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Chemical Reactions and Processes under Flow Conditions
By S.V. Luis, E. Garcia-Verdugo The Royal Society of Chemistry
Copyright © 2010 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-973-9
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 reaction rate may change the limitation to the pore diffusion. For faster exothermic reactions, the temperature gradient across the particle or fluid film might become the controlling factor. In the case of very fast chemical reactions, the mass transfer in the external fluid film becomes the rate limiting step.
An important difference that distinguishes fluid-solid reactions from their catalytic counterparts is that, in non-catalytic systems, a solid is also involved as a reactant. Continuous consumption of the solid phase during the reaction leads to structural changes of the solid bed morphology, and the reactor system is always in the transient state.
The rate of the overall process for external mass transfer limitation can easily be obtained from the knowledge of mass transfer around solid particles and several correlations for fixed or moving solid particles are reported in the literature. The challenges in the mathematical description of these types of reactors concern the molecular diffusion in the pore systems of a solid phase. Continuous changes of a solid's morphology (pore shrinkage or closure, swelling, sintering, softening or cracking of the particles) affect the effective diffusivity with the progress of reaction.
The primary consideration in the design and analysis of such systems is the mode of contact of the phases. Fixed, fluidised and moving bed techniques appear to be the most common mode of phase contacting. Horizontal moving bed, pneumatic conveyers, rotating cylinders and flat hearth furnaces are less common.
In catalytic fluidised bed reactors, the problems of inhomogeneity of the fluidised bed when the gas phase is used as a fluidising agent could be overcome by using an external magnetic force and magnetisable catalyst particles. Fluidisation of magnetisable particles by a gas stream in the presence of a uniform applied magnetic field oriented parallel to the flow prevents the hydro-dynamic instability that otherwise leads to bubbles and turbulent motion within the medium. The fluidised emulsion phase expands uniformly in response to gas flow velocity.
1.2.2 Two-phase Gas–liquid Continuous Industrial Reactors
Chemical reactions between a gas and a solute dissolved in a liquid are very common in industry. Examples of important processes performed in gas-liquid reactors include:
• absorption of acid gases;
• oxidation of organic compounds by oxygen or air;
• chlorination;
• hydrogenation of organic compounds.
In such reactions, a gaseous component(s) is dissolved in the liquid phase where it reacts with other reagent(s). In the catalytic reactions (homogeneous catalysis), the liquid phase contains a catalyst together with liquid reactant(s). Slightly different scenarios may occur for a biphasic (liquid–liquid) mode of operation. For example, a liquid reagent will be dissolved in the other liquid phase containing the catalyst.
The fundamental analysis of two-phase reactors is complex due to the coupling of simultaneously occurring diffusion and reactive processes. In addition, the hydrodynamic conditions of the reactive two-phase system are difficult to define.
For a chemical reaction taking place in the laminar film and bulk liquid, starting from elementary mass balance of a reactant A, the expression for calculating the overall reaction rate can be developed as shown in eqn (1.1):
[MATHEMATICAL EXPRESSION OMITTED]] (1.1)
where: cAb = bulk concentration of A in the liquid phase; H = Henry constant; Ha = Hatta number; kAg = mass transfer coefficient in the gas phase; kA1 = mass transfer coefficient in the liquid phase; Pa = partial pressure of A in the gas phase; x = distance from the interface; z = dimensionless length z = x/δ 1; δ1 = thickness of the laminar layer.
Hatta number, or more precisely Ha2, is a dimensionless number being a ratio of the maximum rate of the reaction in the liquid laminar film and the maximum rate of transport through the liquid film. For a first order chemical reaction is defined in eqn (1.3) as follows:
[MATHEMATICAL EXPRESSION OMITTED]] (1.2)
where: DA1 = diffusion coefficient of A in the liquid phase; kA = reaction rate constant; kA1 = mass transfer coefficient in the liquid phase.
For analysis of such coupled fluid-fluid systems (which may include two liquid phases), it is useful to distinguish between three regimes of reaction rate which are characterised by different Ha values and the enhancement factor E (Table 1.1). The mass transfer rate between two phases is compared with that for pure physical adsorption via enhancement factor (E) as shown in eqn (1.3):
E = rate of reaction or flux of A / maximum rate of mass transfar of A through liquidfilm (1.3)
For slow reactions (Ha < 0.3), the overall rate of mass transfer is not enhanced by the chemical reaction (which takes place mainly in the bulk of reaction phase), and the enhancement factor becomes approximately 1. For the intermediate range of Hatta number (0.3 < Ha < 3), the overall rate of mass transfer is improved by the chemical reaction (E = Ha/tanh(Ha)). In the case of high Hatta number (Ha > 43), the reaction is very fast and proceeds only within the laminar boundary layer (E = Ha)
In gas-liquid reactions, yield and selectivity could also be affected by mass transfer, the nature of gas-liquid contact and the residence time distribution. Table 1.2 gives orders of magnitude of mass transfer parameters for various two-phase reactors.
