January, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
91
Viscosity of Beet House Sirups' A. N. Bennett and A. R. Nees REWARCH LABORATORY, THE GREATWRSTJ~RN SUGARCOMPANY, DENVER, COLO.
Scope of Present Investigation ISCOSITY plays an important part in practically all of the physical and chemical phenomena which take place The present investigation was undertaken for the purpose of in solutions. It is of importance in the processes of diffusion, crystallization, dissolution, and flow of solutions. Its determining the viscosities of pure and impure beet sugar effect is therefore felt in practically every step of the sugar solutions a t the higher concentrations, that is, of saturated manufacturing process. I n the battery it influences the dif- and supersaturated solutions. Two types of sirups were investigated at various purities, fusion of the sugar from the cossettes and the flow of the juice from cell to cell. It plays its part at the various filter sta- ranging from that of final molasses to pure sugar. Sirup of tions, and is a factor of prime importance in the operation of the first type was produced in a non-Steffen beet sugar factory the pans, crystallizers, and centrifugals. As a parting shot and that of the second type was from a Steffen factory. The effect of pH on viscosity was also investigated, as well it controls the ease with which the final molasses can be as the effect of raffinose on pumped from the house. the viscosity of sucrose soluAn accurate knowledge of tions. the viscosity of the various The design and use of a falling sphere viscometer s i r u p s encountered in the for the determination of the absolute viscosity of pure Apparatus sugar industry is therefore and impure sugar solutions has been described. very desirable. P a r t i c u The viscosity over a wide range of temperature and A s t u d y of the various larly is this true in the case concentration has been determined for the following t y p e s of viscometers and of the supersaturated solutypes of solution: pure sucrose; pure sucrose and t h e i r adaptability to the tions from which the sugar raffinose; non-steffenized and steffenized sirups; problems in hand led to the is crystallized. and sirups resulting from the barium process for reconclusion that the falling covering sugar from molasses. sphere type would prove Historical The effect of pH on the viscosity of molasses has been most satisfactory. Many investigated. The determination of the investigators have used this viscosity of sugar solutions type of viscometer and their which are amreciablv suDerexperience has shown it to saturated and are a t high temperatures presents many diffi- be particularly suitable for investigating solutions of high culties. Practically no reliable data covering this range have viscosity. Ladenburg (12) used this type in determining the viscosity been published. Considerable work has been done on the viscosities of pure sugar solutions at concentrations up to 60 of turpentine under pressure. Sheppard (17) investigated the per cent. The very excellent results reported by Bingham viscosity of nitrocellulose solutions by this method, as did also and Jackson (1) cannot be applied to the crystallization proc- Gibson and Jacobs ( 7 ) . Fischer (6),Roubinck ( I @ , and Fischer, L. ( 5 ) also used this method for determining the ess, as the maximum concentration was 60 per cent. Kucharenko (IO,11) has made several attempts to deter- viscosity of various solutions. mine the viscosities of saturated and supersaturated sugar The falling sphere viscometer is quite simple in principle solutions in connection with his studies on the rates of crystal- and does not require an elaborate set-up to insure fairly aclization. In no case has he obtained results that can be relied curate results. The essential parts of the apparatus designed upon. He admits that the accuracy of his results are open to and fabricated in this laboratory are a glass viscometer tube question (11). Kucharenko used a t least three different and a suitably controlled constant temperature bath. methods for determining viscosity, but in every case there The tube and its support are shown in Figure 1. It is apwas something fundamentally wrong, either with the a p proximately 2.35 cm. in diameter and 65 cm. long and has a paratus or the method of calculating the results. stopcock on the lower end for the purpose of withdrawing t h e Orth (2, IS) did considerable work on the viscosities of pure balls after a determination has been made. The distance beand impure sugar solutions. He covered all purities from 60 tween the timing marks is 30 cm. A brass cap, properly to 100 and concentrations from 65 to 85 per cent and tempera- grooved, fits snugly over the upper end of the tube. Actures from 20" to 80" C. However, where they can be com- curately centered in this cap is a short piece of thick-walled pared, his results do not agree with those