Sherwood Data System

The autosampler system is controlled by an IBM computer system, as is the series of pumps for the cell and the sample wash pot. An Archer single board computer (Sherwood Data System) programmed via ASCII strings along an RS232 interface, controls the pumps and the autosampler, setting up a stable representative sample which is then measured by using the standard Solomat software and hardware. 

Figure 9-73 presents some of the data of Fellinger [27] as presented in Reference 40 for Hqg for tho ammonia-air-water systems. This data may be used with the Sherwood relations to estimate Hl and Hg values for other systems. 

Mass transfer across the liquid-solid interface in mechanically agitated liquids containing suspended solid particles has been the subject of much research, and the data obtained for these systems are probably to some extent applicable to systems containing, in addition, a dispersed gas phase. Liquid-solid mass transfer in such systems has apparently not been studied separately. Recently published studies include papers by Calderbank and Jones (C3), Barker and Treybal (B5), Harriott (H4), and Marangozis and Johnson (M3, M4). Satterfield and Sherwood (S2) have reviewed this subject with specific reference to applications in slurry-reactor analysis and design. 

Values of all of these parameters must be available or estimated if we are to determine the global reaction rate. Some of these quantities can be evaluated from standard handbooks of physical property data, or generalized correlations such as those compiled by Reid and Sherwood (87). Others can be determined only by experimental measurements on the specific reactant/catalyst system under consideration. 

A direct-contact gas cooler system operates as follows Approximately 35,000 lb/hr of bone-dry air is passed over hot trays. The air is heated from 150°F to 325°F as it passes over the trays. It exits from the unit with a due point of 105°F. The hot air is sent to a direct-contact cooler, where its temperature is reduced back to 150°F. During the cooling stage, the air is dehumidified with water that is heated frpm 75°F to 105°F. The unit is rated at 3.5 inches of water pressure drop (a) Determine the number of diffusion units needed for this operation and (b) Establish the required dimensions for the direct-contact cooling tower (Hint Use standard low-pressure-drop data from the literature. Some of the older literature give pressure drop data for simple fill. See Sherwood, T. K. and C. E. Reed [6]. 

There is ample experimental evidence to show that the efficiencies of different components in a multicomponent system are not all equal. The first clear statement of this fact can be found in a paper by Walter and Sherwood (1941) who, on the basis of an extensive experimental study of Murphree vapor and liquid efficiencies for absorption, desorption, and rectification operations, concluded The results indicate that different efficiencies should be used for each component in the design of absorbers for natural gasoline and refinery gases. Since the publication of their paper many others have provided additional data to confirm this view [see Krishna and Standart (1979) for a list of references]. We review some of these data below. 

The following data were obtained by Chamber and Sherwood [Ind. Eng. Chem., 29, 1415 (1937)1 on the absorption of ammonia from an ammonia-air system by acid in a wetted-wall column 0.575 in. in diameter and 32.5 in. long. 

The amount of additional information needed to be able directly to take into account heat and mass transfer in Model 4 is high. Using the two-film theory, information on the film thickness is needed, which is usually condensed into correlations for the Sherwood number. That information was not available for Katapak-S so that correlations for similar non-reactive packing had to be adopted for that purpose. Furthermore, information on diffusion coefficients is usually a bottleneck. Experimental data is lacking in most cases. Whereas diffusion coefficients can generally be estimated for gas phases with acceptable accuracy, this does unfortunately not hold for liquid multicomponent systems. For a discussion, see Reid et al. [8] and Taylor and Krishna [9]. These drawbacks, which are commonly encountered in applications of rate-based models to reactive separations, limit our ability to judge their value as deviations between model predictions and experimen-... 

For this gas-film-controlled system, the rate of mass transfer is proportional to gas rate to approximately the 0.8 power. This is similar to data on the isothermal evaporation of pure liquids from a pipe wall into a turbulent air stream. There, Gilliland and Sherwood showed that the mass transfer rate is proportional to the gas mass velocity to the 0.83 power [4]. For systems evaporating water into an air stream, the effect of the liquid rate is similar to that for absorption. Because a large percentage of the heat is transferred by the vaporization of water, it is reasonable to find that the effect of gas and liquid rates is similar to that for other gas-film-controlled mass transfer operations. 

The design becomes somewhat more complex when a mixture of compounds is involved. However, the components of the mixture are usually mutually soluble in the liquid phase, and, as a first approximation for related solvents, it can be assumed that the mixture follows Raoult s law, i.e., the partial pressure of each component in the product gas will be equal to the vapor pressure of the pure component at the gas outlet temperature times its mole fraction in the liquid phase. For more precise calculations and more complex liquid mixtures, it is necessary to use vapor-liquid equilibrium (VLB) data for the specific system. The estimation and correlation of VLB data are discussed in various chemical engineering texts, such as Perry s Handbook (Perry et al., 1984), Reid and Sherwood (1966), and Prausnitz (1969). 

Heat and mass transfer coefficients are usually reported as correlations in terms of dimensionless numbers. The exact definition of these dimensionless numbers implies a specific physical system. These numbers are expressed in terms of the characteristic scales. Correlations for mass transfer are conveniently divided into those for fluid-fluid interfaces and those for fluid-solid interfaces. Many of the correlations have the same general form. That is, the Sherwood or Stanton numbers containing the mass transfer coefficient are often expressed as a power function of the Schmidt number, the Reynolds number, and the Grashof number. The formulation of the correlations can be based on dimensional analysis and/or theoretical reasoning. In most cases, however, pure curve fitting of experimental data is used. The correlations are therefore usually problem dependent and can not be used for other systems than the one for which the curve fitting has been performed without validation. A large list of mass transfer correlations with references is presented by Perry [95]. 

Some typical compilations of experimental data are shown in Figs. 7.39 and 7.40 for heat and mass transfer, respectively. These data indicate that for low values of the Reynolds number, the Sherwood or Nusselt number tends to fall below the limiting value of 2 established for single particles or for packed-bed systems [viz. Eqs. (7.3.18), (7.3.23), etc.]. It was suggested by Kunii and Levenspiel [47] that this apparent anomaly may be resolved by considering that even in these systems some fraction of the gas passed through in the form of bubbles. Thus an incorrect driving force has been used in the calculation of the transfer coefficients. 

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