Pure Hydrocarbons from Petroleum J
SEPARATION OF STRAIGHT-RUN FRACTIONS BY DISTEX PROCESS’
JOHN GRISWOLD, D. ANDRES2, C. F. VAN BERG3, AND J. E. KASCH4 The University of Texas, Austin, Texas
era1 approach opens a new vista in regard to separating pure hydrocarbons from petroleum. Three general modifications of solvent separation operations are now applied to petroleum hydrocarbon fractions-solvent refining, azeotropic distillation, and extractive distillation or Distex. Fundamentally, all three modifications accomplish a separation by altering the activity coefficients or producing Raoult-law deviations such that there is an appreciable difference between the volatility of the compound OF compounds to be separated and that of the other material of the mixture. Then by a counterflow contacting treatment between two phases, certain types of compounds are concentrated in one phase while the other types concentrate in the second phase. Solvent refining is a liquid-phase operation conducted a t a temperature low enough so that immiscibility occurs; the separation takes place by counterflow contacting of a liquid aolvent phase with a liquid hydrocarbon phase. The separation is nearly independent of molecular weight and boiling point of the compounds. Extract and raffinate pprtions consist of materials having low and high hydrogen/carbon ratios, respectively. This operation is therefore uniquely suited to the treatment of materials of wide boiling range such as lubricant fractions, kerosene, or gasoline. The Distex operation consists of fractional distillation in the presence of a selective agent or solvent, and is conducted a t a temperature a t which liquid and vapor phases coexist. As will presently be shown, a narrow-boiling straight-ruii fraction may be separated into paraffin, naphthene, and aromatic portions. The foreign component or agent may be any of several hundred organic compounds, including all of the “selective solvents” used in solvent refining. As a matter of economical operation and ease of separating the solvent from the hydrocarbon extract and raffinate, the solvent should have a considerably higher boiling point than the hydrocarbons. With all solvents likely to be used industrially, hydrocarbons have positive activity coefficients whose relative values depend chiefly on molecular hydrogen/carbon ratio. Thus, paraffins have the highest activities, naphthenes have appreciably lower activities, and aromatics have the lowest. Activity coefficients of aromatics are usually between a third and a half of the values for paraffins for compounds of about the same molecular weight when compared at the same concentrations of solvent. The solutions are low boiling, and when the boiling points of one or more hydrocarbons and solvent are within 10’ to 30” C. of one another, minimum-boiling hydrocarbon-solvent azeotropes usually appear. The principal difference betwean azeotropic distillation and Distex is that in the former a solvent must be selected whose boiling point is close to that of the mtiterial to be separated, whereas in the latter the boiling point of the solvent is unimportant so long as it is well above that of the hydrocarbons. The Distex operation thus has one more degree of freedom than azeotropic distillation, since the phase compositions are not fixed by either temperature or pressure alone. This permits the solvent in the liquid reflux to be maintained a t LL desired optimum concentration. Various aspects of industrial azeotropic and extractive distillations have been published (1,8, 6, 11, 12).
The Distex process is described with its application to the separation of straight-run fractions consisting of paraffins, naphthenes, and aromatics. Details are given of the apparatus and procedures used to resolve the narrow-boiling hexane fraction reported previously. This type of operation has been designated “extractive distillation” where applied to toluene purification, and lately to butadiene purification. The authors’ term “Distex” has been used for a number of years to designate the same type of operation for separations in general, such as paraffin-naphthene. Because of war censorship restrictions, it is published here for the first time.