of Bingham and glass tube, 0.25 cm. in diameter and 7 cm. long. When t h e Jackson and are consequently of doubtful value. cap is in place this small tube extends about 4.5 cm. into t h e Powell (15) determined the viscosities of sucrose, levulose, viscometer tube and about 2.5 cm. below the surface of t h e and dextrose solutions. The maximum concentration investi- solution to be tested. It serves the dual purpose of freeing gated was 40 per cent. Consequently his results are of no the sphere from air bubbles and of insuring its falling through value in studying the process of crystallization. the center of the column of solution. Among the many others who have studied the viscosities of The water bath is shown in Figure 2, which gives the disugar solutions may be mentioned Green (8),Burkhardt ( 4 ) , mensions of the bath proper. It rests on three leveling screws and Hosking (9). Bingham and Jackson report the results of which, with the aid of a plummet attached to the side of the these investigators. Here again the work was all done a t rela- bath. make it possible to keep the bath and viscometer tube in tively low concentrations. a vertical position. In the bottom of the bath is a socket in which the end of the tube support rests. This support carries, Received September 16,1929. Presented before the Division of Sugar near the top, an arm with two small lugs which engage two. Chemistry at the 78th Meeting of the American Chemical Society, Minneapolis, Minn., September 9 t o 13, 1929. holes on opposite sides of the upper rim of the bath. By this.
V
92
INDUSTRIAL AND EhiGINEERING CHEMISTRY
arrangement the tube is held in a rigid, vertical position. The bath is provided with a motor-driven propeller, which insures proper circulation of the water, with an electrical heating unit, and with a special mercury thermometer for controlling the temperature. There are two glass windows on opposite sides of the bath for observing the fall of the sphere through the solution under investigation. The bath holds two viscometer tubes, so that two solutions can be brought to the desired temperature at the same time, Preparation and Analysis of Solutions
T h e preparation and analysis of the sirups present more difficulties than the actual determination of t h e v i s c o s i t y . It is necessary to have the solutions as nearly optically clear a s possible. T h e bwt results are obtained b y mixing t h e sirup a t about 60 per cent dry substance with a considerable quantity of finely-divided paper pulp. The mixture is then filtered by vacuum through a Buchner funnel. The clear filtrate is adjusted to a pH of about 7.5, to prevent ;inversion of sugar, and then evapoFigure 1-Viscometer Tube and Support rated under vacuum to the A,:cap and guide tube: B. clamps; desired concentration. If C, bracket with lug proper care is taken one batch of sirup will suffice for determining the viscosity over the entire concentration range to be examined. The following concentrations were found to be satisfactory for giving smooth viscosity curves, 72, 75, 78, 80, 82, 83, 84, and 85 per cent dry substance. It is extremely important to know the actual dry substance. This is by no means a simple determination, particularly in sirups of low purity, and it requires the greatest care. The dry substance method of Brown and Sharp (3) was used throughout this investigation. The determination of the true sugar was made by means of the double enzyme method as given by Paine and Balch (14). The density was determined by means of a Walker specific gravity bottle for very viscous liquids. The pycnometer was suspended in the bath that controlled the temperature of the viscosity tube, so that these two determinations were always made a t the same time and temperature. Details of Operation
Although the operation of the falling sphere viscometer is simple, several details of manipulation must be observed t o insure good results. The tube is filled with the sugar solution at room temperature to within about 3.5 cm. of the top and is covered with the cap and guide tube, the tube extending about 1 cm. below the surface of the solution. By moving the cap to one side, the solution is overlaid with oil. This prevents evaporation and the formation of crystals at the surface. Castor oil was found to be very satisfactory for this purpose, but any heavy mineral or vegetable oil will do. As no oil gets into the guide tube, the ball comes in contact only with the solution to be tested. The upper end of the guide tube is closed with a small piece of rubber tubing except when the determination is actually being made. The tube is heated to the highest temperature at which the
Vol. 22, No. 1
