1230
X, X,
INDUSTRIAL AND ENGINEERING CHEMISTRY
= m-cresol fraction in aqueous layer, solvent-free basis = pcreol fraction in aqueous layer, solvent-free basis
Subscripts A = aqueousphsse 0 = organic phase Parentheses = concentration
Vol. 42, No. 6
(5) Jantzen,
E.,“Dasfraktionerte Destillieren und dss fraktionerte Verteilen Deohema Monographie,” Band 5, No. 48,pp. 81117, Berlin, Verlag Chemie, 1932. (6) Scheibel, E. G., Ckem. Eng. Progress, 44,681 (1948). (7) Schenck, L. M., and Bailey, J. R., J. Am. Ckem. Soc., 61,2613 (1939).
(8) Schutze, H.G., Quebedeaux, W. A., and Lochte, H. L., IND. ENG.CHEM.,ANAL.ED., 10,675 (1938). (9) Standardization of
LITERATURE CITED
(1) Axe,
W.N., J . Am. Ckem. Soc., 61, 1017 (1939). (2) Axe, W.N., and Bailey, J. R., Ibid., 61,2609 (1939). (3) Boyd, D.R.,J. Chem. Sac., 107,1539 (1915). (4)Glenn, R.A+,and Bailey, J. R., Ibid., 2612 (1939).
Tar Products Tests Committee, London, “Standard Methods for Testing Tar and Its Products,” Rsvised Section 7, pp, 237-8 B, September 1943. (10) Stevens, D.R.,and Nickels, J. E., IND.ENG.CHEY.,ANAL. ED., 18,260 (1946). RECEIVED Januwy 14,1950.
Desorption of Unreacted Isoprene from Synthetic Rubber Latex J
EFFECT OF PRESSURE, AGITATION, AND LATEX DEPTH ORRINGTON E. DWYER AND J. A. BAUMA”’ University of Rochester, Rochester 3, N. Y.
In the manufacture of synthetic rubber by the emulsion process, the monomers are usually copolymerized to 70 to 80% conversion for product quality and economic reasons. Thus it is necessary to remove and recover the unreacted monomers for re-use in the polymeriaers. This investigation has been concerned with the removal of unreacted isoprene from an isoprene-styrene synthetic rubber latex with the unreacted styrene present. The removal of the isoprene from the latex is essentially a Aash distillation or desorption process, usually referred to in the industry aa “venting.” The isoprene, in its escape from the tiny polymer particles, must diffuse through the aqueous filnl separating the particles from the vapor in the vapor buhbles and above the latex. The experimental data indicate that the venting process can be represented by the empirical equation
The operating variables of temperature, pressure, latex depth, and degree of agitation were.studied, and the numerical values of ZC‘, fl, and OL in the above equation have been determined for a variety of operating conditions. Venting runs carried out a t 40” and 60” C. showed that the rate of desorption was greatly dependent on the temperature. The venting rate was found to vary approximately inversely as the square of the absolute pressure. It was found that increase in latex depth causes an appreciable decrease in the venting rate. However, the effect is less pronounced a t greater depths and a t greater driving forces. Agitation has a great promoting effect on venting rate. Under certain conditions it is possible to increase the venting rate severalfold by using mechanical agitation. 1 Present address, Cslco Chemical Division, American Cyanamid Company, Bound Brook, N. J.
S
YNTHETIC rubbers of the GR-S type are copolymers of an aliphatic diene and an aromatic compound containing a vinyl group. In this paper, the authors are dealing with an engineering operation in the manufacture of a synthetic rubber in which the diene is isoprene and the aromatic compound is styrene. The polymerization reaction for such elastomers is usually stopped a t around 75 to 80% monomer conversion by adding an inhibitor, such as hydroquinone, to the reacting charge. If the reaction is carried much beyond this, the reaction rate becomes uneconomically slow and excessive cross linking of the molecular chains results in a less elastic polymer. At the end of the reactioqthe latex consists of minute polymer-monomer particles stabilized with oriented soap molecules attached to their surfaces. The aqueous phase is practically all water, containing no significant amount of monomers and often less than 0.1% inorganic solids. The monomer-polymer particles, very much smaller than the monomer droplets in the original emulsion, are estimated to be in the neighborhood of 600 to 800 A. in diameter (6, 8). For both practical and economic reasons, the unreacted monomers must be removed from the latex, before it is coagulated, and recovered for re-use in the polymerizers. In practice, the unreacted monomers are recovered separately; the low boiling diene is recovered in a two-stage flash distillation or venting process; and the high boiling aromatic later is stripped from the latex in a multiplate steam distillation column. The second stage of the venting process and the steam distillation column are usually operated under considerable vacuum. A description of the recovery of the unreacted butadiene and styrene from synthetic rubber latex, as carried out in a plant scale batch process, has recently been given by Johnson and Otto ( 6 ) . Although the volatilities of the monomers are widely different, the simple flash distillation in the venting chambers results in each recovered monomer being contaminated with a small amount of the other, dependingon the type of monomers and the venting and stripping conditions; consequently in practice, venting conditions are chosen so that over-all plant performance is a t an optimum.
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
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1231
a 0.25-inch bronze needle valve for controlling the pressure in the
I
A
B C D E
F
G
-----
E
Q
I ! !
