|Title of Invention||
"AN IMPROVED HYDROCARBON GAS SENSING INSTRUMENT"
|Abstract||The invention relates to an improved hydrocarbon gas sensing instrument. The sensor comprising a metal (Ru, Pd, Pt, Ag and Au) impregnated tin oxide thin film. The film is provided to detect the hydrocarbon gases in ambient and flowing gases. The sensor is specific to hydrocarbon gases and is negligibly affected by the presence of common automobile exhaust like NOX, CO, industrially used gases such as H2, NH3, H2S and fuel gases like petrol and diesel vapor, with in the operational temperature range from about 250- 350°C. The present sensor instrument has other application like monitoring hydrocarbon level in laboratories, mines and industrial smokestacks, detecting the adulteration in the petrol, diesel and may be used in environment upto 500°C.|
|Full Text||The present invention relates to an improved hydrocarbon gas sensing instrument. Particularly, it relates to an improved hydrocarbon gas sensing instrument using metal impregnated tin oxide thin film as a hydroc arbon sensor.
More particularly, it relates to the improved gas-sensing instrument useful for sensing hydrocarbons including those present in commercially used Liquid Petroleum Gas (LPG). Still more particularly, it relates to an instrument wherein a metal impregnated tin oxide thin film is used for sensing the leakage of liquefied petroleum gas (LPG) and other similar hydrocarbons.
This gas sensing instrument, working on the principle of change in electrical properties as a result of the adsorption of hydrocarbons on the sensor element using a metal impregnated ceramic material can be profitably used to sense other hydrocarbons with higher sensitivity value. Hydrocarbon gases are commercially exploited as fuels for several industrial and domestic applications. For example, Liquid Petroleum Gas (LPG) is a commonly employed fuel that contains lower hydrocarbons such as propane and butane in households for cooking purposes. Similarly, Natural gas is also employed in several industries. These gases if not handled properly are hazardous due to explosion possibilities. The lower explosive limits (LEL) for propane and butane are 2.1 and 1.8 vol. % respectively and more significantly some of the hydrocarbons are physiologically toxic (for e.g. toluene and benzene are carcinogenic) even in parts per million (ppm) amounts in the total volume. However, many of these hydrocarbons per se have no smell and for those hydrocarbons with smell itself, it is very difficult to distinguish e.g. benzene and toluene. Therefore, there is a growing demand to design efficient instruments using sensing materials, which can selectively sense the gases. Efficient hydrocarbon sensors
are also useful in developing automotive emission control systems since methane, ethane etc. are also present in the exhaust emissions from automobiles along with carbon monoxide, nitric oxide and sulfur dioxide.
Different types of techniques are generally used for gas sensing. These include measurement of conductivity, e.m.f. limiting current, heat of combustion, optical absorption etc., wherein the amount of gas present in the environment is directly inferred from these measurements. Different types of catalytic gas sensors can also be used for detecting hydrocarbons such as methane and ethane based on the measurement of the heat of oxidation of the combustible gas in the presence of a catalyst. Reference may be made to Fr 2,452,108, JP 61,262, wherein, a platinum coil encapsulated by a porous alumina/thoria bead containing dispersed catalysts such as platinum and palladium is used. The sensor is known as pellister. Temperature is measured using thermistors and from the heat change the amount of hydrocarbon can be sensed. The key component in such hydrocarbon sensors is a material, which selectively responds to the hydrocarbon gas. Thus the synthesis of such materials followed by the demonstration of hydrocarbon sensitivity is an essential prerequisite in fabricating hydrocarbon sensors. The drawbacks are long term drift, poisoning by humidity and temperature effects associated with such devices are present in the prior art. There are also limitations due to their lack of selectivity since these devices respond to every oxidizable gas such as CO, hydrogen, alcohol, LPG etc.
References may also be made to JP 59,353, JP 57, 175, 948 and JP 59,97,047 wherein sensors used for detecting hydrocarbons are obtained by doping SnO2 - In2O3 with Pd SnO2 In2O3 with Pt and Pd SnO2 -Ti02 with Nb and Pd. Other oxide materials such as
V205, ZnO, WO3 and Fe2O3 are also quite sensitive to hydrocarbons after appropriate
doping. The drawbacks are large cross-sensitivity and the lack of specificity inspite of
the use of doping or additives, besides, devices based on the materials are quite expensive
due to the use of noble metals such as Au, Pd or Pt in amount varying 1 to 8% by weight.
References may also be made to US Patent No. 4,535,315 dated Aug. 13, 1995 and US
Patent No. 5,470,756 dated Nov. 28, 1995 wherein tin oxide having a large specific
surface area with Pt, Pd catalysts of about 4-8% is described for LPG sensing. The
drawbacks are the sensitivity to LPG is small ranging from 0.6 to 20, devices based on
the materials are quite expensive due to the use of noble metals such as Pd or Pt.
The main object of the present invention is to provide an improved hydrocarbon gas
sensing instrument, which obviates the drawbacks as detailed above.
Another object of the present invention is to use ruthenium impregnated tin oxide thin
Still another object of the present invention is to provide an instrument using a cheaper
gas sensor with better selectivity and sensitivity.
In the drawings accompanying this specification Figure 1 represents an gas sensing
instrument which comprises of a sample tube (A) in the preferred form although any open
tube at 200°C to 300°C in which the sample is kept for resistance measurements of gas
can be used, a heater (B) and sample holder (C), on which the sensor (D) is mounted, a
thermocouple (E) which is connected to a temperature controller (F) to measure and
control the temperature of the sample, and a multimeter (G) to measure the resistance.
