T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
7x2
b y displacement of air with about 5 0 0 cc. of the sample of air and benzene vapor. This was a quantity far from sufficient t o displace all the original air, and the results are low. I n Expt. 6, the flask was filled by displacement of mercury and the result is much better. I n No. 7 , a bulb containing a known weight of benzene was broken in a j liter flask slightly evacuated. Air was then admitted and the whole shaken up well by means of a small quantity of mercury in the bottom of the flask. After this, a 340 cc. determination flask was filled by displacement of air with the 5 liters of air and benzene vapor which were driven over by displacement with water. PRELIMINARY RESULTS Difference in Pressure EXPT. NO.
.............................. .............................. ............................ ..............................
1 2 3 .............................. 4.. 5
.............................. ..............................
6
7
Experimentally determined Calculated Cm.Hg Cm.Hg 0.67 0.85 0.94 1.06 2.40 3.00 2.30 3.00 2.00 2.60 1.38 1.50 2.31 2.45
I n the final series of determinations a weighed quantity of benzene, less t h a n t h a t required for saturation, was introduced directly into the determination flask. This was accomplished by means of a second small, sealed bulb (Fig. IV) which contained a weighed amount of benzene. I t s position could be regulated by means ,of t h e attachment screw A, so t h a t on pushing on the rod the small bulb broke, leaving FIG I V the larger one above it intact, t o be broken later. I n this way the actual pressure developed b y the weighed amount of benzene could be measured, and the pressure could be checked by breaking the large bulbs according t o the method already described. F I N A L RESULTS B C D Percentage Pressure difference Pressure measured between Percentage Wt. calc. from Pressure b y new B and difference E X P T . of benzene gas laws developed method average between No. G. Cm. Cm. Cm. Cand D Cand D 4.0 5.0 1.00 0.95 1.01 0.0140 l..... 2.0 0.0 1.46 1.46 1.43 2..... 0.0204 7.0 4.0 2.28 2.20 2.40 3..... 0.0344 12.0 0.2 3.80 3.79 4.32 4..... 0.0618 11.0 1.3 4.56 4.50 5.12 5..... 0.0726 16.0 1.3 4.54 4.48 0.0763 5.34 6..... 0.3 0.2 5.66 5.65 0.0815 5.63 7. 8.0 3.6 5.29 5.11 5.64 0.0819 s..... 7.0 0.0 5.21 5.21 0,0824 5.59 8.0 1.7 5.41 5.50 5.91 0,0839 6.0 0.4 5.69 5.67 6.04 0.0868 8.0 1.4 6.42 6.41 7.02 0.1015 18.0 0.3 6.74 8.20 6.73 0.1175 A
....
u
-
Av., 7.4 Av., 1.5
DISCUSSION OF RESULTS
The agreement between the pressures of benzene actually developed and those determined b y the new method is satisfactory. The average deviation is I . 5 per cent, but better agreement could undoubtedly have been secured by working in a thermostat, as t h e temperature of the room varied considerably. The differences between the pressures actually developed and those calculated from t h e weights of ben-
Vol. IO, No. g
zene, t h e volume of the flasks being known, are fairly large; mean deviation, 7 per cent. I n each case t h e pressure developed was less t h a n the calculated. These deviations may be attributed t o two causes : I-Impurities in t h e benzene and impurities collected from the interior surface of the flask. These impurities when dissolved in the last traces of benzine might lower its vapor pressure until i t would cease t o evaporate, being in equilibrium with t h e pressure in the flask. Naturally this tendency would increase as the amount of benz-ne pressure in t h e flask increased. 2-Divergence of the benzene vapor from the simple gas laws caused perhaps by polymerization, but the results show t h a t this effect is not very large for pressures approaching saturation. SUMMARY
I-A differential pressure method for the quantitative estimation of vapors in gases has been described. 11-Experimental results are given of the trial of this method for the estimation of quantities of benzene vapor in air. DEPARTMENT OP CHEMISTRY, UNIVERSITY OF MANITOBA WINNIPEG, CANADA
THE APPLICATION OF THE DIFFERENTIAL PRESSURE METHOD TO THE ESTIMATION OF THE BENZENE AND THE TOTAL LIGHT OIL CONTENT OF GASES B y HAROLD S. DAVIS, MARY DAVIDSONDAVISAND DONALDG. MACGREGOR Received March 27, 1918
INTRODUCTION
