PapervsPlastic

PapervsPlastic - ‘Cli E ., class and home problems The...

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Unformatted text preview: ‘Cli E ., class and home problems The object ofthis column is to enhance our readers' collection ofinteresting and novel problems inn chemical engineering. Problems ofthe type that can be used to motivate the student by presenting a particular principle in class, or in a new light, or that can be assigned as a novel home problem, are requested, as well as those that are more traditional in nature and which elucidate difficult concepts. Please submit them to Professors James O. Wilkes and Mark A. Burns, Chemical Engineer- ing Department, University of Michigan, Ann Arbor, MI 48109-2136. ,a _.._. “ummnmumeHW__/ ENVIRONMENTAL lMPACT OF PAPER AND PLASTIC GROCERY SACKS A Mass Balance Problem with Multiple Recycle Loops D. T. ALLEN, N. BAKSHANI University of California Los Angcles, CA 90024 Environmental issues are becoming increasingly important in the design of chemical processes and chemical products. Incorporating these issues into an already crowded chemical engineering cur— riculum is a challenge. One way to address this challenge is to develop entire courses dedicated to environmental issues. An alternative strategy is to develop homework and design problems that can be used in existing chemical engineering courses, illus- trating both fundamental engineering principles and environmental issues For the past year we have been developing such problems for the chemical engineering curriculum. One of the problems developed for the mass and energy balances course is given below. The problem illustrates the concept of recycle, a topic normally N. Baksham’ is a research fellow in the Chemical Engineering Deparlmenl. Unlversr'ly of Calllamia, los Angeles. He holds a BS and MS in metallurgy" cal engineering from New Mexico Insulate of Tech- nology and a PhD in applied earth sciences from Stanford Universily. Current loreresls include the process engineering tools required for pollution prevenlion in manufacturing and service indus— fries. David Allen is an associate prolessor or Chemi- cal engineering at flie University of Callfomla, Los Angeles. He received his BS degree, with distinc. lion. from Cornell Univeislly {19?9) and his MS and PhD degrees from California Institute of Tech- nology (1981 and I983). He has also held visiling appointments at lhe Calllomia Inslilule of Tech— nology and the Department of Energy, covered in a mass and energy balance course, and the problem exposes students to the issue of product life-cycle analysis. Specifically, the problem compares paper and plastic grocery sacks based on energy requirements and environmental impacts. The prob— lem is divided into five sections: Background malaria] A problem slalcmcnl Open-ended questions for discussion A solution Sflfiwl“? References and suggcslions for further reading Sections 1-3 and 5 can be distributed to the students as a homework assignment. The prob- lem solution takes between two and three hours for most students. BACKGROUND At; the supermarket checkout stand, consumers are asked to choose whether their purchases should be placed in unbleached paper grocery sacks or in polyethylene grocery bags. Many consumers make their choice based on their perception ofthe relative environmental impacts of these two products. The analysis framework for this problem will be the mass flow diagram shown in Figure 1. For the problem, we can simplify Figure 1 considerably. First, consider the recycle loops. Almost all recycled gro- cery sacks are returned to the raw material formula- tion stage, so we can ignore the product recycle and remanufacture loops. This simplification leads to the mass flow diagram shown in Figure 2 (and Figure 3). (9 Copyright Chi; flit-[sum anSEH L992 Chemical Engineering Education two’le M' “‘5’” ° 4’- omissions ch“ as These two figures define our life—cycle analysis frame— work for comparing paper and plastic grocery sacks. In the figures we have listed the air emissions generated per unit of production for both plastic and paper grocery sacks. Before a quantitative compari~ son between the two products can be made, however, I onuugy ) annrgy {(W 0‘ t, nnrDy I Inmgr “5M ow arm” a v4" product use manufacmrn omission: natural resources manual: manutanum raw malarial; accui swan a". nmmsmni product tricycle EMISSIONS pleducl lomanuiacluling {Z mam-mi: rncyclo Figure 1. The life cycle for manufactured goods: on analysis template BASIS: 10130 in: cl Faiyomylune [9E] Sack: since wmgm e! I PE sack - 0.2632 01.. . . —. i : a: . w. 2-. mm m Pg “m 1' 6mm“ an“ EnmngaS Em pol sack (comma law armorial monsoon and atom: capo...” Ennrgy: 464 on. par luck outsqu: manululum, yoducl manuiaclurn