Introduction exothermic process

Processes An Introduction— A process that proceeds without external intervention is said to be a spontaneous process. A nonspontaneous process cannot occur without external intervention. If a process is spontaneous in one direction, then it is nonspontaneous in the reverse direction. Some spontaneous processes are exothermic, and others are endothermic, so the criterion for spontaneous change cannot be based on enthalpy changes alone. The direction of spontaneous change involves changes in another property called entropy. Entropy provides a measure of the number of ways a given quantity of energy can be dispersed, or distributed, among the particles of the system. 

Reactor heat carrier. As pointed out in Chapter 7, if adiabatic operation is not possible and it is not possible to control temperature by indirect heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flowrate (i.e. product of mass flowrate and specific heat capacity). This will reduce temperature rise for exothermic reactions or reduce temperature decrease for endothermic reactions. The introduction of an extraneous component as a heat carrier effects the recycle structure of the flowsheet. Figure 13.6a shows an example of the recycle structure for just such a process. 

In another example [39, 40], it was demonstrated that gas-phase platinum cluster anions, Pt ( = 3-7), efficiently catalyze the oxidation of CO to CO2 in the presence of N2O or O2 near room temperature in a full thermal catalytic reaction cycle. At the end of the process, the intact cluster is regenerated, and each step is exothermic, and occurs rapidly at thermal energies. In these experiments, the produced Pt clusters are first thermalized by collisions with a buffer gas. When either O2 or NjO is introduced downstream in the flow tube, Pt O and PfyO ions are formed in rapid exothermic reactions [41]. With the introduction of CO into the gas cell, the Pt clusters reappear, as observed with mass spectrometry. From these experiments, it was concluded that neutral CO2 is stoichiometrically formed on the clusters, and that negligible fragmentation of the metal cluster for n > 4 occurs. Further experiments revealed that, at low energies. 

As discussed in the general Introduction to this volume, the initiation of reaction in explosives is commonly believed to be thermal in origin with reaction starting at local sites called hot spots. The hot spots form when one of several possible processes concentrates the energy fed into the explosive into a small region, typically in the range 0.1—10//m diam. Thermal decomposition takes place at the hot spots, and because of the exothermic nature of the decomposition, the rate of decomposition rapidly increases and a thermal explosion results. Suitable hot spots may be formed in various ways for example ... 

Virtually all nitration reactions involve electrophillic attack by nitronium ions, NO2. Consequently, reactions can be regulated by controlling the concentration of nitronium ions in solution. The conditions required for the nitration reaction vary greatly with the reactivity of the aromatic substrate. The nitration mixture required for introduction of the second nitro group into benzene to prepare dinitrobenzene and concentrated nitric and sulfuric acids at 95°C is unsuitable for dinitration of alkylated phenol because it provides the conditions for an uncontrollable exothermic reaction. The DNBP process described above was based on controlling the nitration conditions and has the advantage of having very few side reactions because it is a two-phase system with nitration reactivity based on mass transfer between phases. 

In fact, the absolute value of the enthalpy associated with the overall process of dissolving hydrocarbons in water at 25 C, A//soiution. is an order of magnitude smaller than that of the heat of their evaporation, which is always endothermic (Table 1). This must mean that solvatation (hydration) of the hydrocarbons is exothermic and of the same order of magnitude as the heat of evaporation. It can therefore be concluded that the new bonds created in the presence of hydrocarbons (Af/cw) are enthalpically more beneficial than the hydrogen bonds between the water molecules that break down as a consequence of the introduction of a hydrocarbon chain into water... 

Fires in mines can be caused by many factors but one of the major causes is spontaneous combustion ( spon com ). Spon com occurs when air is allowed to percolate through organic materials, including coal. Through a progressive series of adsorptive, absorptive, and chemical processes, heat is produced, which causes the temperature of the material to rise. As we discussed in the introduction to this chapter, fires occur when the temperature of the material reaches its minimum self-heating temperature, where a continuous exothermic reaction is sustained and the material goes into thermal runaway. 

Originally, industrial synthesis of methanol was over a zinc oxide-chromium oxide catalyst that was operated at a nominal pressure of about 35 MPa (350 atm) and temperatures up to about 450°C. This catalyst unfortunately had a tendency to promote the exothermic methanation reaction (CO + 3H2 CH4 + h20) under certain conditions, which led in some instances to severely overheated reactors. This characteristic plus the high cost of compression and relative nonselectivity of the high-pressure process made it uneconomical following the introduction of low-pressure synthesis in the 1960s. 

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