Method and apparatus for analysis of fluid mixtures

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for the analysis of fluid mixtures, and more particularly to the analysis of hj drocarbon mixtures.

In laboratory and industrial applications it is frequently necessary to analyze a fluid mixture to determine the concentration of its constituents. This is of particular importance in connection with computer control and optimization of fluid processes in the petroleum-refining industry where the economy of various processes is often predicated upon accurate information of the composition of fluid mixtures undergoing process treatment or the concentration therein of selected fluid components.

Certain fluid mixtures, particularly the hydrocarbons, are difficult to analyze or separate due to the similarity of the physical properties of the fluid components. Thus, the components of the fluid mixture may have similar boiling points, similar adsorption characteristics, or they may be composed of similar nonpolar molecules. Such similarities render the analysis or separation of these fluid mixtures by presently known physical methods, such as chromatography or fractionation, extremely difficult.

An example of a fluid mixture difficult to analyze is found in connection with the catalytic reforming process for the octane improvement of fuels. The process charge stream usually comprises a mixture of aromatics, naphthenes, and paraffins. Both the naphthenes and the paraffins may be distinguished from the aromatics by chromatographic analysis. But the naphthenes cannot be easily distinguished from the paraffins due to the similar physical properties of these components. One method by which such a mixture may be analyzed is by use of a mass spectrometer. However, this technique does not lend itself to online continuous process use in its present state of development. Also, the use of prior art chromatographic analysis techniques to analyze the fluid mixture are unsatisfactory due to the inability of this method to distinguish between certain fluid components having similar physical properties.

For the purpose of computer or automatic control of the catalytic reforming process, it is important to continuously analyze the charge stream for its naphthene content since the naphthenes are the main reactants in the process. The naphthene content of the charge stream is therefore a key variable of the process. Hence, if information thereof is continuously made available to a computer control loop controlling other variables of the process, the process may be optimized, improving its economy and the quality of the product.

The invention as herein disclosed provides a solution to the aforementioned problems by a unique and novel method and apparatus for the continuous analysis of fluid mixtures suitable for many process control applications.

SUMMARY

Briefly stated a preferred aspect of the invention provides a method for continually monitoring a component of interest of a fluid mixture having physical properties sufficiently similar to those of other components of the fluid mixture rendering it difficult to separate or distinguish the fluid component of interest therefrom. The method includes reacting a sample of the fluid mixture in a chemical reaction which affects at least one component of the fluid mixture so as to alter at least in part the chemical structure and at least one physical property thereof rendering the fluid component of interest distinguishable from the fluid mixture, and detecting the fluid component of interest. One version of the method includes the step of separating at least a portion of the fluid component of interest from the mixture and then the fluid component of interest thus separated is detected. In one aspect of the method for determining the concentration of the fluid component of 60 interest in a fluid mixture which includes one distinguishable component the reaction converts at least one component of the mixture to the distinguishable component, the separating step includes separating the distinguishable component from the mixture subsequent to the reacting step, and the method includes the further steps of detecting the distinguishable component in an unreacted sample of the fluid mixture and generating a signal representative of the concentration in the mixture of the fluid component of interest in accordance with a predetermined relationship relating the detected distinguishable component of the unreacted sample and the reacted sample, with the concentration of the fluid component of interest in the fluid mixture. In a further aspect the method is adapted to monitor the naphthene content of a hydrocarbon mixture comprising paraffin, aromatic, and naphthene components.

