RICHARD C. CHANG, HERBERT M. SCHOEN, and C. S. GROVE, JR. Syracuse University, Syracuse, N. Y.
Bubble Size and Bubble Size Determination Quick Freezing of Foam
b Stabilizes fresh 1 Holds bubble
foam as it i s generated size
and
bubble
size distribution
unchanged
h
Provides data on relation between dispersity and other physical properties
A L T H O U G H a large amount of work has been done on foams in general, investigations of dispersity of foams and the relationships between dispersity of foam and other physical properties are rather scarce. Distribution of bubble sizes in a very stable froth was studied by Sibree (4, who used a photomicrographic technique. Clark and Blackman (7), using a similar technique to study foams, found that the larger bubbles grow and the smaller bubbles shrink as a function of time. During their study of foam structure Clark and Blackman ( 2 ) observed a relationship between foam dispersity and the opacity of the dispersed system, caused by the scattering of incident light by multiple reflections and refractions. Later they found that the loss of light on transmission through a layer of foam can be expressed as a function of the degree of dispersion of the air. They made photomicrographs of foam at various degrees of dispersion under static conditions at atmospheric pressure and then calibrated light transmission data with these photomicrographs as standards. They claimed that this method can be used to measure “specific surface” of foam. Specific surface was defined as the total surface area in square centi-
meters of the gas-liquid interface in 1 ml. of foam. However, when Clark and Blackman’s light transmission method is applied to a foam system, which changes during standing, errors may be introduced because time is required to take the photomicrographs and the light transmission readings. Stenuf (6,7) modified Clark and Blackman’s technique and studied foam dis-
p.-I
71
0.4w.
A-
BBM5bF&rpR
7
r5ci.W.
/.oCM
I / Figure 1.
Freezing apparatus
persity under flow conditions. He compared the light transmission to the expansion ratio and to actual photomicrographs of the flowing foam. Another method of obviating the instability of foam, used by Sovitskaya ( 5 ) , was the “quick-freezing” technique with liquid oxygen. Photomicrographs were taken after the foam was frozen, and the bubble sizes were determined from them. She found that freezing and thawing did not affect the dispersity and that the frequency distribution of the bubbles was nearly the same in the surface layer as in any other section of the foam mass. Experimental Apparatus and Techniques
Freezing Apparatus. Because foams are usually unstable, bubble sizes and bubble size distribution change on standing. I n order to study these properties, fresh foam was stabilized by quick freezing as it was generated. The rate of freezing is important, because bubble size and bubble size distribution may change while the foam is being frozen. Several freezing chamber designs were tested, the final design, shown in Figure 1, being a modification of Sovitskaya’s apparatus. The freezing chamber, A , is a 1.9 X 1.0 X 0.4 cm. brass sheet. B is a VOL. 48, NO. 11
NOVEMBER 1956
2035
of the chamber to prevent evaporation and condensation of moisture during the freezing process. The freezing chamber is constructed with only two sides, instead of four, to allow any excess foam to flow out when the chamber is covered. The time required for complete freezing is 15 to 30 seconds (average of 16 observations). Photomicrographic Apparatus and The photomicrographs Techniques. were taken with a photomicrographic camera at magnifications of 20 and 40 diameters. Foam in the freezing chamber was placed directly under the camera, as shown in Figure 2, and a picture was taken within a few seconds to represent the unfrozen foam. Another picture of the same foam was taken about 1 minute later, when complete freezing was assured. The bubble sizes were computed from the negatives with the aid of a comparator. Averages were calculated for a number of plates before and after freezing. Statistical analysis was applied to evaluate the difference between averages of the tw-o groups. To photograph the bubble distribution within the foam mass, the frozen foam was sliced with a razor blade at several different depths. Expansion Ratio and Foam Dispersity. A Blackmer sliding vane pump was employed in studying the relationship between expansion ratio and foam
Figure 2. Photomicrographic camera and freezing apparatus
solid brass bar, attached to the bottom of A , which serves as the cooling conductor. The freezing apparatus is precooled by dipping the brass bar in liquid oxygen; then the freshly generated foam is delivered into the freezing chamber. A small glass plate is quickly placed on top
