US3345858A - Continuous chromatographic analysis - Google Patents

Description

Oct. 10,1967 E. R. FENSKE CONTINUOUS CHROMATOGRAPHIC ANALYSIS 2 Sheets-Sheet 1 Filed Jan. 28, 1964 Figure Measuring 0e Sample Out I Reference Cell Sample In /v vf/v r05.- E llsworrh R. Fens/re oam 60s ATTORNEYS United States Patent Ofiice 3,345,858 Patented Oct. 10, 1967 3,345,858 CONTINUOUS CHROMATOGRAPHIC ANALYSIS Ellsworth R. Fenske, Palatine, Ill., assignor to Universal Gil Products Company, Des Plaines, 111., a corporation of Delaware Filed Jan. 28, 1964, Ser. No. 340,640 19 Claims. (Cl. 73-23.1)

ABSTRACT OF THE DISCLOSURE Chromatographic analysis of a fluid mixture utilizing as column packing a solid selective sorbent such as molecular sieves, incorporating a modified procedure which provides a continuous output signal, notwithstanding intermittent sample injection, proportional to concentration of selectively sorbed component(s) in said mixture. The method is particularly adapted to determining total nparafiin content of a mixture of normal and non-normal hydrocarbons. The column is first presaturated with nparaflins. Carrier gas is passed continually through the column, and fixed volume samples of the analysis mixture are periodically injected into the carrier gas stream. Thereafter, the n-paraffin content is reflected by a continuous elevated baseline signal proportional to n-paraffin concentration, which accurately tracks changes in said concentration as between a plurality of the series of samples. The non-normals of each sample are preferably eluted as a single lumped peak superimposed upon the baseline signal.

This invention relates to a method for analyzing mixtures of fluid components and more particularly to the analysis of such mixtures by an improved vapor phase chromatography technique utilizing as the component separating medium a solid sorbent capable of retaining one or more components of the mixture and rejecting one or more other components thereof. More specifically the present invention is directed to a method for the rapid analysis of either or both of two broad classes of components present in a sample mixture: (1) those components which are selectively sorbed by the sorbent and (2) those components which are relatively less sorbed or rejected by the sorbent. Those of the first class are analyzed continuously.

The theory and application of vapor phase chromatography is well known. Much of the work in this art has been directed to the development of high resolution apparatus capable of separating and quantitatively measuring'very closely related compounds in a complex multicomponent system. Many sophisticated column designs have been developed which are capable of handling sample, volumes in the microliter or even nanoliter range; other designs'embody elaborate flow schemes, multiple columns, novel column packing materials, ultra-sensitive detectors, advanced electronic circuitry, and the like, all with the objective of providing ever higher resolution and sensitivity. For the most part such complicated analytical equipment can be utilized to advantage only in the laboratory by or under the supervision of a skilled technician. Equipment of this type has generally proven unsatisfactory, when transferred to the chemical plant or petroleum refinery and utilized as a process stream analyzer, because it lacks reliability under adverse environment and needs considerable maintenance and calibration. Even the highly engineered process stream chromatographs heretofore available for use in field installations do not provide the complete answer and in fact have at least four substantial drawbacks: first, conventional chromatographs hav excessive capability, by which is meant they supply a more thorough analysis of a'multi-component stream than is needed by a plant operator who is usually interested in only one key component or group of components. Second, they require a relatively long time to elute a given sample, hence the interval between analyses is often unduly delayed which cannot be shortened even though the peaks not of interest are mechanically or electronically suppressed. Third, the output signal of a conventional chromatograph, being a train of unidirectional peaks, is inherently discontinuous and therefore unsuited to online process control unless it is taken through a peak-picker or integrator and memory device,

' which auxiliary apparatus adds to system cost and reduces when done manually, necessitates elaborate and expensive computing circuitry when done electronically, and in any case the frequency of the determinations remains limited by sample elution time.

The present invention largely eliminates the aforesaid disadvantages of prior art chromatographic analyzers through the use of a specially pretreated solid sorbent as the chromatographic column packing material. Various solid sorbents of specific composition and structure are known which, upon contact with a mixture of components which differ in their relative sorbency on the solid, have the ability to selectively sorb one or more components thereof and to pass or reject one or more other components, thereby providing a means for segregating the individual components according to their structure. The physical or chemical mechanisms by which particular sorbents are operative for particular multi-component mixtures may differ. For example, typical sorbents operative by reason of their adsorptive properties include silica gel, activated charcoal, aluminosilicates such as the various clays and activated silica gels including, for example, Attapulgus clay, montmorillonite, dehydrated composites of alumina and silica activated by heating to a temperature slightly below the fusion point of the composite, and activated alumnia; sorbents operative by molecular occlusion are the dehydrated metal aluminosilicate hydrates wherein the metal constituent is an alkali or alkaline earth metal, commonly referred to as molecular sieves. These and other types of sorbents, as well as processes employing them, are well known in the art, and are more completely described, for example, in US. Patent 2,985,589. Any of the above sorbents may be employed in the present invention.

