A NEW APPROACH FOR MEASURING WATER AND HEAVY HYDROCARBON DEW POINTS OF NATURAL GASES BY USING A THREE-CHANNEL LASER INTERFEROMETER

Transcription

1 23rd World Gas Conference, Amsterdam 2006 A NEW APPROACH FOR MEASURING WATER AND HEAVY HYDROCARBON DEW POINTS OF NATURAL GASES BY USING A THREE-CHANNEL LASER INTERFEROMETER Main author A.M. Derevyagin S.V. Seleznev, A.R. Stepanov, A.G. Agaltsov NPF Vympel Company, Saratov V.A. Istomin VNIIGAZ, Moscow Region Russia

2 ABSTRACT A novel technology is used for dew point s control. Three-channel laser interferometer allows simultaneously detect three phases on an instrument mirror: liquid phase (water or suppercooled water, water solutions), solid phase (ice or gas hydrates) and hydrocarbon phase. Condensation technique of dewpoint measurement is applied. A new industrial device CONG-Prima 10 was developed. Performance specification of CONG- Prima 10 Analyzer: measurement range of water dew point temperature from -50 to +30 o C; Measurement range of HC dew point temperature from -30 to +30 o C; threshold of admissible absolute error for dew point temperature are ±0.25 o C for water and ±1 o C for hydrocarbons, maximum allowable working pressure of gas analyzed is 25 MPa; Measurement time is about min. The CONG-Prima 10 Analyzer was tested and certified by the State Standard Body of the Russian Federation (in the city of Irkuitsk, the State humidity standard is in the State Standard Institute of this city). The tests were conducted with the help of the following reference means: i) humidity gas generator ( Rodnik-2 reference dynamic generator) with an absolute error of dew point reproduction being ±0.1 0 C (for measurement of range and error); ii) high-precision unit (UVT 103-A-2001), with an absolute error of dew point reproduction being no more than ±0.1 0 C, working pressure being up to 10 MPa (for measurement of working pressure range). The special laboratory test results show that the Analyzer error does not exceed 0.1 o C. This value is comparable (equal) with the accuracy of reference means. Pressure influence on the analyzer s reading has not been found. The difference of 0.2 o C between the readings of the Analyzer in above-zero region without and under pressure can be explained by the fact that the measurements were made on different reference units with an accuracy of ±0.1 o C. Industrial CONG-PRIMA 10 Analyzer may be used for dew point s control of different compositions natural gases. Brief discussion of industrial and laboratory applications is presented. Also the Analyzer may be used for research purposes: for studying gas hydrate equilibria, detection methanol content in gas phase, kinetics of condensation from gas phase.

3 TABLE OF CONTENTS 1. Abstract 2. Body of Paper 3. References 4. List Tables 5. List of Figures

4 A NEW APPROACH FOR MEASURING WATER AND HEAVY HYDROCARBON DEW POINTS OF NATURAL GASES BY USING A THREE-CHANNEL LASER INTERFEROMETER A.M. Derevyagin, S.V. Seleznev, A.R. Stepanov, A.G. Agaltsov NPF Vympel Company, Saratov, Russia V.A. Istomin VNIIGAZ, Moscow Region, Russia 1 INTRODUCTION Process flows of associated oil and natural gases contain different admixtures in solid (mechanical impurities), liquid (absorbents, compressor oil, etc.) and gaseous state (water, methanol, vapors of absorbents and heavy hydrocarbons) a content of which define quality indices of sales gas. Temperatures of water and heavy hydrocarbon dew points are generally accepted criteria for gas quality assessment. Instruments based on condensation method of dew point measurement are known both in Russia and other countries as reliable instruments for monitoring quality of natural and associated gases. They are one of the basic components being used in systems of automation and optimization of processes in oil and gas industry. Presence of extraneous impurities in gas phase during condensation process very often leads to its accumulation on a sensitive element (mirror) of dew point s device. Therefore the correct measurement of dew point s temperatures becomes impossible. Various systems of gas clearing (filters, splitters, membranes) are used to support sensitive element in operation. In turn the application of such systems can lead to distortion of test gas due to sorption-desorption processes on filtering elements and as a result dew point s measurement may be distorted. The analysis shows that the realization of condensation method for natural gas with some impurities requires a solution of some complicated problems, including i) selective measurement of water dew point at the presence of higher hydrocarbons and glycols condensed at higher temperature in natural gas; ii) increase in sensibility of track on locating condensed water on an analyzer mirror during simultaneous condensation of other attached foreign materials; iii) complicated and unreliable design, large size and weight and power consumption. So the new approach for measuring of dew points is needed. We present an advanced gas quality control technology by means of both laser interferometer and condensation methods. Practical realization of the technology in industrial device (CONG-PRIMA 10 Analyzer) is discussed. The device may be used for dew point s control of different compositions natural gases. 2 PHYSICAL PRINSIPLES AND MEASURING PROBLEMS OF GAS DEW POINTS The analysis of methods for measuring gas humidity has shown that direct measuring procedures, of which the most promising is condensation method or method of gas dew points, are of primary priority. The method lies in measuring temperature to which a film of tested gas adjacent to a cooled surface is to be cooled in order to bring it to a state of saturation (by water and other attached foreign materials) under working pressure. This temperature was named as a temperature of dew

