d
EXPERIMENTAL AND THEORETICAL VAPOR-LIQUID RELATIONSHIPS E. J . BEARCE' AND 9. A. GERSTER Z'niversity of Delaware, Newark, Del.
Vapor-liquid equilibrium relationships were experimentally determined at 100 ', 150 O , and 200 O F. for the furfural-water system by the boiling point method. By this procedure the pressure at which a liquid of known composition just began to boil was carefully meas~ired. Such data permitted calculation of the activity coefficients and partial heats of solution for each of the two components of the binary system. Attempts were made to fit the extremely nonsymrnetrical activity coefficient-composition plots with various theoretical equations, but only the more complex types of equations were applicable over the entire composition range. The simple 3-suffix 3Iargules equations could be ad.iusted to fit the experimental data over limited concentration ranges, while special 4- and 5suffix Margules-type equations recently proposed bj Wohl (23) appear promising.
therefore decided to determine experimentally the isothermal vapor-liquid equilibrium relationships for furfural-water a t three temperatures of interest in extractive distillation: 100O , 150", itncl 200" F. E:XP EKIMENI'A L
C n o r c ~OF THE METHOD. Because of the nonidcality of the hinaiy mixture and the large difference in boiling points of the two substances, the relatively simple boiling point method was chosen for this study. In this method, the pressure is dptermined a t which a liquid mixture of known composition and a t some constant temperature just begins to boil. Carlson and Colburn (3)have shown that when this information is available, the activity coefficient for one component, y , , may then bu coinputed from the relationship
P = NCREASED use of furfural as a solvent, iii the refining and dewaxing of lubricating oils (IO,go) and in the separation of Cqhydrocarbons by extractive distillation (9,8, 18) has increased the demand for precise physical data for this material. Previous work in these laboratories has been reported on the solubilities and vapor-liquid characteristics for several single C4 hydrocarbons in dry furfural as well ads for mixtxres of these hydrocarbons in furfural (4, f4). These data, are valuable in design a,nd evaluation of performance of extractive distillation columns employing these substances. However, according to Happel (8),small amounts of water in the furfural solvent are usually employed in this latter operation t o permit the use of lower t,emperatures in tho solvent stripper reboilers. Vapor-liquid equilibrium determinations for several pairs of Cd hydrocarbons in wet furfural arc now being carried out in t,hese laboratories, and a knowledge of the simple furfural-water bina.ry equilibrium characteristics appears desirable. Such information can be used to estimate the effect of water upon hydrocarbon-furfural equilibrium relationships as well as t.0 provide a sound basis for correlating the results of the hydrocarbon-furfural-water s y s t e m now under study. A survey of the literature for existing furfural-water equilibrium data shows that all such determinations were made a t constant pressure. Mains (16)and Ridgeway and Furnas ( 1 7 ) obtained data for t#his syst#em a t atmospheric pressure. The Phillips Petroleum Compa,ny ( 1 6 ) reported data at 62.4 pounds per square inch total pressure, while Highhill (9) determined boiling points and vapor pressures of furfural-wader mixtures a t pressures around 27 mm. of mercury in t,he high furfural concentration range. However, because of the wide difference in boiling points of the two substances--32-3' E'. for furfural and 212" F. for watgr a t 1 atmosphere---such constant pressure data are not too applicable for the calculation of activity ooefiicients, which are a function of temperature. Carlson and Colburn (3)have shown the advantages of using act,ivity coefficients to interpret and extend experimental equilibrium data, and Wohl (81) has summarized the sound thermodynamic relationships involving the use of isothermal activity coefficients which may be used for correlating and predicting vapor-liquid equilibrium data. Values of heats of solution may also be computed if the change of activity coefficient with temperature is accurately known ( 5 ) . It was 1
Present address, Sun Oil Company, blarcus Hook, Pa.
