Hydrate-based Desalination Research Articles

Thermodynamic investigation of hydrate-based CO2 capture from simulated flue gas with new mixed promoters

Abstract CO2 hydrate-based desalination (CHBD) has been developing for decades to meet the global demands of decreasing carbon dioxide (CO2) emissions. In this work, the CO2 was captured from the simulated flue gas which consists of 18.30 mol% carbon dioxide and 81.70 mol% nitrogen in the presence of tetra-n-butyl ammonium bromide (TBAB) + cyclopentane (CP) + glucoamylase. Then the phase equilibrium data of CO2 hydrate were measured by the method of isochoric pressure-search. Among the seven cases with same concentration of TBAB (0.29 mol%) and CP (5.00 vol%) and different glucoamylase proportions (ranging from 0.00 to 20.00 wt%), the optimum concentration of glucoamylase in the mixed promoters was 3.00 wt%. The phase equilibrium data was calculated by the modified van der Waals–Platteeuw (vdW–P) model with a modification of vapor pressure of water in the empty hydrate lattice. The Peng–Robinson equation of state was used to calculate the fugacity of gas. The maximum average absolute deviation was 4.09% between the calculated results and the experimental results. It revealed that the calculated results were in good agreement with the experimental results.

Effect of porous activated carbon particles soaked in cyclopentane on CP-hydrate formation in synthetic produced water

Hydrate-based desalination (HBD) could be an interesting alternative to more conventional processes for desalinating high-salinity waters such as those produced during oil & gas extraction. The present study investigates the hydrate formation stage of a recently patented HBD process, in which porous activated carbon particles (PACP) are used as guest (here cyclopentane (CP)) reservoirs and carriers for hydrate formation. The effects of the CP content (0.5–1 g of CP per gram of PACP), of the PACP particle size (400 and 700 μm), the stirring rate (0–360 rpm), and the salinity of the synthetic produced water (3.5–16 wt% NaCl) on hydrate formation were investigated in batch reactor experiments. The results showed that the CP-soaked PACP drastically enhanced both the hydrate formation kinetics and the water-to-hydrate conversion, even in the case with the highest water salinity (16 wt% NaCl). The best compromise (high kinetics and conversion) was obtained with the largest particle diameter (700 μm) loaded with 0.8 g of CP per gram of PACP. Stirring increases the initial formation kinetics but slightly reduces the final water-to-hydrate conversion, determined at the end of a 78h-experiment. This study demonstrates that using porous particles soaked in cyclopentane is a promising option for boosting hydrate formation in an HBD process for desalination of high-salinity waters.

Gas Hydrate-Based Process for Desalination of Heavy Metal Ions from an Aqueous Solution: Kinetics and Rate of Recovery

Effective desalination of heavy metal ions from industrial effluents is a challenge mainly due to the existing methods of separation technologies that are energy-intensive, have poor economics of scale, and generate a large amount of sludge. The application of gas hydrate-based technology for the desalination of heavy metals is a promising approach because it generates no sludge and is a relatively green process. In a hydrate-based desalination approach, suitable hydrate-forming guests, a sII hydrate former, interact with water by weak van der Waals forces to produce solid hydrate crystals by excluding the salts and other impurities from an aqueous heavy metal ions solution. As5+, Pb2+, Cd2+, and Cr3+ are common heavy metal ions found in industrial effluents that were individually chosen to prepare a 1000 ppm salt solution. In this work, natural gas was used as the hydrate-forming gas along with cyclopentane (CP) because of its immiscibility in water. The presence of CP also reduces the operating conditions for hydrate formation. CP was used at two different concentrations (6 and 1 mol %), and the kinetics of hydrate formation was further improved by the addition of edible surfactant lecithin to the hydrate-forming solution. The gas uptake kinetics, water to hydrate conversion, and rate of water recovery were studied. Superior kinetics of hydrate growth were observed with 6 mol % CP compared to 1 mol % CP. Also, the addition of a benign additive, lecithin, enhances the kinetics of hydrate formation, resulting in efficient desalination of salt ions. The kinetics of As5+ desalination was the fastest among those of the four selected metal ions.

Investigation of the Effect of NaCl on the Kinetics of R410a Hydrate Formation in the Presence and Absence of Cyclopentane with Potential Application in Hydrate-Based Desalination

Serious issues regarding the scarcity of freshwater encouraged researchers to work on various methods for seawater desalination. Hydrate-based desalination (HBD) is a novel method that has been rec...

