I n d . Eng. Chem. Res. 1987, 26, 1659-1663
1659
Starch-Based Blown Films. 2 Felix H. Otey, Richard P. Westhoff, and William M. Doane* Northern Regional Research Center, Agricultural Research Service, US.Department of Agriculture, Peoria, Illinois 61604
Urea and polyols were added to starch-poly(ethy1ene-co-acrylicacid) ( E M ) formulations to facilitate preparation and to improve economics and quality of starch-based films designed for biodegradable agricultural mulch. A urea-containing blend was evaluated in a pilot-plant run. Principal benefit of the urea was to improve the gelatinization of starch a t low levels of water, thus allowing direct extrusion of a uniform film from a semidry blend (-16% HzO) and avoiding the need to premix starch-EAA in a heavy-duty mixer with large amounts of water prior to extrusion processing. Initial tensile strengths of urea-containing film were generally lower than those made by the premix method, but after water soaking to remove the urea, the tensile strengths were nearly equal to those made without urea. Glycerol and starch-derived polyols can be added to starch-EAA systems to increase percentage of biodegradable component without adversely affecting physical properties of the films. Earlier we reported (Otey et ai., 1980) a technique for blending gelatinized starch with poly(ethy1ene-co-acrylic acid) (EAA) to produce flexible blown films that contain high levels of starch. Films made from this system require no plasticizer, yet they remain flexible even after exposure to water and drying. Although the mechanism is not known, we envision that as internal bonding within the starch molecules is reduced during gelatinization and extrusion, new hydrogen bonds are formed between the EAA carboxyl groups and the starch hydroxyls. These bonds may retard retrogradation of the starch to a more brittle structure. Regardless of the mechanism, this approach provided the first known system containing high levels of gelatinized starch that can withstand the strain of extrusion blowing into flexible films. Since our initial report, considerable interest has been expressed in the potentia1 of using the starch-EAA system to produce biodegradable films, especially for agricultural mulch and disposable bags. The U.S.currently uses an estimated 1.25 X los lb of low-density polyethylene (LDPE) film for agricultural mulch with a projected growth to 1.90 X lo8 lb by 1990. Adding a strong alkali, such as sodium hydroxide, to the starch-EAA blend causes extrusion-blown films to have semipermeable characteristics capable of separating aqueous solutes according to molecular size (Otey and Westhoff, 1984). Preliminary evaluations on gas permeability of starch-EAA film suggest potential applications for food packaging because its oxygen permeability is much less than for LDPE film. One major concern to potential manufacturers was the need to premix the starch and EAA in a heavy-duty mixer with large amounts of water before extrusion processing and blowing into films. We have now found that quality films can be made, without this premixing step and with much lower initial moisture content, by incorporating urea into the system. Early in the present study, Rex Plastics, Thomasville, NC, conducted a pilot run on this urea-containing material in a production-size blown-film extruder. This paper describes both the laboratory and pilot-plant preparation of blown film and the effects of adding urea and reduced water content on film properties. The effects of adding poly01 plasticizers, aging for 12 months, and soaking in water are also reported.
