Distillation Principles of

Distillation Principles of Formaldehvde Solutions J

STATE OF FORMALDEHYDE IN THE VAPOR PHASE RI. W. HALL1 AND EDGAR L. PIRET Un,iversity of Minnesota, Minneapolis, M i n n .

T h e fractional distillation of formaldehyde solutions involves processes of reaction kinetics as well as the usual problems of vapor-liquid equilibria and physical transfer operations. The vapor phase densities of mixtures of formaldehyde and water and of formaldehyde and a series of clcohols have been investigated. The vapors apparently consist of an equilibrium mixture of monomeric formaldehyde and methylene glycol, or of monomeric formaldehyde and the hemiformal of the alcohol. The degree of dissociation of the glycol or hemiformal varies with the superheat and with the solvent. The equilibrium constants and the heats of reaction of monomeric formaldehyde with the solvent vapors have been estimated from the vapor density data. A new hypothesis is presented concerning the various distillation phenomena observed in formaldehyde systems.

T

HE literature on the distillation of aqueous formaldehyde solutions has been reported in a previous paper (10) together with the results of an investigation of the phenomena involved. The latter included studies of the variation of boiling points with concentration, liquid-vapor equilibria a t 740- to 760-mm. pressure, and a demonstration of the effect>sof fractional distillation on enrichment of the vapors. Some explanations were presented for the discrepancies which exist in the literature concerning these properties. A favorable liquid-vapor equilibrium involving the methylene glycol-water system was eliminated as the cause of vapor enrichment. The basic phenomena causing the extraordinary vapor enrichment which occurs during the fractional distillation of aqueous formaldehyde solutions are, however, still uncertain. The present investigation on the state of formaldehyde in the vapor phase was therefore undertaken in the hope that some light might be shed on the causes of this enrichment. Auerbach and Barschall (1) in 1907 performed some vapor density measurements on paraformaldehyde which indicated that the paraformaldehyde molecules had dissociated for the most ,part into unhydrated monomeric formaldehyde and water vapor. Kalker (14) in 1931 concluded on the basis of Clapeyron equation calculations that the vapor above aqueous solutions probably is the monomolecular unhydrated formaldehyde. However, Zimmerli (15, 16) in 1927 expressed his opinion that methylene glycol (formaldehyde monohydrate) exists in the vapor phase, and boils a t about 96" C. and that fractionation gives a vapor relatively rich in methylene glycol. Bond (3) in 1933 also stated that vapors from aqueous solutions distilled a t atmospheric pressure contain little or no unhydrated formaldehyde. Hence there still seems to be a question as to the state of formaldehyde above aqueous solutions of 0 to 40% concentration, and apparently this problem has not been studied by direct experimental methods. 1 Present address, Minnesota Mining & Manufacturing Company, S t . Paul, Minn.

The state of formaldehyde in the vapor phase above its alcoholic solutions has also not been reported. The success of alcohol in preventing precipitation of polymer in concentrated aqueous solutions may be the result of its greater affinity for formaldehyde ( 7 ) . If this is true, it would suggest that alcohol and formaldehyde would also show a lesser tendency towards dissociation in the vapor phase than methylene glycol. Vapor density measurements on alcoholic solutions may therefore complement the data obtained on the aqueous solutions. MATERIALS U S E D AND METHODS OF ANALYSIS

MATERIALS USED. The methanol, n-propanol, n-butanol, nhexanol, isopropyl alcohol, and isobutyl alcohol were of analytical reagent grade. The ethyl alcohol was absolute alcohol. All the alcohols except ethyl alcohol were redistilled and the middle fraction was taken for the preparation of the formaldehyde solutions. The samples of paraformaldehyde used throughout this work were from the lot previously reported (10). The same method of preparation of the aqueous formaldehyde solutions was used (IO). The alcoholic formaldehyde solutions were prepared by refluxing the 96% paraformaldehyde and the alcohol until the paraformaldehyde had dissolved. These solutions were then cooled, filtered, and aged for 7 days before analysis. The water samples for the control run were made from distilled water which was boiled for 15 minutes t o expel dissolved gases and then cooled just before insertion into the sample tube. METHODOF ANALYSIS.The sulfite method of Lemme (9) was again (10) used in this investigation for the formaldehyde analyses. In the case of the higher alcoholic solutions, sufficient methanol.was added during the titration to prevent any separation of two phases. APPARATUS AND EXPERIMENTAL PROCEDURE

The average molecular weight of the vapor can be determined by vapor density measurements in which a known weight of sample is vaporized into a bulb of known volume a t a measured pressure and temperature. The pressure is determined a t various temperatures and the various degrees of dissociation can be calculated. The method used in this study is similar to that described by Daniels, Mathews, and Williams ( 4 ) ,except that a mercury seal is used instead of a glass diaphragm. The apparatus is shown in Figure 1. The bulb, A , is made from a single-necked Claisen flask. A steel sphere, B , 15 mm. in diameter, is added t o the bulb so that it can be used to break the sample tube, F . A piece of glass tubing, C, 10 mm. in diameter, is fused to A and used for supporting the apparatus. Tube D is of 2-mm. capillary tubing bent in the form of a U-tube. The end of D is connected to a vacuum pump, to a small air bleeder, and to the external manometer, H . Tube E is a piece of 7-mm. glass tubing in which F is inserted. I n the case of the data shown in Table IV, two tubes, E , were fused to A , in which the water and the paraformaldehyde samples were inserted separately. Before F is inserted, sufficient mercury is added to A so that it can be drained into D and used to form a seal in D after evacuation. This seal in D is used as a balancing manometer to equalize the external pressure and the pressure inside the apparatus, which is read on H . B is then rolled into A , F is inserted into E,E is sealed off, the apparatus is evacuated completely, the mercury is run into D, and the capillary end of F

1277

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

1278

I

l l

I1

I 1

Vol. 41, No. 8

buret. The temperature is measured to 0.1" C. The pressure difference in the system is measured with a mercury manometer xhich is read to 0.5 mm. For the data given in Tables I to 111 and V I to SV,approxi. mately 0.1-gram samples of water, aqueous formaldehyde solutions, or alcoholic formaldehyde solutions, respectively, were prepared in 3-nim. glass tubes having one end sealed and the other end drawn out to a capillary. When the tubes were filled, the capillary ends of the small tubes were heated quiokly to boiling to dispel air and then sealed and weighed to 0.1 nig. The more concentrated aqueous formaldehyde composition for the data in Table IV was obtained by means of samples of distilled rvatttr and paraformaldehyde sealed in separate sample tubes.

l l

] J

From t,he measured volume, the temperature, the sample weight, and the manometer reading, corrccted for mercury vapor pressure, a,re calculated the degree of dissociation, a , the equilibrium constant, IC,, and the heat of reaction, - A H , of the monomeric formaldehyde with the solvent vapors. The perfect gas laws are assumed and any deviation from the calculated line for 10070 dissociation is interpreted as being caused by the exietence iii the vapor phase of undissociated methylene glycol OF of t,hc alcoholic hemiformal. The assumption of t,he simplest n.

r lgure

*

I,

1 apor

1.

