The evaporation of pure liquid drops and the drying of drops containing suspended and dissolved solids in an atmosphere of superheated vapor were studied. Changes in the weight and temperatore of approximately 2 @liter drops of four food products, five miscellaneous materials, and pure water were measured as evaporation and drying proceeded a t different drying temperatures. Evaporation of water was found to take place more slowly in superheated steam than in air. However, the medium, in which faster drying occurred, depended upon the material being dried. No major differences between the final products were observed for these two drying media except that some materials yielded denser particles in superheated steam than in air.The potential of superheated vapors, which are chemically identical to the moisture being removed from a solid, as a drying medium has long been recognized (7). The number of commercial applications has, however, been somewhat limited ( 2 , 4, 11, 23). Drying with superheated vapors has numerous advantages over drying with gases, and that advantage most often cited is the considerable improvement in thermal efficiency ( 4 , 7, 2 2 ) . Chu et al.( 4 ) pointed out that the problem of dust collection is considerably simplified when the excess vapors to be removed from the system are passed through a total condenser. It was also noted that drying with superheated vapors provides an inert environment in which to process materials which are readily oxidized and that, when dealing with flammable solvents, the danger of an explosion within the dryer is eliminated. Walker et al. (23) illustrated the use of superheated vapors as an effective drying medium when the material being dried is a poor conductor of heat. Also, the volume of the exhaust from a drying system will be much smaller for superheated vapor than for gases. A major disadvantage in using superheated vapors is the difficulty in handling solids which are sensitive to temperatures above the boiling point of the solvent at the pressure existing in the diyer. The practical limitation of constructing and financially justifying satisfactory equipment in which condensation will not occur and from which the dry product can be removed without loss of the vapors ( 1 4 ) also exists.A pure liquid which evaporates into a gas or dissimilar vapor attains dynamic equilibrium at a temperature somewhat below the dry bulb temperature because of the combined resistances to heat and mass transfer in such a drying medium. When the liquid evaporates into a medium of its own vapor, the resistance to mass transfer becomes vanishingly small, and the temperature of the liquid then approximates that of its saturated vapor at the ambient pressure ( 4 , 5, 21, 2 4 ) . Similar thermal effects would be expected during the first period of drying when dissolved solids are present. This has been observed for the drying of single drops in air ( 3 , 15 18), even though the length of this period of drying may indeed be very short.Numerous experimental studies of the rates ...

No abstract

No abstract

No abstract

A general analysis of continuous recycle systems and models is presented by considering the fluid history inside the systems. The history of a fluid element is expressed in terms of the number of cycles it completes, its residence time in the system and the total time it resides in a specific section of the systems. Concepts from probability theory are used to derive expressions for the number of cycles distribution (NCD), the residence time distribution (RTD), and the total regional residence time distribution (TRRTD) and their means and variances. Applications of these distributions in various processes are considered. Relationships between the number of cycles, the residence time, and the total regional residence time are expressed in terms of the covariances and correlation coefficient of pairs of these variables. These relationships allow the estimation of one variable for a given value of the other, and thus provide a mathematical means to completely describe the history of fluid in a recycle system. Part I. Single Unit SCOPEThis article presents a general analysis of continuous recycle systems and recycle flow models. The history of the fluid in a recycle system is considered in terms of the time a fluid element resides there, the number of cycles it completes, and the total time it spends in specific sections of the system. Three new concepts are introduced: the joint distribution of number of cycles and residence time, the number of cycle distribution (NCD), and total regional time distribution (TRRTD). These, together with the residence time distribution (RTD), the joint parameters of cycle time, number of cycles, and regional residence time derived here provide a mathematical means to characterize and analyze recycle processes and models.Continuous recycle operations are commonplace in the chemical industry. Many processes such as catalytic cracking, particle coating, granulation and crystallization are based on recirculation. In many large reactors, a recycle stream is used to enhance mixing or improve selectivity.In addition, recycle flow models are commonly used to represent flow in large vessels and describe deviations from ideal mixing (Gibilaro 1971). At present, only the residence time distribution (RTD) is used to characterize recycle systems and construct recycle models.While the RTD is a useful tool in building and verifying recycle models and in characterizing certain recycle processes, there are many systems where the RTD is not an important process parameter. For example, in particle coating (Mann 1972, Mathur and Epstein 1974) the number of cycles a particle completes will determine the amount of coating accumulated. The number of cycles distribution (NCD) provides a means to express the coating uniformity. In systems where different regions are maintained at different conditions, the system performance may be more accurately related to the total time a fluid element resides in a certain region, rather than its residence time in the system as a whole. This is the case, for example, in con...

No abstract

No abstract

The use of vaneless disk atomizers for the partial cleaning, screening, and fractionation of wood pulp fibers was investigated. Studies with paper pulp and rayon fibers indicated separation to occur at the disk edge by ejection of dewatered fibers with classification primarily dependent on fiber diameter. Very shallow disks having keen-edged chamfers with lips of extreme widths effected sharp fractionation of rayon fibers. A simple mechanistic model based on fiber inertia and surface forces described the separation process reasonably well and indicated extension to other particle geometries. Mechanical entanglement of irregular particles and flow instabilities limited slurry concentration and processing rates. IntroductionClassification of slurried solids in the process industries on any significant scale is based usually on differences in particle size and/or density and occasionally on shape. Screening techniques augmented by mechanical vibration and/or hydraulic pulsation generally are used to separate particles according to size. Hydraulic schemes based on inertia or relative velocity, supplemented many times by mechanical assistance, commonly are used to separate particles according to density. Sharp fractionation of particles which deviate from three-dimensional symmetry and/or lack rigidity is more difficult. This problem is particularly acute in the processing of wood and paper pulps, whose useful portions are composed of fibers rendered moderately flat and quite pliable by beating and milling. Larger nonpulped and partially pulped wood particles are removed by screening with provisions to preclude blinding and entrapment, while high-density contaminants are separated hydraulically by cyclones. Reasonably successful fractionation of clean fibers according to length by low-pressure screening techniques and hydrocyclones requires concentrations to be as low as 0.1-0.6 wt %, while high-pressure screening permits concentrations to increase to ca. 1.5 wt % (Britt, 1970). Fiber fractionation by suitable control of flow conditions in discrete fluid fields, e.g., the Johnson fractionator (Olgbrd and Axenfalk, 1972), is limited to concentrations below 0.3 wt %.A new technique to clean, screen, and fractionate pulp suspensions of higher concentrations based on the use of a high-speed, rotating, vaneless disk of the design normally used in spray drying has been proposed (Moller et al., 1979). Fractionation is realized by collection of various axial zones of the circular particle cloud near the disk periphery, as shown in Figure 1. These disks resemble an inverted saucer which terminates in a short hooplike skirt whose border is chamfered to produce a sharp edge at the inner surface, as shown in Figure 2. For the processing of sediment pulps containing 3 wt % solids, it generally was found that (i) fine cellulose and clay particles were concentrated near the top of the cloud, (ii) good pulp fiber was concentrated near the middle and upper middle of the cloud, and (iii) sand grains, nonfibrous wood particles, and bar...