--
Figure 1. Apparutus for Venting Studies H = Handcontrolvalve
A.C. motor, 860 r.p.m. Relay Condenser water, 0' to
- -. Agitator 1-r
Constanttern eraturcbnth Ventingcham%er Transtat
JKL
-
= Manometer
Thermometer Condeneer M = Coldtrap N = Vaporventline P = Water-jacketed buret Q = Gas-meanuringburet P
chamber during a run, and was connected to the condenser by meaxw of heavy neoprene tubing. Both the vapor line and the lead to the manometer, J , were wound with Chromel rasistance ribbon to prevent condensation of vapors. To prevent contact between the copper and the Chromel ribbon, the vapor line waa insulated with a layer of porcelain cement. The temperature of the vapor line and the manometer lead line were both controlled by voltage regulation with the Transtat, G. In the 16-inch glasa condenser, L,ice water was used as the coolant. It was modified slightly by adding a bulb a t the lower end to separate the condensate from any uncondensed vapor. This uncondensed vapor paased to an acetone-dry ice cold trap, M,where it was condensed and whence it was immediately returned to the separating bulb to join the main condensate stream. Noncondensable vapors from the cold trap were collected in the gas-measuring buret, Q, with mercury sealing fluid. From the condenser, the Condensate flowed through a three-way stopcock into one of two 50-ml. measuring burets, which were jscketed and cooled with ice water; the entering and exit temperatures were about 0.5" and 1.0"C., respectively. Cooling water was supplied from tank C, containing a mixture of water and crushed ice, and was pumped in parallel to both the buret jackets and the condenser, using a l/lWhp. Eastern Industries midget centrifugal pump. Satisfactory vacuum was obtained by means of a water aspirator connected to the system at a glass tee located between the cold trap and the gas buret. Agitation was provided by a rotating propeller on the end of a vertical shaft which passed through a packing gland in the center of the top plate of the venting chamber. The s/le-inch steel shaft waa driven by a l/zo-hp. 860-r.p.m. induction motor;
The venting of isoprene from an isoprene-styrene-latex mixture can be considered as a molecular diffusional process, in which the isoprene and any small amounts of styrene must first diffuse to the surface of monomer-polymer particles and thence through the aqueous film separating the particles from the vapor in the escaping bubbles. Since the vapor bubbles contain no components other than those diffusing into them, no diffusional resistance is offered by the gas film enveloping each bubble. It would be expected that bubble formation would be an important step in the release mechanism and that any factor which would promote bubble initiation would cause an increase in venting rate. Dean ( 1 ) and Hatcher and Sage ( 4 )point out that one important factor aiding bubble formation is agitation. So, from the standpoint of diffusion of the isoprene through the aqueous film and also from the standpoint of bubble initiation, agitation would appear to be a very important variable. In this study, the more important operating variables of pressure, temperature, degree of agitation, and latex depth have been, in greater or less degree, investigated. Basically, the one outstanding feature that distinguishes the venting operation under consideration here from the conventional liquid-vapor desorption operation is that in the former the interfacial area through which diffusion takes place is wholly dependent on the other operating conditions. In other words, there is no stripping gas or vapor entering the system and no packing or wetted wall to help determine the liquid-vapor interface. EXPERIMENTAL
Apparatus. The apparatus assembly is shown in Figure 1. The venting chambers, F, the heart of the apparatus, is shown in greater detail in Figure 2 and will be fully described, rn the geometry and design of the chamber and stirrer are held to be factors of some importance. Referring to Figure 1, the venting chamber waa immersed in a constant temperature bath, E,which consisted of a cylindrical glass jar 18 inches high and 12 inches in diameter, equipped with a 500-watt resistance heater, a thermoregulator and relay, I?, and a laboratory impeller-type stirrer. The vapor line, leading from the top plate of the venting chamber to the condenser, was made of */rinch copper tubing, contained
-
Figure 2. Venting Chamber Glans cylinder, 4.75-inch diameter b 10.5 inches long B = Upper trass plate, 5.5 x 5.5 x '/a inches C - Lower brans plate, 5.5 X 5.5 X #/B inches D - Propeller, 45' pitch, 1.25-inoh diameter E - Soft neoprene gasket 1/10 inch F - Neoprene stopper No. b G - Glase tubing, 7' mm., Pyrex No.
A
7740
H - Glass stopcock, 2-mm. hore J - Packing, candle wicking Baturated with Dow Corning atoDcwk grease K Fluted copper fitting L Vapor line */a-inch Copper tubing M Packinggdnd N = Hokeneedlevalve Viineh P Brass rods. 0.2dinch diameter by 14inchealong
---
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1232
speed variation waa obtained by using different size pulleys on the shaft. This shaft was supported at ita upper end by a caatiron bearing and, a t the point where it entered the chamber, by the packing gland. Candle wicking, saturated with Dow Corning silicone stopcock grease, was found to provide a leakproof packing when the shaft waa lubricated with a few drops of heavy lubricating oil. The propeller waa made of sheet copper and was 1.25 inches long and 0.5 inch wide with the faces pitched at a 45' angle with the horizontal. The direction of flow through the propeller was upward. The venting chamber, described completely in Figure 2, consisted of a glass cylinder 4.75 inches inside diameter by 10.5 inches long with two a/B-inch braas plates, connected by four tie rods, which formed the ends. These brass plates were grooved to hold gaskets cut from l/lpinch soft neoprene sheet packing, and the assembly waa tightened by means of wing nuts on the tie rods. A I/kinch Hoke brass needle valve, attached to the bottom plate, waa used'both for charging and draining the venting chamber. The venting chamber was held and supported by bolting the top plate to a rack support. The top plate also held connections for the manometer, vapor vent line, and agitator. All glass parts were made of heat-resistant glass.
EQUILIBRIA FOR ISOPRENE-STYRENETABLB I. LIQUID-VAPOR POLYMER SYSTEM
(Styrene concentration in latex, 0.16 gram/gram polymer)
40" C. I/P 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24
60' C.
mm. 66
100 145 189 233 276 319 361 401 438 476 610
yi
0.900 0.949 0.970 0.979 0.984 0.988 0.990 0.992 0.993 0.994 0.995 0.996
114 206 297 385 470 663
0.880
776 832
0,938 0.960 0,972 0.979 0.984 0.987 0.989 0.990 0,992
...
...
. I .
...
mm. 200 376 627 667 797 926 1048 1166
... ... ... ...
80' C. Ui
0:922 0.949 0.963 0.971 0.976 0.980 0.983
.. .. .. ... ...