-. . . . -4ppnratus . uissociation

is broken off as shown in Figure 1 by dropping B against E'. The upright flask with D approximately balanced is then inserted into an oil bat,h which is stirred by an electric mixer, G. The bath is heated to approximately 150" C., H being varied t o maintain D level at all times. The apparatus is then cooled until the desired temperature is reached. During the first experiments, the bat,h was maintained a t t'his temperature for 15 minutes for each reading, but because the readings,did not change measurably after 30 seconds, most of the readings in later experiments were made after maintaining the bath constant for only 2 to 5 minutes with occasional check readings made with the bath constant for 15 minutes. Readings of pressure on manometer H and of temperature on thermometer J are then taken. Aft.er the readings are discontinued, the tip of E is ground off, the mercury and the broken sample t'ube, F , are removed, and A is cleaned and dried. An extension to E is then fused on, and the next run is st,arted as before. E is sealed off each time within 2 mm. of the same over-all distance to a line scratched on E , so that the volume remains constant, within 0.2 mi. The capacity of A in each of the two pieces of aplmratus ernployed is approximately 300 and 500 ml., respoct,ively. The volume is calibrated to 0.2 ml. by filling with water Irom an Exax

T ~BIJ: 11. VAPORPH.ISEDIbsocIAmoh 011 30.87, FORMALDEHYDE-WATER SYSTEM Observed Partial Pressure, 1Im. I l p Corrected for t I n Vapor

.. ..

29.2 29.2 147.4 146.2 146.0 124.5 124.4 101.0 81.5 74.0 72.0

Time a t Each Temp., Xin.

5 2

5 i

15 > i

2

2 12

j 1

.. ..

20 10 2

2 12 13 2 5 15 2 2 2 2 2 3

2

2

2 2 2 .?I

5 15 2 10 2

PHASEDISSOCIATIOS OF DISTILLEDFVATER

Bath Temp., O C.

Observed Partial Pressure, M m . Hg, Corrected for Hg Vapor

L'i.0

24.8 153.0 144.5 141.0 140.0 135.0 112.0 110.5 110.0 94.0 91.0 86.0 85.0 83.0 81.0 '80.0 79.5 78.2 77.6 76.5 76.1 74.1 69.6 60.4 59.2 53.7 53.0

.

464.4 455.8 451,6 450.7 449.2 419.5 418.0 417.0 399,s 395.3 3O0,4 388.4 386.4 365.9 349.4 342.5 333.5 325.0 315.0 305.6 280.0 231.5 154.0 145.5 111.3 108.0

5-1". I I a n.1- ..,,l..mn l v l l l l l

A V

S'apor

Fractional Dissociation, a

Equilibrium Constant K p , Atm.

...

...

TABLE 111. 'l'imr a t Each Temp., Min.

0.0 0.0 0.0 0.0 0.0

.....

,

.

. . , . .

Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condenmd

Xeight of watei sample 0.0956 gram Calculated partial pressure at 0' C., OCTO dissociation, 297.4 mm. Hg

... . .

..

20 20

15

15 1; a

10 3 3 15 10 10 10 10 10

a

Vapor Equilibrium Constant K O ,A t m . '

0.0

20.;, 225.0 224.0 224.0 212.0 212.0 ... 199.1 0 '374 7.78 188. D . . 185.0 . . 184.0 70.0 181.5 179. 0 67.0 64.0 161.0 67.0 178.5 74.5 185.5 177.0 66.5 Condensed 175.0 66.0 Condensed 65,s 173.0 Condenird 142.5 60.5 Condensed 60.0 Condensed 136.0 50.5 89.5 Condensed 50.3 87.0 Condensed 50.2 Cnnri?ns~ii 86.0 50.0 85.0 Condcn-od Flask volume 304.0 ml. Weight of sample 0.536 firam % .~ H C H O in s a m d e 30.8 Calculated partial pressure a t O n C., OTo dissooihtion 1 1 5 ~ min. 4 FIg 100% dissociation 146.2 mm. Hg

15 15 15 20 20 1.5 15 15

2

TABLE I.

Fractional Dissociation,

V.4POR PHASE DIssocranoK OB' 30.87, FORJIALDCHYDE-WATER SYSTFM

Observed Partial Vapor Bath Pressure, Rlm. H g , Fractional F:quilibrium Temp., Corrected for Hg Uissociation, Constant, O C. Vapor a K p , Atm, 25.7 0.0 ... ... 25.7 15.5 152.6 424.9 0 : 997 , . . 422,7 150.5 0.996 ,.. 402 3 0.994 ... 130.2 0.995 392.2 120.0 0.976 lii:66 381.0 110.0 0.978 17.1 371.2 100.0 0.973 13.55 361.8 91.2 0.970 12.1 360.3 90.0 Condensed 337.5 80.0 Condensed 344.4 81 0 0.942 s , 93 353.4 85.0 0.970 12.1 360.3 90.0 0,986 3.33 331.9 84.0 Condrnsed 324 4 78.5 293.4 Condensed 76.2 Flask volume 303.8 nil. T%-eightof sample 0,0999 gram 7oHCHO in sample 30.8 Calculated partial pressure a t 0' C , , 0 % dissociation 215.2 mm. Hg 272,7 mm. He 10070 dissoriation I

INDUSTRIAL AND ENGINEERING CHEMISTRY

June 1949

12'19

hydrate seems reasonable iri view of the low vapor pressure exerted by higher hydrates which should allow but very little of the higher hydrates to exist in the vapor phase. The equilibrium constant of dissociation, K,, in atmospheres, can be readily calculated froin the value of a , the fraction of methylene glycol dissociated, as follows: Let a = fraction of methylene glycol or alcoholic hemiformal which has dissociated. Let X = moles of excess water (or alcohol) per mole of formal-

Mrthylene glycol

+ water

HCHO

1-0

ff+X

a

The total number of moles at equilibrium = 1

+ + X. (Y

The ecluilibrium constant, Kp,in atmospheres, then br>conws:

\i+TTl For example, froin data in Table 11, ~

0 240

Values of K , were calculated only for those cases in which the observed partial pressure and the calculated 100% dissociation pressure diflered by 1.0 mm. or more.

AQUEOUSCONPOSITIONS.A control run was made with distilled water, the data of which are shown in Table I arid Figure 2. These data fall on both the accepted partial pressure curve of water and the calculated line for 0 % dissociation. Four runs were made with aqueous formaldehyde compodtions, the data of which are shown in Tables I1 to V and Figures 3 and 4. Formaldehyde concentrations of 30.8 and 61.4%, respc.rtively, were used. The data cover a pressure range of 50 to 720 niin. of mercury. The broken straight lines drawn in Fiyuies 2, 3, aiitl 4 are foi the calculated pressures. As an example, in Figure 3, curve9 1 and 2 are calculated on the basis of 100% dissociation of methylene glycol. Curves 3 and 4 ale calculated on the basis of 0%

Time a t Each Temp., Min.

Iv. VAPOR

DISSOCIATION O F 61.4% FORMALDEHYDE-WATER SYSTEM

Eigure 2.

