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INDUSTRIAL AND ENGINEERING CHEMISTRY
residue starch shows a 5 per cent lower “conversion limit” with &amylase than the parent starch, by our method of calculation (1) the residual starch liquefies at a faster rate in malt diastase conversions similar to those used in practice to produce thin-boiling starch pastes for the sizing of such products as textiles and paper. EXPERIMENT 3. Such a conversion is described as follows: 28.35 grams of starch are suspended in 280 cc. of water t o which are added 0.4 gram of a commercial diastase preparation known as Diastase E (Rohm & Haas Company). The water is so preadjusted that the final mixture for conversion will have a pH of 6.0 to 6.2. The conversion mixture is brought to 78” C. in 10 minutes with stirring, and held a t this temperature for 30 minutes. It is then transferred to a metal beaker, immersed in boiling water immediately, and cooked for 15 minutes in the boiling water bath. The
Vol. 34, No. 10
viscosity of the paste is determined by a method used in industrial practice, known as the Scott test. Two hundred cubic centimeters of the paste are transferred to a metal cup provided with an orifice standardized for starch pastes, and the time in seconds is noted for the first 100 cc. of paste to run through the orifice a t the temperature of boiling water. This specific Scott viscosity after conversion, for the residual starch mentioned above, was 43.5 seconds; for a sample of cornstarch not pretreated but simply converted as above, 51.5 seconds.
Literature Cited (1) Kerr, R.W., and Schink, N. F., IND.ENQ.CREM., 33,1418(1941). (2) Kerr, R. W., and Trubell, 0. R., Cereal Chem., 18, 530 (1941). (3) Kerr, R.W., Trubell, 0. R., and Severson, G. M., Ibid., in press. (4) Meyer, K., Bretano. W.,and Bernfeld, P., Helv. Chirn. Acta, 23, 845 (1940).
Supersaturation in Sugar Boiling Operations CONTINUOUS AUTOMATIC MEASUREMENT ALFRED L. HOLVEN California and Hawaiian Sugar Refining Corporation, Limited,
Crockett, Calif.
HE desirability of securing a continuous and automatic measurement of supersaturation during sugar boiling
T
operations has long been recognized in the sugar industry. Claassen’s work ( I ) on beet sugar and later Thieme’s adaptations (16) to cane sugar have served to emphasize the need for further developments in this field. However, practical application of graining and boiling procedures in accordance with fundamental principles of supersaturation has been handicapped in both the cane and beet sugar industry by lack of a suitable method of automatically and continuously measuring the supersaturation of sugar solutions. Recognition of the need for what might be termed a “supersaturation meter” was expressed by Kukharenko ( 1 2 ) over twenty years ago: “In order to solve the problem of continuous crystallization in sugar manufacture, one must invent a device for the direct, accurate, and instantaneous determination of the coefficient of supersaturation of the mother liquor boiling in the vacuum pan.” Lack of an instrument for direct measurement of supersaturation was also commented upon by Webre (17’) in referring to the control of sugar boiling in which he stated: “The difficult part of this work lies in the fact that to date no one has been able to devise a n instrument that would show supersaturation directly.” Such references are typical of the opinions of many investigators involved in the development of sugar boiling operations. Because of this well recognized need of some means for measuring and controlling supersaturation of sugar solutions during crystallizing operations, many investigators have
attempted to develop an instrument which would meet the rather exacting requirements. One of the earliest instruments to be employed in sugar boiling was the Brasmoscope originally devised by Curin (3) and later developed by Claassen ( 2 ) . This instrument consisted merely of a mercury vacuum gage and a thermometer by means of which vacuum and boiling temperatures could be determined. From such measurements boiling point elevations could be calculated, and by reference to tables these, in turn, could be expressed as concentrations. A direct determination of the elevation of the boiling point without the necessity of calculations involving vacuum became the subject of a patent issued to Langen (12) in 1909. I n Langen’s device two opposing thermocouples were employed; one was in the boiling massecuite while the other was in a pilot boiler in which water was boiled under the same pressure as that prevailing in the vacuum pan. Ho-ivever, all that was achieved by either the Brasmoscope or Langen’s apparatus was a measure of the concentration of the mother liquor and not its supersaturation. To convert concentration into coefficient of supersaturation required calculation-, in which the effect of absolute pressure also had to be taken into account. Several other investigators have described devices m hich will measure the boiling point elevation of sugar solutions. However, like the Langen apparatus, these instruments indicate only concentration of the mother liquor and not supersaturation. A measure of sucrose concentration cannot be used by itself as a means of sugar hoiling control because con-
October, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY ABSOLUTE
BOILING
FIGURE 1.
