Resumen de A comparative study of traditional and novel humidity sensing technologies in natural gas
This thesis was carried out inside of the temperature and humidity laboratory (LabTH) of the Instituto Nacional de Técnica Aeroespacial (INTA), which is the Spanish Designated Institute (DI) for the field of humidity and, therefore, holder of the national standards in such quantity. Among many other duties of a DI are to promote and participate in national and international R&D projects whose goals set were in line with those scored in the last European Roadmaps for the hygrometry field, for this particular case.
For these reasons, and as a part of such participation in research projects, it is essential to emphasize that part of this thesis was developed inside of the framework of the joint research project “ENG01-Gas. Characterization of Energy Gases” which running under the European Metrology Research Program (EMRP), with the main target of supporting the natural gas industry to overcome its current big challenges.
Moisture content is a key factor inside the natural gas (NG) industry. Not only does decrease the efficiency of the final product when the water content is high, but it can also trigger potentially dangerous conditions due to the formation of hydrates, that can block pipelines and damage pumping devices and produce other negative effects. For these reasons, accurate measurement and control of humidity is essential in all processing step of natural gas.
However, current practice for such moisture measurements is for the devices to be calibrated at atmospheric pressure and in nitrogen or air as the matrix gas, conditions that vary significantly from those present in the actual industrial process, so their performance in real industrial conditions could be much smaller than expected and even insufficient. Besides, technologies traditionally used by NG industry, such us, condensation hygrometers and metal oxide capacitive probes, have some drawbacks that not always allow to measure and control humidity of NG with the accuracy and reliability desired.
For all of that, metrology plays a very important role developing and validating new highly accurate measuring methods to achieve the industrial uncertainty required. In this sense, a total of 14 hygrometers, based on 6 different measurement technologies were studied. Techniques traditionally used, such as: condensation hygrometers, electrolytic and capacitive sensors based on metal oxides; were compared to novel and promising measuring technologies, which constitute the state of the art in the world of hygrometry, such as: microwave resonators, absorption spectroscopy and polymeric capacitive sensors.
Comparative research of all these technologies was performed in two industrial facilities to simulate the most similar actual conditions in which they are used. First part of comparison was carried out in a laboratory using NG coming from the national distribution grid at absolute pressures up to 6 MPa directly and for the useful humidity range for the NG industry. The second part was performed in a drying plant associated to an underground gas storage (UGS) facility during all its production processes (injection and extraction) and during the intermediate shutdowns.
The properly facilities needed for this comparative study were designed and assembled according to the standards and good practice guides about the sampling and handling of NG at high pressures.
Measurement protocols were developed and various properties of the equipment were evaluated, thus validating relative performance of the different technologies with each other. Characteristics of the dynamic behaviour of the equipment (response times to changes in humidity and variations due to changes in pressure), as well as studies of short-term drift, hysteresis and linearity, were performed.
Most instruments exhibited good behaviour regarding response time, hysteresis and under sudden pressure changes. In spite of the differences exhibited by certain equipment, due to factors and inconveniences explained, the consistency exhibited by the results demonstrates the validity of the experimental and analysis methods used, demonstrating the utility of all technologies for the requirements of the gas industry, with particular aspects in all cases.
It must be highlighted the good behaviour of spectroscopic technology in every properties studied and, therefore its good performance in any condition evaluated. By the other hand, it must also be remarkable the promising outlook of the microwave resonator, a prototype still in development phase, as a humidity sensing technology in many applications.
In addition, it participated in the development and validation phase of the quasi-spherical microwave resonator by evaluating its performance with real NG, for a water content range between 25 ppmv and 100 ppmv, and at a pressure of 0.75 MPa. Its performance was quite satisfactory showing itself as a promising technology in the field of hygrometry, but with issues yet to be developed for real industrial applications.
As a proof of direct cooperation carried out in this thesis between manufacturers of measuring instruments, public research entity (INTA) and the Spanish national gas distributor (Enagás), some of the immediately outcomes were: renovation of part of the drying plant of Serrablo (Enagás) with a modern sampling system and, acquisition one of the spectroscopic instruments to be used in the most critical points of the national distribution grid.
Summarizing, this thesis improved the knowledge of the humidity measurement in non-conventional gases traditionally used in national metrology institutes (NMIs) or DIs, increased the knowledge of the measurement technologies that resulted in the implementation of new improvements and updating in commercialized measurement equipment, and a much more accurate and reliable humidity control especially during the gas drying process.
