Desalination 274 (2011) 91–96

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Desalination

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A new apparatus for seawater desalination by gas hydrate process and removalcharacteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+)

Kyeong-nam Park a, Sang Yeon Hong a, Jin Woo Lee a, Kyung Chan Kang a, Young Cheol Lee a,Myung-Gyu Ha b, Ju Dong Lee a,⁎a Korea Institute of Industrial Technology, 421 Daun-dong, Jung-gu, Ulsan 681–802, Republic of Koreab High-Technology Component & Material Research Center, Korea Basic Science Institute, 30 Jangjeondong, Geumjeong-gu, Pusan 618–230, Republic of Korea

⁎ Corresponding author. Tel.: +82 51 974 9274; fax:E-mail address: julee@kitech.re.kr (J.D. Lee).

0011-9164/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.desal.2011.01.084

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 November 2010Received in revised form 28 January 2011Accepted 28 January 2011Available online 23 February 2011

Keywords:Seawater desalinationGas hydrateRemoval of dissolved minerals

Potential application of gas hydrate-based desalination was suggested with a novel apparatus design. Theequipment continuously produces and pelletizes CO2 hydrates by a squeezing operation of a dual cylinderunit, which is able to extract hydrate pellets from the reactor containing hydrate slurries. Removal efficienciesfor each dissolved mineral from seawater samples was also tested by Inductively Coupled Plasma AtomicEmission Spectroscopy (ICP-AES) analysis. In a single-stage hydrate process, 72–80% of each dissolvedmineral was removed in the following order: K+NNa+NMg2+NB3+NCa2+. Our results also showed that ionrejection by the hydrate process strongly depends on the ionic size and charge. This study illustrates that thesuggested method and apparatus may solve the separation difficulty between hydrate crystals andconcentrated brine solutions, thus it can be applied for more effective desalination processes.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Many countries in the world suffer from a shortage of fresh water,mainly due to increased population as well as the large expansion inindustrial activities. Over the last several decades, seawater hasbecome an important source of fresh water because it is one of themost abundant resources on earth. Traditional desalination plantsbased on the multi-stage flash (MSF) distillation and reverse osmosis(RO) processes have evolved into reliable and established processes.Although the desalination technologies are mature enough to be areliable source for fresh water from the sea, a significant amount ofresearch and development has been carried out to constantly improvethe technologies and reduce the cost of desalination [1,2]. Weinvestigated the potential applications of gas hydrate technologiesfor more economical desalination processes.

Gas hydrates are nonstoichiometric crystalline inclusion compoundsformed by water and a number of small molecules at suitabletemperature and pressure conditions. These compounds exist in threedistinct structures, structure I (sI), structure II (sII) and structure H (sH),which contain differently sized and shaped cages [3]. The desalinationprocess by gashydrate is based ona liquid to solid phase change coupledwith a physical process to separate the solids from the remaining liquidphase.

Gas hydrate formation as a step in developing amethod to producepotable water from seawater was proposed in the 1940s and receivedconsiderable attention in the 1960s and 1970s [4]. In the 1960sKoppers Co. developed a hydrate desalination process based onpropane [5]. Sweet Water Development Co. was another companywhich explored the hydrate process. Both companies operated pilot-plant facilities with support from the Office of Saline Water in theUnited States [4]. Barduhn and associates investigated a variety ofhydrate forming compounds and studied kinetics and separations incontinuous flow systems, and also reviewed the developments of thedesalination process by freezing and gas hydrate formation [6–8].

In the 1960s and 1970s, several process configurations weredeveloped and demonstrated at pilot-plant scale but were neverrealized at an industrial scale. This was due to the difficulty ofseparating the crystals from the concentrated brine solutions and theremoval of dissolved hydrate former from the recovered water [9–11].

More recently, the Bureau of Reclamation in the US sponsored apreliminary study [12], followed by a pilot test conducted at theNatural Energy Laboratory of Hawaii [13]. The test was somewhatsuccessful, although a wash columnwas never built and tested as partof the operation. Estimates of water cost arising from the test were$0.46–0.52/m3 with favorable public financing and $0.59–0.68/m3

with private financing. Problems with the test including difficulty inseparating the crystals, and materials compatibility led to a follow-onprogramwhich included tasks to determine the filterability of hydratecrystals, the design and operation of a wash column, and surveyingalternate higher temperature hydrate formers [14]. Ngan andEnglezos (1996) investigated recovery of water from pulp mill

92 K. Park et al. / Desalination 274 (2011) 91–96

effluents and 2.5 wt.% NaCl solutions through propane hydrateformation in an apparatus in which hydrate nucleation, growth,separation, and melting occur in one vessel [15]. The average reductionin the salt content of the recovered water from the NaCl solutions wasfound to be 31%. Further improvement in the purity could beaccomplished by washing with water.