In accordance with this invention there is provided a means for passing a continuous flow of carrier gas, such as helium, first through a reference thermal conductivity cell, then successively through a separating column and a measuring thermal conductivity cell. A sample injection means is provided for injecting controlled volume samples of the fluid mixture to be analyzed directly into the carrier gas stream between the reference cell and the separating column. The reference cell and measuring cell are incorporated in a suitable bridge circuit which drives a potentiometric recorder. These elements are conventional. I have discovered, however, that if the column is packed with a solid sorbent capable of selectively sorbing one or more, but less than all, of the components present in the sample, and further if the sorbent is substantially presaturated with at least one of the selectively sorbed components, then a true continuous analysis for the selectively sorbed material can thereafter be obtained. The presaturation of the sorbent may be effected by contacting it with the selectively sorbed component or components, either in pure form or in admixture with relatively less sorbed or non-sorbed components, under sorption conditions prior to loading it into the separating column; presaturation may also be accomplished in situ, after the sorbent is loaded into the column, by repetitive sample injection until substantial saturation is achieved. Consider first that the column is filled with fresh or only partially saturated sorbent, carrier gas flow is established at the proper rate, column temperature is stabilized at the proper level, and the measuring bridge circuit parameters are adjusted to provide an electrical zero output in the recorder. Assume that the fluid mixture to be analyzed is a binary mixture consisting of one selectively sorbed component and one relatively less sorbed component. A first sample is injected and a short while later a peak is recorded corresponding to passage of the less sorbed component through the measuring cell; the recorder pen returns to zero and remains there since the selectively sorbed component originally present in the sample is captured by the sorbent and is not stripped or desorbed by the carrier gas. The same response will be obtained after each of a number of successive sample injections until the sorbent becomes saturated with respect to the selectively sorbed component. When saturation occurs the selectively sorbed component begins to elute at a uniform rate from the separating column, even though another sample has not yet been injected, and the recorder pen will suddenly drive upscale a substantial distance above electrical zero corresponding to passage of the selectively sorbed component through the measuring cell. This response is not a peak but is a stable baseline elevated above electrical zero which con tinues for a substantial period of time following the last previous sample injection, and significantly the magnitude of such elevation has been found to be proportional to the amount by weight of selectively sorbed component present in the sample. If, now, successive samples of the fluid mixture are injected periodically, and the concentration of selectively sorbed component in each is different, the analyzer output will have the form of a peak, soon after each Sample injection (due to the less sorbed component) superimposed on a continuous time-varying elevated baseline, the latter accurately tracking the varying content of the selectively sorbed component in the series of samples. The deflection above electrical zero after the less sorbed component peak provides a continuous measure of the selectively sorbed component content of the sampled binary system. The analyzer in effect converts a sampled data input to a continuous output.

Consider, next the case where the fluid mixture to be analyzed is a multi-component mixture consisting of two or more selectively sorbed components and one relatively less sorbed component. After a number of sample injections the sorbent becomes saturated with selectively sorbed material which thereafter elutes at a uniform rate and without discrimination or chromatographic separation as among the several selectively sorbed components. The analyzer output has the same form as when the fluid mixture is a binary system except that now the elevated baseline, after passage of the less sorbed component through the separating column, is proportional to the total selectively sorbed component content of the mixture. The analyzer in effect treats the multi component mixture as a pseudobinary system.

Consider further the case where the fluid mixture to be analyzed comprises at least two selectively sorbed components and at least two relatively less sorbed components. After the sorbent is saturated with selectively sorbed material, continued repetitive sample injection results in a continuous elevated baseline developed by the continuous, non-discriminatory elution of the several selectively sorbed components. If the temperature of the separating column is sufficiently low, chromatographic separation of the less sorbed components will nevertheless occur so that the analyzer output, following each sample injection, is a series of peaks, corresponding to the specific components contained in the less sorbed class, deviating from the elevated baseline. While this type of analysis is frequently useful and is included within the scope of the invention, it is also desirable and possible to make the analyzer respond as though the fluid mixture were binary. This is accomplished by raising the column temperature to the point where all the less sorbed components are eluted from the column almost immediately and without discrimination, while the selectively sorbed components continue to elute uniformly, also without discrimination, thereby achieving a lumping effect. The analyzer output, following each sample injection, now takes the form of a single narrow peak or spike deviating above the elevated baseline, the latter being proportional to the total selectively sorbed component content of the sample. When operated in this manner, e.g., to perform a pseudobinary analysis, the analyzer distinguishes only between two broad classes of components: those which are selectively sorbed, and those which are relatively less sorbed. Such information is often invaluable to the plant operator who is seeking to optimize the performance of a particular piece of hardware or refinery process unit, such as a distillation column or catcracker, and who is not interested in a compound-by-compound analysis of a stream. The pseudobinary analysis has the additional advantage that elution time per sample is generally at a minimum so that higher sampling frequencies may be used. The column temperature necessary to achieve the lumping effect is dependent on several variables such as the type of sorbent, the specific compounds in the mixture to be analyzed, the carrier gas flow rate, etc., but in any event can be ascertained by routine experimentation with a particular systern.