YSPlXl
+
YZP*Z*
(1)
iniismuch as the factors total pressure, P , liquid composition, 21and 5 2 , and vapor pressures, P I and Pz, are all known or measurable, while the activity coefficient for the other component, yz, may be estimated as 1.0 if that component is present in amounts approaching 100 mole 7 0 . For the case of interest xhere it is desired to know the activity coefficient for water out of a liquid containing mostly furfural, the method is extremely accurate, because the water with its greater vapor pressure and large activity coefficient has a partial pressure in the vapor phase which is very nearly equal to the total pressure. L)EscRIwIox OF APPARITUS. The equilibrium chamber itself was a 1-liter, three-necked, borosilicate glass flask fitted with a glass reflux condenser. Agitation of the equilibrium liquid mixture was provided by a flat paddle, '/z X l s / ~ f i X 5/32 inch thick, inserted into the flask through a packing gland and driven by a variable-speed motor. An electric heating coil, immersed directly in the liquid of the flask, consisted of 2 feet of 24-gage Chrome1 -4 resistance wile wound into a l/rinch diameter coil. Although the flask was immersed in a constant temperature bath, this internal coil was required to supply heat quickly to the equilibrium mixture after boiling had begun. A second smaller coil consisting of only 3/rinch length of the same resistance wire was also immersed near the surface of the equilibrium liquid to supply a point source of heat t o inrite boiling. Also immersed in the equilibrium liquid was a S/ie-inch diameter stainless stcel thermocouple well which contained a calibrated copper-constantan thermocwuple accurate to * 0.1' F. The equilibrium flask m s immersed in a constant temperature oil bath which was controlled to within k0.5' F . of the desired temperature. A copper tubing line connected the top of the reflux condenser to the vacuum system consisting of a Cenco bIegavac pump, reservoir, and pressure-measuring instruments. Low pressures were measured to within 0.1 mm. of mercury by a Zimnierli gage and cathetometer, pressures above 80 mm. of mercury by a Wallace and Tiernan precision mercurial manometer. MATERIALS.The furfural was technical grade and was obtained from the Quaker Oats Company. This materia1 was redistilled by simple vacuum distillation a t approximately 20 mm. of mercury where the distillation temperature ranged around 140' F. Batches of 1000 ml. were distilled, the first 200 ml. and the last 200 ml. being discarded. The furfural was used within a few days of being redistilled. The water employed was distilled water. OPERATION.After about 700 ml. of a furfural-water mixture of desired composition had been added to the equilibrium flask, the oil bath was brought to the desired temperature. AS soon as the equilibrium liquid had reached this same temperature, the smaller
1418
July 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
500 400
640 630
rrl
300
200
610
100
600
1419
perature where the liquid first clouds is recorded and corresponds to a definite water concentration. For mixtures of high water concentration where the method is not applicable, known quantities of dry furfural were added to the unknowns before analyzing. RESULTS
Values of the total pressure a t which VAPOR PRESSURE OF WATER I 1 various furfural-water mixtures start to 590 boil were determined under three fsothermal conditions: a t loOD, 150°, and e IO 200" F. The liquid mixture composition was varied within the two single-phase liquid regions which were reported by W I Griswold (6) and are reproduced for convenience in Table I. The original data TEMPERATURE: 150" F are tabulated in Table I1 and are plotted 195 I graphically in Figures 1 and 2, where the VAPOR PRESSURE OF WATER 1 initial boiling pressure is plotted against I90 I I liquid mixture composition. The end I 0 values of these plots correspond to t h e 53 vapor pressures of each pure substance %t 50 the designated temperature. Values of vapor pressure for furfural reported b y 40 Miner Laboratories (15) were augmented TEMPERATURE: 100' F at the lower temperatures by