Experimental Section Materials. Starch was industrial grade corn starch from CPC International, Englewood Cliffs, NJ, EAA was a copolymer of 20% acrylic acid and 80% ethylene with the trademark of Primacor 5981 from Dow Chemical USA., Midland, MI. LDPE was a general purpose grade 3404B
from Chemplex Company, Rolling Meadows, IL. Amaizo 742D (Fluidity 20) and 745D (fluidity 80) were acidmodified hydroxyethylated starches from American Maize-Products Co., Hammond, IN. Armoslip E is an erucamide slip agent from Armak, Chicago, IL. Pilot-Plant Preparation. Figure 1 is a flow digram of the processing steps used to make the film. Starch (14.0 lb dry; 15.6 lb air-dried basis) and EAA pellets (15.75 lb) were mixed at room temperature in a ribbon blender as a solution of water (1200 mL), concentrated NH40H (1320 mL), and urea (5.25 lb) was slowly added over 15-20 min. (CAUTION: The level of ammonia vapor given off was too high for worker safety even with good ventilation. In small laboratory runs, satisfactory products have been produced with one-fourth this concentration of NH40H. Anyone attempting to conduct a large-scale run should first determine a safe concedtration level appropriate for the scale chosen to run.) Mixing was continued for 2 h, with the top taken off the ribbon blender to allow escape of excess NH,. The solid mixture was then extruded into strands in a Brabender Plasti-Corder Type PLV-300 at a barrel temperature of 7$82 OC. The screw of the extruder was 1.9-cm (3/4-in.)diameter; it had 25 flites, a length: diameter ratio of 251, and a compression ratio of 2:l. The die had 24 holes of 1/32-in.diameter. Since we do not have a pelletizer, the stranded material was passed through a Wiley mill to provide & product that could be fed through a commercial blown-film extruder. On the basis of a yield of 38 lb, the product contained about 8% moisture. The product was extrusion-blown into film by Rex Plastics on a Hartig extruder having a 2-in.-diameter screw with a 201 length-to-diameter ratio (40 in. long), a 10-hp drive operated at about 40 rpm, a 4-in.-diameter die set at 240 OF, and a barrel with four heating zones set at 215, 220, 230, and 235 O F . The cylindrical bubble was about 20 f t high and passed through a nip roll and then to a take-up roll. The bubble diameter was varied to achieve a lay-flat width (double thickness) of 1 2 in. going to the take-up roll. Small-scale Semidry Mixing Method. In a typical example, air-dried starch (40 g dry; 44.8 g air-dried basis) and EAA and optionally LDPE (45 g) were mixed with a spatula as a solution of 8.1 mL of water, 8.1 mL of concentrated NH,OH, and 15 g urea was slowly added at room temperature. The solid mixture was then extruded into strands 2 or 3 times through the Brabender extruder described above at & barrel temperature of 75-90 "C. When LDPE was included, the barrel temperature was about 120-125 "C. The extrudate was then blown into film by passing it through the same extruder, but the die was
This article not subject to U S . Copyright. Published 1987 by the American Chemical Society
1660 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table I. Blowing Conditions for Pilot-Plant Run extruder, "C av thickness, run zone 1 zone 2 zone 3 die, "C rpm torque, mag mil 0.5-in. Laboratory Blown Film Die 77 79 80 A 76 112 113 20 2500-3000 1.5 B 105 108 C 115 121 124 124 38 1000-1500 2.9 D 124 131 130 130 48 1000 1.3 7
102
104
1100
116
av lay-flat width, cm
blowing ratio
comments
12.0 14.7
6.0 5.6 7.4
too cold good film good film too hot
30.5
1.9
good film
4-in. Commercial Blown Film Die 40 1.7
11.1
Zone 4 set at 113 "C.
Table 11. Strand Extrusion Conditions and Toraue Values for Semidry Mixtures formulation* extruder, "C' run" starch, 70 EAA, % LDPE, 70 urea, % zone 1 zone 2 zone 3 60 0 0 77 80 81 3 40 6 40 45 0 15 79 81 82 45 0 15 75 79 82 i 40 10 80 86 88 8 40 50 0 5 81 9 40 55 0 86 90 11 40 40 10 10 120 12 40 30 20 10 124 13 40 25 25 10 125 14 40 20 30 10 125 17 40' 60 0 0 76 81 82 0 0 77 77 82 18 4d 60 n
rpm 80 80 80 80 80 30 30 30 30 80 80
torque, magd 1st pass 3rd pass 5000 7500 2000 4000 1500 4000 2000 4500 4000 5500 500 1500 300 1000 400 1100 1000 1500 4000 6000 5500 6000
"Run numbers correspond to those in Table 111. bFormulated with 16.2 mL of 50/50 solution of concentrated NH,0H/H20 by volume/100 g of solid. Runs 11-14 contained slightly less liquid-see Table 111. 'If only one temperature reported, the extrusion was conducted with a one zone barrel. dApproximate values observed by operator during runs. eAmaizo 745D (80 fluidity). fAmaizo 742D (20 fluidity). Fornulatior: Starch, air driec EAA pellets Urea dissolved in H,O cnd NH,OH
I
over CaCl,, while 10 were kept at 50% RH. Instron measurements were made as quickly as possible after removal from the sealed containers.
