THE SORPTION OF WATER VAPOUR BY ACTIVATED CHARCOALS PART. I. APPARATUS-TECHNIQUE-NATURE OF CHARCOALS USED BY P. G. T. HAND AND D. 0. SHIELS
1. Introduction
The work to be described in this, and succeeding papers forms part of an investigation carried out in this laboratory under the direction of Professor A. J. Xllmand from the commencement of 1 9 2 1 , and still in progress. Its primary purpose has been the establishment of the experimental facts concerning the sorption of vapours by charcoal with a view, of course, of arriving a t an understanding of the essential nature of the phenomenon, and of an explanation of the marked differences shown between the behaviour of the vapurs of water and of other substances. I n addition, whatever the use adsorbent charcoals may be put to, the omnipresence of water vapour makes a study of its sorption relations, in particular, of direct practical importance. When this work was commenced, the only previous publications on water vapour of importance known to us were those of Schmidt and Hinteler,’ Bachmann,2 and Lowry and H ~ l e t t . During ~ the course of the work, there ,~ have been published or become available to us papers by R a k o v ~ k yGustafson,j Hallstrom,6 Berl and Andress,’ Isobe,s G u ~ t a v e rKatz’O ,~ and Coolidge.” Two general experimental methods have been employed. I n the one (“dynamic” or “streaming” method), a current of air or of nitrogen charged with a known partial pressure of water vapour, by bubbling through sulphuric acid solutions of definite concentrations a t 2 j”C, was passed through a column of charcoal contained in a silica tube at 2 j°C =t0.05’ until constancy of weight had been reached. The charcoal had been previously outgassed at a definite temperature, and subsequently saturated with the pure dry gas at zs°C. The purpose of the experiments in the nitrogen stream was to investigate whether or not the presence of oxygen in charcoal has a specific effect on the sorption process. I n th’e second (“static” or “vacuum”) method of experimentation, water vapour, free from any admixed gas, was admitted to the previously evacuated Z. phgsik. Chem., 91, 103 (1916). * Z anorg Chem., 100, I (1917). 3 J. Am. Chem., Soc., 42, 1393 (1920). J. Russ. phys. Chem. SOC.,49,371 (1917). 6 Arkiv. Kemi. Mineral Geol., 7, No. 22, I (1919) Dissertation Helsingfors (1920). Z. angew. Chem., 34, 369 (1921). Chem. J. (Japan), 1, 99 (1921). Kolloidchem. Beihefte, 1C, 185 (1922). In Proc. Amsterdam .4cad., 26, 548 (1923). 1’ J. Am. Chem. Soc., 46, 596 (1924).
’
P. G. T. HAND AND D. 0. S H I E L S
442
charcoal (contained, as before, in a silica or glass tube a t zs0C), and the change in weight determined when the pressure, read off on a mercury manometer, had become constant. A series of such experiments, by either method, extending from zero water vapour pressure up to saturation a t 2 5 O C enabled the sorption isotherm to be plotted. In almost all cases these isothermals were determined with falling as well as rising water vapour pressure, i.e. desorption as well as sorption isotherms were investigated. In a few cases (with the “static” method) experiments were made at temperatures other than 2 5 O C .
‘I
2.
Dynamic Experiments
The main apparatus for the air stream experiments is shown in Fig. l a , while the necessary modifications for working in nitrogen are seen in Fig. I b. As certain details of the procedure in the two cases differ it will be as well to describe first those items which are common to both methods. (a) Charcoal tube: The charcoal is contained in a silica U,Fig. 2 C, fitted with glass L pieces ground a t each end (gl gz, g, g4). The former fit the U by means of silica grindings surmounted by cups W1 W Z(filled with Everett’s vacuum wax). The taps are well-ground vacuum taps lubricated with Ramsay grease. (b) Evacuation of charcoal: the U containing the charcoal is heated in an electric furnace so arranged that the arms of the U project about 5 cms. from the furnace. The wax seals are protected by a thick asbestos sheet, and asbestos wool packing (vide Fig. 3A (e) ). 93 and g4fit into ground cups of the evacuation leads, and the joints are made tight by vacuum wax. The evacu-
SORPTION OF WATER VAPOUR BY ACTIVATED CHARCOALS
443
