Flow Method for Determination of Desorption Isotherms and Pore Size Distributions Bruce P. Semonian’ and Milton Manes” Chemistry Department, Kent State University, Kent, Ohio 44242
The method consists of loading a suitable gas or vapor on the adsorbent sample to near saturation, followed by complete desorptlon in a constant flow of carrier gas. A mlnlcomputer Is used for practically continuous recording of the effluent composition and sample temperature and for subsequent calculations. The adsorbate loading corresponding to each adsorbate partlal pressure is calculated by backward lntegratlon of the effluent composition curve. Adsorbate tailing is circumventedby increaslng the temperature wfth decreasing loading. A practically continuous desorption isotherm is calculated from the nonlsothermal data by application of the Polanyl adsorptlon potential theory. The method has been tested on n-butane desorptlon from activated carbons and gives good agreement with statlc data; it should be equally applicable to other adsorbates on activated carbons and In all likelihood to other adsorbents.
The principle of the calculation is illustrated in Figure 1, which is a schematic strip chart recording of the sample temperature and the equilibrium partial pressure &s a function of time. (This is the data that would be fed to the computer.) In this illustration, the initial temperature drops somewhat below the bath temperature because of evaporative cooling at high initial partial pressures. It then becomes steady at the initially constant bath temperature for 10 to 20 min and then rises monotonically as the bath is heated until all the adsorbate is removed. We determine an adsorption isotherm point (i.e., the loading and the partial pressure at some fixed temperature) at time Q as follows: (1)The amount of adsorbate is proportional to the area OPQ under the partial pressure curve (the proportionality constant is readily calculated from the flow rate and the gas law); (2) the partial pressure P’, at temperature T’ is calculated from P and T (the measured values) by Equation 1
The original objective was to design a computerized apparatus for the rapid determination of gas desorption isotherms on activated carbon samples and the consequent calculation of their pore-size distributions. Since on activated carbons one can calculate the isotherm of one gas from experimental data on another by use of the Polanyi adsorption potential theory (1,2),it was anticipated that the adsorbate gas could be chosen for experimental convenience. Moreover, since the time required for adsorption prolongs the determination of static isotherms, even on automated systems, our first efforts were directed toward a flow system. One may, in principle, determine a desorption isotherm by loading a fixed amount of adsorbate on the sample, sweeping it out in a constant flow of carrier gas, and monitoring the effluent gas composition with a suitable detector, such as a thermal conductivity detector. The calibrated detector gives the gas composition which, together with the total pressure, gives the partial pressure; the amount desorbed may then be estimated by a suitable integration involving the carrier gas flow rate and the effluent composition. This method has the disadvantages of requiring an inconvenient measurement of the initial loading and of having poor precision as the loading approaches zero. These difficulties were circumvented by recording the effluent composition as a function of time to zero loading, followed by a backward integration from zero to any desired loading; it was thereby unnecessary to have any precise measurement of the initial loading. Another anticipated difficulty was in the isothermal removal of final traces of adsorbate, where the expected low partial pressures (“tailing”) would lead to imprecise pressure measurements and long desorption times. The solution of this problem consisted in steadily raising the sample temperature to increase partial pressures and desorption rates; the measured partial pressures were in effect corrected to isothermal partial pressures by use of equations based on the Polanyi adsorption potential theory. Finally, the sample was intermittently shaken to keep the loading uniform, i.e., to prevent the sample from acting as a column.
T log ( P J P )= T’ log
‘Present address, Department of Chemistry, University of Georgia, Athens, Ga. 30602.
