\
.-
High-Pressure Rectification
,?
61'
I I. fl-Pentane-N- Heptane Systern L. W. T. CUMMINGS, F. W. STONES, AND M. A. VOLANTE Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Mass.
N
OT WITHSTAXDING the importance of the
Dew point and boiling point curves have been determined f o r three mixtures qf AT-pentane and N-heptane, from a pressure of 6 atmospheres through the critical region. Vapor and liquid densilies hate also been measured at the twophase boundary. Several constanl-pressure, vapor-liquid equilibrium diagrams hare been determined f r o m the data, and their applicability to the problem of high-pressure rectiJication is pointed out.
process of high-pressure rectification of hydrocarbons, there have been no data published on vapor-liquid equilibria for any m i x t u r e of practical importance over a c o m p l e t e range of composition. The solution of the problem of highp r e s s u r e design has been initiated with the collection of data on the binary system composed of h'-pentan; and N-heptane. The general limitations imposed by pressure were discussed in Part I of this paper (2).
APPARATUS The apparatus used in the exploration of the two-phase region in which rectification can be carried out is shown diagrammatically in Figure l. The hydrocarbon mixture under consideration was confined over mercury in the closed end of a fused quartz U-tube as shown in Figure 2, and in detail in Figure 3. The inside diameter of the tube was 0.5 cm. and the outside 1.1 cm. The open end
heating coil of resistance wire was immersed below the liquid and was supplied with current t h r o u g h suitable resistances. The hydrocarbon mixture was visible through the quartz tube and glass vapor bath, and its condition, as well as the position of the mercury level in the quartz tube, could readily be determined visually. A mec h a n i c a 11y o p e r a t e d solenoid actuated a steel ball which moved through the hydrocarbon mixture at regular intervals insuring the attainment of equilibrium. The auartz tube was filled with reanent mercury in small increments stljrting with the tube in an inverted position, and adsorbed gas was removed from the walls by intermediate boiling. During the process of filling the tube, the steel ball was held in closed arm by means of a steel wire. The individual components of the hydrocarbon mixture were injected separately into the quartz tube with the glass injector pipet (Figure 4) which was essentially a U-tube, partially filled n-ith clean reagent mercury. Air filled the space above the righthand arm, and the mercury leveling bottle attached to this arm made it possible to adjust the mercury level as desired. The right-hand arm was a capillary tube 39 cm. long and had a total volume of approximately 0.25 cc. The left-hand arm of larger bore had a conical tip ground to fit a steel cone. A hypodermic steel tube, 0.056 cm. inside diameter and approximately 120 cm. long, was force-fitted into the cone which was sealed to the glass with glue. The hypodermic tubing and cone were cleaned with acetone and dried before being attached to the left arm of the injector. The right or measuring arm of the injector was calibrated by weighing successive increments of mercury, removed through the two-way stopcock. The hydrocarbon to be injected was drawn through the hypodermic tubing into the left arm of the injector as the mercury was drawn from
CONDENSER
FIGURE 1 of the quartz tube was conical and ground to fit a steel disk, and the joint was made tight with deKhotinsky cement. The disk, with the quartz tube, was bolted into the steel-to-quartz connecter, using an aluminum washer. This connection is similar to that developed by Keyes (4). The connecter was joined to.the mercury-oil reservoir and dead-weight gage with 3/le- and 1/8-inch Shelby steel tubing. The line between the steel-to-quartz connecter and the reservoir was filled with mercury through a long steel hypodermic tube inserted through the normally blanked connection in the high-pressure tee. The reservoir was 9 cm. in diameter, and a change in the mercury level in the quartz tube produced a negligible change in the mercury level in the reservoir. The hydrocarbon mixture in the quartz tube was maintained a t a constant temperature by a one-piece gIass vapor bath surrounding the tube, as shown in Figure 2. Pure substances were charged to the vapor bath to produce the temperatures desired. Intermediate temperatures were obtained by adjusting the pressure a t which the particular pure substance was boiled. The 728
FIGURE2. VAPORBATH
July, 1933
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 321 I S T R Y
that arm i n t o t h e m e a s u r i n g arm. The hydrocarbon thus introduced into the injector was kept boiling in a deaerator to remove d i s s o l v e d air. Air adsorbed on the hypodermic tubing and the left arm of the injector was washed out by repeatedly drawing in deaerated h y d r o c a r b o n and expelling it again into the deaerator. The hydrocarbons were dried by di s t i 11i n g into the deaerator over sodium.