Hydrate-based desalination (HyDesal) process employing a novel prototype design

Water and energy are intricately interlinked and therefore it is imperative to develop sustainable technologies to strengthen water-energy nexus. One such industry where the energy – water nexus can be addressed is at Liquefied natural gas (LNG) regasification terminals. At the LNG regasification terminals, LNG is converted back to natural gas by heating using seawater which gets cooled during the process. This cooled seawater is dumped back to the sea leading to wastage of cold energy. One potential technology that can harvest and utilize this waste cold energy is the hydrate-based desalination (HyDesal) process. HyDesal process is based on a liquid to solid phase change by employing a suitable hydrate former for the phase change. While the HyDesal process was proposed almost 70 years ago, it was never commercialized primarily due to slow hydrate formation, inefficient hydrate crystal separation from brine and refrigeration. To address the above challenges, we proposed an innovative HyDesal process utilizing LNG cold energy with enhanced water recovery based on the unusual behavior of propane. In this study, we present the design and demonstration of a prototype for the HyDesal process. With our innovative prototype design, we carried out water recovery experiments and salt rejection studies. Water recovery of 34.85 (±0.35)% and salt rejection of 87.5 (±1.84)% was achieved with 3 wt% NaCl solution.

Evolving artificial intelligence techniques to model the hydrate-based desalination process of produced water

Abstract In this study, two artificial intelligence models based on an adaptive neuro-fuzzy inference system (ANFIS) and a support vector machine (SVM) technique have been successfully developed to predict the desalination efficiency of produced water through a hydrate-based desalination treatment process. A genetic algorithm as an evolutionary optimization method has been used to determine the optimal values of SVM model coefficients. To this end, compressed natural gas and CO2 hydrate formation experiments were carried out, and the desalination efficiency of produced water was measured and utilized for model training and validation. After model development, graphical and statistical analysis approaches have been applied to evaluate the performance of suggested models by a comparison of model predictions with measured experimental data. For the ANFIS model, the coefficient of determination (R2) and average absolute relative error (AARE) are 0.9927 and 0.58%, respectively. The values of AARE and R2 for the SVM model are obtained 0.35% and 0.9985, respectively. These statistical criteria confirm excellent accuracy and robustness of intelligent models in predicting the desalination efficiency of produced water through the hydrate-based desalination treatment process. Furthermore, the Leverage statistical technique has been carried out to define the outliers. The obtained results demonstrate that all experimental data are reliable and both ANFIS and SVM models are statistically valid.

Desalination and Li+ enrichment via formation of cyclopentane hydrate

Hydrate-based desalination is an attractive and energy-effective method. In order to obtain high water recovery and purity, the desalination and Li+ enrichment performance were examined using single-stage desalination via graphite-promoted cyclopentane (CP) hydrate formation. LiCl, LiBr and LiI aqueous solutions with concentration ranging from 0.01 to 0.6 mol/L were used as brine. The rate for CP hydrate formation is determined by concentration, independent of the type of used salts. Halide ions were found to have the different capability in hindering CP hydrate-based desalination, with an order of Cl− < Br− < I−. The desalination efficiency was improved with increased concentration. Water recovery of 40% can be obtained in 1 h reaction, demonstrating the high efficiency of hydrate-based desalination. The desalination efficiency remains constant at around 70% over 24 h reaction, which can be dramatically enhanced by multi-stage desalination. The graphite powder, as the promoter for hydrate formation shows impressive recycling stability. The finding in this study would be of great help for developing effective desalination processes to produce fresh water and enrich high-value salts via CP hydrates.

Hydrate-based desalination process enhanced via graphite

Desalination via hydrate processes is a novel and attractive method for desalting seawater. However, it is a great challenge to form hydrates with large amount in a short time, especially in the presence of high concentrations of salts. In this study, we used cheap and easily separated graphite particles to promote hydrate-based desalination. It was confirmed that graphite has no impact on the structures and compositions of cyclopentane hydrates. The dosage of used graphite can dramatically increase the conversion rate of water to hydrate, without degradation of the desalination efficiency. Due to the presence of graphite, the ratio of NaCl solution to cyclopentane has little impact on the conversion rate. A multi-stage desalination via hydrate formation was demonstrated, which can produce high quality freshwater with a purity of 99.76% meeting the drinking water standard. The findings in this study will help to rationally select and design the desalination process based on hydrates.