viscosity is to be determined and kept a t this point until the solution is free from air bubbles and has attained a uniform temperature. An hour is sufficient to attain a uniform temperature but at the higher concentrations it sometimes requires 2 or 3 hours to free the solution from air bubbles, When the solution is in the proper condition a ball is introduced into the guide tube and is timed as it falls between the two marks on the viscosity tube. Aluminum balls are used for determining the lower viscosities and steel balls for higher viscosities. The steel balls are ordinary 1/16-inchball bearings. The aluminum balls were made to order and are of approximately the same diameter. There is considerable variation in the size of the aluminum balls. Consequently, in making a determination, 10 balls were timed and the average time was taken in calculating the viscosity. As the steel balls were quite uniform, only 5 balls were timed for any one test. When the viscosity is high it is possible to have as many as 5 balls in the tube at the same time. When this is done care should be taken that there is a t least 5 cm. of solution between adjacent balls. This can be regulated by starting the balls down the guide tube a t proper time intervals. It is usually necessary to push the ball beneath the surface of the solution in the guide tube. The weight of the ball is not sufficient to break through the surface film. When the determination has been completed at the one temperature the bath is lowered to the next desired temperature by running in cold water and the procedure is repeated. By allowing 1 hour for the solution to become uniform, 5 or 6 temperatures can be investigated in 8 hours. The temperatures used in our work were 80", 65", 50", 40°,and 30" C. When working with dark solutions it was necessary to use an electric light placed behind the bath in order to see the balls, It wm possible to see through the darkest of best molasses by using a 6-volt stereopticon lamp.
Front View
S i d e VICW
r1': i
rE . e €
&-loom
Figure 2-Constant-Temperature Bath
m.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January, 1930
Calibration of Viscometer
I n accordance with Stokes' law the velocity of a sphere falling through a liquid is given by the equation
v = 2-9 gr2 so -n where V is the velocity, r the radius, S the density of the sphere, and u the density and n the viscosity of the liquid. This equation applies only when the liquid is infinite in extent in all directions. When the liquid is contained in a limited space, the walls and end of the container have an effect on the fall of the sphere. Ladenburg (12) discussed these effects mathematically and modified Stokes' equation as follows :
where R is the radius of the tube, h is the total height of the liquid, (1+ 2.4 is the correction for the wall effect, and
i)
( 1 + 3.3 i ) the correction for the end effect.
'
The equa-
tion for viscosity thus becomes 2prV.T
n =
- o)t
9L (1 +O2.4;)
(1
+ 3.3 ;)
(3)
93
the density was calculated for every 10 degrees from 0" to 100" c. The tube constant when using aluminum balls was found to be 0.0384 + 0.0001 and for the steel balls 0.0397 * 0.0001. I n standardizing the tube the same set of balls was used for all tests. I n making the subsequent determinations on the various solutions each set of balls was weighed and the time of fall corrected for the variation in weight from that of the standard set. If we assume that all the balls have the same density, it can be calculated from Ladenburg's equation that a variation of 1 per cent in weight will cause a variation of 0.62 per cent in the time of fall. This correction was applied to all determinations and was justified, particularly when using the aluminum balls. Different sets of these balls varied as much as 2.5 per cent in weight. With the steel balls the maximum variation was less than 0.5 per cent and the correction applied is of less importance. Viscosity of Pure Sugar Solutions
It is difficult to obtain the viscosity of pure sucrose solutions that are appreciably supersaturated, particularly a t the higher temperatures, owing to the fact that they crystallize readily. At the lower temperatures a supersaturation of 10 per cent can be maintained for some time without the formation of crystals.
where t is the time of fall over the height L. This equation has been shown to hold experimentally by Gibson and Jacobs Lr (S (7)' provided is 0.08 or less and nt u, is small. When spheres of equal radius are used in tubes of the same dimensions a simple equation
is obtained. Thus the viscometer can be standardized by means of a liquid of known viscosity and a tube constant obtained. Then the equation for viscosity becomes 1z
= K(S
- o)t
(5)
where K is the tube constant and is equal to 2er2
Vl5c05lty
Figure 3-Viscosity
in Equation 3. As r, R, and L vary with the temperature, K will also vary and the variation can be calculated from the foregoing expressions. The variation is so small, 0.1 per cent for 40"! that it can be neglected. Castor oil was used for standardizing the viscometer. The viscosity of the oil as determined by the Bureau of Standards is as follows: TEMPERATURE
c.