Bath Temp.,

C.


26.0 0.0 13.5 26.0 716.1 157.9 718.1 158.0 712.6 355.0 152.0 708,O 140.7 688.6 652.3 119.5 632,6 108,5 622.7 104.0 612.7 100.0 608.8 98.5 601.3 97.5 593.8 96.7 96.2 587.8 95.5 072.8 556.3 94.6 537.3 93.8 526.3 93.2 Condensed Condensed 513.8 92.7 91.8 Condensed 498.8 0,995 91.3 683.5 138.0 Flask voliime 524.2 ml. Weight of water sample 0.1204 gram Weight of paraform sample 0.2142 grams 7' H C H O in paraform 96.0 Calculated partial pressure a t 0' C. 0% dissociation 232.85 mm. Hg 100% dissociation 455.3 mm. Hg . . I

..

5 2 10 5

E

2 2 2 2 2 2 2 2 2 2 2 2 2 13

Vapor Equilibrium Constant. K p , Atni.

360

-

400

440

MM. HG

Vapor Phase Dissociation of Distilled Water 0. Watnr (Table 1)

dissociation of methylenc glycol. The circles and triangles show the experimental data obtained. It is apparent that the data fall very close to the 100% dissociation lines except near the boiling point. In order to avoid a large error in weighing the small sample rrquired for the data a t the lowest pressure (Table V), i t was deemed in this case to be advantageous to use a different method for obtaining the samples. After the run a t the highest pressure (Table IV) was completed, the oil bath was reheated to 138" C.,

PHASE

Observed Partial Pressure Mm. Hg, Corrected for Hg Vapor

320

PARTIAL PRESSURE

TABLEV. TABLE

280

VAPORPHASE DISSOCIATION OF 61.4% FORMALDEHYDE-WATER SYSTEM

Time

Observed Partial Vapor Bath Pressure Mm. Hg Fractional E uilibrium Temp., Correcteb for H g Dissociation, !'onstant, C. Vapor a K p , Atni. 65.3 138.0 0,995 ... 5 63.9 127.5 1.01 ... 2 107.0 60.6 1.01 92.5 58.3 1.01 2 103.0 59.7 0,993 5.'68 92.0 57.8 2 0,989 3.48 2 55 4 0.982 78.0 ... 6 57.0 52.0 0.977 ,.. 2 50.5 50.0 0.962 0.89 49.0 2 46.0 0.949 0.60 2 48.0 Condensed 44.0 2 45.5 43.0 Condensed 41.5 2 40.5 Condensed 2 39.5 41.0 Condensed 2 38.0 40.0 Condensed 35.0 39.0 Condensed 37.5 32.0 Condensed 2 35.5 29.0 Condensed Flask volume 524.2 ml. Total weight of original sample 0.3346 gram (Run 3, Table IV) 61.4 LT, . _ HCHO in total commosition 61.4 Composition of sample is same as in Table IV, but a portion of vapor has been removed. Fraction of vapor remaining, assuming no change in degree of dissociation 65.3/683.8 0.0955 Calculated partial pressure a t 0' C. 0% dissociation 22.25 mm. Hg 100% dissociation 43.5 mm. Hg

at Each

Temp., Min. 5

...

;


1280

zot 01 280

I

I

440

360

I

520

I

Figure 3.

1

I

600

PARTIAL PRESSURE - MM. HG

680

780

Vapor Phase Dissociation of Methylene Glycol

7 1;' 7/ /, f-j

'0

80

40

Table

30.8 61.4

I11 IV

100% YO Corve

1 2

3

4

Experimental Data

0

A

a t jvhich temperature the previous data had shown that vaporization was complete and the degree of dissociation was about 99.5%. About 90% of the vapors were then rapidly removed from the bulb by evacuation while the temperature was maintained constant a t 138" C., after which the bulb was again sealed off by means of the mercury in the small balancing manometer. Readings were then taken as the oil bath was cooled to room temperature. The data for this run are shown in Table 1 7 . The calculations were based on the assumption that no change in the degree of dissociation occurred during the partial evacuation of the vapors and that the vapors were 99.5% dissociated. The cooling curves shown in Tables 11, 111, and V were interrupted after the partial pressure curve of aqueous formaldehyde was reached and several readings were taken as the samples were ieheated to a point where nearly complete dissociation had again occurred. Data on the cooling curve were then continued. Equilibrium was therefore approached from both directions. These data indicate that equilibrium was reached, for both sets of data fall on the same curve. The same procedure was used in the subsequent experiments with the alcohols and the data again shom-ed equilibrium to have been attained. The data for the aqueous formaldehyde conipositions indicate that over a pressure range of 50 t o 720 mm., the vaporized methylene glycol when heated 10" C. or more above its boiling point shows dissociation approaching 100% into the components, monomeric formaldehyde and water vapor. At and slightly above the boiling point, the data deviate from the line calculated for completely dissociated vapor. Examination of Tables I1 and V indicates that near the boiling point, the methylene glycol fraction, (1 - a),is of the order of 5% of the total vapor. The variation of the vapor equilibrium constant, K,, with the reciprocal of the absolute temperature, l / T ° K . , is shown in Figure 5 . The equilibrium constant for the temperature range of 40" to 160" C. is given by the expression:

120

Figure 4.

- MM. HG200 160

PARTIAL PRESSURE

240

Vapor Phase Dissociation of 41ethylene Glycol

Calculated Dissociation HCHO, 9%

Vol. 41, No. 6

HCHO, % 30.8 61.4

0.0032

Table I1 V

0.0030

Calculated Dissociation 100% 0% Curve 1 2

0.0028

3 4

0.0326

Experimental Data

2

0.M24

I/T OK

Figure 5. Effect of Temperature ob Vapor Equilibrium Constant of FormaldehydeWater System


June 1949

VAPOR PHASE DISSOCIATION O F 39.35% FORMALDEHYDE-METHANOL SYSTEM

1281

TABLE VI. Time at Each Temp., Min.

Bath T:rnZ., 30.0 30.0 154.0 152.5 139.0 108.0 117.5 122.0 129.5 137.0 96,O 94.0 90.0 84.5 83.5 80.5 78.5 77.2 74.0 70.0 68.6 68.5 59.5 56.0 55.0 52.5 49.7 42.5 36.2 33.8 33.8 33.0

Observed Partial Pressure, Mm. Hg, Correated for Hg Vapor 0.0 44.0 244.2 243.4 233.7 206.6 214.3 219.7 224.3 232.9 191.8 190.3 187.4 177.5 175.0 171.5 169.5 168.5 163.0 158.5 154.0 153.0 143.5 138.5 132.0 120.0 104.0 79.5 64.5 59.0 59.0 57.0

Q

Vapor Equilibrium Constant, K p , Atm.

.. .

...

0 : 897 0.898 0.878 0.775

1.62 1.63 1.27 0.52 0.63 0.81 0.82 1.32 0.29 0.28 0.26 0.17 0.15 0.14 0.12 0.12 0.092 0.078 0.061 0.057 0.037 0.028


0.800 0.832 0.836 0.812 0.681 0.676 0.666 0,585 0.561 0.535 0.520 0.515 0.470 0.439 0.394 0.382 0.311 0,266

16C

'14C

...

Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed

Flask volume 305.2 ml. Weight of sample 0.0902 gram 72 HCHO in samule 39.35 .As paraform oontains 4% water, the solution will contain 4 X 39.35/100 or 1.6% water. Calculated partial pressure a t 0" C. 0% dissociation 97.06 mm. Hg 100% dissociation 162.9 nim. Hg

The molar heat, - A H , of the reaction of monomeric formaldehyde with water vapor can be estimated from the equilibrium constant, K,, by means of the Clapeyron equation. The calculated value obtained is 14,800 calories per mole. This value compares favorably with the heat of reaction of monomeric formaldehyde with liquid water as determined by Delepine (6) and Walker ( I S ) . who both obtained values of about 15,000 calories per mole. ALCOHOLICCOXPOSITIONS. Vapor density measurements were also made on formaldehyde compositions in methanol, ethyl alcohol, n-propanol, n-butanol, n-hexanol, isopropyl alcohol, and isobutyl alcohol. The data are shown in Tables VI to XI1 and Figures 6 and 7. Duplicate runs, Tables XI11 to XV, made with the methanol, ethyl alcohol, and n-butanol compositions show nearly identical results. The alcoholic compositions used in this investigation contained traces of water from the 96% paraformaldehyde used in the preparation of the samples. The data on the dissociation of the aqueous formaldehyde compositions indicate, however, that in the vapor phase water and formaldehyde exist largely as separate and distinct entities when heated a few degrees above the boiling point, Therefore it is probable that traces of water will cause little interference with the accuracy and significance of the dissociation measurements of the hemiformals-derived paraformaldehyde and the various alcohols. The associated forms are probably the hemiformals. The data, shown in Figures 6 and 7 , indicate that the hemiformal systems exhibit varying degrees of dissociation depending on the degree of superheat and the alcohol used. Under similar conditions of superheat, the dissociation becomes greater as the molecular weight of the alcohol increases, but remains nearly the same with both straight- and branched-chain alcohols of the same molecular weight.

lec

100

3 IBO W pz

c'a 6C E W

n.

3 40

t-

eo

C

40

BO

120

240

200

160

mRTIAL PRESSURE- MM. HG

Figure 6.

Vapor Phase Dissociation of n-Alcohol Hemiformals

Alcohol Methanol E t h y l alcohol Propanol Butanol Hexanol

C a l c u l a t e d Dissociation 100 % 0% Curve 1 6 2 7 3 8 4 9 5 10

Experimental Data

::

8

A

TABLE VII.

VAPORPHASE DISSOCIATION OF 37.85% FORMALDEHYDE-ETHASOL SYSTEM

Time a t Each Temp., Min. 15 15 5 2 10 5 2 2 5 5 5 5 5 2 2 2 2 2 2 2 2 5 2 2 2 2 2 2

Bath Temp.,

c.

21.5 21.5 148.5 132.5 120.0 117.5 110.5 107.0 104.0 98.0 94. 90.0 86.5 77.0 65.5 63. B 55.0 113.0 111.0 108.0 62.0 59,5 57.5 56,5 55.5 55.0 38.5 37.5 38.5 30.0

Observed Partial Pressure M m H g Correotgd foi. H i Vapor 0.0 16.0 187.2 179,8 172.8 171.8 167.5 165.2 163.2 159.7 156.3 152.3 149.4 141.9 133.5 131.0 100.6 169,O 168.0 165.6 130.5 127.5 117.0 110.5 105.0 101.0 53.0 51.0 42.5 34.0

.

a

Vapor Equilibrium Constant, K p , Atm.

0 : 994

...


.

0.991 0.973 0.974 0.959 0.950 0.941 0.930 0.908 0.878 0.859 0.810 0.758 0.735 0,963 0.963 0,950 0,736 0.707

I

.

...

Condensed

Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed

4.27 4.21 2.67 2 13 1.76 1.43 0.97 0.72 0.60 0.39 0.26 0.22 3.01 2.99 2.13 0.22 0.18

Flask volume 305.3 ml. Weight of sample 0.0819 gram % HCHO in sample 37.85 As paraform contains 4Ye water, the solution will contain 4 X 37.85/100 or 1.5% water. Caloulated partial pressure a t Oo C. 0% dissociation 64.1 mm. Hg 100% dissociation 121.55 mm. Hg

INDUSTRIAL A N D EN G INEERING CHEMISTRY

1282

TABLE

Time at Each Temp., Min.

..

TABLE rx.

VIII.

V A P O R PHASE DISSOCIATION O F 32.15% FORMALDEHYDE~~-PROFA~OL SYSTEM

Bath Temp., C. 24.0 24.0 145.5 121 . o 119.0 113.5 106.8 93.0 81.5 74.0 71.0 66.0 62.5 61.8 61.0 59.0 55.8 54.4 50.4 45.0 34.5 22.0

Observed Partial Pressure, M m . Hg, Corrected for I-Ig Vapor 0.0 8.0

102.1 151.2 150.8 147.6 144.6 137.8 131.4 125.0 122.5 118.0 111.0 107.0 104.5 96.0 82.5 76.0 62.0 48.5 27.0 11.5


Yap01 Equilibrium Constant, Kp Atm.

... 1.'dol 0.987 0.990 0.978 0.969 0.946 0.915 0,85S 0.836 0.791

.

I

.

... ...

... ...

Conderised Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed

4.48 3.08 1.62 0.94 0.40 0,40 0.28

VAPOR PHASE D I ~ ~ O C I A T 0 8 7I O 27.35% X F0RMALDhHYDE-n-BUTANOI, SYSTEN

Tim? a t Each Temp >fin."

.. 2 5 10 2

165,O

147.0 145.0 141.0 138 5 132.0 116.0 101.0 93.0 78.0 80.0 93.0 88.5 84.5 83.0 80.0 78.5 76.0 71.0 69.5 68.5 66.5 A4,5 60.0

2

5

1 2 2 2 2 4 2 2 2 2

2 2 2 2 2 2 2 2

58.0

57.0 56.0

5 5

43.0 34,o 31.0

10

TABLE X. VAPORPHASE DISSOCIATION OF 21.45% FORMALDEHYDE-~-HEXAKOL SYSTEM rime

at Each

Temu..

a1iL. ..

Bath Temu..

6.

22.0 22.0 156.5 137.5 135.0 126.0 111.5 103.0 93.0 90.0 87.5 84.0 83.0 80.3 78.5 76.2 73.5 69.0 64.8 60.1 57.5 45.0

Observed Partial Pressure, Mm. Hg, Corrected f o r H E Vapor 0.0 1.0 76.9 74.3 73.5 71.6 68.5 66.7 64.3 63.4 61.9 60.9 58.9 53.9 49.5 45.0 40.0 32.5 27.0 22.5 20.0 12.0

Fractional Diasociation. a

Vapor Equilibrium Constant. K,, -4tm.