(I”#‘)
where P’, and P, are the saturation pressures of the vapor at T’ and T. In this fashion we can calculate the entire (desorption) isotherm. If the adsorbate is nitrogen, we can calculate the pore-size distribution directly by use of a number of published calculation methods (3). If the adsorbate is some other gas, we can (as noted earlier) readily calculate the nitrogen isotherm on the sample by use of the Polanyi adsorption potential theory (1,2) (in the absence of chemisorption and of molecular sieving effects). For purposes of predicting the adsorption behavior of an activated carbon in both vapor- and liquidphase applications it is more convenient to plot the results as Polanyi correlation curves, i.e., in terms of the volume adsorbed vs. the function ( T / V )log ( P a l p )where , V is the molar volume of the adsorbate at its boiling-point. We shall present our data as correlation curves, emphasizing that all of the presentation methods noted above are essentially equivalent and, in any case, readily interconverted by a relatively simple computer program.
EXPERIMENTAL Reagents. The n-butane (99.5%)and nitrogen (99.9%)were used as supplied. The activated carbon sample was CAL lot 2131
(Pittsburgh Activated Carbon Co.). Apparatus. Flow Diagram. Figure 2 is a flow diagram of the gas system, together with specification of the equipment. The flow of gases is as follows, when determining an isotherm: Tank nitrogen goes through precision pressure controller 1 (Brooks Model 8601, 1’ taper needle), toggle valve 2 (Whitey Catalog 06521, flow controller 3 (Brooks 8744), and 3-way switching valve 4 (Whitey Ball valve B41X2), to the sample in the sample cell 5. The exit stream from the cell (now containing nitrogen and butane) passes through another 3-way switching valve 6, and needle valve 7 (Nupro),from which the pressure drops essentially to atmospheric pressure. The exit stream now goes to the sample side of the thermal conductivity detector (TCD) 8 and then vents to the atmosphere. Another branch of the nitrogen flow passes through another flow controller 9 and to the reference side of the thermal conductivity cell. The butane is loaded onto the sample from butane tank 10 through toggle valve 11 and flow controller 12, through 3-way ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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Figure 2. F!ow diagram of the apparatus: (1) pressure controller; (2) toggle valve; (3)flow controller; (4) 3-way switching valve; (5) sample cell; (6) 3-way switching valve; (7)needle valve; (8)thermal conductivity ceii; (9) flow controller; (10) butane tank; (1 1) toggle valve; (12) flow controller; (13)3-way switchi valve; (14) needle valve; (15)pressure gauge \ switching valve 13, through the TCD cell, and then to the atmosphere through needle valve 14. For calibrating the TCD, a measured flow of n-butane (measured by a soap-film meter at the vent exit of switching valve 13) is introduced into the nitrogen stream. The nitrogen flow rate is measured by a soap-film meter at the exit of the TCD. The exit needle valve 14 serves as a snubber to keep the system at a fixed superatmospheric pressure. Computer. The computer was a PDP lab 8/e, with 8K of core memory, a 10-bit bipolar Analog to Digital Converter (A/D) and a High Speed Reader-Punch (Digital Equipment Corporation (DEC)). Programs were written in R/T BASIC and PAL-111 coded Assembly (copyrighted by DEC). Interaction with the computer was made through an ASR-33 Teletype. E f f l u e n t Analysis. The TCD was a Gow-Mac Model 24-393, semi-diffusion type, with a Model 24-510 temperature controller and a power supply unit with a special continuously variable attenuation low output impedance amplifier. The detector output was passed through a n-filter network, T = 0.55 second, before being sent t o the A/D converter. Sample Cell. The essentialfeatures of the stainlesssteel sample cell are shown in Figure 3. The cell body (B) was a '/&. Nupro SS-4F-60 in-line filter, with the filter unit removed. The filter (E) was taken from a smaller in-line filter unit (SS-2F-60),drilled to accommodate the thermocouple well (D), and inserted into the cell head (F),which was a standard Swagelok reducing coupling. The filter, of 60-pm grade, effectively kept fines from leaving the cell. The cell was opened for sample changes by unscrewing the cell head (F). The sample rested on a glass wool plug. The remaining fittings were all conventional Swagelok '/*-in. fittings. The thermocouple well was 1/16-in.stainless steel tubing, closed at the sample cell end and silver soldered through the filter element to seal the sample compartment. The sample cell was connected to the gas-flow system with 1/16-in.stainless steel tubing, which was sufficiently flexible for shaking the cell and sample during the run, typically at one sharp shake per second. Procedure. Sample Pretreatment. The determination of the desorption isotherm was preceded by exposure of the sample to
