changer costs. For greater distances, the transportation costs for common fluids can also become significant. A general formulation of the heat exchanger network synthesis problem would introduce new variables for the coordinates (x, y , z ) for each heat exchange site. The source and destination of each stream undergoing heat exchange would also be included in the problem description. The problem is then to select both the structure of the heat exchanger network and the location of each exchanger in the network. The problem can be made discrete if the heat exchangers are constrained to be located a t the source or destination of one of the streams undergoing the match. With this scheme each energy match could occur a t one of four locations. This considerably expands the number of solutions possible. In addition, the residuals from each match now have a location associated with them. This location must be carried along in the analysis so that the next level of matching can be evaluated. The general trends expected when transportation costs are high is that the network will decompose into clusters. The local streams will be matched and the outlying streams will be serviced by utilities. Here we have assumed that the utilities are available everywhere in the space. If this is not the case, the utilities and their locations may be added to the stream list and the expanded problem solved in the same manner. Conclusions A tree hearch procedure is introduced for the solution of heat exchanger network synthesis problems. A general set of energy matches is defined so that all possible networks can be generated. Several previous investigators have not
included all possible matches and have overlooked less costly solutions. The expanded matches give rise to cyclic networks in which a stream can match twice with another stream. The cost of the networks is insensitive to changes in structure for systems that have high energy recovery levels. An upper bound on energy recovery is defined and used to limit the search for nearly optimal systems. Acknowledgments Part of this work was carried out a t Tufts University, Medford, Mass. Dr. J . J. Siirola of Tennessee Eastman Company made valuable suggestions regarding this work. Literature Cited Arnold, L. R . , Bellmore. M. Oper. Res., 22, 383 (1974). Ellwein. L. B., Oper. Res., 22, 144 (1974). Hwa, C. S.,AlChE Int. Chem. Eng. Sym., Ser.. No 4 (1965) Kesler. M. G., Parker. R . O., Chem. Eng Prog. Sym. S e i No. 92. 111 (1969) Kobayashi, S . , Umeda, T . , Ichikawa, A,, Chem. Eng. S o . , 26, 1367 (1971). Lee, K. F., Masso, A. H.,Rudd, D. F . , Ind. Eng. Chem., Fundam.. 9, 48 (1970). Masso, A. H . , Rudd. D. F.,AlChE J . , 15, 10 (1969) McGalliard, R. L., Westerberg. A. W., Chem. Eng. J . . 4, 127 (1972) Menzies, M. A., Johnson, A. I . , Can. J . Chern. Eng.. 50, 290 (1972) Nilsson, N . J.. "Problem Solving Methods in Artificial Intelligence." McGraw-Hill. New York, N.Y., 1971. Pho,T. K., Lapidus, L . , A I C h E J . , 19, 1182 (1973) Powers, G. J., "Non-Numerical Problem-Solving Methods in ComputerAided Design," in "Computer-Aided Design," p 327, Vlietstra and Wielinga, Ed.. North Holland, 1973. Siirola, J. J.. "Status of Heat Exchanger Network Synthesis," Paper No 42a, presented at 76th National AlChE Meeting, Tulsa. Okla., 1974.
Receiuedjor reciew July 31, 1974 Accepted December 16,1974
Reaction Variables in the Air Blowing of Asphalt Luke W . Corbett Exxon Research and Engineering Company, Linden, N e w Jersey 07036
In the manufacture of air-blown asphalts, the selection of flux source and the consistency of the flux are among the most important variables in determining the properties of the finished product. Reaction velocity is also dependent upon flux source as well as upon air rate and its dispersion. These variables, therefore, are significant when developing process designs or techniques of manufacture. The mechanism of air blowing is deduced by compositional analysis, in which naphthene-aromatics convert to polar-aromatics and they i n turn to asphaltenes. Composition also shows that desirably higher penetrations result when the content of saturates plus unreacted naphthene-aromatics is relatively high, Differences due to flux source and in their blown products thus may be related to these compositional features.
Introduction The air blowing of asphalt, sometimes referred to as air conversion, provides products with properties that are unattainable by other means. These properties have made asphalt adaptable to roofing, waterproofing, adhesive, and sealing applications, as described by Abraham (1960) and Traxler (1961). This has resulted in the development of standard specifications for built-up roofing and waterproofing (ASTM, 1973), and specifications covering the use of blown asphalt in various industries (Krchma, 1965).