I t is unnecessary t o dwell on the importance of t h e aromatic hydrocarbons, particularly benzene and toluene, a t t h e present time, and on the necessity of increasing their output in every possible way. A great need has been felt, by those engaged in the commercial production of these substances, for methods of analysis requiring only small samples of gas and giving a rapid estimation of the content of these vapors either collectively or individually. Such methods would make i t possible t o find the conditions of .production necessary t o obtain the maximum concentration of each aromatic substance and would also permit the efficiency of absorption processes t o be tested a t every point. A method widely used a t present for t h e estimation of these vapors requires t h e following steps: r-Their absorption from a large measured quant i t y of t h e gas b y means of a suitable solvent. 2-The distilling and fractionating of the solution thus obtained according t o a definite scheme of operation. The absorption process most widely adopted for works purposes is t h e passage of the gas through a train of wash bottles filled with absorbing oils.* The technique of this process as widely used in t h e United States has been fully described b y F. W. S ~ e r r . ~ 1 H.G. Colman, J . Gas Lighting, 129 (1915). 314-315; H.W.James, 3. SOC.Chem. Ind.,.88 (19161,236. 2 R. Lessing, J . SOC Chem. I n d . , 86 (1917), 103. 8 Mcl. and Chem. Eng., 17 (1917),548, 586, 642.
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Absorption processes of a different kind have been proposed by R. Lessingl and H. H. Gray.2 Other methods3 for the estimation of these vapors in gases are based on the fact t h a t the vapors condense t o liquids with negligible vapor pressures a t low temperatures, while the permanent gases do not. I n the preceding paper there is described a differential pressure method for the estimation of a vapor in a mixture of inert gases. The present paper4 embodies the results of investigations on the application of this method t o the estimation of a vapor in a mixture of other vapors and inert gases. I n particular, we have studied the application of this process t o the estimation of benzene and of the total light oil content in coal gas or coke oven gases. RATE
OF
DEVELOPMENT
OF
THE
DIFFERENTIAL
PRESSURE
We have investigated more carefully the rates a t which the vapor pressures are developed in the separate flasks, after the bulbs containing the liquid or solid are broken. The rates of development of the vapor pressure from liquid benzene a t 18” and from solid benzene a t 0 ” were measured, with the results given below. About I g. of benzene was used in each case, in a 300 cc. determination flask, without shaking. Benzene (liquid) 18.6’
TIMETressure
Min. C m . H g 0 0 1.6 2 3.5 5 4.9 9 6.85 23 49 7.18(Pm) ( a )
K
Benzene (solid) 0’ TIME Pressure Min. Cm.Hg K 0
-0:6;4 -0.058 -0.055 -0.056
5
IO 15 25 35
( a ) POO= saturation pressure at 1 =
0
0.67 1.23 1.54 1.90 1.99 2.21(Pm) . --,
-4:bil -0.035 -4.035 -0.034 -0.028
OO.
If the rate of development of t h e pressure is proportional, a t each instant, t o the undeveloped pressure in the flask, and if we represent the time in minutes by t and the pressure in centimeters of mercury by p , then:
d_p = -K(P, dt stant).
- 9) a n d -I log ( I - -)P t Pa,
The actual value of K would depend on the conditions of temperature, the surface of benzene exposed, the volume of the flask, etc., which conditions must approach constancy for any given determination. As will be seen from the tables given above, the values of K calculated in each case from the experimental results are sensibly constant. I t thus appears t h a t the pressure is an exponential function of the time. Consider a differential pressure apparatus containing air in each side, in which bulbs of benzene are broken a t the same time. The pressures will develop exactly alike so t h a t a t no time is there any difference of pressure indicated. Now suppose, on one side, there is a n original pressure of benzene. Then a difference of pressure slowly develops. This can be demonstrated t o be a n exponential function of the time similar t o t h a t of the pressure in either flask and it can be shown t h a t a t any time Difference i n pressures Maximum difference in pressures Pressure develoDed in either flask Maximum pressure developed in t h a t flask This important result shows the great advantage of the differential pressure method. Consider the determination, a t ordinary temperature, of the benzene pressure in a sample of gas containing I cm. pressure of benzene. When the benzene pressure on each side is 95 per cent developed, the differential pressure reading will be 0.95 cm. instead of the correct I . 00 cm., a n error of 5 per cent. On the other hand, if instead of the differential pressure method, a method is used in mihich only one flask is employed, and the pressure obtained by breaking the benzene bulb is subtracted from the saturation pressure a t t h a t temperature, the error is much greater. For now a t 95 per cent saturation, the numerical value of the error in the result is 5 per cent of the saturation pressure (about I O cm.) or about 5 0 per cent of the original vapor pressure in the gas.
