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Solubility of inorganic salts in sub- and supercriticalhydrothermal environment: Application to SCWO

processesThomas Voisin, Arnaud Erriguible, David Ballenghien, David Mateos, André

Kunegel, François Cansell, Cyril Aymonier

To cite this version:Thomas Voisin, Arnaud Erriguible, David Ballenghien, David Mateos, André Kunegel, et al..Solubility of inorganic salts in sub- and supercritical hydrothermal environment: Applicationto SCWO processes. Journal of Supercritical Fluids, Elsevier, 2017, 120, Part 1, pp.18-31.�10.1016/j.supflu.2016.09.020�. �hal-01417006�

Solubility of inorganic salts in sub- and supercriticalhydrothermal environment: application for SCWO and

SCWG processes

T. VOISINa,b,c,d,∗, A. ERRIGUIBLEa,c,∗, D. BALLENGHIENd, D.

MATEOSd, A. KUNEGELb, F. CANSELLa, C. AYMONIERa,∗aCNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600, Pessac, France.

bFrench Environment and Energy Management Agency,20 avenue du Grésillé-BP 90406, 49004 Angers Cedex 01, France.cUniversité de Bordeaux, Bordeaux INP, CNRS, I2M-UMR5295,site ENSCBP, 16 avenue Pey-Berland, Pessac Cedex, France.

dINNOVEOX, 14 avenue Neil Armstrong, 33700 Merignac, France.∗

The supercritical water oxidation process (SCWO) is of great interest today in recycling toxicand/or complexed chemical wastes with very good e�ciency. When reaching the critical condi-tions (374°C, 22.1 MPa), polarity collapses and water becomes a very good solvent for organiccompounds. However, these interesting properties for organics turn to be problematic regardingdissolved inorganics. Commonly present in the aqueous waste, those inorganics precipitate easilywhen approaching the critical domain, leading to plugs in the process. In order to better under-stand the precipitation of salts in supercritical water, their solubility behaviour is of main interest.However, lots of relevant data are still missing in the literature. The aim of this review is to sum-marise most of the existing data regarding salt solubility in sub- and supercritical water as well asthe di�erent set up and methods developed over the past 50 years, including predictive theoreticalmodeling.

IntroductionLiquid water, as we know it in the normal conditionsof temperature and pressure, is a strong polar solvent.This property designates water as a very good solventfor inorganic salts such as alkali and metallic salts orcomplexes, and a very bad one for organic compounds(oils, petroleum derivatives, polymers. . . ). But in partic-ular conditions of high temperature and pressure (374°C,22.1 MPa), water becomes supercritical, meaning a sin-gle phase is formed between liquid and vapor (c.f. Figure1), resulting in intermediate properties (c.f. Figure 2).One of these particularities is that the water polaritycollapses, leading to an overturn of the solubility abili-ties. More precisely, the polarity of a compound is mainlyquanti�ed with its dielectric constant ε which decreasesquickly with the temperature. This leads to the super-critical water polarity to be as low as non-polar solvents(like hexane. . . ) so that water can now easily dissolveorganic compounds, but with the drawback of inorganicprecipitation.This is the foreground for the Supercritical Water Ox-

idation process (SCWO). Once the organic componentsof the waste have been dissolved, liquid oxygen is intro-duced in order to oxidize most of the compounds intobasic inert molecules. The drawback of this phenomenonis that most of the inorganic compounds become non-soluble in the media, leading to a massive precipitationinto solids which can induce plug formation inside the

∗ cyril.aymonier@icmcb.cnrs.fr

Figure 1: Pressure-temperature phase diagram of purewater (adapted from [1])

tubular reactors. Despite the fact that SCWO processeshave been studied over the past 30 years, salt precip-itation and plugging is paradoxically commonly knownbut the phenomenon is not well characterized. Besides,studies on salt precipitation and solubility in supercriti-cal water are way scarcer.

The aim of this review is to sum up the experimentalmethods that have been developed in order to measuresalt solubility in sub- and supercritical water as well asthe theoretical modelling proposed for predictive calcula-tion. After a short presentation of the SCWO processesand technologies, the notion of salt types will be intro-duced in a second part regarding water-salt equilibria.Consecutively, the third part concerns experimental set-ups for salt solubility measurement, with a historical re-view of the research in the �eld and a summary of all thesolubility data available in sub- and supercritical water.

2

Then, a part concerning salt mixtures and the in�uenceon solubility will follow. And the last part will presentthe di�erent theoretical and semi-empirical models forpredictive calculation of inorganic salt solubility in sub-and supercritical conditions.

Figure 2: Changes in the dielectric constant of wateraccording to the temperature, at 25 MPa. Comparisonwith common solvent values at room pressure and

temperature [2].