experimental 30 determinations by Mertes (13) of these laboratories. The resulting smoothed VAPOR PRESSURE OF WATER 49 20 data from both sources were used in this 1.00 0.99 0.98 0.97 paper and are given for convenience in X, , MOLE FRACTION WATER IN LIQUID Table 111. VAPOR PRESSURE OF FURFURAL Figure 2. Isothermal Boiling PresCALCULATIONO F AcTIvIrrY COEFFIsures for Furfural-Water Mlxtures 0 0.05 0.10 0.15 0.20 0.25 0.30 CIENTS. The fundamental relationship of in High Water Concentration Range Equation 1assumes ideality of the vaporsp X , , MOLE FRACTION WATER IN LIQUID and under nonided conditions, some Figure 1. Isothermal Boiling Presprocedure must be used for taking into account gas law desures for Furfural-Water Mixtures in Low Water Concentration Range viations and pressure effects on the liquid. Methods proposed by Benedict et al. (1) and Scatchard and Raymond (19) were shown by Mertes and Colburn (14) to be applicable for the case "bubble" coil in this liquid was connected to a source of eleqof relatively small deviations. Following the nomenclature tricity and the pressure was varied by application of vacuum until of ( I d ) , Equation 1 may be suitably modified as follows: boiling just started. The equilibrium liquid in the flask was COPtinuously agitated to minimize local superheatlng and to re&sP = Y l P l X d Z I YzPnXdz2 (2) solve quickly the liquid returning from the reflux condenser. The pressure within the flask was slightly increased and decreased where z represents the gas law deviation and liquid pressure facseveral times above and below the boiling .prespre and then brought back t o the condition where boiling Just began ,or tor of each component as defined by the equation stopped. I n each case the pressure and temperature of the mixture were recorded. When several check values of boiling pressure had been obtained, two liquid samples of 16 ml. were withdrawn through 1-mm. glass tubing by a syringe. Values of liquid specific volume for water, v,, were obtained from ( I I ) , while values of liquid specific volume for furfural, u2, The use of the smaller bubble coil was found necessary t o initiwere obtained from (9). The second virial coefficients, B, and ate bubble formation. Without the bubble coil, the mixture Bz,were computed in the manner followed by Mertes and Colburn had a tendency t o bump or t o start boiling violently with an ( I C ) , utilizing critical constants and the generalized equation of accompanying large temperature drop. The larger heating coil Wohl (2.3). Values of z for each componenb were then computed in the equilibrium liquid was used mainly t o bring the mixture for the range of operating conditions covered in this study and quickly t o the boiling point a t the start of a run, but was also are given in Table IV. Because the z values ranged only from used intermittently t o bring this liquid back t o the desired 0.96 t o 1.01, it is evident that they could have been neglected temperature after a particularly long period of violent boiling had without introducing serious error. caused a lowering of the liquid temperature. The importance of withdrawing the liquid sample from the flask under equilibrium boiling conditions cannot be overemphasized. Use of the composition of the liquid charge as the TABLE I. MISCIBILITYLIMITS FOR SYSTEM FURFURAL-WATER AS REPORT~D BY GRISWOLD (6) composition of the equilibrium liquid causes error, especially at Solubility of Water in Solubility of Furfural low concentrations of one of the components. Furfural a1 8-Phase i n Water at %Phase
4
I
4
e
+
The cloud point method described ANALYTICAL PRO~EDURE. by Griswold et aZ. (7) was used to analyze the furfural-water liquid mixtures. I n this method, a mixture of Wesson oil and 1-hexanol is added to the unknown liquid, after which the temperature of the composite mixture is slowly lowered. The tem-
Temperature, F.
Point Mole Fraction Water i n Liquid, zi
Point, Mole Fraction Furfural in Liquid, 2%
100 150 200
0.268 0.364 0.489
0.020 0 026 0 036
INDUSTRIAL AND ENGINEERING CHEMISTRY
1420 TABT,E
11. BOILINGP O I N T DATAFOR
Run
Mole Fraction Water in Liquid,
NO.