PRESSURE
POINT OF
- CM
WATER
1235
HG
- 'C
EFFECT OF PRESSURE ON BOILIXQ POINT ELEVATIONS OFSUCROSE SOLUTIONS t
The degree or coefficient of supersaturation is an important factor in controlling the evaporativeprocesses by which sugar is recovered in crystalline form. There has been no satisfactory means for directly measuring or controlling the degree of supersaturation of boiling sugar solutions. 4 new method is developed here, based on the hitherto unrecognized fact that supersaturation may be calculated by a mathematical formula derived in these investigations. This formula is based on the discovery that, a t all pressures encountered in usual sugar boiling practice, a plot of boiling points of sugar solutions of any degree of supersaturation against the corresponding boiling points of water a t these same absolute pressures yields a straight line. The development of these graphs and their adaptation to an automatic instrument for continuously recording and controlling the degree of supersaturation is described in detail.
centration gives no measure of the tendency of a solution to deposit or dissolve crystals. This is aptly illustrated by the fact that any high-density sugar solution may be highly supersaturated at low temperatures but undersaturated a t high temperatures. Since the tendency for sucrose to crystallize from solution is a function of its supersaturation rather than its concentration, none of the foregoing instruments furnished a direct measure of the index most urgently needed as a basis of sugar boiling control. In addition to these devicee, some investigators have attempted to utilize the conductivity method as a means of sugar boiling control. Owing to their inability to furnish any record of true supersaturation, neither the Brasmoscope, conductometric methods, devices based on boiling point elevation, nor even the vacuum pan refractometer have received widespread application in the sugar industry. They offer neither a means of automatic control of supersaturation nor a simple method of measuring supersaturation for manual control. Heretofore the principal obstacle retarding development of supersaturationmeasuring equipment has been that no general law or mathematical formula by which supersaturation could be expressed as a function of temperature and absolute pressure has been known. Therefore i t has been necmsary to calculate supersaturation by comparing the measured sucrose concentration with the normal solubility of sucrose at the prevailing temperature. This difficulty was overcome in the present investigation by discovery and development (6) of a simple mathematical relation between supersaturation, on the one hand, and the temperature and absolute pressure under which the sugar solution is being boiled, on the other. As both boiling point and absolute pressure FIGURE2. REPRESENTATIVE CONSTAKT SUPERSATURATION LINESFOR SOLUTION OF 100 Pea CENTPTJEITIcan be automatically determined, it is
A STJQAR
INDUSTRIAL AND ENGINEERING CHEMISTRY
1236
O F ABSOLUTEPRESSURE TABLE I. EFFECT R . P.E.
% Solids
-76.0 1.8 2.3 3.0 3.8 4.3 5.1 6.1
Si
Cm. Hg50 1.00 55 1.22 60 1.50 65 1.86 67 2.03 70 2.33 73 2.70 75 3.00 78 3.54 80 4.00 82 4.55 84 5.25 86 6.14 88 7.33 90 9.00
7.0 8.3
9.4 10.6 12.1 14.0 16.6 19.6
The terms used are defined as follows: B. 1’. E.= boiling point elevation, 0 C.; SI sucrose per unit water’ = boiling point of solution = B. P. of water I3. P. E.;s o b . a t T‘ normal solubility pf 8ucrose a t calculated bolling point from Herzfeld‘s tables; d = solubility per unit water; SIIR = supersaturation cyffioient. b Figures i n parentheses are boiling points of water at the absolute pressures indicated. E
-+
ON BOILINQ POINTS OF SUCROSE SOLUTIONS O F VARIOUS SUPERSATURATIONS”
Soly. at T’ B . P . E. T‘ 70 S -35.49 Cm H g (80’ C.)b 78.7 3.69 1.6 81.6 78.8 3.71 2.1 82.1 2.7 82.7 79.0 3.76 79.2 3.80 3.5 83.6 79.3 3.83 4.0 84.0 79.4 3.85 4.7 84.7 79.6 3.90 5.7 85.7 79.8 3.95 6.6 80.6 80.1 4.02 7.8 87.8 80.4 4.10 8.9 88.9 80.6 4.16 10.0 90.0 81.0 4.26 11.4 91.5 81.4 4.37 13.3 93.3 82.0 4.56 15.8 95.8 82.6 4.75 18.7 08.7 1.3 1.8 2.4 3.1 3.6 4.2 5.1 5.9 7.0 8.1 9.1 10.5 12.2 14.6 17.4
B.P.E.