Part I

Supercritical wateroxidation process(SCWO)

The main interest of the SCWO process is to treat acertain type of waste. Chemical liquid wastes are mostof the time characterized by their Chemical Oxygen De-mand (COD), expressing the amount of organic matterin the liquid phase. High organic rate (> 600 g.L-1) likesome of the petroleum derivatives are often sent for incin-eration whereas low organic rate (< 25 g.L-1) are reservedfor biological oxidation in sewage treatment plants. Thisleaves an open window for the hydrothermal oxidativeprocesses like SCWO.The principle of the process is to take advantage of the

properties of supercritical water, which is both a goodsolvent of organic molecules. In order to assure a com-plete oxidation, liquid oxygen, air, or hydrogen peroxideis also injected in the process. The working conditionsare usually a pressure of 25 MPa and temperatures from300°C to up to 600°C.One of the advantages of the SCWO process is that it

does not produce fumes or ashes, but mainly pure gases

(CO2, N2) and liquids (H2O, acids/basis), depending onthe composition of the waste. Furthermore the amountof NOx produced is negligible. As oxidation reactions areexothermic, it enables an autothermal process (meaningthe heat released from oxidation is enough for keepingthe process at the right temperature). For quite highCOD (> 100 g.L-1) energy can be gained during the op-eration. Another main interest comes from the fact thatSCWO process can easily reach 99.99% of waste degrada-tion e�ciency in very low residence time (few minutes)[3�5]. Figure 3 (a) shows the main products obtainedwith a SCWO process, depending on the composition ofthe waste.

(a)

(b)

Figure 3: (a) Scheme of the main products obtainedwith SCWO processes. (b) General scheme of a SCWO

process

A �rst limitation of SCWO processes is to deal withthe corrosion resulting from the combination of high pres-sure and high temperature water, an oxidizing agent andcorrosive species such as ions, heteroatoms or acids. Thisissue imposes SCWO plants to operate with corrosion re-sistant alloys such as super-alloys (Hastelloy-C, Inconel625, ...) [6�8]. Another method consists in using a pro-tective oxidation layer (such as titanium oxide) as a linerinside a stainless steel reactor.As shown on Figure 3 (a), the second main limita-

tion for SCWO plants comes from inorganic materials

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(metals or salts) which precipitate into solids and canremain stuck in the process, leading to plugs in the reac-tor. Over the past 30 years, research has been focused onthe design of reactors, with the aim of solving the plug-ging problem[9�12]. Examples of designs are presentedin Figure 4.

A large number of papers has been published aboutSCWO processes, dealing with reactor designs, scale con-trol or waste treatment e�ciency. A 2001 study [9] sumsup most of the existing technologies for SCWO, but alsothe di�erent remaining issues regarding corrosion, plug-ging and e�ciency. A few years later, two reviews [13, 14]focused on the salt precipitation phenomenon in SCWO.Salt precipitation is a recurring subject in SCWO pro-cesses and lots of works have been done on the engineer-ing of systems and reactors to try to counteract this draw-back. One example is the MODAR reactor (c.f. Figure 4(c)), also called reverse �ow reactor, and is described byseveral works [10, 15, 16] and patents [17, 18]. Reverse�ow reactor principle is to work with a supercritical zone(upper area of the reactor) where oxidation and wastedegradation occurs, and a subcritical zone (lower area)where solid salt can be re-dissolved and be taken out asa concentrated brine. Some other SCWO reactors havebeen designed in order to reduce the corrosion and pre-cipitation issues by keeping the reactor's wall cooler thanthe supercritical zone with an injection of compressed airor cold water (c.f. Figure 4 (a)). Examples are the wall-cooled hydrothermal burner [19], the transpiring-wall re-actors [20�22] or the cold wall reactor [23]. One companywho is currently running a SCWO process at industrialscale is INNOVEOX in Europe, and is using a multi-injection tubular reactor, consisting in injecting pressur-ized oxygen at di�erent stages of the process (c.f. Figure4 (b)).

At a smaller scale, some other prototypes have beenimagined for SCWO applications. One has been devel-oped, using centrifugal forces to split the two phasesbased on the di�erences in weight and density betweensolid salts and supercritical water. Usually called vor-tex or cyclone reactors, several variety of reactors havebeen developed following this process, such as the cen-trifuge reactor [24], the hydrocyclone [25] or rotationalspin reactor [26]. Another interesting design is the sono-chemical reactor [27] (c.f. Figure 4 (d)), combining anultrasound probe to activate oxidation reactions at lowertemperatures to reduce the precipitation and corrosionissues.

Regarding the oxidative process e�ciency, the reviewswritten by BRUNNER [5, 28, 29] , in 2009 are focusedeither on the destruction of organic biomass (lignin, cel-lulose. . . ) or on the corrosion, depending on the solventused, and the di�erent available materials meeting the re-quirements to prevent it. One year later, a more speci�cwork was done [30�32] on the salt precipitation problem,analysing di�erent salt behaviors, with binary mixturesas well but few solubility measurements. A recent workon the salt issue [12] is focused on the deposition of sticky

salts (Na2CO3, Na2SO4. . . ) and the means to avoid it.

With respect to the commercially full scale process, arecent review [11] proposes an overview of most of theexisting SCWO plants, with the description of the typeof waste treated, the capacity and the type of reactorsused. This work links up with a previous thesis doneon the subject [3] in 2000. In this thesis, many di�erentchemical wastes and compounds have been treated andanalyzed in regard to the oxidative e�ciency as well asa sum up of the di�erent type of process, reactors andmaterials was done.

Figure 4: Some of the reactors speci�cally designed forSCWO in order to avoid plugs with precipitation

(adapted from [3]).

