Adsorption of Vapors A New Apparatus, and Di f o r the Lower Ketones, Benzene, and Hexane
07
Activated Carbon SAMUEL JOSEFOWITZ ANI DONALD F. OTHMER
Figure 1. Schematic Diagram of Apparatus
Polytechnic Znstitute of Brooklyn Brooklyn, N . Y .
A . Supporting header tube B. Evacuating mechanism (diffusion pump and Cenco Presso-Vac pu:np) C . Adsorption tube D . Pressure regulating tube containing liquid (surrounded b y constant, low temperature bath) E . Manometer P . Spring of beryllium copper G . Platinum bucket H I Adsorbent thermometer HI Spring thermometer Ha Pure liquid thermometer I . Cathetometer J. Constant temperature bath K . Desorption furnace L. Trap immersed in freezing bath for preventing mercury vapors passing to left or solyent vapors passing to right M . Vent line with calcium chloride tube N . Flexible connection of diffusion pump to system 0. Buret for addition of liquid t o he adsorbed
A n apparatus was 'designed and a techmque was developed by which the phenomenon of equilibrium adsorption could be studied fur controllwork in industrial processes with relative ease and by which accurate, reproducible data could be obtained i n a minimum time. Data are presented for a wide experimental range for the adsorption of acetone, methyl ethyl ketone, methyl isobutyl ketone, hexane, and benzene. Several activated carbons made from coal were studied; practically all previous carbons have come from wood, coconut shells, other n u t shells, and fruit pits.
0
F THE experimental procedures for studying'gas adsorption*
the gravimetric method is the most accurate a n 3 has been favored since the development by XlcBain and Bakr ( 1 ) of a compact and convenient weighing apparatus. They used a helical quartz spring t o weigh a small sample of solid on which the vapors were adsorbed. Sawyer and Othmer (6) modified this, using a beryllium-copper spring, t o give a n apparatus somewhat easier to operate. It was, however, still rather difficult t o assemble and disassemble; and improvements have been made in the present modification. T h e apparatus and method are simple and allow a rapid and reproducible determination of adsorption data. Because of these advantages, the technique is suitable for control work in evaluating different production batches of an */ adsorbent. An apparatus (Figures 1 and 2 ) was designed t o be used up to temperatures of 550" C. and for pressures below the vapor pressure at room temperature of the solvents investigated. It may also be readily adapted for studying the adsorption of permanent gases.
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Figure 2.
Photograph of Adsorption Appari
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stant temperature. A Pyrex tube 25 mm. in diameter and 320 mm. long was closed at one end and had a 29/42 T joint on t,he other end to fit the corresponding joint of the supporting header tube. A side arm was sealed to connect a 15-ml. buret, 0, with a 2-mm. mercury-sealed stopcock, which admitbed t,he pure liquid t o be adsorbed. During a run, this tube was surrounded by a constant temperature bath of methanol contained in a vacuum thermos flask and agitated by an electric stirrer. The tempcrat,ure in this bath i w s always held below room temperature and controlled within 0.2' C. by adding small pieces of dry ice to the bath. [An alternate vapor pressure tube (not' shown) was designed to be used in connection with low boiling liquids or noncondensable gases. It was a Pyrex ampoule acting as a reservoir having an appropriate joint a t one end and a spring-loaded ground-glass stopcock and Pyrex nipple at the other end. A heavy rubber tube was attached to this nipple and led t o a 1-liter mercury leveling bulb. By removing or admitting mercury to thb gas ampoule, the gas pressure could be maintained constant. h side arm vias sealed 18 mm. beloiv the 7 joint leading through a groundglass stopcock to a joint,. A 2-liter gas ampoule