C. Judson King

Pressurized freezing for retarding shrinkage after drying: A quantitative interpretation

Abstract Certain gases, when dissolved at high pressure in foods and biological materials, give a structure of gas bubbles inflating cells after freezing, depressurization, and thawing. This phenomenon has been found to reduce shrinkage after subsequent drying. The effect is interpreted quantitatively in terms of the needs for (a) a sufficient solubility of the gas in the water of the tissue at high pressure so that the Bunsen coefficient is high enough (at least 1.5 cm3/cm3 for maximum effect), and (b) a sufficiently low permeability, expressed as the product of atmospheric solubility and diffusivity in water (equal to or less that of air for maximum effect).

Precipitate flotation of copper as the sulfide, using recyclable amphoteric surfactants

Abstract An experimental study was made to identify surfactants which are effective for removal of copper from dilute aqueous solution (100–500 ppm) by precipitate flotation as the sulfide, and which at the same time can be recovered from the CuS product for recycle. Batch flotation experiments confirmed that a cationic surfactant was necessary for flotation of CuS from such dilute suspensions; however, no satisfactory way could be found for recovering cationic surfactants from the CuS. This led to consideration of amphoteric surfactants, which are cationic at low pH and anionic at high pH. It was found that a change to negative, rather than simply neutral, charge was required for efficient surfactant recovery. It was further found, through the assistance of experiments in which the CuS suspension was agitated with solvents, that certain functional groups which interact chemically with the CuS surface should also be absent from the surfactant molecule. Following this logic, Amphoterge K-2 (Lonza Chemical Co.) was identified as a suitable surfactant, provided CuS was precipitated with S 2− in excess. Tests established that 95% of adsorbed Amphoterge K-2 could be recovered by raising the pH to 11 and boiling the suspension for one hour, followed by decanting. Surfactant thus recovered was effective in a second flotation test. Foamate solids settled rapidly; such behavior would help reduce the consumption of chemicals for the pH change. Column flotation studies were made using Amphoterge K-2 for removal of Cu 2+ present at 100 ppm and pH = 2. High removals of CuS could be obtained at concentrations of surfactant above about 25 ppm, for which conditions a substantial fraction of the surfactant remains in solution rather than being adsorbed onto the CuS. The recovery of CuS would be improved by introducing the surfactant in a separate feed, below the feed of CuS suspension. Adding some surfactant in the CuS feed, as well as in a lower feed, gave an even better recovery of CuS (99.8%) at sufficiently high surfactant loadings.

Upflow vertical tube evaporation with interface enhancement; pressure drop reduction and heat transfer enhancement by the addition of a surfactant

Abstract The heat transfer performance of a series of full-sized, commercial distillation tubes of the double-fluted type was evaluated in an upflow vertical tube evaporator, under process conditions that are realistic for large-plant multiple effect operation. This tube series comprised two of each aluminum-brass tubes, 1O-ft. long and 3-, 2-, 1.5- and 1-in. diameter, tested in parallel pairs. Fresh water and sea water were used as feeds recirculated to the evaporator, with and without the addition of a surfactant for interface enhancement: heat transfer performance was approximately doubled with the larger diameter tubes but the smaller tubes did not respond well. It was concluded that differences in tube diameter can account for the apparent contradictions in the literature, on the basis of our tube pressure drop data. Some conclusions pertinent to the design of multiple-effect vertical tube evaporators are also drawn.

Maximum droplet growth when cooling a saturated solution

Abstract The maximum size of a droplet or spherical solid particle which grows from an initially saturated, homogeneous binary solution as the temperature decreases is predicted through an approximate, closed solution valid for solutes of moderate solubility (up to 1 per cent). The dimensionless radius-time relationship is presented for various levels of the parameters, which involve the activation energy for the temperature dependence of the diffusivity, the heat of solution, and the initial and final temperatures of the system. The decrease of diffusivity with time is shown to hinder the growth process, offsetting the simultaneous increase in driving force.