I
I
/
dispersity. A flowsheet showing this method used for generating foam of variable expansion ratio is presented in Figure 3. Foam liquid (6y0 Mearlfoam), preheated to a desired temperature, was pumped from the solution tank by pump A at a constant rate of 11.4 gallons per minute, as measured by a rotameter. iVhen the foam liquid was delivered into a tee, air was also sucked into the tube by the adjustable speed of the Blackmer sliding vane pump, B, and thus foam was generated. ,4s the speed of pump B was varied, the amount of feed air differed accordingly. Thus, foams of different expansion ratios were obtained. The foam was then passed through a column packed with Berl saddles to a height of 40 inches. The packed column served as a homogenizer. Foam samples obtained were frozen immediately, and a photomicrograph of each sample was taken. This study was also carried out using a centrifugal pump instead of the Blackmer sliding vane pump, to generate the foam. A flowsheet of this generating equipment is shown in Figure 4. Foam liquid was pumped from the solution tank by means of the two centrifugal pumps. The flow rate of the solution was measured by means of a rotameter. The liquid feed rate was controlled by means of the valve located between the pump and rotameter. The foam solution and air were mixed in a tee and the foam formed was passed through one of the two packed columns. Both columns were constructed of l1/z-inch standard iron pipe packed with Berl saddles. One column was 9 inches high (homogenizer I), the other 36 inches high (homogenizer 11). The foam generated was delivered to a reservoir tank. Two pressure gages were used to measure the pressure drop across the column, AP = PI - Pz,in Figures 3 and 4. Foam formed during each test was photomicrographed and the developed plates \vere enlarged twice on paper prints. Bubble sizes and bubble size distribution \vere evaluated from these positive prints. Foam Agents Used
1
PE*d?///vg.
PUMP
Figure 3.
2036
4
8
PZ€D
PUMP
Flowsheet of Blackmer vane pump foam-generating system
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ultravon '\I' Heptadecyl benzinimidazol compound (Ciba Co.) Saponin AK 500 Plant glucosides (A. K. Peters Go.) Unox Phf-1000 Mixture of sodium heptadecyl sulfate. butyl Carbitol, monoethanolamine, morpholine, and sodium nitrate (Union Carbide and Carbon) Sodium lauryl sulfate (Raymond Labs.) Mearlfoam Protein hydrolyzate (Mearl Corp.)
AQUEOUS FOAMS
Figure 5. Outer surface of drained foam of 3% sodium laurylsulfate( X 4 0 )
&PA.
Figure 4.
Flowhseet of centrifugal pump foam-generating system
Results and Discussion
rather than bv any real difference is roughly about 94 oui of 100. Results obtained with Ultravon W a t other concentrations, and with 4% Mearlfoam, are summarized in Table 11. One pair showed identical average bubble size, while the other three pairs varied slightly. Again, the differences were not statistically significant. Bubble Size Distribution in Outer Surface and in Foam Mass. In this series, after a photomicrograph of the outer surface of each sample was taken, two inner surfaces were obtained by cutting with a razor blade. Bubble sizes and bubble size distribution were measured on these pictures (Table 111). Photomicrographs of outer and inner
Effect of Freezing on Bubble Size and Bubble Size Distribution. Table I lists the bubble size groups of five samples of foam generated with 0.5% Ultravon W, before and after freezing. When the plates taken before freezing were compared with those taken after freezing, three out of five gave identical values for the average bubble sizes. The other two samples showed slight variations in bubble size during freezing, but the differences were too small to be statistically significant. The t value (3) of the difference between the two groups was calculated to be 0.064, indicating that the probability of producing such a difference in these samples by chance
Table 1. Effect of Freezing [Material. 0.5% Ultravon W concentration (by weight)] 26-
26
76
Figure 6. Cut surface of foam of Figure 5 (X40)
Grouped Bubble S i z e , Microns 76125176- 225- 275125 176 126 276 516
. Av. Diarn ,
516576
576-
426
L’Na
P
11 11 3 2 5 5 7 7
2 2
274 274 273 269 368 368 244 244 181 165
92.5 92.5 87 88 73.5 73.5 102.5 102.5 107.5 104.0
Figure 7. of Figure
Second cut surface of foam
5 (X40)
Figure 8. Outer surface of freshly prepared foam of 0.3% Saponin AK500 ( X 4 0 )
N o . of Bubbles Beforeb 210 85 25 After 100 85 25 Before 148 34 16 After 139 31 21 Before 232 32 23 After 232 32 23 Before 67 99 14 After 67 99 14 Before 84 31 4 After 67 40 2 a Total number of bubbles. Before and after freezing.
