Physics
Thermodynamic and Kinetic Control
Thermodynamic control refers to the outcome of a reaction being determined by the stability of the products, while kinetic control involves the influence of reaction rates on the outcome. In a thermodynamically controlled process, the most stable product is favored, whereas in a kinetically controlled process, the product formed is determined by the rate of the reaction. These concepts are fundamental in understanding chemical and physical processes.
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7 Key excerpts on "Thermodynamic and Kinetic Control"
eBook - ePub- Stanislaw Sieniutycz(Author)
- 2016(Publication Date)
- Elsevier(Publisher)
Chapter 9Thermodynamic Controls in Chemical Reactors
Abstract
In this chapter we discuss various theoretical and practical aspects of chemical reactor analysis, synthesis, and optimization. In particular we stress the role of dynamic modeling and optimal control methods in optimization of chemical, electrochemical, and biological reactors. We analyze in sequence: stoichiometric coefficients, driving forces of transport and rate processes, nonlinear macrokinetics of chemical processes, chemical Ohm’s law, heterogeneities, stability, instabilities, limit cycles, chemical fluctuations, and the role of chaos and fractals in the chemical word. Particular attention is given to the power yield problems in chemical engines (the second part of this chapter).Rate and transport equations in the first part contain terms exponential with respect to Planck’s potentials and temperature reciprocal. In each elementary step, of transport or rate nature, kinetic mass action law leads to the identification of two competing unidirectional fluxes. While they are equal in the thermodynamic equilibrium, their nonvanishing difference off the equilibrium constitutes the observed flux of the resulting rate of the process. A generalized affinity emerges as a suitable driving force. Correspondence with Butler–Volmer kinetics is shown in electrochemical systems. Approaching the thermodynamic equilibrium the theory converges to the standard linear kinetics described by the Onsager’s theory.In the second part we develop the method of chemical power maximization, similar to that developed earlier for thermal machines. With the thermodynamic knowledge and dynamic optimization (especially by the method of dynamic programming), kinetic limits are estimated for the optimal work function W max that describes integrated power output from the system and generalizes familiar reversible work W rev
eBook - PDFExperimental Organic Chemistry
A Miniscale & Microscale Approach
- John Gilbert, Stephen Martin(Authors)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
In the procedures that follow, you will have the opportunity to determine whether reactions are being conducted under thermodynamic or kinetic control using sev-eral experimental techniques. Figure 13.2 Reaction profile that predicts different products from kinetic and thermodynamic control of competing reactions. H Y W + X Z Y H Z Reaction Progress Potential Energy H ‡ Z H ‡ Y Y ‡ Z ‡ ∇ ∇ ∇ ∇ Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 446 Experimental Organic Chemistry ■ Gilbert and Martin 13.2 F O R M A T I O N O F S E M I C A R B A Z O N E S U N D E R K I N E T I C A N D T H E R M O D Y N A M I C C O N T R O L The principle of kinetic and thermodynamic control in organic chemistry may be illustrated by studying the competing reactions of semicarbazide ( 1 ) with two car-bonyl compounds, cyclohexanone ( 2 ) and 2-furaldehyde ( 4 ), as shown in Equations 13.3 and 13.4, respectively. In this case, one compound, semicarbazide, reacts with two different compounds, cyclohexanone and 2-furaldehyde, to give two differ-ent semicarbazones, 3 and 5, respectively. Both products are crystalline solids that have distinctive melting points by which they may be easily identified. Thus, it is easy to determine experimentally which compound is the product of kinetic control and which is the product of thermodynamic control.