___ Ribbon Blender
I
1 Exrruder with Figure 1. Flow diagram of processing steps.
replaced with a heated 0.5-in. blown-film die. Premixing Method. The premixing study was conducted primarily to determine effects of several formulation variables including moisture, LDPE, and plasticizers on film properties. The procedure was essentially the same as reported earlier (Otey et al., 1980). A mixture of airdried corn starch (11% moisture) and various amounts of water, EAA, and other additives was heated with stirring at 95-100 "C in an 85-mL-capacityBrabender mixer (Type R.E.E.-6) or a 1-qt sigma blade mixer. When LDPE was included, the temperature was increased to about 127 "C near the end of the mixing period. The resulting dough was then extrusion processed into strands several times until the moisture level was reduced to achieve a torque of 1000-1500 (m-g)at 40 rpm, and finally the strands were passed through the blown-film die to produce films. Film Evaluation. Tensile measurements were made on the films with an Instron a t a crosshead speed of 2 cm/min. Ten 0.635 X 10 cm strips were cut from each film. Five were soaked in water for 2-4 h and lightly blotted with a paper towel, and after the strips equilibrated overnight a t 50% relative humidity (RH), their tensile properties were measured. The remaining five specimens were tested without soaking. All films were stored at 50% RH during drying and aging. Effect of humidity on film tensile properties was evaluated by cutting 30 film specimens and allowing 10 each to stand overnight, in sealed containers, over water and
Results and Discussions Pilot-Plant Run. Prior to large-scale extrusion blowing, small samples of the extruded, ground particles were evaluated on the 0.5-in. laboratory blown-film die to establish acceptable temperature settings. Temperatures reported for runs B and C, Table I, appeared to be within an acceptable range for allowing the easiest control of the bubble. Based on these runs, the temperatures reported for run 7 were selected for the large-scale run. The experienced operators at Rex Plastics established the 20-ft bubble with no problem, and they produced a more uniform film thickness (1.7 mil, standard deviation 0.22) and a better blowing ratio (1.9) than we are able to obtain with the 0.5-in. laboratory die [blowing ratio = (2 LF)/C where LF is the lay-flat width of the bubble (12 in.) and C is the circumference of the die]. Two problems with the largescale run were observed. After the run was near completion, a white deposit began developing on the die which caused occasional small holes in the bubble. Also, passing the bubble through the nip roll caused the double layer of film to stick together (known as blocking). Both die buildup and blocking are common for certain commercial films and are usually overcome with additives. These problems appeared to be resolved on our small equipment by adding 0.125 pph Armoslip E. Small-scale Semidry Mixing Method. Table I1 lists some representative extruder conditions selected by the operator for the initial extrusion of the semidry blends into strands. This process both melts the synthetic polymer and gelatinizes the starch and blends the composite into nearly clear strands. Considerably less torque is required when urea is present and when partially hydrolyzed hydroxyethyl starch (run 17) is substituted for ordinary starch. Properties of films made from these products are
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1661 Table 111. Effect of Semidry Blending vs. Premixing with and without Urea 1-2-months aged (initial)b mix time water formulation at dry aged soaked run starch, % EAA, % LDPE, % urea, % liquid, mL" "C, min TS EL TS EL 50 2200 60 60 0 0 189 90 2300 1 40 50 2100 50 2 40 60 0 0 88 25 2300 60 0 0 16.2 0 1700 40 1700 40 3 40 60 2200 40 4 40 45 0 15 189 90 900 45 0 15 16.2 20 1300 100 2700 40 5 40 45 0 15 16.2 0 900 120 2700 20 6 40 90 2300 30 45 0 15 16.2 0 1600 7c 40 8 40 50 0 10 16.2 0 1200 130 2300 30 55 0 5 16.2 0 1700 30 2300 40 9 40 10 34 51 0 15 16.2 0 900 160 1200 90 11 40 40 10 10 14.4 0 1400 80 2200 20 0 1700 50 2500 20 12 40 30 20 10 21.6 0 1400 50 2100 20 13 40 25 25 10 9.2 0 1700 50 3100 20 14 40 20 30 10 7.2 0 1700 20 2400 10 15 40 10 40 10 7.2 16 40d 60 0 0 63.6 30 0 0 16.2 0 2000 40 2000 50 17 40d 60 0 0 16.2 0 1500 30 1700 50 18 40e 60 19 40e 50 0 10 12.0 0 1800 50 1900 20
