A.
E. HANDLOS and A. C. NIXON
Shell Development Co., Erneryville, Calif.
Vapor Pressure of Phosphoric Acid at High Temperature and Pressure
ALL
catalytic reactions, in which a solid catalyst is used, involve an equilibrium between the catalyst and the reactants. In some cases the equilibrium involves solution of the reactants in a liquid film on the catalyst, in which the reaction takes place. In other cases the reactants are adsorbed on the surface and the reaction takes place in the adsorbed layer. The equilibrium is usually difficult to measure under operating conditions since high temperatures and pressures are often involved. It is customary to try to correlate the activity of the catalyst with the surface area measured by determining the amount of an inert gas adsorbed on the surface under arbitrary conditions. This correlation is oftentimes misleading since the surface area measurements are made under pressure and temperature conditions very different from those in use. The method described in this articleweighing the catalyst at operating conditions on a torsion pendulum balanceprovides a means of carrying out measurements at high temperatures and pressures. The method has been tested by measuring the absorption of water on a catalyst at 300' C. and pressures up to 1300 pounds/square inch absolute. The catalyst used in this test is one widely employed for the polymerization and hydration of olefins. However, in many other catalytic reactions water is also present either as a reactant, a conditioning agent, or a product. The adsorption of the water on the catalyst or its reaction with components of the catalyst has an important effect on the behavior of the reaction. Measurements need not be confined to reactions involving the presence of water. I n catalytic cracking, catalytic reforming, catalytic dehydrogenation, and catalytic hydrogenation, adsorption of the reactants of the catalyst must play a n important role. I t is possible to follow the change in adsorption of the reactants under conditions of reaction. This might show why changes in the method of preparation or in operating conditions produce changes in catalyst efficiency. Alternatively, by application of the present method and the use of indifferent gases such as helium or
1960
nitrogen, it is possible to follow the change in apparent surface area with temperature and pressure ; this should make possible much better correlations with catalyst activity. The catalyst for which data are given is phosphoric acid supported on Celite. Phosphoric acid on quartz or kieselguhr catalysts has been used extensively for the polymerization oi' C S and Cd olefins ( 7 , 4 ) . A phosphoric acid catalyst is also used for the hydration of ethylene to ethanol (6). This catalyst was an interesting one to study since, according to Langlois ( 4 ) and Egloff (7), the condition of the phosphorus acid on the catalyst is not known. The acid could be simply extended by the support or an interaction could occur between the acid and the support. Langlois describes the performance of a phosphoric acid on quartz support in terms of orthophosphoric acid while Egloff indicates that the active material in a phosphoric acid in kieselguhr catalyst is a silicophosphate. Application of the present method allows a decision to be reached on this point. The principle of the torsion pendulum has been applied previously to the solution of problems of this type. Seeliger (8, 9 ) and, more recently, Gelewitz and Thomas (3) applied the same principle to measurements of solid-vapor equilibria at atmospheric temperatures and pressures. However, the present work is the first application of this method at high temperature and pressure and was performed in 1947 without knowledge of the papers by Seeliger. As an example of the possibilities of our method, the adsorption of water on a pelleted Celite at 300" C . and pressures up to 1300 pounds/square inch absolute were first measured. Subsequently, the adsorption of water, nitrogen, and waternitrogen mixtures on a pelleted Celite which was impregnated with phosphoric acid was measured at the same temperature and pressures. Although the method was satisfactory, no decision could be made as to whether the water was adsorbed or whether a solution of phosphoric acid existed on the solid. A tentative explanation is now possible because data on the vapor pressures of phosphoric acid solutions at somewhat
INDUSTRIAL A N D ENGINEERING CHEMISTRY
lower temperatures as a function of concentration have been published (2). By extrapolation of these data, agreement with our data can be obtained which suggests that at 300' C., the metaphosphoric acid is probably the stable form of phosphoric acid on Celite at low partial pressures of water in the vapor phase. This is also the stable form in the absence of a support (5). Experimental
Solid-vapor equilibrium data are usually reported as the mass of vapor per unit mass of solid as a function of the partial pressure of the adsorbate in the vapor phase. In this study, a direct measurement of the mass of the solid phase is obtained by application of the principle of the torsion pendulum. The bob of the pendulum is the solid sample and its supporting bucket. The period of oscillation is a measure of the moment of inertia, I , of the system and thereby of the mass of the solid phase. As long as the elastic limit of the suspension wire is not exceeded and the damping is small. the period of the system is given by: T = 27r
The period of the torsion pendulum is then independent of the amplitude of swing. With granular or pelleted solids, a sufficiently large quantity must be taken to average out the inhomogeneity of the solid. If the bucket which confines the solid sample is a flat-bottomed cylinder, the moment of inertia is independent of the height of packing. The possibility of damping, proportional either to the angular velocity--viscous damping-or to some other power of the velocity or acceleration-carried mass effect discussed by Seeliger-must also be considered. No estimate of the order of magnitude of damping influences was available. I n preparation for this eventuality, an electronic timing circuit was developed. With this device the swings of the pendulum were recorded through a photocell and multivibrator circuit
'indow
Figure 1.
