Aggregation of Na2SO4 Nanocrystals in Supercritical Water

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Aggregation of Na2SO4 nanocrystals in supercritical water Thomas Voisin, Arnaud Erriguible, Guillaume Aubert, and Cyril Aymonier Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05011 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Industrial & Engineering Chemistry Research

Aggregation of Na2SO4 nanocrystals in supercritical water Thomas VOISIN a, b, c, Arnaud ERRIGUIBLE c *, Guillaume AUBERT a, Cyril AYMONIER a *

a

CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600, Pessac, France.

b

French Environment and Energy Management Agency, 20 avenue du Grésillé-BP 90406,

49004 Angers Cedex 01, France. c

Université de Bordeaux, Bordeaux INP, CNRS, I2M-UMR5295, site ENSCBP, 16 avenue

Pey-Berland, Pessac Cedex, France. Email: [email protected] and [email protected]

Abstract Supercritical water oxidation processes (SCWO) have been developed as an alternative technology to treat toxic and/or complex chemical wastes with a very good efficiency. However, one main limitation of SCWO processes comes from the precipitation of inorganic compounds which can lead to clogging and interruption of the continuous process. Unfortunately few information are available in the literature regarding the precipitation mechanism and the salt particle properties in supercritical water (T ≥ 374°C, P ≥ 22.1 MPa). Then, this work intends to study the formation of salt aggregates from one common salt: disodium sulfate (Na2SO4). A specific and dedicated experimental set-up is presented and succeed in recovering salt powders from supercritical precipitation. Several analyses are performed on the salt samples to obtain aggregate sizes, morphologies and size distributions. ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Based on the experimental results and a previous work, a numerical modeling of the salt precipitation and aggregation is reported to acquire information on the possible aggregation mechanism.

Keywords: Supercritical water, Supercritical water oxidation, Salt precipitation, Aggregation, Numerical modeling

1. Introduction Supercritical water has become an important research field over the past decades 1–4, thanks to the numerous possible applications given by its specific properties. In supercritical conditions (T ≥ 374°C, P ≥ 22.1 MPa) water exhibits intermediate properties among which is a high diffusivity and reactivity with a very low surface tension, viscosity and polarity. Consequently, these properties induce large changes in solubility, as organic compounds become soluble whereas inorganic compounds no longer are 5. This solubility switch is of main interest for applications like material synthesis

6–9

or recycling of industrial wastewater

and chemical wastes 1,2,10–12. This process called SuperCritical Water Oxidation (SCWO), and displays high chemical degradation rates and efficiencies, combined with very short residence times. But the solid precipitation of inorganic salts, commonly present in wastewater, remains the main limitation of the SCWO processes, as the salt deposition into the reactors leads to clogging and interruption of the process 13,14,15,16. Lots of efforts have been deployed to solve this precipitation problem, mainly through the development of new designs of reactors 11,17. However, no universal solution has been found to solve the salt precipitation problem. In a previous work

18

, we proposed to bring a fresh

look at this problem with a focus on the understanding of the salt precipitation phenomenon in supercritical water. This was obtained with solubility measurement, salt crystallite sizes

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through in situ measurements and modeling of the nucleation and growth of particles. Yet, crystallites of tens of nanometers are not be the reason why clogging occurs in SCWO process, but this is the aggregation of these crystallites in larger particles which is at the origin of the obstruction of reactors. Thus, the objective in this publication is to propose a first report on the aggregation phenomenon of primary salt particles precipitated in supercritical water. The present study is focused on one particular salt, sodium sulfate (Na2SO4), as reference system. As solid salts re-dissolve into water as soon as the temperature is decreased, salt aggregates cannot be recovered easily for analysis. Thus, the first part of this work will be dedicated to the experimental set-up built in order to be able to recover salt aggregates after precipitation in supercritical water. Recovered salt will then be analyzed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and granulometry to determine aggregate characteristics. These experimental data will then be used to fit parameters for the aggregation coefficients, to perform the numerical modeling of the salt aggregation under supercritical conditions. The simulation aims to model, based on simple assumptions regarding aggregation and breakage mechanism, the total precipitation phenomenon of Na2SO4 in supercritical water. This modeling implies to take in account nucleation, growth, aggregation and breakage of salt precipitates from the formation of the nanocrystals to the final micron-sized aggregates.

2. Experimental section 2.1.Recovery of salt aggregates and characterization As it was introduced previously, being able to recover the salt powder after precipitation in supercritical conditions is of main interest, as most aggregate analyses need to be done ex situ.



