Scaling trend of coal chemical saline wastewater in falling film evaporator: experimental solid–liquid equilibrium and numerical simulation

The resource utilization of high-salt wastewater could help alleviate water shortages and solve water environment problems (Badiuzzaman et al. 2017; Xu et al. 2018). In recent years, high-salt wastewater from the coal chemical industry has received widespread attention (Fang et al. 2018; Hou et al. 2019). The demand for water and the high volume of high-salt wastewater produced by the coal chemical industry seriously restricted the development of the industry (Lefebvre & Moletta 2006; Cui et al. 2017). Coal chemical wastewater was divided into high saline wastewater and organic wastewater, and its high saline wastewater salt components were mainly Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, ⁠, and ⁠, which form salts with high solubility, that are difficult to treat, polluting, and have high economic gain from reuse (Zhao & Liu 2016). Usually, most of the coal chemical wastewater was treated by a zero discharge process, and after membrane concentration, inorganic ions (Na⁺, Ca²⁺, Cl⁻, etc.) were separated from the brine by evaporation technology (Son et al. 2021). Among the more mature evaporation technologies, multiple effect distillation (MED) and mechanical vapor recompression (MVR) were processing technologies with falling film evaporator as the core, and their excellent evaporation characteristics gave them an indispensable position in the field of desalination (Kouhikamali et al. 2014). However, high-salt wastewater may have caused equipment scaling in the evaporation process, which not only affected the heat transfer performance of the equipment but also caused a waste of resources (Zhao et al. 2019; Lu et al. 2020). Thus, it was necessary to study the problem of equipment salt scaling in coal chemical high-salt wastewater salt extraction technology to achieve more efficient recycling of inorganic salts.

In the coal chemical treatment process, the common types of scaling in evaporation tubes were particle scaling and crystallization scaling (Epstein 1979). Among them, particle scaling has been extensively studied by numerical simulation for scale formation mechanism (Najibi et al. 1997; Pääkkönen et al. 2012; Song et al. 2019), while precipitation crystalline scaling of some typical aqueous salt systems (formed by inorganic ions such as Na⁺, Ca²⁺, Cl⁻) in coal chemical wastewater and prediction of scaling sites has not been studied enough. The formation conditions of crystallization scaling in the evaporation process depended on the supersaturation of the solution in the flow state (Belmiloudi 2011). Therefore, it was important to master the phase equilibrium relationship of the solution and the characteristics of the solution flow in the tube for the judgment of the critical conditions of the crystallization point. Many researchers had reported on the saturation of the solution and the flow characteristics under actual working conditions. Assarsson (1953) analyzed the sodium salt system at a certain temperature using phase diagrams and obtained data on the solubility of the sodium salt in different systems. With the in-depth study of the phase equilibria, Yang et al. (2010, 2011) specifically studied the solid–liquid phase equilibria of the ternary system NaCl-CaCl₂-H₂O at 323.15 K and 348.15 K. It was found that there were large differences in the content of the components in the system at different temperatures, and the co-saturation point was shifted. Rathgeber et al. (2019) enriched the phase equilibrium data of brines by studying the solubility phase diagram of the NaCl-CaCl₂-H₂O system in the undersaturated range. Lu et al. (2017) studied the solubility of sodium salt systems in multiple temperature ranges and found that the solubility of NaSO₄ in coal chemical wastewater was greater in low-temperature environments than in pure water. The study of phase equilibrium in the multi-temperature range of the NaCl-CaCl₂-H₂O ternary system, which was typical for the coal chemical industry, had not been reported in the literature, which limited the universality of phase equilibrium data research results.

