Heat consuming reactions

The CSB investigation determined that BP Amoco was unaware of the hazardous reaction chemistry of the polymer because of inadequate hazard identification during process development. This lack of awareness is a commonly cited cause of reactive incidents within the CSB data. The BP Amoco incident also involved an endothermic (or heat consuming) reaction rather than the more commonly recognized exothermic (or heat producing) runaway chemical reaction. 

The physico-chemical reactions that occur during the batch melting comprise different types of heat consuming reactions, such as the release of moisture and crystal water (from for instance caoline) from the batch first solid-state reactions between the different raw materials and the decomposition of carbonates, causing the release of CO2 and the formation of eutectic melts. The eutectic melts form by reactions between network modifier and a... 

The third characteristic of interest grows directly from the first, ie, the high thermal conductance of the heat pipe can make possible the physical separation of the heat source and the heat consumer (heat sink). Heat pipes >100 m in length have been constmcted and shown to behave predictably (3). Separation of source and sink is especially important in those appHcations in which chemical incompatibilities exist. For example, it may be necessary to inject heat into a reaction vessel. The lowest cost source of heat may be combustion of hydrocarbon fuels. However, contact with an open flame or with the combustion products might jeopardize the desired reaction process. In such a case it might be feasible to carry heat from the flame through the wall of the reaction vessel by use of a heat pipe. 

Conversion of Ammonia. Ammonia [7664 1-7] mixed with air and having an excess of oxygen, is passed over a platinum catalyst to form nitric oxide and water (eq. 10). The AH g = —226 kJ/mol of NH consumed (—54 kcal/mol). Heats of reaction have been derived from heats of... 

For the electrochemical cell reaction, the reaction free energy AG is the utilizable electrical energy. The reaction enthalpy AH is the theoretical available energy, which is increased or reduced by an amount TAS. The product of the temperature and the entropy describes the amount of heat consumed or released reversibly during the reaction. With tabulated values for the enthalpy and the entropy it is possible to obtain AG. ... 

If a constant-temperature bath is not available, a bucket of water, initially at 25°, serves to dissipate the heat of reaction. At higher temperatures the potassium permanganate is rapidly consumed, presumably by reaction with the acetone. 

Oin experimental technique of choice in many cases is reaction calorimetry. This technique relies on the accurate measurement of the heat evolved or consumed when chemical transformations occur. Consider a catalytic reaction proceeding in the absence of side reactions or other thermal effects. The energy characteristic of the transformation - the heat of reaction, AH i - is manifested each time a substrate molecule is converted to a product molecule. This thermodynamic quantity serves as the proportionality constant between the heat evolved and the reaction rate (eq. 1). The heat evolved at any given time during the reaction may be divided by the total heat evolved when all the molecules have been converted to give the fractional heat evolution (eq. 2). When the reaction under study is the predominant source of heat flow, the fractional heat evolution at any point in time is identical to the fraction conversion of the limiting substrate. Fraction conversion is then related to the concentration of the limiting substrate via eq. (3). 

The conservation of energy, however, differs from that of mass in that energy can be generated (or consumed) in a chemical process. Material can change form, new molecular species can be formed by chemical reaction, but the total mass flow into a process unit must be equal to the flow out at the steady state. The same is not true of energy. The total enthalpy of the outlet streams will not equal that of the inlet streams if energy is generated or consumed in the processes such as that due to heat of reaction. 

If the process involves a reaction, the heat generated or consumed is computed from the heat of reaction per kmol of product (at 25° C) and the kmols of product produced. 

The total emission In the commercial heat treatment of 5 to 8 hours at 170 to 160°C varied from 0.4 to 1.2% for CO2 and 0.05 to 0.2% for CO and 0.04 to 0.1% for total acids based on dry board. Some of this emission might emanate from pyrolysis of higher molecular weight material condensed and deposited on the walls of the heat treatment chamber. The heat of formation of this CO2 and CO Is about half the total heat release measured. Part of the oxidation products might remain in the solid phase within the board material, e.g. as bound carbonyl and carboxylic groups, partly followed by heat consuming dehydration reaction. 

All chemical reactions involve heat exchange. Reactions that release heat are called exothermic, and those that consume heat are called endothermic. Heat exchange is measured as the enthalpy change AH (the heat of reaction). This corresponds to the heat exchange at constant pressure. In exothermic reactions, the system loses heat, and AH is negative. When the reaction is endothermic, the system gains heat, and AH becomes positive. 

The recombination reactions consume free radicals to create stable species, resulting in a net reduction of radicals. Since these recombination reactions are very exothermic, they cause the temperature to increase. The lower panel of Fig. 16.11 shows the contribution of various reactions to the temperature rise. Specifically, it shows the contribution of each reaction i to the heat-of-reaction term in the thermal-energy equation (Eq. 16.98) ... 

The mixture is stirred at a rapid rate and heated for 30 minutes to 126°, whereupon the black slurry thickens. Only approximately 1 ml. of the sodium hydroxide is consumed in this initial heating. The reaction is then allowed to proceed rapidly at 128 to 139° for 2 hours. In the course of the reaction the mixture becomes light tan and appreciably less viscous, and the theoretical amount of sodium hydroxide is required to neutralize the hydrogen chloride which is evolved. Very little additional hydrogen chloride is liberated during another hour of heating. 

In process design calculations, it is usually more convenient to express the heat of reaction in terms of the enthalpy per mole of product formed or reactant consumed. Since enthalpy is a state function, standard heats of reaction can be used to estimate the AH at different temperatures by making a heat balance over a hypothetical process ... 

Unfavorable bottled wine storage conditions may cause the appearance of haze or deposits in white wines. Excessive heating of a bottle of white wine may precipitate protein, causing a haze or cloud excess cold may crystallize potassium bitartrate, creating a layer of small crystals in the bottle. Such haze and precipitates in wines may or may not have negative sensory effects but often affect adversely consumer reaction to the wine. 

The change in enthalpy (A H°) is the heat of reaction—the amount of heat evolved or consumed in the course of a reaction, usually given in kilojoules (or kilocalories) per mole. The enthalpy change is a measure of the relative strength of bonding in the products and reactants. Reactions tend to favor products with the lowest enthalpy (those with the strongest bonds). 

Any steps where a reactive intermediate is consumed without another one being generated, (heat content H) A measure of the heat energy in a system. In a reaction, the heat absorbed or evolved is called the heat of reaction, AH°. A decrease in enthalpy (negative AH°) is favorable for a reaction, (p. 140)... 

There are relatively few chemical reactions capable of heating matter to temperatures greater than 3000°K. Table II contains a list of some of these reactions and the theoretical flame temperatures attainable. These reactions have two characteristics in common (1) high exothermic heats of reaction and (2) stable molecular products with low heat capacities, since dissociation consumes energy and results in additional products which must be heated to the flame temperature. 

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