Automatic Apparatus for Determination of Nitrogen Adsorption and Desorption Isotherms E. V. BALLOU and 0. K. DOOLEN Gulf Research & Development Co., Pittsburgh, f a .
b To improve the effectiveness of pore size distribution studies as a research and process tool, an apparatus has been built which automatically adds or removes constant volume increments of nitrogen from an adsorbent sample system. The pressure is measured by a nulling-type differential pressure gage. The incremental doses of gas and the pressures are recorded. AIthough the recorded data are discontinuous, a large number of points may b e obtained on a single branch of the isotherm. The data from the smoothed curve are used to characterize the pore size distribution of the sample, with the calculations of the Barrett, Joyner, and Halenda method handled by a digital computer. The calculated pore size distribution of representative samples as a function of the data obtained from various time cycles for adsorption and desorption was studied. The deviation from pressure equilibration in the faster runs was greatest at relative pressures corresponding to filling or emptying of the pores of the material.
T
pore size distribution of smallpored materials, such as catalysts, has been successfully characterized by interpretation of the data from nitrogen isotherms ( 1 ) . An automatic apparatus is described, which can be used to obtain and record these data. The time-consuming operations for this measurement are considerably reduced, thus increasing the scope and use of the measurement. The pressure-volume record comes in suitable form to be read a t the points necessary for computations. Operation may be automated in several ways, characterized by the method of introducing or removing known quantities of adsorbate from the system-Le., by continuous injection a t controlled and known rates as described by Innes ( S ) , or by continuous injection of increments. Another type of apparatus described by Klevens and coworkers (4) records the mass adsorbed, but not the pressure. An incremental method, using a relatively simple and practical system, which is applicable to either the adsorption or desorption cycle was chosen for this Tvork. It provides a means whereby small standard volume HE
532
ANALYTICAL CHEMISTRY
increments of adsorbate may be injected or removed from the system regardless of the pressure conditions prevailing a t the sample environment. Recording the gas pressure in the system simultaneously with a chart step for each gas dose provides the complete data for the isotherm. I n automatic operation the time cycle for dose addition and removal is preset. The recorded data can be corrected for the dead volume of the system to give the standard volume of gas adsorbed a t a series of pressures. I n practice, the number of standard nitrogen doses a t relative pressures corresponding to definite Kelvin radii are read from the smoothed chart curve. These data, lvith the dead space factor for the sample, the vapor pressure of the liquid nitrogen bath, and the sample neight are sent to a digital computer which calculates the volume points of the isotherm, the B E T area, and the pore size distribution of the Barrett, Joyner, and Halenda method (1). It is believed that the Barrett, Joyner, and Halenda method is as complete a treatment as can be obtained, based on the assumptions of cylindrical pores and the validity of the Kelvin equation. Because the apparatus allon s a choice of preset dosing volumes and times, it can be used to determine the rate of adsorption and desorption. I n a porous system the rate is. a t least partially, a function of the pore geometry; hence, the isotherms a t various rates provide an ausiliary means for obtaining informamation on pore structure. APPARATUS
The apparatus has two essential components, the doser and the nulling pressure gage, which allow automatic and recorded operation. The doser mechanism is unique in that a fixed number of moles of gas is introduced or withdrawn, regardless of the pressure conditions prevailing in the vapor in equilibrium n i t h the sample. The pressure measuring device is well suited to this use because the vapor pressure in equilibrium with the sample is mechanically sensed without a change in the volume of the system, and the pressure thus sensed is converted to a voltage suitable for recorder operation. These tn-o components are combined n i t h the appropriate relay systems ana other electronic circuitry for repetitive opera-