In order to meet these specifications, a manufacturer of blown asphalt initially concerns himself with the conversion qualities of the base stock used, hereafter termed flux. These qualities, in turn, are dependent to a large extent upon the crude source from which the flux is derived, as brought out by past investigations (Chelton, e t ai, 1959; Greenfeld, 1964; Hughes, 1962; Hoiberg, 1950; Thurston and Knowles, 1936). Other variables, such as flux consistency, blowing temperature, air rate, and catalysts if used, are also known to effect product characteristics as well as processing methods, but perhaps to a lesser Ind. Eng. Chem. Process Des. Dev., Vol. 1 4 , No. 2, 1975
181
degree (see Campbell, 1966; Constantinides, et al., 1957; Gunderman and Kloss, 1965; Ivanyakov, et al., 1973; Kuperschmidt, et al., 1973; Nelson, 1954; Pikalov and Eser, 1973). Because there is considerable variation in design and operation of air-blowing processes, both a t laboratory and plant scale, there appears to be a need for a better understanding of the physical and chemical variables involved. The purpose of this paper, therefore, is to consider the most important independent variables (those predetermined for the operation) with respect to the most important dependent variables (those contingent upon the operation). It is a further purpose of this paper to present experimental data on a series of laboratory air-conversions under controlled conditions, to permit close observation of the reaction rates and the effects of air blowing on composition and temperature susceptibility properties. Experimental Section The Variables of Air Blowing. It is of some aid to an understanding of the overall process if one identifies the independent variables which are selected or controlled in the operation and the dependent variables which are the consequence of the unit operation. Figure 1 illustrates how these variables may be categorized and placed somewhat in the order of their relative importance. A general appraisal of the literature indicates that these are important with respect to either the properties of the blown product or on how the process may be controlled or operated (Goppel and Knotnerus, 1955; Levinter, et al., 1964; Senolt, 1969; Nakajima e t al., 1971). The work described below also will shed light on the interactions of some of these variables. An experimental study was set up for lab-scale conversions in which the independent variables were fixed, using three flux sources, two levels of flux consistency, two levels of reaction temperature, and specific control of air-dispersion. Use of two catalysts was also studied. A target softening point (220°F) for the blown product was selected, and the blowing time extended for the period necessary to reach that softening point. Blown products were sampled and evaluated as to composition and physical properties. Apparatus for Laboratory Air Blowing. The apparatus used in most of this work is shown in Figure 2. Temperature of the process was controlled within *3”F, stirring speed within f10 rpm, and air rate was calibrated to a given level using a wet test meter. Because of the relatively low head of asphalt through which air is passed in laboratory units, a means for agitation or dispersion of the air is generally applied. Background for the development of this system is the mass transfer studies by Calderbank (1958) and its interpretation relative to blowing of asphalt by Senolt (1969). MacLean (1943) as well as Rushton and Oldshue (1953) recognized the appropriateness of the turbine in effecting dispersion of air in a liquid mass, demonstrating the usage of the turbine in many and various applications. Condensable blowing losses were measured by weighing the overhead lines, trap, and condenser before and after the conversion. Noncondensables were estimated by balancing the flux charge, its blown products, and the condensables above. Whereas most of the data herein were developed with the apparatus in Figure 2, an additional apparatus shown in Figure 3 was used to conduct heat of reaction experiments. This involved the conversion of a flux using a homogenizer to disperse a controlled rate of air in a given mass of asphalt in which heat was added to balance off the amount of heat lost through radiation. The tempera182
Ind. Eng. Chem.. Process Des. Dev., Vol. 14, No. 2, 1975
I
INDEPENDENT VARIABLES
DEPENDENT VARIABLES
I
J
FLUX SOURCE O R COMPOSITION
1
~
;“,J:;;;;;INT-
RELATIONSHIP
1
1I I 1 -I II I--1 I I
AIR R A T E OR DEGREE OF DISPERSION
~
CATALYSTS, IF USED
~
EXOTHERMIC HEAT O F REACTION
EFFLUENT LOSSES
Figure 1. Major variables in the air conversion of asphalt. SERVODYNE DRIVE SYSTEM COI. -porrnn l”lIlYrnL”l co
,1/4
Fisher 8 Porter
014 S S SH4FT
C
A
, l/E
THERMOCOUPLE
2 TUREINE E A D E 3/4 from Blrn
/1/4
014 4IR SPIDER
48’’
i?lR CONDENSER
i
I
Figure 2. Laboratory apparatus for air conversion of asphalt.