Some typical examples of two-phase (gas-liquid or liquid-liquid) reactors (see Figure 1.4) include:
• stirred tanks reactors;
• bubble or spray columns;
• packed columns;
• Venturi reactors
The presence of the homogeneous catalyst mainly influences the rate of chemical reaction; however some other effects may appear if the catalyst has interfacial properties (e.g. in micellar and phase transfer catalysis).
1.2.3 Three-phase Catalytic Reactors
Three-phase continuous catalytic processes involving gas, liquid and a solid catalyst are widely used in industrial practice including the manufacturing of commodity chemicals. The most common example includes liquid phase catalytic hydrogenations, which have been carried out industrially for a very long time. Other process examples include:
• desulfurisation;
• hydrocracking;
• refining of crude oil products in petrochemistry;
• synthesis of butynediol from acetylene and formaldehyde;
• reduction of adiponitrile to hexamethylenediamine.
The liquid phase, often acting as a solvent in such types of reactors, not only dissolves the reactants, but also provides a liquid layer around the catalyst particles, which may help to:
(i) avoid deactivating deposits (i.e. guarantee higher catalyst effectiveness);
(ii) achieve better temperature control due to higher heat capacity of liquids; and
(iii) modify active catalytic sites to promote or inhibit certain reaction pathways.
In most applications, the reaction occurs between a dissolved gas and a liquid phase in the presence of a solid catalyst. However, in some cases, when a large heat sink is required for highly exothermic reactions (e.g. in the Air Products methanol synthesis process), the liquid is an inert medium and the reaction takes place between the dissolved gases at a solid interface.
In practice, in implementing three-phase reaction systems, several alternatives are available for bringing the three phases into contact. Generally, one can classify these systems based on whether the catalyst is suspended in the reactor (more precisely in the liquid phase — slurry reactors) or is present in the form of a packed bed of catalyst particles (fixed bed reactors).
Reactors with the catalyst dispersed in a liquid phase may exist in three forms: (a) bubble columns, (b) mechanically stirred tanks, and (c) three-phase fluidised beds (see Figure 1.5).
Reactors with the catalyst placed in a fixed bed mode can operate as (a) trickle bed reactors, and (b) packed bubble column reactors. In the first mode, the gas phase comprises the continuous phase of the reactor; in the second, two-phase (gas–liquid) flow occurs through the fixed bed of catalyst particles (see Figure 1.6).
When comparing the various possible reactors offered to users for a given application, it is necessary to consider both the main characteristic features of each type of reactor (Table 1.3) and a number of appreciation criteria of varying importance in order to perform a given reaction in the reactor of choice effectively (Table 1.4).
The modelling and design of three-phase reactors, including the various mass transfer and reaction steps of the process is shown in Figure 1.7.
The first stage of the modelling process considers the following five steps:
(i) Transport from the bulk gas phase to the gas-liquid interface [eqn (1.4)]:
[MATHEMATICAL EXPRESSION OMITTED]] (1.4)
where: ai = gas-liquid interfacial area/volume of bed; cAi(g) = bulk gas phase concentration of A; cAi(g) = interfacial concentration of A; mol/gcat·S = unit of reaction rate r'a in moles per unit of the mass of catalyst and per time unit; εb = bed porosity (gas + liquid); ρc = density of catalyst pellet.
(ii) Equilibrium at the gas/liquid interface [eqn (1.5)]:
[MATHEMATICAL EXPRESSION OMITTED]] (1.5)
(iii) Transport from interface to bulk liquid [eqn (1.6)]:
[MATHEMATICAL EXPRESSION OMITTED]] (1.6)
where: cAi = concentration of A in liquid at interface.
(iv) Transport from bulk liquid to external catalyst surface [eqn (1.7)]
[MATHEMATICAL EXPRESSION OMITTED]] (1.7)
where: ap = external specific area of pellet; cAs = concentration of A at solid-liquid interface; kc = liquid-solid mass transfer coefficient.
(v) Diffusion and reaction in pellet (assuming second order surface reaction) [eqn (1.8)]
[MATHEMATICAL EXPRESSION OMITTED]] (1.8)
where: cBs = concentration of B at solid-liquid interface; k = η = effectiveness factor defined as the ratio of the reaction rate to the rate in the absence of diffusion.
Combining equations and rearranging gives eqn (1.9), which allows the calculation of the so-called combined mass transfer resistance [expression in the denominator of eqn (1.9)]:
[MATHEMATICAL EXPRESSION OMITTED]] 1.9)
(Continues...)
Excerpted from Chemical Reactions and Processes under Flow Conditions by S.V. Luis, E. Garcia-Verdugo. Copyright © 2010 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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