Steady State Simulation of Continuous Stirred Tank Reactor (CSTR) System Using Aspen Plus
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PDF (Chapter 5
~---=---5 __ Heterogeneous Catalysis 5.1 I Introduction Catalysis is a term coined by Baron J. J. Berzelius in 1835 to describe the property of substances that facilitate chemical reactions without being consumed in them. A broad definition of catalysis also allows for materials that slow the rate of a reac tion. Whereas catalysts can greatly affect the rate of a reaction, the equilibrium com position of reactants and products is still determined solely by thermodynamics. Heterogeneous catalysts are distinguished from homogeneous catalysts by the dif ferent phases present during reaction. Homogeneous catalysts are present in the same phase as reactants and products, usually liquid, while heterogeneous catalysts are present in a different phase, usually solid. The main advantage of using a hetero geneous catalyst is the relative ease of catalyst separation from the product stream that aids in the creation of continuous chemical processes. Additionally, heteroge neous catalysts are typically more tolerant ofextreme operating conditions than their homogeneous analogues. A heterogeneous catalytic reaction involves adsorption of reactants from a fluid phase onto a solid surface, surface reaction of adsorbed species, and desorption of products into the fluid phase. Clearly, the presence of a catalyst provides an alter native sequence of elementary steps to accomplish the desired chemical reaction from that in its absence. If the energy barriers of the catalytic path are much lower than the barrier(s) of the noncatalytic path, significant enhancements in the reaction rate can be realized by use of a catalyst. This concept has already been introduced in the previous chapter with regard to the CI catalyzed decomposition of ozone (Figure 4.1.2) and enzyme-catalyzed conversion of substrate (Figure 4.2.4). -
Reaction Kinetics
1 Reaction Kinetics Dr Claire Vallance First year, Hilary term Suggested Reading Physical Chemistry, P. W. Atkins Reaction Kinetics, M. J. Pilling and P. W. Seakins Chemical Kinetics, K. J. Laidler Modern Liquid Phase Kinetics, B. G. Cox Course synopsis 1. Introduction 2. Rate of reaction 3. Rate laws 4. The units of the rate constant 5. Integrated rate laws 6. Half lives 7. Determining the rate law from experimental data (i) Isolation method (ii) Differential methods (iii) Integral methods (iv) Half lives 8. Experimental techniques (i) Techniques for mixing the reactants and initiating reaction (ii) Techniques for monitoring concentrations as a function of time (iii) Temperature control and measurement 9. Complex reactions 10. Consecutive reactions 11. Pre-equilibria 12. The steady state approximation 13. ‘Unimolecular’ reactions – the Lindemann-Hinshelwood mechanism 14. Third order reactions 15. Enzyme reactions – the Michaelis-Menten mechanism 16. Chain reactions 17. Linear chain reactions The hydrogen – bromine reaction The hydrogen – chlorine reaction The hydrogen-iodine reaction Comparison of the hydrogen-halogen reactions 18. Explosions and branched chain reactions The hydrogen – oxygen reaction 19. Temperature dependence of reaction rates The Arrhenius equation and activation energies Overall activation energies for complex reactions Catalysis 20. Simple collision theory 2 1. Introduction Chemical reaction kinetics deals with the rates of chemical processes. Any chemical process may be broken down into a sequence of one or more single-step processes known either as elementary processes, elementary reactions, or elementary steps. Elementary reactions usually involve either a single reactive collision between two molecules, which we refer to as a a bimolecular step, or dissociation/isomerisation of a single reactant molecule, which we refer to as a unimolecular step. -
Steady State, Quasi-Equilibrium, and Transition State Theory
Manuscript ID: ed-2016-00957b.R2 (Revised) Some Considerations on the Fundamentals of Chemical Kinetics: Steady State, Quasi-Equilibrium, and Transition State Theory Joaquin F. Perez-Benito* Departamento de Ciencia de Materiales y Quimica Fisica, Seccion de Quimica Fisica, Facultad de Quimica, Universidad de Barcelona, Marti i Franques 1, 08028 Barcelona, Spain 1 ___________________________________________________________________________ ABSTRACT: The elementary reaction sequence A I Products is the simplest mechanism for which the steady-state and quasi-equilibrium kinetic approximations can be applied. The exact integrated solutions for this chemical system allow inferring the conditions that must fulfil the rate constants for the different approximations to hold. A graphical approach showing the behavior of the exact and approximate intermediate concentrations might help to clarify the use of these methods in the teaching of chemical kinetics. Finally, the previously acquired ideas on the approximate kinetic methods lead to the proposal that activated complexes in steady state rather than in quasi-equilibrium with the reactants might be a closer to reality alternative in the mathematical development of Transition State Theory (TST), leading to an expression for the rate constant of an elementary irreversible reaction that differs only in the factor 1 ( being the transmission coefficient) with respect to that given by conventional TST, and to an expression for the equilibrium constant of an elementary reversible reaction more compatible -
Multidimensional Reacting Flow Simulations with Coupled Homogeneous and Heterogeneous Models
