Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Subcritical gases

All of these equations predict that the amount adscffbed increases monotonically with increasing pressure. Experimental adsorption isotherms attmn a masdmum value in the amount adsorbed and then fall to zero. At the critical temperature of the gas, the maximum in the isotherm occurs at a pressure of about 10 bars and the zero occurs at a much higher pressure of several hundred bars. For subcritical gases, the maximum occurs at lower pressure and for supercritical gases the maximum occurs at higher pressure. [Pg.44]

Po for subcritical gases may increase, decrease, or change very little as the temperature is raised. [Pg.1036]

Subcritical gases, such as organic vapors, are much more highly soluble in polymers, and, consequently, the above behavior is observed only at very low pressures (or concentrations in polymers). As the penetrant pressure is raised, and the polymers are increasingly plasticized by the penetrant gas, the permeability, diffusion, and solubility... [Pg.1036]

Thus, at subcritical, gas-like conditions, there is no restricted diffusion. As the density increases beyond the critical point, the extent of pore-diffusion restriction increases. [Pg.5]

The adsorption isotherm of microporous adsorbents have often been modeled by the Dubinin-Astakhov model. In this approach, which is based on a so-called pore-filling of an adsorbent by a subcritical gas (T < T ), the total adsorbed density is expressed as ... [Pg.273]

Saim and Subramaniam [38] and Ginosar and Subramaniam [39] also found that the in situ extraction of the coke compounds by near-critical or supercritical reaction mixtures prevents pore plugging that otherwise occurs at subcritical (gas-like) conditions. Although the coke laydown decreased at supercritical (liquid-like) conditions, the isomerization rates were lower and deactivation rates were higher due to pore diffusion limitations in the liquid-like reaction mixtures. It was therefore concluded that near-critical reaction mixtures provide an optimum combination of solvent and transport properties that is better than either subcritical (gas-like) or dense supercritical (liquid-like) mixtures for maximizing the isomerization rates and for minimizing catalyst deactivation rates. These findings indicate that catalytic reactions which require liquid-like reaction media for coke extraction and heat removal, yet gas-like diffusivities for enhanced reaction rates, can benefit from the use of near-critical reaction media. [Pg.399]

Besides shear-induced phase transitions, Uquid-gas equilibria in confined phases have been extensively studied in recent years, both experimentally [149-155] and theoretically [156-163]. For example, using a volumetric technique, Thommes et al. [149,150] have measured the excess coverage T of SF in controlled pore glasses (CPG) as a function of T along subcritical isochoric paths in bulk SF. The experimental apparatus, fully described in Ref. 149, consists of a reference cell filled with pure SF and a sorption cell containing the adsorbent in thermodynamic equilibrium with bulk SF gas at a given initial temperature T,- of the fluid in both cells. The pressure P in the reference cell and the pressure difference AP between sorption and reference cell are measured. The density of (pure) SF at T, is calculated from P via an equation of state. [Pg.56]

Battelle (Seifert and Giesbrecht 1986) and BASF (Stock 1987) each conducted studies on exploding fuel jets, the former on natural gas and hydrogen jets, and the latter on propane jets. The methane and hydrogen jet program covered subcritical outflow velocities of 140, 190, and 250 m/s and orifice diameters of 10, 20, 50, and 100 mm. In the propane jet program, outflow conditions were supercritical with orifice diameters of 10, 20, 40, 60, and 80 mm. The jets were started and ignited after they had achieved steady-state conditions. [Pg.77]

Figure 4.6. Decay of peak overpressure with distance for ignited subcritical 10-mm diameter hydrogen gas jets at various velocities, Uq. A = mean value. Figure 4.6. Decay of peak overpressure with distance for ignited subcritical 10-mm diameter hydrogen gas jets at various velocities, Uq. A = mean value.
F2 = coefficient of subcritical flow, see Figure 7-29 T = relieving temperature of inlet gas or vapor, °R P = upstream relieving pressure, psia, = set pressure + allowable overpressure + atmospheric pressure, usually 14.7 psia), psia P2 = backpressure on valve, psia W = required flow through valve, lbs/hr V = vapor flow required through valve, standard cu fl/min at 14.7 psia and 60°F... [Pg.449]