5 point while the instruments based on this measuring method were named as condensation instruments. Condensation method makes it possible to obtain, normally and in full measure, reliable information about water content in gas. At present there is a spectrum of condensation-type instruments manufactured by different companies in the world market. However, some instruments are out-of-date and allow only manual measurement thus leading to subjective estimation of dew point (for example, Kharkov-1M and Chandler instruments widely used in Russia). The other instruments are based on physical principles that do not allow measurement of two dew points by water and hydrocarbons at a time (for instance, Mishel hygrometer). Another instruments have been developed only as experimental models (for example, devices based on microwave registration of water content in gas phase). The analysis shows that the realization of condensation method requires a solution of some complicated problems, including 1. Selective measurement of water dew point at the presence of higher hydrocarbons and glycols condensed at higher temperature in natural gas; 2. Increase in sensibility of track on locating condensed water on an analyzer mirror during simultaneous condensation of other attached foreign materials; 3. Complicated and unreliable design, large size and weight and power consumption. The analysis of the existing physical methods for detecting the presence of condensing moisture on a mirror (i.e. on a sensor) shows that optical method of registration is the most promising. This choice is based on simplicity of measurement, reliability and stability to real heavy enough operating conditions (i.e. operation in HC gases containing besides water and HC vapors such admixtures as methanol, glycol, compressor oil and mechanical impurities). Usually an optical device includes such components as a source of light (light emitting diode LED), a light receiver (photodiode) and an object for analysis (condensation mirror or sensor). The principle of the method is simple and can be described as follows. Light emitted by LED reflects from the mirror and hits upon the photodiode. During cooling water condenses on the mirror (as a film and condensation mist near the mirror s surface) leading to the dispersion of light that is fixed by the photodiode. However, this method has one essential disadvantage, namely, rather low sensibility of an optical track to thin films of water. This disadvantage is especially negative when measuring small moisture content in gas (i.e. under high gas pressure and low dew point temperature, below 10 o C). This problem can be solved by two different ways: 1. Fiber-optic technology based on the principle of complete disturbance of internal reflection when a film of registered matter appears on the external surface of optical fiber. 2. Replacement of LED with laser emitting source and development of laser interference measuring techniques. As for fiber-optic technology, Vympel Company has found an original design option that lies in using a complete disturbance of internal reflection of light in optical fiber when condensate film appears on the condensation mirror (sensor). The external polished surface of a bend serves as a mirror, with an area of such mirror being 0.25 mm 2. This option has allowed to increase sufficiently sensibility of the optical track. It should be noted that the response of the new optical track is observed at a water film thickness of 0.1 to 0.15 mcm. The unique feature of the design option lies in the fact that quartz optical fiber performs three functions at a time. They include a condensation mirror, a device registering water film (by decreasing photo signal level when condensate film appears and when a part of light flux comes out from the optical fiber) and a waveguide itself. This physical principle (see, figure 1) has been practically realized in a family of commercial instruments, such as CONG-Prima 2, CONG-Prima 4, CONG-Prima 4P [1, 2].

6 Figure 1. Optoelectronics-based principle of measurement and sensor (condensation mirror) design TEC Peltier thermoelectric cooler; LED light emitting diode; Photodiode light receiver recording a level of optic signal passing through fiber-optic cable Nowadays, more than 300 industrial devices CONG-Prima 2 and 4 are used in Russia and CIS and some ones at Western Europe also. The practical experience show than it will be desirable to improve sensitivity and selectivity of the condensation method when detecting simultaneously both dew points on water moisture and hydrocarbons in dirty multicomponent natural gases. We choose laser interference technique for realization of the purpose (CONG-Prima 10). In laser interference measuring technique a silicon plate located on the Peltier thermoelectric cooler serves as a sensor (mirror). To register the processes taking place on the sensor during its heating and cooling an optical system (Figure 2) consisting of a source of coherent light (laser) 1, an optical track, a directing prism 2 and photodiodes 3,5 and 6 is used. The latter method is based on the effect of complete polarization during the reflection at interface of media (i.e. the entire light becomes refracted). This effect appears when light falls on the surface at so-called Brewster angle, Figure 3. A laser beam comes out from the directing prism and falls on the sensor (mirror) at Brewster angle. Three photodiodes register simultaneously changes in the intensity of a reflected beam. In the absence of condensate film on the mirror the entire impinging light becomes refracted. Therefore, values of optical signals coming to the photodiodes are very small. When the condensate film forms, the displacement of media interface, change in media refraction coefficient and, as consequence, appearance of two reflected signals that can reinforce or attenuate each others depending on the condensate film thickness take place. The dynamics of this process is fixed by the photodiode installed opposite the directing prism (direct signal). A value of the signal considered depends on a length laser wave and condensate film thickness. These data allow in principle to measure the condensate film thickness in time, at least at its initial stage of formation (given a length of light wave emitted by laser).