Equilibrium bomb
on a waxed tape alongside of a reference 60-cycle a.c. recording. This equipment was unnecessary. Damping effects were negligible and 100 to 400 periods could be observed with inappreciable decline in the amplitude of the oscillations, Only a stop watch was necessary to record the time. Equation 1 for the period is satisfactory. The moment can be separated into the contribution of the support, I,,, and the solid itself, I,. T = Cz/Zo + I ,
=
4 A
+ BW
(2)
where A , B, and C are constants at a given temperature for the particular suspension wire and bucket assembly. The pendulum and equilibrium bomb are shown in Figure 1. The torsion wire was an alloy of 90 wt. % platinum and 10 wt. 7 0 iridium, 0.020 inch in diameter and 9 inches long. The wire was held by three-jawed chucks at the bucket support and the head of the equilibrium vessel. This alloy was chosen because of its chemical inertness. It is similar to Monel in its mechanical properties. No evidence of creep was observed during use. The borosilicate glass bucket was hung from its support by two pins. The bucket support was
Gage
1 1
;-Air
Bath
made from soft iron so that it could be actuated from the outside of the equilibrium vessel, and was gold plated to prevent corrosion. A platinum mirror was attached to the bottom of the bucket. Mirrors on glass made by chemical deposition, evaporation, and sputtering were not satisfactory in an atmosphere of high temperature steam because the films peeled from the surface. Types 304 and 416 stainless steel were used for the equilibrium vessel. The window of one-inch-thick borosilicate glass, through which the mirror and bottom of the bucket were observed, was sealed by a Bridgman-type closure. The gasket, a ring of gold wire 0.020 inch in diameter, was annealed in place several times. A U-shaped electromagnet, made of soft iron and wrapped with Chrome1 A wire insulated with glass cloth, was placed around the equilibrium vessel. The swing of the bucket was observed by the reflection from the mirror of a narrow beam of collimated light. The entire assembly for the measurement of solid-vapor equilibria is shown in Figure 2. The charging bomb, the equilibrium vessel with the pendulum, and the valves were placed in a n air thermostat. The charging bomb was filled with water before each series of steam runs. The temperature was obtained by observation of a calibrated thermometer suspended inside the bath. The indicated temperature was 300" C., corresponding to 1231 pounds/square inch absolute, but inspection of the maximum steam pressures shows that the effective temperature must have been somewhat higher-e.g., 1433 pounds/ square inch absolute equals 310' C. I t is possible that the bomb, heated by radiation, may have had a higher temperature than the air since the heat losses from the air bath were high at 300" C. A difference of 10 degrees between the reported temperature and the actual temperature is therefore possible. The pressures were measured with a calibrated Heise gage. Calibration of the pendulum was accomplished by loading the bucket with various weighed amounts of Celite pellets a t zero pressure. The period was about 2 seconds with a total load of 50 grams. Excellent agreement with Equation 2 was obtained. For a catalyst load of approximately 50 grams, a deviation of 0.0010 second in period was experienced, corresponding to 0.2 gram or 0.4y0 of the weight. Celite V I I I , a diatomaceous earth with a clay binder produced by the Johns Manville Corp., New York, was used. The sample was pelleted into cylinders 6/32 inch in diameter and "16 inch long. The support was impregnated with phosphoric acid by soaking in 60% acid; after draining the excess liquid, the sample was dried overnight at 140" C.
