Supercritical Temperatures Research Articles

Evaluation on hydrothermal gasification of waste tires based on chemical equilibrium analysis

Massive amounts of waste tires are produced globally, which brings great challenges to the disposal and recycling of used tires. Hydrothermal gasification is a promising option for recycling waste tires. The hydrothermal gasification of waste tires was evaluated based on the chemical equilibrium analysis along with the response surface methodology (RSM) in terms of subcritical temperature range (250–300 °C), transition temperature range (350–400 °C), supercritical temperature range (550–600 °C), supercritical pressure (22.5–30.5 MPa) and feedstock concentration (5–20 wt%). CH4 yield at 350 °C reached a maximum, 41.575 mmol/g. H2 yield increased from 0.0283 to 53.602 mmol/g with increasing the temperature from 250 °C to 600 °C. CH4 yield at the supercritical temperature increased with lifting the feedstock concentration, while H2 yield decreased. The optimal parameters regarding maximum H2 and CH4 yields in the subcritical temperature range were 300 °C, 22.5 MPa and 12.5 wt%, respectively, while they in the supercritical temperature range were 550 °C, 30.5 MPa and 5.4 wt%, respectively. RSM was more suitable for predicting H2 yield in the hydrothermal gasification of waste tires at subcritical and supercritical temperature ranges, but it was available for predicting CH4 yield in three temperature ranges. This study can provide basic data for the hydrothermal treatment of waste tires.

Mitsubishi Chemical is licensing the HydroPRS process of Mura Technology

In this chapter, the interaction of aqueous solutions with the materials of the experimental or production plants is discussed that can lead to major problems due to corrosion. The aqueous solutions contain many reactive compounds, for example, chloride ions, under oxidative or reductive conditions, that can cause corrosion. Corrosion includes all reactions of components of an aqueous reaction mixture with the walls of the equipment of experimental or production facilities, like pipes, valves, fittings, and autoclaves.Corrosion in hydrothermal and supercritical water processes depends on the properties of the reaction mixture, on the properties of the construction materials, and on the construction of the processing equipment. In general, no satisfying solution to corrosion problems can be achieved with addressing only one aspect. On the contrary, only a comprehensive adjustment of all aspects influencing corrosion can lead to a process configuration that can be operated successfully and for a reasonable extended time span.Various types of corrosion occur, such as general corrosion, intergranular corrosion, pitting corrosion, and stress corrosion cracking. General corrosion occurs mainly in the near critical and supercritical temperature range in a region, where the density is high. Corrosion rates below T = 320 °C are very low. At temperatures T > 320 °C and pressures higher than the critical pressure, corrosion rates increase considerably. At a pressure of P = 24 MPa, high corrosion rates drop down to the level observed at lower temperatures just around the critical temperature of water, while at a pressure of P = 38 MPa, corrosion rates remain high up to a temperature of about T = 450 °C, since at that temperature density reaches the limit for corrosion due to ionic processes.In this chapter, first corrosion phenomena are described as occurring in high-temperature aqueous systems. Then, the parameters influencing corrosion are discussed in some detail together with corrosion mechanisms. The effect of corrosion is illustrated by reporting on a number of detailed investigations on corrosion in aqueous systems. A detailed discussion of measures that can be used against corrosion follows and shows the necessity of combining several approaches and ends in a compilation of means against corrosion.

