US3661527A - Method and apparatus for volatility and vapor pressures measurement and for distillation analysis - Google Patents

Abstract

A microanalyzer for determining volatilities, vapor pressures, and for performing distillation analyses of organic materials wherein a sample of the material is supported in a removable flow-through probe on a support and heated while flowing carrier gas over the sample to saturate the carrier gas while monitoring the volatiles yields with a flame ionization detector.

Description

O Umted States Patent [151 3,661,527 Eggertsen et al. [451 May 9, 1972 54 METHOD AND APPARATUS FOR [56] References Cited VOLATILITY AND VAPOR PRESSURES UNITED STATES PATENTS 3,230,046 l/ 1966 Beroza ..23/ 230 3,45 l ,780 6/1969 Prescott et al.. [72] Inventors: Frank T. Eggertsen, Orinda; Elmer E, 1 3,532,270 10/1970 Schoen, Jr. ..73/29 X Seibert, El Cerrito; Fred H. Stross, Orinda, all of Calif. Primary E.\'aminerMorris O. Wolk Assistant E,\'aminerR. E. Serwin 1 Assigneel Shell Oil p y, New York, NY Attorney-T. E. Bieber and J. H. McCarthy [22] Filed: Feb. 24, 1970 [57] ABSTRACT [21] App]. No.1 13,417

A microanalyzer for determining volatilities, vapor pressures, 52 us. Cl. ..23/230 PC, 23/232 E, 23/253 PC, if Ti of odrganic materials 23/254 E 73/29 w erem a samp e o t e matena IS supporte m a remova e flow-through probe on a support and heated while flowing car- [5l Int. Cl. ..GOln 7/16 rier gas over the sample to saturate the carrier gas while moni- [58] Field of Search ..23/230, 232, 253, 254; 73/29 toring the volatiles yields with a flame ionization detector.

14 Claims, 6 Drawing Figures INTEGRATOR /35 36 f RECORDER AMPLIFIER 2 cHANNEL TEMPERATURE PROGRAMMER MIXING CHAMBER PKTE'NTEDIAY 9 m2 SHEET 1 OF 4 INVENTORS:

F.T. EGGERTSEN E E SEIBERT F. H. STROSS PATENTEDHAY 91.972 3,661,527

sum 2 0r 4 FIG. 2

'I'EMPERATURE 26 50 90 I20 RECORDER IMV I 5 MV I 20 MV i 200MV I 4.4 I I I F I I I I I so- I I I I I I I I I I I 4.4 I I E. I I 2 I I I I N2 FLOW RATES, ML/MIN. 5 6| 5 20- '4 4 {I 62 f u 4.4

E E I "E B T T I0 Mm FIG. 3

INVENTORS:

F. T. EGGERTSEN E. E. SEIBERT F. H. STROSS CHART DIVISION PATENTEDMAY 9|972 3,661,527

SHEET 3 BF 4 TEMPERATE,C 25 3o 40 so so I l l I l v l.l8 CHA 80 VAPOR PRESSURE (MILLITORRFM N FLOW,ML/MlN PROBE FLOW= 4.| ML mm HEATING RATE= lC/M|N X5 7 (SEE TABLE I, COLUMN 6) O Q THERMOCOUPLE Z 9 4o 2 2 Q [L] El DE ECT R 3 T O 45.2M mm 0 2 m l E o lo 20 so 40 so so TIME, MINUTES VAPOR PRESSURE CURVE FOR CETANE FIG. 4

o no 20 so 40 SOMINUTES l I I TEMPERATURE O44TORR TORR DISTILLATION CURVE FOR A HEAVY on.

1 SAMPLE c DETECTOR FIG. 5

I 35% :24%: 20% I I I I INVENTORS: 6% F. T. EGGERTSEN l- E.E.SE|BERT N.B.P.: 3oo4oo 500 600 F. H. sTRoss D|ST.T: |o5 I77 246 3|9 METHOD AND APPARATUS FOR VOLATILITY AND VAPOR PRESSURES MEASUREMENT AND FOR DISTILLATION ANALYSIS RELATED APPLICATION The present invention is an improvement and modification of the analyzing method disclosed in the applicants copending application entitled Microanalyzer for Thermal Study, Ser. No. 617,337, filed Feb. 20, 1967, and now U.S. Pat. No. 3,574,549.

