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the addition of stannous chloride is due to the action of this reagent on ammonium molybdate itself. This fact, together with the above doubt as to the final development of the same color intensity a t different temperatures from a given phosphate concentration, points to the desirability, though possibly not absolute necessity, of making readings at 1 or 2 minutes following the addition of reducing agent. If this practice be adopted, the elimination of errors arising from temperature differences can be effected by any one of several alternative procedures. The glass color plates can be standardized by a given technic at various temperatures, or having established values for them a t one temperature, all solutions to be tested can be brought to the same temperature before the addition of the reducing agent; or, finally, having standardized the color plates at one temperature, allow sufficient time to elapse with unknowns of different temperatures to permit the color to intensify to the same point as in the standard. Conclusions
The use of permanent glass color standards offers a means of eliminating one of the unsatisfactory features of the blue molybdate method for the colorimetric determination of phosphorus. The color of these glass standards matches the color of the reduced phosphate solutions very well. A degree of accuracy sufficient for practical purposes can be attained if certain details pertaining to the technic of the determination are followed. It is first of all necessary to establish values for each of the ten circular color plates, for they do not vary uniformly in color.
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Stannous chloride solutions, although protected from the air by layers of mineral oil, will nevertheless gradually become oxidized. Since it is essential that approximately the same quantities of this reagent be used in the tests as were used in the standardization of the color plates, the strength of this solution should be checked now and then unless it is stored under conditions known to preclude oxidation. It has been found that the rate of the reduction reaction is measurably affected by such changes in temperature as may occur in some laboratories. Unless laboratory temperatures are uniform, some one of the alternative procedures suggested in the text should be adopted in order to eliminate possible errors arising from this source. When freshly prepared solution standards are used in place of permanent standards, neither the deterioration of stannous chloride nor temperature differences are serious considerations, since these influences are obviously operative on both standard and unknown. At the present time nothing can be stated regarding the permanency of these glass standards. In order to guard against the possible gradual fading which may conceivably take place, it is suggested that the values found for each color standard be checked occasionally. The use of permanent standards makes it possible to save considerable time in the routine determination of phosphorus with but little sacrifice in accuracy. Literature Cited (1) Truog and Meyer, IND. ENG. CHEM.,Anal. Ed., 1, 136-9 (1929).
Analysis of Gaseous Hydrocarbons A Short-Cut Method' R. Rosen and A. E. Robertson STANDARD OILDEVELOPMENT Co., P.0.Box 486, ELIZABETH, N. J.
As a result of the study of a number of synthetic mix- and isomers as well as the corresponding normal saturated tures of gaseous hydrocarbons, a master graph is presented hydrocarbons, a study was made of their effect upon the whereby from an ordinary distillation analogous to an nature of the distillation curve, and a correction curve for Engler distillation of petroleum products, the analysis of this effect is presented. Analysis of gases may be completed in 60 to 75 minutes, certain gaseous hydrocarbon mixtures may be obtained. A simplified apparatus and procedure are described for and only 200 to 250 cc. of liquid nitrogen are required for the analysis of gaseous mixtures by the proposed short- an analysis. The technic of analysis is easily acquired. Because of its economical features, the short-cut method cut method. A section of the master graph is given in detail, the proposed offers considerable promise as a routine method range being that corresponding to the composition of of control analysis for the operation of various equipment, certain refinery gases. The application of the short-cut once the master graphs, or detailed graphs covering the range desired, are prepared. method to the analysis of these gases is discussed. Since the refinery gases considered contain unsaturates . . . . . .. . . . . . . .
vv
ITH the increasing importance of the lighter hydrocarbons either as gases or as constituents in petroleum products, the necessity for rapid and accurate methods for their analysis into their individual constituents is generally recognized. The use of fractionation equipment such as the Podbielniak apparatus (8, 3) for the analysis of gaseous mixtures is quite prevalent in the oil industry. This method at best requires a skilful operator, consumes 2 to 5 liters of liquid air or nitrogen, and requires 3 to 8 hours for a complete analysis. Furthermore, large volumes of gas must be used for the analysis of samples Presented before the Division of Petro1 Received March 17, 1931. leum Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931.
