report of the polymer core c o w e committee
Polymer Principles for the Chemical Engineering Curriculum Chemkal Engineering Core Course Subcommittee,' ACS Polymer Education Committee, 1980-1982 At some time or other in their careers, most chemical engineers will encounter the subject of polymers. Whether they work in research, production, design, or marketing, the probability is high that they will need some understanding of what polymers are and what they can do. Recognition in academia of this fact of life is documented in a survey by Peppas of Purdue reported in Chemical Engineering Progress (April 1981, p. 23). Out of 94 chemical engineering departments answering his survey, 69 offered a t least one course in oolvmers. I t seems to be a safe eeneralization that polymers d o Ieceive some attention fromchemical engineers although i t is d i s a ~ ~ o i u t i that n e about 25% of all chemical engineering departments db not offer even one course in polymers. The matter of integrating polymer topics with standard courses is not in so happy a situation. The popular textbooks in four areas of chemical engineering contain few explicit references to polymers. The specific situation in each of these four major areas is summarized in Appendices 1 through4. In each summary, suggestions are made regarding appropriate topics and a few typical problems are suppplied. As pointed out in each area summary, there certainly are many opportunities for introducing polymer topics in the standard courses. Insertion of polymer-oriented topics in each area can take the form of Homework problems Derivations Demonstrations Laboratory experiments (U.O. Lab, for example) Projects Other areas, besides those mentioned in the Appendices that might also receive attention are: Process Control Chemical Processes Material Science Unit Operations Laboratory Process and Plant Design and Economies Appendices In each aooendix.. sueeestions are made on how to introduce polym& topics in a major area of the chemical engineers curriculum. Also included are examoles of textbooks in current use as well as a few typical problems. uu
Appendix 1. lntroductoty Chemical Engineering ( F . Rodriguez) The first course in chemical engineering usually emphasizes the laws of conservation of matter and energy. Balancing chemical equations and applying the ideal gas law are
' Members: Robert E. Cohen; Donald Ross Paul; Nicholas Peppas;
Ferdinand Rodriguez. Chairman; Stephen L. Rosen; Montgomery T. Shaw; Leslie H. Sperling;and Matthew Tirrell.
often included. A major example of the application of combined mass and energy balances is humidification, which also demands an understanding of humidity charts. Much of the background material is contained in elementary courses in chemistry and physics that most students have already had in high school. Many colleges place the introductory chemical engineering course in the sophomore year so that college-level mathematics as well as chemistry and physics can be prerequisites. The manner of incorporating polymers into such a course can range from the trivial to the elegant. Polymers as such are not even mentioned in the currently popular texts. However, the concepts of moles, reacting ratios, and product yields can use monomers and polymers just as easily as simple organic compounds. On the other hand, a full course in organic chemistry cannot be assumed as a prerequisite in most schools. A major advantage of polymers as examples in the introductory course is the familiarity that the student has with the terminology beforehand. Polyethylene, polystyrene, poly(viny1 chloride), epoxy, and polyester are part of evervone's vocabularv and are used casuallv in newsoaoers and television. hil lei he oxidation of HCI to-chlorine is not a orocess which the averaee undergraduate has witnessed. the oxidation of polystyrene is as immediate as a n egg carton burning in a fireplace. Some specific topics and examples are: The mole, units, stoichiometry. The examples usually used are taken from inorganic chemistry. However, the reactions of addition polymerization and polyesterificationare not too difficult for the average student who has had at least a year of college-level chemistry. The high molecular weights of familiar polymers can add particular interest to the concept of the mole and of the size of individual molecules. Material balances. Polymerization reactions with recycle of unreacted monomer, combustion of polymers in waste disposal, and polymer modification (acetylation of cellulose) all are good examples. Energy balances. The heat of polymerization often is large for ethenie monomers. It gives rise to practical prohlems in adiabatic situations. Heat removal hv boilinn solvent or monomer can he handled without introdu&ng concepts of heat transfer in the usual sense. Combustion is another kind of energy balancing problem which can use polymeric ingredients. Combined analyses. Most textbooks aimed st the freshmansophomore audience do not include much in the way of process analysis. Rudd, Powers, and Siirola in "Process Synthesis" do include a chapter on detergent production with an emphasis on flow-sheetdevelopment and analysis. Polymers offer the advantage over a product like detergents in that the material produced can utilize simpler chemistry and still he readily identified in familiar consumer products.