BACKGROUND OF THE INVENTION In many industrial processes, particularly, chemical processes, it is necessary to know the volatility of various materials and the possibility of volatile contaminants being present in the materials. For example, in the manufacture of various plastics it is desirable to know the vapor pressure of various constituents in the plastic to determine the quality of the plastic. Also in the case of plastic strips that are impregnated with various insect-killing formulations, it is desirable to know the vapor pressure of the impregnated plastic to determine the useful life of the formulation. Similarly, in the case of materials such as oils or asphalts, it is essential to know the vapor pressure or amount of low-boiling components in order to determine whether the material will meet specifications, as for example in the case where the asphalt is used as a roofing compound and certain fire limits must be met.

Conventional methods for measuring vapor pressure are generally time-consuming and experimentally difficult, especially at pressures below 1 torr. The classical gas saturationgravimetric method depends upon measuring the mass of the volatile substance that is carried away by a fixed volume of a saturated carrier gas. The determination of low vapor pressures by this method requires rather long flushing periods in order to vaporize a sufficient quantity to be weighed precisely. Another problem is that of ensuring that the carrier gas is saturated. The effusion method, in which a measurement is made of the mass flow through an orifice, is slow because here also the loss of weight is determined. Methods which depend upon a direct measurement of pressure, for example with a manometer or MacLeod gage, are generally unsuitable for low pressures because of condensation and adsorption effects and other difficulties. None of the conventional methods are wellsuited for rapid measurements over a wide temperature range by a temperature-programming technique.

SUMMARY OF THE INVENTION The present invention solves the above problems by providing a relatively low cost easily operated apparatus that will determine either the vapor pressure of a material, or will pro vide yield vs. temperature curve from which distillation data can be derived. The device consists of a flow-though probe member that includes means for holding a small quantity of the material to be tested. The probe is provided with means for supplying a metered flow of carrier gas to the probe with the probe in turn being placed in a furnace. The furnace is provided with a heating means in order that its temperature may be raised in a predetermined manner or programmed rate. The furnace also includes an inlet in order that additional carrier gas may be supplied to the system. The total quantity of carrier gas supplied to the probe and the furnace is set at a predetermined level, the gas being discharged from the furnace directly into a suitable detector that may be operated at an elevated temperature. A flame-ionization detector is preferred, since it can be operated at an elevated temperature and is extremely Sensitive to hydrocarbons and other organic materials. The discharge from the furnace is passed to the flame-ionization detector with the signal from the flameionization detector and the temperature of the probe being recorded in relation to time. The recording can then be related directly to the vapor pressure and quantity of the materials evolved as the sample is heated.

The flow-through probe is preferably a tubular member having a gas inlet at one end and an outlet at the opposite end. The opposite end is also provided with a removable cap in order that a sample mounting means may be inserted. The sample may be supported on an inert granular material or, in a I preferred embodiment on a screen formed of an inert material such as stainless steel. The screen, carrying a thin film of sample,.is inserted in the end of the probe and the probe inserted in the furnace and heated. As the gas flows past the screen, it will become saturated with the vapors evolved from the sample and the detector thus gives a true indication of the vapor pressure of the material. A vapor-liquid equilibrium is maintained with the total sample on the screen, and distillation data can be obtained from the resulting yield vs. temperature curve. The results obtained by this method are in agreement with those by conventional vacuum flash distillation. Such analyses are not possible with gas saturation methods previously reported because they utilize larger samples and are not designed to produce a vapor-liquid equilibrium with the total sample or to utilize temperature programming.

The new distillation analysis method has certain advantages over conventional flash distillation. In particular, it extends the analysis to very high boiling material (up to about 700 C.) without thermal cracking; this results from the very low vapor pressures that can be employed. The practical limit in conventional flash distillation is about 500 C. normal boiling point. Also the new method is well-suited for automation to save considerably on operator time.