It will be recognized that the method of this invention differs over prior art chromatographic analyzers in at least four important respects:

(1) At least one of the components of the analysis mixture is eluted from the column continuously, notwithstanding intermittent sample injection, and at least one other component is eluted discontinuously, in contrast to conventional chromatographs wherein each specific component elutes discontinuously as a binary mixture with the carrier gas.

(2) If the analysis mixture contains two or more selectively sorbed components, these are eluted from th column continuously and Without discrimination. This technique deliberately provides for a zero or negligible separation between the specific selectively sorbed components, as opposed to the philosophy of the prior art which has sought to achieve high resolution between all specific components of the analysis mixture.

(3) If the analysis mixture contains tWo or more less sorbed components, in a preferred embodiment of the invention these components are eluted simultaneously or substantially so, within a very short time after sample injection, and their time of passage through the measuring cell is short in comparison to the interval between sample injections.

(4) Elution time per sample is significantly less than that required by conventional chromatographs.

Advantages of the instant technique include the following: a continuous output signal is developed from a discontinuous input, attaining closer tracking of a varying component concentration and easier marriage with standard analog control elements to form a closed loop system; a single component or group of components develops a unitary signal proportional to their weight concentration, which signal is continuous for selectively sorbed components and may be lumped into a single peak for less sorbed components, thereby eliminating the computing circuitry required for automatic component readout in high resolution work; the relatively short elution period permits a higher sampling frequency and therefore increased accuracy and reduced dead time; the use of a solid sorbent as the chromatograph column packing material eliminates problems of stability and substrate boilaway arising from conventional packings comprising an inert solid carrier coated with a high-boiling organic liquid substrate.

It is, therefore, a broad embodiment of this invention to provide a method for analyzing a mixture of fluid components, at least one of which is selectively sorbed by contact with a solid sorbent (such as, for example, silica gel, activated charcoal, activated alumina, or molecular sieves) and at least one other component is relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed component until the sorbent is substantially saturated with respect to the selectively sorbed component, passing a sample of said mixture together with a carrier gas stream through a separation zone containing the pretreated sorbent, and passing the efiluent from said zone to a measuring cell.

A more specific embodiment of this invention is directed to a method for analyzing a mixture of fluid compounds, at least one of which is selectively sorbed by contact with a solid sorbent and at least two other compounds are relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with the selectively sorbed compound until the sorbent is substantially saturated with respect to the selectively sorbed compound, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing samples of said mixture at periodic intervals into the carrier gas stream at a point upstreamfrom the separation zone,

the temperature of said zone being sufficiently high so that all of said compounds are maintained in the vapor phase therein and said less sorbed compounds are eluted from the separation zone substantially simultaneously, and passing the effiuent from the separation zone to a measuring cell.

A preferred solid sorbent is a material characterized as a dehydrated metal aluminosilicate hydrate wherein the metal constituent is an alkali or alkaline earth metal, also known as molecular sieves. These sorbents are particles having pore diameters in the range of 3 to 7 A. and are capable of selectively sorbing a variety of compounds depending upon the specific pore diameter range and size distribution. The molecular sieves may be prepared by c0- precipiating or otherwise combining alumina, silica, and a metal oxide such as an oxide or hydroxide derivative of an alkali metal such as sodium, lithium or potassium or of an alkaline earth metal, such as calcium, magnesium, barium or strontium to form a uniform and intimately dispersed mixture of silica, alumina and the metal oxide or hydroxide, removing excess water from the mixed oxides and thereafter heating the recovered dispersed mixture to a temperature at which the water of hydration will be driven oif by evaporation from the particles. The resulting dehydrated mixed oxides are preferably calcined at a temperature of from about 400 to 800 F. to improve rigidity and strength of the particles. Methods for the preparation of molecular sieves are well documented in the literature, as set forth, for example, in U.S. Patents 2,882,244 and 2,882,243.

