A photo of the stirring device used in this study (b) Schematic diagram of the stirring mechanism. Comparison between hydrate dissociation enthalpies of proposed coolants in this study and those from literature.
Introduction
Introduction
- Cold storage using sensible heat of chilled water
- Cold storage using fusion heat of Ice
- Cold storage using hydrates
- Gas hydrates
- Gas hydrate structures
- Semi-clathrate hydrates
- Gas hydrate applications
Estimates of the amount of gas hydrate deposits in the world are uncertain, however, compared to all other fossil fuel reserves, the energy in these hydrate deposits is significant. The study of gas hydrates became more interesting after the rise in energy prices in the 1970s, and over 7000 papers on natural gas hydrates have been published since then (Makogon et al., 2007). Various procedures to improve the cold storage characteristics of clathrate hydrates are discussed.
Application of Hydrates in Cold Storage Systems: Review of Experimental
Application of Hydrates in Cold Storage Systems
- Phase equilibrium studies
- Kinetic studies
- Effect of additives
- Effect of gas solubility
- Effect of mechanical stirring, ultrasound and magnetic fields
- Effect of crystallizer flow rate
- Different configurations of cold storage process
- Gas hydrates as slurries in secondary refrigeration
- Gas hydrates as a working fluid
Different classes of PCSs have been investigated for secondary cooling applications, including clathrate hydrate slurries (CHSs), microencapsulated PCM slurries (MPCMS), shape-stabilized PCM slurries (SPCMS), and PCM emulsions (PCME) (Youssef et al., 2013 ). A comprehensive review of CO2 clathrate/semi-clathrate hydrate has been carried out by Eslamimanesh et al.
Theoretical Models
Theoretical Models
- Estimation techniques
- Thermodynamic models
- Activity approach
- Fugacity approach
- Kinetic model
- Apparent rate constant during hydrate growth
- Water to hydrate conversion
- Storage Capacity of gas hydrate
The existence of the different equilibrium phases can be determined using Gibbs theory of free energy (Sloan and Koh, 2008). A detailed derivation of the vdWP model equations using statistical mechanics can be found in the work of Sloan and Koh (Sloan and Koh, 2008). They determined that the use of the Kihara cell potential is the most appropriate approach in such calculations and proposed a procedure to incorporate it into the hydrate equilibrium predictions.
Furthermore, when approaching 𝑟 = 𝑅𝑚− 𝑎𝑗 with larger r values, the cell potential approaches minus infinity, resulting in the divergence of the Langmuir adsorption coefficient. The fugacity of the metastable β-phase, fw, can be obtained using the following equation (Waals and Platteeuw exp( RT f. A brief overview of the kinetic models has been presented by Sloan and Koh (Sloan and Koh, 2008) ).
Experimental Procedures and Equipment for Gas Hydrate Phase Equilibrium
Experimental Procedures and Equipment for Gas Hydrates Phase Equilibrium
- Visual and non-visual techniques
- Visual isothermal method
- Non visual isothermal method
- Isobaric method
- Isochoric method
- Hydrate Phase Equilibrium apparatus
The cell pressure is monitored as a function of the system volume at constant temperature (AA). Consequently, a drop in pressure indicates that the pressure is above equilibrium conditions and hydrate formation is restarted. The isobaric temperature search method is based on the visual observation of the hydrate dissociation.
Usually, in the isochoric method, hydrates are formed by lowering the temperature of the system. No visual observations or complicated calculations are required, resulting in reliable determination of the hydrate dissociation data. The temperature and pressure of the system were measured using the thermocouple and pressure transducer.
Experimental Apparatus and Procedure Used in This Study
Description of the experimental apparatus and procedure
- Materials
- Experimental apparatus
- The high pressure equilibrium cell
- Hydraulic hand pump
- Agitation of the cell contents
- The liquid thermostated bath
- Temperature controllers
- Temperature Probe
- Pressure Transducer
- Leak test
- Calibration of Measuring Devices and determining Experimental Accuracies
- Calibration of Pressure transducer
- Vapour pressure test
- Experimental procedures
- Gas hydrate dissociation measurement
- Kinetic measurements
- NIST uncertainty analysis of the experimental hydrate dissociation data
- Uncertainties estimation
- Reporting uncertainty
A Pt100 (platinum temperature probe) with an uncertainty of ± 0.03 K was connected to the cell to measure the cell temperature. A WIKA pressure transducer with an accuracy of 0.05% of full scale was used to measure cell pressure. The pressure transducer was connected to the cell body to measure the cell pressure.
