Review on descaling and anti-scaling technology of heat exchanger in high-salt wastewater thermal desalination

High-salt wastewater refers to wastewater with a total dissolved solid mass concentration of more than 3.5%, and salt content of more than 1% (Fang et al. 2022). With the continuous expansion of industrial enterprises, a large amount of complex high-salt wastewater has been produced (Liu et al. 2021a). Direct discharge of high-salt wastewater will cause serious damage to the ecological environment, so ‘zero liquid discharge’ is currently the main target of wastewater treatment. The main treatment methods of high-salt wastewater include thermal concentration treatment (Vysokomornaya et al. 2015; Guo et al. 2021), membrane separation and concentration treatment (Wu & Chen 2020; Yalcinkaya et al. 2020; Reddy et al. 2022), biological treatment (Zhang et al. 2022), and other combined treatment technologies. Among them, thermal concentration treatment is a promising method due to its advantages of low energy consumption, environmental friendliness, and high purity of salt-separated products (Liu et al. 2021b, 2021c; Chen et al. 2022), in which, heat exchanger is the crucial equipment. At present, the commonly used evaporation technologies include mechanical compression evaporation (MVR), multi-effect evaporation (MED), and multi-effect flash evaporation (MSF).

Although the principle of heat exchanger fouling has been extensively studied by fluid mechanics, the comprehensive analysis of descaling and anti-fouling technologies in the chemical and environmental industries is still lacking. The existing research is insufficient to guide practical engineering, especially in treating high-salt industrial wastewater where the heat exchanger scaling remains a serious bottleneck affecting the heat exchange effect. The review focuses on the studies in the field of industrial wastewater directly related to descaling/anti-scaling (scale inhibition) of metal surfaces (especially heat exchangers), which is different from traditional metal corrosion inhibition approaches, and suggests that in future industrial development, a combination of chemical and physical scale inhibition technology can significantly increase the scale inhibition efficiency to more than 95% through synergistic effect. This work identifies critical knowledge gaps and highlights future research perspectives.

Chemical descaling agents will cause damage to the metal tube of the heat exchanger during the descaling process, which is why it is necessary to accurately calculate and strictly control the amount of chemical agents. The actual project feedback shows that the heat transfer effect of the heat exchanger decreases obviously over time with the application of chemical descaling technology in long-term operation (Qian et al. 2014).

The pickling descaling process produces three types of waste: used acid, filter-pressed sludge, and rinsed water. Nowadays, the use of hydrochloric acid is discouraged due to increasingly strict environmental protection laws. As a result, much emphasis has been placed on the use of alternate descaling methods, such as mechanical descaling (Chattopadhyay et al. 2009).

Mechanical descaling technology can thin the metal heat exchange surface and, in serious cases, cause mechanical fracture of the heat exchange tubes. In general, previously it was mechanically descaling such as water retro jetting or scrubbing with a soft bristle brush by dismantling the plate heat exchanger (Singh et al. 2020). Mechanical descaling devices are only suitable for straight and unobstructed pipes, not for irregular pipes. In addition, the mechanical descaling process relies on manual labor, so it is difficult to realize a full automation application. What is worse, after cleaning by this method, the parts need to be reassembled, and the assembly process will cause pipe leakage. At present, mechanical descaling technology aims at a scale that is difficult to remove by conventional methods.

Both chemical and mechanical descaling technologies have a certain influence on the performance of metal heat exchanger tubes. The anti-scaling technology can protect the performance of metal heat exchange surfaces and prolong the service life of equipment. The existing anti-scaling technologies for inhibiting inorganic salt scaling have been quite mature, and the relevant research shows a linear growth trend (as shown in Figure 5). These technologies mainly use chemical and physical methods to prevent scaling.

The mechanism of chemical scale inhibitors is mostly studied by experimental methods, mainly based on lattice distortion theory (Zhang et al. 2017), complex capacitive effect (Liu et al. 2016), and crystal dispersion effect (Chen et al. 2020a). Chemical scale inhibitors transform hard scale into soft dirt that can be easily washed away by changing the surface morphology of scale crystals, thus inhibiting the formation of inorganic salt crystals (Chen et al. 2018). Experimental studies (Popov et al. 2016; Chen et al. 2020a, 2020b) have shown that chemical scale inhibitors adsorb scale particles through lattice distortion, chelation solubilization, and dispersion. Table 1 describes the scale inhibition process of calcium carbonate scaling under the action of three mechanisms. Under the action of scale inhibitors, loose and soft particles become the main components of precipitation.