'One specific embodiment of the invention, using molecular sieves as the chromatographic column packing material, provides a method for analyzing a mixture of hydrocarbons containing at least one normal aliphatic hydrocarbon and at least one non-normal hydrocarbon se- 6 lected from the group consisting of branched chain and cyclic hydrocarbons. The normal aliphatic hydrocarbon is selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elutes continuously from the column. The non-normal hydrocarbon is passed by the sieves and elutes as a band soon after sample injection. The normal aliphatic hydrocarbon may be saturated or unsaturated, e.g., a normal paraifin, or a straight chain olefin, diolefin or polyolefin, and may contain from 1 to about 22 carbon atoms. The non-normal hydrocarbon may be a branched chain parafiin, olefin, diolefin or polyolefin containing from 4 to about 22 carbon atoms, a cycloparaflln or cycloolefin containing from 4 to about 22 carbon atoms, or a mononuclear or polynuclear aromatic hydrocarbon containing from about 6 to about 22 carbon atoms. Since the components undergoing chromatographic separation are in the vapor state, hydrocarbons containing more than 22 carbon atoms cannot generally be analyzed by this method because they are too difiicultly vaporizable, even with the substantial reduction in hydrocarbon partial pressure afforded by excess amounts of carrier gas, and tend instead to thermally decompose and deposit coke upon the column packing. When the normal aliphatic hydrocarbon is a normal paraflin it may therefore contain from 1 to 22 carbon atoms, and preferably is one containing from 4 to 18 carbon'atoms. The non-normal hydrocarbon may contain from 4 to about 22 carbon atoms, and preferably is one containing from 4 to 18 carbon atoms. The lumping effect is most readily achieved when the carbon number spread of the analysis mixture does not exceed about 6 to 8 carbon numbers.

This invention is especially well suited to analyzing kerosene fractions comprising one or more normal paraffins containing from 10 to 16 carbon atoms, and one or more non-normal hydrocarbons containing from 6 to 16 carbon atoms. For an analysis mixture of this type the temperature of the molecular sieve column should be at least 600 F. and preferably somewhat higher, for example, in the range of 610650 F., in order to obtain continuous non-discriminatory elution of the normal parafiins, on the one hand, and non-discriminatory lumped elution of the non-normal hydrocarbons on the other. Below about 600 F. some separation of the C -C fraction will begin. A typical analysis mixture may comprise a major proportion of normal parafiins and a minor proportion of non-normal hydrocarbons, in which case the height or area under the peak due to non-normals, superimposed on the continuous elevated baseline due to normals, will provide a measure of the total non-normal content of the sample. Another typical analysis mixture may comprise a major proportion of non-normals and a minor proportion of normals, in which case the displacement above electrical zero after passage of the non-normal peak will provide a continuous measure of the total normal parafiin content of the sample.

Another specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of water in a mixt-ure of water with one or more of the following less sorbed materials: methane, ethane, propane, isobutane, hexane, oxygen, hydrogen, nitrogen, air and natural gas. The less sorbed materials are passed by the sieves and elute from the column as a compact band soon after sample injection. The water vapor is selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elutes continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the less sorbed materials, is proportional to the water content of the sample.

Another specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of hydrogen sulfide or mercaptans in a mixture of hydrogen sulfide or mercaptans with one or more of the following less sorbed materials: methane, ethane, propane, normal butane, isobutane, normal pentane, isopentanes, normal hexane, normal heptane, normal octane, hydrogen, carbon dioxide, and natural gas. The less sorbed materials are passed by the sieves and elute from the column as a compact band soon after sample injection. The sulfur compounds are selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elute continuously to develop an elevated baseline signal the height of which, after passage of the peak due to less sorbed materials, is proportional to the sulfur compound content of the sample.

Another specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of carbon dioxide in a mixture of carbon dioxide with one or more of the following less sorbed materials: methane, ethane, ethylene, propane, butane, pentane, nitrogen, hydrogen and carbon monoxide. The less sorbed materials are passed by the sieves and elute from the column as a compact band soon after sample injection. The carbon dioxide is selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elutes continuously to produce an elevated baseline signal the height of which, after passage of the peak due to the less sorbed materials, is proportional to the carbon dioxide content of the sample.

Still another specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of normal unsaturated C C aliphatic hydrocarbons in a mixture of one or more of such unsaturated hydrocarbons with one or more of the following less sorbed materials: methane, ethane, oxygen, hydrogen and nitrogen. The less sorbed materials are passed by the sieves and elute from the column discontinuously soon after sample injection. The un saturated hydrocarbon is selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elutes continuously to produce an elevated baseline signal the height of which, after passage of the peak due to the less sorbed materials, is proportional to the unsaturated aliphatic hydrocarbon content of the sample.

Still another specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of isobutane in a mixture of isobutane with one or more C -C parafiins. The C C par-afiins, being relatively less sorbed, are passed by the sieves and elute from the column as a compact band soon after sample injection. The isobutane is selectively sorbed by the sieves and, after saturation of the sieves therewith, elutes continuously to produce an elevated baseline signal the height of which, after passage of the peak due to the less sorbed C C paraffins, is proportional to the isobutane content of the sample.

A still further specific embodiment of the invention, using molecular sieves as the column packing material, provides a method for the continuous analysis of mononuclear aromatic hydrocarbons in a mixture of such mononuclear aromatic hydrocarbons with polynuclear aromatic hydrocarbons. The polynuclear aromatics, being relatively less sorbed, are passed by the sieves and elute from the column as a compact band soon after sample injection. The mononuclear aromatics are selectively sorbed by the molecular sieves and, after saturation of the sieves therewith, elute continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the polynuclear aromatics, is proportional to the mononuclear aromatic content of the sample.