Therefore, the temperature of the equilibrium cell was kept at the constant temperature of 298.15 K during the pressure calibration. To this end, the temperature of the cell was first set to the constant value of 298.15 K. During the heating process, the pressure of the system increases due to the dissociation of gas hydrate.
Results and Discussion
Results and discussion
- Experimental results
- Thermodynamic study
- Kinetic results
- Modelling results
- Summary of the experimental results
- Conceptualized cold storage process: application of the equilibrium and
- Storage performance
The temperature dependence of the hydrate dissociation enthalpy is shown in Figure 6.13 for the coolants investigated in this study and some pure coolants in the literature. Most of the coolants studied in this paper show a comparable hydrate dissociation enthalpy to those in the literature. In this study, the initial conditions in the hydrate stability zone were chosen to ensure the formation of clathrate hydrate.
It was found that SDS solution increases the rate of hydrate formation of R406A up to a concentration of 500 ppm. The structure of the gas hydrate was also predicted using the model developed in this study. The maximum apparent rate constant for the hydrate formation in the presence of pure water was obtained for R407C.
The highest values of the apparent rate constant in the presence of pure water were obtained for R407C. The gas hydrate dissociation conditions of refrigerant mixtures were evaluated using parameters obtained for pure refrigerants.
Recommendations
Gas hydrates as a source of energy
The hydrate formation rate of R406A increased in the presence of 400 ppm SDS solution and decreased in the presence of 600 ppm SDS solution (Figures C13 and C14). The hydrate nucleation rate for R427A was increased in the presence of 400 ppm SDS solution (Figure 16). The storage capacity of R406A increased in the presence of 400 ppm SDS solution and decreased in the presence of 600 ppm SDS solution (Figure C.25).
For R408A, the storage capacity increased in the presence of a 400 ppm SDS solution and decreased in the presence of a 600 ppm SDS solution (Figure C.26). The apparent rate constant of R507C hydrate formation decreased in the presence of SDS solution (Figure C.30). There was no significant change in the apparent rate constant of R404A in the presence of 400 ppm SDS solution (Figures C.33).
The apparent hydrate formation rate constant of R406A increased in the presence of 400 ppm SDS solution and decreased in the presence of 600 ppm SDS solution (Figures C.34). For R408A, the apparent hydrate formation rate constant increased in the presence of both 400 and 600 ppm SDS solutions.
Marine carbon dioxide sequestration
Gas storage
Because a single volume of gas hydrate can contain 184 volumes of gas per volume of water under standard conditions, they are considered a decent medium in gas storage and transportation applications. The main advantage of gas storage and transportation using gas hydrate over conventional approaches such as liquefaction is safety and lower process volume. The first step is usually achieved by mixing gas and water at a suitable temperature.
In order to promote the hydrate formation nucleation, the application of surfactants has been suggested (Zhong and Rogers, 2000). In the second step, the temperature of the system is set to about 258 K at atmospheric pressure to stabilize the gas hydrate during transport (Taylor et al., 2003). The lower working pressures and higher working temperatures than those of the liquefaction and compression processes make this new method as practical as conventional processes.
Gas separation
Chemical additives that do not participate in the hydrogen-bonded water molecules of the hydrate structure such as tetrahydrofuran (THF), cyclopentane, acetone, etc. salts such as TBAB and tetra-n-butylammonium borohydride (Shimada et al., 2003, Shin et al., 2009, Sun et al., 2010). Kinetic additives that have no effect on the thermodynamics (pressure and temperature) of hydrate formation promote the rate of hydrate nucleation by lowering the surface tension of water thereby increasing the diffusivity of gas molecules in water to initiate hydrate nucleation.