7 Figure 2. Laser interference-based principle of measurement and sensor (condensation mirror) design 1 Source of coherent light (laser). 2 Directing prism serving for exact directing a laser beam on the mirror at the Brewster angle. 3 - Photodiode (rear side) intended to fix scattered light intensity. When water (drops or crystals) condensates, the light increases its intensity. 4 Mirror. Mirror is a silicon plate that absorbs a laser beam when condensate is absent. The mirror temperature is controlled by a thermoelectronic battery (Peltier elements) and measured by a heatsensing device having a size comparable with a size of the mirror. 5 Photodiode (front straight) intended to fix scattered light intensity (direct signal). When HC (film) condensates, the light increases its intensity. 6 - Photodiode (front side) intended to fix scattered light intensity. When water condensates (drops or crystals), the light increases its intensity. The experiments show that the processes of water and heavy hydrocarbons condensation on the mirror (silicon plate) essentially differ. HC condensate is uniformly distributed on the mirror (sensor), while in the process of water condensation a film of liquid aqueous phase is growing to a certain level followed by the formation of microdrops (due to high surface tension of water). Later these drops may be crystallized with the formation of ice or gas hydrates (depending on thermobaric conditions). These processes are followed by a sharp increase in scattered light and attenuation of reflected light with simultaneous loss of laser emission coherence. The photodiodes 5 and 6 fix the formation of drops or crystals (in case of water freezing or hydrating). They also fix a change in scattered light; therefore, they are located from a side of the directing prism and of the photodiode that fixes a direct signal. Figure 3. Principle of operation of gas dew-point laser Analyzer 1 Impinging light; 2 - Reflected light; 3 - Refracted light.

8 When drops or crystals of liquid appear, the most change occurs on the photodiodes 5 and 6 with a value of signal on the photodiode 3 being decreased. When HC film forms and grows, a change of scattered signal on the photodiodes 5 and 6 is insufficient. At the same time a value of direct signal sharply changes depending on a HC film thickness. Thanks to the understanding of physical processes taking place on the mirror and analysis of direct and scattered signals one can clearly determine what component of gaseous phase (water or HC) condenses on the mirror. Thus, this technique allows simultaneous measurement of gas dew point temperature both by HC and aqueous phases (water, ice and gas hydrates) and precise measurement of gas dew point temperature by HC being below a temperature of water dew point. Based on the principles of laser interferometry a CONG-Prima 10 Analyzer has been developed. At present the unit has successfully passed laboratory and metrological tests and has been prepared for full-scale production. Figure 4. Initial information sensing device of KONG-Prima-10 Analyzer 1 Shell; 2 Pressure seal; 3 - Heat-sensing device; 4 Directing prism; 5, 6, 7 Photodiodes; 8 Cooler; 9 Silicon plate (mirror); 10 Laser; 11 Optical track. In figure 5 the process of water condensation at mirror of CONG-Prima-10 Analyzer is presented.

9 Figure 5. Kong-Prima-10 Analyzer s sensor and condensing mirror during the process of moisture condensation 3 NATURAL GAS DEW POINT LASER ANALYZER CONG-PRIMA 10 At present, a three-channel laser interference Analyzer has successfully passed laboratory and numerous commercial tests. The CONG-Prima 10 Analyzer provides: measurement of gas dew point temperatures by water and/or hydrocarbons; recording of measured dew points and average hour values of pressure and temperature of monitored medium (when additional pressure and temperature transducers are connected to the unit) and entering these data into volatile memory of the Analyzer; continuous digital indication of the measured dew points ( o C) and values of pressure (MPa), temperature ( o C) of monitored gas medium and recalculated humidity values (in mg/m 3 ) (when additional pressure and temperature transducers are connected to the unit); transmission of information on the Analyzer s operational state to another information-measuring systems via RS-485 interface; conversion of dew point temperature values ( o C) into another humidity units (mg/m 3 ) and conversion of measured dew point values from an actual pressure into contract pressure (e.g MPa according to Russian standard of gas quality). The main components of the Analyzer are a dew point transducer and a block performing general control of the Analyzer including a central controlling block and an interface unit (Figure 6).

