Znd. Eng. Chem. Res. 1991,30, 1841-1846
1841
Effect of Compounding and Starch Modification on Properties of Starch-Filled Low-Density Polyethylene &que L.Evangelists,' Zivko L.Nikolov,*" Wei Sung,t Jay-lin Jane,t and Robert J. Gelinat Department of Food Science and Human Nutrition and Department of Zndwtrial Education and Technology, Center for Crops Utilization Research, Iowa State University, Ames, Iowa 50011
Linear low-density polyethylene (LLDPE) cast films were prepared with native and modified (octenyl succinate) corn starch, and their properties were investigated. The optimum conditions for continuous compounding were 205 "C and 20 rpm. Physical properties of LLDPE containing starch octenyl succinate (SOS) were compared to the cast films that contained native corn starch (NCS). The addition of either starch to LLDPE decreased the tensile strength and elongation and increased the water absorption. In all cases, SOS/LLDPE cast films showed higher tensile strength and elongation values than did corresponding NCS/LLDPE cast films. The biodegradation rate of starch for SOS-containing films was lower than the rate observed for NCS-containing films. Introduction The use of starch as a filler in plastic materials has been of interest for the past 30 years but has only recently received adequate attention. Among the first applications of starch as a filler was its use in the production of polyurethane foams (Dosmann and Steel, 1961;Bennett et al., 1967). Westhoff et al. (1974)also utilized starch as a filler in poly(viny1 chloride) films, while Otey et al. (1974)prepared cast films from aqueous dispersions of starch, poly(viny1 alcohol),glycerol, surfactant, and formaldehyde. Starch had not been considered as a filler by the polyolefin converters until the mid 1970s when the prices of synthetic polymers increased and concern over the buildup of disposable plastics was voiced. Two major technologies for starch addition to plastics have been developed. One is based on the use of gelatinized starch molecules as an integral part of the polymeric structure, and the other is based on the use of granular starch as filler. Otey et al. (1977,1980,1987) developed a process for making films by blending starch with a poly(ethy1ene-coacrylic acid) (EM). The various blends containing up to 50% gelatinized starch were extrusion-blown into films that had the appearance of conventional polyethylene films. Theseafilms have potential application as agricultural mulch and packaging, especially where biodegradation is important (Otey et al., 1980). Recently, Swanson et al. (1988)examined the effect of starch modification on the same films and found that a mixture of low-density polyethylene (LDPE) and EAA polymers filled with hydroxypropyl or acetyl derivatives of starch had higher elongation and often higher tensile strength than did native starch-filled films. An extensive search of potential biodegradable fillers for thermoplastics &closed that only raw starch satisfied the requirements for adequate thermal stability and minimum interference with melt-flow properties and disturbance of product quality (Griffin, 1974). In an effort to develop new commercially viable and degradable products, Griffin (1977a,b)proposed a process for making LDPE blown &ns containing native or modified starches and autoxidants such as unsaturated fatty acids and their derivatives. The biodegradable starch filler provides a nutrient source for microorganisms, and the autoxidanta, in contact with transition metals, generate peroxide and free radicals that To whom all correspondence should be addressed. 'Department of Food Science and Human Nutrition. Department of Industrial Education and Technology.
