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Trace moisture measurement method and research in natural gas

2022-07-22972

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Trace moisture measurement in natural gas

Abstract: The moisture content of natural gas in pipelines has been one of the parameters that must be tested as part of controlling the quality of natural gas. In this paper we will discuss natural gas moisture measurement methods and present typical moisture measurement instruments and their applications.


1. The measurement and control of moisture

Moisture in natural gas pipelines can cause problems such as inaccurate maintenance and testing of instruments and equipment. If the natural gas contains moisture along with H2S and CO2, it will corrode the equipment and instruments. Moisture also causes condensation and rapid expansion due to the Joule-Thomson effect at the condensation point. In addition, moisture will change the calorific value of the gas and form hydroxide in the pipeline. Therefore, the maximum moisture content of natural gas is specified in the storage and transportation process. 

There are separation methods and desiccant methods to remove moisture from natural gas. The desiccant method uses molecular filters or triethylglycol desiccant units to remove water. The separation method involves injecting methanol or ethylene glycol into the pipeline in front of a dehydration unit.

There are two methods of on-line moisture measurement measurement: direct and indirect. The direct measurement methods are electrolysis and laser method, and the indirect measurement methods are dew point chill mirror method and dew point capacitance method.


1.1. Electrolysis (phosphorus pentoxide P2O5)

The sensor pillar is plated with parallel spiral platinum layer as the electrode, the platinum layer is coated with hydrated phosphorus pentoxide film between, phosphorus pentoxide has a strong water absorption, when the chlorine gas fixed flow through the sensor circulation cell, where all the water is absorbed, resulting in phosphoric acid solution, the reaction formula is as follows.

P2O5 + 3H2O → 2H3PO4

At the same time, between the two platinum layers plus DC voltage, that is, the electrolysis reaction, the decomposition of oxygen and chlorine, phosphorus pentoxide reduction, the reaction formula is as follows.

4H3PO4→6H2+3O2+2P2O5 

It is known from Faraday's electrolysis law: the electricity required to electrolyze 1 mole equivalent of substance is 96,500 coulombs, which is also the electricity required to electrolyze 9.01 grams of water, thus it can be seen that the electrolytic current between the platinum wire electrodes is a linear function of the moisture content in the quantitative gas.


1.2. Laser method

This method involves passing light in a specific wavelength range through the sample gas and comparing the detected spectrum with a reference spectrum so that some relative change or absorption of the spectrum at certain wavelengths is measured. The amount of absorption is proportional to the moisture content of the sample gas. Not suitable for contamination with impurity particles.


1.3. Dew point method

When a certain volume of gas is cooled uniformly at a constant pressure, the partial pressure of the gas and the moisture in the gas remains constant until the moisture in the gas reaches saturation, and the temperature in this state is the temperature at which the number of water molecules in the gas reaches dynamic equilibrium to determine the dew point of the gas. The relative humidity is indirectly measured by the dew point based on temperature and pressure, but is greatly influenced by pressure and temperature.


1.3.1 Cold mirror method

The formation of condensation on metal mirrors is directly observed by instrumentation or identified electronically. When condensation is formed at the dew point, the temperature and pressure at the mirror are measured with precision and the moisture content can then be calculated from the water vapor pressure and standard data tables. It is important to note at this point that there are two dew points for moisture: the dew point for water and the dew point for hydrocarbons. It is not suitable for applications where there is contamination by impurity particles or where there is hydrogen sulfide corrosion.

1.3.2 Capacitance method

The capacitive sensor consists of a layer of aluminum trioxide (Al2O3) separated by an aluminum disk and a layer of gold or film capacitance. When the gas passes through the sensor, the moisture content is measured based on the electrical capacity. It is not suitable for applications with contamination by impurity particles or corrosion by hydrogen sulfide.


2. Sampling system

The design of the sampling system is critical to the accuracy and reliability of natural gas moisture content measurements, as the instrument is susceptible to damage from particulate matter, liquid hydrocarbons, amine glycol, methanol or other foreign substances/chemicals.

The sampling system takes a sample of natural gas from the gas pipeline, depressurizes it, keeps it at a constant temperature, and removes the harmful substances and transmits them to the measuring instrument. Sampling system components and materials that require attention.

1. Material: 316L stainless steel;

2. The shorter the sampling line, the better;

3. Pressure reducer: need to pay attention to some of the problems caused by the rapid expansion of the gas through the pressure reducer and the Joule-Thomson effect caused by liquid condensation. In some cases, a preheated sampler or multi-stage pressure reducer can be used to prevent condensation. Preferably, the pressure reducer is installed on the pipeline to increase the volume flow rate, reduce the sample dew point temperature, and reduce the sample lag time;

4. Filters: Polymeric filters are used to remove particles and droplets, glycol and other dirt from the sampling line. Note that the filter element should not leave residual or filtered water;

5. Temperature control: the application of heat tracing lines to prevent the formation of condensation in the sampling system, so that the sampling system temperature is maintained above the dew point temperature of the gas. If the temperature of the sampling system is stable above the dew point temperature of water and hydrocarbons, then the measurement accuracy will reach the best value.


3. Maintenance

The accuracy of the moisture analyzer can be affected by impurities or sampling system and other conditions, the need for timely maintenance of moisture analyzers. Some analyzers can be calibrated and maintained in situ, while some analyzers must be returned to the manufacturer for maintenance and calibration.


4. Conclusion

Most of the measurement problems are in the sampling system, and it is more difficult to measure moisture in contaminated or corrosive gases. It is relatively easy to measure in "clean" natural gas. The accuracy and reliability of the instrument depends on the selection of the equipment, the design of the sampling system, the training of the operator, and the regular maintenance and recalibration of the instrument.



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