Zl
A1 A2 ..3 A4 A5 A6 A7 48 A9 4 10 All -412 A13 A14 A15 A15a 416 A17 A18 A19
A20 A2 1 A22 A23 A24
Bl B2 B3
B4
B5 BO B7 B8 B9 B10 B11 B12 B13 B14 R15
CI c2 c3 c4 c5 C6 c7
ca
C9 C10 c11 (7
12
0 0962 0.1813 0 0325 0.0752
0,1204 0.1138 0.0243 0.0484 0.0783 0.0831 0.1053 0.1472 0.2037 0.0198 0,0450 0.0429 0.07oO 0,1064 0 . I630 0.2148 0.2058 0,2383 0.9818 0,9900 0 9951
AKD VAPOR-LIQUID
SYSTEM
Temperature of Boiling Liquid, F.
Total Activity Coefficient Pressure of Boiling for Water, System, 71 = hlm. H g Abs. z~Pyi/Pixi
Teniperature, looo F. 37.30 100.0 49.90 100.0 20.10 99.9 35.85 99.9 42.90 100.2 43.20 100. I 17,48 100.0 23.41 100.1 29.60 99.9 35.90 100.0 39.10 100.0 44.50 100.0 50.10 99.9 14.86 100.0 24.07 100.0 23.50 100,o 30.10 100.0 39.90 99.9 46.40 99.9 52,30 100.1 50.47 100,o 62.87 100,o 52.3 100.0 52.1 100.0 51.1 100,o
0.2192 0,2866 0.9898 0.9935 0.9845 0,9792
Temperature, 150D F. 57.80 150.0 83.31 150.0 102 . 3 150.0 121.2 149.9 136.0 150.0 148.7 149,Q 150.0 159.7 72.7 150.0 115.5 150.0 181.4 150.0 190.4 149,B 200.3 149.9 199.5 149 . 9 207.7 149.9 208,6 150.0
0.1549 0.1958 0.9895 0.9958 0 9840 0.9823 0.9723
Temperature, 200' F. 166.5 200,o 230.2 200,o 277.3 199.9 335.9 200.0 333.2 199.9 199.9 409.4 421.4 200,o 548.6 200. 0 610.8 199.9 623.8 200.0 630.0 200.0 635.2 200,l
0.0246 0.0491 0,0658 0.0925 0 1198 0,1434 0.1658 0.0397 0.0793
EQUILIBRIUM
FURFURALWATER
8.94 5.12 9.70 8.49 6.51 6.94 I O . 81 7.91 6.52 7.68 6.70 5.55 4.58 12.61 8.80 8.96 7.45 6.79 5.26 4.55 4.57 4.16 1.002 1.001 1.000+
Activity Coefficient for Furfural, YZ =
1.03 1.09 1.02 1.04 1.06 1.05 1.01 1.02 1.04 1.04 1.05 1.07 1 11 1.01 1.02
I
1.04 1.05 1 06 1.08 1.10 1.15 9.7 43.7 26.9 29.5 23.2
. coefficients for Lvater i n the ]OK n-ittc,r conre computed accortiing to Fkluntioii 2 , assuming the activity c.oc~fficic~nt for furfu~,alto be 1.0. The resulting activity coefficient, values were then plotted against liquid coniposition and a smooth curve was drawn extending beyond the miscibility limit of water in furfural (.o the cornposition (!orre spondirig to pure iyater. where the activity coefficient.for water is unity. The activity coeficients for furfural werr then cvoniputed by Equation 2 from the experimental d a h i t r i d the a h v i t y c'oeficient plot just drawn for water. The next step \\-as t,o find a. suitablr equation t o fit the relationship between the activity coefficients just coinputed and the liquid composition, so that a more accurate ieprescntation of these values could be obtained in the immiac:ible regions where experimental d a h were lacking. As s h o l w below, the four-suffix van 1,aar equations were found to fit the experimental dat,a, and values of y2 from these equations were then used instead of the value of 1.0 previously assumed t o make a second final calculation of the water activity coefficients. This final adjustment did not affect the absolute values of the activity coefficients more than a few per cent. A similar procedure was followed for the furfural act,ivity coefficients. Computed results are summarized in Table I1 and plotted as Figure 3.
