Carbon Dioxide Solvent Research Articles

Controlling the Properties of n-Alkanethiolate Self-Assembled Monolayers on Gold by Using Supercritical Carbon Dioxide−Ethanol Mixtures as Solvents

We report the use of ethanol−supercritical carbon dioxide mixtures as solvents in the formation of n-alkanethiolate self-assembled monolayers (SAMs) on gold. Results from reflectance−absorption infrared spectroscopy and electrochemical impedance spectroscopy reveal that the SAMs formed from these environmentally friendly solvent mixtures are highly crystalline and exhibit large resistances against the diffusion of redox probes. Increasing the percentage of ethanol in the solvent results in SAMs with larger average cant angles and lower charge-transfer resistances, consistent with a greater density of defect sites within the film. The ability to control the defect density within the SAM by simply altering the ethanol−carbon dioxide solvent composition represents a straightforward and environmentally friendly strategy for tuning the properties of these molecular films.

Simulation of particle formation during the rapid expansion of supercritical solutions

Experimental and theoretical results for the rapid expansion of supercritical solutions (RESS-process) are presented. Experiments were carried out with the solvents carbon dioxide (CO 2) and trifluoromethane (CHF 3) and the organic solutes cholesterol, benzoic acid and griseofulvin. It is shown that for each of these solutes it is possible to produce particles in the size range of 200 nm which leads to improved dissolution. Furthermore, to demonstrate the higher bioavailability of the micronized pharmaceutical griseofulvin as of the original material, results of a dissolution experiment are shown. Besides the experiments the flow and particle formation and growth in the expansion unit (inlet–capillary nozzle–supersonic free jet) of the RESS-process is modelled numerically. The model to calculate the flow field consists of mass, momentum and energy balances as well as the extended generalized Bender equation of state (egB-EoS). The flow is assumed to be steady, one-dimensional, viscid and non-adiabatic. The general dynamic equation (GDE) is included to model the particulate phase. The results of the modelling show that particle formation occurs in most of the cases mainly in the supersonic free jet and that the main mechanism for particle growth is coagulation. Furthermore, it can be inferred from the high particle number concentration at the Mach disk that particle growth by coagulation continues in the subsonic part of the free jet (expansion chamber). Experimental results with variation of additional air supply to the expansion chamber are presented to confirm the theoretical results.

In-line catalytic purification of carbon dioxide used in analytical-scale supercritical fluid extraction

Supercritical fluid extraction analyses are often compromised by trace impurities present in the solvent carbon dioxide. These impurities, commonly used as lubricants in the specialty gas industry, can produce significant background levels, increasing limits of detection and quantification. This problem is especially severe when electron capture detection (ECD) is used for trace concentrations of analytes (e.g., polychlorinated biphenyls and chlorinated pesticides). In this study, an in-line catalyst-based purification system was successfully employed to remove ECD-responsive contaminants from CO2. Low-purity (98%) "Bone Dry" CO2 was purified to levels cleaner than a very-high-purity grade of CO2 specified at less than 10 ppt ECD-responsive contaminants. Purification was successfully applied to extremely sensitive on-column experiments as well as higher flow rate off-line experiments. In addition to lowering limits of detection and quantification, significant cost savings can be realized by purifying inexpensive, low-purity CO2 instead of relying on much more expensive, prepurified CO2.

Acid Gas Extraction of Pyridine from Water

Pyridine was extracted from aqueous solutions initially containing 5 or 15 wt % pyridine by using liquid or supercritical carbon dioxide at 10 MPa as a solvent in a mechanically agitated countercurrent extraction column. The lowest pyridine concentration in the raffinate was 0.06 wt %, whereas the pyridine concentration in the extract was 86−94 wt %. From the initial amount of pyridine, 96−99% was transferred from the feed stream to the extract by using relatively small solvent-to-feed ratios of 2.8−4.6 (kg of solvent/kg of feed). The measured distribution coefficients for the water/pyridine/carbon dioxide system ranged from 0.3 to 1 (weight units), depending on the initial pyridine concentration in water. Carbon dioxide is a particularly suitable solvent for the extraction of pyridine from concentrated aqueous solutions. The efficiency may be the result of an acid−base interaction between weakly basic pyridine solute and weakly acidic carbon dioxide solvent in an aqueous environment.