mound usn Almnsr Emminnsi 0.0145 0:. our task natural losuwces law malonais acquwlmn producl deposal Recycle Atmospheric Emission: : 000115 oz. pol sack [unturned HM macro! Emmet:ch and p'G‘JJEE (I'll-553‘? Figure 2. The life cycle for man ufoctured goods: polyethylene { PE ) grocery socks (Source: Franklin Associates. Ltd—see suggestions for further reading.) BASIS: a of Paper sack: - 60.7904? at 33.395 acts “‘5” “l ‘ Pie“ mi ~ 2"“ “1 Energy: 72: on. par cack (combined raw material amt-salon and produce arousal) Energy: 905 am par sack malarial: mnulacsura. wwucl manulaswm product use Mmos. Emusmnsz 90515 02. par sack nnmral (accurao: raw malarial: pmflucl acqumlacn uxnposal Rncynin Atmospneac Emssians: use oz. per sa:k (combined raw material acqu'sxien and new: czsmsall Figure 3. The llfe cycle for manufactured goods: paper grocery socks (Source: Franklin Associates. Ltd—see suggestions for further reading.) Spring 1992 we must consider how the products are used. Al- though both are designed to have a capacity of 1/6 barrel, fewer groceries are generally placed in plas— tic sacks than in paper sacks, even if the practice of double~bagging (one sack inside the other), used in some stores, is taken into account. There is no gen~ eral agreement on the num— ber of plastic grocery sacks needed to hold the volume of groceries usually held by a pa— per sack. Reported values range from 1.2 to 3. In this problem we will use a value of 2.0 plastic grocery sacks re- quired to replace a paper gro~ cery sack. PROBLEM STATEMENT a) Using the data in Figures 2 and 3, determine the amount of energy required and the quantity of air pollutants re— leased per 1,000 lb of produc- tion of plastic sacks. Also de— termine the amount of energy required and the quantity of air pollutants released for the quantity of paper sacks capable of carrying the same volume of groceries as the 1,000 lb of polyethylene sacks. Both the air emissions and the amount of energy required are functions of the recycle rate, so perform your calculations at three recycle rates, 0% recycled, 50% recycled, and 100% recycled. 1)) Plot the results of part a) for both types of sacks. Com— pare the energy requirements and atmospheric emissions of the paper and plastic gro~ cery sacks as a function of re- cycle rate. c) Based on your results, dis— cuss the relative environmen— tal impacts of the two prod ucts. Note that in part b) of the problem, you compared the quantity of air emissions re leased. As shown in Table 1, the qualitative characteristics 83 of the air emissions due to paper sacks are different than those due to plastic sacks. In your discussion you should consider whether or not it is valid to compare directly the mass of atmospheric emissions due to the two products. (1) The material and energy requirements of the plastic sacks are primarily satisfied using petroleum, a non-renewable resource. In contrast, the paper sacks rely on petroleum only to a limited extent and only for generating a small fraction of the manufac» turing energy requirements”! Most of the energy requirements of pulp and paper manufacturing are met by burning wood chips. Compare the amount of petroleum required for the manufacture of two plastic sacks to the amount of petroleum neces sary to provide 10% of the energy required in the manufacture of one paper sack. Assume 0% recycle, and that 1.2 lb of petroleum is required to manufacture 1 lb of polyethylene. The higher heating value of petro» leum is 20,000 BTU/1h. Questions for Discussion 1) Is 100% recycle really feasible for the products being analyzed or for any consumer products? Consider at least two points in your analysis: con— taminants on or within the sacks, and mechanical wear and tear of the grocery sacks. 2) In this problem you have con- sidered only two choices for deliv- ery of groceries: paper sacks and plastic sacks. Can you suggest other alternatives? SOLUTION a) The energy requirements and to- tal atmospheric pollutants for both paper and polyethylene (PE) grocery sacks, extracted from Figures 2 and 3 of the problem statement, are listed in Table 2. All values pertaining to PE sacks are based on 1,000 lbs of product, or 80,790 PE sacks. Values for the paper sacks are based on 60,790/2 : 30,395 sacks, the number required to hold an equivalent vol- ume of groceries. b) The data from part (a) are plotted in Figures 4 and 5. These figures 84 show the effect of recycle rate on energy require— ments and atmospheric pollutants. At 0% recycle, PE sacks (on an equal-use basis, two PE sacks per paper sack) require approximately 20% less energy than paper sacks. However, as the recycle rate in— creases, this difference in energy requirement de~ creases linearly. At recycle rates above 80% there appears to be no significant