Article pubs.acs.org/jced
Equilibrium Hydrate Dissociation Conditions of CO2 + HCFC141b or Cyclopentane Mao Wang, Zhi-Gao Sun,* Cheng-Hao Li, Ai-Jun Zhang, Juan Li, Cui-Min Li, and Hai-Feng Huang School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China ABSTRACT: Equilibrium hydrate dissociation conditions (vapor + hydrate + liquid water + liquid HCFC141b or cyclopentane) formed from CO2 + water + HCFC141b or cyclopentane are determined using an isochoric method in this work. HCFC141b and cyclopentane are reported to form the structure II hydrates. Phase equilibrium pressures of the experimental systems of CO2 + water + HCFC141b or cyclopentane are all lower than that of CO2 + H2O at any given temperatures. The presence of HCFC141b has more effect on lowering CO2 hydrate equilibrium pressures at lower temperatures than that of cyclopentane, but HCFC141b and cyclopentane have the similar effect on CO2 hydrate equilibrium pressures at higher temperatures.
1. INTRODUCTION Gas hydrates are solid crystals forming at certain pressure and temperature. Water molecules form cavities via hydrogen bonds. The guest molecules, such as propane, CO2, and so forth, are trapped in water cavities. Guest molecules link with water molecules through van der Waals forces. Gas hydrates structure generally depends on guest molecules and the conditions of pressure and temperature. There are three kinds of structures of clathrate hydrates.1 Previous study showed that clathrate hydrates were applied to various domains such as flow-assurance in the oil and gas industry, gas storage and transportation and cold storage.2−7 CO2 hydrate can be used to store cold energy as its latent heat is about 500 kJ/kg. The latent heat of CO2 hydrate is bigger than that of ice. However, CO2 tends to form structure I hydrates under higher pressure conditions (>1 MPa). Additives can be chosen and added to improve CO2 hydrate formation conditions.8−11 HCFC141b and cyclopentane were reported to form the structure II hydrate, which cold form hydrate at near atmosphere pressure.12,13 HCFC141b hydrate and cyclopentane hydrate can be used for cold storage, gas storage, and separation.14−22 HCFC141b and cyclopentane may also be used to reduce phase equilibrium pressure conditions of CO2 hydrate. Hydrate equilibrium data of CO2 + HCFC141b/ cyclopentane + H2O are insufficient at present. Equilibrium hydrate dissociation conditions of the systems of CO2 + HCFC141b/cyclopentane + H2O were measured in this work.
Figure 1. Schematic diagram of the experimental apparatus. APT, absolute pressure transducer; RTD, resistance temperature detector; V1−V8, valves.
process can be observed from the windows. A magnetic stirrer is used to agitate the test materials inside the cell. The stainless steel cell was put into a thermostatic bath to control the experimental temperature. The thermostatic bath can work in the temperature range from −20 to 100 °C, whose accuracy is ±0.05 °C. The experimental pressure was measured using an absolute pressure transducer, whose pressure range was from 0 to 10 MPa. The accuracy of pressure transducer was ±20 kPa. The experimental temperature was measured by platinum resistance thermometer, whose accuracy was ±0.1 °C. Pressure and temperature were recorded using Agilent 34970A data acquisition system.
2. EXPERIMENTAL APPARATUS AND PROCEDURE Experimental Apparatus. The experimental apparatus is shown in Figure 1, which is previously described by Sun et al.23,24 The experimental rig has a high-pressure cell, which is made of stainless steel. The volume of the cell is about 300 cm3. The pressures of the cell can run up to 20 MPa, which has two lateral plexiglass windows. Hydrate formation and dissociation © XXXX American Chemical Society
Received: April 22, 2016 Accepted: July 13, 2016
A
DOI: 10.1021/acs.jced.6b00333 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Experimental Procedure. The materials used to form hydrate were shown in Table 1. Distilled water, HCFC141b
3. RESULTS AND DISCUSSION The experimental method was verified by measuring hydrate phase equilibrium dissociation conditions of CO2 + H2O, compared to literature data of Robinson and Metha as well as Larson.1 Hydrate phase equilibrium data of CO2 in pure water were shown in Table 2. As shown in Figure 2, the measuring data in this work was in accord with that from literature.1 It showed that the above experimental apparatus and procedure were validated.
Table 1. Test Materials Used in This Worka component
purity
supplier
HCFC141b cyclopentane CO2 water
≥99.5% ≥95% 99.99%
Zhejiang Zhonglong Refrigerant Co., Ltd. Sinopharm Chemical Reagent Co., Ltd. Suzhou Hongyun Gas Co. distilled
a
The purity of HCFC141b, cyclopentane, and CO2 are mass fraction, mass fraction and mole fraction, respectively.