Thermodynamic, structural, and kinetic studies of cyclopentane + CO2 hydrates: Applications for desalination and CO2 capture

This study examines the thermodynamic, structural, and kinetic characterizations of cyclopentane (CP) + CO2 hydrates in the presence of NaCl for their potential applications in desalination and CO2 capture. Powder X-ray diffraction (PXRD) demonstrated that CP molecules fully occupied the large (51264) cages of CP + CO2 hydrate (structure II), whereas approximately 62% of the small (5 1 2) cages were captured by CO2 molecules. To determine accurate hydration numbers and dissociation enthalpies of CP and CP + CO2 hydrates, cage occupancy of guest molecules revealed by Rietveld refinement of PXRD patterns was combined with integrated areas of endothermic heat-flow curves obtained by a high-pressure micro-differential scanning calorimeter (HP μ-DSC). The addition of CP contributed to a significant stabilization of CO2 hydrate due to CP molecules’ occupation of the large (51264) cages and a subsequent structural transformation to sII. The shift in the equilibrium curves of CO2 + CP + NaCl hydrates to lower-temperature and higher-pressure regions was more significant at higher NaCl concentrations. The cage-filling behavior of guest molecules and the growth patterns revealed by collected time-dependent Raman spectra indicated that the presence of NaCl retarded the growth rate of CO2 + CP hydrate. The experimental results covering hydrate phase equilibria, dissociation enthalpies, hydration numbers, guest distributions, and enclathration behaviors of CO2 + CP hydrates form fundamental reference data for understanding cage-specific guest occupation and growth behaviors of gas hydrates in the presence of salts. The results will also be useful when designing hydrate-based desalination and CO2 capture technologies using CP.

Improved Kinetics and Water Recovery with Propane as Co-Guest Gas on the Hydrate-Based Desalination (HyDesal) Process

Water is a key resource for sustainable development and plays a crucial role in human development. Desalination is one of the most promising technologies to mitigate the emerging water crisis. Thermal desalination and reverse osmosis are two of the most widely employed desalination technologies in the world. However, these technologies are energy intensive. Clathrate-hydrate-based desalination (HyDesal) is a potential energy-efficient desalination technology to strengthen the energy–water nexus. In our previous study, we proposed a ColdEn-HyDesal process utilizing waste Liquefied Natural Gas (LNG) cold energy based on a fixed-bed reactor configuration. In this study, we evaluated the effect of 10% propane in three different gas mixtures, namely, nitrogen (G1), argon (G2), and carbon dioxide (G3), as hydrate formers for the HyDesal process. The achieved water recovery was very low (~2%) in the presence of NaCl in the solution for gas mixtures G1 and G2. However, high water recovery and faster kinetics were achieved with the G3 mixture. To improve the water recovery and kinetics of hydrate formation for the G2 gas mixture, the effect of sodium dodecyl sulfate (SDS) was evaluated. The addition of SDS did improve the kinetics and water recovery significantly.

Progress and trends in hydrate based desalination (HBD) technology: A review

Abstract The shortage of freshwater boosts the development of seawater desalination technology. As a novel method, the hydrate based desalination technology has been put forward for decades and achieved considerable development in the past years. This review focuses on the experimental progress at the aspects of the hydrate former choice, formation promotion and ion removal efficiency and conceptive innovation of hydrate separation and energy utilization. It should be noted that gaseous hydrate former with low formation pressure and insoluble liquid hydrate former are worthy for further study. Besides, the water migration caused by propane deserves to be investigated much more deeply for the potential value of wide application. Moreover, the utilization proposal of LNG cold energy brings more possibility of commercial application. In a word, the hydrate based desalination technology is hopefully an environment friendly, low-cost and widely used desalination technology in the near future.

Lattice Boltzmann modeling for hydrate formation in brine

In this study, we develop a numerical model based on Lattice Boltzmann (LB) method for simulating hydrate formation in brine, aiming at bridging the gap between microscopic mechanism and macroscopic experimental observations. The model employs convection-diffusion equations, coupled with salt exclusion and salinity-dependent hydrate formation kinetics, and is applied into an experiment of cyclopentane-hydrate formation in brine. Numerical simulation results indicate: 1) the predicted water conversion under wide salinity conditions can match well with experimental measurement and their deviations are less than 9%, much lower than the measurement error; 2) the salt exclusion by hydrate formation decreases the concentration of guest molecule and lowers the hydrate formation rate. The rapidly increased hydrate surface area offsets such effects of the enhanced salt concentration and promotes the rapid formation of the hydrate at the initial stage. Then, the constantly decreased concentration of guest molecule and enhanced concentration of the salt ions dominate the hydrate formation rate and gradually slow down the hydrate formation process; 3) the hydrate morphology significantly depends on guest molecule diffusion length. Comparatively, the effect of salinity on it is small and can be reasonably ignored. This LB model could have a potential application for analyzing the formation mechanism of oceanic hydrate and predicting the efficiency of hydrate-based desalination.