ture rise in the asphalt while air blowing was then used as a measure of heat output, over a given period of time and for a measured increase in softening point. In order to control temperatures within a given nominal range around 500”F,air was applied for a 15-min period and then replaced with nitrogen for the next 15 min, while minor cooling was applied to control the starting temperature for each cycle. The above cycle was then repeated until the asphalt was converted to a 220°F softening point product. Heat of reaction experiments, similar to this in principle, have been reported by Lopatinski (1971) and Smith and Schweyer (1963, 1967), the latter showing significant differences in the rate of heat emission as dependent upon flux source. Flux Feedstocks. In order to illustrate the effect of flux source on composition, crude sources A, B, and C were selected to provide a general cross section of types. From each of these, fluxes a t two levels of consistency or viscosity (ASTM D 2170-67) were prepared by straight reduction to grade. Table I shows the levels of viscosity together
BLOWN A T
A1
FLUX
--t
4 50 F __
500 F
220 18
220 17
.
__
AEORATORY HOMOGENIZER JHERMOMETER
40
60 PA
80 AT
100 93
S.P.'F P e n @ 77-F
J&).
BLOWN A T - +
%?
FLUX
4
x
500 F
THERMOCOUPLE
Figure 3. Laboratory apparatus for measuring heat of reaction.
Table I. Properties of Flux used for Laboratory Air Conversion
S.P. F ?ell
77F
103
220
220
183
14
il
Figure 4. Compositional changes, asphalt A .
Flux Soft pt, PenetrationKin.vis. Flash pt, source "F at 77°F at 210°F "F Ai B, c1
A?
BZ 0
93 84 88 103 103 101
CZ n.a., not applicable.
n. a." n.a. n. a . 183 182 199
978 756 740 1795 1912 1440
580 485 570 595 560 670
with their penetration a t 77°F (ASTM D 5-73), softening point (ASTM D 36-70), and flash point (ASTM D 92-72). Compositional Method. Each of these fluxes, as well as the products after the several air blowing treatments, were fractionated to study their composition. Asphalt composition, as used here, is defined as the weight percentage of the four generic fractions present, namely, the saturates, the naphthene-aromatics, the polar-aromatics, and the asphaltenes. The method used for separation of asphalt into its fractions has been shown to be applicable to all types of asphalt as well as to other high boiling petroleum fractions (Corbett, 1969). Catalyst Addition. Although the primary purpose of this work was to study the variables of noncatalytic conversions, a catalyst, when used, must be considered to be an independent variable. For purposes of illustration, phosphoric anhydride which was patented by Hoiberg (1950) and ferric chloride by Abson (1930), were used experimentally. The commercial technique of their use is described by Shearon and Hoiberg (1953). In the laboratory the catalyst was premixed with a 5% aliquot portion of the flux a t about 400"F, followed by returning the aliquot to the main batch just prior to reaching the ultimate blowing temperature, Discussion of Results In the discussion below, each of the independent variables in air blowing is considered separately, noting the effects of that variable on the several dependent variables shown in Figure 1. Effect of Flux Source. 1. The softening point-penetration relationship of the blown products is shown to be an important dependent variable. This is illustrated in Table I1 which lists penetrations a t 77°F for three softening point levels from each of the flux sources. Products from
Table 11. Penetration of Air-Blown Products at Various Levels of Softening Point Penetration at 77°F f o r Conver sion softening point of Flux temp, Catalyst source "F at 0.5% 130°F 190°F 220°F A, A, B, Bi C1 C1 A, A, B, B,
C2 Cz B2
Bz Ci Ci
450 500 450 500
450 500 450 500 450 500 450 500 500 500 500 500
pzos FeC1, pZ05
FeC1,
63 60 62 60 53 50 54 52 50 47 49 46
25 24 23 23 18 17 22 21 19 17 16 13
18 17 17 16 12 11 14 14 14 13 10 9 17 19 16 23
