Modeling of Chemical Kinetics and Reactor Design
SCOPE This valuable reference volume conveys a basic understanding of chemical reactor design methodologies that incorporate both scale-up and hazard analysis. It shows readers how to select the best reactor for any particular chemical reaction, how to estimate its size, and how to obtain the best operating conditions. An understanding of chemical reaction kinetics and the design of chemical reactors is very important to the chemist and the chemical engineer. Engineers share interests in fluid mechanics and transport phenomena, while the chemist deals with the kinetics and mechanisms of reactions. The chemical engineer combines the knowledge of these subjects for the better understanding, design, and control of the reactor. The recent accidents that have occurred in the chemical process industries with inherent fatalities and environmental pollution have imposed greater demands on chemical engineers. Consequently, chemical reactor design methodologies must incorporate both control and hazard analysis. However, the design of chemical reactors is still essential for its proper sizing, and is included in various types of process simulators. In an industrial problem, it is essential to select the best type of reactor for any particular chemical reaction. Additionally, it is necessary to estimate its size and determine the best operating conditions. The chemical engineer confronted with the design of various reactor types often depends on the scale of operation and the kinetics. Many excellent texts have appeared over the years on chemical reactor design. However, these texts often lack sections on scale-up, biochemical reactor design, hazard analysis, and safety in reactor design methodology. The purpose of this book is to provide the basic theory and design and, sometimes, computer programs (Microsoft Excel spreadsheet and software) for solving tedious problems. This speeds up the work of both chemists and engineers in readily arriving at a solution. The following highlights some of the subjects that are covered in this text. xiv MIXING An important unit operation in chemical reaction engineering, mixing, finds application in petrochemicals, food processing, and biotechnology. There are various types of fluid mixing such as liquid with liquid, gas with liquid, or solids with liquid. The text covers micromixing and macromixing, tracer response and residence time distribution (RTD), heat transfer, mixing fundamentals, criteria for mixing, scale of segregation, intensity of segregation, types of impellers, dimensional analysis for liquid agitation systems, design and scale-up of mixing pilot plants, the use of computational fluid dynamics (CFD) in mixing, and heat transfer in agitated vessels. BIOCHEMICAL REACTION This is an essential topic for biochemists and biochemical engineers. Biochemical reactions involve both cellular and enzymatic processes, and the principal differences between biochemical and chemical reactions lie in the nature of the living systems. Biochemists and biochemical engineers can stabilize most organic substances in processes involving microorganisms. This chapter discusses the kinetics, modeling and simulation of biochemical reactions, types and scale-up of bioreactors. The chapter provides definitions and summary of biological characteristics. CHEMICAL REACTOR MODELING 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 mathematical expressions show how physical xv processes interact with chemical processes. The proposed model must represent the flow behavior of an actual reactor, which is realistic enough to give useful information for its design and analysis. The text reviews different reactor flow models. SAFETY IN CHEMICAL REACTION Equipment failures or operator errors often cause increases in process pressures beyond safe levels. A high increase in pressure may exceed the set pressure in pipelines and process vessels, resulting in equipment rupture and causing major releases of toxic or flammable chemicals. A proper control system or installation of relief systems can prevent excessive pressures from developing. The relief system consists of the relief device and the associated downstream process equipment (e.g., knock-out drum, scrubber, absorbers, and flares) that handles the discharged fluids. Many chemical reactions (e.g., nitration) in the chemical process industry result in runaway reactions or two-phase flow. This occurs when an exothermic reaction occurs within a reactor. If cooling no longer exists due to a loss of cooling water supply or failure of a control system (e.g., a valve), then the reactor temperature will rise. As the temperature rises, the reaction rate increases, leading to an increase in heat generation. This mechanism results in a runaway reaction. The pressure within the reactor increases due to increased vapor pressure of the liquid components and gaseous decomposition products as a result of the high temperature. Runaway reactions can occur within minutes for large commercial reactors and have resulted in severe damage to a complete plant and loss of lives. This text examines runaway reactions and two-phase flow relief. SCALE-UP The chemical engineer is concerned with the industrial application of processes. This involves the chemical and microbiological conversion of material with the transport of mass, heat and momentum. These processes are scale-dependent (i.e., they may behave differently in small and large-scale systems) and include heterogeneous chemical reactions and most unit operations. The heterogeneous chemical reactions (liquid-liquid, liquid-gas, liquid-solid, gas-solid, solid-solid) generate or consume a considerable amount of heat. However, the course of xvi such chemical reactions can be similar on both small and large scales. This happens if the mass and heat transfer processes are identical and the chemistry is the same. Emphasis in this text is on dimensional analysis with respect to the following: Continuous chemical reaction processes in a tubular reactor. Influence of back mixing (macromixing) on the degree of conversion and in continuous chemical reaction operation. Influence of micro mixing on selectivity in a continuous chemical reaction process. Scale-up of a batch reactor AN INTEGRATING CASE STUDYAMMONIA SYNTHESIS This book briefly reviews ammonia synthesis, its importance in the chemical process industry, and safety precautions. This case study is integrated into several chapters in the text. See the Introduction for further details. Additionally, solutions to problems are presented in the text and the accompanying CD contains computer programs (Microsoft Excel spreadsheet and software) for solving modeling problems using numerical methods. The CD also contains colored snapshots on computational fluid mixing in a reactor. Additionally, the CD contains the appendices and conversion table software.