Procedure. Preparatory to making a run, a small amount of water was introduced into the venting chamber to saturate the air therein, and the vacuum was turned on. Kext, the Chrome1heating lines were turned on. The vapor lines were heated to a temperature a few degrees higher than that obtained in the chamber during the run. When the water in the chamber was practically all evaporated, the needle valve in the vapor line was closed, thus isolating the venting chamber from the rest of the system. By this procedure the partial pressure of air in the chamber was r e duced to about 1 mm. or less. Readings were then taken of the venting chamber manometer, room temperature, and barometric pressure. Introduction of the latex was the next step. A 500-ml. buret with a short length of neoprene tubing attached to the bottom was filled with latex to a point about the zero mark. This level was t8henbrought down to zero a t the same time filling the neoprene tube with latex. The tube was immediatcly connected to the Hoke needle value in the bottom plate of the chamber, and then this valve and the buret stopcock were opened allowing the vacuum to pull in the latex. Vigorous foaming occurred immediately but soon subsided. When the latex level fell to the 500ml. mark on the buret, the needle valve and the stopcock were closed and the buret and tubing were removed. After the latex was charged, the constant temperature bath was raised from below, to immerse the chamber, and the water therein wa8 quickly brought up to temperature by introducing live steam. The bath stirrer was turned on, and when the operating temperature was reached, the steam was shut off and the automatic tem-
Vol. 42, No. 6
perature control system put into operation. To prevent condensation in the manometer leg, the height of the mercury was adjusted, by raising or lowering the leveling bottle, so that the mercury level on the chamber side was just up to the heated section of the glass tubing. The chamber stirrer was then turned on. About 3 hours were required for the system to reach equilibrium-that is, a constant pressure in the venting chamber. The manometer reading, room temperature, and barometric pressure were taken at that time in order that the sum of the initial partial pressures of isoprene and styrene could be determined. This pressure, with the aid of the pressure-concentration relationship, established the initial composition of the latex. As an alternate method of operation, just as, or before, operating temperature was reached, the venting chamber was slightly vented to remove any noncondensables present, before establishing equilibrium. Both methods were about equally satisfactory. When the isoprene-styrene-latex system had reached its equilibrium premure, the condenser and the receiving buret were conditioned for the venting operation. It was necessary to introduce enough water into each buret so that the lower level of the hydrocarbon layers fell above the lower limit of the graduated scale. Ice water was pumped through the buret jackets and the condenser; the cold trap was filled with dry ice and acetone, and the stopcock in the vacuum line was turned off leaving the condensing end of the venting system a t a pressure corresponding closely to the vapor pressure of water a t O D C., a much lower pressure than that existing in the venting chamber. The venting operation was actually begun by opening the needle valve in the vapor line and simultaneously starting the timer. The pressure in the venting chamber rapidly dropped, and in a minute or two the predetermined operating pressure was reached. The venting pressure was easily controlled by manual operation of the needle valve. Practically complete condensation of the vented materials, as evidenced by no significant accumulation of material in the acetonedry ice trap, maintained a low pressure in the condenser and burets. Furthermore, accumulation of noncondensables in the collection buret was very slight. At the very beginning of a run the pressure in the condenser rose because of the large amount of vapor coming over, but it soon leveled off and remained constant throughout the remainder of the run. Condensate was collected and measured in either or both bureta. Readings of total hydrocarbon condensate, time, venting temperature, buret cooling-water inlet and outlet temperatures, room temperature, agitator speed, and venting chamber pressure were taken every 5 minutes for the first hour, then every 10 minutes for the remainder of the run. A run was terminated when the condensation rate of the hydrocarbons became so low that it was difficult to determine the venting rate of isoprene therefrom. Agitator speed was measured with a tachometer. VAPOR-LIQUID EQUILIBRIA
Vapor-liquid equilibria for the system under consideration have recently been obtained at the University of Rochester (3) and on the same latex that was used in this investigation. Since isoprene and styrene are only very slightly soluble in water, the aqueous phase of the latex can be ignored, and from an equilibrium standpoint the system can be assumed to be isoprene-styrene-polymer. The equilibrium data used in correlating the experimental venting data are presented in Table I. The data for 80" are less reliable t8hanfor the other two temperatures, but it is believed that they are sufficiently accurate for the use to which they are to be put. The equilibrium partial pressure of water above latex may be taken the same as the vapor pressure of water, without significant error. RESULTS
The scope of this investigation, for which 37 satisfactory run8 were made, is indicated in Table 11.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
1233
TABLE11. SCOPEOF INVESTIGATION TX??'
Latex Depth,
R.P.M.
40
Mm. Hg
4.37
250-a48 260-426 328 328
515
60
I 4.37
0
\1::'
265
ti0
Preasure Range
Crn.
500-344 294-554 302
4.37 4.37 4.37
No.
Runs 8 7 4 3
6
8 1
The original experimental data are presented below, along with a sample calculation showing how the data for a given run were treated. The final calculated results for each run are not presented in tabular form but are shown graphically in Figures 3-9, and 11. The experimental results have been rorrelated by the following empirical equation I#- 41, UMHl
- E' = K ' v ~ r S ( p f - pi)* d0
(1)
In this equation, the rate coefficient, K', is dependent on temperature and degree of agitation; exponent @ is a constant, independent of temperature and degree of agitation; and exponent 01 is quite dependent on agitation, but the effect of temperature is not as clear-cut. At this point, it can be stated that, in general, the rB term in Equation 1 takes care of variation, in interfacial area with changes in venting pressure and also change in rate of bubble initiation with pressure; exponent 01 indicates the extent to whirh self-agitation, caused by ebullition, is present. The value of @ is taken as -2.0, but a was found to increase from about 1.0 a t high agitation to about 2.5 to 3.0with no mechanical agitation. Table I11 summarizes the values which were obtained for K', 01, and 6 for the different operating conditions.
VALUESOF K', a, AND 0 TABLE111. OBSERVED Temp., C. 40
Stirrer Rate. R.P.M. 515
60 265
60
Latex Depth,
Crn.
K'
4.37 4.37 8.37 13.11
2.45 0.00146 0.000133 0.0000195
ii%] 2.91
0.0285 0.170 1.37
1.37
4.37 4.37 4.37
a 1.05
8 -2.0 -2.0
Figure 3.