Another aspect of the invention provides apparatus in novel combination for continually monitoring a component of a fluid mixture including a microreactor for subjecting a sample of the fluid mixture to the aforementioned chemical reaction, and detection means for detecting the fluid component of interest. Also included are valve and conduit means for transmission of the fluid mixture through the apparatus. In one version of the apparatus the detection means include a chromatograph column and a detector. In a preferred embodiment of the apparatus for monitoring the naphthene content of a hydrocarbon mixture heating means are also provided for maintaining suitable operating temperatures in the microreactor, a noble metal reforming catalyst is employed in the microreactor for converting naphthenes of the fluid mixture to aromatics, and ft.ft'-thiodipropionitrile or other suitable polar substrate is employed in the chromatograph column for separating the aromatic component from the paraffin and naphthene components of the fluid mixture. In a further embodiment a unique analog computer is provided coupled with the detector for computing the concentration of the fluid component of interest in the fluid mixture.

In view of the foregoing it is an object of the invention to provide an improved method for the analysis of fluid mixtures.

Another object of the invention is to provide a method for detection of a component of a fluid mixture difficult to distinguish from the mixture by employing a chemical reaction producing identifiable components.

Another object of the invention is to provide a method for determining the concentration in a fluid mixture of a component difficult to distinguish from the mixture.

Another object of the invention is to provide a method for monitoring the naphthene content of a fluid mixture of hydrocarbons.

Another object of the invention is to provide embodiments of apparatus to fulfill the aforementioned objectives.

These and other objects, advantages and features of the invention will be more fully understood by referring to the following descriptions and claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating an embodiment of apparatus for practicing the invention.

FIG. 2 is a schematic block diagram of a computer which may be used in conjunction with the apparatus of FIG. 1 to compute the concentration of the fluid component of interest in the fluid mixture tested.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fluid mixture being tested is introduced into the apparatus through a conduit 10 from a source S,. Typically, this fluid mixture can be associated with one of many hydrocarbon refining processes. Thus, for example, the source S_t can be a sample stream of the fresh feed of a catalytic-reforming process for the octane improvement of fuels which usually includes a mixture having components of naphthenes, aromatics, and paraffins. It is to be understood that as used herein the term fluid component shall signify any group of one or

3,607,073 more similar chemical compounds such as, for example, a component of naphthenes, a component of paraffins, etc.

The fluid mixture being tested passes through the conduit 10 and enters an electrically operated selection valve 11. For calibrating the device a calibration standard fluid mixture of known composition is provided from a source S₂ which is coupled with the selection valve 11, by a conduit 12, connected therewith. The selection valve 11, when in its first operative position, permits passage therethrough of the fluid mixture being tested, and when in its second operative position, permits passage therethrough of the calibration standard. Coupled with the outlet port of the selection valve 11 is a conduit 14 which is in turn connected to the inlet port of a samplemeasuring valve 15 which incorporates an outlet port through which flow measured samples of the fluid mixture being tested. The sample-measuring valve 15 also incorporates a vent port in fluid communication with its inlet port for uninterrupted flow of the fluid mixture therethrough and out of its vent port. The latter port is connected by a conduit 16 to a second sample measuring valve 17 which also incorporates a measured sample outlet port and a vent port, the latter port being connected to a vent conduit 18 for flow of the fluid mixture out of the apparatus. In this configuration either the fluid mixture being tested or the calibration standard is present at all times in the bodies of the sample measuring valves 15 and 17, the respective fluid flowing out of the apparatus through the vent conduit 18.

The sample measuring valves 15 and 17 are conventional constant volume liquid-sampling valves which when actuated measure and release to their respective sample outlet ports single samples of predetermined amounts of the fluid mixture present therein. Sample valves providing samples ranging from 1 to 50 microliters can be used in the apparatus. However, for the analysis of napthhenes it is preferred to use sampling valves which provide liquid samples of about 10 microliters.