eBook - PDF- D.A. Taylor(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
6 Process and kinetic control Closed-loop control systems may be divided into kinetic and process control. A third category of electrical control, consisting of voltage and current regulators and feedback amplifiers, will not be considered separately as these units normally form part of kinetic and process control systems. A kinetic control system is used to control displacement, velocity or acceleration of a mechanical component. Kinetic control systems are often referred to as servomechanisms or simply servos. Process control is used to maintain at some desired value a variable such as temperature, pressure, flow or liquid level. Either control system may be continuous or discontinuous in its action. PROCESS C O N T R O L All control systems share certain features. They all function by moving information around the system. The information is used to produce a control action in which energy is transferred to or from the controlled object. Process control systems have the extra feature in that the controlled object is moved from one place to another. Usually the controlled object is a material undergoing a process of physical or chemical change. There are often a number of important variables in the process with feedback loops used to control them. In many cases it may not be possible to obtain a direct or immediate feedback of the controlled variable. The control system must then be modified accordingly. In every system the effect of the time taken for the material to move from one place to another must be taken into account. As an example, consider a boiler. Water is converted into steam which leaves the boiler and passes to a turbine. After leaving the turbine the exhaust steam is condensed back into water and returned to the boiler to continue the cycle. The temperature of the steam cannot be measured at the place where the water receives the energy 215
eBook - PDFQuantum Thermodynamic Processes
Energy and Information Flow at the Nanoscale
- Guenter Mahler(Author)
- 2014(Publication Date)
- Jenny Stanford Publishing(Publisher)
The same holds for the temperature, a thermal control parameter: Coupling the system to a thermal reservoir of some given temperature an isothermal process can easily be implemented. This is because in equilibrium the system and reservoir are on the same temperature (zeroth law of thermodynamics). Such control features can be applied even in the quantum domain (cf. Section 5.1). The analogue to the gas in a cylinder with movable piston is a single particle in a box of varying size V . Decreasing the size (i.e., modifying the particle potential) will increase the energy of the quantized levels. For fixed occupation of levels the average energy of the particle will thus increase: This represents a positive pressure p . ( p is the negative partial derivative of the average energy with respect to volume V .) But where does the changing volume V come from? It could be re-interpreted as an effective dynamics—in turn based on quantum theory. From a more detailed point of view the external agent can thus be included in the theoretical model as another The Big Questions 175 quantum system. We thus attempt to replace the non-autonomous control by pertinent design requirements (cf. [Schr ¨ oder (2010); Abah (2012)]). Limitations encountered in such preliminary models have to do with the fact that interactions in quantum mechanics typically lead to entanglement, that is, the effective local subsystem dynamics looses coherence. Mechanical driving, however, requires the persistence of coherence on time scales very long compared to any observation time, that is, the validity of a classical limit, cf. Section 3.3.8. 4.2.5 What is the Difference between Work and Heat? An important aspect of thermodynamic processes is their energetic impact. For the first law of thermodynamics work has to be distinguished from heat: This law states that the internal energy of a system can change in two ways: by adding work or by adding heat.
eBook - PDFChemistry
Structure and Dynamics
- James N. Spencer, George M. Bodner, Lyman H. Rickard(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
640 Chapter Fourteen KINETICS 14.1 The Forces That Control a Chemical Reaction 14.2 Chemical Kinetics 14.3 Is the Rate of Reaction Constant? 14.4 Instantaneous Rates of Reaction 14.5 Rate Laws and Rate Constants 14.6 The Rate Law Versus the Stoichiometry of a Reaction 14.7 Order and Molecularity 14.8 A Collision Theory Model of Chemical Reactions 14.9 The Mechanisms of Chemical Reactions 14.10 Zero-Order Reactions 14.11 Determining the Order of a Reaction from Rates of Reaction 14.12 The Integrated Form of Zero-, First-, and Second-Order Rate Laws 14.13 Determining the Order of a Reaction with the Integrated Form of Rate Laws 14.14 Reactions That Are First-Order in Two Reactants 14.15 The Activation Energy of Chemical Reactions 14.16 Catalysts and the Rates of Chemical Reactions 14.17 Determining the Activation Energy of a Reaction 14.18 The Kinetics of Enzyme-Catalyzed Reactions Special Topics 14A.1 Deriving the Integrated Rate Laws 14.1 The Forces That Control a Chemical Reaction The previous chapter showed how thermodynamics can explain why some reactions occur but not others. In that chapter, we saw that thermodynamics is a powerful tool for predicting what should (or should not) happen in a chemical reaction. It is ideally suited for answering questions that begin, “What if . . . ?” Thermodynamics predicts, for example, that hydrogen should react with oxygen to form water. It also predicts that iron (III) oxide should react with aluminum metal to form aluminum oxide and iron metal. Both of these reactions can, in fact, occur. We can experience the first by touch- ing a balloon filled with H 2 gas with a candle tied to the end of a meter stick. We can demonstrate the other reaction by igniting a mixture of Fe 2 O 3 and pow- dered aluminum. This doesn’t mean that H 2 and O 2 burst spontaneously into flame the instant the gases are mixed.
