurately. Frequently the rates are... 

In an energy balance over a volume element of a chemical reactor, kinetic, potential, and work terms may usually be neglected relative to the heat of reaction and other heat transfer terms so that the balance reduces to ... 

As shown on the CD-ROM that accompanies this book (following the links HYSYS — Chemical Reactors Setting Up Reactors —> CSTR or ASPEN —> Chemical Reactors Kinetic Reactors — CSTRs RCSTR), analysis of this process shows the possibility of multiple steady states. For example, at a water flow rate of 400 kmol/hr, the following steady states are obtained (1) conversion of 83% with an effluent temperature of 62°C, (2) conversion of 45% with an effluent temperature of 44°C, and (3) conversion of 3% with an effluent temperature of 25°C. The intermediate steady state at 45% conversion is unstable, while the other two steady states are stable. Furthermore, a controllability and resiliency (C R) analysis on this process is carried out in Case Study 21.1, where a design involving a single CSTR is compared with one utilizing two CSTRs in series. ... 

As discussed in the previous chapter, the two most common conversion variables employed in chemical reactor/kinetic studies are a and X. The term a is employed to represent the change in the number of moles of a particular species due to chemical reaction. The most conunonly used conversion variable is X, and it is used to represent the change in the number of moles of a particular species (say A) relative to the number of moles of A initially present or initially introduced (to a flow reactor). Thus,... 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

The concept of macroscopic kinetics avoids the difficulties of microscopic kinetics [46, 47] This method allows a very compact description of different non-thennal plasma chemical reactors working with continuous gas flows or closed reactor systems. The state of the plasma chemical reaction is investigated, not in the active plasma zone, but... 

Stanley M. Walas, Ph.D., Professor Emeritus, Department of Chemical and Petroleum Engineering, University of Kansas Fellow, American Institute of Chemical Engineers (Section 7, Reaction Kinetics Section 23, Chemical Reactors)... 

Hill, An Introduction to Chemical Engineeting Kinetics and Reactor Design, Wiley, 1977. 

Walas Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959 Butterworths, 1989, pp. 153-164), and Rase Chemical Reactor Design for Process Plants, vol. 1, Wiley, 1977, pp. 178-191). 

A number of factors limit the accuracy with which parameters for the design of commercial equipment can be determined. The parameters may depend on transport properties for heat and mass transfer that have been determined under nonreacting conditions. Inevitably, subtle differences exist between large and small scale. Experimental uncertainty is also a factor, so that under good conditions with modern equipment kinetic parameters can never be determined more precisely than 5 to 10 percent (Hofmann, in de Lasa, Chemical Reactor Design and Technology, Martinus Nijhoff, 1986, p. 72). 

Hofmann, Tndustrial process kinetics and parameter estimation , in ACS Advances in Chemlstiy, 109, 519-534 (1972) "Kinetic data analysis and parameter estimation , in de Lasa, ed.. Chemical Reactor De.sign and Technology, Martinus Nijhoff, 1986, pp. 69-105. 

Cropley, J.B., Systematic Errors in Recycle Reactor Kinetic Studies, Chemical Engineeiing Piogiess, February 1987, 46-51. (Model building, experimental design)... 

Knowledge of the rate is important to design chemical reactors for industrial production. It is also important for optimizing the production and to define the safety limits of operation. As was mentioned in the introduction, various transfer processes can influence chemical rates. The recognition of such interference is of primary importance during any study of kinetics, especially in those studies that will serve as the basis of design for production reactors. 

The two main principles involved in establishing conditions for performing a reaction are chemical kinetics and thermodynamics. Chemical kinetics is the study of rate and mechanism by which one chemical species is converted to another. The rate is the mass in moles of a product produced or reactant consumed per unit time. The mechanism is the sequence of individual chemical reaction whose overall result yields the observed reaction. Thermodynamics is a fundamental of engineering having many applications to chemical reactor design. 

Fig ure 4-16. Trickle-bed (tubular reactor) for hydrodesulfurization. (Source J. M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill, Inc., 1981.)... 

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

This response time should be compared to the turbulent eddy lifetime to estimate whether the drops will follow the turbulent flow. The timescale for the large turbulent eddies can be estimated from the turbulent kinetic energy k and the rate of dissipation e, Xc = 30-50 ms, for most chemical reactors. The Stokes number is an estimation of the effect of external flow on the particle movement, St = r /tc. If the Stokes number is above 1, the particles will have some random movement that increases the probability for coalescence. If St 1, the drops move with the turbulent eddies, and the rates of collisions and coalescence are very small. Coalescence will mainly be seen in shear layers at a high volume fraction of the dispersed phase. 

G.H. Charles, An Int. to Chemical Engin. Kinetics Reactor Design, John-Wiley Sons, 1997. 

For this purpose, cylindrical channels have been assumed. In randomly packed fixed beds the porosity is about 0.4, from which the relationship dp = 2.25 d is obtained. Since the focus is on heterogeneously catalyzed gas-phase reactions, it is important to not only ensure comparable conditions from a hydrodynamic point of view, but also as far as chemical reaction kinetics is concerned. Therefore, it is assumed that both reactors contain the same amount of catalyst. 

Most accidents in the chemical and related industries occur in batch processing. Therefore, in Chapter 5 much attention is paid to theoretical analysis and experimental techniques for assessing hazards when scaling up a process. Reaction calorimetry, which has become a routine technique to scale up chemical reactors safely, is discussed in much detail. This technique has been proven to be very successful also in the identification of kinetic models suitable for reactor optimization and scale-up. 

Modelling can at least facilitate the determination of the most effective scale-up program. Information from three fields is needed for modelling (1) chemical kinetics, (2) mass transfer, and (3) heat transfer. The importance of information for different processes has been qualitatively evaluated (see Table 5.3-5). Obviously, sufficiently accurate information on heat transfer is needed for batch reactors, which are of great interest for fine chemicals manufacture. Kinetic studies and modelling requires much time and effort. Therefore, the kinetics often is not known. Presently, this approach is winning in the scale-up of processes for bulk chemicals. The tools developed for scale-up of processes for bulk chemicals have been proven to be very useful. Therefore, the basics of this approach will be discussed in more detail in subsequent sections. 

A survey of the mathematical models for typical chemical reactors and reactions shows that several hydrodynamic and transfer coefficients (model parameters) must be known to simulate reactor behaviour. These model parameters are listed in Table 5.4-6 (see also Table 5.4-1 in Section 5.4.1). Regions of interfacial surface area for various gas-liquid reactors are shown in Fig. 5.4-15. Many correlations for transfer coefficients have been published in the literature (see the list of books and review papers at the beginning of this section). The coefficients can be evaluated from those correlations within an average accuracy of about 25%. This is usually sufficient for modelling of chemical reactors. Mathematical models of reactors arc often more sensitive to kinetic parameters. Experimental methods and procedures for parameters estimation are discussed in the subsequent section. 

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