SEPARATION PROCESSES

PHILLIP C. W ANKAT Purdue University West Lafayette, IN 47907

s EPARATION PROCESSES AND MASS transfer have long been an integral part of chemical engi­

neering education. At Purdue University two graduate electives in separation processes and one elective in mass transfer are offered. The graduate students also all take a course in trans­port processes, which covers the basics of mass transfer.

One of - the separation electives (ChE 558, Equilibrium Stage Separation Processes) is a dual level senior / graduate elective. This course covers multicomponent distillation, absorption and extraction and an optional section on chroma­tography. C. Judson King's text Separation Pro­cesses is used, and the course has been taught for the last nine years (see Ref [l] for details).

The mass transfer elective (ChE 624, Mass Transfer) has been taught as a special topics course. Recently, this course has emphasized the fundamentals of multicomponent systems, mass

Phil Wankat received his BSChE from Purdue and his PhD from Princeton. He is currently a professor of chemical engineering at Purdue. He is interested in teaching and counseling, has won several teach ing awards at Purdue, and is a part-time graduate student in Education. Phil's research interests are in the area of separation process with particular emphasis on cyclic separations, two-dimensional

separations, preparative chromatography, and high gradient magnetic

separation.

io8

transport through membranes, convective mass transfer, and the macroscopic mass balance. Ap­plications emphasized have been in turbulent diffusion, mass transfer at phase boundaries, mass transfer with simultaneous chemical reaction, fixed bed sorption, transport through polymers, and biomedical devices.

The third elective in this area (ChE 623, Separation Processes) is a much newer course and has only been taught twice in its current form. This course was designed to cover subjects not covered in the other two electives, and to do it in different ways. The result is a unique elective in separation processes which is the subject of this article.

CO URSE PHILOSOPHY

J N DESIGNING CHE 623, a course philosophy had to be developed and followed. The first tenet

was that as a special topics elective it is difficult to say something must be part of the student's education. Thus, I was willing to initially consider almost any subject as long as it was in the general area of separation processes. However, the second tenet limited the first since I decided not to allow substantial overlap with either ChE 558 or ChE 624. Thus, distillation, absorption, extraction and fundamental mass transport theory would not be covered.

My third decision was to spend close to half the semester on operating techniques for adsorp­tion, chromatography and ion exchange. The major reason for this choice was selfish : this is my research area (and I want to tell the world about my research) and it is an important class of separation techniques which I believe will be­come increasingly more common in the future. Because of my enthusiasm the students also be­come interested and, in addition, it helps train my graduate students for their research.

The fourth decision was to allow the students to be selfish and to pick areas that interest them

© Copyright ChE Division, ASEE, 1981

CHEMICAL ENGINEERING EDUC~TION

PERIOD(S)

1 2 3-6

7 (R)

8

9 (R) 10 11 12 (R) 13 (R) 14 15 16

17-18

19 20-22 23 24 (R)

25-26 27 (R) 28-29 30 (R) 31-34 35 (R) 36-42 43-45 (R) Finals

TABLE 1 Preliminary Course Outline

SUBJECTS

Introduction Overview and classification schemes [2, 3] Fixed beds: Phenomena [ 4 ], column balances [5, 6], local equilibrium theory [5, 6] Discussion of separations literature and of topics for second half of course Sorbex process [7] and two-dimensional analogy [8] Discussion of experimental papers Moving feed point chromatography [9] Parametric pumping [4, 10] Discussion of theoretical papers Library search methods Pressure swing adsorption [11, 12] Cycling zone adsorption [4, 13, 14] Application local equilibrium model to ion exchange [5, 15] Solution for local equilibrium with dispersion and linear system analysis [16, 17] Constant Pattern Solutions [6] Thomas Solution Method [6, 16, 18] Test No class, Individual meetings on course project Topics selected by class No class, Individual meetings Topics selected by class No class, Individual project reports Topics selected by class No class, Individual meetings Topics selected by class Student presentations of course projects 2nd test (not a final)

(R) Periods spent on separations research and class research project.

for the second half of the semester. Thus I let the class pick the topics, subject only to the first two constraints.

