INDUSTRIAL AND ENGINEERING CHEMISTRY
956
dispersion of these plasticizers may be attributed to their similarity in structure to the GR-S unit. The esters of the aromaticaliphatic alcohols had the same order of plasticity for the uncured stocks as had tall oil, but the moduli of the cured stocks were lower than the cured tall oil stocks. This was especially outstanding with the cinnamyl ester which produced a cured stock having half the modulus of the tall oil stock, The aromaticaliphatic esters gave higher ultimate elongations than the tall oil stocks. These observations indicated a generally greater softening of the cured stocks. The marked superiority of the aromaticaliphatic esters in flex-crack resistance may have indicated that the esters most closely related to the GR-S unit have the property of reducing the tendency for cracking under dynamic flex. However, it may be argued that stocks plasticized with esters of aromatic-aliphatic alcohols showed greater resistance to flex cracking because these stocks had low moduli. Hence the work done per flexing cycle on each test piece was less for these lowermodulus stocks, This argument does not appear to apply in the case where the methyl phenyl carbinyl ester is compared with Witco MR No. 38, for here the moduli of the stocks are of the same order yet the ester is superior in flex crack resistance. While the lowering in modulus of the aromatic-aliphatic esters on the cured stocks was greater than tall oil, these stocks retained their hardness and did not lose their resilience. I n tensile strength and tear resistance the aromatic-aliphatic esters gave poorer re-
Val. 3!7, No. 10
sults than tall oil but still compared favorably with some of the commercial types of plasticizers. With respect to the other classes of esters in Table I the aliphatic esters were found to be about as greasy on the mill as tall oil. There was some decrease in plasticizer incorporation time. I n general there was no significant improvement in the physical properties of the stocks. These remarks apply about as well for the esters of polyhydroxy alcohols. Attaching phenolic types to the tall oil molecules did tend to improve the plasticizing action of this material. I n general they were less greasy on the mill and easier to incorporate. Otherwise the physical properties of the stocks were of the same order as tall oil. The physical properties of the cycloaliphatic esters were of the same order as the phenols. ACKNOWLEDGMENT
The authors wish to thank H. F. Schwarz and H. R. Spielman for their help in obtaining the data on the physical properties of the stocks. LITERATURE CITED
(1) Breokley, J., Rubber Age (N. Y.), 53,331 (1943). (2) Carlton, 6. A., and Reinbold, E. B., India Rubber World, 108,
141 (1943). (3) Schwarz, H.F., Zbid., 110,412 (1944).
(4) Vila, G.R., IND. ENO.CREM.,34, 1275 (1942).
Two-Component Equilibrium Curves for Multicomponent Fractionation FRANK J. JENNY ~ ~ MICHAEL 1 3 J. CICALESE Hydrocarbon Research, Znc., New York, N . Y .
A
GRAPHICAL method for the solution of multicomponent
fractionation problems was presented in an earlier paper (9). A modification of the graphical solution using a two-component structure is presented whereby equilibrium curves are drawn which simulate the McCabe-Thiele structure for twocomponent systems. Figures 1 and 2 show the new form of equilibrium curve for multicomponent problems which is based on plotting the mole fraction of the light key component and lighter in the vapor phase, y, against the mole fraction of the light key component and lighter in the liquid phase, x. Figure 1 shows the equilibrium curve and operating lines for the theoretical minimum reflux ratio of example 1, for which a detailed algebraic solution is available (9). Figure 2 presents equilibrium curves for reflux ratios varying from the theoretical minimum to total or infinite reflux. METHOD OF CONSTRUCTION
To illustrate the method used in constructing two-component equilibrium curves, the example given in the previous paper is reviewed briefly. The material balance on the column follows:
CHa CaHs CaHa NC4Hs NCrHa NCaHu _.
Feed, Molee/Hr. 26 9 25 17 11 12
Dist Gau MoledHr.’ 26 9 24.6 0.3
100
59.9
-
.... .... -
Bottoms
MO~W/H;.
.... ....
0.4 16.7 11.0 12.0
-
40.1
The light key component in this separation is CaHs and the heavy key component is NCIHXO. “Light key component and lighter” is defined here as the summation of CHI, CzHs, and C3H8. A complete solution of the problem a t a reflux ratio of 1.5 to 1 gave the following equilibrium values for the concentrations of the light key component and lighter throughout the column: Light Ke Component and lighter X
Stripping section Reboiler Tray B-1 Tray B-2 Tray B-3 Tray B-4 Tray B-5 Tray B-6 Tray below feed tray Feed tray Fractionating seation Tray above teed tray 2nd tray above feed tray Tray A-4 Tray A-3 Tray A-2 Tray A-1 Condenser
1/
0.0274 0.0516 0.0865 0.135 0.195 0.268 0.350 0.480 0.555 0.418 0.518 0.886 0.792 0.873 0.931 0.968
0.707 0.772 0.872 0.924 0.957 0.979 0.995
These values me plotted as curve b of Figure 2. The values in the region of the feed tray are slightly different from those presented in the original paper because the feed tray temperature has been changed from 205’ to 210”F.in order to obtain a more optimum feed tray location. Figure 1shows that a t the theoretical minimum reflux ratio the operating lines touch the equilibrium curve at two distinct points, one in the fractionating or enriching section, point A , and the
October, 1945
Y
INDUSTRIAL AND ENGINEERING CHEMISTRY
M O L E F R A C T I O N IN
A new method of plotting equilibrium curves for multicomponent systems is based on the use of equilibrium values for the “light key component and lighter”. This method simulates the McCabe-Thiele structure for two-component systems. It has been in use for over five years, and its application to the solution of industrial problems has been useful and time saving. Illustrative examples are included. The proposed method of applying the two-component structure presupposes that a complete solution is available for a specific problem and that another solution is desired i n which conditions are not greatly changed from the original problem.
