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An Experimental Procedure And Apparatuses For Measurement Of Density Of Porous Particles
J. Rouqucrol, F. Rodriguez-Rcinoso, K.S.W. Sing and K.K. Unger (Eds.) Characterization of Porous Solids 111 Studies in Surface Scicnce and Catalysis, Vol. 87 0 1994 Elscvicr Scicncc B.V. All rights rcscrvcd.
339
AN EXPERIMENTAL PROCEDURE AND APPARATUSES FOR MEASUREMENT OF DENSITY OF POROUS PARTICLES B.Buczek, E.Vogt Faculty of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Cracow, Al. Mickiewicza 30, Poland
An apparatus known as Bulk Densimeter was used to measure the apparent density of coarse porous particles by powder densimetry method, and of fine particles by a comparative method. These techniques for the measurement of particle density are competitive for the conventional mercury densimetry approach. Both methods enable lll be measured. Their advantages are a the density of porous particles less than 1 ~llto result of the simplicity of the determinations, which are based on bulk density measurements. The measurements made a standard conditions show high accuracy and reproducibility of the results.
1. INTRODUCTION
The apparent density is one of the basic features of both adsorbents and catalysts. The knowledge of the apparent density together with the true density enables the calculation of the volume of the pores included in the particles of a solid. This is the simplest estimation of a solid’s porous structure based on densimetric measurements. The increasing interest in new techniques of density determination is the result of limitations in standard methods and the need to determine these characteristics more accurately. In this work, two apparatuses were used to measure the apparent density of coarse adsorbents by the powder densimetry method and of tine particles by a comparative method. One of the apparatuses, known as a Powder Characteristic Tester, was made by Hokosawa Micromeritics Laboratory; the other, a Bulk Densimeter, was made by the Faculty of Energochemistry of Coal and Physicochemistry of Sorbents of University of Mining and Metallurgy. The use of these apparatuses led to a high reproducibility of the results and a standarization of the conditions of measurement.
340
Figure 1 . Powder Characteristic Tester 1 - main unit, 2 - dispersibility measuring unit, 3 - amplitude, 4 - vibrating plate, 5 - spatula assembly, 6 - pan base, 7 - tap holder, 8 - rheostat, 9 - timer. 2
,
/---
1
----IT-
Figure 2. Bulk Densimeter 3 1 - standard 100 cm cup, 2 - extension piece, 3 - pan, 4 - tapper, 5 - timer, 6 - starter.
341
2. EXPERIMENTAL AND RESULTS For the purpose of measuring the apparent densities, the Powder Characteristic Tester accessory, originally designed to measure bulk density, was employed. The bulk density is the mass of particles making up a bed divided by the volume of the bed. In order to perform a bulk density measurement, a calibration cup is filled with excess material, by means of an extension piece attached to increase its volume. The entire set is then placed in an automatic tapper which is adjusted to vibrata with a frequency of 1 Hz for 180 s. A constant level is maintained by adding more material as the previous material becomes more densely packed. After the measurement is over, the excess material from the above calibration cup is removed by means of a ruler. The bulk density is calculated from the known material mass contained in the known cup capacity.
2.1. Determination of apparent density by powder densimetry (1,2) The density was measured using a 100 cm3 standard cup as a powder pycnometer. As the pycnometric medium powders, which according to Geldart's classification belong to easily (dry goods) granular substances ones and hence do not indicate cohesive properties, have been used. The grain diameter of the pycnometric medium powders should be much smaller than porous size of particles, but larger than the largest porosity. In order to find the density values of porous silica gel, active carbon and a molecular sieve, the pycnometcr was filled with the said materials, their amounts corresponding to 30% of the pycnometer capacity. The remaining portion was packed with an appropriate powder pycnometric fluid so as to be sure of the best dispersion of the adsorbent particles within the cup capacity. Subsequent procedures were similar to the bulk density measurement method. Knowing the mass (m ) and bulk density (p,) of the pycnometric powder, as well P as the adsorbent mass inside the pycnometer (ma), its apparent density (p ) was "P calculated from:
where Vn is the pycnometer capacity. The apparent density values thus obtained for adsorbents and non-porous glass beads are listed in Table 1 . Every determination was repeated several times and good repeatability was achieved (k0.02 * 1O'3 kg/rn3). For reference purposes, the apparent density values of the same adsorbents as found by the mercury displacement method and by water pycnometry for glass beads are also included.
