Elsevier

Fluid Phase Equilibria

Volume 329, 15 September 2012, Pages 55-62
Fluid Phase Equilibria

Development and testing of a new apparatus for the measurement of high-pressure low-temperature phase equilibria

https://doi.org/10.1016/j.fluid.2012.05.024Get rights and content

Abstract

A new apparatus for the study of high-pressure phase equilibria at low temperatures using an analytical method was designed, assembled and tested. The apparatus was specially developed for the study of multi-phase equilibria in systems containing hydrocarbons, water and hydrate inhibitors, at temperatures ranging from 213 K to 353 K and pressures up to 40 MPa. The core of the apparatus is a variable-volume equilibrium cell, equipped with a 360° sapphire window and connected to an analytical system by three capillary samplers.

The quality of the apparatus was confirmed through several tests, including the study of the system methane + water. An equilibrium point for the quaternary system methane + n-hexane + methanol + water is also presented.

Highlights

► A new apparatus for the study of high-pressure phase equilibria at low temperatures using an analytical method is described. ► The experimental set-up was tested, namely through the study of the binary system methane + water. ► A test was made with the quaternary system methane + hexane + methanol + water. ► The comparison of the obtained results with literature values confirms the quality of the new apparatus.

Introduction

Vast quantities of so-called “production chemicals” are used by the natural gas industry in order to facilitate production from reservoirs and transport in pipelines. Examples of such chemicals are hydrate inhibitors such as methanol or glycols, injected to the natural gas well stream to prevent the formation of gas hydrates during transportation and further processing. The relevance of the problem increases with the exploration of arctic fields and the tendency towards long distance offshore pipelines placed on the seabed. The blocking of a pipeline due to the formation of hydrates translates into high economic losses, not only due to the direct costs for solving the problem, but also due to the consequent disruption in the production.

Hydrate inhibitors are usually transferred in a dedicated pipeline from land and injected offshore into the well stream at the choke [1]. Arriving onshore is a mixture of gas, condensate, water and inhibitor with all the components distributed through all the phases in equilibrium. After separation, the gas undergoes further treatment such as dehydration, and the inhibitor is treated before being recycled back to the gas field. For the optimisation of these and other processes, the precise knowledge of the phase equilibria involving these compounds is crucial, especially in cases of very low concentrations, e.g.: the amount of glycol present in the gas phase.

In order to provide an idea of the amount of money spent on hydrate inhibitors, Sloan [2] mentioned the example of the Canyon Express project in the Gulf of Mexico with a combined tieback gas production of 500 MMSCFD of dry gas. The costs involved in the use of methanol as hydrate inhibitor are approximately 18 million USD per year, already considering a saving of 4–5 million USD per year supported by the recovery and recycling of the methanol.

Hydrate inhibitors are used in large excess, to account for wide safety margins. A reduction of 5% in the amount of methanol used, based on a better knowledge of its phase equilibria, could lead to additional savings close to 1 million USD per year in costs of methanol only.

Computational methods for prediction of phase equilibria have made considerable progress in recent years, but the experimental measurement of phase equilibrium remains an indispensable source of data, especially where theoretical methods fail, e.g. for high pressures, in the characterisation of complex systems, with high concentrations of acid gas or in the estimation of the partition of the inhibitor between the vapour, aqueous and organic phases [2]. Continuous developments in science and industry require data for new types of compounds, or simply for new temperature and pressure ranges. The oil and gas industry is a good example of this. As oil prices increase, new locations become profitable for drilling, often meaning new product compositions, more extreme reservoir pressure and temperature and/or new climatic conditions. In especially sensitive areas, environmental regulations may furthermore impose limitations on emissions, requiring higher efficiency separation processes, or may even prohibit the use of some of the common production chemicals, which then have to be replaced with new and more environmentally friendly compounds.

Even for simpler systems, which have been studied at various times over the years by different researchers, the data is sometimes scarce in particular ranges of pressure and temperature. In other cases, the abundance of data reveals considerable discrepancies in the results obtained by different research groups, as recently demonstrated for example by Folas et al. [3], who made an analysis of literature values for the system methane + water.

Computational models, molecular simulations and correlation methods can be used to reduce the number of experimental data points to be measured, but experimental data will always have a decisive role in the validation of theoretical methods and in the adjustment of parameters in correlations [4].

The importance of reliable and precise experimental phase equilibrium data is unanimously recognised by the scientific community as well as by industry [5], although the real accuracy of published experimental data does not always correspond to the accuracy claimed by the authors for the apparatus used. Reliable and precise measurements are difficult to achieve and often the quality of the staff performing and supervising the experiment can have a higher impact on the results than the equipment itself [6]. Experience is very important, since experimental difficulties and mistakes are rarely published, though they are an essential part of the know-how for measuring high-quality data. A survey of the European Federation of Chemical Engineering on industrial needs for thermodynamics and transport properties [7] showed that there is a clear need for qualified laboratories with experienced staff to provide experimental measurements.

The present work aims to provide a contribution to the generation of high-quality data, through developing new experimental equipment for the study of phase equilibria. Here an attempt is made not only to describe the apparatus, but also to underline some of the more critical aspects of the development process.

The need for a full characterisation of all the phases in equilibrium, including the quantification of residual amounts, and the interest in measurements at very low temperatures (so as to replicate polar conditions for example) was confirmed in discussions with the industry. With this in mind, it became clear that the preferred method for the phase equilibria studies would be an analytical isothermal method [6], [8], [9], [10]. This implies a higher complexity of the equipment, with the necessary development of an analytical method and a sampling procedure. Nevertheless, analytical methods allow a better understanding of the complex equilibrium systems under study.

Section snippets

The new apparatus

A new experimental set-up for the measurement of multi-phase equilibria at temperatures ranging from 213 K to 353 K and at pressures up to 40 MPa was developed, using an analytical isothermal method [6], [8], [9], [10].

The most immediate challenges in the development of the apparatus were related to the desired temperature range of application. Working at temperatures below 230 K imposes severe limitations on parts and components to be used.

Other challenges were related to the use of an analytical

Chemicals

In order to confirm the quality of the results provided by the new apparatus, different tests were performed. A first series of tests focused on the quality of the temperature and pressure measurements. Two substances were tested, ethane and carbon dioxide.

In a second series of tests, performed to evaluate also the sampling and the analytical method, experiments on the binary system water + methane were performed. Milli-Q deionised water was used.

Finally, a few measurements were made with the

Discussion

The results of the tests performed in this newly developed apparatus were very satisfactory, not only in demonstrating the quality of the experimental set-up, but also in providing an essential insight on what the important aspects of the experimental procedure are, in order to obtain good results.

The study of the binary and quaternary systems was not exhaustive, due to time limitations of the project. They were intended to demonstrate the proof of principle of the method and to test the

Acknowledgements

The authors are grateful to the Danish Technical Research Council for the financial support to this work, as part of the project “Gas Hydrates – from Threat to Opportunity”.

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    Current address: Bayer Technology Services GmbH, Property Data & Thermodynamics, Geb. B310, D-51368 Leverkusen, Germany.

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