Selection and Sequencing of Separations

Abstract Selection criteria are reviewed for choosing the most promising methods of separation for accomplishing a given separation of chemical components. The degree of separation attainable is governed by the separation factor, which in turn is related to molecular or ionic properties. Various separation processes are contrasted with regard to general desirablilty of using them, both on an industrial scale and on a laboratory scale. Heuristic rules for synthesizing sequences of separations to create multiple products are also considered. Criteria which have proven useful for generating separation sequences on an industrial processing scale are contrasted with those which may be useful for choosing sequences of separation steps in the laboratory.

Guest Editorial: Research on Drying

I am pleased to congratulate Arun Mujumdar on the occasion of the twenty-fifth anniversary of the founding of Drying Technology. With the start of this journal and the earlier launching of the International Drying Symposia (IDS), Professor Mujumdar created vehicles that invigorated the field of drying research and brought it to a much greater and broader level of activity. When one considers the very large capacity and myriad applications of drying, the amount of activity on drying research still seems less than is warranted by the importance of the field, but the research would be far less without Drying Technology and IDS. Drying processes are among the largest consumers of energy in industry. There are good avenues for research to lessen energy consumption in drying. The volume of research on energy conservation in drying over the years has been much less than, for example, that on energy conservation in distillation. Yet the importance is comparable. Possible reasons for this striking difference are that the equipment for distillation is less varied than that for drying and that distillation research classically received much attention from large petroleum and chemical companies, as well as from the Fractionation Research Institute. Applications of drying and types of dryers are more diverse and scattered than those for distillation, with the result that the private sector has not invested as much in research. There is therefore a cogent argument for more public sector support of research on drying. The U.S. Department of Energy and U.S. Department of Agriculture and comparable organizations in other countries are logical venues for that purpose. Research on drying has tended to be product specific and dryer specific. This fact has limited the growth of general understanding. There can and should be more generic research on drying. For example, the mechanisms of water movement through most media being dried are still not well understood, yet with knowledge of transport mechanisms one has a much better insight into how to control structure, migration of solutes, drying rates, and the loss or retention of properties such as texture, flavor, aroma, and color. Since drying is one of the simplest, yet effective, means of preservation of foods, pharmaceuticals, beverages, and other consumer products, such knowledge can extend and improve applications considerably. Another general challenge in food and beverage applications is to retain flavor and aroma in substances that are far more volatile than water, even though the water is being vaporized. Since equilibrium is not favorable, the approach must be through selective transport, taking advantage of the much smaller size of the water molecule and creating local environments that trade upon molecular size for mobility. Much has been learned about how to do this in freeze and spray drying, but many opportunities for understanding and advances remain for those and other applications. The inverse need pertains when an odorous substance is being dried to a product that should be odor free, leading to approaches that minimize transport barriers to enable full escape with the water vapor. Yet another need is to know how to control the structure and morphology (shape, form, and configuration) of a dried product. This knowledge is useful to control bulk density, porosity, lightness or darkness of appearance as relates to exposure of pores at the surface, layering of substances for products that are mixtures, tendencies of the product to be crystalline or amorphous, flowing properties of powders as they relate to surface smoothness, loss or retention of non-aqueous volatile substances during bursting, wetting properties, and rehydration tendencies. Rehydration can also occur in unwanted situations. For amorphous materials, the phenomena of stickiness and structural collapse have been explained in terms of viscous flow. With this understanding, more research could be carried out on product formulation and on ways to control drying conditions to result in desired structural morphology and lack of stickiness, with applications particularly to specialized biological and pharmaceutical products and=or to freeze drying. These are but a few of the avenues of drying research that are promising and that are not product specific or dryer specific. More research of this sort will provide an underpinning of fundamental understanding that will be powerful in improving and controlling product quality, enabling new and better designs and configurations of dryers, saving energy, and extending the use of drying as a method of stabilization and preservation.