Yet another specific embodiment of the invention, using as the column packing material a solid adsorbent such as dehydrated silica gel, activated charcoal, activated alumina, etc., furnishes a method for the continuous determination of mercaptans or amines in a mixture of mercaptans or amines with a parafiinic hydrocarbon fraction. The parafiins, being relatively less adsorbed, are passed by the adsorbent and elute from the column as a compact band soon after sample injection. The polar mercaptans or amines are selectively adsorbed and, after saturation of the adsorbent therewith, elute continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the parafiins, is proportional to the mercaptan or amine content of the sample.

Still another specific embodiment of the invention, using as the column packing material a solid adsorbent such as dehydrated silica gel, activated charcoal, activated alumina, etc., furnishes a method for the continuous determination of olefins in a mixture of olefins and paraffins. The parafiins, being relatively less adsorbed, are passed by the adsorbent and elute from the column as a compact band soon after sample injection. The olefins are selectively adsorbed and, after saturation of the adsorbent therewith, elute continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the paraflins, is proportional to the olefin content of the sample.

Still another specific embodiment of the invention, using as the column packing material a solid adsorbent such as dehydrated silica gel, activated charcoal, activated alumina, etc., provides a method for the continuous determination of aromatic hydrocarbons in a mixture of aromatic hydrocarbons and non-aromatic hydrocarbons. The non-aromatics, being relatively less adsorbed, are passed by the adsorbent and el-ute from the column as a compact band soon after sample injection. The aromatics are selectively adsorbed and, after saturation of the adsorbent therewith, elute continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the non-aromatic hydrocarbons, is proportional to the aromatic hydrocarbon content of the sample.

In still another embodiment of the invention, employing as the column packing material a solid adsorbent such as dehydrated silica gel, activated charcoal, activated alumina, etc., there is provided a method for the continuous determination of water vapor in a mixture of water vapor with air, nitrogen or other inert gas, or paraffinic hydrocarbons. The air, nitrogen, inert gas or parafiinic hydrocarbons, as the case may be, being relatively less adsorbed, are passed by the adsorbent and elute from the column as a compact band soon after sample injection. The water vapor is selectively adsorbed and, after saturation of the adsorbent therewith, elutes continuously to develop an elevated baseline signal the height of which, after passage of the peak due to the less adsorbed materials, is proportional to the water vapor content of the sample.

The invention is specifically described in connection with the accompanying drawings of which:

FIGURE 1 is a schematic diagram of a process stream analyzer embodying the invention.

FIGURE 2 is a schematic diagram of a bridge circuit for deriving an output signal from the apparatus of FIGURE 1.

FIGURE 3 is a plot of a typical output signal developed by the method of this invention.

With reference to FIGURE 1, there is shown in partial section a linear sample valve 10 which comprises outer stator plates 11 and an inner reciprocable plug 16 in fluid-tight contact with plates 11. Stator 11 is provided with ports 12, 13, 14 and 15. Plug 16- is provided with three transverse ports 17, 18 and 19; intermediate port 18 is of somewhat smaller diameter and is the sample or metering port. Plug 16 is driven to either of its two adjusted positions by means of a shaft 20 which is connected to an upper diaphragm operator 21 and also to a lower diaphragm operator 23. Actuating air is alternately supplied (through suitable automatically programmed valving, not shown) through line 22 to operator 21 or through line 24 to operator 23; when one of lines 22 and 24 is under pressure, the other line is simultaneously vented. With operator 23 pressured, plug 16 is moved upwardly into the position shown which is the sample port refill position: ports 12, 18 and 13 are in serial fluid communication, ports 14, 19 and 15 are in serial fluid communication, and port 17 is inoperative. With operator 21 pressured, plug 16 is moved downwardly into the sample injection position: ports 12, 17 and 13 are now in serial fluid communication, ports 14, 18 and 15 are in series fluid communication and port 19 is inoperative. Sample valve 10 is enclosed by an electrically heated, temperature controlled jacket 25 which maintains the valve at an elevated temperature suflicient to preheat, partially vaporize or completely vaporize the sample, as desired. A circulating sample stream of the process fluid to be analyzed is introduced through inlet line 26, filter 27 and line28 to valve 10; it passes through either port 18 or port 17 and is Withdrawn through line 29, flow controller 30 and line 31, from which it may conveniently be returned to a point in the process under lower pressure than the inlet. The volume and length of tubing runs of the sample loop should be kept as small as practicable to minimize transport lag. A source of carrier gas 32 is connected to the analyzer by line 33. The carrier gas is preferably helium, but may also be neon, argon, hydrogen, nitrogen, C or other inert gas differing substantially in thermal characteristics from the components of interest in the sample stream. From line 33 the carrier gas is taken through a flow controller 34, line 35, reference thermal conductivity cell 36 and line 37 to valve it passes through either port 18 or port 19 and then through line 38 to separating solumn 39 which contains a suitable solid sorbent. Vapor eflluent from column 39 flows through a measuring thermal conductivity cell 40 and is then vented through line 41. It will be appreciated that other types of detection devices may be substituted for the thermal conductivity cell, for example, this may be an ionization detector or a beta ray detector. Cells 36 and 40 and column 39 are encased in an electrically heated, temperature controlled detector block 42 which holds the column at a temperature sufliciently high to vaporize all of the components in the sample and, where the sample mixture contains more than one selectively sorbed component or more than one less sorbed component, sufliciently high to prevent any appreciable separation as between the selectively sorbed components and, preferably, also as between the less sorbed components. The sample valve, detector block and carrier gas flow controller are mounted in a temperature controlled housing 43 adapted to provide an interior temperature in the range of 60-120 F.

