High pressure vapor-liquid equilibrium measurements of methane and water mixtures using nuclear magnetic resonance spectroscopy

Abstract

Gas composition, which can vary from location to location in natural gas pipelines, constrains the allowable operating conditions and compressor package design. Compressor systems are designed such that they provide the optimal balance between efficiency and gas throughput with safety margins to maintain component lifetime. The presence of liquid in the compressor can lead to excessive wear of intake and discharge valves and impact performance. To prevent ingestion of liquid slugs, operating conditions and separation equipment must be selected appropriately using mixture dew point calculations from commercially available mixture property prediction software such as NIST-REFPROP. NIST-REFPROP is highly reliant on mixture Vapor liquid Equilibrium (VLE) data to predict phases. Thus, there is a need for low uncertainty VLE data for gas mixtures at pressures (1 - 10 MPa) and temperatures (<0 – 100 °C) experienced within natural gas infrastructure, especially for mixtures containing H2O, which would lead to more accurate dew point calculations and allow designers to maximize system performance without compromising component wear and tear. For a mixture comprised completely of hydrocarbon species, VLE calculations at high pressures are accurate as the interaction parameters between the constituents are close to unity and there is typically a wealth of low-uncertainty data available. However, when H2O is present in natural gas significant intermolecular interactions cause the mixture VLE to deviate from ideality. In order to accurately model the VLE of these mixtures, the energy associated with these interactions must be known and accounted for in the calculations. As such, high quality experimental VLE data are needed to improve and validate the thermodynamic models. Nuclear magnetic resonance (NMR) spectroscopy allows for high-quality data collection for water containing samples. This thesis provides the groundwork for using NMR spectroscopy to conduct low-uncertainty VLE measurements of water-hydrocarbon mixtures. Two NMR spectrometers were investigated, and methods were developed to accurately characterize the temperature, pressure, vapor phase and liquid phase molar composition of methane-water systems at equilibrium, the five conditions required for VLE measurement. Preliminary results for low pressure (0-2.06MPa) samples of methane and water showed that the liquid phase methane compositional data taken utilizing NMR spectroscopy significantly deviated from the NISTREFPROP model, revealing the lack of low uncertainty VLE data required to determine the needed interaction parameters for methane and water systems. Future work should target the collection of the high-fidelity methane-water VLE data, and NMR spectroscopy has the potential to perform this task.