Effect of Pressure on Venting Rate
From the nature of things, it would be expected that diffusion of the isoprene through the aqueous film would be the controlling step in the escape mechanism-that is, the diffusional resistance encountered by the isoprene in the polymer particle would be small compared to that encountered in the aqueous film. Although the venting process may be considered to be primarily a diffusional one, diffusion is apparently not the whole story. Rate of bubble formation seems to be an important factor and under certain conditions of operation may conceivably be the controlling factor in determining the venting rate. The equilibrium partial pressure of isoprene, $in Equation 1, was determined from the data presented in Table I simply by knowing the over-all concentration of isoprene in the polymer particles. This means that if there were a significant isoprene concentration gradient through the-particles, the partial pressure of the isoprene above the particles would be something less than that obtained by awuming no gradient a t all. From the experimental data, there is no way of ascertaining whether or not there is an appreciable isoprene concentration gradient set up in the polymer particles and, therefore, whether or not the aqueous film offers essentially all of the rasistance to isoprene transfer. Brit he-
-2.0
With p equal to -2.0, us will be shown later, from Equation I, a log-log plot of
for a given run should give a straight line having a slope of a. This is the method which is used for presenting and plotting the experimental data, for from swh plots values of a and K' are readily obtained. DISCUSSION
The venting process ran be considered as a diffusional one, in which the isoprene first diffuses from the interior to the surface of the rubber particles and then through the aqueous film surrounding each bubble of vapor. The tiny rubber particles are adjacent to, and possibly to some extent concentrated in, the aqueous film.
loo
100
+,*- PJ Figure 4.
Correlation with Pressure
INDUSTBIAL AND ENG INEERING CHEMISTRY
1234
(tf-k),mmHa
Figure 5. Plot of Equation 1 to Evaluate K' and CY
cause of the extremely small size of the particles, the presence of appreciable styrene in the particles, and the nonpolar nature of the organic phase, one would expect that the diffusion resistance encountered by the isoprene in the aqueous film would be the major, if not essentially the only, resistance. The results presented in this paper were obtained on a batch process, whereas commercially the operation is usually performed in a continuous process. Theoretically, this should have no effect on the accuracy of the experimental results or on their applicability to the design of continuous venting equipment. There is this one drawback, though, to the use of batch operation-the experimenter cannot run his equipment until steady conditions are reached. When this is possible, more precise data are usually obtained. Pressure. Presumably, pressure has two important effects: the first of these is the effect on bubble formation and the second is the effect on bubble size which, in turn, affects the interfacial area. Since these effects are additive in the sense that they both cause a decrease in the venting rate with increase in pressure they are taken care of by the TB termin Equation 1. The effect of pressure on ventmg rate is demonstrated in Figures 3 and 4. Figure 3 shows what happens when -(dX,/&)(Z/V) is plotted against (p: p i ) . Separate lines are obtained, depending on the venting pressure, Figure 4 shows these same data replotted with -(dNi/d6)(n*/VL) as the ordinate; all the data points are fairly well represented by a single line. Similarly, Figures 5, b, and 7 each present a series of runs, where pressure is the variable, under different conditions of operation. The exponent -2 on 7r was obtained by plotting K ' d against r for runs carried out a t different pressures. These plots are shown in Figure 8. Values of K'ap were obtained from the type plot shown in Figure 3 by drawing the best straight line through the data points for each run. The lines were all drawn with the same slope, which was the best slope for a given set of runs. Actually, the three lines in Figure 8 would fit the data a little better if they were drawn with a slope of -2.2 instead of -2.0. But the data are not numerous enough, the pressure range not great enough, nor the accuracy high enough to warrant the trouble of using the 2.2 exponent when the simple square suffices.
Vol. 42, No. 6
From the standpoint of the effect of pressure on the interfacial area and assuming spherical vapor bubbles, the rate would be expected to vary as the -2/s power of the pressure. This indicates that the predominating effect of pressure is not its effect on bubble size but its retarding effect on bubble formation. It is not expected that pressure would have a significant effect on the diffusion aspect of the venting process, since here one is dealing primarily with diffusion through a liquid film. As for the effect of pressure, it can be summed up simply by saying that the venting rate of isoprene varies inversely as the square of the pressure, this effect being independent of temperature and degree of mechanical agitation. Temperature. The effect of temperature on rate of venting has been more thoroughly studied in another similar investigation a t the University of Rochester ( 8 ) . Figures 4 and 5 show plots of Equation 1 for data obtained a t 40" and 60' C. under otherwise similar operating conditions. At 40°,CY was found to be equal to only 1.05, whereasat 60", it was equal to 1.60. It isapparent that temperature is a very important variable. However, it would be expected that the relative increase in venting rate with increaee in temperature would be somewhat the same for all driving forces. In other words, with reference to Equation 1, one would expect the exponent CY to vary relatively little with temperature and K' to vary considerably with temperature. In fact, this was what was found in the other study (2). From a diffusion standpoint, one would expect an increase in temperature to cause an increase in the venting rate because of its effect on the diffusivity, and hence on K'. On the other hand, the exponent a is visualized as being a measure of the degree of selfagitation produced by the formation of the vapor bubbles in, and the rise of the bubbles through, the latex and hence should not be greatly affected by temperature. Temperature increase causes an increase in the interfacial area due to increase in the size of the vapor bubbles, but this effect will not be large for the temperature range of practical interest. Also, temperature increase causes a decrease in the viscosity of the latex, thereby decreasing the resistance of the aqueous film to isoprene transfer. Comparison of Figures 8b and 8c shows that temperature has little if any effect on the value of p in Equation 1. Agitation. In the process under study, there are two types of agitation: mechanical agitation and self-agitation. The former is that produced by the rotating impeller, and the second is that produced by the ebullition. Self-agitation is a maximum a t the beginning of a run when the venting rate is high and dwindles to insignificance a t the end of a run when the venting rate is low. This variation in degree of self-agitation during a run is taken care of by the exponent CY on the driving force. Figure 9 shows the tremendous effect of mechanical agitation on the venting rate; a t a driving force of 200 mm., the venting rate is found to vary with the agitation rate as follows:
-
