Reaction phase-contacting principles

This section starts with a classification of phase-contacting principles according to the type of catalytic bed. Advantages and disadvantages of the reactor types are explained, followed by a discussion of criteria for reactor selection and an overview of purchasable microreactors for catalytic gas-phase reactions. 

Tenets (i) and (ii). These are applicable only where the reactant undergoes no melting and no systematic change of composition (e.g. by the diffusive removal of a constituent) and any residual solid product phase offers no significant barrier to contact between reactants or the escape of volatile products [33,34]. When all these conditions are obeyed, the shape of the fraction decomposed (a) against time (f) curve for an isothermal reaction can, in principle, be related to the geometry of formation and advance of the reaction interface. The general solution of this problem involves intractable mathematical difficulties but simplifications have been made for many specific applications [1,28—31,35]. 

The superconducting 1-2-3 phases are known to be chemically sensitive, and in fact are bought to be metastable compounds imder all conditions of temperature and oxygen partial pressure (1). Thus, it is imperative that any material in intimate contact with 1-2-3 phases does not react with the component oxides to form more stable compounds, since such a reaction will destroy the superconducting material. This is especially important for thin fflm applications, since the amount of superconductor is small compared to that of the substrate, and the diffusion path for potential solid state reactions, i.e., the film thickness, is very short. In this paper we will concentrate on searching for materials that should be most stable in contact with the 1-2-3 superconductors, since they appear to be more sensitive to chemical disruption than the more recently discovered Bi- and Tl-based phases. The principles and much of the data presented here can be applied to any oxide superconductor, whether it is presently known or yet to be discovered. 

The calculation principle on which the assessment of design for such reactors is based is a substitution of the multi-phase reaction system by a quasi-single-phase model. In two-phase systems both reactants have to get into contact at a certain place. Consequently a reaction and a transport phase are distinguished. If the mass transfer rate from the transport to the reaction phase is veiy fast compared to the actual reaction rate, the process in total is dominated by the reaction kinetics. In order to discriminate this situation from one taking the mass transfer into account, it is referred to as micro-kinetically dominated In this ease all formal kinetic laws presented for homogeneous systems may be applied directly. 

Multiphase catalytic reactors are employed in nearly 80% of industrial processes with annual global sales of about 1.5 trillion, contributing around 35% of the world s GDP [17]. Microreactors for multiphase reactions are classified based on the contact principles of gas and liquid phases continuous-phase contacting and dispersed-phase contacting [18]. In the former type, the two phases are kept in continuous contact with each other by creating an interface. In the latter case, one fluid phase is dispersed into another fluid phase. In addition, micro trickle bed operation is reported following the path of classical chemical engineering. The study of mass and heat transfer in two-phase flow in micro trickle bed reactors still remains as a less... 

There are a number of possible explanations for the formation of more than one photodimer. First, due care is not always taken to ensure that the solid sample that is irradiated is crystallographically pure. Indeed, it is not at all simple to establish that all the crystals of the sample that will be exposed to light are of the same structure as the single crystal that was used for analysis of structure. A further possible cause is that there are two or more symmetry-independent molecules in the asymmetric unit then each will have a different environment and can, in principle, have contacts with neighbors that are suited to formation of different, topochemical, photodimers. This is illustrated by 61, which contrasts with monomers 62 to 65, which pack with only one molecule per asymmetric unit. Similarly, in monomers containing more than one olefinic bond there may be two or more intermolecular contacts that can lead to different, topochemical, dimers. Finally, any disorder in the crystal, for example due to defective structure or molecular-orientational disorder, can lead to formation of nontopochemical products in addition to the topochemical ones formed in the ordered phase. This would be true, too, in those cases where there is reaction in the liquid phase formed, for example, by local melting. 

However, care must be exercised in using molecular sieves for drying organic liquids. Appreciable amounts of impurities were formed when samples of acetone, 1,1,1-trichloroethane and methyl-r-butyl ether were dried in the liquid phase by contact with molecular sieves 4A (Connett Lab.Practice 21 545 1972). Other, less reactive types of sieves may be more suitable but, in general, it seems desirable to make a preliminary test to establish that no unwanted reaction takes place. For the principles of synthesis and identification see R. Szostak Molecular Sieves, Chapman Hall, London 1988, and for structure, synthesis and properties see R.Szostak Handbook of Molecular Sieves, Chapman Hall 1992. 