tion of the dosers and proper movement of the recorder chart and pen, and with the appropriate glass vacuum system, together n-ith pumps. gages, and temperature-control baths to handle the sample evacuation and sample-vapor environmental conditions. (The apparatus is being developed commercially by the American Instrument Co. of Silver Spring, Md.) The gas dosers and the arrangement of the lines to the sample and to the nulling pressure gage are shonn in Figure 1. All stopcocks remain in the position shown during both the adsorption and desorption runs; the electric valves on the inactive doser remain closed, A block diagram of the control system for the electric doser valves and the recorder is shown in Figure 2. Both the handling system and the detailed electric circuitry are conventional. However, the doser system and the nulling pressure gage differ somewhat from conventional models. Gas Dosers. The basic functional parts of a gas doser are two Skinner electric valves ( S o . CBDA-1130) connected by a T. T h e remaining branch of the T connects to a jacketed gas volume and a manometric mercury switch. The pressure-sensing end of the mercury switch is a transparent plastic cylinder 21/2 inches in diameter and l'/s inches high. The cylinder is divided into an upper and a lower half, inside each of which a shallow cone opens toward the middle. The cavity thus formed has orifices a t each cone apex. The cylinder halves are fastened together with screws and sealed a t the midsection with rubber O-rings. On the adsorption doser, a thin rubber membrane fits without tension between the O-rings and transmits pressure changes but prevents mercury surges. It was practical to omit the rubber membrane on the desorption doser. I n operation, the bottom half of the cylinder is filled with mercury, which leads to a vertical glass capillary with platinum or tungsten contacts. Between doses, the mercury in the capillary just touches the top contact. K h e n the programmer for the electronic circuits calls for doser operation, an electric valve opens to allows gas to leave the doser volume bulb (Hoke No. 431 throttling valves on either side of the electric valves are used to regulate the gas flow to a tolerably slow rate). As the pressure in the doser volume decreases, the mercury column in the manometric switch falls until i t clears the second contact. At this
ADSORPTION
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Figure 1. Gas-handling system of automated adsorption apparatus
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point in the cycle, the outlet valve closes and the inlet valve to the doser opens. Rising pressure in the doser volume forces mercury upward to the top contact in the glass capillary. The relay circuits then close the inlet valve to the doser, and a holding circuit is set up to keep both valves closed until the programmer calls for another doser cycle. The action of the valves stabilizes the size of the gas dose introduced to, or taken from, the sample-vapor system. The number of moles of gas per doser cycle is dependent on the magnitude of the doser volume, which may be adjusted with a mercury well, and on the pressure differential over which the mercury switch operates-Le., the vertical distance between the first and second contacts. It is not dependent on the pressure conditions above the sample. I n practice, the size of the gas increment per doser cycle was determined by injecting or removing gas from a thermostated 1;olume. Although the description above fits either the adsorption or desorption doser, each nil1 necessarily operate in opposite direction relative to the adsorbent sample. Also. the adsorption doser mercury switch operates against a pressure head greater than nitrogen vapor saturation pressure, while the desorption doser mercury switch operates against a vacuum above the top contact. The programmer for the doser cycle
PROGRAMMER
It MERCURY COLUMN CONTACTS
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1 h
Figure 2. Control system for automated adsorption apparatus
ELECTRIC VALVES FOR
INCREMENTAL CHART ADVANCE
X-AXIS POWER SUPPLY
could be any of a variety of timing devices. It was convenient to use a loop of Type 1A ladder chain, running over a sprocket driven by a 1-r.p.m. clock motor. One side of each of selected links is bent to trip a microswitch adjacent to the sprocket. This simple programming device allon s a wide variation of dosing time cycles and easy change-over to a different cycle when a new run is started.