flux source A have consistently higher penetrations and thus better temperature susceptibility (Pfeiffer, 1950), which to a large extent is highly desirable. 2. Product composition is known to change substantially during air blowing (Nyul, et al., 1959; Hughes, 1962; Corbett, 1963; Tucker and Schweyer, 1965). Asphaltenes always increase significantly while naphthene-aromatics always show a decrease. Polar-aromatics also decrease, and thus it is reasoned that the naphthene-aromatics are being converted to polar-aromatics and they, in turn, to asphaltenes. Data in Figures 4, 5, and 6 consistently show this pattern of change. I t will also be noted that the saturates display very little or no change during the blowing conversion. Flux source C has the highest content of naphthene-aromatics, and upon air blowing, results in the largest increase of asphaltenes and the largest decrease of naphthene-aromatics. 3. Reaction velocity is conveniently expressed by two parameters. The first is the average conversion rate from Ind.Eng.Chem., Process Des. Dev., Vol. 14, No. 2, 1975
183
5
FLUX
S.P. F Pen 77 F
84
FLUX
B2
BLOWN AT+
450-F __
220 17
J
BLOWN AT + 4 5 0 F
__
_500 _
F
FLUX
F 77 F
S.P. Pc81
220 16
2
BLOWN A T
450'F
500°F __
~
88
220 12
FLUX
500 F ~-
4
BLOWN A T +
220 11
45O'F __
500'F -
SA
NA
PA
AT S.P. F Pc! 77 F
103
220
220
le2
14
13
S P
2
Table 111. Pseudo-reaction Velocity Constant for Conversion of Flux to 220°F Softening Point as Dependent on Flux Source, Temperature, Air Rate, and Catalyst
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No
Yes Yes Yes Yes
P,O, FeC1, PZO, FeC1,
0.0030 0.0049 0.0038 0.0058 0.0028 0.0043 0.0030 0.0047 0.0027 0.0041 0.0015 0.0022 0.0073 0 .oo 10 0.0043 0.0112 0.0048 0.0134
flux to end product, measured as the average softening point change per minute of reaction time (LSP/min). This provides a comparative measure of the rate of conversion, as based on a given apparatus under controlled conditions and technique. The second parameter is that known as the Holmgren (1954) pseudo-reaction velocity constant, commonly used in both unpublished and published work (Lockwood, 1959; Wright, 1962). The reaction constant, K, is determined from the equation: In RlJRo = K P S z t , in which R1 and Ro are the final and original softening points, P is the mole percent of oxygen in the air, S is the gas space velocity in cubic feet per minute per ton, and t is the time in hours. This provides a constant identifiable with a given flux source but based on a specific apparatus on which the measurements were made. It provides an opportunity to vary flux source and 184
101
220
1)O
220 9
10
Table IV. Exothermic Heat of Reaction when Converting Fluxes A and C to 220°F Softening Point
Con Pseudoversion A i r reaction Flux temp, rate Turbine Catalyst constant, source "F l./min used at 0.5% ri 2 2 2 2 2 2 2 2 2 2 2 2 4 2 2 2 2 2
F 771
Figure 6. Compositional changes, asphalt C.
Figure 5 . Compositional changes, asphalt B.
450 500 450 500 450 500 450 500 450 500 450 500 500 500 500 500 500 500
I
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2, 1975
Flux source A,
Cycle no. 0 1 2 3 4 5 (5.9)
6 7
Soft. pt, "F
A
94 110 126 144 170 198 (220) 223
ASP/min Constant, K AH/lb AH/lb/"F A H/lb/hr
Temp, Soft. "F Pt, "F 16 27 30 31 37 (34) 39
8 9 10 11 12 (12.2) 1.45 00.0181
30 0.24 5.1
Flux source C, A Temp,
"F
92 99 107 114 122 129
32 32
36 32 34
139 149 160 171 190 207 217 (220)
32 34
36 34 42 40 40 (8) 0.69 00.0087 75 0.59 6.2
other experimental variables, in order to obtain a measure of the effect of such variables. Figure 9 shows how a l S P / m i n varies with flux source, while Table I11 shows the corresponding variation in constant K . 4. Heat of reaction experiments, as conducted with the Figure 3 apparatus, were limited to the use of two flux sources only. We find in Table IV that flux A1 converts about twice as fast as flux C1, but the amount of exothermic heat liberated per pound from flux AI, or per pound per degree of softening point, is roughly half that from flux CI. By calculation it is then found that flux C1 liberates slightly more heat on an hourly basis and appreciably more on an overall flux-to-end-product basis. Thus it ap-
FLUX
9
CAT. BLOWN+ WlTH.5%
p
a
Fd13 -
~
S.P. F Pen 7 7
F
FLUX
220 17
103 182
c1
CAT. BLOWNWITH .5s
.-
220
15, , 1
19
FK13
'2'5
-
30
40
50
SATURATES P L U S NAPHTHENE A Q O I I A T I C S IN ELO\\N ASPHALT AT 2 2 0 - F SOFTENIUG POINT
S.P. F Pcii
Figure 8. Correlation of the content of saturants plus naphthene. aromatics with penetration at 77°F.
