Akihiro Yamasaki

Formation experiments Of CO2 hydrate from the two-phase flow of liquid CO2 and water were carried out using static mixers with different types of mixing elements to elucidate the effects of mixing functions of the static mixer on CO2 hydrate formation. The mixing principle of the Kenics type static mixer is composed of three mixing functions, flow division, flow reversal, and radial mixing. In this study, three types of mixing elements in addition to the basic Kenics type static mixer were applied for CO2 hydrate formation; each mixing element lacked at least one mixing function among three mixing functions. The observed behaviors Of CO2 hydrate formation could be classified into six patterns: (1) hydrate chunks, (2) CO2 drops agglomerated through the hydrate film, (3) dispersed liquid CO2 drops covered with hydrate film, (4) mixture of tiny liquid CO2 drops and hydrate particles, (5) mixture of hydrate chunks and agglomerated CO2 drops, and (6) mixture of agglomerated and dispersed CO2 drops. Occurrences of the above patterns depended on the type of the mixing element as well as the flow rates of liquid CO2 and water. From the observations, flow division was suggested to be important for the formation of hydrate chunks but not essential, and flow reversal was essential for the formation of dispersed hydrate particles. The formation mechanisms Of CO2 hydrate with the static mixer are discussed according to the results.

Process design for a new injection method of liquid CO2 using a static mixer was conducted based on laboratory experimental results on the formation process of liquid CO2 drops covered with hydrate film by a Kenics-type static mixer, and numerical simulation of the liquid CO2 drops at 500 and 1500 m. The Sauter Mean Diameter (SMD) of the liquid CO2 drops covered with hydrate film was dramatically decreased with the use of the static mixer; empirical equations were obtained for the SMD, and also the maximum and minimum diameters of the liquid CO2 drops for a given flow velocity (Weber number, We). The ascending and dissolving behavior of a liquid CO2 drop with hydrate released in the ocean at an intermediate depth was numerically simulated, and the maximum drop diameter to avoid evaporation of the drop before complete dissolution was estimated. Based on these results, scaling up of the static mixer was conducted by assuming a disposal process Of CO2 emitted from a 100-MW thermal power plant, and the mixer diameter was determined as a function of the given SMD. Moreover, the power consumption of the static mixer was evaluated and found to be almost negligible. (c) 2005 Elsevier B.V. All rights reserved.

A method for predicting the location of a dissociation condition on an H-L-w-V line under isochoric operation was presented. To establish the method, the governing equations for the H-L-w-V coexistence under isochoric conditions were derived. Here, a liquid and a vapor phase were expressed by the PR EOS + MHV2 model and a hydrate phase by the van der Waals-Platteeuw model. The molar volume of the vapor phase was calculated from the equation of state, and a simple expression for the molar volume of the hydrate phase was derived. Then, to prove the validity of the proposed method, experimental studies about the dissociation process of the hydrates were performed. The temperature and pressure traces in the hydrate dissociation process, including the location of the dissociation condition, were successfully predicted by the proposed method. In addition, the thermodynamic consistency among the phase models was discussed. It was pointed out that agreement between the calculated and experimental results about the H-L-w-V equilibrium line did not ensure thermodynamic consistency among the phase models. (c) 2005 Elsevier B.V. All rights reserved.

A method for determining quadruple points of a two-component system containing a simple hydrate phase is proposed. This method utilizes the quasi-static change of the system along three-phase equilibrium lines and was proved to be able to determine the quadruple points as accurately as the conventional method. By using this method, even though some preparation is necessary, a quadruple point can be determined in just a single experimental run. The behavior of the system near the quadruple points was also examined experimentally, for both the quasi-static and the irreversible change cases. At the quadruple points, the temperature and pressure of the system were kept constant for a while, as at the triple point of water. In both cases, the representative point of the state of the system passed through the quadruple point on a p-T diagram. © 2004 Elsevier B.V All rights reserved.