The last three decisions were concerned with the way the course was taught. Since the lecture is an efficient method for presenting new informa­tion, I decided that most of the content would be transmitted by lecture. An assigned text was not used, partially since there is no text covering the diverse topics of this course, but also because I wanted the students to get a feel for the separa­tions literature. So a combination of textbooks, journal articles, and handouts was used. Finally, I wanted the students to get an idea of what re­search in separations is like. This goal was achieved with a course project which consisted of a small, theoretical research project on an un­solved problem.

The ways in which these decisions were imple­mented is discussed in detail below.

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We first started with ordinary adsorption and then considered simulated

counter-current operation and the related moving feed point chromatography.

COURSE SCHEDULE

T o MEET THE OBJECTIVES discussed above, the preliminary schedule shown in Table 1 was

handed out the first day of class. Note that during the first half of the course a variety of operating methods for adsorption, chromatography and ion exchange were covered, and that this portion of the schedule is listed in detail. The schedule for the second half of the semester was left open and was filled in only after considerable discussion with the students.

Throughout the semester, time was allotted for discussion of the research literature in separa­tions, and for the research project. Individual meetings with the students were scheduled and time was set aside for student presentations at the end of the semester.

ADSORPTION, CHROMATOGRAPHY AND ION EXCHANGE COVERAGE

THE COURSE OUTLINE FOR coverage of adsorp-tion, chromatography and ion exchange is

shown in Table 1. First we looked at the basic equations of change for a packed bed in detail [4, 5, 6]. Then the logical order to make assump­tions was discussed [5, 6] and the solution by the method of characteristics for the local equilibrium model was developed [5, 6]. Once this basic model had been developed, the local equilibrium model was used to explain and contrast a variety of operating methods. We first started with ordinary adsorption [5, 6] and then considered simulated counter-current operation [7] and the related moving feed point chromatography [9]. The students further explored these methods with the local equilibrium model by solving homework problems which are not in the literature. As an aside we discussed how analogous two-dimensional separators could be constructed and analyzed [8].

We then discussed a variety of cyclic operating methods. Both direct and recuperative mode para­metric pumping [ 4, 10] were discussed. The com­mercially important pressure swing adsorption system [ 4, 11] was studied and the limits of ap­plicability of the local equilibrium model were demonstrated [12]. Single and multicomponent

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cycling zone adsorption [4, 13, 14] were then ex­plored. Finally, the local equilibrium model was used to study binary ion exchange [5, 15], and differences and similarities with Langmuir ad­sorption were highlighted. Homework assignments developed from my research were used to further investigate these subjects.

Having looked at a variety of operating methods we then studied several other mathe­matical models. First the linear local equilibrium model with dispersion [16, 17] was introduced, and the use of superposition in the solution of linear problems was studied. Then constant pattern methods [6] were explained, and the section was completed with the Thomas solution method [6, 16, 18]. Again homework assignments provided practice.

We discussed nucleation and crystal growth, crystal size distributions, and crystallization equipment.

Four homework assignments with a total of twenty problems were passed out and a one hour closed book test was given. Students were given an equation sheet in advance so they did not have to memorize equations.

In the past we covered interacting multicom­ponent analysis by the local equilibrium method, and very briefly discussed numerical methods. Be­cause of time constraints these areas were not covered this semester. In the future I would like to include two or three classes on numerical methods. Obviously, other topics could be included. The selection used here satisfied my purposes. The material was covered at a rapid but digestable pace.

TOPICS SELECTED BY CLASS

Roughly half of the lecture periods were left open for topics to be selected by the class. Since students are not accustomed to selecting their own topics, I lead them through the selection pro­cess. The need to select topics was discussed during the first class period and in the second class period a variety of separation methods were briefly discussed. During period seven the students were to browse through a variety of journals and look at articles on separation methods. Then they developed and turned in a first list of topics of interest.

I took these first lists and made a master list

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which was returned to the students. They then gave me enlarged second lists of their interests and I again made a master list and distributed it. The third time I asked for a list with items rank ordered. I collated these lists and decided what to cover during the remainder of the semester. The two topics of interest to the majority of the students were membrane separations and crystal­lization. In addition, I decided to cover molecular sieves, activated carbon and affinity chromatogra­phy, which were all requested by one or two students. These latter topics were connected with the first half of the semester and could be covered quickly. Although we were not able to include all of the student requests, at least one topic from each student's list was discussed.