SI/S
.
Q
-23.3 0.271 0.329 0.390 0.490 0.530 0.605 0.692 0.760 0.880 0.975 1.005 1.232 1.403 1.607 1.895
6.3
-
-9.20 C m . Hg (50° C.)a. 72.4 2.62 51.3 72.5 2.64 51.8 52.9 72.6 2.65 72.8 2.68 53.1 72.9 2.69 53.6 54.2 73.0 2.71 73.2 2.73 55.1 73.3 2.75 55.9 73.5 2.77 57.0 73.8 2.81 58.1 74.0 2.85 59.1 74.2 2.88 60.5 74.6 2.94 62.2 75.1 3.02 64.6 75.7 3.12 67.4
1.5
2.0 2.6 3.4 3.9 4.5 5.5
0.382 0.462 0.566 0.094 0.755 0,860
0.990 1.000 1.278 1.424 1.598 1.822 2.090 2.425 2.885
7.5 8.6 9.7 11.1 12.9 15.4 18.3 1.3 1.7 2.3 3.0 3.4 4.0 4.9 5.7 6.8 7.8 8.8 10.1 11.8 14.2 16.9
Development of Supersaturation Formula The term “supersaturation coefficient”, as originally proposed by Claassen, is equal to SI/S,where S is the solubility of sucrose in the normally saturated solution at the prevailing temperature, and SI is the amount of sucrose actually dissolved; S and Sl are both expressed as sucrose per unit water. In the absence of any recognized fundamental relation by
,
0 SUPER-SATURATION
,0,s
____---
Soly. a t T‘ % S Cm. H g (70’ C.!a-
71.5 72.0 72.6 73.4 73.9 74.5 75.5 76.3 77.5 78.5 79.7 81.1 82.9 85.4 88.3
76.5 76.6 76.7 76.0 77.0 77.2 77.4 77.6 77.8 78.0 78.3 78.6 79.0 79.6 80.2
3.25 3.27 3.29 3.33 3.35 3.38 3.42 3.46 3.50 3.54 3.61 3.67 3.76 3.90 4.05
-5.48 Cm. €In (40’ C.) b41.3 70.6 2.4 41.7 70.7 2.41 42.3 70.8 2.42 43.0 70.9 2.44 43.4 71.0 2.45 44.0 71.1 2.48 44.0 71.3 2.49 45.7 71.4 2.50 46.8 71.6 2.53 47.8 71.8 2.55 48.8 72.0 2.57 50.1 72.2 2.60 51.8 72.6 2.65 54.2 73.0 2.70 56.9 73.5 2.77
Si/S 0.308 0.373 0.456 0.559 0.606
0.690 0.790 0.867 1.011 1.130 1.260 1.430 1.632 1.880 2.220
F3.P.E. -14.80 1.4 1.9 2.5 3.2 3.7 4.4 5.3 6.1 7.3
8.3
9.4 10.8 12.5 15.0 17.8
SoIy. a t T‘ ___
% S Cm . Hg (60’ C.)b-
T’
61.4 61.9 62.5 63.2 63.7 64.4 65.3 66.1 67.3 68.3 69.4 70.8 72.5 75.0 77.8
74.4 74.5 74.6 74.8 74.9 75.1 75.2 75.4 75.6 75.9 76.1 76.4 76.8 77.4 77.9
2.90 2.92 2.94 2.97 2.99 3.01 3.03 3.06 3.10 3.14 3.18 3.24 3.31 3.42 3.52
0.506
0.620 0.762 0.829 0.947 1.085 1.200 1.403 1.570 1.770 2.020 2.320 2.710 3.250
0.345 0.418 0.510
0.627 ~.._
0.679
0.774
1.2 1.6 2.1 2.8 3.3 3.8 4.7 5.4 6.6 7.5 8.5 9.8 11.4 13.8 16.5
31.2 31.6 32.1 32.8 33.3 33.8 34.7 35.4 36.6 37.5 38.5 39.8 41.4 43.8 46.5
68.8 68.9 6‘3.0 69.1 60.2 69.3 69.5 69.6 69.8 70.0 70.2 70.4 70.6 71.1 71.6
2.21 2.22 2.23 2.24 2.28 2.26 2.28 2.29 2.31 2.33 2.36 2.38 2.40 2.46 2.52
0.892 0.980 1 . 1.4~. 1 1.274 1.430 1.630 1.885 2.140 2.555
--
__ 3.15 Cm . rrg (300 c . ) -
c
0.417
SdS
0.452 0,550 0.673 0.830 0.902 1.031 1.182 1.310 1.531 1.718 1.928 2.205 2 . 860 2.980 3.570
which supersaturation coefficients could be correlated with boiling temperatures, i t has been necessary heretofore to compute supersaturation by reference t o tables or graphs, a function which no automatic instrument could be expected to perform. An investigation was therefore made in which initial efforts were directed toward a study of possible relations between supersaturation coefficients a t various boiling points, in the hope of developing a mathematical formula to serve as the basis for the development of a suitable instrument for measurement and control purposes. In view of the manifold applications of the Duhring rule (4),which others ( l a , 19) liave found to have a sound thermodynamic basis, i t was decided to explore the possibility of applying its principles to the boiling points of cane sugar solutions of various degrees of supersaturation. I n its original form the Duhring rule states that if the temperature of one substance is plotted against the temperature a t which another similar substiinre has the same vapor pressure, the resulting graph is a straight line. Therefore it pointed to the possibility that at various absolute pressures there might be a linear relation between the boiling points of aqueous cane sugar solut8ionsand water. Accordingly, the boiling points of cane sugar solutions of various purities, degrees of supersaturation, and absolute pressures 1i:tve been computed. The basic data used for such computations have been selccted from the following sources:
SOLN. -
/ FIGURE3. GRAPH ILLUSTRATING MANNER IN WHICH SCPERSATURATION MAY BE REGARDED AS A FUNCTION OF TEE BOILINQ POINTS O F WATER b N D SUUAR LIQUORS UNDER EQUAL ABSOLUTEPRESSURE
.
ROILINGPOIST ELEVATION OF PURE CAKESUGAR SOLUTIOXS AT These data arc based on Claassen’s values (I) as mentioned by Tliicme (1;) and checked in ,this laboratory (6). The accuracy of these values is further verified by Internatitinat Ciitical TalJles ( 9 ) , Katilcuberg (IO),and more recently by Tressler, Zimmermnn, and Willits (f/7)* k r ~ i O s r I I E i i I CP R s s s u r t E .
I. I;? I
I”
1
possible to secure not only a measurement of concentration but, what is more important, a continuous measurement of the coefficient of supersaturation as well. The accuracy with which supersaturation can be measured by this system is unaffected by variations in either absolute pressure, boiling temperature, or purity of the sugar liquor, since the effect of all of these factors may be readily compensated for. I n such respects i t offers distinct advantages over methods previously employed.
I
VoI. 34, No. 10
BoIl.ISti P O I N T ELEVATIoN O F IMT’URJ2 CANE SUGAR SOIaUTIONS AT A’I’MOSI~HIZILIC PRESSURE. These values are taken from Thietne’s “Studies on Sugar Boiling” (IO). CT OF AIWOLUTEPRESSURE O N BOILIXG P O I N T ELEV.4TION OF Suaan SOLUTIONS. These data are based on a previous investigation ( 5 ) and are shown in Figure 1. Attention is particulizrl directed to the fact that these data are valid for impure as wefi us pure sugar solutions. TOTAL SOl.IDS CONCENTRATION O F SATURATED SUGAR SOLUTIOKS OF VARIOUSPURITIES. Data for 100” Brix purity sugar solutions are Herzfeld’s values reported by International Critical Tables (7). These values conform to the formula (14):
October, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1237
Y i. 64.1835 f 0.13477X f O.O005307X* where Y = per cent sugar X = temperature, C. h
For impure cane sugar solutions Thieme's solubility values are accepted (16). VAPORPRESSUREOF WATERAT VARIOUSTEMPERATURES. Values inlnternational Critical Tables (8)are used. Table I shows the data calculated a t various absolute pressures for a 100' purity sugar solution a t concentrations between 50 and 90 per cent solids. Similar calculations have 80', 70', 60°, and 50' been made for liquors a t QO', purity. The preparation of these tabulations involved a tremendous number of arithmetical calculations which were independent,ly checked to avoid errors. Intermediate supersaturation values were judiciously interpolated by graphic means in order to avoid any sacrifice in precision. The graph of Figure 2 is typical of the fifty to sixty graphic supersaturation lines derived by such means. The straightness of these supersaturation lines is somewhat surprising and indicates that the slope of any line which is representative of the boiling points of a cane sugar solution of any given degree of supersaturation may be assumed to be a constant over the range of temperatures, pressures, and purities encountered in usual sugar boiling practice. The significance of this statement is illustrated in Figure 3, a typical Duhring diagram in which the boiling points for seve r d degrees of supersaturation are plotted against the corresponding boiling points of water as a standard. The line which passes through the origin a t an angle of 45' to the ooordinate axes is the DUhring line for pure water-i. e., 0.0 supersaturation. Consider the line representing a supersaturation of 1.0. One point on this line is K , the point a t which the supersaturation line of 1.0 intersects the 45' Diihring line for water. The second point by which the line is determined is established by using the boiling point of water, T,, as the ordinate and the boiling point of the sucrose solution, T,,as the abscissa. Since the line is straight, its direction is definitely established by the two points, K , an&the intersection of T , and T.. The slope of this constant supersaturation line is the tangent 6f the angle 8 which is equivalent to (T,- K)/
(T.- K ) .