A fluid is described as supercritical or subcritical if its temperature is above or below its critical temperature. Above the critical temperature the liquid and vapor phases are indistinguishable, the densities of the two phases become identical and the substance is described as a fluid, the physical properties of which are intermediate between those of a liquid and a gas [75]. [Pg.284]

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... [Pg.60]

Recently the term, gas-expanded liquids (GXLs) [6-8] has been used to describe these unique mixtures while others use the term subcritical mixtures to describe the phase of matter. No matter what term is used to describe these mixtures one point should be clear all of these mixtures are liquids not supercritical fluids. Eurthermore, there is no discontinuity observed in moving from the supercritical condition to a liquid. However, EEL mixtures and supercritical fluids are two different phases of matter. [Pg.424]

Lee and coworkers distinguish between sub-critical, critical and supercritical external flash intensities to produce photochemical reaction. For subcritical intensities, free radical production is too low to produce the sequence of events described above. Critical intensities do create these events. At very high, supercritical flash intensities, free radical production thruout the gas volume is so large and subsequent reactions are so rapid that the entire gas volume explodes in what amounts to a constant volume expln Urtiew et al (Ref 7) studied the reaction between silane (SiH4) and tetrafluorohydrazine (N2F4) with cis-2-butane as an inhibitor. The... [Pg.156]

Figure 2.8 Representative supercritical (T = 31 OK) and subcritical (T = 280K) Van der Waals isotherms for C02, showing the liquid-gas (L + G) condensation plateau (P = 52 atm) for T = 280K, and outlining the 2-phase liquid-gas coexistence dome (dotted line) topped by the critical point (x) at Tc = 304K, Pc = 73 atm. Figure 2.8 Representative supercritical (T = 31 OK) and subcritical (T = 280K) Van der Waals isotherms for C02, showing the liquid-gas (L + G) condensation plateau (P = 52 atm) for T = 280K, and outlining the 2-phase liquid-gas coexistence dome (dotted line) topped by the critical point (x) at Tc = 304K, Pc = 73 atm.
When compressed at subcritical temperatures, the gas condenses, (hat is, macroscopic clusters or droplets are formed under the influence of the attractive forces. During condensation, the pressure remains constant while the volume decreases, giving rise lo an infinite compressibility in the Iwo-phase region. At the criticai point the system is on the verge of condensation and the compressibility is also infinite. [Pg.430]

Richter, P., B. Sepulveda, R. Oliva, K. Calderon, and R. Seguel (2003). Screening and determination of pesticides in soil using continuous subcritical water extraction and gas chromatography-mass spectrometry. J. Chromatogr. A, 944 169-177. [Pg.270]


See other pages where Subcritical gases is mentioned: [Pg.44]    [Pg.3]    [Pg.1035]    [Pg.1036]    [Pg.216]    [Pg.334]    [Pg.631]    [Pg.631]    [Pg.328]    [Pg.261]    [Pg.2026]    [Pg.630]    [Pg.630]    [Pg.363]    [Pg.219]    [Pg.2394]    [Pg.301]    [Pg.311]    [Pg.365]    [Pg.328]    [Pg.371]    [Pg.421]    [Pg.895]    [Pg.230]    [Pg.265]    [Pg.169]    [Pg.187]    [Pg.176]    [Pg.214]    [Pg.15]    [Pg.36]    [Pg.402]    [Pg.144]    [Pg.219]    [Pg.268]    [Pg.50]    [Pg.481]    [Pg.48]    [Pg.13]   
See also in sourсe #XX -- [ Pg.3 ]




SEARCH



© 2024 chempedia.info