When valve 10 is inthe sample port refill position, process fluid entering lines 26 and 28 flows through port 18 and is returned through lines 29 and 31 to the process or to a suitable sump or drain. Carrier gas is admitted through lines 33 and 35 to reference cell 36, passes through port 19, line 38, column 39, measuring cell 40, and vent line 41. When valve 10 is in the sample injection position, plug port 18 is moved down to register with stator ports 14 and and the sample trapped therein is swept out by the carrier gas into column 39 and thence to measuring cell 40. Plug port 17 moves down into registry with stator ports 12 and 13 to provide continuity of sample loop flow during the sample injection portion of the operating cycle. Frequency of sample injection may be varied from as much as 10 times per minute to once every ten mniutes depending on the mixture to be analyzed, type of sorbent, length of column, column temperature, etc. The response time of the analyzer itself can frequently be made less than about five seconds after the sample is injected. One sample injection every one-two minutes is usually quite adequate for process monitoring and control applications and is preferred to allow time for operation of auxiliary apparatus such as a peak picker or bridge zeroing servo. Notwithstanding the intermittent nature of sample injections, a continuous output signal with respect to the selectively sorbed component or components is readily obtained.

In FIGURE 2 there is illustrated a suitable two-element bridge circuit 50 for obtaining an output signal. One side of the bridge contains a fixed resistor 51 connected in series with a temperature sensitive resistive filament 56 within reference cell 36. The other side of the bridge contains the series combination of potentiometer 52 (coarse zero adjustment), fixed resistor 53, potentiometer 54 (fine zero adjustment) and a temperaure sensitive resistive filament 55 within measuring cell 40. Both sides of bridge 50 are connected through a voltage divider 57 (attenuation adjustment) across which is connected a potentiometric recorder 58. A constant current source 59 supplies regulated current to bridge 50 through a milliammeter 60. Obviously there are many other functionally equivalent circuits which could be used; for example, a four-element bridge would increase the sensitivity two-fold.

As an example of the operation and response of the invention, apparatus of the above-described construction may be employed to analyze a hydrocarbon stream consisting of about 60i5% by weight of C -C normal paraflins and about 40i5% by weight of C -C nonnormal hydrocarbons (branched chain alkanes, aromatics and naphthenes). The separating column is 5 inches in length x A inch in diameterand contains about 1.9 grams of molecular sieves (a dehydrated calcium aluminosilicate hydrate having a pore diameter of about 5 A.). The volume of the sample injection port of the liner sample valve is 3 microliters. Operating conditions are established as follows:

Sample valve temperature 320-330 F. Detector block temperature 625 6'35 F.

The response of the analyzer is shown in FIGURE 3 in which the recorder output is plotted as a function of time. The abscissa represents electrical zero, which is determined by the measuring bridge circuit parameters and may or may not coincide with chart zero. The arrows marked I indicate successive sample injections. During the initial portion of this run, embracing peaks 61 and 62, each of which correspond to the total non-normal hydrocarbons lumped together, the molecular sieve sorbent is not as yet substantially saturated with normal parafiins, and the normals are therefore occluded by the sorbent and do not emerge from the column. The recorder pen returns to the electrical zero baseline after each of nonnormal peaks 61 and 62 has passed. At point 11, however, the sorbent has finally become saturated with the normal paraflins, which commence to elute continuously thereafter, and the analyzer output increases rapidly. Peaks 63 and 65 each represent the total non-normal hydrocarbon content of the respective sample, but they are now superimposed on an elevated baseline 64. In other words, even though the sample injections occur at rather wide intervals of 60 seconds, the recorder pen does not return to zero but draws a substantially continuous stable line 64, except for the intermittent non-normal peaks. The height of line 64 above electrical zero is proportional to the total normal paraffin content by weight of the sampled stream and, surprisingly, follows small variations of the normal hydrocarbon content of successive samples with high accuracy. The non-normal hydrocarbons elute almost immediately without discrimination and all the normal parafiins elute continuously, also without discrimination.