342
Table 1 Apparent density of coarse adsorbents by the powder densimetry method Adsorbent (diameter range) Silica gel (2.5-6.0 mm) Active carbon (2.0 mm) Molecular sieve (0.8-1.2 mm) Glass beads (0.4-0.6 mm)
Density ( kg/m3 * bronze steel
) using
zinc
mercury
1.12
1.13
1.16
1.17
0.63
0.63
0.64
0.64
0.92
1.01
1.08
1.10
2.85
water
2.90
2.2. Determination of apparent density by the comparative method (3,4) The density of fine porous particles was measured by a method based on the assumption that the minimum packed bed voidage is the same for similarly shaped particles of a narrow size range. Density measurements by this method involve determining the bulk density of the porous particles to be tested (p ba )and the bulk density of the reference material (p br ) of a known apparent density (p ). Thus: aPr
Where k is a factor defining the correlation between the shapes of the porous particles and the reference material. The factor k is equal to one for spherical particles. For other shape particles’ k will determine experiment. The method was applied to determine apparent densities of narrow fractions of fine porous particles obtained via disintegration and sieving of coarse adsorbents. In addition, the density of a cracking catalyst was measured. The reference materials were the powders originally used as pycnometric fluids in powder densimetry experiments. The experimental results obtained by the comparative method are shown in Table 2. The apparent density values obtained by the above method was compared with the density values resulting from the mercury displacement method (Table 1) by calculating the differences in both determinations. Such comparison is reasonable for these adsorbents. Their structure contains no closed pores, which might be made available in the disintegration process and thus may alter the percentage of pores in the particle volume. For the cracking catalyst the difference was calculated using the supplier’s data.
343 Table 2. Apparent density of fine particles by the comparative method
Particles
(diameter range)
Density ( kg / m3 *
Silica gel: 0.04-0.08 lfl~ll Active carbon: 0.02-0.04 mm Molecular sieve: 0.2-0.4 mm
Cracking catalyst: 0.040 - 0.056 lfl~ll
0.056 - 0.071 mm 0.071 - 0.090 lfl~ll 0.090 - 0.125 ~lllll
Difference (%)
)
1.14 0.67 1.11
-2.56 4.69 0.91
using glass beads
using bronze beads
using glass beads
using bronze beads
1.55 I .56 1.50 1.51
1.59 1.58 1.55 1.54
1.30 2.00 -2.00 -1.30
3.90 3.30 1.30 0.60
3. CONCLUSION mercury method
L r~
20
comparative method
, 5
P
I
Dowder densimetrv method
I
I
I
50
100
200
I I
500
I
1000
____+
I
1
5000 dP [pml
Figure 3.Application range of various methods to measure the apparent density of porous particles. The mercury displacement method finds wide application for the apparent density determination of coarse particles. However, its applicability is limited to particle sizes over 0.8 mm. Attempts have been made to extend this range by conducting measurements at a pressure over the atinospheric value. However, in order to obtain reliable
344
results, such a method would require a porous particle structure which would be different from its intergranular porosity. The mercury method is also useless for particles of porous metals and catalysts which form amalgams with mercury. The results indicate that both the powder densimetry and the comparative method allow the density of porous particles to be determined which would otherwise be impossible to measure by the mercury method. Furthermore, by using very fine powders as pycnometric fluids, some disadvantages of mercury displacement are eliminated.
REFERENCES B. Buczek, D. Geldart, Powder Technol., 45 (1986) 173. B. Buczek, Chemia Analityczna, (in Polish), 32 (1987) 969. M.A. Hooker, D.H.T. Spencer, private commun., National Coal Board, (1979). 4. A.R. Abrahamsen, D. Geldart, Powder Technol., 26 (1980) 35. 5. B. Buczek, E. Vogt, Przemysl Chemiczny, (in Polish), submitted. 1. 2. 3.