5 10 15 20
VISCOSITY Poises 38.96 23.90 15.12 9.86
TEMPERATURE
c.
O
25 30 35 40
VISCOSITY Poises 6.69 4.61 3.25 2.34
The density of the castor oil over this same temperature range is as follows: TEMPERATURE
c.
5 10 15 20
DENSITY 0.971 0.967 0.963 0.960
TEMPERATURE
c.
25 30 35 40
DENSITY 0.956 0.952 0.949 0.945
The density of the balls a t 25' C. was 2.741 for the aluminum balls and 7.768 for the steel balls. From these figures
-
POlScS
of Pure Sugar Solutions
The sugar used was the highest commercial grade obtainable and had a purity of 99.95. It was not further purified. The concentrations investigated varied from 66.6 to 76.2 per cent and the temperatures from 25" to 70" C. The results were plotted, using the logarithms of the viscosity. Table I was compiled from the curves. Table I-Viscosity SUGAR IN SOLUTION 25' C. Percent Poises 1.05 1.27 1.56 1.97 2.49 3.21 4.20 5.60 7.52 10.3 14.2 20.0
30' C. Poises 0.78 0.93 1.12 1.39 1.74 2.21 2.85 3.74 4.97 6.70 9.08 12.6
of Pure Sugar Solutions VISCOSITY AT: 40' C. 50° C. 6 0 ° C. Poises Poises Poises
..
0 : 64
0.77 0.94 1.15 1.43 1.80 2.31 3.01 3.95 5.30
.. .. ... .
0:65 0.79 0.97 1.20 1.51 1.93 2.49
.. .... 0: 58 0.70 0.86 1.06 1.33
7 0 ° C.
Poises
.. .. .. .. .. .... ....
0:64 0.78
From these results the viscosity of sugar solutions saturated a t the various temperatures were determined as shown in Table 11. Typical curves showing the relations between concentration and viscosity and temperature and viscosity of pure sugar solutions are shown in Figures 3 and 4.
INDUSTRIAL AND ENGINEERING CHEMISTRY
94
The results given in Table I can be compared with those obtained by Green (8) a t only one point, namely, 65 per cent and 25' C. Our result is 1.05 poises, as compared to Green's 1.04. This is considered a very good agreement. Table 11-Viscosity SUGAR IN
SOLUTION^ 25' C. Per cent
Poises
30' C. Poises
of Saturated Sugar Solutions VISCOSITYAT: 40' C. 50" C. Poises Poises
60' C. Poises
70° C. Poises
According t o Hertzfeld's solubility data.
Viscosity of Sugar Solutions Containing Raffinose
Raffinose is present in all sirups from which beet sugar is manufactured. There are practically no reliable data on the viscosity of raffinose or of sucrose-raffiose solutions. As it required too much of the sugar to work with pure raffinose solutions, sucrose solutions containing different amounts of raffinose were examined. The three solutions used contained 66.62 per cent of total sugars and 3.44, 6.89, and 13.76 per cent of raffiose. Determinations of the viscosity were made a t 25" and 30" C. and these viscosities were compared with those of pure sucrose solutions. The raffinose used was made in this laboratory from beet molasses and contained as its only impurity about 1 per cent sucrose. The results obtained are given in Table 111. Table 111-Viscosity
Viscosity of Beet House Sirups
Sirups were made by adding the proper amount of sugar to molasses to produce a sirup of the desired purity. The first series was made from a non-steffenized molasses. It was found impossible to obtain a clear solution of this molasses by simple filtration. It was therefore treated with Norit at the rate of 5 per cent on dry substance. This produced satisfactory sirups. Previous work in this laboratory has shown that this carbon treatment would lower the viscosity of 60 purity molasses about 10per cent. This effect decreases as the purity of the sirup increases, carbon having no effect on the viscosity of pure sugar solutions. The lowering of the viscosity is due to the adsorption of colloids by the carbon. The carbon-treated molasses had a true purity of 61.14 and contained 2.64 per cent raffinose on dry substance.
of Sucrose-Raffinose Solutions
(Total d r y substance, 66.62 per cent) 1.0
VISCOSITY AT:
SOLUTION Pure sucrose 3 . 4 4 per cent raffinose 6 . 8 9 per cent raffinose 13.76 per cent raffinose
Vol. 22, No. 1
25' C. Poises 1.440 1.503 1.577 1.698
30' C. Poises 1,040 1.083 1.134 1.222
When these viscosities are plotted using as ordinates the sucrose percentage on total dry substance (the sucrose purity of the solution), straight lines are obtained as shown in Figure 5.
