712
I n d . E n g . Chem. R e s . 1988,27, 712-716
enhance compaction characteristics. 2. Little or no strength will develop in phosphogypsum without some degree of chemical stabilization formed by the hydration of cement or by the pozzolanic activity of fly ash. 3. With low amounts of stabilizers or after curing for short periods, low strengths were obtained, and the type of base has no significant effect on strength. Under these conditions, the strength was found to increase with the amount of neutralizer (pH), the curing period and/or the amount of cement. At a high cement contents (lo%),the strength developed when Ca(OH), was used as a neutralizer was much higher than that developed with NaOH. The use of NaOH can produce deleterious results in the form of hairline cracks. This is attributed to the migration of water-soluble Na2C03to the surface and/or the formation of an expansive gel. Using Ca(OH),, on the other hand, allows the formation of CaC03 which is known to contribute to strength development. 4. The deleterious effect of NaOH as neutralizer appears to be magnified when fly ash was used as a stabilizer. With increased NaOH contents, more hairline cracks were generated and complete failure occurred after curing for long periods (28 day). The use of Ca(OH), was beneficial in that a pronounced increase in strength was achieved in mixtures with pH values higher than 5. 5. Data have shown that a possible reversal in strength may occur at curing periods more than 28 day. This may be due to the formation of Ettringite produced by the further reaction of hydrated calcium silicate and sulfate in the presence of aluminates. This reaction may be accelerated by using a cement rich in C3A or with a higher surface area such as Type I11 portland cement. These results will be reported in the near future. On the other hand, a cement low in C,A can be expected to retard the formation of Ettringite. Accordingly, stabilization with Type V (i.e,, sulfate resistant) portland cement may be beneficial.
Acknowledgment The authors thank Mobil Chemical Company, CEMR, The Bureau of Mines, and Texas A&M University AUF for supporting the Research program on Utilization of Industrial Wastes. Literature Cited Borris, D. P. Proceedings of the Internal Symposium on Phosphogypsum, The Florida Institute of Phosphate Research, Vol. 1,pp 1-6, 1980. Deussner, M.; Heinz, D.; Ludwig, U. Abstracts of Papers; American Ceramic Society, 87th Annual Meeting Proceedings, Cincinnati, OH, May 1985; American Ceramic Society: Columbus, OH, 1985. Diamond, S., Ed. Symposium on Fly Ash Incorporation in Cement and Concrete; Material Research Society: Philadelphia, 1981; pp 112-123. Gadalla, A.; Rozgonyi, T.; Saylak, D. Particulate and Multiphase Processes; Ariman, T., Veziroglu, T. N., Eds.; Hemisphere: Washington, New York, London; 1987; Vol. 2, pp 161-172. Goers, W. E. Proceedings of the International Symposium on Phosphogypsum, The Florida Institute of Phosphate Research, Vol. 1, pp 37-52, 1980. Ghosh, S. N. Advances in Cement Technology; Pergamon: Oxford, United Kingdom, 1983. Gregory, C. A. M. S. Thesis, Texas A&M University, College Station, 1983. Kouloheris, A. P. Proceedings of the International Symposium on Phosphogypsum, The Florida Institute of Phosphate Research, 1980, Vol. 1, pp 8-35. Lea, F. M. The Chemistry of Cement and Concrete; St. Martin's: New York, 1956. May, A.; Sweeney, J. Proceedings of the Znternational Symposium on Phosphogypsum, The Florida Institute of Phosphate Research, Vol. 2, p 481,-1980.. McKerral. W. C.: Ledbetter. W. B.: Teaeue. D. J. "Analvsis of Flv Ashes Produced in Texas" Texas Transportation Research Re"port, 240-1, 1979. Young, J. T. Cements Research Progress; American Ceramic Society: Columbus, OH, 1984. Yung, Chung Che M.S. Thesis, Texas A&M University, 1985. I
Received for review April 21, 1987 Accepted November 17, 1987
Rubber Additives Derived from Guayule Resid William W. Schloman, Jr. Central Research Laboratories, T h e Firestone Tire & Rubber C o m p a n y , Akron, Ohio 4431 7
