Carbon, Vol. 30. No. 7, pp. ICO7-IOI Printed in Great Bntain.
I, 1992 Copyright
WATER ISOTHERM MEASUREMENTS MICROPARTICLES OF CARBON
ooO8-6223192 $5.00 + .oO 0 1992 Pergamon Press Ltd.
GLENN 0. RUBEL U.S. Army Armament, Munitions, and Chemical Command, Chemical Research, Development, and Engineering Center, Aberdeen Proving Ground, MD 2 IO lo-5423 (Received 3 February 1992; accepted in revisedform
new method, single particle electrodynamic levitation SPEL, has been used to measure the water isotherm for microparticles of porous carbon. SPEL is an ultrasensitive gravimetric analyzer that uses electric fields to levitate single microparticles in a flowing gas stream. The quantity ofadsorbed vapor is directly proportional to the electric field intensity required to suspend the particle at the center of the levitater. Water isotherms for nanogram microparticles of BPL and ASC carbon are measured at 25°C using SPEL and compared to bulk carbon isotherm data. It is shown that SPEL can successfully differentiate the ASC and BPL carbon water isotherms and replicate standard isotherm data with only a limited number of single particle measurements. Hysteresis effects are also measurable with SPEL.
carbon, water vapor adsorption, microparticles, electrical levitation,
1. INTRODUmION Microporous carbons and impregnated microporous carbons are used extensively by industry and the military in filter systems for air purification. The activated carbons have a high capacity for vapor adsorption due to their high porosity, large specific surface area, and high degree of surface activity. Because water vapor can affect the adsorption of hazardous vapors onto the carbon adsorbents, considerable research has been conducted on the measurement of water isotherms for porous carbons[ 1,2]. The measurement of water isotherms for porous carbons has also been used to analyze the pore structure of carbon, although recent efforts have been directed toward the use of hydrocarbon vapors for microporosity determinations[ 31. A common experimental procedure for isotherm analysis is the gravimetric method where water vapor adsorption is determined from the weight change of the adsorbent bed. The variety of gravimetric techniques varies from the large macroscopic studies where adsorption tubes containing grams of carbon adsorbent are used to the smaller microscopic studies involving electrobalances where milligram quanties ofcarbon are required. The time required for the isotherm measurement is directly related to the quantity of carbon used in the study, varying from days for the adsorption tube studies to hours for the electrobalance studies. It would be attractive both economically and scientifically to develop new methods that reduce the measurement time for the carbon isotherms. It is the object of the current study to present a new method for isotherm determinations that reduces the total isotherm measurement time and accurately reflects the adsorption capacity of the adsorbent. The new isotherm method, single particle electrodynamic levitation SPEL, levitates individual microparticles of carbon in an electric field. The quantity of
adsorbate is determined from changes in the electric field intensity which exactly balances the weight of the particle. Nominally, nanogram quantities of carbon are used in these studies, consequently measurement times are significantly reduced. This gravimetric method is used initially to measure the water isotherm of silica gel, a desiccant with a well-established water isotherm. This reliability experiment is used to demonstrate the accuracy and sensitivity of the SPEL method. Additional water vapor adsorption and desorption experiments are conducted on BPL and ASC carbon and variations in the isotherms are discussed. It was of particular interest to determine whether the isotherm measurements for a small set of carbon particles could faithfully reproduce the isotherm data for bulk carbon.
The single particle levitation apparatus as shown in Fig. 1 has been described in detail earlier. Similar in principle to the quadrapole, SPEL uses a central hyperboloidal electrode to establish a time-dependent electric field to cause the particle to execute bounded oscillatory motion. Top and bottom endcap electrodes are connected to a controllable direct current voltage supply that establishes a static electric field in the vertical direction. When the force due to the static electric field equals the particle weight, the particle comes to rest at the chamber center. Under these conditions, the particle weight is directly proportional to the electric field intensity and vapor adsorption can be determined from the change in the levitation voltage. Normally the static electric field intensity is manually controlled to position the charged particle at the center of the levitation cell. This is accomplished by viewing the particle with a telemicroscope through a 3%mm objective lens. However, automatic control of the levitation voltage
G. 0. RUBEL WHITE
TELEMICROSCOPE WITH SCANNING GRATICULE
Fig. 1. Schematic of single particle electrodynamic levitater and associated ancillary equipment.
