Short-Cut Method of Hydrocarbon Analysis

Met. Eng., 33, 667. (1926). (3) Gluud, W., and Breuer, P. K., Ges. Abhandl. Kenntnis Kohle, 2,. (4) Kester, E. B., IND. ENQ. CHEW., 24, 1121 (1932). (...
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ANALYTICAL EDITION

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corresponding fractions of high-temperature tar acids is offered. ACKNOWLEDGMENT The authors wish to extend their thanks to the Combustion Utilities Corporation for permission to publish the results of this work and in particular to 8.P. Burke, formerly director of research, for his suggestions and guidance.

LITERATURE CITED (1) Avenarius, R., 2. angew. Chem., 36, 165 (1923). (2) Curtis, H. A . , and Beekhuis, H. A., Chem. & Met. Eng., 33, 667 (1926).

Vol. 6 , No. 1

(3) Gluud, W., and Breuer, P. K., Ges. Abhandl. Kenntnis Kohle, 2, 236 (1917). (4) Kester, E. B., IND.ENQ.CHEW.,24, 1121 (1932). ( 5 ) Morgan, J. J., and Meighan, M. H., Ibid., 17, 626 (1925). (6) Morgan, J. J., and Soule, R. P., Chem. & Met. Eng., 26, 927 (1922). (7) Peters, W. A,, Jr., and Baker, T., IND.ENQ. CHEM.,18, 69 (1926). (8) Steinkopf, W., and Hopner, T., J. prakt. Chem., 113, 137 (1926). (9) Vavon, G., and Zaharia, N., J. usines gaz, 53, 534 (1929). (IO) Wehuizen, Rec. trav. chim., 37, 276 (1918). (11) Weindel, A,, Brennstof-Chem., 3, 245 (1922).

RicExvm September 21, 1933.

Short-Cut Method of Hydrocarbon Analysis 11. Application to Analysis of Stabilizer Bottoms R. ROSENAND A. E. ROBERTSON, Standard Oil Development Company, P. 0. Box 485, Elizabeth, N. J. A short-cut method of hydrocarbon analysis ap- stabilizer bottoms may be analyzed. I n the use of plicable to certain types of routine samples has these graphs, corrections must be applied for the been developed to reduce the fime and expense of presence in such samples of certain constituents not such analyses. Its application to the analysis of present in the synthetic samples, and correction stabilizer gas and reflux has previously been de- charts f o r this purpose have been prepared by calscribed, and its further application to stabilizer culating the effect of these components on the distillation curves. bottoms is described in this paper. Synthetic samples, covering the range of comThis method, applied to stabilizer bottoms, checks positions normally found for stabilizer bottoms, microfractionation analysis within 0.5 per cent on have been made up and run by the short-cut the propane, 1 per cent on the butane, and 2 per method employing the changes in apparatus and cent on the pentane and hexane plus heavier hyprocedure necessary for samples of this type. From drocarbon fractions. It offers distinct advantages the resulting distillation curves, graphs have been from the standpoint of ease of operation and prepared by the use of which routine samples of economy in time and materials.

I

N VIEW of the fact that control of refinery equipment

requires frequent analyses of samples of similar composition, it was felt that a rapid, economical, and accurate method of analysis would be of great value. Such a method, based on a distillation analogous to the Engler distillation for naphthas, was described by Rosen and Robertson (7) and is now being used by several laboratories for analyzing stabilizer gas and reflux. I n the present paper its application to the analysis of stabilizer bottoms is described. So-called graphic methods of analysis are limited in application because they are based on physical characteristics of the mixture to be analyzed, and the number of components which can be determined is only one more than the number of determinable physical characteristics. In the short-cut method the analysis is obtained by considering temperatures at different per cents-off on a distillation curve. Thus, for three-component mixtures or mixtures which can be reduced to three components, such as stabilizer gas and reflux, it is necessary to consider temperatures a t two different per cents-off, while for four-component systems, or mixtures which can be so treated, such as stabilizer bottoms, it is necessary to employ temperatures a t three different per cents-off. Several graphical methods for the analysis of hydrocarbons and similar mixtures have been described in the literature. Colman and Yoeman (4) supplied this principle to the analysis of mixtures of benzene, toluene, and xylene.