Another category of thermodynamic promoters (semiclathrate hydrates) generally comprises environmentally friendly tetra-n-butylammonium salts in which part of the cage structure is broken to accommodate the large tetra-n-butylammonium molecule. This property hydrates the semiclathrate, resulting in higher storage capacity compared to promoters such as THF. Although promoters such as THF can significantly promote the hydrate formation process, due to their high volatility, a significant amount of promoter would be lost during the corresponding storage/separation/transport processes (Eslamimanesh et al., 2012).
Desalination of sea water
The kinetic additives used include anionic surfactants sodium dodecyl sulfate (SDS) and linear alkylbenzene sulfonate (LABS), cationic surfactant cetyltrimethylammonium bromide (CTAB) and nonionic surfactant ethoxylated nonylphenol (ENP) (Sun et al., 2010, Sloan and Koh, 2008). THF and TBAB are well-known thermodynamic promoters that have been used on a non-industrial scale to date. The resulting hydrate can be separated later to obtain clean water and refrigerant that can be reused in the system.
Nemours' studies were carried out to design an efficient and economical desalination process using gas hydrates (Parker, 1942, Briggs et al., 1962, Javanmardi and Moshfeghian, 2003, Ngema et al., 2014b). Although the results indicate that the process may not be economically viable compared to conventional methods (Englezos, 1993, Chun et al., 2000), it has been found that the use of hydrate promoters can significantly reduce the energy costs of the process. .
Food industry
PRSV equation of state
Huron and Vidal and (HV) Mixing Rules
The originality of Huron and Vidal's combination of an EoS and an activity (the redundant Gibbs model) has encouraged a family of matching methods over the past twenty years. Of superior benefit is the use of predictive GE models in these EoS-GE models, such as the UNIFAC style, as this makes the EoS-GE algorithm fully predictive.
Mixing Rules of Modified Huron-Vidal (MHV)
UNIFAC Activity model
Kinetic results
- Initial conditions
- Pressure change and induction time
- Effect of SDS solution on the induction time
- Storage capacity (SC)
- Effect of SDS solution
- Apparent rate constant
- Effect of SDS solution
The pressure variation before and during the formation of gas hydrates is described in this section. The effect of the initial pressure as well as the addition of SDS solutions on the induction time of gas hydrate formation are studied and shown in figures C.3 to C.16. SDS solution inhibited gas hydrate formation and nucleation rate for three refrigerants R410A, R407C and R 507C (Figures C.9 to C.11).
No effect of 400 ppm SDS solution was observed for 408A and a stimulatory effect in the presence of 600 ppm SDS solution (Figure C.15). Storage capacity during R410A hydrate growth at an initial temperature of 284.8 K and pressures: ♦, 1 MPa; ■, 1.1 MPa in the presence of pure water; solid lines, trend lines. The apparent rate constant (Kapp) of hydrate formation as a function of time is shown in Figures C.27 to C.35 for the coolants studied.
Modeling results
- Modeling parameters
- Hydrate model
- Fluid phase model
Pure component parameters used in PRSV EoS (Stryjek and Vera, 1986), UNIFAC volume and surface parameters, and interaction parameters used in the UNIFAC model are reported in Tables C.2 through C.4. Hydrate Phase Diagram of R143a (C2H3F3); Experimental data: ●, (Hashimoto et al., 2010a); ■, upper quadruple point, (Hashimoto et al., 2010a); Q2, quadruple peak; solid lines, model predictions using the Kihara approach, dashed model predictions using the Parrish and Prausnitz (1972) approach. Hydrate phase diagram of R141b (C2H3Cl2F); Experimental data: ●, (Liang et al., quadruple trough, (Liang et al., 2001); Q1, quadruple trough; solid lines, model predictions using the Kihara approach, line model predictions of the line using the Parrish and Prausnitz (1972) approach.
Hydrate phase diagram of R410A (mixture of R32/R125a with mole percent, respectively) Experimental data: ●, this work; ●, (Akiya et al., 1999); solid lines, model predictions using the Kihara approach, dashed lines model predictions using Parrish and Prausnitz (1972) approach.Hydrate phase equilibrium of R427a, experimental data: ●, (this work); solid lines, model predictions using Kihara approach.Hydrate phase equilibrium of R406A, experimental data: ▲, (this work); lines, model predictions by using Kihara approach.