the quartz tube, and a small a m o u n t of the hydrocarbon was expelled to remove any air in the end of the tubing. With the two-way s t o p c o c k closed, the tubing was pushed into the closed end of the quartz tube and the required amount of the first component was injected. The second c o m p o n e n t was injected in a similar m a n n e r from a second injector pipet. The quartz tube was calibrated along the length of its closed arm by successive injections of benzene over mercury from the injector pipet. The displacem e n t of the m e r c u r y in the quartz tube was measured x-ith a cathetometer. The dead-weight gage used was of the
729
quartz tube. KO oil head correction was necessary, as the level of the oil in the mercury-oil reservoir was the same as that in the gage. The determination of the mercury level in the quartz tube also made possible the calculation of the specific volumes of the mixture. I n addition to the mercury level correction, the barometric pressure was added to the pressure indicated by the gage, and the vapor pressure of mercury was subtracted to obtain the true pressure exerted by the mixture. The fact that mercury is a good conductor of heat inrolved the possibility that the mixture in the quartz tube was a t a temperature below that of the bath owing to the conduction of heat by the mercury in the bath to that exposed to the air a t room temperature. It was found, however, that the pressure measurements a t a given temperature were the same when one leg of the quartz tube was in the vapor, bath as when the whole tube was immersed in a liquid, constanttemperature bath, indicating that within the precision of the apparatus the loss of heat through the mercury column produced a negligible effect on the temperature equilibrium The S-pentane and S-heptane were obtained from Eastman Kodak Company, the former being made synthetically. The hydrocarbons were washed with cold concentrated sulfuric acid, dilute sodium hydroxide, and finally with water. After drying, they were fractionated over sodium. The normal boiling points of the N-pentane and Ai-heptane were 36.0" and 98.4" C., respectively. The individual compo-
phere. The variation of the room temF1(;URE 3. DEperature, the p r e s s u r e s used, and the T~'lL OF Q U A R T Z TCBE v i s c o s i t y of the oil affect the g a g e (Dimensions i n centiconstant less than the precision of calibration, and corrections for these effects meters) were not applied. The temperature was measured with n calibrated chromelcope1 thermocouple, with the cold junction in ice water. The potential was measured with a Leeds and Xorthrup type K potentiometer. The hot junction was placed at the top of and adjacent to the quartz tube, a cotton plug filling the space above it.
IWESTIGATIOK OF TWO-PHASE REGXOS Following the injection of the components of the mixture into the quartz tube, the apparatus was assembled. Temperature equilibrium was obtained in the quartz tube at a given temperature level after about 2 hours. The pressure of the liquid mixture at the temperature in question was determined by adjusting the weights on the dead-weight gage until a pressure change of not more than 0.027 atmosphere would make a small bubble of gas disappear :tnd reappear. This condition, in which the gaseous phase of the mixture is just disappearing, represents one limit to the two-phase region at the temperature in question. The other limit, or the dew point, is the condition under which the liquid phase is just beginning to form and was measured by increasing the volume under which the mixture was confined until it was entirely gaseous and then adjusting the premure until a small change would make a small amount of liquid form and disappear. The presence of liquid was manifest b y a n iridescence inside the quartz tube or by a wet spot where the steel ball contacted with the side of the tube. It was found that the effect of entrainment could be avoided when it was desirable t o repeat a measurement a t a smaller volume by compressing a t a temperature a t which the mixture remained gaseous. The position of the mercury level was determined with a cathetometer t o make the necessary correction t o the observed pressure, owing to the difference in the level of the mercury in the quartz tube and in the mercury-oil reservoir. Allowance was made for the lower density of the mercury in the
nents were tested for purity after injection into the apparatus by isothermal compression. The pressure change during condensation was negligible,
RESULTS AKD DISCUS~ION The border curves for the three mixtures of Ai-pentane and hr-heptane studied are shown in Figure 5. The data are given in Table I. The vapor pressure curves of the components flanking the diagram are the data of Young (9). The critical temperature and cricondentherm of each mixture is indicated on the diagram by Toand T,, respectively. The upper line of each border curve up to the critical temperature is the locus of the boiling points of the mixture, and the lower line the locus of the dew points. The pressure-temperaturecomposition diagram has the general characteristics predicted for paraffin hydrocarbon mixtures from Kuenen's data (5) on unknown mixtures of ethane and butane.
INDUSTRIAL
730
AND ENGINEERING
CHEMISTRY
Vol. 23, No. 7
TABLEI. PRESSURE, TEMPERATURE, COMPOSITION, AXD SPECIFIC VOLUMEDATA PRESSURE OF
N-PEXTANETEMP MIXTCRE SP. VOL.