12 12 20 25 25 25 6 6 9
7
14 14 21 20 26 26 15 15 17 16
13 13 17 18 20 20 19 19 16 23
12 12 14 13 4 4 16 16 15 6
,. .. 1
2
1 1 1 3
3
1
Figure 9. Cut surface of foam of Figure 8 (X40) VOL. 48, NO. 11
NOVEMBER 1956
2037
Table II.
Effect of Freezing on Difference Surfactants Grouped Bubble S i z e , Microns
Material
25-76
76-1?25
126-176
175-2$26
226-275
49 44 54 78 11 10
34 39 49 91 6
37 33 47 62
19 21 24 32
20 21 9
5
5
10
4
6
3 6
iVo. of Bubbles
Beforea 0 . 1 % Ultravon W After 0.1% Ultravon W Before 4% Mearlfoam After 4% Mearlfoam Before 1% Ultravon W After 1% Ultravon W a Before and after freezing.
Table 111.
5
276-526
325-376
376-426
425-475
6
5 2
3 2
Av.
t
ZN D i a m . , p
Value
189 185 183 268 40 47
0.2 0.2 1.04 1.04 0.07 0.07
16 16
7
.. ..
.. ..2
2 3
3
..
..
..
. e
4
2
a
4
161.0 159.0 119.0 112.0 189.0 1187.0
Bubble Size Distribution o f Outer Surface and of Inner Surface after Cut Grouped Bubble S i z e , Microns
Surface
25-50
60-100
100-160
160-200
200-260
250-300
300-350
560--400 400-450
460-500
BN
Av.
Diam.,
p
No. of Bubble8 Outer Inner
.. .. .. .. I
Outer Inner Outer Inner Outer Inner Outer Inner
.
.. ..
12
..
1 4 4
2
2
7
1
5 3
17 7
5 4 2 8 11
18 26 26 16
39 45 34 30
..1 2
..
1
..
1
2 4 12
7 18 50 27 29
3% Sodium 6 6 2 2 3 5 6 2
Lauryl Sulfate 9 8 6
6 2 2
5 4 5
5 5
1
1
6 2 3
5 5 4
5 ”.
5 9
.. ..
..
*.
.. ..
.. ....
*.
e .
e .
1
..
....
..
..
..
0.37, Saponin AI( 500 9 * * 1 6 1
3 3 1 3 1 3
..
m .
..
