O-FID analytical equipment and method

Abstract: An anlytical equipment and method for the selective detection of individual oxygenated compounds in complex organic mixtures, in particular fuels, comprising, downstream an equipment of chromatographic separation, a cracking reactor, a catalytic hydrogenation microreactor and a FID detector. The cracking reactor is provided with a catalyst under the form of metal elements, made of palladium, inserted inside the capillary tube though which flows the gas under test. Thanks to the present equipment, the analytical process, based on the specific response of oxygenates to the method of flame ionization detection (O-FID), foresees that the cracking reaction takes place at temperatures below 1000 degrees C and, consequently allows to keep the cracking reactor at stand-by temperatures below 700 degrees C.

Description: The present invention relates to an analytical equipment and method for the selective detection of individual oxygenated compounds in complex organic mixtures, specially for fuels of the recent type that use ogygenated compounds, instead of lead, to increase their antiknock value (octane rating). The already known analytical equipments of the type considered in the present invention are based on the specific response of oxygenates substances to the methods of flame ionization (O-FID) detection and comprise, downstream an equipment of chromatographic separation, a cracking reactor constituted by a capillary tube through which the gas under test flows, said tube being made of a platinum and rhodium alloy that is heated to high temperatures as well as, downstream, a catalytic hydrogenation microreactor and a flame ionization detector (FID). In the cracking reactor, which is directly connected with the capillary column of chromatographic separation, the cracking reaction takes place, during which the individual oxygenated compounds, selectively eluted in sequence by the capillary column, are transformed into carbon monoxide according to the known reaction: C n H m O x → x CO + m/2 H 2 + (n-x)C while hydrocarbons constituting the organic matrix are converted into carbon and hydrogen. As known (see Werner Schneider et al. "Journal of Chromatography" 245, 1982, 71-83; G.R.Verga and A. Sironi "Journal of High Resolution Chromatography & Chromatography Communications" Vol. 11, March 83, pages 248-252) said reaction takes place, in the equipments of the prior art, at temperatures of 1100-1300 degrees C, since the presence of a carbon layer (under the form of graphite) on the internal walls of the platinum/rhodium capillary tube is required for the correct performance of each analysis. The absence of said arrangement could involve, besides the already mentioned reaction, other reactions that would generate compounds different from the desired ones (for instance CO 2 instead of CO) and, consequently errors or in any case lack of analytical precision. In known equipments the carbon layer on the walls of the platinum/rhodium capillary tube is obtained by continuously enriching the carrier gas with some parts per million (ppm) of hydrocarbon, by means of an appropriate doping system, or by injecting into the equipment a suitable quantity of hydrocarbon before each analytical cycle. Furthermore, in the time intervals between a series of analyses and the subsequent one, the cracking reactor is never brought to room temperature but kept at temperatures not going below 700 degrees C. This precaution is necessary in order to avoid long idle times between one analysis and the following one and to avoid considerable sudden changes of temperature in the cracking reactor that would decrease the operating life of the capillary tube. High operating and stand-by temperatures of the cracking reactor as well as the need of introducing a carbon excess cause a gradual deterioration of the platinum/rhodium capillary tube, that therefore must be replaced every 1000 hours of operation on the average: the replacement of the capillary tube involves dead times and outstanding costs, mainly because of the high cost of rhodium. An object of the present invention is now to provide an analytical equipment of the afore defined type, for the selective detection of individual oxygenated compounds, that provides a higher average operating life of said capillary tube of the cracking reactor and/or a consequent operating cost that is lower than that of the equipments of the prior art. This object is achieved by means of the present invention that concerns an analytical equipment for the selective detection of individual oxygenated compounds in complex organic mixtures, in particular fuels, of the type comprising, downstream an equipment of chromatographic separation, a cracking reactor consisting of a capillary tube through which the gas under test flows, said tube being made of a platinum/rhodium alloy and heated to high temperatures, a microreactor of catalytic hydrogenation and a flame ionization detector (FID), characterized in that said capillary tube contains one or more metal catalysts, in particular made of palladium and/or its alloys. The presence of said one or more catalysts can be obtained in different ways, for example and with no restricting purposes, palladium may be present as thin layer on the internal surface of the platinum/rhodium capillary tube, or may be introduced into said tube under the form of small particles, powders, sponges, wire gauzes