Methods for the graphical analysis of gasolines and natural gasolines from the A. S. T. M. distillation curve have been described by Smith (8), Pocock and Blair ( 6 ) ,and Blair and Alden (I), and appear to have considerable application for such hydrocarbon mixtures. Stabilizer bottoms or similar light naphthas consist of propane, butane, pentane, and hexane plus heavier hydrocarbons having compositions varying between the two following extreme type analyses: TABLEI. WEIGHTPERCENTOF STABILIZER BOTTOMS SAMPLE 1

2

PROPANH~ BUTANE 3

0

42 22

HEXANH~ PLUSHEAVIER PENTANE H Y n R o C A R B o N s 45 10 24 54

The analysis of samples of this type by the short-cut method necessitated changes in apparatus, changes in procedure, and preparation of curves applicable to the analysis of four-component mixtures covering the above composition range. The method of attack was to develop an apparatus and procedure applicable to this type of sample, to make up synthetic mixtures representing various concentrations of the different components, to run these mixtures by the apparatus, and to prepare the graphical analysis curves from the distillation curves thus obtained. The propane and butane used in this investigation were the same purified hydrocarbons used in the investigations by

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INDUSTRIAL AND ENGINEERING CHEMISTRY

Rosen and Robertson (7). The pentane and hexane were obtained from the fractionation of natural gasoline. Microfractionation analyses showed that these hydrocarbons were not pure, but in making u p synthetic mixtures allowance was made for the impurities. The pentane and hexane contained iso-compounds, but no unsaturates. APPARATUS

The apparatus used in this work is shown in Figure 1. The graduated buret B and the leveling bulb A are for measuring the gases to be used in the preparation of the synthetic mixtures, the gas containers being attached a t &. The distillation bulb C is surrounded by test tube D, while the thermocouple leads E connect the thermocouple resting on the bottom of bulb C to the potentiometer. The open-end manometer F shows the

FIGURE 1. APPARATUS FOR DISTILLING LIQUID SAMPLES pressures on the distillation bulb C. The tube L,the bubbler M , and the reservoir 0 are so arranged that any desired pressure ma be automatically maintained on the bulb C. The closedendTmanometer G is so constructed that the readings are great1 magnified for accurate reading. It is made of 2-mm. tubing w i d about 12 inches (30 cm.) of the bottom made of 6-mm. tubing, and partly filled with mercury. Above the mercury (point 2'2) on the closed side is a light hydrocarbon oil having substantially zero vapor pressure. The manometer is evacuated on the closed side above the oil. In Figure 1 the manometer is represented as it would be if the receiving bulb J were evacuated, the oil in the closed side just reaching up into the small part of the manometer to the point 8. Now as pressure builds up in J , as gas is admitted into this receiver, the level 2'1 will dro oil level up on the other side much more than The oil level at S is used as the indication of ressure, and it will move about 4 mm. for each mm. of pressure guilt up in J , if the manometer is constructed as described. It is necessary to calibrate the manometer G and since, in the apparatus shown in Figure 1, the volume of J plus the additional volume of lines and free space above the mercury level in M is different a t different pressures, owing to the drop in the mercury level in M , this was taken into account in calibrating G, so that at any pressure one division corres onded to a definite volume of gas entering J and was equal to tEe amount which would build up 1 mm. pressure when the mercury level in M was even with

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The inner tube R in the distillation bulb C is for the purpose of facilitating the boiling of the sample and was found necessary because of the tendency of the sample t o superheat, thus destroying the smoothness of the distiIlation curve. This tendency tvas much more marked in the case of stabilker bottoms than in that of the lighter stabilizer gas and reflux. It was found necessary to distill stabilizer bottoms at 200 mm. pressure instead of at 760 mm., because a t the latter pressure only a small percentage of the sample would distill over below room temperature, and room temperature cannot be exceeded in this type of apparatus without condensation in the lines. Accordingly, the automatic pressure regulator L, M , 0 was found necessary because at the lower distillation pressure the small variation in pressure, inevitable in manual regulation, corresponded t o a much larger temperature change than was the case when the distillation was carried out at atmospheric pressure. The special manometer G was substituted for the ordinary closed-end manometer t o obtain greater accuracy, made necessary by the fact that a t 200 mm. pressure a much larger expansion bulb J was necessary for the same size sample than a t 760 mm. This apparatus differs from the original short-cut apparatus in the way heat is supplied t o distillation bulb C. Since, in the course of distilling stabilizer bottoms a t 200 mm., it is necessary for distillation bulb C t o be warmed to approximately room temperature, it was impractical to warm it by exposure t o atmospheric temperature as was found satisfactory in the original short-cut procedure. The arrangement adopted was to enclose C in test tube D and surround this with a Dewar flask, filled with acetone cooled down with solid carbon dioxide, in which was inserted a n immersion heater.

t k %:%!!??

