Critical Temperature Of Carbon Dioxide Research Articles

A novel self-condensing transcritical CO2 power cycle with a vortex tube: Thermoeconomic assessment study and comparison

The low critical temperature of carbon dioxide (CO2) requires low-temperature heat sinks for condensation, which limits the application of the transcritical carbon dioxide (tCO2) power cycle system. This paper proposes a novel self-condensing tCO2 power cycle system with a vortex tube, which uses the vortex tube to achieve CO2 condensation without an external low-temperature heat sink. Firstly, the detailed thermodynamic and economic models of the proposed tCO2 power cycle system are established and verified. Then, the effects of decision variables on the thermoeconomic performances of the system are studied through parametric analysis. Finally, the performances of the self-condensing tCO2 power cycle system and the simple regenerative supercritical carbon dioxide (sCO2) power cycle are optimized and compared by single-objective and multi-objective optimizations. The single-objective optimization results reveal that the optimal thermal efficiency, exergy efficiency, and the levelized cost of electricity of the self-condensing tCO2 power cycle system are 41.42%, 69.01%, and 0.0739 $/(kW·h), which are 3.06%pt (percentage point) higher, 3.89%pt higher and 3.90% lower than those of the simple regenerative sCO2 power cycle system. The multi-objective optimization results show that compared with the simple regenerative sCO2 power cycle system, the self-condensing tCO2 power cycle system improves the thermal and exergy efficiencies by 1.81%pt and 2.41%pt at the expense of 3.37% higher in the levelized cost of electricity. Exergy destruction analysis reveals that the high temperature recuperator has the largest exergy destruction, which should be improved first.

Thermal efficiency gains enabled by using CO2 mixtures in supercritical power cycles

The present paper explores the utilisation of dopants to increase the critical temperature of Carbon Dioxide (sCO2) as a solution towards maintaining the high thermal efficiencies of sCO2 cycles even when ambient temperatures compromise their feasibility. To this end, the impact of adopting CO2-based mixtures on the performance of power blocks representative of Concentrated Solar Power plants is explored, considering two possible dopants: hexafluorobenzene (C6F6) and titanium tetrachloride (TiCl4). The analysis is applied to a well-known cycle -Recuperated Rankine- and a less common layout -Precompression-. The latter is found capable of fully exploiting the interesting features of these non-conventional working fluids, enabling thermal efficiencies up to 2.3% higher than the simple recuperative configuration. Different scenarios for maximum cycle pressure (250–300 bar), turbine inlet temperature (550–700 °C) and working fluid composition (10–25% molar fraction of dopant) are considered. The results in this work show that CO2-blends with 15–25%(v) of the cited dopants enable efficiencies well in excess of 50% for minimum cycle temperatures as high as 50 °C. To verify this potential gain, the most representative pure sCO2 cycles have been optimised at two minimum cycle temperatures (32 °C and 50°C), proving the superiority of the proposed blended technology in high ambient temperature applications.

Evaluation of the property methods for pure and mixture of CO2 for power cycles analysis

The use of a binary cycle coupled to the complete reinjection of non-condensable gases could provide a valid answer to the improvement of the sustainability of geothermal plants. In recent years, the interest in the use of CO2 as a working fluid in transcritical cycles has increased. However, the low critical temperature of carbon dioxide (≈30 °C) requires the cooling cycle, temperatures below 15 °C, which are not always available. In this work, to overcome this limitation and obtain a higher critical temperature and a lower maximum pressure for more flexible applications of transcritical binary cycles, the possibility of using a second component, mixed with CO2, has been evaluated. For this purpose, the following fluids have been proposed: R1234yf, R1234ze(E), n-butane, n-hexane, n-pentane, and propane, with a minimum considered carbon dioxide molar content of 60%. To carry out a cycle analysis, the knowledge of the thermodynamic properties of CO2-mixtures is fundamental; however, suitable equations of state under the appropriate conditions for these blends have not been clearly defined yet. Therefore, in the first part of this paper, different EoS for predicting thermodynamic properties of pure CO2 and CO2-based mixtures are analyzed and compared with reference data obtained from works published in the literature. However, because of the lack of experimental data of the selected blends, the values of density, enthalpy and entropy, obtained with the selected EoS, are compared with NIST REFPROP results. The EoS involved in the evaluation are cubic-type (PR, PR-Twu, PRSV, RK, SRK, GCEOS), Virial-type (LKP, BWRS), Helmholtz-type (SW), and SAFT-type (PC-SAFT). In a power cycle, the fluid works under different conditions, involving several possible states across the components. So the influence of the different EoS on each power cycle's key component for pure CO2 and two selected CO2-based mixtures has been investigated. The thermodynamic results show that the CO2-based mixtures, in a transcritical configuration, can achieve efficiencies higher than the sCO2 power cycle.