The commercial viability of natural rubber from the desert shrub guayule will depend on production of high-value byproducts to offset farming and processing costs. Guayule resin, the nonrubber extractables, can be chemically modified to produce solid derivatives which enhance the physical and chemical properties of rubber compositions. These derivatives affect sulfur vulcanization in a manner related to the nature of the modifying agent. Sulfurized guayule resin reduces vulcanizate hysteresis. Guayule resin condensed with polyamines or amine-terminated polyethers improves green strength and vulcanizate tear strength. The future of the desert shrub guayule (Parthenium argentatum Gray) as a domestic source of natural rubber (NR) remains the subject of speculation. The economics of guayule as a commercial crop is not firmly established. In many economic assessments, byproduct credits are assumed to make significant reductions in net farming and processing costs (Eagle, 1981; Weihe and Nivert, 1983; McFadden and Nelson, 1981). To justify such assumptions, at least two criteria must be met. First, is the byproduct available from commercially acceptable shrub processing? A byproduct can have no practical application This work was presented at a meeting of the Rubber Division, American Chemical Society, Montreal, Quebec, Canada, May 1987. 0888-5885/88/2627-0712$01.50/0
if the material represents too small a fraction of the isolable feedstock biomass (McLaughlin, 1985). Second, have the byproduct's physical or chemical properties been established? A byproduct's composition can often define its potential applications. Without direct demonstration of utility in a particular application, a byproduct has little practical value other than its energy content. Guayule processing yields three significant byproduct types: resin, low molecular weight rubber, and bagasse (residual biomass). Resin, the nonrubber shrub extractables, is a ubiquitous and unavoidable processing byproduct. For every kilogram of total rubber produced, there will be 1kg or more of resin. Resin contains a wide variety of secondary metabolites (Schloman et al., 1983). The 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 713 Table I. Sulfurized Guayule Resin Physical Properties resin feed" activator total S content, w t % sp,d O C none 11 56 nonpolar nonpolar none 26 67 nonpolar none 31 74 whole none 33 77 whole TMTDb 33 85 whole ZMBT' 32 74 whole none 47 91
" Nonpolar
= hexane-soluble resin fraction; whole = unfractionated resin. bTMTD = tetramethylthiuram disulfide, 2.3 wt % based on resin. ZMBT = zinc 2-mercaptobenzothiazole,3.7 wt %
based on resin.
Table 11. Di- and Polyamine-TreatedGuayule Resin" Physical Properties resin feedb aminec total N content, wt % sp, O C TEPA 4.4 74 whole polar TEPA 4.6 92 whole DETA 2.8 69 MDA 1.4 72 whole whole PPDA 1.9 80 Polar = hexane-insoluble resin fraca Products of method A. tion; whole = unfractionated resin. TEPA = tetraethylenepentamine, DETA = diethylenetriamine, MDA = methanediamine, PPDA = p-phenylenediamine.
Softening point.
composition of guayule resin varies seasonally (Schloman et al., 1986). Resin is obtained by acetone extraction of milled shrub or coagulated latex (Eagle, 1981) or by coagulation of rubber from a combined rubber-resin miscella (Engler and McIntyre, 1984; Beattie and Cole, 1986). Coagulation can be carried out in a manner which fractionates the crude rubber, yielding a final product with acceptable physical properties (Beinor and Cole, 1986). Under these conditions, the low molecular weight fraction of the rubber, representing 15-25% of the total rubber produced, can be isolated as an additional byproduct. Few high-value uses have been demonstrated for guayule resin. The prooxidant properties of resin may be useful in applications calling for rubber peptizers. Resin-based coatings with good water resistance have been prepared (Belmares et al., 1980). Chemically modified resin or resin fractions have been used to enhance the properties of unvulcanized rubber compositions (Kay and Gutierrez, 1985). Reported here is the use of chemically modified guayule resins and resin fractions to improve a range of physical properties in both unvulcanized and vulcanized rubber compositions. The chemical modifications include sulfurization and condensation with polyfunctional amines and amine-terminated polyethers.