and filaments. Therefore using said palladium-based catalysts it is possible to considerably reduce the operating and stand-by temperatures of the cracking reactor, obtaining as a consequence an increase of the average operating life of the capillary tube, corresponding to about 5 times the average duration obtainable according to the known technique. Another advantageous feature of the present invention, related to the first one, concerns the reduction of rhodium content in the alloy forming said capillary tube, which involves a significant decrease in the operating costs of the analytical equipment: the rhodium percentage in the alloy constituting the capillary tube may be reduced up to actually zero. A third advantageous feature concerns the fact that, during the stage of activation of the palladium catalysts, it is possible to incorporate carbon into the atomic lattice of palladium itself. In this way the use of the continuous hydrocarbon doping system results unnecessary, since the required carbon amount is provided by the very samples to be analyzed and is properly retained by the palladium catalysts. The invention also concerns an analytical method that, by means of an equipment such as the one described hereinabove, foresees the introduction of the sample to be analyzed into the equipment of chromatographic separation through an injector of the splitting type, and its subsequent passage into a separating capillary column under the action of a carrier gas (for instance N 2 ). The individual oxygenated compounds, selectively eluted by the separation capillary column, find themselves, each in a different moment, in the cracking reactor where the homonymous already mentioned reaction takes place. According to an innovative aspect of the present invention, the cracking reaction, in the equipment presented herein, takes place at temperatures below 1000 degrees C and, more precisely, at a temperature of approximately 800 degrees C. Consequently, also the stand-by temperatures of the cracking reactor can be kept below 700 degrees C and, more precisely, at a temperature of approximately 500 degrees C. The present invention will be now described, for illustrative and non limiting purposes, with reference to the accompanying drawings, wherein: figure 1 is a schematic drawing of an O-FID analytical equipment according to a possible embodiment; figure 2 is a partial sectional longitudin al view of a cracking reactor according to a possible embodiment; figure 3 is a sectional longitudinal view of the device that, according to a possible embodiment, incorporates the catalytic hydrogenation microreactor and the FID detector; and figure 4 is a magnified cross- sectional view of the capillary tube of the cracking reactor according to a possible embodiment of the invention. With reference to figure 1, the analytical equipment illustrated therein is essentially constituted by an equipment of chromatographic separation, downstream which there are provided a cracking reactor 6 and a device 8 that, in the specific configuration shown, comprises a reactor of catalytic hydrogenation and a FID detector. The equipment of chromatographic separation comprises a line 25 for air feeding directly connected with said device 8, a line 26 for carrier gas (e.g. N 2 ) feeding, along which there is provided a series of filters 1, 2, 3 to purify carrier gas from possibile oxygen compounds (for example, CO, CO 2 , O 2 , H 2 O) connected with an injector 4 of the splitting type, a line 27 for auxiliary gas feeding (e.g. H 2 ), on which there is provided a filter 7 (for O 2 and H 2 O) connected with the device 8 and with the line 26 upstream the injector of the splitting type 4, and a separation capillary column 5. With reference to figure 2, the cracking reactor 6 is enclosed in a container having side walls 14 made of marinite or other similar material, closed on the top by a couple of metal plates 12a, 12b and on the bottom by another couple of metal plates 13a, 13b; the metal plates 12a, 13a are electrically connected to one another and both to an electrode 16a projecting from the upper metal plate 12a, as well as plates 12b, 13b are electrically connected to one another and both to an electrode 16b projecting from the upper metal plate 12b. Each of the lower metal plates 13a and 13b, separated by electrically insulating means 17, presents a connector 18, linking the cracking reactor to the separation capillary column 5 and to the catalytic hydrogenation microreactor, incorporated, according to a particular embodiment, in the device 8 together with the FID detector, the sealing being ensured by gaskets 15 generally made of graphite. Inside, the cracking reactor 6 essentially comprises a capillary tube 9 heated to high temperature obtainable, for instance, by Joule effect through low voltage electric power supplied to the electrodes 16a and 16b, a thermocouple 10 being provided to sense a signal depending on the temperature, and means 11 electically and thermally insulate the capillary tube 9 from the external environment. Figure 3 shows a longitudinal sectional view of the device 8 that, according to an embodiment of the invention, incorporates a catalytic hydrogenation microreactor and a FID detector. Said catalytic hydrogenation microreactor essentially consists of a short capillary tube 21 of the PLOT type made of glass, lined with a inner layer of aluminium oxide in which nickel is adsorbed as