Multiphase reactors examples

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

Mass transfer steps are essential in any multiphase reactor because reactants must be transferred from one phase to another. When we consider other multiphase reactors in later chapters, we will see that mass transfer rates fiequently control these processes. In this chapter we consider a simpler example in the catalytic reactor. This is the first example of a multiphase reactor because the reactor contains both a fluid phase and a catalyst phase. However, this reactor is a very simple multiphase reactor because the catalyst does not enter or leave the reactor, and reaction occurs only by the fluid reacting at the catalyst surface. 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

Let us consider a simple example of multiphase reactors where the reaction A B occurs in a CSTR, but now A enters the reactor in phase a but does not react until it enters phase P, where the homogeneous rate is itC. As an example A could be an ester in an oil phase O, which will hydrolyze into an alcohol and an acid when it comes in contact with a water... 

Listed below are examples of multiphase reactors used in industry. For each reactor, answer the following. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

Emulsion polymerization is usually carried out isothermally in batch or continuous stirred-tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small ( 0.5 fim) polymer particles, which are the locus of the reaction, are suspended in a continuous aqueous medium. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. In industrial practice, such a reactor often shows sustained oscillations. Solid-catalyzed olefin polymerization in a slurry batch reactor is a classic example of a slurry reactor where the solid particles change size and characteristics with time during the reaction process. 

These systems are common in liquid extraction and also in a multiphase reactor with an organic and an aqueous phase. Common sources of pollution are incomplete separation and contamination due to trace organics in the aqueous phase. An example is in alkylation reactions (e.g., n-butane reaction with olefins to form isooctanes). Strong acids, such as sulfuric and hydrofluoric acids, are used as catalysts, and the recovery and the recycle of acid need to be optimized in order to reduce the waste generation. 

Bioreactors In biotechnology, although single-phase reactors are used (e.g., see the enzyme reactor described above), multiphase reactors are predominant. There are characteristics of bioreactors that set them apart from the t) ical chemical reactors, such as, for example, the presence of biomass that can remain in suspension but can also form biofilms on any surface. Formation of biofilms can either be a desired or an unwanted side reaction, depending on the objective of the application. 

Examples of multiphase reactors (a) trickle-bed reactor, (b) countercurrent packed-bed reactor, (c) bubble column, (d) slurry reactor, and (e) a gas-liquid fluidized bed. [From Reactor Technology by B. L. Taimy. Kirk-Othmer Encyclopedia of Chemical Technology, vol. 19, 3rd ed., Wiley (1982). Reprinted by permission of John Wiley and Sons, Inc., copyright 1982.]... 

Discussions on flow modeling so far have been more or less restricted to singlephase reactors. However, in a broad range of application areas, multiple phases are involved in chemical reactions (see examples cited by Ramachandran and Choudhari, 1983 Doraiswamy and Sharma, 1984 Kunii and Levenspiel, 1991 Shah, 1991 Dudukovic et al, 1999). Reactors carrying out such reactions are generically termed multiphase reactors. There are several types of multiphase reactors and several methods are available to classify these reactors. One of the simplest methods of... 

Apart from the flow regimes, several other issues control the performance of these multiphase reactors. For example, in a gas-liquid reactor, the rate of mass... 

The standard, when one considers multiphase reactors, has become more complex over the years. Most can be classified as reactions over heterogenous catalysts. The catalytic activity occurs in one phase, the solid phase, while transport of the reactants occurs in a gas or liquid phase, or both. A common example is the catalytic converter for automobile exhaust gas. The key steps for a packed bed reactor are ... 

An application of microfluidic reactors is the development of a membraneless fuel cell. Two streams, one containing a fuel such as methanol, the other an oxygen-saturated acid or alkaline stream, are merged without mixing. The laminar flow pattern in the narrow channel helps to maintain separate streams without the use of membrane separators. Opposite walls function as the electrodes and are doped with catalyst. Ion exchange, protons for the add system, takes place through the liquid-liquid interface. This is an example of a solid-liquid-liquid-solid multiphase reactor. ... 

The present chapter applies the fundamental principles of CRE to specialized examples of practical interest. Many standard texts on CRE start with a detailed presentation of reactors for homogeneous reactions and then extend the treatment to multiphase reactors. In the present chapter, homogeneous reactors constitute only a small part of the treatment and are covered under one of several case studies included. The bulk of the treatment is devoted to multiphase reactions and reactor characteristics. A broad outline of the scope of the chapter is presented in Eigure 11.1. 

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