5-6-months aged
TS
EL
2100 2400 1700 700 1000
50 90 30 100 90
water soaked TS EL 2300 50 2100 60 1600 40 1800 20 2400 40
1400 1600 1800 900 1500
50 90 50 140 70
2300 2100 2300 2000 2400
40 30 60 60 40
1500 1300
60 80
2200 2400
20 20
2400 1800 1600 1200
80 50 50 50
2400 1900 1600 1400
30 60 50 20
dry aged
ORuns 1, 2, 4, and 16 contained 11 mL of concentrated NH40H/100 g of solids. Total liquid in all other runs was 50/50 by volume H,O/NH,OH. bTS = tensile strength, psi; EL = percentage elongation; dry aged at 50% RH and 25 "C. Water-soaked samples allowed to stand in water for 4 h and dry aged 24 h before testing. 'Pilot-plant run. dAmaizo 745D. 'Amaizo 742D.
reported in Table I11 under the same run numbers. Run 7, for example, is the pilot-scale preparation. Our intent for adding urea to these systems was to improve the rate of starch gelatinization a t low water levels. Urea is known to contribute to the gelatinization of starch with the effect that either less water or less heat is needed. For example, a solution of 2.25 parts urea and 4.5 parts water will gelatinize 1 part starch very quickly a t room temperature. In water alone, starch must be heated to about 60 "C to initiate gelatinization. We extruded blends of starch, urea, and low levels of water in an earlier study on the use of starch for controlling the release of urea fertilizer (Otey et al., 1984). Starch gelatinization is a difficult term to clearly define. Zobel(l984) has an excellent review on the subject. When starch is heated in excess water to progressively higher temperatures, the granules hydrate and lose their birefringence, and the slurry begins to thicken. Collectively, these events are referred to as gelatinization. The success of blending starch and EAA, or any other polymer, depends on increasing the compatibility of the two polymers. One approach lies in breaking the inter- and intrachain hydrogen bonding of the starch molecules and forming new bonds between the starch and added polymer. In the present study, the formation of hydrogen bonds between starch hydroxyls and EAA carboxyls provides such an approach to achieve compatibility. We envision that maximum compatibility would require total disruption of all internal bonding within the starch. However, from a practical standpoint, this may not be possible for ordinary corn starch. Differential scanning calorimeter studies by %bel have indicated that major structure changes continue to occur beyond the birefringence endpoint, probably due to the slow disruption of persistence crystal structures. Even though the urea at low levels of water may not totally disrupt the crystal structure, it does allow the production of films a t considerable economic benefits over the premixing method, where large amounts of water and extended mixing were used to disrupt the starch. The small amount of ammonium hydroxide used in our study may also influence the gelatinization of starch. Infrared analyses of films reveal typical C=O absorption for both carboxylic acids and carboxylate, indicating that some