Mixtures other than hydrocarbons, such as those set forth above, may be analyzed in similar fashion. In all cases a continuous output signal corresponding to the selectively sorbed material is obtained by utilizing a solid sorbent which has been substantially saturated with respect to the selectively sorbed material. Where the analysis mixture comprises more than one selectively sorbed component, the temperature of the separating column is sufficiently elevated so that little or no separation of the selectively sorbed component will occur. And in a preferred embodiment of the invention, when the analysis mixture comprises more than one less sorbed component, the temperature of the separating column is held sufficiently high so that no significant separation of the less sorbed components will occur and these are therefore lumped together in a single peak which elutes soon after sample injection. In this manner a unitary signal is developed which is proportional to the weight concentration of a single component or a group of components which is selectively sorbed by the solid sorbent and there is also developed a unitary peak corresponding to the weight concentration of all the less sorbed components. The elution time per sample is markedly reduced over that provided by conventional chromatographic analyzers, therefore permitting a higher sampling frequency, increased accuracy and reduced dead time. The invention also permits a very high order of accuracy and sensitivity with respect to the selectively sorbed material, because by using larger sample volumes and/or more frequent sample injections, the effective concentration of sorbed material in the separating zone effluent may be multiplied many fold, whereby concentrations of the order of 15 p.p.m. Will produce a detectable output signal without the need for ultrahigh signal amplification.

I claim as my invention:

1. A method for analyzing a mixture of fluid components at least one of which is selectively sorbed by contact with a solid sorbent and at least one other component is relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed component until the sorbent is substantially saturated with respect to the selectively sorbed component, passing a carrier gas stream through a separation zone containing the pretreating sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, passing the effluent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed component in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples.

2. The method of claim 1 wherein said sorbent is silica gel.

3. The method of claim 1 wherein said sorbent is activated charcoal.

4. The method of claim 1 wherein said sorbent is activated alumina.

5. The method of claim 1 wherein said sorbent is a dehydrated metal aluminosilicate hydrate.

6. A method for analyzing a mixture of fluid compounds, at least one of which is selectively sorbed by contact with a solid sorbent consisting of a dehydrated metal aluminosilicate hydrate and at least one other compound is relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed compound until the sorbent is substantially saturated with respect to the selectively sorbed compound, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, passing the effluent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed compound in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples.

7. A method for analyzing a mixture of fluid hydrocarbons, at least one of which is selectively sorbed by contact with a solid sorbent consisting of a dehydrated metal aluminosilicate hydrate and at least one other hydrocarbon is relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed hydrocarbon until the sorbent is substantially saturated with respect to the selectively sorbed hydrocarbon, passing a carrier gas stream through a separation Zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, passing the efliuent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed hydrocarbon in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples.

8. The method of claim 7 wherein said selectively sorbed hydrocarbon is a normal aliphatic hydrocarbon and said less sorbed hydrocarbon is a non-normal hydrocarbon selected from the group consisting of branched chain and cyclic hydrocarbons.

9. The method of claim 7 wherein said selectively sorbed hydrocarbon is a normal paraflin and said less sorbed hydrocarbon is a non-normal hydrocarbon selected from the group consisting of branched chain and cyclic hydrocarbons.

10. The method of claim 9 wherein said normal paraffin contains from 1 to 22 carbon atoms and said nonnormal hydrocarbon contains from 4 to 22 carbon atoms.

11. The method of claim 9 wherein said normal paraffin contains from 4 to 18 carbon atoms and said nonnormal hydrocarbon contains from 4 to 18 carbon atoms.

12. The method of claim 9 wherein said normal paraffin contains from 10 to 16 carbon atoms and said nonnormal hydrocarbon contains from 6 to 16 carbon atoms.

13. A method for analyzing a mixture of fluid compounds, at least one of which is selectively sorbed by contact with a solid sorbent and at least two other compounds are relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed compound until the sorbent is substantially saturated with respect to the selectively sorbed compound, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, the temperature of said zone being sufliciently high so that all of said compounds are maintained in the vapor phase therein and said less sorbed compounds are eluted from the separation zone substantially simultaneously, passing the effluent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed compound in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples, and said baseline signal having superimposed thereon a single peak for each of said samples corresponding to the total less sorbed compounds contained in such sample.

14. A method for analyzing a mixture of fluid hydrocarbons, at least one of which is selectively sorbed by contact with a solid sorbent consisting of a dehydrated metal aluminosilicate hydrate and at least two other hydrocarbons are relatively less sorbed by the sorbent, which comprises pretreating a mass of said sorbent by contact with said selectively sorbed hydrocarbon until the sorbent is substantially saturated with respect to the selectively sorbed hydrocarbon, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, the temperature of said zone being sufiiciently high so that all of said hydrocarbons are maintained in the vapor phase therein and said less sorbed hydrocarbons are eluted from the separation zone substantially simultaneously, passing the effluent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed hydrocarbon in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples, and said baseline signal having superimposed thereon a single peak for each of said samples corresponding to the total less sorbed hydrocarbons contained in such sample.