n-ith nicrcurysealed stopcocks a t both ends fitted into this joint and contained 0.01 I 1 0 20 40 60 80 100 120 140 160 180 200 the sample of the gas being investigated.] TEMPERATURE O F CARBON('GI MANOMETER. The manometer, E, was a Pyrex tube 7 mm. in Figure 3. Isobars on.Carbon A of Acetone outside diameter att,ached by means of a 12/30 T mercury-sealed joint'. The open end was bent and Dassed down into a small bottle filled kith clean cotton. The column oi mercury (triple-distilled) was measured nith a stainless steel scale; and corrections were made for temperature and atmospheric pressure. SPRING. The spring, F , was of beryllium-copper, as Sawyer and Othmer ( 5 ) and Martin ( 2 ) showed that this alloy is an ideal material. Those used were manufactured by the Instruments Specialties Company, Inc., Little Falls, N.J., of wire 0.178 mm. in diameter viith a coil 8.28 nim. in diameter. The sensitivity of a representative one of 306 turns was 242 mm. per gram. I t was suspended by a Pyrex hook from a cross bar sealed in the joint of the supporting tube, A . At the lower end a Pyrex fiber hook 20 cm. long was used to carry the adsorbent and its container, which when empty, was EQUILIBRIUM V A P O R PRESSURE OF A C E T O N E -(MM.) about 30 em. above the bottom of the Figure 4. Isotherms on Carbon A of Acetone adsorotion tube. P L ~ T I X UBUCKET. M A platinum bucket, G, approximately 2 em. high and 7 mm. in diameter carried the adsorbent. A 3 X 2.5 HEADER TUBE. The supporting header tube, A , was carried em piece of platinum foil 0.001 inch thick was wrapped around by rubber-lined adjust'able clamps which held i t to a rigid steel frame, cushioned by rubber washers t,o minimize vibration. This a 7-mm. carbon rod. A fine platinum wire tied it together. One end was closed over a rounded end of a Pyrex rod 7 mm. in outside tube, of Pyrex 25 min. in outside diameter, was equipped with diameter. Two small holes were made approximately 2 mm. from four standard taper joints: (1) manometer joint -$- 12/30. (2) the top, one piercing the lap joint. A fine platinum wire formed Vapor pressure tube joint, 29/42. Cross bars were sealed in the bail. for suspension of thermometer. (3) Adsorption tube joint -$29/42. Cross bars were sealed in for suspension of thermometer and spring. (4) Mercury trap joint -$- 29/42. A stopcock 15 mm. in diameter of bore was sealed in bet'rveen the adsorption tube and the mercury trap joint to allow the evacuating mechanism to be shut off. EVACUATING MECHASISI\.Z.The evacuating mechanism, B, connected by means of a semiflexible joint, included a mercury diffusion pump and a Cenco Presso-Vac backing pump and was able to reach pressures of 10-6 mm. A trap, I,, surrounded by a dry ice-acetone bat,h was fitted between the flexible joint and the joint of the supporting tube in such a way that it could easily be removed for cleaning without breaking any permanent. joints. This trap condensed any mercury which might have evaporated into the adsorption chamber, and prevented any vapors from the adsorption chamber from reaching the backing pump. A vent cock and calcium chloride drying tube, M , allowed the evacuating mechanism to be put under atmospheric pressure after the main stopcock was closed and the adsorption apparatus was isolated. ADSORPTION TUBE. The adsorption tube, C, was of 28-mm. Pyrex, 60 em. long, closed a t one end and with a 29/42 -$- joint at the other end. I t enclosed the weighing spring, the bucket of adsorbent, and the thermometer, H I , for the measurement'of the adsorbent temperature. PRESSURE-REGULATIXG TUBE. The pressure-regulating tube, D,maintained a constant vapor pressure (alyays less than atFigure 5. Isosteres on Carbon A.of Acetone mospheric) of the adsorbate by keeping a liquid sample a t con-
r
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CARBON TEMPERATURE
'C. 0
Figure 6 . Heats of Adsorption on Carbon A of Acetone TEMPERATURE
Figure 7.
