Abstract:
A chemical sensor utilizing a substrate and a fluoropolymer coating is disclosed. Transducers may be connected to the substrate to generate an alternating potential across the substrate, which in turn causes the substrate to resonate due to the converse piezoelectric effect. The polymer coating absorbs the analyte, thus changing the mass of the sensor, and accordingly changing its resonant frequency. The transducers detect this change in resonant frequency to indicate to the operator that the analyte is present. The use of amorphous copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD), and tetrafluoroethylene (TFE) allows for improved sensitivity and responsiveness while also allowing for robust characteristics enabling the sensor to be used in a variety of environmental conditions.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to systems for monitoring environmental contaminants and, more particularly, to systems for monitoring fugitive emissions from process equipment. 
     BACKGROUND OF THE INVENTION 
     Industrial plants which handle volatile organic compounds (VOCs) typically experience unwanted emissions of such compounds into the atmosphere from point sources, such as smoke stacks, and non-point sources, such as valves, pumps, and fittings installed in pipes and vessels containing the VOCs. Such VOCs include, but are not limited to, aromatics (e.g., benzene, toluene, ethylbenzene, and xylenes), halogenated hydrocarbons (e.g., carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene), ketones (e.g., acetone, and methyl ethyl ketone), alcohols (e.g., methanol, ethanol, and propanol), ethers (e.g., dimethyl ether and methyl t-butyl ether), and aliphatic hydrocarbons (e.g., natural gas and gasoline). 
     Emissions from non-point sources, referred to as fugitive emissions, typically occur due to the leakage of the VOCs from joints and seals. Fugitive emissions from control valves may occur as the result of leakage through the packing between the valve stem and the body or bonnet of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, which results in greater fugitive emissions than valves employed in less demanding service. 
     While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a means to identify and reduce fugitive emissions, and to comply with the more stringent regulation of emissions. For example, the Environmental Protection Agency (EPA) has promulgated regulations for specifying the maximum permitted emission of certain hazardous air pollutants from control valves, and requires periodic surveys of emissions from control valves. 
     Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual method is time consuming and expensive to perform, and can also yield inaccurate results due to ineffective collection of the fugitive emissions from the equipment being monitored. If measurements are made on a valve exposed to wind, emissions from the valve may be dissipated before the analyzer can properly measure the concentration of the emissions. Also, if the analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, thereby distracting plant personnel from other duties. 
     Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The length of the survey interval may be monthly, quarterly, semi-annually, or annually, with the required surveys becoming less frequent if the facility operator can document a sufficiently low percentage of control valves exhibiting excessive leakage. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year. In a large industrial facility, where the total number of survey points can range from 50,000 to 200,000, a reduced number of surveys can result in large cost savings. By installing automated fugitive emission-sensing systems on valves subject to the most demanding service conditions, and thus, most likely to develop leaks, compliance with the EPA regulations can be more readily achieved for the entire facility. 
     However, employing chemical sensors in an industrial environment requires designing sensors that perform satisfactorily in the presence of high relative humidity across a broad temperature range. The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. A provision also must be made to enable periodic calibration of the chemical sensors. The output signals from the fugitive emission sensing system must be suitable for input into plant monitoring and control systems typically found in process plants. This permits simple and inexpensive integration of the sensing system into existing plant process control systems. 
     The fugitive emission sensing system must be inexpensive to manufacture, and use a power source that is readily available in a typical process plant in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to risk of explosion, requiring electrical equipment to be intrinsically safe or of an explosion-proof design. It also must be able to operate in harsh environments, including areas subject to spray washing, high humidity, high and low temperatures, and vibration. The system also must be simple and reliable, in order to keep maintenance costs to a minimum. 
     In certain applications, the sensors used to detect fugitive emissions are provided in the form of piezoelectric-based sensors having high sensitivities to surface mass changes, such that when an alternating potential is applied across the sensors, changes in resulting wave characteristics in the sensors, specifically the resonant frequency, indicate the presence of the analyte. More specifically, the sensors typically include a quartz crystal substrate with an outer layer made of material selected to most effectively absorb the analyte. Such outer coatings are selected to increase sensitivity, while reducing acoustic wave damping effects. In addition, such materials should be environmentally robust to accommodate the aforementioned wide temperature ranges, humidity ranges, and high levels of dust particles and other contaminants. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a sensor is provided which may include a substrate, at least two electrodes connected to the substrate, and a layer of fluoropolymer positioned over the substrate and at least one of the electrodes. 
     In accordance with another aspect of the present invention, the fluoropolymer is a copolymer of 2-2-bistrifluoromethyl 1-4,5-difluoro-1,3-dioxole. The copolymer may comprise tetrafluoroethylene. 
