In situ Raman spectroscopic investigation of flux-controlled crystal growth under high pressure: A case study of carbon dioxide hydrate growth in aqueous solution
Introduction
Natural gas components like methane, ethane, propane, carbon dioxide, nitrogen and hydrogen sulfide are known to form hydrates at high pressure and low temperature. The growth kinetics of crystal from solution under high pressure is of great interests in industrial and geological processes (e.g., geological CO2 sequestration as a means to mitigate the global warming [1] and production of CH4 from CH4 hydrates with the injection of CO2 [2]). However, only limited accurate data exist for the hydrate crystal growth rate [3]. The growth rate and the morphology of the crystal are controlled by the surface reaction as well as transport of mass and heat, usually depend both on the diffusion rate and the reaction rate. To better understand the phenomena of crystal growth in solution under high pressure and improve the processes designed for the crystal growth under high pressure, it is necessary to develop high-pressure in-situ non-invasive techniques to measure the solute concentration distribution in the solution around a growing crystal and determine the ratio of surface flux to mass transport flux, because the rate-dominant process depends on the ratio of surface flux to mass transport flux. Furthermore, an accurate value for the reaction rate constant of hydrate formation is required for the design of proper reactor for a large-scale hydrate production [4].
Kinetic studies of hydrate growth are often carried out in a controlled environment using a semi-batch stirred tank reactor, where liquid water and hydrate former gas are in contact at suitable temperature and pressure to form gas hydrate. On the basis of experimental and theoretical analyses, models were established to describe the formation kinetics. For example, a fugacity model, proposed in the pioneering work of Englezos et al. [5], can be used to determine the reaction rate constant of hydrate formation. Skovborg and Rasmussen [6] limited hydrate growth to a mass-transfer problem and considered the transport of gas molecules from the gas phase to the liquid water phase as the rate-determining step in the overall hydrate formation process. Hashemi et al. [7] modified the gas hydrate growth model of Englezos et al. [5] based on a concentration driving force, where the equilibrium concentration at the hydrate surface was determined at the surface pressure and temperature. In most of these studies, heat and mass transfer resistances were designed to be negligible by using high agitation rates in semi-batch stirred tank reactors, even though the growth or dissolution of crystal may be controlled either by mass or heat transfer or by interface reaction [8]. On the other hand, the role of hydrate intrinsic kinetics has been more recently suggested to play a smaller role in hydrate growth in real systems than heat and mass transfer effects [1]. In addition, although attempts have been made to use particle size measurement techniques in recent studies, for example, Herri et al. [9] used turbidimetry measurements to characterize kinetic inhibitors during the crystallization of methane hydrate, and Clarke and Bishnoi [10] used a focused beam reflectance method probe to measure the particle size distribution to obtain the intrinsic rate constant, accurate measurement of the amount of hydrate crystal is still a challenge for studying gas hydrate kinetics.
In the present study, a new reactor has been used for hydrate growth, in which the size of crystal, the transfer of CO2 and the consumption of CO2 and water can be accurately determined. CO2 hydrates are formed in a capillary high-pressure optical cell, a section of which can be fully filled by the crystal. The length of the sections for hydrate crystal and solution were measured as a function of time. An in situ measurement technique based on Raman spectroscopy was established to directly measure the CO2 concentration distribution in the solution around a growing hydrate crystal, in order to determine the mass transfer rate and analyze the rate-determining process. Growth rates of carbon dioxide hydrate in water were determined at 20 and 40 MPa and 275.15, 278.15, 280.15, and 283.15 K. The conversion of the rate-determining step from mass transfer to interfacial reaction was observed.
Section snippets
Experimental apparatus and procedures for growing single crystal
A capillary high-pressure optical cell (HPOC, [11], [12]) in combination with a Linkam CAP500 heating–cooling stage was used for the Raman spectroscopic study of mass transport flux in the aqueous solution during the hydrate growth at high pressure and low temperature (Fig. 1). The HPOC was constructed from a round cross-sectional flexible fused silica capillary tube with 375 μm OD, 50 μm ID and about 25 cm in length. One end of the tube was sealed with a hydrogen flame and the other end of the
Measured carbon dioxide hydrate growth rates in aqueous solution
The progresses for the growth of carbon dioxide hydrate in pure water at 20 MPa and 275.15, 278.15, 280.15 and 283.15 K, and at 40 MPa and 280.15 K were observed within the stability field of hydrate and water in the binary system CO2–H2O (Fig. 3).
As shown in Fig. 2, the single hydrate crystal fully filled a section of the HPOC, which gradually became longer with time. Fig. 4 shows the changes in length of the sections for the hydrate crystal and aqueous solution in capillary tubes during the
Conclusions
We developed a high-pressure in-situ non-invasive technique to measure the solute concentration distribution in the solution around a growing crystal, in order to study the rate-dominant process of crystal growth coupled surface reaction and mass transport. Carbon dioxide hydrate growth in water were observed at 20 and 40 MPa and 275.15, 278.15, 280.15 and 283.15 K. The growth rate is diffusion controlled when diffusion flux is low, and becomes interfacial reaction controlled after the diffusion
Acknowledgments
We are grateful to Dr. Rui Sun and Dr. Qingcheng Hu for their critical reviews and helpful comments. We thank Ms. Peixiao Mao for her kind help with the experiments. This work was partly supported by the National Sciences Foundation of China (Nos. 41102154, 41176047), the Programme of Introducing Talents of Discipline to Universities (No. B14031), Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences), Ministry of Education (TPR-2014-04), and the Knowledge
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