An accurate model for prediction of gas hydrate formation conditions in mixtures of aqueous electrolyte solutions and alcohol

Abstract

ammerschmidt (1 934) developed an empirical method for predicting the hydrate formation temperature for natural gas H constituents in the presence of alcohol. ('Alcohol' is used here to describe alcohols and polyhhydroxy alcohols, or glycols.) Subsequently, several other models have been suggested, for example, Anderson and Prausnitz (1 986) and Moshfeghian and Maddox (1 993).In the presence of electrolytes, the method of Englezos and Bishnoi (1 988) is applicable. Their procedure uses the activity model developed by Pitzer and Mayorga (1 973) to predict the water activity coefficient in the presence of electrolytes. Their model is not recommended for hydrate forming gases with high solubility in the water phase. Javanmardi and Moshfeghian (1996) and later Javanamardi et al. (1 998) took a different approach and incremented the pure water hydrate formation temperature to predict conditions of incipient hydrate formation in the presence of electrolytes. Englezos (1 992), by using flash calculation algorithms and the fugacity model of Aasberg-Peterson et al.(1 991), predicted the temperature for incipient hydrate formation in single aqueous electrolyte solutions. Javanmardi and Moshfeghian (2000) used the same activity model to predict the hydrate formation temperature in mixed aqueous electrolyte solutions. Yousif and Young (1994), based on modification of the Hammerschmidt model, developed a simple model to predict the hydrate temperature suppression of aqueous mixtures of salts and a single alcohol. Their correlation is restricted to the prediction of the hydrate condition for a specified gas mixture; however, we used it to predict hydrate formation conditions for other systems. In the Nasrifar et al.( 1 998) work, the hydrate formation temperature in the presence of mixed electrolytes and alcohols is calculated by adding a correction to the hydrate formation temperature in the presence of inhibitors. Therefore, their method is a two-step procedure.In the present work, a new method is suggested for direct prediction of hydrate temperature in the presence of both aqueous electrolyte solutions and a single alcohol. The model is based on the extension of the water activity model to the above systems. The suggested method is A new thermodynamic model for calculating the hydrate formation temperature of different hydrate formers in aqueous solutions of both electrolytes and a single alcohol has been presented. This method uses a generalization of the Aasberg-Petersen model for water activity. For calculation of water activity in the presence of electrolytes, the effect of alcohols was taken into account without using any new fitting parameters. The results are in good agreement with published experimental data.

Measurement and prediction of gas hydrate and hydrated salt equilibria in aqueous ethylene glycol and electrolyte solutions

“…Most of the existing models are based on activity coefficient with a number of tuned parameters and empirical equations. As for gas hydrates, several thermodynamic hydrate prediction models have been described in the literature (Clarke and Bishnoi, 2002;Vu et al, 2002;Javanmardi et al, 2001), and experimental dissociation data for mixed methanol and electrolyte solutions have been reported by several researchers (Jager et al, 2002;Bishnoi and Dholabhai, 1999). However, predictions of existing models for salt-methanol solutions can be in error by up to 4.5-5.6 K (Matthews et al, 2002).…”

Thermodynamic modeling of gas hydrate formation conditions in the presence of organic inhibitors, salts and their mixtures using UNIQUAC model

“…Nasrifar et al [10] and also Nasrifar and Moshfeghian [11,12] applied the Margules equation along with the Pitzer-Mayorga electrostatic model. Javanmardi et al [13] applied the Margules equation along with a Debye-Hückel type model. Moradi et al [14] applied the UNIQUAC-NRF model along with the Pitzer-DebyeHückel model.…”

“…Up to now, although thermodynamic models were used successfully for either salts or alcohols containing solutions, they were inaccurate for mixtures of alcohols and salts. One of the ways to computing the water activity coefficient is g-f approach; in the simplest method, the water activity coefficient is calculated by summing the contributions from gases, electrolytes and alcohol so that interactions of gas-electrolyte, gas-alcohol, and electrolytealcohol are neglected [10][11][12][13][14]. In these studies, the water activity coefficient in the aqueous electrolyte systems is generally calculated by summing electrostatic and short-range interaction terms.…”

“…Moradi et al [14] applied the UNIQUAC-NRF model along with the Pitzer-DebyeHückel model. In these works [10][11][12][13][14], it was assumed that no interaction is taking placed between electrolyte and alcohol. Jager et al [15] showed that the influence of the mixed inhibitors on hydrate formation is greater than the sum of single inhibitors effects.…”

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“…Since the driving force for hydrate dissociation in the presence of thermodynamic inhibitors is not known, models have been developed based, for example, on a change in partial pressure 3 (Masoudi and Tohidi, 2005) or a change in the activity of water (Javanmardi et al, 2001). There are deficiencies in these approaches which underscore the need to identify the driving force.…”