Reactor chemical transformation

Reactor design is particularly critical, because reactors involve chemical transformations and often potentially significant energy releases. 

Figure 1 Illustrates two general MOCVD reactor configurations, the horizontal reactor and the axlsymmetrlc vertical reactor. The reactant gas (ASH3, Ga(CH3)3 and Al( 013)3) enters cold and heats up as It fiows toward the substrate where a solid film of AlGaAs Is being deposited. The chemical transformations Involved In the deposition process may occur both In the gas phase and on the surface of the growing film.

The starting point, therefore, for any evaluation of the degree to which any process maybe considered sustainable requires a full inventory of all the inputs and outputs. For chemicals production, this necessarily requires the focus to be on the engineered process, not simply on the chemical transformation that occurs in the reactor. While necessary, the latter is not sufficient. 

Other advantages of the tubular reactor relative to stirred tanks include suitability for use at higher pressures and temperatures, and the fact that severe energy transfer constraints may be readily surmounted using this configuration. The tubular reactor is usually employed for liquid phase reactions when relatively short residence times are needed to effect the desired chemical transformation. It is the reactor of choice for continuous gas phase operations. 

The rates at which chemical transformations take place are in some circumstances strongly influenced by mass and heat transfer processes (see Sections 12.3 to 12.5). In the design of heterogeneous catalytic reactors, it is essential to utilize a rate expression that takes into account the influence of physical transport processes on the rate at which reactants are converted to products. Smith (93) has popularized the use of the term global reaction rate to characterize the overall rate of transformation of reactants to... 

Today, a large body of work on microwave-assisted synthesis exists in the published and patent literature. Many review articles [8-20], several books [21-23], and information on the world-wide-web [24] already provide extensive coverage of the subject. The goal of the present book is to present carefully scrutinized, useful, and practical information for both beginners and advanced practitioners of microwave-assisted organic synthesis. Special emphasis is placed on concepts and chemical transformations that are of importance to medicinal chemists, and that have been reported in the most recent literature (2002-2004). The extensive literature survey is limited to reactions that have been performed using controlled microwave heating conditions, i.e., where dedicated microwave reactors for synthetic applications with adequate... 

Operating with chemicals and pressurized containers always carries a certain risk, but the safety features and the precise reaction control of the commercially available microwave reactors protect the users from accidents, perhaps more so than with any classical heating source. The use of domestic microwave ovens in conjunction with flammable organic solvents is hazardous and must be strictly avoided as these instruments are not designed to withstand the resulting conditions when performing chemical transformations. 

Additional applications of this technology for rapid lead discovery and lead optimization have been reported [87, 90-93]. It should also be noted that a variety of chemical transformations, in particular in the area of transition-metal catalyzed reactions, have been performed with this or related equipment (Chapt. 11) [25]. Other monomode microwave reactors using related concepts to introduce high-throughput were recently introduced by CEM Corp. (Discover or Explorer line of products, Fig. 12.7.) [81]. At the time of writing this review no published synthetic applications using this microwave reactor were available. 

A large drawback of batch reactors is that a great deal of time is spent not performing chemical transformations a batch process is inherently inefficient in terms of plant usage. Also, as the reaction proceeds and more product is formed, the composition of the batch changes, altering the physical properties of the reaction including viscosity, heat capacity and gas solubilities, all of which... 

The considerable interest in the design of container molecules [26, 27, 30], for their potential application as nano-scale chemical reactors has particularly received much attention [97]. In this sense, supramolecular catalysis allowing the chemical transformation of a substrate selectively entrapped within a molecular receptor, will behave as a chemical reactor [98]. One way to obtain a supramolecular catalysis, is to design a molecular receptor containing a lipophilic cavity allowing selective substrate binding, and specific sites for metal ion coordination [97, 99]. Attempts in this direction have... 

Many chemicals of commercial interest are produced by reactions which are known to be complex. In such cases, treatment of reaction kinetics requires careful attention often, more than one stable species is produced in substantial amounts. Both the conversion of reactants and the distribution of reaction products must then be taken into account when designing a reactor. In this chapter, we shall consider a general approach to complex reaction schemes and product selectivity. Attention will be given to the implications for choice and operation of chemical reactors there will be no attempt to give a comprehensive description of particular kinetic studies. A reaction will be regarded as complex if chemical transformations between constituent species involves more than one mechanistic step. 