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A Figure 3. Sample Cell. (A) Gas inlet; (B) cell body; (C) sample; (D,D) thermocouple well; (E) filter; (F) cell head; (G) gas outlet n-butane, followed by heating in a nitrogen stream at 35OOC for about 10 min to remove the n-butane and any other adsorbed gases. Loading and Desorption. The pretreated sample and cell were first swept free of other gases by a n-butane flow of 70-100 cm3/min for about 5 min. Valve 6 (Figure 1)directed the effluent stream to snubbing valve 14, which was adjusted to pressurize the system to 5 psig. The flow system was then closed at valves 6 and 13 by turning them midway between their two open positions. The sample cell was then immersed in an ice bath in order to lower the partial pressure of the n-butane, at constant loading. The computer data acquisition program was activated, and the nitrogen sweep initiated by opening valve 13. The effluent stream passed through valve 6 and the snubbing valve 7, which had previously been adjusted to maintain the desired system pressure at the set carrier gas flow rate. Following a brief initial nitrogen sweep in the ice bath, which sufficed for substantial reduction of the adsorbate partial pressure, the sample cell was transferred to the heater bath, originally at room temperature. Following further reduction in the adsorbate partial pressure, the sample temperature was gradually raised to 350°C and the exit gas composition allowed to return to the original nitrogen baseline; this terminated the run. At this point the data on loading and temperature vs. time were punched out on paper tape. The entire loading and desorption procedure typically took about 30 min. Calculations from Data. The data from the run, on paper tape, were fed back to the computer, and reprinted in reverse order on another tape. The inverted tape was then fed to the computer, which then calculated the (liquid) volumes adsorbed as a function either of isothermal partial pressure (i.e., as an isotherm) or else as a function of the butane adsorption potential ( I , 2). The result was a printout on the teletypewriter of either the isotherm or a Polanyi correlation curve. Our calculation was not carried through to the conventional pore-size distribution.
RESULTS Figure 4 illustrates the reproducibility of successive runs on a single carbon sample, The sample (Pittsburgh Activated Carbon Co. CAL Lot 2131) has been the subject of a long series of studies in both liquid and gas phase ( 4 ) . The plot of volume adsorbed vs. ( T / V log P,/P) is a Polanyi correlation curve (1,2);as noted earlier, it is in many ways more informative than a pore-size distribution. The ordinate is plotted on a linear rather than a logarithmic scale to spread out the upper points. For clarity, only about one-tenth of all of the data points are plotted. A comparison of the data with the results of static isotherms is shown in Figure 5. The solid curve is drawn from static isotherms of ethane and propane (5). The coincidence is quite
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good except at the bottom and top, where the dynamic points are somewhat lower and may well be better. For example, the curve at low capacity was calculated from adsorption data on methane and ethane. Since then, Schenz and Manes (6) have found evidence of molecular sieving for hexane on this carbon in the very low capacity region, and the same could be true for butane. At the higher loading levels, a single static point determined by direct weighing of the carbon sample coincided with the dynamic data. The data thus far obtained therefore show excellent reproducibility and at least very good accuracy.