Experimental Section Shrub Origin and Processing. Whole shrubs were harvested from irrigated stands maintained at Sacaton, AZ. The shrub was shipped to pilot plant facilities in Akron, OH, where it was chopped in a hammer mill and flaked by two passes through a 2-roll mill set to zero clearance. The flaked shrub was exhaustively extracted with acetone to remove resin. Acetone was distilled from the resin solution with the aid of a nitrogen sparge. Resin Partition. A 2:l (w/w) mixture of hexane and resin was rapidly stirred for 30 min and then allowed to settle. The supernatant was decanted and desolventized by means of a nitrogen sparge to yield the resin nonpolar fraction. The hexane-insoluble residue containing the resin polar fraction was similarly desolventized. Resin Derivatization. General. Ring-and-ball softening points (sp) were determined in accordance with ASTM method E 28-67. Elemental microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, TN. Resin Sulfurization. In a reactor equipped with a mechanical stirrer, 150 g of guayule resin or resin fraction was heated to 140 "C. Sulfur and any activators (2-5% based on guayule material charge) were then added. The temperature of the mixture was raised to 155-160 OC and maintained at this level for 90 min. During this time, a nitrogen stream was passed through the mixture by means of a sparge tube so as to facilitate the distillation of volatile byproducts. Table I summarizes the physical properties of representative sulfurized resin and resin fractions prepared with varying sulfur levels.
Table 111. Polyether-Treated Guayule Resin Physical Properties resin feeda polyether SP, "C whole Jeffamine D-230 67 polar Jeffamine D-230 77 polarb Jeffamine D-230 71 polar Jeffamine T-403 51 polar Jeffamine ED-600 27 Polar = hexane-insoluble resin fraction; whole = unfractionated resin. Product of method B; all others, products of method A.
Table IV. Rubber Test Recipe component NR soln SBR, 23.5% bound styrene N330 carbon black medium aromatic process oil zinc oxide stearic acid antioxidanta acceleratorb sulfur derivatized guayule resin
level, phr' 50 50 50 3 3 2 1 1 1
various
a Santoflex 13, Monsanto. N -tert-Butyl-2-benzothiazolesulfenamide. cParts by weight per 100 parts of rubber.
Resin Condensation with Polyamines or Polyethers. Method A. In a reactor equipped with a mechanical stirrer and a Dean-Stark distilling receiver with reflux condenser, 300 g of guayule resin or resin polar fraction and 150 g of toluene were heated to 110 OC. Cross-linking agent was then added to the heated solution. The quantities of cross-linking agent were adjusted to provide equivalent levels of reactive end groups (NH,), typically 0.14 mol/100 g of guayule material. The mixture was refluxed until water evolution was complete. Desolventization with a nitrogen sparge to 225 OC was followed by a steam sparge at 200-225 OC until volatile byproduct distillation was complete. Method B. In a reactor equipped with a distillation head, a mixture of 300 g of guayule resin or resin polar fraction and cross-linking agent was heated to 165-170 OC. The pressure in the reactor was reduced to about 3-10 kPa by means of a vacuum pump to facilitate distillation of about 10 g of volatile byproducts. The pressure in the reactor was returned to atmospheric and the reaction mixture treated with a steam sparge at 210-225 "C until volatiles distillation was complete. Tables I1 and I11 summarize the physical properties of representative polyamine- and polyether-treated guayule resins. Physical Testing of Rubber Compositions. The guayule resin derivatives were evaluated in a blend of natural and styrene-butadiene rubbers (NR-SBR) (Table IV) (Schloman and Davis, 1986a-c). Additive levels are expressed as parts by weight per 100 parts of rubber (phr). Green strength properties of the unvulcanized stocks were determined by using an Instron Model TTD tester. Green
714 Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988
strengths are reported as uncured tensile strength. Building tack properties were also determined on the Instron tester. Test pieces were nylon fabric backed plaques which had been pressed together 15 min at 90 "C before measurement. Cure characteristics, including scorch time (t,2), cure time (t'90), cure rate index [CRI = 100/(t'90 - t,2)], and maximum torque or cure state (MHR),were determined with a Monsanto oscillating disk curemeter in accordance with ASTM method D 2084-81. Tensile strengths of the cured compounded stocks were determined in accordance with ASTM method D 412-80, meof cured compounded stock thod B. Tear resistance (T,) specimens cut with die B were determined in accordance with ASTM method D 624-81. Dynamic properties of the cured compounded stocks were evaluated as the hysteresis loss factor (tan 6). This was determined at 23 "C by using the MTS Model 830 elastomer test system.