Performance analysis of a self-condensation compressed carbon dioxide energy storage system with vortex tube

The development of low-carbon society needs a high renewable share in energy sector. Energy storage technologies play a crucial role to handle the relevant issues caused by the fluctuant renewables penetration. Compressed carbon dioxide energy storage is recognized as a promising technology thanks to the favorable thermophysical properties of carbon dioxide. In order to realize the condensation process of low pressure carbon dioxide without the support of extra cold source, a self-condensation compressed carbon dioxide energy storage system with vortex tube is proposed. The vortex tube acts as the condensation device in such system. Results indicate that the condensation process is more sufficient when the vortex tube inlet temperature close to the critical temperature of carbon dioxide. Under design condition, the system energy density, roundtrip electrical and exergy efficiencies are 5.43 kWh/m3, 53.45% and 61.83%, respectively. Meanwhile, the pressure increasement of low pressure storage tank has a positive effect on roundtrip efficiency, but a negative effect on energy density. The pressure increasement of high pressure storage tank has positive effect on both energy density and roundtrip efficiency. Higher compressor isentropic efficiency results in higher roundtrip efficiency and reduced energy density, whereas higher turbine isentropic efficiency implies raised roundtrip efficiency and energy density. The increase of ambient temperature can enhance both the roundtrip efficiency and energy density.

Supercritical carbon dioxide as a new working medium for pneumatic launch: A theoretical study

Compared with the conventionally gaseous or liquid working media, the specific internal energy of supercritical carbon dioxide (SCD) is higher at the same temperature and pressure, and the critical temperature of carbon dioxide is close to room temperature, making SCD a potential new working medium for pneumatic launch. To analyze the feasibility of this conception, an analytical model of a pneumatic catapult is established on basis of the conservations of mass and energy. The model consists of a high-pressure chamber and a low-pressure chamber connected by multiple valves, and there is a movable piston in the low-pressure chamber that can push an aircraft to accelerate. The effects of the launch readiness state of SCD in the high-pressure chamber, the initial volume of the low-pressure chamber and the valve control on the movement of the aircraft are analyzed. It is found that there is a restrictive relation between the temperature and pressure of the launch readiness state of SCD, i.e., there is a maximum allowable launch readiness pressure when the launch readiness temperature is fixed. If this restrictive relation is not satisfied, the working medium in the low-pressure chamber will drop to its triple point within a few milliseconds, leading to a launch failure. Owing to this restrictive relation, there is an optimal launch readiness state of SCD with the highest working capacity for any allowable launch readiness temperature. The pressure of the low-pressure chamber will decrease significantly as the initial volume increases, leading to a decreased acceleration of the aircraft. The acceleration can be controlled below a critical value by a designed sequential blasting technique of multiple valves. The calculated results show that a 500 kg aircraft can be accelerated from 0 to 58 m/s in 0.9 s with 36 kg of carbon dioxide. This research provides a new technique for the controllable cold launch of an aircraft.

Density, Speed of Sound, Compressibility, and Excess Properties of Carbon Dioxide + n-Dodecane Binary Mixtures from 10 to 70 MPa

Density and speed of sound of carbon dioxide + n-dodecane binary mixtures were measured at pressures from 10 to 70 MPa and at two temperatures around the critical temperature of carbon dioxide (303...

Heat rejection pressure optimization for a carbon dioxide split system: An experimental study

Recent studies indicate carbon dioxide (R744) as a valid alternative to classical substances such as HFCs used in vapour compression plants. However a transcritical refrigeration cycle is needed because the critical temperature of carbon dioxide is usually near the ambient temperature. Consequently the carbon dioxide refrigerator performances are significantly influenced by the heat rejection pressure. In this paper an experimental investigation on working optimization for a “split-system” to cool air in residential applications is presented: by varying the heat rejection pressure an optimum working condition has been found at different ambient temperatures. Furthermore a simplified model to predict the optimum heat rejection pressure is shown and a comparison with experimental results is carried out. Both the model validation and the experimental results suggest that the heat rejection pressure optimization is an convenient method to improve the performance of a carbon dioxide split system. Finally an algorithm based on the aforementioned model has been proposed in order to control an electronic back pressure valve by means of a PLC.