In addition, the influencing factors for fouling in different operating conditions of the evaporation process should not be neglected, which were mainly reflected in the working fluid characteristics and operating parameters (Zhao & Chen 2013). Gourdon et al. (2010) studied the evaporation process of aqueous NaCO₃ and Na₂SO₄ solutions in a vertical falling film evaporation tube and analyzed the fouling behavior of different working fluids, and the results showed that the crystal mass distribution depended on the composition of the solution. Izadi et al. (2011) studied fouling experiments on heat exchanger tubes with different seawater samples and confirmed that the difference in fouling behavior was influenced by the different compositions of the working fluid. Lv et al. (2020) studied the fouling behavior of Na₂SO₄ and mixed salts in heat exchanger tubes and analyzed the effect on fouling thermal resistance at different operating parameters. The results showed that the fouling thermal resistance was determined by the variation of Na₂SO₄ content and solubility. Therefore, for saline wastewater with a complex multi-salt system, the increasing salt concentration in the evaporation process might cause equipment blockage and lead to dangerous accidents (K. Li et al. 2018; D. Li et al. 2018). And the phase equilibrium speculation of fouling sites in the evaporation process was a very important way to guide the solution to the scaling problem, and it was necessary to study it to tackle the problem of salt fouling prevention.

Combining the characteristics of multi-salt phase equilibrium and evaporation system, the phase equilibrium of the ternary system NaCl-CaCl₂-H₂O at 303.15–373.15 K was studied in this paper, taking the saline wastewater of the coal chemical industry as an example. Meanwhile, the flow characteristics of saline wastewater in a falling film evaporation tube were simulated using a multiphase flow volume of fluid (VOF) model for the prediction of tube scaling trends and verified by an experimental setup. The experimental results and simulation data provided a basis for the prevention of salt scaling problems and the purification of NaCl and CaCl₂ from high-salt wastewater in the coal chemical industry.

The reagents used in the experiment, NaCl and CaCl₂, were all from Tianjin Kemiou Chemical Reagent Co., Ltd, with purity and purity: mass fraction ≥99.0%, and both were analytical pure reagents. The water used for sample preparation and experimental process analysis was deionized water, with a pH of about 6.6 and conductivity <1 × 10⁻⁵ s · m⁻¹. The CO₂ was removed by boiling before use.

The instruments used in the experiment were an electronic balance (AL104, Mettler-Toledo Company, accuracy 1.0 × 10⁻⁴ g); ultra-pure water machine (UPT-II-20T, Sichuan Youpu Ultra-pure Technology Co., Ltd); precision constant temperature air blowing drying oven (DHG-9053J, Shanghai Sanfa Scientific Instrument Co., Ltd); digital density meter (JL3002LD, Guangzhou Techland Materials Technology Co., Ltd, accuracy 2.0 × 10⁻⁴ g · cm⁻³); constant temperature water bath oscillator (THZ-82, Changzhou Zhibo Instrument Manufacturing Co., Ltd, accuracy ±0.1 °C); and X-ray powder diffractometer (XRD, D8 Advance, Bruck, Germany).

The salt components of high-salt wastewater of the coal chemical industry were mainly Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, ⁠, and ⁠, among which the cations were Na⁺, Ca²⁺, and the anions were mainly Cl⁻. Therefore, in this paper, the isothermal solution equilibrium method was used to study the solubility data of the ternary system NaCl-CaCl₂-H₂O (Bian et al. 2018; K. Li et al. 2018; D. Li et al. 2018). According to the common saturation point of the ternary system, experimental points were selected to calculate its composition, and the samples were accurately weighed and put into glass bottles, deionized water was added, sealed, and placed in a constant temperature water bath oscillator in the temperature range of 303.15–373.15 K (in the vibration state). Whereafter, the supernatant was removed periodically, weighed and recorded, and then chemically analyzed until the equilibrium standard was reached when the content of each chemical component remained constant. The equilibrated liquid phase and wet slag were chemically analyzed and the density of the liquid phase was determined using a digital densitometer, while the equilibrated solid phase was identified by an XRD.

The Cl⁻ ion concentration was measured by titration with a standard solution of silver nitrate under near-neutral conditions with an indicator of potassium chromate (Sheykhi & Moore 2012) with a relative error ≤± 0.3%. In the presence of ammonia buffer solution and chromium black T indicator, Ca²⁺ concentration was measured by EDTA standard solution (Deng & Li 2008) with a relative error ≤± 0.4%. According to the electric neutrality of electrolyte solution, the concentration of Na⁺ ions was calculated by the subtraction method (Yang et al. 2015). Balance determination was carried out for each group of samples three times, and the results were averaged to ensure the accuracy of the determination results.

