Hydrate-Based Desalination Using Cyclopentane Hydrates at

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Hydrate-Based Desalination Using Cyclopentane Hydrates at Atmospheric Pressure Hongfei Xu,†,‡ M. Naveed Khan,†,§ Cor J. Peters,*,†,§ E. Dendy Sloan,† and Carolyn A. Koh*,† †

Center for Hydrate Research, Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States ‡ Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States § Department of Chemical Engineering, Khalifa University of Science and Technology (Petroleum Institute), Abu Dhabi, U.A.E. S Supporting Information *

ABSTRACT: 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 efficiency of 81% (the salinity decreased from 3.5 to 0.67 wt %) was achieved by using a three-step separation method, including gravitational separation, filtration, and a washing step. Washing the hydrate sample using filtered water decreased the salinity from 1.5 wt % in the solid hydrates before washing to 1.05 wt % after washing. sH, which have different crystallographic structures.4,8−14 The first theoretical activity for desalination via hydrate formation was carried out as early as 1942, using R-23 as the hydrate former.15 Max and Pellenbarg in 200016 and Max in 200617 patented several column crystallizer designs for hydrate desalination. Desalination in the crystallizers was proposed to involve injection of hydrate-forming gas into seawater under hydrate formation conditions that would be present at the bottom of the ocean. The hydrate crystals would be expected to form and rise to the top of the water column, where they would dissociate (at lower pressures) and be separated to give fresh water. Recently, Park et al.3 reported the development of a dual-cylinder reactor to “squeeze out” salt water and form hydrate pellets; thus hydrate formation and separation could be achieved using a single reactor. In addition to various theoretical and experimental activities related to high-pressure desalination, some work has been performed using atmospheric-pressure hydrate formers. Lowpressure hydrate-based desalination can avoid the complexities

1. INTRODUCTION Many countries and regions in the world, such as the Middle East and North Africa, cannot exclusively rely on groundwater resources. Rapidly increasing population and enhanced living standards together with the expansion of industrial and agricultural activities are placing pressure on existing water resources. To avoid water shortages due to increases in water demand, there is a need to expand the sources of water.1 Over the last several decades, desalination of seawater and brackish water has become an important source of fresh water to meet the expected water demand. Traditional methods to accomplish desalination include multistage flash distillation, electrodialysis, and reverse osmosis, but reverse osmosis is the most dominant method because of its low operating cost.2 However, research is still required to improve the desalination technologies, which are limited mostly by the cost of the process.3 In this current work, a hydrate-based method was studied for its potential applications in seawater desalination. Clathrate hydrates are nonstoichiometric inclusion compounds composed of molecules of suitable size enclathrated inside hydrogen-bonded water cages. Depending upon the chemical properties (mainly size) of the hydrate formers (guests), the water molecules (host) arrange themselves into different crystal structures and cages that are stabilized by the hydrate formers.4−7 Three common clathrate hydrates are sI, sII, and © XXXX American Chemical Society

Special Issue: In Honor of Cor Peters Received: September 12, 2017 Accepted: February 19, 2018