220 23
220
88
I6
77F
Figure 7. Compositional changes when using catalysts.
Table V. Process Losses as Dependent on Flux Source, Temperature, and Use of Catalysts Effluent measurements (wt % of charge) ConConver sion densFlux temp, Catalyst able source "F at 0.5% oils 4 50 500 450 500 450 500 4 50 500 4 50 500 4 50 500 500 500 500 500
-
1.0 9-
8-
Non-
H, in H,O"
condensables Total %
0.16 0.26 0.97 1.71
0.05 0.17 0.14 0.08
0.30 0.55 0.37 2.13
0.51 0.98 1.48 3.92
0.41 0.08 0.17 0.17 0.72
0.12 0.13 0.16 0.10 0.20
0.96 0.05 0.51 0.14 0.09
1.49 0.26 0.84 0.41 1.01
0.23
0.10
0.61
0.94
...
1.1
...
. . . . . .
. . . . . .
. . . . . .
PzO, 0.82 0.16 0.37 1.35 FeC1, 0.38 0.17 0.29 0.84 PzO, 0.31 0.17 0.75 1.23 FeC1, . . . . . . . . . . . . a Calculated Hz loss from the flux, uia HzO measurement.
pears that flux source is an important variable when considering the overall exothermic heat to be handled. 5. Product losses consist of (a) condensable oils which are derived from the front end distillate fractions naturally present in the flux, (b) water which is the reaction product when flux hydrocarbons are dehydrogenated, and (c) noncondensables consisting of gaseous reaction products. In Table V it will be noted that fluxes from source B, which have the lowest flash points and probably have lower distillation cut-points, yield more losses than the other two flux sources. Water collected from the reaction is reported as hydrogen derived from flux hydrocarbons, assuming that all of the oxygen comes from the air used in blowing. Effect of Flux Consistency. (1) The softening pointpenetration relationship is indicated in Table I1 by higher
CATALYST
Figure 9. Rate of conversion as dependent upon flux source, temperature, flux consistency, and catalyst.
penetrations which result when the flux from a single source has a lower viscosity. This is an important consideration because flux consistency needs to be regulated in order to meet desired end product softening point and penetration. (2) Product compositions always show a higher content of plasticizing components, i.e., saturates plus naphthene-aromatics, when produced from lower viscosity fluxes. This is consistent with differences in end product penetrations as indicated by the relationship in Figure 8, where the higher contents of saturates plus naphthene-aromatics consistently provide higher penetrations. (3) The reaction velocity measured as ASP/min is consistently higher for lower viscosity fluxes from the same source (see Figure 9) and the same is true for reaction velocity constant (see Table 111). (4) Heat of Reaction was not studied relative to flux consistency. (5) Process losses are typically higher for lower viscosity fluxes from a given source. The reason for this is that lower viscosity fluxes are generally cut at lower distillation temperatures, thus allowing for slightly more volatiles to be present, which may be stripped or blown out during the blowing operation. Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 2,1975
185
Effect of Temperature of Reaction. (1) The softening point-penetration relationship is affected to a small degree by blowing temperature, resulting in slightly higher penetrations when lower temperatures are used (see Table 11). (2) Product compositions are only slightly affected by blowing temperature, although it is noted that slightly more asphaltenes are made when using a higher blowing temperature. Other differences are too small to set any pattern. (3) Reaction velocity measured by ASP/min becomes greater as the temperature is increased, as would be expected (see Figure 9). The reaction velocity constant follows the same pattern. (4) Heat of reaction was not studied with respect to the temperature of reaction. ( 5 ) Product losses are always higher a t the higher