• (1)

  Mathematical models of flow and heat transfer

There were three types of multiphase flow models in numerical simulation, including the VOF model (Liu et al. 2020), mixture model (González-Teruel et al. 2020), and Eulerian model (Dolejší & Svärd 2020). Among them, the VOF model was a surface tracing method under a fixed Eulerian grid, where different fluid components were traced by solving the same momentum set equation for the volume fraction of each fluid phase (Yang et al. 2008). The VOF model was suitable for multiple flow processes (e.g., stratified flow) that required one or more interfluent fluid interfaces, and the evaporation process of falling film in a vertical tube belonged to the category of the stratified free-surface flow of gas–liquid two-phase, so the VOF model was used to simulate the formation of falling film in a vertical tube.

Here, Q_L was the heat flow rate.

• (2)

  Physical model construction and determination of boundary conditions

The falling film evaporation tube was modeled by 3D modeling software, and its structural parameters are shown in Table 1.

• (3)

  Problem assumptions and solution settings

In this study, the model was simplified with the following main assumptions: the pressure applied to the cross-section inside the evaporation tube was uniform; the effect of vapor on the falling film inside the tube can be neglected; the thermodynamic equilibrium was maintained inside the evaporation tube; and the mixture of NaCl and CaCl₂ was an incompressible Newtonian fluid. In addition, the solution settings for the numerical simulation are shown in Table 2.

• (4)

  Independence detection

Solubility data had been reported for binary systems (NaCl-H₂O, CaCl₂-H₂O) at different temperatures. As shown in Table 3, the solubility data of the experimental data in this paper and the binary system in the literature are listed, and the experimental methods and data in this paper were found to be reliable by comparison.

The equilibrium liquid phase composition, wet slag composition, physical and chemical properties density, and equilibrium solid phase composition for the ternary system NaCl-CaCl₂-H₂O at temperatures 303.15–373.15 K are listed in Table 4.

Figure 3(a) and (b) show the equilibrium phase diagram and equilibrium data of the NaCl-CaCl₂-H₂O ternary system at temperatures of 303.15–373.15 K, respectively. Combining Figure 3(a) and (b), it can be seen that w(NaCl) gradually decreased as w(CaCl₂) increased in the equilibrium system. This was because the dissolution of CaCl₂ in saturated NaCl solution led to an increase in the concentration of Cl⁻, which was influenced by the homoionic effect and shifted the dissolution equilibrium of NaCl, resulting in a gradual decrease in w(NaCl).

The solubilities of NaCl and CaCl₂ increased slightly with the increase in temperature and the solubility curves shifted. And at w(NaCl) = 15–28%, w(NaCl) and w(CaCl₂) showed regular changes with temperature (see Figure 3(b)). This was mainly because of two positive soluble salts containing the same anion, when the content of one salt was relatively low, the effect of temperature on dissolution was greater than the effect of the same ion.

Based on the specific parameters of physical properties of brine (Sharqawy et al. 2011), the physical parameters (density, specific heat, thermal conductivity and viscosity) of four saline wastewaters (10% NaCl + 9, 10, 11 and 12% CaCl₂) at different temperatures were fitted in this paper, as shown in Table 5, where x was the temperature and y was the value of physical parameters at different temperatures. The physical properties of the four saline wastewaters were imported into Fluent software and the solver parameters (see Table 2) were set for the numerical simulation of the evaporation process.

Figure 7(a) depicts the distribution of the liquid film temperature along the tube length for the four saline wastewaters at a tube wall heating temperature of 380.15 K.

It can be seen that the axial temperature of the saline wastewater in the tube first increased rapidly and then tends to stabilize, which was because the temperature difference between the liquid film and the heating wall of the tube at the entrance was large, resulting in a large heat transfer rate and a rapid increase in the liquid film temperature. When the temperature difference between the liquid film and the heating wall of the tube decreased, the heat transfer rate became smaller and the growth rate became slower. In contrast, the highest temperature at the outlet of different components was 377 K and the lowest temperature was 370 K. The main reason was that the lower the salt content of the wastewater, the higher its thermal conductivity, and the faster the temperature in the evaporation tube will rise.