SCWO process exhibits very interesting properties andbene�ts, in terms of e�ciency, energy consumption orresidence time. But some key issues remain a limitingfactor for its industrial development, such as corrosionand/or salt precipitation. Despite the great investmentput into the reactor designs research 30 years ago, node�nitive solution has been found. However, research onsalt behavior under sub- and supercritical water condi-tions has experienced a second wind with the recent in-terest in using supercritical water for biomass conversion,and material recycling [33�35].

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Part II

Water-salts equilibriaand types of saltsAs introduced, the general trend for inorganic salts isto precipitate when reaching supercritical conditions forwater. However, phase behavior with water varies a lotdepending on the nature of the salt. Lots of works havebeen done to try to classify salts regarding to their behav-ior in supercritical water. First, SCOTT & VAN KONY-NENBURG [36] classi�ed 6 types of binary �uid phasediagrams, using the Van Der Waals equation of state.Di�erences between the diagrams are characterized bythe immiscibility or miscibility domains and the con�gu-ration of the critical lines between the two critical pointsof the pure compounds. A seventh type has then beendiscovered [37] with the Lennard-Jones equation of state.Following this work, VALYASHKO [38�40] used some ofthese diagrams which enable the description of solid-�uidinteractions, such as salt-water systems. However, dueto the large di�erence in critical temperature betweeninorganic salts and water (in comparison with two �u-ids for example), unusual phase equilibrium can occurwhen reaching the critical domain. In order to betterbuild these diagrams, some speci�c assumptions to thewater-salt systems have been added (taken from [38]):

1. The melting temperature of the pure nonvolatilecomponent (salt) is higher than the critical tem-perature of the volatile component

2. No solid-phase transition (polymorphism, solid-solution...) or azeotropy in liquid-gas equilibria isconsidered

3. Liquid immiscibility is terminated by the criticalregion at high pressures and cannot be representedby more than two separated immiscibility regionsof di�erent types

4. All geometric elements of phase diagrams, their re-actions and shapes (but not the combinations ofthese elements) can be illustrated by existing ex-perimental examples.

Using a continuous topological transformation approach,the results reveal two main behaviors for salts (type Iand type II) , which are split into many sub-diagrams.Operating small changes on thermodynamical parame-ters allow to process a continuous transformation fromone diagram to another[38, 41].These thermodynamic binary phase diagrams nicely

classify most of the salts according to their behavior insupercritical water. Following this classi�cation, type Isalts present a continuous solubility curve at supercriti-cal temperature which does not cross the critical curve,whereas type II salts present an intersection between the

solubility curve and the critical curve, leading to two crit-ical endpoints in this domain (c.f. Figure 5).

These two general diagrams can then be complicatedwith immiscibility domains occurring in some cases.MARSHALL [42] made a di�erence between type I andtype II salts saying that the �rst group generally has ahigher solubility in supercritical water than the others,and classi�ed most of inorganic salts according to thiscriteria (c.f table I). In addition, VALYASHKO [43] clas-si�ed them according to their melting temperatures (c.f.table I). Type I salts have a melting temperature be-tween 800°C and 1000°C whereas type II salts have theirmelting temperature above 700-800°C. What is interest-ing is that the two classi�cations are not in oppositionwith each other, but complementary. It is also impor-tant to notice that according to this classi�cation, for agiven cation (anion), the salts solubility in supercriticalwater will increase with the size of the associated anion(cation).

Table I: MARSHALL and VALYASHKO classi�cationsfor type I and type II salts.

As appearing in the di�erent classi�cations, sodiumchloride (NaCl) is a very good illustration of a type Isalt, with a high solubility in water, even at sub-criticalor supercritical conditions (compare to other salts), andthe appearance of gas-liquid like equilibrium. The bi-nary diagram at high pressure and high temperature forNaCl is well de�ned (c.f. Figure 6) and has been checkedwith several experimental data [44]. The lower limit ofthe diphasic zone has also been directly observed, but itsupper limit remains theoritically �xed to a certain tem-perature from which a unique supercritical phase remainsand salt precipitates.

A good example of a type II salt is sodium sulfate(Na2SO4), which presents an intersection of the solu-bility curve with the critical curve, leading to a simple

5

Figure 5: P-T-x diagrams for a type I salt (a) and a type II salt (b). For a type I salt, the binary critical curve iscontinuous and distinct from the saturation curves, whereas for the type II the binary critical curve is interrupted bythe saturation curves, leading to critical end points P and Q. Abbreviations: A, volatile compound ; B, nonvolatilecompound ; TP, triple point ; CP, critical point ; EV , eutectic vapor coordinates ; EL, eutectic liquid coordinates ;

P, A-rich S-L-V critical end point ; Q, B-rich S-L-V critical end point (from [13]).

