Catalysts reaction, mass transfer

At steady state, the rate at which reactants are supplied to the external surface of the catalyst by mass transfer must be equal to the rate at which they are consumed by the catalytic reaction. Per unit mass of catalyst, the rate of disappearance of species A is then given by... 

It is interesting to note that the chlorinated ethylenes do not appear to follow this trend of increasing rates with increasing chlorination. (Lowry and Reinhard 1999 Schreier 1996) This may be due in part to the extremely fast rates of these reactions, which increase the relative importance of mass transfer limitations. For very fast reactions, mass transfer of compounds to the catalyst surface, rather than the intrinsic catalytic reaction rate, may determine the rate of disappearance of hydrocarbons and the resulting apparent rate constants. 

In fast reactions, mass transfer or intraparticle diffusion becomes controlling. Thinner catalyst coatings, Taylor flow, etc. can be applied to optimize these... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

The spatial distribution of composition and temperature within a catalyst particle or in the fluid in contact with a catalyst surface result from the interaction of chemical reaction, mass-transfer and heat-transfer in the system which in this case is the catalyst particle. Only composition and temperature at the boundary of the system are then fixed by experimental conditions. Knowledge of local concentrations within the boundaries of the system is required for the evaluation of activity and of a rate equation. They can be computed on the basis of a suitable mathematical model if the kinetics of heat- and mass-transfer arc known or determined separately. It is preferable that experimental conditions for determination of rate parameters should be chosen so that gradients of composition and temperature in the system can be neglected. 

Gas-liquid multiphase catalytic reactions require the reacting gas to be efficiently transferred to the liquid phase. This is then followed by the diffusion of the reacting species to the catalyst. These mass transfer processes depend on bubble hydrodynamics, temperature, catalyst activity, physical properties of the liquid phase like density, viscosity, solubility of the gas in the liquid phase and interfacial tension. 

In addition to the rates of olefin reactions, mass transfer also plays an important role in determining the extent of propylene conversion and the product distribution from SAPO molecular sieves. Restrictions on molecular movement may be severe in the SAPO catalysts, due to pore diameters (4.3 A for SAPO-34) and structure (one-dimensional pores in SAPO-5 and SAPO-11). The deactivation of SAPO-5 and SAPO-11 catalysts may be more directly related to mass transfer than the coking of SAPO-34. Synthesis of large or highly-branched products, having low diffusivities, inside the pores of SAPO-5 or SAPO-11 essentially block internal acid... 

In catalytic reactions mass transfer from the fluid phase to the active phase inside the porous catalyst particle takes place via transport through a fictitious stagnant fluid film surrounding the particle and via diffusion inside the particle. Heat transport to or from the catalyst takes the same route. These phenomena are summarized in Fig. 8.15. 

The global transformation rate of a gas-liquid reaction catalyzed by a solid material is influenced by the mass transfer at the gas-liquid boundary and the liquid-solid boundary. Mass transfer and surface reaction occur in sequence, and for fast chemical reactions, mass transfer may affect the reactant concentration on the catalyst surface and, as a result, the reactor performance and the product selectivity. For a gaseous reactant, three mass transfer steps can be identified [113] (1) the direct transfer from the bubble through the thin liquid film near the wall to the catalyst surface (characterized by k aQg), (2) the transfer from the caps (i.e., front and back end) of the gas bubbles to a dissolved state in the liquid slug (characterized by and (3) the transfer of dissolved gas... 

Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective utilization of the catalyst, one usually suspends a powdered or granular form of the catalyst in the liquid phase. This type of reactor is useful when one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid (e.g., in the hydrogenation of various oils). The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Mass transfer limitations on reaction rates can be quite significant in those instances where three phases (the solid catalyst and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

If the PT catalyst in both the phases is in extractive equilibrium (i.e., there is equilibrium distribution of catalyst) and mass transfer resistances are absent or neglected completely, then ipQY and ipqx are each equal to 1. In other words, the reaction is in regime 1, and no further modeling besides determining the intrinsic kinetics is required. 

Selectivity, which is one of the most important characteristics of an industrial process, depends on several parameters temperature control, residence time distribution, gas and liquid hold-ups, catalyst loading, catalyst type, mass transfer rates etc... If homogeneous side reactions are awkward, fixed beds give better results.But if the desired product can react further on the catalyst, small catalyst particles have to be preferred to avoid concentration gradients in the pores and slurry reactors are the best. In this last case, the poor residence time distri-... 

Kinetics of Cas-Liquid Reactions on Solid Catalysts 293 Mass transfer coefficients ... 

A proper resolution of Che status of Che stoichiometric relations in the theory of steady states of catalyst pellets would be very desirable. Stewart s argument and the other fragmentary results presently available suggest they may always be satisfied for a single reaction when the boundary conditions correspond Co a uniform environment with no mass transfer resistance at the surface, regardless of the number of substances in Che mixture, the shape of the pellet, or the particular flux model used. However, this is no more than informed and perhaps wishful speculation. 

M ass Transfer. Mass transfer in a fluidized bed can occur in several ways. Bed-to-surface mass transfer is important in plating appHcations. Transfer from the soHd surface to the gas phase is important in drying, sublimation, and desorption processes. Mass transfer can be the limiting step in a chemical reaction system. In most instances, gas from bubbles, gas voids, or the conveying gas reacts with a soHd reactant or catalyst. In catalytic systems, the surface area of a catalyst can be enormous. Eor Group A particles, surface areas of 5 to over 1000 m /g are possible. 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

As the polymer molecules form and dissociate from the catalyst, they remain ia solution. The viscosity of the solution increases with increasing polymer concentration. The practical upper limit of solution viscosity is dictated by considerations of heat transfer, mass transfer, and fluid flow. At a mbber soflds concentration of 8—10%, a further increase in the solution viscosity becomes impractical, and the polymerisation is stopped hy killing the catalyst. This is usually done by vigorously stirring the solution with water. If this is not done quickly, the unkilled catalyst continues to react, leading to uncontrolled side reactions, resulting in an increase in Mooney viscosity called Mooney Jumping. 

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