Correlation and estimation of solubilities of fatty acids in supercritical carbon dioxide using solution model approach

In this study, solid solubility data of five fatty acids in supercritical carbon dioxide (CO₂) at different temperatures and pressures are correlated using a two-parameter solution model developed from the regular solution model coupled with the Flory⿿Huggins equation. The developed solution model with fewer parameters yields correlated results comparable to those from commonly used semi-empirical equations. In addition, both parameters in the solution model can be further generalized with the chain length of fatty acids and a new predictive solution model is proposed for solubility prediction. The predictive solution model proposed in this study provides better predicted results and yields average deviation in predicted solubilities of 22.1%. To further apply this solution model to other compounds, solid solubility data of three triglycerides in supercritical CO₂ at 313 K are also correlated. After model simplification and generalization, a new predictive solution model for triglycerides is also proposed, which yields average deviation in predicted solubilities of 29.8%. These results demonstrate that the solution model used in this study is applicable for correlation and prediction of solid solubilities of structure-related compounds in supercritical CO₂.     

Equilibrium solubilities of a p-toluenesulfonamide and sulfanilamide mixture in supercritical carbon dioxide with and without ethanol

Supercritical separation processes for a multi-component mixture of solutes are of practical interest. In this study, the experimental equilibrium solubilities of two solute mixtures, p-toluenesulfonamide (p-TSA) and sulfanilamide (SNA), in supercritical carbon dioxide (SC CO₂) were measured at temperatures of 308, 318 and 328 K and pressures in the range of 11.0-21.0 MPa using a dynamic flow method. The effect of cosolvent on the multi-component system was investigated by the addition of a 3.5 mol% ethanol. In the ternary system (p-TSA + SNA + CO₂), the solubility of SNA increased as compared to its binary system (SNA + CO₂), while the solubility of p-TSA decreased. In the quaternary system (p-TSA + SNA + ethanol + CO₂), a significant solubility enhancement was observed for both p-TSA and SNA. The selectivity, which is thought to imply the intermolecular interactions between p-TSA and SNA, was also enhanced by the presence of ethanol so that the two solutes could be separated by a max. purity of 99.4%. The influence of the hydrogen bond interaction on solubility was discussed. The equations of Chrastil, Méndez-Santiago and Teja, and their modified forms were used to correlate the experimental data.     

Development of activity coefficient model based on COSMO method for prediction of solubilities of solid solutes in supercritical carbon dioxide

A COSMO base activity coefficient model was newly developed to predict the solubilities of solid solutes in supercritical carbon dioxide. This activity coefficient model describes that the system is composed of the surface segments on the solvent molecule and vacancy unlike the conventional model based on COSMO method. The density change of supercritical fluid can be represented by the change of the surface area of the vacancy. This prediction model is referred to “COSMO-vac (vacancy)” model. The solubilities of 16 pharmaceuticals in supercritical carbon dioxide were predicted by COSMO-vac model. The averaged deviations between the logarithmic experimental and predicted results are smaller than unity. Furthermore, the predicted results for the solutes composed of only C, H and O atoms are better than those for the solutes including the other atoms. The percentage of the predicted results within the order of the experimental data at the pressure over 15 MPa is higher than that at the pressures below 15 MPa.     

Thermodynamic modeling of solubilities of various solid compounds in supercritical carbon dioxide: Effects of equations of state and mixing rules

No one can ever deny the significance of calculations of solubilities of industrial solid compounds in supercritical CO₂ in separation processes. In this work, the Peng-Robinson (PR) and the Esmaeilzadeh-Roshanfekr (ER) equations of state (EoS) along with several mixing rules including the Wong-Sandler (WS), the covolume dependent (CVD) and the van der Waals one (VDW1) and two (VDW2) fluid mixing rules are applied to evaluate the solubilities of 52 mostly used solid compounds in supercritical carbon dioxide. Besides, the Van-Laar excess Gibbs energy (G^ex) model is applied in phase behavior calculations by the WS mixing rule. The optimal values of the proposed thermodynamic model parameters are evaluated using the DE (differential evolution) optimization strategy. The absolute average deviations of the model results from 1776 experimental data points and the optimal values of the adjustable parameters of the model are reported to investigate the capabilities of combinations of each equation of state with different mixing rules in calculations of the solubilities. The results indicate that the combination of the ER EoS with the WS mixing rule leads to more accurate results (AAD = 9.0%) compared with other ones.     