Textbooks for Introductory Chemical Engineering: Felder, R M., and Rousseau. a. W., "Elementary Plinciplea of Chemical Praee-." Wiley, 1978. Henly, E. J., and Rosen, E. M.."MateMI and Energy Balance Computations." Wiley, 1969. Volume 62
Number 12 December 1985
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Sample problems for junior-level courses are: Viscous heating: Hill, 1977. Whitwell,J.C..andToner,R.K.,"ConseruationofMaassndEne~y,"MeGraw-Hill,1973.
Sample problems suitable for Introductory Chemical Engineering (taken from Rodriguez, F., "Principles of Polymer Science," 2nd ed., McGraw-Hill, 1982): 11 Propylene is polymerized inn slurry renrtor by an equimolnr
mtxturr of slummun rrr~thyland trrnnium retrarhlorid~.T ~ P catalyst residue remains with the polymer in a hydrolyzed form. If a customer specifies a maximum ash content of 0.10 wt $ in product. what product;vity (rnolebol monomer converted per mole of rntnlyst~must br achieved? Assume the ash is entrr~lyAI?O, nnd 'I'iO?. 2, In a n adrnbntir tnbulnr renetur, stgrrne is converted partially to polymer at high pressure and the mixture of monomer and polymer is sprayed into a vacuum chamber with evaporation of monomer and recovery of polymer. If the heat of vaporization is 355 Jlg and the monomer enters the tube, at 50°C, what fraction can be converted to polymer per pass and still allow polymer recovery at 50°C? The beat of polymerization is 16.7 kcal/mole. Appendix 2. Transporf Phenomena and Unit Operations (M. T. Shaw)
T h e principles of heat, mass, and momentum transport, including their applications in hatch and staged unit operations, are normally taught in several courses extending over t h e junior and senior years. Most departments bring transport phenomena and unit operations together with other subject matter in a senior design course, which can be a n appropriate setting for complex problems involving polymers. However, t h e polvmer suhiects should be introduced in the junior-level Eouise on transport phenomena. Unit operations analysis is often included in such courses, h u t the needs of separate U.O. courses ie.g., using ~ c ~ a ahn de Smith) should be addressed. Examination of t h e problems and examples in three popular transport phenomena texts (Bennett and Myers; Bird, Stewart, and Lightfoot; and Geankoplis) reveals t h a t polymer-related subjects are treated, h u t very infrequently. In fact, problems dealing with food processing are described more frequently than polymer-related topics. By analogy, polymer processing operations could be similarly presented; however, authors should also he encouraged t o deal with important polymer topics in separate sections. Suggested subjects in various areas are: Momentum transfer. Drag flow, nan-Newtonian flow of melts and
solutions, drag reduction. Heat transfer. Continuous vulcanization, blown-film lines, eatru-
sion coating, spinning, viscous heating, melting.