precipitation behavior at subcritical conditions, without3-phase equilibrium (see Figure 6). Some recent experi-ments have been performed on the subject, towards im-provement of gasi�cation processes [30�32]. The set upis a Modar like reactor, with a subcritical part at thebottom and a supercritical zone on the top. The aimof this set up is to study the di�erent salt behavior, de-pending on their type, by looking at their ability to berecovered from the bottom part of the reactor. Experi-ments were performed with binary mixtures of water-saltsystems, with type I or type II salts. The �rst trend ob-served is that type I salts (K2CO3, K3PO4, K2HPO4,KH2PO4, NaNO3, KNO3 and Ca(NO3)2) precipitate insupercritical water and can be recovered as a brine so-lution, whereas type II salts (Na2CO3, Na2SO4, K2SO4

and Na3PO4) precipitate but directly stick to the reactorwalls and rapidly plug the process, disabling any recov-ery as a brine. But even if the behavior follows the sametrend for a salt type, di�erences still exist due to the sol-ubility variations. This is seen in the nitrate compounds,the solubility of NaNO3 is higher than the one of KNO3,itself higher than Ca(NO3)2 (meaning Ca(NO3)2 is easilyrecovered than NaNO3 because it precipitates more eas-ily). As a similar result, KH2PO4 is easily recovered incomparison with KNO3. These results seem to be on theopposite trend than the one predicted by MARSHALL[42] and VALYASHKO [43] classi�cations (solubility in-creases, for a given anion, with the size of the cation andvice versa).

In the end, this classi�cation in types of salt cannot

give all the information regarding solubility at supercriti-cal conditions but it can be used as a trend for the generalbehavior.

Figure 6: Binary diagrams at 25 MPa for water-NaCl(left) and water-Na2SO4 (right). L=liquid phase;

V=vapor phase; S=solid phase (from [13])

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Part III

Solubility measurementsfor inorganic salts in sub-and supercritical water

Inorganic materials, like metallic or inorganic salts, arehighly soluble in water for temperatures under 200°C.This solubility is strongly dependent on the dielectricconstant of water which is dropping down when reachingthe supercritical domain. Thus, inorganics precipitate,leading to the well-known problem of reactor plugging.The initial studies for salt solubility at high tempera-tures and pressures were �rst done for geological purpose[45, 46] in the 40s. These �rst analyses were performedwith vapor pressure measurements at di�erent tempera-tures, up to 650°C. Various salted solutions of potassiumbased salts [45] and sodium based salts [46] were investi-gated.

Then, several authors have investigated the phase equi-librium of the NaCl-H2O system at high temperaturesand pressures, including the supercritical domain, withvapor pressure measurement [51�55]. Following the keeninterest in the phase equilibria for the binary systemNaCl-H2O, a comparison of all the data of the previousstudies was carried out [56], looking at the trends and dis-agreements between the results. Experiments were alsoperformed to complete the lack of data near the criti-cal temperature of water and re-determine some vapor-liquid-halite phases where disagreement existed. In orderto better understand the salt deposition (mainly sodiumbased salts) occurring in turbines and steam machines,which induces fatigue cracking, wearing and mechanicalfailures, the solubility of sodium chloride in steam at hightemperature (450-500°C) and pressure (5-10 MPa) wasmainly studied [57, 58]. The water steam was analysed ina continuous process with a �ame spectroscopy detectorat the end, in order to detect the sodium concentrationaccording to time. Among all the previous cited papers,it was the only one to provide details about experimentalsetup and procedure. Other continuous processes can befound in previous works [59�61] where solubility measure-ments were made using chloride titrations (colorimetricor volumetric analysis). Alternative methods for NaClconcentration measurements used the titration of Na+

ions by ion-chromatography, or even 22Na radioactivetracers in batch system [62, 63], where this method isused as an in situ analysis.

In addition to all researches on sodium chloride solu-tions in sub- or supercritical water, some experimentshave also been performed on sodium sulfate solutions, inthe same range of temperature, pressure, and with anequivalent geological purpose. Studies on Na2SO4 sol-ubility in steam have also been performed [59, 64, 65].Salt concentration measurements were mostly done us-

ing a 35S radioactive tracer in continuous systems.

In 1993, the �rst paper about experimental resultsfor the solubility of salts in sub- and supercritical wa-ter, towards the SCWO application, was published byARMELLINI and TESTER [47] . This work summa-rizes most of the previous results on sodium chlorideand sodium sulfate solubility in steam, comparing themeasurement methods and editing the temperature andpressure domains screened by the past researches. A de-tailed description of the experimental setup and methodis given, as well as a �tting of the data with a Gibbs-Helmholtz model which is compared with some other the-oretical solubility models.

In opposition to the past experimental methods, whichusually consisted of heating a brine solution and thenanalysing the salt concentration remaining inside, thisnew setup (c.f. Figure 7 (a)) proposes a di�erent ap-proach: a pure water solution is introduced, pressurizedand heated up, then the water passes through a tube �lledwith salt crystals in order to dissolve them and saturatethe water to the maximum concentration. Then the so-lution is analysed using ICP measurements. The interestin using such a method is that it prevents the pipes fromplugging, but it can only be used with low saturation con-centration value due to the limited amount of salt insidethe pipe.

This kind of process has also been used by other au-thors [48, 66] for several inorganic salts. In opposition,some use the reverse process, consisting in heating a brinesolution in order to precipitate the salts. Thus, the re-maining solution is at the saturation concentration forthe chosen conditions of pressure and temperature. Thismethod is particularly useful when analysing salts witha low melting temperature, as with nitrate salts [49] (c.f.Figure 7 (c)). Following this, LEUSBROCK et al. [67](2009-2010) used this alternative method for di�erentsalts, without limitation to the low melting temperatureones (c.f. Figure 7 (b)).