RECORDER
Nulling Pressure Gage. A servooperated differential pressure gage (Figure 3) senses t h e pressure difference between a pair of bellows, which are mechanically connected back to back. The ends of the bellows assembly are rigidly mounted, while their junction is attached to a lever system. The lever system k attached to the armature of a differential transformer, so that any bellows movement results in a VOL. 32, NO. 4, APRIL 1960
533
AMPLi FiER
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MICROMETER
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SYSTEM PRESSURE
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3. Nulling pressure gage
voltage output from the transformer. The voltage is amplified to drive a conventional two-phase servomotor m-hich is geared to a micrometer screw. The linear output of the micrometer is in turn coupled to a helical spring, the other end of which is attached to the lever s j stem so as to return the bellows to their balanced position. This system allows the gage to sense and indicate a pressure change with a negligible change in volume. h mechanical counter geared to the motor gires the pressure reading. Three thousand units of the counter cover the range from vacuum to atmospheric pressure. A 10-turn potentiometer, also geared t o the motor, acts as a “command pot” for a self-balancing potentiometric recorder. The response time of the manometer is 12 seconds for a change of one atmosphere. This is close to full scale on the recorder, which has a much faster response time. As used in the apparatus, the pressure gage is referred to a nitrogen vapor pressure thermometer in the liquid nitrogen bath adjacent to the sample. Auxiliary Apparatus. T Ro electromechanical counters indicate t h e number of ingoing and outgoing doses. T h e self-balancing potentiometric recorder was designed a n d built at this laboratory. The chart drive was altered to advance the chart 0.1 inch each time a gas doser empties. It was necessary to maintain the level of the liquid nitrogen above the sample. This was done by a sensing bulb and a N i t e r reservoir flask, using a design described by Faeth and TT’illingham ( 2 ) . EXPERIMENTAL
Preliminary to the adsorption and desorption runs the usual sample pretreatment and cooling steps are taken. The chart scale is adjusted, and dead space is determined with helium. Once the operating slvitches are thrown, there is
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ANALYTICAL CHEMISTRY
no further manual manipulation during the adsorption run. I n this apparatus the change to the desorption run involves reversing three electrical s~ itches, after n hich desorption proceeds a t the preset rate to about 0.2 relative pressure. The gas in each incremental dose can be varied from about 1 cc. a t standard temperature and pressure to a n amount limited only by the inconvenience of fabricating and thermostating the volume used in the doser. I n practice, 1.5 cc. a t standard temperature and pressure per dose was adequate to give a large number of points in the isotherm without being unduly time-consuming in application to a variety of samples. Reasonably detailed studies m r e made in the isotherm data for three samples: reference sample 8 of silicaalumina cracking catalyst (distributed by the Bone Char Research Project, Inc., c o W. A. Bemis, Treasurer, Revere Sugar Refinery, 333 RIedford St., Charlestown 29, Mass.), Houdry 8-46 silica alumina, and Alcoa alumina Type H-44. The first of these samples had a narrow pore distribution around 24 A. radius TT hile the others had a n-ider pore distribution i7-ith maxima above this figure. T n o automatically timed cycles were used for each sample. as follor+s: Fire doses of gas 17-ereadded or n ithdralvn a t 45-second intervals followed by a n equilibration period of 3 minutes before the nest cycle; and the doses of gas n ere added or withdrawn a t 45-second intervals. I n addition, the gas doses were added without a n automatically timed cycle as follow: I n a “free cycle” operation of the electric valves, the time between succeeding doses varied from about 10 to 40 seconds, depending on the back pres-
01
02
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-_07 -3 0 8
06
09
10
Figure 4. Recorder chart tracing of nitrogen on silica alumina reference sample 0.85-gram sample, 1 .O CC. a t STP per dose Left. Low relative pressure region adsorption Right. High relative pressure region adsorption and desorption
sure of the system. The term “free cycle” indicates that each valve cycle follorved immediately upon the completion of the preceding cycle. I n a semimanual operation ten to twenty doses were added or withdravn, and the doser was manually shut off until pressure equilibration n as observed manometrically.
As the samples nere of different size and pore volume, the effect of nonequilibrium timing cycles was not dlrectly comparable between samples. The sample weight and standard dose size are given with the data in Table I. RESULTS