     In accordance with another aspect of the invention, a method of detecting volatile organic compounds using a sensor is provided. The sensor comprises a substrate, at least two electrodes connected to the substrate, and a coating positioned over the substrate and at least one of the electrodes. The coating is a fluoropolymer. 
    
    
     These and other aspects and features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a fugitive emissions sensing system employing the present invention; and 
     FIG. 2 is a schematic diagram of a chemical sensor circuit including one embodiment of the chemical sensor of the present invention. 
    
    
     While the invention is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents, falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and with specific reference to FIG. 1, a fugitive emissions sensing system utilizing the present invention is generally depicted by reference numeral  20 . However, it is to be understood that the present invention is primarily directed to a chemical sensor  22  (FIG. 2) which can be employed in a variety of applications, including applications separate from the fugitive emissions sensing system  20 . 
     By way of overview, FIG. 1 is a block diagram of an illustrative fugitive emissions sensing system  20  employing the chemical sensor  22 . An emission source  24  is shown, from which a sample stream  26  is drawn into sample retrieval system  28 . The sample retrieval system  28  includes an accumulator  30 , a sensor chamber  32 , and an ejector  34 . A chemical sensor array  36  is located within the sensor chamber  32 . The sample stream  26  is drawn from the accumulator  30  into the sensor chamber  32 , exposing the chemical sensor array  36  to the sample stream  26 . The chemical sensor array  36  contains one or more of the chemical sensors  22 . The sample stream  26  then passes into the ejector  34 . A compressed air source  40  provides compressed air  42  to the ejector  34 , creating a pressure drop within the ejector  34  which draws a sample stream  26  through the sensor chamber  32  and into the ejector  34 . The compressed air  42  and sample stream  26  are mixed within the ejector  34  and exhausted to atmosphere as a mixture  44 . 
     The gas sensor array  36  is connected to a sensor interface circuit  50 , which processes the signals from the sensor array  36  and provides the process signals to a microcontroller  52 . The microcontroller  52  stores the data from the sensors  22  in a memory  54 , and uses the sensor data received from the fugitive emissions sensing system  20  to initiate control actions to reduce or eliminate the emissions. For example, the microcontroller  52  could close a valve upstream from the emissions source  24  to stop the flow of fluid through the emissions source  24  in order to stop emissions caused by the leakage of the fluid. Alternatively, the microcontroller  52  could alter operating conditions of the emissions source  24  itself to reduce or eliminate the fugitive emissions. The microcontroller  52  may use a communication interface circuit  56  to provide control signals to the upstream valve, the emission source  24 , or any other equivalent that may be used to reduce or eliminate the emissions. 
     It can therefore be seen that the fugitive emissions sensing system  20  may be used to detect the presence of, or measure the concentration of, various types of fluids emitted from the emissions source  24 . The system may be used to detect hazardous, toxic or polluting substances emitted from the source, or to detect leakage of non-hazardous substances, the loss of which may be a cause of concern. The fugitive emission sensing system  20  may be used to detect emissions from any kind of source, particularly industrial process equipment from which hazardous substances may leak. Examples include control valves, block valves, pumps installed on lines carrying hazardous gases, agitators, screw conveyors, or other equipment installed on process vessels containing hazardous fluids, heat exchanges, reactors, etc. When emissions are detected by the fugitive emissions sensing system  20 , this data may be used by the fugitive emissions sensing system  20  to control the process in such a way as to reduce or eliminate the emissions. 
     As indicated above, the chemical sensor array  36  may include one or more chemical sensors  22  responsive to a particular analyte or fugitive emission being monitored. In the embodiment depicted in FIG. 2, the chemical sensor  22  is a quartz crystal microbalance (QCM) sensor, but can be another type of piezoelectric acoustic wave devices, including surface acoustic wave (SAW) devices, acoustic plate mode (APM) devices, and flexural plate wave (FPW) devices. Alternatively, fiber optic sensors and electrochemical sensors may be used. 
     As shown in FIG. 2, the chemical sensor  22  may be connected to an oscillator circuit  62  for monitoring emissions. In an alternative embodiment, the chemical sensor  22  could be connected to a network analyzer. More specifically, the oscillator circuit  62  may include NAND gates  64  and  66 , and an AND gate  68 , connected in series. A resistor  70  may be connected between the output of the NAND gate  66  and the circuit power supply voltage  72 , and a resistor  74  may be connected between the output of NAND gate  66  and circuit power supply voltage  72 . A resistor  75  may be connected across the NAND gate  64 , connecting a first input to the output. A select signal  76  may be connected to the second input of the NAND gate  64 , and the same select signals may also be connected to an input of the AND gate  68 . An enable signal  78  may be connected to an input of the NAND gate  66 . When the select signal  76  and the enable signal  78  are both high, the NAND gates  64  and  66  act as high-gain inverting amplifiers and cause an oscillator  80  to oscillate between high and low voltage, producing an oscillating square wave output. The oscillating voltage from the oscillator output  80  may be transferred through the AND gate  68  and applied across the chemical sensor  22  causing the chemical sensor  22  to physically resonate. 
