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Thus, several authors have reported that a darker the relative importance of the different mechanisms. Therefore, the tubers must balanceresults: Bird et al. In the French frying process the blanching operation consists in placing the potato strips on conveyor belts that carry them through a hot water bath. Smith, In order to design the operating conditions of the Whereh is the heat transfer coefficient at the sphere-fluidinter- process, in both cases,it is necessary to know the reducing face and k the potato thermal conductivity.

The specific weight of where DG is the apparent diffusion coefficient of reducing sugars each tuber was determined by using a picnometer. In this equation by replacing the potato sphere with an acrylic one, 3.

Then, In Eq. Heat and mass transfer with simultaneous The reaction rate term in Eq. Potato spheresof 2. Stirring was Califano and Calvelo, in which the rate of glucose production strong enough to secure uniformity of heat transfer coefficient.

This kinetic Temperaturesat the center of the sphereand in the blanchingbath behavior may be extended to the production of reducing sugarson were recorded and initial and final concentrations of reducing sugars the assumption that fructose might not play an inhibiting role on were measured. Duplicates of experiments were performed in order the reaction.

Moreover, for proposing Eq. Temperatures were Solution method comparedwith those predicted by an unsteadystateheat transfer Eq. Once the concentration profile was obtained as a function of Table 1 shows the change in the reducing sugar content time by means of an IBM Series 1 Computer, averageconcentration during the heating of potatoes without blanching. Thus, experiments on heating of potato Time spheres were carried out, and the results obtained evaluated by T, "C Tr 'C 1s Initial Final kg?

Kennebec potatoes harvest were previously placed in a I tronically controlled resistances and introduced into a 0. IIhe potato sphere was placed at the center of the This design was developed in order to obtain a uniform Each potato sphere of 2. The air temperature was also recorded.

The A satisfactory corre- values reported in the literature for different enzymes lation was obtained between the experimental thermal capable of causing a reducing sugar increase in similar histories and those calculated by using the model. Different pairs of values of the pre-exponential factor A and the activation energy E, in the kinetic equation Heat and mass transfer with simultaneous for reducing sugar generation were fed into the model, generation of reducing sugars calculating the variance s2 and the sum of residuals S.

Results on changes of the reducing sugar content during in each case Box et al. The blanching temperature Tf ranged from The average initial and final water contents of the samples were The error of the average was f 0. As it may be tions and the corresponding heat and mass transfer proper- observed, there exists a good agreement between both sets ties. Several values of the apparent diffusion coefficient of glucose in potato were supplied to the program for each run of data.

The scattering between the experimental concen- and the coefficient which minimized the variance and the trations and those predicted by the model with the adopted sum of residues was chosen. The heat transfer parameters fed to the program were those already reported. A satisfactory agreement between experimental thermal histories and those predicted by the model was obtained. The fluid properties were evaluated at the bath temperature; CT is the total molar concentration of water.

Values of kL ranged from 3. The apparent diffusion coefficient obtained changed with temperature according to the Stokes-Einstein equation as proposed. Z-Experimental final concentrations in terms of predicted Stahl and Loncin reported an effective diffusion coefficient of 6. The corre- sponding diffusivity in water was 8.

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This tortuosity factor value shows very good agreement with that obtained from the reported blanching experiments. A sharp decreaseof the concentration near the potato Moreover, as heat transfer in this system is faster than surface is observed due to the diffusion of the reducing mass transport, very low values of Lewis number an sugars to the blancher.

Effect of changes in the blanching temperature A slight maximum is also shown in Fig. The value obtained was in accor- 70 - dance with the data reported by other authors in similar I 5 I 10 I 15 I systems. The tortuosity factor agreed with the value calcu- t min lated from data reported by Stahl and Loncin on 1 Fig.

However, such increments reducing sugar content, due to its influence on the simul- do not affect the surface concentration of reducing sugars taneous mechanisms of generation and mass transfer to the in the same way. Thus, the effect a tends to decrease the blanching bath.

Above that temperature the starch This increment seems of little significance when com- gelatinization process is also significant.

Editors , Ch. Applied SC. Certari asupres calitatiflortehnol- ogie a le siuriIor zonate de cartofi in vederea prelucrariprin deshi- taneous generation of sugars is not taken into account dratare si prefire. Lucrari stinntifice 8: Burton, H. Development amino systems.

Nature Potatoes, their changes and needs in storage. Farm periods of time in order to achieve the same concentration Building Digest 3 14 : It provides an introduction to mass transfer applied in biology and medicine.