It is interesting to notice that one set up [50] com-bines the precipitation and the dissolution of the salts(Na2SO4 and Na2CO3) to analyse the solubility in super-critical water (c.f. Figure 7 (d)). In fact, sodium sulfateand sodium carbonate salts are commonly considered as�sticky salts�, which precipitate easily on reactor walls.Using a �ow conductivity cell at the end of their setup,the salt concentration is known continually according totime. A brine solution is passed through the system,pressurized and heated up. The conductivity will de-crease while the salt precipitate in the reactor; when thesteady state is reached (conductivity drops to a plateau),pure water is passed through, dissolving the salt inside.At this point, the pure water is saturated to a concentra-tion similar to the one of the exiting brine before, thusconductivity remains the same until there is not enoughsalt to saturate the water anymore. This method enablesthe comparison of solubility values between solubilisationand precipitation methods.

All the solubility data regarding inorganic salt in sub-

7

Figure 7: Schemes of di�erent experimental set up for salt solubility measurement. (a) Set ups for the �salt bed�method used by ARMELLINI & TESTER [47] ; (b) Structure for salt bed or precipitation method used by

LEUSBROCK et al. [48] ; (c) Precipitation apparatus used by Dell'ORCO et al. for low melting temperature salts[49]; (d) SCWO plant for salt deposition & salt bed method used by KHAN & ROGAK[50].

and supercritical water are represented in the right col-umn of Figure 8. From one data set to another, pres-sure may vary. In order to have a global representa-tion of the data, solubility values (in ppm) have beenrepresented according to the water density, which takesinto account pressure and temperature variations, and isoften the key parameter for salt solubility. It appearsfrom these graphics that chloride salts (NaCl, KCl,...)have very close solubility values when changing the al-kali cation, but alkaline earth compound seems to havea slightly lower solubility. Similarly, nitrate salts havevery close solubility values, independently of the counter

cation. Comparatively, sulfate salts appear to have lowersolubility values and phosphates seem to have large solu-bility dependency according to their hydration level andcounter cation. Few data are available regarding othertypes of salts.

Many other methods can be used to measure the sol-ubility of salts. When studying molecular ions (SO2−

4 ,CO2−

3 . . . ), spectroscopy can be used as an in situ analy-sis to follow the ion concentration. This kind of methodincludes the use of sapphire windows in order to pointthe analysing light through the media. This kind of setup is used to detect into which type of phase the inor-

8

Figure 8: Presentation of all the solubility data available in the literature for inorganic salt in sub- and supercriticalwater as a function of the water density.

9

Salt Authors & year Temperature (°C) Pressure (MPa) Reference

KCl HIGASHI et al. (2005) 370 - 400 9 - 12 [66]LEUSBROCK et al. (2009) 395 - 405 18 - 24 [67]

LiCl LEUSBROCK et al. (2009) 388 - 419 19 - 24 [67]

NaNO3P. Dell'ORCO (1995) 450 - 525 25 - 31 [49]

LEUSBROCK et al. (2009) 390 - 406 17 - 23 [67]

KNO3P. Dell'ORCO (1995) 475 25 - 30 [49]

LEUSBROCK et al. (2009) 400 - 410 20 - 24 [67]

LiNO3P. Dell'ORCO (1995) 475 25 - 30 [49]

LEUSBROCK et al. (2009) 390 - 405 18 - 24 [67]CaCl2 LEUSBROCK et al. (2010) 395 - 415 19 - 23 [68]MgCl2 LEUSBROCK et al. (2010) 393 - 395 19 - 23 [68]

Na2HPO4 LEUSBROCK et al. (2010) 395 - 408 21 - 23 [69]NaH2PO4 LEUSBROCK et al. (2010) 398 - 422 20 - 24 [69]K2HPO4 WOFFORD et al. (1995) 400 - 450 25 - 31 [70]MgSO4 LEUSBROCK et al. (2010) 385 - 401 19 - 23 [69]KOH WOFFORD et al. (1995) 423 - 525 22 - 30 [70]

Na2CO3KHAN & ROGAK (2004) 380 - 440 24 - 25 [50]

LI et al. (1999) 450 24 - 27 [71]H3BO3 LI et al. (1999) 425 - 450 24 [71]CaCO3 MARTYNOVA et al. (1964) 440 24 [72]NaOH URUSOVA M. A. (1974) 400 28 [73]

Mg(OH)2 MARTYNOVA et al. (1964) 440 24 [72]

NaCl

GALOBARDES et al. (1981) 400-550 1 - 10 [58]ARMELLINI & TESTER (1993) 450 - 550 10 - 25 [47]

IGASHI et al. (2005) 350 - 400 9 - 12 [66]LEUSBROCK et al. (2009) 380 - 420 17 - 24 [67]

Na2SO4

RAVICH & BOROVAYA (1964) 320 - 370 20 - 25 [74]ARMELLINI & TESTER (1993) 500 25 [47]

HODES (1997) 342 - 363 25 [75]DiPIPPO et al. (1999) 320 - 365 20 - 25 [76]ROGAK et al. (1999) 370 - 505 25 [77]

SHVEDOV et al. (2000) 350 - 375 19 - 30 [78]KHAN & ROGAK (2004) 382 - 397 24 [50]

Table II: Presentation of all the solubility data available in the literature for inorganic salt in sub- and supercriticalwater.

ganic compound precipitates [79] or to develop an in situturbidity measurement set up [49] in order to detect thenucleation point of precipitates and the di�erent phasesfor inorganic salt mixtures.