The apparatus also includes flow control means to permit a selection of flow rates through the sample while maintaining a constant total flow to the detector. The choice of flow-rates allows a rapid check of the completeness of saturation of the carrier gas and extends the pressure range over which accurate measurements can be made. A flow control means for introducing air between tests is provided in order to clean the apparatus by air oxidation, thus minimizing holdup of material due to adsorption effects.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more easily understood from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawings in which:

FIG. 1 is a schematic drawing of an apparatus constructed according to this invention;

FIG. 2 is an elevation view of a probe used in the apparatus of FIG. 1;

FIG. 3 is a chart recording of the vapor pressure of a sample by isothermal heating;

FIG. 4 is a chart recording of the vapor pressure of a sample by temperature programming; and

FIGS. 5 and 6 are chart recordings of a distillation analysis of a heavy oil.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2, there is shown an apparatus suitable for carrying out the method of the present invention. The apparatus comprises a furnace member 10 having a flowthrough probe member 11 inserted in one end thereof. The opposite end of the furnace is joined directly to the flameionization detector 30 in order that the vapors evolved from the sample may be conveyed directly to the detector, thus minimizing loss by adsorption or condensation. The furnace is provided with a heating element 12 surrounding its outer surface with the heating element being coupled to a temperature programmed power supply 13. The power supply should include means for programming the rise in furnace temperature to coincide with any desired heating cycle. The inlet of the flow-through probe is supplied with an inert carrier gas, for example, nitrogen from a source 18. The source of nitrogen should have sufficient pressure so that it may be throttled to maintain the desired flow rates through the furnace and the probe. As flow controls there are shown four flow controllers 14-17, with three of the controllers 15-17 being coupled to throttle valves 20-22. The throttle valves are coupled to a common flow line 23 which discharges into the probe and a mixing chamber 24. The flow control means 14 is directly coupled to the inlet of a mixing chamber 24 which in turn is coupled to a two-way valve 26 that communicates with the inlet 27 of the furnace. The three flow controllers 15-17 are preset to supply metered flow rates of nitrogen to the flow-through probe. For example, they can be preset to provide 1,5 and ml per minute flow rates. Likewise, the flow controller of 14 should be adjustable so that the flow through the furnace plus the flow through the probe is equal to a predetermined value, for example, 30 ml per minute. This nitrogen flow scheme insures that the flame ionization detector operates under constant nitrogen flow conditions and has constant response characteristics at all probe flow rates. The provision for variable probe flow rates permits rapid changes in flow through the probe to check for complete saturation of the carrier gas; and also allows distillation analyses to be made at various selected rates.

The mixing chamber 24 is provided for diluting predetermined quantities of standard gas samples which are introduced to calibrate the detector. The mixing chamber has an inlet means 25 through which the standard samples may be injected. For example, .the inlet may be a conventional septum and the known quantities of materials may beinjected by the use of a gas-tight hypodermic syringe. Alternatively, a calibrated gas sample valve may be used to inject samples.

The detector 30 is a conventional hydrogen flame-ionization detector wherein the sample is burned with hydrogen at the jet 33. The flame-ionization detector shown here is'of the type described in the above-referenced copending application. The air and hydrogen fed to the detector are preferably pure in order to prevent possible variations arising from organic impurities. The flame-ionization detector 30 is provided with a heating element 31 surrounding its outer surface which maintains the temperature of the detector substantially constant, for example, at 500 C. A high detector temperature minimizes condensation and adsorption of any of the vapors that are evolved from the sample and thus prevents contamination or plugging of the detector. The signal from the detector is coupled by means of a lead 34 to an amplifier 35 whose output is integrated by an integrator 17 and also recorded on a dual-channel recorder 36. The second channel of the recorder is used to record the temperature that is measured by means of a thermocouple disposed in the flowthrough probe. The thermocouple is coupled by a lead 38 to the second channel of the two-channel recorder.