High pressure multiphase equilibria in aqueous systems of carbon dioxide, a hydrophilic organic solvent and biomolecules

A binary mixture of water and a water-soluble organic solvent which at atmospheric pressure reveals no miscibility gap, can be forced to split into two liquid phases by pressurization with a `near-critical' gas. Since both coexisting liquid phases are water-rich, the liquid phase split might be used, for example, for the recovery of delicate, high value products by means of extraction in biotechnology. An overview is given on the phase behavior of ternary systems water–hydrophilic organic solvent–carbon dioxide, focusing on the high pressure three phase equilibrium LLV. Exemplary experimental results for such phase equilibria are presented, discussed and compared with predictions from an equation of state. Furthermore, new experimental results for the partitioning of vanillin and caffeine to the coexisting liquid phases in high pressure three-phase LLV equilibrium in water–acetone–carbon dioxide at 313 and 333 K are presented together with a simple method for correlating these results.

Oldshue–Rushton Column in Supercritical Fluid Extraction

The performance of a mechanically agitated Oldshue–Rushton column using carbon dioxide solvent at 10 MPa and 313 K to extract ethanol from aqueous feed was investigated. The overall mass-transfer coefficient K od a, the height equivalent to a theoretical stage (HETS), dispersed phase holdup, as well as the column capacity data were measured as a function of solvent-to-feed ratio and rotor speed. The values of overall mass transfer coefficient K od agenerally ranged from 0.009 to 0.012 s−1, and the values of the HETS ranged from 0.44 to 0.74 m. The total throughput of the Oldshue–Rushton column without agitation was approximately 68m3·h−1·m−2, whereas at 300 rpm the total throughput was approximately 48 m3·h−1·m−2.

Measurements and modeling of cloud point behavior for polypropylene/ n-pentane and polypropylene/ n-pentane/carbon dioxide mixtures at high pressure

The phase behavior of polypropylene (PP) in n-pentane and n-pentane/carbon dioxide solvent mixtures has been studied using a high-pressure variable volume view cell. Cloud point pressures for polypropylene ( M w=50,400) in near-critical n-pentane were studied at temperatures ranging from 432 to 470 K for polymer concentrations of 1 to 15 mass%. Furthermore, cloud point pressures for polypropylene ( M w=95,400) in near-critical n-pentane were studied at temperatures ranging from 450 to 465 K for polymer concentrations of 1 to 8 mass%. Cloud point pressures were also measured for PP ( M w=200,000, 3 mass%) in n-pentane at temperatures ranging from 450 K to 465 K. The cloud point pressures for PP ( M w=50,400) in n-pentane/CO 2 mixtures were determined for PP concentrations of 3.0 mass% and 9.7 mass% with CO 2 solvent concentrations ranging from 12.6 mass% to 42.0 mass% at temperatures ranging from 405 K to 450 K. All of the experimental cloud point isopleths were relatively linear with approximately the same positive slope indicating LCST behavior. The experimental cloud point pressures were relatively insensitive to the concentration and molecular weight of polypropylene. At a given temperature, the cloud point pressure of the PP/ n-pentane/carbon dioxide system increased almost linearly with increasing carbon dioxide solvent concentration (for carbon dioxide concentrations less than 30 mass%). The Sanchez–Lacombe (SL) equation of state was used to model the experimental data.