P2 *

The receiving bulb J is of 3-liter capacity. The mercury trap H is placed in the vacuum line. The arrangement shown at L, M , 0 is for the purpose of maintaining a constant pressure on J. At the beginning of the run, J , M , and 0 are evacuated and as the sample in distillation bulb C (Figure 1) gradually builds up pressure (with stopcock XI open to L ) , the mercury in L, which, if there were no pressure in C, would stand even with point P2,is pushed down toward PI. Now when, with stopcock X Z open to N , the pressure in C in mm. of mercury equals the vertical distance between PI and Pz, gas will bubble up through the mercury in M and escape into receiver J . As va or collects in J the pressure therein gradually builds up so that t i e mercury level in M drops, the excess mercury flowing over at Pz into reservoir 0. Meanwhile the pressure a t PI does not change, always remaining the vertical distance between PI and Pz. At the end of a determination when J is evacuated, stopcock XZis opened to 0 directly and the reservoir raised so that the mercury therein flows back into M . The apparatus is then ready for another determination.

FIGURE 2. APPARATUS FOR DISTILLING EITHER LIQUIDOR GASEOUS SAMPLES

It is entirely practical to construct a single apparatus capable of running stabilizer gas, reflux, or bottoms. Figure 2 is the diagram of the apparatus constructed in the laboratory for this purpose. The measuring bulb B (500 cc. capacity) and drying tube H are provided for measuring and introducing gas samples. The other parts of Figure 2 serve the same purpose as the corresponding parts of Figure 1. For convenience J (3000 cc. capacity) is mounted on to of M (25 mm. inside diameter), while the latter is made more &an 760 mm. long, so that with J evacuated the distillation may be carried on at atmospheric pressure, which is the pressure used in the analysis of stabilizer gas and reflux. However, it is not necessary that M be more than 300 or 400 mm. long, since the distillation of stabilizer gas can be started with sufficient pressure in J , which added to the length of M will equal 760 mm. The manometer G is of the closed-end reservoir type, since it was felt that such a manometer would be accurate enough for routine analysis. The volume of bulb C and lines from X to P1is about 11 cc. At the bottom of distillation bulb Cis a triple junction copperconstantan thermocouple made of No. 36 B & S wire, the cold

ANALYTICAL EDITION

14

10

M 60 70 00 BO PRESSURE BUILT UP IN MM. HG

20 30 4 0

100

110

120

FIGURE 3. TYPICAL DISTILLATION CURVESFOR SYNTHETIC LIQUIDSAMPLES Propane, weight per cent Butane, weight per cent Pentane, weight per cent Hexane, weight per cent Total pressure mm. Millivolts 10 ber cent off Millivolts' 40 per cent off Millivolts' 70 per cent off Temperature, lOpercentoff, O C. Temperature 40per cent off, C. Temperature: 70per cent off, ' C.

CURYE32 CURVE^ 33 5.0 0.0 30.0 30.0 43.4 46.6 21.6 23.4 126 112 -0.80 -0.50 -0.34 -0.26 40.13 +0.18 -21.4 -13.3 -9.0 -6.3 +3.5 4-4.7

junction of which is maintained at ice temperature. The millivolt readings are taken on a portable potentiometer reading to tenths of a millivolt. A single-junction thermocouple could be used with a more sensitive potentiometer.