*
attack the polyethylene chain. This technology is currently used in the U.S.to make various types of starch-based degradable plastics containing between 6 and 9% starch. The process technologies reported by Griffin are described mainly in the patent literature, and the lack of detailed information hampers the critical evaluation and comparison of the various processes and fillers. In this work we investigated physical properties of linear lowdensity polyethylene (LLDPE) cast films that contained starch odenyl succinate aluminum complex (SOS)(Figure 1)and compared them to the properties of films containing native corn starch (NCS). The starch modification with octenyl succinate reduces the hydrophilic nature of the starch surface (Caldwell, 1952). The effect of starch modification on the biodegradation of starch in the films will also be discussed. Experimental Section Materials. Industrial-grade native corn starch was a gift from American Maize-Products Co. (Hammond, IN). Starch octenyl succinate with trademark Dry-flo was also a gift from National Starch and Chemical Corp. (Bridgewater, NJ). All starch samples were dried under vacuum at 100 "C to a final moisture content of less than 0.5% and were stored in a desiccator before use. Appropriate precautions should be taken in handling the dried starch on a large scale because of ita explosive nature. The moisture content was measured by using the Karl-Fischer titration method according to ASTM E203-75. Linear low-density polyethylene (LLDPE) PE 2045 was obtained from Dow Chemical Co. (Midland, MI) and was used as base resin. Bacillus amyloliquefaciens (B. amyloliquefaciem) aamylase was purchased from Sigma (St. Louis, MO). Film Preparation. A 1:9 (w/w) mixture of starch and LLDPE was compounded by using a Brabender (Hackensack, NJ) PL 2000 Plasti-Corder drive and a Brabender twin-screw mixer, Model 15-02-000.The twin intermeahing screws of the mixer were single-flighted counter-rotating screws 42 mm in diameter with interrupted mixing zone. Mixing studies were performed at 145,165,185,205,225, and 245 "C with three screw speeds of 20,40,and 60 rpm at each temperature. The extruded strands, 3.2 mm (1/8 in.)in diameter, were forced-air cooled by passage through a cooling trough and were pelletized by using a Brabender Granutizer Model 12-72-000.The pelletized mixture was immediately transferred to a desiccator and stored until needed for film preparation. The pelletized batches of approximately 750 g each were extruded into cast film through a 15.2 X 0.5 cm (6 X 0.18 in.) ribbon die using a
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1842 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
-0OC
R' I -R -COO-
R' -OOC-~-COO-
A/3+
GRANULE
R' I -0OC -R -COOR': Octenyl R: (CH2-CH) Figure 1. Structure of starch octenyl succinate aluminum complex.
31.8 mm (11/4in.) Brabender extruder Model 125-25 HC. The single-stage mixing screw of the extruder had a compression ratio of 3:l. The screw speed was set at 15 rpm, and the four heating zones of the extruder were set at 150, 160,170, and 190 "C, respectively; the ribbon die was also held at 190 "C. The film was pulled through a pair of water-cooled rolls on a Brabender Univex Model 05-92000-00 take-off at a speed of 2.3 m/min. To study the effect of starch content on film properties, the two types of starches (NCS and SOS) were mixed with LLDPE to final starch concentrations of 5,10,15,20, and 25% (w/w). Pelletized batches were prepared for each combination using the twin-screw mixer at 205 "C and 20 rpm. Cast films were prepared by using the same conditions as those described above. Testing of Cast Films. Tensile strength and elongation were tested by using a slight modification of the ASTM D882-83 method. Test samples for tensile strength and elongation measurements (five samples per treatment) were prepared by cutting the films in the machine direction using a 1.3 X 15.2 cm (1/2 X 6 in.) template. The width and thickness of the samples were measured by using a digital caliper. The average thickness of cast films was 127 pm. Before the tensile strength and elongation were measured, the test samples were conditioned for at least 44 h in a 50% relative humidity chamber maintained by using a supersaturated solution of magnesium nitrate. Tensile strength and elongation measurements were performed immediately after the conditioned samples were taken out of the humidity chamber. Tensile strength measurements were made by using an Instron Model 4502 (Park Ridge, IL) universal testing instrument with wedge action grips and rubber-lined grip faces. The initial grip distance was 5.1 cm (2 in.), and the test speed was set at 51 cm/min (20 in./min). Peak loads and elongation at break were recorded. Water absorption was measured by using 2.5 X 7.6 cm (1 X 3 in.) long film strips following the general recommendations outlined in the ASTM D570-81 