P
ETROLEUM is an abundant source of hundreds of individual hydrocarbons. However, only relatively few of its known hydrocarbons are prepared in commercial quantities because of the technical difficulties involved in their separation. Fractional distillation is inadequate to separate a pure hydrocarbon from a mixture when (a) another hydrocarbon is present that boils within about 3” C. of it, or ( b ) another close-boiling hydrocarbon forms a nonideal solution with it. Straight-run fractions through CCcontain only paraffins and naphthenes that can be completely resolved by fractionation, Straight-run CCfractions cannot be completely resolved by this operation; cyclopentane and neohexane boil only 0.2’ C. apart, and benzene forms minimum-boiling azeotropes with the paraffins and naphthenes whose boiling points are somewhere near 80’ C. (9, 13). Straight-run Cr fractions may contain a number of pairs of hydrocarbons whose boiling points are within 1” C. of one another. I n general, the boiling points of hydrocarbons having the same number of carbon atoms increase as the hydrogen/carbon ( R / C ) ratio decreases, so that C* and C, naphthenes and aromatics boil higher than some C, and Ca parafEins and naphthenes, respectively. Toluene forms nonideal solutions with nonaromatics whose boiling points are near 110” C. Abnormally low relative volatilities occur in toluene-nonaromatic binaries-always at high concentrations of the lower-boiling component. This makes it virtually impossible to fractionate out small amounts of toluene from lower-boiling nonaromatics, and t o fractionate out small amounts of higher-boiling nonaromatics from toluene. ‘EXTRANEOUS-COMPONENT SEPARATIONS
‘
Within the past few years increasing attention has been paid to the problem of physical separation of mixtures by means of a selective foreign or extraneous component, or “solvent”. The gen1 Earlier articles in this series appeared in Volume 35, pages 117-19, 247-51,864-7 (1943),and 36,pages 1119-23 (1944). * Present address, Lubri-201 Corporation, Cleveland, Ohio. a Present address, Humble Oil & Refining Company, Baytown, Texas. 4 Present address, Petroleum Administration for War, New Interior Building, Washington, D. C.
65
66
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
Since the purpose of the solvent is to spread the volatilities of close-boiling hydrocarbons, its selectivity is of prime importance. While some compounds are more selective than others, dozens of compounds are sufficiently selective to be useful, and almost any polar liquid in which the hydrocarbons are soluble is somewhat selective (15 ) . However, economic considerations of availability and cost, chemical stability, corrosiveness, and toxicity limit the industrial list to relatively few compounds. Of these, all the solvents used in lubricating oil refining are feasible, such as Chlorex (2,2' dichloroethyl ether), furfural, phenol, and nitrobenzene. Vapor-liquid equilibrium between hydro,carbons in a solvent depends upon the factors: properties of solvent, vapor pressures (or boiling points) of pure hydrocarbons, hydrocarbon structure, and concentrations. h valuable index to vapor-liquid behavior may be obtained by determining and comparing experimental activity coefficients of individual hydrocarbons in the solvents. But as a subsequent paper will show, individual activities of mixed hydrocarbons in a solvent depend upon the composition and concentration of the hydrocarbon mixture in a manner not yet predictable. At present, direct experimental vapor-liquid equilibria may not be dispensed with. Heptane and methylcyclohexane were chosen t o evaluate the effectiveness of severa1 solvents for paraffin-naphthene separation.
ESSB 78MOL % @
Figure 1. Effect of Solvent Concentration on Vapor-Liquid Equilibrium at Atmospheric Pressure (n-Heptane-Rlethylcyclohexane)
H e a t e r and Vaporizer Detail
Vol. 38, No. 1
FIGURE 2
ANILINE and REFLUX HEATER (SEE DETAIL)
VARIABLE SPEED TRANSMISSION
Aluminum cyl.cast oround a coil of V i 6 " o . d seamless sloe1 tubing. Cartridge type healer inserted into hole thru ContBr.
Crank ond worm pear to adjust belt tension, N i c h r m e wirein fibsrQlass broided
DISTEX COLUMN SPECIFICATIONS: I " r t d . iron pipe * 48" pocked aocllOn, I/8" metal helices 4 heoter sectians lagging of 80% mogne$ia
EXPERIMENTAL APPARATUS FOR DISVEX PROCESS
LIQUID L E V E L
LEGEND % 'valve 8 :bewing zgear pump size i-8
& '
T. sthermocouple 8OTTOM
PRODUCT
TAKE-OFF MANIFOLD
DISTEX COLUMN
w, i w o t l a
'January, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
67
EXPERIMENTAL VAPOR-LIQUID EQUILIBRIA
.