C l a U l D ,
%
957
other in the stripping section, point D. Between these two points is a region representing the fractionation in the vicinity of the feed tray. Point C corresponds to equilibrium on the feed tray and point B corresponds to equilibrium on the bottom tray of the fractionating section. Section A B corresponds to the bottom trays of $he fractionating section in which section components heavier than the heavy key component are refluxed back completely. At point A, the heavy key component goes through its theoretical maximum concentration. Section CD corresponds to the top trays of the stripping section in which section components lighter than the light key component are completely stripped. A t point D the light key component goes through its theoretical maximum concentration. The equilibrium curvc is discontinuous between points B and C, representing the crossover from the stripping to the fractionating section. Figure 2 shows the effect of reflux ratio on the position of the equilibrium curve a t constant total pressure. For the example illustrated, a t total reflux the equilibrium curve ( d , Figure 2) approximates the equilibrium curve for a two-component system
958
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
in which the two components are the light and heavy key components of the multicomponent problem. At lower reflux ratios the equilibrium curve becomes more concave with respect to the diagonal. APPLICATION OF CURVES
The proposed two-component structure bas found widespread application in the design of industrial fractionating columns during the past five years. The method has also been of considerable merit in aiding students of distillation to visualize the mathematical significance of multicomponent tray calculations in much the same manner as the McCabe-Thiele method has served for strictly two-component systems. Shiah (4) also published a twocomponent method for multicomponent systems. However, Shiah did not include in his method the effect of change in position of equilibrium curve with changing reflux ratio. Figure 2 illustrates that for the general problem of multicomponent fractionation, i t is not always permissible to neglect this change in position of the equilibrium curve. Similar methods were also proposed by Obryadchakoff (3)and Brown, Singer, and Wilson (1).
An obvious and useful application of the two-component structure is in determining the effects of relatively small changes in reflux ratio and/or changes in terminal conditions. Thus, in the example given, if i t were dwired to change the terminal conditions from 1.0 to 2.0% propane in the bottoms, and a h
Vol. 37, No. 10
change the butane content in the overhead from 0.5 to 1.0% at a reflux ratio of 1.5 to 1,new operating lines could be drawn in and the number of trays required could be determined graphically by using equilibrium curve b of Figure 2. Before stepping ofi the number of trays from curve b for the new case, a small correction is made at either end of the equilibrium curve for the revised terminal Conditions, based on a bubble point calculation on the bottoms liquid and a dew point calculation on the overhead vapor. An exact solution of this problem gives 11.9 calculated theoretical trays (exclusive of a reboiler and partial condenser) compared to 12.1 theoretical trays determined graphically. Similarly, a n exact solution for the original terminal conditions, for a reflux ratio of 2.0 to 1,gives 12.7 calculated theoretical trays compared to 11.9 theoretical trays determined graphically from equilibrium curve b of Figure 2 which was calculated for a reflux ratio of 1.5 to 1. The short-cut method of stepping off the required number of theoretical trays at a reflux ratio of 2 to 1 from curve b gives a solution which is slightly low. This is t o be expected since the equilibrium curve will become somewhat flatter at the higher reflux ratio. Actually with a reflux ratio of 1.75 to 1 the shift in the equilibrium curve is so slight than an exact solution gives the same number of trays, 13.6,as that obtained by using curve b. This application of the two-component structure presupposev that a complete solution is available for a specific problem and that another solution is desired in which conditions are not greatly changed from the original problem. A commercial application is illustrated by another problem. In an alkylation unit the effluent from the reactor contain8 a considerable percentage of isobutane since the alkylation reaction is favored by a large excess of this component. T h i s s t r e a m is generally charged directly to an isob u t a n e column where substantially all the isobutanc in a high degree of purity is takrir overhead and recycled to the reacntor. Since the fractionation required is a relatively sharp one b e t w e e n closeboiling key oompoiients (iso- and n-butane), the isob u t a n e column is generally a large arid expensive tower. and ita high coilsumption of utilities is an importmt factor
INDUSTRIAL A N D ENGINEERING CHEMISTRY
October, 194!3
in the over-all economic picture. Consequently, considerable attention is paid to the design of such a column in the selection of the optimum reflux ratio. By utilizing the two-component equilibrium curves, much of the tedious algebraic calculations have been eliminated. The following molal material balance for an isobutane tower, taken from the design of a commercial unit, is used to illustrate the method of application: Feed
Molw/hr. C:Ha ieo-C~Hle n-C&e Alkylate
98.9 1272.8 467.4 251.0
2090.1
Overhead
M~IWIHL. 98.0 1203.5 161.1
Bottom
Mole&.