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DOI: 10.1021/acs.jced.7b00815 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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temperature for this study was set at −2 °C, which is 0.1 °C (calculated by Multiflash) above the freezing point of the corresponding salinity. Salt solutions with a concentration of 3.5 wt % were used for these experiments. Figure 1b shows a schematic of the apparatus using a spray injection technique. The desalination setup using a spray nozzle was composed of a column crystallizer, a spray nozzle (Spraying Systems Co.), a Ruska pump, a freezer, and a chiller (VWR, model 1197D). First, 1000 mL of salt solution was loaded into the column crystallizer and cooled to the desired temperature by the chiller, and then CyC5 was injected. Cyclopentane was sprayed into the column crystallizer every 1 h using the Ruska pump, which created a pressure drop of approximately 100 psi across the spray nozzle. The sizes of the CyC5 droplets were less than 5 μm, as analyzed using a ZEISS Axiovert S-100 microscope (Figure S1 in the Supporting Information). Twelve sprays of CyC5 were injected into the column, with the total volume of CyC5 being approximately 300 mL. Desalination experiments were carried out for a range of different salinities (0.17, 0.35, 0.65, 1.5, 3.5, 4.5, and 5 wt % NaCl). 2.2. Separation of Hydrates and Unreacted Saline. Hydrate slurries were formed in the crystallizer, which contained solid hydrates and unreacted saline. Separation procedures were applied to collect the solid hydrates, including gravitational separation and vacuum filtration. Gravitational separation applies the density difference between hydrates and saline to concentrate low-density hydrates at the top. After all of the CyC5 was injected, hydrates were kept in the crystallizer for additional 8 h to anneal. Then the unreacted saline was drained from the bottom outlet of the crystallizer. Although some unreacted saline was removed after the gravitational separation, some unreacted saline was trapped in the solid hydrates. Thus, the remaining hydrates were transferred to a Buchner funnel for the vacuum filtration procedure to further remove the trapped saline. The Buchner funnel was jacketed to facilitate cooling during filtration in order to avoid melting the hydrate crystals. Two types of filter disks (Kimble F grade (pore size = 4.0−5.5 μm) and C grade (pore size = 40−60 μm)) were used to determine the effect of porosity. Two filtration durations were studied (5 and 30 min). After the removal of the saline solution, a filter cake formed after the filtration process. 2.3. Washing. To remove trapped salt water between/on the surfaces of the hydrate crystallites in the filter cake, a washing step was performed. The procedure for this step included breaking up the filter cake and mixing the powders with 25 g of deionized (DI) water (or the recycled water from the filtration step) to wash out the trapped saline in the filter cake. Then the filtration process was performed to remove the liquid in the newly formed mixture.

associated with high-pressure processes and potentially reduce the cost of desalination. Kishimoto et al.18 studied cyclopentane (CyC5) hydrate crystal growth at the interface between a salt solution and cyclopentane. Hydrate conversion and separation of unreacted hydrate former are major problems associated with most of the hydratebased desalination processes. This work presents details on CyC5 hydrate formation carried out using column crystallizers with two injection methods: tubing injection and spray injection. The hydrate formation process is followed by a three-step process to separate hydrate crystals, including gravitational separation, filtration, and washing. This study determines the effects of temperature, agitation speed, and droplet size on hydrate formation in the column crystallizers. Furthermore, the effects of the gravitational separation, filtration, and washing processes on crystal separation are investigated to guide future directions for improving the separation processes associated with hydrate-based desalination.

2. EXPERIMENTAL SETUP AND METHODOLOGY 2.1. Hydrate Formation. Two hydrate formation setups were used in the hydrate formation process. The first

Figure 1. Schematic of the experimental setup: (a) injecting CyC5 through spray tubing (1/16 in.) and (b) injecting CyC5 through a spray nozzle.

experimental setup consisted of a column crystallizer, which included an agitator and an injection unit that comprises a syringe pump (New Era Pump System Inc., model NE300) and freezer (model NE300), as shown in Figure 1a. Cyclopentane was injected from the bottom of the reactor, while the temperature of the crystallizer was maintained using a circulating ethylene glycol solution in the jacketed crystallizer. The column crystallizer was made of acrylic plastic and had a capacity of 1.5 L (i.d. = 7 cm and o.d. = 9.5 cm). The salt solution (1 L) was precooled with the circulating glycol solution to the experimental temperature prior to the injection of cyclopentane. CyC5 was pumped into the bottom of the column crystallizer at a flow rate of 0.4 mL/min using the syringe pump. Before entering the column crystallizer, CyC5 was passed through a 1/16 in. tubing. In order to avoid long induction times of cyclopentane hydrate, CyC5 hydrate crystal seeds (1 g) were added into the column crystallizer to initiate the nucleation. The effect of agitation speed (300, 400, or 600 rpm) on hydrate formation was then studied. The operation

3. MATERIALS For the desalination experiments, salt solutions were prepared by mixing sodium chloride (Macron Chemicals) and DI water. An atmospheric-pressure hydrate former, cyclopentane (SigmaAldrich, 98%), was chosen. CyC5 has a low solubility in water (less than 0.01 wt % at 20 °C). Its boiling point is 49.25 °C,19 which makes it less volatile compared with other low-pressure formers, such as tetrahydrofuran. The salt water freezing point and hydrate equilibrium temperature at atmospheric pressure are functions of salinity. In the absence of salt, the equilibrium temperature of CyC5 hydrate at atmospheric pressure is 7.7 B



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Figure 4. Effect of agitation speed on water conversion into hydrates.

