View cell

Although any of the designs mentioned above will provide the location of phase boundaries (versus temperature and pressure), it is also important to know the compositions of the two phases in equilibrium. Note that while tie lines (lines connecting phases in equilibrium on T-x or p-x diagrams) are horizontal for simple binary mixtures, this is not true for phase separation in multicomponent systems (most notably polymer-fluid systems where the polymer sample contains chains of various lengths). Consequently, ports which allow withdrawal of samples following phase separation and equilibration are an important feature of view cells. Such ports also allow for the measurement of partition coefficients of solutes between, for example, aqueous and CO2 phases. 

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Samples can be removed for analysis, phase volumes can be measured to determine mixture composition and molar volumes (70), and phase boundaries can be measured. Many different configurations of view cells have been proposed. Some are capable of pressures ia excess of 100 MPa (14,500 psi). The cell coateats may be viewed safely through the sapphire wiadow by use of a mirror, video camera, or borescope. 

Pig. 9. Schematic drawiag of a typical stainless steel variable-volume view cell having a movable internal piston. The outside diameter of the cell is 5.08 cm ... 

Palsson B. The challenges of in silico biology - moving from a reductionist paradigm to one that views cells as systems will necessitate changes in both the culture and the practice of research. Nature Biotechnol 2000 18 1147-1150. 

Figure 10.8 The monomers are polymerized inside a high-pressure view cell (a). After reaction, the scC02 is vented and the monolith removed from the cell (b)...

Fig. 2. Diagrammatic summary of the method of mounting and viewing cells in culture dishes on an upright microscope.

The view-cell reactor is made of titanium and has two sapphire windows, a gas inlet valve and an outlet valve, as shown in Figure 3. The view cell is interfaced with a pressure transducer, a thermocouple, and a pressure relief valve. The pressure and temperature are computer-monitored during the reaction. 0.6 ml of 50 wt% H2O2/H2O (10.41 mmoles), 0.20ml of pyridine (2.47 mmoles), or some other base, was dissolved in 5 ml of acetonitrile or methanol, and was added to the reactor. 2.2 ml of supercritical CO2 was charged after lOOmg of propylene (2.38 mmoles) had been added to the reactor. The reactor was heated with a band heater at 40 - 70°C for 3, 6, 12, and 24 hr reaction periods. Following a batch conversion experiment, the amounts of products formed were determined by GC and GC/MS. 

Figure 3. The view-cell reactor and diagram of system Acknowledgements...

The regioselective course of the Diels-Alder reaction in supercritical carbon dioxide was investigated. The analysis failed to confirm the previously reported dramatic effect of reaction conditions on Diels-Alder regiochemistry. The results highlighted the importance of verifying phase behavior when sampling CO2 reaction mixtures and the utility of a view-cell reactor that allows direct monitoring of phase behavior (Ren-slo et al., 1997). 

New solvents should be examined for compatibility with photochemistry. Laser light scattering is used with a variable-volume view cell in an almost fully automated setup for accurately determining the phase behavior of pure or mixed fluids. The automated light-scattering techniqne yields good data, is relatively quick, and is non-labor inten-... 

Measuring P-V-T Phase Behavior with a Variable Volume View Cell 141... 

Fig. 17. Cross-section of the view cell at the spectroscopic level. The IR beam is directed either through the ATR crystal (bottom, solid lines) or through the transmission windows (upper part, dashed lines). The four mirrors are mounted on a motor-driven mobile attachment (76).

The danger of unnoticed phase changes occurring in the saturators which can be monitored by using a view cell ... 

Figure 7. Comparison of the phase in the mixtures of methanol-CO, (A) and water-CO, (B) using a view cell at 60 °C and 34.0 MPa.

The phase equilibrium of the binary system stearyl alcohol/C02 was measured in a high pressure view cell (own design) according to the synthetic method by Teipel et al. [6] and presented in Fig. 2. The curves represent the border between the homogeneous and two phase region above the curve a homogeneous solution is present, below two phases exist. The measurements were carried out for different concentrations of stearyl alcohol in CO2 represented by the different curves. It is seen that 2.1 wt.% of stearyl alcohol are complete soluble in SC-CO2 at 333 K and 20 MPa. 

The equipment used in all experiments consisted basically of a C02 cylinder, a 20-mL view cell with three sapphire windows for visual observations, an absolute pressure transducer (Smar LD 301) with a precision of 0.012 MPa, a portable programmer (Smar HT 201) for pressure data acquisition, and a syringe pump (ISCO 260D). The equilibrium cell contained a movable piston, which permitted pressure control inside the cell. Figure 1 presents schematic diagram of the experimental unit. 

