ure fro JoaSGRISWOLD AND C. F. V.4N BERG' T h e Gniversity of Texas, Austin, Texas
T h e Distex process has been applied to the separation of a narrow-boiling heptane fraction (Skellysolve C) from mid-continent natural gas, and pure n-heptane has been prepared by tw-o operations. The Distex pilot plant and it5 performance are described at different reflux ratios in separating the heptane fraction into paraffinic, naphthenic, and aromatic portions, Pure heptane was obtained from the paraffinic portion by batch fractionation.
The solvent-reflux preheatcr was of similar construction biii contained 12 feet of 3:'16-inch copper tubing. The feed preheatcr consisted of 1/*-inch steel tubing and a cartridge heater cast i i i t r , an aluminum block. The solvent stripping r ) r recovery colnmn consisted of 2-iiieI standard pipe packed Ii-ith four feet of 1!8-inch card teeth. It was conipeiisated for heat loss by a 300-watt capacity heater The top 15 inches Jvere left' bare t o poi-ide the small amount of reflux neceFsary to separate the solvent'. The stripper rehoiler \!-as of 2-liter capacity and was provided with a 6:jO-\\-atilioatcr. Solvent take-off was manually controlled. All cloctrical heater. n-ere controlled by iiidividual Variace, and the input t o any heate:. could be measurcd by a nattmeter shunted into the circuit. 1roii~constantan thermocouples were provided to measure the stripper arid all stream temperatures. A lox-range portable potentiomctci: \vas used which gave temperatures accurate t o *O.S' C.
T
HE preceding article of this series (4)described essentials of the Distes operation 11-ithits application to the separation of a narrow-boiling hexane fraction (Skellysolve B). It was demonstrated that pure Cghydrocarbons could be obtained by separating the fraction into paraffin and naphthene portions by distillation in the presence of a selective solvent and then fractionating each portion t o recover the pure compounds. The original Distes unit was a column, 1 inch in diameter with a 48-inch packed section. In order to run larger samples and obtain bet,ter separations in an apparatus more closely approaching commercial equipmenr, a Distex pilot plant with a screen-plate column and integral solvent recovery column was constructed (Figure 1). The main column ITas of 2-inch standard pipe and had 150 screen plates (called "column 2" in an earlier article, 6). Details of the columns arid heaters are shown in Figure 2. The main column consisted of six sections of twenty-five plates each. It n-ai wrapped first xith Fiberglas tape, then wound with six heating sections of Sichrome wire strung through Piberglas tubing and insulated with standard pipe lagging. Each heating element had a capacity of about 200 watts and was controlled by an individual S'ariac. An iron-constantan thermocouple was mounted at the center of each section, projecting to the interior of the column. Each section also included a manometer connection at its top and a feed or takeoff connection a t its center. Figure 2 shows the column as used for solvent opcration. The bottom take-off cup was equipped with a sight glass. An airactuated lowrange flowmeter i ~ a s adapted to the use of a level controller. IS'hen a reboiler was employed, the column tended t o surge, or flood and drain. This difficulty !vas eliminated by a hydrocarbon vaporizer consisting of a coil of 5 feet of '/B-incli o.d. stecl tubing mounted in a steam chamber. The outlet temperature was controlled by adjusting the steam pressure. 1 Present addrecs, Humble Oil &- ReGning Company, Baytown, Texas. 2 Earlier articles in this series appeared i n Volume 3 5 , pages 117-19, 247-51, 854-7 (1943), Volume 36, pages 1119-23 (1944), and Volume 38, pages 61-6 (1946).
Figure 1. Distew Pilot Plant
170
February, 1946
171
INDUSTRIAL AND ENGINEERING CHEMISTRY
Viscose gear pumps made by Zenith Manufacturing Company were used for feed, solvent, reflux, and bottoms streams. They were driven through variable-speed drives (California Transmission Company Cub model), giving a rate adjustment of 3.3 to 1. When a greater or smaller rate was required, a pump of a different capacity was used. All pumps were driven from a common countershaft by a l/C-horsepower motor and speed reducer, so that the shaft turned a t 100 r.p.m. Flow rates of 0.5 to 500 ml. per minute could be obtained and held constant. The same column was used for batch fractionation by substituting a 4-gallon Ivelded steel still pot (heated by ring heaters) in place of the take-off cup and bottoms vaporizer. Figure 3 is the flow diagram for solvent operation, showing steel pipe condensers, storage, and accumulator vessels. Hydrocarbon feed was introduced into the column a t a point depending on its composition and the separation desired. The top of the column was bared of insulation, and heat loss condensed nearly all of the solvent out of the overhead vapor. The vapor was condensed and a definite fraction was pumped back through the vaporizer. Plate efficiency of the column on heptane-methylcyclohexane decreased from 58% (at 16 ml. of liquid per minute) to 40% immediately below the flood point of 140 ml. per minute, as reported previously (6). Plate efficiency tests for Distex operation using same hydrocarbons in approximately 80 mole yo aniline are summarized in Table I. Top and bottom samples from the 150-plate column were so nearly pure heptane and methylcyclohexane that they could not be analyzed with sufficient accuracy for plate computation of the entire column. Samples taken from the fiftieth and hundredth trays and analyzed by the method reported previously (4) gave plate efficienciesof 28 to 30%, which is about two thirds of the d u e for the hydrocarbons alone. It is interesting t o note that the flood point of the column (as measurcd by liquid rate) was not decreased for the solvent operation. The hydrocarbon capacity is much lower in the solvent operation.
