REACTOR FOR APPLICATION TO A GAS CHROMATOGRAPHIC SYSTEM

Abstract: The invention relates to a reactor wherein compounds are thermally decomposed. The reactor comprises a reactor shell (46), a base (42), a guide tube (48) located centrally to the reactor shell (46) and a reaction tube (52).

Description: TECHNICAL FIELD This invention relates to a reactor and, more particularly, to a reactor for application to a gas chromatography system. BACKGROUND ART Chromatography generally involves a separation process for redistributing molecules in a mixture from a thin material phase to a second bulk material phase. The two phases in contact may be liquid-liquid, liquid-solid, vapor-solid or vapor-liquid. Where the bulk phase is a gas stream, the term gas chromatography is used to describe the process. The gas stream is then analyzed to obtain information specific to the transfer molecules. Typical detection systems include thermal conductivity, flame ionization and argon detectors. More recent detectors include electrolytic conductivity detectors providing an electrical signal which is functionally related to the presence of a selected element. Electrolytic conductivity detectors have long been available for investigating the properties of electrolytes in solutions. Such devices in the prior art typically include electrode surfaces with a continuous phase liquid electrolyte therebetween. Conventional use of electrolytic conductivity detectors in a gas chromatograph system generally follows this prior art approach and separates any gas phase from the liquid phase prior to the detector cell which provides an output related to the electrolyte. An early typical electrolytic conductivity detector, the Coulson detector, is depicted in the Journal of Gas Chromatography , April, 1965, at pages 134-137. As therein depicted the electrolytic conductivity detector cell provides a liquid collecting and gas venting section for separating liquid from the gas flow and thereafter introducing the liquid to an electrode volume where the liquid electrolyte is used to form a continuous conductive path between electrode structures in the sidewalls. Yet another electrolytic conductivity detector is depicted in U.S. Patent No. 3,934,193 to Hall. The Hall detector also separates the flow into a gas flow and a liquid flow and the liquid flow is captured between two concentric electrode surfaces for conductivity measurement. Thus, in both detectors, a slow moving volume of liquid is formed having a relatively long dwell time with respect to chromatographic events. The Coulson conductivity cell is typically a complex glass device which is difficult to interface with many chromatographs. Further, gases can accumulate in the detector section between electrodes. These gas bubbles can be difficult to displace and the liquid phase tends to merely divert about the gas phase bubble rather than to sweep away the bubble. The Hall conductivity detector is a substantially more rugged detector and easier to interface than the Coulson detector but the separating chamber is subject to deterioration. The relatively large electrode area per fluid volume ratio produces good sensitivity to the conductivity of the electrolytic fluid but can require frequent cleaning to maintain the volume cleanliness. Thus, it would be desirable to provide an electrolytic conductivity detector which is not subject to blockage by gas accumulation and which is substantially self cleaning, while retaining the large signal-to-noise ratio which is desired for sensitive results. It will be appreciated that the signal-to-noise ratio can be affected by short term variations in the fluid flow. Such short term variations are typically induced by the pump which moves the solvent liquid, used to dissolve portions of the gas phase, through the system and through the electrolytic conductivity detector. The signal-to-noise ratio could desirably be improved by reducing or eliminating the induced flow fluctations. It will also be appreciated that gases evolving directly from the sample being measured are reacted with an oxidant or a reductant to form reaction products suitable for use in combination with an electrolytic conductivity detector. Thus, a reactor is typically furnished having a quartz or nickel reaction tube within a heated volume. Access to the reaction tube is desirable to check for reaction tube cleanliness and tube integrity. However, prior art reactor assemblies generally require removal of the reactor from the analysis system in order to change the reaction tubes. Further, the integrity of the reaction tube is often jeopardized by the system used for supporting the reaction tube within the reactor chamber. An objective is to improve access to a reactor tube assembly for reducing the likelihood of reactor tube detriment on system performance. US-A-4 295 856 describes a reactor comprising a reactor shell, a cap having an aperture, a shell floor, a base having an aperture, a tube nut, a guide tube, a ferrule and a reaction tube located centrally to the reactor shell. US-A-4 066 411 discloses threaded fitting means for an N-nitroso compound detection system. SUMMARY OF THE INVENTION The invention relates to a reactor for thermally decomposing compounds, comprising a reactor shell having a shell floor and a removable cap, a base adjoining shell floor, a reactor core positioned within the reactor shell, and a reaction tube contained within the reactor core, characterized in that a tube nut is threadedly engaged in a first threaded aperture extending through removable cap, a tube fitting nut extends from the interior of reactor shell through a second threaded aperture in the shell floor and base, a first ferrule is positioned within said tube nut and a second ferrule is positioned within said second threaded aperture below said tube fitting nut, the reactor core extends from within said first threaded aperture to said tube fitting nut, and the reaction tube extends from said first ferrule through the interior of reactor core to said second ferrule. These objectives and other