Henry G. Schwartzberg

Modelling deformation and flow during vapor-induced puffing☆

Abstract An equation describing bubble expansion in pseudoplastic fluids was modified to provide a differential equation describing vapor-induced pore expansion in foams forming in ‘molten’ starch. Correlations for (1) the rheological properties of molten starches as functions of shear rate, water content, temperature and prior specific mechanical energy input; (2) equilibrium water partial pressures for such melts; (3) net latent heats; and (4) the diffusivity of water were used in conjunction with the pore expansion equation, mass and enthalpy balances, and equations describing diffusive transfer of water in shells surrounding pores, to model vaporinduced puffing of starch-based particles. Properties such as initial pore radius, popping temperature, surface tension and initial moisture contents and hypothetical correlations for the flow yield stress and wallrupture stress were used to permit the model to conform to known puffing characteristics of popcorn. The differential equations involved were solved by finite-difference procedures. Parameters in property correlations and unknown property values were adjusted to provide computed expansion times, expansion ratios, residual moisture contents and fractions of open pores that agreed with observed values for popcorn at different initial moisture contents.

Ice Crystal Size Changes During Ripening in Freeze Concentration

Crystal sizes were measured photographically versus elapsed time for non‐uniform ice crystal populations mixed at equilibrium temperatures with sugar solutions. The mass of ice did not change, but crystals smaller than a “neutral diameter,” Dn, tended to melt because of their excess surface energy. This subcooled the solution with respect to larger crystals, which consequently grew. This process, “ripening,” which slowly increases mean crystal sizes and Dn is used to produce ice crystals large enough to be cleanly separated from freeze concentrated solutions. “Sequential analysis” was used to determine Dn and follow changes in the diameters, Di, of fractions of the ice crystal populations. Rates of change in Di for both growth and melting were roughly proportional to (I/Dn ‐ 1/Di. Overall mass‐transfer coefficients were not markedly different for growth and melting, and decreased from roughly 2 mm/s to roughly 0.5 mm/s as sugar concentration increased 10% to 42%. In 10% sucrose solutions, mass‐transfer coefficients decreased from roughly 2 mm/s to roughly 1 mm/s when gelatin was added at levels which increased from 0% to 0.5%. Ripening in freeze concentrated liquid foods apparently can be accelerated by manipulating initial crystal size distributions and temporarily removing high molecular weight components.

Predicting Temperature vs Time Behavior During the Freezing and Thawing of Rectangular Foods

Based on the effective heat capacity characteristics of biological materials during freezing we have derived a short‐cut equation for predicting their temperature vs time behavior during freezing and thawing. The equation can be used in the low Biot number range for rectangular bodies with similar or dissimilar heat‐transfer coefficients at their different surfaces. It has been experimentally and numerically tested for rectangular blocks of chopped beef with two or four exposed sides for Biot numbers ranging between 0 and 3. In this range it predicts temperature vs time behavior with errors of less than 5 to 10% in terms of overall processing time. By using numerical methods, the approach used can be extended to handle problems involving time‐varying boundary conditions and many types of non‐linear, physical‐property behavior.

Mass transfer in a countercurrent, supercritical extraction system for solutes in moist solids

Abstract Solutions of partial differential equations (PDE) that describe mass transfer in conventional continuous, countercurrent solid-liquid extraction systems also successfully describe mass transfer in nearly continuous, contercurrent extraction systems where supercritical fluid (SF) is used to extract solutes from moist solids. These solutions can also be used to describe mass-transfer behavior in absorbers where solutes are transferred from SF to showers of liquid drops if information about drop diameters and velocities, effects of circulation in drops and, most important, axial dispersion is available. The variables involved include: dimensionless concentrations, Ficks number, F = Dst/a2 , the stripping factor, a the Peclet number, UL/Da and the mass-transfer Biot number, Bi. In extractors, a slightly greater than 1.0 are needed to provided efficient extraction without excessive circulation of solvent; in absorbers, α should be < 1.0. α, Pe and Bi are used in PDE solutions to determine F and extrac...

Batch Coffee Roasting; Roasting Energy Use; Reducing That Use

The operation of batch, gas-fired coffee roasters equipped with afterburners that destroy roasting-generated volatile organic compounds (VOCs) and carbon monoxide is described. Overall heat and material balances are used to analyze and calculate energy use in such roasters and energy use reductions achievable by (a) transferring part of the heat now discharged in stack gas to gas streams in or entering the roaster or to coffee entering the roaster or (b) bypassing afterburners when VOC production is low.

New needs and old needs.

o what extent and how should biotechnology be incorporated into chemical engineering education? Should T b iochemical engineering remain solely a specialized sub-discipline, or should some knowledge of biology and its engineering aspects form part of every new chemical engineer’s body of skills? Needs for specialized biochemical engineers will, no doubt, continue to grow; but so will the need for chemical engineers in general to become more knowledgeable and skilled in many areas dealing with biotechnology. We may soon have to decide how to provide such skills. To provide necessary background knowledge, chemical engineering departments should strongly consider adding a course in biochemistry and one in cellular biology, or a course combining elements from both of these subjects, to their curricula. At the very least, such courses should be available as electives. Departments experiencing dimculty in finding space to schedule such courses may want to consider replacing the second semester of Organic Chemistry with a course in Biochemistry. Should chemical engineering departments go beyond this? I think so. Most chemical engineering programs provide a total of two to four technical electives in the Junior and Senior years. Therefore, if suitably trained faculty are available, most chemical engineering departments could offer bioengineering electives in the Junior and Senior years. In fact, it appears that an increasing number of departments are doing so right now. Moreover, it probably would be worthwhile to incorporate some coverage of biological topics in more general chemical engineering courses. Can this be done without overloading students and without creating undesirable conflicts between new needs and old needs? To add this new material, what old material, if any, should be removed? When we examine chemical engineering curricula and what chemical engineers actually do, we find gaps and inconsistencies. Some chemical engineers require a knowledge of multicomponent distillation or catalytic reactor design in their daily work, many do not. Others require knowledge of specialized topics, such as furnace and boiler design, corrosion, solids handling, structural design, etc., which are rarely covered in undergraduate programs. Undergraduate programs do not and can not fully prepare students for the range of tasks they are likely to encounter in their professional career; and much of the specialized material covered in such programs may not be used at all by students after they graduate. Education is a lifelong process; and a great deal of specialized material has to be learned on the job. Our main tasks should be to provide students with skills and background knowledge which facilitate such learning. This does not mean that specialized topics should not be covered in undergraduate programs, but at the same time, they are not sacred cows. Such topics provide exemplary material which reinforces basic training and analytical skills in chemical engineering. The precise choice of topics may be somewhat less important than the reinforcement process itself. There is no reason why judiciously chosen topics from biochemical engineering and biology in general should not be part of such reinforcement. The underlying framework for chemical engineering includes: material and energy balances; thermodynamics and energy conversion processes; phase and reaction equilibria; heat, mass and momentum transfer; kinetics and process economics. These areas are also important for biological processes. We should be able to incorporate biologically-based material id many standard chemical engineering courses. One could, for example, readily cover microbial growth and inactivation and enzyme kinetics in courses on kinetics; treat oxygen and carbon dioxide transfer in fermentors in courses dealing with mass transfer; and cover substrate utilization and metabolic heat production in courses dealing with material and energy balances. Other examples could deal with cooling and gas turnover requirements during post-harvest storage; heat transfer in cans during retorting, or the freezing and thawing of foods. One might also be able to use biologically-based problems, such as, problems in pharmacokinetics, rather than problems relating to processing plant design in chemical engineering analysis courses.