Phase behaviour of tri-n-butylmethylammonium chloride hydrates in the presence of carbon dioxide

The phase behaviour of the system water?Ctri-n-butylmethylammonium chloride (TBMAC)?CCO₂ was investigated by pressure-controlled differential scanning calorimetry in the range 0?C10?mol% TBMAC in water and at CO₂ pressures ranging from 0 to 1.5?MPa. In the absence of CO₂, an incongruent melting hydrate, which estimated composition corresponds to TBMAC·30H₂O, crystallizes at temperatures below ?13.6?°C and forms with ice a peritectic phase at approximately 3.9?mol% TBMAC. In the presence of CO₂ at pressures as low as 0.5?MPa, curves evidenced the presence of an additional phase exhibiting congruent melting at temperatures that are strongly pressure dependent and significantly higher than those of hydrates obtained without CO₂. This new phase, whose enthalpy of dissociation and CO₂ content increase slightly with CO₂ pressure, was identified as a mixed semi-clathrate hydrate of TBMAC and CO₂ of general formula: (TBMAC?+?xCO₂)·30H₂O.     

A method for the characterization of emulsions, thermogranulometry: application to water-in-crude oil emulsion

Emulsions are used in a wide range of applications and industries. Their size distribution is an important parameter because it influences most of the emulsion properties of emulsions. Several techniques of characterization are used to determine the granulometric distribution of emulsions, but they are generally limited to dilute samples and are based on complex algorithms. We describe a method that allows characterization of the droplet size distribution of emulsions using thermal analysis (thermogranulometry). This method permits the use of very concentrated samples without any dilution or perturbation of the system. We first define our method by a thermodynamic and kinetic approach. We studied a real system, i.e., crude oil emulsions, which form very concentrated, viscous, and opaque emulsions with water. We present a correlation between the size of droplets and their freezing temperature, corresponding to our system. Then we compare the size distributions obtained by our method with those derived by direct microscopy observations. The results obtained show that thermogranulometry may be an interesting method of characterization of emulsions, even for concentrated systems.     

DSC and PVT measurements

The dissociation of gas and model hydrates was studied using a classical thermodynamic method and a calorimetric method, in various aqueous media including pure water, high concentration calcium chloride solutions and water-in-oil emulsions. Methane hydrate dissociation temperatures vs. pressure curves were determined using pressure vs. temperature measurements in a constant volume cell (PVT), and high pressure differential scanning calorimetry (DSC), at 5 to 10 MPa gas pressure and at temperatures ranging from -10 to +12°C. PVT and DSC results are in good agreement, and concordant with data available in literature. From a thermodynamic point of view, there are no measurable differences between bulk solutions and emulsions. From a kinetic point of view, due to the considerable surface of interface between the two phases, emulsions allow the formation of much greater amounts of hydrate than solutions, without any agitation. Model hydrate of trichlorofluoromethane was studied in 9 to 27 mass% calcium chloride solutions in emulsion in oil, using DSC under atmospheric pressure, at temperatures ranging from -20 to +5°C. A diagram of dissociation temperature vs. salt concentration is proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modernization and validation of an isoperibol rotating bomb calorimeter for the measurement of energies of combustion of sulphur compounds

A rotating bomb calorimeter belonging to the Laboratory of Chemistry and Process Engineering (UCP) at ENSTA ParisTech since the 1960s was modernized and re-set in order of service for the measurement of heats of combustion of sulphur-containing compounds. The parts of the calorimeter that were replaced concerned essentially the thermal regulation, firing system and data acquisition, while most of the mechanical organs were found to work perfectly. The apparatus was tested extensively and calibrated using standard benzoic acid. The standard energy of combustion of thianthrene was measured to validate the operating protocol for sulphur compounds.     