method. The samples were conditioned in an oven at 50 "C for 24 h after which the initial weights were obtained. The conditioned strips were soaked in distilled water at room temperature for 24 h. Excess water was removed from the surface of the samples by blot-drying with paper towels prior to final weight determination. The amount of water absorbed was calculated as a percent of the initial sample weight. Statistical Analysis. The data on tensile strength, elongation, and water absorption of the cast film samples were analyzed by using the statistical analysis system (SAS, 1987) program. Treatment effects were determined by using analysis of variance (ANOVA). Least significant difference (LSD) at a = 0.05 was used as a method of mean separations. Enzymatic Degradation of Starch in Starch-Filled Plastics. The accessibility of the native and modified
starch in the cast film samples to enzymatic attack was studied by using bacterial a-amylase. B. amyloliquefaciens a-amylase was selected for its ability to hydrolyze raw native (insoluble) starch. The enzyme activity was determined by measuring the production of reducing sugars using the Somogyi-Nelson method (Nelson, 1944) with maltose as standard. One unit (v)of a-amylase activity was defined as the amount of enzyme that hydrolyzes 1 pmol/min of a-1,4-glucosidicbonds in starch at 37 "C and pH 6. Plastic films containing starch were cut into 1.2 X 1.2 cm squares weighing approximately 100 mg each, weighed to f O . l mg, and were transferred to 250-mL beakers. A 46-mL aliquot of 0.05 M acetate buffer and 4 mL of enzyme stock solution containing 60 U/mL were added. The beakers were then covered with aluminum foil squares and were incubated in a Lab-Line (Melrose Park, IL) orbital shaker/water bath at 37 "C and 100 rpm. Controls were prepared in the same manner, except that 4 mL of buffer was substituted for the enzyme stock solution. The reaction mixtures were analyzed for total carbohydrate (Dubois et al., 1956) every 24 h for the first several days (the period when most degradation occurs) and every 48 h thereafter. At the end of each 24- or 48-h interval, 1mL of reaction mixture was removed for total carbohydrate analysis. The residual reaction mixture was then discarded and, after the plastic samples were washed, was replaced with fresh reagent solutions. Between reagent changes, the films were washed in the beaker, twice with distilled water followed by once with 70% ethanol.
Results and Discussion Effects of Mixing Temperature and Screw Speed on the Physical Properties of the Films. One of our goals in this work was to evaluate the effects of modified starch on cast film properties and to compare them with the properties of cast films containing native starch filler. To obtain meaningful comparisons, one needs to use films that are of the same quality and that are devoid of additives that could obscure their comparison. For that reason we first attempted to optimize the compounding conditions using 10% starch filler. The biggest problem encountered in making the starch-filled films was the presence of bubbles in the film despite using starch with less than 0.5% moisture content. Although moisture associated with the starch is believed to be the main cause of bubble formation (Griffin, 1977a,b), we assumed that at starch moisture levels below 0.5% water vapor bubbles could be virtually eliminated by adjusting the processing conditions. We decided against adding any desiccant in order to avoid introducing additional variables that could complicate the films' evaluation. In preliminary experiments we observed that bubbles were present in the extruded pellets (Evangelista, 1990). Therefore, the effect of the mixing temperature and screw speed on the number, distribution, and the size of bubbles in 10% starch-filled LLDPE films was investigated. In all experiments extrusion conditions were kept constant and are given in Materials and Methods. The results, presented in Figure 2, indicate that the size and number of bubbles that appeared in the films were dependent on the screw speed and mixing temperature during compounding. Cast films, made from pellets that were mixed at 145 or 165 "C and at screw speeds of 40 or 60 rpm contained the greatest number of large bubbles. The number and the size of bubbles were reduced by increasing the mixing temperature and/or lowering the screw speed. A decrease in screw speed at any temperature seemed to have a more pronounced effect on bubble formation than did an increase in temperature. The optimum
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1843 900 SCREW SPEED
LSD=
COMPOUNDING TEMPERATURE ("C) 1
I
I
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I
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Figure 2. Photomicrograph (50X magnification) of iodine-stained 10%SOS/LLDPE cast films compounded at different temperatures and screw speeds.