Vol. 6 , No. 1

I n the course of the work this procedure was used to make up base stocks of various compositions ranging from a pentane-hexane ratio of 0.5 to 2. After the synthetic mixture was prepared as described above, tube D was placed around C and the whole surrounded with a Dewar flask containing acetone cooled with solid carbon dioxide to below the boiling point of the sample. A 10-ohm heater was placed in the bottom of the Dewar flask, and for a 1-pint (0.473-liter) flask, one ampere of current passed through the heater. The amount of heat was regulated so that the actual distillation was completed in 30 to 40 minutes. With J , MI and L evacuated, the height of the reservoir 0 was adjusted to maintain a pressure of 200 mm. on distillation bulb C. When the distillation was started, simultaneous readings were taken on the temperature of the boiling liquid in C and the manometer G, respectively. These readings plotted against one another gave the distillation curve. When the temperature in C approached room temperature] the distillation was stopped by closing stopcock XI,evacuating J , and opening stopcock XI to allow passage of the gas directly from C to J . The additional pressure built up in J was noted. If any liquid remained in C the evacuation was repeated. The total additional pressure built up in J was added to the distillation curve to make up the total pressure built up by the sample. On the basis of the total pressure the temperatures at different per cents-off were determined. Typical synthetic distillation curves are shown in Figure 3. The procedure for making routine distillations on the apparatus shown in Figure 2 is much the same as that just described. To analyze a liquid sample, 0, J, and C are evacuated and C is surrounded with solid carbon dioxide. The bomb containing the sample is connected with the valve on the underside by means of a short piece of rubber tubing to stopcock X, which is opened to allow evacuation of the line to the valve on the bomb. With XI closed the valve on the bomb is opened slightly until about 3 cc. of liquid sample collect in C. The stopcock X is now closed and the sample bomb removed. From this point, the procedure is the same as that described above.

PROCEDURE The general procedure followed for making up synthetic mixtures was to make up a base stock of pentane and hexane and then to introduce some of this into the distillation bulb C (Figure 1). To this the desired amount of propane and butane was added. To make the base stocks a glass bulb fitted with a stopcock was evacuated and weighed. A measured amount of hexane was introduced into this bulb and the weight taken; pentane was then introduced and the bulb weighed again. From these weights and the analysis of the pentane and hexane, the exact composition of the base stock was calculated. Some of this stock was then quantitatively introdueed into the distillation bulb of the apparatus through stopcock X. This was done by cooling the bulb C with solid carbon dioxide, inverting the glass bulb containing the base stock, connecting it to stopcock X with rubber tubing, and after opening X (with the lines evacuated), opening the stopcock on the glass bulb slightly; thereby all the base stock coming from the bulb was collected in C, leaving the lines clear. Stopcock X was then closed and the bulb containing the base stock removed and weighed. The weight of the base stock introduced was thus obtained by difference, and from this weight the amount of propane and butane necessary for any desired composition was calculated, measured in B, and added to the sample.