Interfering contaminants in carbon dioxide solvent used in the supercritical fluid extraction of polychlorinated biphenyls

In supercritical fluid extraction (SFE), contaminants in the CO 2 can also be concentrated, interfering with analyte quantitation and increasing analyte detection limits. As an example, we measured levels of electron capture-responding contaminants in three grades of CO 2, using off-line GC–ECD. In all cases, the contaminant is chlorotrifluoroethylene (CTFE) grease, which significantly affects congener specific analysis of polychlorinated biphenyls (PCBs). For total PCB levels of 30–300 ng/ml, the PCB/CTFE grease ratio should be ≥0.6 to allow quantitation of the 21 largest PCB peaks to within 10–20% of the true value. Accurate quantitation of coplanar congeners or total PCB mass, both sums of many small peaks, requires PCB/CTFE≥6.

Technical Feasibility of CO2 Flooding in Weyburn Reservoir - A Laboratory Investigation

Abstract Miscible or near-miscible flooding with CO2 or hydrocarbon gases will significantly increase, and extend the production life of, Saskatchewan light and medium oil reserves. These pools are approaching their economic limit of production under conventional technology (primary and secondary recovery methods). Responding to industry needs, a comprehensive study by Saskatchewan Research Council is assessing the suitability of the CO2 flooding process for reservoirs in southeast Saskatchewan. This paper presents the details of a laboratory study to evaluate the applicability of CO2 flooding for a southeast Saskatchewan reservoir, Weyburn. The evaluation is based on the measurement of CO2 minimum miscibility pressure for the Weyburn reservoir fluid, of PVT properties for reservoir fluid CO2 mixtures, and of oil recovery behaviour from core flood tests conducted in the near-miscible region. Laboratory studies showed that the tertiary CO2 injection at over 12 MPa into the waterflo'oded reservoir is expected to produce miscible flooding conditions. At this pressure of injection, over 60 mol % CO2 was dissolved in the reservoir fluid. This reduced the viscosity of the fluid nearly fourfold to 0.70 mPa.s and swelled the oil about 45%. Coreflood studies indicated that the microscopic displacement efficiency of the process increased with injection pressure in the near-miscible region. It is believed that the geology of the Weybum reservoir will prove advantageous for near-miscible operation. However, this aspect needs to be further investigated. Introduction Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limit of production under conventional technology. Primary and secondary methods together recover about 21% of the initial oil-in-place (IOlP). Although the remaining light and medium oil-in-place in Saskatchewan is 1,373 million cubic metres(1) the remaining reserves producible under primary and secondary recovery are only 59 million cubic metres. This latter number would rise by 20 million cubic metres if medium oil in southwest Saskatchewan, not considered here, were included(1). The development of tertiary recovery techniques is, therefore, essential to increase the production life of these reservoirs and to maintain current production. Application of the miscible or near miscible oil displacement process using CO2, hydrocarbon or other gases as injection fluids is expected to increase the LMO reserves threefold and extend the production life of these pools by two decades(2). Miscible (or near-miscible) flooding with carbon dioxide or hydrocarbon solvents is considered to be one of the most effective enhanced oil recovery (EOR) processes applicable to LMO reservoirs. A comparison of effectiveness of CO2 and hydrocarbon gases as miscible solvents has been discussed(3). CO2 floods minimize gravity segregation compared with the hydrocarbon solvents. In addition, CO2 generally costs less than hydrocarbon miscible solvents in the United States(3). There is a limited supply of CO2 in Saskatchewan and Alberta for EOR purposes. However, CO2 can be imported from the U.S. to be implemented in miscible displacement projects in Canada. Current industry interest in CO2 miscible flooding is high, as evidenced by the growing level of activity in field testing(4) and increasing production from this method worldwide.

Concealed air leak associated with large tidal volumes in partial liquid ventilation.

