Reaction, heat isotherm

Most chemical reactions are greatly affected by temperature. The previous chapters discussed reactions at isothermal condition, however, industrial reactors often operate under non-isothermal condition. This is because chemical reactions strongly depend on temperature, either absorbing (i.e., endothermic) or generating (i.e., exothermic) a large amount of heat. 

Worz et al. stress the possibility of carrying out very fast reactions with large reaction heat in micro reactors [110]. They often use the terms isothermal operation or isothermicity to describe adequately the carrying out of a reaction with a heat that is taken out of the processing volume immediately upon release. In practice, they often refer to a temperature increase of 1-2 °C as a limit for fulfilling the criterion of isothermal operation. 

The small reaction volumes in micro reactors and the large specific surface areas created are seen as beneficial to cope with the problems caused by the release of the large amounts of heat, as mentioned above [37, 38]. Delicate temperature control is expected for micro-reactor operation isothermal processing is said to be achievable even when high reaction heats are released [94]. Small size should increase process safety and suppress unwanted secondary reactions [37, 38]. 

Figure 5.4-35. Baseline and reaction heat flows calculated during heating and isothermal periods (reprinted with permission from Landau et al. (1994). Copyright (1994) American Chemical Society).

Energy balances are needed whenever temperature changes are important, as caused by reaction heating effects or by cooling and heating for temperature control. For example, such a balance is needed when the heat of reaction causes a change in reactor temperature. This is seen in the information flow diagram for a non-isothermal continuous reactor as shown in Fig. 1.19. 

In Illustration 10.2 we saw that when one uses a series of stirred tanks for carrying out an exothermic reaction under isothermal conditions there may be occasions when the heat require-... 

As discussed in Section 2.3.1.2, SEDEX [103, 104] and SIKAREX [106] instruments are also used isothermally. In the case of the SIKAREX, the temperature of the sample is held by a heating coil at constant temperature by establishing a constant rate of heat exchange to the jacket (held about 50 to 100°C below the sample temperature). By measuring the electrical input, a negative copy of the reaction heat profile is obtained. Typical sensitivity of the equipment is 0.5 W/kg operating with a sample size of 10 to 30 g and in a temperature range of 0 to 300°C. 

For exothermic reaction, heat is released and particles are hotter than the surrounding fluid, hence the nonisothermal rate is always higher than the isothermal rate as measured by the bulk stream conditions. However, for endothermic reactions the nonisothermal rate is lower than the isothermal rate because the particle is cooler than the surrounding fluid. 

Absoption of reaction heat is required, such that isothermal conditions can be approached. 

STRs in the most recent fine chemical hydrogenations [26]. The L/S slurry is circulated back at high flow in a loop connected to a Venturi. The local underpressure in the neck causes gas to be sucked in the intense turbulence achieves a very large interfacial area between tiny bubbles and the slurry. An external heat exchanger on the loop enables an almost unlimited heat removal, convenient for extremely high exothermic reactions, and isothermal operations. On the other hand, JLRs are restricted to a batch mode and can only accommodate catalysts compatible with the pump (low hard ness, low attrition). 

In the case of the nonisothermal first-order exothermic reaction heat is auto catalytic, for it raises the temperature and provokes an increase of reaction rate, yet is itself a product of the reaction. In the Gray-Scott scheme, B is plainly autocatalytic and its degeneration by the second reaction plays the role of the direct cooling in the non-isothermal case. This reaction appears in the chemical engineering literature in 1983,16 and is the keynote reaction in Gray and Scott s 1990 monograph on Chemical Oscillations and Instabilities.17 A justification of the autocatalytic mechanism in terms of successive bimolecular reactions is the subject of Chapter 12. 

In current industrial practice, reactive processing carried out in non-isothermal conditions, for both inherent and other reasons, such as changes in temperature at the surface of an article during the process cycle. Inherent reasons are the existence of inner heat sources, which can be of chemical origin (enthalpy of reaction), heat of phase transition from crystallization of a newly formed polymer or heat dissipated due to the flow of a reactive mass. 

Figure 4.11 shows three possible isothermal trajectories in the CTT diagram. Curing at Tg > Tgoo (trajectory a) leads to complete conversion. However, due to the high values of both the reaction heat and the polymerization rate, it is usually not possible to keep isothermal conditions when curing at high temperatures. The temperature of the sample continuously increases as it is... 