As a summary, dealing with quantitative salt solubilitymeasurements in sub- and supercritical conditions oftenlead in using two di�erent methods. One consisting of theprecipitation of a brine, while the other saturates purewater through a column �lled with salt crystals. But evenif these methods have been used several times by di�erentresearch groups, solubility data remains quite scarce fora lot of inorganic salts. Moreover, these studies onlyconcern binary mixture with water, without in�uence ofany other species present in the solution. This leads usto the next topic, concerning salt mixtures.

Part IV

Solubility with saltmixtures

Despite the fact that some of the common salt's behaviorunder sub- and supercritical conditions are quite known,very few studies investigated the behavior of a mixtureof two salts, and the e�ect that one can have on the solu-bility. One of the �rst studies on mixtures [76] managedto reconstruct a part of the ternary phase diagram of theH2O � NaCl � Na2SO4 mixture under supercritical con-ditions, up to 550°C and 25 MPa (c.f. Figure 9). For theternary �gures, they used previous data [44, 47] to buildthe ternary phase diagram with a continuous topologi-cal method. These results correlate with VALYASHKO'sprediction [38, 40] that the presence of a type I salt witha type II would increase the type II solubility with tem-perature.

Most of the experimental work regarding salt mix-

10

Figure 9: Example of the H2O - NaCl - Na2SO4 ternarydiagram at 400°C, 25 MPa, from DiPIPPO et al.

tures has been done towards the improvement of thegasi�cation processes [30�32, 80], with experimental setup built in order to analyse and separate brines andsalt mixtures. In fact, for the last decade, a lot of re-search has been turned towards supercritical water gasi�-cation (SCWG) of biomass for hydrogen and/or methaneproduction[1, 34, 81]. Key challenges for this kind ofprocess are similar with SCWO, except that catalystsmay be used. In this case, some inorganic ions can de-grade or destroy the catalysts. It is then of great in-terest to be able to trap these poisonous ions. UsingModar like reactors, the aim of the set up is to improvethe selective salt separation. In order to study this sep-aration, various tests were performed with type I�typeI, type I�type II and type II�type II mixtures [30�32].First, looking at type I � type I possible interactions,no changes can be highlighted and the mixture seems tobehave like the solution containing one type I salt. Incontrast, type I � type II and type II � type II salt com-binations show interesting results. For example, with themixture Na3PO4/K2SO4 (type II � type II), as the saltsprecipitate, instead of recovering a Na3PO4/K2SO4 so-lution at the outlet, Na2SO4/K3PO4 (type II � type I)mixture is obtained, meaning that the formation of a typeI salt in equilibrium with a type II salt is favored. Someexchanges between common ions of di�erent salts can alsobe seen with NaNO3/K2CO3 (type I � type II) where atthe end, Na2CO3 is mainly recovered. Regarding themixtures of two types II salts (which cannot recombinedinto a type I salt), the behavior remains the same as abinary solution: precipitation and plugging occurs. Asa conclusion of all the works about the behavior of salt

mixtures, the authors declare that �it is not possible topredict the separation performance of a given salt mixturejust by knowing the separation performance of the singlesalt solution� [32].

In the meantime, KRUSE et al. [80] published theirresults on another study on the improvement of super-critical gasi�cation and the ways to catch poisonous saltsfor metal catalysts. Beginning with a liquid or solid brineinside a semi-continuous reactor (c.f. Figure 10), it isshown that some ions can be trapped inside, depend-ing on the brine used. For example, a potassium brine(KHCO3) is deposited inside before running the exper-iment. A brine solution containing sodium ions is theninjected through the apparatus. The analysis performedon the exiting solution shows that the sodium concen-tration is lower than the initial concentration, and thatpotassium ions are mainly present at the outlet. Severalother similar experiments have been done following thesame procedure, with di�erent kinds of brine or solid bedinside the reactor. These results tend to show the sameconclusions as the previous papers [30�32] which is theability of particular brines to trap some ions by playingon the solubility di�erence between two salts, and themechanism by which certain salts are favored.

Earlier in the decade, a study was performed on thesolubility of Na2SO4 and Na2CO3 under supercriticalconditions [50], but also the individual solubility of bothsalts. According to this work, solubility remains the sameabove supercritical conditions, but is slightly reduced forNa2SO4 at near critical conditions, because of the pres-ence of Na2CO3 in the solution. This in�uence on thesolubility of one salt on the other would be due to thefact that they have a common ion. This leads to an excessin the concentration which would favor the precipitationof the less soluble salt (meaning Na2SO4), according tothe common ion e�ect. Common ion e�ect is a direct im-plication of Le Chatelier's principle, which implies thata chemical equilibrium is favored in the direction of theproducts if the reactant concentrations are in excess. Inthe case of the salt mixture, the presence of Na+ ionscoming from Na2CO3 increases the sodium concentra-tion for the precipitation of Na2SO4, leading to a smallincrease in the products side, thus a decrease in sodiumsulfate solubility.