Current ventilator strategies aim at maintaining an open lung and limiting both peak inspiratory pressures and tidal volumes to avoid alveolar distension. Perfluorocarbons, as well as being excellent solvents for oxygen and carbon dioxide, have the unique properties of being able to recruit dependent lung regions and improve pulmonary mechanics. Optimal ventilator strategies for partial liquid ventilation (PLV) have not yet been clearly defined. In the surfactant-depleted rabbit model, an approach involving a large tidal volume (VT) (15 ml/kg) and lung filled to FRC with perfluorocarbon (PFC) was compared with strategies involving a moderate VT (9 ml/kg) and partially filled lung (6 ml/kg), a moderate VT (9 ml/kg) and lung filled to FRC with PFC, and a large VT (15 ml/kg) and partially filled lung (6 ml/kg). PEEP was maintained at 5 cm H2O except in the moderate VT, partial-filling group, in which a PEEP of 9 cm H2O was used to maintain the rabbits for the duration of the experiment. Oxygenation was satisfactory in all groups, and peak inspiratory pressures were not significantly different. However, five of the 13 animals in the large-VT, PFC-filled lung group died of a pneumothorax prior to completion of the experiment. Of the eight animals in this group surviving the experiment, two had radiographic evidence of pneumothoraces, with an additional three animals having autopsy evidence of air leak. Of the 22 animals in the other groups, all survived with the exception of a single rabbit in the large VT, partial-filling group, which had both radiographic and autopsy evidence of air leak. We conclude that there is a significant risk of barotrauma in a PLV strategy in which a large VT is used in association with a lung filled to FRC with perfluorocarbon. Adequate gas exchange can be achieved with alternative ventilation strategies in combination with PLV.

Solvatochromic characterization of the liquid phase in liquid—supercritical CO2 mixtures

AbstractThe solvatochromic dye phenol blue (N, N‐dimethylindoaniline) is used to characterize the solvent strength (polarity) of the saturated liquid phase in a series of solvent–carbon dioxide binary mixtures. Data were obtained at 35 and 55°C and at pressures up to ∼70 bar. Five solvents were investigated—acetone, cyclohexane, methanol, THF, and toluene. The polarity of the liquid phase decreases significantly with increasing pressure due to the increasing carbon dioxide content of this phase at equilibrium. For example, the polarity of acetone saturated with carbon dioxide at 35°C and ∼60 bar is equivalent to the polarity of pure cyclohexane at ambient pressure. The local environment about the dye is significantly richer in the polar liquid component than the bulk composition would indicate. The degree of enrichment reflects concentration effects at low pressure, and both concentration and pressure effects at high pressure where the mixtures are highly compressible. The NRTL model of Renon and Prausnitz is able to predict these local compositions with reasonable accuracy except at CO2‐rich conditions where compressibility effects are important.

Mass transfer in a countercurrent spray column at supercritical conditions

Mass transfer characteristics were determined for countercurrent extraction in a spray column with supercritical carbon dioxide solvent. Feeds were 5 and 10 vol% aqueous ethanol and iso-propanol at 25–65°C and 6.89–15.51 MPa. Results obtained in a 22 mm inside diameter column showed enhanced extraction efficiencies over comparable liquid-liquid systems. The models based on conventional liquid-liquid extraction were modified to correlate overall mass transfer coefficients for supercritical conditions. The constant value in the model of measuring individual mass transfer coefficient was influenced by the density of carbon dioxide and the system used.

RISM Integral Equation Study of Local Solvation Behavior of Naphthalene in Supercritical Carbon Dioxide

We applied the extended RISM integral equation theory to investigate the local solvation behavior of a naphthalene solute in a supercritical carbon dioxide solvent. A ten-site model potential for naphthalene (by Sediawan, Gupta, and Mclaughlin) and a three-site potential for carbon dioxide (by Murthy, Singer, and McDonald) were used to elucidate local orientation of the carbon dioxide solvent molecules around the solute. Important physical effects of the quadrupole of carbon dioxide as well as molecular geometry of naphthalene are, therefore, taken into account. To gain insight into preferential orientation of carbon dioxide around a naphthalene molecule in a supercritical carbon dioxide solvent, we used a novel supermolecule approach by which potential of mean force surfaces of a carbon dioxide−naphthalene pair were calculated. Effects of molecular shape of solute and solute−solvent attractive interactions on solute partial molar volume were examined.