6
4
I
I
1
8
10
1L
Steam Pressure, PoundjSquare Inch Absolute X
Figure 3. Adsorption of water on Celite and impregnated Celite at
300" C. and cooled over Drierite. The sample contained 10.4% phosphorus and some residual water. Eighty-four per cent of the phosphorus could be leached with hot water and titrated potentiometrically as orthophosphoric acid. Some of the acid evidently reacted with the clay binder to form aluminum phosphate. During the course of the experimental work, an additional 3 to 4y0 of the acid reacted with the support. Additional water was lost when the sample was held in a vacuum a t 300" C. The total phosphorus content of the sample was unchanged. Measurement of the surface area by nitrogen absorption with the Brunauer-Emmett-Teller method gave a surface area of 6 to 10 square meters/gram.
Results The water adsorption of the unimpregnated Celite was determined. Then the water adsorption on the Celite impregnated with phosphoric acid was studied. Data were obtained with water vapor alone, with mixtures of water vapor and nitrogen, and with nitrogen alone. Finally, a second run was made with water vapor alone to determine whether the sample had been changed by the 50 hours of treatment at 300" C. represented by these experiments. The unimpregnated Celite pellets were exposed to a series of steam pressures at 300" C., with the results shown in Figure 3. Equilibrium was reached within 10 minutes after each change in
I
0
1
2
Cc. Nitrogen at STP/PNz
3
-*
. 4
X 1 O2
Figure 4. Effect of water on adsorption of nitrogen on impregnated Celite VOL. 48, NO. 10
4
OCTOBER 1956
196 1
d
2 E7
2
0 Pt-essure l n c r e a s m g >-Pressure First C y c lD e e c r e a s i n g } Second First C y c l e
400
330
-
L
0
200
,
1
400
600
600
I
I
1000
:LO@
Steam Pressure, Pound/Square Inch Absolute
Figure 5. Water absorption on impregnated Celite showing hysteresis loop
steam pressure and was reproducible to &0.47c whether approached from the high pressure side (desorption) or from the low pressure side (adsorption). The data fit a Freundlich adsorption isotherm fairly well. Next, the water adsorption of the impregnated pellets was measured at 300" C. Steam injection was started a t about 150' C. t o prevent excessive dehydration of the acid. Again, equilibrium was reached within 10 minutes after each change in steam pressure, and longer soaking caused no drift in the adsorption. I n one case, a steam pressure of 745 pounds/square inch absolute was maintained for 16 hours without any evidence of drift after the first 10 minutes. The amount of water adsorbed on this sample a t each steam pressure is also given in Figure 3. The dry weight of the sample, corresponding to zero steam pressure a t 300' C., was measured after the first cycle of water adsorption data was run. T h e corresponding phosphorus content of the dry sample was 11.67c. The amount of adsorption (Figure 3) is based on this dry weight since the degree of hydration of the phosphorus oxides is not known. The interpolation which is shown is not real but indicates the choice of zero point. The water adsorption of the impregnated Celite with the bomb filled with mixtures of nitrogen and water vapor was measured next. The total pressure for the nitrogen and steam adsorption data is 1030 pounds/square inch absolute. Adsorption of nitrogen on the dry catalyst was measured over a range of zero to 1260 pounds/square inch absolute and is represented by the single data point on the axis of Figure 4. For the steam-nitrogen adsorption points, the sample was equilibrated with water vapor and then pressured with nitrogen. The increase in adsorbate is small compared to the water adsorption. The steam and nitrogen data are also shown in Figure 3. In Figure 4, the nitrogen adsorption is apparently depressed in the presence of a small amount of water and then rises again as further dilution of the phosphoric acid on the support takes place. I t is difficult to
