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ROGER S. PORTER and JULIAN F. JOHNSON California Research Corp., Richmond, Calif.
Extractive Distillation
. . . by
Circular Gas Chromatography This simple, new technique gives usable separation factors for extractive distillation. Effect of temperature is easily shown
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X T R A C ~ V E distillation yields many otherwise unobtainable separations. An extractive solvent aids separation by altering relative volatilities. The process is useful in reducing distillation requirements of theoretical plates, reflux ratio, and heat input. The use of extractive distillation requires a knowledge of vapor-liquid equilibria for ternary and higher component systems. Such data are scarce for multicomponent systems. The absence of data is due principally to the tedious, complex experimental procedures ( 7 7). Recently, it has been shown that gas chromatography can give the required data (6, 7, 70, 75). Separation factors, or alpha values, for extractive distillation reduce to vapor pressure ratios for components over the liquid phase. In gas chromatography the rate of travel of a component along the column is a function of its vapor pressure over the partitioning liquid. Therefore, extractive distillation alpha values may be calculated as the ratio of corrected gas chromatographic retention times. Corrected retention times are component elution times minus elution times for air or other insoluble gases. The relationship between extractive distillation alpha values and gas chromatographic data has been derived rigorously (6, 7, 70). Gas chromatography gives a limiting alpha value, as measurements are made near infinite solvent, or partitioning liquid, concentration. This limiting value indicates the maximum separation achievable. Similarly, solvent concentrations are high in successful extractive distillations (76). Conventional gas chromatography employs partitioning liquids of a much lower volatility than those of the compounds to be separated. This limits its usefulness, as the solvents normally employed in extractive distillations have an appreciable volatility. This difficulty has been ovetcome Sy the use of circular gas chromatography which permits the use of volatile partitioning liquids (8).
Limiting separation factors are reported here for five hydrocarbon pairs using aniline as the partitioning liquid over the temperature range 25' to 150' C.
Method A schematic diagram of the apparatus is shown below. The pump used to circulate the helium carrier gas was a modified Model 4100 Rota-Flex or a Bantam Dyna-Vac, Model 4080 (ColeParmer Instrument and Equipment Co., Chicago, Ill.). The air bath thermostat was stirred and regulated by a Resistotrol (Hallikainen Instruments, Berkeley, Calif.). The thermal conductivity detector employed contained 8000 ohm (nominal) thermistors. The bridge and
INJECTION BLOCK
recorder were of conventional design. Columns were constructed of '/,-inch outside diameter copper tubing. The inert support was 20- to 30-mesh size (Johns-Manville C-22) insulating brick. Aniline (Baker's Analyzed Reagent grade) was applied to the brick in an acetone solution to give 28.770 by weight of aniline on brick. This concentration is sufficient to give reliable separation factors uninfluenced by the nature of the solid support (5-7). Equivalent separation factors were obtained at 14.4 and 28.7 weight 7 0 concentration of aniline for the system n-heptane-methylcyclohexane. Care was taken in preparing and preserving the column packing. Any contamination problem incurred here will be in the direction of errors
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HEATER! = PUMP MOTOR
OVEN METER
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TRAP LEADS TO =BRIDGE a RECORDER DETECTORS
Circular gas chromatograph permits use of volatile partitioning liquids VOL. 52, NO. 8
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AUGUST 1960
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found in actual extractive distillations. The helium carrier gas can be expected to cause no abnormal effect on components in the gas and liquid phases (6, 7). The influence of column pressure, always beIow 2 atmospheres, should likewise be negligible. The aniline vapor pressures a t test temperatures ranged from 0.01 to 0.3 atmosphere (72). Column lengths ranged from 2 to 10 feet. The columns were divided into two equal lengths so that pump and detector could be placed opposite one another in the cycle. This positioning minimizes detection of pressure surges. Separation factors were independent of column length over the range tested. Column efficiencies were lower for shorter columns, because of the greater percentage of dead volume. A change in column efficiency should not influence alpha (5-7). Alpha values were found to be independent of carrier gas flow rates. which varied from 15 to 100 ml. per minute. This agrees with other work (7, 10). Column flow rates were measured by a rotameter in the column cycle. During actual determinations, the rotameter was excluded from the system because