DISCUSSION We consider first some of the details of the procedure and then the advantages and general applicability of the method. The semi-diffusion type TCD was originally chosen as a compromise between the conflicting requirements of rapid response and of minimum sensitivity to variations in flow rate. The relatively long response time of 7 s was sufficiently short for this determination. The response had some sensitivity to flow rate; this gave no serious problems, because of the
constancy of the flow as set by the flow controller. Because of the relatively high adsorbate concentrations, the TCD operated at its lowest sensitivity range; even toward the conclusion of a run, the increasing temperatures kept the butane partial pressure relatively high for rapid depletion. This enabled operation of the cell block at room temperature and the filaments at not much higher temperatures, so that there was no need to prevent their exposure to air. Moreover, the TCD calibrations remained stable even when the detector was left on for weeks, Furthermore, the nitrogen stream through the sample side could be discontinued while loading the sample and immediately resumed on desorption without disturbing the baseline. The principal disadvantage of the detector cell was that its response was nonlinear a t higher concentrations with a loss of sensitivity; this set an upper limit of about 20 vol '30butane. The system was pressurized during the run in order to keep the initial butane concentration below this limit. The particle size of the sample is probably not critical. We originally worked with samples at 200-320 mesh. Other coarse granular samples were lightly broken up with no attempt at fine grinding. In all likelihood, samples at 50 mesh or below may be expected to equilibrate with the gas stream more rapidly than, for example, the response time of the thermal conductivity detector. The sample size (-0.5 g) sufficed to minimize bypassing of the gas stream, which would have given an erroneouslylow measurement of the equilibrium adsorbate gas pressure. It turned out that our computer did not have quite enough memory for both storing all of the data (typically 10 points/min for about 30 min) and carrying out the calculations. The separate procedures for recording and calculating the data were therefore expedients to circumvent this limitation and are not essential to the method. Sample pretreatment was necessitated by the finding of significant differences between the first run on a sample and the mean of consequent runs, which were of good reproducibility. The reason for the divergence of the first run has not been established with certainty. One possibility is that the n-butane may displace traces of hydrogen, to which the thermal conductivity detector would be very sensitive. The divergence was removed by the pretreatment procedure, which apparently was more effective for getting rid of interfering gases than simply heating the sample in a nitrogen atmosphere. Pretreatment for lower boiling adsorbates such as nitrogen may not be necessary. We now consider the advantages and applicability of the method. The primary advantages over static methods (including computerized static equipment (A computerized and automated static system is commercially available from the Micrometrics Instrument Co.), which was not available at the initiation of this study) are speed, simplicity of construction, and the generation of practically continuous data points; all of these advantages are attributable to the flow system. In addition, the sample preconditioning prevents degassing on introduction of adsorbate gas, a phenomenon that one cannot ordinarily detect with static systems where one measures only pressure and not composition. The effect may be quite significant, particularly at low pressures. On the other hand, static systems are of obvious advantage in determining adsorption rather than desorption isotherms (particularly with the occurrence of hysteresis) and for the determination of chemisorption rates. The described method should be well adaptable to samples of lower surface areas than activated carbons. For such substances the desorption might not require any temperature increase. One may envision the application of some combination of: (a) increasing sample size; (b) reducing the flow ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
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rate of carrier gas; and (c) having increased detector sensitivity. In all such cases the sample size would have to be large enough to scrub the carrier gas stream sufficiently well to approach equilibrium both of the sample with the gas stream and of the gas stream with the sample, Le., to minimize bypassing of the sample by a significant portion of the exit gas. The present method is well adapted for activated carbons, for which the Polanyi-based estimation method is quite accurate for transposing adsorption data over a wide temperature range. It is also readily adapted to the more conventional use of nitrogen as the adsorbate at cryogenic temperatures, with helium or hydrogen as the carrier gas. Moreover, although it is here illustrated for activated carbon, it should be readily applicable to any solid of relatively high surface area and
heterogeneoussurface for which we can use the Polanyi theory (or any other workable method) to calculate an isotherm from nonisothermal data.
LITERATURE CITED (1) R. J. Grant, M. Manes, and S. B. Smith, AIChE J., 8, 403 (1962). (2) R. J. Grant and M. Manes, Ind. Eng. Chem. 3, 221 (1964). (3) S. J. &egg and K. S. W. Sing, “Adsorption, Surface Area and Porosity”, London and New York, 1967. (4) M. R. Rosene, M. Ozcan, and M. Manes, J. phys. Chem., 80, 2586 (1976). (5) M. Manes and L. J. E. Hofer, J. Phys. Chem., 73, 584 (1969). (6) T. W. Schenz and M. Manes, J . Phys. Chem., 79, 604 (1975).