I'
I
\
'\
STEAM T R A P
TO MANOM
1I T.C. WELL.
ow.
HE
ELEME
TO CONDENSER
440W. CARTRIDGE
OPERATIONAL CONSIDERATIONS
The Distex operation on a pilot plant scale is difficult because all flow rates and temperatures must be maintained constant to obtain satisfactory separation. As noted, all streams were pumped and means were provided for close control of temperatures by electrical and steam heaters. A pressure differential
Figure 2.
Distex Column and Stripper Detail
B72
INDUSTRIAL AND ENGINEERING CHEMISTRY
of about 23 mm. of mercury prevailed over each 25-plate section Then the column was operating properly. Pressures a t each section were observed by open-end manometers mounted on a panel. For operation at 80 mole % solvent in the liquid reflux, the column temperature a t any point is the boiling point of the hydrocarbon mixture present at that point in 80 mole % solvent. This temperahre may be predetermined by observing the boiling point of a known mixture in any type of boiling-point or of vaporliquid equilibrium apparatus. The column temperature gradient then depends upon the compositions of overhead and bottoms products, This gradient may be estimated beforehand and checked while the column is in operation by noting the stream rates of hydrocarbons , and solvent at top and bottom. Plate calculations may be made for the separation of key hydrocarbons (of different type), using an appropriate value of oi on the solvent-free basis. The most troublesome separation in the Distex operation to resolve a narrow-boiling straight-run
-El--
fraction (into portions according t o type) is between low-boiling naphthenes and higher-boiling paraffins. This separation determines the minimum hydrocarbon reflux ratio as discussed later. The aniline point temperature (using equal volumes of hydrocarbon and aniline) is easily determined, and LYas used as a control test on the performance of the Distex column. Aniline points of Ce and C I paraffins range from 68" to 78" C., those of the Ca and C, naphthenes are between 31" and 48" @., and those of aromatics are below -30" C. The Distex overhead then contains the least naphthene when its aniline point is highest, the Distex hydrocarbon bottoms contains the least paraffin when i t s aniline point is lowest, and the column gives the best type separation when the aniline point spread is greatest, Although no attempt mas made to obtain primary Distex products completely frec from solvent, the products contained only traces, and since the solvent was aniline, there was no interference with the aniline point test. ANALYSIS OF H E P T A N E FRACTION
Skellysolve C was selected as the narrow-boiling heptane fraction. It is a commercial product manufactured from midcontinent natural gas by the Skelly Oil Company. Inspection tests follow:
--
I~,I5,etc. C O L U M N S E C T I O N S 2 SOLVENT 8 REFLUX HEATER 3 -CONDENSER 4
-
COOLER
i - FEEE-HEATER
6 - B O T T O M S TAKE-OFF C U P 7 -VAPORIZER 8 - CONTQ3L V 4 L V E 9 STRIPPER IO MANOMETER OUTLETS II LIQUIO LEVEL CONTROL
-
a
1
I2 13
-- PSRO-OLDV dF CN TT
I N S T R d.M ENl PUMP
PUMP 14 - R E F L U X P U M P 1 5 - F E E D PUMP 16 - B o ? T O M R E C Y C L E P U M P 17
-
PROOUCT R E C E I V E ;
17
7
OVERHEAD PRODUCT
1
ATM V E t i 1
A Ik 4
SOLVENT STORAGE
$
4
+ BOTTOM
Figure 3.
Distex Flow Diagram
Vol. 38, No. 2
PRODUCT
A.S.T.,31. distillation, ' F. Initial b.p. 190 5% over 194 10% over 196 20% over 196 30% over 196.5 40 % over 198 50% over 198.5 607, over 199.5 70% over 200 80% over 202 90% over 204 9570 over 207 End point 233
Residue %R Recovery, LOSS,
ng
%
Gravity, 'A. P. I. Aniline point, ' C . Bromine No.
99.0 0.8 0.2 1.40127 63.2 55,O