Mole % 55.8
C. 125.6 146.2 150., 173.4 191.0 216.6 224.9 230.3 219.4 229.7 232.3 231.7 232.3 222.9 212.0 185.6
'
74.7
25.5
,
175.1 159.0 233.5 233.7 233.7 233.6 233.6 174.6 131.6 156.4 184.4 171.8 200.1 215.6 220.1 220.0 219.5 219.5 215.1 217.9 217.9 185.2 151.4 174.1 174.4 174.4 174.4 156.4 185.0 185.0 200.6 200.6 217.7 217.6 217.5 220.1 211.0 210.9 220.0 155.6 174.5 184.4 200.5 232.9 211.7 233.1 247.1 252.1 248.7 253.3 253.3 245.1 233.6 221.1 253.4 253.5 242.8 242.8 251.2 251.0 200.9 200.8 192.7 192.1
Atm.
6.43 9.39 10.05 14.46 18.79 27.07 29.81 31.87 27.97 31.68 32.50 30.15 30.24 25.64 13:41 11.04 8.18 32.61 32.64 32.55 32.44 31.09 14.86 8.77 13.53 20.96 17.29 26.33 32.40 33.33 33,47 33.45 32.30 29.68 33.22 31.23 17.25 9.41 14.39 14.53 18.12 14.50 13.51 20.96 17.21 26.36 22.70 32.95 30.95 32.97 32.61 30.46 27.32 32.53 7.24 9.95 11.64 14.96 23.50 17.53 23.64 28.18 29.82 28.72 30.15 29.34 25.33 21.10 17.30 30.19 29.33 26.64 24.33 28.01 29.52 14.98 12.30 10.50 13.16
CONDITION OF MIXTURE
Cc./gram
1.82
... ... ...
1.92
Boiling Boiling Boiling Boilinn
point point point ooint
2.47 2.69 3.14
...
...
3.44 6.85 6.90 9.78 12.86 23.20 29.07 38.18 4.00
4.13 6.22
...
6.26
2.04
... ... ...
...
2.39 2.95 3.97 3.97 3.70 5.51 7.34
...
6.21 16.93 33.33
...
... 20:69
...
1a:sg 2.35 11.65
...
6.04
...
5.20 2.63
...
5.28 1.95
... ...
... ...
Dew point Dew point Dew point Dew noint D e n point Dew point Dew point Boiling point Boiling point Dew point Retrograde condensation Retrograde condensation Boiling point Boiling point Boiling point Boiling point Boiling point Boiling point Boiling point B d i n g point Boiling point Boiling point Dew point Dew point Boiling point Dew point Dew point Dew point Dew point Dew point Boiling point Dew point Boiling pojnt Boiling point Dew point Boiling point Dew point Boiling point Dew Doint BoiliGg point Retrograde condensation Boiling point Dew point Retrograde condensation Boiling point Boiling point Boiling point Boilinr Doint
The temperatures reported are accurate to 0.1' C. and the mixture compositions to 0.1 mole per cent. The specific volumes of these mixtures are shown in Figure 6. The curves for the pure components flanking the diagram are the data of Young (9). The loci of the critical temperatures and the cricondentherm are indicated by T,and T,, respectively. The liquid specific volumes are accurate within a t least 2 per cent. The vapor specific volumes are more precise, the average value being about 1 per cent. The volume of a given single-phase mixture is not the sum of the volumes of the components, and the deviation from volume additivity increases with increasing temperature.
SPECIFIC VOCUMC Of LIQUID AND R-PENTANC VAPOR FOR ANDMIXTURES WHWTANC Cf
____
,..
2.60 3.09 3.66 3.09 3.91 5.50 ii:i7 ... 3.89 5.65 2.88 9.52 6.06 3.37 24:94 30.30
...
*
--
Boiling point Retrograde condensation Retrograde condensation Dew point Dew Doint Dew point Retrograde condensation Retrograde condensation Boiling point Dew point Dew point Boiling point Boiling point Dew point Dew point Boiling point
Mixtures rich in pentane with critical temperatures between 197.2' and 229.5' C. have critical pressures greater than that of pure pentane. The maximum pressure exceeds the critical pressure of pentane by approximately 0.5 atmosphere. This maximum pressure indicated the existence of a double discontinuity in the x-y diagram a t constant pressure within this small pressure range. The linear relation between the critical temperature of the mixtures and their compositions proposed by Pawlewski ('7) does not represent the data, and it is improbable that the envelope curve can be described exactly by any simple relation. Retrograde condensation was observed in these mixtures over a temperature range of about 1' c. The pressures were measured within 0.03 atmosphere.