In the present study, hydrate-based desalination is suggested withnovel apparatus design to easily extract dehydrated high-density gashydrates from a reactor containing hydrate slurries. Removalefficiency for each dissolved mineral was also tested for real seawatersamples by ICP-AES analysis. This study illustrates that the suggestedmethod and apparatus may solve the separation difficulty betweenhydrate crystals and concentrated brine solutions, thus it can beapplied for effective desalination processes.

2. Experimental section

2.1. Apparatus

Fig. 1 shows a schematic of the new apparatus design, whichcontinuously produces and pelletizes gas hydrates by a squeezingoperation of a dual cylinder unit. The apparatus can be divided intofive parts: hydrate reactor with dual cylinder unit, water and gas feedline, circulation pump with bubble generator and the hydratedischarge unit. The main part of the apparatus is the jacket-typehydrate reactor, which is made of 316 stainless steel and has aninternal volume of about 1800 cm3. In addition the reactor has a dualcylinder unit, two viewing polycarbonate windows, a water-levelsensor and a drain valve. In order to temperature control, a refrigerantfluid (water–ethylene glycol solution) is circulated between theduplex jacket assembly and an external refrigerator (model HD-03A,Hyundai Eng. Co., Korea). Three pressure transmitters (Trafag, model8251, Switzerland) and five copper-constantan thermocouples(Omega, T-type, USA) are employed for the pressure and temperaturemeasurements of the hydrate reactor, gas tank and water feed tank.The pressure of the gas and water feed tanks are kept higher than thehydrate reactor which enables the supply of gas and water to thehydrate reactor through the control valves with a PID controller.

In this apparatus, a high pressure circulation pump (Samec,multiple-stage electric pumps, Italy) and century tube type bubblegenerator are used to mix the reactor contents. Preliminary experi-ments showed that the average bubble size was around 10 μm, whichwas measured by optical microscopic analysis through the windows.

Fig. 1. Schematic of the experimental apparatus fo

As shown in Fig. 2, the hydrate reactor includes a dual cylinder unitalong with pistons, which is vertically movable in a connection pipe.The connection pipe has plurality of passing holes having differentsizes and shapes. The bigger sized passing hole (main hole) provideschannels for the hydrate slurries to flow into the connection pipe.When sufficient amounts of hydrate slurries are drawn into theconnection pipe, the upper piston moves downwards to squeeze theslurries. Subsequently, the hydrate slurries are compressed andsqueezed by the strokes of the dual cylinder operation, and thenresidual waters flow out through the plurality of small passing holes.The pistons are operated by hydraulic pressure ranging from 50 to150 kg/cm2. In this study, 50 kg/cm2 of hydraulic pressure wasapplied. Such a squeezing stroke is repeated several times to obtaina sufficient thickness of hydrate pellet. After the process of pelletizing(or compressing), the upper and lower piston move downward at thesame speed to extract the hydrate pellet. As shown in the Fig. 1, thegas hydrate pellet is extracted by a discharging operation beneath thereactor. An actual extracted hydrate pellet sample is shown Fig. 3.After the gas hydrate pellet has been extracted, the upper and lowerpistons are returned to the initial state to begin a subsequent process.

2.2. Experiments

For the desalination experiments, seawater sampleswere obtainedfrom the southeast coast of Korea (35° 5′ 1″ north, 128° 47′ 11″ east)andwere delivered to hydrate formation experiments. Carbon dioxide(CO2, 99.8%, SEM Co. Korea) was chosen as the hydrate forming gas,because the dissolved CO2 in drinkingwater after desalination processis not harmful. When the whole system reached the desiredtemperature, the gas tanks and hydrate reactor were flushed at leastthree times with the hydrate forming gas (CO2) to remove anyresidual air. Subsequently a volume of 1000 ml of seawater wasinjected and the reactor was filled with hydrate forming gas until thedesired pressure was obtained. The experimental pressure andtemperature conditions were 2.9 MPa and 280 K, respectively.

