Elsevier

Fluid Phase Equilibria

Volume 530, 15 February 2021, 112873
Fluid Phase Equilibria

Development of a new method for measurement of the water dew/frost point of gas

https://doi.org/10.1016/j.fluid.2020.112873Get rights and content

Abstract

The water content of gas is of importance in a wide variety of areas such as oil and gas, carbon capture and storage, medical, nuclear, food and hydrogen cells. In cases where pressurised gas is being used or transported in pipelines (i.e. natural gas, carbon dioxide, compressed air....) changes in temperature or pressure may result in water condensing. The condensed water may cause a number of issues such as corrosion, ice and hydrate formation. In order to avoid water condensing the gas needs to be dried to a level where condensation will not occur at any temperature/pressure conditions encountered. There are a wide variety of methods and equipment available for making water content measurements in laboratory or industrial processes. The available devices vary in terms of different parameters such as accuracy, long-term stability, sensitivity to contaminants, response time, pressure rating, initial and running costs. This paper introduces a new method that can potentially be incorporated into equipment for use in both laboratory and field applications for accurate dew/frost point measurements at a wide range of pressures. Initial measurements have been made for nitrogen, methane and natural gas and the results have been compared with literature data and model predictions.

Introduction

Accurate measurements of water content are required for two main reasons. Firstly, in order to provide data that can be used to develop and validate thermodynamic models used for optimisation of drying units required for safe and economical design of pipelines and facilities. Secondly, in order to continuously monitor water content allowing early warning of the presence of “off-spec” gas that may lead to significant problems. There are a number of instrument technologies currently available for measurement of water content of gases including capacitance sensors, quartz crystal microbalance (QCM),electrolytic cell, CaC2-GC, fibre optic sensor, Karl Fischer titration, chilled mirror, chilled surface acoustic wave sensor and Tuneable Diode Laser Absorption Spectrometer (TDLAS). Detailed descriptions of the different technologies have been presented [1] and comparisons of different equipment have also been made [2], [3], [4], [5], [6], [7].

As there have been a number of reviews comparing the attributes of the different technologies it is not necessary to repeat the findings in detail. However,it is worth mentioning a few of the main points relating to the most usedequipment, in order to set out how the new method may compare, should it prove a robust and reliable approach. The chilled mirror is considered asthe most reliable method as it directly measures the dew/frost point, a thermodynamic property [8,9]. As such it is used as a NIST-referenced humidity transfer standard [10]. In cases where components other than water may condense on the mirror such as alcohols or hydrocarbons the measurements may not be accurate. In addition, if water soluble salts are present and accumulate on the mirror surface erroneous readings will occur. As with chilled mirror, Karl Fischer is also regarded as an absolute or fundamental method. The test gas is passed through a small volume of absorbent solution, which is subsequently titrated with Karl Fischer reagent [11,12]. Capacitance sensors use changes in the impedance, measured as a function of water molecules adsorbed to the porous dielectric of a capacitor [8,13,14,16]. Capacitance sensors are, in general, less accurate than chilled mirror devices. They tend to drift and can be slow to react. In addition, the presence of alcohols may have an influence on the measured values. On the plus side, they are very sensitive to changes in water content, can be used at pressure and are less expensive than most otherdevices. QCM devices use changes in the resonant frequency of a hygroscopic polymer coated QCM to measure water content [8,10,13,[14], [15], [16]]. They are fast to react although they are expensive and may require regular maintenance. TDLAS sensors work by applying the Beer-Lambert Law to a beam of light at the water absorption frequency, passed through the gas [17]. TDLAS hygrometers are quick to react and accurate, however they cannot be used at pressure, need to be recalibrated if the background gas changes and are expensive. Fibre optic or Fabry-Perot hygrometers work by measuring a shift in the spectrum of reflected light dependenton the amount of adsorbed water in a hygroscopicFabry-Perot filter [18]. They can be used at line pressure, however can be slow to respond, are expensive and may drift over time. It is difficult to compare the accuracy of all devices because different values are quoted by different manufacturers. Accuracies are also indicated in terms of dew/frost point or ppmV or as a percentage value. As there is not a linear relationship between dew/frost temperature and ppmV, care should be taken when make comparisons. In the case of most chilled mirror instruments on the market the dew/frost point is displayed along with the calculated ppmV value adjusted for the pressure of the measurement cell. Overall, reviews consider the chilled mirror to be the most accurate with a stated value of between ±0.1 and 0.5K Table 1 summarises uncertainty figures, for the methods described above, from two different reviews. It should be mentioned that this is not a complete list of all available types of hygrometer.