In Figure 9 the lines for the various agitation rates tend to converge at high venting rates. This is due to the fact that as the mechanical agitation rate is increased the self-agitation effect diminishes. In other words the self-agitation effect gets "drowned out" a t high mechanical agitation rates, thus decreasing the value of a. The limiting value of a,as mechanical agitation is increased, would presumably be 1. The effect of agitation rate on CY is shown in Figure 10, where the approach to unity a t high agitation rates is apparent. In the venting of unreacted isoprene from synthetic rubber latex, it is strikingly clear that agitation is a key variable and one
June l9gO
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
Figure 6. Plot of Equation 1 to Evaluate K' and ci
that should receive much attention in the design of commercial venting equipment. The great beneficial effect of agitation is undoubtedly ita promoting effect on bubble initiation. There is also the helpful effect of reducing the effective aqueous-6lm resistance with increased rates of agitation. Latex Depth. As the lines in Figure 11 show, the venting rate per unit volume varies inversely as some function of the latex depth, when no mechanical agitation is uaed. A wholly satisfac-
1235
tory explanation cannot be given for the adverse effect of increase of latex depth. It is assumed to be due chiefly to the fact that as depth is increased degree of self-agitation is decreased. In other words, the deeper the latex, the calmer the ebullition. At any rate, this explanation ties in with the fact that a t high driving forces where self-agitation is the greatest, the effect of latex depth is not as great as a t low driving forces. Also, it is evident from Figure 11 that increase in latex depth becomes less effective at the greater depths. Had the effect of latex depth on the venting rate been studied where appreciable mechanical agitation was present, it is believed that the differences exhibited in Figure 11 would not, have been found. The effect of latex depth without mechanical agitation was studied because no mechanical-agitation is used in plant scale venting systems; at least that was the case when the present study was begun. Another partial explanation of the adverse effect of depth increase is that the surface area, through which some of the material transfer occurs, decreases per unit volume of latex. With increased latex depth the vapor bubbles are, on the average, in contact with the latex longer. This should tend to have the opposite effect of that described above. It is speculated that this is not actually the case because occlusion or retentioil of partially stripped latex particles in the aqueous film around each vapor bubble prevent richer particles from coming in active contact with the vapor bubbles. The spread of the data is such that there is appreciable overlapping for the three latex depths investigated-namely, 4.37, 8.74,and 13.1 om.-and it is not possible to draw quantitative conclusions regarding the effect of this operating variable on the venting rate, except what has already been said and except what the experienced engineer can conclude from the experimental results presented in Figure 11. Foam and Coagulation. The amount of foam above the venting latex varied during a run, of course, being greateat a t the beginning when the isoprene evolution rate was the highest. Also, the depth of foam varied with the temperature, pressure, agitation rate, and latex depth. The foam depth will vary directly as the ratio of the latex volume to the top surface of the latex, other factors remaining unchanged. However, under the worst conditions the foam depth aeldom exceeded 1.6 to 2 inches after the initial surge of vapor had subsided and steady venting conditions had been established. Usually, under steady operating conditione the foam depth was less than an inch. Aa for the formation of coagulum during a run, no special study was made of this, but by visual inspection of the latex no significant amount of coagulation was apparent. A slight skin formation did occur on the impeller and the walls of the venting chamber; this was removed after about 4 to 6 runs. Accuracy of Result& The experimental data presented, subject to the inherent variability of most rate data from diffusional 0 R.W.
515 RPM.
511 RP.M
h
t n (0)
Figure 7. Plot of Equation 1 to Evaluate K' and a
(b)
Figure 8. Evaluation of Constant, fl
(4
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1236
Vol. 42, No. 6
As for the accuracy of the experimental meaaurements, it is estimated that the temperature control was accurate to *0.2’ C., the pressure waa accurate within 1%, and the stirrer speed was accurate within 2%. The burets used for measuring the condensate were calibrated and found to be much more accurate than the work actually required. Relatively poor data near the end of some runs resulted from mathematical handling of small quantities, and the data in this region were not taken too seriously in drawing the curves, in some instances. Furthermore, owing to unattainment of steady venting conditions at the very beginning of a run, the first one or two data points were omitted in thc correlations. AS for the over-all accuracy of the data and the method of correlation, it is estimated that venting rates calculated by means of Equation 1, using the constants in Table 111, agree with experimental rates within 5 t o 6%.
TABLEIV. LATEXANALYSIS Per Cent Total soliaS Free isoprene content Free styrene content Combined styrene in polymer Polymer content Hydroquinone (inhibitor) Density a t 2 5 O C.
a6.2 6.25 4.0 2a.o 94.6 0.07 0.952
K./CC.
SUMMARY AND CONCLUSIONS
Effect of Agitation on Venting Hate
Figure 9.
operations, show a fair amount Of scattering* The nature of the experimental operation also contributed to a certain amount of variability from run to run. One source of error in the final calculated results was the determination of the instantaneous venting rates by graphical means. The PaPhiCal determination of point slopes of curves, no matter how carefully done, is always subject to some error. The data were obtained by three different operators, successively, and each operator had to discard a few runs while orienting himself with the apparatus. Once in a while, a run for no apparent reason would give wild results, a phenomenon not rare to chemists and engineers working with synthetic rubber latex. An estimate of the accuracy of the experimental data can be obtained by inspection of Figures 3 through 9, and 11. Referring to Figure 3,runs B10,B11, B18,and B19 were
IO
The results presented here are interpreted to show that the venting of unreacted isoprene from isoprene-styrene latex is a diffusional process in which bubble formation is a very important factor. Bubble formation is greatly accelerated by agitation and is retarded by increase in pressure. From the data available, it is difficultto ascertain the effect of temperature on bubble formaappear to be to the overwhelmtion, but it ing effect of agitstion and the pronounced effect of pressure. In using the rate coefficients presented for the of mercial venting equipment,, it is difficult to compare the degree of agit,ation in the experimental equipment with that which might
-.. ._.--
-R
P H
Figure 10. Effect of Agitation Rate on a
all made at about the same pressure and therefore theoretically the data points for all four runs should fall pretty close to a single line. Actually the data for runs B10,B11, and B19 do agree very well. The points for run B18 are somewhat high. Good agreement is also found between runs A13 and A22 which were carried out under practically the same conditions as shown in Figure 6. The same is true of runs B32,B33,and B34 shown in Figure 5.
