DISCONTINUITIES I N T H E SORPTION PROCESS L. J. BURRAGE King's College, London, England Received October 18, 108.3 INTRODUCTION
In view of the discontinuous nature of desorption isothermals, it appeared to be of interest to establish the presence of these discontinuities in the sorption process. The new technique for the determination of isothermals (see preceding paper), of which brief mention has already been made (1), has shown that when disturbing factors are absent these isothermals consist of a series bf rectangular steps. The presence of disturbing effects of various kinds all cause these steps to become curved, in some cases the actual number being increased by the addition of small breaks. The reasons for this will not be discussed here as this subject is dealt with in a further paper in course of preparation. EXPERIMENTAL
A series of experiments was carried out to determine whether this steplike structure or any indication thereof could be obtained in the sorption process, despite the probability of drift causing disturbances owing to the cleaning-up effect. Since in other experiments it has been found that the presence of carbon dioxide is a troublesome factor, which often partially vitiates the results, it was decided to carry out three experiments, using (1) a charcoal which had a high capacity for carbon dioxide, (2) one which was fair and (3) one which was poor in this respect, viz. : (1) Charcoal A (see reference 2) (2) Charcoal L 1 (Steam activated soft wood. Apparent density 0.445) (3) Charcoal N 1 (Zinc chloride activated almond shell. Apparent density 0.416)
The procedure adopted was the reverse of the usual retentivity process (3). A column of dry charcoal 60 mm. in length was employed and air saturated with moisture at 11.5 mm. pressure was passed a t 400 cc. per minute and 15OC. The air stream was interrupted at very small intervals of time and the adsorption curve constructed, portions of which are shown in figures l a and lb. 41
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L. J. BURRAGE
FIG.la Irt..'
FIG.l b DISCUSSION
Figures l a and l b show quite conclusively that in spite of small fluctuations, the adsorption-time curves consist of a series of straight lines. These give rise to sorption isothermals shown in figure 2. The pressure is calculated in the following manner: From the slopes of the linear portions of the retentivity curve, the weight of vapor adsorbed in
43
DISCONTINUITIES IN THE SORPTION PROCESS
a given time is obtained. Knowing the pressure, and therefore the quantity of vapor in the charging stream in the same period of time, the difference between this and the amount adsorbed represents the weight of vapor in the effluent gas, this giving the pressure of vapor over the end of the charcoal column. These pressures are correct but the isothermals are approximate, since in each case the quantity is not a “true” value but an average one, for it represents the total water in the column expressed as milligrams per gram of charcoal and not that amount corresponding to the last layer of the charcoal column. This in no way vitiates the results, as the object of the present investigation was to demonstrate that sorption takes place in rectangular steps. This is shown by the pressure remaining constant over a given quantit.y range, irrespective of whether the quantity concerned is an average or true value. Figure 2 shows clearly that the sorption isothermal 12
9
. 6
E
.E
xn n
(F
A3 G
3
zoo
)w
0. i n
5w
mq/gram
FIG.2
occurs in rectangular steps, the individual points being omitted except for those marking the beginning and end of these steps. Instead of ignoring the slight deviations from the straight lines in figures . l a and lb, the average rate of adsorption between each two successive points was calculated and assumed to hold a t the middle point of the time interval (figures 3a and 3b). If there were no disturbing factors the rate of adsorption along a step would be constant, the rate-time diagrams appearing in rectangular steps. Figures 3a and 3b, which greatly magnify the errors, show that (1) is the least rectangular, (2) more so, and (3) fairly well defined. These results fall in the inverse order of their capacity for carbon dioxide, as one might have expected. The carbon dioxide evolved during each run was noted qualitatively by passing the efluent gas through baryta water and noticing the change in depth of turbidity. In this connection it is of interest to note that the sudden increase in the rate of
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L. J. BURRAGE
adsorption (figure 3b, point *) was definitely associated with a greatly increased evolution of carbon dioxide.
FIQ.3e
I a)o
3P
'
w
liter.
25.
770
FIG.3b
In figure 2, the pressure figures for the lowest step in each case can be neglected, as several smaller steps whose pressures are, close together are
DISCONTINUITIES IN THE SORPTION PROCESS
45
represented here and one cannot accurately determine them. In general there is a tendency for the quantity per step to be small a t low pressures, larger at intermediate and smaller again a t higher pressures, similarly t o that found in desorption isothermals. To facilitate comparison with the pressures a t which the steps occur in desorption isothermals to be submitted in a later publication the pressures a t which the steps occur have been tabulated. Charcoal 1 4 . 1 mm. 5 . 8 mm. 9 . 9 mm. 10.8 mm. 11.2 mm.
Charcoal W 5 . 1 mm. 6 . 2 mm. 8 . 2 mm. 1 1 . 5 mm.
Charcoal 3 5 . 5 mm. 6 . 7 mm. 7 . 8 mm. 8 . 2 mm. 9 . 2 mm. 10.1 mm. 11.1 mm.
SUMMARY
Sorption isothermals have been shown to consist of a series of rectangular steps similar to desorption isothermals. The effect of the carbon complex (GO,) on the rate of adsorption has been noted. The pressures a t which steps have been found to occur have been tabulated for different charcoals. REFERENCES
(1) ALLMAND, BURRAQE, AND CHAPLIN: Trans. Faraday SOC.28,218 (1932). (2) HANDAND SHIELS:J. Phys. Chem. 32,441 (1928). (3) ALLMAND AND BURRAQE: J. SOC.Chem. Ind. 47,372 (1928). BURRAQE: J. Phys. Chem. 34,2202 (1930).