Process Research of Solution‐Enhanced Dispersion by Supercritical Fluids to Prepare Energetic Material Nano‐Capsules

AbstractNano‐capsule energetic materials can improve the mechanical properties, combustion/explosion properties, and component compatibility of solid propellants and reduce their toxicity, water absorption capacity, and sensitivity. The following approaches have been performed to obtain nano‐capsule energetic materials. First, the structure of solution‐enhanced dispersion by supercritical fluids (SEDS) core device (nozzle and hanging basket) were modified to improve the atomization performance, eliminate the vortex, and avoid the repeated growth of crystals. The ultrafine RDX particles with average particle size of 705 nm and distribution range of 400–1200 nm were prepared by using the improved devices. Second, the optimal process parameters for the preparation of ultrafine RDX by SEDS were obtained. Reducing the solution concentration or the supercritical CO2 temperature will be beneficial to obtain dry RDX particles with smaller particle sizes. When the solution concentration ratio was 10 g (RDX)/100 mL (DMF), the temperature of supercritical CO2 was 35 °C, and the flow rate ratio of supercritical CO2 to solution was greater than 10 (kg/h): 2 (ml/min), the experiment results were good. Third, to facilitate the detection of shell elements, fluororubber (F26) and RDX were used as raw materials to prepare nano‐capsules. Transmission electron microscopy image showed that the RDX‐F26 particles featured a core‐shell structure with diameters that ranged between 200 nm and 300 nm, spherical shape, and uniform wall thickness. The X‐ray photoelectron spectroscopy showed that the core was RDX, the shell was F26, and the thickness of the shell was 14.6 nm. As the capsule wall material, the F26 also inhibited the overgrowth of RDX crystal in the course of crystallization successfully. Sensitivity analysis showed that the RDX‐F26 nano‐capsules had lower impact and friction sensitivities than the refined RDX. These results show that the use of improved SEDS process is suitable for the preparation of energetic material microcapsules.

Atomic-level insights into transition mechanism of dominant mixing modes of multi-component fuel droplets: From evaporation to diffusion

For a multi-component hydrocarbon mixture under supercritical conditions, especially for fuels injected into compression ignition engines, the mechanism for the transition of the dominant mixing mode from evaporation to diffusion is not well established. In this paper, evaporation processes of a six-component hydrocarbon fuel (13.16 mol% toluene, 13.81 mol% n-decane, 22.30 mol% n-dodecane, 24.60 mol% n-tetradecane, 14.66 mol% n-hexadecane and 11.47 mol% n-octadecane) droplet in nitrogen environments were studied using molecular dynamics (MD) simulations, in comparison with those of three-component and single-component fuel droplets. The ambient pressure ranged from 2 MPa to 16 MPa and the ambient temperature ranged from 750 K to 1350 K. Results indicated that the transition characteristics of the mixed fuel were not the linearly weighted average of the physical properties of individual components in the mixture based on their mole fractions. The reason why there is a limitation on the maximum transition temperature when diffusion dominates the fuel-ambient gas mixing process under high pressures has been discussed. The average resultant force on a fuel atom of an individual component increases with increasing pressure or decreasing temperature at the supercritical temperature, and diffusion will gradually dominate the mixing process of the fuel. The clustering behavior of fuels under supercritical conditions has also been discussed.

Enhanced photoacoustic effect for simultaneous imaging and drug release using phase-transition mesoporous silicon nanoprobe

Probe-assisted integration of imaging and therapy into a single modality provides tremendous opportunities in biological applications. In this study, phase-transition-mediated cavitation was used for simultaneous photoacoustic imaging (PA) and triggering drug release. For this purpose, we developed unique phase-transition mesoporous silicon nanoprobes (ICG/PFC/PTX@MSNs) consisting of indocyanine green (ICG), perfluorocarbon (PFC), and paclitaxel (PTX). For high-dose laser irradiation, the encapsulated ICG absorbs laser energy, providing localized heating well over the supercritical temperature of PFC. Then, liquid PFC was subjected to a liquid-to-gas phase transition, which generated stronger PA signals and promoted fast drug release. The enhancement of ICG/PFC/PTX@MSNs for PA imaging was demonstrated in in vitro and in vivo experiments. The average PA signal based on the phase-transition mechanism was ∼3 times higher than that of the traditionally used thermal expansion mechanism. Furthermore, the rapid drug release based on the same phase-transition-mediated cavitation mechanism can be achieved simultaneously; eventually, ∼80.4% of the total encapsulated drug were released. The hematoxylin–eosin stained section of tumor tissues from the ICG/PFC/PTX@MSN group showed many dead cells with condensed nuclei and pyknosis. This method not only promotes efficacy of chemotherapy but also makes accurate imaging-guided chemotherapy possible.