Connected with the outlet port oi the sample measuring valve 15 is a conduit 19 which is in turn connected with a vaporizer 20 where the samples are vaporized in the presence of a suitable carrier gas such as hydrogen which is introduced into the vaporizer 20 through a conduit 21 connected with a source of the gas S₃. Generally, many carrier gases can be used provided the gas is of a composition which does not adversely affect to a substantial extent the chemical reaction to which the fluid mixture is subjected. However, the use of hydrogen is preferred in the analysis of the catalytic reforming fresh feed to help prevent coking of the reforming catalyst used in the microreactor discussed below. The vaporized mixture passes from the vaporizer 20 through a conduit 22 connected therewith and enters a microreactor 23. The microreactor 23 includes a section of tubing packed with a suitable catalyst to effect a chemical reaction of the fluid samples converting a major portion of the fluid component of interest thereof into a component distinguishable or separable from the fluid mixture. For the analysis of naphthenes in fluid mixtures comprising at least naphthenes and aromatics, or naphthenes and paraffins, the microreactor 23 is preferably comprised of about 5 feet of about 3/16-inch diameter steel tubing filled with 35-80 mesh platinum-reforming catalyst. To further stimulate the reaction of microreactor is in turn mounted within a temperature-controlled enclosure, such as a regulated oven 24, which maintains the microreactor at an elevated temperature. The choice of this temperature and the operating pressure of the apparatus depend upon the fluid mixture being analyzed and are generally not critical. However, these operating conditions should be chosen such that the desired chemical reaction will take place and is most favored thermodynamically. For the analysis of naphthenes the preferred value of the microreactor temperature is approximately in the range of about 200° to 400° C. when the pressure therein is maintained in the vicinity of 45 p.s.i.g. In this case, a major portion of the naphthene component of the samples is converted to aromatics which can be separated from the paraffin and other dissimilar components. Thus, the increase in the aromatic component is indicative of the naphthene content of the samples.

Having thus reacted the samples in a manner rendering separable or distinguishable the fluid component of interest, its concentration in the samples can be determined by separating, or detecting, the fluid component of interest in any of a number of ways, such as by absorption spectroscopy or gas chromatography. For the analysis of naphthenes it is preferred that the detecting be performed with a gas chromatograph column employing a suitable polar substrate.

For this purpose a chromatograph column 25 and an electrically operated conventional backflush valve 26 are provided. The outlet flow of the microreactor 23 passes through a conduit 27 connected with a first port 26a of the backflush valve 26 which incorporates five further ports, 26b through 26/, respectively, to enable forward or reverse flow through the chromatograph column 25. The fifth port 26e of the backflush valve is connected with a vaporizer 28 by a conduit 29. The vaporizer 28 is in turn connected with the inlet end of the chromatograph column 25 by a conduit 30. The third port 26c is connected with the exit end of the chromatograph column by a conduit 31; the fourth port 26d is connected with an outlet conduit 32; and the second and sixth ports 26b and 26/ respectively, are externally connected to each other by a loop conduit 33.

When the backflush valve 26 is in its first operative position its first port is in fluid communication with its second port, its third port is in fluid communication with its fourth port and its fifth port is in fluid communication with its sixth port. With the valve in this operative position the fluid mixture including the carrier gas from the microreactor entering the backflush valve through its first port passes therethrough and out of its second port, reenters the valve through its sixth port, passes out of the valve through its fifth port, passes through the conduit 29, through the vaporizer 28 and and thence passes through the chromatograph column 25 in a forward direction. The fluid from the chromatograph column passing through the conduit 31 reenters the backflush valve through its third port, and exits therefrom through its fourth port, and passes through the outlet conduit 32.

When the backflush valve 26 is in its second operative position its first port is in fluid communication with its sixth port, its second port is in fluid communication with its third port, and its fourth port is in fluid communication with its fifth port. When the backflush valve is in this operative position the chromatograph column is backflushed by a flow of the carrier gas in the conduit 27 entering the backflush valve through its first port and exiting through its sixth port, reentering the valve through its second port, exiting through its third port and thence passing through the conduit 31, the chromatograph column 25 and the vaporizer 28 in a reverse direction, reentering the backflush valve through its fifth port and exiting therefrom through its fourth port to the outlet conduit 32. The outlet conduit 32 is in turn connected with a pressure regulator 34, for regulating the pressure in the apparatus. For the analysis of naphthenes the preferred pressure is in the range of 30 to 60 p.s.i.g. The pressure regulator is in turn connected with a suitable gas chromatography detector, such as a thermal conductivity detector 35, for detecting the effluent of the chromatograph column. The electrical signals developed by the detector 35 are transmitted to a computer 38 which is further discussed in reference to FIG. 2.