Scale inhibitors can be classified into natural polymer scale inhibitors, inorganic phosphate scale inhibitors, organic phosphoric acid scale inhibitors, polycarboxylic acid polymer scale inhibitors, and ‘Green’ scale inhibitors. Many phosphorous compounds are toxic and expensive. Additionally, some phosphonate compounds are less thermally stable than polymeric scale inhibitors under environments of high temperature and pressure (Husna et al. 2022). A good scale inhibitor should be able to be used over a wide range of temperatures and pressures, so phosphate scale inhibitors are gradually being replaced.

‘Green’ scale inhibitors have received increasing attention due to their environmental friendliness (Wang et al. 2017). Li et al. (2019a) studied four scale inhibitors: polyacrylic acid (PAA), hydrolyzed polymaleic anhydride (HPMA), polyepoxysuccinic acid (PESA), and polyaspartic acid (PASP). Under the action of scale inhibitor, the higher the Ca²⁺ concentration in solution, the better the scale inhibition effect.

PASP was developed in the 1990s, inspired by the metabolic processes in marine animals (Frankel & Berger 1949). PASP has chelating ability and dispersed carboxyl groups, enabling it to chelate with cations (Ca²⁺, Mg²⁺, and Al³⁺) (Xu et al. 2011). Oxygen ions in the PASP molecule and Ca²⁺ on the calcite surface were completely matched, and the adsorption effect of the scale inhibitor on the calcite surface was obviously improved, thus achieving the purpose of inhibiting CaCO₃ scale formation (Cheng et al. 2020). The modified PASP has certain corrosion inhibition properties and can effectively prevent the corrosion of metal ions on the surface of carbon steel (Chai et al. 2019), because the adsorption of PASP on the metal surface conforms to the Langmuir adsorption isotherm (He et al. 2020).

Betz Laboratory (Brown et al. 1992) found that the commonly used detergent PESA had a strong adsorption effect on Ca²⁺, Mg²⁺, and Ba²⁺, and developed it as a scale inhibitor. PESA has strong alkali resistance and is an excellent scale inhibitor in water with high hardness. Li et al. (2012) suggested that the synthesis of PESA was greatly affected by pH value and temperature. The optimized polymerization temperature was 90 °C and the polymerization time was 2 h, respectively. After 10 h, the scale inhibition performance of the 20 ppm sample reached more than 95%. Zeng et al. (2012) simulated the coulomb interaction between PESA and CaSO₄(001) at different temperatures using the MD method. The chemical bonds were easily formed between Ca²⁺ and oxygen atoms of the PESA carboxyl group. The radial distribution functions of O(carbonyl of PESA)–H(H₂O), O(CaSO₄) –H(H₂O), and O(CaSO₄) –H(PESA) indicated that the solvent had an effect on the anti-scaling performance of CaSO₄.

Environmentally friendly scale inhibitors have excellent biodegradability and can be biodegradable into non-toxic chemicals. At present, it has been found that PASP/PESA and its derivatives can effectively inhibit the formation of Ca-scale, but whether they can adapt to strong acidic and alkaline conditions remains to be determined and evaluated through practical experiments. Table 2 summarizes the environmentally friendly scale inhibitors studied in the last three years. The application dose affects the scale inhibition efficiency.

Polymeric scale inhibitors are widely used in the treatment of wastewater because of their enhanced thermal stability and better environmental compatibility. According to recent research, the scale solubility increases as the temperature increases, and the scale inhibition efficiency also improves to some extent. However, at temperatures above 90 °C, the effectiveness of scale inhibitors decreases dramatically (Jiang et al. 2022). Therefore, the chemical anti-scaling technology can be adapted to high-salt wastewater thermal desalination. Considering that the object of scale inhibitor treatment is relatively simple, small-scale experiments must be conducted before engineering application.

Non-chemical water treatment technologies are attractive options so the use of scale inhibitors or other chemical-involved processes can be avoided or minimized (Lin et al. 2020). The physical anti-scaling performance is better than mechanical descaling technology (Huang et al. 2023). Physical cleaning can reduce the thermal scaling resistance of heat exchangers (Pogiatzis et al. 2015; Koo et al. 2016). However, it has strong corrosion to the equipment and the operation process is complicated. Förster et al. (1999) experimentally proved that pulsation was a powerful tool to mitigate the formation of scale layers. Theoretically, physical scale inhibition is achieved by applying physical fields (which can be regarded as pulsation) to accelerate the collision and precipitation of scale ions in solution (Liu et al. 2018). Physical anti-scaling technology has great potential due to its environmental protection, energy saving, easy maintenance, and low cost. The most commonly used technologies are ultrasonic scale inhibition, electrostatic water treatment technology, and magnetic water treatment technology.