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Figure 4. Effects of compoundingtemperature and screw speed on elongation of (a) LLDPE, (b) 10% SOS/LLDPE, and (c) 10% NCS/LLDPE cast films. Screw speed: 20 rpm (CHI), 40 rpm (001,and 60 rpm (A-A).
c
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Mixing temperature ("c) Figure 3. Effects of compoundingtemperature and screw speed on tensile strength of (a) LLDPE, (b) 10%SOS/UDPE, and (c) 10% NCS/LLDPE cast films. Screw speed 20 rpm (0-o),40 rpm (0O), and 60 rpm (A-A).
compounding conditions were temperatures above 205 "C and mixing speeds of 20 and 40 rpm. Similar conclusions were reached for 10% NCS/LLDPE films (data not shown). It appears that the size of the bubbles and their number is related to the pressure drop at the die and the residence time in the mixer. The low number of bubbles observed in the films compounded at temperatures above 165 "C and screw speeds below 60 rpm could be explained by the combined effects of low melt viscosity and longer residence time. These conditions might have allowed dissolved volatiles and generated water vapor to be vented in the feed zone before the polymer melt moved deeper into the mixer.
The starch particles in the cast films were stained with iodine and their distribution under various processing conditions was examined microscopically. Qualitative examination of the stained films showed that both starches were distributed evenly throughout the film irrespective of the mixing conditions. A typical distribution of starch particles is shown in Figure 2. Our compounding studies also showed that the mixing temperature should be kept below 245 "C (Evangehta, 1990). At that temperature the extruded pellets and cast films had a yellow to brown color, depending on the residence time in the mixer. The highest discoloration occurred at a mixing speed at 20 rpm followed by 40 and 60 rpm, and it was attributed to a thermal degradation of the starch. The discoloration was higher for 10% SOS/LLDPE than for 10% NCS/LLDPE films under the same processing conditions. The results summarizing the effect of compounding temperature and the screw speed on the tensile strength of the various cast films are presented in Figure 3. The tensile strength of LLDPE films did not change significantly with temperature and screw speed (Figure 3a). The highest tensile strength values were obtained from films that were compounded at 225 "C. The addition of 10% starch decreased the tensile strength of LLDPE almost 30%. The screw speed at mixing temperatures below 205 "C significantly affected the tensile strength of the films containing 10% starch. A screw speed of 60 rpm had a detrimental effect on tensile strength at lower compounding temperatures (Figure 3b,c), which could be explained by the presence of bubbles in the films, as discussed previously. The tensile strength of NCS/LLDPE cast films appeared to be more sensitive to changes in screw speed than were those of SOS/LLDPE films. A
1844 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 1.o
60 1
LSD= I 0.5
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Starch content ( X ) Figure 6. Tensile strength of SOS/LLDPE (0) and NCS/LLDPE (m) cast films with different starch concentration.