Solid-11quid-gas (SLG) equilibrium temperatures and pressures were measured in a constant-volume view cell. A known amount of porphyrin was loaded into the cell, which was then attached to the solvent delivery system and heated to the desired operating temperature. Once thermal equilibrium was obtained, solvent was metered into the cell until the pressure within the cell reached the SLG pressure. Equilibrium was obtained when the pressure stablized and all three phases could be observed. This pressure and the corresponding temperature were recorded as one point on the SLG equilibrium line for the binary mixture. Additional points were obtained by setting a new temperature and repeating the procedure. 

Bubble points for gas-liquid equilibrium were measured at constant temperature by observing the pressure at which the equilibrium gas phase disappeared upon injection of small amounts of solvent into the view cell. The equilibrium composition of the liquid phase was obtained from the known composition in the cell. Other pressures and corresponding compositions at this temperature were obtained by repeating the procedure for different porphyrin loadings. 

Dew points were also measured using the procedure described above, except that the disappearance of the liquid phase was observed as solvent was added to the view cell. Dew point measurements were limited by low porphyrin concentrations in the gas phase, which required loading very small amounts of porphyrin into the cell. 

The accuracy of the pressure and temperature measurements was verified by measuring the vapor pressure curves and critical points for pentane and for toluene. Vapor pressures were measured by observing the formation of a liquid phase as pentane or toluene was injected into the constant-volume view cell under isothermal conditions. The observation of critical opalescence was used to determine the critical point. The measured vapor pressures and critical points are given in Table I. Vapor pressures deviate from... 

D. Steiner, K. Dodierty, and R, Carroll. Golgi/granule processing of peptide hormone and neuropeptide precursors a mini re view./. Cell Biochem. 24 121 130 (1984). 

Figure 8.5. Schematic diagram of pilot-scale supercritical CO2 processing system. 1. anhydrous milk fat (AMF) flow meter 2. AMF pump 3. CO2 pump 4. CO2 loop 5. flow loop 6. entrainment vessels 7. view cell 8. separator 1 9. separator 2 10. separator 3 11. separator 4 12. CO2 meter 13. dry test meter (Reproduced with permission from Bhaskar et al., 1993).

Phase equilibria and pressure-temperature coordinates of critical points in ternary systems were taken with a high-pressure apparatus based on a thermostated view cell equipped with two liquid flow loops which has been described in detail elsewhere [3]. The loops feed a sample valve which takes small amounts of probes for gas-chromatographic analysis. In addition to temperature, pressure and composition data, the densities of the coexisting liquid phases are measured with a vibrating tube densimeter. Critical points were determined by visual oberservation of the critical opalescence. 

Samples were taken from the precipitated bottom phase, the solvent phase, and the C02 vapour phase. The sampling procedure for the bottom phase depended on whether the precipitate was a liquid (triglyceride oil), solid (lecithin) or a mixture of both components. Samples of liquid bottom phase were withdrawn from valves V3 and V16 positioned at the base of the view cell, with valve VI shut. Valves V5, V6 and V2 were opened to ensure constant pressure inside the view cell during sampling. Samples of solid lecithin were recovered by first removing the solvent phase under pressure as above, followed by depressurization, and then solvent washing of the apparatus. Lecithin was retained on a filter placed above valve V3. For mixtures of lecithin and Soya oil, the two sampling procedures were combined. The Soya oil was first removed under pressure, followed by the solvent phase. The apparatus was then depressurized and solvent washed. Samples of the solvent phase at pressure were also taken via valve V4 and collection vessel CV3, to measure the C02 content, and the concentration of solute still in solution. 

Tubing connects the chamber and a high-pressure view cell, which is thermostated to the same temperature as the chamber. In order to observe swelling behavior, some of the solid material is also placed into the view cell. It thus is in contact with the gas at the same pressure and temperature as the material in the suspended vial. The interior of the view cell may be observed through a video camera. The video images are scanned into a PC and analyzed for changes in size. 

The entire system is monitored by PC. Pressure and temperature within the chamber and the view cell as well as the weight of the sample are recorded continuously. 

The sample is filled into the vial which then is attached to the suspension device. Then the high-pressure chamber is firmly closed and heated or cooled to system temperature. More of the sample material is placed within the view cell. The apparatus then is evacuated to remove air and to obtain a reliable value for initial mass. The thermostated dense gas is charged into the chamber and the view cell until system pressure is attained. Pressure build-up only takes a few seconds because of the small volume of the chamber. The weight of the sample rises as gas dissolves in or is being adsorbed to the sample and assymptotically nears its equilibrium value. At the same time, the dissolution of the gas in the solid leads to volume changes, a process observed with the sample particles enclosed in the view cell. Their size is recorded with the camera and evaluated with the help of a PC. The apparatus is evacuated again at the end of each experiment to remove the entire gas. 

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