/.I
1.2
1.3
A6
A6
h7
POISOS
The viscosities of sirups of 74.9 and 88.2 purity were also determined. The temperature range covered was from 40" to 80" C. and the concentration range was from 74 to 86 per cent dry substance. When the viscosities for any given dry substance are plotted with sirup purities as ordinates straight lines result. The data in Table IV were obtained in this manner.
74%h
PURITYD. S. Per cent Poises 60 70 80 90 100
60 70 80 90 100
of Carbon-Treated Non-Steffen Sirups
;q. gag,
VISCOSITY
23.
Poises
Poises Poises A t 40' C. 16.9 4.35 8.06 2.50 18.3 4.56 8.62 2.61 9.19 19.8 4.80 2.73 9.77 21.2 5.03 2.84 2.95 5.26 10.3 22.6 At 50' C. 3.76 7.26 1.31 2.10 4.00 7.77 1.35 2.21 4.24 8.28 2.31 1.39 8.79 2 . 4 2 4 . 4 8 1.44 9.29 2.52 4.70 1.48 At. 60° C. 3.59 1.21 2.00 0.77 3.76 1.25 2.07 0.79 3.92 1.29 2.14 0.81 4.07 1.32 2.21 0.83 0.85 1.35 2.28 4.23 At 70' C. 0.74 1.15 1.99 0.75 1.19 2.05 0.76 1.23 2.10 0.77 1.27 2.15 0.78 1.31 2.20 A t SOo C. 0.49 0.73 1.15 0.497 0.745 1.18 0.505 0.76 1.20 0.51 0.775 1.23 0.52 0.78 1.25 D. S. stands for dry substance. ~~
From the data given in Tables I and I11 it can be shown t h a t the first sucrose-raffinose solution has the same viscosity as a 66.83 per cent sucrose solution, that the second has the same as a 67.06 per cent sucrose solution, and that the third has the same as a 67.40 per cent sucrose solution. From these data it is calculated that 1 per cent raffiose has the same effect on the viscosity of a solution as does 1.06 per cent sucrose. This is true under the conditions of this experiment, but it may not hold at higher temperatures and concentrations.
6
Figure 5-Viscosity of Sucroee-Ra5nose Solutions (Total Sugars in Solution 66.62 Per Cent)
Table IV-Viscosity
V i s c o s i t y - Poises Figure 4-Viacosity of Pure Sugar Solutions
/.C
1.7
I/lSCOSlfy
60 70 80 90 100 60 70 SO
90 100 60 70 80 90 100 0
Poises 39.6 43.4 47.1 50.8 54.5 16.7 17.0 18.3 19.5 20.8
Poises 112 124 136 148 160 38.8 42.0 45.2 48.4 51.6
86 % D. S . Poises 400
120
~
7.19 7.66 7.92 8.29 8.66
16.3 17.1 17.8 18.5 19.2
3.61 3.74 3.87 4.01 4.14
7.66 7.82 8.07 8.33 8.58
1.99 2.05 2.10 2.16 2.22
3.83 3.96 4.0s 4.21 4.34
43.1 46.1 49.2
18.1 18.8 19.4
8.44
8.83
9.22
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1930
The data for 100 purity (pure sugar) were obtained by extrapolation. Where these results can be compared with those actually determined on pure sugar solutions (Table I) they show fair agreement. It is felt that the probable error of these extrapolated results is not greater than 3 per cent up to and including a concentration of 78 per cent. Above this concentration no claim is made for accuracy, but these results are included in the table as a matter of record. Table V gives the viscosities of a 75 purity sirup saturated at various temperatures. The concentrations of the saturated sirups were determined by Brown and Sharp ( 3 ) . Table V-Viscosity of 75 Purity Sirup Saturated a t Various Temperatures DRY
SUBSTANCE Per cent
TEMPERATURE O
c.
