Pressure, critical effect

Variation in the pressure of the reacting gas can affect corrosion processes in two ways. In the cases more usually met with in practice, in which the corrosion rate is controlled by diffusion processes in the surface film of corrosion product, the influence of gas pressure on corrosion rate is slight. If, however, the dissociation pressure of the oxide or of a constituent of the scale lies within the range involved, the stability of the corrosion product will be critically dependent on the pressure. The effect of temperature is, however, far more critical and thus, in practical cases, pressure variations rarely decide the stability of corrosion products. 

Sizing, safety relief, 436, 437-441 API liquid valve, 444 Balanced valves, 441 Conventional valves, 438 Critical back pressure, 440 Effects of two-phase flow, 437 Hydraulic expansion, 441 Rupture disks, 434 Sub-critical flow, 449 Slurry flow, process pipe, 142-147 Regimes, 143... 

Bochniak, D. J., Subramaniam, B. Fischer-Tropsch Synthesis in Near-Critical n-Hexane Pressure-Tuning Effects. AIChE J. 1998,44, 1889- 1896. 

Risks linked with chemical processes are diverse. As already discussed, product risks include toxicity, flammability, explosion, corrosion, etc. but also include additional risks due to chemical reactivity. A process often uses conditions (temperature, pressure) that by themselves may present a risk and may lead to deviations that can generate critical effects. The plant equipment, including its control equipment, may also fail. Finally, since fine chemical processes are work-intensive, they may be subject to human error. All of these elements, that is, chemistry, energy, equipment, and operators and their interactions, constitute what we call process safety. 

We must emphasize one essential peculiarity until recently practically the whole of the critical effects in CO oxidation (and in the other catalytic oxidation reactions) were obtained at normal ( 760 torr) or almost normal pressures. In ultrahigh-vacuum experiments, these effects have been observed, and have induced scepticism. Over the last several years the situa-... 

Let us illustrate this by the example of critical effects. For CO oxidation these effects were observed quite often at normal pressures but extremely rarely in the high-vacuum region (see Sect. 1). Meanwhile the above calculated data show that the critical effect can also be observed in this region. Steady-state diagrams (Figs. 2 and 3) demonstrate intervals for the conditions under which critical effects do exist. This contradiction has existed until recently. 

On the other hand, it is clear that the "ideal models cannot describe the behaviour of complex catalytic reactions in complete detail. In particular, we cannot quantitatively explain the values of the self-oscillation periods obtained by Orlik et al. Secondly, for example, we have failed to describe the critical effects obtained by Barelko et al. in terms of model (2)—(3) corresponding to the two-route mechanism with the parameters taken from ref. 49 or ref. 142. Our calculated reaction rates proved to be at least two orders of magnitude higher than the experimental values. Apparently our models must be considerably modified, primarily in the region of normal pressures. It is necessary to take into account the formation of unreactive oxygen forms that considerably decrease the rate of C02 generation, the dependence of the reaction parameters on the surface composition and catalyst volume and finally the diffusion of oxygen into the catalyst. 

These mediators can produce a number of effects including bronchiolar constriction, capillary dilatation, or urticaria (i.e., hives). In severe episodes of type I reactions a life-threatening anaphylaxis can develop in humans due to extreme bronchoconstriction and precipitate hypotension. Epinephrine is the principal drug used in the acute management of these critical effects since it achieves (1) an elevated blood pressure via activation of alpha receptors in peripheral resistance blood vessels and (2) relaxation of bronchiolar smooth muscle via activation of (32 receptors in the lung. Relief from the dermatological problem (i.e., hives) is also achieved via vasoconstriction of capillaries in the skin that reduce permeability, and, hence, fluid accumulation. Penicillin is a classic example of a drug that can cause a type I reaction. 

As we have mentioned in the Introduction, the location of the critical point of the lowest density liquid-liquid transition of real water is unknown and both scenarios (critical point at positive or at negative pressure) can qualitatively explain water anomalies. Recent simulation studies of confined water show the way, how to locate the liquid-liquid critical point of water. Confinement in hydrophobic pores shifts the temperature of the liquid-liquid transition to lower temperatures (at the same pressure), whereas effect of confinement in hydrophilic pores is opposite. If the liquid-liquid critical point in real water is located at positive pressure, in hydrophobic pores it may be shifted to negative pressures. Alternatively, if the liquid-liquid critical point in real water is located at negative pressure, it may be shifted to positive pressures by confinement in hydrophilic pores. Interestingly, that it may be possible in both cases to place the liquid-liquid critical point at the liquid-vapour coexistence curve by tuning the pore hydrophilicity. We expect, that the experiments with confined supercooled water should finally answer the questions, concerning existence of the liquid-liquid phase transition in supercoleed water and its location. 

DJ Bochniak, B Subramaniam. Fischer-Tropsch synthesis in near-critical n-hexane Pressure-tuning effects. AIChE J 44 1889-1896, 1998. 

Results obtained at 25° and 100 °C for both sample sources indicated that although the impurity variation had little effect on the compressibility factor—0.005% at 25°C and 0.006% at 100°C for our range of pressures—its effect was systematic. Impurities had a more dramatic effect at conditions characteristic of the critical region. This is illustrated in Figure 3 for ethylene density results obtained at 25°C by us and others (2,15). For samples with reported purities of 99.99% or better, the... 

Critical Flux or Pressure and Effect of Membrane Pore Size ... 

One of the most important properties of an SCF is that its physical properties can vary dramatically as a result of relatively small changes in temperature and pressure. This effect is most pronounced around the critical point, where the density of carbon dioxide is approximately 0.46 g/mL. If the pressure is doubled, the density of the fluid increases dramatically, reaching a density comparable to that of liquid carbon dioxide (Figure 4.3). 

Up to now the safety evaluation of a secondary sodium leak accident has been based on the severest temperature-driven pressure rise, assuming the largest scale of sodium leak. This was chosen since the loss of integrity of the building due to the pressure rise in the room has a critical effect on loop separation and therefore accident limitation. 

At first glance, one might consider the effect of compressed CO2 on the phase behavior of multi-component polymer systems to be a simple combination of the known effects of liquid solvents and hydrostatic pressure. Solvent effects are primarily enthapic in nature and typically manifest in upper critical solution behavior. Common solvents mitigate unfavorable interactions between dissimilar segments and enhance miscibility. In blends, the addition of highly selective solvents, e.g. a non-solvent for one component, can lead to precipitation of the unfavored species at high dilution. In block copolymers, the effect of selective solvents is less clear, but studies to date reveal a collection of the solvent at the domain interface, selective dilation of one phase, and stabilization of the disordered phase via depression of the UODT. The systems we have studied each exhibit a lower critical transition. For these specific systems, previous work indicates the hydrostatic pressure suppresses free volume differences between the components and expands the region of miscibility. 

---