is also possible through an electro-optical feedback circuit that uses laser light scattering to detect the particle position in the levitater. The telemicroscope was equipped with a scanning graticule that permitted diameter measurements to an accuracy of 5%. Particle charging was accomplished in the following manner. For the dielectric, silica gel (Grade 42, Davison Chemical), the silica granules were first crushed in a mortar until the silica gel appeared powdery. The powder was mixed with methyl alcohol to a liquid consistency. The colloidal mixture was drawn into a pipette tube with an orifice diameter of 1 mm. A platinum wire, connected to a 20K direct current power supply, is immersed in the liquid until the tip of the wire is flush with the orifice of the pipette. When the power supply voltage was increased to approximately 8K, a spray of charged colloidal
particles was generated. The spray of charged particles was directed over the top ofthe levitater and typically more than one particle was captured by the electrodynamic field of the levitater. A glass rod was used to remove all particles except the primary particle of interest. Finally the top endcap of the levitater was placed in position and the static electric field intensity was adjusted until the particle was visible in the telemicroscope. For the activated carbons, induction charging was used to charge the microparticles of carbon. Because carbon is electrically conductive, microparticles of carbon in contact with ground will attain a net charge in the presence of an electric field. The electric field is generated using a coaxial arrangement where a wire, connected to a power supply, is centered in a copper tube connected to ground. The carbon powder, generated by crushing the carbon pellets in a mortar, is deposited on the copper tube and the wire voltage is increased to 5K volts. Aspiration of air through the tube directs the charged carbon powder into the levitater. The BPL carbon was supplied by Calgon Carbon Corporation. Both silica gel granules and the carbon pellets were dried at a temperature of 110°C at atmospheric pressure for a time period ofthree hours. Thus our regeneration procedure follows closely the techniques used by Mahle and Friday. The gas environment of the particle was controlled by passing compressed air through the levitation cell at a flow rate of 2 cc/s. For the water isotherm measurements, the relative humidity of the gas stream was controlled by mixing a “dry” air flow with a water-saturated air flow. By varying the relative flow rates of the two separate flows, a precise control of the gas dew point was achieved. The gas dew point was monitored with an EG&G dew point hygrometer.
Fig. 2. Comparison of water vapor adsorption by silica gel as measured by single particle method (0) and by bulk method (x) at 25°C. Average single particle diameter is 42 micrometers.
Water isotherm measurements
Figure 2 compares the water loading of silica gel as reported by Davison Chemical and that measured by SPEL. The water loading, expressed in terms of grams of water to grams of adsorbent, is derived from the levitation voltage according to the relation V,lV,
the error bars are comparable to the size of the data points. It is noted that the SPEL isotherm slightly underestimates the data reported by Davison Chemical. However, this is believed to be due to a normalization offset resulting from slightly different starting relative humidities. Another interesting finding is that the actual water isotherm of silica gel, as measured with SPEL, was independent of the number of times the granules were successively ground. In the present experiments, the number of pestle grinds varied from 3 to 15 for the silica gel and for this variation no change in the water isotherm is observed. This is perhaps not surprising because the majority of water adsorption for silica gel is occurring in the micropores, the di-
3. RESULTS AND DISCUSSION
Water loading = 1 -
where VDis the levitation voltage for the dry adsorbent particle. The isotherm generated by SPEL represents five separate runs averaged together. However, the reproducibility of the isotherms is such that
al. / /
Fig. 3. Comparison of water vapor sorption by ASC carbon as measured by single particle method (0) and by bulk tube method (x) at 25°C. Average single particle diameter is 5 1 micrometers.