containing only small percentages of one component-e. g., dry gases (4). I n view of this situation, a simple and rapid method of hydrocarbon analysis, which would consume little or no liquid air or nitrogen, would find considerable application in the industry for routine control purposes. The object of this work was to develop such a method embodying the features enumerated. Hydrocarbon analysis by present methods involves fractional distillation in a suitable microfractionating column. The fractionation is conducted preferably a t atmospheric pressure and the individual hydrocarbons are taken overhead a t their corresponding boiling points. The percentages of the hydrocarbons are determined directly from the fractional-distillation curve plotted for the temperature of the
INDUSTRIAL A N D h’NGINEERING CHEMISTRY
July 15, 1931
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overhead vapor against the amount of distillate. I n contrast to this, the method proposed here involves ordinary distillation of the condensed gaseous mixture at atmospheric pressure, in a manner analogous to an Engler distillation of petroleum products. A method applying this principle and applicable to the analysis of mixtures of benzene, toluene, and xylene was described by Colman and Yeoman ( 1 ) . A similar method applicable to the analysis of natural gasoline was reported by Smith ( 5 ) .
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Figure 2
The first part of the paper presents a proposed shortcut method developed on the basis of synthetic mixtures of pure saturated hydrocarbons. It gives the first of a series of graphs whereby, from an ordinary distillation, the analysis of ternary mixtures of the hydrocarbons ethane, propane, and butane over a limited range may be obtained. The second part of the paper shows the application of this method and the use of the graph for the analysis of refinery gases. It also discusses the effect of unsaturated hydrocarbons and isobutane when they are present in addition to the normal saturated hydrocarbons considered. Description of Method
The method of analysis proposed is limited in its present form to the analysis of three component mixtures, although four components may be determined if one is methane or “fixed gases,”-i. e., gases that do not condense a t the temperature of boiling nitrogen, as carbon monoxide, hydrogen, nitrogen, and oxygen. I n the development of the method, synthetic ternary mixtures of the pure hydrocarbons ethane, propane, and butane, covering a considerable range of composition, were made up. These gaseous mixtures were condensed and distilled by the procedure given. Distillation curves repre-
285
senting temperature against percentage by volume distilled off were obtained in this manner. From these curves, the temperatures at two chosen percentages off were taken and plotted against each other on ordinary graph paper. Lines were drawn through all points having the same concentration of one component and another set of lines drawn through all points having the same concentration of another component. A graph showing these two sets of lines is sufficient to define any composition of the ternary mixture for the range covered. Since it was impractical to prepare a detailed graph covering the entire range of composition for the ternary mixture ethane, propane, and butane, sufficient data were obtained to prepare a skeleton graph covering the entire range. I n addition, however, a section of this graph that covers the range of certain gases encountered in refinery practice was prepared in detail. RAWMATERIALS-The hydrocarbons ethane, ethylene, propane, propylene, butane, and isobutane, used in the preparation of all synthetic mixtures throughout the work, were available in 3-pound (1.36-kg.) steel cylinders. Analysis of these gases by an improved microfractionating column showed that from 1 to 2 per cent of fixed gas was present. This was removed by cooling the cylinders by means of liquid nitrogen to temperatures a t which the vapor iressure of the hydrocarbon is less than 1 mm., and then pumping the cylinders with a high-vacuum pump for an hour. I n this manner the percentage of fixed gas was reduced to about 0.2 per cent and correction for this residual gas was made in preparing the synthetic mixtures. The hydrocarbons were then distilled by an apparatus described in this paper to give the distillation curves plotted in Figure 1. These curves show that practically the entire volume of each gas is distilled off at a c o n s t a n t t e m p e r a t u r e . Since it has been shown by Washburn (6) that constancy of pressure during isothermal vaporization is an excellent criterion for purity, it is evident from Figure 1 that the hydrocarbons used in this work possessed a high degree of purity, DESCRIPTION OF APPARATUS-The apparatus used in the preparation of the synthetic mixtures and in their distillation is shown in Figure 2. The portion to the left represents the equipment used in the preparation of synthetic mixtures. It consists of the 100-cc. waterjacketed buret, 0, graduated in tenths of a cubic centimeter and equipped with the leveling bulb PI. The water-jacketed buret, A’, of 900 cc. capacity is graduated in 100 cc. Pz is the leveling bulb for N. The reservoir of 1000 cc. capacity for storing s y n t h e t i c samples, M , is e q u i p p e d with leveling bulb Pa. The three connections a t Q designate the lines leading to the steel cylinders containing the pure hydrocarbons. The portion of the figure to the right represents the e q u i p m e n t used in making the distillations of the synthetic mixtures. It consists of tube D in which the gas is condensed and subsequently distilled, and XI to X18 are various stopFigure 3