     In order to appreciate the significance of this resonance, it is first important to understand that the chemical sensor  22  utilizes the converse piezoelectric effect. By way of background, the piezoelectric effect holds that a mechanical stress applied to the surfaces of various crystals, including quartz, affords a corresponding electrical potential across the crystal having a magnitude proportional to the applied stress. The electrical charge generated in the quartz crystal under stress is due to the shift of dipoles resulting from the displacement of atoms in the crystalline material. The converse piezoelectric effect holds that application of a voltage across certain crystals, including quartz crystals, results in a corresponding mechanical strain in the crystal. In quartz, this strain or deformation is elastic. It follows that an alternating potential across the crystal causes a vibrational motion in the quartz crystal, i.e., the aforementioned resonance. The chemical sensor  22  therefore includes a crystal substrate  82  which interacts with the oscillating circuit  62 , and in turn causes the oscillator circuit  62  to oscillate at the resonant frequency of the chemical sensor  22 . Thus, the frequency of the oscillator output  80  will vary as the resonant frequency of the chemical sensor  22  varies. 
     The resonant frequency of the chemical sensor  22  can vary based on a number parameters, including the mass, size, shape, and cut of the quartz crystal substrate  82 . Quartz crystal exhibits a natural resonant frequency that is a function of the mass and structure of the crystal. The precise size, type of cut, and thickness of the quartz crystal substrate  82  are selected to result in a particular resonant frequency. For example, an AT-cut crystal with a nominal resonant frequency of 8-30 megahertz is suitable for gas sensor applications. Suitable quartz crystal substrates may be obtained from Standard Crystal Corporation of California. Other types of suitable materials to serve as the substrate include lithium niobate (LiNbO 3 ), which is particularly suited for a surface acoustic wave (SAW) based-sensor; and aluminum nitride (AIN), which is particularly suited for a thin film resonator based-sensor. 
     In order to apply the alternating potential across the substrate  82 , first and second electrodes  84  and  86  are connected to the crystal substrate  82  and may be constructed of gold-on-chromium, although other suitable corrosion-resistant conductors may be used, possibly including aluminum, palladium, gold, chromium, and graphite. The electrodes  84  and  86  may serve as both the conductors for generating the alternating current across the crystal substrate  82 , and as transducers for sensing parameters related to changes in resonant frequencies resulting in the crystal substrate  82 . 
     As indicated above, the resonant frequency of the chemical sensor  22  is a function of the total mass of the device. Therefore, the mass of any coating provided around the crystal substrate  82  also affects the total mass of the device, and thereby affects the resonant frequency of the chemical sensor  22 . The coatings provided about the crystal substrate  82  are selected to absorb molecules of the analyte. When analyte molecules are absorbed by the coating, the mass of the coating is slightly increased, which in turn increases the mass of the entire sensor  22 , and thus changes the resonant frequency of the sensor  22 . The resonant frequency of the chemical sensor  22  is also a function of the viscoelastic properties of the coatings and mechanical stresses caused by temperature effects in the sensor mounting structure. However, these effects are either negligible or can be compensated for. Thus, a very sensitive chemical detector may be constructed by selecting a coating that has a chemical affinity with the particular analyte of interest. The quantity of molecules absorbed and deposited, and the resulting change in the operating frequency of the oscillator circuit  62 , is a function of the concentration of the analyte being measured in the environment surrounding the chemical sensor  22 . The frequency changes linearly with changes in analyte concentration, within certain limits. 
     Thus, a change in the concentration of the analyte may be measured by measuring the change in the frequency of the oscillator output  80 . The chemical sensor  22  may be calibrated by exposing the chemical sensor  22  to known concentrations of the analyte and recording the resulting frequency of the oscillator output  80 . The chemical sensor  22  may then be used to measure the absolute concentration of the analyte by comparing the measured frequency to the aforementioned recorded values. 