Qian et al. (2018) established a dynamic experimental device for scaling prevention and heat transfer enhancement of acoustic cavitation sewage heat exchangers. The results showed that with the increase in flow rate and acoustic cavitation time, the scale inhibition rate gradually increased. The highest descaling rate was 85.7% and the scaling rate was 12.6 kg/(m²h). Park & Lee (2018) applied 23 kHz sinusoidal ultrasound intermittently to drillships, which caused cavitation in adjacent water and ultimately prevented the sedimentation of marine scaling organisms at 214 dB of sound. Generally speaking, ultrasonic treatment has a better anti-scaling effect in treating low-hardness water.

Most experimental results demonstrated that ultrasonic irradiation inhibited scale formation by affecting the size and morphology of crystals. However, Basheer et al. (2021) held a different view, arguing that ultrasonic radiation did not affect the nature, morphology, and size of the formed calcium carbonate phases.

The authors suggest that the different conclusions might be due to the different sequences of samples processed by ultrasound. In both experiments of Dalas (2001) and Basheer et al. (2021), the salt solutions were treated with ultrasound separately and then mixed. If the salt solution is mixed before the treatment of ultrasound, it is likely to be found that ultrasound can change the morphology of the crystals. The way to further improve the efficiency of ultrasonic descaling is to perform an in-depth study on the cavitation mechanism and to investigate the effects of multiple synergized parameters on descaling efficiency.

Flow and temperature affect scale inhibition technologies. Analyzing these factors can aid in selecting technology for practical engineering. Huang et al. (2023) investigated the effect of ultrasound on preventing scaling at different temperatures. Their simulation results showed that increasing the temperature led to an increase in sound pressure on the solid–liquid surface of the pipe. Combined with Equation (7), it can be seen that when other liquid parameters remain unchanged, the cavitation threshold decreases with the increase of liquid temperature in the pipe. Thus, the study concluded that the descaling effectiveness improved with a rising temperature. Qu et al. (2019) investigated the effect of different water flow rates on ultrasonic descaling technology. The finite element method was used to analyze the pipe model containing flowing water, and the sound pressure distribution in the pipeline was determined. As shown in Table 3, when the water flow velocity in the pipeline was 0 m/s, the scale removal performance was better; on the contrary, when there is water flow, the scale removal performance is weakened.

The experimental results showed that the higher the temperature, the more obvious the ultrasonic scale inhibition effect, and the smaller the flow rate, the weaker the ultrasonic scale. Therefore, when using ultrasonic scale inhibition in practical projects, priority should be given to higher temperatures and lower flow rates.

Under the action of the magnetic field, the surface tension of water decreases and the pH value increases. In addition, the shear viscosity of magnetized water increases and the magnetic field inhibits the scale formation (Esmaeilnezhad et al. 2017). Magnetic field water treatment technology promotes the unified formation of crystal nuclei through the external magnetic field so that a large number of CaCO₃ microcrystals can be formed rapidly in water, producing aragonite crystals that will not adhere to the surface of the tube. Yamamoto et al. (2019) developed a scale removal system utilizing a superconducting magnet that can remove the iron oxide scale from the boiler feed water in thermal power plants. In this research, researchers succeeded in homogenizing the distribution of captured particles in the filter stacks by controlling the applied magnetic field strength. Yu et al. (2011) compared various types of descaling instruments and found that when a magnetic field is applied to the descaling instrument it has better scale inhibition and descaling effect, the inhibition rate reach 69.3%.

Table 5 summarizes the advantages and disadvantages of physical anti-scaling technology. At present, relatively mature physical scale inhibition technology is mainly applied to non-phase change thermal equipment, and the application condition is generally below 100 °C.

Compared with chemical anti-scaling technology, physical anti-scaling technology has certain requirements for equipment and high one-off cost investment. Besides, there are also requirements for the hardness of high-salt wastewater and the temperature of the process. However, the technology does not have high requirements on the operation of workers, which is conducive to industrial applications. For example in the chemical, mining, coal, and petroleum industries, ultrasonic scale inhibition technology and magnetic water treatment technology have been put into use.