Vapor-liquid equilibria for heptane-methylcyclohexane in several solvents were determined in a glass Othmer-type equilibrium still. Heat loss from the vapor chambet was compensated and phase separation in the condensate chamber was prevented by separate electrical-resistance wire heaters wrapped around the chambers. The heptane was obtained pure from California Chemical Company. The methylcyclohexane was obtained from Rohm & Haas. It was purified by treatment with nitrating acid and then distilled in a laboratory column. A heart cut was taken of about 80% of the charge. All solvents were redistilled through the laboratory column, and the boiling points, refractive indices, and densities of 80% center cuts were in satisfactory agreement with literature values. Determinations at reduced pressures were made with the O t h e r apparatus equipped with a vent condenser through which ice water was circulated and which was connected to a vacuum pump and manometer. Determinations a t superatmospheric pressures were, made with the steel equilibrium apparatus re ported elsewhere (8). The ternary equilibrium samples (two hydrocarbons in a solvent) were analyzed as follows: all of the hydrocarbon and a small amount of solvent were distilled out of a weighed-sample and charged to a washing buret made by sealing a 50-ml. glass analytical buret tube to the top of a 125-ml. separatory funnel. The last traces of solvent were removed by washing with acid and/or alkali and then with water. The funnel was then filled with distilled water so that the hydrocarbon was entirely in the graduated buret. The hydrocarbon volume and temperature were recorded. Treatments used to remove the solvents were: two washes of concentrated sulfuric acid for Chlorex and nitrobenzene; concentrated hydrochloric acid and then concentrated sulfuric acid for aniline and aminocyclohexane; 20% sodium hydroxide followed by neutralization with sulfuric acid for phenol; 50% sulfuric and then concentrated sulfuric acid for furfural; all were followed by water washes. Determinations on known mixtures showed a constant over-all mechanical and solution loss of hydrocarbon amounting to approximately 0.3 ml. and independent of the size of sample and its composition. This correction was added to all observed volumes of the extracted hydrocarbon samples. The weight of the heptane-methylcyclohexane sample and its analysis were obtained from known density and refractive index data, respectively (3). A Bausch & Lomb precision oil refractometer gave refractive indices corresponding t o an analytical accuracy of 0.1 mole %. From the experimental y-x values, relative volatilities (a) of heptane to methylcyclohexsne were calculated on the solvent-free basis;
where x and y are mole fractions of heptane in liquid- and vapor-phase hydrocarbons, respectively. The results are summarized in Tables I and 11. The barometric aniline data in Table I are plotted in Figure 1. Inspection of the data indicate the following:
1. There is no advantage in Distex operation a t super- or subatmospheric pressures over operation a t atmospheric pressure,
FIGURE 3 FLOW DIAGRAM .ABORATORY S C A L E DISTEX PROCESS
TABLE I. VAPOR-LIQUID EQUILIBRIA AT 745
i:
5 MM.
FOR
n-HEPTANE-METHYLCYCLOHEXANE-SOLVENT
Solvent Aniline
Mole 92
% ' T oC.
78
124 121 121 121 122
7.9 24.0 42.4 64.7 88.1
10.4 30.5 50.3 72.0 91.4
1.35 1.39 1.42 1.40 1.44
1.4Q
58
110 113 113 114
89.0 64.4 35.7 9.8
91 . o 69.6 41.1 12.0
1.27 1.27 1.26 1.26
1 265
... ... ... ... ...
16.3 63.4 76.0 44.1 35.4
20.8 60.9 80.9 51.2 42.5
1.35 1.36 1.34 1.33 1.35
1.35
... ...