The effect of excess phosphate on the solubility of hydroxyapatite

The solubility of hydroxyapatite (HA) is critical in fields such as medicine, dentistry and geochemistry. Previously, it had been found that it was apparently slightly increased with 1 mmol/L excess phosphate present. This study was to determine the solubility of HA over the pH range 3.3–5.4 with a series of concentrations of such ‘excess’ phosphate with solid titration (ST), and to identify the precipitate formed at equilibrium with SEM, TEM, EDX, FTIR and XRD. For [PO₄]_XS=0–0.3 mmol/L, results followed closely the already-reported ST ‘low’ solubility isotherm. At [PO₄]_XS=0.5 mmol/L, the solubility surface switched abruptly to a ‘high’ position that could not be reconciled with either the ‘low’ isotherm or conventional calculations. Thus, HA solubility is critically dependent on the presence of excess phosphate. Such excess may account for the discrepancy between ST and excess-solid results, although the crystallographic explanation is as yet lacking.     

Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

Solubility of Na2SO4, Na2CO3 and their mixture in supercritical water

The solubility of many salts in water decreases dramatically with temperature in the vicinity of the critical point of pure water. Examples of these salts are sulfates of sodium, potassium, lithium and sodium carbonate. These salts are usually produced during supercritical water oxidation (SCWO) and contribute to fouling. The solubility of Na₂CO₃ and Na₂SO₄ has been determined in pure form and in the presence of each other, for the temperature range relevant to SCWO. The experimental procedure was to pass the salt solution through a tube at constant temperature. After a brief initiation period during which no salt sticks to the tube, the salt above the solubility limit deposited on the tube surface. The solution leaving the section was thus at the solubility limit. A rapid decrease in the salt solubility was observed just above the pseudo-critical temperature. For supercritical conditions, the solubility of each salt in the form of a mixture was quite close to the solubility of pure salt. At the highest fluid density considered (480 kg/m³) the presence of Na₂CO₃ reduces the solubility of Na₂SO₄, as might be expected from the “common-ion effect”.     

Continuous salt precipitation and separation from supercritical water. Part 1: Type 1 salts

The precipitation and separation performance of various binary type 1 salt-water mixtures was systematically studied for the first time in a continuously operated laboratory plant. The aim was to find a field of operation for the salt separator where salts can be separated with high efficiency. Experiments with aqueous solutions of the salts NaNO₃, KNO₃, Ca(NO₃)₂, K₂CO₃, KHCO₃, (NH₄)₂CO₃, K₃PO₄, K₂HPO₄, KH₂PO₄, NaCl, KCl, NH₄Cl and (NH₄)₂SO₄ were carried out at 30 ± 0.5 MPa varying the salt separator temperature from sub-critical to supercritical. For most of these salts separation efficiencies ranging from 80 to 97% were obtained. For the nitrates the separation efficiency increased in the order NaNO₃ < KNO₃ < Ca(NO₃)₂, whereas for potassium salts the separation efficiency of the phosphates was significantly higher than that of KNO₃. Considerable hydrolysis of the phosphate and the hydrogen phosphate salts in supercritical water was found, although this had no negative influence on the phosphate separation efficiency. It was found that the ammonium salts decompose in supercritical water, probably to ammonia and the corresponding mineral acids, leading to reduced separation of the ammonia due to its high solubility in supercritical water.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Transformation of petroleum asphaltenes in supercritical water