= K (a con-
J . SOC.Chem. I n d . , 36 (1917), 103. J . Chem. SOC..111 and 112 (1917). 179. 8 St. Claire Deville, J . des Usines a Gaz, 1889; Lebeau and Damiens, Compt. Rend., 166 (1913), 144, 325; Burrell, Seibert and Robertson, U.S. Bureau of Mines, Technical Paper 104 (1915); Burrell and Robertson, THIS JOURNAL, 7 (1915), 669; H. F . Coward and F. Bailey, Manchester L i t . and Phil. SOC.,24 (1916) 4 After the work OH the present paper was completed, the comprehensive article from the U. S. Bureau of Standards on the “Recovery of Light Oils and the Refining of Toluol” appeared in THISJOURNAL, 10 (1918), p. 25, is an article b y R. P. 51. In the same number of THISJOURNAL, Anderson on the “Determination of Benzene Vapor.” This contains a summary of the various methods which have been employed for the estimation OB benzene vapor in gases and proposes a method on which he has done preliminary work. I n this method the benzene content is to be estimated by measuring the increase in volume of the gas mixture when placed in contact with liquid benzene. He also points out that a similar method differing only in detail had been developed by the Soci&tB Roubaisisnne d’Eclairage par le Gaz and R. R. L. H. Forrieres. For an account of the specifications of the German patent on this method see J . Soc-Chern. I n d . , 33 (1914), 129. 1
2
7 I3
ESTIMATION OF BENZENE VAPOR I N THE PRESENCE OF TOLUENE VAPOR
Our next work on this subject concerns the estimation of benzene when toluene vapor is also present. A known amount of toluene vapor was introduced into the air of one of the flasks. Bulbs of benzene were afterwards broken on each side. If now the toluene did not dissolve in the benzene, the same benzene pressure would be developed in each flask and the manometer reading would not change. On the other hand, if the toluene dissolved in the benzene t o any extent, the pressure developed in the flask containing the toluene would be less than in the other. We were well aware t h a t the toluene would dissolve in the benzene t o some extent, but as the flasks used had a capacity of 1 5 0 cc. t o 350 cc. and as the quantity of benzene used wag less t h a n I g. in each case, it
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
seemed probable t h a t the toluene would not appreciably dissolve out from the gas before equilibrium h a d been reached b y the evaporating benzene. Otherwise, the toluene would first have t o diffuse from all parts of the flask into the outer layer of liquid benzene and then from this layer through the main body of the liquid itself. Experiment has shown t h a t these ideas were erroneous; the toluene rapidly dissolved out from the gas in the flask into the liquid benzene. It seems impossible t h a t this should be due t o simple gas diffusion; it must rather be caused by convection currents in the gas. We attempted t o prevent such currents by surrounding the liquid benzene with fine copper gauze, but without success. Below are given summaries of a few of these experiments, some performed a t ordinary temperatures and others a t the temperature of melting ice. The following points are t o be noted in interpreting the results: I-The benzene bulb in the flask containing the toluene was always broken first, so t h a t the benzene pressure was always a little higher on t h a t side a t first. Accordingly, the toluene really dissolved out more quickly than would appear from the table. 2-The rates a t which t h e difference in pressures developed in the several experiments are not strictly comparable, because it was impossible t h a t the interval between the breaking of the bulbs should be the same in each case, and t h a t the contents of t h e bulbs should be uniformly distributed on breaking. 3-In comparing the final differences in pressure, i t must be remembered t h a t the magnitude of these dePended on the amount of liquid benzene left in the flask. If this w a s large, the lowering of its pressure caused by the dissolved toluene was small, and vice versa. This effect is discussed in detail later in the paper. 4-111 the experiments in which solid benzene was used, there was sufficient toluene present in the flask to cause the benzene in t h a t flask t o melt.
