Article pubs.acs.org/EF
Kinetics of Methane Clathrate Hydrate Formation in Water-in-Oil Emulsion Xingang Li,†,‡ Chao Chen,†,‡ Yingnan Chen,†,‡,§ Yonghong Li,†,‡,§ and Hong Li*,†,∥ †
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China National Engineering Research Center of Distillation Technology, Tianjin 300072, China § Key Laboratory for Green Chemical Technology of State Education Ministry, Tianjin 300072, China ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ‡
ABSTRACT: The methane hydrate formation kinetics in water-in-oil (w/o) emulsion was measured using the pressure− volume−temperature (PVT) method during isochoric, isothermal processes at an initial pressure of 6.80 MPa in an agitated reactor. The effects of agitation rates (n) of 300−1100 rpm, average water droplet diameters (d̅) at 30% water cut, and temperatures (T) of 269.15−277.15 K on the induction time and hydrate formation rate were systematically studied. The experimental results show that the induction time of hydrate formation initially decreased as the agitation rate increased and then increased when higher agitation rates were used. The results did not reveal any influence of the average diameter of the water droplets on induction time. Increasing the temperature increased the induction time. The rate of hydrate formation increased as the agitation rate increased and the average diameter and temperature decreased. The total methane hydrate formation was affected by the average diameter and temperature, and the hydrate formation rate was enhanced in w/o compared with many other methods. A mathematical model of hydrate formation kinetics in w/o was established based on crystal growth theory and the mass transfer of methane. The effects of the amounts of hydrate in a w/o system on the mass-transfer coefficient of the oil/ water interface were considered by introducing an empirical equation. The parameters of the model were determined by correlating the experimental data with the model data. The agreement between the experimental data and the calculated results of the model was satisfactory.
1. INTRODUCTION Gas hydrates are crystalline compounds that are not stoichiometric and are composed of water and gas. The gas molecules (guests) are trapped in a cagelike structure composed of hydrogen-bonded water molecules (hosts).1 Recently, many studies have been conducted regarding gas hydrates, because of their ability to plug petroleum transport pipelines2−4 and their possible applications in gas separation,5−9 desalination,10,11 refrigeration,12 transportation and the storage of natural gas,13 carbon dioxide sequestration,14 and so on. The success of these potential applications has been hindered by technological problems associated with hydrate formation, including a slow formation rate, low conversion, and the economics of scaling up the process.15 Many methods have been used to enhance the hydrate formation rate, such as adding zeolite in pure water,16 using a surfactant dry solution,17 using a fixed-bed column filled with sand,18 or adding a surfactant.19−22 The absorption-hydration hybrid method uses a water-in-oil (w/o) emulsion that contacts guest molecules and not only enhances the contact area between gas and water but also increases the guest molecule concentration at the w/o interface.7,9 Both aspects are beneficial for the rapid formation of hydrates. The research results of Ma et al.23 on the kinetics and phase behaviors of catalytic cracking dry gas hydrate in a w/o system have shown that the hydrate formation rate can significantly increase in a w/o emulsion, relative to pure water. For gas-hydrate equilibrium, the separation efficiency of the gas mixture is more effectively enhanced in the w/o emulsion than © XXXX American Chemical Society
in pure water. Furthermore, the small hydrate particles formed in the w/o emulsion were dispersed in the oil phase without agglomeration. Therefore, the absorption−hydration hybrid method is a promising technology for separating gas and transporting and storing natural gas. Studying the kinetics of gas hydrate formation in a w/o emulsion could provide basic data and improve our understanding of hydrate growth mechanisms, which could be useful for establishing a mathematic model to simulate the hydrate growth process. This type of model is important for applying hydrate technologies to natural gas transportation and storage, guaranteeing the flow of petroleum through transport pipelines, and gas separation, because it could be used to simulate the growth of hydrate. Few studies have been conducted regarding the formation of gas hydrate in w/o emulsion systems in stirring reactors. Turner et al.24 showed that the gas consumption rate increased when the impeller speed was increased from 400 rpm to 500 rpm under supersaturation. However, no clear trend was observed in the gas consumption rate when the water fraction was increased from 35% to 100%. Shi et al.25 studied the growth and formation of natural gas hydrates at 2 MPa, 277.15 K in a w/o emulsion, and showed that the natural gas consumption increased when the water cut increased from 10% to 25%. Talatori et al.26 studied the rate of mixed gas hydrates Received: December 26, 2014 Revised: March 5, 2015
A
DOI: 10.1021/ef5028923 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Schematic diagram of the experimental apparatus.
