Article pubs.acs.org/jced
Solubility of Aluminum Sulfate in Near-Critical and Supercritical Water J. Rincón,* R. Camarillo, and A. Martín Department of Chemical Engineering, Faculty of Environmental Sciences and Biochemistry, Universidad de Castilla La Mancha, Avda. Carlos III, s/n 45071 Toledo, Spain ABSTRACT: The solubility of aluminum sulfate in sub- and supercritical water (SCW) has been measured using a flow system at temperatures and pressures ranging between (619 and 675) K and (15 and 29.2) MPa, respectively. Experimental solubility values (in terms of mole fraction) varied from 1.61·10−5 to 2.94·10−5 under the conditions studied in this work. These data have been fitted by response surface methodology (RSM) obtaining values for an average percentage deviation (APD) of 6.2 %. To extend temperature and pressure ranges analyzed, a theoretical solubility model based on the Flory−Huggins approach was applied, leading to worse predictions than the previous model. Subsequently, some additional empirical and semiempirical approaches were also used for the description of the solubility. As a result, a very simple approach (a polynomial function of fluid density) reached an accuracy even better (APD = 5.8 %) than the more complex polynomial model derived from response surface methodology. conversion of biomass,8−10 the formation of particles,11,12 or its use as a reaction medium in industrial processes.13,14 It should also be highlighted that, although several design aspects have been proposed to avoid these problems,15−18 including different reactor models and/or separation of the inorganic substances before the process step, the implementation of these new designs requires a deep understanding of the solubility of the inorganic species at the SCW processing conditions. Thus, considering this need and the fact that only limited solubility data of inorganic salts (a particular group of inorganic species) are available in the vicinity of the water supercritical point,4,19 the solubilities in sub- and supercritical water of aluminum sulfate are investigated in this paper. The interest in this salt comes from the fact that it can be present in industrial wastewaters capable of being decontaminated through SCW oxidation (for example, in the aqueous effluents from pulp, paper, and dry cleaning industries) and, unlike other sulfate salts, such as sodium or magnesium sulfates,5,19 no solubility data of aluminum sulfate have been published in the literature. In particular, the aim of the work has been not only to investigate the solubility of the salt in near-critical and supercritical water but also to describe its solubility using the most appropriate method from those available in the literature.
1. INTRODUCTION The physicochemical properties of sub- and supercritical water (SCW) are significantly different from those exhibited by the fluid at ambient conditions.1,2 Due to this fact, and particularly to its transition in solvation behavior, changing from polar to nonpolar, SCW has the ability to dissolve organic chemicals, but inorganic compounds are much less soluble and tend to precipitate. As a result, scaling and plugging in installations operating with SCW may occur. In other words, SCW processing may be affected by two important technical constraints, equipment fouling and changes on heat transfer conditions near the reactor walls, both due to the precipitation of inorganic species.1,3 Regarding the origin of inorganic compounds in SCW processing, there are two primary sources.4,5 First, dissolved or suspended inorganic substances may be present in the water itself (e.g., sodium chloride, potassium phosphate, etc.), and they can be fed directly to the process. As the fed stream is heated to supercritical conditions, the solubilities of inorganic compounds are greatly reduced, the solutions become supersaturated, and solids precipitate from them. A second source of these compounds may be their production during the specific SCW application. For example, as a result of the oxidation of organic wastes, inorganic acids (as hydrochloric and sulfuric acids) are produced in the SCW oxidation process, and since these acids are neutralized with caustic soda or another base, salts (as sodium chloride and/or sodium sulfate) that precipitate under supercritical conditions are produced.6,7 It should be noticed that, although these problems have been first encountered in the SCW oxidation process, the presence of inorganic substances in SCW is expected to have a major influence on future commercial applications, such as the © 2012 American Chemical Society
2. EXPERIMENTAL SYSTEM AND METHODS 2.1. Materials. The solubility experiments were performed using deionized water and aluminum sulfate ACS grade, Received: April 2, 2012 Accepted: June 18, 2012 Published: June 28, 2012 2084
dx.doi.org/10.1021/je3003942 | J. Chem. Eng. Data 2012, 57, 2084−2094
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Al2(SO4)3·18H2O, provided by Panreac (Panreac, Montplet & Esteban, S.A., Barcelona, Spain). The salt purity was guaranteed to be (98.0 to 102.0) % (ACS specification). The chemicals used in the analyses (magnesium chloride, sodium acetate, potasium nitrate, acetic acid, barium chloride, and sodium sulfate) were also provided by Panreac (Panreac, Montplet & Esteban, S.A., Barcelona, Spain). All of them were analytical grade. 2.2. Experimental Setup. Figure 1 shows the scheme of the experimental installation used for the solubility measure-
with inert quartz beads to provide a higher crystallization surface. The cell temperature is kept at the desired value by a digital controller (DC) that regulates the electric current through a resistor (R) surrounding the equilibrium cell. The temperature can be measured with an accuracy of 0.1 K, but the control system of the equipment allows temperature variations in the cell in the range ± 3 K. To prevent any entrainment of small particles, a glass wool filter is installed at the exit of the cell. The collection module is the part of the facility where fluid depressurization and sample collection occur. The most important parts of this module are the micrometering valve (MV) and the effluent collection flask (CF). The micrometering valve is used for fluid expansion and subtle flow adjustment. To prevent crystallization of the dissolved salt before the collection device, the length of tubing between the upper part of the equilibrium cell and the micrometering valve was insulated with glass wool and made as short as possible. The effluent device was a glass device cooled in an ice bath. 2.3. Procedure. The experimental procedure for the solubility measurement was as follows. First of all, an aqueous solution of known salt concentration (about 5.3·10−5 mole fraction of aluminum sulfate) was prepared. Then, the equilibrium cell was filled to 3/4 its volume with inert quartz beads 1 mm diameter. Next, the metering valve was closed, and after setting pressure (using the BPR) and temperature (with the DC) at the desired values, 1 mL/min of the salt solution was fed into the cell with the pump (i. e., the solution residence time in the cell was about 135 min, as calculated from the fact that the cell was filled about 3/4 its volume with the beads and considering that the porosity of the quartz beads bed was 0.39). The flow through the cell went from the bottom to the top. The metering valve was kept closed until the set pressure and temperature were reached in the cell. Once this occurred, the metering valve was opened so the salt aqueous solution could flow through the system. From this time, when the feed entered the equilibrium cell, it became oversaturated due to both the conditions in the cell and the salt concentration of the feed. As oversaturation occurred, the salt precipitated until a phase equilibrium was reached. The quartz beads within the cell provided a high crystallization surface so that the kinetic limitations of the precipitation step could be neglected. The resulting exiting stream was saturated at the temperature and
Figure 1. Experimental setup for solubility measurement.