ation outfit consisted of a Kraus mercury vapour pump backed by a Cenco Hyvac Oil Pump. Pressures are measured by means of a McLeod gauge, inserted in the evacuation lines. In most of this work two evacuation temperatures are used viz. z70°C and 8ooOC. (c) Vacuum weight of charcoal: In determining this the usual precautions are taken to check the volume of the empty container by filling with mercury a t a known temperature and weighing; checking for the free-space air over the charcoal which necessitated determining the density of the charcoal, reference to which will be made later, and also determining the loss in weight of the charcoal during evacuation. All weighings are carried out with a T--tube counterpoise. The weights used were checked by means of the Kohlrausch method. (d) Saturation of charcoal with dry gas: This is carried out by attaching the U to the ground cup G S 2 Fig. Ia, allowing the contents t o come to the temperature of the thermostat and then passing a stream of 'dry gas over the charcoal a t a rate of 60-80 ccs. per min. Equilibrium being judged to be set up when the change in weight of the U was not greater than f 0.1 mgr. Knowing the free-space volume above the charcoal a correction for the amount of gas in this was made, and hence the weight of the sorbed gas obtained. (e) Experimental details using an air stream: Dry Air: Air from cylinder A Fig. Ia is passed through trap TI to (I) the gas washer \VI, containing conc. sulphuric acid and thence by way of flowmeter Fm to the soda lime train S1 S2 Ss and S4. The rate of flow is roughly adjusted by means of the reducing valve fitted to the cylinder. and finally adjusted by taps between W1 and A. Leaving S4 the air passes via trap Tz through a sulphuric acid pumice tower M, filled from F1, and finally, via two-way tap tl over phosphorus pentoxide, contained in P, into the thermoatat by way of lead coil LS2 to two-way tap t a ; finally issuing at GS2. The U is attached a t this point by means of g3,Fig. 2c, and connected to a mercury manometer N from g4 by means of rubber tubing. By carefully opening the container taps the flow of air is admitted to the charcoal. The manometer is used to ascertain the excess pressure, over atmospheric, of the air in the free >pace. Usually 2-3 mms. Dryness of dzr: In order to obtain perfectly dry air two extra gas bubblers containing conc. sulphuric acid were introduced between S4 and TZFig. Ia. To check the dryness of the air a stream a t 100cc. per min. was passed through the apparatus and thence over a weighed quantity of pure phosphorus pentoxide contained in a U-tube fitted with ground-in glass tap heads, connected of course to GS2. The exit side of the C was connected to an absorption bulb containing conc. sulphuric acid. After two hours the U-tube, weighed against a counterpoise, shewed no increase in weight. P u n t y of air. This was tested by attaching a small bubbling apparatus, containing lime water to GS,, and passing air through the train at 100 ccs. per min. for a number of hours. S o precipitate of calcium carbonate was observed.
P. G. T. HAND AND D. 0. SHIELS
444
(2) Air charged with water vapour: I n this case the air-stream is diverted at two-way tap tl via two-way t 2 through trap T3,containing a shallow layer of conc. sulphuric acid, into which dips a capillary inlet tube, and thence to the glass coil saturators XI and X2. These saturators are filled with sulphuric acid solutions of such strength that a water vapour pressure is obtained of approximately I mm. below the vapour pressure of the acid solutions of the saturators situated within the thermostat. Leaving the two “external” saturators the moist air travels through the “splash catcher” T3,through the lead coil I&, to the “internal” glass coil saturator O1by way of safety trap T4. From 01the air stream passes by way of two-way tap ts into the “internal” saturator OS, and thence by way of traps Te and Tj, through two-way tap t r to GSS, where it enters the charcoal tube. The two internal saturators are filled with sulphuric acid of predetermined strength to give a known water vapour pressure a t 2 j°C. The volumes of XI and X pare both 2 0 0 0 cc., while O1 contains 400 cc. of acid, and OS1 2 5 cc. During any run with moist air the parts of the U exposed above the water level, indicated by a dotted line, are kept a t 26OC A I O C by means of a metal box, B, which is heated from below by means of a micro burner Q. Eflciency of Saturators: I n order t o determine whether the air leaving the final saturator O2was saturated with respect to the solution contained therein, the amount of water sorbed by the charcoal for a given solution was determined. iln additional saturator, Fig. zD, was introduced after 0 2 . The Utube containing the evacuated charcoal, after saturation with dry air, was connected to the extra saturator and air again passed over the charcoal via the saturating train and the extra saturator. The value thus obtained for the weight of water sorbed agreed very well with the value obtained without the extra saturator.
