Photoelectric Device for Measuring the Specific Surface Area of Powdered Materials
Abstract Generated abstract
A photoelectronic device is described for measuring the specific surface area of nonporous powdered materials in the range of 160 to 3500 cm²/g. The instrument determines the time required for a fixed volume of air to pass through a powder layer of constant thickness under standardized pouring conditions, using photoresistors, a micromanometer, and an automatically driven scale to register the pressure drop interval. Tests on pulverized-coal ash samples examined the relation between scale rotation, particle size distribution, and specific surface area, with calibration against a T-3 surface-area meter. For the reported ash measurements, discrepancies from the T-3 results did not exceed about 4 to 5%, while the measurement time per sample was reduced to 25 to 30 seconds.
Full Text
PHOTOELECTRONIC DEVICE FOR MEASURING THE SPECIFIC SURFACE AREA OF POWDERED MATERIALS
K. N. POPOV
(Presented by the Department of the Physicotechnical Faculty)
We have developed a photoelectronic device intended for measuring the specific surface area of powdered materials that have no internal porosity, whose specific surface area lies within the range of \(160\text{–}3500\ \mathrm{cm}^2/\mathrm{g}\).
Fig. 1. Schematic diagram of the device: 1 — photoresistances; 2 — measuring bridge; 3 — electronic amplifier; 4 — SD motor; 5 — scale; 6 — sample under investigation; 7 — collector; 8, 10 — electromagnetic valves; 9 — micromanometer; 11 — cam press; 12 — SD-54 motor for sample delivery; 13 — sample-feed mechanism; 14 — shutter; 15 — knife; 16 — vessel.
The principle of its operation is based, as in V. V. Tovarov’s surface-area meter [1], on measuring the time required for a definite volume of air to pass through a layer of powder. The time is measured automatically with the aid of a photoelectronic circuit.
The schematic diagram of the device is shown in Fig. 1.
An important unit of the device is the sample-feeding mechanism, which consists of two hoppers inserted one into the other, a conveyor, and a discharge hopper. A shutter 14 is mounted in the bottom of the receiving hopper. In all measurements it is turned through a constant angle. The receiving hopper is installed at a specified height above the conveyor belt.
The sample under investigation is poured into the receiving hopper and then, when the shutter is turned, is discharged onto the belt.
This hopper design ensures identical pouring conditions for all samples.
With the aid of knife 15, installed above the conveyor, the thickness of the samples under investigation is made constant, equal to \(h\). Tubes are brought from collector 7 beneath the level of the sample; the measured volume of air passes through them.
At constant layer thickness and identical pouring conditions for the samples, the resistance of the layer will depend on the particle-size composition. The coarser the fractional composition of the sample, the lower its hydraulic resistance and the greater the rate of pressure drop in vessel 16.
The pressure in the vessel is measured by a micromanometer, which before the start of measurement is set so that the column of displaced liquid blocks the light fluxes falling from the illuminating lamps onto the photoresistors \(\Phi C_1\) and \(\Phi C_2\) (photoresistors of type \(\Phi C\)-K2)—[2].
Pressing the “measurement” button opens the air discharge through the sample. As soon as the liquid column passes \(\Phi C_2\), the measuring bridge goes out of balance. The voltage from the measuring diagonal of the bridge is amplified by an electronic amplifier and fed to a relay that switches on the SD motor. When the liquid column passes \(\Phi C_1\), the bridge is balanced—the motor stops rotating. As a result, the angle through which the scale turns depends on the time taken by the liquid column to traverse the section between \(\Phi C_1\) and \(\Phi C_2\).
The measurement is carried out in the following order: the sample under investigation is fed from the hopper by the feed mechanism by switching on the SD-54 motor; by turning the press handle, pressure is created in the vessel; with the “Zero setting” button the scale is set to zero; then, by pressing the “measurement” button, the measurement is made.
The measured time obeys the relation
\[ t=\frac{T\cdot \alpha}{360}=0.02055\alpha, \tag{1} \]
where \(T = 7.4\) sec is the time in which the scale turns through \(360^\circ\) (the instrument constant);
\(\alpha\) is the angle of rotation of the scale when measuring the sample, in degrees.
The investigations were carried out on ash samples taken from the sampling devices of pulverized-coal boilers at a thermal power station. The moisture content in the samples was not more than 0.2% by weight.
The ash was sieved on sieves with mesh sizes of 60, 88, 120, and 200 μ. The experimentally obtained dependence of the scale rotation angle on the mean particle diameter of the samples, for three tests, is shown in Fig. 2.
\(\alpha_{xx}\) is the angle of rotation of the scale in the absence of a sample on the belt, i.e., when collecting air through the tubes into the atmosphere, in degrees.
In measurements on samples of a broader class with respect to granulometric composition, the following results were obtained—Table 1.