ments. It is a flow system (Basic-02, Mervilab) that includes three sections: impulsion module, equilibrium module, and collection module. In the impulsion module an aqueous solution of known salt concentration is fed from a reservoir into the installation by a high-performance liquid chromatography (HPLC) pump (PU2080, Jasco). The pressure is regulated by a back-pressure regulator (BPR) within ± 0.2 MPa and checked by a manometer (PI-1) with an accuracy of 0.01 MPa. The feed solution was kept oxygen-free by bubbling nitrogen into the salt aqueous solution in the reservoir. The equilibrium module essentially consists of a cylindrical equilibrium cell (EC) with a 250 mL volume (basically a stainless steel tube 34 mm i.d. and 275 mm long). It is filled
Table 1. The Two-Factor Central Composite Design Matrix and Results for Aluminium Sulfate Solubility factor levels experiment no.
experiment order
1 2 3 4 5 6 7 8 9 10 11 12
8 7 5 4 1 2 12 3 6 11 10 9
P/MPa 17.0 27.2 17.0 27.2 15.0 29.2 22.1 22.1 22.1 22.1 22.1 22.1
± ± ± ± ± ± ± ± ± ± ± ±
T/K
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
responsea
coded factors 627 627 667 667 647 647 619 675 647 647 647 647
± ± ± ± ± ± ± ± ± ± ± ±
3 3 3 3 3 3 3 3 3 3 3 3
XP
XT
−1 1 −1 1 −1.41 1.41 0 0 0 0 0 0
−1 −1 1 1 0 0 −1.41 1.41 0 0 0 0
YAS·105 1.65 2.90 1.83 2.34 1.61 2.60 2.94 2.11 2.86 1.97 2.35 2.45
± ± ± ± ± ± ± ± ± ± ± ±
0.26 0.45 0.28 0.36 0.25 0.40 0.46 0.33 0.44 0.31 0.36 0.38
a
The solubility data reported in the last column are the arithmetic mean of the solubility values determined in the two samples analyzed in each experiment. 2085
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manufacturer recommendations. The standard uncertainties in the molar masses were 0.0003 g·mol−1 for water and 0.02 g·mol−1 for aluminum sulfate.22 Lastly, all of these uncertainty data and an error propagation analysis21,23 allowed to estimate the uncertainty in the mole fraction of aluminum sulfate. It was found to be equal to 15.5 % [i.e., ur(YAS) = u(YAS)/YAS = 0.155]. Obviously, it is clear from this section that the aluminum sulfate solubility has been derived in this work from the sulfate anion concentration. However, it should be noticed that the aluminum cation concentration was also determined by atomic absorption spectroscopy and found that, within the experimental error, no excess amount of one of the species aluminum or sulfate was present in the samples (in relation to that amount corresponding to aluminum sulfate molecular formula). Finally, note that the pH of the effluent solution from the equilibrium cell was not measured so the probable occurrence of parallel hydrolysis reactions could not be determined.19 2.4. Analytical Method for Sulfate Concentration Measurement. The concentration of sulfate ions in the collected samples was determined using a turbidimetric method (ASTM standard method D516-88). In this method the sulfate ion solution is converted to a barium sulfate suspension, and the absorbance of the suspension is measured with a UV−vis spectrophotometer (Zuzi, model 4001/52) at a wavelength of 390 nm. As indicated above, the relative uncertainty in the mass of aluminum sulfate determined using this method was found to be 3 %. 2.5. Analysis of Metals. The concentration of aluminum and corrosion products (chromium and nickel) in the collected samples was also determined. The metallic content analyses were performed by atomic absorption spectroscopy using a Varian Spectra 220 FS spectrophotometer (uncertainty