I
1
I
!
,
I
160 180 200 220 240 260 TCh4PW.ATUPL-DECAEES CWTIGRADC
The data of Figure 5 represent a solid whose coordinates are pressure, temperature, and composition, exterior to which rectification, involving as it does interaction of a liquid and gas, cannot occur. The conditions represented by the interior of the solid, however, permit of separation by rectification, since both phases exist. The temperature-composition diagrams shown in Figure 7 enclose a t their respective pressures the region in which two phases exist. The data of Figure 7 were determined graphically from Figure 5 , and values are given in Table 11.
73 1
INDUSTRIAL AND ENGIYEERING CHEMISTRY
July, 1933
discussion, may best be obtained from the data through the pressure-composition diagram. TABLE111. CONSTAST-PRESSURE VAPOR-LIQUID EQUILIBRIA N-PENTANE IS: Liquid Vapor Mole per cent
TEMP.
C.
PRESSURE 10.00 ATM.
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
MOL PER CENT VPENTANE
TABLE11. N-PEBTANE IK:
TEMP.Vapor Liquid C.
Mole per cent
PRESSURE 10.00 ATM.
126.1 138.9 150.5 154.5 169.7 174.9 189.7 202.7
100.0
...
100.0 74.7 55.8
...
25.5
...
74.7 55.8 25.5 0.0
... ,
..
. 0.0 ..
'FEXPERATURE-COMPOSITIOSDATA N-PEKTANE IN: Y-PEXTABE IN: T E M P . Vapor Liquid TEMP. Vapor Liquid C. Mole per cent C. M o l e per cent PRESSURE 20.00 ATM.
184.6 181.4 193.6 195.2 209.3 221.0 230.3 248.5
100.0
... 74.7 . .. 55.8 ... 25.5
0.0
100.0 74.7
...
55.8
. .. ...
25.5
23.6 40.8 54.5 65.6 74.5 81.8 87.5 92.3 96.4
190.8 180.2 170.7 162.2 154.6 147.7 141.6 136.0 130.9
N-PEKTANE IN: Liquid Vapor Mole per cent
N-PENTANE IN: TEMP. O
C.
PRESSURE 20.00 A I M .
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
16.9 32.0 45.6 57.6 68.0 76.5 83.5 89.7 95.1
235,9 226.1 216.8 208.1 199.7 191.9 184.6 177.7 171.0
Liquid Vapor Mole per cent
TEMP.
C.
PRESSURE SO.20 ATM.
25.5 30.0 40.0 50.0 60.0 70.0 80.0 90.0
25.5 33.1 45.7 57.3 67.6 77.0 85.3 93.5
253.5 248.8 239.5 230.7 222.3 214.1 206.3 198.6
Below the critical pressure of heptane the equilibrium relation is continuous through all concentrations, as exemplified by the relations shown a t 10.00 and 20.00 atmospheres. However, a t pressures above the critical of the less volatile component, the equilibrium relation exists through restricted ranges of concentration and terminates in the concentration a t which the curve and diagonal intersect. For example, a t 30.20 atmospheres total pressure, intersection occurs a t a composition of 25.5 mole per cent pentane, with the result that separation by rectification is possible only through the range of concentration from 25.5 to 100 mole per cent pentane. This limitation increases from the critical pressure of the less volatile component to the maximum pressure. There is a small pressure range in this system of about 0.5 atmosphere between the critical pressure of pentane and the maximum
PRESSURE 80.20 ATM.