15. The method of claim 14 wherein said selectively sorbed hydrocarbon is a normal aliphatic hydrocarbon and said less sorbed hydrocarbons are non-normal hydrocarbons selected from the group consisting of branched chain and cyclic hydrocarbons.

16. The method of claim 14 wherein said selectively sorbed hydrocarbon is a normal paraffin and said less sorbed hydrocarbons are non-normal hydrocarbons selected from the group consisting of branched chain and cyclic hydrocarbons.

17. A method for the continuous chromatographic analysis of the total aliphatic hydrocarbon content of a mixture of fluid hydrocarbons containing from 4 to 18 carbon atoms, at least one of which is a normal aliphatic hydrocarbon selectively sorbed by contact with a solid sorbent consisting of a dehydrated metal aluminosilicate hydrate and at least two other hydrocarbons are non-normal hydrocarbons selected from the group consisting of branched chain and cyclic hydrocarbons which are relatively less sorbed by the sorbent, which method comprises pretreating a mass of said sorbent by contact with said normal aliphatic hydrocarbon until the sorbent is substantially saturated with respect to the normal aliphatic hydrocarbon, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, the temperature of said zone being sufli-ciently high so that all of said hydrocarbons are maintained in the vapor phase therein and said nonnormal hydrocarbons are eluted from the separation zone substantially simultaneously, passing the effluent from said zone to a measuring cell, and developing from said cell "a substantially continuous elevated baseline signal, the

magnitude of which is proportional to the concentration of selectively sorbed normal aliphatic hydrocarbon in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples, and said baseline signal having superimposed thereon a single peak for each of said samples corresponding to the total non-normal hydrocarbons contained in such sample.

18. A method for the continuous chromatographic analysis of the total normal paraflin content of a mixture of fluid hydrocarbons, at least one of which is a normal paraflin containing from 10 to 16 carbon atoms and which is selectively sorbed by contact with a solid sorbent consisting of a dehydrated metal aluminosilicate hydrate and at least two other hydrocarbons are non-normal hydrocarbons seleoted from the group consisting of branched chain and cyclic hydrocarbons containing from 6 to 16 carbon atoms and which are relatively less sorbed by the sorbent, which method comprises pretreating a mass of said sorbent by contact with said normal parafiin until the sorbent is substantially saturated with respect to the normal paraffin, passing a carrier gas stream through a separation zone containing the pretreated sorbent, introducing fixed volume samples of said mixture at periodic intervals into the carrier gas stream at a point upstream from the separation zone, the temperature of said zone being sufiiciently high so that all of said hydrocarbons are maintained in the vapor phase therein and said non-normal hydrocarbons are eluted from the separation zone substantially simultaneously, passing the efiluent from said zone to a measuring cell, and developing from said cell a substantially continuous elevated baseline signal, the magnitude of which is proportional to the concentration of selectively sorbed normal parafiin in said sampled mixture and which responds to changes in said concentration as between a plurality of the resulting series of samples, and said baseline signal having superimposed thereon a single peak for each of said samples corresponding to the total non-normal hydrocarbons contained in such sample.

19. The method of claim 18 wherein the temperature of the separation zone is above about 600 F.

References Cited UNITED STATES PATENTS 5/1961 Boeke 73-23.1

JAMES J. GILL, Acting Primary Examiner. RICHARD C. QUIESSER, Examiner,

Claims (1)

1. A METHOD FOR ANALYZING A MIXTURE OF FLUID COMPONENTS AT LEAST ONE OF WHICH IS SELECTIVELY SORBED BY CONTACT WITH A SOLID SORBENT AND AT LEAST ONE OTHER COMPONENT IS RELATIVELY LESS SORBED BY THE SORBENT, WHICH COMPRISES PRETREATING A MASS OF SAID SORBENT BY CONTACT WITH SAID SELECTIVELY SORBED COMPONENT UNTIL THE SORBENT IS SUBSTANTIALLY SATURATED WITH RESPECT TO THE SELECTIVELY SORBED COMPONENT, PASSING A CARRIER GAS STREAM THROUGH A SEPARATION ZONE CONTAINING THE PRETREATING SORBENT, INTRODUCING FIXED VOLUME SAMPLES OF SAID MIXTURE AT PERIODIC INTERVALS INTO THE CARRIER GAS STREAM AT A POINT UPSTREAM FROM THE SEPARATION ZONE, PASSING THE EFFLUENT FROM SAID ZONE TO A MEASURING CELL, AND DEVELOPING FROM SAID CELL A SUBSTANTIALLY CONTINUOUS ELEVATED BASELINE SIGNAL, THE MAGNITUDE OF WHICH IS PROPORTIONAL TO THE CONCENTRATION OF SELECTIVELY SORBED COMPONENT IN SAID SAMPLED MIXTURE AND WHICH RESPONDS TO CHANGES IN SAID CONCENTRATION AS BETWEEN A PLURALITY OF THE RESULTING SERIES OF SAMPLES.

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