Figure 7(b) shows the temperature diagram of the liquid film at 2 mm from the wall for the saline wastewater (10% NaCl + 9% CaCl₂). It can be seen that the temperature of this component at the inlet containing saline wastewater near the wall increased rapidly, which indicated that the liquid film near the wall was the main region in the heat transfer process. In addition, in the area containing the presence of vapor in the evaporation tube (0.5–2 mm radial range), the temperature changes didn't differ much.

According to the results in Figure 10, the salt-containing wastewater (10% NaCl + 9, 10, 11, 12% CaCl₂) evaporated continuously during the evaporation process leading to the increase in salt concentration, and, finally, the salt-containing wastewater precipitated NaCl crystals at 718 mm, 942 mm, 966 mm and 1,000 mm in the tube, respectively. Among them, at the crystallization points A, B, C, and D, w(NaCl) was 16%, and the corresponding w(CaCl₂) was greater than the phase equilibrium data of the system when the salt-containing wastewater reached the crystallization conditions and the crystallization ability of NaCl was better than CaCl₂, and thus NaCl crystallized and precipitated. For the wastewater with lower salt content, its thermal conductivity was higher, the liquid volume fraction decreased faster, and crystallization occurred earlier in the evaporation tube compared to other components.

The experimental process was to transport the salt wastewater of a certain concentration in the storage tank through the pipe to the storage tank at the top of the tube by the pump, and the salt wastewater was evenly distributed on the inner wall of the tube through the liquid distributor downward under the action of gravity. Among them, the descending film evaporation tube was a custom-made device (made of flexible steel pieces), which could be disassembled at a specific location to measure the scaling rate, the outer wall of the tube was wound with electric heating wire for heating (the heating section was 1,000 mm), the heat insulation layer was used outside the electric heating wire for heat insulation treatment, and the temperature was measured by a thermocouple. To ensure the accuracy of the experimental data and empty the air inside the tube, the descending film evaporation device was operated continuously and then verified experimentally.

In this paper, the solid–liquid phase equilibrium of the NaCl-CaCl₂-H₂O ternary system at different temperatures (303.15–373.15 K) by the isothermal dissolution method and the falling film flow of four saline wastewaters in the falling film evaporation tube were simulated by the VOF model. The conclusions are as follows.

• (a)

  Based on the phase equilibrium data, the phase diagram and schematic diagram of the system at multiple temperatures (T = (303.15–373.15 K)) were determined, and the results showed that the mesostable phase diagram of the system was simple and showed a regular variation of w(NaCl) and w(CaCl₂) with temperature when w(NaCl) was 15–28%. The phase diagram of the multi-temperature brine system in this paper provided a detailed equilibrium relationship of solubility, which was important for solving the challenges of salt recovery and the prevention of salt scaling in coal chemical wastewater.

• (b)

  Based on the phase equilibrium data, the flow characteristics of four salt-containing wastewaters (10% NaCl + 9, 10, 11, 12% CaCl₂) were simulated, and the risk of precipitation of NaCl crystals for the four salt concentrations during evaporation was obtained at tube lengths of 718 mm, 942 mm, 966 mm, and 1,000 mm, respectively, and verified by a falling film evaporation experimental setup, which proved the reliability of the predicted points. The combination of phase equilibrium and numerical simulation provided a novel method for the accurate determination of scaling sites and trends.

This study was financially supported by the scientific research fund project of the National Natural Science Foundation of China (Grant No.52070123), Natural Science Foundation of Shandong Province (Grant No.ZR2020ME224), Key Scientific and Technological Innovation Project in Shandong Province (Grant No.2019JZZY020808), and Project of Shandong Province Higher Educational Young Innovative Talent Introduction and Cultivation Team [Wastewater treatment and resource innovation team].

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.