WO1996006349A1

WO1996006349A1 - Improved flame ionization detector

Info

Publication number: WO1996006349A1
Application number: PCT/US1995/010895
Authority: WO
Inventors: Loren T. Mathews; James L. Medici
Original assignee: California Analytical Instruments, Inc.
Priority date: 1994-08-23
Filing date: 1995-08-23
Publication date: 1996-02-29

Abstract

A flame ionization detector consisting of a burner assembly (10) which includes a precombustion chamber (54) for mixing the fuel and sample gas, a combustion chamber (56) having a burner nozzle (60) for fluid communication with the precombustion chamber. An annular collector ring (70) and a spring loaded electrode collector (73) to which a potential is applied for the collection of ions are disposed in the combustion chamber (56). The collector (70) is connected to an electrometer (78) for the amplification of a current created by the migration of carbon ions to the collector (70) during combustion. Critical flow orifices (14, 24 and 34) disposed in the lines (12, 22 and 32) are used to deliver fuel gas, air, and sample gas to the flame ionization detector. The sample line (32) communicates with a bypass line (42) and capillary (44) for a passage of a major portion the sample gas directly to an exhaust vent (58).

Description

IMPROVED FLAME IONIZATION DETECTOR

Field of the Invention

This invention relates to flame analyzing methods and more particularly to an improved detector therefore.

Background of the Invention

Flame Ionization Detectors (FID) are the most commonly used type of detector for gas chromatography and for air impurity measuring devices, such as devices for measuring auto emissions and the like. Conventionally these detectors utilize a mixture of a fuel and the sample gas being tested and ignite the mixture in the presence of polarized electrodes. Ignition of the sample gas causes the molecule being detected to become ionized where upon the electrons are atracted to the positive electrode while the ions are collected on the negative electrode, commonly referred to as the collector. The migration of ions and electrons creates a small ionic current between the electrodes which is related to the quantity of the sought for material in the combustion mixture. Accordingly, as the quantity or the composition of the sample gas changes, a variation in the ionic current can be detected and correlated with the chemical changes.

Inconsistencies in the flow of the fuel and sample gas can mask the small ionic current or, at the least, produce errors in the read out of the ionic current. Consequently great care is taken in the manufacturer of the detector and calibration both at the factory and during use to insure that such inconsistancies are avoided to the greatest extent possible.

The sensitivity of the FID is also dependent on the potential placed on the electrodes. Conventionally the collector is operated at a potential of around 180 volts. Although it is known that raising the collector potential will increase the sensitivity of the FID, the design of the collector normally limits the operating potential to 180 volts as stated above. Increasing the potential above this point increases the time required to zero the detector between runs. Accordingly it would be desirable to provide an

SUBSTITUTE SHEET (RULE 26 inexpensive FID in which the flow rates and pressures of the fuel sample and air gases are closely and continuously controlled in the detector while at the same time providing a design which would allow the detector to be ran at higher collector voltages thus increasing its sensitivity.

Summary of the Invention

The device of the present invention is a Flame Ionization Detector (FID) which is rugged, inexpensive to manufacture and relatively easy to maintain. The device utilizes the flame ionization method for the determination of total carbon in a sample gas. In its operation the sample gas is passed through a flame in the presence of polarized electrodes. The migration of the carbon ions to the collector electrode creates a small current which is proportional to the carbon content in the combusted sample gas.