Pressure water vapour
Mgs/grm water sorbed
4 saturators 1j6.4~
13.40 mms 13.36 mms
5 saturators 156.38
Completeness of the saturation of dry air with respect to the solution through which i t passed is confirmed by the very small change in concentration of sulphuric acid after passage of air for many hours. Run No. I
Hrs. of passage of air
Concentration of Solution %HPSOI
0
24 2
3
0
37.01
25
36.99
0
30.94
127
30.96
SORPTION OF WATER VAPOUR BY ACTIVATED CHARCOALS
445
(3) Determination of water vapour pressures: The vapour pressures of the solutions were determined from a concentration-pressure curve plotted from the data of Regnault and Sore1.l The concentrations of the solutions were determined by means of density measurements using a I O co. pyknometer which was weighed against a sealed counterpoise. Two determinations for each solution being made, agreement being within 0.j mgr. The concentrations were obtained from Domke’s* figures, plotted as a concentrationdensity curve (4) Possible carriage forward of sulphuric acid: Small quantities of sulphuric acid will, no doubt, be carried over to the charcoal in the course of time. To test this, a stream of air was passed through the saturators at 80 ccs. per min., and thence into a flask containing I j o cc. I N BaC12. After j hours a very faint turbidity was observed. A check solution containing I . j x IO-^ gm 98% H2S04in I j o cc. I K BaClZ gave a readily noticeable precipitate. By comparison it was estimated that the turbidity in the test run represented less than 5 X IO+ gm HDSOI. This turbidity was obtained after passage of air through 615: sulphuric acid solution (the most concentrated solution used). Generally speaking, the time of streaming through the more concentrated solutions is much less than in the case of the weaker solutions. Equilibrium being more rapidly obtained at low vapour pressures than a t higher ones; so that the average amount of acid carried over could not be per hour. As some charcoals were exgreater than about I X IO-^ gm posed to various humid streams for 600 hours the maximum quantity of acid carried over would not be greater than 6 X IO-^ gm, or for a charge of 3 gms charcoal z mgr/gm. ( 5 ) Changing saturating solutions: The saturators O1 and O2 are completely filled with the required solution from FD and Fa. The solutions are then drawn off through Y1and Yp, well mixed and the process repeated, after which the saturators are filled to the required height. The traps T4T, and Ts are washed, three times, by blowing over solution from O1and Oz, making use of soda-lime tubes Q1 and QD to prevent contamination with carbon dioxide. Tyithdrawal of the washings is effected via C1and Cz, which are finally sealed after leaving a small quantity of liquid a t the bottom of T, and T5. (f) Experimental details using a nitrogen stream. ( I ) Dry Xitr‘ogen: The apparatus used in the nitrogen stream experiments is shewn in Fig. Ib. The dotted line Ll L1 denotes the line of demarcation to the lcft of which the apparatus is similar, except for a few minor details, to the air stream apparatus. Xitrogen is obtained from Cylinder A, and is 99.j7$ pure. The usual traps, gas washer, flow meter, and pumice-sulphuric acid tower are present. (TI W1, Fm and If). TI is a trap while lVZ and UTa contain conc. sulphuric acid. SI, Sz, S3 and S4 constitute the sold lime train. From Sq the dry nitrogen passes over a mixture of electrolytic 1
Landolt-Bbmstein: “Tabellen,” 426-427 ( 1 9 1 2 ) . Landolt-Romstein: “Tabellen,” 265-266 (1912).
P. G. T. HAND AND D. 0.SHIELS
446
copper foil, and copper wire gauze (60 mesh) contained in two conibustion tubes, CT, inserted in an electric furnace maintained at 4o0°-4500C. This acts as the deoxidizer. Leaving this the gas flows through the cooling coil LSS, and passes by way of two-way tap tl to the phosphorus pentoxide tube, through the thermostat, to the C tube. In order to work in as pure a nitrogen stream as possible, a special attachment to the ground joint connection GS2 was used where by the dead space between the inserted limb of the U-tube and tap tal Fig. Ia, could be evacuated and flushed out with the pure gas before it was allowed to flow over the charcoal. This procedure was carried out every time the U-tube was attached to the apparatus. Purity of Oxygen: To test the amount of oxygen present in the gas stream, a method was worked out for the estimation of small amounts of oxygen in gaseous mixtures.' The point of attachment of this apparatus to the streaming apparatus is not shewn in the diagram Fig la. A large number of tests were carried out on the dry nitrogen stream; results showed that the amount of oxygen present was of the order of less than I part in 1.5 X 106 by volume. Dryness of Nitrogen: This was tested in exactly the same manner as in the air stream method; after 8 hours run at 80 ccs. per min. no increase in the weight of the Pz O5 tube could be detected. Nitrogen charged with water vapour: I n this case the flow of gas is (2) diverted a t tl (Fig. Ib) through tz to the usual saturation line as shewn in Fig. Ia. Purity of Nitrogen charged with water vapour: A few of the tests taken on the moist gas are as follows: ( I ) Fresh charge of copper in deoxidizer (100gms). Freshly made up acid. Apparatus swept out with nitrogen for z hours, copper not heated. After flowing for 3 hour over copper a t 4o0°-4500C. Oxygen detected: I part in 1.4x 1 0 4 parts K2. (2) Fresh charge of copper in deoxidizer. Freshly made up acid. Apparatus swept out with nitrogen for z hours, copper not heated. ilfter flowing 13 hours over heated copper. Oxygen detected: I part in 6.6 X 1 0 5 parts Nz. Time of flow Nz over deoxidiser Hrs. IO0
140 I 60
Oxygen detected
1/1.4 X 1/1.4 X I / I . ~X
Kate of flow ccs./rnin.