G. 0. RUBEL
ameters of which are on the order of nanometers. For these sizes, grinding is not expected to affect the geometry of the pores. Mahle and Friday analyzed the water adsorption characteristics of AK and BPL carbon using the tube adsorption method and determined the pore size distribution using a modified Sircar isotherm model. They showed that the mean micropore diameter of both carbons is on the order of a nanometer, thus grinding of the carbon pellets is not expected to affect the water adsorption isotherms. Figure 3 shows a comparison of the water adsorption for ASC carbon as measured by SPEL and with the tube adsorption method. The SPEL isotherm data represents the average of five separate microparticle measurements. Other than a small overestimation of the data, the SPEL water adsorption curve follows closely the data of Mahle and Friday. The slight difference in the water adsorption data could be due to differences in the starting relative humidity of the respective experiments. Hassan et al. have suggested that lot variations and different regeneration schemes can lead to isotherm differences like those in Fig. 3. Because the same carbon regeneration schemes were used, the later explanation is dismissed. Figure 3 also shows an interesting difference in the water vapor desorption branch of the ASC carbon as measured by SPEL and the tube method. The two methods give almost identical results for water desorption from 100 to 50% relative humidity, however, below 50% relative humidity the SPEL desorption branch diverges significantly from the tube branch. The SPEL desorption branch exceeds the tube desorption branch for relative humidities as low as 10%. In fact, while the tube method shows a dra-
matic increase in water desorption below 50% relative humidity, the SPEL desotption branch shows a constant desorption rate. Mahle and Friday argued that the nonclosure of the water isotherm loop was due to impregnants in the carbon, such as copper, silver, and chromium salts, that react with the water vapor once adsorbed. Thus the difference in the water desorption branches shown in Fig. 3 could be due to different quantities of impregnants in the respective carbon samples. Furthermore, the divergence of the desorption branches at the lower relative humidities might suggest that the impregnants in the SPEL sample are concentrated in the smaller micropores. Because it is known that the water isotherms of BPL and ASC carbon are significantly different, it was of interest to use SPEL to measure the water adsorption isotherm of BPL carbon. For these measurements, the BPL carbon is unimpregnated. Figure 4 shows the adsorption branch for BPL carbon measured with SPEL and as reported by other investigators[2,4]. The SPEL method has identified clearly the basic characeristics of the BPL carbon isotherm, i.e., retarded water adsorption at the lower relative humidities and the rapid water uptake above 40% relative humidity. Deviations between the SPEL isotherm and other isotherm data is confined to the larger relative humidities. As suggested by Hassan et a1., such deviations could be due to variations in the specific surface area of the carbon from various lots. Nevertheless, it is remarkable that from such a small sample set (five particles), one can generate a BPL carbon water isotherm similar to the isotherm data for bulk carbon. These preliminary studies on the use of SPEL to measure the water isotherms of microparticles of car-
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Fig. 4. Comparison of water vapor adsorption by BPL carbon as measured by single particle method (0) and by bulk tube (+) and electrobalance (x) methods at 25°C. Average single particle diameter is 45 micrometers.
Water isotherm measurements for carbon
Fig. 5. Three different classes of water isotherms measured for ASC carbon using the single particle method. Isotherm 0 is statistically most probable.
bon suggest that it is feasible to use this microscopic technique to determine the isotherm of bulk carbon. However, as a caveat it must be stressed that this statement applies only to the present study. Even for the samples analyzed in this study, some heterogeneity was observed. Figure 5 shows a typical set of water isotherms for microparticles of ASC carbon which reflect the divergent character of some of these particles. The dramatic differences in the isotherms suggests that the ASC carbon sample contains carbon material with substantially different microporosity. Fortunately in the present case, the statistical percentage of these outliers is small. For more heterogeneous samples, a greater number of microparticle measurements must be conducted before the SPEL measurements converge to the bulk measurements. While this circumstance would diminish the attractiveness of the SPEL as a new and rapid method for isotherm determination, it is believed that knowledge gained through SPEL about microscopic sample heterogeneity could assist modelers in the development of improved microporosity models. Furthermore, by
studying individual nanogram microparticles, microphysical transport can be investigated more easily than in the tube sorption studies. It is believed that such microphysical data could strengthen existing lumped models that use parameterization methods to describe the carbon microphysics. Finally, measurement times are significantly reduced as compared to bulk sorption studies, requiring approximately one hour to complete a single isotherm measurement. REFERENCES 1.
M. M. Dubinin and V. V. Serpinski, Dokl. Akad. Nauk. SSR 99, 1035 (1954). J. J. Mahle and D. K. Friday, CRDEC-TR-018, U.S. Army Chemical Research, Development and Engineering Center, A.P.G., MD (1988). X. Lu, M. Jaroniec, and R. Madey, Langmuir 7, 173 (1991). N. M. Hassan, T. K. Ghosh, A. L. Hines, and F. K. Loyalka, Carbon 29,681 (1991). G. 0. Rubel, J. CoNoidInt. Sci. 81, 188 (1981). Davison Chemical, IC-16-782, Ind. Chem. Dept., Baltimore. MD.