Liquid phase mass transfer problems

Inspection of Eqs. (14-71) and (14-78) reveals that for fast chemical reactions which are liquid-phase mass-transfer limited the only unknown quantity is the mass-transfer coefficient /cl. The problem of rigorous absorber design therefore is reduced to one of defining the influence of chemical reactions upon k. Since the physical mass-transfer coefficient /c is already known for many tower packings, it... 

Two-phase mass transfer and heat transfer without phase change are analogous, and the results of mass-transfer studies can be used to help clarify the heat-transfer problems. Cichy et al. (C5) have formulated basic design equations for isothermal gas-liquid tubular reactors. The authors arranged the common visually defined flow patterns into five basic flow regimes, each... 

Many heterogeneous catalytic organic reactions are run in the liquid-phase, and liquid phase reactions present special mass transfer problems. Diffusion barriers exist between the gas and the liquid and between the liquid and the solid, so there are gas-liquid-solid diffusion barriers. When these barriers are too large, the true chemical rate at the surface is not observed. 

Yet another way to detect mass transport problems is with a newly developed poisoning technique.24,26,49,50 This technique works for liquid-phase hydrogenations and possibly for other reactions that are poisoned by CS2. It takes advantage of the fact that CS2 poisons Pd and Pt linearly until all reaction stops. If mass transfer problems exist, the initial linear decrease in rate occurs at a slope less steep than the slope of the chemically controlled rate (Fig. 1.7). If no mass transport problems exist, the rate decreases linearly from the start with no change in slope. Therefore a plot of rate versus amount of CS2 reveals the existence or absence of mass transport problems 49... 

So far only propene and butene are hydroformylated commercially using the RCH/RP process. A reason which has been postulated for this is the decreasing solubility in water with increasing number of C atoms in both the starting alkene and the reaction products (Figure 5.4) and the associated mass-transfer problems in the relatively complicated gas-liquid-liquid, three-phase reaction. 

The mass transfer coefficient for ozone can be calculated from both the liquid and gas phase mass balances as described by equations 3-26, 3-27 or 3-28. Difficulties arise with the liquid phase mass balance if a reaction is present. The reaction rate under the operating conditions investigated must be used, considering especially the cL prevalent in the system. Since this can be very difficult to assess, and use of inaccurate reaction rates leads to inaccurate kLa, application of the gas phase mass balance is an elegant way to avoid this problem. 

The latter strongly depends on the specific reaction mechanism, the stoichiometry, and the presence or absence of parallel reaction schemes (69). The rate expressions for Rt usually represent nonlinear dependences on the mixture composition and temperature. Specifically for the coupled reaction-mass transfer problems, such as Eqs. (A10), it is always essential as to whether or not the reaction rate is comparable to that of diffusion (68,77). Equations (A10) should be completed by the boundary conditions relevant to the film model. These conditions specify the values of the mixture composition at both film boundaries. For example, for the liquid phase ... 

There is, however, a mass transfer problem of demixing at lower temperatures caused by high viscosities. Concentrated polymer solutions tend to take hours to form two distinct liquid phases. A solution to this problem is the use of the lower critical solution temperature. Because of their thermodynamic nature, all polymer-solvent mixtures tend to form two liquid phases ( LL ) with low viscosities, at higher temperatures (LCST) as depicted in Figure 3. 

Mass transfer is one subject that is unique to chemical engineering. Typical mass transfer problems include diffusion out of a polymer to provide controlled release of a medicine, diffusion inside a porous catalyst where a desired reaction occurs, or a large absorption column where one chemical is transferred from the liquid phase to the gas phase (or vice versa). The models of these phenomena involve multicomponent mixtures and create some tough numerical problems. 

Mass-transfer problems can be solved by two distinctly different methods, one utilizing the concept of equilibrium stages, the other based on diffusional rate processes. The choice of the method depends on the kind of equipment in which the operation is carried out. Distillation, leaching, and sometimes liquid extraction are performed in equipment such as mixer-settler trains, diffusion batteries, or plate towers, which contain a series of discrete processing units, and problems in these areas are commonly solved by equilibrium-stage calculations. Gas absorption and other operations which are carried out in packed towers and similar equipment are usually handled using the concept of a diffusional process. All mass-transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

The quantity kg is sort of the odd-man-out in most work on slurry reactors (and even also for fluid-bed and gas-liquid reactors). If the bubble (gas) phase consists of pure reactant only, then a mass-transfer resistance in a film inside the bubble loses its meaning and kg drops out of the problem. Even in the case of mixed gas-phase components, gas phase mass-transfer coefficients are so much larger than their liquid-phase counterparts that the gas-phase transport rate would seldom be of importance in determining the overall rate of chemical reaction. 

Another interesting selectivity problem arises when there are two different reactants in the supply phase, say A and C, that both react with the liquid phase reactant B, forming P and Q respectively, where the formation of Q is undesired. An example of practical importance is the selective absorption of hydrogen sulfide from an inert gas containing also carbon dioxide, in an alkaline solution (containing, e.g., alkanol amines). Conditions can be such that carbon dioxide (C) reacts rapidly with the alkanol amine, whereas hydrogen sulfide (A) reacts instantaneously. The consequence is that the absorption rate of hydrogen sulfide is practically determined by the gas phase mass transfer rate, and Ae rate of carbon... 

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