A Laboratory Miscible Displacement Study For the Recovery of Saskatchewan's Crude Oil

Abstract Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limits of production under waterflooding, leaving 70 to 80 per cent of the initial oil-in-place (IOIP) in the reservoirs. The miscible displacement process using carbon dioxide, hydrocarbon, and other gases as solvents appears to be one of the most promising methods for these reservoirs. A laboratory study was conducted to evaluate the applicability of various solvents for the recovery of oil from a southeast Saskatchewan reservoir. The physical, chemical, and phase behavior (PVT) properties of wellhead and reservoir fluids were determined. Slim tube displacement tests were conducted between the reconstituted Pinto reservoir fluid and three solvents: carbon dioxide, ethane, and wellhead gas from the Steelman gas plant. Tests were carried out at various pressures and the reservoir temperature of 56 °C. The minimum miscibility pressures (MMPs) determined for the above three systems demonstrated that the MMP for ethane was much lower than that for carbon dioxide, which was lower than that for Steelman gas. The MMPs for ethane and carbon dioxide were below the reservoir fracture pressure. Introduction Most of Saskatchewan's light and medium oil (LMO) reservoirs have reached their economic limit of production under current technology. Primary and secondary methods together recover about 21 % of the initial oil-in-place (IOIP). Although the remaining light and medium oil-in-place in Saskatchewan is 1135 million cubic metres(1), the remaining producible reserves under primary and secondary recovery are only 65 million cubic metres. Medium oil in southwest Saskatchewan has not been considered here, but if it is included then up to 17 million cubic metres of additional oil (through primary and secondary recovery) may be recovered(1). The development of tertiary recovery techniques, therefore, is essential to increase the production life of these reservoirs and to maintain current production. Application of the miscible oil displacement process using CO2, hydrocarbon, or other gases as injection fluids is expected to increase the LMO reserves threefold and extend the production life of these pools by two decades(2). Miscible flooding with carbon dioxide or hydrocarbon solvents is considered to be one of the most effective enhanced oil recovery (EOR) processes applicable to LMO reservoirs. Current industry interest in CO2 miscible flooding is high, as evidenced by the level of activity in field testing(3,4) and CO2 source development in the United States. CO2 has a viscosity similar to hydrocarbon miscible solvents. Both types of solvent affect the volumetric sweep out because of unfavorable viscosity ratio. However, CO2 density is similar to that of oil. Therefore, CO2 floods minimize gravity segregation compared with the hydrocarbon solvents. In addition, the supply cost of CO2 is generally more favorable than hydrocarbon miscible solvents in the United States(3). However, there is no cheap source of CO2 in Canada yet. It should be noted that, since the world oil price collapse in 1986, the number of EOR projects in the United States has decreased markedly from 512 in 1986 to 366 in 1988, whereas the number of CO2 miscible projects has increased from 38 in 1986 to 49 in 1988, a 29% rise during these two years(4).

Kopplung einer einfachen sub- oder superkritischen Extraktion mit normalen Analysenmethoden

Substances can be extracted in pressure resistant hydraulic tubes with carbon dioxide, ammonia, or other compressed solvents and the extracts can be analyzed by conventional methods. The elevated pressure is indicated by a small gauge and after extraction reduced to normal pressure by a small capillary, which can be connected to the bulk volume of a syringe in order to collect the extract in this syringe. The extract is then transferred into an analytical apparatus, e.g. a gaschromatograph. The method is demonstrated by the gas chromatogram of clove oil from different preparation methods.