A reliable control of the reaction course can be obtained by isothermal operation. Nevertheless, to maintain a constant reaction medium temperature, the heat exchange system must be able to remove even the maximum heat release rate of the reaction. Strictly isothermal behavior is difficult to achieve due to the thermal inertia of the reactor. However, in actual practice, the reaction temperature (Tr) can be controlled within 2°C, by using a cascade temperature controller (see Section 9.2.3). Isothermal conditions may also be achieved by using reflux cooling (see Section 9.2.3.3), provided the boiling point of the reaction mass does not change with composition. 

Another interesting application of micro reactors is to use them as calorimeters. They may show excellent performance in terms of sensitivity [9-12]. Moreover, their performance in terms of heat exchange allows study of the kinetics of fast exothermal reactions under isothermal conditions. Such a development was realized by Schneider [13, 14], who studied such a reaction with a power of up to 160 kW kg-1. This type of calorimeter is simple to use and determines the reaction kinetics in a short time, with very small amounts of reaction mass, and without any hazard for the operator. 

Performing a reaction under isothermal conditions is somewhat more complex. It requires two temperature probes, one for the measurement of the reaction mass temperature and a second for the jacket temperature. Depending on the internal reactor temperature, the jacket temperature is adjustable. The simplest method is to use a single heat carrier circuit to act either on the flow rate of cooling water or on the steam valve. With a secondary heat carrier circulation loop, the temperature controller acts directly on the heating and cooling valves by using a conventional... 

Based on the investigation of reaction, heat transfer and mass transfer of the KD306-type sulfur-resisting methanation catalyst [9-11], the non-isothermal one-dimensional and two-dimensional reaction-diffusion models for the key components have been established, and solved using an orthogonal collocation method in this paper. The scope is to study the catalyst intraparticle reaction-diffusion processes that involve parallel, non-first order, equilibrium-restrained reactions. 

Figure 3.58 shows the results of the numerical calculation of the temperature distribution for this well in an isothermal reaction regime. Isothermal conditions were realized by applying appropriate heat sinks close to the well. 

For comparison to the isothermal conditions in the well, an adiabatic calculation with the same well geometry was executed. This time, all uncoated faces were set to be adiabatic while the coated channel faces obeyed the above-given boundary conditions. Figure 3.61 shows that there is a tremendous temperature increase if the exothermal reaction heat is not allowed to leave the control volume. 

As the system is adiabatic, no heat is lost and the enthalpy change of the system is zero. We can, hypothetically, conduct the same process by many other routes, e.g., those indicated in Figure 7. Route a consists of two steps the reaction conducted isothermally at temperature TA... 

An accurate study of the temperature profile structure in film and capillary samples involves considerable technical difficulties, which accounts for the lack of direct information on the role of the isothermal and nonisothermal mechanisms in the systems considered. However, some features of the structure are evident from the cinegram of Fig. 9. It shows that the wave front traveling in a capillary is noticeably ahead of the zone of intense reaction-heat release, marked by violent boiling of liquid helium in the cryostat. This observation allows the conclusion that here the fore part of the wave front is located in the not yet heated portion of the sample that is, small degrees of... 

However, when a fresh sample of the resin was heated isothermally at 185 °C, there was an exothermic process evident that peaked within the first minute and reached completion within approximately 4 to 5 min. Thus, it is evident that the cure mechanism may change with change in reaction conditions, revealing the complexity of the whey-based resin thermosetting process. 

Figure 6. Isothermal contours in two-dimensional reactive flow, Galerkin calculation. Heat generation by viscous dissipation and reaction heating.

As noted above, normalization of the carboxylate ionization is likely the source of the reaction heat observed in region IV of the heat capacity isotherm. The carboxylate ionization process must contribute to the enthalpy isotherm in the low-hydration region. 

Isothermally Operated Reactors. In an isothermal reactor the temperature of the reactant stream is constant in axial direction. Hence this stream does not take up reaction heat (in the case of an exothermic reaction) and all heat generated within the bed must be transported radially to the reactor wall. If the bed radius is too large and the effective heat conductivity of the bed too low, a radial temperature profile will develop with appreciable differences between the centre of the bed and near the wall. The temperature profile will be more pronounced as the radial distances are longer and as fluid velocities are lower, hence, wide and short reactors are likely to suffer most from radial temperature inhomogeneity. 

The previous discussion focused on simultaneous diffusion and reaction in isothermal catalyst pellets. Since A//, is significant for many industrially relevant reactions, it is necessary to address how heat transfer might affect solid-catalyzed... 

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