Effects of Low Pressure

The operation of mass spectrometers under vacuum leads to various consequences. First, vacuum alters the retention times of analytes compared with those obtained under the same chromatographic conditions using a detector of the type FID, NPD, or BCD (refer to Chapter 1 dedicated to gas chromatography). Indeed, these detectors work under atmospheric pressure or slight overpressure. The depression in the mass spectrometer leads to aspiration of the carrier gas and consequently of the analytes that the gas conveys. The aspiration spans several dozens of centimeters from the extremity of the capillary column connected to the mass spectrometer. 

The acceleration of elution translates into a decrease of the compound retention times compared to those obtained in FID, for instance. Obviously, the shorter the column is, the more the phenomenon is pronounced. It is not problematic because the elution order of the compounds is conserved. One must nevertheless keep in mind that, when comparing two chromatograms (one in GC, the other in GC-MS), the analyte retention times change even though the chromatographic conditions are identical. 

Another consequence of vacuum is of a practical nature. Users of GC-MS generally work with a single capillary column because changing columns generally 

A deactivated silica capillary connected to the analytical column at the transfer line level can allow a column change without stopping the pumping. There are also valve systems that allow changing the column without pausing the pumping but these systems often cause absorption of the analytes at the level of the valve. One must therefore check, before using this kind of system, that the valve components are passivated to avoid adsorption of the molecules. 

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Table 5.4 Effect of low pressure on stripping toluene from residue...

Baum, E. Lewis, T. Toomer, R. (1978). The lateral motion of charge on thin films of polyethylene terephtalate. Journal Phys. D Appl. Phys., Vol.ll, pp. 963-977 Draughn, R. Catlin A. (1968). Effect of Low Pressure on Surface Charge of Electrets. 

EFFECT OF LOW PRESSURES ON THE ROOM TEMPERATURE TRANSITIONS OF POLYTETRAFLUOROETHYLENE. 

Kylian O, Hasiwa M, Rossi F. Effect of low-pressure microwave discharges on pyrogen bioactivity. IEEE Trans Plasma Sci 2006 34(6) 2606-10. 

The vacuum in the mass spectrometer also influences the retention times of the molecules. The gas is drawn in under the effect of depression (see Section 2.2.3 covering the effects of low pressure). 

The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

Griskin et reported that there is no apparent effect of steam pressure on the rate of oxidation of Cr-Ni steels at temperatures between 600°C and 650°C at 10.1-20.2 MPa. Similar observations for Cr-Mo and Cr-Mo-V steels between 500°C and 600°C have been made by Wiles" . She compared low-alloy steel samples exposed to 101 kPa steam with power plant components that had operated for up to 150000b in steam at 17.25 MPa and found no significant difference in the oxidation rates (Fig. 7.11). 

We now have the foundation for applying thermodynamics to chemical processes. We have defined the potential that moves mass in a chemical process and have developed the criteria for spontaneity and for equilibrium in terms of this chemical potential. We have defined fugacity and activity in terms of the chemical potential and have derived the equations for determining the effect of pressure and temperature on the fugacity and activity. Finally, we have introduced the concept of a standard state, have described the usual choices of standard states for pure substances (solids, liquids, or gases) and for components in solution, and have seen how these choices of standard states reduce the activity to pressure in gaseous systems in the limits of low pressure, to concentration (mole fraction or molality) in solutions in the limit of low concentration of solute, and to a value near unity for pure solids or pure liquids at pressures near ambient. 

Table 1 illustrates the effect of hydrogen pressure on the selectivity to MIBK based on the initial rate of MIBK production and the rate of IPA production. Palladium gives very high selectivity to MIBK, typically in excess of 93% with the selectivity improving significantly with decreasing pressure. This result is of particular importance since the CD process for MIBK production is carried out at relatively low pressure (< IMPa). In contrast, alternative one-step processes for MIBK production are carried out in the liquid phase in trickle-bed reactors at pressures as high as 10 MPa. 

The effect of hydrogen pressure in the reaction network and kinetics of quinoline hydrodenitrogenation has been matter of debate. Some controversial results and explanation were raised by the proposal of light hydrocarbons formation [78], The lack of observation of these hydrocarbons in previous experiments was explained by the low pressure employed and the deviations observed of the mass balances in these experiments were an evidence for the formation of lights HCs. The controversy is not clear yet and might be the subject for further investigations. 

Effects of hydrogen pressure, materials yield strength, and temperature on the fracture toughness (fCIH) of three low-alloy vessel steels (a) pressure, (b) yield strength, and (c) temperature. 

In general, the pressure of a reaction system can increase for three reasons (1) evaporation of low boiling chemicals, (2) formation of gaseous by-products as a result of the desired reaction, and (3) production of gases as a consequence of undesired reactions or decompositions. For normal operations, it is imperative to know how deviations in operating conditions affect the gas production. Further, the effect of increased pressure on the reaction rate must be determined to avoid uncontrollable pressure increases in confined systems. 

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