18.6 34.2 56.6 76.0
23.3 41.7 63.1 80.8
1.33 1.37 1.31 1.33
1.33
13.6 43.7 61.2 80.5 27.8
17.1 50.5 67.3 84.6 33.6
1.31 1.31 1.31 1.33 1.32
1.32
79.7 60.6 45.9 30.7 16.5
83.6 66.2 51.8 36.3 20.2
1.30 1.27 1.27 1.29 1.28
1.28
20.4 37.5 56.6 79.1
22.9 41.2 60.2 81.4
1.16 1.17 1.16 1.16
1.16
Furfural
78.4 78.5 80.0 79.5 79.5
Phenol
80.0 82.4
81.1
81.5 Nitrobenzene
81.5 82.0 82.1 82.5 81.5
Chlorex
80.5 80.5 81.0 81.0 81.5
Aminocyclohexane
0
136 138 140 142
Hydroaarbona (Solvent-Free Baaia) Mole % n-heptane Rel. volatility Liquid Vapor a Av. 91.8 94.0 1.405 71.1 79.0 1.53 44.9 55.4 1.52 1.52 27.3 1.52 19.8
75.6 76.2 76.6 77.0
... ...
... ... ... ... ..* ... ... ... ... ...
...
... ... ...
Not included in average value.
2. Distillation in the presence of a high concentration of selective solvent greatly reduces the number of plates required to separate a paraffin from a close-boiling naphthene. 3. The effectiveness of the solvent increases as solvent concentration (in the liquid) is increased. 4. The solvent-free relative volatility for the system heptanemethylcyclohexane-aniline is substantially independent of hydrocarbon composition a t constant solvent concentration.
VAPORIZER
INDUSTRIAL AND ENGINEERING CHEMISTRY
68
Vol. 38, No. 1
stant solvent concentration) that an average value may be found and used in the conventional equations as noted. From the preceding data and the fact that considerable work and data were already available on the properties of aniline-hydrocarbon mixtures, aniline was used in further investigations. Since the solvent is not very effective a t low concentrations and an inordinate amount of solvent must be handled a t high concentrations, there exists a definite economic solvent concentration. The concentration of 80 mole % aniline was chosen somewhat arbitrarily as being near the optimum. Boiling points, calculated relative volatilities, and Distcx 01 values for compounds of interest in a Ce petroleum fraction are given in Table 111. The Distex relative volatilities were detcrmined with purified materials using the general techniques described earlier. Solution abnormalities and azeotropes betn-een hydrocarbons do not occur in the presence of the solvent, since hydrocarbon activity coefficients and their vapor-liquid equilibria are controlled primarily by the solvent rather than by the presence of other hydrocarbons. The Distex separation of aromaticnonaromatic hydrocarbons is much easier than the corresponding paraffin-naphthene separation, as indicated by the comparative Distex cr values. I -
m a
SOLVENT OISTEX SEClION
Figure 4.
SOLVENT RECOVERY
SECTION
Generalized Flow Diagram of a Distex 'Unit
It follonw from observation (4)that an appropriate solventfree value of a: may be used in conventional distillation equations ( 5 , 1 7 )for plate and reflux ratio calculations for a Distex operation t o separate heptane from methylcyclohexane with aniline, provided the solvent concentration in the column is maintained substantially constant. While it is known that this relation does not apply to all systems, in a number of cases the solvent-free 01 for close-boiling key components of a hydrocarbon mixture in a solvent ia so slightly affected by hydrocarbon composition (at con-
TABLI:
TI. FOR
I T . 4 P O R - h Q C I D EQUILlBlZI.4 AI' Tr J R I O U S PRESSURES TA-HEPTANE - k f E T H Y L C Y C L O I I E X h > E-SOLVENT
Mole Solvent .Aniline
%
T o C.