-OC
Isosteres on Carbon A of Benzene
THERMOMETERS. Three thermometers were used. A big? temperature fhermometer, H,, 0 O to 550 C., graduated every 2 , was suspended in the adsorption tube by means of a Nichrome wire, so that the bulb was approximately 1 em. below the bucket when loaded, to give the adsorbent temperature. After each run, the thermometer was recalibrated and readings were corrected. Two other thermometers with a range of - 50 O to 50 O C. in 1O C. were carefully calibrated. One, Hz, was suspended in the upper part of the vapor pressure tube and the other, Ha, was in the pure liquid in tube D. The cathetometer, I, was mounted rigidly in CATHETOMETER. a 2-inch steel pipe fitted solidly into the top of the laboratory bench. The microscope with a focal range of 5 om. was focused on the front of the lowermost coil of the spring, illuminated from behind. The vertical motion of the microscope was 50 mm.; the vernier scale could be read directly to 0.005 mm. and interpolated to 0.001 mm. With a spring sensitivity approximating 0.25 mm. per mg. of weight, changes of 0.00002 to 0.00003 gram could be . measured. No measurements less than 0.0001 gram were made, however. BATH. The constant adsorption temperature bath, J , consisted of a vacuum thermos flask containing peanut oil in which the lower part of the adsorption tube, containing the platinum bucket, was immersed during a run. A high speed electric stirrer ensured uniform temperature. Through a 7-mm. Pyrex U-tube was threaded a 500-watt Nichrome resistance wire coil to act as the heater. It was controlled by a variable transformer. FURNACE. The desorption furnace, K*, was a 30-mm. Pyrex tube around which a 500-watt Nichrome coil had been wound, all enclosed in a 4 X 9 X 4:5 inch cabinet made of l/a-inch Transite board. Heating was controlled by a variable transformer.
3
3
4
5 6
I I I I I I I I I 8 1000 20003 4 5 6 8 DO00 2ObOO VAPOR PRESSURE OF ACETONE (rnrn.1
Figure 8. Isosteres on Carbon A of n-Hexane, Methyl Ethyl Ketone, and Methyl Isobutyl Ketone
MATERIALS
Three commercial activated carbons manufactured from bituminous coal were used. They were identified only as A, B, and C, and we;e supplied by the Chemical Sales Corporatipn. They were all through 16 and on 30 standard screen mesh, and had apparent densities of 56.8, 51.0, and 56.0 grams per 100 ml. and chloropicrin life of 55, 56, and 55, respectively. About twentyfive pieces were selected a t random from larger samples. On
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was approximately 0.01 The mercury diffusion I r69 1 6 0r was then started and all to pump for 3 hours t o 1 pressure of 10-6 mm. I4 0 140 large stopcock was closec pumps were shut off, t h nace was turned off slowl~ I20 the apparatus was allow 12 0 come to room temper 6 LL (The vacuum held i nitely, showed that all E 100 100 and cocks were tight.) a PURGING. When the In ratus had reached room g 80 80 perature, 2 ml. of pure 0, were introduced into D 0 slowly. The liquid vap 60 60 immediately and was ad2 by the carbon. Aft1 minutes, the degassing 40 tion was repeated, an 0 005 0.10 0.15 0.20 Q25 0.30 0.05 0.10 0.15 0.20 0.25 0.30 0.35 carbon was then cons GRAMS BENZENEIG. C GRAMS N-HEXANE/a C ready for use. The new of the spring was noted ADSORBATE CONCENTRATION the exact weight of thc degassed adsorbate wa determined. This purg eration was probably nc essary, since the spring r 140 in all cases after the desorption was identim that after the first deso 120 DATATAKING.A d methanol bath was around the vapor p tube and 12 ml. of purc were slowly introduced. of the adsorbate cor immediately, because low temperature of thi thus most of the bi of the spring by sudd 60 sorption was eliminatec 200 " desorption iurnace v i : : I 40 0 1 I 1 I F 1 T F C 4 placed by the oil bat1 4 0 1 1 1 1 1 r l i ) l / i I v, 0 0.05 0.10 0.15 020 0.25 0.30 0.05 0.10 0.15 0.20 0.25 0.30 the temperature of t z_ GRAMS M.E.K.I p C GRAMS M.I.K. f p C sorbent was adjusted, the temperature of thi ADSOZBATE CONCENTRATION pressure tube. Read the spring deflection we Figure 9. Heats of Adsorption on Carbon A of Benzene, n-Hexane, 3Iethy1 Ethyl every 10 minutes; an Ketone, and Rlethyl Isobutyl Ketone librium was considerec attained when three suk readings were identical, within 1 to 1.5 hours after anv