If the supersaturation increases and the absolute pressure remains unchanged, the temperature of the boiling water will remain the same as before-namely, T,. However, because of its increased concentration of solids, the boiling point of the massecuite will have risen to some new value which may be represented by Ti. The location of the constant supersaturation line corresponding to these conditions is likewise established by two points-namely, the focal point K and the point T,T: (boiling point of water as ordinate and boiling point of sugar liquor as abscissa under conditions of absolute pressure and purity as specified). This new line, which will be assumed to represent a supersaturation of 1.4, has a slope equal to tangent $'which, in turn, is equivalent to (T, - K ) /
(Ti- K ) .
As a third example, consider the situation which arises if the massecuite is boiling a t a temperature of T., as in the first example, but under a somewhat higher absolute pressure. Obviously such a change represents a reduction in supersaturation because of the lower boiling point elevation and correspondingly lower sucrose concentration of the mother liquor. In this case the temperature of water boiling at the higher absolute pressure will have increased to some value such as Ti,and in this general example the inhrsection of TL and T, may indicate a supersaturation of 0.6, for instance. Not until the liquor has been concentrated to some higher value, such as that represented by Ti,will a supersaturation coeffi-
SLOPE
OF CONSTANT
SUPER-SATURATION LINES
As TANGENT 0
FIGURE4. SLOPESOF CONSTANTSUPERSATURATION LINES FOR SUGARSOLUTIONS OF VARIOUS PURITIES
cient of 1.0 be restored. Similarly, if boiling of this liquor is continued under the increased absolute pressure represented by TL,a supersaturation of 1.4 will not be reached until the sugar liquor has been concentrated sufficiently to raise its boiling point to some still higher value represented by T:. All supersaturation lines do not intersect a t exactly the same point although the intersections for any particular purity come so close together that no serious error is introduced by making such an assumption. Furthermore, since constant K appeais as a unit in both the numerator and denominator, slight deviations in its magnitude have practically no effect on the resulting quotient or tangent 8 value. For all practical purposes K may be assumed to have a value of -6.0" C. for pure suerose solutions within the usual industrial working range of supersaturlttion. Nevertheless, in order to determine the slopes of constant supersaturation lines more accurately, the exact K value a t which individual supersaturation lines intersect the 45" Diihring line for pure water was determined graphically and checked algebraically. Inspection of data obtained by such means shows that the deviations in K may be expressed as a function of the supersaturation and purity of the sugar liquor being boiled. Consequently it has been possible to introduce simple compensation factors (referred to later) which resolve even slight deviations to a common intersection point, K ; the general relation and the supersaturation coefficients determined from it are thereby simplified and their accuracy is increased. Without going into a more detailed discussion of all developments involved in this subject, the most significant conclusions may be briefly summarized as follows : 1. Constant superaaturation lines are suhstantially linear for all supersaturations within the range OF absolute pressures encountered in sugar boiling practice. This holds true not only for solutions of pure sucrose but for im ure sugar solutions as well. 2. Constant supersaturation ines converge almost to a common point of intersection, and deviations that do exist are of such a nature that they can be compensated for easily. 3. Any degree of sucrose supersaturation may be represented as a line whose slope or tan 0 value serves as an index of the degree of supersaturation.
The summarized results of this work are represented by Figure 4 which shows the slopes of supersaturation lines of various purities, expressed as tan 8.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1238
Automatic Measurement of Supersaturation The previous discussion has shown how supersaturation is equivalent to a ratio of the differences between the boiling point of water and a reference temperature and the boiling point of the sugar solution and a reference temperature when both are boiling a t the same absolute pressure. The means for securing a measure of the coefficient of supersaturation from the above developed principle wilt appear in the following discussion.