@-h) , mm. H9. Figure 11. Effect of Latex Depth on Venting Rate
I N D W S T R I A L A N D E N G I N E E R I N G CHEMISTRY
June 1950
n 1.e
w I
be p r e s e n t i n 1a r ge commercial venting systems. Thecommercial equip ment would be so much greater than the experim e n t a l equip m e n t t h a t it would be extremely hazardous to assume that the geometr r i c-s i mi 1a r i t y rule (7) could be applied. This means that the effect of agitation in large venting veaeela must be estimated until rate coefficients on l a r g e r equipment are available. I n the meantime, the r e s u l t s on the effect of agitation on isoprene v e n t i n g can serve aa an excellent guide. As for the effects of temperature and pressure, it is expected that they would be much the same for both experimental and commercial equipment sizes. In other words, the rate coefficients for the c a s e s of zero mechanical agitation should be fairly dependable for engineering and design Purpo~S. In the design of v e n t i n g equipment f o r the recovery of unreacted isoprene, the agitation should be 88 great 88 possible, the temperature should be aa high as possible, and the pressure should be as low as possitde. These are
1237
the three important operating variables. Their optimum values will depend on practical and economic considerations. It is understood that, currently, venting tanks in standard copolymer plants are not equipped with agitators. It is suggested that if the recovery of isoprene is to be undertaken to any great extent in such plants, serious consideration be given to the installation of agitation equipment. Although the recovered monomer in the present investigation is isoprene, the general qualitative conclusions regarding the effect of the four operating variables on venting rate should be applicable to all monomers. Based on the resulta of this work on the venting of isoprene from isoprene-styrene latex, it is specifically concluded that: 1. The ventingprocess can be well represented by Equation 1, where the exponent @wasfound to be independent of temperature and agitation and to have a b l u e of -2.2, although for the sake of expediency a value of -2.0 was used in the correlation work; u is believed to be primarily a function of the degree of agitation. 2. The operating variables which affect the venting rate are temperature, pressure, degree of agitation, and latex depth. The fist three of these are the most important, from the standpoint of their relative effects. 3. The venting rate is greatly affected by temperature. However, this effect waa found to be influenced by the driving force. Moderate changes in temperature, say 10"C., were found to cause a severalfold increase in the venting rate a t the higher driving
forces. 4. The venting rate varies inversely as the 2.2 power of the pressure. 5. Agitation has a tremendous effect on the venting rate, it being possible to increase the venting rate severalfold with impeller speeds up to lo00 r.p.m. 6. Venting rate is decreased with increase in latex depth. However, for a given increase in depth the effect is very much less at large depths than at small depths. For example, at 60" C. and no agitation, the venting rates for 8.74cm. and 13.1-cm. depths are practically the same when the driving force is above 500 mm. 7. Under the operating conditions under which the experimental data were obhined, foaming and coagulation tendencies were both considered to he very slight and of no practical consequence. SAMPLE CALCULATION-RUN
NO. A10
Initial vacuum at 23' C. (uncorrected) = 736 mm. Hg 753
- 736 13 54 =
19 mm. pressure
13.80 Vapor pressure of water at 23' C. = 21.0 mm. Hg. Therefore, it can be assumed that there waa no air present in the venting system. Equilibrium gage pressure at 60 'C. (uncorrected) = 293 mm. Hg Equilibrium total preasure = 749
+ 293 113.60 3.54 = 1041 mm. Hg
Vapor pressure of water at 60" C. = 149 mm. Hg p:
+ p:
= 1041
- 149
892 mm. Hg
And from a plot of the data in Table I, ZIP = 0.220. Since there are 117 grams of polymer in the 500-ml. charge, 0.220 X 117 = 25.75 grams of isoprene in latex at equilibrium.
Now,calculate the amount of isoprene in the vapor space at equilibrium: p!v = nRT 892 X 0.99 24m 760
u3; X 82.1 X 333
68.1
1238
INDUSTRIAL AND ENGINEERING CHEMISTRY
At e = 5, grams isoprene in latex = 32.95 - 10.96 - 1.74 = 20.25. At e = 5, grams of isoprene vented from the latex from e = 0 is obtained thus:
TABLE VI. EXPERIMENTAL DATA Initial yentVacuum Barometria Agitaon Pressure Vacuum tion 1n.a Temp., Chamber, Run Rate, Mm. No. C. Mm. Hga 1st 2nd R.P.M. Hg5 c. 0 A8 60 428 745.4 746.3 728 20 0 752.9 748.2 60 385 A10 736 23 0 60 449 A12 747.6 747.6 731 25 0 A13 60 322 748.7 747.6 726.5 23 1050 60 440 739.2 741.2 A15 717 24 60 450 264 749.2 749.2 A16 724 24 264 60 436 734.7 735.6 A17 713 27 60 493 A21 0 753.0 751.5 733 25 0 60 347 A22 756.6 60 396 758.6 0 A23 ,. 515 B5 60 397 753.0 ., 515 60 202 740.6 B10 515 60 199 Bll ... 747.1 515 60 299 B13 ,., 738.0 60 203 515 ,. 756 .. B18 Bl9 515 60 200 748 B20 515 60 41 1 747 60 458 751 515 B21 B23 515 40 489 739 515 B24 40 485 745.3 *. B25 748 40 488 515 515 B31 739 40 486 ... 515 B32 750 40 441 *. 748 B33 515 40 442 744 B34 515 40 442 515 B35 750 40 403 c2 264 60 407 .. 746 260 60 407 , 749 c3 260 60 408 c4 750 260 60 409 ... c5 , * . 749 0 744 60 416 c10 0 60 416 744 C13 0 60 421 C14 748 0 60 427 755 C16 0 60 428 756 C17 0 60 418 746 C18 .., 0 60 418 746 c19 a Actual manometer reading. b From this point on, column gives correated (pf p:) values, exoluding p t
... . .. ... ...
... . .... .. .. .... ... .. . ... ... ...
... .. . .. .