condition had been chan various occasions it was necessary t o make a duplicate loading though some systems required as long as 3 to 3.5 hour from the same larger sample, and reproducible results were weight of vapor adsorbed was then calculated from the dc always secured. However, other samples, presumablv prepared of the spring. by the same method a t different times, differed somewhat. The temperature of the liquid (and hence the pressur All liquids used as adsorbates were C.P. materials which had system) was maintained constant. The carbon tcm been further purified by distillation in a laboratory rectifying was changed, alternately increased and decreased within column. They were degassed by violent boiling immediately of 40" to 200 O C. Frequent checks were made by appro before being introduced into the apparatus. These included desired temperature from above and below. I n no c acetone, methyl ethyl ketone, methyl isobutyl ketone, n-hexane, a n y hysteresis observed, although equilibrium was reach1 and benzene. what faster by starting at the higher temperature and OPERATION the carbon. (Thus, adsorption seemed faster t h a n desi Several runs were interrupted, the carbon degassed, a1 Before each run, all stopcocks and joints were PREPARATION. runs made, which indicated by the reproduction of 1 removed cleaned, and greased. The weighing spring was calibrated $sing standard weights in the bucket, and a straight-line that complete equilibrium had alwavs been reached. calibration was always obtained. A reading was taken with the bucket empty, and then filled with adsorbent t o about 3 mm. from RESULTS its rim (approximately 0.18 gram of carbon), The thermometers and the adsorption tube mere fastened in place. The desorption The data for all sg-stems were obtained a t constai furnace was mounted around the adsorption tube with the bucket temperatures in D-Le., equilibrium pressures-and t h in its center, A dry ice-acetone bath was placed around the merwere plotted as the logarithm of the adsorbate conccntrr cury trap. sus the temperature. Four to six isobars were deterr DEGASSING.The backing pump was started, and the temperature of the bucket and carbon Ti-as raised slowly until thermomeach system, ranging from pressures of a fex millimct eter HI read 500" C. [An asbestos sheet at 5 cm. and another at the vapor pressure of the adsorbate a t room tcmperatr 20 cm. above the furnace prevented any heat radiation from afisobars for acetone on carbon A are indicated in Figure fecting the spring, the bottom of which was the length (20 em.) of for the other systems are not reproduced here. the Pyrex fiber from the hot bucket.] After 0.5 hour the pressure
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April 1948
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
A e r o ~ lplot was made to obtain the adsorption isotherms, which were plotted a t 20' C. intervals as adsorbate concentration versus the adsorption pressure. Only the isotherms of acetone on carbon A (Figure 4)are reproduced here. Another cross plot was made of the isosteres (lines of constant concentration of adsorbate on adsorbent) by plotting the logarithm of the adsorbate pressure a t constant concentration versus a temperature scale calibrated from the logarithm of the vapor pressure of acetone as a reference substance (4). The steps used were as follows: (1) A sheet of standard logarithmic paper had the temperatures indicated corresponding to ths vapor pressures of acetone taken along the X axis'. ( 2 ) Ordinates corresponding to these temperatures were drawn. (3) Values of the data of equilibrium adsorption pressures were plotted on these temperature ordinates according to the logarithmic calibration of the Y axis; and the lines were drawn connecting these points. These lines so obtained were straight for all systems (Figures 5, 7, 8), indicating the excellent mutual correlation of the data. The slopes of these isosteres were then plotted versus adsorbate concentration and the instantaneous heats of adsorption calculated from these slopes and plotted a t 50'. loo", 150",and 200" C. versus adsorbate concentra&ionin Figures 6 and 9. This series of graphs represents a complete picture of the adsorption systems investigated; a further correlation of these data is presented elsewhere ( 3 ) .