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5
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water absorption of (a) LLDPE, (b)10% SOS/LLDPE, and (c) 10% NCS/LLDPE cast films. Screw speed: 20 rpm (0-O), 40 rpm (0O ) , and 60 rpm (A-A),
Storcn content (%) Figure 7. Elongation of SOS/LLDPE (0)and NCS/LLDPE (m)
maximum tensile strength for 10% starch-filled cast films was obtained at 185 "C and 20 rpm. Mixing temperature and screw speed did not significantly affect the elongation of LLDPE films (Figure 4a), but they did reduce the elongation of 10% starch/LLDPE films. The addition of 10% NCS or 10% SOS decreased the elongation values of LLDPE films by 8% (Figure 4b,c). The screw speed had a greater impact on elongation than did the mixing temperature. As was the case with the tensile strength, a combination of low temperature and high mixing speed reduced elongation more than did high temperature and low mixing speed. Maximum elongation was obtained at 185 "C and 20 rpm for both starches. In examination of the effect of compounding parameters on water absorption by LLDPE films (Figure 5a), no definite trend was observed. Adsorption values fluctuated with temperature, and the screw speed did not affect the absorption of water. The addition of 10% starch to LLDPE increased water absorption from 0 to 2 % (Figure 5b,c). Starch granules that were closer to the surface or along the cut edges probably absorbed most of the water. Only mixing temperature had an effect on water absorption of 10% starch/LLDPE cast films. Absorption increased with mixing temperature; absorption values were significantly higher at 225 and 245 "C, where discoloration of the films due to starch thermal degradation was observed. Therefore, higher absorption values measured at those temperatures were probably due to a higher hygroscopicity for thermally decomposed starch. Although compounding LLDPE and starch at 185 "C and 20 rpm produced cast films with the highest tensile strength, films mixed at 205 "C were devoid of bubbles and possessed a smoother texture. Mixing conditions of 205 "C and 20 rpm were, therefore, adopted as optimal for cast
films and were used in subsequent studies. Effect of Starch Content on the Properties of LLDPE Films. Cast films with starch contents ranging from 5 to 25% were prepared using master batches mixed at 205 " C and 20 rpm. The tensile strength of LLDPE films decreased significantly with increasing starch concentration (Figure 6). At 5% starch concentration, there was a 26% decrease in tensile strength for SOS/LLDPE film and a 30% decrease for NCS/LLDPE film. The tensile strength decreased by an additional 8-1070 for every 5 % increase in starch concentration for both the native and modified starch films. This decrease in tensile strength is probably due to the discontinuity created by the starch granules in the LLDPE film and by the poor interfacial interaction between the starch granules and the LLDPE matrix. The latter results in stress applied to the film not being passed effectively between the LLDPE and the starch granules. In all cases, SOS/LLDPE films consistently showed higher tensile strengths than did NCS/LLDPE films but were significantly higher only at the 5, 10, and 25% starch levels. Modification of the surface of the starch granules with octenyl succinate apparently reduces the interfacial tension between the starch granule and LLDPE and thereby improves the tensile strnegth of the 5 and 10% SOS/LLDPE film. These results support previous observations that reducing the hydrophilic nature of the starch granule surface increases the starch-polymer bond (Griffin, 1977a,b). The elongation of SOS/LLDPE and NCS/LLDPE films decreased with increasing starch concentration, but it was not as striking as that of tensile strength (Figure 7). Except at the 5% starch level, SOS/LLDPE films had significantly higher elongations than did NCS/LLDPE films.
cast films with different starch concentration.
Ind. Eng. Chem. Res., Vol. 30,No. 8, 1991 1845 I
8 1
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Figure 8. Water absorption of SOS/LLDPE (0) and NCS/LLDPE (H) cast films with different starch concentration.
OY 0
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Time (days) Figure 9. Enzymatic hydrolysis of starch in SOS/LLDPE films at 15% ), 37 OC and pH 6.0. SOS content: 5% (O-O), 10% (t. (A-A),20% (A-A),and 25% (0-0).
Water absorption of SOS/LLDPE and NCS/LLDPE films increased proportionally with starch content (Figure 8). There was no significant difference in water absorption between the two film types up to 15% starch concentration. However, the NCS/UDPE cast filmsabsorbed more water than did the SOS/LLDPE films when more than 15% starch was present. The difference in water absorption observed at starch concentrations above 15% might be due to the different r a t e of hydration of modified and native starch. This difference probably exists at lower starch levels as well, but it would be more difficult to detect. Starch Hydrolysis of NCS- and SOS-Containing Films. The starch degradation kinetics over the 2-week incubation at 37 OC are presented in Figures 9 and 10 for SOS/LLDPE and NCS/LLDPE films, respectively. The values plotted in these figures represent only starch lost by hydrolysis, as corrections were applied to each film for starch loss by leaching into the buffer solution. The initial rates of hydrolysis for NCS/LLDPE films were approximately two times higher than were the corresponding rates for SOS/LLDPE. The fractions of the initial starch content that had been hydrolyzed after the 2-week incubation period ranged between 10 and 13% for NCS-containing films and between 3 and 5% for SOS-containing films. These findings suggest that modification of the starch surface with octenyl succinate hinders starch biodegradation. This could be due to a reduced accessibility of the modified starch granules as compared with NCS when embedded within the polyethylene matrix. In order to test
2
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8 1 0 1 2 1 4
Time (days) Figure 10. Enzymatic hydrolysis of starch in NDS/LLDPE filme at 37 "C and pH 6.0. NCS content: 5% (O-o),10% (0-01, 15% (A-A), 20% (A-A), and 25% (0-0).