85.0 80.8 79.0 83.1 80.9 80.7 78 6 82.2 81.4 82.1 76.5 75.1 74.5 82.5 82.5 86.9 82.2 88.1 67.0 44.a 94.0 72.0 76.2 88.6 77.1
50 64 80 79 91 101 101 105 110 122 150
91.6
Phenol
Nitrobenzene None
75.7 69.7 58 5 74.0 92.0 65.5 81.5 91.6 63.0 73.9 93.0 0
200
260 280 200 250 200 100 150 190 240 260
Pressure, Lb./Sq. I n . Abs. 141 mm. 201 mm. 301 mm. 293 mm. 397 mm. 494 mm. 499 mm. 538 mm. 697 mm. 748 mm. 33.5 31.5 31.3 28.5 29.0 26.0 28.0 27.0 35.0 40.0 43.5 81.5 76.5 56.5 156 113 165 264 101 88 54 200 165 125 91 68 31 14.7 50 107 253 325
Hydrocarbons (Solvent-Free Basis) Mole % 78 heptane Rel. vola-~ tility a Liquid Vapor 43.4 50.8 1.35 42.8 $0.4 1.36 1.36 43.1 50.7 1.38 42.9 50 8 42.5 50.2 1.36 42.3 50.2 1.38 43.9 51.3 1.35 42.2 50.2 1.39 43.1 51.1 1.38 1.40 41.8 50.2 Rfi.2 62 9 .. 1.32 1.31 25.1 30 5 1.30 26.9 31 2 28.5 37 8 1.37 1.36 38.1 45.6 37.1 44.9 1.38 1.36 23.5 29.4 71.0 77.6 1.41 42.8 48.8 1.25 44.4 48.3 1.17 36.3 44.1 1.89 40.8 46.2 1.25 42.9 49.0 1.28 40.6 48.2 1.36 41.0 46.5 1.25 41.6 48.6 1.32 42.7 48.1 1.24 63.9 68.0 1.20 1.21 43.4 47.9 44.8 50.9 1.28 40.3 48.6 1.39 1 20 43.8 48.4 43.9 49 8 1,27 39.0 45.8 1.32 1,18 43.1 47 3 42.3 47.8 1.25 41.7 48.7 1.33 1.083 (7) 45:6 4?:7 1.09 45.4 47.7 1.10 1.11 45.4 47.9 1.04 45.6 47.1
SEPARATION OF PETROLEUM HEXANE FRACTlON
I n the Distex operation, the solvent is added near the top of the column and removed from the bottom as a solution containing the extract product hydrocarbons. This requires a continuous flow type of apparatus and complicates its adaptation to laboratory experiment. I n other words, the laboratory apparatus must be a pilot plant. Figure 2 is a drawing of the apparatus used to separate the hexane fraction. The column consisted of an enriching section with the equivalent of one equilibrium stripping contact furnished by the reboiler. The column was constructed of 1-inch standard pipe with a packed section of 48 inches of inch x 22 gage one-turn Xichrome helices. Heat loss was compensated by four electrical windings, which also permitted control of column temperature gradient. Eleven iron-constantan thermocouples were installed a t the locations shown; each was connected through a single multipoint switch and cold junction to a lorn-range potentiometer, giving temperatures accurate to 0.5" C.< General details of the reflux preheater, reboiler, vaporizer, condenser, and bottoms cooler are shown on Figure 2 Separate pumps were used for solvent, reflux, and feed. These were viscose pumps purchased from Zenith Manufacturing Company. Flow-rate control was obtained by the variable-speed transmissions shown and also by changing the gears driving the pumps.
TABLE111. RBLATIVEVOLATILITIES OF BIXARYCO HYDROCARBON SYSTEMS AT 1 ATMOSPHERE B.P., O C. a 2,2-Dimethylbutane 49.7 2,3-Dimethylbutane 58.0 1.33b 2,3-Diniethylbut~ne 58.0 2-Methylpentane 60.2 1.07b 2-Methylpentane 60.2 3-Methylpentane 63.2 1.10a 3-Methylpentane 63.2 n-Hexane 68.8 1.2Ob 2-Methylpentane 60.2 n-Hexane 68.8 1.33'~ n-Hexane 68.8 Methylcyolopentane 71.9 1.106 Methylcyclopentane 71.9 Cyclohexane 80.7 l.33b Cyclohexane 80.7 1.5-016c 80.1 (azeotrope) Beneene Methylcyclopentane 71.9 1.85-0.97 (9) 80.1 (azeotrope) Benzene a 80 mole % aniline. b Calculatd Calculated from Zquations 5 and 6 (7). From data a t 70' C. (f4). d Experimental determination by E. E. Ludwig. Experimental determination by J . E. Kasch.
Distex a"
... ,..
... ... I . .