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account for this behavior for one might reasonably assume that the sorption of nitrogen on the solid would be suppressed as the acid becomes more and more dilute-the increase in weight at the water-nitrogen ratio of 1.8 is beyond our experimental error and represents a nitrogen uptake much greater than the solubility of nitrogen in water. However, the adsorption of water is certainly not affected in any major way by the presence of the nitrogen which is in accord with the experience of Pidgeon and Van Winsen (7), who found that the adsorption of water by asbestos was indifferent to the presence of air. As a final test, a second steam adsorption was run to determine whether the Celite had changed during the above experiments, These data (Figure 5) indicate a 10 to 15yolower water adsorption than the first steam cycle. The appearance of a hysteresis loop in this cycle does not indicate that equilibrium was not reached, since this phenomenon is well knoivn in studies of other gassolid systems. The failure to observe a hysteresis loop in the first water cycle was presumably due to the lack of sufficient data in the high pressure region. Discussion
From the reported adsorption surface area of Celite, it can be calculated that the water adsorption in the impregnated material, even a t low steam pressure (200 pounds/square inch absolute), corresponds to a film of water about 30 molecules in thickness. I t seems reasonable to regard this as a homogeneous film of phosphoric acid rather than to consider that the Celite adsorbs the phosphoric oxides, or some form of strong phosphoric acid, and that the water is laid down on top of, or beside, the oxides without interacting with them. With the assumption of a homogeneous film of acid, the strength of the acid at the various steam pressures could be calculated if it Tvere known at a single point. Data have been reported ( 2 ) for the vapor pressure of water over strong phosphoric acid. Although obtained a t pressures lower than those of interest here, the reported values slightly overlap the pressure range of this study if they are extrapolated to 300" C. Figure 6 is obtained in the following way. From an analysis of the impregnated support and the weight of the sample charged to the apparatus, the phosphorus content of the sample is known. At zero vapor pressure of water, the sample consists of the support and pyro-, ortho-, ormetaphosphoric acid, or phosphorus pentoxide. T o compare the Ivater absorption data with the vapor pressure data of Fontana which are given in terms of orthophosphoric acid, the water absorption data as strength of acid are plotted against the steam pressure,
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
50 10
1
100
I IC00
Vapor Pressure of Pound/Square
Water, Inch Absolute
Figure 6. Comparison of water absorption data on impregnated Celite for various phosphoric acids as initial material
each curve based on a different initial degree of hydration at zero pressure. O n these terms metaphosphoric acid (HPOz) corresponds to 122 weight 7 0 ortho- (HsPOd) and pyrophosphoric acid (HhP207) to 110%. The correspondence between the two sets of data is remarkably good if the acid on the dry support is assumed to have been metaphosphoric acid (HPOa). Furthermore, this curve appears to be a very logical continuation of that for the extrapolated literature data, if the Celite were acting as an inert support for a film of phosphoric acid. Acknowledgment The authors are pleased to acknowledge the invaluable assistance of D. C. Waldman in making the measurements, and helpful discussions with R. L. Maycock and the late R. W. Millar. Nomenclature
A , B, C = constants K = proportionality constant in Equation l I = moment of inertia, g./sq. cm. I, = moment of inertia of bucket assembly moment of inertia of solid sample period of oscillation of pendulum, sec. U7 = weight of solid sample, grams
I, T
= =
Literature Cited (1) Egloff, G., Welnert, P. C., World Petroleum Congr. Proc., 3rd Congr., Hague. 1951. Sect. IV. DD. 201-14. ( 2 ) Fontpna,'B. J.,' J . Am. &%I. Soc. 73, 3348 (1951). (3) Gelewitz, E. W., Thomas, €1. C., R e v . Sei.Znstr. 25, 55 (1954). (4) Langlois, G. E., Walker, J. E., World Petroleum Conpr., Proc., 3rd Conpr., Hague, 1951, Sic;. IV, pp. 191-250. ( 5 ) Mellor, J. W., "A Comprehensive Treatise in Inorganic and Theoretical Chemistry," vol. 8, p. 978, Longmans, Green, New York, 1928. (6) Nelson, C. R., Courter, M. L., Chem. Eng. Progr. 50, 526-31 (1954). ( 7 ) Pidgeon, L. M., Van Winsen, A., Can. J . Research 9, 153 (1953). (8) Seeliger, R., Physik 2. 22, 563 (1921). (9) Seeliger, R., 2. Physik 4, 189 (1921).
RECEIVED for review August 2, 1955 ACCEPTED April 10,1056