Results and Discussion Chemical derivatization effects profound changes in both the chemical and physical properties of guayule resin. Many classes of resin components, including terpenes, sesquiterpenes, fatty acid triglycerides, and polyphenolics, are reactive toward sulfur. Resin keto isoprenoids can be condensed ("cross-linked") with reactive amines. Guayule resin is a tacky gum which becomes a free-flowing liquid at temperatures above about 50 "C. Sulfurization or condensation with polyamines or amine-terminated polyethers yields solid, truly resinous products. Materials with softening points below about 70-75 "C tend to aggregate on storage. As discussed below, the softening points of guayule resin derivatives can be satisfactorily adjusted by choosing an appropriate guayule resin fraction or varying the amount and type of derivatizing agent. Rubber compositions are formulated to achieve a specific property or range of properties. These properties include the building tack and green strength of unvulcanized (uncured) compositions. Building tack and green strength contribute to maintaining the physical integrity of tires or rubber mechanical goods before curing. The vulcanization process itself can be described in terms of the scorch time (the induction period before cross-link formation), the cure time (the total time to a desired level of maximum cross-link formation, typically go%), and the maximum rheometer torque or cure state (composition viscosity). The cure rate index (CRI) is a measure of the rate of cross-link formation. Accelerators are added to increase the rate of sulfur vulcanization. However, adequate scorch time is desirable to ensure processing safety. Vulcanizate properties such as tensile strength, tear strength, and hysteresis are complex functions of the type of polymer and the number and type of cross-links, among other factors (Coran, 1978). The desirability of a particular property can depend on the application (Studebaker and Beatty, 1978). Increased tensile strength can be related to improved abrasion resistance, an important property in tire treads. Low hysteresis is important in goods subject to repeated deformation, such as tires and air springs. Tear resistance is an important property in belt or tread stocks. The survey evaluations reported here were made by using simple drop-in formulations. Recipes were not adjusted to optimize cure or a particular compound performance. Nevertheless, the guayule resin derivatives produced significant improvements in various properties of unvulcanized and vulcanized rubber compositions. Sulfurized Resin. Cross-linking vegetable oils with 10-25% sulfur can yield both liquid and solid products. The solid derivatives find primary application in rubber
Table V. Sulfurized Guayule Resin: Effect on Building Tack of Unvulcanized Stocks additive total S building additive content, tack, additive level, phr phr kN/m 3.4 none sulfurized resin, 33% S (no 0.5 0.2 3.4 activator) 0.8 0.3 3.5 1.4 2.0 0.6
sulfurized resin, 47% S (no activator)
0.7 0.8 0.3
2.7 2.5 3.4
Table VI. Sulfurized Guayule Resin: Effect on Cure Rate (CRI) and Cure State, 150 "C cure additive time, min state, additive level, phr scorch cure CRI dN.m 11.2 21.2 10.1 37.4 none 37.0 sulfurized 0.5 10.2 19.2 11.1 39.4 resin, 33% S 0.8 9.2 18.2 11.1 38.2 (no activator) 1.4 8.8 17.5 11.5 sulfurized resin, 47% S (no activator)
2.0 0.6
8.3 9.8
16.4 12.3 19.3 10.5
39.3 37.3
Table VII. Sulfurized Guayule Resin: Effect on Tensile Strength and Dynamic Propertiesn tensile hysteresis additive strength, loss additive level, phr MPa factor 18.6 0.214 none 0.5 14.6 0.202 sulfurized 0.8 17.8 0.200 resin, 33% S 2.0 21.0 0.193 (no activator) sulfurized resin, 47% S (no 0.6 17.1 0.193 activator) "Stocks cured 30 min a t 150 "C.