After considerable reading, an outline and reading list for the second half of the course was developed (Table 2). We started by discussing the characteristics of molecular sieve adsorbents [19, 20] and of activated carbon [21] and solvent recovery by activated carbon [22]. Activated carbon was the one topic where students did not like the assigned reading [21]. Affinity chroma­tography was covered with an emphasis on princi­ples and not the specific reactions [23].

Seven class periods were devoted to membrane separations. We started by reviewing all types of membrane separators [24], and studied reverse osmosis and ultrafiltration in detail. Osmotic pres­sure [25] was briefly discussed since everyone had

TABLE 2

Outline of Topics Selected by Class

PERIOD(S)

25 26 28 29 31

32

33

34 and 36 37 38-39

40-41 42

SUBJECT

Molecular Sieve Adsorbents [19, 20] Activated Carbon Adsorption [21, 22] Affinity Chromatography [23] Introduction to Membrane Separations [24] Osmotic Pressure [25] and start concentra­tion polarization [26] Concentration polarization without gelling [26, 27] Concentration polarization with gelling [26, 28] Transfer inside the membrane [29] Equipment and cascades [28, 30, 31] Crystallization from solution: Nucleation and crystal growth [32, 33, 34) Crystal Size Distributions [32-35) Crystallization equipment and operation [32, 34, 36]

Note: Missing class periods were used for research project purposea and are listed in Table 1.

CHEMICAL ENGINEERING EDVONflQN

I-

The two topics of interest to the majority of the students were

membrane separations and crystallization.

forgotten this portion of their physical chemistry. Then the mathematical analysis of concentration polarization both without [25, 27] and with gelling [26, 28] was covered. We switched to irreversible thermodynamics to study transfer inside the mem­brane [29]. Finally we discussed membrane equip­ment and cascades [28] with additional examples of cascades presented in class [30, 31].

The student-selected topics were finished with five periods on crystallization from solution. We discussed nucleation and crystal growth, crystal size distributions, and crystallization equipment. The two basic references [32, 33] were supple­mented by other sources [34-36].

The student-selected topics section included three homework assignments with a total of a dozen problems and a second closed book test was given. I again gave the students equation sheets before the test since this approach seemed to work well.

One difficulty inherent in letting students select the topics is that the professor may not know any­thing about the topic. This was certainly the case for crystallization, and I am not an, expert in membrane separations. I was aware of this po­tential problem ahead of time, and warned the students of its possibility. Throughout the se­mester I spent considerable time reading up on the various topics, and put crystallization last so that I would have more time to prepare. Since the course is in my research area, I was willing to devote extra time to reading and learning. My lack of expertness was only apparent a few times, and the students were quite understanding. Over­all, this portion of the course went very well.

SEPARATIONS RESEARCH AND RESEARCH PROJECT

SINCE ONE OF THE MAJOR course goals was to introduce the students to separation research,

a considerable amount of effort was devoted to the course project. To combat the nemesis of student research projects, procrastination, I de­veloped a pattern of exercises, small projects, and check points which culminated in the final written paper. The eleven classes labeled (R) in Table 1 are part of this pattern.

The pattern started with browsing through journals and then. listing (without reading) a

FALL 1981

total of 15 articles on subjects of interest. The students then read a recent experimental article of their choice. This article was then analyzed in detail starting with the bibliographic citation and the purpose of the study. The methods, results and authors' conclusions were described and finally the student presented his evaluation of the study. In class the students were divided into small groups and informally discussed the papers they had read. The same procedure was repeated for theoretical papers. This activity was very popular with the students. They felt they learned a lot in the presentations, but weren't anxious because the presentations were informal and ungraded. The written papers were collected and graded.

The class heard a librarian lecture on library search methods. As an assignment they were asked to find certain articles from vague citations and to list articles citing given papers or authors. This was a useful activity, but the presentation was at a somewhat too low level.