Aggregate analyses usually consist in using several techniques, such as Scanning Electron Microscopy (SEM), Dynamic Light Scattering (DLS) and granulometry. But these methods are rarely adapted for in situ analyses, especially in supercritical water conditions. Few research work managed to recover some salt for analysis, but with the loss of temperature and pressure control

19-21

. Thus, losing control on temperature or pressure during drying can

interfere with the aggregate sizes distribution and morphology, for example by dissolution into water or precipitation due to steam formation. The experimental set-up we propose here enables to recover a sufficient amount of salt aggregates for analysis. The concept is to remove the water inside the reactor in the studied conditions, after precipitation, by addition of pressurized nitrogen. Indeed, after precipitation, the remaining water inside the reactor needs to be removed, in order to be able to cool down the system without re-dissolving all the solid salt in water. Flowing pressurized nitrogen enables to substitute water with N2, so that temperature can be decreased and the system depressurized while maintaining the solid salt inside the reactor. 2.1.1. Experimental set-up The experimental set-up used designed, built and developed (c.f. Fig. 1) is composed of a batch reactor with an internal volume of 20 mL, made in Inconel 625 to prevent corrosion. The sealing of the whole system is insured by a metal-metal contact between the top and the body of the reactor with the addition of a highly resistant elastomer o-ring seal (KALREZ®). The removable top holds one inlet pipe for the introduction of nitrogen and one outlet pipe with a sintered filter, so that water can be removed without dragging the salt aggregates away. Switchable valves are placed in different locations to insure a good control on the system; the pressure is set using a micrometric valve and the control panel of the syringe pump (ISCO) in a constant pressure mode. Two pressure gauges are placed at the inlet and outlet to detect if any clogging occurs during experiment. The whole batch reactor is heated up thanks to five


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heating cartridges (140 W) connected to the temperature control and three K-type thermocouples are located at the surface of the reactor. The safety of the set-up is guaranteed using a rupture disk (max. 34 MPa).

Figure 1: Scheme of the experimental set-up. 2.1.2. Protocol The protocol of the experiments is composed of two steps: •

reaching supercritical conditions in the reactor to precipitate salt particles,

•

flowing pressurized nitrogen in the system to remove water from the reactor, while keeping temperature and pressure constant.22



Nitrogen gas is chosen for its high compressibility and low thermal capacity as well as its high chemical stability and neutrality. Initially, 5 mL of Na2SO4 aqueous solution at 1 mol/L, is introduced into the reactor in order to reach 25 MPa at 400°C. Sodium sulfate solubility in these conditions is 4 mg/kg. Several experiments have been run in the same conditions but with different initial concentrations (0.1 mol/L, 0.5 mol/L and 1 mol/L). The specificity is to avoid any steam formation during heating, as it would induce precipitation. The system is pressurized with nitrogen during heating, to keep water in liquid state until the supercritical conditions are reached. Experimental work performed with sodium sulfate and pressurized gas showed no influence of the gas phase on the salt solubility values. Therefore it is assumed that using N2 gas in the present system does not change the way Na2SO4 precipitates. Once final temperature is reached, the system is maintained in conditions for 5 min to insure the salt precipitation. Then, the outlet switchable valve is opened, leading to a slight depressurization of about 1 MPa, instantly counterbalanced by the syringe pump flowing nitrogen at 25 MPa. The N2 flow is maintained until no water can be seen at the outlet of the set-up. The system is then closed, cooled down and depressurized. The salt powder is recovered at the bottom of the batch reactor. The recovered powders do not require any drying step, as all the water has been removed. Samples are then analyzed with XRD, SEM and granulometry. XRD were performed on a standard apparatus (PANalitycal X'pert MPD Bragg-Brentano, Cu Kα1, α2). For SEM and granulometry measurements, powders were previously grinded manually and exposed to ultrasounds to ensure homogeneity and reproducibility. For granulometry analysis, aggregates have been dispersed in ethanol knowing that sodium sulfate solubility is very low in it. Particle size distributions were measured in number. 2.2.Results and discussion


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The XRD characterization of the powders shows that Na2SO4 does not contain any water in the crystallographic structure, and the detected phase corresponds to the room temperature stable phase V of sodium sulfate (c.f. Fig. 2).

Figure 2: XRD pattern of the Na2SO4 powders with the stable phase V. Applying Scherrer calculations on two well resolute peaks of this XRD result, an average crystallite size of 35 ± 2 nm is obtained. This result is larger than the one obtained from in situ measurements (average size of 20 nm

18

), but can be explained by the difference in set-up

configurations and heating rates (about 15 min for the present batch reactor, compared to few seconds for the sapphire capillary of in situ experiments). SEM analyses were performed on the powders, looking for aggregate sizes and possible morphology trends. SEM pictures on Figure 3 (a to d) represent micrographs of aggregates at different scales showing that small aggregates with a size inferior to 1 µm coexist with larger ones with a size above 10 µm.