Recorded Pressure-Volume Data. Figure 4 s h o w a tracing of the chait record of a nitrogen adsorption isotherm in t h e low and high relative pressure regions, as obtained for t h e first automatically timed cycle. The equilibration points a t the end of each cycle are marked by circles. The sample was the reference sample 8 of silica alumina. Figure 5 is a photograph of a complete adsorption curve. The smoothed curve through the points on the chart is read a t 64 pressures and these data are sent to the digital computer for calculations. Validity of Data from Automated Operation. T h e first object of the tests at various time cycles was t o determine if the d a t a obtained from operation a i t h a preset timing cycle could give a reasonable picture of pole size distribution ivhen compared to data from isotherm points a t observed equilibrium. I n Table I, the calculated B E T area, the total pore volume to 300 il. radius, the surface areas as calculated
4 corder Figure chart 5. Complete record of adresorption and desorption of nitrogen on silica-alumina reference sample
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60
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Figure 6. Pore size distributions from automatically obtained data Left. Reference sample No. 8 silica-alumina Middle. Houdry S-46 silica-alumina Right. Alcoa H-44 alumina
from the sum of increments of pore areas b y the Barrett, Joyner, and Halenda method, and the mean pore radius are listed for each test. The program for the digital computer does not artificially adjust calculations to impose total pore area agreement with the BET area. Table I includes figures for reference sample 8 which were obtained by manual operation of a conventional volumetric gas adsorption apparatus. Although the desorption branch of the isotherm alone is customarily used for pore volunie distribution calculations, the adsorption isotherm data were also treated here to compare more fully the data from automated operation with those from manual equilibration. The values obtained by a n automated timing cycle, which included a short equilibration period following each series of doses injected or removed, were very close to those obtained by manual equilibration. A somewhat faster cycle with these samples also gave reasonable values for many purposes. Results from the free cycle test were probably unsuitable except to obtain a preliminary figure for Table
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Figure 7. Detailed comparison of pore size distribution data Nitrogen desorption
-Equilibrium values
--- Automated cycle No. 1. the pore volume of a given sample. I n Figure 6, the pore size distribution plot is given for each sample as obtained
Physical Data from Nitrogen Isotherms on Various Samples Sum of
Pore Mean Areas, Pore Pore BET A ~ Sq. ~ Meters/ ~ , T'olume, Radius, Cc./Gram 8. Sq. fi/Ieters/ Gram Time Cycle Gram Ads. Des. Ads. Des. Ads. Des. Reference Sample No. 8 of Silica-Aluminaa 1. hlanual equilibrationb 613 . . . 648 0.74 . . 23 2. Manual equilibratione 606 598 634 0:75 0.74 27 24 3. 5 doses 45 sec. apart, then 3-min. waitc 594 580 652 0.74 0.75 27 23 4. Continuous doses at 45-sec. intervalse 590 574 680 0 75 0.75 28 22 5. Free cycle, 10- to 40-sec. intervalse 550 524 728 0.73 0.75 3 1 20 Houdry S-46 ,4luminad 1. Manual equilibration 292 278 310 0.42 0.44 33 27 2. 5 doses 45 sec. apart, then 3-min. wait 290 275 315 0.42 0.44 34 26 3. Continuous doses at 45-sec. intervals 286 270 331 0.41 0.44 35 25 4. Free cycle, 10- to 40-sec. intervals 277 258 373 0.40 0.44 38 22 Alcoa H-44 illuminae 1. Manual equilibration 234 236 261 0.41 0.41 39 32 2. 5 doses 45 sec. apart, then 3-min. wait 233 230 270 0.40 0.41 39 31 3. Continuous doses at 45-sec. intervals 226 222 280 0.40 0.40 41 29 4. Free cycle, 10- to 40-sec. intervals 215 202 282 0.38 0.40 44 28 a 1.05 cc. of STP gas per dose to 0.85-gram sample. On conventional volumetric apparatus. On automatic doser. 1.5 cc. of STP gas per dose to 0.75-gram sample. e 1.5 cc. of STP gas per dose to 1.4-gram sample. VOL. 32, NO. 4, APRIL 1960
535
from desorption data with the automatic doser using the first automatically timed cycle. A portion of the pore size distribution curve for the silica alumina reference sample is enlarged in Figure 7. The plots obtained from the data of the the first timing cycle and from manual equilibration are shown. The values agree reasonably well, even in this detailed curve. Rates of Adsorption. Because t h e volumes adsorbed at a preset rate of addition or removal of nitrogen differ from t h e equilibrium values, the deviation should indicate a property, characteristic of t h e system. Four effects may contribute t o this deviation : The heat of adsorption effect which causes the sample to be temporarily warmer or cooler than the liquid nitrogen bath, The gaseous flow or diffusion into, or out of, the capillaries of the sample. The rate of condensation or evaporation onto, or from, the surface. The surface flow or liquid adsorbate in adjusting to new meniscus curvatures as the pores fill or empty.