of the EAA carboxyls have reacted with ammonium hydroxide. We have not optimized the amount of ammonium hydroxide needed. Runs made with 2 and 4 mL of concentrated ammonium hydroxide in 14.2 mL of water per 100 g of solids each gave films with properties comparable to those reported in Table 111, where 8.1 mL of concentrated ammonium hydroxide and 8.1 mL of water were used. With added urea and with repeated passes through the extruder, films could be made without ammonium hydroxide. At this stage of our study, we find that at least small amounts of ammonium hydroxide, or comparable base, are necessary for ease of blowing and film quality. Earlier, we (Otey and Westhoff, 1984) reported that addition of NaOH in place of ammonium hydroxide yields films with greater transparency, and they have semipermeable properties. The NaOH probably increased the degree of starch gelatinization and thus improves transparency. Data reported in Table I11 reveal the effects of premixing formulations on film properties in high levels of water and extruding semidry blends with and without urea and LDPE. Premixing ordinary corn starch and EAA with large amounts of water prior to extrusion processing gave films with considerably better properties (runs 1 and 2) than the semidry extrusion technique (run 3), and there was little effect of water soaking on film properties. The semidry mix was much more difficult to blow into film, and the film had a slight papery feel. Once urea was added to a semidry mix, the films were easily prepared and no longer had a papery feel. Their dry aged tensile strengths were slightly lower due to the plasticizing effect of urea and lower EAA content (runs 6-8), but their appearance and percentage elongation were much better than those (run 3) made by the semidry mix method without urea. Once urea was added to a semidry mix, the films were easily prepared and no longer had a papery feel. Premixing with high and low levels of water did not appear to affect properties of these urea-containing formulations (runs 4 and 5 ) . Data suggest that when urea is present, extent of mixing is more critical than the amount of water. This is further supported by the slightly better film properties obtained from the pilot-plant run (run 7 ) where the 4-ft extruder
1662 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 Table IV. Effect of Humidity on Film Propertiesn over CaCl,* 50% RH 100% RH run TS EL TS EL TS EL 40% Starch; 60% EAA' 3 1900 10 1500 40 900 140 3d 1600 20 1400 50 800 110 18' 1500 20 1400 40 800 140 50 800 70 Id 2800 10 2200 40% Starch; 45% EAA; 15% Ureac 3100 10 1100 130 700 3100 10 1200 90 500 2500 10 1900 30 700
6 7 7d
190 150 70
a Aged 16 h at indicated RH. Values are average of 2-3 runs on 10 samples each. bTS = tensile strength, psi; EL = percentage elongation. 'Film composition. dSoaked in water 4 h and aged 4 days before testing. e Amaizo 742D starch.
was used. After they were soaked in water, which removed the urea, tensile strengths were comparable to those made by premixing without urea; however, their percentage elongation was lower. Adjusting the formulation (run 10) such that water soaking leaves a film with a 40/60 starch/EAA composition gave a film with higher percentage elongation than runs 1-3 but with generally lower strength after water soaking. As increasing amounts of LDPE (runs 11-15) were substituted for E M , film properties were not greatly affected until the 40% LDPE level was reached. We have not been able to prepare useful films with LDPE as total replacement for EAA. LDPE is not only incompatiblewith starch but it has only fair compatibility with EAA. Use of hydroxyethylated (HE) starch (runs 16-19) appeared to offer promise in terms of easier film preparation and better properties. In general, we find slightly better properties for the acid-modified HE starch, Amaizo 745D (runs 16 and 17). Effect of Humidity on Film Properties. Tensile measurements, even within the same run, sometimes gave erratic results at different times of measuring, leading us to evaluate the influence of humidity on physical prop-