104
70-80
10'
70-80 70-80
104
In many instances 100-zoo hours elapsed before the charcoal reached equilibrium. To keep the oxygen concentration down to the limits found in the foregoing tests the deoxidizer was recharged every 60-100 hours. It would therefore be safe to assume that the maximum concentration of oxygen present is not greater than I/I . 4 X IO'. Hand: J. Chern. Soc., 123,2573 (1923).
SORPTION O F WATER VAPUOR BY ACTIVATED CHARCOALS
447
(g) Record of a typical run: To obtain data to plot the results as an isotherm an experiment must be composed of a number of “runs.” that is, a series of determinations of equilibrium points for various partial pressures of water vapour. I n the cases where the velocity of sorption was fairly rapid, i.e. a t low pressures, the stream was passed over the charcoal until no change in the quantity of water sorbed took place over a fairly lengthy period, usually 5-7 hours. I n the case of runs taking 2 0 0 hours or so equilibrium was judged when the change in weight was not greater than 0.I mgr/gm per hour. As an example of how a “run” is carried out the following figures, taken from a trial run, will make clear the main details. Nitrogen-water vapour Stream: American coconut. Charcoal : As taken from stock. State of Charcoal: 10-12. Mesh size: 800°C. Temp. of outgassing: Evacuated Wt. of charcoal: 1.3754 pms. 2.1526 gms. charcoal Evacuated Wt. of container 14.63 ccs. Internal vol. of U-tube:
+
(I)
Saturation with dry Nitrogen.
I
Rate ccs/min
775.5
50
22a (2)
3 Press S2 mm
2
Hrs
Water vapour pressure.
I
2
Hrs.
Rate ccs/min
3 Total Press. mm
75
752.0
24
70 70 70
22.5
70
752‘0 753.5 762.3 764.5 762.4 762.0
0.75 5.5
47.5
16
70
22.5
60
138.75
5
6
W t , of S1 in free space gms.
Wt. of U contents gms.
+
2.1996
0.0158
7
wt. of N1 sorbed gms.
0.0312
14.08 mms.
4 5 Press.Sz \Vt.ofX’p+Water Vap. infreespace mm gms
6 7 Wt. of C wt. of contents water sorbed. gms g m
2.3920 2.4188 2.5023
2.5123 2.5165
748.32 747.92
0.01544 0.01543
Wt. of water absorbed
=
2.5184 2.5199
0.3191 0.3206
233.15 mgs/grm.
In the above table: Column I refers to the times of exposure of the charcoal to th‘e flow of gas. “ z indicates the rate of flow of the gas. 3 shows the total pressure over the charcoal a t the moment of detachment of the U-tube, prior to weighing.
P. G. T. HAND AND D. 0. SHIELS
448 " "
"
4 gives the partial pressure of the nitrogen over the charcoal. 5 shews the weight of nitrogen water vapour in the free space of the U-tube. 6 gives the weight of the U-tube its contents, as weighed against its counterpoise. 7 gives the weight of water vapour sorbed by the charcoal, on the assumption that none of the gas originally present has been displaced.
+ +
c
7
3. Static Experiments (a) Charcoal tube: The container (Fig. PB),either silica or glass, consists of a bulb 8 cc. capacity, a well-fitting end on vacuum tap, and a side tube ending in a ground joint. (b) Vacuum weight of charcoal: The method of obtaining the evacuated weight of the charcoal is very similar to the method used in the streaming method. -4more detailed account here will suffice for both methods, static and streaming.
SORPTIOS O F W.4TER VAPOUR BY ACTIVATED CHARCOALS
449
The container, with tap in position, and shut off, is weighed, using a sealed counterpoise of the same material and having an external volume to within I cc. of the container. The charcoal is introduced, the tap replaced in the shut position, and the whole reweighed. S e x t the container tap is greased, and inserted in the shut position and the whole reweighed. An example will shew the idea of the method. .Imerican Coconut
Charcoal
Density (after 8oo°C Evacuation) Internal Volume of container S o . I (silica) I. Wt. of container air -!- charcoal L, 2. air iI)--( 2 ) = Approximate Wt. of Charcoal Volume of charcoal = I 669 cc. 3. Kt. of air in container T. = I ; 5°C. Bar. = 740 I nim. 4. K t . of air in free space above charcoal T. = 18.o'C. Bar = ;lo I mni (1)-(4) = Kt. of container charcoal
+
[(
(2)-(3)
5.
+
1 b
=
''
''