the latter hypothesis, granular NCS and SOS were each suspended in a buffer solution and hydrolyzed with aamylase at 37 "C. Starch concentrations were 0.3% (w/v). Similar initial rates of hydrolysis were obtained for both NCS and SOS, thereby lending support to the hypotheses of reduced accessibility (data not shown). The reduced accessibility of SOS in the cast film to enzymatic attack parallels our observations of increased tensile strength and elongation for the same films. Except for the 5% and 10% SOS/LLDPE films, approximately 1.5% of the starch originally present was found to have leached from the control samples following the 2-week incubation period. The 5% and 10% SOS/ LLDPE f i i had already lost 1% of the total starch after 24 h of incubation and had lost an additional 4 and 1% , respectively, at the end of incubation period. This might be explained by the two aberrant films having more starch granules exposed on the film surfaces than did the other film samples. Such variations in starch distribution might explain the apparent similarity of water absorption between NCS and SOS cast films at 5 and 10% starch loadings. In general, both the rate and the extent of starch biodegradation were relatively low. It appears that for more efficient degradation of plastic films synergistic activities of chemical degradation and photo- and biodegradation should be implemented whenever possible.
Conclusions The work reported here indicates that the addition of starch alters the physical and mechanical properties of polyethylene films. Optimization of compounding conditions was necessary to produce bubble-free films. Compounding starch and LLDPE at 205 "C and 20-rpm screw speed resulted in bubble-free cast films containing up to 15% NCS and up to 20% SOS. The addition of starch to LLDPE decreased the tensile strength, elongation, and increased water absorption. In all cases, SOS/LLDPE cast films showed higher tensile strength and elongation values but lower levels of biodegradation than did the NCS/ LLDPE cast films Acknowledgment
This research was supported by the Iowa Corn Promotion Board, the ISU Center for Crops Utilization Ressarch, the Iowa State Legislature, Iowa Department of Agriculture and Land Stewardship, and the Iowa Agriculture and Home Economics Experiment Station. This is journal paper No. 5-14306 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, Projects 2860 and 0178.
I n d . E n g . Chem. Res. 1991,30, 1846-1855
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Nomenclature EAA = poly(ethy1ene-co-acrylicacid) LDPE = low-density polyethylene LLDPE = linear low-density polyethylene NCS = native corn starch SOS = starch octenyl succinate Registry No. PE2045, 26221-73-8; water, 7732-18-5; corn starch, 9005-25-8; starch octenyl succinate, 52906-93-1.
