Pressure toward decreased concentrations

Solutions of ruthenium carbonyl complexes in acetic acid solvent under 340 atm of 1 1 H2/CO are stable at temperatures up to about 265°C (166). Reactions at higher temperatures can lead to the precipitation of ruthenium metal and the formation of hydrocarbon products. Bradley has found that soluble ruthenium carbonyl complexes are unstable toward metallization at 271°C under 272 atm of 3 2 H2/CO [109 atm CO partial pressure (165)]. Solutions under these conditions form both methanol and alkanes, products of homogeneous and heterogeneous catalysis, respectively. Reactions followed with time exhibited an increasing rate of alkane formation corresponding to the decreasing concentration of soluble ruthenium and methanol formation rate. Nevertheless, solutions at temperatures as high as 290°C appear to be stable under 1300 atm of 3 2 H2/CO. 

Webb et al. [20] used this approach to study alkene hydroformylations. They found a lower rate, yet higher selectivity towards the desired products, when compared to the reaction in IL alone. This increase in selectivity can be attributed to the partitioning of the product to the gas phase, which reduces its contact with the catalyst, and prevents further reaction. In order to have higher reaction productivity it is necessary to obtain a higher substrate concentration in the IL phase. This can be achieved by decreasing the CO2 partial pressure, which decreases its solvent power, and leads to reactant partitioning more into the IL phase. Webb et al. also found that, under certain conditions, the system could operate continuously for several weeks without detectable catalyst degradation. 

For chemical reactions, we have repeatedly assumed that a small but essentially constant concentration of the transition state is in equilibrium with the reactants. It is the concentration of the transition state that determines the magnitude of the rate constant. In Section 2.8, we dealt with the effects of temperature on the rate constant, but it should also be apparent that pressure can affect the value of fe if the transition state occupies a different volume than that of the reacting species. If the transition state occupies a smaller volume than the reactants, increasing the pressure will shift the equiftbrium toward the formation of a higher concentration of the transition state, which will increase the rate of the reaction. If the transition state occupies a larger volume than the reactants, increasing the pressure will decrease the concentration of the transition state and decrease the rate of the reaction. As will be discussed in Chapter 5, the effect of internal pressure caused by the solvent affects the rate of a reaction in much the same way as does the external pressure. 

Figure 3.2 shows the equilibrium conversion of methanol steam reforming as a function of the S/C ratio of the feed [25]. It is obvious that the maximum hydrogen concentration in the reformate is gained at S/C 1. However, to minimize the carbon monoxide concentration in a practical system, a surplus of steam is required. Therefore, in practice, systems operate at S/C ratios ofbetween 1.3 and 2.0. Elevated pressure also decreases the selectivity towards carbon monoxide [49]. 

The molecular composition of sulfur vapor is a complex function of temperature and pressure. Vapor pressure measurements have been interpreted in terms of an equiHbtium between several molecular species (9,10). Mass spectrometric data for sulfur vapor indicate the presence of all possible molecules from S2 to Sg and negligible concentrations of and S q (H)- In general, octatomic sulfur is the predominant molecular constituent of sulfur vapor at low temperatures, but the equihbrium shifts toward smaller molecular species with increasing temperature and decreasing pressure. 

Ca.ta.lysts, At ambient temperatures, only a relatively small amount of ethanol is present in the vapor-phase equiUbrium mixture, and an increase ia temperature serves only to decrease the alcohol concentration. An increase in pressure helps to shift the equiUbrium toward the production of ethanol because of a decrease in the number of molecules (Le ChateUer s principle). On the other hand, reaction velocity is low at low temperatures. Hence it is necessary to use catalysts and relatively high temperatures (250—300°C) to approach equiUbrium within a reasonably short time. 

When an electric field is applied, Na+ drifts toward the cathode. Therefore the concentration of Na+ in the A phase decreases with time. The concentration of Na+ in the B phase also decreases with time (Fig. 2, right). In order to maintain electrical neutrality in the B phase, H+ is provided from COOH groups. On the other hand, the concentration of Na+ in the C phase remains constant under an electric field. The concentration of Na+ in the D phase increases with time. OH- should be provided to maintain electrical neutrality in each phase. As shown in Fig. 2, we should focus mainly on the concentrations of Na+ and H +. The osmotic pressures at the anode and cathode sides at a time t are given by Eqs. 14 and 15. 

The work described by Marshall (16), together with the vapour pressure studies on 1 1 and 1 2 electrolytes up to 300 C reported by Lindsay and Liu (17) and recent theoretical work by Silvester and Pitzer (21) and by Helgeson and Kirkham (22) provide a good understanding of the behaviour of simple electrolytes over wide ranges of temperature and concentration. However, as just seen, the behaviour under SVP conditions above 300 C becomes decreasingly well defined towards the critical point. 

Pressure oscillations in the first arrangement depended on the equivalence ratio of the flow in the annulus and decreased with velocities in the pilot stream greater than that in the main flow due to decrease in size of the recirculation zone behind the annular ring and its deflection towards the wall. Increase in swirl number of the second arrangement caused the lean flammability limit to decrease, and the pressure oscillations to increase at smaller values of equivalence ratio. Unpremixedness associated with large fuel concentrations at the centre of the duct increased the pressure oscillations. Pressure oscillations caused the position of flame attachment to move downstream in both flows with a decrease in amplitude of oscillations. 

The principle ofLe Chatelier summarizes the conclusions that may be drawn from the illustrative examples in this chapter "Whenever a stress is placed on a system at equilibrium, the equilibrium position shifts in such a way as to relieve that stress." If the stress is an increase in the partial pressure (concentration) of one component, the equilibrium shifts toward the opposite side in order to use up part of the increase. If the stress is an increase in the total pressure, the stress may be partially relieved by a shift toward the side with the smaller number of gaseous moles if there are the same number of gaseous moles on each side, no shift will occur, and no stress will be relieved. If the stress is an increase in temperature, the stress is partly relieved because, for an endothermic reaction, the equilibrium constant increases and the equilibrium shifts to the right for an exothermic reaction, the equilibrium constant decreases and the equilibrium shifts to the left. A catalyst places no stress on the system and causes no shift in the equilibrium position. 

At constant pressure and temperature, the equilibrium is shifted away from the side subjected to an increase in concentration of any constituent, or toward the side subjected to a decrease in concentration of any constituent. 

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