Mass transfer to a continuous

Case B Cas-Liquid Mass Transfer to a Continuous Tank Reactor with Chemical Reaction... 

J. Mass Transfer to a Continuous Phase from a Single Spherical Drop... 

The liquid is transferred to a continuous extractor (Note 10) and extracted with ether until the supernatant layer of ether remains colorless (about 2 hours). The ethereal extract is discarded (Note 11). The aqueous solution is transferred to a 1-1. beaker and acidified by the cautious addition of 60 ml. of 121V hydrochloric acid (Note 12). The solution is returned to the extractor, which is attached to a tared round-bottomed flask. The solution is extracted with ether until no more -hydroxyphenylpyruvic acid is obtained (Note 13). The undried ether solution is evaporated to dryness on a boiling water bath to give crude p-hydroxy-phenylpyruvic acid as a pale-yellow crystalline mass. The mass is broken up with a spatula, and the flask is kept over potassium hydroxide in a vacuum desiccator until its weight is constant. The yield of crude acid is 6.9-7.2 g. (92-96%). It melts at 210— 215° (dec.) (Note 14). 

For steady-state mass transfer through a stagnant layer of fluid Eq. (21.19) or Eq. (21.24) can be used to predict the mass-transfer rate provided B-j- is known. However, this is not a common situation, because in most mass-transfer operations turbulent flow is desired to increase the rate of transfer per unit area or to help disperse one fluid in another and create more interfacial area. Furthermore, mass transfer to a fluid interface is often of the unsteady-state type, with continuously changing concentration gradients and mass-transfer rates. In spite... 

The reaction mixture in ethyl acetate is then transferred to a 100-ml reactor, purged under a nitrogen atmosphere, 340 mg of Lil is added, and the whole mass is then heated, with mechanical stirring, on an oil bath, up to ethyl acetate reflux temperature. The heating is continued for 5 hours, until the disappearance of the epoxide (II), as evidenced by the thin-layer chromatography. 

Most studies on heat- and mass-transfer to or from bubbles in continuous media have primarily been limited to the transfer mechanism for a single moving bubble. Transfer to or from swarms of bubbles moving in an arbitrary fluid field is complex and has only been analyzed theoretically for certain simple cases. To achieve a useful analysis, the assumption is commonly made that the bubbles are of uniform size. This permits calculation of the total interfacial area of the dispersion, the contact time of the bubble, and the transfer coefficient based on the average size. However, it is well known that the bubble-size distribution is not uniform, and the assumption of uniformity may lead to error. Of particular importance is the effect of the coalescence and breakup of bubbles and the effect of these phenomena on the bubble-size distribution. In addition, the interaction between adjacent bubbles in the dispersion should be taken into account in the estimation of the transfer rates... 

The proposed technique will be used here to illustrate the case of interfacial heat and multicomponent mass transfer in a perfectly mixed gas-liquid disperser. Since in this case the holding time is also the average residence time, the gas and liquid phases spend the same time on the average. If xc = zd = f, then for small values of t, the local residence times tc and td of adjacent elements of the continuous and dispersed phases are nearly of the same order of magnitude, and hence these two elements remain in the disperser for nearly equal times. One may conclude from this that the local relative velocity between them is negligibly small, at least for small average residence times. Gal-Or and Walatka (G9) have recently shown that this is justified especially in dispersions of high <6 values and relatively small bubbles in actual practice where surfactants are present. Under this domain, Eqs. (66), (68), (69) show that as the bubble size decreases, the quantity of surfactants necessary to make a bubble behave like a solid particle becomes smaller. Under these circumstances (pd + y) - oo and Eq. (69) reduces to... 

Mass transfer processes involving two fluid streams are frequently carried out in a column countercurrent flow is usually employed although co-current flow may be advantageous in some circumstances. There are two principal ways in which the two streams may be brought into contact in a continuous process so as to permit mass transfer to take place between them, and these are termed stagewise processes and continuous differential contact processes. 

In a liquid-liquid extraction unit, spherical drops of solvent of uniform size are continuously fed to a continuous phase of lower density which is flowing vertically upwards, and hence countercurrently with respect to the droplets. The resistance to mass transfer may be regarded as lying wholly within the drops and the penetration theory may be applied. The upward velocity of the liquid, which may be taken as uniform over the cross-section of the vessel, is one-half of the terminal falling velocity of the droplets in the still liquid. 