Integrated experimental and thermodynamic investigations into the water content of gas have been ongoing for the past 20 years at Heriot-Watt University. A number of measurement options for water content have been evaluated, including GC, capacitance aluminium oxide, silicon and polymer capacitance sensors, TDLAS and chilled mirror. Data generated using the developed capabilities have been presented in several publications [19], [20], [21], [22], [23], [24]. In 2015 some of the published data was validated by new measurements made in other international laboratories, the publication [25] stated that the good match between our data and those from other laboratories directly influenced a decision on drying requirements for the Alaska Stand Alone Pipeline translating to a cost saving of millions of US dollars. Although successful in generating accurate data using existing technology, it has become apparent that there is a requirement for an alternative method that can be used to make accurate measurements over a wide range of water contents, pressure and temperature conditions and compositions. This has been the driver to seek a new approach.

Section snippets

New method

The method is based upon a relatively simple principle, similar to a chilled mirror, in that it measures the dew or frost point which can then be used to calculate the water content [9]. This method is different to all other documented methods, as far as the authors are aware. In the case of modern chilled mirror devices, the presence of water/ice on a chilled polished surface is detected by monitoring reflected light. In the proposed method, the presence of water/ice is detected by changes in

Experimental equipment and methods

The experimental equipment (Figure 2) is comprised of a temperature controlled, variable volume equilibrium cell and a heated line and valve through which gas equilibrated with water, ice or hydrates is passed to the temperature controlled flow tube and then to the capacitance hygrometer. A flow meter is located after the hygrometer in order to monitor the flow rate of gas controlled by the heated valve. The equilibrium cell is a 300 mL, Titanium piston vessel rated to 69 MPa. The cell is

Test fluids

For the example test (Figure 1) with CO2 a Spectra-Seal® 104 ppmV (±5%) water in CO2certified standard supplied by BOC Ltd, was used. Nitrogen and Methane, with purities as shown in Table 2 below were used. Natural gas with a composition shown in Table 3, supplied by BOC Ltd, was used. Distilled water was used in all tests.

Thermodynamic modelling

A thermodynamic model, Heriot-Watt PVT (HWPVT),was used to help validate the experimental measurements along with literature data. The model uses the CPA-EoS to determine componentfugacities in fluid phases. The CPA-EoS combines the well-known Soave-Redlich-Kwong (SRK) EoS for describing the physical interactions with the Wertheim's first-order perturbation theory, which can be applied to different types of hydrogen-bonding compounds. The hydrate phase is modelled by using the solid solution

Nitrogen

Measurements for the frost point of nitrogen equilibrated with water, at different pressures, were made at 274.1 and 283.1 K. The water contents calculated from the frost point temperature at atmospheric pressure are given in Table 4 and plotted along with literature data and predicted values in Figure 3. As can be seen, for the data and predictions at 283.1K, there is a good agreement between experimental data and model predictions in all cases apart from the highest pressure value reported by

Discussion

As discussed in the introduction, accurate measurements of the water content of gases are important to give data for validation of thermodynamic models and to monitor levels in different facilities. This enables safe design and operation, avoiding problems such as corrosion and the formation of ice or hydrates. The existing technologies, available for making these measurements, have a number of pros and cons. This paper presents a new approach based upon a simple principle, which may have

Conclusions

This paper presents a new approach to measurement of water dew/frost point in gases. Experimental measurements have been made, using a prototype set-up, for nitrogen, methane and natural gas. The results are in good agreement with both literature data and model predictions thus validating the new method. The estimated accuracy of this method (± 0.1 K) is equivalent to chilled mirror hygrometers, which are currently considered to be the most reliable devices for water dew/frost point

Acknowledgements

This work is being conducted in support of projects being supported by Galp Energia, Linde AG, Petrobras, Petronas, Equinor and Total which is gratefully acknowledged.

Valderio de Oliveira Cavalcanti Filho acknowledges financial support from Petrobras through his PhD grant.

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