Observed Partial Pressure, 1Im. Hg, Corrected for Hg Vapor 0.0 3.5 127 0 129.5 126.0 124.0 123.5 122.0 120.2 114.9 109.2 104.8 97.4 98.4 104.8 102.4 100.5 99.5 98.5 98.0 95.5 93.5 88.5 83.5 77.0 71.0 61.0 52.0 49.0 46.0 25.5 12.0 8.5


Vapor Equilibrium Constant K,, Atin.'

...

0:9+5 1,000

3.41

1.033

1.012 1.018 1,005 1.011 1,000 0.975 0.932 0.870 0.877 0.931 0.912 0.898 0.883 0.882 0.878 0.840

I

.

,

...

. .

... ..,

. .

2.93 0.97 0.43 0.46 0.95

Condensed Condensed Condensed Condensed Condensed Condensed Condenscd Condensed Condensed Con d en 3 e d Condensed Condensed

0.71 0.99 0.50 0.48 0.46 0.32

Flask volume 305.0 ml. Weight of sample 0.0745 gram 70 HCHO 27.35 As paraforin contains 4% mater, the solutlon will contain 4 X 27.35/100 or 1.1 % water.

...

..I

1.018 1.006 1.000 0.986 0.975 0.95s 0.942 0.905

Bath

Tti~., 20.5 20.5 161.4

'0

Flask volume 303.3 nil. Weight of sample 0.0840 gram % H C H O in sample 32.15 A s paraform contains 4% water, the solution will contain 4 X 3J.15/100 or 1.3Ye mater. Calculated partial pressure a t 0" C. OY0 dissociation 55.2 mm. H g 100% dissociation 105.4 mm. H g

Vol. 41, No. 6

TABLE XI.

... Cnndensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed

0.92 0.68 0.40

Flask volume 305.3 i d . Weight of sample 0.0577 gram % HCHO in sample 21.45% .Is paraform contains 4% water, the solution will contain 4 X 21.45/100 or 0.85% water. Calculated partial pressure at 0' C. 0% dissociation 26.0 mm. H g 100% dissociation 49.0 mm. H g

The variation of the vapor equilibrium constants, K,, with the reciprocal of the absolute temperature, 1 / T " K., is shown in Figures 8 and 9. The equillibrium constant for the temperature range of 50" to 160" C. is given by the expressions in Table XVI. The molar heat, - A H , of the reaction of monomeric formaldehyde with alcohol vapor was estimated from the equilibrium constant, K,, by means of the Clapeyron equation. The calculated value obtained is about 14,800 calories per mole for each of the alcohol systems. This value compares favorably with the heat of reaction of monomeric formaldehyde with liquid methanol, propanol, and butanol as determined by Walker ( I S ) , who obtained values of about 15,000 calories per mole for each alcohol system. Examination of both Figures 8 and 9 shows that under the same temperature conditions the degree of dissociation seems to rise to a maximum (higher K,) with the propyl hemiformals and

VAPORPHASE DISSOCIATIOX O R 32.2%

FORMALDEHYDE-ISOPROPYI, i h C O E I O I , S Y R T E X

Time a t Earl], Temp., blm. , .

...

10 2 5 2 2

2

2 2

Bath Temp., O C. 22.2 22.2 144.5 148.0 141.0 112.0 110.5 104.0 99.0 93.0 88.0 81.0 80.0 75.0 64.0 55.0 53.0 49.5 47.0 43.0 42.0 40.0 37.5 31.9 30.2 27.0 23.5

Observed Partial Pressnre, 4lm.Hg, Corrected for Hg Vapor 0.0 9.0 120.3 120 I8 119.3 111.0 110.0 108.2 107.2 104.8 103,s 100.5 100.0 , 97.5 93.5 80,O

88.0 85.5 83.5 80.0 73 0 68.0 58.0 42.5 37.0 31.6 25.5

Fractional Dissociation. a

Vapor Equilibrium Constant. K,, Atiii.

...

:

. .

1 024

1.028 1.025 1.024 1.014 1.014 1.021 1.010 1.010 0.992 0.985 0.964 0.932 0.900 0.890 0.854 0.823 0.763

...

.. 1'.78

0.86

Condensed Condensod Condensed. Condensed Condcnsod Condensed Condonmd

0.63 0.47 0.32 0.25 0.16

Flask volume 305.3 ml. Weight of sample 0.0620 gram % H C H O in sample 32.2 A5 paraform contains 4% water, the solution will contain 4 X 32.2/100 or 1,37& water Calculated partial pressure a t Oo C. 0% dissociation 40.8 mm. Hg 100% dissociation 78.07 mm. Hg

then decreases with further increase in the molecular weight of the alcohol. -4comparison of the aqueous and alcoholic data in Figures 5, 8, and 9 shows that, especially a t the lower temperatures the hemiformals are dissociated to a lesser degree (lower K,) than methylene glycol, indicating that the affinity of formaldehyde for alcohol

June 1949


TABLE XII. VAPORPHASE DISSOCIATION OF 27.5% FORMALDEHYDE-ISOBUTYL ALCOHOL SYSTEM Time a t Each Temp., 31in.

Observed Partial Vapor Bath Pressure Mm. HE Fractional Equilibrium Temp., Correcteh for Hg' Dissociation, Constant, C. Vapor a Kp, Atm. 24.2 0.0 ... 24.2 6.0 ... 154.6 140.6 0 : 994 ... 140.2 149.5 1.013 ... 146.3 139.1 1.013 127.9 117.0 0.988 ... 116.8 94.0 0.927 1.00 112.5 87.0 0.889 0.60 B 106.0 75.5 0.836 0.35 68.0 101.0 2 0.784 0.23 90.5 2 62.5 Condensed Condensed 60.7 83.5 2 59.5 79.0 Condensed 2 55.5 Condensed 64.3 2 57.5 53.5 Condensed 3 2 50.4 48.5 ,.. ... 2 46.8 40.0 ... ... 4 41.0 29.0 ... ... 5 22.5 8.5 Flask volume 305.3 inl. Weight ofsample 0.0832 gram 7" HCHO in sample 27.5 As paraform contains 4% water, the solution will contain 4 X 27.5/100 or l.lVc water. Calculated partial pressure a t Oo C. 0% dissociation 47.5 mm. Hg 1 0 0 ~ dissociation o 90.0 mm. Hg

...

...