The transformation of petroleum asphaltenes in supercritical water was studied. The experiments were performed in autoclave at temperature 380 °C and pressure 226 atm with stirring for 3 h, medium density was about 0.33 g/cm³. The reaction resulted in the formation of gas products, about 4.3%, and an insoluble residue (coke) with about 48.6% yield. The remaining products were separated into fractions by consecutive dissolution in hexane (30.0%), benzene (10.6%), and chloroform (5.7%). The properties of the obtained products were studied with FT-IR spectrometry and ¹H NMR spectroscopy. The method of simulated distillation was used to demonstrate that the fractional composition of the hexane-soluble part of the products is close to the fractional composition of a mixture of the diesel fraction and vacuum gas oil of the corresponding oil in 1:1 ratio. The obtained data support the conclusion that asphaltene cracking proceeds in SCW, with most probable main processes being dealkylation of substituents in the aromatic fragments of molecules and aromatization. This leads to formation of gaseous products and hexane-soluble fraction consisting of lighter aliphatic and aromatic compounds, as well as carbonized solid residue.     

Non-catalytic Oppenauer oxidation of alcohols in supercritical water

Non-catalytic Oppenauer oxidation was applied for alcohols, such as benzyl alcohol (4) and benzhydrol (1), in the presence of an excess amount of carbonyl compound, formaldehyde (5a), as an oxidant with and without water. Oppenauer oxidation took place in both reactions of 4 and 1 to afford the oxidation products, benzaldehyde (6) (95%) and benzophenone (2) (64%), concomitant with relatively small amounts of reduction products, toluene (7) (1%) and diphenylmethane (3) (13%), respectively, at 400 °C for 10 min without water in an SUS 316 batch-type tubular reactor. Lower yields of oxidation products 6 (68%) and 2 (30%) were obtained in supercritical water under the conditions of 400 °C, 10 min, and 0.35 g/mL water density, while the formation of the reduction products 7 and 3 was completely suppressed. Thus, water was indispensable for the clean and highly selective Oppenauer oxidation of 4 and 1 to yield 6 and 2.     

烃类在超临界水中的化学转化

超临界水是一种新型反应介质,烃类在超临界水中化学转化效率高。对烃类在超临界水反应制氢气、重油改质和合成含氧化合物方面的研究进展进行了综述,同时简要介绍了各种技术产生的背景,对研究重点进行了必要评述,展望了该领域的发展前景。     

Reforming of methanol and glycerol in supercritical water

Reforming of pure glycerol, crude glycerin, and methanol (pure and in the presence of Na₂CO₃) in supercritical water was investigated. Continuous experiments were carried out at temperatures between 450 and 650 °C, residence times between 6 and 173 s, and feed concentrations of 3-20 wt%. For methanol the gas products are mainly H₂, CO₂, and CO. The carbon-to-gas efficiency and the observed activation energy for pure methanol are higher than for methanol with Na₂CO₃. This can be explained by assuming different decomposition mechanisms for pure methanol and methanol with Na₂CO₃. For glycerol, H₂, CO, CO₂, CH₄, and higher hydrocarbons are produced. The carbon-to-gas efficiencies of crude glycerin and pure glycerol are comparable. Overall, 2 of the 3 carbon atoms present in glycerol end up in carbon oxides, while 1 carbon atom becomes C_xH_y. The overall mechanism of glycerol decomposition involves the dehydration of 1 mole of H₂O/mole glycerol. For both, methanol and glycerol at carbon-to-gas efficiencies below 70%, the gas yields (mole/mole feed) and carbon-to-gas efficiency correlate well.     