ANALYTICAL EDITION
286
cocks used as indicated. Tube D is equipped with a thermocouple which rests on the bottom of the tube. Thermocouple leads, L, are sealed to this tube by means of deKhotinsky cement. The thermocouple is single junction and is made of No. 36 copper and constantan wires. The temperature is indicated by means of a precision potentiometer, although one reading to tenths of a millivolt may be used if a triple-junction thermocouple is substituted for the single junction. Crushed ice in R thermos bottle is used for the cold junction.
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The fixed gas and methane are pumped off through a trap, F , while the sample is surrounded by liquid air. The sample, if not synthetic, is introduced through B, a drying tube filled with Desicchlora (anhydrous barium perchlorate). C is a calibrated flask for measuring the volume of the sample introduced, and J is a calibrated expansion bulb for collecting the gas during the distillation. H is a closed-end manometer equipped with a mercury seal, for indicating the pressure in the bulb J , and E is an open-end manometer to measure the pressure in the distillation tube D. GI is a leveling bulb for bulb C. G2 is a leveling bulb for expansion bulb .I, and is provided so that, if desired, the sample may be removed from the apparatus by mercury displacement for further examination, after the distillation is finished. I n such a case, the sample would be removed through stopcock x 1 3 .
PROCEDURE FOR MAKINQ SYNTHETIC MIXTURE-Prior to making up a synthetic mixture, the burets 0 and N and storage bulb M (Figure 2 ) are filled with mercury, and the lines from X2 to Q are evacuated. A gas is now released from its cylinder into the burets 0 and N displacing the mercury therein. The mercury levels are adiusted to the desired marks by manipulating the proper leveling bulbs, making sure that the gas is under slight pressure. The excess pressure is vented to the atmosphere through stopcock XI64 The gas is now transferred to bulb M , care being taken that the lines are left a t atmosphere pressure. The lines are evacuated before introducing another gas. I n this manner, each of the gases is introduced in the volume required to make up the synthetic mixture desired. PROCEDURE FOR ANALYsrs-The apparatus from x3 to ,I (Figure 2) is evacuated. With stopcock X4 closed, the sample is transferred from storage bulb M , or from outside container through stopcock X 1 , to the flask C. The sample is held under slight pressure while the level is adjusted to the mark with leveling bulb Gl. Excess pressure is released by opening stopcock X s to Xe to the atmosphere. Stopcock x 6 is closed, and tubes D and F are surrounded with Dewar flask& containing liquid nitrogen or air. Stopcocks X j to Xll are-closed, while stopcock 1 6 is opened to tube D and the sample is allowed to condens If pressure shows on the manometer, E, the presence of
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fixed gas or methane is indicated since the vapor pressure of ethane is only 0.1 mm. mercury a t the boiling point of nitrogen. The fixed gas is readily removed by pumping with the mercury vapor pump, K , upon opening stopcocks X,to X Q . When methane is present, it is necessary to remove it from solution. This is accomplished by warming tube D until the vapor pressure of the liquid reaches atmospheric pressure, cooling the tube with liquid nitrogen to recondense the sample, and pumping off .the released methane. This process is repeated until the removal of all the methane is assured-i. e., until no pressure is indicated on the manometer at the temperature of boiling nitrogen. When this has been completed, the stopcock Xg is closed, stopcocks X ? and X i are opened, and the liquid nitrogen removed from tube F. Tube F is warmed to room temperature. If ethane should condense in F, it is distilled back into tube D after which stopcock X S is closed. During the removal of fixed gas and methane, trap F is used to prevent any ethane and heavier hydrocarbons from being pumped off. An aluminum cylinder (Figure 3) bored to fit snugly around tube D is cooled by immersion in liquid nitrogen to a temperature below the boiling point of the mixture to be examined and placed around tube D. The cylinder is surrounded by a Dewar flask and covered with cotton felt a t the top to insure uniform heating of the still. Heat input to the cylinder, and hence to the still, is controlled by lowering the Dewar flask and thus increasing the surface of the cylinder which is exposed to room temperature. The distillation is conducted a t atmospheric pressure by cracking stopcock XI as the pressure tends to exceed atmospheric. The distillate is collected as a gas in the previously evacuated calibrated expansion bulb. The temperature of the liquid being diitilled is given by the thermocouple, which is made of very fine wire to prevent conduction of heat, a precision potentiometer being used for reading the temperature.