catalyzing element and heated by means 22 to obtain the temperatures required for the hydrogenation reaction of the carbon monoxide, the latter being supplied by the cracking reactor 6 through the connector 23. Said FID detector consists of a nozzle 19 directly connected to said hydrogenation microreactor, sealing being ensured by gaskets 20 generally made of graphite. Figure 4 shows a cross sectional view of the capillary tube 9, inside which there are inserted, according to one of the possible embodiments of the invention, metal filaments of palladium 24 braided together. After the replacement of the platinum/rhodium capillary tube 9 or eventually after a prolonged off period of the analytical equipment, it is or may be necessary to perform the activation of the palladium catalyzing elements 24 by means of a series of subsequent heating and cooling cycles in hydrogen atmosphere and, then, the hot saturation with hydrocarbon for carbon incorporation into the palladium atomic lattice. During the use of the analytical equipment, the necessary carbon amount incorporated in palladium is supplied with the injection of the very samples to be analysed, thus allowing to avoid the presence of a doping system needed in the equipments of the known technique. The presence of a catalyzing agent, under the form of said metal elements in palladium, inside the capillary tube 9 made of a platinum and rhodium alloy, makes a high rhodium content in the alloy superfluous, since average operating temperatures are 800 degrees C. Furthermore, according to an advantageous feature of the present invention, the capillary tube 9 can even be realized without any rhodium content. According to a further advantagenous feature of the present invention, the presence of said catalyzing elements 24 made of palladium allows a significant reduction of the operating temperatures in the cracking reactor 6, thus extending the average operating life of the capillary tube 9 and consequently decreasing the operating costs of the analytical equipment, moreover allowing to avoid the presence of a continuous hydrocarbon doping system.

Claims: An analytical equipment for selective detection of individual oxygenated compounds in complex organic mixtures, in particular fuels, of the type comprising, downstream an equipment of chromatographic separation: a cracking reactor consisting of a capillary tube through which the gas under test flows, said tube being made of a platinum and rhodium alloy and heated to high temperatures, a catalytic hydrogenation microreactor and a flame ionization detector (FID), characterized in that said capillary tube contains one or more catalyzing metal elements. An analytical equipment according to claim 1, characterized in that said catalyzing metal element or elements are made of palladium and/or its alloys. An analytical equipment according to claim 2, characterized in that said catalyzing palladium element or elements are present under the form of a layer on the internal surface of said capillary tube. An analytical equipment according to claim 2, characterized in that said catalyzing palladium element or elements are introduced into said capillary tube under the form of particles, powders, sponges, wire gauzes or filaments. An analytical equipment according to one of the preceding claims, characterized in that said capillary tube is made of a platinum and rhodium alloy with low rhodium content. An analytical equipment according to claim 5, characterized in that the rhodium percentage is nil. An analytical equipment according to one of the preceding claims, characterized in that carbon is incorporated into the catalyzing metal elements made of palladium. An analytical method for the selective detection of individual oxygenated compounds in complex organic mixtures, based on the specific response of oxygenates to the method of flame ionization detection (O-FID), by using an equipment of the type described in at least one of the preceding claims, characterized in that, during the reaction, the cracking reactor is kept at a temperature below 1000 degrees C. An analytical method according to claim 8, characterized in that, during the reaction, the cracking reactor is kept at a temperature around 800 degrees C. An analytical method according to claim 8, characterized in that, during the stand-by time between an analysis and the subsequent one, the cracking reactor is kept at a temperature below 700 degrees C. An analytical method according to claim 10, characterized in that, during the stand-by time between an analysis and the subsequent one, the cracking reactor is kept at a temperature around 500 degrees C. An analytical method according to one of the claims 8 to 11, characterized in that the catalyzing palladium elements are initially activated by thermal cycles in hydrogen atmosphere and then submitted to hot saturation with hydrocarbon, said catalyzing elements incorporating carbon in the palladium atomic lattice. An analytical method according to one of the claims 8 to 12, characterized in that the necessary carbon amount in said catalyzing elements during each analytical cycle, in absence of a doping system, is supplied by the same samples to be analysed.

Inventors: SIRONI ALBINO, VERGA GIUSEPPE, SIRONI, ALBINO, VERGA, GIUSEPPE
Applicants: THERMOQUEST ITALIA SPA, THERMOQUEST ITALIA S.P.A.
Application year: 1992
Main classification: G01N33
IPC codes: G01N31, G01N33, B01J19, G01N27, G01N30