Figure 2. CyC5 hydrate equilibrium temperatures and water freezing points at different salinities.

°C.20 Figure 2 shows the equilibrium temperature of the hydrate and the water freezing point as functions of salinity at 1 atm, which were calculated using the software Multiflash.

4. RESULTS AND DISCUSSION 4.1. Hydrate Formation from Salt Water. It was found that during hydrate formation in the column crystallizer, the

Figure 5. Effect of operation temperature on water conversion into hydrates.

Figure 3. Conceptual picture representing hydrate desalination. Yellow spheres represent salt ions.21

salinity of the seawater in the column increases gradually as water is converted into hydrates (as salt ions are excluded from the hydrate lattice),21 as illustrated in Figure 3. The effects of agitation speed and temperature on the conversion of water into hydrates were studied using the column crystallizer (with tubing injection), as shown in Figures 4 and 5, respectively. The conversion of water into hydrates was calculated by using equations S.1−S.6 in the Supporting Information. It was evident that water to hydrate conversion increases with increasing agitation speed and amount of subcooling (lower operating temperatures). This is as expected since these conditions would increase the mass transfer between water and gas and the driving force for hydrate formation. In order to convert more water into hydrates, a spray nozzle was used to inject finely divided droplets (approximately 5 μm

Figure 6. Spray injection can increase conversion of water into hydrates.

in diameter) into the column crystallizer. Higher conversion of water into hydrate can be promoted by increasing the contact area between CyC5 and water. Figure 6 compares the conversions of water into hydrates for the systems using the tubing injection and spray injection methods. The operating conditions for these experiments were −2 °C and atmospheric pressure with 3.5 wt % salt solution. It was apparent from the C



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Figure 7. Effect of initial salinity on conversion of water into hydrates and the theoretical maximum conversion. The theoretical maximum conversion was estimated using Multiflash (commercial phase equilibrium software).

Figure 9. Comparison of experimental results for different filtration operations. F30 and F5 denote filtration using the F-grade funnel for 30 min and 5 min, respectively; C30 and C5 denote filtration with the C-grade funnel for 30 min and 5 min, respectively.

results that the creation of fine droplets leads to an increase from 18.9% to 35.0% water to hydrate conversion. In addition, salt solutions with different salinities were used to determine the effect of salinity on conversion of water into hydrates. The water conversion was compared with the theoretical maximum conversion, which was calculated by assuming no mass transfer limitations. It should be noted that the theoretical maximum conversion is hard to reach because of mass transfer limitations. Cyclopentane hydrates form at the interface between water and cyclopentane droplets. If a cyclopentane droplet is fully covered by a hydrate shell, it is difficult for the cyclopentane within the shell to contact water

for further reaction. Figure 7 shows that as the initial salinity increases, water to hydrate conversion decreases. This is attributed to the increase in salt concentration shifting the stability region for hydrate to lower temperatures. 4.2. Water Purification. Initial salt water and hydrate samples were analyzed for salinity changes using a digital conductivity meter (Thermo Scientific Orion Star pH/ conductivity benchtop multiparameter meter). Water purification was carried out via a three-step process, and the removal efficiency was calculated for each separation step.

Figure 8. Schematic of the gravitational separation process in the column crystallizer. D



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Figure 11. Salinity change during the recycled water washing process.

