Homogeneous Liquid Reactions

Homogeneous liquid reactions For homogeneous liquid reactions, the activity can be... 

An established school preparation of 2-propanone (acetone) involves the small-scale (and rather exothermic) oxidation of the alcohol with dichromate(VI). It was observed in several laboratories that when the acidified dichromate solution was added to the alcohol in small portions (1-2 cc) rather than dropwise as specified, small sparks or incandescent particles were produced which sometimes survived long enough to escape from the neck of the flask. This also happened if the alcohol and/or the oxidant solution were diluted with extra water, with old or new samples of alcohol, and if air were displaced from the flask by carbon dioxide. It is therefore important not to exceed the specified dropwise rate of addition of oxidant solution. It is very unusual for glowing particles to be produced from a homogeneous liquid reaction system. 

The kinetics of a homogeneous liquid reaction are studied in a flow reactor, and to approximate plug flow the 48-cm long reactor is packed with 5-mm nonporous pellets. If the conversion is 99% for a mean residence time of 1 sec, calculate the rate constant for the first-order reaction... 

Floyd, T.M., Schmidt, M.A., Jensen, K.F., A silicon microchip for infrared transmission kinetics studies of rapid homogeneous liquid reactions. Micro Total Analysis Systems, Proceedings 5th TTAS Symposium, Monterey, CA, Oct. 21-25,2001, 277-279. 

The profiles of temperature and composition shown in Fig. 23-3 are not of homogeneous liquid reactions, but are perhaps representative of all kinds of reactions. Only in stirred tanks and some fluidized beds are nearly isothermal conditions practically attainable. 

In recent years, the main focus of the patents has shifted from the development of novel microstructured device to the well-defined application of the device to proper reaction process. Homogeneous liquid reactions are occupying a large part of patents. The main improvement of these patents is to achieve the faster reaction in microreactor than batch reactor or to conduct the reaction with hazardous chemicals without human exposure. Microreactors also can be applied to multiphase and gas-phase reactions. Recently, the carbonylation reaction using carbon monoxide in microreactor has been reported [15]. The polymerization process can be also controlled with microreactors. The microreactor leads to the concise synthesis of polymers with a narrow molecular weight distribution. 

Homogeneous catalysts. With a homogeneous catalyst, the reaction proceeds entirely in the vapor or liquid phase. The catalyst may modify the reaction mechanism by participation in the reaction but is regenerated in a subsequent step. The catalyst is then free to promote further reaction. An example of such a homogeneous catalytic reaction is the production of acetic anhydride. In the first stage of the process, acetic acid is pyrolyzed to ketene in the gas phase at TOO C ... 

In terms of cost and versatility, the stirred batch reactor is the unit of choice for homogeneous or slurry reactions and even gas/liquid reactions when provision is made for recirculation of the gas. They are especially suited to reactions with half-lives in excess of 10 min. Sam-... 

Nonisotnermal Operation Some degree of temperature control of a reaction may be necessary. Figures 23-1 and 23-2 show some of the ways that may be applicable to homogeneous liquids. More complex modes of temperature control employ internal surfaces, recycles, split flows, cold shots, and so on. Each of these, of course, requires an individual design effort. 

Although they are termed homogeneous, most industrial gas-phase reactions take place in contact with solids, either the vessel wall or particles as heat carriers or catalysts. With catalysts, mass diffusional resistances are present with inert solids, the only complication is with heat transfer. A few of the reactions in Table 23-1 are gas-phase type, mostly catalytic. Usually a system of industrial interest is liquefiea to take advantage of the higher rates of liquid reactions, or to utihze liquid homogeneous cat ysts, or simply to keep equipment size down. In this section, some important noncatalytic gas reactions are described. 

Few mechanisms of liquid/liquid reactions have been established, although some related work such as on droplet sizes and power input has been done. Small contents of surface-ac tive and other impurities in reactants of commercial quality can distort a reac tor s predicted performance. Diffusivities in liquids are comparatively low, a factor of 10 less than in gases, so it is probable in most industrial examples that they are diffusion controllech One consequence is that L/L reactions may not be as temperature sensitive as ordinary chemical reactions, although the effec t of temperature rise on viscosity and droplet size can result in substantial rate increases. L/L reac tions will exhibit behavior of homogeneous reactions only when they are very slow, nonionic reactions being the most likely ones. On the whole, in the present state of the art, the design of L/L reactors must depend on scale-up from laboratoiy or pilot plant work. 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

Homogeneous liquid phase inorganic oscillatory reactions. D. O. Cooke, Prog. React. Kinet., 1978, 8,185-229(184). 

Homogeneous, liquid-phase reactions may also be important in trickle beds, and a strictly homogeneous term has been included in Equation (11.42) to note this fact. There is usually no reaction in the gas phase. Normally, the gas phase merely supplies or removes the gaseous reactants (e.g., H2 and H2S in hydrodesulfurization). ... 

Application Homogeneous reactions Liquid-Solid reactions Gas-Liquid-Solid reactions Gas-Liquid reactions Liquid-Liquid reactions... 

An accident occurred when 2-propanol reacted with potassium dichromate in a sulphuric medium. The aqueous solution of dichromate and sulphuric acid were added in small quantities. The homogeneous liquid was throwing up glowing particles (CrOj particles ). The same reaction proved not to be dangerous when this process was carried out drop by drop. 

An nth-order homogeneous liquid phase reaction is carried out in a batch tank reactor. 

Extending the formalism for ET in homogeneous phase, reactions at liquid-liquid interfaces can be described in terms of a series of elementary steps initiated by the approach of reactants to the interfacial region and the formation of the ET precursor complex [1,5,60],... 

The influence of temperature on gas-liquid reactions is more complex than homogeneous reactions. As the temperature increases,... 

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