...

is milch greater than for water. These data may explain to some extent the solubilizing role played by alcohols when added to aqueous formaldehyde solutions, as this greater affinity should also exist in the liquid phase ( 7 ) . APPLICArlON OF HYPOTHESIS TO DISTILLATIOY I'HENOMENA

It has been demonstrated (IO) that when a mixture of formaldehyde and water vapor, obtained by boiling an aqueous formaldehyde solution at atmospheric pressure, is subjected to partial condel sation, the formaldehyde content of the uncondensedvapor is gre?ter than that of the entering gas mixture. the ratio of condensate to distillate determining the degree of vapor enrichment. This fact is supported by the scattered data of other investigations. It has also been demonstrated that a favorable liquid-vapor equilibrium involving the methylene glycol-water system is probably not the cause of vapor enrichment. From this it would follow that during partial condensation there is a resistance to the transfer of formaldehyde from the vapor phase to the liquid phase, so that water is preferentially condensed. This is in agreement with Walker's stated views (12). Data presented in this investigation indicate that the formaldehyde vapor above aqueous solutions at and above the boiling point consists largely of the dissociated products, monomeric formaldehyde, and water vapor, in equilibrium with a small amount of methylene glycol vapor. The data obtained on the water-formaldehyde system, although not conclusive, are supported by the very distinct and larger effects found in the alcoholic mixtures. It has been demonstrated conclusively (a, 6, 11) that formaldehyde in aqueous solution exists to a very large extent in the form of methylene glycol and higher hydrates and that the methylene glycol is the only hydrate exerting any appreciable vapor pressure. It is apparent then that the methylene glycol existing in the liquid phase must necessarily dissociate either in the liquid phase or in the vapor phase during the vaporization process in order for the formaldehyde to exist in the vapor phase in the dissociated form. It is also apparent that the dissociated monomeric formaldehyde existing in the vapor phase must necessarily combine with liquid water or with water vapor a t some stage during or after partial condensation in order for the formaldehyde to exist again in the aqueous condensate as methylene glycol or as a higher hydrate.

1283

TABLE XIII. VAPORPHASE DISSOCIATION OF 39.35% FORMALDEHYDE-METHANOL SYSTEM

Time a t Each Temp., Min.

..

5 2 2 2 . 5 2 2 2 2 2 2 2 5 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 15 15

Bath Tzmp., C. 24.0 24.0 156.0 149.0 145.5 143.0 136.5 126.0 122.0 120.0 115.0 109.5 105.5 100.0 96.0 88.0 80.0 70.0 65.0 62.5 59.5 64.0 51.0 49.5 48.0 46.0 44.5 41.5 39.0 35.5 29.5 29.5

Observed Partial Pressure hlm. Hg Correcteh for H h Vapor

a

Vapor Equilibrium Constant, Kp Atm.

...

...


Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed Condensed

Flask volume 305.0 ml. Weight of sample 0.0866 gram % HCHO in s a m d e 39.35 As paraform contains 4% water, the solution will contain 4 X 39.35/100 or 1.6Ve water. Calculated partial pressure a t 0' C. 0% dissociation 93.4 mm. Hg 100% dissociation 156.8 mm. Hg

TABLE XIV: VAPORPHASE DISSOCIATION OF 37.85% FORMbLDEHYDE-ETHYL ALCOHOL SYSTEM Time a t each Temp., Min.

Observed Partial Vapor Bath Pressure M m Hg, Fractional Equilibrium Temp., Correcteb- for' Hg Dissociation, Constant C. Vapor a Kp, Atm.' 0.0 24.0 , . 24.0 15.0 ... 13 147.3 216.4 0 : 994 ,,. 131.5 15 208.3 0,993 ... 201.2 10 121.5 0.972 4.80 15 110.0 192.0 0.937 1.93 20 187.2 102.5 0.926 1.58 181.3 95.5 5 0.899 1.07 2 178.0 91.6 0.883 0.90 172.5 2 86.0 0.852 0.65 167.0 8 1 . 5 0,814 0.46 1 157.5 73.0 0.749 0.29 154.5 70.5 0.727 0.26 145.0 63.0 2 0.652 Condensed 0.16 138.5 61.0 2 2 131.5 59.8 Condensed 2 123.5 58.3 Condensed 55.2 2 112.0 Condensed 2 52.3 923.0 Condensed 76.5 46.3 2 Condensed 2 61.0 40.9 Condensed 34.0 2 42.0 Condensed Flask volume 305.3 ml. Weight of sample 0.0950 gram % HCHO i n sample 37.85 As paraform contains 4% water, the solution will contain 4 X 37.85/100 or 1.5% water. Calculated partial pressure a t O o C. 0 % dissociation 74.15 mm. Hg 100% dissociation 141.1 mm. Hg

;

It has been postulated by iVaIker (12) that the above reactions take place in the liquid phase a t a rate which is so slow as to be controlling. During the vaporization step, methylene glycol would be decomposed in the liquid phase with the liberation of monomeric formaldehyde. During the condensation step, monomeric formaldehyde would be absorbed in liquid water. The discovery made in this work of the existence of an equilibrium in the vapor phase among water, methylene glycol, and monomeric formaldehyde has made possible the consideration of a new hypothesis to explain the phenomena occurring during the

Vol. 41, No. 6


1284

FORMALDEHYDE- NORMAL ALCOHOL SYSTEM

I

@

i

- METHANOL (TABLES 6,131

- ETHANOL (TABLES 7,14) - PROPANOL (TABLE 8 ) 0 - BUTANOL (TABLES 9,151

~

x

-

3 2

I

0.9 0.6

i

2 0.4 I

Y" 0.2 01

0

I

40

1

80

I

I

le0

160

I

200

PARTIAL PRESSURE - MM. HG

I

240

0.I 0.08

Figure 7.

Vapor Phase Dissociation of Isoalcohol Hemiformals

Calculated Dissociation 100 % 0% .4lcohol Curve Isopropyl alcohol 1 3 Isobutyl alcohol 2 4

0.06 0.04

Experimental Data

8

distillation of aqueous formaldehyde solutions. -4t equilibrium, the concentrations of methvlene glycol, formaldehyde, and water in the vapors must satisfy a nuniber of conditions. Each of these vapors must be in equilibrlum w t h the liquid, and satisfy, let us say as a n approximation, Raoult's or Henry's law. However, these concentrations must also simultaneously satisfy the chemical equilibrium relationships between the components of the vapor, as determined by the value of the equilibrium constant, K , . The degree of dissociation of the methylene glycol can therefore determine to an important extcnt the total formaldehyde content of the vapor in equilibrium with the liquid. The late of dissociation or of association, in the vapor or in the liquid phase, can also become important in determining the amount of total formaldehyde that is transferred t o or from the vapor phase in a nonequilibrium distillation, condensation, absorption, 01 other process. Depending upon experimental conditions, the ti ansfer of formaldehyde from the vapor phase to the liquid phase can take place by absorption of nionomeric formaldehyde in liquid water or by the absorption of the small amount of methylene glycol follou ed by subsequent adjustment of the vapor equilibrium. I n view of the larger association of formaldehyde with alcoholic vapors, it appears that this factor can he important in processes involving alcohols such as in the gas phase oxidation of methanol to formaldehyde or in processes vihere alcohols, water vapor, and formaldehyde are included among the components. The fractional distillation of formaldehyde is therefore a process involving not only the usual problems of vapor liquid equilibria, but in addition problems in reaction kinetics. A knowledge of the mechanisms involved and of the rate of the reactions can be important to the economic design of equipment and to the use of formaldehyde in processing. It is difficult, however, with the available experimental evidence, to show