difference in energy requirements for PE and paper sacks. Therefore, on the basis of energy alone, paper sacks would be considered competitive with PE sacks, at high {>80%) recycle rates. The plot for total atmospheric emissions shows a similar declining difference between the prod« acts, with increasing recycle rates. At 0% recycle, "TABLEr ' ofAtmospheric Emissions for Paper and Plastic ' I' Grocery Sacks 3 (Source: Franklin Associates, Ltd.) Atmospheric Pollutants For Use (lb) - Emissions for 2 Polyethylene Sucks " ‘2: Recycling I 100% Recycling- Emissions {or 1 Paper Sack . - ' F 0% Recycling 100% Recycling '- 2.8 x 10"1 " 8.01: 10'4 ' 3.9 x 10"i _ 10.6 x 104 6.5 x 10" 24.6 x 10“ 9.2 x 10" 4.9 x 3.0:4 ' 0'3 x 104 . "1.7 x10“ _ 3. "3.2 x 104 I 2.7 x 10* 13.3 x 10* 0.6 x 104 7.0 x 10“ _ ' 0.0 0.1 x 107‘ 0.1 x 10" :_'-:-0.0 0.3mm, 0.2x104. . _ " -.-- 4.5 x 103‘ '3' 0.0 . f: "0.0 0.0 (5;. :._-'0.0_ Chemical Engineering Education total atmospheric emissions are 60—70% lower for PE sacks; this difference gradually declines to 40% at 100% recycle. 0) PE sacks generate lower amounts of atmospheric emissions at all recycle rates—a fact that may be significant if there are no qualitative differences be— tween the emissions. However, the emission compo— sition data of Table 1 show both quantitative and possible qualitative differences in the emissions as- signed to PE and paper. In the case of paper sacks, the amount of particulates, nitrogen oxides, and sul- fur oxides is higher than for PE. As might be ex- pected, higher levels of hydrocarbon emission are assigned to PE sacks. These hydrocarbons are also very likely to be qualitatively different from the hy— drocarbon emissions generated by paper-sack pro— duction. It would be difficult to assess the respective environmental impacts of the hydrocarbon emissions without a much more detailed description of the emissions. Also, lack of emission data from other sources within the life cycle (La, incineration and emissions from landfills) makes the comparison of PE and paper sacks incomplete and any comprehen— —'—0‘-— Polyethylene Sacks —--0— Paper Sacks :1 a 40 2 E >’. lg, 30 : LU 20 u 29 40 so so 100 Recycling Rate Figure 4. Energy requirements for grocery socks. Basis: 66,970 polyethylene socks, 30,395 paper sacks. 2m u a, “W Papanndts a ——o-— Polyozhlunn Sade. 15a of I: O E 3?. g 100 11.5 .9. '6 "5. so in D E ct o 2 u 4 u 6 e a D 1 o 0 Recycling Rate as Figure 5. Atmospheric emissions for grocery socks. Basis: 60.970 polyethylene socks. 30,395 paper socks Spring 1992 sive comparison difficult. d) Petroleum requirements of polyethylene sacks: Fuel: 39.5 x 106 BTU 11b petroleum _ 0 032 lb petroleum 60,790 sacks 2X 104 BTU “ ' sacli Material lb petroleum sack [0.2632 02M 11h sack 16 oz Total : 0.052113 petroleumlsack )(1.2)=0.020 Petroleum requirements of paper sacks: Fuel: 49.5x10G BTU 0 1) 11h petroleum _0 008“) petroleum 30,395 sacks ‘ 2:4104 BTU ‘ ' sack Two polyethylene sacks require more than an order of magnitude more petroleum than a paper sack. Sample Answers for the Questions for Discussion 1) The term "100% recycle" implies that all of the material in a grocery sack can be recovered, but complete material recovery is generally impossible to achieve. In the case of polyethylene and paper sacks, manufacturers invariably print identification labels or advertisements on the sack. The printing is usually done with an ink or dye that is undesirable in the remanufacturing process and is not easily removed. In addition, a variety of consumer items, such as foods and beverages, can contaminate the sacks in a similar manner. In both cases, the con- taminants could lower the quality of remanufactured sacks to a point where the sacks are unusable. There- fore, in order to meat quality specifications, some of the reCycled material containing the contaminants at concentrated levels is removed as a purge stream, and additional raw material and energy are required. 2) Many nations have adopted the reusable grocery sack concept with significant success, where success is measured by the number of people actively prac- ticing the concept. Shoppers may reuse their du- rable sacks made out of nylon, jute, or thick cotton- string netting hundreds of times. The effect of gro- cery sack reuse as opposed to sack recycle is illus- trated in Figure 1. Sack reuse is represented by the product recycle loop; note that there is less energy, atmospheric emissions, and waste associated with the product recycle loop than with the materials recycle loop. All material and manufacturing steps are bypassed for the life of the sack. However, be- cause the manufacture of typical durable grocery sacks involves an order of magnitude more energy 85 use and emissions than the manufacture of a paper or plastic