TEMPERATURE "C IOX OFF

FOR ALL FIGURE 4. 10 AND 40 PERCENTTEMPERATURES SYNTHETIC SAMPLES

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15

INDUSTRIAL AND ENGINEERING CHEMISTRY

The method adopted was to prepare graphs representing plane sections of a solid figure, thus limiting each graph to temperatures a t two per cents-off. Figure 4 shows all the 10 and 40 per cent temperatures plotted against each other for all the synthetic samples, while Figure 5 shows all the 40 and 70 per cent points similarly plotted. The lines in each system of curves on this figure represent constant concentration of propane and butane. Figures 6, 7, 8, 9, and 10 are the curves in Figure 4 plotted separately. Figure 5 could also be divided into several curves, but this was not considered necessary since such a series would not differ greatly. It is to be noted that the graphs in Figures 6, 7, 8, 9, and 10 each have a corresponding graph in Figure 5 and that R all p o i n t s i n each 4 graph represent the s a m p l e s m a d e up 9 with a base s t o c k FIGURE 5. 40 AND 70 PERCENTTEMPERATURES having the pentaneFOR ALLSYNTHETIC SMLES hexane ratio given with the graph. The procedure for distilling a gas sample is the same as These graphs are TEMPEFtAiWRE*C 10% OFF that described by Rosen and Robertson (7) except that a used to analyze unfew minor changes are made necessary b y the automatic k n o w n s a m p l e s , FIGURE 7. INDIVIDUALCURVE, 4. PENTANE-HEXANE RATIO, pressure regulator. I n addition, just before beginning the The procedure is to FIGURE I.J distillation it is believed preferable to measure the fixed gases plot t h e 40 and 70 EXTRAPOLATTON DATA and methane over into J by opening XI directly to J, rather per c e n t temperaCorrection than to pump directly from C as described in the above paper. tures a g a i n s t one Pentane-hexane 10% 40% c. c. another on Figure 5. 1.3 4-0.2 f0.4 PREPARING GRAPHS These temperatures 1.4 f0.1 f0.2 1.6 0.0 0.0 As previously explained, it is necessary to consider tem- are plotted first be1.6 -0.2 -0.4 1.7 -0.3 -0.8 peratures at three different per cents-off in order to form a cause t h e correbasis for the analy- sponding graphs do sis of four-com- not intersect as do the 10 to 40 per cent graphs (Figures 4 and ponent m i x t u r e s 5). This gives a pentane-hexane ratio, and the 10 and 40 per There still remains cent-off temperatures are then plotted on the 10 to 40 per t h e p r o b l e m of cent graph corresponding to this pentane-hexane ratio. For choosing t h e b e s t example, if a sample shows 40 and 70 per cent temperatures three per cents-off of -4 and $13.6’ C., respectively, these values are plotted and the best method on Figure 14 (derived from Figure 5 as described below) of p l o t t i n g t h e s e and fall on the curve corresponding to a pentane-hexane ratio temperatures ob- of 1. This value corresponds to Figure 8 of the 10 to 40 tained from runs on per cent graphs; so this graph is used to obtain the propane synthetic mixtures. and butane concentrations of this sample. The procedure I n this work the in cases where the 40 and 70 per cent temperatures fall on a temperatures a t 10, curve in Figure 14, for which there is no corresponding 10 to TEMPERATURE ‘C 10% OFF 40, and 70 aer cent 40 per cent graph, will be described below. The pentane and FIGURE6. INDIVIDUAL CURVE,FIG- have been chosen as hexane concentration may be obtained from the following URE 4. PENTANE-HEXANE RATIO,2 b e i n g t h e m o s t formula: EXTRAPOLATION DATA feasible. The 10 fi 10C@rection per cent point was % pentane = (100-P-B) Pentane-hexane 1+R 40% chosen because it is c. a c, +o.a +o.8 as near t h e b e g i n %hexane = - (100-P-B) 1.8 1.9 f0.2 f0.4 ning of the distilla1 +R 2.0 0.0 0.0 tion as it is safe to assume that the un- when R is the pentane hexane ratio, P is the per cent of evenness due to slight superheating and other irregularities propane, and B is the per cent of butane. incident to starting the distillation have been smoothed out. The 70 per cent point is as near the end of the curve as it is CORRECTIONS practical to take, since with heavy samples the 70 per cent I n analyzing routine samples by the above procedure it point may be near or past the end of the curve. The 40 per cent point was chosen because it is approximately half-way was found necessary to correct for the effect of butylene and between the other two and apparently gave as good graphs as isobutane on the distillation curves, since only normal butane any other point. I n addition to the above considerations, was used in making up the synthetic mixtures. Graphs giving these three per cents-off gave graphs which permitted the the corrections for the 10 and 40 per cent points for this effect are shown in Figure 11. greatest accuracy in the determination of propane.

k

E

F

. I C

O

.

16

ANALYTICAL EDITION

These curves were prepared by the calculation of two sets of distillation curves, one for samples containing only normal butane in the butane fraction, and one for samples containing 53 per cent butylene in the butane f r a c tion. This concentration of butylene was used because it FIGURE 8. INDIVIDUAL CURVE, gave a butane fracFIGURE 4. PENTANE-HEXANE RATIO, t i o n h a v i n g a n 1 a v e r a g e boiling EXTRAPOLATION DATA point of -3.2" C. Correction 10% 40% Pentane-hexane which was t h e c. c. a v e r a g e boiling 0.832 $0.4 $0.9 0.909 +0.2 +0.5 point observed for 1.000 0.0 0.0 this fraction in 1.100 -0.1 -0.2 1.200 -0.2 -0.4 r o u t i n e samples. The calculation of the curves was undertaken to avoid making numerous synthetic runs. These calculations were carried to the 40 per cent point, so that the correction a t 10 and 40 per cent could be obtained. While it is realized that such a calculated curve might not be the same as an experimental one of the same composition, it is felt that, inasmuch as the curves being compared are calculated in the same manner, the butylene would have the same effect on the experimental curves as on those calculated. The method employed for calculating the distillation curves is based on Murray's formula (5) for calculating equilibrium between liquid and vapor hydrocarbon mixtures : Cl =

p 100

LC

+ + L (1 - >) where ~

.8

ist6

? f0 Y"

8-2

3 -20 -18

-I6 -14 -12 -10 -8 -6 TEMPERANRE YI 10% OFF

-4

C1

=. liquid moles of

any h y d r o carbon L = sum of all liquid moles C = total moles of a component in the mixture P, = vapor pressure of a pure component P = absolute p r e s s u r e of t h e system