More particularly the device of the present invention comprises a burner housing which includes a pre-combustion chamber for mixing the fuel and sample gas and a combustion chamber which is in fluid communication with the pre-combustion chamber by means of a burner jet. The combustion chamber is also in fluid communication with a source of hydrocarbon free air which is delivered into the combustion chamber adjacent the burner jet. A collector ring is disposed in the combustion chamber to which a potential is applied for the collection of ions. The collector ring is connected to an electrometer for the read out of the current created by the migration of carbon ions to the collector ring. The collector ring is connected to a power source which imposes a potential on the collector ring and which polarizes the collector ring with respect to the burner jet so that the collector ring and the burner jet function as the polarized electrodes for the FID of the present invention. The sample gas, the fuel gas and the hydrocarbon free air are delivered to the burner housing through critical flow orifices so that the flow rate is uniform and constant throughout the operation of the detector. In addition the detector includes pressure regulators upstream of the critical flow regulators to insure the proper pressure for the sample gas, fuel and hydrocarbon free air. A by-pass flow line communicates with the sample inlet line and opens to the vent. This by-pass flow line improves the response time of the detector. Diagnostic means are provided for monitoring the flow pressures of the sample gas, fuel and hydrocarbon free air as well as the flame temperature in the combustion chamber. Ignition of the fuel air mixture is provided by a conventional ignition device. The provision of a collector ring in the device of the present invention permits higher voltages to be applied to the collector which increases the sensitivity of the device without substantially increasing the time for the detector to return to zero between runs. The critical flow orifices in the inlet lines for the fuel, hydrocarbon free air and the sample eliminate the need for adjustment of the sample, fuel and air gas ratios to optimize the flame by the user of the device. The device of the present invention is easily disassembled and cleaning is a simple operation which can be carried out with relatively little or no operator skill.

These and other features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the drawings.

Brief Description of the Drawings

FIG. 1 is a schematic flow diagram of the device of the present invention;

FIG. 2 is a side sectional view of the burner assembly of the device of the present invention; and FIG. 3 is a top sectional view taken along line 3-3 of FIG 2.

Description of the Preferred Embodiment

Referring to FIG. 1, the device of the present invention comprises a burner assembly 10 to which hydrogen containing fuel gas is introduced through a fuel line 12. A critical flow orifice 14 is disposed in the fuel line 12. A solenoid operated shut off valve 16, a pressure relief valve 18 and a filter 20 are also provided in the line 12. The preferred fuel is a mixture of hydrogen and helium (40% hydrogen/ 60% helium) which is normally maintained in a pressurized container prior to introduction to the burner assembly 10. Valve 18 can be adjusted to allow use of an alternative fuel, such as pure hydrogen. Hydrogen free air is introduced to the burner assembly 10 through an air line 22 and a critical flow orifice 24. The air line also contains a solenoid operated shut off valve 26, a pressure relief valve 28 and a filter 30. A sample gas enters the system through a sample line 32 and critical flow orifice 34. An electronically controlled proportional pressure relief valve 38 and a filter 40 are disposed in the sample line 32. A close coupled by-pass flow line 42 communicates with the sample line 32 for conducting a major portion of the sample gas through a capillary 44 and bypass flow meter 46 to exhaust. A pressure transducer 25 in each of the lines, 12, 22 and 32 senses line pressure and relays a signal to a controller board (not shown).

The critical flow orifices, 14, 24 and 34 control respectively the flow rate of the incoming fuel, sample gases and air to produce the optimum flame throughout the life of the detector. Such close control of the flow rates of the incoming gases eliminates the need to optimize each burner to achieve the best conditions for combustion. The flow rate of the sample through the critical flow orifice 24 is small compared to the flow rate of the incoming sample to the critical flow orifice. Consequently at the completion of a run a substantial amount of time is required to bleed the sample through the critical flow orifice so that it may be replaced by a new sample to be tested. This can result in lengthy lag times between runs. This problem is overcome by the provision of the by-pass line 42 and capillary 44 through which a major portion of the sample flows to the exhaust. Accordingly only a small amount of the sample is drawn through the critical flow orifice 34 in the sample line 32 and the time lag necessary to clear the sample line for a new sample gas is substantially reduced.

