Choice of a suitable reactor

Choosing a suitable reactor for a gas-liquid reaction is a question of matching the characteristics of the reaction system, especially the reaction kinetics, with the 

Type of contactor Specific area a m2/m3 Liquid hold-up eL (fraction) 

You are asked to recommend, with reasons, the most suitable type of equipment for carrying out a gas-liquid reaction between a gas A and a solution of a reactant B. Particulars of the system are as follows  

Rate constant of the reaction (A + B - Products second order overall) = 0.03 m /kmol s. 

For bubble dispersions (plate columns, bubble columns, agitated vessels) take kL as having a range from 2 x 10 4 to 4 x 10 4 m/s. For a packed column, take kL as having a range 0.5 x 10 4 to 1.0 x 10 4 m/s. 

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The advantages and disadvantages of various types of reactor outlined in this section are summarized in Table XI. The choice of a suitable reactor generally depends upon these characteristics, as well as cost, safety, and availability considerations. 

The choice of a suitable reactor for gas-liquid reaction or absorption is very often a question of matching the reaction kinetics with the capabilities of the proposed reactor. The specific interfacial area a, liquid holdup /3, and mass-transfer coefficients ki and k (or kifl and k fl) are the most significant characteristics of a reactor. A synthesis of published values of the mass-transfer parameters will be given in this section. 

For the choice of a suitable reactor, the heat of reaction under standard conditions ArH is, among others, of decisive importance. Since the ArH generally can not be measured directly in a calorimeter (industrially relevant reactions do not proceed with 100% yield), they must be determined via the experimentally readily accessible heats of combustion of the reactants AcH . 

Issue choice of a suitable reactor for manufacturing scale operation. This reaction scheme is ... 

Restrictions which may exist for the choice of a commercial reactor need not be imposed at the development stage. In some cases, a reactor of one type may be best for acquiring data in model characterisation, whereas a reactor of another type might be more suitable for full-scale production. (The cautions expressed in Sect. 4 must be taken into account.) Continuous flow back-mixed reactors can be very useful for kinetic studies because the absence of concentration gradients can reduce uncertainties in concentration measurements. When these reactors have attained a steady state, many of the problems associated with stiffness (see above) can be avoided. 

A chemical reactor is a vessel in which reactants are converted to products through chemical reactions. This vessel takes many shapes and sizes depending upon the nature of the chemical reaction. The choice of a suitable laboratory reactor depends upon the nature of the reaction system (fluid-solid catalytic, fluid-solid noncatalytic, fluid-fluid, etc.), the nature of the required kinetic or thermodynamic data, or the feasibility of operation. The important parameters for a successful reactor design are the following ... 

Solubilities and diffusivities of gas are practically always required for design of gas-liquid process and obtaining solubility and diffusivity data for the gas-liquid system under consideration may be a chalenging problem so wide is the range of solutes and solvent the chemical engineer or researcher may encounter. Moreover the choice of a suitable gas-liquid contactor is also a question of matching these data, those concerning the reaction kinetics and the physical kinetics characteristics of the proposed reactor, i.e., specific gas-liquid interfacial area, heat and mass transfer coefficients and gas or liquid holdup. Some considerations on solubility and diffusivity will be proposed in part 1 of this review and on gas-liquid mass transfer in part 2. 

The first major decision in the choice of a reactor for gas-liquid reactions taking place in the liquid phase is based on the optimal usage of the total reactor volume, i.e. the choice of the parameter P, which is the ratio of the liquid-phase volume to the volume of the diffusion layer (see Section 8.4.2). When reactions are slow compared to the mass transfer from the gas to the liquid, sparged stirred tanks and bubble columns are preferred, as these reactors have the largest bulk liquid volume. On the other hand, fast reactions for a large part take place in the diffusion layer, so in this case spray columns and packed columns are more suitable. 

