Interface water-liquid carbon dioxide

The miscibility of water and hquid carbon dioxide is very poor and an intermediate solvent has to be used to allow the replacement of water by carbon dioxide. In a procedure initially developed to prepare representative samples for electron microscopy, water is replaced by ethanol through exchanges with alcoholic solutions of increasing concentration. The alcogel prepared by a final exchange with absolute ethanol (Fig. 3c) is introduced in a pressure vessel in which liquid CO2 is admitted and replaces ethanol in the gel. The C02-impregnated gel is compressed and heated above the critical point of CO2 (31.05°C, 73.8 bar). Release of pressure above the critical temperature allows CO2 to be extracted without the formation of any liquid-vapor interface and a dried aerogel is formed (Fig. 3d). 

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

Carbon dioxide is absorbed in water from a 25 per cent mixture in nitrogen. How will its absorption rate compare with that from a mixture containing 35 per cent carbon dioxide, 40 per cent hydrogen and 25 per cent nitrogen It may be assumed that the gas-film resistance is controlling, that the partial pressure of carbon dioxide, at the gas-liquid interface is negligible and that the two-lilm theory is applicable, with the gan film thickness the same in the two cases. 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

Thus, when deahng with gas transfer in aerobic fermentors, it is important to consider only the resistance at the gas-liquid interface, usually at the surface of gas bubbles. As the solubihty of oxygen in water is relatively low (cf. Section 6.2 and Table 6.1), we can neglect the gas-phase resistance when dealing with oxygen absorption into the aqueous media, and consider only the liquid film mass transfer coefficient Aj and the volumetric coefficient k a, which are practically equal to and K a, respectively. Although carbon dioxide is considerably more soluble in water than oxygen, we can also consider that the liquid film resistance will control the rate of carbon dioxide desorption from the aqueous media. 

Supercritical fluids are unique solvents and reaction media due to liquid like density and gas like viscosity. Diffusion is not limited by any interface. Under ambient conditions hydrocarbons and water are nearly unmiscible. Phase equilibrium changes significantly in the supercritical region of water (Tc = 647 K, pc = 22.1 MPa). Hydrocarbons and supercritical water become miscible at any ratio, whereas supercritical carbon dioxide and hydrocarbons still have a broad miscibility gap [4],... 

C Using solubility data of a gas in a solid, explain how you would determine the molar concentration of the gas in the solid at the solid-gas interface at a specified temperature. 14-32C Using Henry s constant data for a gas dissolved in a liquid, explain how you would determine the mole fraction of the gas dissolved in the liquid al the interface at a specified tempemture. 14-33C What is permeability How is the permeability of a gas in a solid related to the solubility of the gas in that solid 14-34 Determine the mole fraction of carbon dioxide (CO2) dissolved in water at the surface of water at 300 K. The mole fraction of CO in air is 0.005, and the local atmosphere pressure is 100 kPa. 

Phenylgermanic acid anhydride, (CgHsGeOjaO.—Equimolecular proportions of mercury diphenyl and germanium tetrachloride in dry xylene are heated in a sealed Pyrex bulb for two days, then diluted with dry ether and filtered. The solid residue is pure phenylmercuric chloride, and the filtrate is treated with benzene, and finally with water containing a few drops of ammonium hydroxide. The granular precipitate which separates at the liquid interface is removed and dried at 115° C. The anhydride is a white, fluffy, amorphous solid, with no definite melting-point, soluble in excess of alkali, and reprecipitated by carbon dioxide, insoluble in water and organic solvents. 

The yeast cell wall confers certain important properties from the point of view of brewing. Thus, some brewing yeasts rise to the surface of the fermenting wort towards the end of fermentation (top yeasts) while others sediment (bottom yeasts). This distinction is a reflection of differences in composition of the yeast cell wall, although the chemical nature of these differences is not known. The ability of top yeasts to accumulate at the liquid-air interface can be demonstrated in water [51]. Shaken in a very clean tube, the yeast persists so well at the interface that what appears to be a type of skin is visible at the meniscus. Bottom yeasts fail to form such a skin and this simple test is therefore valuable in practice for distinguishing between top and bottom yeasts. Other factors are undoubtedly involved in head-formation such as the transport of yeast clusters to the surface on the interfaces of carbon dioxide bubbles. Chains of cells or loosely-packed floes are particularly favoured by such flotation. 

The substitution/displacement of solvent molecules located within the solid matrix by dense CO2 is the basis for drying of highly porous materials. Supercritical carbon dioxide is not miscible with water, but completely miscible with most organic solvents and, therefore, very suitable for such a process. The specific volume of the C02-solvent mixture changes when the phase is changed from liquid to gas within the porous structure. In case part of the liquid remains in the structure, the surface tension at the solid-liquid interface creates capillary forces that destroy the structure, that is, delicate structures tend to break up and porous structures collapse. 

With very slow reactions (such as between carbon dioxide and water) the dissolved molecules migrate well into the body of the liquid before reaction occurs so that the overall absorption rate is not appreciably increased by the occurrence of the chemical reaction. In this case, the liquid film resistance is the controlling factor, the liquid at the interface can be assumed to be in equilibrium with the gas, and the rate of mass transfer is governed by the molecular CO2 concentration-gradient between the interface and the body of the liquid. At the other extreme are very rapid reactions (such as those of ammonia with strong acids) where the dissolved molecules migrate only a very short distance before reaction occurs. The... 

Similar MC calculations were used by Trout s group to study the carbon dioxide-liquid water interface at 220 K and 4 MPa near the phase boundary of a carbon dioxide hydrate (273 K and 4MPa). Nucleation was achieved by seeding the system with a cluster of carbon dioxide hydrate. It was found that a small cluster with diameter <9.6 A dissolved into the solution readily. A hydrate crystal started to grow, however, when a hydrate cluster twice that size (19.3 A) was implanted into the system. The crystal eventually spanned the whole system (Figure 22). Thus the critical nucleus size for hydrate nucleation is estimated to be about 19 A consisting of approximately 200 water molecules. This is a considerably smaller number than that estimated from the local harmonic model of around 600 molecules. The theoretical results refuted the labile cluster hypothesis.This hypothesis speculates the agglomeration... 

The performances of the Nafion and the hydroxide-ion-conduction membrane cells were compared for two cathode solutions (1) D1 water (18 Mil cm) saturated with carbon dioxide by continuous bubbling of the pure gas at a pressure of 1 atm and (2) 1 M sodium bicarbonate solution. As shown in Figure 10.13, the cell voltages with the hydroxide-ion membrane were found to decrease substantially upon changing the electrolyte from C02-saturated D1 water to 1 M sodium bicarbonate. However, the performance of the Nalion-based cell did not alter significantly for the same change in the cathode solution. This phenomenon can be ascribed to differences in ionic contact at the electrode/manhrane interface, as hydroxide-ion membrane is cross-linked and could not be hot pressed to improve the ionic contact. Thus, the use of a liquid electrolyte could substantially improve the ionic contact between the catalyst layer and the hydroxide-ion-conduction manbrane. 

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