Table 2. Equilibrium Temperatures and Pressures for CO2 Hydrate in Pure Watera
and cyclopentane were weighed using a CP225D electronic balance, whose accuracy was ±0.01 mg when the measuring range was less than 80, and the accuracy was ±0.1 mg when the measuring range was from 80 to 220 g. The equilibrium hydrate dissociation conditions were determined using an isochoric method heating step by step.23−25 The cell was rinsed three times with distilled water. After the cell was dry, a vacuum pump was used to evacuate the cell. Water and HCFC141b or cyclopentane was charged into the cell. In this work, an excess amount of HCFC141b or cyclopentane was put into the cell to ensure the presence of HCFC141b or cyclopentane in any conditions. The weight ratio of HCFC141b to water and cyclopentane to water were 1:2.07 and 1:3.04, respectively. Then CO2 was filled into the stainless steel cell. The test fluids of CO2 + HCFC141b/ cyclopentane + H2O were comprised of three components (CO2, water, HCFC141b or cyclopentane) and four phases (vapor, H2O, CFC141b/cyclopentane, and hydrate). There is only one free degree for the experimental system of CO2 + HCFC141b/cyclopentane + H2O according to Gibbs phase rule. Therefore, the equilibrium conditions of pressure and temperature do not relate to the compositions of the experimental system in this work. The cell was put into the thermostatic bath after the test materials were all charged into the cell. The test materials were cooled to form hydrate. The hydrate formation may be determined by temperature rise and rapid pressure drop as heat was released and CO2 was encapsulated into hydrate cage when hydrates formed. The hydrate formation also was seen from the windows visually. The speed of magnetic stirrer was about 300 rpm during all the experimental process. When a lot of hydrates were observed from the windows, the temperature of the test materials was raised to dissociate all the hydrate. The procedure of hydrate formation and dissociation was done two times. Then the test materials were cooled for hydrate formation. After the hydrate formed in the cell, the test materials were heated step by step (0.1 °C in each step) to dissociate the hydrate. Sufficient time (3−5 h) was given in order to attain phase equilibrium state at each step. When the temperature of the test materials was raised, an obvious pressure increase was watched at each step as some hydrate dissociated in the cell. Once all hydrate dissociated completely, a smaller pressure increase was watched. The parameters of temperatures and pressures were logged continuously and the P−T curve of equilibrium data was plotted. Hydrate equilibrium dissociation point was confirmed from P−T curve, where the slope of P−T curve markedly altered. Hydrate dissociation procedure was also observed and confirmed from the windows.
T/K
P/MPa
276.45 278.15 279.05 280.25 280.85 281.55 282.05
1.81 2.25 2.51 2.94 3.19 3.55 3.83
a
The overall errors of T and P are u(T) = 0.2 K and u(P) = 20 kPa, respectively.
Figure 2. Hydrate equilibrium conditions for CO2 + H2O.
The isochoric method heating step by step was used to gain the equilibrium hydrate dissociation conditions of CO2 + HCFC141b/cyclopentane + H2O. The equilibrium hydrate dissociation data were shown in Table 3 and Table 4. The equilibrium data were also drawn in Figure 3 and Figure 4. Table 3. Hydrate Equilibrium Temperatures and Pressures for CO2 + HCFC141b + Watera T/K
P/MPa
282.35 283.25 284.15 285.35 286.15 287.75 288.65 289.75
0.11 0.20 0.30 0.49 0.65 1.01 1.35 1.86
a
The overall errors of T and P are u(T) = 0.2 K and u(P) = 20 kPa, respectively.
B
DOI: 10.1021/acs.jced.6b00333 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
temperatures. Figure 3 presented that there was a significant shift to lower pressures with the additive of HCFC141b. The higher temperature, the larger difference of hydrate equilibrium pressures between the system of CO2 + HCFC141b + H2O and the system of CO2 + H2O. Figure 4 presented the four-phase of equilibrium hydrate dissociation conditions of CO2 + cyclopentane + H2O. The equilibrium hydrate dissociation temperatures of CO2 + cyclopentane + H2O were higher than that of CO2+ H2O at a certain pressure. It meant that cyclopentane was also enclathrated in hydrate cages. Cyclopentane molecules occupy large cages of structure II hydrates described by Fan et al.13 Structure II hydrate formed in the system of CO2 + cyclopentane + H2O.22,26 The shift tendency of hydrate phase boundary was the same as that of CO2 + HCFC141b+ H2O. As an example, when T was 281.55 K, the hydrate equilibrium pressure was 0.149 MPa for CO2 + cyclopentane + H2O, while the hydrate equilibrium pressure was 3.55 MPa for simple CO2 in pure water system. Figure 4 also showed that the equilibrium data in this work was accord with that from ref 26. Figure 5 compared the equilibrium hydrate dissociation conditions of CO2 + HCFC141b + H2O and CO2 +
Table 4. Hydrate Equilibrium Temperatures and Pressures for CO2 + Cyclopentane + Watera T/K
P/MPa
281.55 281.95 282.75 283.85 284.75 285.45 286.05 286.95 288.25 289.15 290.25
0.15 0.18 0.25 0.35 0.50 0.61 0.70 0.89 1.20 1.48 1.92
a
The overall errors of T and P are u(T) = 0.2 K and u(P) = 20 kPa, respectively.
Figure 3. Hydrate equilibrium conditions for CO2 + HCFC141b + H2O.
Figure 5. Comparison of hydrate equilibrium data of CO2 + H2O + HCFC141b and cyclopentane.
cyclopentane + H2O. It was very interesting that the boundary shift of the hydrate phase equilibrium of CO2 + HCFC141b + H2O was more than that of CO2 + cyclopentane + H2O at lower temperatures (