1) A polypropylene melt for whichK = lo4d~n.s'/~andn = 112 is injection-molded through a runner of diameter 0.3 em and length 7.5 cm at a rate of 20 em3/$. If melt enters the runner at 250PCand the runner surface is at 250°C, estimate the average melt temperature leaving the runner. Take the thermal properties to be those of polypropylene. (Middleman, 1977) Extrusion (as a unit operation): 2) Design a screw extruder for pelletizing low-density polyethylene at a rate of 10,000 lbh. The head pressure required to pump the melt across the pelletizing plates is 1250 psi. The feed comes from a reactor at 500°F. Assume a Newtonian viscosity of 0.05 lbrs/in2 and a density of 48 lb/ft3. Because the screw must be gravity fed, the channel depth in the feed
section should he no less than 2 in. Neglect the effect of flieht " clearance, and assume isothermal operation. Your answer must be in terms of screw geometry and frequency of screw rotation. (Tadmor and Gogos, 1979) 0
Appendix 3. Chemical Engineering Thermodynamics (L. H . Sperllng) Many parts of polymer science have a strong basis in thermodynamics. Here are a few examples. The theorvof rubberelastieitv. Like the kinetic thearvofeases. rubber elastkity theory has its basis in entropic phenomena. The central orohlemis howtocalculate thenrunher ofconforrnation~a .. .-.-~ ~ netwbrk chain portion can assume as a function of elongation. An important equation resulting from this calculation is ~~~
~
~
~~~~~
~~~~
~
where c i s the stress, n is the number of network chains per unit volume, a is the final length over the initial length of the sample (LIL,), and R T the gas content times the absolute temperature. Examples related to chemical engineering include the manufacture and properties of rubber tires, gaskets, helting, and toys. Phase diagrams. Composition-temperature phase diagrams may he of two basic types: polymer molecule-diluent (plasticizer) or polymer-polymer types. The latter, which often exhibit a lower critical solution temperature, are becoming highly important in the engineering of impact-resistant plastics, thermoplastic elastomers, and some types of hot-melt adhesives. Fundamentally, the concentration of each species, their molecular weights, heats of miring, and grafting level, if any, all play an important part. Thermodynamics of polymerization. The free energy of polymerization has a somewhat different form from that in smallmolecule reactions because the many small molecules are being reacted to produce one long chain. Examples include the step polymerization of nylon, and the chain polymerizationof polystyrene (both of which follow different kinetic schemes). Of significant thermodynamic interest is the ceiling temperature, above which the free energy of polymerization is positive, thus leading to an unzipping of chain-formed polymers. It is to be expected also that many suggestionscoming from the Subcommittee for Physical Chemistry would be applicable. RePorts have already been ~uhlishedbv Mandelkern and hv Mattice in separate articles (J. HEM. EDUC.,55,177; 58,911).
Mass transfer. Devolatilization, drying of pellets, film casting. Unit operations. Coagulation, extrusion, devolatilization.
Textbooks for Chemical Engineering Thermodynamics: Balzhiser, R E., et d., "Chcmiesl Engineering Thermodwsmies."Prentice-Hall,1972. Prausnitz, J. M., "Molecular Themodynsmies of Fluid-Phase %uilibrium." PrentieeHall, 1969. Sandier, S. I.. "Chemical Engineering Tbermdynamier." Wiloy, 1977.
Textbooks for Transport Phenomena and Unit Operations:
Smith, J. M., and Van Ness. H. C., "Introduetlon to Chemical Engineering'llermodynsmin," 3rd ed.,McGraw-Hill. 1975.
Sample problems for junior-level courses: Foust, A. S., et sl.. "Principles of Unit Operations." Znded.,Wiley, 1980. Geankoplis. C. J., "Transport Pmnsseand Unit Opprationa," Allyn and Bamn. 1978. McCabe, W. L.,Smith,J. C.,andHsrriotL P.,"UnitOperafionsafChemidEngineering," 4th d., MeCraw-Hill, 1985. Treybal, R. E.. ''Mass Transfer Operations." 3rd ed., McGraw-Hill, 1980. Welty, J. R., Wicks, C. E., and W i h n . a. E., "Fundamentals of Momentum, Heat, and MmTransfer." 2nd ed.,Wiley, 1976.
1080
Journal of Chemical Education
1) The theory of rubber elasticity and the theory of ideal gas dynamics show that the two equations
G = vRT PV = nRT
where G = shear modulus u = concentration of network chains
share certain common thermodynamic ideas. What are they? (L.H. Sperling, August 6,1982, copyright protected.)
