Method and apparatus for gas analysis

The invention relates to a method and an apparatus according to the preambles of the independent claims related thereto.

Observing of impurities in gas is important particularly in use of ultra clean gases, such as e.g. in use of cooling gases for military missiles, in maintenance and service of gas networks in industry, in gas factories, in laboratories etc. In most cases the analysis is nowadays based on cryostatic cooling of a process gas, whereby the impurities of the gas are "revealed" e.g. due to the same blocking a gas flow channel in an analyzer when the gas gets frozen before the boiling point of the gas to be analyzed is reached. In this connection in the cooling particularly a cryoanalyzer has been found cost efficient, the operating principle of which corresponds to a so called JT-cooler (Joule-Thomson cooler) , which is based on cryocooling of gas by leading a gas e.g. from a gas generator, a gas bottle or a gas network through a dryer and/or a filter to an discharge cell in zero pressure, in which the pressure of the gas quickly decreases absorbing energy. The discharge cell is vacuum isolated and inside the same is a feed pipe, which gets cooled in a JT-cooler operating e.g. on the principle shown in figure 1 in less than two seconds in practice to the boiling point of the gas to be analyzed due to a drop in temperature, which takes place by influence of expansion of the gas when it gets discharged from the feed pipe.

The type of cooler described above is being used in connection with operating processes of most heterogeneous gases, such as e.g. oxygen, argon or zero air, in which case the meaning is to discover impurities in the process gas in a way that impure gas 11 MAY 2012

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may not enter the actual operating process in order to avoid production interruptions and/or even device damages. By using of an impure gas in a critical operating process, there has been caused in practice significant technical, economical and operational damages .

In connection with cryocooling, it is furthermore known to use monitoring arrangements based on measurement of a counterpressure of a cryoanalyzer, in which case the gas, being exhausted from the discharge cell, is being led to a mass flow measuring instrument, such as a rotameter, whereby a momentary value of the flow defined by the same is being registered at adequately short time intervals. The pressure of the gas to be fed into the discharge cell is being measured simultaneously and its momentary values are registered at corresponding time intervals.

Functioning of the type of cryoanalyzer described above is based on calibrations that are performed at certain time intervals by using clean gas. Because the accuracy of mass flow measuring instruments does not stay constant in practice e.g. depending on the operating process and on gas to be analyzed, the calibration of this kind of counter pressure monitoring arrangements needs to be performed at very short time intervals for the sake of certainty. A consequence of a calibration, being performed carelessly or insufficiently, is that there is no certainty of adequate purity of the gas, in which case there is a risk that an excessively impure gas may enter the operating process.

It is the aim of the method and apparatus according to the present invention to achieve a decisive improvement in the problems described above and thus to raise essentially the level of prior art. In order to carry out this aim, the method and apparatus according to the 11 ZUlZ

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invention are mainly characterized by what has been presented in the characterizing parts of the dependent claims related thereto. As the most important advantages of the method and apparatus according to the invention may be mentioned simplicity and efficiency of the configurations applicable for the same as well as use thereof, in which case by extremely simple principles and measures it is made possible to maintain optimal purity of a process gas in a way that not any kind of risk will be caused due to impurity of a gas to its operating process. On the simple operating principle of the invention, thanks to the detector arrangement monitoring in a concrete manner the temperature of the expansion process of the gas, it is possible to make sure adequate purity of the gas to be analyzed in an extremely simple way. When the expansion temperature, being measured at the point where the gas gets discharged, gets below e.g. when analyzing nitrogen temperature ~ -185 °C, the purity of the gas is confirmed, because the analyzed gas does not have e.g. methane, the boiling point of which is -161,6 °C and the melting point -182,5 °C (the boiling points e.g. water, carbon dioxide and other carbon hydrides being clearly above -100 °C) . By using advantageously e.g. resistance temperature detectors, such as PT 100, PT 1000 and/or the like, one and the same analyzing process can be exploited uninterruptedly over a very long time period without a need for calibrations to be performed at certain time intervals thanks to the very good measuring accuracy and constancy of the detectors in question, in addition to the above having also a very profitable purchase value.

The invention makes thus possible optimization of operating costs and technical functioning in utilization of operating processes of most heterogeneous gases without a risk of an impure gas entering the operating process, in which case first of all not any production interruptions will be caused thereto due to calibration measures that need to be performed at unnecessarily short intervals. There will not be caused a risk, either, if there is no time/ willingness to carry out a calibration to be performed at any given time.

Other advantageous embodiments of the method and apparatus according to the invention have been presented in the dependent claims related thereto.

In the following description, the invention is illustrated in detail with reference to the appended drawings, in which in figure 1

is shown generally the operating principle of a Joule-Thomson -cooler, in figure 2

is shown the operating principle of the method and apparatus according to the invention, and in figure 3

is presented a gas expansion process temperature curve as a function of time.