For the analysis of naphthenes in hydrocarbon mixtures the preferred configuration of the chromatograph column 25 comprises a length of about 10 feet of about %-inch diameter steel tubing packed with beta, beta'-thiodipropionitrile deposited by conventional techniques on 60-80 mesh support, such as Chromosorb P. In this configuration the column holds up the aromatic component of the samples while permitting the saturates and other components to pass through first. Then, when the column is backflushed the held-up aromatics are detected as a group passing from the chromatograph

3,607,073 column in a reverse direction to the detector 35; the area under the curve defining the detected thermal conductivity response pulse occurring during backflushing of the aromatics being proportional to the aromatics content of the respective fluid samples.

The analysis of the foregoing fluid mixture involves converting .he naphthenes to aromatics. Should the fluid mixture initially contain an appreciable amount of aromatics it is necessary to provide a sampling by the chromatograph column of the fluid mixture in the unreacted condition. Therefore, for the analysis of those fluid mixtures wherein one component is converted to another by the microreactor, it is necessary to provide the capability in the apparatus for direct analytical sampling of the unreacted fluid mixture. Hence, a flow of samples of the fluid mixture is provided from the sample measuring valve 17 through the vaporizer 28, connected with the sample-measuring valve 17 by a conduit 36, and from the outlet of the vaporizer 28 to the conduit 30 at the inlet end of the chromatograph column 25. The carrier gas is introduced into the vaporizer 28 through the conduit 29 connected with the backflush valve 26, which receives a flow of the carrier gas from the source S₃ passing through the vaporizer 20 and the microreactor 24. The hydrogen source S₃ is connected with the detector 35 through a conduit 39 to provide a reference against which the thermal conductivity response of the chromatograph column effluent is measured by the detector 35.

The calibration characteristic, that is, the predetermined relationship between the thermal conductivity response measured by the detector 35, and the concentration of the fluid component of interest in the fluid mixture tested may be determined analytically. However, such a procedure is extremely difficult since all of the fluid component of interest may not be converted by the microreactor. Hence, a more suitable procedure is the use of a calibration standard fluid mixture of well-known composition having a concentration of the fluid component of interest similar to, or in the expected range of, the concentration of the fluid component of interest in the fluid mixture being tested. The calibration standard is introduced from a source thereof S₂, through the conduit 12, through the selection valve 11, which when in its second operative position permits a flow of the calibration standard into the sample measuring valve 15, and thence into the balance of the apparatus.

The calibration characteristic of the apparatus is initially determined by two runs of the standard therethrough. In the first run the standard passes through the microreactor and through the chromatograph column for analysis. In the second run the calibration mixture is introduced directly into the chromatograph column for analysis. Once the first calibration is determined, subsequent, or periodic calibrations need be performed only by passing the calibration standard through the microreactor since any subsequent changes in the operating characteristics of the apparatus take place primarily in the microreactor due to changes in the catalyst activity with age.

A time-cycle controller 37, which includes conventional program-timing elements, is provided to control the actuation of the various electrically operated valves through an appropriate timing sequence. Generally, the timing sequence is not critical provided that there is sufficient residence time of the samples in the microreactor and the chromatograph column.