Seawater Desalination by Hydrate Formation and Pellet Production Process

In order to improve the efficiency of CO2 hydrate-based desalination, cyclopentane (CP) was used as secondary hydrate guest to alleviated temperature requirements for hydrate formation. The cations (Na+, K+, Mg2+, Ca2+) and the anions (Cl− and SO42−) were analyzed by inductively coupled plasma optical emission spectrograph (ICP-OES) and ion chromatography (IC), respectively. Dissociated water from the extracted hydrate pellets showed that the removal efficiency of each ion was 55.72-85.52%.This study illustrated that the apparatus may solve the separation difficulty between hydrate slurry and concentrated seawater, and improve ion removal efficiency, thus it can be applied for more effective desalination and solution concentration.

Investigation of thermodynamic and kinetic effects of cyclopentane derivatives on CO2 hydrates for potential application to seawater desalination

In recent decades, gas hydrates have received significant attention due to their potential roles in energy and environmental applications. The formation of gas hydrates involves a process to assemble variously shaped frameworks made of hydrogen bonded networks of H2O molecules, and thus ionic species, such as Na+, K+, Mg2+, Ca2+, Cl−, SO42−, etc., can be excluded from the solid gas hydrate phase. Therefore, gas hydrates can potentially be utilized in desalination technology. For potential application to a hydrate-based desalination (HBD) process, three cyclic compounds, cyclopentane (CP), cyclopentanone (CP-one), and cyclopentanol (CP-ol), were selected, focusing on a comparison of thermodynamic and kinetic effects. From the thermodynamic view, the study confirmed that the hydrate formation was favorable in the order of CP, CP-one, and CP-ol in the presence of CH4 or CO2. For the kinetic comparison, however, CP-ol and CP-one showed a relatively shorter induction time than CP. The conversion yield of CO2 hydrate of CP-one was more than double those of CP and CP-ol hydrates, which is ascribed to the unique hydration of the ketone during the hydrate formation. The findings suggest that CP-one could be a promising candidate for the HBD process.

Thermodynamic and kinetic influences of NaCl on HFC-125a hydrates and their significance in gas hydrate-based desalination

In this study, HFC-125a was selected as a hydrate-forming guest for gas hydrate-based desalination. The thermodynamic and kinetic effects of NaCl on HFC-125a hydrates were investigated with a primary focus on phase equilibria, gas uptake, dissociation enthalpy, and dissociation behavior. The equilibrium curve of HFC-125a hydrate shifted to higher pressure regions at any given temperature depending on the concentration of NaCl. The presence of NaCl also reduced the gas uptake and conversion to hydrates, because of the enrichment of NaCl in the solution during gas-hydrate formation. Even though NaCl did not affect the dissociation enthalpy of the HFC-125a hydrate, the thermograms obtained using a high-pressure micro-differential scanning calorimeter (HP μ-DSC) demonstrated that HFC-125a + NaCl hydrates started to dissociate at lower temperatures due to NaCl in unconverted solutions. Rietveld refinement of powder X-ray diffraction (PXRD) patterns indicated that the HFC-125a hydrate (sII) was transformed into Ih as it dissociated. The dissociation of HFC-125a + NaCl hydrates was retarded and completely ended at higher temperatures compared to the pure HFC-125a hydrate by the sodium chloride dihydrate (NaCl⋅2H2O). Overall, these results could facilitate a better understanding of HFC-125a hydrates in the presence of NaCl; further, they might also be useful in the design and operation of hydrate-based desalination plants using HFC-125a.

Thermodynamic stability and formation kinetics of CHClF2 hydrates in the presence of NiCl2

A hydrate-based technology is suggested for the desalination of industrial effluents, especially electroplating effluents including NiCl2. To decrease the required energy for operating a hydrate-based desalination (HBD) process, chlorodifluoromethane (CHClF2) was used as a guest for hydrate formation at relatively mild conditions. The phase equilibria of CHClF2 hydrates formed in aqueous NiCl2 solutions were investigated to identify the effect of NiCl2 on the thermodynamic stability of CHClF2 hydrates. The dissociation phase boundary of CHClF2 hydrates in the presence of NiCl2 was shifted to higher pressures and lower temperatures as the salinity of aqueous solutions increased. To confirm the feasibility of NiCl2 separation from wastewaters via the HBD process, X-ray diffraction and Raman spectroscopic measurements were performed. The results showed that the Ni2+ and Cl− ions did not participate in the hydrate framework, which revealed that the HBD process could be used to remove NiCl2 from effluents. The formation kinetics of CHClF2 hydrates were analyzed by a new kinetic model adopting transient time-dependent apparent rate constants, which significantly depends on the sub-cooling temperature and the concentration of NiCl2 in the aqueous solutions. Thermodynamic approximations regarding the Ni2+ and Cl− ion exclusion from the clathrate framework and the full occupancy of CHClF2 molecules in the large cages of structure I clathrate hydrate were confirmed by comparing the calculated Ni2+ concentrations in the liquid phase in equilibrium with CHClF2 clathrate hydrates with the experimental measurements. These results provide good information for separating metal ions from wastewater streams by HBD processes.