40 50 60 70
VISCOSITY Poises 4.41 3.73 3.83 4.47 5.50
75.80 77.68 80,OO 82,46 84.85
80
The viscosity passes through a decided minimum at about 55' C. This is characteristic for the viscosity curves of sirups of any purity. However, the minimum is less pronounced as the purity increases and, further, the temperature corresponding to the minimum viscosity increases with the purity. Table V I gives the densities of the sirups whose viscosities are shown in Table IV. Table VI-Densities PURITY Per cent
74% D. S."
60 70 80 90 100
1.3908 1.3846 1.3779 1.3701 1.3612
60 70 80 90 100
1.3845 1.3783 1.3719 1.3644 1.3554
60 70 90 100
1.3785 1.3723 1.3659 1.3583 1.3494
60 70 80 90 100
1.3725 1.3662 1.3602 1.3520 1.3429
60 70 80 90 100
1.3660 1.3598 1.3534 1.3459 1.3364 D. S. stands for
SO
a
of Non-Steffen Sirups
DENSITY 78% D. S. 82% D. S.
At 40" C. 1,4188 1.4123 1,4052 1 3966 1.3878 At 50' C. 1,4127 1.4061 1,3990 1.3909 1.3817 At 60' C. 1.4067 1,3400 1,3930 1,3848 1.3757 At 70' C. 1,4006 1.3939 1.3872 1,3785 1,3692 At 80' C. 1,3942 1.3877 1.3806 1,3724 1.3627 d r y substance
86% D. S.
1,4469 1,4400 1.4321 1,4232 1,4139
1.4749 1.4677 1.4592 1.4497 1,4402
1,4409 1.4339 1.4264 1,4174 1.4080
1.4691 1.4617 1.4533 1.4439 1.4343
1.4349 1.4277 1,4201 1.4113 1,4019
1.4631 1,4554 1,4472 1,4378 1,4282
1,4287 1.4216 1,4142 1.4051 1.3954
1.4568 1,4493 1.4412 1,4316 1.4217
1,4224 1.4156 1.4079 1,3989 1.3889
1.4506 1.4434 1.4351 1,4254 1.4152
Steffenized Sirups
The sirups of the second series were made from a steffenized molasses. It was not necessary to treat these sirups with carbon, as filtration with paper pulp produced satisfactory solutions. The same concentration range was covered, but the temperature range was extended to include 30" C. Owing to lack of time, sirup of this type was not as thoroughly investigated as that of the non-Steffen type. Only two purities were run. The results obtained are given in Table VII. Difficulty was experienced in determining the dry substance of these sirups, particularly a t the higher concentrations. Sirups of this type contain an appreciable amount of matter that decomposes readily on drying. There is, consequently, some doubt concerning the accuracy of the viscosity results, but they are given as a matter of record.
95
Table VII-Viscosity
Per cent 57 77 57 77 57 77 57 77 57 77 57 77 a
Poises
of Steffen Sirups
Poises
Poises At 30' C. 21.3 10.5 6.31 12.1 25.4 At 40' C. 4.81 8.88 5.22 9.89 2.95 At 50' C. 4.08 2.45 1.54 2.57 4.60 At 60' C. 1.36 2.13 0.87 1.38 2.29 At 70' C. 1.23 0.82 1.32 0.56 0.83 At 80' C. 0.76 0.53 0.79 0.39 0.54 D. S. stands for d r y substance.
Poises
E.3.
