Design of Multiphase Reactors

Vishwas G. Pangarkar

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9781118807569
• eBook ISBN: 9781118807767
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Chemie- & Biochemietechnik

Details simple design methods for multiphase reactors in the chemical process industries

• Includes basic aspects of transport in multiphase reactors and the importance of relatively reliable and simple procedures for predicting mass transfer parameters
• Details of design and scale up aspects of several important types of multiphase reactors
• Examples illustrated through design methodologies presenting different reactors for reactions that are industrially important
• Includes simple spreadsheet packages rather than complex algorithms / programs or computational aid

1
Evolution of the Chemical Industry and Importance of Multiphase Reactors

1.1 Evolution of Chemical Process Industries

Multiphase reactors have been at the cutting edge of technology development in the chemical industry. This premier status of multiphase reactors can be best appreciated in the context of the evolution of the chemical industry itself. It is therefore appropriate to discuss specific aspects relating to the growth and progress of the chemical process industries. This introduction starts with the evolution of the modern chemical industry and discusses the importance of green and sustainable methods and the inevitability of catalysts and multiphase catalytic reactors for carrying out highly efficient catalytic reactions.

The chemical process industries took a long and arduous road of development from modest beginnings through processes such as brewing and distillation; manufacture of soap, sugar, and paper, etc. in small-scale units. Most of the development was based on serendipity and empiricism rather than application of sound chemical engineering principles. In view of the poorly defined methodology, the advancement was slow. The seventeenth and eighteenth centuries witnessed practically no scientific progress that could bring about significant improvements in chemical engineering principles required for rational design.

In the early 1920s, a need was felt to have a unified approach for different disciplines of chemical engineering, and thus, the concept of unit operations (Walker et al. 1923) was introduced. Subsequently, the concept of unit processes (Groggins 1958), which allowed treatment of individual reaction types on a unified basis, was added. For example, hydrogenations, esterifications, nitrations, etc. were organized on the basis of related thermodynamics, kinetics, and, to a lesser extent, the hardware for each type of process. Groggins showed that unit operations and unit processes are intimately connected through the governing chemical engineering principles. Indeed, from this point onward, chemical engineering has been mathematically defined as ChE = Unit operations + Unit processes.

Extensive research efforts, particularly in the Western world that began in the second half of the twentieth century, laid the foundations of the modern chemical industry. The information generated is, however, still not sufficient for many objectives. For example, a priori design procedures for majority of the process equipment are still lacking. Indeed, in many chemical engineering design problems, we find that experience must supplement pure theory. This is an indication that chemical engineering is still largely an art rather than science, where we can evaluate the parameters exactly. For comparison, in electrical engineering, we can precisely calculate the drop in voltage, given the resistance of a conductor and the current it carries. It would be difficult to do the same for the pressure drop in a simple two-phase pipe flow. We need to resort to empirical/semiempirical approaches in a majority of the cases because of lack of sufficient knowledge of the phenomena involved. This example should serve as an indicator of the difference in chemical engineering and other basic engineering disciplines.

In the initial stages, the chemical process industries were restricted to inorganic chemicals (sulfuric acid, nitric acid), sugar, paper, fertilizers, etc. Most hydrocarbons were derived from coal.