1.454
... 2.1d 3.2e
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
January, 1946
The flow may be more easily followed by reference to Figure 3. The hydrocarbon feed was pumped through the vaporizer and entered the column as vapor immediately above the reboiler. The bottoms leaving the reboiler (principally cyclic hydrocarbons in aniline solution) passed through the cooler and into a receiver. The hydrocarbons were stripped from the bottoms, and the solvent was recovered in a separate still. The overhead vapors were completely condensed, part being removed as product and the remainder being returned as reflux to the incoming solvent. The solvent-reflux stream passed through the preheater and into the top of the column. The electrical heaters and vaporizers were individually controlled with Variac transformers. Heat input to the reboiler was measured by a wattmeter. The meter reading was used to calculate vaporization rate after subtraction of the equivalent of heat loss, which was experimentally determined a t the various temperature levels encountered in the operation. The column was tested under Distex operation with n-heptanemethylcyclohexane-aniline mixtures, and the number of equivalent theoretical plates (ETP) were also determined using the binary hydrocarbon mixture a t total reflux. The results are given in Table IV. I n this case the number of equivalent theoretical plates for the Distex operation with aniline is about two thirds that for operation on the hydrocarbons alone. The hexane fraction separated was Skellysolve B whose A.S.T.M. boiling range was 148” F. (initial boiling point) to 165’ F. (end point). This waa charged to the Distex column, which was operated with a hydrocarbon reflux ratio ( L I D ) of about 13, and a t a solvent rate that maintained slightly more than 80 mole % aniline in the liquid reflux. The overhead product amounted to 60% of the charge and was almost entirelyparaffinic. The bottoms contained all of the cyclohexane and benzene and all but 3.5% of the total methylcyclopentane present. The benzene was removed from the bottoms product by nitration, and the overhead and bottoms products were separately fractionated in a Podbielniak Heligrid column, a t rates for which the column had between 50 and 60 ETP. From these distillations, samples of 99% pure n-hexane and 98% pure methylcyclopentane were obtained. Complete details of the inspection tests on the stock, all distillation curves, and the complete analyses of stock and Distex fractions were reported earlier (20). These results indicate that the Distex operation is effective and feasible for the paraffin-naphthene separation and the preparation of pure hydrocarbons of those types as
69
well as of pure aromatics from petroleum fractions. A recently developed “selectivity index” calculated from critical solution temperatures of hydrocarbons in a solvent (6)confirms the conclusion that aniline is one of the most effective solvents for paraffin-naphthene-aromatic separations. The preceding data and experiments demonstrate that in narrow-boiling hydrocarbon mixtures, there are differences in the activities of various types of hydrocarbons in a solvente.g., between paraffins, naphthenes, and aromatics. The differences in activities between types are of greater influence on vaporliquid equilibria than are differences in vapor pressures of individual components of narrow-boiling mixtures. Algebraically, a’ab
=
PbYb Pay.
where a’ = relative volatility of two hydrocarbons of different type in a solvent; P., Pb = vapor pressures of pure hydrocarbons; yo, yb = activity coefficients of respective hydrocarbons.