Poises
48.8 59.3
Poises
137 161
18.6 21.4
495 513
45.8 52.4
7.82 8.97
141 148
17.6 19.5
3.83 4.26
7.90 8.47
2.08 2.25
3.99 4.10
1.21 1.27
2.15 2.20
High-Raffinose Sirups
These sirups are similar to those produced in the barium process for recovering sugar from beet molasses. They are relatively high in raffinose, the final molasses containing as much as 20 per cent on dry substance. A typical analysis follows: sucrose, 47.42; raffinose, 14.08; dry substance, 72.55; true purity, 65.36; raffinose, 19.41 per cent on dry substance. It is practically impossible to determine the true dry substance of these sirups by drying. Some of the impurities decompose readily, causing a continued and appreciable loss in weight. I n view of this fact, the dry substance was determined by refractometer. The figure thus obtained is somewhat higher than the true dry substance, the spread between the two increasing as the purity decreases. As the exact relation between refractometer and true dry substance is not known, it is impossible to correlate the viscosities of sirups of this type with those of the Steffen and non-Steffen sirups. The viscosities were determined at three different purities, 65, 80, and 90. The results obtained are given in Table VIII. Table VIII-Viscosity
of High-RafEnose Sirups VISCOSITY
PURITY74% D. S.a Per cent Poises
76% D. S. 78% D. S. Poises Poises A t 40° C. ~~. 6.34 12.3 5.86 11.2 5.53 10.7 At 50' C. 3.01 5.50 2.74 5.01 2.60 4.76 At 60' C. 1.58 2.71 1.46 2.50 1.37 2.38 At 70' C. 0.92 1.49 1.36 0.84 0.80 1.30 At 80' C. 0.58 0.89 0.54 0.82 0.52 0.78 for dry substance. ~
65 80 90
3.57 3.30 3.11
65 80 90
1.79 1.62 1.52
65 80 90
1.01 0.91 0.86
65 80 90
0.62 0.56 0.54
65
80 90 a
D. S. stands
80% D. S. Poises
82% D. S. Poises
26.5 24.0 22.5
62.2 56.0 52.5
10.8 9.95 9.33
22.1 20.7 19.5
~
5.01 4.68 4.45
9.73 9.12 8.75
2.64 2.40 2.28
4.92 4.52 4.27
1.52 1.38 1.28
2.81 2.52 2.37
Effect of pH on Viscosity
Molasses is so highly buffered that it is impossible to change its pH without materially changing the nature of the dry substance and consequently the purity of the solution. To lower the pH from 9.0 to 7.2 it requires about 0.35 per cent hydrochloric acid on dry substance. It thus becomes apparent that the determination of the effect of pH, per se, on the viscosity of molasses presents marked difficulty.
INDUSTRIAL AND ENGINEERING CHEMISTRY
96
A non-Steffen molasses was treated with sufficient barium hydrate to raise the pH from about 7.0 to 9.0. Besides adjusting the pH to the desired value, this treatment removed some of the sulfates present, thus preventing their crystallization on concentration. The viscosity of this molasses was determined a t 50" and 60" C. and over a concentration range from 76 to 80 per cent. The pH was then reduced to 7.2 by means of hydrochloric acid in one portion and sulfur dioxide in another. The viscosity of these two samples was determined over the same range. The results are given in Table IX. Table IX-Effect
of pH on Viscosity of Molasses VISCOSITY
PH 9.0
7 , 2 with HC1 7 . 2 with SOP
76% D . S.O Poises 2.04 2.07 2.08
1.14 9.0 7 . 2 with HCl 1.15 7 . 2 with Son 1.18 D. S. stands for dry substance.
78% D . S. Poises At 50° C. 3.50 3.53 3.55 At 60' C. 1.85 1.89 1.90
80% D . S.
Poises 6.34 6.41 6.50 3.15 3.21 3.27
It is seen that there is an apparent slight increase in the viscosity as the pH is lowered. The effect is small and is practically within the limit of error of the determination. It might well be accounted for by small errors in the determination of the dry substance. Discussion of Results
The viscosity of sirups made from a non-steffenized molasses is lower than the viscosity of a pure sugar solution of the same dry substance. This difference increases as the concentration increases and as the temperature decreases. This lowering in viscosity is due to the inorganic impurities present and is partially counteracted by the raffiose, which has a greater effect on viscosity than does sucrose.