From the mid-1950s onward, petroleum crude took over from coal as the principal supplier of hydrocarbons. The major impetus to the organic chemicals sector came from the availability of inexpensive petroleum crude in large quantities. Products derived from petroleum crude had capability to undergo a variety of complex reactions to yield different products that the evolving society required. Chemical engineering became a much more complex profession than in the 1920s. Some basic changes were occurring but were not obvious. The refining and petrochemical industries started producing specialized products with well-defined functions/properties. Products such as high-octane gasoline and specific lubricants were considered as commodity products notwithstanding their careful formulation that gave the specific desired end result. These formulations also underwent changes brought about by various considerations such as environmental impact. An example is that of replacement of tetraethyl lead in gasoline by more benign antiknock compounds. The chemical process industries were slowly shifting from the commodity/bulk chemicals to specialty/functional products (Cussler and Moggridge 2001). This paradigm shift called for more specialists than generalist chemical engineers. According to a 2004 survey (Jenck et al. 2004), “the global chemical industry represents a significant part of world trade and economic activity with 10 million employees and a combined turnover of some USD 1600 billion excluding pharmaceuticals, and at USD 2200 billion including pharmaceuticals, representing 4–5% of world income. It contributes 9% of world trade whilst emitting only 4% of global carbon dioxide.” Evidently, the carbon dioxide emissions of the chemical process industries are an insignificant fraction of the total global carbon dioxide emissions. In spite of these highly revealing statistics, most chemical majors are curtailing greenhouse gas emissions as a part of overall sustainability measures (McCoy 2008). Indeed, processes for utilizing this liability (CO₂, generated by other sectors) for value-added products are receiving increasing research attention (Section 1.3.1.1). A recent review by Muller et al. (2014) discusses the thermodynamic feasibility of potential reactions for converting CO₂ to value-added chemicals. This review points out the severe thermodynamic limitations imposed by the low energy level of CO₂. The following conclusion have been derived: Thermodynamically, favorable routes for producing useful chemicals require (i) high-energy reactants such as epoxide that overcome the low energy level of CO₂, (ii) in situ hydrogenation of the intermediate, or (iii) formation of at least two water molecules per mole of CO₂. Such efforts are indicative of an industry that is conscious of its societal responsibility despite the fact that it bears a very small burden of the CO₂ generated.

The chemical process industries have been at the receiving end of the regulatory authorities not because of their greenhouse gas emissions but due to their toxic emissions. Both of these prompted a drive for sustainable processing. The sustainability aspect needs to be dealt with in some detail. The rapid expansion of the petroleum refining and petrochemicals industry through a laissez-aller approach resulted in unbridled consumption of vital resources with simultaneous generation of hazardous waste. The development of the industry was disorganized with little attention being paid to the damage caused to the ecosystem. Over the past five to six decades, release of toxic wastes in all forms (solid/liquid/gaseous) has caused serious damage to the ecosystem. Rachel Carson’s 1962 book Silent Spring was the first recorded warning of the catastrophic nature of the rapid, unsavory expansion. Rachel Carson argued that man is not above nature but is an integral part of it and hence must ensure peaceful coexistence with all species involved. The chemical and allied industries were the main culprits in the eyes of the society that suffered episodes such as the “Love Canal” and “Bhopal” tragedies. As a result of the severe criticism, the chemical industry is now closely looking at safety, health, and environment (SHE) issues while developing a new process or designing a new plant. However, the SHE aspects as practiced are related to decisions that are essentially of short-term nature to avoid contingencies typified by “firefighting” situations. In the 1980s, a more mature approach “sustainable development” was advocated as a long-term objective. Our Common Future published by the World Commission on Environment and Development defined sustainable development as “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” This definition abhors senseless consumption and waste creation. A complementary definition of sustainable development given in 1991 in Caring for the Earth: A Strategy for Sustainable Living by IUCN, UNEP, and WWF was “Improving the quality of human life while living within the carrying capacity of the supporting eco-system.” The 2002 World Summit on Sustainable Development e...

Table of contents

• Cover
• Title page
• Copyright page
• Dedication
• Foreword
• Preface
• 1 Evolution of the Chemical Industry and Importance of Multiphase Reactors
• 2 Multiphase Reactors: The Design and Scale-Up Problem
• 3 Multiphase Reactors: Types and Criteria for Selection for a Given Application
• 4 Turbulence: Fundamentals and Relevance to Multiphase Reactors
• 5 Principles of Similarity and Their Application for Scale-Up of Multiphase Reactors
• 6 Mass Transfer in Multiphase Reactors: Some Theoretical Considerations
• 7A Stirred Tank Reactors for Chemical Reactions
• 7B Stirred Tank Reactors for Cell Culture Technology
• 8 Venturi Loop Reactor
• 9 Gas-Inducing Reactors
• 10 Two- and Three-Phase Sparged Reactors
• Index
• Download Workbook files
• End User License Agreement