For the analysis of naphthenes in the fresh feed of catalytic reforming processes it is preferred that the time cycle controller 37 provide for the following functions: the injection of a first sample of the fluid mixture being tested into the microreactor for reaction therein, permitting the sample to pass to the chromatograph column and reside therein for a sufficient time interval for adsorption of the aromatics and elution of the saturates content of the sample in the forward direction, thereafter, reversing the flow through the chromatograph column to backflush the aromatics content of the sample and detecting the aromatics content thereof. Subsequently, a second sample of the fluid mixture is injected directly into the chromatograph column and the aromatics detection sequence above is repeated. For the aforementioned analysis of naphthenes a preferred timing sequence is as follows: 1) for all normal operation, that is, absent calibration, the selection valve 11 is kept in its first operative position; 2) the sample measuring valve 15 is actuated for about 30 to 120 seconds while a single sample of the fluid mixture is injected into the microreactor; 3) while the sample is reacted and passes through the backflush valve into the chromatograph column, the backflush valve is maintained in its first operative position for a period of about 5 to 15 minutes and then is actuated to its second operative position permitting backflushing of the chromatograph column; 4) the backflush valve then remains in its second operative position for a period of about 7 to 20 minutes while the aromatics pass therethrough and are detected as a group; 5) the backflush valve is then returned to its first operative position; 6) the sample-measuring valve 17 is then actuated for about 30 to 120 seconds to release a single sample of the fluid mixture, which bypasses the microreactor entering into the chromatograph column; 7) the backflush valve 25 remains in its first operative position for a period of about 5 to 15 minutes permitting forward flow through the chromatograph column and elution of the saturates; 8) then the backflush valve 25 is actuated to its second operative position permitting reverse flow through the chromatograph column and detection of the aromatics as a group; 9) then the backflush valve 25 is returned to its first operative position, and the operating cycle connecting with step number 2 above is repeated. For the initial calibration of the apparatus the selection valve 11 is actuated to its second operative position and the aforementioned operating cycle commencing with step number 2 is executed with respect to the calibration standard fluid mixture. The periodic calibration checks, for checking the catalyst aging, may be performed as often as once a day or once a week depending upon the severity of use of the apparatus. For this calibration check the sequence of steps numbers 2 through 5 are executed while the selection valve 11 is maintained in its second operative position. After the calibration check the selection valve 11 is returned to its first operative position and the normal sequence of operation is reinstated.

The output signals from the detector 35 are transmitted to the computer 38, in which is programmed the predetermined calibration relationship between the detected signals and the concentration in the fluid mixture tested of the fluid component of interest. The computer 38 also is responsive to the time-cycle controller 37 so that is can interpret each of the thermal conductivity signals generated by the detector 35 in proper order. I have found that the following equation expresses the aforementioned predetermined relationship, based upon the results of the calibration run, when a first component of interest of the fluid mixture is analyzed for which is converted to a second component of the fluid mixture by the chemical reaction:

Υ^ΝΧζ-ΧιΊΚΖι-Ζι) (1 ) where:

.¾ = percent of the second component in the fluid mixture being tested after the reaction,

Λι = percent of the second component in the fluid mixture being tested before the reaction, z₂ = percent of the second component in the calibration standard mixture after the reaction,

Zi = percent of the second component in the calibration standard mixture before the reaction, ⁼ percent of the fluid component of interest in the calibration standard mixture as determined independently such as by mass spectrometetry, y = percent of the fluid component of interest in the fluid mixture being tested.