Estimation of casing material corrosion rates for supercritical geothermal development

Supercritical geothermal development method has been proposed to increase the use of geothermal resources; the associated challenges include high temperatures and pressures, as well as an acidic environment, which increase the corrosion rate of casing materials. Here, we estimated material corrosion rates assuming that the temperature and pressure in a developmental environment were 500 °C and 60 MPa, respectively. Wells WD-1a in the Kakkonda geothermal field, Japan, and Icelandic Deep Drilling Program-1 (IDDP-1) were used as model cases for the concentrations of volcanic gasses. Geochemical calculations were conducted to obtain distributions of pH values from room temperature to 500 °C at 60 MPa. Corrosion rate distributions for the metallic materials were derived for the temperature recovery period after drilling and production and post-production periods inside the well. Consequently, the pH remained approximately constant at temperatures ≤ 300 °C and increased as the temperature exceeded 300 °C. In the supercritical temperature and pressure region, the pH increased as the pressure decreased, and it maximized at a 0.35 g/cm3 fluid density. The IDDP-1 model exhibited lower pH values than the WD-1a model; the H2S/CO2 ratio and HCl gas also increased. Furthermore, from room temperature to 300 °C, the corrosion rates were found to increase with the temperature and maximized in the subcritical regions at 300–350 °C. The corrosion rates then decreased as the temperature increased in the supercritical temperature regions. In the possible developmental processes, the corrosion rates tended to be high at temperature of 300–350 °C (~2,000 m) during the temperature recovery period after drilling, whereas the corrosion rates tended to increase near the wellhead during fluid production. The corrosion rate maximized near the wellhead immediately after the production period ceased at a well depth of 1,000–1,500 m the following week.

Methane adsorption on the Zr-BDC metal-organic framework structure at supercritical temperatures and pressures

Methane adsorption by the sample of the metal-organic framework structure (MOF) Zr-BDC (BDC is benzene-1,4-dicarboxylate) containing zirconium ions was studied in a pressure range of 0.1–40 MPa at 303, 313, 323, and 333 K. The excess adsorption isotherms Γ(P,T) and adsorption isotherms of total amount a(P, T) were calculated, and the adsorption volume W was determined. The adsorption isosteres were constructed on the basis of these isotherms, and the isosteric qst(a,T) and initial q(0,T) heats of adsorption were calculated. The average isosteric heats of adsorption q st * (a, T) were determined taking into account the dependence of qst(a,T) on the amount adsorbed. These values were compared with the average heats of adsorption q(E) calculated with allowance for the characteristic energy of adsorption E(T), which were determined, in turn, from the adsorption isotherms a(P,T). Two calculation methods were used: via the characteristic curve and via the adsorption isotherm presented in the linear coordinates of the Dubinin—Astakhov equation. In both cases, new parameters were introduced in the adsorption equation to determine E(T): “Ps” that replaces the vapor pressure at saturation and critical temperature Tc. The Zr-BDC MOF sample is a microporous adsorbent, which can be used for methane storage.

Oxidation Behavior of an Austenitic Steel (Fe, Cr and Ni), the 310 H, in a Deaerated Supercritical Water Static System

