Exotherms shape

Reaction (13.4) is exothermic and reversible, and begins at about 700 K by Le Chatelier s Principle, more iron is produced higher up the furnace (cooler) than below (hotter). In the hotter region (around 900 K), reaction (13.5) occurs irreversibly, and the iron(II) oxide formed is reduced by the coke [reaction (13.6)] further down. The limestone forms calcium oxide which fuses with earthy material in the ore to give a slag of calcium silicate this floats on the molten iron (which falls to the bottom of the furnace) and can bo run off at intervals. The iron is run off and solidified as pigs —boat-shaped pieces about 40 cm long. 

The oxychlorination reaction is very exothermic and the catalyst is very active, which makes it necessary to mix the catalyst with an inert diluent to avoid overheating in a fixed-bed reactor. A low surface area, spherically- or ring-shaped alumina or chemical porcelain body can be used as a diluent with the ring-shaped catalyst. The density of the inert material should be similar to the catalyst to avoid segregation during loading, and the size should be slightly different to allow separation of the inert material from the spent catalyst. 

Influence of reactor size/shape on process performance via heat transfer. Heat transfer between a reaction mixture and its surroundings strongly depends on the size of the reactor. The amount of heat evolved during an exothermic reaction is proportional to the volume of the reaction mixture (Qgcncrated V), ie. the effective volume of the reactor, whereas the amount of heat removed from the mixture is proportional to the heat-transfer surface area (firemovod A,). Morc precisely, the amount of heat transferred is given by ... 

Cylinders have the advantage that they are cheap to manufacture. In addition to varying the shape, the distribution of the active material within the pellets can be varied, as illustrated in Figure 6.7. For packed-bed reactors, the size and shape of the pellets and the distribution of active material within the pellets can be varied through the length of the reactor to control the rate of heat release (for exothermic reactions) or heat input (for endothermic reactions). This involves creating different zones in the reactor, each with its own catalyst designs. 

As a vessel of a given shape increases in size, both the surface area and the volume increase, but they do not increase at the same rate. For a sphere the surface area is a function of the diameter squared and the volume is a function of the diameter cubed. This is also true for a cylinder whose height is a multiple of its diameter. The polymerization of styrene is an exothermic reaction. The amount of energy released at any time is dependent on the volume of the reactor, and the rate of removal of that heat is dependent on the surface area. Unless the heat is removed, the temperature will rise and the reaction rate will increase. The result will be an uncontrolled reaction that not only may ruin the batch but could also damage the reactor and might cause a tire or explosion to occur. 

Other reactions will have somewhat different forms for the curve of Qq versus T. For example, in the case of a reversible exothermic reaction, the equilibrium yield decreases with increasing temperature. Since one cannot expect to exceed the equilibrium yield within a reactor, the fraction conversion obtained at high temperatures may be less than a subequilibrium value obtained at lower temperatures. Since the rate of energy release by reaction depends only on the fraction conversion attained and not on the position of equilibrium, the value of Qg will thus be lower at the higher temperature than it was at a lower temperature. Figure 10.2 indicates the general shape of a Qg versus T plot for a reversible exothermic reaction. For other reaction networks, different shaped plots of Qg versus T will exist. 

An example of an idealized DSC curve with an exotherm peak is represented in Figure 2.13. The shape of the DSC curve depends on the reaction order, the occurrence of autocatalytic decomposition, and on parameters such... 

In DSC instruments, heat production (q) can be determined directly as a function of temperature. The shape of the heat production curve is also important for hazard identification. A sharp rise in energy release rate (i.e., a steep slope of the exotherm), whether due to a rapid increase of the rate constant with temperature or to a large enthalpy of reaction, indicates that the substance or reaction mixture may be hazardous. Figure 2.14 illustrates an example of a DSC curve with a gradual exothermic reaction, while Figure 2.15 is an example of a steep exothermic rise. 

In the Bowes and Cameron test [133], the stability of the powder at constant (uniform) ambient temperature is investigated. Cube-shaped baskets, made of wire gauze, are filled with the substance and placed in an oven that is controlled at the desired constant temperature. The temperature in the center of the cube and of the oven are continuously recorded. By testing at different temperatures, and using a number of cube dimensions, the thermal stability of the powder can be established, that is, the determination of the temperature below which the exothermic decomposition of the powder does not result in a runaway. Bowes [133] has given a number of theoretical calculations for scaling up the test results. 

Up till now anionic mercury clusters have only existed as clearly separable structural units in alloys obtained by highly exothermic reactions between electropositive metals (preferably alkali and alkaline earth metals) and mercury. There is, however, weak evidence that some of the clusters might exist as intermediate species in liquid ammonia [13]. Cationic mercury clusters on the other hand are exclusively synthesized and crystallized by solvent reactions. Figure 2.4-2 gives an overview of the shapes of small monomeric and oligomeric anionic mercury clusters found in alkali and alkaline earth amalgams in comparison with a selection of cationic clusters. For isolated single mercury anions and extended network structures of mercury see Section 2.4.2.4. 

An intrinsic, exothermic water-gas shift reaction occurs in the steam reformer reactor. The combined reaction, steam reforming and water gas shift, is endothermic. As such, an indirect high temperature heat source is needed to operate the reactor. This heat source usually takes the shape of an immediately adjacent high temperature furnace that combusts a small portion of the raw fuel or the fuel effluent from the fuel cell. Efficiency improves by using rejected heat from other parts of the system. Note that the intrinsic water-gas shift in the reactor may not lower the... 

Eq.(30) is a straight line and Eq.(27) is an S shaped curve as shown in Eigure 2. This plot shows the curve of heat generated and the line of heat removed versus temperature in an exothermic CSTR. The three steady-states are the points of intersection Pi, P2, P3, of the curve Qg and the line Qr-... 

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