Rheological study of CO2 hydrate slurry in a dynamic loop applied to secondary refrigeration

An original rheological characterisation of CO2 hydrate slurries in flow is proposed in the present paper. Hydrate slurries are two-phase fluids composed of hydrate crystals in suspension in a liquid phase. One of the applications of CO2 hydrate slurries relates to secondary refrigeration, principally due to the high latent heat of melting of CO2 hydrates used as phase change materials and the expected lower cost of gas hydrate production compared to scrapped and brushed surface exchangers (classically used in the field of refrigeration and air-conditioning). In order to be considered as suitable for refrigeration applications, CO2 hydrate slurries must, however, fulfil other criteria such as satisfying flowing conditions. An experimental loop was used in this work to produce CO2 hydrate slurries by gaseous CO2 injection in pre-cooled water and to measure pressure drops as a function of volumetric flow rates for various fractions in solid particles. As most of multiphase solid-liquid fluids, CO2 hydrate slurry can be theologically characterised by its pseudo-homogeneous apparent viscosity, strongly depending on the mass fraction, the type and the Organisation of the solid particles in the liquid phase related to following parameters: nucleation, growth, shape, size distribution, agglomeration, etc. In order to improve the description of the theological behaviour of the studied slurry, an empirical model based on the method of the capillary (Ostwald) viscosimeter was applied from our experimental data and provided a Herschel-Bulkley (HE) type equation integrating the solid fraction of the slurry. The results of the empirical HB model were compared to a classical pseudo-homogeneous pressure drop model. (C) 2008 Elsevier Ltd. All rights reserved.