Multidimensional Reacting Flow Simulations with Coupled Homogeneous and Heterogeneous Models A Dissertation Presented to the Faculty of the School of Engineering and Applied Science University of Virginia In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Mechanical and Aerospace Engineering by Ryan Frederick Johnson September 2014 Approval Sheet This dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical and Aerospace Engineering Ryan Frederick Johnson This dissertation has been read and approved by the Examining Committee: Prof. Hossein Haj-Hariri, Committee Chairman Prof. Harsha K. Chelliah, Dissertation Advisor Prof. Leonid Zhigilei Prof. Houston G. Wood Prof. James McDaniel Accepted for the School of Engineering and Applied Science: Dean, School of Engineering and Applied Science November 2014 The coupling of homogeneous (gas phase) and heterogeneous (solid phase) reactions is found in various phenomena, from the burning of wood in a fireplace to the ablating heat shield of the space shuttle flying at hypersonic speeds. With the goal of understand- ing the complex coupling mechanisms in such flows, this dissertation focuses developing a numerical model of low-Mach number reacting flows with coupled homogeneous and heterogeneous reactions. Numerical studies were performed in collaboration with the Air Force Institute of Technology, with the objective of guiding experimental design to investi- gate high-energy laser interactions with surfaces. The surfaces considered so far are porous carbon samples due to the well-established literature on heterogeneous reaction rates. In this study, the reacting carbon surfaces considered were two flow configurations: par- allel flow over a reacting boundary layer (flat plate) and impinging stagnation point flow (stagnation flow). -
Fundamentals of Chemical Reactor Theory
UNIVERSITY OF CALIFORNIA, LOS ANGELES Civil & Environmental Engineering Department Fundamentals of Chemical Reactor Theory Michael K. Stenstrom Professor Diego Rosso Teaching Assistant Los Angeles, 2003 Introduction In our everyday life we operate chemical processes, but we generally do not think of them in such a scientific fashion. Examples are running the washing machine or fertilizing our lawn. In order to quantify the efficiency of dirt removal in the washer, or the soil distribution pattern of our fertilizer, we need to know which transformation the chemicals will experience inside a defined volume, and how fast the transformation will be. Chemical kinetics and reactor engineering are the scientific foundation for the analysis of most environmental engineering processes, both occurring in nature and invented by men. The need to quantify and compare processes led scientists and engineers throughout last century to develop what is now referred as Chemical Reaction Engineering (CRE). Here are presented the basics of the theory and some examples will help understand why this is fundamental in environmental engineering. All keywords are presented in bold font. Reaction Kinetics Reaction Kinetics is the branch of chemistry that quantifies rates of reaction. We postulate that an elementary chemical reactionI is a chemical reaction whose rate corresponds to a stoichiometric equation. In symbols: A + B C + D [1] I for our purpose we will limit our discussion to elementary reactions 1 Stenstrom, M.K. & Rosso, D. (2003) Fundamentals of Chemical Reactor Theory and the reaction rate will be defined as: -r = k · (cA) · (cB) where k is referred as the specific reaction rate (constant). -
CHAPTER 5:Chemical Kinetics
CHAPTER 5:Chemical Kinetics Copyright © 2020 by Nob Hill Publishing, LLC The purpose of this chapter is to provide a framework for determining the reaction rate given a detailed statement of the reaction chemistry. We use several concepts from the subject of chemical kinetics to illustrate two key points: 1 The stoichiometry of an elementary reaction defines the concentration dependence of the rate expression. 2 The quasi-steady-state assumption (QSSA) and the reaction equilibrium assumption allow us to generate reaction-rate expressions that capture the details of the reaction chemistry with a minimum number of rate constants. 1 / 152 The Concepts The concepts include: the elementary reaction Tolman's principle of microscopic reversibility elementary reaction kinetics the quasi-steady-state assumption the reaction equilibrium assumption The goals here are to develop a chemical kinetics basis for the empirical expression, and to show that kinetic analysis can be used to take mechanistic insight and describe reaction rates from first principles. 2 / 152 Reactions at Surfaces We also discuss heterogeneous catalytic adsorption and reaction kinetics. Catalysis has a significant impact on the United States economy1 and many important reactions employ catalysts.2 We demonstrate how the concepts for homogeneous reactions apply to heterogeneously catalyzed reactions with the added constraint of surface-site conservation. The physical characteristics of catalysts are discussed in Chapter 7. 1Catalysis is at the heart of the chemical industry, with sales in 1990 of $292 billion and employment of 1.1 million, and the petroleum refining industry, which had sales of $140 billion and employment of 0.75 million in 1990 [13]. -
Chemical Kinetics 1 Announcements Course Format: the Overall Format of the Course Is Similar to CHEM 1000
Announcement CHEM 1001N quizzes Surnames A - M: Stedman Lecture Hall D Surnames N - Z: Accolade East 001 Those who will be writing in Accolade East 001 should go and find that room NOW because they will not be given extra time on the day of the quiz if they show up late. Accolade East is still under construction and the only entrance that appears to be accessible is on the northeast corner. This building is across the street from Shulich, near the Student Services Centre. Room 001 is in the basement. CHEM 1001 3.0 Section N Chemical Kinetics 1 Announcements Course format: the overall format of the course is similar to CHEM 1000 Textbook: Petrucci, Harwood and Herring (8th edition) Section N specific material: www.chem.yorku.ca/profs/krylov/ Attending lectures and tutorial is mandatory Questions will be addressed during and after lectures Quizzes will be held during tutorial times on January 31, February 21, March 7, and March 21 CHEM 1001 3.0 Section N Chemical Kinetics 2 Chemical Kinetics Problem Set: Chapter 15 questions 29, 33, 35, 43a, 44, 53, 63, 80 CHEM 1001 3.0 Section N Chemical Kinetics 3 Semantics • Adjective “kinetic” originates from Greek “kinetikos” that, in turn, originates from Greek “kinetos’ which means “moving”. • Noun “kinetics” is used in a singular form only. Example: “The kinetics of this process is fast” • In general, the word “kinetics” is used in physical and life sciences to represent the dependence of something on time. Example: “Kinetics of substance uptake by a cell was measured” • Chemical kinetics is a branch of chemistry which is concerned with the rates of change in the concentration of reactants in a chemical reaction.