It should be noted that the time cycle indicated above is merely the suggested time cycle for the analysis of naphthenes and appropriate adjustments may be made with corresponding adjustments of the length of the microreactor and the chro3,607,073 matograph column. Furthermore, similar adjustments can be made for the analysis of other fluid mixtures. Thus, for example, increasing the length of the microreactor or of the chromatograph column results in an increased residence time of the fluid mixture in these respective items, whereby, the tim- 5 ing sequence should be modified accordingly. It is also to be noted while the various automatic valves have been described as electrically operated that pneumatically operated valves may be used in their place. In this instance the time cycle controller would be coupled to conventional electrical-to-pneu- 1® matic valve operators. Also to be noted is that various of the conduits illustrated in FIG. 1 may be eliminated by joining some of the equipment items. Thus, for example, the samplemeasuring valves and the vaporizers may be combined into a $ single injector assembly. It is also to be noted that since the device is calibrated as discussed above the reaction in the microreactor need not be complete, that is, the reaction need not affect the entire amount in the fluid mixture of the component reacted. Also, the reaction may affect more than one 2θ component in the fluid mixture. Thus, for example, in a mixture containing naphthenes, paraffins, and aromatics, a portion of the paraffins may be reformed while a portion of the naphthenes are converted to aromatics. The device, after calibration, is nonetheless able to solve for the naphthene con- 25 tent of the mixture. Similarly, the device may be used to solve for a component of the mixture other than the one reacted. For example, in analyzing a mixture of naphthenes and paraffins to determine the paraffin content, the device can be calibrated to account for the reaction of the naphthenes and 30 the paraffin component may be solved for inferentially. It is also to be appreciated by one skilled in the art that the device can be adapted to analyze a variety of fluid mixtures by appropriate modifications of the catalyst used in the microreactor and the packing material of the chromatograph column. 35 Thus, for example, a noble metal may be used as a catalyst in conjunction with a silicone rubber as substrate material for the analysis of mercaptans as hydrogen sulfide in mixtures of hydrocarbons. Another example is the identification of olefins and/or aromatics in a hydrocarbon mixture by hydrogenation 40 of said olefins and/or aromatics.

Referring now to FIG. 2 which is a schematic block diagram of a computer which can be used as the computer shown in FIG. 1, the signals from the detector 35 are transmitted to an integrator 50 which also receives timing signals from the time 45 cycle controller 37 so that the integrator is able to distinguish, by time displacement, the electrical pulses received from the detector during the timed sequence of operation. The integrator 50 integrates the area under each of the thermal conductivity response pulses and provides four corresponding output signals, namely, x₂, X;, z₂, and Zi, as defined in reference to equation (1) above. The x₂ and xl signals are transmitted to a subtraction element 51 which subtracts the latter, x,, from the former, x₂, and provides an output signal corresponding to this $$ difference, namely, x₂—x_t. This signal is in turn transmitted to a multiplication operator 52. The z₂ and Zi signals from the integrator are transmitted to a subtraction element 53 which subtracts the latter, Zi, from the former, z₂, and provides an output signal corresponding to z₂—Zi- This signal is in turn ¢9 transmitted to a division element 54. For developing a signal corresponding to the percent of the fluid component of interest in the calibration mixture a standard voltage supply 55 is provided for applying a constant voltage to a potentiometer 56 which is manually set to a position corresponding to the 65 value Y,. The K, signal, thus developed, is transmitted to the multiplication element 52, which multiplies the Yi signal by the difference signal from the subtraction element 51, and provides an output signal corresponding to the product, namely, Y ι(λ₂—Xi). This signal is transmitted to the division opera- 70 tor 54 which divides this signal by the difference signal from the subtraction element 53, and provides an output signal corresponding to the quotient. The output signal from the division element 54 therefore corresponds to the concentration of the fluid component of interest in the fluid mixture being 75 tested in accordance with equation (1). This signal can be transmitted to a display device or a chart recorder and can be utilized to control the process by application thereof to suitable process control equipment.

It is to be appreciated by one skilled in the art that while electrical computation elements have been discussed above, pneumatic computation elements can be used in their place quite advantageously. It is also to be appreciated by one skilled in the art that a digital computer can be utilized to perform the computational functions of the analog computer of FIG. 2. In this instance, in place of the time-cycle controller, the computer can be preprogrammed to control all the valveswitching functions, including the calibration cycles, and the computer can synchronize the integration steps with the operating sequence.

While the invention has been described with a certain degree of particularity, it can, nevertheless be seen by the examples hereinabove set forth that many modifications and variations of the invention may be made without departing from the spirit and scope thereof.