Literature Cited Bennett, F. L.; Otey, F. H.; Mehltretter, C. L. Rigid urethane foam extended with starch. J . Cell. Plast. 1967, 3, 369-373. Caldwell, C. G. Free-flowing starch esters. U S . Patent 2 613 206, 1952. Dosmann, L. P.; Steel, R. N. Flexible shock-absorbing polyurethane foam containing starch and method of preparing - - same. US. Patent 3 334934, 1961. Dubois, M.; Giles, K. A,; Hamilton, J. K.; Rebers, P. A.; Smith, R. Colorimetric Method for Determination of Sugars and Related Substances. Anal. Chem. 1956,28, 350-356. Evangelista, R. L. Effect of starch on mechanical and physical properties of starch-filled polyethylene. M.S. Thesis, Iowa State University, Ames, IA, 1990. Griffin, G. J. L. Biodegradable fillers in thermoplastic. Adu. Chem. Ser. 1974, 134, 156-170. I
Griffin, G. J. L. Biodegradable synthetic resin sheet material containing starch and a fatty acid material. U.S. Patent 4 016 117, 1977a. Griffin, G. J. L. Synthetic resin sheet material. US. Patent 4021 388, 1977b. Nelson, N. A photometric adaptation of the Somogyi method for the determination of glucose. J. Biol. Chem. 1944, 153, 375-380. Otey, F. H.; Mark, A. M.; Mehltretter, C. L.; Russell, C. R. Starchbased film for degradable agricultural mulch. Ind. Eng. Chem. Prod. Res. Deu. 1974, 13,90-92. Otey, F. H.; Westhoff, R. P.; Russell, C. R. Biodegradable films from starch and ethylene-acrylic acid copolymer. Ind. Eng. C h e n . Prod. Res. Deu. 1977,16, 305-309. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-based blown films. Ind. Eng. Chem. Prod. Res. Deu. 1980, 19, 592-595. Otey, F. H.; Westhoff, R. P.; Doane, W. M. Starch-based blown films. 2. Ind. Eng. Chem. Res. 1987,26, 1659-1663. S A S I S T A T guide for personal computers, 6th ed.; SAS Institute: Cary, NC, 1987. Swanson, C. L.; Westhoff, R. P.; Doane, W. P. Modified starches in plastic films. Proceedings of the Corn Utilization Conference II, Columbus, OH, 1988; National Corn Growers Association: St. Louis, MO, 1988. Westhoff, R. P.; Otey, F. H.; Mehltretter, C. L.; Russell, C. R. Starch-filled polyvinyl chloride plastics: preparation and evaluation. Ind. Eng. Chem. Prod. Res. Deu. 1974, 13, 123-125.
Receiued f o r reuieur December 1, 1990 Accepted March 4, 1991
PROCESS ENGINEERING AND DESIGN Dynamics of Homogeneous Azeotropic Distillation Columns Henrik W. Andersen,? Lionel Laroche,*and Manfred Morari* Chemical Engineering 210-41, California Institute of Technology, Pasadena, California 91125
The dynamics of azeotropic distillation columns are significantly more complicated than those of simple binary columns. For changes in the internal flows, we initially observe a response in overall separation with a time constant that is similar to the one observed in binary distillation. However, we also observe a slow time-constant response, which is due to changes in the relative amount of entrainer in the extractive section. The fact that the gains for these two effects have different signs causes overshoot and, depending on the operating point, may also cause inverse responses with significant time constants. None of these phenomena are observed in binary distillation. They can be modeled by a simple second-order model. We also examine the open-loop dynamics for configurations aimed at dual-composition control. Just as in binary distillation there is a "high gain direction" that is obtained for changes in the external flows. The dynamics of this direction are hardly affected by changes in the operating point. The "low gain direction" is obtained for changes in the internal flows, just as in binary distillation, and also for changes in the entrainer feed flow rate. The properties of this direction change significantly with the operating point. For some operating points we observe overshoot and for others multivariable right-half-plane transmission zeros not observed in binary distillation. 1. Introduction
Separation of binary azeotropes is an important process in the chemical industry. The separation is characterized by the fact that it is impossible to perform by means of
* T o whom correspondence should be addressed. Chemical Engineering, Building 229, The Technical University of Denmark, 2800 Lyngby, Denmark. Procter & Gamble, Canada.
*
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binary distillation. The most common approach to solve this problem is to add a third component, the entrainer, which enables the separation of the components forming the binary azeotrope. Depending on the thermodynamic properties of the mixture, the separation can be performed either entirely by distillation (the homogeneous case) or partly by decanting two liquid-phases (the heterogeneous case). Previous work in the area of azeotropic distillation has focused on the prediction of thermodynamic data, 0 1991 American Chemical Society