339
AN EXPERIMENTAL PROCEDURE AND APPARATUSES FOR MEASUREMENT OF DENSITY OF POROUS PARTICLES B.Buczek, E.Vogt Faculty of Energochemistry of Coal and Physicochemistry of Sorbents, University of Mining and Metallurgy, 30-059 Cracow, Al. Mickiewicza 30, Poland
An apparatus known as Bulk Densimeter was used to measure the apparent density of coarse porous particles by powder densimetry method, and of fine particles by a comparative method. These techniques for the measurement of particle density are competitive for the conventional mercury densimetry approach. Both methods enable lll be measured. Their advantages are a the density of porous particles less than 1 ~llto result of the simplicity of the determinations, which are based on bulk density measurements. The measurements made a standard conditions show high accuracy and reproducibility of the results.
1. INTRODUCTION
The apparent density is one of the basic features of both adsorbents and catalysts. The knowledge of the apparent density together with the true density enables the calculation of the volume of the pores included in the particles of a solid. This is the simplest estimation of a solid’s porous structure based on densimetric measurements. The increasing interest in new techniques of density determination is the result of limitations in standard methods and the need to determine these characteristics more accurately. In this work, two apparatuses were used to measure the apparent density of coarse adsorbents by the powder densimetry method and of tine particles by a comparative method. One of the apparatuses, known as a Powder Characteristic Tester, was made by Hokosawa Micromeritics Laboratory; the other, a Bulk Densimeter, was made by the Faculty of Energochemistry of Coal and Physicochemistry of Sorbents of University of Mining and Metallurgy. The use of these apparatuses led to a high reproducibility of the results and a standarization of the conditions of measurement.
340
Figure 1 . Powder Characteristic Tester 1 - main unit, 2 - dispersibility measuring unit, 3 - amplitude, 4 - vibrating plate, 5 - spatula assembly, 6 - pan base, 7 - tap holder, 8 - rheostat, 9 - timer. 2
,
/---
1
----IT-
Figure 2. Bulk Densimeter 3 1 - standard 100 cm cup, 2 - extension piece, 3 - pan, 4 - tapper, 5 - timer, 6 - starter.
341
2. EXPERIMENTAL AND RESULTS For the purpose of measuring the apparent densities, the Powder Characteristic Tester accessory, originally designed to measure bulk density, was employed. The bulk density is the mass of particles making up a bed divided by the volume of the bed. In order to perform a bulk density measurement, a calibration cup is filled with excess material, by means of an extension piece attached to increase its volume. The entire set is then placed in an automatic tapper which is adjusted to vibrata with a frequency of 1 Hz for 180 s. A constant level is maintained by adding more material as the previous material becomes more densely packed. After the measurement is over, the excess material from the above calibration cup is removed by means of a ruler. The bulk density is calculated from the known material mass contained in the known cup capacity.
2.1. Determination of apparent density by powder densimetry (1,2) The density was measured using a 100 cm3 standard cup as a powder pycnometer. As the pycnometric medium powders, which according to Geldart's classification belong to easily (dry goods) granular substances ones and hence do not indicate cohesive properties, have been used. The grain diameter of the pycnometric medium powders should be much smaller than porous size of particles, but larger than the largest porosity. In order to find the density values of porous silica gel, active carbon and a molecular sieve, the pycnometcr was filled with the said materials, their amounts corresponding to 30% of the pycnometer capacity. The remaining portion was packed with an appropriate powder pycnometric fluid so as to be sure of the best dispersion of the adsorbent particles within the cup capacity. Subsequent procedures were similar to the bulk density measurement method. Knowing the mass (m ) and bulk density (p,) of the pycnometric powder, as well P as the adsorbent mass inside the pycnometer (ma), its apparent density (p ) was "P calculated from:
where Vn is the pycnometer capacity. The apparent density values thus obtained for adsorbents and non-porous glass beads are listed in Table 1 . Every determination was repeated several times and good repeatability was achieved (k0.02 * 1O'3 kg/rn3). For reference purposes, the apparent density values of the same adsorbents as found by the mercury displacement method and by water pycnometry for glass beads are also included.