features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified block diagram of a gas chromatograph with an electrolytic conductivity detection system. FIGURE 2 is a cross section of a reactor according to one embodiment of the present invention. FIGURE 3 is an enlarged cross section showing reaction tube sealing and securing components. DETAILED DESCRIPTION Referring first to FIGURE 1, there is depicted in block diagram form a gas chromatography system having an electrolytic conductivity detector cell for signal derivation. Gas chromatograph 10 generates a sample gas flow from a selected material within gas chromatograph 10. The output sample gas is combined with reaction gas 12 for input to reactor 14. The output from reactor 14 generally contains inorganic compounds that will support electrolytic conductivity, such as HCl, SO₃, NH₃, or CO₂. Thus, the reaction gas 12 may be an oxidant or a reductant with respect to the effluent gas from chromatograph 10 to obtain appropriate compounds for the conductivity detector. The gaseous output from reactor 14 is provided to detector cell 16 and combined with a suitable solvent from solvent reservoir 18, as hereinafter described. Detector cell 16 provides a solvent return 25 to solvent reservoir 18 and provides an output signal to conductivity amplifier 24 to obtain an output from device 26. Solvent may be added to the system from reservoir 18 by the action of pump 20. The solvent may also flow through ion exchange resin bed 22 for solvent purification and removal of extraneous electrically conductive materials prior to reuse. Ion exchange bed 22 is conveniently designed, according to one embodiment of the present invention, to include a pneumatic reservoir for damping pressure and flow changes resulting from the action of pump 20. The response of detector cell 16 is a function of the product from reactor 14 and the specific solvent from reservoir 18 which are mixed and passed through detector cell 16 to return line 25 for reuse. Further, the output of reactor 14 may be determined by the reaction temperature which is effected by temperature controller 28. The overall gaseous input is further determined by vent timer 30 actuating solenoid vent valve 32 for venting detrimental system gases during a measurement cycle. Referring now to FIGURE 2, there is depicted an improved reactor for use in the present system. Chromatograph input 34 is directed through fluidic valve 40 and combined with reaction gas from entry tube 38. Vent tube 36 is provided for venting solvent which may be passed through chromatographic input 34 at the start of a measurement cycle when vent valve 32 may be actuated. Thus, the desired chromatographic effluent is mixed with the reaction gas at fluidic valve 40 and passed into reaction tube 52. The reactor shown in FIGURE 2 conventionally comprises a base 42, shell floor 44, reactor shell 46 and reactor cap 50. Heating elements (not shown) are included within reactor shell 46 for heating reactor protective core 48 about reaction tube 52. However, an improved sealing and support system is provided for reaction tube 52 to enable tube 52 to be removed by components wholly accessible through cap 50. Reaction tube 52 is secured at its base by tube fitting nut 56 compressing ferrule 54 and forming a generally leak-tight seal therewith. The ferrule is preferably a reducing ferrule to accommodate a 1/16th inch o.d. reaction tube 52 which can then be removed from the fitting by a small movement of tube fitting nut 56. The upper reaction tube fitting assembly is more particularly shown in FIGURE 3. Reactor tube nut 58 is conveniently formed to threadedly mate with removable ceramic reactor cap 50 to support reaction tube 52. Tube nut 58 is machined to accept ferrule 59 about reaction tube 52 which is sealingly compressed by union 60. In this manner reaction tube 52 is supported and sealed within tube nut 58 wherein relatively fragile 1/16th inch o.d. quartz tubes may form reaction tube 52. The reactor assembly shown in FIGURE 2 may then be connected to an output tube, such as scrubber 62, by the action of tube nut 64 threading on union 60 to sealingly compress ferrule 63. It is readily apparent that the use of reducing ferrules for supporting and sealing about reaction tube 52 and the mating engagement of nuts 56 and 58 with reactor base 42 and reactor cap 50, respectively, permit reaction tube 52 to be easily accessed through removable cap 50 and removed for inspection and/or replacement without removing the entire reactor 14 (FIGURE 1) from the system. The output from reaction tube 52 may be passed to scrubber 62 before passing to reaction product tube 65. Scrubber 62 is optionally provided for removing undesired components of the reacted gas. For example, SO₂ and SO₃ can be removed by a CaO scrubber; HCl can be removed by a AgNO₃ scrubber.

Claims: A reactor for thermally decomposing compounds, comprising a reactor shell (46) having a shell floor (44) and a removable cap (50), a base (42) adjoining shell floor (44), a reactor core (48) positioned within the reactor shell (46), and a reaction tube (52) contained within the reactor core (48), characterized in that a tube nut (58) is threadedly engaged in a first threaded aperture extending through removable cap (50), a tube fitting nut (56) extends from the interior of reactor shell (46) through a second threaded aperture in the shell floor (44) and base (42), a first ferrule (59) is positioned within said tube nut (58) and a second ferrule (54) is positioned within said second threaded aperture below said tube fitting nut (56), the reactor core (48) extends from within said first threaded aperture to said tube fitting nut (56), and the reaction tube (52) extends from said first ferrule (59) through the interior of reactor core (48) to said second ferrule (54). The reactor of Claim 1, wherein: the base has located therein a heating element. The reactor of Claim 2, wherein: the base has located therein a thermocouple.

Inventors: HALL, RANDALL C.
Applicants: O.I. CORPORATION
Application year: 1985
Main classification: G01N30
IPC codes: G01N27, G01N30, G01N31