Table 1
| Sample No. | Sieve-opening size, µm | Fraction yield, % | \(x_{\text{grad}}\) |
|---|---|---|---|
| \(12/2\) | \(<60\) | 27 | 67 |
| \(12/2\) | 60 | 16.7 | 67 |
| \(12/2\) | 88 | 15.2 | 67 |
| \(12/2\) | 120 | 23.0 | 67 |
| \(12/2\) | 200 | 17 | 67 |
| \(11/2\) | \(<60\) | 36 | 98 |
| \(11/2\) | 60 | 20 | 98 |
| \(11/2\) | 88 | 15 | 98 |
| \(11/2\) | 120 | 18 | 98 |
| \(11/2\) | 200 | 10 | 98 |
| \(6/2\) | \(<60\) | 46 | 108 |
| \(6/2\) | 60 | 19 | 108 |
| \(6/2\) | 88 | 16.7 | 108 |
| \(6/2\) | 120 | 13.7 | 108 |
| \(6/2\) | 200 | 4.5 | 108 |
| \(9/2\) | \(<60\) | 54 | 113 |
| \(9/2\) | 60 | 15.5 | 113 |
| \(9/2\) | 88 | 13.4 | 113 |
| \(9/2\) | 120 | 12.5 | 113 |
| \(9/2\) | 200 | 4.7 | 113 |
Table 2
| Sample No. | \(\gamma\), g/cm³ | \(x_{\text{grad}}\) | \(t\), sec | Specific surface area \(S\), cm²/g: device readings | Specific surface area \(S\), cm²/g: T-3 readings |
|---|---|---|---|---|---|
| \(12/6\) | 2.34 | 85 | 1.75 | 2410 | 2460 |
| \(12/6\) | 2.34 | 83 | 1.71 | 2385 | 2405 |
| \(11/6\) | 2.31 | 106 | 2.18 | 2685 | 2565 |
| \(11/6\) | 2.31 | 105 | 2.16 | 2675 | 2610 |
| \(10/6\) | 2.36 | 133 | 2.74 | 3010 | 3020 |
| \(10/6\) | 2.36 | 129 | 2.65 | 2960 | 3100 |
We also carried out an investigation of the dependence of the readings on the specific surface area \(S_{\text{sp}}\) of the samples. The specific surface area was determined on a T-3, Giprocement, surface-area meter [3].
The specific gravity \(\gamma\) was determined by the pycnometer method [4].
In measurements on samples of a broader class with respect to granulometric composition, the following results were obtained—Table 1.
Table 1
| Sample No. | Sieve-aperture size, µm | Fraction yield, % | $x_{\mathrm{grad}}$ |
|---|---|---|---|
| 12/2 | <60 | 27 | 67 |
| 12/2 | 60 | 16.7 | 67 |
| 12/2 | 88 | 15.2 | 67 |
| 12/2 | 120 | 23.0 | 67 |
| 12/2 | 200 | 17 | 67 |
| 11/2 | <60 | 36 | 98 |
| 11/2 | 60 | 20 | 98 |
| 11/2 | 88 | 15 | 98 |
| 11/2 | 120 | 18 | 98 |
| 11/2 | 200 | 10 | 98 |
| 6/2 | <60 | 46 | 108 |
| 6/2 | 60 | 19 | 108 |
| 6/2 | 88 | 16.7 | 108 |
| 6/2 | 120 | 13.7 | 108 |
| 6/2 | 200 | 4.5 | 108 |
| 9/2 | <60 | 54 | 113 |
| 9/2 | 60 | 15.5 | 113 |
| 9/2 | 88 | 13.4 | 113 |
| 9/2 | 120 | 12.5 | 113 |
| 9/2 | 200 | 4.7 | 113 |
Table 2
| Sample No. | $\gamma$, g/cm³ | $x_{\mathrm{grad}}$ | $t$, sec | Specific surface area $S$, cm²/g: instrument readings | Specific surface area $S$, cm²/g: T-3 readings |
|---|---|---|---|---|---|
| 12/6 | 2.34 | 85 | 1.75 | 2410 | 2460 |
| 12/6 | 2.34 | 83 | 1.71 | 2385 | 2405 |
| 11/6 | 2.31 | 106 | 2.18 | 2685 | 2565 |
| 11/6 | 2.31 | 105 | 2.16 | 2675 | 2610 |
| 10/6 | 2.36 | 133 | 2.74 | 3010 | 3020 |
| 10/6 | 2.36 | 129 | 2.65 | 2960 | 3100 |
We also investigated the dependence of the readings on the specific surface area $S_{\mathrm{sp}}$ of the samples. The specific surface area was determined on a Giprotsement T-3 surface-area meter [3].
The specific gravity $\gamma$ was determined by the pycnometer method [4].
In measurements on ash samples, the scale of the device was calibrated according to formula [2]
\[ S = A \cdot \sqrt{t}, \tag{2} \]
where \(S\) is the specific surface area, \(\text{cm}^2/\text{g}\);
\(A\) is a device constant depending on the properties of the powder under study: specific gravity, particle shape, etc.;
\(t\) is the time of rotation of the scale, sec.
In the figure: \(\alpha\), degrees; \(\Phi\), microns; ●—experiment No. 1; ○—experiment No. 2; ◇—experiment No. 3; \(\alpha_{xx}\).
Fig. 2. Dependence of the angle of rotation of the scale on the mean particle diameter in the sample.
As a reference powder, one of the samples was taken with a specific surface area \(S_{\text{ref}} = 3020\ \text{cm}^2/\text{g}\), determined on a T-3 surface-area meter. The angle of rotation of the scale for the reference powder was \(\alpha_{\text{ref}} = 133^\circ\).
The time was determined by formula (1).
The device constant for ash was found to be \(A = 1822\).
The results of measurements on samples with different specific surface areas are presented in Table 2.
The discrepancy between parallel measurements on the device and on the T-3 surface-area meter does not exceed 4–5%. The duration of measurement for one sample is 25–30 sec, which is 10–15 times faster than on the T-3.
REFERENCES
- V. V. Tovarov, Zavodskaya laboratoriya, XIX, 5, 620, 1953.
- M. M. Grinshtein. Photoresistances in Industrial Automation Devices. GEI, Moscow, 1962.
- Instructions for Determining Specific Surface Area on the T-3 Surface-Area Meter, Giprocement.
- N. M. Pavlushin and G. G. Sentyurin. Laboratory Manual on Glass Technology. Promstroiizdat, 1957.