49.2 Ib2:3
595:9 , . .
... ~-
IT. VALUES O F 2 FOR FURFURAL A X D Fv.4TER AT VARIOUS T E X P E R ~ T C AND R E SAT REPREBEKTATIVE V ~ L K E SOF TOTAL PRESSGRE Furfural P , mm. Hg B
-
I elriperatiire, E.
7 .
100
150
1 08 1,12 1.16 1.1; 44.7 77.6 9 0 . .5
1 03
"
1.L
4.7 10.5 22.2 42.0 76.8 132 215
1 .05
R , 57 4.90 4.19 4.77 4.25 3.66 3.00 1.001 1.000 1.002
WATER
TABLE
LO2 1.03
1.03 1.04 1.04 1.05 1.08 1.10 1.12 1.03 1.05 1.17 1.25 40.4 70.9 50.2 41.7
1,002
VAPOR PRESSURES O F FURFURAL (1%)AND (11) AT VARIOUS TEMPER-4TURES
Temperature, F. 68 100 125 150 175 200 225 250
zzZ'ydP~2
7.53 6.50 6.39 5.63 4.99 4.62 4.35 6.65 6.15 3.81 3.10 1,001 1.000 1.001 1.002
1.004
TABLE111.
Vol. 42, No. 7
200
0 60 0 210
1,001 0.993 1,002 0.983
0 630
1.00~ 0.963
TVater P , mm.
-~
Wg
2
0 60 0 210 0 630
1.002 1,000 1.004 0.999+ 1.010 0.990
SAUPLECALCLJLATIO~Y FOR RUNB-4 Teinp. at equilibrium (TI,O K. Mole fraction water in liquid (zl) Mole fraction furfural in liquid ( 2 2 ) Total pressure a t boiling point (Pj, mm. Hg Vapor pressure of water ( P I ) . mm. Hg (Table 111) Vapor pressure of furfural ( P z ) , inin. H g (Table 111) Activity coefficient for furfural ( y t ) (Figure 3) Liquid specific volume for water ( E , ) , ml./gram mole (11) Liquid specific volume for furfural (v?), ml./gram mole ( 9 ) Virial coefficient for water (Bi), ml. /gram mole (calculated from Equation 3 of 14) Virial coefficient f o r furfural (Bz), ml./gram mole (calculated from Equation 3 of 14) Gas constant (H),(mi.) (mm. Hg)/(' K.) (grain mole) z1 = Exg.io [(Pi- P)(i>i- Bi)I '12.303 K T I (192.3 - 121 2)(18 38 42&+] = 1,0015 = Exp.,o __(2 303)(62.365)(338 6) (22.2 121.2)(87.35 1794.8) = 0.991 22 = Exp.io (2.303)(62,36,5)1338,6j
338. 6 0 .0928 0.9075 121.2 192.3 22.2 1.02 18.38 87.35
-425.4
-1794,8 62,365
+
-
121.2
= _ =
+
-
(1,02)(22.2)(0.907.5)/O.YYl (192.3)(0.0925)/ 1.002
5.66
If the activity coefficient for furfural yz had been assumed as equal to 1.0, the value of 71 would be 5.68, and if both z, and z2 had also been taken as equal t.o 1.0, 71would be 5.67. C:.u,prI.iimox OF H E a w OF SOLCTION.A knowledge of the heat effects upon mixing furfural and water is desirable wherever these materials are used in chemical process work. For complex systems such as the hydrocarbon-furfural-water systems employed in certain extractive distillation operations of commercial importance, furfural-water heat of solution data are useful in est,imating enthalpy values of such mixtures. Differential heats of solution may be computed from activity coefficient dat,a by the uie of the equation
where L, = partial or differential heat of solution of component 1 dissolved in a liquid of composition for which y1 applies, B.t.u./pound mole of component 1 added T = partial or differential heat of solution of component 1 dissolved in a liquid of composition for which applies, B.t.u./pound ?ole of component 1 added T = absolute temperature, R. R = gas constant, 1.987 B.t.u./(pound mole)(' R). The logarithms of the activity coefficient values for water from Figure 3 were plotted against the reciprocal of the absolute temperature at constant liquid compositions of z1 = 0, 0.1, and
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
July 1950
22 5
LL
LL
1421
the partial heat of solution of furfural in pure water was computed. The integral heats of solution were computed by graphical integration of the partial heats of solution plotted as a function of composition. Computed values of both partial and integral heats of solution are presented in Table V. Both heat effects represent heat absorbed during the solution process. The absolute values of heat of solution are about one fifth as large as the molar latent heats of vaporization of each component, indicating the necessity for including heat of solution effects in design or correlation calculations involving furfural-water mixtures.