The invention relates first of all to a method for gas analysis, whereby a gas, such as nitrogen, oxygen, argon, zero air or like, is being analyzed by an analyzer 1 operating by a cryogenic cooling process based on expansion of gas, in order to observe contamination of the gas, such as existence of water, carbon hydrides, carbon dioxide and/or like therein. The analyzer comprises a vacuum isolated expansion 11 MAY 2012

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chamber la and a flow arrangement lb, such as a spiral pipe, being led therein, whereby the gas to be analyzed, when getting discharged from the above into the expansion chamber in a zero pressure gets liquefied, whereby the contamination of the gas is being observed by monitoring means 2 monitoring liquification of the gas. The liquification of the gas is being monitored e.g. on the principle shown in figure 2 by a temperature detector assembly 2a in the expansion chamber la, in order to observe the contamination of the gas based on an expansion temperature Tl discovered by the same to exceed essentially the boiling point of the gas to be analyzed, such as the temperature being corresponding or higher than a melting point of a certain impure gas or gas mixture.

As an advantageous embodiment of the method, the liquification of the gas to be analyzed is being monitored visually through an at least partly transparent casing lal of the expansion chamber la.

Furthermore as an advantageous embodiment, the analyzer 1 is being calibrated by cooling with the same a clean gas to its boiling point, such as nitrogen to temperature 77, 35 K /-195, 8 °C . In figure 6 there has been presented a temperature curve as a function of time for cryocooling in analyzing of nitrogen. In the curve there has been marked also the boiling points of water, carbon dioxide and certain carbon hydrides. In case the gas to be analyzed has such impurities, the impurities when getting liquefied in the temperature at the boiling point thereof and after that further getting frozen in the melting point thereof, decrease for their part the expansion temperature of the gas to be analyzed in a way that the expansion temperature Tl that is characteristic to the clean gas when getting discharged will not be reached. When analyzing other 11 MAY 2012

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gases, the analyzer will naturally be calibrated based on the boiling points thereof, whereby e.g. the boiling point of argon is -185, 88 °C and oxygen -182,9 °C . Furthermore as an advantageous embodiment the analyzation pressure of the gas to be analyzed is being adjusted to 100 - 160 bar.

As a further advantageous embodiment of the method, the expansion temperature Tl of the gas to be analyzed is essentially higher than the boiling temperature of the gas in question, the pressure of the gas is being decreased essentially lower than the analyzation pressure, profitably e.g. 10-20 bar, whereafter a melting flow of the gas is maintained, until desired melting temperature, profitably e.g. 10 - 20 °C, will be achieved for restarting the gas analysis process after checking of the gas source and the dryers and/or filters coupled therewith etc. and taking care of necessary reparation/replacement measures.

Furthermore as an advantageous embodiment in connection with the method, a control unit 3 is being exploited that is at least in a data transmitting connection with the analyzer 1, by means of which an operating process of the gas is being controlled based on measuring information of the analyzer 1.

The invention relates on the other hand also to an apparatus for gas analysis, which comprises an analyzer 1 operating by a cryogenic cooling process based on expansion of gas for analyzing a gas, such as nitrogen, oxygen, argon, zero air or like, in order to observe contamination of the gas, such as existence of water, carbon hydrides, carbon dioxide and/or like therein. The analyzer comprises a vacuum isolated expansion chamber la and a flow arrangement lb, such as a spiral pipe, being led therein, whereby when the gas, getting discharged from the above into the expansion chamber in 11 MAY 2012

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a zero pressure, gets liquefied, in order to observe contamination of the gas by monitoring the liquification of the gas by monitoring means 2. The expansion chamber la is provided with a temperature detector assembly 2a e.g. on the principle shown in figure 2, in order to observe the contamination of the gas based on an expansion temperature Tl, discovered by the same to exceed essentially the boiling point of the gas to be analyzed, such as the temperature being corresponding or higher than a melting point of a certain impure gas or gas mixture.

As an advantageous embodiment of the apparatus,, the expansion chamber la comprises an at least partly transparent, such as glass pipe structured, casing lal in order to enable visual monitoring of the liquification of the gas to be analyzed.

The apparatus comprises furthermore advantageously a control unit 3 that is at least in a data transmitting connection with the analyzer 1 in order to control an operating process of the gas based on measuring information of the analyzer 1. Furthermore as an advantageous embodiment in practice, the temperature detector assembly 2a comprises one or several resistance temperature detectors, such as PT 100, PT 1000 and/or like. It is clear that the invention is not limited to the embodiments presented or described above, but instead it can be modified within the basic idea of the invention in very many ways. It is thus first of all clear that in applying the method, also other type of coolers can be exploited instead of analyzers based on

JT-cooler, by means of which the boiling point of a gas to be analyzed at any given time can be achieved. Furthermore the operating diagram presented in figure 11 MAY 2012

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2 is simply of exemplary nature, which is why dryers, filters or control, regulating, by-pass and/or back flow valves etc. that are needed for processes in practice are not shown.