The aim of this work was to study the corrosion behavior of a Fe-Cr-Ni alloy (310 H stainless steel) in water at a supercritical temperature of 550 °C and a pressure of 250 atm for up to 2160 h. At supercritical temperature, water is a highly aggressive environment, and the corrosion of structural materials used in a supercritical water-cooled nuclear reactor (SCWR) is a critical problem. Selecting proper candidate materials is one key issue for the development of SCWRs. After exposure to deaerated supercritical water, the oxides formed on the 310 H SS surface were characterized using a gravimetric analysis, a metallographic analysis, and electrochemical methods. Gravimetric analysis showed that, due to oxidation, all the tested samples gained weight, and oxidation of 310H stainless steel at 550 °C follows parabolic rate, indicating that it is driven by a diffusion process. The data obtained by microscopic metallography concord with those obtained by gravimetric analysis and show that the oxides layer has a growing tendency in time. At the same time, the results obtained by electrochemical impedance spectroscopy (EIS) measurements indicate the best corrosion resistance of Cr, and (Fe, Mn) Cr2O4 oxides developed on the samples surface after 2160 h of oxidation. Based on the results obtained, a strong correlation between gravimetric analysis, metallographic analysis, and electrochemical methods was found.

Creating Cloud-Fracture Network by Flow-induced Microfracturing in Superhot Geothermal Environments

Superhot geothermal environments with temperatures of approximately 400–500 °C at depths of approximately 2–4 km are attracting attention as new kind of geothermal resource. In order to effectively exploit the superhot geothermal resource through the creation of enhanced geothermal systems (superhot EGSs), hydraulic fracturing is a promising technique. Laboratory-scale hydraulic fracturing experiments of granite have recently demonstrated the formation of a dense network of permeable fractures throughout the entire rock body, referred to as a cloud-fracture network, at or near the supercritical temperature for water. Although the process has been presumed to involve continuous infiltration of low-viscosity water into preexisting microfractures followed by creation and merger of the subsequent fractures, a plausible criterion for cloud-fracture network formation is yet to be clarified. The applicability of the Griffith failure criterion is supported by hydraulic fracturing experiments with acoustic emission measurements of granite at 400 °C under true triaxial stress and at 450 °C under conventional triaxial stress. The present study provides, for the first time, a theoretical basis required to establish the procedure for hydraulic fracturing in the superhot EGS.

Machine-learning free-energy functionals using density profiles from simulations

The formally exact framework of equilibrium Density Functional Theory (DFT) is capable of simultaneously and consistently describing thermodynamic and structural properties of interacting many-body systems in arbitrary external potentials. In practice, however, DFT hinges on approximate (free-)energy functionals from which density profiles (and hence the thermodynamic potential) follow via an Euler–Lagrange equation. Here, we explore a relatively simple Machine-Learning (ML) approach to improve the standard mean-field approximation of the excess Helmholtz free-energy functional of a 3D Lennard-Jones system at a supercritical temperature. The learning set consists of density profiles from grand-canonical Monte Carlo simulations of this system at varying chemical potentials and external potentials in a planar geometry only. Using the DFT formalism, we nevertheless can extract not only very accurate 3D bulk equations of state but also radial distribution functions using the Percus test-particle method. Unfortunately, our ML approach did not provide very reliable Ornstein–Zernike direct correlation functions for small distances.

Correlation and prediction of thermodynamic properties of dilute solutes in water up to high T and P. IV. Simple fluids He, Ne, Ar, Kr, Xe, Rn, H2, N2, O2, CO, CH4 - Re-parametrization

Fugacity coefficients of dilute aqueous solutions of six noble gases and simple fluids H2, N2, O2, CO, CH4 have been re-evaluated from ambient conditions (298 K, 0.1 MPa) to high temperatures (up to 2000 K) and water densities (up to 1500 kg m–3). The main goal of the reevaluation is to provide an agreement within 1% with experiment-based values of the vapor-liquid distribution constants, KD, which are known for simple fluids (except Rn) in water with high precision all the way to the critical point of water at 647.096 K. As before, the approach is based on a correlation developed for the function, A12∞=V2∞/κTRT, and includes known constraints: second virial coefficients at low water densities, hard sphere behavior at high water densities, near-critical principles, and corresponding-states relations. A comparison with literature data at subcritical (Henry's and vapor-liquid distribution constants) and supercritical temperatures (fugacity coefficients) is made.