coolant bath, a pressure control system, and a data logger. The vessel has a diameter of 50 mm, a height of 103.3 mm, and an effective volume of 200 mL and was designed to work safely under pressures of up to 10 MPa (Beijing, Century SenLong experimental apparatus Co., Ltd., China). One pressure transducer and two Pt100 thermal resistance detectors were installed on the top detachable cover of the vessel. A pressure transducer (Model CYB-20SJ, Beijing, WESTZH Science and Technology Co., Ltd., China) with an accuracy of ±0.01 MPa was used to measure the pressure in the vessel. Two Pt100 thermal resistance detectors with a precision of ±0.1 K were used to measure the temperatures of the gas and liquid phases, respectively. The vessel temperature was controlled by immersing the vessel in a circulating coolant bath (Model W503B, Tianjin, BiLon Lab Equipment Co., Ltd., China). The coolant bath temperature was maintained from 253.15 K to 368.15 K with an accuracy of ±0.1 K. In addition, a vent was used to release the inert gases, unreacted gas or the gas from the decomposing hydrates. Two pressure gauges with a precision of ±0.1 MPa were used to measure the pressure of the gas supply. The pressure and temperature data were recorded every 30 s, using a data acquisition system (KingView 6.53, Beijing, Wellintech Co., Ltd., China). 2.2. Experimental Materials. Methane (99.99%) was obtained from Tianjin Liufang Gas Industry Corporation, and decane (98%), Span 80 (pharmaceutical grade), and Tween 80 (pharmaceutical grade) were purchased from Aladdin Industrial, Inc. Double-distilled water was used for all emulsions. 2.3. The Preparation of Emulsions. Mixtures of different types of surfactants often exhibit synergism in their effects on the properties of a system.33 The mixture of Tween 80/Span 80 had the best synergistic effect on stabilizing a water-in-octane nanoemulsion with HLB = 9.34 Considering the structural similarity between octane and decane, the same mixture of Tween 80/Span 80 and the same HLB value were used. The quantities of water and decane were measured using a graduated cylinder. A mixture of Tween 80 and Span 80 with a mass ratio of 0.783 (the HLB value of the mixture of Tween 80/Span 80 was 9) was used as a surfactant and weighed on a balance with a precision of ±0.1 mg. The emulsion was prepared by stirring a mixture of decane, water, and surfactant
containing methane, ethane, and propane in 50% and 80% water cut w/o emulsions in a stirred, constant-volume, highpressure cell and observed higher rates of hydrate formation with higher emulsion stability. The rate of hydrate formation at 50% water cut was greater than that at 80% water cut. Xiang et al.27 studied the hydrate formation/dissociation of natural gas in (diesel oil + water) emulsion systems using five water cuts: 5%, 10%, 15%, 20%, and 25%. These authors showed that the natural gas consumption due to hydrate formation at a high water cut was significantly higher than that at low water cuts. Mu et al.28 showed that the hydrate growth rate increased when the initial pressure was increased from 6.48 MPa to 8.76 MPa, the water cut was increased from 5% to 30% (by volume), and the temperature was decreased from 278.2 K to 274.2 K. However, little research has been conducted regarding the effects of agitation rate, the diameter of water droplets at constant water cuts, and temperatures ranging from −5 °C to 5 °C on the rate of hydrate formation in w/o emulsions in a stirred reactor. Different hydrate formation models have been proposed to explain the mechanisms of hydrate formation in w/o emulsions. These models include the hydrate shell model,24,25,29−31 a model that is based on crystal growth theory and coupled with a normal distribution of induction times to account for the formation of microsized droplets,32 a model that considers heat transfer and reactions on the surfaces of water droplets,28 and the Kolmogorov−Johnson−Mehi−Avrami (KJMA) model.26 In this study, the formation kinetics of methane hydrate in w/o emulsions were measured using the pressure−volume− temperature (PVT) method during isochoric, isothermal processes in an agitated reactor. When the initial pressure was 6.80 MPa, the influences of the agitation rate, average diameter of the water droplets, and temperature on hydrate growth were examined. Basic data from the experimental investigation were used to generate a kinetic model. A mathematical model with three parameters was established to calculate the hydrate formation rate in w/o emulsions.
2. EXPERIMENT 2.1. Apparatus. The apparatus employed in this study is shown in Figure 1 and consists of a stainless steel vessel, a B
DOI: 10.1021/ef5028923 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
To determine the amount of hydrate formation, the experimental data were processed as described below. By ignoring the solubility of methane in water, it was assumed that methane was distributed in the gas, oil, and hydrate phases. Thus, the amount of methane encapsulated in the hydrate phase at time t was expressed as follows:
for 10 min with an IKA homogenizer (T 25 Digital) at 10 000 rpm. The average sizes of the water droplets in the emulsion were measured using a Zetasizer Nano-ZS laser nanoparticle size analyzer that was manufactured by Britain Malvern Instruments. Three different surfactant concentrations were added to the emulsions. Three types of 30% water cut emulsions were used and denoted as 1#, 2#, and 3#. The water in decane emulsion preparation details are listed in Table 1.
NmH(t ) = NmT − [NmG(t ) + NmO(t )]
where NmH(t) denotes the amount of methane in the hydrate phase at time t (given in moles); NmT denotes the total amount of methane in the system (in moles); NmG(t) denotes the amount of methane in the gas phase at time t (in moles); and NmO(t) denotes the amounts of methane in the oil phase at time t (in moles). At the beginning of hydrate growth, we assumed that the amount of methane in the hydrate phase was zero. The total amount of methane in the system was expressed as follows:
Table 1. Information Regarding the Water-in-Decane Emulsion Preparations emulsion No.
water cut (vol %)
mass fraction of Span 80 in water,a ω (wt %)
stirring speed, n (rpm)
stirring time (min)
average water droplet diameter, d̅ (nm)
1# 2# 3#
30 30 30
2 6 10
10000 10000 10000
10 10 10
2338.0 1387.6 1170.2
a
(1)
NmT = NmG(0) + NmO(0)
The mass ratio between Tween 80 and Span 80 was 0.783.
(2)
where NmG(0) and NmO(0) denote the amount of methane in the gas phase and oil phase at the beginning of hydrate growth, respectively (in moles). Substituting eq 2 into eq 1, the amount of methane encapsulated in the hydrate phase at time t can be expressed as follows:
2.4. Experimental Procedure. Before the experiment, the vessel was cleaned with distilled water and ethanol before drying. Then, a 100-mL w/o emulsion was added to the vessel. The vessel was sealed with the detachable top cover and immersed in the coolant bath. Nitrogen was added to the vessel until the pressure of the system reached 6.80 MPa. Next, nitrogen injection was stopped by turning off valve V2. If the system pressure decreased