Vol.
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No.
g
this pressure, however, was developed in the first 15 min., yet the flasks had not been shaken and t h e liquid benzene lay undisturbed on the bottom. When i t is considered t h a t the difference in pressure was due t o toluene dissolved in t h e benzene and t h a t this toluene had t o be absorbed out of the gas in a 300 cc. flask into about I cc. of liquid benzene on t h e bottom, one cannot but be struck by the comparative speed of the process of absorption. It may well be t h a t i t is not so difficult t o wash these aromatic hydrocarbons from gases as has often been supposed, a conclusion of great importance for industrial absorption plants. VARIATIONS PENDING
IN ON
THE THE
DIFFERENTIAL QUANTITY
PRESSURE,
DE-
O F LIQUID USED
If the gas t o be analyzed contains only one vapor, together with inert gases which are insoluble in liquid of the same composition as the vapor, the differential pressure developed will not vary with the amounts of liquid used. On the other hand, suppose the vapors of two completely miscible liquids are present. If a n attempt is made t o estimate one of these vapors by the differential pressure method, with bulbs containing its liquid, the actual difference in pressure developed between the two flasks will depend on the amount of liquid used, as the following considerations show: Consider a mixture containing inert gases and two vapors, benzene and toluene, for example. Let a bulb of pure benzene be broken in each of the flasks of the apparatus. Then in the one, A, containing t3ure air, the full Dressure of benzene for t h a t temperature is developed. I n the flask B, containing the vapors and inert gases, t h e following conditions exist at equilibrium:
I-The toluene has dissolved in t h e benzene until its solution in the benzene gives a vapor pressure of toluene equal t o the residual vapor pressure of toluene in the gas. has lowered the vapor 2-The pressure of the benzene. Consequently a difference in pressure has been deDifference in Pressures veloped between flasks A and B, made up of the folDeveloped between Flasks V. P. of Toluene Time in Minutes Kind of in Flask 1-3 3-6 7-10 10-15 20-30 60 80 lowing factors: ater ria^ No. Temp. in Bulbs Cm. Cm. Cm. Cm. Cm. Cm. Cm. Cm. (a) The original vapor pressure (unsaturated) of 1 22- Liquid benzene 2.37 0.30 0.74 1.54 1.90 2.41 . . . . 2 24' Liquid benzene benzene in B. surrounded by copper gauze 0 27 0 . 2 2 0 . 2 9 0.39 .. ( b ) The original pressure of the toluene vapor which 2:41 3 22' Liquid benzene 2:05 0:80 1 :io 2.00 . . 4 2 3 O Liquid benzene 0.24 . . 0:39 .. 0: s 2 o,45 . . o:46 . . . . o : ~ 4o.98 ,:io is now dissolved in the benzene in B. 5 23O Liquid benzene 6 ' 0 Solid benzene (c) The lowering of the vapor pressure of the 0.075 G. 0 . 2 2 , . 0.04 0 . 1 2 0 . 1 5 .. 0.32 0.34 . . . . . . . . . . . . . . . . liquid benzene in flask B b y the dissolved toluene. Toluene 0.006 G. 7 0' Solid benzene Pressure developed between A and B = a b c. 0.090 G. 0.32 . . 0 . 3 3 . . . . 0.62 . . . . Toluene 0.009 G. . . . . . . . . . . . . . . . . Consider now the variations in the final differential 8 O n Solid benzene 0.98 G. 0.24 . . . . 0.27 . . . . . . . . pressure between A and B, caused b y using different 9 0 ' Solid benzene 0.81 G. .. quantities of liquid in flask B. As these quantities are Toluene0.008 G. O:i4 :'. 0:il :: .. O:i4 0:57 :: Factor ( 6 ) becomes larger, and approaches 10 Oo Air saturated with toluene . . . . . . . . . . 0.23 0.41 .. aincreased, limiting value, which would represent the dissolving I n most of the observations recorded in this table, of all the toluene in the liquid benzene. At the same the difference in pressure had ceased t o increase a t time, Factor (c) approaches a value infinitely small. The difference in pressure developed between the t w o the time of Dhe last observation. The greater part of '
+ +
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
flasks a t this limiting value P, is equal t o the sum of the original vapor pressures of benzene and toluene in the gas. If now the quantities of liquid benzene are decreased, Factor (a) approaches a value infinitely small, which would exist if the quantity of liquid benzene used were so minute t h a t only an infinitely small quantity of toluene could be dissolved from the gas. I n this case, Factor (c) increases t o a limiting value, equal t o t h e lowering of the vapor pressure of benzene caused by the dissolving of sufficient toluene t o give a vapor pressures of toluene from the solution equal t o t h a t originally present in the gas. The difference in pressure developed between the two flasks a t this limiting value, P o , is equal t o t h e sum of the original vapor pressure of benzene in t h e gas and the lowering of t h e vapor pressure of benzene from a solution of toluene which gives a vapor pressure of toluene equal t o . t h a t in the original gas. The approximate form of this curve is shown in Fig. I.