Continuous salt precipitation and separation from supercritical water. Part 2. Type 2 salts and mixtures of two salts

Using a continuously operated laboratory plant for the catalytic hydrothermal gasification of biomass featuring a supercritical water salt separator we investigated the separation performance of three different binary type 2 salt-water mixtures and three ternary salt-water mixtures that consisted either of two type 1 salts or two type 2 salts dissolved in water. It turned out that a concentrated salt brine could not be recovered at the salt separator for the binary type 2 salt-water mixtures of Na₂CO₃, Na₂SO₄, and K₂SO₄. These salts precipitate as solids from supercritical water and thus lead to salt deposits inside the salt separator vessel.The ternary mixtures of two type 1 salts dissolved in water (KH₂PO₄-K₂HPO₄-H₂O and three different mixtures of NaNO₃-K₂CO₃-water) exhibited a separation performance similar to the binary solutions of type 1 salts that were discussed in Part 1 of this article. However, the mixtures showed separation performances that were different from the corresponding single salt solutions.It was also possible to recover a concentrated brine when feeding solutions containing the two type 2 salts Na₂CO₃ and K₂SO₄. For these mixtures a certain amount of the type 1 salt K₂CO₃ might form in supercritical water leading to salt separation efficiencies up to 95% for these mixtures.     

超临界水降解聚丙烯的工艺研究

采用间歇式管式反应器进行了超临界水降解聚丙烯实验,研究了影响聚丙烯降解的因素。实验结果表明,在温度400－450℃、压力23－35MPa及反应时间60－120min的条件下,超临界水能有效地降解聚丙烯。反应温度和反应时间是影响聚丙烯降解的主要因素,温度越高、时间越长,聚丙烯降解越彻底;聚丙烯颗粒度越小降解速率越快,粉末原料在温度400℃、反应时间60min时,以油相产物为主;在温度450℃、反应时间120min时,有利于得到气相产物。     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

超临界水在降解废弃物及资源化中的应用

综述了超临界水的特性及其在降解废旧塑料、橡胶、纤维素等废弃物及资源化中的应用现状。与传统热分解方法相比,超临界水可实现对高分子废弃物的快速、有效分解,通过分解反应条件的控制,可以控制产物组成,是一种很有前途的废弃物资源化技术。     

Solubility of supercritical gases in organic liquids

Chrastil (1982) [6] demonstrated that the solubility of a substance in a supercritical fluid (SCF) can be correlated with the density of the pure supercritical gas. Therefore, Chrastil's equation permits calculation of the supercritical phase composition of binary SCF + substance mixture based on the knowledge of the supercritical gas density and avoiding the use of equation of state based models.In this work, it is demonstrated that the supercritical fluid density also defines the liquid phase composition of binary systems; a density-dependent relationship is presented to calculate the solubility of supercritical gases in organic liquids. The isothermal solubility of several gases commonly employed in supercritical processing, such as carbon dioxide, methane, and ethane, in different organic liquids, including alkanes, alkenes, alcohols, acids, ketones, esters, terpenes and aromatic compounds, was successfully correlated as a function solely of the pure supercritical fluid density. As an application, pressure vs. composition phase diagrams of binary SCF + substance mixtures were obtained circumventing the use of equation of state models.     

Catalytic supercritical water oxidation of tri-n-butyl phosphate: Process optimization by response surface methodology and cytotoxicity assessment

Catalytic supercritical water oxidation (SCWO) of an organophosphate flame retardant, namely tri-n-butyl phosphate (TNBP) was studied. Firstly, copper oxide nanoparticles (NPs) were synthesized in SCW and their properties were characterized by various analyses. Afterwards, their catalytic performance was investigated under different conditions including reaction temperature (400–500 °C), TNBP volume percentage in the feed (1–4%), oxidant ratio (0–2) and reaction time (50–150 min) based on response surface methodology (RSM). The synthesized CuO NPs had an average particle size of 30 nm with a narrow distribution. According to RSM analysis, the reaction temperature and time are the most significant factors; whereas, the impact of the other factors, especially TNBP volume percentage in the feed, was found to be negligible. Overall, excellent performance was achieved under optimal conditions found by the RSM, which was reaction temperature of 500 °C, TNBP volume percentage of 4%, oxidant ratio of 1.5, and reaction time of 90 min. The TOC removal efficiency as an indicator of TNBP degradation was about 99%. Finally, in vitro cell viability assays for the cytotoxicity evaluation of fresh and SCW-treated solution were applied. The results of MTT showed that SCWO converts TNBP into by-product that did not induce any cytotoxicity.