0.02

0.0032

0,0028

0.0030

I/T

0.0026

0.0024

O K

Figure 8. Effect of Temperature on l a p o r Equilibrium Constant of Formaldeh~de-n-.4lcohol System

conclusively n-hich mechanism is controlling. Further sbudies on this system are therefore needed. The vapor enrichment 11-hichoccurs during partial condensation can probably be accounted for on the basis of a slow rate of adjustment of the vapor equilibrium, so that m-ater vapor is preferentially condensed and falls away as reflux before all the inonomeric formaldehyde in equilibrium with i t has reacted to form t,he higher boiling methylene glycol vapor or before all the monomeric formaldehyde can be absorbed in the liquid condensat,e. The resulting vapors are therefore rich in formaldehydc. The presence of a chemical equilibrium in the vapor seems to offer an adequate explanation for most of the other puzzling distillstion phenomena which are characteristic of the formaldehydewater system. For instance, one would expect t,he first member of the glycol series to boil a t about 180' to 200" C. If a mixture of water and methylene glycol were vaporized viithout decomposition, the vapor sliould contain only a small amount of t.he higher boiling met,liylene glycol. However, the weight. per cent of total formaldehyde actually existing in the vapor phase a t equilibrium above a 2.1% solution is nct,ually about 2.1% (IO). D a h from t,he present investigation indicate that at equilibrium, under atmospheric conditions and a t the boiling point, methylene glycol vapor is dissociated into mononieric formaldehyde and water vapor t>othe extent of about 95%. The amount of met,hyfene glycol present in the vapor, as might be expected from the boiling points, is therefore much less than 2.1%. A 2.1'3, solution boils near 100" C. and under these conditions of temperature, concentration, and pressure, a definite degree of dissociation exist.s. -4Iower temperature, and therefore a loiver equilibrium constant (Figure 5 ) will result in a low percentage of total formal-

June 1949


TABLEXVI. EFFECTOF TEMPERATURE ON VAPOREQUILIBRIUM CONSTANTS OF FORMALDEHYDE-ALCOHOL SYSTEMS

TABLEXV. VAPOR PHASE DIWOCIATIOI~ O F 27.35% FORMALDEHYDE-n-BUTANOL SYSTEM Time a t each Temp., Min.

1285

-

Observed Partial Vapor Pressure, Mm. Hg, Fractional Equilibrium Corrected f o r Hg Dissociation, Constant, C. Vapor a K p , Atm. ... 0.0 24.5 ... 1.5 24.5 ... 116.1 164.0 ih 0 :969 2.46 115.1 163.0 0 0.974 2.88 112.2 151.0 8 ... .. 111.0 143.0 5 109.4 137 0 5 0:963 1. a 3 102.8 118.0 15 0.967 2.06 102.0 114.0 2 0.967 1.48 97.7 100.0 10 0.923 0.75 92.9 87.5 2 0.920 0.71 91.9 84.0 2 0.905 0.58 90.9 82.0 2 0.897 0.52 90.0 80.5 2 0.877 0.51 88.0 76.0 2 0.850 0.31 85.0 69.0 2 0.832 0.26 8 4 . 0 68.0 2 0.815 0.23 83.0 67.0 2 Condensed 78.0 65.0 2 Condensed 74.0 64.0 2 Condensed 9 PI?, 0 70.0 Condensed 65.0 6i.o 2 Condensed 5 9 . 0 6 0 . 0 2 Condensed 55.0 58.0 2 Condensed 5 0 . 0 57.0 2 Condensed 42.5 54.5 2 Condensed 38.0 53.0 2 32.0 Condensed 2 49.5 Condensed 24.5 44.5 2 16.5 Condensed 35.5 5 Condensed 13.5 33.0 2 Condensed 10.5 29.5 ? Condensed 8.5 27.0 Condensed 7.0 25.0 Flask volume 305.0 ml. Weight of sample 0.067 gram % HCHO 27.35 As paraform contains 4% water, the solution will contain 4 X 27.35/100 or 1.1%water. Calculated partial pressure a t 0' c. 0% dissociation 38.9 mm. Hg 100% dissociation 73.1 mm. Hg Bath Temp.,

..

.

:

Logio K p = -3200/T f 8 . 2 Logio K p = -3200/T 8 8 Logia K p = - 3200/T 4 8 . 9 L o g i o K p = -3200/T 4- 8 . 8 LogioKp = -3200/T f 8 . 8 Loglo Kp = 3200/T 9.5 LogioKp = -3200/T f 8 . 7

1. Methanol system 2. Ethanol system 3. Propanol system 4. Butanol system 5. Hexanol system 6. Isopropyl alcohol system 7. Isobutyl alcohol system

+

-

+

glycol vapor and 95% of its dissociated products, monomeric formaldehyde and water vapor. When heated 10' C. or more above its decomposition temperature, methylene glycol dissociates almost completely, so that the vapor consists essentially of monomeric formaldehyde and water vapor which remain in the vapor phase as separate and distinct entities. The vapor equilibrium constant, K p , of aqueous formaldehyde compositions for a temperature range of 40" to 160" C. is given by the expression, log&, = -3200/T 9.8. The approximate molar heat of reaction of monomeric gaseous formaldehyde and water vapor, calculated from this expression by means of the Clapeyron equation, is 14,800calories. When vaporized, alcoholic hemiformals also dissociate into a vapor equilibrium containing the hemiformal, monomeric formaldehyde, and alcohol vapor. The alcoholic compositions exhibit varying degrees of dissociation depending on the degree of superheat and the alcohol employed. Under similar conditions of superheat, dissociation becomes greater as the molecular weight of the alcohol increases, becoming almost 1 0 0 ~ aot temperatures 10" C. or more above the decomposition temperature when hexanol is the solvent. Under similar conditions of superheat, the degree of dissociation is nearly the same with straight- and branched-chain alcohols of the same molecular weight. Under

+

IO

dehyde in the vapor. This explains the results obtained when these solutions are distilled a t reduced pressure; it has been shown (10) that on distillation a t about 20" C. a t 20-mm. pressure, the vapors from a 2.1% formaldehyde solution contain only 0.16% formaldehyde. On the other hand, as the temperature and pressure in the system are raised above atmospheric conditions, the equilibrium constant increases. Distillation of aqueous formaldehyde solutions would then result in a higher concentration of total formaldehyde in the evolved vapors, the vapor containing more formaldehyde than a t atmospheric pressure. This is confirmed by the experiments of Ledbury and Blair (El), who show that the formaldehyde content of the vapor increases with temperature and pressure; a 2.101, formaldehyde solution gives a vapor of 5.4,7.4,and 7.6y0a t 40,80,and 100 pounds per square inch gage, respectively. It would also follow that as the temperature and pressure in the system are raised, the equilibrium constant will increase t o the point where it will become less effective as the controlling resistance to the transfer of formaldehyde. The efficiency of pressure distillation should therefore fall off when carried t o its extreme limit. This is borne out by the data mentioned above (8, 10). This hypothesis also predicts that at a temperature and pressure somewhat above atmospheric conditions, the vapors evolved from solutions of low concentrations of formaldehyde should be richer than the liquid, even when determined under liquid-vapor equilibrium conditions. Liquid-vapor equilibria therefore would be much less important in the distillation of formaldehyde-water mixtures than the effect of the vapor phase reaction, for the latter apparently is a hi-ge factor in determining the composition of the vapors. SUMMARY AND CONCLUSIONS

