Article pubs.acs.org/EF
Profiling the Concentration of the Kinetic Inhibitor Polyvinylpyrrolidone throughout the Methane Hydrate Formation Process Jason Ivall, James Pasieka, Dany Posteraro, and Phillip Servio* Department of Chemical Engineering, McGill University, Montréal, Québec H3A 0C5, Canada ABSTRACT: Gas hydrate inhibition is a prime focus of industrial hydrocarbon research as pipeline blockages can be costly and dangerous. Historically, many chemical species have been studied for their effects on the hydrate formation process. One of the most investigated compounds in the kinetic hydrate inhibitor (KHI) category is polyvinylpyrrolidone (PVP). While the effects of PVP on hydrates are well-documented, the mechanism that defines its function is still not completely understood. To obtain further insight into its behavior, bulk liquid samples in a PVP-containing system were extracted at six specific times throughout a methane hydrate formation process. The effect of PVP loading concentration was also investigated. It was found that as time progressed, concentration of PVP in the liquid decreased, suggesting that PVP binds to the surface of growing hydrates. Furthermore, this decrease in concentration was more prevalent in situations where lower initial PVP loadings were used.
1. INTRODUCTION Gas hydrates are multi-component crystalline structures formed from water and small volatile molecules. Examples of such hydrate-forming compounds include hydrocarbons (for example, CH4 and C2H6), carbon dioxide (CO2), and nitrogen (N2). Under suitable thermodynamic conditions, namely, mild temperatures and high pressures, a rigid hydrogen-bonded network develops between water molecules to form cavities that enclose the other components. Further interactions between the guest molecules and the host cage, in the form of weak van der Waals forces, contribute to the final stability of the crystal lattice.1 The size and ratio of the encaged molecules are the largest determinants in the type of hydrate structure formed. The three most common hydrate structures encountered in industry and academia are structure I (sI), structure II (sII), and structure H (sH).2 The study of gas hydrates can be divided into three main disciplines. First, the high gas storage capacity of hydrate crystals has inspired the emergence of novel industrial technologies. These techniques seek to exploit this property for the transport and storage of gases as well as the sequestration of carbon dioxide.3 Second, the vast quantities of naturally occurring methane hydrates located in permafrost formations and subsea sediments have motivated research and technology dedicated to extracting these immense energy sources.4,5 Finally, the bulk of hydrate research has been devoted to addressing the problem of flow assurance associated with hydrate formation within hydrocarbon transmission lines. This issue has been a longstanding problem in the oil and gas industry since the discovery of its occurrence in the 1930s.6 To avoid hazards to equipment, personnel, and the environment as well as to minimize operational delays, industry has committed substantial monetary investments into hydrate research and mitigation techniques.7 The most widely used practice remains the injection of thermodynamic inhibitors (TIs), such as methanol and glycol, into pipeline fluids.1 These additives alter the activity of the system to shift incipient hydrate-forming © XXXX American Chemical Society
temperatures and pressures to more extreme conditions that will not likely be experienced within pipelines.8 While this method offers the advantage of guaranteed hydrate prevention, the toxicity, low recovery, and large volumes of inhibitors required to offset the operational conditions (10−50 wt %) have urged the development of alternative chemical approaches.8,9 The discovery of low-dosage hydrate inhibitors (LDHIs), additives that act by a more targeted approach and can be used in quantities of less than 1 wt %, was born from these considerations.10 Kinetic hydrate inhibitors (KHIs) are a subclass of LDHIs that interfere with the nucleation and growth processes of hydrate formation.11 Generally, hydrate formation can be described in three distinct steps: saturation, induction, and growth.12 A schematic of this process can be seen in Figure 1. Initially, the gas-phase hydrate former dissolves into the aqueous phase until the system becomes saturated. This point occurs in the figure at tEquilibrium. After this stage, gas slowly continues to dissolve, creating a supersaturated system and
Figure 1. Typical methane consumption plot with a representation of the time points where liquid samples were extracted. Received: January 20, 2015 Revised: March 23, 2015
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DOI: 10.1021/acs.energyfuels.5b00145 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
also investigated. The results of this study could forge a more holistic understanding of the system during various stages of hydrate formation. This information could also provide further insight into the inhibitor-adsorption hypothesis and hopefully accelerate subsequent inhibitor development.
metastable state, where hydrate formation is possible. During this interval, small hydrate nuclei form and dissociate until a critical radius size is achieved. The formation of this stable hydrate crystal defines nucleation, from which the growth phase ensues. The nucleation event occurs at the point tTurbidity in Figure 1, and the time elapsed between saturation and nucleation is commonly referred to as the induction period. During the growth stage of hydrate formation, the kinetics of a system are defined by a combination of heat, mass, and reaction rate limitations.2 In the early portion of the growth process, for a well-mixed system, the heat- and mass-transfer resistances are comparatively smaller than at the later stages and the influence of the reaction rate on system kinetics is at its greatest.13 This term comprises both the intrinsic adsorption rate of guest incorporation into the hydrate crystal, a term fixed by the thermodynamic state of the system,12 as well as the total surface area of the particles. Generally, for uninhibited systems, a linear gas consumption profile with respect to time is observed, indicating that growth is not solely under mass- or heat-transfer control. KHIs are known to both increase the time required for the attainment of a critical-sized nucleus, i.e., the induction time, as well as to reduce the rate of gas consumption during growth.11 Therefore, while equilibrium conditions are not significantly affected, crystal formation is both delayed and restricted throughout the residence time of the fluid in the pipeline.7 The most studied class of KHIs is the polyvinyllactam family. These water-soluble polymers consist of a polyvinyl backbone and variable pendant lactam groups and are typically between 10 and 40 kDa in length.11 Polyvinylpyrrolidone (PVP) was among the first KHI to exhibit a definitive inhibitory effect and commonly serves as the standard for experimental comparison.7 The monomer of PVP polymer is shown in Figure 2. PVP and
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. Figure 3 provides a schematic of the experimental apparatus used in this study. A complete description of the equipment and instruments can be found in previous works.14 Briefly, methane hydrates are formed in a 600 cm3 316 stainless-steel semi-batch stirred crystallizer. A top-mounted electric mixer set to a rate of 750 rpm provides continuous agitation of the liquid throughout the experiment. This stir rate is chosen as beyond this level the liquid begins to slosh. The system is maintained at a constant pressure through a control valve and reservoir assembly, while the temperature is controlled via submersion of the crystallizer in a chilled glycol/water bath. Pressure is monitored by Rosemont 3051S1 scalable pressure transmitters accurate to ±0.025% of a given span, and temperature is monitored by general-purpose resistance temperature probes (Omega, class A accuracy of ±0.154 K at the experimental temperature). The temperature and pressure readings are relayed to a National Instruments data acquisition system (NI-DAQ 7) and displayed by LabVIEW software. Bulk liquid samples are extracted via connection of a 60 mL BD syringe to the liquid sample port situated at the base of the crystallizer. A 0.2 μm (nominal) high-pressure stainless-steel filter (Norman Filters) fitted within the transfer line prevents hydrate particle flow-through into the sample. 2.2. Procedure for Hydrate Formation and Sample Extraction. PVP solutions (0.007, 0.07, and 0.7 wt %) are prepared using PVP10 powder (average molecular weight of 10 kDa, SigmaAldrich) without further purification. Reverse-osmosis (RO) water (0.22 μm filter, conductivity of 10 μS, and total organic content of
