Net-exothermal

The surface decomposition process of AP propellants is net exothermic, with heat absorbed at the AP and fuel surfaces by endothermic pyrolysis but with more heat liberated in the gas phase close to the AP crystals. In detail, decomposition at the AP surface consists of endothermic pressure-independent solid-to-gas phase dissociative sublimation, followed by exothermic pressure-dependent gas-phase oxidation. The over-all reaction for the AP decomposition is pressure dependent. 

Figure 2.25. Separation of the net exothermal effect (characterized by a temperature increase AT) into components of polymerization ATp and crystallization ATc.

For the Cu-containing system Galactose oxidase, we conclude that the unpaired spins initially are located on the Cu atom and on the axial tyrosine, whereas the equatorial, cysteine-linked tyrosine obtains the unpaired spin upon proton transfer from substrate to axial Tyr. The computed barrier for the rate determining step (H-atom transfer from substrate to Tyr-S-moiety) is in excellent agreement with experimental data, whereas the charge transfer between Cu(II) and substrate ketyl anion is less exothermic than estimated experimentally. A question is however raised regarding the experimental estimates, based on the computed data and the overall net exothermicity of the substrate reaction. 

Using ammonia as the starting material for nitric oxide preparation bypasses the enormous energy requirement for elemental nitrogen bond dissociation, and in so doing achieves the product of interest via a net exothermic (thermodynamically favorable) process. 

To harness the energy from glucose, cells must break the bonds in which the energy is stored. The net exothermic reaction that takes place when the bonds in glucose are broken is similar to the combustion of hydrocarbons. 

For solutions that exhibit positive deviations from ideality, the excess enthalpy is positive that is, the heat of mixing is endothermic. For separation processes involving such solutions, the net heat of mixing reduces the minimum rate of work from that for ideal solutions. The rate of heat rejection is equal to the sum of the net exothermic heat of separation and the minimum rate of work. In Example 17.2, the net exothermic heat of separation is 2,887,000 Btu/hr and the rate of heat transfer from the process to the surroundings is 16,393,000 Btu/hr. 

The other processes mentioned above (ATR, POX and CR) are all net exothermic reactions but rely upon cryogenicaUy separated oxygen - an expensive and energy intensive process. 

The kinetics of the hydrolysis of D-glucosyl, D-mannosyl, D-galactosyl, and L-fucosyl a- and 3-phosphates between pH 1-6 have been determined. For the mono-anionic species the /3-anomers were hydrolysed 3-7 times faster than the a-anomers. Extended Hiickel calculations have shown that the difference in enthalpy between cyclic AMP and acyclic related phosphate esters is due to larger net exothermic solvation enthalpy in the former, ascribed to extra stabilization of the products of hydrolysis, due to their ability to form more stable hydrogen bonds with water. ... 

A Figure 13.4 Analysis of the enthalpy changes accompanying the solution process. The three processes are illustrated in Figure 13.3. The diagram on the left illustrates a net exothermic process < 0) that on the right shows a net endothermic process... 

The net exothermic reaction needs high temperatures for reasonable reaction rates. 

In addition, several organoactinide complexes also catalyze the regioselective hydrothiolation of alkynes with various thiols (Scheme 14) [46,47]. Bond enthalpy considerations for the unexplored reaction predict net exothermicity for RSH addition to alk5mes, aUenes, and alkenes mediated by organoactinide complexes. While alkyne insertion into the An-S bmid (step ii) is predicted to be exothermic. 

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

Between 50 and 60% of the formaldehyde is formed by the exothermic reaction (eq. 23) and the remainder by endothermic reaction (eq. 24) with the net result of a reaction exotherm. Carbon monoxide and dioxide, methyl formate, and formic acid are by-products. In addition, there are also physical losses, hquid-phase reactions, and small quantities of methanol in the product, resulting in an overall plant yield of 86—90% (based on methanol). 

Coke gasification occurs just outside the raceway area where gaseous oxygen is no longer available to completely combust the CO to CO2. This reaction goes essentially to completion at temperatures between 1500 to 2100°C. The net heat effect is exothermic, as shown in equation 1. The endothermic equation (eq. 2) allows control of the temperature in front of the tuyeres by controlling the moisture in the hot blast. 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

Catalysis is utilized in the majority of new paper filter cure ovens as part of the oven recirculation/bumer system which is designed to keep the oven interior free of condensed resins and provide an exhaust without opacity or odor. The apphcation of catalytic fume control to the exhaust of paper-impregnation dryers permits a net fuel saving by oxidation of easy-to-bum methyl or isopropyl alcohol, or both, at adequate concentrations to achieve a 110—220°C exotherm. 

The second context is the process reac tor. There is a potential for a runaway if the net heat gain of the system exceeds its total heat loss capabihty. A self-heating rate of 3°C/day is not unusual for a monomer storage tank in the early stages of a runaway. This corresponds to 0.00208°C/min, 10 percent of the ARC s detection limit. ARC data for the stored chemical would not show an exotherm until the self-heating rate was 0.02°C/min. Therefore, onset temperature information from ARC testing must be used with considerable caution. 

As stated earlier, catalytic cracking involves a series of simultaneous reaction.s. Some of these reactions are endothermic and some are exothermic. Each reaction has a heat of reaction associated with it (Table 4-4). The overall heat of reaction refers to the net or combined heat of reaction. Although there are a number of exothermic reactions, the net reaction is still endothermic. 

By thermodynamic convention, l Hp < 0 for exothermic reactions, so that a negative sign is attached to the heat-generation term. When there are multiple reactions, the heat-generation term refers to the net effect of all reactions. Thus, the term is an implicit summation over all M reactions that... 

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