10 Figure 6. General view of central controlling block (above); General view of Analyzer operated by interface unit (below: left - power supply source, in center dew point transducer, and right interface unit) The Analyzer has two design options: 1. One dew point transducer. The Analyzer is operated by the interface unit; 2. Up to four dew point transducers simultaneously operated by the central controlling block. Dew point transducer is of explosion-proof construction. Interface unit and controlling block are designed for operation in explosion-proof zones.

11 Performance specification of CONG-Prima 10 Analyzer Measurement range of water dew point temperature Measurement range of HC dew point temperature Threshold of admissible absolute error for dew point temperature: Water HC Maximum allowable working pressure of gas analyzed Measurement time Output signals Size: Dew point transducer Interface unit Central controlling block (Monitor for central controlling block) o C o C ±0.25 o C ±1 o C (pure propane) 25 MPa min Digital, according to EIA RS-485; Analog 4-20 ma 120x285x240 mm 190x482x518 mm 273x482x518 mm (356x482x102 mm) The CONG-Prima 10 Analyzer was tested and certified by the State Standard Body of the Russian Federation (in the city of Irkuitsk, the State humidity standard is in the State Standard Institute of this city). The tests were conducted with the help of the following reference means: Humidity gas generator ( Rodnik-2 reference dynamic generator) with an absolute error of dew point reproduction being ±0.1 o C (for measurement of range and error) High-precision unit (UVT 103-A-2001), with an absolute error of dew point reproduction being no more than ±0.1 o C, working pressure being up to 10 MPa (for measurement of working pressure range). The test objectives lay in determining Analyzer s accuracy, measurement range and pressure influence on Analyzer s reading, Tables 1 and 2. The test results show that the Analyzer error does not exceed 0.1 o C. This value is comparable (equal) with the accuracy of reference means. Pressure influence on the Analyzer s reading has not been found. The difference of 0.2 o C between the readings of the analyzer in abovezero region without and under pressure can be explained by the fact that the measurements were made on different reference units with an accuracy of ± 0.1 o C. Table 1. Determination of range and accuracy of the CONG-Prima 10 Analyzer during measurement of dew point temperature by water Actual dew point temperature, τ a, o C Measured dew point temperature, τ m, o C Analyzer error τ = τ m - τ a, o C Table 2. Determination of working pressure range during measurement of dew point Actual dew point temperature, τ a, o C Measured dew point temperature, τ m, o C Analyzer error τ = τ m - τ a, o C Pressure in measuring chamber, MPa

12 Table 3. Industrial places where CONG-Prima 10 Analyzers are installed Place Italy, Concern «Snam» Germany, company «RURGAZ». Gas transportation system (Dorsten) Russia, «GAZPROM», «Yamburggazdobicha», Zapolyarnoe gas field Russia, «GAZPROM», «Yugtransgaz», The Blue Stream gas pipeline Russia, «GAZPROM». «Yugtransgaz», Boundary point of Turkmenistan gas (intake from Kazakhstan to Russia) Russia, «GAZPROM». «Mostransgaz». Boundary point of Turkmenistan gas intake from Russia to Ukraine (Valuiki place) Range of measured dew point on moisture Range of measured dew point on СН Conditions and some features of operation Gas from North Sea fields, which containing С 6+ hydrocarbons, gas after preliminary conditioning in Germany Gas from North Sea fields, high- BTU gas with high contents of С 6+ hydrocarbons (Geshergas) Gas after DEG conditioning. Directly on pipeline without additional system of filtration and purification. Aerosol DEG in dried gas is exist Completely conditioned gas Wet gas without treatment Wet gas after preliminary treatment 4 Brief review on scientific research application of CONG-Prima 10 Analyzer First of all we would like to note the following unique features of the CONG-Prima 10 Analyzer as an industrial unit and as a tool for precision scientific research (in the field of condensed state physics). Separated analysis of liquid (water and hydrocarbon) phase condensed on the mirror and solid phase by reflected light at different angles, i.e. the process of condensation and three its types of phases are visualized in parallel and independent via its receiving channel; As compared to the CONG-Prima 4 Analyzer, the sensitivity of CONG-Prima 10 unit increased by an order of magnitude makes it possible to enhance accuracy of dew point measurement, to study and visualize the process of phase condensation near a cooled surface; By its physical essence, the Analyzer is a highly sensitive miniature laser interferometer allowing direct (interference) measurement of condensate film thickness on the mirror and thus analysis of an initial stage of condensed phase generation on the cooled surface and its sublimation kinetics. Schematic diagram of a stationary unit and its pneumatic circuit for conducting gas hydrate experiments is illustrated in Figure 7.