New methods of acid hydrolysis of cellulose and plant raw materials

Prospects for the development of hydrolysis production are determined by the relevance of industrial use of plant biomass to replace the declining reserves of fossil organic raw materials and increasing demand for ethanol, especially for its use as automobile fuel, protein-containing feed additives that compensate for protein deficiency in feed production, and other products. Based on the review of the research results presented in the scientific literature, the analysis of modern methods of liquid-phase acid hydrolysis of cellulose and various types of plant raw materials, including those that differ from traditional ones, is performed. The main directions of increasing its efficiency through the use of new catalytic systems and process conditions are identified. It is shown that the most promising methods for obtaining monosaccharides in hydrolytic processing of cellulose and microcrystalline cellulose, pentosan-containing agricultural waste and wood, are methods for carrying out the process at elevated and supercritical temperatures (high-temperature hydrolysis), the use of new types of solid-acid catalysts and ionic liquids.

Numerical Investigation of Boundary Grid Effect on Heat Transfer Computation of RP-3 at Supercritical Temperature of Helical Tube Wall

To get reliable computational results, the RNG k-e turbulence model with enhanced wall treatment was validated to solve the heat transfer of supercritical RP-3 in a helically coiled tube, and models of the thermo-physical properties of RP-3 were optimally chosen. Most significantly, the grid independence was validated by two-step procedure, and the effect of boundary grids of the supercritical-temperature wall on the computational accuracy was well studied. Through adjusting boundary-layer girds’ size, four regions (increased, pseudo-convergence, decreased and convergence) of the outlet temperature Tout were obtained and analyzed. The results showed that the maximum computation errors of Tout and the pressure differential between the inlet and outlet ΔP reached 20.65% and 98.15%, respectively, indicating that boundary grids have a significant influence on computation of flow and heat transfer. Based on this, a dimensionless distance from the wall-adjacent cell to the wall y+ = Prw−1/1.78 (Prw denotes Wall Prandtl number) was recommended as a convergence point. The variation laws of viscous length scale y were discussed under different structural parameters, operation parameters, and helical lengths. An explicit model of y* was proposed to calculate the height (y) of the first boundary layer grids and refine boundary grids efficiently. A modified model for coefficient of friction factor Cf was proposed based on Rogers’s, and Nusselt number Nu was proposed based on an analogy of momentum and heat transfer. The above models about y*, Cf and Nu could apply to both the entrance region and the whole tube length, and showed good performance when Reynolds number was extended to above 70 000, or whenever the outlet temperature is below or above the critical point.

The effect of flame straightening on the microstructure and mechanical properties of different strength steels

In many cases, flame straightening is unavoidable after welding for the reduction of deformation. Due to the not very concentrated heat source, the process can cause significant changes in the microstructure, especially in high strength and wear-resistant steels. Due to their different physical properties, the effects vary depending on the flammable gases (acetylene, propane). The situation is complicated by the fact that the manual technology carries a risk of overheating, which can have detrimental effects on the mechanical properties. During our experiments, three steels are investigated (S355J2 + N, XAR400, S960QL). The thermal cycles for the physical simulations were determined by thermocouple measurement during real experimental conditions. Three peak temperatures (1000 °C, 800 °C and 675 °C) and two types of industrial cooling conditions (air and water cooling) were studied. The samples were examined by optical microscopy tests, hardness testing and Charpy V-notch impact tests. During straightening the XAR400 showed high sensitivity to softening even in the lower temperature range, while hardening occurred in the S960QL steel at a higher peak temperature values during water cooling. The inter- and supercritical temperature should be avoided in all steels; however, the subcritical temperature can be beneficial to the toughness properties of the S960QL and XAR400.