compositions as fillers or processing aids, providing some cure activation properties. Guayule resin treated with up to 25% sulfur improves tack and green strength in unvulcanized rubber compositions and activates cure. In contrast, unsulfurized guayule resin reduces tack and green strength (Kay and Gutierrez, 1985). Low-sulfur guayule resin derivatives have the disadvantage of low softening points (Table I). Increasing the level of incorporated sulfur above about 25% raises the softening point enough to yield resinous products which remain free-flowing solids. Activating the sulfurization reaction with an accelerator such as tetramethylthiuram disulfide (TMTD) can further increase the softening point without affecting the level of sulfur incorporation. Tables V-VI1 summarize the performance of NR-SBR stocks containing various levels of sulfurized guayule resins. Additive levels in Table V are expressed two ways: as parts by weight of sulfurized resin, and as parts by weight of sulfur contributed by the sulfurized resin. The high-sulfur derivatives offered little or no improvement in compound tack (Table V). Consequently, green strength effects were not evaluated. The derivatives activate cure (Table VI), decreasing scorch time and cure time. As a result, the cure rate index is higher. These trends appear to be related to the sulfur content contributed by the additive. Cured stock tensile properties are retained only at higher additive loadings (Table VII). The most favorable characteristic of cured stocks containing sulfurized guayule resin is the reduction in hysteresis. This property improves with increased levels of sulfurized resin.
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 715 Table VIII. TEPA-Treated Guayule Resin: Effect on Building Tack and Green Strength of Unvulcanized Stocks peak green building strength, additive sp, "C level, phr tack, kN/m MPa none 2.23 0.75 74 1 2.53 1.27 3 2.98 1.40 92 1 2.62 1.11 3 2.89 1.53 Table IX. TEPA-TreatedGuayule Resin: Effect on Cure Rate (CRI) and Cure State, 150 O C cure additive time, min state, sp, "C level, phr scorch cure CRI dN-m 9.6 16.3 14.9 36.0 none 74 1 7.8 13.8 16.7 43.6 3 4.4 8.6 23.8 42.4 43.3 7.6 13.3 17.5 92 1 3 4.6 8.8 23.8 43.7 Table X. TEPA-TreatedGuayule Resin: Effect on Tensile StrenPth. Tear Strenath. and Dynamic ProDerties tensile tear additive strength, strength, hysteresis sp, OC level, phr MPa" kN/mb loss factorb none 21.9 49.6 0.181 74 1 18.6 51.6 0.170 3 16.6 54.5 0.167 92 1 20.0 52.1 0.212 3 15.5 54.0 0.200 "Stocks cured 23 min at 150 "C. *Stocks cured 30 min at 150 "C.
Polyamine- or Polyether-Treated Resin. Guayule resin condensed with diamines or higher alkyleneamines yields resinous products with acceptably high softening points (Table 11). The product nitrogen content can be tailored by the choice of cross-linking agent. Cross-linking the guayule resin polar fraction gives a notable improvement in product softening point. Tables VIII-X summarize the performance of NR-SBR stocks containing high nitrogen content derivatives prepared with tetraethylenepentamine (TEPA). These give consistent improvements in building tack and green strength (Table VIII). They also provide substantial reductions in scorch time and increases in the cure rate index (Table IX), consistent with their amine character. In cured stocks, there is a trend toward decreased tensile strength (Table X). The derivatives reduce hysteresis in isolated instances but have no consistent effect on performance. However, these derivatives can substantially increase vulcanizate tear strength. Guayule resin treated with amine-terminated polyethers (Jeffamines, Texaco Chemical Co.) is relatively low softening (Table 111). Long-chain (Jeffamine ED-600) and branched polyethers (Jeffamine T-403) are particularly unsuitable as reactants because of this. The derivatives are higher softening when prepared from the guayule resin polar fraction instead of the unfractionated resin. Tables XI-XIII summarize the performance of NR-SBR stocks containing various levels of guayule resin or resin polar fraction treated with Jeffamine D-230. While these additives offer no consistent improvement in building tack, they do increase green strength (Table XI). Furthermore, the resin derivatives are effective cure activators, decreasing scorch time and increasing CRI (Table XII). This behavior is similar to that reported for poly(alky1ene oxides) in SBR and has been related to the level of oxyalkylene residues in the activator (Hall, 1955). Poly-
Table XI. Jeffamine-Treated"Guayule Resin: Effect on Building Tack and Green Strength of Unvulcanized Stocks peak green building strength, additive sp, "C level, phr tack, kN/m MPa none 2.23 0.75 67 1 2.32 1.16 2 2.06 1.16 5 1.88 1.26 77 1 2.14 0.81 2 2.41 1.00 5 2.41 1.10 Jeffamine D-230.