Figure 3: SEM pictures of Na2SO4 powder after precipitation in supercritical water at different magnifications. (a) *40, (b) *250, (c) *800 and (d) *2500. There is no defined morphology for the Na2SO4 aggregates and their sizes appear to be various. The size distributions obtained from granulometry analyses show that the majority of aggregates have a size of 0.5 µm, with few bigger sizes between 1 and 10 µm. Several experiments have been run in the same conditions but with different initial concentrations (0.1 mol/L, 0.5 mol/L and 1 mol/L). From the different distributions obtained (c.f. Fig. 4 (a)), it can be seen that concentration has very little influence on the size distributions.


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(a)

(b)

Figure 4: Size distributions of Na2SO4 aggregates obtained. (a) Aggregate size distributions of Na2SO4 for different initial concentrations. (b) Aggregate size distributions of Na2SO4 with application of ultrasounds for different durations. Another test on size distributions' evolution was performed using ultrasounds within the granulometry apparatus, in order to see if large aggregates can easily brake. Ultrasounds were applied for 1 or 2 minutes. Results shown in Figure 4 (b) indicate that there is no influence on the aggregate sizes, as the distribution remains identical. The fact that only little numbers of aggregates actually brake due to ultrasounds, suggests a strong interaction and cohesion within sodium sulfate aggregates. In summary, the experimental semi-batch set-up presented enables to recover, in a controlled way, Na2SO4 salt powder after precipitation in supercritical water. It appears that distributions are composed of a great number of small aggregates around 0.5 µm, and some larger aggregates between 1 and 10 µm. Initial concentration does not seem to influence final aggregate size or distribution and the sizes obtained after manual grinding seem stable, as ultrasound show little influence on the size distribution. Now that all the required



experimental data have been obtained, we can focus on the development of a numerical modeling of the precipitation and aggregation of Na2SO4 nanocrystals in supercritical water.

3. Numerical modeling The aim of this section is to perform the numerical modeling of the precipitation of Na2SO4 in supercritical water, taking into account the aggregation phenomenon. The objective is to propose an aggregation mechanism in order to fit the experimental results, which would then improve our understanding of the salt precipitation/aggregation phenomena in supercritical water. To fulfill this goal, the present model is based on the nucleation & growth model of the precipitation of Na2SO4 developed in our previous work

18

, with the addition of the

aggregation and breakage terms. 3.1.Aggregation model The aggregation phenomenon of particles is a generic term for a great number of different mechanisms. Since the initial aggregation models developed by Von Smoluchowki 23, several works contributed to the improvement of the equations governing the various aggregation mechanisms

24,25

. However, few research works concern the modeling of precipitation and

aggregation of salt 26, and none in supercritical water conditions. The numerical modeling proposed here, consists in solving the Population Balance Equation (PBE) of particles, with aggregation and breakage terms using the Quadrature Method of Moment (QMOM). Temperature and pressure conditions are taken as close as the experimental one, using a smoothed temperature gradient (c.f. Fig. 5) from 320°C to 400°C and a pressure of 25 MPa. We consider that the concentration and the temperature are homogeneous in the reactor.


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Figure 5: Temperature profile used for the numerical modeling compared to the experimental one. The global PBE expresses the population density function n(L,t) according to the following equation:

, ,

, , , ,

(1)

where G stands for the growth rate, L the particle size and t the time. The right terms represent the birth and death rates for aggregation (Ba(L,t) and Da(L,t)) and breakage (Bb(L,t) and Db(L,t)). To solve this PBE, we use the QMOM method, adapted for aggregation and breakage problems, introducing the moment approximation:



, ≈

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(2)

The coefficients wi and Li are the weights and abscissa of the sum, respectively. The nodes number N is taken as 3 in our work and determined using the Product Difference algorithm 25. The PBE then becomes a system of 2N momentum equations that need to be solved:

0

1 $ # # &% '# 2

(3)

#

'# # ( ) ( +,- 0, … ,5

#

A 7th equation is solved to ensure the mass conservation of salt during the calculations, taking into account the salt particle density ρp and the shape factor kv:

01

334 56 7

(4)

The B0 term represents the nucleation rate of particles, whereas βik is the aggregation coefficient, ai the breakage coefficient and bi(j) the fragment function of the aggregate breakage (m0 = total number of particles, m1 = total length of particles, m2 = total surface of particles, m3 = total volume of particles). Only homogeneous primary nucleation is considered according to the following expression:


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3 @ 16H @ B07 > 8 98 : ;

Aggregation of Na2SO4 Nanocrystals in Supercritical Water

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