Numerical Simulation of Multiphase Reactors with Continuous Liquid Phase

Chao Yang, Zai-Sha Mao

• Publisher: Academic Press
• Year: 2014
• Print ISBN: 9780080999197
• eBook ISBN: 9780124115798
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería química y bioquímica

Numerical simulation of multiphase reactors with continuous liquid phase provides current research and findings in multiphase problems, which will assist researchers and engineers to advance this field. This is an ideal reference book for readers who are interested in design and scale-up of multiphase reactors and crystallizers, and using mathematical model and numerical simulation as tools. Yang and Mao's book focuses on modeling and numerical applications directly in the chemical, petrochemical, and hydrometallurgical industries, rather than theories of multiphase flow. The content will help you to solve reacting flow problems and/or system design/optimization problems. The fundamentals and principles of flow and mass transfer in multiphase reactors with continuous liquid phase are covered, which will aid the reader's understanding of multiphase reaction engineering. - Provides practical applications for using multiphase stirred tanks, reactors, and microreactors, with detailed explanation of investigation methods - Presents the most recent research efforts in this highly active field on multiphase reactors and crystallizers - Covers mathematical models, numerical methods and experimental techniques for multiphase flow and mass transfer in reactors and crystallizers

Chapter 1

Introduction

To meet the growing need for bulk chemicals in the national economy and in human life, chemical engineers have been trying to develop the best methodology for scaling up all types of reactors for diversified products. Historically, a larger scale reactor was tentatively designed after a series of cold model experiments and hot model tests. Even though these tests were done carefully and the design was backed up with a wealth of valuable intellectual experience, such a scale-up remains quite risky, because a new or large-sized reactor is the result of extrapolation based on tests in limited scopes of reactor configuration and experimental conditions. Better extrapolation would result from a basis of scientific laws that have been proved universally true in many industrial tests in addition to numerous natural phenomena. Mathematical models of chemical reactors are believed to be a sound scientific basis for such extrapolations. A practical model of a reactor is very complicated: phenomenologically involved with multiphase flow, macro- and micromixing, heat and mass transfer, and complex chemical reactions; mathematically with algebraic, ordinary and partial differential equations with strong nonlinearity and mutual coupling. Fortunately, we can utilize numerical simulation to solve such models, and tentatively guide the scale-up of chemical reactors to successful commercial operation. This explains why we are advocating the approach of mathematical modeling and numerical simulation so ardently, both in chemical engineering fundamental research and in industrial innovation.

Many interesting methods may be complemented with mathematical modeling and numerical simulation – for example, optimized operation for higher productivity or better product quality, upgrading the performance of reactors already on the process line, etc. There is one further comment here on scaling-up a reactor. Strictly speaking, we are not sure if the present reactor type and configuration are suitable for a larger scale reactor, as judged by our previous experience on extrapolation. A chemist can conduct a reaction successfully in a lab beaker (a small stirred tank), but this does not mean all commercial reactors for the same reaction should be conducted in large beakers. Using the approach of mathematical modeling and numerical simulation, we can conduct many virtual (numerical) tests of several reactor types and configurations on different scales, with the confidence that the capability of such a first-principles-based approach can achieve an optimized extrapolation of reactors. This approach may ultimately resolve the methodology of reactor renovation and innovation. The approach relies heavily on an in-depth quantitative understanding of the mechanisms occurring in chemical reactors for building the mathematical models and the various numerical techniques for solving the established models, as itemized and exemplified in this book.

This book is primarily focused on chemical engineering sciences and technologies, and aims to be a reference book for scientists and engineers in the fields of chemical reaction engineering, mass/heat transfer, hydrodynamics, crystallization, etc. The book will provide design, optimization, and scale-up concepts and numerical methods for multiphase reactors and crystallizers such as stirred tanks, loop reactors and microreactors for different application purposes. There are five subsequent chapters on various topics relevant to multiphase reactors with liquid phases.

Chapter 2 deals with the multiphase flow and interphase mass transfer on a particle scale. The mechanism of multiphase flow and mass transfer on the mesoscale is vital to the design and scale-up of reactors and crystallizers. The orthogonal boundary- fitted coordinate system-based simulation and level set method are improved to compute the motion and mass transfer of bubbles and drops, and also the mirror fluid method for motion of solid particles is developed. Thereafter, the modified cell model is proposed to examine the flow and transport behavior of particle swarms. The study on the motion and mass transfer of a solute to/from a single drop with a surfactant adsorbed on the interface and the Marangoni effect is expounded to better understand the liquid extraction and reaction processes. Also, the principal research results for the transport process of a spherical particle in pure extensional and simple shear flows are introduced in this chapter.