erties. Table IV lists physical properties of aged (9-12 months) films after they were kept at near 0% RH (over CaC12),50% RH,and 100% R H (over water) for about 16 h. During aging, films over water expanded about 2% and their percentage elongation increased dramatically, while those over CaC12 shrank 2-5% and their percentage elongation decreased to near 10-20%. We assume the moisture serves as a plasticizer on the starch portion of the film. Changes in humidity seem to have more effect on the urea-containing films than those made without urea, especially at high and low humidity. Soaking films in water for 4 h reduces the effect of high humidity, which suggests that the starch may retrograde slightly during 4 days of aging after the soaking period. The effect of high moisture can be an advantage for mulch applications where the soil moisture will cause the films to relax and follow the contour of the row. This property could be especially important for films that have lost some percentage elongation during extensive storage before mulch application. Effect of Polyols and LDPE. Prior to the work with urea, a study was made with the premix technique in which increasing levels of poly01 plasticizer, starch, and LDPE were evaluated. The combination of glycerol and glycol glucoside (Westhoff et al., 1979) was shown in an earlier study to be an effective plasticizer for starch-poly(viny1 alcohol) films. Also, we were trying to reduce the cost and increase the level of biodegradable materials in the film. Table V reflects the effects of these variables on film properties after aging and water soaking. Each formulation was mixed in a Brabender mixing chamber at 95 "C for about 25 min and then extruded into strands (two to four passes) until moisture loss increased torque to about 1000 mg. The strands were then extrusion-blown into films. Data should be interpreted in terms of trends within a series rather than of any individual formulation. In each series, increasing starch level had little effect on strength but significantly reduced percentage elongation, a property that was further reduced with 1year of aging. Addition of poly01 plasticizer improved initial percentage elongation slightly but generally had little effect after films
Table V. Effect of Starch Level, LDPE, and Poly01 Plasticizer on Film Properties after Aging and Water Soaking" 1-2-months aged (initial) 12-14-months aged formulation dry aged water soaked dry aged water soaked LDPE, % EAA, % TS EL TS EL TS EL TS EL starch, % Starch and EAA Only 20 80 2300 130 2200 170 2400 90 2000 60 70 2300 70 2300 120 2500 70 2100 60 30 40 60 2300 50 2100 50 2300 50 2000 30 50 50 2000 20 2000 20 2100 10 2000 20 20 30 40 50
20 30 40 50
20 30
40 50
Contain 20% (Based on Starch) Plasticizer (1Part Glycerol/3 Parts 200 2300 180 76 2200 150 2200 120 64 2200 2200 50 2100 50 52 40 2500 30 40 2400 Starch, EAA, and LDPE 2400 160 2400 2100 80 2100 2200 60 2200 10 1600 1800
40 35 30 25
40 35 30 25
38 32 26 20
Contains LDPE and 20% Plasticizer based 38 2100 150 1800 32 2000 80 1900 26 2200 20 2300 20 2500 10 2400
130 70 60 10
Glycol Glucoside) 2000 70 2200 100 2300 30 2500 10
2100 2200 2600 2600
120 60 20 10
1900 1600 1600 1600
80 60 20 10
2000 1600 1700 1400
140 60 40 10
on Starch 120 1800 60 1600 30 1900 10 2100
90 40 20 0
1600 1700 2000 2000
100 30 10 10
"Prepared by premix method; mixed at 95 OC with 60-80 mL of H,0/100 g of formulation for 20 min, 18 mL of concentrated NH,OH/100 g EAA was added, and mixing was continued for another 5 min. All dry aging conducted at 50% relative humidity and 25 OC. Water-soaked samples allowed to stand in water for 2 h and then aged 24 h before testing. TS = tensile strength, psi. EL = percentage elongation.
Ind. Eng. Chem. Res. 1987,26, 1663-1667
1663
were aged and water soaked. Substituting half of the EAA with LDPE caused some loss in both strength and percentage elongation with aging and water soaking; however, cost reduction may outweigh these slight property losses. Also, addition of poly01 plasticizers does not greatly affect properties of these starch-EAA-LDPE systems.
properties. All films containing 40% or more biodegradable material develop mold growth and lose at least 50% of their physical properties within a few months of soil contact. The optimum balance in film properties may require breaking up the film by disking into the soil followed by microbial decay of the buried film.