Eagle Lake Viking Miscible Displacement Laboratory Studies

Abstract A series of displacement tests was conducted on preserved reservoir core plugs to assist in estimating waterflood and miscible flood performance potential for the Eagle Lake Viking Reservoir in Saskatchewan. The reservoir rock is a heterogeneous sandstone, comprising thin interbeds of sand and shale with the sand lenses containing widely different amounts of smectite clays. To assess miscible recovery efficiencies in this complex sand, displacement tests were conducted with first-contact miscible propane and carbon dioxide solvents. Using standard waterflood relative permeabilities and a dual permeability model, all miscible displacements were matched by simulation with a Todd-Longstaff-type miscible mixing model. Introduction The Eagle Lake Viking Reservoir is located in southwest Saskatchewan as shown in Figure 1. It is a sandstone formation of Mississippian age. Discovered in 1957, one half of this 12.7 million m3 field was put on waterflood ten years later in 1967. Unfortunately, because of its low permeability, it has been possible to only inject 0.14 pore volumes of water in the subsequent 2O-year period. As a result, in the thirty years since discovery, only 15% of original oil- in-place has been produced. The cause of poor performance appears to be due to the abundant shales and days. Thin section analyses, however, show that bands of higher permeability are also found throughout the pay zone and often exist as thin to very thin sand stringers which are relatively free of days. In 1985, operations engineers decided that the half of the field which was still under primary depletion should be placed under some form of secondary recovery. Because the waterflood had been less than satisfactory, it was decided to examine miscible flooding as a potential alternative. Hence, a core flooding study using carbon dioxide as a flooding agent was designed with the following objectives;quantify carbon dioxide oil displacement efficiency;compare carbon dioxide with waterflood oil recovery; andcompare the potentially complex carbon dioxide miscible oil displacement process with a more conventional hydrocarbon solvent process using propane. To extend the initial objectives for reservoir design purposes, a fourth objective was to:obtain waterflood and miscible displacement parameters for use in numerical simulation of field applications, Reservoir Fluid and CO2 Miscibility Pressure The live oil used in this study was obtained by recombining separator gas and liquid to a bubble point pressure of 6509 kPa at 22 °C. Fluid densities were measured as a function of pressure at the reservoir temperatures of 22 °C and were matched with the Peng-Robinson(l) equation-of-state. The reference 2 characterization procedure was used and viscosity predictions were made with the Jossi, Stiel and Thodos correlation. Table 1 illustrates the accuracy or this prediction which involved proprietary correlations for heavy end parameters. This set of density and viscosity data was used in the simulation study. The oil composition is also shown in Table 1. To determine the first-contact miscibility pressure for CO2, oil and CO2 were blended in a visual cell at 22 °C. These results and the Peng-Robinson equation-of-state matches are presented in Table 2.

Response of the rabbit lung as a criterion of safety for fluorocarbon breathing and blood substitutes.

From the first liquid breathing experiments until now, the lung, not surprisingly, has played a central role in the evolution of fluorocarbon blood substitutes. The first breathable fluorocarbon, a mixture of F-alkylfurans(FC75), bp 102 degrees C, while a poor solvent for the lung's lining and a good solvent for oxygen and carbon dioxide, proved to cause a characteristic gas/vapor microbubble embolism following intravenous administration as an emulsion. Higher boiling fluorocarbons, e.g. F-tributylamine (FC47), bp 174 degrees C, do not produce such gas-vapor emboli. However, intermediate boiling compounds such as F-decalin (PP5), bp 141 degrees C, produce lungs which, although they certainly appear not to contain microbubble emboli, do not collapse when the thorax is opened. Such hyperinflated non-collapsible lungs (HNCL) occur in the rabbit after the intravenous infusion of F-decalin emulsions as well as after the intratracheal infusion of F-decalin neat liquid. F-decalin induced HNCL retain their appearance and low specific gravity for many weeks, gradually returning toward normal after many months. F-methyl decalin, bp 165 degrees C, does not cause HNCL after intravascular or intratracheal administration. Fluorocarbons having boiling points between 140 degrees C and 165 degrees C are being tested in order to find a perfluorinate with the highest transpiration rate, and hence vapor pressure, compatible with an acceptable body dwell time. We have given fluorocarbons intratracheally to 75, intravenously to 221 and both intratracheally and intravenously to 8 rabbits. Free radical trapping agents, antineutrophil, antiinflammatory and other drugs have been administered without appreciable decrease of HNCL. Fluorocarbon critical solution temperature, lipid solubility, emulsifiability, and other physicochemical properties may mediate the pulmonary effect. One method of preventing and treating low dose F-decalin-induced HNCL in rabbits is described.