Chapter 3 deals with the numerical simulation of multiphase stirred tanks, which are the most used reactors or crystallizers in continuous, batch, or fed-batch modes. Good mixing in stirred tanks is important for minimizing investment and operating costs, providing high yields when mass/heat transfer is limiting, and thus enhancing profitability. Multiphase flow and transport in stirred tanks demand more intensive attention with combined numerical and experimental approaches. In this chapter, we present extensive experimental and numerical simulation results of recent developments for stirred tanks. Multiphase flows (including two- and three-phase flows) are discussed in detail based on numerical methods using the Eulerian multifluid approach and RANS (Reynolds average Navier–Stokes)-based turbulence models (e.g., k–

model). Novel surface aeration configurations are introduced for better gas dispersion and high pumping capacity, and the hydrodynamic characteristics of multi-impellers and numerical simulation of gas hold-up in surface-aerated stirred tanks are also addressed. Some new advances in numerical simulation are also presented. The algebraic stress model (ASM) and large eddy simulation (LES) are recommended for future research on multiphase flows in stirred reactors.

Chapter 4 deals with the hydrodynamics and transport in loop reactors. Airlift internal loop reactors are commonly used in petrochemical, hydrometallurgical, energy, environmental, and bio-engineering processes due to their excellent advantages of simple structure, high gas–liquid mass and heat transfer rates, good solid suspension, homogeneous shear distribution, and good mixing. Although great achievements have been made on loop reactors, the design and scale-up of these reactors still remain difficult due to the nature of complex multiphase flow. In this chapter, investigations of the flow and mixing characteristics by experiments and computational fluid dynamics simulations are presented on airlift reactors with very high and low height-to-diameter ratios. Also, as an intrinsic element of the new technology of coal liquefaction in China, an internal airlift loop reactor pilot test is introduced on the feasibility of replacing the bubble column reactor on the industrial process line of direct coal liquefaction.

Chapter 5 deals with the preliminary investigation of numerical methods and experiments for flow and mixing in two-phase microreactors. The miniaturization of chemical engineering devices has recently brought significant changes, and the progress in microreactors opens doors to more efficient, economic, and safer process intensification. The selectivity of fast chemical reactions depends on the quality of macro- and micromixing. In this chapter, the flow, pressure drop, mass transfer, and mixing of two-phase flow in microchannels with different wetting properties are investigated for different flow patterns. Immiscible two-phase flows, thermal transfer, and mass diffusion in microchannels are numerically studied by a lattice Boltzmann method based on field mediators.

Chapter 6 deals with the mathematical models and numerical simulation of solid– liquid crystallizers. Crystallizers are widely used to produce fine and bulk chemicals. Most of the theoretical and experimental studies are aimed at understanding important mechanisms in the crystallization process in order to stabilize process control, and ultimately to obtain products with desired crystal size distribution (CSD), morphology, and mean size. In this chapter, numerical simulations towards predicting the full CSD directly in a more practical crystallization reactor are presented in a Eulerian framework, and nucleation, growth, and aggregation are considered. The effects of aggregation, feeding concentration, agitation speed, mean residence time, and the CSDs of different locations are studied numerically. Reaction crystallizations are mixing-sensitive multiphase processes, so macro- and micromixing in crystallizers and some other multiphase reactors are also presented.

Chapter 2

Fluid flow and mass transfer on particle scale

Abstract

A single particle (bubble, drop, or solid particle) in an infinite continuous phase is a simplified model used to probe the law of multiphase flow and transport processes in complex multiphase systems, and it has been studied extensively by both experimental and numerical simulation. Interfacial instability such as the Marangoni effect on a sub-drop scale, which plays a significant role in heat and mass transfer, is being approached numerically with encouraging success. Different numerical methods, such as orthogonal boundary-fitted coordinate system-based simulation and the level set method, are adopted to simulate the motion and interphase mass transfer of a drop or a bubble and investigate the respective effect of particle size, deformation, surface active agent, etc. on the simplified model for summarizing transport rules. Also, the mirror fluid method and the cell model are respectively used to stud...

Table of contents

• Cover
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter 1: Introduction
• Chapter 2: Fluid flow and mass transfer on particle scale
• Chapter 3: Multiphase stirred reactors
• Chapter 4: Airlift loop reactors
• Chapter 5: Preliminary investigation of two-phase microreactors
• Chapter 6: Crystallizers: CFD–PBE modeling
• Index