Referring now to FIG. 2, there is shown an elevation view in section of the flow-through probe shown in FIG. 1. More particularly, the probe is formed from a tubular body 40 having a shoulder 41 formed on one end. A nut 42 is used for coupling the sample-containing portion 43 of the probe to the tubular body 40. The sample-containing portion 43 of the probe is tubular in shape and provided with a small diameter tubular extension 44 that extends partially through the tubular body 40. The tubular section 44 is provided for coupling the gas flow from the probe inlet to the sample holder 43. The opposite end of the sample holder is closed by means of a threaded end cap 45 having a small diameter central opening 46 through Y which the gas exits. The complete probe body may be formed of suitable material, for example, an inert metal such as stainless steel or the like. The small diameter tube 44 in combination with the small diameter exit 46 and disposition of sample asa thin film in the probe insures excellent contact between gas and sample, thus promoting vapor-liquid equilibrium throughout the volatilization.

A thermocouple 50 is disposed in the sample-holding portion 43 of the probe and is coupled by means of lead 51 to one channel of the dual-channel recorder shown in FIG. 1. The sample holder, serves to contain the sample support which may be when granular solid, for example small lengths of silver wire or a gas chromatography support. Preferably, at least for liquids, the sample is supported on a piece of screen material, designated 52 in FIG. 2. As explained above, the use of a screen metal for holding a sample, allows the use of relatively thin films of the sample that insure rapid attainment of vaporliquid equilibrium throughout the whole sample. This allows accurate measurement of the vapor pressure more rapidly and conveniently than with prior art devices. Also distillation analyses can be made as will be explained later.

EXAMPLE I. VAPOR PRESSURE OF CETANE The vapor pressure of cetane was measured over the temperature range 26 to 140 C., using both isothermal and temperature programming techniques. The conditions of analysis were:

Sample: 5 mg. cetane/500 mg. silver granules, or l8 mg.

cetane/26 mg. C-22 firebrick.

FID Temperature: C.

N flow rate: 1,5, or 10 ml/minute through probe; 30

ml/minute total FIG. 3 is a chart record for example 1 of the vapor pressure of cetane at several temperatures and with various flow rates through the probe member. At the bottom of the record there is shown a dotted line that indicates the no-flow base line. From heights of the recorded detector signals obtained it is seen that the response is directly proportional to the flow rate through the probe, at least up to 10 ml. per minute. It is concluded that the gas flowing through the probe was completely saturated by the vapors from the sample. The exact vapor pressure can be obtained from the following formula:

wherein .n, moles per minute of the volatile substance evolved, n moles per minute of N passing through the probe, P barometric pressure. The quantity n, is obtained by multiplying the signal height by a response factor, moles per. minute per chart division, previously determined by calibration. The flame ionization detector responds to organic carbon, the carbon response factor being quite uniform, or calculable, for various compounds. Accordingly, the response factor may be determined for a standard gas sample (e.g. butane), from which the factor for another compound may be computed. Alternatively, theresponse factor for the sample may be determined directly by evaporating a known amount of material in the system and measuring the integrated response (peak area). l i

The vapor pressure results obtained for cetane agree well with the literature values as shown in Table I.

Sample supported on silver wire. Sample supported on C-22 firebrick granules.

EXAMPLE 2. VAPOR PRESSURE OF SQUALANE Similarly the vapor pressure of squalane, a high boiling liquid (C l-l was determined with the sample supported on short lengths of silver wire as well as on the screen and by temperature programming at 5 C. per minute. The results, given in Table II, show that comparable values are obtained by the two techniques of supporting the sample, thus showing that the convenient screen support gives gas saturation.

TABLE II Vapor Pressure of Squalane m torr at (C.)

"Values reported by l. M. Heilbmn et al, J. Chem. Soc. p. 3 I 32 1926) EXAMPLE 3. DISTILLATION ANALYSIS OF AN OIL The apparatus of FIG. 1 can also be used to perform simple one-plate distillations and obtain results similar to those from conventional vacuum flash distillation. The method involves heating 1-2 mg of sample supported on a screen while monitoring the vapors swept out by a flow of nitrogen (usually at 5 ml per minute). The detector temperature was 500 C. The heating rate is controlled to maintain a preselected detector signal level corresponding to a vapor pressure of usually 1 torr or less. The cumulative yields at selected intervals during the volitilization, corresponding to distillation cut points are determined by integrating the area of the recording. Typical volatization curves are shown in FIGS. 5 and 6 together with distillation data derived from the curves.

The distillation temperatures and yields corresponding to selected normal boiling points were determined in the following manner.