Phase equilibria of natural flavours and supercritical solvents

To study the possibility of separation of cineole from limonene (two of the main components of eucalyptus oil) we measured the vapour-liquid equilibrium of the systems containing either d-limonene or 1, 8-cineole in the supercritical solvents carbon dioxide (at 318.2 and 323.2 K) and an azeotropic mixture of ethane+carbon dioxide (at 303.2 K). The results obtained show that both solutes have similar solubilities in the supercritical solvents studied here.

C Characterization For Fluid Properties Predictions

Abstract A simple iterative procedure has been developed for characterizing the C7+; fraction of a hydrocarbon fluid with only one pseudocomponent. This procedure applies the Peneloux, Rauzy and Freze shifting factors and Watson's heavy end correlations to the PengRobinson equation of state. The procedure requires the following experimental data:bubble point pressure;compositions of gas and liquid resulting from a room condition flash of theservoir fluid;flashed gas-oil ratio, andreservoir fluid density at pressures above the bubble point pressure. When the equation of state is correctly tuned, the results match not only the reservoir fluid phase behaviour and fluid density, but also the phase behaviour of mixtures of reservoir fluid blended with hydrocarbon and carbon dioxide solvents. Introduction In order to predict the fluid properties and phase behaviour of hydrocarbon fluids, either in the presence or absence of solvents, people have been increasingly turning to equations of state (EOS). Unfortunately a great deal of misinformation about EOS usage has been created because of misunderstandings regarding their applications and limitations. As an example, it is sometimes believed that an EOS can be used to generate reservoir fluid properties such as density and viscosity without first tuning the EOS to laboratory data. This is not so. A reservoir fluid is a complex multicomponent mixture, the properties of which depend significantly upon the interaction of the various components. Because every reservoir oil has its own unique composition, these interactions vary from one oil to the next. Hence, experimental data is always required for the proper tuning of an equation of state. This data should include low-pressure gas and liquid analyses as opposed to high-pressure liquid analysis. As will be shown later, high-pressure liquid analyses are less accurate. In order to employ an EOS, one must characterize the C7+ fraction of the reservoir fluid. In this context, characterization is defined as the determination of the critical temperature, critical pressure, acentric factor and the interaction parameters. There are two approaches to this characterization:the application of a continuous component model(1–3); andthe grouping of components into one or more pseudocomponents(4–13). This report describes a technique which, using only one pseudocomponent, falls into the latter group. Two procedures are used for determining C7+ pseudocomponent parameters. In one procedure these parameters are continuously adjusted until the EOS can match measured equilibrium data, e.g. bubble point pressure, gas-oil ratio, density and equilibrium compositions. This is a very tedious task. A common alternative procedure involves the determination of these parameters using correlations based on one or more measureable property, such as specific gravity, or molecular weight, or normal boiling point of the pseudocomponent. Although many correlations exist(4–13), there is little instruction available regarding the over-all procedure from data acquisition through to tuned EOS. Furthermore, little exists in the literature regarding the need to ensure that all aspects of the tuning procedure are internally consistent.

Phase separation in alcohol/liquid carbon dioxide solvent systems facilitates critical point drying.

Carbon dioxide and methanol or ethanol, although miscible, form alcohol/CO2 solutions that do not easily mix with additional pure liquid CO2. If the CO2 inlet is situated at the top of a critical point drying apparatus chamber, pure CO2 will entirely displace the alcohol/CO2 phase (which is more dense) while keeping the chamber filled with liquid. This unexpected phenomenon is invaluable in critical point drying delicate biological tissues which remain continuously immersed, avoiding surface or convection currents. By providing an objective criterion for intermediate solvent displacement, the protocol also eliminates ambiguous 'flushing' steps.