The temperature of the sample is sensed by a thermocouple connected to a temperature
controller. The sample (D) is placed in the sample holder and the wires corning from the
sample holder are connected to a multimeter (G). Air is then passed in a controlled manner through (H) and the change in resistance of sample is measured at various concentrations of the gases by controlling the quantity of the gas injected in the sample tube either in open air or under a flow of air or nitrogen through (I). Figure 2 represents the selectivity histogram wherein sensitivity (Y-axis) indicates the ratio of change in conductance after introduction of sensing gas to the original conductance of the sample at the same temperature, with 1000 ppm concentration of different gases (X-axis) at an operating temperature of 250°C for Ruthenium impregnated (0.67wt.%) tin oxide thin film. The gases are (1) CO, (2) Kerosene, (3) Alcohol, (4) Petrol, (5) LPG, (6) H2, (7) NH3, and (8) Diesel. The sample shows a maximum sensitivity of 215 towards LPG followed by petrol and Fk.
The inventors of the present invention have, during the course of their research observed that the use of metal impregnated tin oxide thin film sensors prepared by the dilute solution route using spray pyrolysis technique enhances the selectivity and the sensitivity of the instrument. The metal impregnation of the tin oxide creates surface states (donor-acceptor levels) in the band gap leading to improved sensitivity and selectivity. When the gas sensing equipment deploying the modified metal impregnated tin oxide which basically works on the principle of the change in electrical resistance due to adsorption of hydrocarbon gases at a controlled temperature is operated in the presence of hydrocarbon gases diluted in air by using the standard two probe technique. The sensor is specific to hydrocarbons and is negligibly affected by the presence of common automobile exhaust
NO and CO.
Accordingly, the present invention provides an improved hydrocarbon gas sensing instrument which comprises a sample tube (A) heating by element heater (B), a sample holder (C) being placed under the above tube (A) characterized in that a sensor (D) being on the said sample holder and on which the said sensor (D) is mounted, a thermocouple (E) which is connected to a temperature controller (F) to measure and control the temperature of the sample, and a multimeter (G) to measure the resistance.
In an embodiment of the present invention the sensor is made of sensing material which is prepared using metal impregnated tin oxide thin film material prepared by the process fully described in our co-pending patent application no. 0136del2001.
The temperature of the sample is sensed by a thermocouple connected to a temperature controller. The sample (D) is placed in the sample holder and the wires coming from the sample holder are connected to a multimeter (G). Air is then passed in a controlled manner through (H) and the change in resistance of sample is measured at various concentrations of the gases by controlling the quantity of the gas injected in the sample tube either in open air or under a flow of air or nitrogen thorough (I).
In particular suitable electrical contacts were provided to the instrument by phosphor bronze/stainless steel using conducting paste of Ag, Au, Pt or RuO2. The conductivity is measured at a temperature ranging from 200-350°C with or without hydrocarbon diluted with air in the range of 10 to 10,000 ppm of volume.
The sensitivity, response and recovery times of some of the sensors are shown in Table 1. These parameters demonstrate the superiority of our material for efficient sensing of LPG.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
The thin film tin oxide sensors with 0.42wt% Ruthenium impregnation is tested for gas sensitivity after forming two ohmic contacts using conducting silver paste as described earlier. These films show a sensitivity of 2500 at an operating temperature of 250°C for 1000 ppm of Liquid Petroleum Gas. The sensitivity is defined as the change in conductance on exposure of gas to the original conductance.
The Ruthenium impregnated tin oxide thin film prepared by spraying 0.005M RuC13 in tin chloride solution in ethanol i.e. with 0.62wt% of ruthenium embedded after giving ohmic contacts is tested at temperature of 250°C. The film shows a sensitivity of 820.
The Ruthenium impregnated tin oxide thin film prepared by spraying 0.001M RuC13 in tin chloride solution in ispropanol i.e. with 0.38wt% of ruthenium impregnated after giving ohmic contacts are tested at temperature of 250°C. The film shows a sensitivity of
The main advantages of the present invention are:
1. An improved instrument with very high sensitivity towards hydrocarbon (LPG).
2. Highly selective instrument for sensing hydrocarbon gas.
3. Low cost due to use of thin film material.
4. Instrument requires low power due to use of the thin film gas sensing material.
1. An improved hydrocarbon gas sensing instrument which comprises a sample tube
(A) heating by element heater (B), a sample holder (C) being placed under the
above tube (A) characterized in that a sensor (D) being on the said sample holder
and on which the said sensor (D) is mounted, a thermocouple (E) which is
connected to a temperature controller (F) to measure and control the temperature
of the sample, and a multimeter (G) to measure the resistance.
2. An improved hydrocarbon gas sensing instrument wherein the sensor is ruthenium
impregnated tin oxide thin film.
3. An improved hydrocarbon gas sensing instrument substantially as herein
described with reference to the examples and drawing accompanying this
|Indian Patent Application Number||135/DEL/2002|
|PG Journal Number||34/2008|
|Date of Filing||22-Feb-2002|
|Name of Patentee||COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110 001, INDIA.|
|PCT International Classification Number||G01N 27/12|
|PCT International Application Number||N/A|
|PCT International Filing date|