temperature of reaction, undoubtedly due to the distillation effect discussed in the previous section. Effect of Air R a t e o r the Degree of Dispersion. (1) The softening point-penetration relationship shows no significant difference with respect to air rate used. ( 2 ) Product compositions also show no significant difference. (3) Reaction velocity in terms of the reaction velocity constant are greatly affected by doubling the air rate, as shown for flux A1 in Table 111. This is expected because air rate is a factor in the Holmgren equation, described previously. In the same table one experiment using flux A1 was carried out, with and without the use of the turbine. This resulted in a reaction constant about five times that when the turbine was not used, thus indicating the effectiveness of the turbine in improving the degree of dispersion. (4) Heat of reaction was not studied with respect to air rate. ( 5 ) Product losses were not considered with respect to the variable of air rate, although it is generally conceded that higher air rates would tend to increase the distillation effects. Effects of Catalysis. (1) The softening point-penetration relationship is significantly affected when catalysts are used. Table I1 shows that both ferric chloride and phosphorus pentoxide provide higher penetrations in the blown products with ferric chloride being more effective. ( 2 ) Product compositions, shown in Figure 7, illustrate again that a higher content of plasticizing components provides higher penetrations. Here again ferric chloride is more effective than phosphorus pentoxide, suggesting that the chemical mechanisms involved must be different. (3) The reaction velocity in terms of ASP/min is two to three times higher when ferric chloride is used, while the reaction velocity constant is about double for that catalyst compared with phosphorus pentoxide. (4) Heat of reaction was not studied with respect to catalysts. (5) Product losses were not significantly different when using these two catalysts. Conclusions 1. Flux source has a significant effect on all of the dependent variables, Le., on softening point-penetration, on the composition of the blown product, on the rate of reaction, on the heat of reaction per degree softening point rise, and on process losses. 2 . Lower flux consistency or viscosity yields blown asphalts with higher penetrations a t the same softening point level. Flux consistency has only a slight effect on the other variables. 3. The temperature of the blowing reaction affects all variables in some degree, having a significant effect on the rate of reaction with minor effects on penetration or process loss. 4. The independent variables of higher air rates and better degree of dispersion significantly increase the rate 186
Ind. Eng. Chem., ProcessDes. Dev., Vol. 14, No. 2, 1975
of reaction but have little or no effect on the other dependent variables or product properties. 5 . Catalysts significantly increase the penetration, with ferric chloride being slightly more effective than phosphoric anhydride. Ferric chloride significantly increases the rate of reaction. 6. Composition analysis supports the concept that the changes taking place during air blowing involve conversion of naphthene-aromatics to polar-aromatics and they in turn to asphaltenes. Composition studies also serve to show that higher penetrations in blown products are probably due to a higher proportion of saturates plus naphthene-aromatics. Differences in flux source and in their blown products are thus explained by these compositional features. Nomenclature A1 = flux from crude source A a t a lower viscosity level B1 = flux from crude source B a t a lower viscosity level C1 = flux from crude source C a t a lower viscosity level A2 = flux from crude source A a t a higher