In recent years, composite technologies of water treatment have attracted the attention of many researchers for much better effect of treatment. The synergistic effect of ultrasonic field, EF, and magnetic field can improve the scale inhibition performance and expand the application range.

According to the results of the above study, it can be seen that the synergistic effect of ultrasound with electric and magnetic fields, respectively, plays a more minor role. Especially, when ultrasound is acting together with an EF, the auxiliary function of ultrasound is less than 10% effective in enhancing the scale inhibition rate, as shown in Figure 20. Although the ultrasonic action cannot have a large impact on the scale inhibition efficiency from a quantitative point of view, it can be found that the ultrasonic action greatly changes the morphology of calcium carbonate crystals and can transform their crystal structure toward aragonite based on the experimental results characterization, as shown in Figure 21 and Figure 23. This is because of the formation of a large number of crystal nuclei in solution from ultrasonic treatment, there are no sufficient ions existing in solution for the continued growth of nuclei, and the supersaturation of the solution may be decreased, thus further influencing the precipitation of crystal on heat transfer surface (Geng et al. 2021).

Under the action of anti-scaling technology, the crystallizing process occurs in three stages: formation of an unstable phase, transformation to a metastable phase, and then development of a stable phase (Zhang et al. 2007). The function of scale inhibitors is realized mainly by controlling the crystallizing process at the second stage, while the function of physical fields is realized mostly by the unstable phase formed in the first stage.

Table 6 displays the characteristics of chemical and physical scale inhibition techniques. It is evident that the current techniques mainly target the calcium carbonate scale. Chemical scale inhibitors require a significant input within a short timeframe, leading to increased costs. In contrast, physical scale inhibition technology requires minimal subsequent maintenance, but its effectiveness requires to be improved. Empirical evidence supports the efficacy of chemical anti-scaling technology during the early stages of scale formation, while physical anti-scaling technology can take over a month to become effective.

The synergistic action between physical fields and scale inhibitors deforms CaCO₃, making it more slippery and dispersive (Li et al. 2013). Moreover, the addition of a magnetic field can enhance the chelation of scale inhibitors on Ca²⁺. When the scale inhibitor interacts with the physical field, the scale inhibition performance is mainly affected by the scale inhibitor. However, the introduction of magnetic fields and electric fields can reduce the number of scale inhibitors to a certain extent and reduce the pollution of water bodies by scale inhibitors.

According to the existing research, the synergistic effect of scale inhibitors and physical fields can solve the vast majority of heat exchanger scale formation problems. Combining the two technologies can effectively address selectivity and efficiency issues in engineering, with practical significance. However, more solid dissolves in the high-salt wastewater and the scaling composition is complex, the effectiveness of the synergistic inhibition method in practical engineering needs to be further verified and investigated.

In the environmental field, industrial wastewater treatment has been plagued by heat exchanger fouling problems for a long time. This review mainly introduces the traditional and novel descaling and anti-scaling technologies for heat exchangers in the treatment of high-salt wastewater. The damage to heat exchangers caused by descaling technologies commonly used in industrial wastewater treatment is identified, while new scale inhibition technologies to solve the scaling problems are proposed. In this review, the chemical and physical anti-scaling technologies are introduced and their limitations are pointed out. Finally, on the basis of this study, the advantages, disadvantages and adaptation scenarios of various techniques are integrated and the feasible techniques that are most suitable for the environmental field are put forward. The effective combination of scale inhibitor and physical field is the future development trend of scale inhibition technology.

Different industrial processes and working environments cause different scale inhibition problems. There is a lack of customized solutions and more refined research is needed to meet diverse needs. In addition, current anti-scaling technologies rely on chemicals, which have environmental impacts and safety risks. Non-chemical descaling and anti-scaling technologies need to be studied to be more environmentally friendly. What is worse, the physical fields may cause secondary water pollution to a certain extent, and the subsequent steps after evaporation and desalination treatment deserve more attention. The research of descaling and anti-scaling technology involves not only fluid flow problems but also crystallization problems. Research and development of anti-scaling methods may include the design of metal surfaces or substrates that are effective in preventing scale growth and enhancing scale inhibition.

This work is supported by the National Natural Science Foundation of China (No. 52370149), Fundamental Research Funds for the Central Universities (2022-4-YB-06), Shandong Excellent Young Scientists Fund Program (Overseas) (2023HWYQ-080), and Shandong Provincial Natural Science Foundation (ZR2021QE290), China.

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

The authors declare there is no conflict.