sack, the consumer must use the sack at least ten to twenty times before an environmental benefit is achieved. CONCLifiSION Assessing the total environmental impact of any product is a difficult process, involving evaluations of processing steps ranging from raw material acqui— sition to post-consumer waste disposal. Comparing the environmental impact of competing products is even more complex. Making comparisons between products usually involves making trade—offs between very different environmental impacts. The purpose of this problem is to illustrate the difficulties involved in comparing the total environn mental impact of different products. Paper and pies» tic grocery sacks were used as a case study. To com~ pare paper and plastic grocery sacks we found that we must evaluate the trademffs between energy use, pollutant emissions, and the depletion of natural resources. Plastic sacks appear to result in less at— mospheric emissions and require less energy. On the other hand, paper sacks rely on a renewable re— source for material and energy. Thus there is no clear, environmentally superior product. The con- sumer is left with a difficult choice, and as illus- trated in the problem this choice must be made with incomplete information. REFERENCES 1. Hocking, M.B., "Paper versus Polystyrene," Science, 251, 504 (1991) Suggestions for Further Reading ' Resource & Environmental Profile Analysis of Polyethylene and Unbleached Paper Grocery Socks, flanking Associates, Ltd, Prairie View, KS (1990) - Federal Office of the Environment, "Comparison of the Ef- fects on the Environment from Polyethylene and Paper Carrier Bags," Bismarckplatz 1, 1000 Berlin 33, RFG, En- glish version. August (1988) 0 Riggle, 1)., "Recycling Plastic Grocery Bags," Biocyclc, p 40, June(1990) Cl Second Law of Thermodynamics Continued from page 81. The extra cost of erasing these digits exactly cancels any energy gain elsewhere in the system. The conundrum of Maxwell‘s demon has been re- solved by applying the concepts of thermodynamics of irreversible computation. In our discussions, we assumed the behavior of the demOn to be completely deterministic, La, one instruction is completed before it goes on to the next 86 instruction. What is not so clear is what would hap- pen if' the demon could wander a little, i.e., if the demon knew its instructions but was not quite sure of the order in which to carry them out. The demon would then proceed from one step to another, going forward or backward, in a somewhat random fash» ion. In the long run, this might allow the demon to extract some work. There is no doubt what the outcome of the above argument is going to be, but it is a loophole which has yet to be closed. REFERENCES E. Maxwell, J.C., Theory of Heat, 4th ed. Longinans, Green & Co. London, 328 (1875) 2 Bonnet, C.H., Sci. Am.. 255(11), 108 (1987) 3 Maddox, J., Nature, 345, 109 {1990) 4 Peterson, 1., Sci. News, 137, 378 (1999i 5 Smoluchowski, M., Z. Phys. (1912) 6 Smoluchowslci, M., Lecture Notes, Leipzig (1914} 7. Sziiard, L., Z. Phys, 53, 840 (1929) 8. Bennett, C.H., IBM J. Res. Den, 1‘7, 525 (1973) 9 Londoner, R., IBM J. Res. Den, 3, 183 (1961) 10 Zurclt, W.H., Nature, 341, 119 (1989) 11 Landauer, R., Nature, 335, 779 (1988) 12 Caves, C.l‘vI., Play. Rev. Letters. 64, 2111 (1990) D Liquid-Liquid Processes Continued from page 71. information is obtained by the Stirred Transfer Re— actor, which is a modified Lewis cell. The interfacial area between the contacted liquid phases needed for the estimation of mass transfer and reaction rates is calculated from information about the drop size dis— tribution and the disperseduphase volume fraction- The former is obtained by the Microphotographic Technique andfor the Laser Capillary Spectropho- tometer Technique and the latter by the Ultrasonic Technique. Tracer concentration measurements by the La— ser Photometric Technique yield information about flow properties, i.e., axial mixing parameters in both phases. Drop size-concentration bivariate distribu~ tions are obtained by the Laser Capillary Spectre“ photometry Technique. This information is extremely valuable in model discrimination and parameter es- timation of models describing droplet breakage and coalescence. It also prevides information on dispersed phase mixing. Finally, the Ultrasonic Technique is also employed for the control of the dispersed—phase volume fraction in extraction columns to secure non- flooding optimum Operation. REFERENCES 1. Flett, 13.8., The Chemical Eng, 32, i (1981) 2. 'I‘avlarides, L.L., J.-H. Bee, and C.K. Lee, Sep. Sci. and Chemical Engineering Education ...
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PapervsPlastic - ‘Cli E ., class and home problems The...

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