FIGURE 9. INDIVIDUAL CURVE, FIGURE 4. PENTANEHEXANE RATIO,0.666 A graph f o r t h e r a p i d EXTRAPOLATION DATA s o l u t i o n of M u r r a y ' s Correction Pentane-hexane 10% 40% formula was prepared by c. c. Cerini (2). T h i s g r a p h , -0.2 -0.7 0.588 however, was not accurate -0.1 -0.4 0.624 0.0 0.666 0.0 enough for the purpose de$0.2 +0.5 0.714 $0.9 0.769 f0.4 sired, since it was designed to take care of all values of L. Accordingly a simplified nomographic chart shown in Figure 15 has been constructed to cover only high values of L. A distillation was considered as a series of small flash distillations. To calculate a distillation curve, the mole per cent of each component for the original mixture was first calculated, and from this the boiling point of the mixture was determined by assuming a boiling temperature and calculatingthe vapor pressure 0

0

Vol. 6, No. 1

of the sample for this temperature. From the vapor pressures at two temperatures near the boiling point, the latter was ascertained by plotting the logarithm of the vapor pressure against the reciprocal of the absolute temperatures, drawing a straight line through the two points, and noting where it crossed the line corresponding to the pressure of the system. All vapor pressure values for pure compounds were obtained from the chart prepared by Brown and Coats (3). It was now assumed that a temperature rise of 1' or 2' took place and that a certain amount of vapor was distilled off. P,/P was now calculated for the new temperature, a value of L assumed, and C,/C obtained for each component from Figure 15. Since C is known, C, for each component is thus obtained. These added together should give the value of L assumed; otherwise a new value of L must be assumed and the rocess repeated. When a value of L is found which will equal t i e sum of the Cl values calculated from it, 100-L is taken as the per cent evaporated for the rise in temperature chosen. The mole per cent for each component is now calculated,being CI/L for e a c h component. To check the above calculation, the vapor pressure of the partly distilled sample may now be calculated at the new temperature and should equal P. Another rise in temperature is now assumed and the above calculation repeated. Each of these calculations gives a point on FIGURE10. INDIVIDUAL CURVE, the distillation curve, FIGURE 4. PENTANE-HEXANE RATIO, the temperature be0.3 ing plotted against EXTRAPOLATION DATA the t o t a l mole per Correction cent weathered off, Pentane-hexane 10% 40% In this work a temc. 0 c. p e r a t u r e rise was 0.500 0.0 0.0 0.526 +0.1 4-0.4 chosen which would 0.555 t0.2 +0.7 give an L value of about 95, since it was believed that such a value would approximate distillation conditions. The larger the value of L, the more nearly are distillation conditions approached, but the calculation becomes more tedious as the value of L increases. 0

-

A comparison of calculated and experimental distillations is shown in Figure 16. The reason that curves A and B diverge is probably that B contains isopentane and isohexane, while the vapor pressure data used for calculating A were for normal hydrocarbons only. Since it was desired to determine the effect of butylene, it was not believed that the curves should necessarily coincide as long as they were similar. To apply these corrections, the observed 10 and 40 per cent temperatures are plotted on the proper graph and observed propane and butane percentages obtained. These are plotted on Figure 11 to obtain 10 and 40 per cent corrections, which are algebraically added to the original values and the correct propane and butane concentrations obtained from the 10 to 40 per cent graph. In the exam le given above, if the 10 per cent temperature was -15' C . &is would be plotted against the 40 per cent temperature of -4' C. on Figure 8, and propane and butane values of 1.8 and 29.5 per cent, respectively, obtained. These, however, are not the correct values for the sample, but have been obtained only for purposes of plotting on Figure 11 t o obtain the 10 and 40 per cent corrections. When this is done the 10 and 40 per cent corrections of +2.7 and $0.7" C. are respectively obtained. These corrections applied t o the above 10 and 40 per cent temperatures give corrected values of -12.3 and -3.3" C., respectively. These corrected values are then plotted on Figure 8 t o obtain final correct values of 0.0 and 30.5 per cent for the propane and butane fractions.