This e�ect from one salt on another, commonly called�salting in� e�ect if increasing the solubility and �salt-ing out� when decreasing it, has been studied more re-cently [82]. Using a sealed inconel crucible and Dif-ferential Scanning Calorimetry (DSC), di�erent ternarysalt mixtures were studied by detecting the precipita-tion phenomenon, immiscibility limit and supercriticalhomogenization. This allows them to highlight saltingin or salting out e�ects, looking at the changes in theprecipitation temperatures. The study was focused onthree type I salts (K2CO3, Na2HPO4 and K2HPO4) andthree type II (Na2SO4, K2SO4 and MgSO4) and smallsalting in e�ects are observed for Na2SO4/K2SO4 andNa2SO4/Na2CO3 mixtures. Regarding MgSO4 mixtures

11

Figure 10: Scheme of the semi-continuous reactor usedto trap poisonous ions, for gasi�cation applications

(adapted from [80]).

with either Na2SO4 or K2SO4, di�erent e�ects occur de-pending on the molar ratio (salting in and salting oute�ects) between the salts. Surprisingly, a complicatedbehavior between K2CO3 and Na2CO3 does not allowthe quanti�cation of any salting e�ect, and no precipica-tion is observed for phosphate salts (which could be inopposition with the solubility values for phosphate saltsfrom the litterature [69], but the temperatures were ac-tually not high enough to precipitate). Besides the factthat the work is not performed at constant pressure, itbrings interesting information concerning salt mixtures.

The results enable to establish a trend in the ionssolubility (comparing the precipitation temperature) inthe following decreasing order : K+ > Na+ > Mg2+forcations and HPO4

2− > SO42− > CO3

2− for anions. Thissolubility scale means that for example, Na2CO3 salt willbe less soluble than Na2SO4 which will be less solublethan K2SO4. This solubility scale is in opposition withthe trend from MARSHALL and VALYASHKO (TableI). The possible explanation given by the authors comesfrom the consideration of the ionic radius. A small ionwith a single charge, like Na+, will lead to a small ionicradius, thus a strong electrostatic interaction with otherions leading to clustering and ion association prior to pre-cipitation. Whereas a bigger ion like K+ with a largerradius will have a weaker electrostatic interaction andwill be less in�uenced by the drop in dielectric constantwith temperature. Regarding Mg2+, the atom may bebigger, but as the ionic charge is also bigger, the result-ing ionic radius is smaller than K+ or Na+ and it is verysensible with changes in dielectric constant. The excep-tion of CO3

2− salts is quite interesting though. Based onits ionic radius, carbonate salts should be more solublethan sulfate ones. One hypothesis is that cation-anion

interactions may be also in�uenced by the number of co-ordination sites, as sulfate ion own 6 sites compared to3 for the carbonate ion, which could explain the changesin solubility.

Initial studies on salt mixture behaviors were quiteconsistent with the predictions of the salt type diagrams,as long as the interactions were not too complex. Butmore recent studies have begun to highlight much moreintricated interactions between salts. Behaviors wouldthen depend on the nature of the salts in presence (typeI or II salt), their solubility limit, and the presence of ashared ion between them. Thanks to the new impetusbrought by the research for supercritical gasi�cation pro-cesses, several interaction mechanisms have been pointedout, and trends begin to appear. Even if there is nogeneral behavior explanation yet, the ion radius theoryregarding salt solubility scale is a good start for a betterunderstanding of salt mixtures precipitation under sub-and supercritical conditions.

Part V

Predictive andsemi-empirical modelsfor inorganic saltsolubility undersupercritical conditionsSolubility data, regarding inorganic salts, is a key factorto understand precipitation, but as it has been shownearlier, experimental data is not always easy to produce.Especially under supercritical conditions, experimentalset up becomes costly and unhandy. Some data con-cerning common salts like NaCl, KCl or Na2SO4 can befound in the literature, but for more uncommon ones,data are most of the time missing. This is where predic-tive models could be very useful. Lots of theories havebeen developed on ion solvation or ion interaction withwater for simple salts. Moreover, thermochemistry pro-vides a very strong panel of key parameters to charac-terize an element or entity. Besides theoretical models,semi-empirical models most of the time �tted on exper-imental data, like Chrastil type equations, are also useddue to their better accessibility. It often requires the useof basic thermodynamic parameters (such as thermal ca-pacity, Gibbs enthalpy. . . ) and are easy to handle and toapply to almost any type of data.

Depending on the product of interest, some correlationmodels will be more suited than other. Enthalpy andCp approach, or Chrastil like models can be adapted forinorganic and organometallic compounds whereas Flory-Huggins and cubic equation approach better �t the or-

12

ganic and polymeric compounds, or �uid and moleculemixtures. The review from 2011 [83] sums up most ofthe papers between 2005 and 2010, dealing with solubilityof solids in supercritical �uids, indicating which correla-tion model is used to �t the data and which temperatureand pressure range is scanned. Way earlier, CHRASTIL[84] published in 1982 a paper about his famous semi-empirical model on solubility in supercritical �uids. Thismodel has then been adapted and modi�ed for variouscompounds and systems. In 1986, PITZER et al. [85]publish a �rst paper on the modeling of the critical be-havior of NaCl solutions. For the next seven years, heproposed several papers on the thermodynamic proper-ties of electrolyte solutions under supercritical conditions[56, 86�94], developing the virial based Equation of Statesmodels for various salts like NaCl, KCl or NaOH.