44
284 283.8 309.8 295.8 300.2 286
29 19 37 29 31 56 28
106.4
86 129 96 88
140.8 139.2 127.2 138.8
115
surfaces of two samples are presented in Figures 5 to 9 as examples. These samples were statistically analyzed for significance. The t values obtained indicated that the deviations of average bubble size were not significant. Relationship between Expansion Ratio and Foam Dispersity. The relationship between the expansion ratio and bubble size distribution, as studied with the Blackmer sliding vane pump using 670 Mearlfoam, is presented in Table IV. Results obtained with the centrifugal pump, using 670 Unox PM-1000 and Mearlfoam solutions a t several concentrations, are summarized in Tables V and VI, respectively. In Table IV, the average bubble sizes obtained with 6% Mearlfoam were found to be inversely proportional to the expansion ratios. This fact is in agreement with Stenuf’s (7) investigation
made using the light transmission method. The change of average bubble size was less noticeable when the expansion ratio increased from 9.7 to 11.0 as compared to the increase from 7.6 to 9.7. This seems to indicate that there is a maximum limit in the ability of a given foam system to reduce bubble sizes. Photomicrographs of the resulting foam produced by the three different expansion ratios are presented in Figures 10 to 12. Using the centrifugal pump, B, Figure 4, the 670 Unox PM-1000 solutions showed results similar to the 670 Mearlfoam solution (using the Blackmer sliding vane pump) when the pressure range was held between 10 and 20 pounds per square inch gage. Within this pressure range the foam was produced in a steady stream, but “slug flow” was observed
when the pressure was increased to 29 pounds per square inch gage. When the 4.5 and 3y0Unox PM--1000 solutions were studied, slug flow was obtained only within the pressure range of 11 to 21 pounds per square inch gage. With this slug-flow type of foam, the relationship between expansion ratio and foam dispersity, as $hewn in Table V, was very different from that obtained with steady flow. Contrary to the results obtained with 670 Unox PiM-1000 solution, a high inlet air pressure resulted in a low expansion ratio, which gave smaller average bubble sizes with Unox PM-1000 at lower concentrations. This apparent anomaly can be explained as follows: When the foam is generated in a steady flow condition, most of the air forced into the pump is “trapped” by the foam solution. When a foam solution of lower concentration is used, the
Figure 10. Foam made with Mearlfoam solution ( X 2 0 )
Figure 11. Foam made with Mearlfoam solution ( X 2 0 ) Expansion ratio 9.7
Figure 12. Foam made with Mearlfoam solution (X20) Expansion ratio 1 1 .O
Expansion ratio 7.6
2038
6%
INDUSTRIAL AND ENGINEERING CHEMISTRY
6%
6%
AQUEOUS FOAMS ~~~
~~~
Table VI.
Average Bubble Size, Expansion Ratio, and Pressure Drop Material. Mearlfoam Packing column height. 9 inches Packing column diameter. Il/pinch std. pipe Packing material. Berl saddle Solution rate. 1.5 gallons per minute
4.6% Concn.
6% Concn.
Photo No. Expansion ratio Av. diameter, fi Inlet air pressure, P Pressure drop ( A P ) , lb./ sq. inch Temp., O C.
3% Concn.
1.5% Concn.
187.00 14.2 167.00 10.00
192.00 35.5 88.00 28.00
193.00 10.35 231.00 11.00
194.00 15.00 215.00 15.00
195.00 21.7 196.00 21.00
197.00 32.00 56.8 37.00
202.00 11.75 275.00 12.00
198.00 23.2 105.00 12.00
199.00 42.0 90.4 37.00
204.00 12.0 280.00 12.00
205.00 17.0 154.5 16.00
207.00 32.0 90.6 28.00
2.00 28.00
12.00 30.00
4.00 28.00
4.00 28.00
6.00 28.5
21.00 28.5
4.00 28.00
7.00 27.6
11.00 28.00
4.00 29.5
5.00 30.00
9.00 30.00
solution can no longer trap all of the air; hence slug flow is obtained. The higher the inlet air pressure, the more the air escapes trapping by the foam solution, resulting in a lower expansion ratio. O n the other hand, a reduction of bubble sizes is caused by the mixing action of the air in the packed column. The higher the air pressure, the greater the mixing action; therefore, in spite of the low expansion ratio, a smaller average bubble size is produced. A steady-flow foam was again obtained with 3% Unox PM-1000 solution when the height of the homogenizing column was increased from 9 to 38 inches. This , was attributed to the greater resistance offered by the longer packing column, which made steady flow possible. Under this condition the results were similar to those obtained with 6% Mearlfoam solutions and Unox P-1 000 solutions at higher concentrations. These foams were produced a t expansions of 7.33, 12.7, and 18.8; here again smaller average bubble sizes were produced a t the higher expansion ratios.