Optimization can be done by proper reactor choice followed by a suitable temperature progression in the case of a batch or semibatch reactor, or by temperature profiling in the case of a tubular reactor. An even more effective way is to optimize reactant concentrations, pressure, and/or temperature by applying certain simple rules of kinetics and manipulation of the chemistry (wherever possible). Hence the combined efforts of chemist and chemical engineer are needed to optimize selectivity in a given complex reaction. 

When the diameter of the bubble (gas) has a value of the same order of magnitude as the diameter of the tube, the fluidized bed will operate in slugging regime, where the bubble occupies the entire cross section of the bed. Depending on the size of the bubble, we might have preferred paths in bed. These cases do not contribute to a good contact of the gas with the suspended particles in the bed, therefore, they should be avoided. The most suitable solution is the choice of a particular type of gas distributor in the inlet of the reactor. 

The technology of choice for on-board electric power on mid-length space vehicle missions (several days to a year), including the important man-moon mission, was the fuel cell. This was because the use of batteries for more than a couple of days proved too heavy, combustion engines and gas turbines required too heavy a fuel supply, and the use of a nuclear reactor was only suitable for missions of a year or more. There was a simple choice of fuel for space fuel cells it was hydrogen because it doesn t require a fuel processor other than storage and pressurization, it is relatively lightweight when stored under pressure, and it was the best fuel for the early-developed alkaline fuel cell. Fuel flexibility was not an issue. 

It must be emphasised that it is unnecessary to correct a heat of reaction to the reaction temperature for use in a reactor heat-balance calculation. To do so is to carry out two heat balances, whereas with a suitable choice of datum only one need be made. For a practical reactor, the heat added (or removed) Qp to maintain the design reactor temperature will be given by (from equation 3.10) ... 

Polymer synthesis is not difficult today. To synthesise a polymer we only need an appropriate quantity of the monomer and the catalyst and a suitable polymerisation reactor and we can obtain a polymer of our choice in terms of the required Molecular weight, structure, crystallinity, etc. 

Flow pattern Next one decides whether a batch or continuous reactor is suitable and, if flow, whether a mixed of unmixed reactor is preferred. Initially one may do calculations for PFTR and CSTR to bracket all flow patterns. This is the subject of Chapters 3 and 4. The choice of catalyst and heat removal method will be very important in deciding the best flow pattern. 

The most important applications of zirconium involve its alloys, Zircaloy. The aUoy offers excellent mechanical and heat-transfer properties and great resistance to corrosion and chemical attack. This, in conjunction with the fact that zirconium has a low neutron absorption cross section, makes this ahoy a suitable choice as a construction material for thermal nuclear reactors and nuclear power plants. Other uses are as an ingredient of explosive mixtures, as getter in vacuum tubes, and in making flash bulb, flash powder (historical), and lamp filaments, in rayon spinnerets, and in surgical appliances. 

Finally, the use of the expensive metal palladium as the metal of choice in the telomerization reaction holds obvious disadvantages for the economic feasibility of large-scale processes. Catalyst recovery and reuse should therefore receive further attention in future studies, for instance, by clever reactor design, or heteroge-nization of the catalyst. Alternatively, the use of palladium might be completely avoided if non-noble metals can be prompted to perform the same reactions when a suitably designed ligand environment is offered. 

The fluidised bed is only one of the many reactors employed in industry for gas-solid reactions, as reported by Kunii and Levenspiel.25 Whenever a chemical reaction employing a particulate solid as a reactant or as a catalyst requires reliable temperature control, a fluidised bed reactor is often the choice for ensuring nearly isothermal conditions by a suitable selection of the operating conditions. The use of gas-solid fluidised beds... 

As the temperature at which significant initiator decomposition takes place depends on the initiator itself, a successful operation of the reactor requires a proper choice of the initiator in many cases, suitable mixtures of different initiators are also used. The reactor performances are often enhanced by a proper use of multiple feed streams of cold ethylene and/or initiator (s). 

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