Table XII. Jeffamine-Treated"Guayule Resin: Effect on Cure Rate (CRI) and Cure State, 150 "C cure time, min additive state, sp, "C level, phr scorch cure CRI dN.m none 9.6 16.3 14.9 36.0 67 1 8.5 14.6 16.4 44.4 2 7.9 13.4 18.2 44.0 5 6.3 10.3 25.0 42.7 77 1 8.2 14.2 16.7 45.6 2 7.9 13.3 18.5 45.0 5 5.7 9.4 27.0 41.7 a
Jeffamine D-230.
Table XIII. Jeffamine-Treated"Guayule Resin: Effect on Tensile Strenath. Tear StrenPth. and Dynamic ProDerties tensile tear additive strength, strength, hysteresis sp, "C level, phr MPab kN/mc loss factor' none 21.9 49.6 0.181 67 1 16.2 52.5 0.167 2 18.1 53.2 0.174 5 16.7 54.9 0.204 77 1 17.6 56.1 0.161 2 22.8 53.5 0.183 5 20.2 56.3 0.193 "Jeffamine D-230. bStocks cured 23 min at 150 "C. cStocks cured 30 min at 150 "C.
ether-treated guayule resins generally reduce cured stock tensile strength (Table XIII). Decreases in hysteresis are seen at 1-2 phr. More favorably, the additives increase cured compound tear strength at all usage levels.
Concluding Remarks Guayule resin is a low-cost material that is well suitid for producing derivatives of value to the rubber industry. These derivatives produce desirable changes in several of the physical and chemical properties of rubber compositions. Chemically modified guayule resin affects vulcanization in a manner consistent with the nature of the cross-linking agent, acting variously as sulfur donors or amine and polyether activators. Several derivatives are good candidates for green strength promoters. Overall, they are less successful as tackifiers. Improvements in cured stock tear strength and hysteresis are particularly noteworthy. Such improvements could be enhanced by appropriate adjustments in the cure system (sulfur, accelerakor) or cure conditions (time, temperature). Acknowledgment I thank J. A. Davis for rubber compound preparation and testing. This work was funded by the Gila River Indian Community under Department of the Navy (NAVAIR) Contract N00019-82-C-0486 and by the Department of the Navy (NAVAIR) under Contract 53-3142-7-6005
Ind. Eng. Chem. Res. 1988,27, 716-718
716
administered by the US.Department of Agriculture.
Literature Cited Beattie, J. L.; Cole, W. M. US Patent 4 591 631, 1986. Beinor, R. T.; Cole, W. M. US Patent 4623713, 1986. Belmares, H.: Jimenez, L. L.; Ortega, M. Znd. Eng. Chem. Prod. Res. Deu. 1980, I9(1), 107. Coran. A. Y. In Science and Technology of Rubber; Eirich, F. R., Ed.; Academic: New York, 1978; ppi91-338. Eagle, F. A. Rubber Chem. Technol. 1981,54(3), 662. Engler, C. R.; McIntyre, D. Dept. of Commerce Report EDA/RED84-43, Feb 1984; pp 7-12. Hall, G. E., Jr. US Patent 2713572, 1955. Kay, E. L.; Gutierrez, R. US Patent 4 542 191, 1985. McFadden, K.; Nelson, S. H. Dept. of Energy Report DOE/ER/ 30006-T1, Sep 1981; pp 78-79.