Various reactor combinations are used. For example, the product from a relatively low solids batch-mass reactor may be transferred to a suspension reactor (for HIPS), press (for PS), or unagitated batch tower (for PS) for finishing. In a similar fashion, the effluent from a continuous stirred tank reactor (CSTR) may be transferred to a tubular reactor or an unagitated or agitated tower for further polymerization before devolatilization. 

This introduces the gas-phase residence time VgjQg as a new parameter. It also introduces an ambiguity regarding the term kgAi(a — Ug). There is no resistance to mass transfer within a pure component so kgAj oo and a — Ug 0. Thus, kgAi(a — Ug) is an indeterminate form of the oo x 0 variety. Its value must continue to equal the rate at which oxygen is transferred into the liquid phase. Equation (11.5) remains true and the pair of simultaneous ODEs become... 

Mass transfer in the continuous phase is less of a problem for liquid-liquid systems unless the drops are very small or the velocity difference between the phases is small. In gas-liquid systems, the resistance is always on the liquid side, unless the reaction is very fast and occurs at the interface. The Sherwood number for mass transfer in a system with dispersed bubbles tends to be almost constant and mass transfer is mainly a function of diffusivity, bubble size, and local gas holdup. 

As Sherwood and Pigford(3) point out, the use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation. Applying the two-film theory of mass transfer, explained in detail in Volume 1, Chapter 10, the concentration gradients for transfer to a desired solute from a raffinate to an extract phase are as shown in Figure 13.19, which is similar to Figure 12.1 for gas absorption. 

Chemical reactions between the analyte and a reagent or combination of reagents can be implemented in continuous systems to obtain a reaction product that is capable of facilitating mass transfer and/or continuous detection that results in increased sensitivity or selectivity. These reactions... 

For a detailed description of the separation processes that may take place at the sensing microzone, the foundation of which is closely related to non-chromatographic continuous separation techniques based on mass transfer across a gas-liquid (gas diffusion), liquid-liquid (dialysis, ultrafiltration) or liquid-solid interface (sorption), interested readers are referred to specialized monographs e.g. [3]). 

Any consideration of mass transfer to or from drops must eventually refer to conditions in the layers (usually thin) of each phase adjacent to the interface. These boundary layers are envisioned as extending away from the interface to a location such that the velocity gradient normal to the general flow direction is substantially zero. In the model shown in Fig. 8, the continuous-phase equatorial boundary layer extends to infinity, but the drop-phase layer stops at the stagnation ring. At drop velocities well above the creeping flow region there is a thin laminar sublayer adjacent to the interface and a thicker turbulent boundary layer between this and the main body of the continuous phase. 

If a particle is suddenly exposed to a step change in the composition of the continuous phase, or if the surface composition undergoes a step change to a new constant value, the rate of mass transfer becomes a function of time even... 

Very few solutions have been obtained for heat or mass transfer to nonspherical solid particles in low Reynolds number flow. For Re = 0 the species continuity equation has been solved for a number of axisymmetric shapes, while for creeping flow only spheroids have been studied. 

To 1035 g. of syrupy arsenic acid (80-85 Per cent, sp. gr. 2.00/200 Note 1) in a 12-inch evaporating dish, is added 828 g. (800 cc.) of aniline (Note 2) in 100-cc. portions meanwhile, the lumps of aniline arsenate which are formed are broken up by rapid stirring with a porcelain spatula. When all the aniline has been added, the powdered solid is transferred to a 3-I. round-bottom flask equipped with a mechanical stirrer, a thermometer reaching to the lower part of the vessel, and a condenser arranged for downward distillation (Note 3) an additional 800 cc. of aniline is added and the flask slowly heated in an oil bath. The bath may be kept at a temperature not exceeding 170-175° as long as there is any considerable amount of unmelted material in the flask. When the contents of the flask have become liquid the temperature of the bath is dropped and the mixture held at 155-160° (inside temperature), with continual stirring, for at least four and a half hours. The mass will have assumed an intense violet color. 

---