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I n order that the results may be comparable, distillations are conducted at 760 mm. instead of atmospheric pressure. This is accomplished by regulating stopcock X ? so that the pressure observed on manometer E is always 760 mm. This regulation is acquired with practice. It is necessary to hold the pressure constant to within a few millimeters in order to obtain check results. The distillation should be conducted a t a fairly constant rate of an average of about 15 cc. of gas per minute, During the distillation, simultaneous readings on the potentiometer and the manometer H are taken as the voltage changes 0.1 or 0.2 millivolt, or as the pressure rises 5 to 10 mm. The distillation is continued while readings are taken. When the distillation is completed, stopcock X ? is opened wide and the total pressure, Pz,on the manometer H is noted. From the barometric pressure, the volumes of the measuring flask, still, and expansion bulb, the pressure, P1, corresponde
July 15, 1931
INDUSTRIAL A N D ENGI NEERING CHEMISTRY
ing to the volume of the sample taken is calculated by the formula
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A graph is now prepared by plotting the temperature against the pressure in millimeters. From this curve, the temperature corresponding to any percentage off may be read. Figuxe 4 is a simplified apparatus recommended for routine analysis of any gaseous mixture by this method. It is similar to the distillation apparatus shown in Figure 2 except that the trap F shown there has been eliminated, since it appears that no appreciable amount of ethane is condensed in it during the removal of fixed gases and methane. The volumes of those parts of the apparatus shown in Figure 4 which affect the nature of the curves obtained upon distillation are as follows : Bulb C Bulb D Bulb J
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RESULTS-A considerable number of synthetic mixtures of the hydrocarbons ethane, propane, and butane were made up and distilled by the method described. From these distillations, the graphs shown in Figures 5 and 6 were prepared. Figure 5 is an outline of the master graph showing all pos-
287
sible compositions of the three hydrocarbons. In its present form, it is not intended for use for analytical purposes by the method described, but merely to indicate the general nature of these curves. Figure 6 gives a section of Figure 5 in detail. It is the first of a series of such sections which it is planned to present, This particular section covers the composition of certain refinery gases and is discussed in the second section of this paper. Several typical distillation curves are shown in Figure 7 . The reproducibility of results is shown in Figure 8, which gives check distillation curves for the same sample. It is to be noted that the two curves are represented by different ordinates so that if superimposed, the curves would almost coincide. An analysis may be completed in 60 to 7 5 minutes as compared to 3 to 8 hours by the fractionation methods. The consumption of liquid nitrogen or air is about 200 to 250 cc., most of which is used in cooling the aluminum cylinder. This compares favorably with ordinary fractionation methods which usually require 1000 to 5000 cc. DIscussIox-It is apparent from Figure 6 that, for the synthetic samples prepared and distiIled in the manner described, an excellent regularity is obtained. By the preparation of the necessary synthetic mixtures, the range may be eqtended in any direction desired, and the same regularity may be expected to prevail over the entire range for the three hydrocarbons considered. Graphs of a similar nature may be developed for ternary mixtures of other hydrocarbons in a similar manner. I n each case, two of the hydrocarbons are obtained by the graph and the third by difference. Although in Figure 5, the temperatures at 10 per cent and 80 per cent off were chosen, and in Figure 6, the temperatures of 10 per cent and 90 per cent off were used, it is obvious that, for any similar charts which may be prepared, the petcentages off that should be chosen are those which will yield satisfactory curves for the range under consideration. For the master graph, it is also obvious that in order to obtain