I-
z
w0 zLL W
$
* 2_
Y
k 9
&,MOLE
FRACTION WATER IN LIQUID
Figure 3. Activity Coefficients for Furfural and Water as a Function of Temperature and Composition 0.2, inasmuch as the slope of the curve represent,s the leftrhand side of Equation 4. These plots are shown in Figure 4, where it is apparent t h a t the slopes of the curves and the resultant partial heat of solution values for water are affected by both temperature and composition over the range studied. Although these changes are slight, they appear to be both definite and consistent and were therefore retained in the correlation. A similar plot of log y z at 5z = has been included in pigure 4,from which I-
C
APPLICATION OF EQUATIONS TO R E S U L T S
Representation of the experimental results by one or more types of theoretical equations is indicated to check the soundness of the data and especially to permit prediction of ternary and more complex equilibrium relations involving these two substances. A complex analysis of such equations available for correlation purposes has been made by Wohl @ I ) , while a typical application of the prediction of ternary equilibrium values from three set's of binary data through the use of these equations has been recently demonstrated ( 4 ) . In this latter study, the threesuffix Margules equations were shown to be especially valuable because of their applicability, their comparative simplicity, and the lack of any restrictions in their use in the ternary form. The ternary two-suffix van Laar equations are restricted, in that all of the binary constants must be related in a specified manner. SIMPLEMARGULES-TYPE EQUATIONS. The binary three-suflix Margules equations were used in the following form to t,est their to represent the data: log
YI =
%:[Ai
limit log log
+-
+ 2 ( A0), -= AA1i ) ~ i l
y1 (SI
(5-4)
7 S: [As 2 (Ai - &)s%\ limit log y z (2% 0) = Az
Y!
(SB)
When constants A1 and A2 in these equations were evaluated as the logarithms of the experimental activity coefficient values extrapolated in Figure 3 back to infinite dilution of each component, the resulting correlation was not satisfactory. Lowering of the values of A1 and A , to 5 and 40, respectively, increased the fit
2.2
0 L LL
A
g2
TABLEV. HEATSOF SOLUTIONFOR FURFURALWATER
2.1
MIXTURES
0 3 LL
5 L L
+ t
2.0
A.
2
+ K
22
1.9
+(3
s
1.8 I. I
+ W z
HLL
Heats Absorbed on Solution of Water i n Furfural over Range 100' t o 200' F. Differential H e a t of Integral H e a t of Mole Fraction Water Solution, B.t.u./Lb. Solution. B.t.u./Lb. i n Liquid, XI Mole of Hz0 Dissolved Mole of Hz0 Dissolved 0 0.10 0.20
Temperature, 100' F. 4830 2930 1830
4830 3900 3050
0 0.10 0.20
Temperature, 150' F. 4830 3870 2950
4830 1370 3900
0 0.10 0.20
Temperature, 200' F. 4830 4990 4900
4830 4900 4980
1.0 0.9
LL W
s 0.8 >t 5 0.7 + 0 E 0
B.
. e-
0.6
3
0.5