Next the students selected a general topic of interest for their research project. They could either select a topic of their own or pick from a list I passed out and when they had selected a topic, they were asked to meet individually with me to discuss it. A citation search and literature review were required.

Halfway through the semester a very specific problem within their general topic area had to be picked. I discussed these problems with each student and requested that they develop a clear and limited problem statement. The specific pro­jects chosen are listed in Table 3. The projects were to involve a theoretical analysis of a problem which had not been solved or use of a new mathe­matical method on a problem which had previously been solved. Four of the seven students worked on problems which I suggested. Two progress reports were required during the second half of the se­mester in order to stimulate continual progress.

TABLE 3

Student Research Projects

Analysis of multicomponent, equilibrium, pressure swing adsorption.

Numerical analysis for supercritical fluid adsorption. Numerical solution for affinity chromatography. Determination of adsorption isotherms by a continuous

flow method. Dynamic behavior of discrete cycling zone extraction. Cylindrical rotating continuous flow electrophoresis. Mathematical modeling of rotary thermal diffusion

columns.

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To combat the nemesis of student research projects, procrastination, I developed a pattern of exercises, small projects, and check points which culminated in the final written paper

To encourage a carefully written paper, an out­line was required a week prior to the oral report. These outlines were commented on and returned to the students. A rough draft of the entire paper was then required when the student presented his oral report on his project and these were graded and returned before the students wrote their final draft.

Despite this structure there was some pro­crastination. However, it was significantly less than I have observed in any other class. Two students ran out of time, but five of the seven projects listed in Table 3 were completed. The projects were all quite ambitious and several had significant results. In my opinion, four of the projects would be totally acceptable as research papers in the open literature if the results were significantly fleshed out. I have encouraged the students to do this. Compared to the previous time I taught ChE 623 when no structure was em­ployed in developing research projects, these re­search projects and oral reports were much more professional and results were much more sig­nificant.

SUMMARY AND CONCLUSIONS

ChE 623, Separation Processes, was designed to include three major threads. The first of these was the study of operating methods for adsorption, chromatography, and ion exchange in a pattern set by the instructor. The second thread was the study of topics selected by the students with the assignments and lectures being developed by the instructor. The third thread was the course project done by each student. A structure was used to dis­courage procrastination on the research project.

The first half of the course was enthusi­astically accepted by the students. They became quite interested in the material, and five of the later research projects were related to that ma­terial. The second half of the course also went well, although the students were somewhat less enthusiastic, perhaps because they were working on their research projects.

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The research project which was structured to encourage work throughout the semester de­creased, but did not prevent, procrastination. The resulting research projects were much better than those turned in after the previous course was offered. I recommend that other professors con­sider a similar paced structure when a course proj,ct is a major part of a course. •

REFERENCES

1. Wankat, P. C., "A Modified Personalized Instruction­Lecture Course," in J. M. Biedenback and L. P. Grayson (eds.), Proceedings of the Third Annual Frontiers in Education Conference, IEEE, NY, 1973, 144-148.

2. Karger, B. L., L. R. Snyder and C. Horvath, ;An Introduction to Separation Science, Wiley, NY, 1973, Chapter 4.

3. Lee, H., E. N. Lightfoot, J. F. G. Reis and M. D. Waissbluth, "The Systematic Description and Develop­ment of Separations Processes," in N. N. Li (ed.) Recent Developments in Separation Science, Vol. III, Part A, CRC Press, Cleveland, 1977, 1-69.

4. Wankat, P. C., "Cyclic Separations: Parametric Pumping, Pressure Swing Adsorption and Cycling Zone Adsorption," CHEM! module to be published by AIChE.

5. Course handout. Mass and Energy Balances and Local Equilibrium Solution. (Copies are available from the author) .

6. Sherwood, T. K., R. L. Pigford and C. R. Wilke, Mass Transfer, McGraw-Hill, NY, 1975, Chapter 10.

7. Broughton, D. B., R. W. Neuzil, J. M. Pharis and C. S. Breasley, "The Parex Process for Recovering Paraxylene," Chem. Eng. Prog., 66 (9), 70, (1970).

