Pressure conditions reaction rates

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

The effect of very high pressure on reaction rates is rarely studied but reactions of PF3 with S02 have been shown to be strongly influenced by application of pressure. Thus, whereas reaction with S02 at 150°C produces elemental sulfur and POF3 in only 5% yield at 670 atm, an 84% yield is obtained when the pressure is increased to 4000 atm (83). Under similar high-pressure conditions POF3 is also obtained when PF3 is treated with 02 (84) or C02 (85). 

The thermodynamic equilibria for the reactions of zirconium with oxygen, water vapor, carbon monoxide, carbon dioxide, and nitrogen have been discussed elsewhere (27). All these reactions can occur in the temperature range of 800° to 1200°C. and down to pressures of 10-8 mm. of mercury. In this range the rate of solution of the compounds formed is sufficient to maintain the zirconium surface in a film-free condition provided the reaction rate is maintained below the rate of solution. At very low pressures the reaction rate is probably proportional to the pressure of the gases present. The critical conditions for the reactions are the pressure and temperature at which the rate of formation of the compound equals the rate of solution in the metal. Although we have not determined these conditions precisely, experience has shown that the metal remains in the proper film-free condition at 800° to 1200°C. at pressures of the order of 1 X 10 mm. of mercury and less. 

Studies of the C—H activation of alkanes on platinum surfaces are limited by the very weakly bound chemisorbed state. The low surface temperatures required to accommodate the molecule are insufficient to activate the C—H bonds. It is interesting that C—H activation is apparently occurring on platinum clusters under ambient low-pressure conditions. The rate of reaction appears to be 1-10% of gas kinetic. This suggests that the activation barrier for alkanes on platinum clusters is low, and does not involve significant steric hindrance. One intriguing possibility is that the cluster inserts into the RC—H bond, forming species such as RC—M —H. 

MCH and in H2 at temperatures below 370°C. Toluene desorption was identified as the rate-limiting step. It was found that at low pressures the reaction rate increases with MCH partial pressure, and at higher pressures there is no effect of MCH partial pressure on the reaction rate. This change in apparent order occurs at 1 mmHg MCH partial pressure approximately. On the other hand, it was also reported (15) that the hydrogen partial pressure has no effect on the reaction rate. These reaction orders close to zero for both reactant under practical conditions indicate that the platinum surface active sites are completely covered by adsorbed hydrocarbons (reactant molecules or intermediate compounds). The reaction proceeds, according to the following steps ... 

The partial oxidation of a liquid hydrocarbon with air is studied in the laboratory, under atmospheric pressure at a temperature of 350 K. The reaction is allowed to proceed to a degree of conversion of 98%. The selectivity is then 85%, so that the yield is 83%. It is found that at higher temperature and pressure the reaction rate increases, but the selectivity is lower. However, it turns out that this is a case of consecutive reactions, so that the selectivity is higher at lower degrees of conversion. It was found by additional experiments that the optimum conditions for a technical process are 10 atmospheres, 450 K and a degree of conversion of only 20%. The selectivity appears to be 95% then. These conditions deviate considerably from those in die original laboratory experiments. For the further development one has to study the reaction under these new conditions. 

The calculation procedure described above and in [gJ gives as a result a complete description of the conditions in the reactor including temperature and concentration profiles, pressure drop, reaction rates, gas enthalpies, equilibrium temperatures, effectiveness factors, etc. Furthermore, radial temperature and concentration profiles in catalyst particles and across the gas film surrounding the particles may printed for selected levels in the catalyst bed. Fig. 10 and 11 show some results obtained by simulating the performance of an adiabatic catalyst bed for the same inlet and outlet conditions (cfr. Table 2, first example) specifying two different catalyst particle sizes. 

Instead of concentrating on the diffiisioii limit of reaction rates in liquid solution, it can be histnictive to consider die dependence of bimolecular rate coefficients of elementary chemical reactions on pressure over a wide solvent density range covering gas and liquid phase alike. Particularly amenable to such studies are atom recombination reactions whose rate coefficients can be easily hivestigated over a wide range of physical conditions from the dilute-gas phase to compressed liquid solution [3, 4]. 

The experimentally measured dependence of the rates of chemical reactions on thermodynamic conditions is accounted for by assigning temperature and pressure dependence to rate constants. The temperature variation is well described by the Arrhenius equation. 

At conditions of high temperature and low pressure, for sufficient catalyst activity and acceptable reaction rates, equiUbrium conversions maybe as low as 5%, necessitating recycle of large amounts of unreacted propylene (101). 

To achieve the goal set above, measurements for reaction rates must be made in a RR at the flow conditions, i.e., Reynolds number of the large unit and at several well-defined partial pressures and temperatures around the expected operation. Measurements at even higher flow rates than customary in a commercial reactor are also possible and should be made to check for flow effect. Each measurement is to be made at point... 

The well-known difficulty with batch reactors is the uncertainty of the initial reaction conditions. The problem is to bring together reactants, catalyst and operating conditions of temperature and pressure so that at zero time everything is as desired. The initial reaction rate is usually the fastest and most error-laden. To overcome this, the traditional method was to calculate the rate for decreasingly smaller conversions and extrapolate it back to zero conversion. The significance of estimating initial rate was that without any products present, rate could be expressed as the function of reactants and temperature only. This then simplified the mathematical analysis of the rate fianction. 

Can we predict the optimum conditions for a high yield of NH3 Should the system be allowed to attain equilibrium at a low or a high temperature Application of Le Chatelier s Principle suggests that the lower the temperature the more the equilibrium state will favor the production of NHS. Should we use a low or a high pressure The production of NH3 represents a decrease in total moles present from 4 to 2. Again Le Chatelier s Principle suggests use of pressure to increase concentration. But what about practicality At low temperatures reaction rates are slow. Therefore a compromise is necessary. Low temperature is required for a desirable equilibrium state and high temperature is necessary for a satisfactory rate. The compromise used industrially involves an intermediate temperature around 500°C and even then the success of the... 

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