It was also found necessary to apply a correction to the 70 per cent temperature in the case of samples high in hexane

January 15, 1934

17

INDUSTRIAL AND ENGINEERING CHEMISTRY

p l u s heavier hydrocarbons, because in such cases this fract i o n contains much m o r e heptane than the c o r r e s p o n d i n g fraction used in making the s y n t h e t i c mixtures. One samp l e of s t a b i l i z e r bottoms with a content of 42.6 per cent of hexane plus heavier hydrocarbons showed 32 per cent of heptane FIGURE11. CHARTFOR CORRECTINGplus heavier hydro10 AND 40 PER CENTTEMPERATURESc a l h n s in this fratFOR EFFECTOF ISOMERSAND UN- t i o n , w h e r e a s t h e SATURATES IN BUTANEFRACTION hexane plus heavier hydrocarbons used in making the synthetic mixtures had only 11 per cent heptane in the Corresponding fraction. Microfractionation and short-cut runs on several stabilizer bottoms samples established the 70 per cent correction curve shown in Figure 12. It is obvious that since the 70 per cent correction can be correlated with the temperature of the 70 per cent point, Figure 5 could be revised to apply the correction automatically. Such a revision is shown in Figure 13. With Figure 13 as a basis, the working graph, Figure 14, used in actual analysis of routine samples was prepared by drawing the heavy lines to approximate the positions of the groups of curves on the former figure and dividing the intervening spaces into equal parts. The method of constructing Figure 14 involves the assumption that for a given pentanehexane ratio, the 40 to 70 per cent points fall on a single curve, if the concentrations of propane and butane are within the range covered by the graphs. This is not strictly true, but was assumed for the sake of simplicity, and does not involve a serious error.

FIGURE13. 40 AND 70 PER CENT TEMPERATURES AFTER APPLYING CORRECTIONS INDICATED IN FIGURE 12

"

It will be observed that if the 40 to 70 per cent tempera-

.'

ture of an unknown s a m p l e does not fall on a point corresponding to one of the 10 to 40 per cent curves, it will be +I necessary to extrapolate between the two curves between k which this temperature falls. g., For this purpose extrapola- 9 tion data are provided with ;.2 each 10 to 40 per cent curve which correct the 10 and 40 per cent temperatures, so that the graph is applicable to the rz sample under consideration. These data were calculated by noting the average difference * in temperature between corresponding points on consecutive 10 to 40 per cent graphs, C6%RRREmlb4Ns; Fd-,a a n d d i v i d i n g the difference into five equal portions. As FIGURE 12. CURVE FOR indicated in the extrapolation CORRECTING 70 PER CENT d a t a , o n e o r t w o of these portions are added to or subIN H ~ F~~~~~~~ ~ ~ tracted from the experimental 10 to 40 per cent temperatures to make the curve nearest them applicable.

;

f

F

E

~

For example, if a sample had 40 and 70 per cent temperatures of +1 and +19.7" C., respectively, a pentane-hexane ratio of 0.769 would be obtained from Figure 14, since this line lies nearest the point. There is no 10 to 40 per cent graph corresponding to this value, but the graph nearest this value is Figure 9 with a pentane-hexane ratio of 0.666 and under "EXtrapolation Data ' with this graph will be found the ratio 0.769 together with 10 and 40 per cent temperature correction values which, when ap lied t o the 10 and 40 er cent temperatures for the sample, maEe the graph a plicabre to its analysis. This procedure was adopted t o avoij the necessity of making up 10

FIGURE14. WORKING GRAPHFOR 40 TO 70 PER CENTTEMPERATURES, PREPARED ON BASISOF FIGURE13

~

~

18 ANALYTICAL EDITION Vol. 6, No. 1 to 40 per cent graphs corresponding to all possible pentane-hexane ratios. corrections presented are based on an average boiling point of -3.2” C. for the butane fraction of the stabilizer bottoms. Accordingly, it is desirable to determine the average boiling ACCURACY OF METHOD point of the butane fraction of the stabilizer bottoms to be The accuracy of this method is limited by that of the micro- analyzed and to alter the 10 to 40 per cent correction curves fractionation column used to analyze the materials required accordingly. It is believed that under ordinary operating in making up the synthetic mixtures. However the analyses conditions the average boiling point of the butane fraction would not change appreciably from day to day. 80