The general principle of two-phase catalysis in polar solvents, for example, in water, is shown in the simplified diagram of Fig. 1. The metal complex catalyst, which can be solubilized by hydrophilic ligands, converts the reactants A + B into the product C. The product is more soluble in the second than in the first phase and can be separated from the catalyst medium by simple phase separation. Excellent mixing and contacting of the two phases are necessary for efficient catalytic reaction, and thus the reactor is normally well stirred. 

All of the studies so far discussed have involved gas-phase reactions. Recently, MacDiarmid et al. 571,57z) suggested that the same principles can be applied to polyacetylene in contact with aqueous solutions. Since 02 is a hard base it would be... 

Membrane bioreactors have been reported for the production of diltiazem chiral intermediate with a multiphase/extractive enzyme membrane reactor [15, 16]. The reaction was carried out in a two-separate phase reactor. Here, the membrane had the double role of confining the enzyme and keeping the two phases in contact while maintaining them in two different compartments. This is the case of the multiphase/ extractive membrane reactor developed on a productive scale for the production of a chiral intermediate of diltiazem ((2R,3S)-methylmethoxyphenylglycidate), a drug used in the treatment of hypertension and angina [15]. The principle is illustrated in... 

Since dissolved gas concentrations in the liquid phase are more difficult to measure experimentally than the liquid reactant concentration, Equation 8 evaluated at the reactor exit 5=1 represents the key equation for practical applications involving this model. Nevertheless, the resulting expression still contains a significant number of parameters, most of which cannot be calculated from first principles. This gives the model a complex form and makes it difficult to use with certainty for predictive purposes. Reaction rate parameters can be determined in a slurry and basket-type reactor with completely wetted catalyst particles of the same kind that are used in trickle flow operation so that DaQ, r 9 and A2 can be calculated for trickle-bed operation. This leaves four parameters (riCE> St gj, Biw, Bid) to be determined from the available contacting efficiency and mass transfer correlations. It was shown that the model in this form does not have good predictive ability (29,34). 

In principle, the chemical composition of water recharging a lithologically homogeneous aquifer will depend upon the mineral phase present in the aquifer, the amount of dissolved carbon dioxide (H2CO3), the amount of aquifer surface area (S) in contact with the hydraulically effective pore volume (V), the temperature at which reaction occurs (T), the contact time (t), and the reaction rate (k). Laboratory experiments may be carried out using specific lithologic media to determine the rates of reaction of these media with water containing dissolved carbon dioxide. If the interrelationships of the above variables can be sufficiently defined, a determination of one of the above aquifer properties can be made, if the others are known, and if a representative water sample from the aquifer is available. 

Timely and up-to-date, this book provides broad coverage of the complex relationships involved in the interface between gas/solid, liquid/solid, and solid/solid...addresses the importance of the fundamental steps in the creation of electrical glow discharge... describes principles in the creation of chemically reactive species and their growth in the luminous gas phase... considers the nature of the surface-state of the solid and the formation of the imperturbable surface-state by the contacting phase or environment... offers examples of the utilization of LCVD in interface engineering processes...presents a new perspective on low-pres.sure plasma and emphasizes the importance of the chemical reaction that occur in the luminous gas phase...and considers the use of LCVD in the design of biomaterials. 

As carried out industrially, the processes pose problems in almost all their aspects. The catalysts generally operate between 800 and 1100 °C and at very high space velocities (>100 000 h ) with contact times of the order of 10" — 10 s the question arises therefore whether the reactions are wholly surface catalysed, or whether surface initiated gas-phase reactions are important. Since there is a considerable reorganization of atoms in reactants during their conversion to products, the nature of the reaction intermediates has been the subject of considerable speculation over many years. Reaction theories for ammonia oxidation were named, prior to 1960, after the principle intermediate proposed, viz. the imide (NH), nitroxyl (HNO), and hydroxylamine (NH2OH) theories. Similarly, alternative theories for the Andrussow cyanide process have proposed methylene-imine (CH2=NH) and nitrosomethane (CH3.NO) as reaction intermediates. Modern techniques might now reasonably be expected to discriminate amongst these hypotheses. 

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