buoyancy forces, resulting in an increase in the hydrate slurry volume fraction and the transformation into larger hydrate aggregates. The unreacted salt solution is then drained from the bottom of the column crystallizer. Cleaner water can be obtained by dissociating the remaining hydrates. 4.2.2. Vacuum Filtration. In order to remove residual salt in the hydrate slurry after the gravitational separation step, a Buchner funnel was used for a further filtration step. The effects of porosity of the filter disks and duration of filtration on the removal efficiency were studied. Figure 9 shows the results for vacuum filtrations under different experimental conditions. The optimized operational conditions for vacuum filtration are to use the F-grade disk and to filter the hydrate sample for 30 min. It was observed during the course of the filtration process that the hydrate slurries transformed into a more solid morphology comprising very large hydrate aggregates. After the solid morphology formed, it was very difficult to remove further trapped salt solution through filtration. 4.2.3. Washing. Figure 10 shows the final salinities of the water after the gravitational separation, filtration, and washing using DI water. The salinity of collected hydrate slurries after the gravitational separation was 3.14 wt %, which was reduced to 1.51 wt % after gravitational separation and filtration. The salinity of the hydrate sample was reduced to 0.67 wt % after the three steps of salt removal, yielding a desalination efficiency of 81%. The same procedure for the removal of saline water from the hydrate slurries was repeated using recycled water or filtered water to wash the solid hydrate sample. Figure 11 shows the final salinities after gravitational separation, filtration, and a washing step using recycled water. The salinity of the collected hydrate slurries after gravitational separation was 3.16 wt %, which was further reduced to 1.50 wt % after gravitational separation and filtration. The salinity of the hydrate sample after the three steps of salt removal was 1.05 wt %, which corresponds to a desalination efficiency of 70%.

4.2.1. Gravitational Separation. Figure 8 illustrates that because of the difference in density between hydrate crystals and the salt solution, hydrate particles formed in the column accumulate at the top of the column. The smaller hydrate particles and the saline solution surrounding the particles constitute the hydrate slurry. The volume fraction of the hydrate slurry increases as a result of the accumulation of more hydrates. As more water is converted into hydrates, the hydrates accumulate at the top of the column because of

5. MULTISTAGE HYDRATE FORMATION−FILTRATION TO PRODUCE FRESH WATER As can be seen from the results in the previous sections, trapped salt water still exists even after long filtration times. The salinity after filtration is higher than the fresh water criterion. In order to produce fresh water, both the washing method and the multistage hydrate formation−filtration process are feasible. Thus, in this section, a multistage hydrate slurry formation− filtration process was studied in order to produce potable water.

Figure 10. Salinity change during the DI water washing process.

Figure 12. Multistage hydrate slurry formation−filtration process. E



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ORCID

Cor J. Peters: 0000-0002-3783-2308 Carolyn A. Koh: 0000-0003-3452-4032 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the current and past CSM Hydrate Consortium members for their support: BP, Chevron, ConocoPhillips, ENI, ExxonMobil, Halluburton, MultiChem, Nalco Champion, One Subsea, Petrobras, Schlumberger, Shell, Statoil, and Total. M.N.K. and H.X. acknowledge the Petroleum Institute, Abu Dhabi for support.

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Figure 13. Salinity after each reaction−filtration stage.

The salinity of potable water is lower than 0.1 wt %. The experimental process is shown in Figure 12. After hydrate formation, the filtration process is used to remove unreacted salt water from the hydrate slurries. The first stage of reaction used a solution with 3.5 wt % salinity. The other stages used a salt solution with the same salinity as the melted hydrate phase of the stage prior to it. The salinity of the melted hydrate phase in each stage of the multistage process is shown in Figure 13. Five stages were needed to reach the criterion of potable water. The experimental temperature was set to be 0.1 °C above the freezing point. An F-grade Buchner funnel was used for the filtration.

6. CONCLUSIONS Cyclopentane hydrate formation in a column crystallizer was investigated at atmospheric pressure. For desalination using tubing injection, it was found that a lower temperature and a higher agitation speed result in more water being converted into hydrates, as expected. In the experiments using the spray injection method, more water can be converted into hydrates because of the fine cyclopentane droplet sizes created by the spray injection. To collect hydrates and remove unconverted water from the hydrate slurries, a three-step separation process was performed, including gravitational separation, filtration, and washing (using DI water or filtered water). After gravitational separation and filtration, the salinity was reduced from 3.5 wt % to approximately 1.5 wt %. Washing the filtered hydrate crystals with DI water or filtered/recycled water reduced the salinity to 0.69 and 1.05 wt %, respectively.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00815. Image of CyC5 in a water emulsion used to analyze the CyC5 droplet size in the emulsion and procedure to calculate the conversion of water into hydrate (PDF)