RECEIVED for review February 2,1977. Accepted March 25, 1977. We thank the Calgon Corporation for support of this work.
Personal Vinyl Chloride Monitoring Device with Permeation Technique for Sampling Leonard H. Nelms,’ Kenneth D. Reiszner, and Philip W. West* Environmental Sciences Institute, Chemistry Department, Louisiana State University, Baton Rouge, Louisiana 70803
A method for measuring the exposure of personnel to vinyl chlorlde has been developed which utilizes the permeation technlque for sampllng. The vinyl chloride that permeates the membrane Is trapped on actlvated charcoal which Is removed for subsequent determlnation by gas chromatography. The monitor is about the sire of a standard fllm badge, weighs less than 35 g, and requires no source of power. The method Is insensitive to temperature and humldlty, and is free of signlflcant Interferences. The method is ideally suited to personal monltoring programs required by OSHA regulatlons, because the anaiytlcal data represent a time-welghted-average exposure and require no further data reduction step.
The health hazards posed by vinyl chloride (chloroethene) vapor in the industrial environment have received broad coverage in both the scientific and public press. The fact that the deaths of some 28 workers have been attributed to chronic exposure to high concentrations of vinyl chloride has resulted in stiff Federal regulations covering this hazard. Current standards (1) call for an action level of 0.5 parts-per-million time-weighted-average (TWA) exposure, which, if exceeded, requires the implementation of an extensive personal monitoring program. This directive permits a maximum allowable 8-h TWA exposure of 1 ppm and a maximum permissible exposure of 5 ppm for no more than 15 min. These regulations create the need for a simple, inexpensive method for measuring time-weighted-average exposures of personnel to vinyl chloride. A method has been developed to meet these needs. The approach employs a sampling device based on the permeation principle, and an analytical finish utilizing gas chromatography. Previously published methods for making vinyl chloride determinations have been of two basic types, area monitors and personal monitors. Area monitors have been widely used ‘Present address, Air Section, Kern-Tech Laboratories, Baton Rouge, La. 70808 994
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
for determination of contaminants in the ambient air of the working environment. Typically, they utilize a gas chromatograph with a continuous automatic sampling system. The system is capable of making a fixed number of determinations per hour and requires data analyses to provide a timeweighted-averagefor concentration. Other area monitors rely on a long-path infrared analyzer for the determination and give a continuous record of the contaminant level. Again, data reduction is necessary to produce a time-weighted-average. Either system represents an initial investment of thousands of dollars for equipment and requires periodic maintenance for sustained operation. While they provide instantaneous readings of the vinyl chloride level in the ambient air, they cannot effectively monitor the air inhaled by the individual worker, which is subject to such localized effects as leaking and venting of equipment. Methods for personal monitoring developed to date have all relied on the collection of vinyl chloride by either drawing air over a suitable adsorber such as activated charcoal (2,3), or drawing air into a gas sampling container at a known constant rate. Most of these methods require the use of a battery-powered pumping system which is often heavy, noisy, and quite cumbersome. Methods employing collection of the air sample within a container also require that the worker wear this bulky container in addition to the aforementioned pump. These various personal monitoring systems are all inconvenient and uncomfortable and, in many cases, may also prove to be hazardous in their own right because of their unnecessary bulkiness. A new sampling method is now proposed which employs a small, light-weight, reusable, and uncomplicated personal monitor that does not require a source of power. The sampling device is a badge having dimensions of 41 mm by 48 mm and a thickness of 7 mm (Figure 1). An internal cavity is covered by a permeable membrane through which vinyl chloride passes at a rate proportional to the external concentration. The vinyl chloride that permeates through the membrane is adsorbed on activated charcoal which is later removed from the device after completion of the exposure, and the amount adsorbed is then determined by gas chromatography. There are no