     The particular coating chosen for the crystal substrate  82  should preferably readily absorb the molecules of the analyte, to provide fast response times and a high degree of sensitivity to the analyte over a broad temperature range, but do so without damping the generated waves. The present invention provides such a coating in the form of a fluoropolymer coating  88 . The fluoropolymer may be a copolymer comprising perfluoro-2,2-dimethyl-1,3-dioxole. The comonomer typically is fluorinated. Useful fluoropolymers are disclosed in U.S. Pat. Nos. 4,754,009 and 5,000,547, the disclosures of which are expressly incorporated herein by reference. An especially preferred fluoropolymer coating  88  is commercially available from Dupont Fluoroproducts, Wilmington, Del., under the tradename TEFLON® AF. TEFLON® AF is a copolymer of 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole (PDD) and tetrafluoroethylene (TFE). 
     A preferred fluoropolymer for coating  88  has the following combination of properties: 
     1. High glass transition temperature of at least 160° C.; 
     2. High moduli, especially at elevated temperatures; 
     3. High strength, especially at elevated temperatures; 
     4. Low creep under compressive load; 
     5. Melt fabricability at moderate temperatures; 
     6. Fabricability into films and coatings by solvent casting; 
     7. Low temperature sprayability; 
     8. Low refractive index; 
     9. Excellent dielectric properties; and 
     10. Excellent chemical resistance. 
     Unexpectedly, the high glass transition temperature fluoropolymer used in the present invention overcomes the undesirable properties inherent in high glass transition temperature polymers. High glass transition temperature polymers typically are unsuitable for sensor applications because of slow and hysteresis responses to analytes. The high glass transition fluoropolymer, however, also has acoustic wave properties superior to conventional low glass transition temperature polymers, like poly(isobutylene) and poly(diphenoxy phosphazene). 
     Accordingly, the fluoropolymer coating improves upon the performance of low glass transition temperature polymers, and overcomes the disadvantages of high glass transition temperature polymers. For example, the fluoropolymer coating  88  has a high glass transition temperature, and does not damp sensor transducers to the same degree as low glass transition polymers. 
     A relatively thick film of the fluoropolymer coating  88 , e.g. a coating of about 1 to about 10 microns, can be deposited on the crystal substrate  82 . A preferred coating thickness is about 2 to about 8 microns, and to achieve the full advantage of the present invention, the coating thickness is about 3 to about 6 microns. Persons skilled in the art are capable of determining the optimum coating thickness from consideration of use temperatures, desired response time, and expected analyte concentrations. A relatively thick coating increases the sensitivity of the chemical sensor  22  because the sensitivity is generally proportional to the thickness of the coating  88 . Moreover, given the aforementioned benefits, use of such a fluorinated copolymer as the coating  88  allows the sensor  22  to be used in a wide range of temperatures without compromising performance. 
     Unlike other high glass transition polymer films, the fluoropolymer coating  88  exhibits fast and reversible responses to volatile organic compounds of low molecular weight. In addition, the fluoropolymer coating  88  is chemically inert and less susceptible to environmental aging, e.g., attacks from ozone and oxidizing gases. This improves the stability and lifetime of the chemical sensor  22 . The fluoropolymer coating  88  is also hydrophobic, such that interference due to water vapor and polar volatile organic chemicals has a low impact on performance. Since it has low-surface energy, the fluoropolymer coating  88  has a low tendency to collect foreign objects, such as dust particles, and thus needs a low degree of care. The fluoropolymer coating  88  is also soluble in a commercial solvent at ambient temperature, thus facilitating application of the coating  88  to the crystal substrate  82  using conventional methods. Suitable solvents for such use include solvents having a mixture of fluorinated hydrocarbons, such as the solvent marketed under the tradename FC-75 FLUORINERT® by 3M Corporation, St. Paul, Minn. 
     In accordance with the present invention, the fluoropolymer coating may be applied to the crystal substrate and electrodes using the following procedure. The crystal substrate and electrodes are first cleaned using acetone and methanol. The TEFLON AF® is then dissolved in a fluorinated hydrocarbon solvent to produce a solution having a concentration of 1-6% TEFLON AF®, by weight. The concentration of TEFLON AF® in the solution is related to the desired coating thickness. The more concentrated the solution, the thicker the resulting coating will be. Approximately 7-10 drops of the solution is then applied to the substrate and electrodes to completely cover one side of the sensor. The coated substrate is then placed on a spin coater, a machine adapted to rotate at variable speed, with the preferred speed range being 500-6000 RPM, for a duration of approximately two minutes. The selected spin rate depends on the targeted coating thickness, with higher spin rates being selected for thinner coatings. After spin coating, the sensor is air dried for approximately one minute, with the aforementioned steps then being repeated for each side of the sensor. The sensor is then cured at a temperature of 100° C. for approximately two hours. Alternatively, if the coating is being applied to surface acoustic wave sensors or thin film resonator sensors, spray-coating and dip-coating techniques may be employed, respectively. 
     From the foregoing, it can therefore be seen that the present invention provides an improved chemical sensor and coating for a chemical sensor.