8
Q ' L i y h t * y L i q u 2 n . - ~ in FIG.
,,1,
B
1
Po-Pa
=AB
11.5
(it
= T 2I>
At 2 5 ' t h e ratio of the vapor pressure of benzene to t h a t of tolueneis about I o : 3 , so that AB = T
(y - I)
or about 7
T
3
Thus AB the Pressure Of 7 originally present in the gas, magnified about -times. 3
Suppose t h a t instead of toluene, the original mixture had contained the vapor of some less volatile liquid, a xylene for example. Then AB would represent the original vapor pressure of the xylene magnified t o a still greater extent than was t h a t of toluene. I t is therefore evident t h a t when t h e differential pressure method, employing bulbs of liquid benzene, is used for a gas mixture containing bensene vapor and the vapors of high boiling compounds, the vapors of these compounds will cause a sharp increase in the differential pressure in those cases in which minute quantities of benzene remain in Flask B. We have tested these conclusions experimentally on a sample of illuminating gas collected in small gas holders. The gas was passed through them in series for a time sufficiently long t o ensure a uniform sample. The apparatus used was of the modified type described in a previous paper. I n an apparatus of this form one of the flasks can first be evacuated and then filled with the gas t o be analyzed. The flasks were of about 150 cc. capacity each. The results of these experiments are given in the table. T h e high values obtained in Nos. 6 and g are undoubtedly due t o experimental error. Wt. of Liquid Differential Benzene' Benzene2 Pressurea No. Grams Grams Cm. 0.0586 l... 2.83 2.34 0.0617 2... 2.33 3 . . . . . . . . . . . . . . . . . . . . . 0.0707 1.20 +... .................. 0.0978 1 .oo J... 0.1092 0.129 (1.29) 6... 0.140 0.97 7... 0.96 8 . . . . . . . . . . . . . . . . . . . . . 0.214 0.314 (1.11) 9... 1.04 1 0 . . . . . . . . . . . . . . . . . . . . 0.957 1.20 1.39 11. . . 1 Represents the weight of liquid benzene broken into Flask B. 2 The calculated weights of benzene which remained at equilibrium some having evaporated into the flask. 8 T h e pressure difference between t h e two flasks, with corrections for initial difference and manometer movement.
EXPT.
P o - P a = AB = Limiting value of lowering of v. p. of benzene-Original v. p. of toluene in gas. I t is now necessary t d obtain a relation between the two factors on t h e right-hand side of the equation, t h a t is, a relation between the original pressure of toluene in the gas and the lowering of the vapor pressure of benzene in a solution of benzene and toluene which gives a vapor pressure of toluene equal t o t h a t in the original gas. Let there be N1 molecules of benzene and Nz molecules of toluene in the solution Let E'b be the saturation pressure of pure benzene a t t h a t temperature Pt, the saturation pressure of pure toluene a t t h a t temperature T , t h e original vapor pressure of toluene in the gas N2 Lowering of v. p. of benzene = Pb N2 NI
+ N1 Pt Lowering of v. p. of toluene NI + Nz N2 Or, v . p. of toluene from solution Pt N1 + Nz =
=
Pb Therefore, Lowering of v. p. of benzene - v. p. of toluene from solution Pt Or substituting in the relation obtained above:
..................