FIGURE5. BASICCIRCUITFOR MEASUREMENT OF SUPERSATURATION
yo. 34, N ~ io .
in the respective Wheatstone bridges) in a simple potentiometer circuit, their ratio, which is indicative of the supersaturation coefficient, can be read directly on slide-mire X a s a measurable value. The influence of variations in the intersection point or constant K is automaticaly compensated for by the inclusion of auxiliary slide-wires SIand S B in each Wheatstone bridge circuit. Furthermore, compensation for the effect of purity is obtained by providing an adjustable high-resistance shunt Sa across the resistance thermometer immersed in the boiling sugar solution. TTith a 100" purity sugar liquor, the full amount of resistance S3is in circuit; and because of its extremely high value, it has only a negligible effect on the resistance of the arm containing resistance thermometer T,. As the purity of the sugar liquor decreases, the boiling point representative of any particular coefficient of supersaturation correspondingly increases. To compensate for this, the amount of resistance in shunt Sa is correspondingly reduced so that the effective resistance of the P'D' arm of the Wheatstone bridge (which includes T , and & in parallel) is reduced to the value which T , would have if immersed in 100" purity sugar boiling a t the prevailing absolute pressure. Thus the supersaturation values for liquors of lower purity are automatically realigned with the original calibrations for 100" purity sugar liquors. A different and more precise method of compensating for variations in purity may be accomplished by connecting an adjustable resistance Sq in series with the measuring slide-wire SI. Essentially this alters the effective resistance of the measuring slide-wire so that it reads proper supersaturations corresponding to the lower purities. Either of these methods of purity compensation may b~ made manually or automatically.
I n using the simplified formula (Tw - K)/(T,- K) = t a n 6 as a basis for development of an instrument €or supersaturation measurement, i t is first necessary to establish a measurable value representative of ( T , - K ) and a second measurable value representative of (T. - K ) . Once these values have been established, they may be opposed in a ratio measuring device, such as a potentiometer; thus a direct measure of tan 8, nThich can be expressed in its equivalent of supersaturation, will be obtained. The manner in which this is accomplished is illustrated by Figure 5, a diagram of the basic circuit employed for measuring supersaturation. I n establishing the quantity indicative of the numerator ( T , - K ) in the above equation, the first requirement is to measure the boiling point of water under the prevailing absolute pressure. The temperature-measuring element T , (Figure 5) is a resistance thermometer, placed in a pilot mater-boiling pan which is connected by a vapor pipe to the vacuum pan. If this resistance is included in one arm of a Wheatstone bridge circuit, W , the other resistances, R,, Rt, and Rs,can be so proportioned with respect to one another by means of Wenner's formula (18) that the potential difference between points E and D is at all times proportional to ( T , - K ) . A potential difference representative of the desired function will thereby result. Likewise, a similar resistance thermometer mounted in the vacuum pan and connected in a second Wheatstone bridge circuit, M , and similarly proportioned with resistances R;,R;, and Rj will provide a quantity representative of the denominator
(T.- K ) .
With the establishment of these two quantities, i t was found that, by opposing the potential differences (developed
FIGURE 6.
ARRANOEMENT FOR ILECORDING SUPERS.4TURATION
By incorporation of the above described circuits into a standard recording instrument, a direct measure of the coefficient of supersaturation is obtained on a suitably calibrated slide-wire. The following example illustrates how this may be accomplished: Assume that a 100' purity sugar liquor is boiling at 79.8" C. under an absolute pressure equivalent to 23.3 cm. of mercury. Under these conditions the resistance thermometer will assume a resistance corresponding to 70' C., the temperature at which water in the pilot pan will boil
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1942
1239
I I FIGURE 7
TYPICAL CHART FROM SUPERSATURATION RECORDER
I
I
I
I
!
