Phase equilibria modeling of inhibited gas hydrate systems including salts: applications in flow assurance, seawater desalination and gas separation

Abstract

Accurate hydrate phase equilibria and vapor-liquid equilibria predictions are critical to the safe and economic design of flow assurance, gas processing, and seawater desalination technologies. Inaccurate predictions of vapor-liquid equilibria can also lead to erroneous hydrate phase equilibria predictions. Hydrate phase equilibria predictions typically use the classical van der Waals and Platteeuw (vdWP) model based on statistical thermodynamics, with some modifications (e.g. CSMGem, with Gibbs Energy Minimization). In this thesis work, the vdWP model with Gibbs Energy Minimization algorithm was developed in Matlab. The developed algorithm was evaluated by first investigating the effect of hydrogen (H2) concentration on the phase equilibria of sH hydrate (i.e. H2O+CH4+H2+methylcyclohexane (MCH) quaternary systems). The predictions were shown to be in close agreement with experimental phase equilibria measurements. Cage occupancies of methane and hydrogen in sH hydrate were predicted to increase with increasing pressure, and the extent of occupation was found to be dependent on the methane: hydrogen ratio in the feed gas. Current hydrate phase equilibria predictions (using different models, e.g. CSMGem, Multiflash and PVTsim) for inhibited systems in subsea pipelines (with salts, e.g. NaCl, KCl, CaCl2, also thermodynamic hydrate inhibitors (THIs), e.g. methanol, monoethylene glycol) exhibit large errors. The unavailability of phase equilibria data and absence of an association equation of state in CSMGem leads to problems in predicting the phase equilibria of associating fluids and inhibited systems. Therefore, the current CSMGem model is not reliable for predicting these inhibited systems. Such thermodynamic calculations are critical to flow assurance and desalination process design. To overcome these limitations this work revisited the fluid model and a new fluid model is proposed for phase equilibria predictions; an association equation of state has been developed and applied to predict fluid phase properties for a wide range of hydrocarbons (low to high MWt), polar components and electrolytes (salts). Five parameters of the association equation of state were determined for the associating components by simultaneous minimization of absolute errors in saturated liquid densities and vapor pressures, with comparisons to experimental data. In order to predict the phase equilibria of gas hydrates without inhibitors, the proposed association equation of state needs to be tuned with vapor-liquid equilibria. In this thesis work experimental hydrate phase equilibria and vapor-liquid equilibria data (over a range of temperature, pressure and composition) were collated and utilized to tune the fluid and hydrate models. Binary interaction parameters were optimized for a range of hydrate formers, including methane, ethane, nitrogen and hydrogen in combination with all other available hydrocarbons. Different equations of state were also used to predict the vapor-liquid equilibria for mixtures of methane, ethane, nitrogen and hydrogen, in combination with other available hydrocarbons and various cross associating systems (including water, methanol, ethanol, monoethylene glycol). The equations of state used in these predictions include: the cubic plus association, Soave Redlich Kwong (SRK) and Peng Robinson (PR) equations of state. In addition to tuning of the fluid model for hydrocarbons, equation of state parameters and binary interaction parameters were optimized for hydrogen sulfide and carbon dioxide mixtures with hydrocarbons and other associative components. The equations of state used in these predictions include: the cubic plus association, Soave Redlich Kwong (SRK), Peng Robinson (PR), Statistical Associating Fluid Theory (SAFT), and Perturbed-Chain SAFT (PC-SAFT) equations of state. Absolute errors were calculated and compared for combinations of binary mixtures including self associating and cross associating systems. Various mixing rules were utilized in the SRK and PR equations of state to investigate their effect on VLE prediction accuracies. Model accuracies were compared with and without optimized binary interaction parameters. Various electrolyte physical models: Debye Huckel, truncated Debye Huckel, Pitzer theory and Bromley activity model were also tested for mean activity coefficient calculations. The Debye Huckel and truncated Debye Huckel models give accurate predictions at lower salt concentrations, but major deviations were observed at molal concentrations of greater than 1 mol/kg. The Bromley activity model and Pitzer theory were found to be good alternatives to the Debye Huckel model and its modifications. For 1:1 electrolytes, the Pitzer theory and Bromley activity model give accurate predictions up to the salt saturation limit. However, for 1:2 electrolytes (CaCl2, MgCl2 and BaCl2), the Pitzer theory, Bromley activity model, and Debye Huckel models were not able to capture these electrolyte contributions. Furthermore, a more rigorous statistical thermodynamics approach has also been developed to accurately predict the activity of the aqueous phase and 1:2 electrolytes. The remaining thesis work investigated other applications of hydrate phase equilibria predictions. Desalination of seawater using gas hydrates is a potential technique to produce potable water. However, poor understanding and control in prior studies of nucleation, growth, and separation have prevented adequate advancement or commercialization of this hydrate desalination process. Thermodynamic modeling was applied to this desalination process to calculate the required driving forces for hydrate formation and dissociation. Hydrate formation experiments were performed using a Jerguson high pressure visual cell. Hydrate formation onset times were found to be three times longer in salt water compared to fresh water (for the same driving force). Hydrate formation onset times were shown to decrease with increasing subcooling, and hydrate memory effects were reduced with increasing replenish time (time lapse between dissociation and subsequent gas re-pressurization). A high pressure desalination apparatus was also designed and constructed to produce and overflow hydrates in the inner annulus of a gas bubble column inside a dissociation reactor. Hydrate nucleation and crystal growth studies were performed in this bubble column reactor. The salt removal efficiency was determined by measuring the conductivity of the recovered water (from dissociated hydrates). The proof-of-concept experiments showed that desalination efficiency was independent of the initial salt content (over the range 3.3 - 7 wt.% NaCl) and hydrate subcooling. Hydrate morphology was also found to be a key factor in determining the amount of overflowing (separated) hydrates, and hence desalination efficiency. Separation of greenhouse gases from flue gas exiting a power plant is a key industrial objective. In comparison to the other separation techniques, a hydrate formation method can be an appropriate choice in terms of fuel efficiency and degree of separation. Phase equilibrium modeling is critical to evaluating a hydrate separation process. In this work, hydrate phase equilibria predictions were performed for carbon dioxide and nitrogen mixtures at various composition ranges to determine the suitable region for separation. The cage occupancies were also determined to select the optimal separation temperature. Recovery and separation factors for carbon dioxide and nitrogen were found to depend on temperature. A process design was also proposed for other gaseous mixtures, including: CH4+CO2, N2+CO2, CH4+H2S and H2+CO2 mixtures.