- Daniel Walgraef, J Martinez-mardones, Carlos Hernan Worner(Authors)
- 2000(Publication Date)
- World Scientific(Publisher)
PHYSICO-CHEMICAL THERMODYNAMICS OF MATERIAL SYSTEMS: A REVIEW OP BASIC CONCEPTS AND RESULTS ARMANDO FERNANDEZ GUILLERMET Consejo National de Investigaciones Cientificas y Ttcnicas Centra Atomico Bariloche-Instituto Balseiro 8400 San Carlos de Bariloche-Argentina. E-mail: [email protected] 1 Introduction 1.1 General Considerations Thermodynamics developed from the study of heat-engines and the relations between heat and work. However, after some time, it was recognized that the study of the effects of the thermal and mechanical interactions between the system and the surroundings provides valuable information on the equilibrium properties of the material systems, and on the reactions or transformations which occur. Today, thermodynamics might be considered as a discipline which deals with (i) a wide class of macroscopic properties of material sys-tems, (ii) the way in which these properties are influenced by the thermal, mechanical and chemical interactions with other systems, and (iii) the reac-tions or transformations involved. One of the key variables in the thermodynamic approach is temperature, and, in a certain sense, thermodynamics may be defined as the science dealing with the forms in which the properties of matter are modified by the changes in temperature. In particular, for material systems, it is interesting to determine the effects of temperature upon the equilibrium properties of a given structure, and to explore the possibility of inducing changes of structure by suitable temperature variations. The question of identifying the most stable structure for given external conditions is usually known as the Phase Stability Problem, which is often considered as a central problem in the study of material systems. Classical thermodynamics developed without referring to any particular model of the structure of matter.
eBook - PDF- Ethirajan Rathakrishnan(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
A typical example for a control mass is a balloon filled with a light gas, such as hydrogen, released in atmosphere. The balloon will gain height and the volume of the balloon will increase with in-crease of altitude. Neglecting the binary diffusion of air and hydrogen through the balloon wall, the hydrogen mass in the balloon can be taken as the control mass system. A control volume or open system is an identified shape, which may change its position in space or time or both but the shape will remain unchanged. The jet engine of an aircraft is a typical open system. The aircraft may run on the runway, take-off, climb, cruise at an altitude, descend, land and run on the runway and come to a halt. In these phases of travel from take-off to landing, the engine changes its position in space, the time rate of change of spacial po-sition varies, and the mass flow rate of air, fuel, and the combustion products through the engine varies, but the shape of the engine remains unchanged. 1.4 Forms of Energy Energy can exist in numerous forms—such as chemical, electrical, kinetic, potential, mechanical, thermal, and nuclear—and their sum constitutes the total energy E of a system. The total energy of a system per unit mass is termed specific energy e . That is, e = E m (kJ/kg) where m is the mass of the system. Thermodynamics provides no information about the absolute value of the total energy of a system. It only deals with the change of the total energy of a system, which is of interest in engineering applications. Thus, the total 10 Basic Concepts and Definitions energy of a system can be assigned value zero ( E = 0) at some convenient reference point. The change in the total energy of a system is independent of the reference point selected. For example, decrease in the potential energy of a falling object depends only on the elevation difference between the initial and final positions and not on the reference level chosen.
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