13 Figure 7 Schematic diagram of experimental assembly for stationary method of hydrate equilibrium investigation 1 Compressed natural gas cylinder; 2 Reducer; 3 Cryostat with saturator; 4 dehydrator (cartridge with silicagel); 5,6,7 Cryostats with separators; 8 - The KONG-Prima-10 reference dewpoint analyzer. The above experimental technique can be applied for [3]: Carrying out experimental study of gas hydrate equilibria based on hydrate-forming gases both for pure gases and binary and multi-component mixtures. The data obtained can be used as an experimental basis for developing improved dynamic and thermodynamic models of low-pressure gas hydrate phases and, first of all, for gas hydrates of structures I and II. Carrying out multi-factor study of the influence of volatile admixtures in natural gas on dew points by all aqueous phases (water, supercooled water, water-methanol solution and other alcohol solutions, DEG, ice and also gas hydrates). Refining data on methanol solubility in natural gas under equilibrium with aqueous solutions of methanol in a wide range of pressures and temperatures. Studying the physics of supercooled water and aqueous solutions at temperatures below zero o C (features of water vapour condensation from natural and other gases; time of the existence of metastable phases as function of temperature, pressure, film thickness, etc.) and for studying kinetics of condensation on a cooling surface different substances (dissolved in gas phase).

14 5 CONCLUSIONS A novel approach for a dew point s control was developed. It gives us possibilities to detect simultaneously three dew points of natural gas: liquid water (stable or metastable), gas hydrate (or ice) and hydrocarbon dew points. Laser multichannel interferometer technique is applied to increase sensitivity and realized measuring different dew points on the same cooling surface. The method is realized in industrial device dew point s Analyser Cong Prima 10. Analyzer "CONG-PRIMA-10" possesses the resistibility to action of different impurities. Also the intellectual monitoring system of a sensitive element (mirror) and system of self-cleaning are incorporated intodevice that allows to use the Analyzer without additional filtration system. Analyzer give us all possibilities of the practical realization global gas dew points monitoring system on gas transmission lines of Russia. The system allows to reveal and remove the reasons of subquality of gas flows in automatic mode and thus to secure the reliability of gas transmission system operation and sales gas quality. Analyser may be also used for research purposes: for studying gas hydrate equilibria and kinetics of condensation, detection of methanol content in gas phase, etc.

15 REFERENCES 1. Derevyagin A.M., Seleznev S.V. and Stepanov A.P. (2002). KONG-Prima-4 Dewpoint Analyzer by Moisture and Hydrocarbons. Nauka i tekhnika v gazovoyi promyshlennosti, (1):5-22 (in Russian). 2. Derevyagin A.M., Seleznev S.V., Stepanov A.P., Agaltsov A.G., Konyakhin A.N. and Istomin V.A. (2003). KONG-Prima-4P Portable Dewpoint Analyzer by Aqueous Phases and Heavy Hydrocarbons: Design Features and Prospects for Technological Application. Nauka i tekhnika v gazovoyi promyshlennosti, (4):37-44 (in Russian). 3. Istomin V.A., Derevyagin A.M. and Seleznev S.V. (2002). A dew point technique for experimental study of gas hydrate systems. Proceedings of the 4 th International Conference on Gas Hydrates, Yokogama (Japan):

16 List Tables 1. Table 1 Determination of range and accuracy of the CONG-Prima 10 Analyzer during measurement of dew point temperature by water 2. Table 2. Determination of working pressure range during measurement of dew point 3. Table 3. Industrial places where CONG-Prima 10 Analyzers are installed

17 List of Figures Figure 1. Optoelectronics-based principle of measurement and sensor (condensation mirror) design Figure 2. Laser interference-based principle of measurement and sensor (condensation mirror) design Figure 3. Principle of operation of gas dew-point laser Analyzer Figure 4. Initial information sensing device of KONG-Prima-10 Analyzer Figure 5. Kong-Prima-10 Analyzer s sensor and condensing mirror during moisture condensation the process of Figure 6. General view of central controlling block (above); General view of Analyzer operated by interface unit (below: left - power supply source, in center dew point transducer, and right interface unit) Figure 7. Schematic diagram of experimental assembly for stationary method of hydrate equilibrium investigation