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REFERENCES

(1) Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J.-M. Advances in seawater desalination technologies. Desalination 2008, 221, 47−69. (2) Bradshaw, R. W.; Simmons, B. A.; Majzoub, E. H.; Clift, W. M.; Dedrick, D. E. Clathrate hydrates for production of potable water. MRS Online Proc. Libr. 2006, DOI: 10.1557/PROC-0930-JJ01-06. (3) Park, K.-n.; Hong, S. Y.; Lee, J. W.; Kang, K. C.; Lee, Y. C.; Ha, M.-G.; Lee, J. D. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Desalination 2011, 274, 91−96. (4) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (5) Carroll, J. Natural Gas Hydrates: A Guide for Engineers, 2nd ed.; Gulf Professional Publishing: Houston, TX, 2009. (6) Englezos, P.; Ngan, Y. T. Incipient equilibrium data for propane hydrate formation in aqueous solutions of NaCl, KCl, and CaCl2. J. Chem. Eng. Data 1993, 38, 250−253. (7) Ballard, A. L.; Sloan, E. D., Jr. The next generation of hydrate prediction: I. Hydrate standard states and incorporation of spectroscopy. Fluid Phase Equilib. 2002, 194-197, 371−383. (8) Jager, M. D.; De Deugd, R. M.; Peters, C. J.; de Swaan Arons, J.; Sloan, E. D. Experimental determination and modeling of structure II hydrates in mixtures of methane + water + 1,4-dioxane. Fluid Phase Equilib. 1999, 165, 209−223. (9) Patchkovskii, S.; Tse, J. S. Thermodynamic stability of hydrogen clathrates. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 14645. (10) Tanaka, H. On the thermodynamic stability of clathrate hydrates. Can. J. Phys. 2003, 81, 55−60. (11) Østergaard, K. K.; Tohidi, B.; Danesh, A.; Todd, A. C.; Burgass, R. W. Gas hydrate equilibria of 2, 3-dimethylbutane and benzene with methane and nitrogen. Chem. Eng. Res. Des. 2000, 78, 731−737. (12) Ripmeester, J. A.; Tse, J. S.; Ratcliffe, C. I.; Powell, B. M. A new clathrate hydrate structure. Nature 1987, 325, 135−136. (13) Ripmeester, J. A.; Ratcliffe, C. I. 129Xe NMR studies of clathrate hydrates: New guests for structure II and structure H. J. Phys. Chem. 1990, 94, 8773−8776. (14) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Gas storage in structure H hydrates. Fluid Phase Equilib. 1998, 150-151, 383−392. (15) Garrison, K.; Slape, R. J.; Snedden, L. L. Freon-12 Hydrate Desalting Process; U.S. Department of the Interior: Washington, DC, 1968. (16) Max, M. D., Hydrate Desalination for Water Purification. U.S. Patent 6,991,722, 2006. (17) Max, M. D.; Pellenbarg, R. E., Desalination through Gas Hydrate. U.S. Patent 6,158,239, 2000. (18) Kishimoto, M.; Iijima, S.; Ohmura, R. Crystal growth of clathrate hydrate at the interface between seawater and hydrophobicguest liquid: effect of elevated salt concentration. Ind. Eng. Chem. Res. 2012, 51, 5224−5229. (19) Rowley, R.; Wilding, W.; Oscarson, J. DIPPR 801 Property Database, Software Package [G]; Design Institute for Physical Property Data, American Institute of Chemical Engineers: New York, 2009.

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(20) Fan, S.; Liang, D.; Guo, K. Hydrate equilibrium conditions for cyclopentane and a quaternary cyclopentane-rich mixture. J. Chem. Eng. Data 2001, 46, 930−932. (21) Khan, M. N. Phase Equilibria Modeling of Inhibited Gas Hydrate Systems Including Salts: Applications in Flow Assurance, Seawater Desalination and Gas Separation. Ph.D. Thesis, Colorado School of Mines, Golden, CO, 2016.

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Hydrate-Based Desalination Using Cyclopentane Hydrates at

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