Possible mechanism of oil formation in a flow of supercritical fluid (using carbon dioxide as an example)

An experimental study of the supercritical fluid extraction of the organic substance with carbon dioxide from rock and soil showed that a supercritical (SC) fluid can dissolve, transfer, and accumulate petroleum-like hydrocarbons including relict ones. It was hypothesized that during the supercritical extraction, the dissolution of organic substances is accompanied by their mechanochemical transformation in the micropores and microcracks of the solid. The resulting changes in the composition, structure, and distribution of the individual hydrocarbons are similar to those in petroleum-mother rocks. We can therefore assume that fast mechanochemical transformation of the dispersed organic matter of sedimentary rocks takes place in a flow of a deep supercritical fluid. This, in turn, would allow one to revise the time scale of oil accumulation from millions to possibly hundreds of years.     

Frontal polymerization of styrene under the conditions corresponding to the transition of the initial styrene-carbon dioxide to the supercritical state

The possibility and conditions of realization of the frontal polymerization of styrene-carbon dioxide mixtures with transition to the supercritical (SC) state are examined. A method for experimental determination of the conditions of transition of the mixture to the supercritical state is developed. The critical temperatures for various initial monomer-to-CO₂ ratios are determined. Numerical simulation of the thermal polymerization of styrene in a tubular reactor is performed. The temperature and concentration profiles along the length of the reactor corresponding to the transition of the styrene-carbon dioxide mixture to the SC state are calculated.     

Some thermodynamic processes of anthracite–carbon dioxide mixture in supercritical fluid state

In the context of the development of the catalyst regeneration procedure via supercritical fluid CO₂ extraction, some thermodynamic properties of the anthracene–carbon dioxide mixture in supercritical fluid state have been studied. Data on anthracene solubility in pure and modified (dimethyl sulfoxide, 5 wt %) supercritical carbon dioxide (SC–CO₂), the heat capacity of anthracene and its mixtures with carbon dioxide, and the heat of solution of anthracene in SC–CO₂ are presented. Anthracene solubility in SC–CO₂ is described satisfactorily using the Peng–Robinson equation of state.     

Improvement of functionality of carbonate macadam via supercritical fluid impregnation with bituminous compounds

A comprehensive process for the deasphaltizing of heavy oil residue and subsequent treatment (impregnation using a solvent in the supercritical fluid state) of a carbonate macadam by bituminous compounds (deasphaltizate) is developed. The purpose is to improve the functionality of the material and, above all, to reduce its water absorption capacity. The process parameters are specified based on the newly obtained data on the thermal capacity of heavy oil residue and deasphaltizate. The results of the process of experimental implementation and the characteristics of impregnated macadam, including the water absorption capacity which decreased from 3.6 to 0.54%, are presented.     

MR measurement of critical phase transition dynamics and supercritical fluid dynamics in capillary and porous media flow

Supercritical fluids (SCF) are useful solvents in green chemistry and oil recovery and are of great current interest in the context of carbon sequestration. Magnetic resonance techniques were applied to study near critical and supercritical dynamics for pump driven flow through a capillary and a packed bed porous media. Velocity maps and displacement propagators measure the dynamics of C(2)F(6) at pressures below, at, and above the critical pressure and at temperatures below and above the critical temperature. Displacement propagators were measured at various displacement observation times to quantify the time evolution of dynamics. In capillary flow, the critical phase transition fluid C(2)F(6) showed increased compressibility compared to the near critical gas and supercritical fluid. These flows exhibit large variations in buoyancy arising from large changes in density due to very small changes in temperature.     

Polymorphism of Risperidone in Supercritical Fluid Processes of Micronization and Encapsulation into Aliphatic Polyesters

The specific features of the transformation of risperidone polymorphs as a result of micronization and encapsulation into aliphatic polyesters (polylactides and polylactoglycolide) have been studied using supercritical (SC) carbon dioxide. It has been shown that the micronization of risperidone, which originally is polymorph A, via the rapid expansion of supercritical solutions (RESS) and the supercritical antisolvent (SAS) precipitation leads to its crystallization in less thermodynamically stable polymorph B. This transition is complete for SAS and only partial for RESS. When these micronized samples are encapsulated into polylactides and polylactoglycolides via the formation of particles from gas-saturated solutions (PGSS) and monolithization with further cryogrinding (MCG), risperidone polymorph B is partially converted back into polymorph A. At the same time, the micronization of initial risperidone polymorph A via cryogrinding and its further PGSS and MCG encapsulation into polylactides or polylactoglycolides does not result in any change in the polymorphic state of risperidone, and it always remains in initial polymorph A.     