Experimental Analysis and Numerical Simulation on the Stability of Geological Storage of CO <sub>2</sub>: A Case Study of Transforming a Depleted Gas Reservoir into a Carbon Sink Carrier

Geological storage of CO2 is one of the most economical, feasible and effective measures to slow down global warming. In this paper, a combined long core model was designed to study the seepage characteristics of supercritical CO2 displacement. Moreover, the stability of permanent storage of cushion gas layers formed by supercritical CO2 injection has been systematically studied. The research results showed that in the supercritical temperature and pressure range of CO2, the front edge of CO2 displacement can form a relatively stable seepage zone. Supercritical CO2 displacement can achieve high gas storage rate and stable CO2 storage. At the same time, the recovery rate of remaining natural gas has been significantly improved. As the injection pressure increased, supercritical CO2 inhibited the reverse diffusion of natural gas molecules. Therefore, the breakthrough of the supercritical CO2 displacement front under high pressure laged behind. However, due to the increase in the density difference of gas molecules, the forward diffusion of supercritical CO2 has been enhanced. Temperature will not significantly affect the displacement and storage effects of supercritical CO2 in gas reservoirs. The increase in injection pressure and reasonable control of the injection rate can delay the breakthrough of supercritical CO2 displacement. These measures are conducive to the stable storage of CO2 and the improvement of remaining natural gas recovery. The implementation of CO2 geological storage is suitable for the later stage of gas reservoir depletion development. The high-density gravitational heterogeneity of supercritical CO2 enables the injected CO2 to form a stable high-density cushion gas layer in the gas reservoir, which can achieve stable CO2 storage for more than 100 years.

Effects of different supercritical temperatures on the modification of aramid pulp surface via coating silica to enhance the properties of EPDM composites

AbstractPoor dispersion is a major disadvantage of aramid pulp (AP) reinforced composites. In this study, nano‐silica (SiO2), as the surface modifier and reinforcing agent, was synthesized on AP surfaces to improve the dispersed character in ethylene‐propylene‐diene elastomer (EPDM) using supercritical carbon dioxide (scCO2). The influence of the temperature of scCO2 on treated AP, configuration of the surface, and mechanical performance of AP/EPDM composites were studied. Based on the results of Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy, SiO2 was chemically grafted onto the AP surface. The SiO2 nanoparticles were uniformly deposited on the AP surface, and the roughness of AP was increased based on the SEM results. The optimum temperature of the supercritical reaction was 100°C considering the results of all the tests. Under the optimum temperature and with 10 phr of AP, the mechanical properties, oil resistance, and dynamic mechanical properties of the SiO2‐AP/EPDM composites were investigated. The tensile strength and tear strength of the SiO2‐AP/EPDM were improved by 121.1 and 134.9%, respectively, compared with that of the pure AP/EPDM composites. The fracture surface of the SiO2‐AP/EPDM composites showed that SiO2‐modified AP had a good dispersion in EPDM, and AP and EPDM had good interfacial bonding. Dynamic mechanical analysis results showed that the storage modulus of the composites was improved greatly compared with the pure AP/EPDM, and the loss factor also increased. The oil resistance of the composites was also improved.

Equation of State of a Cell Fluid Model with Allowance for Gaussian Fluctuations of the Order Parameter

The paper is devoted to the development of a microscopic description of the critical behavior of a cell fluid model with allowance for the contributions from collective variables with nonzero values of the wave vector. The mathematical description is performed in the supercritical temperature range (T > Tc) in the case of a modified Morse potential with additional repulsive interaction. The method, developed here for constructing the equation of state of the system by using the Gaussian distribution of the order parameter fluctuations, is valid beyond an immediate vicinity of the critical point for wide ranges of the density and temperature. The pressure of the system as a function of the chemical potential and density is plotted for various fixed values of the relative temperature, both with and without considering the above-mentioned contributions. Compared with the results of the zero-mode approximation, the insignificant role of these contributions is indicated for temperatures T > Tc. At T < Tc, they are more significant.