(a)

(b)

Figure 11: (a) Comparison of the di�erent models withexperimental data for NaCl from MASOODIYEH et al.[95] ; (b) Extension in the validity domain for the SAA

model.

The limitation of Pitzer EoS on aqueous electrolyte so-lutions is that it requires a large number of characteristicparameters to be able to predict the solubility behav-ior for inorganic salts. Moreover, Pitzer equations aremore convenient to be used for phase diagram determi-nation rather than solubility curves. In order to disposeof a predictive model for solubility under high tempera-ture and pressure, HELGESON et al. [96] (1981) pub-lished a signi�cant paper for their new model, called HKFfor Helgeson-Kirkham-Flowers. This model uses lots ofdi�erent thermodynamic parameters, semi-empirical in-teraction functions, and some variables from the BORNtheory for electrolyte interactions. Several other papershave been published later [97�100] to improve the model,which will then take the name of R-HKF model (R standsfor �Revisited�). Beyond its good abilities to predict sol-ubility, the R-HKF model requires some speci�c thermo-dynamic parameters which can be di�cult to determine(like the molar volume of the salt-solvent mixture ac-cording to the temperature and pressure), especially foruncommon salts. In order to try to dodge this problem,SUE et al. [101] (2002) created a simpli�ed version of theR-HKF model, considering the temperature-density de-pendence of the semi-empirical part of the model, insteadof the temperature-pressure dependence. This model isknown as the SAA model (Sue-Adschiri-Arai) and is eas-ier to handle. Recently, MASOODIYEH et al. [95](2014) published a paper to summarize these di�erentcorrelation models (R-HKF, SAA and Chrastil) and com-pare them with the literature data available to determinetheir precision (c.f. Figure 11 (a)). Without surprise, theR-HKF model provides the best �ts with experimentaldata, but the SAA model is most of the time very closeas well, with very few di�erences with R-HKF.

However, the R-HKF model requires at least 7 parame-ters for each compound involved in the considered chem-ical reaction. In the meantime of simplifying the R-HKFmodel into the SAA model, SUE et al. also extendedthe validity domain to the near- and supercritical region(c.f. Figure 11 (b)), using more recent experimental dataof NaCl and KCl [102�105]. This improvement allowsthe SAA and R-HKF models to predict quite nicely thesolubility of some common salts under supercritical con-ditions. Even though these predictive models well �t ex-perimental data, their use remains quite complex, due tothe number of parameter required, which are most of thetime determined from experimental data. And as soonas experimental data are available, it will become moreconvenient to adopt a Chrastil like model, which is themost common method employed in the literature.

Conclusion

Research on SCWO processes for waste treatment hasexpended a lot during the last 20 years, and many tech-nological gaps have been overpassed. Despite these im-

13

provements, the precipitation of inorganic salt remainsan important issue for most of the SCWO applications.Thanks to the broad development and accessibility ofcorrosion-resistant alloys and sapphire windows technolo-gies, new investigations on the salt precipitation phe-nomena under supercritical conditions become possible.Since, lots of solubility data, phase diagrams and mod-elling have been established in sub- and supercritical con-ditions for the most common salt like NaCl or Na2SO4,but much less data exist for other salts. As a conse-quence, incoherencies appear between experimental re-sults and predictive behaviors, such as the predicted in-crease in solubility of a given cation (anion) with the sizeof the anion (cation), which has been invalidated withthe more recent salt mixture experiments. Moreover, thebehavior of a single salt in water remains a very simplis-tic example compared to the complexity of a real wastetreated in SCWO.Regarding the solubility of salt mixtures, as cited be-

fore, it is for now not possible to predict the behaviorof a given salt mixture just by knowing the behavior ofthe two separated salts. As the salt-water solubility dataunder supercritical conditions are still scarce in the liter-ature, only very few papers are published regarding thestudy of the mixture, due to the complexity to analyse asolubility mixture without knowing the single solubilityof each salt. Despite the great investment put into the

classi�cation of salt in order to highlight a trend for eachtype of salt, the behavior of ternary mixtures remainsvery di�cult to predict and is most of the time speci�cto one application. Still, several types of predictive mod-els have been developed for pure salt solubility in super-critical water conditions, and the experimental approachusing DSC analysis is quite interesting and brings newhypothesis for salt classi�cation depending on their solu-bility value and ionic radius. In addition, the in�uence ofthe oxidative compound or the degradation products likeCO2, on the salt solubility is quite unknown although itspresence in the SCWO process is non-negligible. Thislack of data is quite surprising, as some studies showthat the presence of dissolved salt in water in�uence thesolubility of dissolved gas [106�117], it is very likely thatdissolved oxidative compounds in�uence salt solubility aswell.For future research work, it is probably important to

better understand the in�uence of the presence of otherspecies on the solubility of inorganic salts (salting-in andsalting-out e�ects) under near- and supercritical condi-tions with the aim of understanding the precipitationphenomena in SCWO processes. But it requires �rstsome basic data for pure salts, as it is very probable thatcomplex behaviors like diphasic domains (type I salts),miscibility limits or critical end-points will strongly in-�uence the interactions with other salts or compounds.

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