Referring to FIG. 2, the burner assembly 10 includes a housing 50 which is preferably machined from a solid block of polytetrafluorethylene to have a pair of longitudinally spaced cavities defining a precombustion chamber 54 and a combustion chamber 56. A burner nozzle 60 extends from the precombustion chamber and opens into the combustion chamber 56 adjacent a collector element 70. An exhaust vent 58 communicates with the combustion chamber 56 to carry away the combustion products. The burner vent is open to the atmosphere and, due to complete combustion in the combustion chamber 56 and low associated sample/fuel/air rates, does not require a condensate drain line. The fuel and sample gas mixture are discharged into the cumbustion chamber 56 through the burner nozzle 60 and the air line 22 discharges into the combustion chamber immediatly adjacent the burner nozzle. A glow plug 62 is carried in the wall of the housing for igniting the fuel, air and sample mixture. Power is supplied to the glow plug 62 through a conductor 64 which is in electrical communication with a source of current (not shown). A thermocouple 66 is likewise carried in the housing wall and extends into the combustion chamber 56 for measuring flame temperature. The thermocouple signal is carried to the controller board (not shown) by means of a conductor 68. The collector 70, in the form of a ring or sleeve, is disposed about the inner wall of the combustion chamber 56 and is electrically connected through a line 72 to a power source (not shown) for imposing a potential on the collector. The line 72 also conducts the current created by the flow of ions to the collector 70 during combustion of a sample and is connected to an electrometer of conventional design (not shown) for amplifying the ionization current from the collector. Contact between the collector 70 and the line 72 is provided by electrically conductive spring 73 which compensates for thermal expansion and contraction to maintain electrical contact during operation of the FID. A line 84, also connected to the power source (not shown) provides potential for polarizing the burner nozzle 60. An electrically conductive spring 85 makes electrical contact with the burner nozzle 60 and compensates for thermal expansion and contraction during operation of the FID.

The upper portion of the housing 50 is provided with an enlarged collar 80 through which extend bolt holes 82 for securing the burner assembly 10 in a suitable frame or housing (not shown) .

As illustrated in FIGS. 2 and 3, the collector 70 is an annular stainless steel member disposed about the inner wall of the combustion chamber 56. A potential applied to the collector 70 and the burner 60 polarizes the collector with respect to the burner nozzle so that the collector and the burner nozzle serve as electrodes for the flame ionization detector of the present invention. The annular configuration of the collector 70 allows for the imposition of collector voltages, on the order of 300v which are high enough to substantially reduce the time required to return the collector to a zero condition after being subjected to high hydrocarbon levels. Conventional flame ionization detectors, which utilize wire as a collector element, operate at around 180 volts. The wire collector element of a conventional FID must be properly positioned for optimum performance and is subject to vibration influence and may require re-positioning after shipment. Conventional FIDs utilizing wire collectors are fragile and difficult to clean and it normally requires a skilled technician to dissassemble, clean and reassemble the unit. In operation, highly regulated flows of fuel, sample and air are provided to the burner assembly 10 through the critical flow orifaces 14, 24 and 34 respectively. The fuel and air are delivered to the critical flow orifices at 15 psig and consequently the flame characteristics are predetermined without the need of flame optimization adjustments. In the embodiment illustrated the sample is introduced to the precombustion chamber 54 through the electronically controlled proportional pressure relief valve 38 and the critical flow orifice 34 in the sample line 32. The by-pass flow line permits flow of the sample gas at between about 2.5 and 4 liters per minute while the sample entering the precombustion chamber 54 through the critical flow orifice 34 flows at the rate of approximately 6 cc/min. The fuel and sample gas are mixed in the precombustion chamber 54 and pass into the combustion chamber 56 through the burner nozzle 60. Carbon free air is lead into the combustion zone adjacent the burner nozzle at a flow rate of about 220mL/min to supply oxygen to sustain combustion. The inlet pressure of the fuel and air are monitored by internal pressure transducers 25 to maintain fuel and air pressure at about 25ρsig and they act to close the solenoid operated valves, 16 and 26, in the event that there is a loss of fuel or air pressure. The sample gas pressure is maintained at between about 6psig and 25psig. In addition, the signal from the thermocouple is monitored at the controller board and in the event of a change in flame temperature from ideal, the solenoid valves 16 and 26 and the pressure relief valve 38 are operated to cut off the flow of fuel, air and sample.

