Adiabatic reactions, pressure effects

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

Thermal data where reactions are detected at lower temperatures are obtained from test runs on an ARC or other more sensitive calorimeters. In the ARC, the temperature is raised stepwise and at a much slower effective rate than with the DSC. The ARC is nearly adiabatic and, thus, more nearly approaches plant reactor conditions. Another important advantage is the fact that the reaction pressure is monitored and recorded in the ARC. 

Provided that there is a change in the number of moles upon reaction, an obvious measure of the extent of a reaction is given by the change in pressure. The latter has to be related to the stoichiometry of the reaction by quantitative analysis of the products and reactant or reactants and by material balance. Abnormal pressure effects sometimes occur due to adiabatic reactions, unimolecular reactions which are in their pressure-dependent regions (particularly in flow systems)... 

A more detailed version of the Stranks approach would incorporate the notion due to Sutin [4, 5] that electron transfer within the precursor assemblage ML(f+i)+,ML + occurs over a reaction zone of thickness Sa rather than on hard-sphere contact of the reactants. In that case, a must be treated as pressure-sensitive, compressing along with the solvent. It turns out, however, that allowance for compression of a in Eqs (5.6) and (5.7) can be neglected for adiabatic reactions because it is almost exactly cancelled by a term AVp representing the effect of compression on the pre-exponential part of the expression for [9] (the exponential part generates AV j). Electron transfer, however, could be non-adiabatic -... 

Figure 4.17 Effect of the O/C ratio [here expressed as air ratio X— (0/C)/4] on the equilibrium composition of methane partial oxidation reaction pressure, 1 bar calculated for an adiabatic reactor with a 200 preheating temperature of the methane/air feed [66].

Figure 9.6 Pressure-effect on rates of some self-exchange electron-transfer reactions between metal ions comparison of observed volumes of activation with values calculated from classical Marcus theory for adiabatic reactions. The plot shows calculated and observed AP values (cm mol ) at mid-range of pressure (100 MPa, except 70 MPa for Fe(H20)g ) for adiabatic (filled symbols) and nonadiabatic (open circles) self-exchange in couples with rigid ligands. Solvents (o, ) water ( ) CD3CN (A) (CD3)2CO (V) CD3OD. Key (A,B) (C,D) Cu(dmp)2 (E-G) Ru(hfac)j (H) Fe(C5H5)2 (I-K) Mn(CN-t-Bu)g ...

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

The heat of decomposition (238.4 kJ/mol, 3.92 kJ/g) has been calculated to give an adiabatic product temperature of 2150°C accompanied by a 24-fold pressure increase in a closed vessel [9], Dining research into the Friedel-Crafts acylation reaction of aromatic compounds (components unspecified) in nitrobenzene as solvent, it was decided to use nitromethane in place of nitrobenzene because of the lower toxicity of the former. However, because of the lower boiling point of nitromethane (101°C, against 210°C for nitrobenzene), the reactions were run in an autoclave so that the same maximum reaction temperature of 155°C could be used, but at a maximum pressure of 10 bar. The reaction mixture was heated to 150°C and maintained there for 10 minutes, when a rapidly accelerating increase in temperature was noticed, and at 160°C the lid of the autoclave was blown off as decomposition accelerated to explosion [10], Impurities present in the commercial solvent are listed, and a recommended purification procedure is described [11]. The thermal decomposition of nitromethane under supercritical conditions has been studied [12], The effects of very high pressure and of temperature on the physical properties, chemical reactivity and thermal decomposition of nitromethane have been studied, and a mechanism for the bimolecular decomposition (to ammonium formate and water) identified [13], Solid nitromethane apparently has different susceptibility to detonation according to the orientation of the crystal, a theoretical model is advanced [14], Nitromethane actually finds employment as an explosive [15],... 

A survey, with many references, of 14 classes of preparative reactions involving hydrogen peroxide or its derivatives emphasises safety aspects of the various procedures [11]. Following the decomposition of 100 1 of 50% aqueous hydrogen peroxide which damaged the 630 1 stainless vessel rated at 6 bar, the effect of added contaminants and variations in temperature and pH on the adiabatic decomposition was studied in a 1 1 pressure vessel, where a final temperature of 310°C and a pressure around 200 bar were attained. Rust had little effect, but addition of a little ammonia (pH increased from 1.8 to 6.0) caused the induction period to fall dramatically, effectively from infinity to a few h at 40°C and a few min at 80°C. Addition of sodium hydroxide to pH 7.5 reduced the induction period at 24°C from infinity to about 4 min [12],... 

We see the reason for this increase in Texp and NO in a more rapid rise in pressure and an approach to adiabatic compression. Interest attaches to a circumstance observed in the experiments at p0 = 200 mm in the glass apparatus where the amount of nitric oxide changed when the point of ignition was transferred to the center of the flask P (Fig. 1) the conditions of the chemical reactions in the flame front do not change it is only the conditions of the subsequent compression of the combustion products which are affected. The question has been investigated in detail by Frank-Kaimenetskix [7]. We shall merely observe here that the influence of the conditions of compression of the combustion products on the yield of nitric oxide proves the thermal nature of the reaction, since the compression is effective after combustion, when the reaction of the fuel with oxygen has ended,. Such an influence would be impossible from the point of view of an induced reaction. 

According to the hot-spot theory (Neppiras Noltingk 1950), the homogeneous ultrasound reaction takes place in the collapsing cavitation bubble and in the superheated (ca. 2,000 K) liquid shell around it. Species with sufficient vapor pressure diffuse into the cavity, where they undergo the effect of adiabatic collapse. 

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