191.3 210.4 216.1 225.7 231.9 253.5
100.0 100.0 ... 7 4 . 7 74.7 ... ... 55.8 55.8 .. , 25.5 25.6
0.0
It is observed qualitatively from Figure 7 that, as the pressure increases from 10.00 to 30.20 atmospheres, the region of coexistence of liquid and vapor diminishes. At 30.20 atmospheres the critical state of heptane has been exceeded and the temperature-composition diagram a t this pressure terminates with the mixture containing 25.5 mole per cent pentane. At this pressure, the two-phase region is limited not only by temperature, but also by the composition of the mixture. This limitation by concentration begins a t the critical state of the less volatile hydrocarbon (in this case heptane) and increases to the maximum pressure where it becomes unique. At pressures below the critical pressure of the less volatile component the temperature-composition diagrams of paraffin hydrocarbon mixtures will undoubtedly be found continuous through all concentrations. The temperature-composition diagyams of Figure 7 are a good test of the precision of the data because they combine the measurements of temperature, pressure, and composition on three separate mixtures as well as the vapor pressure determinations of the components by another investigator. The constant-pressure vapor-liquid equilibrium diagrams of Figure 8 were determined graphically from the corresponding temperature-composition diagrams. The data are tabulated in Table I11 and are precise within 0.2 mole per cent. This type of diagram is the usual basis of design of rectification equipment, and the diagrams presented are representative of the behavior of the system. Diagrams a t other pressures may be determined from the data. The constant-temperature equilibrium diagram, of interest in thermodynamic
MOL PER CENT n - W A N E IN LlWlU
pressure in which there is also a discontinuity in the rich end of the diagram. Above the maximum pressure, about 33.5 atmospheres, the equilibrium diagram ceases to exist and separation by rectification is impossible. The equilibrium relation approaches the diagonal of the djagram as the pressure increases, or in other words, vapor in equilibrium with a given liquid contains less of the more volatile component a t high than at low pressures. Increased pressure has the effect of increasing the volatility of the less volatile and decreasing that of the more volatile component. From the point of view of design, the effect of increased pressure on the equilibrium diagram is to increase number of plates which would be required in a rectifying column to effect a given separation. The size of the equipment is not
i32
I N D U S T 1%I A L A N D E N G I R IS E 11 I h G C H E ilI I S T R Y
necessarily increased, because specific volume decreases with pressure. The minimum reflux ratio with a given feed for a given separation becomes greater as the pressure increases. On the other hand, the heat of vaporization decreases wit11 increasing pressure, with the result that the heat consumption for rectification alone tends to decrease with pressure. The important effects of pressure on design will he considered in a subsequent paper. The vapor-liquid eqiiililirium diagram cannot be calculated a t temperatures above the critical of either component by the use of the solution laws in their ordinary form because the vapor pressure of one component loses its significance. Furthermore, in the high-pressure range in question, the deviations of the vapors from the gas laws are large. However, it is not uncommon for engineers to at.tempt to meet the problem by extrapolation of the vapor pressure curves of t.he component in question above the critical point, together with t,he use of Raoult's law, ignoring other deviations due to pressure. A diagram thus calculated is xhown in Figure 9 for a pressiire of 26.0 atmospheres. The discrepancy between this curre and the equilibrium relation as determined experimentally, also shown in the same figure, is too great for the method to be of value, even as an approximation in engineering design. Generalized fugacity charts for hydrocarbon vapors h a i e recently become available (6,s). Using the former of these, the equilibrium diagram at 26.0 at.mospheres was calculated and is plotted in Figure 8 for comparison with the experimental curve. Here, too, it was necessary to extrapolate the fugacity of pentane above its critical temperature. This was done by calculating this quantity from the solution laws,,ernploying the experimental data on the equilibria determ~ned a t the temperatures above the cribical point, of the pentane and drawing a representative curve through the values obtained. A second comparison was made a t 30.2 atmospheres total pressure, and the results are plotted in Figure 10. The agreement in this case was somewhat less satisfact.ory. I n general, it may be stated that the solution laws, corrected for the departure from ideality of the gas phase by the fugacity charts, apply reasonably we11 to these mixtures until pressure* in the neighborhood of the crit.ica1 are approached. I n the critical region these laws break down, as is obvious from the fact that the discontinuity of the s y diagram is incompatible with the solution laws.
\ol 25,No i
This investigation was conducted in the Research Laboratory of Applied Chemistry v i t h the cooperation of the Humble Oil and Refining Company, for which the authors express their appreciation. LITERATURE CITED (1) Boyd, Chem. Eng. DoctoJrk Thesis. Mms. Inst. Teoh.. 1929. (2) Cummings, 1x0. Exo. CKEM.,23, 900 (1931). (3) K W S and D ~ W W , RPU.sei. rnstmLments. 14,491(1927). 14) , . Kcves. Tavlor. and Smith. J . Malh. Phus. Mass. Znst. Tech.. 1. i l l (1922). (5) Kuenen. Proc. Rou. SOC.Edinburgh. 21, 483 (1895-7). (6) Lewis and Luke, Trans. Am. Soc. Mcch. Engis., 54, 55 (1932). (7) PawlewsW, Ber., 15. 460 (1582). (8) . . Selheimer. Soudors, Smith, and Brown, IND. ENS. CHEM.,24, 515 (1932). (9) Young, Proc. Roy. Soe. Dublin, 12,228 (1910). RECEIVSD April IO, 1933. This resenrch is part of B thesis to be submitted to the faculty of M m ~ ~ a ~ b ~lnetitute ~ e t t a of Taohnology in PBrtial fulfilment of the rewiremen18 of the doctor of science degree.