viscosity level Bz = flux from crude source B a t a higher viscosity level C2 = flux from crude source C at a higher viscosity level LSP/min = change in softening point ( O F ) per minute AH = enthalpy of the exothermic reaction, Btu/lb P = original process gas concentration, vol % S = process gas space velocity, ftS/min/ton t = time, hr Ro = softening point (OF) of flux before air blowing R1 = softening point (OF) of air-blown product K = pseudo-reaction velocity constant SA = saturates, see Corbett (1969) NA = naphthene-aromatics, see Corbett (1969) PA = polar-aromatics, see Corbett (1969) AT = asphaltenes, see Corbett (1969) r = correlation coefficient Literature Cited Abson. G.. U.S. Patent (to the Chicago Testing Laboratory) 1,782,186 (1930) Abraham, H., "Asphalts and Ailied Substances." 6th ed. Van Nostrand, New York. N.Y., 1960. ASTM D 312-71, Asphalts for Use in Construction of Built-up Roof Covering, 1973a. ASTM D 449-71, Asphalts for Darnpproofing and Waterproofing, 1973b. Chelton. H. M . ,Traxier, R. M., Romberg. J. W.. Ind. Eng. Chem.. 51, 1353 (1959). Calderbank, P. H.. Trans. Inst. Chem. Eng.. 36, 399 (1958). Campbell. P. G.. Wright, J. R . , Ind. Eng. Chem , Prod. Res. Dev.. 5 , 319 (1966). Corbett, L. W., ASTM Spec. Tech. Pub/., 347, 40 (1963). Corbett, L. W . , "Bituminous Materials; Asphalts, Tars and Pitches," Vol. l i , Part i , p 81, Interscience. New York, N.Y.. 1965. Corbett, L. W., An. Chem.. 41, 576 (1969). Corbett, L. W., Proc. Assoc. Asp. Pav. Tech.. 39, 481 (1970). Costantinides, G , Batti. P.. Chim Ind. (Miian). 2, 96 (1957) Gunderman. E.. Kloss. B., €rdoe/ Kohle, 18, 780 (1965) Goppei. J . M . , Knotnerus. J . , Fourth World Pet. Congr.. / I / , 399 (1955). Greenfeld, S. H.,lnd. Eng. Chem. Prod. Res. Dev., 3, 158 (1964). Hoiberg. A . J.. U.S. Patent (to Lion Oil Co.) 2,450,965 (1948) Hoiberg, A. J., Proc. Assoc. Asp. Pav. Tech., 19, 225 (1950) Holmgren. J. D., Ph.D. Thesis, University of Florida, Gainesvllle, Fla., 1954. Hughes, F. J., Ind. Eng. Chem.. Prod. Res. Dev.. 1, 290 (1962). Ivanyakov, 0. V.. Kaminski, E. F . , Gun, R. E., Neftepererab. Neftekhim. (Moscow), No. 5 , 7 (1973) Kuperschmidt, M . L., Kiryushina, V M., Khim. Tekhnol. Top/. Masel., 18, 35 (1973). King, W. H., Corbett. L. W'., An. Chem., 41, (1969). Krchma, L. C., "Bituminous Materials; Asphalts, Tars and Pitches." I I , Part I , p 585, Interscience, New York, N.Y., 1965. Levinter. M . Kh., Gahakbarov, M. F.. Khim, Tekh. Topliv. Masel, 9, 32 (1964). Lopatinskii, V. A,. Lopatinskii, A Y., Khim. Tekhnol. Topi. Masel. 16, 34 (1971). Lockwood, D.C., Pet. Ref.. 38, 197 (1959) MacLean, G.. U.S. Patent (to Turbo-Mixer Corp.) 2,313,654 (1943) Nakajima. T., Tanobe. C., Sekiyu Gakkai. Shi.. 1 4 , 913 (1971) Nelson, W. L., Oil, Gas J . . 158, (1954) Nyul, G.. Zaker, P., Mozes, G.. Erdoel KohIe. 12, 967 (1959) Pfeiffer. J. Ph., "Properties of Asphaltic Bituman," Elsevier, New York, N.Y.. 1950.
Pikalov. V . N., Eser, V . N., Neftapererab. Neftekhim (Moscow),
No. 6,
8 (1973). Rescorla. A . R., Forney, W. E., Blakey, A . R.. Frino, M . J., lnd. Eng. Chem.. 48, 378 (1958). Rushton, J. H . , Oldshue, J. Y . , Chem. €ng. Pfog.. 49, 161 (1953a). Rushton. J. H., Oldshue. J. Y . , Chem. Eng. Prog., 49,267 (1953b). Senolt, V. H., Bitumen, Terre, Asphalte, Peche, 20, 563 (1969). Shearon, W. H..Hoiberg. A . J . , Ind. Eng. Chem.. 45,2122 (1953). Smith, D . B . , Schweyer, H. E., Ind. Eng. Chem., Prod. Res. Dev.. 2, 209
Thurston, R. R., Knowles. E. C., i n d Eng. Chem., 28,88 (1936). Traxier, R . N.. "Asphalt Its Composition. Properties and Uses." Reinhold, New York. N.Y.. 1961 Tucker, J . R., Schweyer, H. E., ind. Eng Chem., Prod. Res. D e v , 4, 5 1
(1965) Waither, H.. Bitumen, Teere. Asphalte, Peche, 7, 392 (1956). Wright, J. R., Campbell, P. G..ACS (Pet. Div.) (1962).