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Examination of the data obtained and of that previously reported (4, 6) showed that the adsorption capacity as well as the heats of adsorption vary widely for different carbons as well as for different vapors. These differences are shown best b y studying instantaneous heats of adsorption curves, determined from the slopes of the isosteres (4). It is observed that the same adsorbate on different carbons may have either an increasing or a decreasing instantaneous heat of adsorption with increasing adsorbate concentration. A discussion of the adsorption mechanism to account for this phenomenon is beyond the scope of this paper. ACKNOWLEDGMENT
Thanks are due the Chemical Sales Corporation of Pittsburgh, Pa., for supporting this research and to Robert T. Weil of Manhattan College for supplying the cathetometer. LITERATURE CITED (1)
McBain, J. W., and Bakr, A. M., J . Am. Chem. Soc., 48, 690
(1926). (2) Maytin, W., Metals & Alloys, 17, 1203 (1943). (3) Othmer, D. F., and Josefowitz, Samuel, IND.ENG.CHEM.,40, 733 (1948). (4) Othmer, D. F., and Sawyer, F. G., Ibid., 35,1269 (1943). ( 5 ) Sawyer, F. G., a n d O t h m e r , D. F., Ibid., 36,894 (1944).
RECEIVED September 5, 1946. Presented in part before the Division of Industrial and Engineering Chemistry a t the 110th Meeting of,the AYERIC A N CHEMICAL SOCIETY, Chicago, Ill.
Partition Coefficients of Formaldehyde Solution d
HARRY G. JOHNSON1 AND EDGAR L. PIRET University of Minnesota, Minneapolis, Minn. D a t a for systems of formaldehyde, water, and various water-insoluble organic solvents indicate that the lower members of the series of aliphatic alcohols are the solvents best suited for the extraction of formaldehyde from solutions.. In systems involving the alcohols, a temperature investigation indicates that the partition coefficients are straight-line functions of temperature in. the range of 2' to 45" C. However, the temperature effect is not large. In the formaldehyde, water, and alcohol systems the addition of inorganic salts, soluble in water hut insoluble in the alcohols, caused a large increase in the value of the partition coefficients.
F
ORMALDEHYDE is today a most importanl commercial chemical, yet practically no data on partition coefficients for this compound have appeared in the literature in the past 40 years. The present work was carried out to provide such information. An application of these data could lead to the selection of a solvent for the extraction of formaldehyde from its aqueous solutions. These data should also find application among the many and increasing uses of formaldehyde in the synthetic resin and other indu~t~ries. In the selection of a solvent for formaldehyde the following distinction may be made between types of solvents. The solvents may be classified as inert solvents and as reaction solvents. Inert solvents are those which under normal conditions do not form chemical compounds with formaldehyde. Reaction sol1
Present address, General Mills, Inc., Minneapolis, Minn.
vents, on the other hand, are those in which some reversibIe chemical reaction can occur between the formaldehyde and t h e solvent. Included in this group are the alcohols and water. In the case of the alcohols, the formaldehyde and the alcohols combine chemically to yieId hemiacetals or acetals, depending on pH conditions. Walker in his monograph ( 7 ) points out that formaldehyde exists in aqueous solutions as an equilibrium mixture of methylene glycol, CHS(OH)Z, and several low-molecular weight polyoxymethylene glycols of the type HO(CH,O),H. Since the formaldehyde hydrates, or glycols, are relatively unstable and are formed by reversible reactions, the aqueous formaldehyde solutions may be regarded as solutions or mivtures of formaldehyde and water. Because of this compound formation, aqueous solutions of formaldehyde show solvent properties very similar to those of ethylene glycol and glycerol. Also, the solvent properties of formaldehyde solutions are somewhat similar to those of water (8). In the distribution of formaldehyde between water and a water-insoluble reaction solvent, the partition coefficient may favor the organic phase because of the formation of a chemical compound more stable than the formaldehyde hydrates. When reaction solvents are employed the extract probably consists of an equilibrium mixture of methylene glycols, water, and hemiacetals: HOCHzOH
+ C4H90H
CdH90CHzOH
+ HsO
(1)
On the other hand, the partition coefficients for formaldehyde between inert solvents and water favor the aqueous phase;
