Tempering reactions

Figure 9-7 A tempered reaction system showing the important energy terms. The heat losses through the reactor walls are assumed negligible.

By using open or vented cells, data are obtained to determine if the full-scale reactor vent flow will be single or two-phase, and if a tempered or nontempered reaction occurs [192]. A tempered reaction is one that can be controlled through the latent enthalpy of evaporation. 

For vapour pressure systems, the emergency relief system should be designed so that the action of the pressure relief system removes vapour (and therefore latent heat) at a rate fast enough to hold the temperature, and therefore the pressure, constant. This is referred to as a "tempered" reaction system (see Figure 3.3). In most cases, the rate of reaction does not then increase beyond this point. However, it is worth noting that, in some cases, the reaction rate. may continue to rise at constant temperature and pressure if ... 

Fauske [32] represented a nomograph for tempered reactions as shown in Figure 12-35. This accounts for turbulent flashing flow and requires information about the rate of temperature rise at the relief set pressure. This approach also accounts for vapor disengagement and frictional effects including laminar and turbulent flow conditions. For turbulent flow, the vent area is... 

As with the glass apparatus, the pressure Dewar calorimeter can be fitted with a stirrer, connections for additions and sampling, pressure and temperature sensors, and jackets for heating or cooling. It can also be connected to a dump tank for the investigation of tempering reactions (Figure 3.11) (Sections... 

For a tempered reaction system, the heat generated by the reaction is equated to the sensible heat change of the reacting liquid mass as its temperature increases and the heat loss due to the evolution of volatile solvent. The result is (Boicourt, 1995)... 

It is rather slow at moderate temperatures sind the hydrobromic acid formed in the initial stages of the resLCtion inhibits its further progress. By carrying out the reaction at 60-70 or above in the presence of a large excess of water, the inhibition observed at lower temper, atuies does not occur. 

Sodium cyanide does not dissolve m butyl bromide The two reactants contact each other only at the surface of the solid sodium cyanide and the rate of reaction under these con ditions IS too slow to be of synthetic value Dissolving the sodium cyanide m water is of little help because butyl bromide is not soluble m water and reaction can occur only at the interface between the two phases Adding a small amount of benzyltrimethyl ammonium chlonde however causes pentanemtnle to form rapidly even at room temper ature The quaternary ammonium salt is acting as a catalyst it increases the reaction rate How7... 

Vent sizing for two-phase (runaway reactions) flow vent sizing for a tempered system... 

A solvent used in an exothermic reaction is nonvolatile, and moderately toxic. An alternative solvent is less toxic, but also has a much lower boiling point. There is a trade-off between toxic hazards and the potential for tempering the exotherm, but also generating pressure from boiling solvent in case of a runaway reaction. 

Chemical reactions are sometimes conducted in a dilute solution to moderate reaction rates, to provide a heat sink for an exothermic reaction, or to limit maximum reaction temperature by tempering the reaction. In this example there are conflicting inherent safety goals—the solvent moderates the chemical reaction, but the dilute system will be significantly larger for a given production volume. Careful evaluation of all of the process risks is required to select the best overall system. 

Reduction of reaction hazards by tempering a reaction with the use of a more volatile solvent that will boil and more reliably remove the heat of reaction. 

Fig. 8A. The exothermic energy release of shocked but unreacted Ni-Al mixtures shows a profound change. A preinitiation" phenomenon in which reaction temper ture is reduced by over 200 °C is caused by the shock process. A compact composer of composite particles that inhibit mixing shows no such effect [88H01].

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

The reaction rate and therefore the heat production rate can be tempered by diluting the S03 concentration (or partial pressure) in the gas phase and their reducing the S03 flow (kmol/m2,s) to the gas-liquid interface. In other words, the rate of reaction will be controlled by the transport of S03 through the gasphase. Volumetric (or molar) levels of between 2.5% S03 and 7% will be applied in practice 2.5% for delicate alcohol ethoxylate sulfation and 7% for... 

Use diluted S03 in the gas phase to temper the rate of reaction and therefore the reaction temperature at the gas-liquid interface and in the organic phase itself. 

Blum, A. (1983). Breeding programs for improving crop resistance to water stress. In Crop Reactions to Water and Temperature Stresses in Humid, Temperate Climates, ed. C.D. Raper Jr and P.J. Kramer, pp. 263-76. Boulder, Colorado Westview Press. 

When a temperate phage is mixed with sensitive indicator bacteria and plated as described above, the reaction at each focus of infection is generally a combination of lytic and lysogenic responses. Some bacteria will be lysed and produce phage, others will survive as lysogenic cells, and the plaque becomes visible as a partial area of clearing in the bacterial lawn. It is possible to pick off cells from the central areas of these plaques and demonstrate that they carry prophage. 

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