To characterize this effect better, the volume adsorbed with the second timing cycle of doses was compared with the equilibrated adsorption values. The differences were small a t low pressures, great in the medium pressure range, and again small when the pore volume was filled. This was taken to indicate that thermal equilibration i n i s not a n important factor, as the effect of the heat of adsorption a t a fixed volumetric rate of delivery would be greater for adsorption a t low pressures. Instead, the parameters connected with pore filling and emptying appeared to control the speed of the adsorption and desorption process. The differences in volumes adsorbed a t definite pressures are plotted in Figure 8 against the equivalent Kelvin pore radii corresponding t o the pressures. The general shape of the curves from desorption rates is similar to the pore size distribution curves. DISCUSSION
Calculations from Isotherm Data. T h e recorded d a t a from this apparatus are given in terms of gas added or removed and relative pressure. As was shown above, a n automatically timed doser cycle of five doses of 1.0 to 1.5 cc. STP per dose, followed by a 3minute equilibration, yielded data comparable to manual equilibration with these samples. When the pore size of the sample is in a larger radius range, it is necessary to lengthen the equilibration time. Both the dose size and the number of doses per cycle can be varied, but in practice the latter is more convenient to change with this apparatus. As the apparatus can operate on a 24-hour 536
0
ANALYTICAL CHEMiSTRY
2c
40
60
80
20 ANGSTROMS
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basis, a total time of 16 to 20 hours may be applied to a given determination, and still allow successive tests to be made on a daily basis. Blthough the use of incremental, rather than continuous, doses allows application of a n automated method to an adsorption system requiring pressure and volume equilibration, it also limits the data to a number of points. It is customary in manual operations to characterize an adsorption or desorption isotherm by 10 to 20 points. 155th this apparatus it is practical to obtain 50 points on each branch of the isotherm, so that the slopes of the curve can be recorded in detail. It is then possible either to draw a smooth curve through the equilibration points and pick off the necessary pressure points for the pore size distribution calculation or to transcribe data points to punch cards for interpolation and calculation by a digital computer. The recording of the data in completely digitalized form is a further step that could be practically applied to this apparatus. When only the desorption curve is actually used for calculation of pore distribution, the adsorption process may be conveniently set at a faster rate, as long as adequate time is allowed for equilibration a t saturation pressure. Determination would also be more rapid using the adsorption branch of the isotherm for the pore size distribution calculation. Deviation of Pressure-Volume Data from Equilibrium Values with Increasing Rate of G a s Feed or Removal. Wheeler (5) showed that under conditions of small pores and pressures as low as 1 atmosphere, the diffusion rate into the pore structure of the catalyst is dependent on the radius of the pore. Application of this consideration leads to one explanation for the spread of the adsorption peak deviation in Figure 8, compared to the sharpness of the desorption deviation. I n the adsorption process the system pressure continuously increases. This system pressure will correspond to the necessary vapor pressure required to fill pores of successively
Figure 8. Deviation of automated 45-second - p e r dose cycle from equilibrium
large radii as the experiment proceeds. If the diffusion rate into the pore is dependent on the radius, condensation in larger pores occurs more rapidly than in smaller pores. Hence, a condition may occur in which the small pores are still being filled by slower diffusion and condensation. The deviation curve then spreads out over a pressure range. Upon desorption, the reverse of this situation occurs, as the pores of largest radii, from which diffusion is relatively rapid, empty first. The probability is reduced of pore liquid evaporating to a system pressure differing from that to nhich it equilibrates, and a sharp deviation peak results. Any restriction in the radius of entrance to a pore will also contribute to this effect, as the diffusion-controlling radius for the adsorption process may be smaller than the equilibrium radius for pore filling. Cpon desorption, however, the diffusion-controlling radius value will be equal to the the radius corresponding to the equilibrium-emptying pressure of the pore liquid. ACKNOWLEDGMENT
The authors acknodedge the continuous aid and encouragement of K’. D . Coggeshall and R. D. Wyckoff in the original concepts of this apparatus and its subsequent construction. LITERATURE CITED
(1) Barrett, E. P., Jojvner, L. G., Halenda, P. P., J . Am. Chem. SOC.73,373 (1951).
(2) Faeth, P., Willingham, C., “Assembly, Calibration, and Operation of a Gas Adsorption Apparatus for the Measurement of Surface Area, Pore Volume Distribution, and Density of Finely Divided Solids,” Tech. Bull., Mellon Institute of Industrial Research, Pittsburgh, Pa., September 1955. ( 3 ) Innes, IT. B., ASAL. CHERI.23, 759 11951). (4) Klevens, H., Carriel, J., Fries. J., Peterson, A,, Proc. Intern. Congr. Surface Actzuity, 2nd, London, lQ57, 2 , 162. (5) JVheeler, h., in “Catalysis,” P. H. Emmett, ed., Vol. 11, Chap. 2, Reinhold, Seiv Tork, 1955. RECEIVED for review September 21, 1959. Accepted January 11, 1960.