The result is an engineering book which is much more readable and understandable than other books covering these subjects. It provides much more physical insight than conventional books on unit operations. It explores the interactions between mass transfer and chemical reaction, which are omitted by many books on transport phenomena. The earlier editions are good, but this one is better.

The book works well as a text either for undergraduates or graduate students. For a one-semester undergraduate chemical engineering course of perhaps 45 lectures plus recitations, I cover Chapter 2, Sections 3. If there is time, I add Sections If this course aims at describing separation processes, I cover crystallization before discussing membrane separations.

We have successfully taught such a course here at Minnesota for the last 10 years.

For a one semester graduate course for students from chemistry, chemical engineering, pharmacy, and food science, I plan for 45 lectures without recitations.

This course covers Chapters 2 to 9 and Chapters 16 to It has been a mainstay at many universities for almost 30 years. Diffusion and mass transfer are often interesting because they are slow.

Their rate controls many processes, from the separation of air to the spread of pollutants to the size of a human sperm. The study of diffusion is thus important, but it is also fun.

I hope that this book catalyzes that fun for you. Diffusion is a fascinating subject, as central to our daily lives as it is to the chemical industry.

Diffusion is responsible for gas absorption, for the fog formed by rain on snow, and for the dyeing of wool. Problems like these are easy to identify and fun to study. In fact, it is relatively simple. I have more trouble with a shear stress.

I have never clearly explained chemical potentials to anyone. These books fall into two distinct groups that are hard to read for different reasons. Such texts are characterized by elaborate algebra, very complex examples, and turgid writing. Students cheerfully hate these books; moreover, they remember what they have learned as scattered topics, not an organized subject. The second group of books consists of texts on transport processes. They are much more readable than the traditional texts, especially for the mathematically adroit.

Such cases include simultaneous diffusion and chemical reaction. This approach effectively excludes students outside of engineering who have little interest in these other phenomena.

Diffusion Mass Transfer in Fluid Systems

I emphasized physical insight, sometimes at the loss of mathematical rigor. I discussed basic concepts in detail, without assuming prior knowledge of other phenomena. I aimed at the scope of the traditional texts and at the clarity of books on transport processes.

This second edition is evidence that I was partly successful. Had I been completely successful, no second edition would be needed. Had I been unsuccessful, no second edition would be wanted.

Chapter 8 on mass transfer xxi xxii Preface to Second Edition is expanded to even more detail, for I found many readers need more help. Chapters 9—12, a description of traditional chemical processes are new. The result is still useful broadly, but deeper on engineering topics.

I have successfully used the book as a text for both undergraduate and graduate courses, of which most are in chemical engineering. I then cover the material in Chapters 9—12 in detail, for this is the core of the subject. I conclude with simultaneous heat and mass transfer, as discussed in Chapters 19— The resulting course of 50 classes is typical of many offered on this subject. For graduate students, I give two courses in alternate years. Neither requires the other as a prerequisite.

In the second graduate course, on mass transfer, I cover Chapters 8—9, Chapters 13—16, and Chapter These courses, which typically have about 35 lectures, are an enormous success, year after year. For example, for those in the drug industry, I might cover Chapters 11 and I am indebted to many who have encouraged me in this effort. My overwhelming debt is to my colleagues at the University of Minnesota. Mistakes that remain are my fault.

Diffusional Mass Transfer by Skelland a H P

Teresa Bredahl typed most of the book, and Clover Galt provided valuable editorial help. Finally, my wife Betsy gives me a wonderful rich life. After a day it will penetrate upward a few centimeters. After several years the solution will appear homogeneous.

diffusinal mass transfer bye skelland

The process responsible for the movement of the colored material is diffusion, the subject of this book. Diffusion is caused by random molecular motion that leads to complete mixing. It can be a slow process. In general, it varies less with temperature than do many other phenomena. This slow rate of diffusion is responsible for its importance. In many cases, diffusion occurs sequentially with other phenomena.

When it is the slowest step in the sequence, it limits the overall rate of the process.

It limits the speed with which acid and base react and the speed with which the human intestine absorbs nutrients.We discuss the differences between the two models in Section 1.

If this course aims at describing separation processes, I cover crystallization before discussing membrane separations.

These assumptions may seem arbitrary, but they are similar to those made in many other branches of science. The result is an engineering book which is much more readable and understandable than other books covering these subjects.

Lucrari stinntifice 8: Bengtsson and M.

We measure the drug concentration versus time as shown, and we want to correlate these results in terms of as few parameters as possible. Thus, the effect a tends to decrease the blanching bath. The change of concentration in each bulb is a measure of diffusion and can be analyzed in two different ways.