... ... ... ... .. .. .. ...
... ... ... ... ... ... ... ... ... ... ...
... ... ... ... ... ( 1 .
..
Equi-
Latex Charged, M1. 500 500 500 500 500 600 500 500 500 500
librium Pressure Mm.Hgh 227 293 284 256 259 315 321 304 243 257
10.96
....
... ... ...
... ... ...
.. .. ....
.... ... .. ... ... .,..
758 762 755 755 730 757 755 745 777 751 736
600
500 500 500 1000 1000 1000 1000 1500 1500 1500
- (7.20 - 1.74) = 5.50
At e = 5, I / P was found to be 0.173, and from the data in Table I, for I/P = 0.173, y, = 0.990. This means that the assumed value of 0.99 was just right. Of course, it would be expected that under actual venting conditions the isoprene and styrene would not come off in their equilibrium ratio. T h e v e n t i n g vapor would be richer in isoprene than the equilibrium vapor because the diffusivity of isoprene through water would be greater than that of styrene through water. But even the equilibrium values of y%are so close to unity that the actual venting values, if higher, cannot be significantly different. Since y,, at 8 = 5, is 0.99, assume that yL, at e = 10, is 0.988, At e = 5,
.... .. .. .. *...
*. .. .. .. ..
Vol. 42, No. 6
+
p, =
1
[749
- 385
-1
13.54 13.60
149
:.
w: = 7.20 grams
+
total isoprene in system = 25.75 7.20 = 32.95 grams at equilibrium. 13 54 Operating pressure = 749 - 385 = 365 mm. Amount 13.60 of isoprene in vapor when 8 = 5 minutes is 365 - 149 7.20 ____ = 1.74 grams 892 Assume that, when 8 = 5 minutes, the mole fraction of isoprene in the condensate is the same rn that in the vapor-namely, 0.99. The weight fraction will be 0.99 X 68.1 0.99 X 68.1 0.01 X 104.1 = 0*985 The volume fraction will then be 0.985/0.7005 0.985/0.7005 0.015/0.923 = 0’990 The densities of isoprene and styrene at 0 C. are 0.7005 and 0.923 gram per ml., respectively. The above calculation is based on the assumption that there is no volume change on mixing. It will be seen that the mole fraction and volume fraction values are the same. Within the concentration range involved, this assumption is satisfactory. So, for a given increment of hydrocarbon condensated, it will be assumed that its mole fraction isoprene equals its volume fraction isoprene. Furthermore, for a given increment of condensate, it will be assumed that its hydrocarbon analysis will be the same as the existing vapor analysis when the increment is finally condensed. In other words, for the increment of hydrocarbon condensate which condenses between e = 5 minutes and e = 10 minutes, its mole fraction of isoprene will be taken equal to the mole fraction of isoprene in the vapor a t 0 = 10 minutes. The mole fractions of isoprene are 80 close to 1 and do not vary appreciably and the condensate incrementa are so small, that the above assumption is easily justified.
+
+
O
vL
=i
0.500
15 8 - 1.74 -1000 1000 X 0.7
=
i
-
0.99 = 214
0*482
At 0 = 10, Wi = 5.50 pi =
(365
+ 2.62 = 8.12
- 149)0.888 = 213
from the equilibrium relationship, when I / P = 0.153, y i = 0.988. At e =i 100, pi =
1
- 385 G ] - 149 13.60
1
0.975 = 210
At 0 = 100, Total grams isoprene in condensate = 22.07
.‘.Wi
3
22.07
- (7.20
- 1.72)
Grams isoprene in latex = 32.95 - 1.72
16.59
- 22.07
= 9.16
These procedures are followed throughout the entire run, and a set of calculations is made for each time interval. The results of these calculations are shown in Table V. The -dNi/dB values in Table VI were obtained graphically by taking the slope of the ni us. e cruve a t each time instant. About 10% by volume of the total condensate collected during a run was water, usually about 2 to 3 ml. As for the styrene, its concentration in the hydrocarbon vapor was low, only about 2% of the original styrene in the latex being vaporized. ACJLNOWLEDGMENT
This paper is based on the results of an investigation conducted as part of a research program financially supported by the Office of Rubber Reserve, Reconstruction Finance Corporation. The sponsor’s permission to publish these results is much appreciated. The authors also wish to acknowledge the assistance of Lowell T. Burke and Vernon A. Breitenbach who obtained a large portion of the experimental data. Thanks are also due Joseph D. Helwig for
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
1239
TABLE VII. EXPERIMENTAL DATA
1
Time, Min. As indicateda 5 10 15 20 25 30 40 50 60 70
80 90 100
2.5 5 7.5 10 12.5 15 17.5
20
22.5 25 30 35 40 50
60
70 80 90
2.5 5 7.5 10 12.5 15 17.5
20
22.5 25 30 35 40
?!
i2
60 65
70 75 a
A8
31.9 32.8 33.9 34.9 35.15 35.85 (75)
Brdrooarbon Vented for Each A15 A16 18.1 (4)" 8:6 2i:9 19.9 11.5 31.8 25.8 14.2 33.5 29.4 16.3 34.4 32.0 18.1 35.1 33.4 19.0 35.6 34 8 31) 20.9 36.2 35:5 la51 22.3 36.7 23.2 36.9 24.1 (75) 37.2 (75)
3... 03
&a..
A10
....
15:s 19.6 22.1 23.7 25.1 26.1 27.8 28.7 29.7
2i:a 23.3 24.5 25.6 27.3 28.6 29.2
...
30:l
31.0 ai:95
ai:o B5 6.5 10.6 14.5 17.3 19.8 21.2
A12 .,.
25.6
X{El 30.9
B10 2.5 2.85 3.65(7.8) 3.95
B11
5.20
5.20
1.95 2.55 3.30 4.10
...
...
23:2
... 5.95
24:9 25.7
i:20 7.80
S:io
27:2
10.25 11.15
... ...
2i:9
..* .
a .