Phase equilibria of alcohols in supercritical fluids: Part I. The effect of the position of the hydroxyl group for linear C 8 alcohols in supercritical carbon dioxide

The ability to model and predict the behaviour of high-pressure alcohol and carbon dioxide mixtures is important for industrial purposes. The phase equilibria behaviour of four 8-carbon alcohols in supercritical carbon dioxide are measured to determine the effect of the hydroxyl group position on alcohol solubility. Experimental bubble- and dew point data are generated on a high pressure phase equilibrium cell for the systems 1-octanol, 2-octanol, 3-octanol and 4-octanol in supercritical carbon dioxide between 35 °C and 75 °C. 1-Octanol is shown to be the least soluble and, at 35 °C, exhibits a phase transition pressure 85 bar higher than that of 2-octanol. 1-Octanol also exhibits a temperature inversion near the critical temperature of carbon dioxide in the mixture critical region. 2-Octanol possesses marginally higher phase transition pressures than 3-octanol which, in turn, possesses marginally higher phase transition pressures than 4-octanol. This difference in phase equilibria is believed to result from a difference in polarity. Shifting the hydroxyl group from the first to the second carbon atom causes a large decrease in polarity and increase in solubility. Further movements toward the molecule centre result in progressively smaller polarity reductions and solubility increases, producing phase boundaries that coincide or differ minimally.

Molecular Simulation of the High-Pressure Phase Equilibrium of the System Carbon Dioxide−Methanol−Water

The high-pressure phase equilibrium of the system carbon dioxide−methanol−water was studied by molecular simulation at temperatures near the critical temperature of carbon dioxide. The system was modeled by multisite Lennard-Jones plus Coulomb intermolecular potentials with common combining rules for unlike site interactions. Good agreement was observed between experimental and simulated phase equilibrium data. Reasonably accurate predictions were obtained for the two-phase liquid−liquid and three-phase liquid−liquid−vapor coexistences.

Solubilities of Essential Oil Components of Orange in Supercritical Carbon Dioxide

The solubilities of pure limonene and linalool in compressed carbon dioxide have been measured using a flow apparatus at 318.2 K and 328.2 K and pressures ranging from 69 bar to 111 bar. The solubilities were successfully modeled using equations of state (Peng−Robinson, Soave−Redlich−Kwong, 3P1T, Dohrn and Prausnitz nopolar) and a semiempirical equation (Chrastill model). The generalized parameters for the Peng−Robinson EOS were obtained for each system. These parameters were independent of temperature, and they reproduce successfully all data available in the literature. The results show that the solubility of limonene in supercritical carbon dioxide was bigger than the solubility of linalool. The behavior of the systems was discussed by estimation of the best conditions of supercritical extraction. Those conditions were elevated pressure and a temperature near the critical temperature of carbon dioxide.

High-pressure multiphase behaviour of ternary systems carbon dioxide–water–polar solvent: review and modeling with the Peng–Robinson equation of state

Mixtures of water and a hydrophilic organic solvent, e.g. an alcohol, a ketone, or a carboxylic acid, reveal a complex phase behaviour when pressurized with near critical carbon dioxide at temperatures near the critical temperature of carbon dioxide. Although water and the hydrocarbonaceous solvent are completely miscible, by pressurization with carbon dioxide a liquid–liquid phase split is observed, resulting in a ternary liquid–liquid–vapor equilibrium. In some cases even a four-phase equilibrium is observed. Such equilibria are discussed for systems with the organic solvents methanol, ethanol, 1-propanol, 2-propanol, acetone and propionic acid. That phase behaviour is modelled using the Peng–Robinson equation of state with several mixing rules. A short description of the mathematical tools for such phase equilibrium calculations is given. From pure component and binary data alone, the calculations usually result in a qualitative agreement with the experimental data, i.e. typical effects like for example the existence of ternary critical endpoint lines and tricritical points can be predicted. However, in most cases the quantitative agreement with experimental results is poor. It can be considerably improved by fitting some interaction parameters to ternary high-pressure three-phase equilibrium data.