Receiccd for reuieu August 5, 1974 A c c e p t e d J a n u a r y 2, 1975
(1963). Smith. D.
E.,Schweyer.
H. E., Hydrocarbon Proc., 46,167 (1967).
The Cyclic Operation of a Vapor-Liquid Separator Larry M. Joseph*' and Robert H. Kadlec Department of Chemical Engineering, The University of Michigan. Ann Arbor, Michigan 48104
This s t u d y involved an experimental investigation into t h e cyclic operation of a vapor-liquid separator. Cycling was affected by t h e periodic control of t h e outlet vapor flow rate. Separation of t h e ethanol-
water system in dynamic operation, a s compared to steady state at t h e time averaged pressure, served as t h e performance index. T h e exit flow resistance was varied periodically. All other oscillating quantities, including t h e pressure, are responses to t h i s fluctuation. A definite improvement was evidenced in t h e dynamic operation. It was also found that t h e length of t h e cycle time had an effect on performance and there appeared to be an optimum length of cycle.
Introduction Steady-state operation is considered a desired goal in all phases of chemical engineering. This idealized mode of operation, however, is frequently not encountered in practice. Better understanding of the transient behavior of a system might result in a more profitable design as well as improved performance and control. It has been shown in some systems that deliberately operating in a controlled unsteady manner can also result in improved performance. This form of operation is termed cyclic operation since the system is forced to run with cyclicly induced upsets. Cyclic operation is rather novel in chemical engineering. In fact, until quite recently most of the work done in the application of optimization theory was directed solely toward steady-state operation ( e . g . , Rudd and Watson, 1968). There are some operations in chemical engineering which operate in an unsteady manner. Adsorption and batch reaction are examples of such processes. Although both of these operations are cyclic in nature, they are not the type of system which is discussed here. A cyclic operation will be considered as one in which some parameter or parameters of the system are deliberately varied in a periodic fashion with the aim of altering the system performance. The principle of controlled cyclic operation was first applied to the area of distillation by Cannon (1952, 1961), Gaska and Cannon (1961), and Szabo, et al. (1964). This work entailed the periodic control of the vapor flow in an on-off manner. During periods of vapor flow no liquid downflow was allowed. When the vapor was turned off, the liquid was permitted t u flow down the column. To whom ail correspondence should be sent at the Department of Energy Engineering. University of Illinois at Chicago Circle, Chicago. 111.
60680.
Schrodt (1965) reported that operating in this manner can result in a doubling of stage efficiencies and a tripling of the throughput when compared with conventional operation. Schrodt (1967) demonstrated the effectiveness of this mode of operation on a plant size column. Again the performance was better in cyclic mode although the improvement was not as great as laboratory studies indicated. Wankat (1974) has recently reviewed the application of cyclic operation to adsorption, ion exchange, and chromatographic separations. In these processes the cycled parameter is a thermodynamic variable which changes the equilibrium distribution coefficient. Normally these separation processes operate in an unsteady manner. Cyclic operation improved performance over the normal operating mode. A wide variety of areas have been investigated for potential periodic effects. Experimental efforts are typified by the works of Wilhelm, et al. (1966, 1968), and Pigford, e t a!. (1969a,b) on adsorption processes, and by Douglas (1967, 19721, Douglas and Rippin (1966), and Matsubara, et a1 (1973), on reactors. Bailey (1973) has recently reviewed the area of periodic operation of reactors. Theoretical studies in periodic processes were pioneered by Horn and Lin (1967). Cyclic processes with pressure changes have been reported by Skarstrom (1959). Turnock and Kadlec (1971). Kowler and Kadlec (1972a,b) and Shendalman and Mitchell (1972), among others. This study investigated the cyclic operation of vaporliquid separator. The chemical system was ethanol and water. The outlet vapor flow was altered in a cyclic manner which resulted in a constantly varying system pressure. Comparison of the separation of this system with steady state served as the index of performance. Experimental Apparatus Figure 1 is a process flow diagram of the apparatus in a Ind. Eng. C h e m . . P r o c e s s D e s . Dev.. Vol. 14, No. 2, 1975
187
