Article pubs.acs.org/crystal
Measuring the Effect of Multi-Wall Carbon Nanotubes on Tetrahydrofuran−Water Hydrate Front Velocities Using Thermal Imaging James Pasieka,† Nathan Hordy,‡ Sylvain Coulombe,‡ and Phillip Servio*,† †
Department of Chemical Engineering, McGill University, Montréal, Québec H3A 0C5, Canada Plasma Processing Laboratory, Department of Chemical Engineering, McGill University, Montréal, Québec H3A 0C5, Canada
‡
ABSTRACT: Clathrate hydrates are currently being studied for their applications in many areas such as natural gas storage and transportation, component separation, and carbon dioxide sequestration. The ability to increase hydrate production is integral in the success of these innovative technologies. It has been found that the addition of multi-wall carbon nanotubes (MWNTs) to hydrate systems promotes clathrate formation. In order to better understand how this occurs, an analysis of the heat transfer during the formation of tetrahydrofuran (THF) hydrates was performed. Two concentrations of both conventional (hydrophobic) and plasma-functionalized (hydrophilic) MWNTs were added to a THF−water hydrate-forming solution. With the use of infrared imaging, the velocity and temperature of the thermal front during hydrate formation was measured. It was found that in both cases, the presence of MWNTs elevated the velocity of the front for a given system sub-cooling. Furthermore, as the MWNT concentration increased, so did the velocities. The presence of the MWNTs also decreased the sub-cooling required for nucleation.
1. INTRODUCTION Clathrate hydrates are nonstoichiometric crystalline compounds that consist of inclusion molecules encaged by a water lattice. First discovered in 1810 by Sir Humphry Davy, these structures were initially studied as an academic curiosity.1 It was only until the 1930s when it was found that hydrates were blocking oil and gas pipelines that research had found an industrial relevance.2 Current hydrate research areas include the generation of these compounds for the storage and transportation of natural gases, the mitigation of carbon dioxide emissions, and for uses in novel separation techniques.3−7 In order to exploit hydrate structures for their potential use in the aforementioned technologies, it is of great interest to study techniques to expedite their formation. In the past, researchers have studied the effects of adding multi-wall carbon nanotubes (MWNTs) to hydrate systems.8−10 The findings of such investigations were that the addition of the MWNTs increased the yield of hydrate production. The problem with adding such nanoparticles to an aqueous system is that the MWNTs are naturally hydrophobic and settle out of solution.10 In order to overcome this, the MWNTs were often either mixed in with a surfactant or chemically treated.10,11 Recent developments have now allowed MWNTs to be functionalized with oxygen-containing groups using glow discharge plasmas.12,13 The process adds covalently bonded oxygenated functionalities such as carboxyl, carbonyl, and hydroxyl groups to the MWNTs.13 The produced MWNTs are highly dispersible in polar solvents and thus form suspensions in aqueous solutions.13 While the presence of MWNTs has shown enhancement to hydrate production, there is little explanation for the promotion mechanism of the nanotubes.8−10 One possibility is © 2013 American Chemical Society
the increase in mass transfer of the hydrate former in the liquid phase associated with the addition of nanoparticles.14 It is also well-known that adding nanoparticles to conventional heat transfer fluids enhances their effective thermal conductivity.15 Therefore, it is plausible that the increased rate of hydrate formation could be attributed to the solution’s ability to quickly expel the heat of crystallization. This in turn helps in maintaining a lower system temperature and a higher level of sub-cooling. In order to investigate this hypothesis, the heat transfer patterns of tetrahydrofuran hydrates were analyzed. Tetrahydrofuran (THF) is an industrially used organic solvent. It is of particular interest as it is one of the few inclusion molecules that is in a liquid phase at hydrate-forming conditions.16 Furthermore, as a polar compound, it is fully soluble in water. Unlike common gas-phase inclusion bodies such as methane or carbon dioxide, THF becomes enclathrated at atmospheric pressure. The hydrate structure (S-II) formed by THF is also the same as that formed by natural gas.16 These properties make it a compound of interest due to a lack of mass transfer issues when in solution, as well as the elimination of the need to house the system in a high-pressure vessel. It is therefore an ideal hydrate former to study when attempting to understand the mechanisms of hydrate nucleation and growth. With the use of infrared imaging, it is possible to observe the change in temperature caused by the heat of formation. In a subcooled quiescent hydrate-forming solution, the location of the nucleation region can be detected due to its sudden Received: May 17, 2013 Revised: July 8, 2013 Published: July 16, 2013 4017
dx.doi.org/10.1021/cg400767u | Cryst. Growth Des. 2013, 13, 4017−4024
Crystal Growth & Design
Article
increase in temperature. Following this, the thermal front boundary between the recently converted hydrate and the subcooled solution propagates until the entire system is solidified. The objective of this study is to measure the rate of displacement of the thermal front as a function of system sub-cooling for a THF−water hydrate system with and without the presence of MWNTs. Two forms of nanotubes were used in the experiments: untreated, hydrophobic, and plasma-functionalized, hydrophilic MWNTs. The effect of MWNT loading (concentration) was also studied. It should also be noted, that to the best of our knowledge, this is the first time hydrate growth is measured with the use of thermal imaging.
2. MATERIALS AND EXPERIMENTAL APPARATUS The experimental apparatus consists of two 12.7 × 30.48 cm insulated Aavid Thermoalloy Hi-Contact Aluminum Cold Plates connected in series with a Neslab RTE 740 chiller. The cooling fluid used is a 50/50 by volume mixture of ethylene glycol and water. The fluid can be set to any temperature in the range of −40 to 200 °C and can be maintained with an accuracy of 0.01 °C. The cold plates rest on a Newport VH3600-SG4−325A optical table that is used to dampen vibrations. The THF used for the experiments is purchased from Acros and is anhydrous, stabilized, and 99.9% pure. The water is treated via reverse osmosis and is obtained from McGill University’s Chemical Engineering Department. The water passes through a 0.22 μm filter, has a conductivity of 10 μS, and has a total organic content of