Ultrasound assisted supercritical fluid extraction of oil and coixenolide from adlay seed

Oil and coixenolide are important components of adlay seed (Coix lachrymal-jobi L. var. Adlay) with many beneficial functions to human health. In this work, a novel extraction technique--ultrasound assisted supercritical fluid extraction (USFE)--was studied. Effects of operating conditions on the extraction, including extraction temperature (T), pressure (P), time (t), CO(2) flow rate (F) and ultrasonic power (I) were investigated. There are optimum temperatures which gives the maximum extraction yields (EYs) for the supercritical fluid extractions with and without ultrasound. The effect of pressure on EYs for is similar to that of pressure on CO(2) density. Based on the yield of extraction, the favorable conditions for supercritical fluid extraction (SFE) were: T at 45 degrees C, P at 25 MPa, t at 4.0 h and F at 3.5L/h. While ultrasound was applied as in USFE, the following parameters were preferred: T at 40 degrees C, P at 20 MPa, t at 3.5h and F at 3.0 L/h, respectively. The results show that supercritical fluid extraction with the assistance of ultrasound could reduce the temperature, pressure, CO(2) flow rate, as well as time used in the process. Compared with SFE, USFE could give a 14% increase in the yield for extracting oil and coixenolide from adlay seed with less severe operating conditions.     

Supercritical fluid technologies for the advanced processing of plant raw materials

Publications dedicated to the application of supercritical fluids in the advanced processing of plant raw materials for the production of a wide range of products such as ethanol, liquid and gaseous fuels, sugars, organic acids, and oils are reviewed and analyzed. The specific features of the supercritical fluid technologies and their advantages over the existing traditional methods for the processing of plant raw materials are demonstrated.     

基于拟沸腾理论的超临界CO2管内传热恶化量纲分析

超临界流体广泛应用于工程技术领域,其流动传热特性对工程设计具有重要意义,但是,由于超临界流体的物理微观和宏观行为的机理尚不清晰,所以其异常的流动传热特性并未得到很好的解决.普遍认为超临界流体在分子尺度上可分为类气和类液两种不同的特性,直到最近通过实验在宏观上监测到超临界水类液和类气之间的转变,且这一过程与拟沸腾理论一致,使得问题逐渐变得清晰.本文基于拟沸腾理论对超临界CO2异常流动传热行为进行了研究,在假设类液和类气转换过程不均匀的情况下,从经典的量纲分析和亚临界过冷沸腾理论模型出发,提出了一个适用于超临界流体拟沸腾换热过程的分析方法.通过引入表征类气膜生长速度与流体主流平均流速之比π=(qw·ρ1)/(G·Δi·ρg)和表征近壁区类气膜温度梯度π13=(qw·βpc·di)/λg两个无量纲数,来表征拟沸腾如何导致传热恶化,解释了超临界CO2竖直向上加热流动过程中的异常换热特性,即较大的类气膜生长速度使近壁区快速聚集了较多的高温流体,而较大的类气膜温度梯度使类气膜覆盖在壁面.当核心的冷类液不能充分润湿热壁面时,传热恶化.新无量纲数较好的诠释了超临界流体拟沸腾诱导传热恶化机制,为超临界拟沸腾传热研究提供了理论依据.     

地质样品中有机物的超临界萃取研究

建立了以CO2为超临界流体的超临界萃取地质样品中有机物的分析方法,讨论和优化了超临界萃取条件,测定了茂名油页岩中的n-C13-n-C34,并与索式抽提结果作了对比,该方法测定茂名油页岩中的n-C15-n-C34链烷烃,相对标准偏差为（n=4)4.9%-23.8%,n-C13的相对标准偏差为9．2％（n=4),而传统经黄的索式抽提法测得的n=C15-n-C34链烷烃的相对标准偏差为11．2％－37．7％（n=4),n-C13的相对标准偏差为46．4％（n=4),本法与传统方法相比,具有高效,无热降解,无毒,选择性好,精密度好等优点,可直接用于GC,GC－MS的测定。     

Onset of shear thickening in a simple fluid

We report on nonequilibrium molecular-dynamics simulations of the shear-thickening transition in a simple fluid under shear. We relate the shear-thickening transition to the onset of instabilities in the flow profile and to that of dramatic variations in normal stress differences. The dependence of the critical shear rate, which indicates the onset of shear thickening, on density and temperature is rationalized by introducing a ratio between two characteristic times, quantifying the short-time mobility of a particle and the deformation imposed by the applied shear rate, respectively. The shear-thickening transition is shown to occur at a constant value for this ratio for all state points studied. From a structural point of view, this transition is accompanied by the formation of clusters as recently observed in experiments on complex fluids.Received: 26 July 2004, Published online: 21 September 2004PACS: 83.60.Rs Shear rate-dependent structure (shear thinning and shear thickening) - 47.50. + d Non-Newtonian fluid flows - 83.10.Mj Molecular dynamics, Brownian dynamics     