8. Wankat, P. C., "The Relationship Between One­Dimensional and Two-Dimensional Separation Pro­cesses," AIChE Journal, 28, 859 (1977).

9. Wankat, P. C., "Improved Efficiency in Preparative Chromatographic Columns Using a Moving Feed," Ind. Eng. Chem. Fundam., 16, 468 (1977).

10. Pigford, R. L., B. Baker and D. E. Blum, "Equi­librium Theory of Parametric Pump," Ind. Eng. Chem. Fundam., 8, 144 (1969).

11. Skarstrom, C, W., "Heatless Fractionation of Gases Over Solid Adsorbents," in N. N. Li (ed.), Recent Developments in Separation Science, Vol. II, p. 95, CRC Press, Cleveland, 1972.

12. Wong, Y. W., F. B. Hill, and Y. N. I. Chan, "Studies of the Separation of Hydrogen Isotopes by a Pressure Swing Adsorption Process," Separat. Sci. Technol., 15 (3), 423 (1980).

13. Baker, B. and R. L. Pigford, "Cycling Zone Adsorp­tion: Quantitative Theory and Experimental Results," Ind. Eng. Chem. Fundam., 10, 283 (1971).

14. Foo, S. C., K. H. Bergsman and P . C. Wankat, "Multicomponent Fractionation by Direct Thermal Mode Cycling Zone Adsorption," Ind. Eng. Chem. Fundam., 19, 86 (1980).

15. Anderson, R. E., "Ion-Exchange Separations," in P. A. Schweitzer (ed.), Handbook of Separation

CHEMICAL ENGINEERING EDUOA.TION

Techniques for Chemical Engineers, Sect. 1.12, McGraw-Hill, NY, 1979.

16. Lightfoot, E. N., R. J. Sanchez-Palma and D. C. Edwards, "Chromatography and Allied Fixed Bed Separations Processes" in H. M. Schoen (ed.), Nf?/W Chemical Engineering Separation Techniques, Inter­science, NY, p . 125 (1962).

17. Lapidus, L. and N. R. Amundson, "Mathematics of Adsorption in Beds. VI. The Effect of Longitudinal Diffusion in Ion Exchange and Chromatic Columns," J. Phys. Chem., 56, 984 (1952).

18. Thomas, H. C., "Chromatography: A Problem in Kinetics," Annals New York Academy of Science, 49, 161 (1948).

19. Lee, M. N. Y., "Novel Separations with Molecular Sieves Adsorption," in N. N. Li, Recent Developments in Separation Science, Vol. II, (1972), p. 75.

20. Breck, D. W., Zeolite Molecular Sieves, Wiley, NY, 1978.

21. Mantell, C. L., Carbon and Graphite Handbook, Inter­science, (1968), Chapter 13.

22. Wankat, P. C., and L. R. Partin, "Process for Re­covery of Solvent Vapors with Activated Carbon," Ind. Eng. Chem. Process Des. Dev., 19, 446 (1980).

23. May, S. W., and L. M. Landgraff, "Separation Techniques Based on Biological Specificity," in N. N. Li (ed.), Recent Developments in Separation Science, Vol. V., 227-255 (1979).

24. Lacey, R. E., "Membrane Separation Processes," Chem. Eng., Sept. 4, 1972, p. 56-74.

25. Reid, C. E., "Principles of Reverse Osmosis," in U. Merten (ed.), Desalination by Reverse Osmosis, 1966, p. 1-14.

26. Blatt, W. F., A. Dravid, A. S. Michaels, and L. Nelsen, in "Solute Polarization and Cake Formation in Membrane Ultrafiltration" in J. E. Flinn (ed.), Mem­brane Science and Technology, p. 47-74, 1970.

27. Sherwood, T. K., P. L. T. Brian, R. E. Fisher and L. Dresner, "Salt Concentration at Phase Boundaries in Desalination by Reverse Osmosis," IEC Fundamentals, 4, 113, (1965).

28. Porter, M.C., "Membrane Filtration," in P. Schweitzer (ed.), Handbook of Separation Techniques for Chemi­cal Engineers, McGraw-Hill, NY, 1979, Sect. 2.1.