..................
................... .................. .................. ................... . ..................
These results are plotted in Fig. 11. It seems reasonable t o suppose t h a t the elevation of the curve from $3
$
$ 2
;s
t '
& b
1
2
3
4
5
6
7
We/phf of Liquid Rernaminq
8 in
9
IO
11
I2
Flask in Grams
FIQ.I1
B to A is mainly due t o toluene. This curve projected would meet the axis a t a point corresponding t o a vapor pressure of 1.4 cm. The constancy of the pressures in Nos. 7 and 8 would indicate t h a t under these conditions all the toluene and other vapors had
716
T H E J O U R N A L O F I N D U S T R I A L .A N D E N G I N E E R I N G C H E M I S T R Y
Vol.
IO,
No.
Q
dissolved in the liquid benzene. The average value can be in equilibrium with solid benzene. Any smaller of these pressures is about 0 . 9 7 cm.; or the limiting pressures of these vapors will not be affected by the value of the elevation A B due t o toluene is 1.35 - breaking of the bulb containing solid benzene. T h e 0 . 9 7 = 0.38 cm.; that is, the original vapor pressure following considerations make this clear: of toluene in the gas was about 0 . 1 5 cm. Suppose t h a t t h e light oil in the gas, besides benThe gradual elevation of the curve from C t o D is zene, is toluene. We are'then t o calculate the maxiundoubtedly due t o increasing absorption into the mum quantity of toluene t h a t can be in equilibrium liquid benzene of some constituents of the gas other with solid benzene, the temperature being known. For each temperature there is a solution of toluane than the light oils. As this effect was not very pronounced, i t seems probable t h a t in experiments done in benzene which would be in equilibrium with solid in this way when the volume of liquid benzene used is benzene. I t s concentration can be calculated from about 0 . 2 t o 0.5 per cent of t h e volume of the flask, the formula for the depression of the freezing point: the difference in pressure developed represents the total pressure of the light oils in the gas. The following experiment was carried out on iiWhere T = depression of freezing point of solvent luminating gas, in order to'test the solubility of the (benzene) permanent gases in benzene: WI = weight of dissolved substance (toluene) A sample of illuminating gas was shaken u p for a M = molecular weight of dissolved sublong time with two successive portions of straw oil stance (92) (sp. gr. 0.87), such as is used in commercial works = weight of solvent (benzene) t o remove the light oils from gases, until i t seemed K = molecular depression constant of solprobable t h a t these hydrocarbons had been removed vent (4900) from the gas. The left flask contained gas with the so t h a t benzene, etc., removed by oil washing; the right, air. The pressure developed was 0 . 0 2 cm. less in the W2 left than in the right. This result indicates t h a t the If the weight of benzene (W,) be 78, or I gram-moleerrors, caused by the permanent gases in coal gas dissolving in the benzene, are small. The experiment, cule, then the weight of toluene (W,) is 78 X T however, should be repeated with more detail. 53 x 92 0 . 0 1 7 T gram-molecules. But the solution in equilibN E W METHOD FOR T H E ESTIMATION O F BENZENE I N A rium with the solid benzene may be considered as a GAS C O N T A I N I N G ALSO T O L U E N E A N D O T H E R solution of benzene in toluene. Knowing its concenVAPORS tration, the vapor pressure of toluene from this soI n a former paper i t has been shown t h a t the amount lution can be calculated in terms of the vapor pressure of benzene in an inert gas can be accurately estimated of pure toluene a t t h a t temperature, by the modified by the differential pressure method, using bulbs of equation of Raoult : liquid benzene. I t has been shown above, however, AP fll _ t h a t when the gas also contains toluene and other P NI + N z light oils, the following factors must be considered: Where N1 = number of gram-molecules of benzene I-The toluene and other light oils dissolve in the Nz = number of gram-molecules of toluene liquid benzene so t h a t their pressures are almost A P entirely removed from the gas, while a t t h e same - = fractional lowerin-g of toluene vapor P time these dissolved substances lower the vapor prespressure sure of the benzene. 