Phase change in fuel sprays at diesel engine ambient conditions: Impact of fuel physical properties

In mixing controlled Diesel combustion concept, mixing processes are a key phenomenon that significantly effects power, efficiency and emissions. A consequence of fuel-air-mixing is fuel phase change, which is not fully understood yet. In this study, phase change mechanisms of sprays of different n-alkanes at high pressure and temperature conditions are investigated. Mie scattering imaging is utilized to obtain the maximum liquid penetration of Diesel surrogates (hexane, heptane, decane and dodecane) injected into nitrogen atmosphere in a constant volume chamber at fuel supercritical temperature and ambient pressures ranging from sub- to supercritical. Two 1D models are utilized to predict the liquid length based on the required enthalpy for full phase change. The theoretical models are validated against experimental data to identify phase change regimes. Along with ambient conditions, fuel properties significantly influence the phase change mechanism in a spray. Especially fugacity plays an important role regarding the droplet evaporation rate.

Phase transitions of multi-component fuel droplets under sub- and supercritical conditions

For a multi-component hydrocarbon mixture under supercritical conditions, the mechanism and criterion for the transition of the dominant mixing mode from evaporation to diffusion are not well established. In this paper, phase transition processes of three-component hydrocarbon fuel (5.3 wt% isooctane, 25.8 wt% n-dodecane and 68.9 wt% n-hexadecane) droplets in sub- and supercritical nitrogen environments were studied using molecular dynamics, in comparison with those of single-component n-hexadecane droplets. The initial diameters of the droplets were 25.5 nm (three-component) and 26.5 nm (single-component), respectively. Based on the quantitative Voronoi tessellation, a new criterion, which was a combination of two dimensionless critical values of Hc = 0.85 and Wc = 0.35, was proposed to determine the transition of the dominant mixing mode from evaporation to diffusion during fuel-ambient gas mixing. Results indicated that when the ambient pressure ranged from 2 MPa to 10 MPa and the ambient temperature ranged from 750 K to 1200 K,the density difference between the vapor phase and the liquid phase decreased gradually with increasing ambient pressure or decreasing ambient temperature. And the dominant mixing mode gradually transitioned from evaporation to diffusion. Increasing the ambient pressure did not necessarily promote the occurrence of phase transition, while increasing ambient temperature accelerated the phase transition monotonically. Light fuel components increased the minimum pressure of the diffusive mixing zone. A major finding was that under a certain ambient pressure, the supercritical transition temperature had not only a minimum but also a maximum.

Acylglycerol synthesis including EPA and DHA from rainbow trout (Oncorhynchus mykiss) belly flap oil and caprylic acid catalyzed by Thermomyces lanuginosus lipase under supercritical carbon dioxide

Supercritical carbon dioxide (SCCO2) was studied as a medium for the esterification of eicosapentaenoic acid (n-3 C20:5, EPA) or docosahexaenoic acid (n-3 C22:6, DHA) and caprylic acid (C8:0, CA) in structured triacylglycerols (sTAG) using Thermomyces lanuginosus lipase as biocatalyst. Process variables (n-3 long-chain polyunsaturated fatty acid: CA, n-3 LCPUFA:CA content ratio), glycerol content (wt. %), and supercritical time, temperature and pressure were optimized by the Response Surface Methodology through a central composite design of 25–1 + star. Synthesis of sTAG with EPA, DHA and CA under SCCO2 was significantly affected by the n-3 LCPUFA:CA content ratio and supercritical time. MALDI-TOF mass spectrometry revealed that acylglycerols with the highest levels of EPA or DHA content in the sn-2 position were obtained when the following variables conditions were applied: 50% (n-3 LCPUFA:CA content ratio), 40 °C (supercritical temperature), 20 MPa (supercritical pressure), 4 h (supercritical time) and 20.0 wt. % (glycerol concentration). For such experimental conditions, esterification catalyzed by Thermomyces lanuginosus lipase under supercritical carbon dioxide allowed obtaining sTAG synthesized with 54.95% of CA, 11.64% of EPA and 13.77% of DHA.