29. Merten, U., "Transport Properties of Osmotic Mem­branes" in U. Merten, Desalination by Reverse Os­mosis, MIT Press (1966), Pages 15 to 54.

30. Sourirajan, S. (ed.), Reverse Osmosis and Synthetic Membrane, National Research Council, Canada, (1977) .

31. Hwang, S. T. and J . M. Thorman, "The Continuous Membrane Column," AIChE Journal, 26, 558 (1980).

32. McCabe, W. L. and J. C. Smith, Unit Operations of Chemical Engineering, 3rd ed. McGraw-Hill, NY, 1976, Chapter 28.

33. Larson, M. A. and A. D. Randolph, "Size Distribution Analysis in Continuous Crystallization," CEP Symp. Ser., Vol. 65, #95, p. 1 (1969).

34. Randolph, A. D. and M. A. Larson, "Theory of Par­ticulate Process," Academic, NY, 1971, Chapters 4 to 9.

35. Garside, J. and M. B. Shah, "Crystallization Kinetics from MSMPR Crystallizers," Ind. Eng. Chem. Process

FALL 1981

Des. Develop., 19, 509 (1980). 36. Singh, G., "Crystallization from Solutions," in P.

Schweitzer (ed.) Handbook of Separation Techniques for Chemical Engineers, McGraw Hill, NY, 1979, Sect. 2.4.

COAL LIQUEFACTION Continued from page 182.

Present and Developing Methods, in press, Marcel Dekker, Inc., New York (1981).

Bll. Liu, Y. A. and G. E. Crow, "Studies in Magneto­chemical Engineering: I. A. Pilot-Scale Study of High-Gradient Magnetic Desulfurization of Solvent­Refined Coal," Fuel, 58, 345 (1979).

B12. Liu, Y. A. and M. J. Oak, "Studies in Magneto­chemical Engineering: II. Theoretical Development of a Practical Model for High Gradient Magnetic Separation, and III. Experimental Applications of a Practical Model of High Gradient Magnetic Separa­tion to Pilot-Scale Coal Beneficiation," AIChE J., in press (1981).

B13. Eissenberg, D. M. and Y. A. Liu, "High Gradient Magnetic Beneficiation of Dry Pulverized Coal via Upwardly-Directed Recirculating Fluidization," U.S. Patent number 4,212,651, issued on July 15, 1980.

B14. Liu, Y. A., "Novel High Gradient Magnetic Separa­tion Processes for Desulfurization of Dry Pulverized Coal," Chap. 9 in Recent Development in SepMation Science: Volume VI, Norman N. Li, Editor, CRC Press, Boca Raton, FL (1981).

C. SELECTED RECENT THESES FROM THE AUBURN COAL RESEARCH PROGRAM

Cl. McCord, T . H., "A Feasibility Study of Novel High Gradient Magnetic Separation Processes for De­sulfurization of Dry Pulverized Coal" (1979).

C2. Jeng, J. F ., "Determination of a Solvent Quality Index for Coal Liquefaction," (1979).

C3. Fan, C. W., "Heteroatom Removal from Model Com­pounds by Coal Mineral Catalysts," (1979).

C4. Henson, B. J., "Solubilities of H 2 and CO2 in Coal Liquids," (1980).

C5 .. Majlessi, S.H.R., "Synergistic and Phase Behavior Effects Among Aliphatic and Aromatic Compounds in Coal Liquefaction," (1980).

C6. Wagner, R. G., "A Feasibility Study of Novel Con­tinuous Superconducting High Gradient Magnetic Separation Process for Desulfurization of Dry Pul­verized Coal," (1980).

C7. Brook, D., "Effect of Pyrite on Liquefaction Catalysis," (1981).

CB. Crawford, J., "Kinetics of Pyrite-to-Pyrrhotite Transformation," (1981).

C9. Pehler, F. A., "Development and Demonstration of the Auburn Fluidized-Bed Superconducting High Gradient Magnetic Separation Process for Desulfur­ization of Dry Pulverized Coal," (1981).

ClO. Smith, N., "NMR Investigation of Recycle Solvent Quality," (1981).

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