As CO2 gas is consumed during hydrate formation, additional gaswas automatically supplied from the gas tank and a constant pressuremaintained in the reactor with the help of a PID controller and controlvalve. A constant water level was also maintained by a water-levelsensor and control valve.

When the squeezed hydrate pellet was extracted from the reactor,the weight was measured immediately prior to hydrate dissociation.As shown in Fig. 3, dissociated gas bubbles are seen on the surface of

r desalination process by gas hydrate method.

Fig. 2. Front view and cross section of the high pressure gas hydrate reactor. (a) Front view and (b) cross section with the dual cylinder operation.

93K. Park et al. / Desalination 274 (2011) 91–96

the hydrate pellet which was exposed to atmospheric conditions.After it fully decomposed at atmospheric pressure condition, thedissociated water was also weighed to check hydrate conversion.

The salt concentration was analyzed by a digital salt-meter (PAL-ES2, ATAGO Co., LTD. Japan). The detection principle is theconductivity method, and maximum uncertainty is less than 5%. Thecontent of the each dissolved mineral such as sodium, magnesium,calcium, and potassium was analyzed by Inductively Coupled PlasmaAtomic Emission Spectroscopy (ICP-AES). Removal efficiencies foreach dissolved mineral were calculated from the mineral contents ofthe initial seawater and the dissociated hydrate sample. Somesamples of hydrate pellets are reserved in liquid nitrogen for furtherex-situ spectroscopic analysis.

2.3. Raman spectroscopy analysis

A fiber optic based Raman spectrometer (Sentinel, Bruker,Germany) was used to identify the structure of the pure CO2 hydrates.The light source for excitation was an integrated diode laser whosewavelength and power level were 532 nm and 100 mW, respectively.

Fig. 3. Real sample of the extr

A Unilab II probe having a 60 mm working distance was used for theRaman analysis.

2.4. Inductively coupled plasma spectrometer

In order to evaluate desalination efficiency, each dissolved mineral(Na+, Mg2+, Ca2+, K+ and B3+) wasmeasured by ICP-AES (ACTIVA, JYHORIVA). The sample was filtered with a cellulose nitrate membranefilter (Whatman, 0.22 μm) and it was boiled with 1 ml Nitric acid at150 °C for 30 min to eliminate organics. After pretreatment, theconcentration of dissolved ions was measured by ICP-AES. The testresult was assured by Korea Basic Science Institute (KBSI).

3. Results and discussion

For seawater desalination, CO2 hydrates were formed from aseawater sample and the formation reaction proceeded for about150 min after nucleation began. During the hydrate experiments forthe desalination test, CO2 hydrate pellets were successfully producedand extracted from the hydrate reactor. The pelletizing step can beviewed in the video accompanying this work (Video 1).

acted gas hydrate pellets.

Table 1Salinity (dissolved salt content) changes in the liquid and hydrate pellet sample.(Hydrates were formed at 2.9 MPa and 280 K).

Sampling time frombeginning [min]

Salinity changes inseawater sample [wt %]

Salinity in hydratepellet sample [wt %]

Salt removalefficiency [%]

0 3.2 – –

150 3.8 0.7 78.1

Fig. 4. Removal efficiency of each dissolved mineral with its ionic radius.

94 K. Park et al. / Desalination 274 (2011) 91–96

Initial seawater and extracted pellet samples were analyzed forsalinity (dissolved salt content) changes, measured by a digital salt-meter. As the hydrate experiments progressed, the salinity of theseawater in the reactor increased slightly while the salinity of thehydrate pellet decreased. As shown in Table 1, around 78% of salinitywas removed by gas hydrate process. The removal (or desalination)efficiency was calculated as follows:

Removal Desalinationð Þ efficiency ηð Þ = CA0−CA

CA0× 100 ð1Þ

where CA0 is the salinity in the feed seawater and CA is the salinity inthe dissociated hydrate pellet.

ICP-AES analysis was also carried out to investigate the reductioncharacteristics of each dissolved mineral. As mentioned in theexperimental section, the pellets were fully decomposed afterweighing, and then ICP-AES analysis was performed. In the presentstudy, we measured the four most abundant metal ions (Na+, Mg2+,Ca2+ and K+) and boron ion (B3+). In view of the fact that boron canhave a serious impact on the human body and plants [16], the WorldHealth Organization (WHO) has set a preliminary limit of 0.5 mg/L fordrinking water and the EU is also suggesting a guideline of 1.0 mg/L[17]. For that reason, we included boron ion (B3+) analysis.