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of its detrimental dead volume contribution. Figure 1 shows a typical gas chromatographic determination of alpha. Alpha values were calculated from a series of full cycle retention times. Cyclic times were uniformly consistent to =!=5yo except where peak identification was difficult due to overlap. Only one alpha is reported for each test. A statistical analysis indicates precision of better than =!= 10% for 9570 confidence limits. I n conventional gas chromatography, partial fractionation may occur during sample introduction. This leads to errors in retention times. Using a circular column, however, injection fractionation is easily observed by comparing alpha values from the first and subsequent detector passes. Alpha values are derived from cyclic times after the first detector pass. Such times cannot involve injection fractionation. Separation factors were independent of sample size and hydrocarbon ratio. This was checked on methylcyclohexane-n-heptane for mole ratios from 1 to 5 and 5 to 1 and for sample sizes of 10 to 50 PI. T o facilitate identification, hydrocarbon pairs were commonly tested in a 2 to 1 mole ratio with the larger
INDUSTRIAL AND ENGINEERING CHEMISTRY
amount for the compound with the lower vapor pressure over aniline. Consistent alpha values were also obtained when components were injected and measured separately. This adds the requirement that column variables remain the same during tests. By injecting binary mixtures, the derived alphas are automatically independent of such changes. Results and Discussion
Extractive distillation correlations were obtained for five hydrocarbon pairs. Each pair is important in petroleum refining. Table I lists binary mixtures and individual boiling and aniline points. The systems illustrate a variety of conditions from virtually ideal to highly nonideal solution behavior. Results of this work are plotted in Figure 2. The compound with the shortest retention time has the highest vapor pressure over aniline. I t is uniformly listed first in Figure 2. Cyclohexane-Benzene. Extractive distillation has been extensively studied on this system as simple distillation does not produce separation. An azeotrope is formed at 50 to 55 mole yo of benzene, which boils at 77.4 to 77.7' C. ( 7 ) .
CIRCULAR G A S C H R O M A T O G R A P H Y A number of solvents, including aniline, permit high purity cyclohexane to be distilled from benzene (4, 73, 76). To date, there are few data on this system for the change of separation factor with temperature. Figure 2 shows the marked improvement in separation factor as the temperature is lowered. An alpha value of 5.76 a t 20' C. has been reported (70) for this system from gas chromatographic measurements. This is in good agreement with the extrapolated value from Figure 2. For this system alpha values have also been derived for finite aniline concentrations by the gas chromatographic method. This was accomplished by making the column packing a mixture of benzenecyclohexane and aniline on brick. Alpha values were 4.4 and 4.0 a t 30" and 50' C. for 50 weight of aniline. As expected, these values were below the data given in Figure 2, with the largest difference a t the- lowest temperature. Methylcyclohexane-Toluene. Separation of methylcyclohexane from toluene by distillation is difficult despite a 10" C. boiling point difference. This is because of nonideal solutions formed by binary mixtures. Separation factors decrease to l .07 with increasing methylcyclohexane concentrations (2, 76). Aniline aids separation of this commercially important system (9, 76). At the highest solvent concentration previously reported, 75 weight of aniline, an alpha of 2.59 was obtained at 147' C. at atmospheric pressure (2). This is directionally consistent with the gas chromatographic alpha of 3.1 for infinite solvent concentration at the same temperature. The variation of alpha with temperature is shown in Figure 2. A threefold improvement in alpha should be achieved by operating extractive distillation columns at reduced pressure and temperature. This appears to be feasible
Table 1.
Pair
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A =n-HEPTANE 0 =METHYLCYCLOHEXANE SUBSCRIPT DENOTES DETECTOR PASS. ANILINE COLUMN AT 100°C. SEPARATION FACTOR = 1.65
--INJECTION AIR
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TIME,MINUT€S FigureIl.
Record shows periodic passes of components past the detector
because of the relatively high mutual solubility in this ternary system (Table I). However, phase separations a t lower temperatures limit the ultimate alpha (2). n Heptane Methylcyclohexane. These compounds differ in normal boiling points by 2.6" C. Their binary mixtures form virtually ideal solutions with an alpha of 1.07 for all concentrations at atmospheric pressure (3). Aniline provides an excellent extractive separation of this pair ( 2 ) . Equilibrium still data ( 3 ) give a value for alpha a t 134" C. of 1.27 for 58% aniline and 1.52 for 9201, aniline. Data from this work give an alpha of 1.65 at the same temperature. Thus, the agreement is good.
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Properties of Mixtures Studied by Circular Gas Chromatography Boiling Normal Point Binary Aniline Boiling Difference, ' Components Point, O c. Point, ' C. O c.
Cyclohexane Benzene 2 Toluene Methylcyclohexane 3 Methylcyclohexane n-Heptane 4 n-Octane Methylcyclohexane 5 n-Octane Toluene Critical solution temperature. 1
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