The activity coefficients of Equation 2 are over-all Raoult law deviations that include gas law deviations, and are not therrnodynamic quantities. Nevertheless, Equation 2 is adequate for the present purpose. Activity coefficients are seen to be coordinate with or of equal importance to vapor pressures of the pure compounds in their effectson the relative volatility. For narrow-boiling mixtures the volatility (yP)of each paraffin is higher than that of each naphthene, and the volatility of each naphthene is higher than that of each aromatic, when dissolved in a high concentration of solvent. I n wide-boiling mixtures a paraffin may be of sufficiently higher boiling point than a naphthene so that the relative volatility as noted above is reversed. As stated earlier, the activity coefficients depend on the composition of the entire mixture in a manner not yet predictable. Fractionation of a narrow-boiling mixture dissolved in a solvent then effects a primary separation according to hydrocarbon type. Resolution of the portions (each consisting principally of but a single type) may frequently be accomplished by ordinary fractionation. Type separation facilitates ultimate resolution in two ways : The single-type portions contain fewer hydrocarbons than the original stock, and the Distex separation frequently removes compounds of intermediate boiling points. Secondly, solution abnormalities that interfere with fractionation are absent in the single-type portions. The general problem of preparing pure hydrocarbons from petroleum then consists of three steps: (1)preparation of a narrowboiling stock or fraction that contains the desired hydrocarbons; COLUMN (2) separation of the stock into single-type portions by the Distex TARLEIV. DISTEX TESTS ON 48-INCH ENRICHING operation; (3) obtaining the individual hydrocarbons in pure Reflux Ratios R~~ sample Mole % n - H e p s Theoreti- Theoretical state by fractionation of the single-type portions (separately). No. No. Feed Distillate Actual ca1 min.a Plateab A Distex unit consists of a fractionating column with condenser, n-Heptane-Methylcyolohexane in 87 Mole % Aniline (Aniline-Free Basis; reboiler, and a short stripping or solvent recovery column (Figure a = 1.47) 4). The hydrocarbon feed is introduced to the main column a t a 33 8.3 7.8 21 25.4 97.2 22 15.3 95.4 35 13.3 12.4 point determined by its composition and the separation to be 23 18.8 69.0 11 7.0 8.0 made. The solvent is introduced near the top of the column. 82.8 9.1 18.8 19 9,s 85.8 9.6 22 18.8 9.0 The column section above the point of introduction of solvent 18.8 10.8 93.8 80 12.3 serves to fractionate out the solvent from the hydrocarbons, so 94.6 24 16.7 12.3 35 13.8 95.9 46 13.8 16.7 12.5 that the overhead is solvent free. The bottoms are separated into m 16.7 la. 2 96.6 12.90 extract hydrocarbons and solvent in the solvent recovery column, 25 1 18.7 88.0 18 10.0 11.6 2 18.7 96.5 m 11.3 12.40 and the solvent is recirculated to the main column. The considerations governing the hydrocarbon reflux ratio are the same Efficiency Test, Binary Hydrocarbon Mixture, No Solvent (Total Reflux: a = 1.08) aa for conventional fractionation when the proper value of a is Liquid Rate employed, and the entire operation is actually fractional distillaa t Top of Sample Column Mole % n-HePtane Theoretical HETP tion in the presence of an extraneous liquid which cauBes a separaNo. Cc./Min: Still pot Distillate PlatesC Inches’ tion that would not otherwise occur. 1 15 41.6 76.0 18.6 2.6 2 3 4
28 36 42d
41.9 42.9 43.2
76.0
18.4
72.6
16.3
76.7
From Smoker nomogram (16). From Smoker equation (17). C From Fenske equation for total reflux (6). d Rate of incipient flooding. b
17.6
2.6 2.7
4.1
FLOW SHEET FOR PURE HYDROCARBON PLANT
Using Skellysolve B as tan example, a proposed flow sheet has been developed for large-scale continuous separation of its principal hydrocarbons in purities of 99% or better (Figure 5 ) . The
INDUSTRIAL A N D ENGINEERING CHEMISTRY
70
r'
FIGURE 5
:,%%thybutane
Vol. 38, No. 1
COMMERCIAL METHOD FOR SEPARATION OF PURE HYDROCARBONS FROM Cc FRACTION OF PE~ROLEUM
54 2-mothylpentane 90
DISTILLATION No.2.
PROPOSED FLOW SHEET USiNG DiSTEX PROCESS
50 PLATES
Thewetical W t e s at total reflux are given. Moterial buiance in bbls. based on 1000 bbls. of charge,
3 2 ch.
166 mcp 198
DISTILLATION N0.3. 32 P L A T E S IOOObbIS. C g FRACTION
1
1 I
I
I
L
I 2 n-pentane < I cyclcpentone 24 2.3-dimefhylbutone 160 2-methylpentone t l 3-methylpentone 588 n-hexane 166 methylcyclopentane 16 benzene 32 cyclohexane
99 %
.
I O 6 bbIs.,99%
RECYCLE
'
147 n-hexane
41 mcp.
2
188 ANILINE
m I
16 P L A T E S
a
40
DISTEX No I.