The height of the detector signal at which the analysis was made (FIG. 5) was converted to vapor pressure, as described above for cetane, using a calibration factor which varied with the normal boiling point. Thus, the vapor pressure for the cut point corresponding to 200 C. normal boiling point was calculated using the calibration factor for a C hydrocarbon (since the normal boiling point temperature of C is close to 200 C). Similarly, the carbon numbers selected for normal boiling point cuts of 300, 400, 500, 600 and 700 C. were C C C C and C The vapor pressure at a given detector signal level varies inversely with the carbon number.

vapor pressure K/carbon number The constant K includes the carbon response factor for the detector, the nitrogen flow rate and barometric pressure.

Having calculated the vapor pressures corresponding to the selected normal boiling points, the corresponding distillation temperatures were determined from a suitable correlation chart (a chart of this type by Myers and Fenske is published in Industrial and Engineering Chemistry, Vol. 47, pg. 1652, 1955).

In another mode of operation, the temperature is programmed, for example at 10 C. per minute, to produce a curve like that in FIG. 6. In this case, the distillation temperatures are found by determining the intersection points of the recorded curve with vapor pressure curves calculated for the selected normal boiling cut points as shown in FIG. 6.

A comparison of the results of the method of the invention with those obtained with conventional vacuum distillation is shown in the following table:

TABLE III Simulated Vacuum Distillation Yield, %w at normal b.p. of ("C.)

Sample Method 300 400 500 600 C.

T 24 52 87 100 B V 16 40 81 T 9 37 84 99 C V 9 23 53 T 6 21 56 D V 0.4 12 66 T 0.6 7 67 96 V vacuum distillation T= method ofpresent invention From the above comparison it can be seen that the results using the method of this invention are within a few percentage points of the vacuum distillation results.

EXAMPLE 4. VOLATILITY AND OXIDATIVE STABILITY OF A DETERGENT The volatility and rate of oxidative decomposition of a dc tergent formulation, consisting of 15 percent of an alcohol ethoxylate and 85 percent sodium tripolyphosphate, was measured in the apparatus. Such measurements are useful for determining the spray drying temperatures that can be used with different formulations and with different oxidation inhibitors.

To make this test 45 mg. of granular sample was placed in the sample holder. The sample was heated at 4 C. per minute with nitrogen or air flowing through the probe at 5 ml. per minute. The volatiles yields were determined at selected temperatures by integrating the volatilization curves. The oxidative stability is indicated by the temperature at which the curve obtained with air rises above that obtained with nitrogen. The volatiles yields (peak area) and rates of volatilization (height of detector signal) are computed with the aid of calibration factors for the detector.

I claim as my invention:

1. A method for measuring the vapor pressure of micro samples of organic materials, said method comprising:

placing the sample as a thin film on a support member inside a probe, said probe being equipped with means for measuring sample temperature, and having an inlet and outlet for passage of gas;

placing said probe and sample in a furnace;

flowing an inert carrier gas past the sample in said probe at a known rate which is sufiiciently low to saturate carrier gas with sample vapors; flowing additional carrier gas through said furnace and around said probe to sweep sample vapors out of the furnace; v

discharging the total flow of gas from the furnace directly to a flame ionization detector that is maintained at a sufficiently high temperature to avoid condensation of sample vapors;

heating the sample by temperature programming;

determining the detector response as a function of sample temperature by suitable recording means; and

converting the detector response at selected temperatures to vapor pressure by means of detector calibration factors relating the magnitude of the response to the vapor pressure. 2. A method for the distillation analyses of micro samples of organic materials, said method comprising:

volatilizing the sample while making vapor pressure vs. temperature measurements by the method of claim 1; said measurements being obtained from the detector response at selected intervals during the sample volatilization;

integrating the detector response up to said selected intervals, said integrated response serving as a quantitative measure of distillation yield.