....
9.3 816.8 251.0
.... -
-
1513.5
576.6
To illustrate this case, the following simplifying aasumptions are made: (a) Feed enters as liquid a t the boiling point; (a) constant molal reflux is assumed in each section of the column; (c) constaut relative volatility, a,is used as follows: a 2.5 1.28 1.o
0.05
For the solution of this problem, a two-component diagram is drawn first, ulling equilibrium data for iso- and n-butme. This is curve b of Figum3. This curve is then wed as a guide in the selection of a preliminary reflux ratio since the true multicomponent equilibrium curve will be outside this curve but close to it. A reflux ratio of 4 to 1 is selected, and several equilibrium points
959
are calculated for the top and bottom sections of the column by direct equilibrium calculations. Equilibrium points corresponding to the feed tray and the bottom tray of the fractionating section are now calculated in the manner explained in the previous paper, making use of the fact that alkylate and propane will be essentially at their theoretical minimum concentrations on the feed tray and tray above the feed, respectively. The calculated points for the light key component and lighter (propane plus isobutane) are now plotted on an 5 g diagram as in Figure 3. If deemed advisable, additional equilibrium points in the vicinity of the feed tray may also be calculated directly in order to define accurately the position of the equilibrium curve in this section. With the equilibrium curve completely defined and drawn in (curve a of Figure 3), the theoretical number of trays is stepped off in the conventional manner. The solution of the reference problem showed thirty-one theoretical trays plus a re boiler and total condenser which checked exactly with a stepwise algebraic solution. Other solutions within the accuracy of the equilibrium data can be determined for moderate changes in reflux ratio, using the previously determined equilibrium curve. The examples wed for iIlustration in the present paper me based on problems in which a relatively sharp separation is desired between adjacent components. The method has also been applied Ruccessfully to problems involving a split key component.
-
LITERATURE CITED (1) Brown, G.G.,Singer, 8.C., and Wilson, R. R., IND.Em.CEIDY,, 28,824 (1926). (2) Jenny, E'.J., Tram. Am. Inst. C h .E n ~ r e .35, , 636 (1939).
(3) Obryadchakoff,€3. N., IND.EHQ.CHBY..24,1166 (1932). (4) Shiah, C. D., R 4 ~ i m Natural Gasoline Mfr., 21, 182 (1942).
1
0
Thermoplastic Laminates C. W. EURENIUS, R. H. HECHT, WILLIAM KOCH, AND H. C. MALPASS Hercules Powder Company, Wilmington, Del. Thermoplartic laminntee prepared by bonding fillers such am cloth and paper with cellulose aretnte and ethyl cellulose plastics w o r e evaluated in direct comparison to commercial phsitolii4onded laminutea. They were found to POSIWDR a r t urtttwal degree of toughnear, and were readily drawn i n l o contplex shapes with inexpensive equipment when softenrd b y heat. They have the additional advantagesof quic*kand easy fabrication arid an unlimited range of color Imnsihiliiien. Pmpertim nucsh as low-temperature Bexihiliiy, itnpwt smenqth, modulitr, of elantirity, water a h r p t ion, uttcl electrical irtwulrrt ion behavior can be controlled b y proper aelcction uf t&e h n d i i y plastic and the filler mutsriul.
T
HE art of combining plastic materials with fibrous fillers is old, for some threa decades ago fabric-filled phenolics were developctd and placed on the market for electrical insulating and mechanical ww. Other thermosetting binders, such as the ureas and melamine, have expanded into laminar constructions, and during the war the so-called contact resins have come into prominence. All of these materials have their individual sets of properties and charactaktics which tend to direct them into specific
end uses where they logically belong. However, even though they differ in many respecta, they have one thing in common; that is, they are thermosetting and must be polymerized by heat or a combination of heat and pressure. Furthermore, being threedimensional polymers, they have relatively low impact strength and must depend to a large extent on the filler for impact strength or shock resistance. During the past year this company has investigated combinations of celluloRe derivative plastics, which are noted for their inherent toughness, and various types of fillers, such as fabrio, paper, and asbestos paper. The purpose of this article is to present some of the results of this investigation. Two different types of laminar constructions were examined; the differences were primerily in the methods of preparation rather than in the materials employed. Low-pressure combinations of thermoplastics and cloth, which were studied in some greater detail, were p r e pared by prwsing a uandwich lay-up of plastic sheeting and filler. A simultaneous application of heat softens the plastic sufficiently to allow it to flow into and adhere to the cloth under the exerted pressure. These are called low-pressure cosstructions to die tinguish them from the other type which is based upon the successive lay-up of several plies of wet-plastic-impregnated cloth over a form, and which depends upon solvent r e l m e for setting