1
1
L
I
I
I
I
I
I
I
I
I
I
I
1
1
1
I
I
I
1
I
I
I
I
0
saturation is possible by utilizing any one of a number of standard recording instruments that will measure the potential differences established. The manner in which such an instrument is applied in sugar boiling is shown in Figure 6. Several supersaturation recorders based on these principles have been in successful operation for years. At the outset it was felt that the Doint a t which the vacuum I I pan thermometer would have to be installed 1 1 in order to prevent lap: might be critical. I I However, experience has sh&n that, proI I vided the thermometer is installed where I l it is subjected to vigorous circulation and is not overly influenced by hydrostatic head, it can be depended upon to provide instantaneous readings of supersaturation with a high degree of accuracy. The only difficulty encountered was due to breakage of the first resistance thermometers because of their fragility. However, this was overcome easily by construction of more rugged thermometers without sacrifice in sensitivity and by the use of protective shields. Since the installation of these instruments, many comparative checks have been made between actual supersaturations calculated mathematically and those measured by the recorder. The good agreement found indicates the accuracy and reliability of the instruments. The readings have consistently checked well within 0.1" and usually within 0.05" with supersaturation values calculated from temperat u r e a n d a b so 1u t e pressure readings; Table I1 is typical of a large number of such checks. As a simplified means of easily checking the operation of these supersaturation recorders, a slide rule has been devised for calculating supersaturation from the boiling temperature, absolute pressure, and purity of the material, using the same principles on which the recording instruments are based. A typical chart from one of these recorders is shown in Figure 7. I n actual practice these instruments function as continuous supersaturation recorders and have proved to be a convenient and reliable means for applying supersaturation coefficients to the control of sugar boiling operations, particularly during the grain formation period of a strike.
under an absolute pressure equivalent to 23.3 cm. of mercury. The potential setup in this Wheatstone bridge is thus proportional to ( T , - K ) . As T , is equal to 70.0" C., and the reference temperature K under prevailing conditions may be assumed to be -6.0" C., the expression ( T , - K ) becomes the algebraic difference of 70.0" - (-6.0') or 76.0" C. I n a similar manner the potential established in the second m'heatstone bridge, in which the sugar-boiling thermometer is located, will be directly proportional to (T,- K ) or 79.8" - (-e.()"), or 85.8" C., since T,is equal to 79.8" C. and K may be assumed to be -6.0" C. It is apparent, then, that the ratio (Tu- K ) / ( T , - K ) = 76.0/85.8 is equivalent to a tan 0 of approximately 0.885 which, from Figure 4, is indicative of a supersaturation of 1.26 in this case. This result may be checked by reference to available tables and OF CALCULATED SUPERSATURATION VALUESAND TABLE 11. COMPARISON other basic data. Calculation of supersaturaSUPERSATURATION DETERMINED WITH RECORDER DURING ACTUAL tion by Herzfeld's solubility data and the BOILING OF SUGAR sucrose concentration corresponding to a boilObsvd. Obsvd. B. P. B. P. So%s Sucroseb % Suoroseb Calcd. ing point elevation of 9.8" C. (Figure 1) gives U. P. Abs. of Eleva- in Super- per Unit Solids per Unit Super- Recorder the same result. ofoSoln., Pressure. Water, tion satd. Water, in Satd. Water, satn.. SuperC. Cm. Hg C. C: 9oln.o SA So1n.C S SdS satn. By similar data for low-purity cane sugar 3.29 76.8 3.31 6.8 76.7 0.99 0.93 19.9 66.4 72.8 solutions, which for brevity have been omitted, 3.51 3.35 7.3 77.8 1.05 77.0 1 .oo 19.7 66.2 73.5 3.30 1.26 4.15 76.7 it is possible to use these same principles to 1.25 64.0 8.7 80.6 72.7 17.9 1.32 1.32 4.44 77.1 3.36 9.3 81 .6 18.4 64.7 74.0 measure the coefficient of supersaturation by 3.36 1.34 1.35 4.55 77.0 9.6 82.0 18.3 64.5 74.1 77.3 1.43 4.85 3.40 1.40 10.2 82.9 18.4 64.7 '74.9 determination of the corresponding tan 0 value 78.1 1.40 1.41 5.oa 3.56 83.4 68.1 10.6 78.7 21.4 3.65 1.41 1.41 for the entire range from pure aqueous solutions 5.17 78.5 11.0 83.8 69.8 23.1 80.8 1.42 1.45 5.36 78.7 3.70 11.4 84.3 70.1 23.4 81.5 of sucrose down to molasses purity. 78.8 3.72 1.45 84.6 1.48 5.50 11.7 70.2 23.5 81.9
Applications of Supersaturation Equipment The practical application of the abovedescribed basic principle of determining super-
81.7 79.8
23.5 22.6
70.2 69.3
11.5 10.5
84.5 83.2
5.45 4.95
78.7 78.3
3.70 3.61
1.47 1.37
1.43 1.34
3
1240
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited (1) Claassen, Z . deut. Zuclcerind. Ver.. 29, 1161 (1904); 39, 807 (1914); A&. Suikerind., 23, 303 (1915). (2) Claassen, Z . deut. Zuckerind. Ver., 41,809, 825 (1916). (3) Curin, Oe, Ihid., 19, 756 (1894). (4) Duhring, “Neue Grundgesitze der rationeller Physik und Chemie”, Leipsig, 1878. ( 5 ) Holven, ISD. ENG. CHEM., 28, 452 (1936). (6) Holven, U. S. Patents 3,135,511-12 (1939), 2,263,847 (1941); Cuban Patent 11,617 (1940). (7) International Critical Tables, Vol. IT, p. 344, New York, McGram-Hill Book Co., 1927. ( 8 ) Ibid., Vol. 111,pp. 211-12. (9) Ibid., Vol. 111,p. 328. (10) Kahlenberg, J . P h ~ s Chem., . 5, 339 ( l g o l ) .