Conclusions The incorporation of urea into a starch-EAA formulation provides one approach to improving preparation and economics of producing the starch-based film. This approach could allow large-scale continuous production by feeding the various components to a low-energy blender and then to an extruder with a mixing screw to produce pellets or directly into a blown-film extruder. A practical formulation of 40% starch, 20% EAA, 15% urea, and 25% LDPE would have a material cost of 35-40 cents/lb, which is near the price of LDPE. Farmers are reporting removal and disposal costs for nondegradable mulch at 100-200 dollars/acre. If 230 lb of LDPE is used per acre, this removal cost is 43-87 cents/lb of f i i . Hence, considerable economic benefit could be realized by mulching crops with a degradable film that need not be removed. No field studies have been conducted on our starchEAA films but we believe that they will need further modification to provide a balance between optimum rates of deterioration and acceptable physical properties for a range of agricultural mulch applications. Preferably, such films should decompose within 12 months into sufficiently small pieces not to interfere with cultivation. A film composed of 40% Amaizo 742D, 20% sucrose, and 40% EAA appears to meet this criterion when in contact with soil. However, such films have poor initial physical
Acknowledgment We thank R. C. Phelps, M. E. Rivers, and others at Rex Plastics, Thomasville, NC, for conducting the pilot-scale extrusion blowing runs and Sara Walz-Salvador of this laboratory for assistance with the film testing. The mention of firm names or trade products does not imply that they are endorsed or recommended by the US.Department of Agriculture over other firms or similar products not mentioned. Registry No. EAA (copolymer), 9010-77-9; PE, 9002-88-4; NH,OH, 1336-21-6; H2NCONH2,57-13-6;starch, 9005-25-8;hydroxyethyl starch, 9005-27-0; glycerol, 56-81-5; glycol glucoside, 5994-13-8.
Literature Cited Otey, F. H.; Westhoff, R. P.; Doane, W. M. Ind. Eng. Chem. Prod. Res. Deu. 1980,19, 592-595. Otey, F. H.; Westhoff, R. P. Ind. Eng. Chem. Prod. Res. Deu. 1984, 23, 284-281. Otey, F. H.; Trimnell, D.; Westhoff, R. P.; Shasha, B. S. J. Agric. Food Chem. 1984,34,1095-1098. Westhoff, R. P.; Kwolek, W. F.: Otev, - . F. H. StarchlStaerke 1979, 31, 163-165. Zobel, H. F. Starch Chemistry and Technology;Academic: Orlando, FL, 1984; pp 285-309. '
Received for review October 6, 1986 Accepted April 27, 1987
Approximate Lumping Applied to the Isomerization of Methylcyclohexenes Maurice Peereboom Department of Inorganic and Physical Chemistry, Delft University of Technology, 2600 GA Delft, The Netherlands
In view of the very fast isomerization of methylenecyclohexane into 1-methylcyclohexene, the kinetics of the double bond shift isomerization of 1-methylcyclohexene (lMCH), 3-methylcyclohexene (3MCH), 4-methylcyclohexene (4MCH), and methylenecyclohexane (MECH) over y-alumina has been studied on the basis of a pseudo-three-component system in which MECH and l M C H are lumped together. Despite the fact that this system of isomers is only approximately lumpable, it appears that the lumping is a very good one. The kinetic parameters obtained in the temperature range 413-523 K are representative of the behavior of the original four-component system. The Wei and Prater technique appears extremely suitable for calculating the pseudo reaction rate constants, indicating a kinetic model in which the reactions proceed by a series mechanism. In practice it is often expedient to represent the kinetics of reactions in a complex reaction system in terms of lumped (hypothetical) components (e.g., Zhu et al., 1985; Frenklach, 1985). For example, many industrial chemical processes involve complex mixtures that are difficult to analyze in detail, which renders it nearly impossible to determine the kinetics of each of the reactions separately. For that reason, the various species are assigned to a limited number of classes where each class is treated as a single entity. This kind of modeling is also suitable for reaction systems in which, as a result of some very fast interconversions, partial equilibrium is achieved. The 0888-5885/87/2626-1663$01.50/0
choice of the lump compositions has then to be such that the evolution of the lump as a function of time is identical with that of the s u m of the individual components of the lump. A general theoretical lumping analysis for monomolecular reaction systems is presented in Wei and Kuo (1969); more recently, an extension to the lumping of bimolecular reaction systems has been developed by Li (1984). The purpose of the present study is to investigate the lumpability of the isomeric methylcyclohexenes and methylenecyclohexaneand to describe the lumped system in terms of a complex firsborder reaction scheme. Relative rate constants for the lumped system calculated by the Wei 0 1987 American Chemical Society