The model is formulated based on the state equipment network (SEN) representation (Yeomans and Grossmann, 1999). The general characterization of this representation includes three elements state, task and equipment. A state includes all streams in a process and is characterized by either quantitative or qualitative attributes or both. The quantitative characteristics include flow rate, temperature and pressure, whereas the qualitative characteristics include other attributes such as the phase(s) of the streams. A task, on the other hand, represents the physical and chemical transformations that occur between consecutive states. Equipment provides the physical devices that execute a given task (e.g., reactor, absorber, heat exchanger). 

Reducing the number of steps in a chemical transformation can have a profound impact on its economics. Thus, there has been considerable interest in developing 1-step processes for propylene-to-acrylic acid or isobutylene-to-methacrylic acid, which currently involve the intermediate production of acrolein and methacrolein, respectively (9). Sometimes simplification of a process just involves the elimination of a purification step, so that the products of one reactor go straight to another without isolation of an intermediate. 

Traditionally the technique of the medical physicist, magnetic resonance imaging (MRI) has long been used to investigate the internal structure of the human body and the transport processes occurring within it for example, MRI has been used to characterize drug transport within damaged tissue and blood flow within the circulatory system. It is therefore a natural extension of medical MRI to implement these techniques to study flow phenomena and chemical transformations within catalysts and catalytic reactors. 

CHEMICAL ELEMENTS. A chemical element may be defined as a collection of atoms of otic type which cannot be decomposed into any simpler units by any chemical transformation, but which may spontaneously change into other units by radioactive processes A chemical element is a substance that is made up of but one kind of atom. Of the over 100 chemical elements known, only 90 tire found in nature. The remaining elements have been produced in nuclear reactors and particle accelerators. Theoretical physicists do not all agree, but some believe that fission-stable nuclei should exist at atomic numbers 109. 114. and 126. Claims thus lur have been made for the discovery, isolation, or creation of elements up to 110. The element with the highest atomic number officially named and entered into the formal table of atomic weight is darmstudlium (Dx) with an atomic number of 110. 

The possibility of having membrane systems also as tools for a better design of chemical transformation is today becoming attractive and realistic. Catalytic membranes and membrane reactors are the subject of significant research efforts at both academic and industrial levels. For biological applications, synthetic membranes provide an ideal support to catalyst immobilization due to their biomimic capacity enzymes are retained in the reaction side, do not pollute the products and can be continuously reused. The catalytic action of enzymes is extremely efficient, selective and highly stereospecific if compared with chemical catalysts moreover, immobilization procedures have been proven to enhance the enzyme stability. In addition, membrane bioreactors are particularly attractive in terms of eco-compatibility, because they do not require additives, are able to operate at moderate temperature and pressure, and reduce the formation of by-products. 

Section 7 of this Handbook presents the theory of reaction kinetics that deals with homogeneous reactions in batch and continuous equipment. Single-phase reactors typically contain a liquid or a gas with (or without) a homogeneous catalyst that is processed in a reactor at conditions required to complete the desired chemical transformation. 

A catalyst for a particular chemical transformation is selected using knowledge of similar chemistry and some level on empirical experimentation. Solid catalysts are widely used due to lower cost and ease of separation from the reaction medium. Their drawbacks include a possible lack of specificity and deactivation that can require reactor shutdown for catalyst regeneration or replacement. 

Useful chemical transformations are thus confined to the Weisz window on reality. The Weisz window is an important concept from various standpoints. For scale up, it is helpful in determining the limiting reactor sizing to be considered for a given intrinsic catalyst activity, as determined by laboratory experiments. During catalyst development, it provides guidance in assessing whether further catalyst development is necessary. It also indicates the directions for further research e.g., in the case of certain zeolites that were found to be too active... 

A typical chemical plant flowsheet has a mixture of multiple units connected both in series and in parallel. As noted in the previous chapter, the common topology consists of reaction sections and separation sections. Streams of fresh reactants enter the plant by being fed into the reaction section (or sometimes into the separation section) through a heat exchanger network. Here the chemical transformations occur to produce the desired species in one or more of a potentially wide array of reactor types continuous stirred tank, tubular, packed bed, fluidized bed, sparged, slurry, trickle bed, etc. 

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