3. The method of claim 2 wherein said detector is sensitive to organic materials.

4. The method of claim 3 wherein the sample is supported on a screen.

5. The method of claim 3 wherein the sample is heated at a rate to maintain a vapor pressure of less than 1 torr.

6. The method of claim 3 wherein the areas of the recording are integrated to obtain the yields at various cut points.

7. The process of claim 1 wherein the inert gas is nitrogen.

8. The process of claim 1 wherein the sample is less than 10 milligrams and the inert gas flow is less than 100 milliliters per minute. I

9. The process of claim 1 wherein the heat rate is between 1 C. and 10 C. per minute.

10. An apparatus for obtaining measurements suitable for volatility measurements and distillation analyses of organic samples, comprising:

a furnace, said furnace being adapted to receive a sample probe member, and having an inlet opening and an outlet opening for establishing a gas flow therethrough;

a probe member, said probe being adapted for insertion and removal from said furnace and capable of containing the sample of the material to be analyzed as a thin film, said probe in addition having inlet and outlet ports for the passage of gas;

a temperature measuring means mounted in said probe member;

a flow control means for establishing a known inert gas flow past the sample in said probe member and an additional inert gas flow around the probe member and through said furnace;

a detecting means directly connected to the outlet of said furnace for measuring the volatiles yield; and a recording means for recording, in a correlatable manner, the detector response and the corresponding temperature as the sample is volatilized by heating.

1 l. The apparatus of claim 10 wherein said probe comprises an elongated tubular member having a removable end, said removable end having an outlet opening;

a sample holding means for holding a sample of material to be analyzed, said means being insertable into said probe adjacent said removable end.

12. The apparatus of claim 11 wherein said furnace comprises a tubular member having an inlet and outlet ends, said inlet end being adapted to receive said probe.

13. The apparatus of claim 12 wherein said detector is provided with a heating means capable of maintaining said detector above the maximum operating temperatures of the furnace.

14. The apparatus of claim 11 wherein said sample holding means comprises a screen inserted in said probe adjacent said removable end.

Claims (13)

1. 2. A method for the distillation analyses of micro samples of organic materials, said method comprising: volatilizing the sample while making vapor pressure vs. temperature measurements by the method of claim 1; said measurements being obtained from the detector response at selected intervals during the sample volatilization; integrating the detector response up to said selected intervals, said integrated response serving as a quantitative measure of distillation yield.
2. 3. The method of claim 2 wherein said detector is sensitive to organic materials.
3. 4. The method of claim 3 wherein the sample is supported on a screen.
4. 5. The method of claim 3 wherein the sample is heated at a rate to maintain a vapor pressure of less than 1 torr.
5. 6. The method of claim 3 wherein the areas of the recording are integrated to obtain the yields at various cut points.
6. 7. The process of claim 1 wherein the inert gas is nitrogen.
7. 8. The process of claim 1 wherein the sample is less than 10 milligrams and the inert gas flow is less than 100 milliliters per minute.
8. 9. The process of claim 1 wherein the heat rate is between 1* C. and 10* C. per minute.
9. 10. An apparatus for obtaining measurements suitable for volatility measurements and distillation analyses of organic samples, comprising: a furnace, said furnace being adapted to receive a sample probe member, and having an inlet opening and an outlet opening for establishing a gas flow therethrough; a probe member, said probe being adapted for insertion and removal from said furnace and capable of containing the sample of the material to be analyzed as a thin film, said probe in addition having inlet and outlet ports for the passage of gas; a temperature measuring means mounted in said probe member; a flow control means for establishing a known inert gas flow past the sample in said probe member and an additional inert gas flow around the probe member and through said furnace; a detecting means directly connected to the outlet of said furnace for measuring the volatiles yield; and a recording means for recording, in a correlatable manner, the detector response and the corresponding temperature as the sample is volatilized by heating.
10. 11. The apparatus of claim 10 wherein said probe comprises an elongated tubular member having a removable end, said removable end having an outlet opening; a sample holding means for holding a sample of material to be analyzed, said means being insertable into said probe adjacent said removable end.
11. 12. The apparatus of claim 11 wherein said furnace comprises a tubular member having an inlet and outlet ends, said inlet end being adapted to receive said probe.
12. 13. The apparatus of claim 12 wherein said detector is provided with a heating means capable of maintaining said detector above the maximum operating temperatures of the furnace.
13. 14. The apparatus of claim 11 wherein said sample holding means comprises a screen inserted in said probe adjacent said removable end.

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