Continuous salt precipitation and separation from supercritical water. Part 3: Interesting effects in processing type 2 salt mixtures

Characteristics, mechanisms and measurement methods of dissolution and deposition of inorganic salts in sub-/supercritical water

The efficient and harmless treatment of hypersaline organic wastes has become an urgent environmental problem. Compared to traditional thermochemical methods, supercritical water oxidation has been proven to be an efficient organic waste treatment technology due to the advantages of low cost, high degradation rate, no secondary pollutants, etc. However, the solubilities of inorganic salts drop rapidly near the critical point of water, and some sticky salts form easily agglomerates and then adhere to internal surfaces of reactor and pipeline, causing plugging and inhibition of heat transfer. Hence, the characteristics, mechanisms and measurement methods of the dissolution and deposition of inorganic salts in sub-/supercritical water are summarized and analyzed systematically and comprehensively in this work, intending to provide a valuable guide for salt deposition prevention and subsequent research directions. Firstly, a new classification form of inorganic salt is put forward based on melting point. The phase equilibriums of brine systems are then analyzed in detail. Six theories concerning dissolution mechanisms are discussed deeply and various measurement methods of salt solubility are also supplemented. Furthermore, salt deposition characteristics and related measurement technologies are summarized. Notably, a new idea "hydrothermal molten salt" system is reviewed which may provide a solution for salt deposition in sub/supercritical water. Finally, an outlook for the follow-up researches is prospected and some suggestions are proposed.

Review on metal dissolution characteristics and harmful metals recovery from electronic wastes by supercritical water

Supercritical water (SCW) technology can be applied as an efficient and environment-friendly method to recover toxic or complex chemical wastes. Separation and chemical reactions under supercritical conditions may be realized by changing the temperature, pressure, and other operating parameters to adjust the physical and chemical properties of water. However, salt deposition and corrosion are often encountered during the treatment of inorganic substances, which will hinder the commercial applications of SCW technology. The solubility of salt in high pressure/temperature water forms the theoretical basis for studying the recovery of metal salts in supercritical water and understanding salt deposition. Therefore, this work systematically and objectively reviews different research methods used to analyze salt solubility in high pressure/temperature water, including the experimental method, prediction theoretical modeling, and computer simulation method; the research status and existing data of this parameter are also analyzed. The purpose of this review is to provide ideas and references for follow-up research by providing a comprehensive overview of salt solubility research methods and the current situation. Suggestions for more efficient metal recovery through technology integration are also provided.

Sub- and Supercritical Water Liquefaction of Kraft Lignin and Black Liquor Derived Lignin

To mitigate global warming, humankind has been forced to develop new efficient energy solutions based on renewable energy sources. Hydrothermal liquefaction (HTL) is a promising technology that can efficiently produce bio-oil from several biomass sources. The HTL process uses sub- or supercritical water for producing bio-oil, water-soluble organics, gaseous products and char. Black liquor mainly contains cooking chemicals (mainly alkali salts) lignin and the hemicellulose parts of the wood chips used for cellulose digestion. This review explores the effects of different process parameters, solvents and catalysts for the HTL of black liquor or black liquor-derived lignin. Using short residence times under near- or supercritical water conditions may improve both the quality and the quantity of the bio-oil yield. The quality and yield of bio-oil can be further improved by using solvents (e.g., phenol) and catalysts (e.g., alkali salts, zirconia). However, the solubility of alkali salts present in black liquor can lead to clogging problem in the HTL reactor and process tubes when approaching supercritical water conditions.

Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations

Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular-scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na-SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K-SO4 show significantly less clustering under equivalent conditions. In mixed systems (Nax K2-x SO4 ), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure-breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K(+) and SO4 (2-) , compared with the analogous Na(+) -SO4 (2-) interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na-K-SO4 . Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium-sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting-in in ternary hydrothermal solutions of Na-K-SO4 reflects the opposing, but complementary, natures of Na-SO4 versus K-SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.

Application of red mud as both neutralizer and catalyst in supercritical water oxidation (SCWO) disposal of sewage sludge

Red mud was used in the supercritical water oxidation (SCWO) disposal of sewage sludge, not only as a neutralizer for acidic substances produced in situ, but also as a catalyst for decomposition of pollutants.