McLaughlin, S. P. Econ. Bot. 1985, 39(4), 473. Schloman, W. W., Jr.; Davis, J. A. US Patent 4621 118, 1986a. Schloman, W. W., Jr.; Davis, J. A. US Patent 4616068, 198613. Schloman, W. W., Jr.; Davis, J. A. US Patent 4 622 365, 1986c. Schloman, W. W., Jr.; Garrot, D. J., Jr.; Ray, D. T.; Bennett, D. J. J. Agric. Food Chem. 1986, 34(2), 177. Schloman, W. W., Jr.; Hively, R. A,; Krishen, A.; Andrews, A. M. J . Agric. Food Chem. 1983,31(4), 873. Studebaker, M. L.; Betty, J. R. In Science and Technology of Rubber; Eirich, F . R., Ed.; Academic: New York, 1978; pp 367-418. Weihe, D. L.; Nivert, J. J. In Proceedings of the Third International Guayule Conference,Pasadena, Calif., 1980; Gregg, E. C., Tipton, J. L., Huang, H. T., Eds.; Guayule Rubber Society: Riverside, CA, 1983; pp 115-125.
Received for review August 31, 1987 Accepted December 18, 1987
COMMUNICATIONS Concentration Profile Inversion in Distillation Rating Programs with Tray Efficiencies A rating program for a distillation column with a fixed number of trays and with tray efficiencies of less than 100% has been found to display an interesting phenomenon. An inversion in the liquid concentration profile above the feed tray can occur when the rating program is used to calculate the required reflux flow rate to keep product compositions constant as feed composition changes. This requires a modification in the criteria of the rating program to permit a one-tray decrease in liquid composition on the tray above the feed tray. Distillation column simulations are commonly used to analyze two types of problems: design and rating (Buckley et al., 1985). In the design problem, the number of trays required to achieve desired product purities is determined for a given value of the reflux ratio. In the rating problem, the performance of an existing column is analyzed. In this case, the number of trays is fixed, and a trial and error procedure must be used to determine required reflux flows or resulting compositions. In the design of a column, specification of an integer number of trays will generally result in one terminal product which is purer than the design specification. The last step in such a design procedure may require use of a rating program to "fine tune" the reflux flow to produce exactly the desired product concentration. Another common use of a rating program is the determination of steady-state relationships among design variables (e.g., effect of feed concentration changes on reflux flow and reboiler diity to maintain desired product purities). Such steady-state gains (open or closed loop) are important as an initial step in operability analysis. In this report we examine a rating problem in which the number of trays and terminal compositions are fixed. Some interesting results are observed when an attempt is made to calculate the required reflux flow and reboiler duty as the feed composition is changed. To illustrate this situation, the ethanol-water separation at atmospheric pressure is used. ,
Tray Efficiency of 100% First consider ideal trays with a base case feed composition of 20 mol % ethanol. All cases consider a saturated 0888-5885/88/2627-0716$01.50/0
Table I. Design Parameters E, % z NT 0.2 18 100 0.2 72 40 72 40 0.11
NF 2
5 5
RR 1.73 1.74 1.80
QR, 106Btu/h 11.20 11.26 6.30
liquid feed of 10o0 mol/h, distillate composition of 83 mol % ethanol, bottom composition of 1 mol % ethanol, and a partial reboiler. Other design parameters are specified in Table I. Design and rating programs were used to calculate the number of trays and exact reflux flow. Figure 1shows the vapor and liquid compositions in the bottom section of the column, assuming equimolal overflow and 100% tray efficiencies. According to Buckley et al. (1978),if a reflux is guessed that is too small, a premature switch from the stripping to the rectifying operating line can result. This leads to the unrealistic situation illustrated in Figure 2. Such a case will result in negative liquid compositions and failure of the rating program. Buckley et al. suggest that the problem can be solved by making sure that x N F + ~> xNF, i.e., that composition increases as one moves from the feed tray up to the tray above the feed. If this criterion is not met, it means that the guessed value of reflux is too low and reflux should be increased.
Tray Efficiency of 40% Consider the case in which the tray efficiency is 40%. Of course, the total number of trays and the feed tray are not the same as in the 100% efficiency case. 0 1988 American Chemical Society