1
cyclohexone 32 bbk., 99 %
-+ 2-methylpentone
DISTEX No.2. 16 PLATES.
-* cyclopentone)
I i
1188 bbis..
ANILINE
STRIPPER
+
147 n-h 207 mcp 32 ch 16 benzene -
ANILINE STRIPPER
402
operating units consist of three Distex columns and three fractionators. The number of theoretical plates at essentially total reflux and 80 mole yo aniline were calculated using the relative volatilities of Tables I and 111. Not more than fifty theoretical plates are required for any column. The quantities shown are for 1000 barrels of charge. Two primary Distex columns rather than just one are used so that only one sharp separation, rather than two, is required of each column. Fresh charge and hydrocarbon recycle are fed t o Distex No. 1 which takes only paraffins overhead. Any cyclopentane present in the charge accumulates in this column and is eliminated by bleeding out a small portion of the vapor. The Distex overhead is fed to distillation column 1 which separates hexane as a pure bottoms product. The original charge contained only a trace of 3-methylpentane, This accumulates in the fractionator and is eliminated by bleeding out a small portion of the reflux. The overhead consists of the lo.;r.er-boiling isohexanes with any pentane present in the charge. This stream is fed to distillation column 2 which separates pure 2-methylpentane as bottoms product. The minor paraffins are not separated. The bottoms product from Distex No. 1 is fed to Distex No. 2 which separates the residual hexane along with some methylcyclopentane, and the mixture is recirculated to the original charge. The bottoms hydrocarbons containing the cyclics are fed t o Distex No. 3, which separates benzene and heavy impurities as bottoms. The overhead is fed to and separated by fractionator No. 3, into pure methylcyclopentane and pure cyclohexane.
*
166 mcp. 32 ch. 16 benzene 214
-
ANILINE
/I I
DISTEX N0.3. 12 PLATES
J
STRIPPER
ACKNOWLEDGME-YT
The University of Texas Bureau of Industrial Chemistry, under the direction of E. P. Schoch, sponsored the project by the purchase of equipment and by fellowships awarded to J. E. Kasch during the school years of 1939-40 and 1940-41 and by summer session fellowships in 1940 to D. Andres, C. F. Van Berg, and C. A. Walker. C. R. Everett determined the subatmospheric equilibrium of n-heptane-methylcyclohexane-aniline, and M. R. Morrow assisted in the construction of the Distex column. The Phillips Petroleum Company and Shell Development Company donated samples of methylcyclopentane used in the equilibrium determinations. LITERATURE CITED
(1) Benedict, Johnson, Solomon, and Rubin, Trans. Am. Inst.
Chem. Engrs., 41,371 (1945). (2) Benedict and Rubin, Ibid., 41,353 (1945). (3) Bromiley and Quiggle, IND.ENQ.CHEM.,25, 1136 (1933). (4) Colburn and Schoenborn, Trans. Am. Inst. Chem. Engrs. 41, 421 (1945) (5) Fenske, M. R., IND.ENQ.CHEW.,24,482 (1932). (6) Francis, A.W . ,Ibid., 36,764 (1944). (7) Griswold. Ibid. 35. 247. (1943). Griswold; Andres; and d e i n ; Trans. Am. Inst. C h m . Engrs., 39, 223 (1943). Griswold and Ludwig, IND.ENG.Cmx.,35, 117 (1943). Griswold, Van Berg, and .Kasch, I b i d , p. 854.
Happel, Correll, Eastman, Fowle, Porter, and Schutte, Trans. Am. Inst. Chem. E n g ~ .41, , 539-631 (1945).
Lake, G.R.,Ibid., 41,327 (1945). Richards and Hargreaves, IND.ENQ.CEEM.,36,805 (1944). Scatchard, Wood, and Mochel, J. P h p . C h m . , 43,119 (1939). Shiras and Johnson, U.S.Patent 2,357,028(Aug. 29, 1944). Smoker, E.H.,IND.ENQ.CHEM.,34,509 (1942). Smoker, E.H.,T r a m . Am. Inst. Chem. Engro., 34,165 (1938).