Thermodynamic Prospects of Alternative Refrigerants as Solvents for Supercritical Fluid Extraction

Wide-range thermodynamic formulations for four hydrofluorocarbons (R32, R125, R134a, and R152a) and two hydrochlorofluorocarbons (R22 and R123) are employed to derive the properties relevant to the use of these alternative refrigerants as solvents in supercritical fluid extraction (SFE). The calculated solubility parameters indicate that, in the high-pressure, low-compressibility region, the solvating powers of all the six refrigerants are superior to that of carbon dioxide. Regions of near-critical enhancement in compressibility are identified in carbon dioxide and in the refrigerants. A simple thermodynamic analysis suggests the possibility to employ the compressibility enhancement in alternative refrigerants to boost the selectivity of extraction of thermally stable, nonvolatile, environmentally sensitive analytes. In SFE with carbon dioxide, this mode of operation is not practical because of insufficient volatility of such analytes at temperatures near the critical temperature of carbon dioxide.

Modelling of vapour-liquid-liquid equilibria of CO 2 -crude oil mixtures

Abstract At temperatures close to the critical temperature of carbon dioxide, some crude oils form three phases when mixed with CO 2 . The objective of this work was to investigate whether the three-phase behaviour of CO 2 -crude oil mixtures can be predicted with the Soave-Redlich-Kwong equation of state. In the description of the C 7+ -fraction, a previously described characterization method was applied. In this characterization procedure, the critical properties and the acentric factor of single carbon number groups within the plus fraction are estimated on the basis of measurable physical properties. By comparison of experimental and calculated data, it is shown that no, or very little tuning of the EOS parameters is necessary to obtain representative predictions of three-phase regions. In addition it is demonstrated how too severe tuning leads to very erroneous results in the simulation of phase diagrams. Finally, it is shown how the described method fails to predict the three-phase region of a very heavy oil with a small three-phase region at the given temperature.

Lipase Reactions in Supercritical Fluids

The supercritical fluids (SCF) have several excellent solvent properties such as low viscosity, high diffusivity and high dependence of solubility on pressure and temperature. The critical temperature of carbon dioxide is 304.1 K, low enough to treat thermally labile biomaterials without decomposition, and CO2 is non-toxic to human beings. Supercritical carbon dioxide (SCCO2) have been used in extraction of flavor substance from hop and other flavors and also in removal of undesirable substances such as caffeine in coffee beans and in removal of residual organic solvent in powdery medical raw materials.The interest in the applications of SCF is now enlarged from extraction to the other areas such as chemical and enzymatic reactions, material processings, formation of fine particles etc. Most of the enzymes are stable in anhydrous SCCO2, consequently the lipase reactions in SCF have been studied in the examples of modification of the physical property of triacylglycerols, resolution of optical isomers, syntheses of flavor esters etc. In this review the characteristics of SCF as solvent of enzymatic reactions are summarized with the main results so far obtained for the lipase reactions in SCF.

Phase equilibria for the carbon dioxide-ethanol-water system with trace amounts of organic components.

Fluid-liquid and liquid-liquid equilibria were measured for carbon dioxide-ethanol-water mixtures containing trace amounts of ethyl acetate, acetaldehyde, 2-pentanol, 1-propanol, diacetyl, acetic acid and methanol at temperatures near the critical temperature of carbon dioxide. Large separation factors were observed in the extraction of all the trace components except for acetic acid and methanol from ethanol aqueous solutions using supercritical or liquid carbon dioxide as solvent. The separation factor increased with increasing solvent density.A simple empirical model was developed to correlate the experimental data assuming the infinite dilution state for the trace components. The model correlates the separation factors with fairly good agreement with the experimental data.

Phase equilibria in the binary system carbon disulfide-carbon dioxide

Reiff, W.-E., Roth, H. and Lucas, K., 1992. Phase equilibria in the binary system carbon disulfide-carbon dioxide. Fluid Phase Equilibria, 73: 323-338. Vapor-liquid and three-phase equilibrium compositions for the binary system carbon disulfide-carbon dioxide have been measured at eleven temperatures ranging from 273.15 to 473.15 K. At subcritical temperatures the pressure at each temperature ranged from the vapor pressure of carbon disulfide to the vapor pressure of carbon dioxide. Above the critical temperature of carbon dioxide the pressures were extended to the vicinity of the critical region at about 16 MPa. The data were used to investigate the correlative capability of two empirical equations of state as well as the predictive capability of a molecular model based on perturbation theory.