Synthesis of carbon nanotubes/alumina composites in supercritical media

Nanosized composites based on multiwall carbon nanotubes (CNTs) and Al₂O₃ have been obtained for the first time in supercritical (SC) media (water, hexane, and their mixture). For comparison, materials of the same net composition have been prepared by hydrothermal synthesis and sol–gel processing. The composites have been characterized by electron microscopy, X-ray diffraction, and thermal analysis. The structure of the materials synthesized in the SC media depends on the fluid composition. The most uniform composite containing alumina particles that are comparable in size to the CNT diameter and are stabilized on the carbon surface can be obtained in the SC mixture of hexane and water. When water and hexane are used separately, the formation of large alumina crystals on the CNT surface and contamination of the composite by the products of hexane pyrolysis and carbonization are, respectively, observed.     

Atomistic investigation of phase transition in solid argon induced by shock wave transmission

A molecular dynamics (MD) simulation is employed to study the phase transition process in argon induced by shock wave transmission. Deriving the relation between the shock and piston velocities, the theoretical equation of state for argon is presented. Also, argon equation of state is obtained by measuring the quantities directly from simulations to be able to detect the phase transitions. The phase transition is also detected by using argon phase diagram and free energy calculations. A comparison shows good agreement between the theoretical and MD results for the phase transitions. Based on these simulations, it is concluded that under a shock wave transmission with suitable energy, the solid argon experiences a phase transition from solid to liquid and another from liquid to supercritical fluid. By reflecting the shock wave back at the end of its passage, the whole argon may reach the supercritical state.     

Supercritical Fluid Impregnation of Broken Stone with Deasphaltizate Obtained from Oil Residue

Results are presented concerning the studies on obtaining a heavy oil residue, its deasphalting and the impregnation of crushed carbonate stone with deasphaltizate. The heavy oil residue is obtained via the separation of extra-heavy crude oil using a steam thermal method. The deasphalting of the oil residue and the impregnation of crushed carbonate stone are carried out using a propane-butane solvent in the liquid and supercritical fluid state, respectively.     

Fatty Acid Composition and Biological Activity of Supercritical Extracts from Arctic Brown Algae <Emphasis Type="Italic">Fucus vesiculosus</Emphasis>

The fatty acid composition of arctic brown algae was studied using supercritical (SC) fluid extraction. A method for fractionating the SC extract based on the differences in the solubility of its components (fatty acids, polyphenols) was proposed. The composition and biological activity of the fractions were studied.     

Nonmetal-to-metal transition in dense fluid helium

The nonmetal-to-metal transition in dense fluid helium is discussed, which has been, in analogy to metallization of hydrogen, predicted as first-order plasma phase transition using chemical models for the equation of state and plasma composition. However, recent ab initio simulations performed for dense fluid helium indicate that this transition is continuous in the considered regime, without a density jump and latent heat as characteristic of a first-order phase transition. Implications for some astrophysical plasmas are discussed.     

Local moment formation in the superconducting state of a doped Mott insulator

A microscopic theory is presented for the local moment formation near a nonmagnetic impurity or a copper defect in high-Tc superconductors. We use a renormalized mean-field theory of the t-J model for a doped Mott insulator and study the fully self-consistent, spatially unrestricted solutions of the d-wave superconducting (SC) state in both the spin S=0 and S=1/2 sectors. We find a transition from the singlet d-wave SC state to a spin doublet SC state when the renormalized exchange coupling exceeds a doping dependent critical value. The induced S=1/2 moment is staggered and localized around the impurity. It arises from the binding of an S=1/2 nodal quasiparticle to the impurity. The local density of states is calculated and connections to NMR and STM experiments are discussed.     

Supercritical fluid technologies in the chemistry of wood and its components

The advantages of application of supercritical fluid technologies at the stages of complex wood and plant processing are analyzed based on the modern concept of lignin-carbohydrate matrix formation.     

Structure of supercritical water: The concept of critical isotherm as a percolation threshold

Computer simulations are used to study the rearrangements of hydrogen-bonded structures of water upon transition to the supercritical state. It is shown that the destruction of the infinite hydrogen-bonded cluster, i.e., crossing the percolation threshold occurs in the subcritical region and that, at the critical temperature, structural variations reach the point where the fluid acquires the properties of a system with two types of ordering. The existence of tetrahedral clusters in supercritical water is confirmed only at high pressure.