30 40 50 60 70

08

I 2 ~

3

L

c c

-L

ADVANTAGES O F SHORT-CUT METHOD The short-cut method for control purposes offers several advantages over fractional distillation. The time required for a short-cut analysis is about 1.5 hours, including calculations, while a similar analysis by fractionation would take about 3.5 hours for a gas sample and 6 hours for a liquid sample. The operation is simple and does not require as great expertness for good results as microfractionation; furthermore the operation, being largely automatic, is less subject to the human element, which often leads to widely divergent results in I microfractionation , especially in routine analysis. Rosen and Robertson (7) describe the analysis of stabilizer reflux by the shortcut method without the use of l i q u i d nitrogen and it is

known t h a t such a n a l y s i s may be

FIGURE16. COXPARISON OF CALCULATED AND SYNTHETIC DISTILLATION CURVES

used to calculate the amount of butane A and C calculated, B synthetic run in the stabilizer gas. COMPOSITION WEIQRTPIR CENT Since the analysis of Butylene A B C . . 26.5 stabilizer bottoms Butane 50 50 23.5 33.3 33.3 33.3 by the s h o r t - c u t 16.7 16.7 method does not reHexane .. 1617 ,. FIGURE15. NOMOGRAPH FOR CALCULATING LIQUID-VAPOR quire liquid air, the EQUILIBRIA FOR SMALL AMOUNTS VAPORIZED short-cut apparatus, using solid carbon dioxide as the cooling agent, permits complete analytical control over the operawere carefully carried out by an experienced operator and tion of the stabilizer. are believed to be of a high order of accuracy. The short-cut runs, both synthetic and routine, are highly reproducible. ACKNOWLEDGMENT A comparisoii of short-cut and microfractionation analyses The authors are indebted to J. B. Maxwell, of the Enon several routine samples is given in Table 11. gineering Department of this company, for the preparation of the nomographic chart used in calculating the distillation TABLE11. COMPARATIVE ANALYSES curves.

..

2 : : +

MICROFRACTIONATION WEIQHT SHORT-CUT WEIQETPBR CENT PER CBNT SAMPLE Propane Butane Pentane Hexane Propane Butane Pentane Hexane 0.9 25.4 31.2 42.6 1 1.0 25.8 31.8 41.4 0.9 26.3 26.4 46.4 2 0.7 26.3 27.0 46.0 1.1 27.1 29.5 42.3 3 1.4 27.5 29.6 41.5 0.0 26.6 31.0 42.4 4 0.0 26.3 30.6 43.1 3.2 22.4 24.5 49.9 6 3.6 22.0 25.6 48.8 0.0 25.6 29.2 46.2 6 0.0 25.5 28.7 45.8 1.7 43.6 34.7 20.0 7 1.9 44.4 33.6 20.1

LITERATURE CITED (1) Blair, NI. G., and Alden, R. C., IND.ENG.CHEIM., 25, 559 (1933). (2) Cerini, W.F.,Petroleum Eng., 2, No. 5, 84 (1931). (3) Coats, H.B., and Brown, G. G., Univ. of Mioh., Dept. Eng. Research, Cir. Series, NO. 2 (1928). (4) Colman, H.G.,and Yoeman, E. W., J. SOC.Chem. Ind., 38, 57T (1 91 9). \-_ -_ ,-

(5) Murray, W., IND. ENQ.CHDM.,21,917 (1929).

These data show that this method checks the microfractionation column within 0.5 per cent on the propane fraction, 1 per cent on the butane fraction, and 2 per cent on the pentane and hexane fractions. In the preparation of these graphs, every effort was made to avoid complications whenever possible. It would be possible to reconstruct the graphs in Figures 6, 7, 8, 9, and 10 so that the corrections in Figure 11 would be automatically applied, as was done in the case of Figure 5, but it was felt that the 70 per cent correction would probably be fairly constant for all sorts of samples whereas the 10 and 40 per cent corrections wouldprobably not be constant. The 10 to 40 per cent

(6) Pocock, L. A., and Blair, M. G., Nut. Petroleum News, 24, No.20, 37 (1932).

(7) Rosen, R., and Robertson, A. E., IND.ENG.CHEM.,Anal. Ed., 3,284 (1931). (8) Smith, S. S., Oil and Gus J.,28, No.4, 38 (1930). RECEIYED August 17. 1933.

In the article entitled “Asbestos in PermanCORRECTION. ganate Titrations” by Curtis and Finkelstein [IND.ENQ.CHEM., Anal. Ed., 5,318 (1933)l the second line in the second column on page 318 should read “in a typical limestone carrying 54 per cent of calcium as calcium oxide.”