sufficient spread between the curves for the various compositions possible, it is more satisfactory to prepare two or more charts representing temperatures at different percentages off. This is illustrated by Figure 9, &wherethe master graph shown in Figure 5 is drawn for temperatures at 10 per cent and 50 per cent off. This gives a larger spread for the high butane percentages. The accuracy of the method is governed by the accuracy of the graph, which in turn is dependent upon the reliability of the temperature and pressure readings. A difference of one in the percentage of any of the constituents within the range investigated has a distinct effect on the distillation
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curve, and hence its position within the graph. This is illustrated by Figure 10 which shows the effect of a change of 1per cent on the butane content of two synthetic mixtures of otherwise similar composition. Using the apparatus described and operating with the procedure given, it has been shown that check runs on the same sample agree to within 0.5 per cent on any constituent. It follows that the analysis of any ternary mixture of the hydrocarbons included within the range of the graph is re-
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liable to 0.5 per cent, Inasmuch as recent unpublished studies of various equipment for analysis of synthetic gaseous hydrocarbon mixtures show that the maximum accuracy obtainable by the most satisfact paratus is not better h of the constituents than an average of 0.5 per cent present, the results obtained by the proposed method compare favorably with the other equipment. For ternary mixtures fhat do not contain any fixed gas or methane besides the three hydrocarbons that make up the mixture, carbon dioxide snow may be used instead of liquid nitrogen or air. As illustrative of the satisfactory use of carbon dioxide for samples of this type, Figure 11 shows two consecutive runs on the same sample, curve A obtained when liquid nitrogen was used, and curve B when carbon dioxideacetone slush was used. Application of Short-Cut Method
In the study and control of various petroleum processes, a knowledge of the composition of gases involved is most important and a method of analysis suitable for routine purposes is desirable. For example, recently a number of refineries have adopted stabilization equipment, the operation of which may be controlled from the analysis of the overhead gas from the stabilizer or of the stabilizer reflux. Since fractionation of stabilizer gas and stabilizer reflux by a Podbielniak apparatus gave the average analysis shown in Table I, it was the object of this investigation to develop
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the method that has been described with the view of applying it to analyses of gases of this type. It is for this reason that the detailed graph shown in Figure 6 was prepared. Table I-Analysis
by Podbielniak Apparatus STABILIZER STABILIZER GAS REFLUX % by VOI. % b y vol. Fixed gas and methane 8.0 0.0 Ethane, ethylene 25.0 14.0 Propane, propylene 05 0 82 0 Isobutane, butane 2.0 4 0 CONSTITUENT
After the graph was completed, comparative analysis of stabilizer gas or stabilizer reflux samples by the short-cut method and a Podbielniak column revealed that the shortcut method gave results that were too low, particularly on the butane content. I n an attempt to explain this discrepancy, Orsat analyses were made of the individual cuts obtained during the fractionation of average stabilizer gas, and these showed that about 15 per cent of the ethane fraction consisted of ethylene and about 25 per cent of the propane fraction was propylene. Further, fractionation analysis showed that 60 to 80 per cent of the butane fraction was isobutane. Consequently, a study was made of the effect of each of these hydrocarbons, in the concentration given, on the distillation curves of the otherwise similar ternary mixtures described in the first part of the paper. Since for control purposes the butane content of these gases is of primary importance, and, further, since the 90 per cent offpoint practically determines the percentage of butane, only the effect upon the 90 per cent point was considered.
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