342
Table 1 Apparent density of coarse adsorbents by the powder densimetry method Adsorbent (diameter range) Silica gel (2.5-6.0 mm) Active carbon (2.0 mm) Molecular sieve (0.8-1.2 mm) Glass beads (0.4-0.6 mm)
Density ( kg/m3 * bronze steel
) using
zinc
mercury
1.12
1.13
1.16
1.17
0.63
0.63
0.64
0.64
0.92
1.01
1.08
1.10
2.85
water
2.90
2.2. Determination of apparent density by the comparative method (3,4) The density of fine porous particles was measured by a method based on the assumption that the minimum packed bed voidage is the same for similarly shaped particles of a narrow size range. Density measurements by this method involve determining the bulk density of the porous particles to be tested (p ba )and the bulk density of the reference material (p br ) of a known apparent density (p ). Thus: aPr
Where k is a factor defining the correlation between the shapes of the porous particles and the reference material. The factor k is equal to one for spherical particles. For other shape particles’ k will determine experiment. The method was applied to determine apparent densities of narrow fractions of fine porous particles obtained via disintegration and sieving of coarse adsorbents. In addition, the density of a cracking catalyst was measured. The reference materials were the powders originally used as pycnometric fluids in powder densimetry experiments. The experimental results obtained by the comparative method are shown in Table 2. The apparent density values obtained by the above method was compared with the density values resulting from the mercury displacement method (Table 1) by calculating the differences in both determinations. Such comparison is reasonable for these adsorbents. Their structure contains no closed pores, which might be made available in the disintegration process and thus may alter the percentage of pores in the particle volume. For the cracking catalyst the difference was calculated using the supplier’s data.
343 Table 2. Apparent density of fine particles by the comparative method
Particles
(diameter range)
Density ( kg / m3 *
Silica gel: 0.04-0.08 lfl~ll Active carbon: 0.02-0.04 mm Molecular sieve: 0.2-0.4 mm
Cracking catalyst: 0.040 - 0.056 lfl~ll
0.056 - 0.071 mm 0.071 - 0.090 lfl~ll 0.090 - 0.125 ~lllll
Difference (%)
)
1.14 0.67 1.11
-2.56 4.69 0.91
using glass beads
using bronze beads
using glass beads
using bronze beads
1.55 I .56 1.50 1.51
1.59 1.58 1.55 1.54
1.30 2.00 -2.00 -1.30
3.90 3.30 1.30 0.60
3. CONCLUSION mercury method
L r~
20
comparative method
, 5
P
I
Dowder densimetrv method
I
I
I
50
100
200
I I
500
I
1000
____+
I
1
5000 dP [pml
Figure 3.Application range of various methods to measure the apparent density of porous particles. The mercury displacement method finds wide application for the apparent density determination of coarse particles. However, its applicability is limited to particle sizes over 0.8 mm. Attempts have been made to extend this range by conducting measurements at a pressure over the atinospheric value. However, in order to obtain reliable
344
results, such a method would require a porous particle structure which would be different from its intergranular porosity. The mercury method is also useless for particles of porous metals and catalysts which form amalgams with mercury. The results indicate that both the powder densimetry and the comparative method allow the density of porous particles to be determined which would otherwise be impossible to measure by the mercury method. Furthermore, by using very fine powders as pycnometric fluids, some disadvantages of mercury displacement are eliminated.
REFERENCES B. Buczek, D. Geldart, Powder Technol., 45 (1986) 173. B. Buczek, Chemia Analityczna, (in Polish), 32 (1987) 969. M.A. Hooker, D.H.T. Spencer, private commun., National Coal Board, (1979). 4. A.R. Abrahamsen, D. Geldart, Powder Technol., 26 (1980) 35. 5. B. Buczek, E. Vogt, Przemysl Chemiczny, (in Polish), submitted. 1. 2. 3.