Chemical Reactions and Processes under Flow Conditions Edition. ed. Edition

By S.V. Luis, E. Garcia-Verdugo

The Royal Society of Chemistry

Copyright © 2010 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-192-3

Contents

Chapter 1 Engineering Factors for Efficient Flow Processes in Chemical Industries Alexei A. Lapkin and Pawel K. Plucinski, 1,
Chapter 2 Flow Processes Using Polymer-supported Reagents, Scavengers and Catalysts Eduardo García-Verdugo and Santiago V. Luis, 44,
Chapter 3 Zeolites and Related Materials for Developing Continuous Flow Systems Maria J. Sabater Fernando Rey and Jesús Lázaro, 86,
Chapter 4 Microfluidic Devices for Organic Processes Paola Laurino, Arjan Odedra, Xiao Yin Mak Tomas Gustafsson, Karolin Geyer and Peter H. Seeberger, 118,
Chapter 5 Flow Processes in Non-Conventional Media Tânia Quintas and David J. Cole-Hamilton, 163,
Subject Index, 196,

CHAPTER 1

Engineering Factors for Efficient Flow Processes in Chemical Industries

ALEXEI A. LAPKIN AND PAWEL K. PLUCINSKI

Centre for Sustainable Chemical Technologies, Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK

1.1 Introduction

Continuous chemical processes integrated via energy and material flows are forming the basis of a highly successful petrochemical industry. Effectively all petrochemical processes, starting from crude oil heating, hydrotreating, cracking, refining and further synthesis of bulk products are performed in continuous flow reactors and separators. The same applies to other large-scale processes, for example, the synthesis of ammonia and sulfuric acid. The scale of production and the close integration of materials and energy are the key attributes of traditional continuous flow processes that contribute to their remarkable efficiency.

The introduction of continuous flow processes in smaller-scale manufacturing such as speciality chemicals, chemical intermediates, pharmaceutical intermediates, active ingredients in agrochemicals and pharmaceuticals, nutraceuticals, fragrances, surfactants, etc. faces significant challenges due to the reliance of these industries on sunken capital — the existing infrastructure of batch multipurpose plants and the slow introduction of the suitable scale technologies. Only recently have the compact and microreactor systems been developed that could begin to replace the traditional batch multipurpose plants. However, the advantages of continuous processing are clear enough. The processes are generally more efficient than batch ones and offer much higher throughput per unit volume and per unit time. Reactants are introduced continuously, react on contact within a smaller reaction space with better defined temperature and flow fields, and are removed continuously from the reaction space. There is better control of process variables and the risk of side reactions is reduced. The reactor volume is determined by the flow rate and residence time of the materials rather than vice versa; therefore, vessels can be smaller and heat transfer and mixing are easier to control. Waste levels are generally also lower.

The areas in which flow processes have been developing at a rapid pace are biotechnology and biomedicine. In these areas, the closer relation to living systems (which can be said to be 'flow systems' and highly material/energy integrated systems) gives stronger impetus to the exploitation of the functionality of the flow reactors. Such features of small continuous flow systems as the extensive use of in situ analytics, sequential operations, use of weaker fields such as electric and magnetic separations, microwave heating and sonication as well as parallelisation and automation for increase in productivity have already found numerous applications in biotechnology and are rapidly penetrating into chemical processes.

This chapter considers the engineering basis for the design of continuous flow chemical and biochemical reactors at different scales. The emphasis is on new and emerging areas of process intensification (PI), flow chemistry, and compact and microreactors; process engineering of petrochemical reactors is well covered in earlier literature and some aspects are discussed in Chapter 3. One of the main differences between large-scale and micro-scale flow processes, to which we pay particular attention, is the more significant role of surface-fluid interactions and hence the need to account for solid-fluid physico-chemical interactions in the reactor design. The issues of scale up of small-scale flow reactors are also considered.

The process intensification concept that emerged in industry initially aimed to reduce the physical footprint of plants, and hence reduce capital investment and improve safety. This concept is now widely accepted in the broader meaning of the reduction in the overall impact of chemical processes over their entire life cycle. The different tools of PI are shown in Figure 1.1.

In flow chemistry, a chemical reaction is run in a continuously flowing stream; liquids (normally reagent/substrate solutions) are driven through a reactor which is often a capillary or tubing. In recent years, flow chemistry has emerged as a viable means for performing many types of chemical transformations. Within industry, flow chemistry is already having a major impact: large pharmaceutical companies have teams of chemists and chemical engineers active in the field. On the macro scale, flow processes are being developed for the manufacture of active pharmaceutical ingredients where a series of synthesis reactions, work-up steps and crystallisation of the final active pharmaceutical intermediate (API) are performed in a sequence of flow modules as shown schematically in Figure 1.2.

1.2 Heterogeneous Catalytic Flow Processes in the Petrochemical Industry: A Brief Overview

1.2.1 Gas–solid and Liquid–solid Catalytic and Non-catalytic Continuous Processes

Reactions in systems where at least one reactant is solid play a major role in the materials processing industries, encircling a broad range of operations such as extractive metallurgy (e.g. ore leaching), coal gasification (or more generally combustion of solid fuels: coal, lignite, etc.), pyrolysis of lignocellulosic products, incineration of municipal waste and catalyst regeneration. Most of these reactions can be represented by a general stoichiometric equation:

[MATHEMATICAL EXPRESSION OMITTED]

The reactions involving a solid reactant include the following elementary steps (Figure 1.3, shown here as an example of a gas-solid system with solid particle pyrolysis):

(i) external (gas phase) mass transfer;

(ii) diffusion inside the pores (if solid is porous);

(iii) chemical reaction(s) between gaseous and solid reactants (may involve adsorption of reactant(s) and desorption of reaction products);

(iv) diffusion or reaction(s) product(s) from the reaction site towards the external surface of the solid;

(v) external mass transfer of formed reaction product(s) away from the solid interface.

The diffusion of reaction products through the pore system of a solid material and external mass transfer — forming an integral part of the process — are important if the reaction is reversible. Although the process steps listed above occur in series, any one or more of these could be rate limiting.

In slow reacting systems, the overall dynamics will be limited by the surface kinetics (intrinsic rate); the increase of...