2-A less serious error is caused by the gases other ' AP I - = than the light oils dissolving in the liquid benzene. P I 0.017 T I n order t o eliminate these sources of error, we have A t o o C., T = 5 . 5 O conducted investigations on gases b y the differential IOO pressure method, using bulbs filled with solid benzene Therefore, AP -= __ P 109 and immersing the apparatus in a bath below the I t is well known Or, the vaporpressure of toluene from the solution is 9 freezing point of benzene, 5.48'. 109 t h a t the solubility of permanent gases in a solid, such as frozen benzene, is vanishingly small, SO t h a t the of the vapor pressure of pure toluene a t t h a t temperasecond source of error mentioned above is completely ture (about 6 mm.) = 0.5 mm. This, then, represents the possible concentration of toluene vapor in eliminated. Again, the vapor pressure from the solid benzene, equilibrium with solid benzene a t o o C. I n these calculations we have assumed t h a t the at any fixed temperature, is independent of the solusimple law for the depression of the freezing point tion by which i t is surrounded, so t h a t while the solid benzene is present a t equilibrium there can be no given above holds good over the range of temperatures considered. We intend t o test this experilowering of its vapor pressure. mentally. If there are deviations they will probably Moreover, there are certain definite pressures of the light oil vapors which, a t definite temperatures, tend to make the concentrations of toluene vapor
wz
yJ
+
~
.
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
which can be in equilibrium with solid benzene greater t h a n those calculated above. We also expected t h a t the method for the estimation of benzene could be used in the presence of still larger quantities of toluene vapor on account of metastability of the benzene crystals, provided they were well frozen. Similar reasoning will apply t o a gas mixture containing benzene with xylene or other vapors. I n order t o test these conclusions a series of experiments was carried out in the following way: Air saturated with toluene vapor by bubbling i t through toluene in a Geissler absorption tube was collected in a eudiometer. From this a measured quantity was drawn into the left flask of the differential pressure apparatus evacuated sufficiently t o receive it. Determinations were also made with both toluene and xylene vapors present. I n this case quantities of air separately saturated with the vapors were drawn into the eudiometer. These quantities were measured, so t h a t the pressure of each vapor in the flask could be calculated for the temperature of the experiment. The apparatus was then packed in snow, or snow and brine, which was kept well stirred, and the determination carried out in the usual way. Special care must be taken in breaking the bulbs of solid benzene in order not t o fracture the bottoms of the flasks. The rod should be hit, not a hard blow, but a series of taps with a light object (we used the handle of a small screw driver). The bulb soon breaks and t h e solid benzene settles down in small pieces. EXPT.
Temperature of Bath -4.4O
NO.
............ ............ --8.3O -8.0' ............ ............ -6.6' 5 . . .......... -5.8O 6 . . .......... -5.0' 1 2
3 4
Calculated Pressures of Vapors in Flask Cm. Toluene 0 . 0 7 8 Toluene 0.122 Toluene 0.155 Toluene 0.167 Xylene 0.023 Toluene 0.122 Xylene 0.034 Toluene 0,237 Xylene 0.067
I
Pressure Developed Cm. -0.035 -0.085 -0.021 -0,055 -0.218 -0.011
I n not a single case was the differential pressure developed towards the flask containing the vapors. This indicates t h a t these vapors did not dissolve in the solid benzene. On the other hand, a small variable pressure was developed the other way. We are unable t o say whether this was due t o some source of experimental error in t h e particular apparatus used or whether i t was actually caused by the presence of the toluene and xylene. We intend t o investigate this phenomenon further. Tests of this method for t h e estimation of the benzene and of the light oil content were carried out on a sample of illuminating gas collected in small gas holders. The following results were obtained: EXPT. No.
..................... ..................... ..................... 4 ..................... 5 ..................... 6 ..................... 7 ..................... 1 2
3
8
.....................