C2 8.4 12.7(6) 15.3 17.1 18.5 19.9 20.9 21.8 22.5 22.9 23.9
.. .. .. .... .. .. .. I .
e:bO 0:70 7.55 8.00 8.70 9.35 9.35 10.90 11.15 11.90 c3 8.0
13,4 10.9 16.2 16.5 17.8 19.6 20.4 21.2 22.0
23.0 23.6 24.3
.. .. *.
..
.. .. ..
Run, MI.
A17
A13
B13 7.0 10.75 12.8 14.7 15.9 17.25 18.2 19.25 19.8 22.55 21.85 22.7 23.45 24.7
.. .. ....
c4 7.9 11.3 13.5 15.9 17.7 18.4 19.6 20.1
zi:i 22.0
.. ..
....
.... ....
..
...... ..
......
B18
B19
i:0
5:i5 5.90 7.05
8.55 10.25 11.15 12.25
...
BZO
i:30
24:05 25.75
10 35
27:i
:
:
ii:Os
28:i 30.3
12.7
l7:95 19.35
... 13.7
15.25 16.5 18126
...
... ... ..* ... ...
B2 1 13.4 20.8 24.25 20.45 27.95 29.3 29.95 30.82 31.3 32 .O
... ... ... ... ... ...
..
....
. I
13.5 16 25 16.3
2415 28.5 31.0 31.6 33.0 33.8 34.9 35.7 36.3 37.1 (75)
.. 10.3 16.6 19.9
A21
243
30.1 33 .O 34.7 35.3 36.5 37.7 38.7 39.5 39.9 40.2 (81) 40.25
B21 3.1 4.8
14:1 15.6 16.9 18.2 19.5
ii:o
B25 3.0 4.6 6.8 7.8 9.3 10.5
12:9
12:6
14:7 15.8 17.4 18.4
14:1 15.5 16.6 17.6 19.5 20.6
6.9
7.9
20.0 21.1 ... .... ... 22.5 ... ... 22.1 . . . . . . . . . .
c5 8.1 12.3 14.6 16.5 17.9 19.1
c10
2Q:4
2i:0
26:3
2i:7 27.8 29.6
2s:4 30.2 31.5
2i:o 30:l 31.5 32.7
24:6
22: 1 23.0 23.8 24.2
20:7 28.6 30.1
....
.. .*
....
..
014 .-. 10.8 15.3 18.6 21.0 22.8 24.3
C16
aKo
33:9
34:7
33:2
34:s
35:s
30:5
35:l
30:s
37:4
3i:6
30:4
37:4
si:?
..
37:4
A23 8.15 (2.5) 11.65 16.50 19.60 21.80 23.50 24.75 26.70 28.05 28.85 29.80 30.10 30.55
20.0
21.1 22.65 23.6 24.3 25.2 25.7 26.35
...
B23 2.6 3.8 5.7 7.4 8.9 10.1 11.2 12.2
C13
A22 7.5(3) 9.5 14.1 16.6 18.6
...
B31
B32
a . .
B33
B34
B35
216 4.0 5.0
311
7:s
7:2
4:o
9:s
9:o l0:4 11.3
5:1 0:7
13:9
s:2
5:3 6.7 8.8 9.8 11.9 12.5 14.4
2:s 3.5 4.7
218 4.3 5.5
0:2
s:1
1517 18.1 18.8
i:2 10.8
16:7 12.1
.. .... ..
li:7 14.9
1i:1 16.0
20.0
.... *.
. . . . C17
::
* *
15.1 9.5 ... . 16.2 10.5 . . . .. .. .. .. .. ... C18 10.4 16.7
..
c19 9.3 14.4 18.1
36:1
27:5 29.8 31.7 33.2
23:3 25.5 27.0 28.8
42:O 44.4 46.2 48.5
3i:3 38.8 40.5 42.3
3i :o 33.3 35.7 37.4
50:7
45:O
39:3
5i:S
47:5
42:3
54:s
..
*.
..
....
Figures in parentheclecl are actual times readinga were taken, if suoh times are not listed in first column.
performing much calculation work. The latex used in the investigation was especially prepared for the purpose by the Government Synthetic Rubber Laboratories of the University of Akron. For the splendid cooperation of this organization in not only preparing the latex but also in supplying analyses of monomers, latex, and polymer, the authors are most grateful.
VL wi
zut
= !atex volume at time e, liters = lsoprene in vapor space, grams
* isoprene in vapor space under equilibrium conditions, grams Wi = is0 rene vented, grams = mope fraction isoprene in hydrocarbon vapor y, e = time, minutes r = total pressure, mm. Hg a, B enipirical constants
-
NOMENCLATURE
I = isoprene, grams K' rate constant, defined by Equation 1 Ni = gram moles is0 rene in volume VL at time e 71.1 = is0 rene ventex gram moles P = poPymer, grams P? = equilibrium partial pressure of isoprene above isoprenestyrenelatex mixture, mm. Hg pi = partial pressure of isoprene above isoprene-styrenelatex mixture, mm. Hg Pt = equilibrium partial pressure of styrene above isoprenestyrene-latex mixture, mm. Hg Pa = partial pressure of styrene above isoprenestyrene-latex mixture, mm. H p : = equilibrium partiaf pressure of water above isoprenestyrenelatex mixture, mm. Hg s = styrene, grams I
J
LITERATURE CITED
(1) Dean, R. B.,J. Applied Phys., 15,446-51 (1944). (2) Dwyer, 0.E., and Burke, L. T., IND. ENG. CHEM.,42, 1240 (1960). (3) Dwyer, 0.E.,
and Sesonske, A., report submitted to Office of Rubber Reserve, Reconstruction Finance Corporation, Dee. 29, 1947. (4)Hatcher, J. B., and Sage, B. H., IND.END.CHEM.,33, 443-52 (1041). (5) Johnson, C.R., (1940).
and Otto,W. M., Chem. Eng. Progrsss, 45,407-14
(6) Maron, S.H., Case Institute of Technology, private communication, August 1940. (7) Martin, J. J., ! h a s . Am. Inat. Chsm. Engra., 42,777-81 (1946). (8)Semon, W.L.,Cham. Eng. N e w , 24,2900-5 (1046). RE~EIVED January 7. 1950.