Voi. 34, No. 10
(11) Kukharenko, “Vistnik Cukrovoi Promislorosti”, Kiev, 1920; Intern. Sugar J . , 29, 649 (1927). (12) Langen, German Patent 210,543 (1909). (13) Monrad, IND. ENQ.CHEhf,, 21, 139 (1929). (14) Prinsen Geerligs, “Cane Sugar and I t s Manufacture”, p. 6,London, Norman Rodger, 1909. (15) Thieme, Facts About Sugar, 22, 1156, 1208-12 (1927); “Studiea in Sugar Boiling”, tr. by 0. &I. Willcox, New York, Facts About Sugar, 1928. (16) Tressler, Zimmerman, and Willits, J . phys. Chem., 45, 1242 (1941). (17) Webre, IND. Eria. CHEM., 27, 1157 (1935). (18) Wenner, Bur. Standards, Sci. Paper 531 (1926). (19) White, I N D . ENG. CHEhf., 22, 230 (1930). PRESENTED before the Division of Sugar Chemistry and Technology a t the 10.7rd Meeting of the AMERICAN CHEMICAL SOCIETY,Memphis. Tenn.
Densities of Liquefied Petroleum Gases TECHNICAL COMMITTEE, NATURAL GASOLINE ASSOCIATION OF AMERICA Kennedy Building, Tulsa, Okla.
HIS work was d e signed to provide newly determined liquid densities over a relatively wide temperature range for propane, p r o p y l e n e , isobutane, n-butane, 1-butene, and n-pentane which would be useful in the establishment of more accurate densities for them. A literature survey of the liquid densities of these liydrocarf ons had d i s c l o s e d that many density data mere for single temper‘‘Lt ures or for short temperature ranges. I n addition, few citations were found for t e m p e r a t u r es above 68” F. With the exception of n-pentane, the density values were in rather poor agreement. Only one literature reference was a v a i l a b l e indicating the degree to which hydrocarbon mixtures follow the loss of perfect solutions. It WRS decided, therefore, to include in the work the measurement of densities of two different two-component mixtures for a short temperature range.
T
F IC I RE 1. METALPYCNONETER USEDIN EXPERIMENTS
Hydrocarbons Investigated Fractionation of natural gasoline yielded a number of concentrates; each contained approximately 95 per cent of the desired paraffins. Each concentrate mas then fractionated in a column having 20 feet of S/8-inch stone Raschig rings as packing. A cut was taken from the middle of the temperature plateau of each concentrate for the density measurements. Each cut was examined for purity by a special precision weathering test: a weathering range of less than0.5” F. between the 20 and 70 per cent vaporized points was found for all cuts, which indicated a purity of a t least 99.5 per cent. The purity of the propane, however, was 99.8 per cent. Propylene was obtained by the fractionation of vapors from a cracking furnace. A heart cut from the propylene temperature plateau was taken for the density measurements. 1-Butene was prepared by the dehydration of n-butyl alcohol, followed by fractionation of the products of dehydration. Fractionation of both of these olefins was carried out in the column used in preparing the paraffins. The olefins were tested by [ow-temperature fractional analysis and Orsat analysis, and a purity of at least 90.5 per cent was found. Two mixtures were made by blending the pure components; the mole per cent compositions were determined by lowtemperature fractional analysis: hlixt. 1 Mixt. 2
Propane Isobutane n-Butane n-Pentane
Mole % ’ 52.6
Mole %
47.4
62.4 47.6
Equipment and Procedure For temperatures above 0” F. a metal pycnometer was used for density mcavurement (Figure 1). It consisted of a steel cell, A , of approximately 4500 ml. capacity, to the top of which was attached a smaller steel cell, B, of approximately 400 ml. capacity. The two cells were connected by valve C. A smaller valve, D,was connected into the unattached end of the smaller cell. The empty weight of the complete