Contribution of the Sedimentation and Coalescence Mechanisms to the Separation of Concentrated Water-in-Oil Emulsions

Crude oil is, in the vast majority of cases, produced together with formation water in the form of water/oil emulsions. Until recently oil/water separators were simply designed using Stokes' law. In order to improve the design of these separators the two main contributing mechanisms, sedimentation and coalescence, have to be better understood. This article presents a method that distinguishes the respective contributions of the sedimentation and coalescence mechanisms. Two series of experiments have been carried out, the first with a system heavily charged in surfactant that only allows sedimentation of the emulsions droplets. The second with a much lower surfactant concentration allows both sedimentation and coalescence to take place. By comparing the separation velocities of the oil/emulsion interfaces of the two series of experiments, it seems possible to determine the contribution of the two mechanisms towards the separation.     

Modelling and characterisation of diluted and concentrated water-in-crude oil emulsions: comparison with classical behaviour

Water-in-oil type emulsions can be formed during the crude oil production process. The presence of natural surfactants in oil (asphaltenes, resins) and mechanical stirring (piping/well system) produce emulsions, the stability and rheological behaviour of which depend mainly on the chemical composition of the oil and the internal phase concentration. In this work, water (brine 8 g NaCl/cm³) in oil (crude oil) emulsions were prepared and characterised by varying the internal phase concentration (5–80%). Rheological properties are discussed according to the composition of the oil and the temperature of the system. Relative viscosity was modelled following the classical models of Mooney and Krieger and Dougherty, but the best-fitting model for the experimental results was found with an exponential type equation between relative viscosity and volume fraction, as proposed by Richardson. Moreover, we observed that the plastic behaviour determined through the yield stress determination depended not only on the internal phase concentration but also on the temperature. Quantitative analysis of the emulsions’ viscoelastic parameters (storage and loss modulus) was made. In the case of concentrated emulsions (containing over 70% of internal phase), Princen’s theory of the high internal phase ratio emulsions (HIPRES) was verified.     

Dissociation enthalpies and phase equilibrium for TBAB semi-clathrate hydrates of N<Subscript>2</Subscript>, CO<Subscript>2</Subscript>, N<Subscript>2</Subscript> + CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> + CO<Subscript>2</Subscript>

Tetra-n-butyl ammonium bromide (TBAB) semi-clathrate (sc) hydrates of gas are of prime importance in the secondary refrigeration domain and in the separation of gas molecules by molecular size. However, there is a scarcity of dissociation enthalpies under pressure of pure gases and gases mixtures for such systems. In addition, the phase equilibrium of TBAB sc hydrates of several pure gases is not well defined yet as a function of the TBAB concentration and as a function of the pressure. In this paper, dissociation enthalpies and the phase equilibrium of TBAB sc hydrates of gas have been investigated by differential scanning calorimetry (DSC) under pressure. Pure gases such as N₂ and CO₂ and gases mixtures such as N₂ +  CO₂ and CH₄ +  CO₂ were studied. To our knowledge, we present the first phase diagram of TBAB sc hydrates of N₂ for different pressures of gas in the TBAB concentration range from 0.170 to 0.350 wt. Enthalpies of dissociation of TBAB sc hydrates of pure gases and gases mixtures were determined as a function of the presssure for a compound with a congruent melting point whose hydration number corresponds to 26.     

Application of high pressure DSC to the kinetics of formation of methane hydrate inwater-in-oil emulsion

A micro DSC analyzer fitted with special high-pressure vessels was used to investigate the kinetics of methane hydrate formation in the water phase dispersed as a stable emulsion in deep offshore drilling fluids. At high sub-cooling conditions, the peak of hydrate formation is perfectly visible and regular-shaped, and could be fitted by a Gaussian law. The average time for hydrate crystallization of the water droplets’ population was represented as a logarithmic function of the inverse of absolute temperature. At low sub-cooling conditions, the formation appears confused with the baseline; the amount of hydrate formed was thus measured from its enthalpy of dissociation, after periods of formation of variable duration.