Temperature -170
-18'
00 00 00
4.5' 6.5' 4.5," 5.1 5;6' 22' 200
Difference in Pressure Developed Cm. 0.61 0.61 0.58 0.62 0.60 0.88
1.12 0.91 0.98 1.12 0.94 1.01
7=7
The following points are t o be noted: I-The pressure developed was practically constant from -17' t o o o C. This value (0.60 cm.) probably corresponds t o the pressure of benzene vapor in t h e gas. ;.-The first part of Expts. 6 and 7 was in each case carried out in a water bath a t a temperature below the melting point of benzene. The result, 0 . 9 8 cm., obtained a t 5 . I O , when some solid benzene was present on both sides, probably represents the total pressure of the benzene and the light oils, for a t a t e m perature so near the melting point of benzene the latter must have nearly all dissolved out. 3-The latter parts of Expts. 6 and 7 were done a t a temperature slightly above the melting point of benzene and the difference in pressure read after all the solid benzene had disappeared. The high value I . 1 2 cm. obtained for the differential pressure is probably caused by solution in the liquid benzene of constituents from the gas other t h a n the light oils. 4-The mean of the values obtained a t room temperature ( 0 . 9 7 cm.), bulbs of moderate size being used, probably represents the total light oil content in the gas as has already been shown in this paper. To sum up, determinations made a t oo, t h a t is, in melting ice, gave the benzene content in the gas; and those done a t ordinary temperatures with t h e proper proportions of liquid benzene gave the total light oil content. Samples of illuminating gas, collected in small gas holders, were also analyzed by the method outlined above, a t 2 3 O , the following liquids being used in the bulbs : I-Benzene only. 2-A solution of equal volumes of benzene a n d toluene. 3-A solution of equal volumes of benzene, toluene, and xylene. The following results were obtained: Benzene Cm. 1.03 Differential Pressure( 1 ) . (2). . . . . . . . 1 . 0 0 MEAN. 1.02 Total Pressure Developed.. 9.0
.......
....
Benzene and Toluene Cm.
Benzene Toluene Xylene Cm.
0.99
0.99
1.06 1.03 5.0
1.14 1.06 4.0
I n the first case, the toluene and xylene from the gas dissolve in the benzene. I n the second case, t h e pressures of both the benzene and toluene from the solution in the bulbs are greater than their original pressures in the gas; they are estimate$ by the same principle. I n the third case, all three vapors are estimated in this way. It is seen t h a t the mean values of the results are about the same in every case, which confirms the conclusion given above, t h a t when benzene is used in the bulbs a t ordinary temperature, the differential pressure represents the total light oil pressure in the gas. The advantage of using one of the solutions rather than pure benzene in the bulbs is seen in the figures representing the total pressures developed, for in these cases the pressures developed are much smaller, so t h a t the danger of leakage is greatly decreased.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
7 18
PRELIMINARY W O R K CENTRATION
OF
PRODUCED
ON T H E VARIATION I N T H E CONLIGHT
AT
O I L S I N THE
VARIOUS
COAL
STAGES
GAS
OF
CARBONIZATION
We have shown above t h a t t h e differential pressure method, bulbs of liquid benzene being used, gives a n easy and fairly accurate method for the estimation of the light oil content of a gas mixture. I n order t o obtain some idea of t h e change in t h e light oil content of coal gas as carbonization proceeds, we carried out the following experiments on one of t h e retorts of the Winnipeg Electric Railway Gas Plant: Through a hole drilled in the door of the retort, samples of gas were drawn off from time t o time, washed with water, and t h e light oil content estimated. A 340 cc. estimat‘ion flask was filled b y displacement of air with 2 t o 3 liters of gas. This was not sufficient and the results are probably low, though they are comparable with each other, as the treatment was uniform throughout. TIME Min.
Light Oil Content of Gas Cm. H g 0.94
O(a)
0.70 13 0.56 52 0.29 102 0.21 186 227 240Jb) ( a ) Retort charged with coal. ( b ) Carbonization complete, retort emptied.
I t will be seen t h a t the gas which came over the first hour was rich in light oil; after t h a t the content steadily declined. COMMERCIAL P O S S I B I L I T I E S OF T H E M E T H O D P A R T I C U LARLY I N CONTROLLING T H E PROCESS O F A B S O R P T I O N O F L I G H T OILS A T RECOVERY PLANTS
Before the comple