Clathrate hydrate formation in NaCl and MgCl2 brines at low pressure conditions

Hydrate-based desalination (HBD) has been developed to obtain fresh water from seawater in an economic and environmental sustainable manner. As a low-pressure hydrate former for the HBD process, chlorodifluoromethane (R22) hydrates were formed in the pressure range of 1–6 bar under brine environments, aqueous NaCl and MgCl2 solutions. Synchrotron X-ray diffraction and Raman spectroscopy measurements revealed that the R22 hydrates formed with NaCl and MgCl2 show structure I and enclathration of R22 molecules into the large cages. These results also confirmed that the salt ions cannot be encaged in the gas hydrate framework, reflecting the ion exclusion behavior for HBD above the eutectic of water and hydrated salts. The formation kinetics of R22 clathrate hydrate in the presence of salts show that both the initial growth rate and pressure drop of the R22 hydrates heavily depend on the salinity of aqueous solutions. A new theoretical approach adopting the transient time-dependent apparent rate constant of hydrate formation with salts was proposed to predict the formation kinetics of R22 hydrates under brine environments, which was in good agreements with the experimental results. These results provide good information for separating ionic compounds from aqueous solutions by hydrate-based separation processes.

Hydrate-Based Desalination Using Cyclopentane Hydrates at Atmospheric Pressure

The use of a hydrate-based technology in seawater desalination is an interesting potential hydrate application since salt ions would be excluded from the hydrate crystal lattice. In order to better understand the hydrate-based desalination process, experiments have been conducted using cyclopentane (CyC5, sII) hydrates, which can be formed at atmospheric pressure and temperatures below 7.7 °C. The hydrate formation experiments were performed at various subcoolings for aqueous solutions with different salinities in a bubble column. The hydrate formation times decreased and the hydrate conversion increased with increasing subcooling and agitation. Various hydrate-former injection methods were studied, with the most effective method involving spraying finely dispersed CyC5 droplets (around 5 μm in diameter) into the water-filled bubble column. The latter method resulted in a 2-fold increase in seawater conversion to hydrate crystals compared with injecting millimeter-scale CyC5 droplets. A desalination effic...

Desalination of high salinity produced water using natural gas hydrate

Hydrate-based desalination was suggested as a new method in recent years. In this study, compressed natural gas (CNG), mixed with various produced water samples and the hydrate formation reactions were performed at the pressure of about 95 bar and temperature of 274.2 K. After hydrate formation, the concentrated salty water was extracted from the reactor and the formed hydrate was washed with a suitable amount of fresh water to successfully improve desalination efficiency. Then, the hydrate was dissociated to produce fresh water. Desalination efficiencies for dissolved mineral extraction from produced water samples were measured and it has been found that with 3-stages hydrate process, 79.5%-–84.3% of dissolved mineral components will be removed using gas hydrate formation. This study indicated that the CNG hydrate formation for desalination can be applied for produced waters with initial total dissolved solids (TDS) less than 160,000 mg/L.

Investigation of salt removal using cyclopentane hydrate formation and washing treatment for seawater desalination

Salt removal via formation of gas hydrate was investigated by experimental measurement of the salt concentration in filtered water and water retrieved from hydrate crystals. Gas hydrate formation was carried out at 277.15K, using seawater of 3.4wt% salinity and 3mol% cyclopentane to water-cyclopentane mixture. Single-stage hydrate formation followed by filtration removed 63% of the salt ions. Successive washing treatment with 274.15 and 277.15K freshwater was applied with different amounts of washing water, and several major cations and anions in seawater were analyzed in each treatment step. Ions having larger radii showed less salt removal by hydrate formation, and salt removal decreased as the conversion of water to hydrate progressed. However, salt removal was improved by washing treatment to nearly the same degree regardless of the kind of salt ions and the salt removal efficiency improved up to 42% more than that of filtration only. The results presented in this study can be used as basic design values for the development of a hydrate-based desalination process.