The incoming gases in the combustion zone are ignited by the glow plug and the carbon components of the sample undergo ionization that produces electrons and positive ions. A potential of 300 volts is imposed on the collector which serves as the negative electrode of the detector for collecting the ions while the electrons migrate to the burner nozzle. The migration of ions creates a small ionization current which is amplified by the electrometer 78 for processing as a function of the carbon content of the sample. From the foregoing it will be seen that the close regulation of flow rates for the incoming sample error and fuel gases provides flame optimization throughout the life of the detector. The collector ring operates at a high potential as compared to prior art devices which reduces the response time for the device. In addition, response time is further enhanced by the by-pass line and capillary which permits a major portion of the sample gas to be vented while a minor portion is introduced into the burner assembly 10. This eliminates the presence of excess or dead sample which must be purged before attempting a run on a new sample. Waiting time between sample runs is thus substantially reduced. The solenoid operated valves cut off flow of all of the gases in the event of a drop in pressure or flame temperature which provides the additional benefit of avoiding waste of hydrogen free air as well as the more conventional fuel flow cut off safety feature.

The annular collector, as contrasted to the conventional wire collectors, provides ruggedness to the burner assembly 10. In addition the burner assembly 10 can be readily cleaned without danger of damaging or destroying the collector.

Having described the invention, we claim:

Claims

1. A flame ionization detector for the determination of the concentration of a component by measurement of a current created by the combustion of a sample in the presence of ionized electrodes, said detector comprising: a burner body having longitudinally spaced pair of cavities defining a precombustion chamber and a combustion chambe ; a fuel inlet line and a sample gas inlet line in fluid communication with a source of fuel and a source of sample gas respectively, said lines discharging into said precombustion chamber; means in said fuel inlet line and said sample gas inlet line for regulating the flow of a sample gas and a fuel gas into said precombustion chamber; a burner nozzle communicating between said precombustion chamber and said combustion chamber, said burner nozzle discharging into said combustion chamber; an oxygen inlet line communicating with a source of oxygen containing gas, said line discharging into said combustion chamber; means in said inlet line for regulating the flow of said oxygen containing gas into said combustion chamber; an annular collector disposed in said combustion chamber; first conductor means electrically communicating between said collector and a source of electrical power and current amplifying means; second conductor means electrically communicating between said burner nozzle and said source of electrical power; igniter means for initiating combustion of said sample, fuel and oxygen containing gasses in said combustion chamber; and vent means for carrying away the combustion products from said combustion chamber.

2. The flame ionization detector of claim 1 wherein said means regulating the flow of said fuel, sample and oxygen containing gasses is a critical flow orifice disposed in said

SUBSTJTUTE SHEET (RULE 26 fuel, sample and oxygen inlet lines.

3. The flame ionization detector of claim 1 further including a solenoid operated shut-off valve and an internal pressure transducer in each of said inlet lines.

4. The flame ionization detector of claim 1 further including temperature detection means for monitoring the temperature of said combustion chamber.

5. The flame ionization detector of claim 1 wherein said annular collector is maintained at a potential of -300 volts during operation of said detector.

6. The flame ionization detector of claim 1 wherein said annular collector consists of a an electrically conductive ring disposed on the wall of said combustion chamber.

7. The flame ionization detector of claim 1 wherein said annular collector consists of an electrically conductive sleeve coaxially disposed in said combustion chamber.

8. The flame ionization detector of claim 1 wherein said sample inlet line communicates with a sample by-pass line for the flow of a major portion of said sample gas directly to said vent means without entering said combustion chamber.

9. The flame ionization detector of claim 1 wherein said first conductor means includes an electrically conductive spring to provide uniform contact between said conductor and said collector.

10. The flame ionization detector of claim 1 wherein said second conductor means includes an electrically conductive spring to provide uniform contact between said conductor and said burner nozzle.

11. The flame ionization detector of claim 1 wherein said burner body is a machined block of polytetrafluorethylene .