When vaporized, methylene glycol apparently dissociates into a vapor equilibrium containing, a t 100" C., about 5% methylene

8

6 4

3

I

I 0 .a 0.6

0.4

0.3 0.2

0. I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

1

I

0.02

0.0032

0.0030

04028

0.0026

Figure 9. Effect of Temperature on Vapor Equilibrium Constant of FormaldehydeIsoalcohol System 0

Is0 ropylalcohol

0: Isogutyl alcohol

Table XI Table XI1

1286


identical temperature conditions, the degree of dissociation seems to rise to a maximum (higher K p ) with the propyl alcohols and then decreases with further increase in the molecular weight of the alcohol. The molar heats of reaction of gaseous monomeric formaldehyde with alcohol vapor, calculated from the vapor equilibrium constants by means of the Clapepron equation, have been found to be about 14,800 calories for each of the alcohol systems examined. A comparison of the aqueous arid alcoholic data shows thar, at, the lower temperatures especially, alcohol-formaldehyde mixtures are less completely dissociated than the methylene glycol derived from aqueous solutions, indicating that the affinity of formaldehyde for alcohol is much greater than for mater. A new hypothesis is presented concerning the distillation phenomena which occur during the distillation of aqueous formaldehyde solutions. The presence of an equilibrium in the vapor phase and the rate of adjustment in the chemical equilibrium for the system methylene glycol-monomeric formaldehyde-kat,e~ seems to account for many of the phenomena that occur during the distillation of aqueous formaldehyde solut,ions.

Vol. 41, No. 6

LITERATURE CITED

(1) A u e r h a r h , F.,and Barschail, H.. A r b . kaiserl. Gesundh.. 27, 7

(1907). (2) ;iuerbach, F., a n d B a i s r h a l l , H., Arb. Reichsgeszindh., 22, 584

(1905). ( 3 ) B o n d , H. A . . U. 9. Patent l,X)5,03:3 ( d p r i l 2 5 . 1 (4) D a n i r l s , F., M a t h e x - s , J . E-I.,arid Williams, J. IT., Physical Cheiiiiat.r\-," p. 133, Nen; Yolk, McC

Co., 1941. ( 5 ) Delepine, M . , Compt. ~ r n d .124, , 816,1454. 1528 (1897). (6) H i h b e n , J., J. Am. ('kern. S o c . , 53, 2418 (1931). (7) Johnson, H G., a n d P i t , e t , E. L.. 1x1). ENG.C ' r % m , ,40, 743 (1!)1S), (8) Ledbury,W., a n d Blair. E , , Ilept. Sci. I n d . liesearch (Bi.it,), S p e c . R e p t . 1, 40-51 (1927). (9) L e m n i e , G., Chem. % i o , , 27, 896 (1903). (10) P i r e t . E. L., and H a l l , XI.W,, INI).Ihc:. CHEW,40, 661 (lO4S). (11) S c h o u , 8 . d.,J . ('hrnz. P h y s . , 26, 7% (1929:. (12) $Talker, J. F., "Foi~maldeiiyde." A.C.S. >Ioriograph 98,pp. 1h38, 48-60, i 3 . S e w Yoi,k, Reinhold Publishing Corp., 1944. (13) M'niker, J. F.. .I. A m . Chem. Soc.. 55, 28 (14) Kalker. J. F., J . Plius. C h c m . . 32, 1104 (13) Zimmerli, A , , Inn. 1-h-c;.( ' H E X . , 19, 524 (16) Zinimerli, A., U. S . P a t e n t 1,862,179 (March 13, 1928). KECEIT.E;D .July A, 1948. Inrostigation siipportcd i n part by fund by the Graduate Scliool. I-nirernity of 3Ijnnesota.

G. ALLEN CAVE, NATHAN J. KROTINGER, AND JOHN D. JIcCALEB Central Experiment Station, U . S . Bureau of Mines, Pittsburgh, P a .

T h e preparation of TNT, PETN, nitroguanidine. and ammonium picrate of various particle sizes and shapes is described. Methods employed include pebble milling, slow cooling with or without stirring, quick cooling in ice, the addition of the hot solvent to dry ice w-ith stirring, and the addition with stirring of the hot solution to a cold diluent. I t is shown that fine particle sizes may be reproducibl) obtained by methods involving the use of dry ice, or by the addition of a hot solution to a cold diluenk.

HE investigation of the effect of particle size upoii the detonat,ion of compressed explosivc charges requires closely sized samples of the materials. This paper describes the preparation of t,rinitrotoluene ( T S T ) . pentaerythritol tetranitrate (PETE), nitroguanidine, and animoniuni picrate samples of various particle sizes and shapes. There are many rnet,hods for classifying powders in bulk. Of these, conceivably sieving, sedimentation in a liquid column, and gas elutriation were available for use with the niaterials employed. Honxver, sieving, even when effected by hand, n-as ineffective as the materials tended to acquire a space charge and form hard balls in the sieve. Even when t'his effect was not encountered, the classification was inaccurate, depending rather on the second largest cross section of the particle than on the mean diameter. Gas elutriat,ion was also uiisatisfactory as the part'icles tended to clump, particularly a t the lower size ranges. Sedimentation in a liquid column was, in general, a sa.tisfactoi-ymethod of classification. Hindered settling was minimized by control of the concentration of the solid, and tmbulence mas avoided either by careful temperature control or by increasing the cross section of the settling column. The crystal densit'ies of the explosives ranged around 1.6, and practicable settling times could

be obtained by using saturated solutions of the materials in carbon tetrachloride or mixtures of carbon tetrachloride Jyith methylene iodide, iodobenzene, or n-butyl alcohol. DETERBXINATION OF PAK1'ICLE S I Z E

The particle size of prepared material was determined by microscopic examination or by electron microscope photography. Samples for microscopic examination viere prepared by adding a small portion of the material to a drop of olive oil or castor oil on a microscope slide. Aft,er gentle mixing with a glass rod, the cover slip was added and the slide viewed with the microscope. The length and width of particles in the field were determined by a scaled eyepiece which previous calibration had shown to have a scale division length of 0.5 t o 8.3 ,u according to the magnification used. The depth dimension was determined by focusing on first the top, then the bottom of each particle, the difference in height being shown on a micrometer scale attached to the racking mechanism and calibrated so that the nearest 0.5 p could be estimated. The operation vias repeated using a horizontal traverse for a number of particles in the center of t,wo or three fields. For most samples examined, a count of 200 particles gave scarcely any advantage over a couut of 50. For some wide ranges, hoTvever, counts of 100 t o 200 particles were necessary to give reasonable accuracy. The following parameters were then calculated: 2abc where a, b, and c are t'he longest, second long1. V,,,, = -, n est, and shortest dimension of each particle, and n is t'he number of uarticles counted. 2.

Mean particle diameter (size) =

4F

Distillation Principles of

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