The experimental results with the content of each dissolvedmineral, removal (desalination) efficiencies and water conversion tohydrate are summarized in Table 2. The conversion of water tohydrate was calculated as follows

Conversion of water to hydrate %ð Þ =Wpellet−Wwater

� �× hydration number

Wwater

× 100ð2Þ

where Wpellet is the weight of the hydrate pellet and Wwater is theweight of the melted water from the hydrate pellet. The hydrationnumber of 6.2 reported from Udachin et al. [18] was used for theconversion calculation.

As seen in Table 2, the amount of dissolved ions in the seawatersamplewas found in the following order: Na+NMg2+NK+NCa2+NB3+.The concentration of minerals in each hydrate pellet taken at 90, 120,150 min were quite similar to each other. Therefore, the average value

Table 2Experimental results with the content of the dissolved minerals (Na, Mg, Ca, K, and B), rem

Sample Sampling time[min]

Na[mg/L]

Mg[mg/L]

Seawater in reactor 0 10003.82 996.92Extracted CO2 hydrate pellet b 90 2117.52 234.02

120 2256.55 249.98150 2006.66 216.50Average 2126.91 233.50

Removal efficiency [%]c 78.7±1.3 76.6±

a Hydration number of 6.2, reported from Udachin et al. [18], was used for the conversiob Hydrate pellet was extracted from hydrate reactor (at 2.9 MPa and 280 K) and then dec Removal efficiency = CA0−CA

CA0× 100:

was taken for the evaluation of removal efficiency of minerals. As thehydrate experiments progressed, the dissolved minerals were success-fully removed in the following order: K+NNa+NMg2+NB3+NCa2+. Inparticular, the removal of alkali metals (Na+ and K+) was better thanthat of alkaline earth metals (Mg2+ and Ca2+) by the hydrate-baseddesalination process.

When comparing mineral removal efficiency and its ionic radius[19] (Fig. 4), a good linear correlation was obtained. Thecorresponding R2 value was also included in this figure to give anindication of the goodness of fit of the regression. With the exceptionof Ca2+, the order of mineral removal efficiency was stronglyproportional to the ionic radius (R2=0.994) which means largerions are more expelled from hydrate pellet, whereas smaller ions areless expelled for the period of hydrate formation and palletizing step.Another correlation between mineral removal efficiency and its ioniccharge was also obtained as shown in Fig. 5. The data showed anegatively linear correlation (R2=0.953), except for Ca2+. Thisrelationship can be explained by the electrostatic attraction betweenhydrate particles and positively charged ions, because most particlesdissolved in water have negative charge [20], thus hydrate particles(negatively charged) attract stronger positive ions from the solution.It can be concluded that the mineral removal efficiency depended onthe ionic radius and strength of positive ionic charge. This suggeststhat a size effect or electrostatic attraction exists during hydrate-based desalination process. To clarify the removal behavior ofdissolved minerals, further studies such as hydrate-ion interactionor surface analysis such as X-ray diffraction would be required.

As shown in Figs. 4 and 5, the calcium ion (Ca2+) did not show thelinear correlation between ion removal and its radius or charge. This isprobably related to the precipitation of calcium carbonate (CaCO3)caused by the excess use of CO2 as the hydrate forming gas. As carbondioxide dissolved in seawater, carbonate ions (CO3

2−) are produced

oval (desalination) efficiencies and water conversion to hydrate.

Ca[mg/L]

K[mg/L]

B[mg/L]

Water conversionto hydrate a

[%]

403.65 432.74 2.91 –

111.24 82.38 0.83 60.0114.79 87.99 0.78 49.3113.54 84.67 0.72 65.2113.19 85.01 0.78 58.2

1.7 72.0±0.5 80.4±0.7 73.3±1.9 –

n calculation.composed for the analysis.

Fig. 5. Removal efficiency of each dissolved mineral versus its ionic charge.

95K. Park et al. / Desalination 274 (2011) 91–96

and can react with calcium ions that are already in the seawater.Consequently, the carbonate ions cause the precipitation of calciumions in the form of CaCO3 as follows [21]

CO2ðgÞ þ H2OðlÞ→CO2−3 ðaqÞ þ 2H

þðaqÞ

Ca2þðaqÞ þ CO

2−3 ðaqÞ→CaCO3ðsÞ

The magnesium ion (Mg2+) could also compete with CO32− but

MgCO3 is appreciably more soluble than CaCO3 [22], which indicatesthat the CaCO3 particles are more abundant in the test sample. Since acirculation pump was used for the mixing of the reactor contents, thesolid CaCO3 could combine with hydrate particles. This could be thereason why the removal efficiency of calcium ion (Ca2+) was belowthe regression line shown in Figs. 4 and 5.