Results of the tests with 1.5, 3.0, 4.5, and 6.0% Mearlfoam, using the centrifugal pump, showed a relationship between expansion ratio and foam dispersity similar to those obtained using the Blackmer sliding vane pump. The air pressure range used was between 1 0 and 37 pounds per square inch gage. A steady flow was produced in all cases; hence the results agree with those obtained with the 60/, Unox PM-1000 solutions. A decrease in average bubble size was, in all cases, accompanied by an increase in the pressure drop across the homogenizing column, whether the foam was in steady or slug flow. Presumably the smaller bubble sizes caused an increase in the apparent foam viscosity and hence an increase in the pressure drop. These results were in agreement with the findings of Stenuf (6). It is apparent from the above three series of tests that the bubble sizes and bubble size distribution were affected not only by the expansion ratio, nature of foaming agent, and concentration, but
also by the type of generating system, inlet air pressure, and height and/or nature of the packed column. Conclusions The quick-freezing of foam does not significantly change the structure of the foam with respect to bubble sizes and bubble size distribution. Bubble sizes and bubble size distribution of the outer surface of a foam mass and those within the foam mass are almost the same. The occasional small differences are not statistically significant. Expansion ratio, bubble sizes, and bubble size distribution are related. However, the type of generating system, nature of the foaming agent, concentration of the solution, inlet air pressure, and height and/or nature of the refining section are also important in determining the foam dispersity. The pressure drop across the homogenizer (packed column) is directly affected by the bubble sizes and bubble size distribution of the foam. Acknowledgment
Table
IV.
Relationship between Expansion Ratio and Bubble Size
(Material.
6% Mearlfoam.
Figure No.
APO
ab
85-75
10 11 12
11.9 15.5 18.0
7.6 9.7 11.0
46 108 136
a
Liquid rate, 11.4 gallons per minute)
Grouped Bubble Size, Microns 76-125 126-176 176-236 23 82 70
22 49 72
18 10 15
225-275
Av. Diam.,
12 5
I.(
120 95.5 94
e .
Pressure drop, across homogenizer lb./sq. inch. Expansion ratio.
Table V.
Photo No. 01
D av., p PI, lb./sq. inch
literature Cited (1) Clark, N. O., Blackman, M., Trans. Faraday SOC.44, 1 (1948). ( 2 ) Ibid., p. 7. ( 3 ) Davies, 0. L., “Statistical Methods in Research and Production,” p. 58,
Average Bubble Size, Expansion Ratio, and Pressure Drop (Material. Unox PM - 1000) 6% Concn. 215.0 8.38 47.6
gage 10.0 AP = Pi - Pa 3.5 Temp., O C. 26.0 Slugging flow.
217.0 9.45 31.85 20.0 8.0 26.0
Homogenizer I 4.6% Concn. 224a 9.67 46.9 21.0 9.0 26.0
223’ 10.45 67.3 15.0 6.0 26.0
The authors wish to thank the Engineer Research and Development Laboratories of the United States Army, the Naval Research Laboratories, and the Office of Naval Research, under whose sponsorship this research was carried out.
8% Concn. 22ga 9.0 34.0 21.0
9.0 24.8
227‘ 15.8 61.4
11.0 4.0 24.0
Homogenizer 11 3% Concn. 233.0 7.33 66.3
11.0 6.0 26.0
234.0 12.7 52.9 16.0 9.0 26.4
235.0 18.8 47.1 22.0 12.0 26.0
Imperial Chemical Industries, London, 1947. (4) Sibree, J. V., Trans. Faraday SOC. 30,325 (1 934).
( 5 ) Sovitskaya, E. M., Kolloid. Zhur. 13,
.,
309 (1951).
( 6 ) Stenuf, T. J.. unmblshed M.Ch.E. thesis, Syracuse Qniversity, 1951. ( 7 ) Stenuf, T. J., unpublished Ph.D. thesis, Syracuse University, 1953.
RE~EIVED for review November 25, 1955 ACCEPTEDJuly 3, 1956 VOL. 48, NO. 11
NOVEMBER 1956
2039