Vol. 22, No. 1
Sirups made from a steffenized molasses, as a rule, are higher in raffinose and lower in ash than are non-Steffen sirups. Consequently their viscosity is somewhat higher, but nevertheless it is lower than that of a pure sugar solution of the same concentration and temperature. Molasses resulting from the barium process for recovering sugar is extremely high in raffiose and correspondingly low in ash. Its viscosity is appreciably higher than that of either of the other types and is also higher than that of a corresponding pure sugar solution. I n general, it can be said that the viscosity increases as the raffinose increases and as the inorganic constituents decrease. The viscosity of saturated solutions at a given temperature increase as the purity of the sirup decreases (Tables I1 and V). This is because the solubility of sugar increases rapidly with decreasing purity. The viscosity of saturated solutions shows a decided minimum at some definite temperature, depending upon the purity of the sirup. Solutions of pure sugar show the minimum a t about 70" C., 75 purity sirups at 55" C., and 60 purity sirups at 45" C. Literature Cited (1) Bingham and Jackson, Bur. Standards, Sci. Paper %98. (2) Brown, "Handbook of Sugar Analysis," p. 310. (3) Brown, Sharp, and Nees, IND. ENG. CHEM.,20, 945 (1928). (4) Burkhardt, Z . Rubenzuckerind., 1874. (5) Fischer, 2. angew. Chem., 84, 153 (1921). (6) Fischer, Chem.-Ztg., 44, 622 (1920). (7) Gibson and Jacobs, J. Chem. Soc., 117, 473 (1920). (8) Green, Ibid., 98, 2023 (1908). (9) Hosking, Phil. Mag., 49, 274 (1900). (10) Kucharenko, Sucr. Belge, 46, 222 (1927); 47, 244 (1928). (11) Kucharenko, Planter Sugar M f r . , May, June, July, 1928. (12) Ladenburg, A n n . Physik, 28, 9 (1907). (13) Orth, Bull. assocn. chim. SUCY. dist., 29, 137 (1912). (14) Paine and Balch, IND. ENG. CHEM.,17, 240 (1926). (15) Powell, J. Chem. Soc., 105, 1 (1914). (16) Roubinck, Z . Zuckerind. Bbhmen, 38, 578 (1914). (17) Sheppard, J. IND. END. CHEM.,9, 523 (1917).
Thermophilic Digestion of Sewage Solids'*' I-Preliminary Paper Willem Rudolfs and H. Heukelekian NEWJERSEYAGRICULTURAL EXPERIMENT STATION, NEW BRUNSWICK, N. J.
HE role and importance of temperature in the digestion of sewage solids has been repeatedly emphasized during recent years. Attention has been called to the advantages of maintaining the temperature of digestion tanks a t approximately 20" C. during the winter months. The optimum temperature for digestion has been found t o be nearer 26-28' C., and higher temperatures up t o 37" C. have not shown further acceleration of the digestion. The bacteria known as thermophilic organisms have an optimum temperature range of 50" to 60" C. The group contains a variety of organisms some of which may grow a t both 37" and 55" C. (facultative thermophiles), while others only a t 55O.C. (obligate thermophiles). The bacteria that have an optimum growth range between 20' and 37' C. will ordinarily not be active a t 50-60" C. but they are not necessarily killed.
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Presented before the Division of Water, 1 Received May 20, 1929. Sewage, and Sanitation a t the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to May 3, 1929. *Journal Series Paper of the New Jersey Agricultural Experiment Station, Department of Sewage Disposal.
The effect of high temperatures on the digestion of sewage solids has not been fully investigated. As early as 1875 Popoff (2) measured the rate of gas production from canal mud with temperatures as high as 50-55' C. He found that at the thermophilic range 38-55' C. the rate of gas production was faster than a t 16-22' C. When material incubated a t 50-55" C. was later digested a t 20-25" C., the evolution of gas stopped for 4 days. Coolhaas (1) studied the decomposition of salts of fatty acids and carbohydrates by thermophilic bacteria. At 60' C. a great number of salts of fatty acids were converted to methane and carbon dioxide when inoculated with canal mud. The minimum temperature for the thermophilic decomposition was found to be 45' C. and the maximum 69" C. The organisms were spore-forming types and were not the same as those active a t lower temperatures. Cabbage leaves inoculated with feces and incubated a t 60" C. gave more methane than a t 26" C. Out of 20 grams of dry cabbage leaves, 5.3 liters of methane were produced in 20 days. I n view of these results, it was considered of interest to