Extracted hydrate samples were characterized by Raman spec-troscopy. To compare Raman spectra, two hydrate pellets formedfrom pure water and from seawater were investigated, as well as theCO2 gas. Raman spectra of CO2 gas and CO2 hydrates are representedin Fig. 6. The Raman peaks of the intramolecular symmetric C Ostretching vibration mode of CO2 were detected in both gas andhydrate phases and the spectra exhibit the double peaks, whichcorrespond to the well-known CO2 Fermi-diad peaks [23]. Raman

Fig. 6. Comparison of Raman spectra of CO2 gas and CO2 hydrates.

peaks of CO2 gas were observed at 1285 and 1389 cm−1, while thespectra of CO2 in the hydrate phase shifted to the lower frequency side(at 1276 and 1380 cm−1 ) than that of the pure gas state. It was alsonoted that there is no difference between Raman spectra of CO2

hydrate formed from pure water and that formed from seawater. Thisindicates that the dissolved minerals in seawater did not affect thehydrate structure, Raman peak position and intensity.

For the desalination test, it should be pointed out that the desali-nation process was performed by a single-stage of hydrate formation–decomposition without any pre- and post-treatment. In addition, thehydrate pellet was not sufficiently compressed during the palletizingstep, which means unconverted salt water still remained as listed inTable 2. Nevertheless, 72–80% of each mineral was effectively removedby the single-stage hydrate process. If an additional stage of the hydrateprocess was added, approximately 92–97% each dissolved mineralscould be removed from the seawater, based on the calculation from theapplication of same removal efficiency as listed in Table 2. It is alsonoteworthy that the earliest RO membranes reject boron to a level ofabout 50%, thus the issue of boron removal has comeunder the scientificspotlight. Despite the progressive improvement in membrane materialand fabrication, boron rejection by modern seawater RO membranesremains considerably lower than that of sodium chloride which is themain inorganic salt in seawater [24]. In some cases even a two-stage ROprocess is insufficient to meet WHO guidelines for the boron level(0.5 mg/L) [25]. However, a two-stage hydrate process could reject 93%boron, which is sufficient meet to WHO guidelines.

Due to recent developments in membrane technology, the trend inthe desalination industry is to use reverse osmosis (RO) for desaltingseawater [26]. However, the most serious problem in membraneprocesses is the complexity of controlling membrane fouling andscaling. As a consequence, the efficiency of the RO process willdecrease due to the pressure drop leading to decreasing flux and anincrease of operational cost by increasing the energy consumption,additional chemical use and reduced membrane lifetime [26].

4. Conclusion and further work

This study illustrates that the suggested method and apparatusmay solve the separation difficulty between hydrate crystals andconcentrated brine solutions, and also it can be applied for moreeffective desalination processes instead of membrane process, whichcan increase operational cost owing to the fouling and scalingproblems. In spite of the advantages of the hydrate-based desalinationprocess, many challenges still remain. As stated above, the issue of ionrejection by the hydrate process depending on the ionic size and/orcharge should be clarified to understand the ion rejection mechanismof dissolvedminerals, which can also apply tometal ion recovery fromindustrial wastewater effluent and seawater treatment as well.

In order to commercialize this desalination concept, key technol-ogies such as rapid hydrate conversion and treatment of largeramounts of water should be developed in conjunction with properlydesigned equipment. In addition, further desalination research with adifferent hydrate forming gas (such as HFCs, SF6 and C3H8) should becarried out, since these gasses are known to from hydrates underrelatively mild conditions (i.e., lower pressure and higher tempera-ture), which can reduce energy consumption during hydrate-baseddesalination process. It is our hope that the principles of hydrateprocess and methodologies discussed in this paper will contribute toimproving desalination processes in the near future.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.desal.2011.01.084.

Acknowledgments

The authors wish to thank Professor Englezos, University of BritishColumbia for helpful suggestions. Financial support for this work was

96 K. Park et al. / Desalination 274 (2011) 91–96

provided from Ministry of Knowledge and Economy (MKE) through“Energy Efficiency & Resources, KETEP Program (2010 T100200399)”and Ministry of Land, Transport and Maritime Affairs (MLTM) inKorea.

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