Fast Exothermic Reactions

The Hammond Postulate implies that the transition stah of a fast exothermic reaction resembles the reactants (se( reaction energy diagram at left). This means that it wil be hard to predict the selectivity of competing exothermi( reactions both barriers may be small and similar even i one reaction is more exothermic than the other. 

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

The main driver was to develop a laboratory-scale micro-channel process and transfer it to the pilot-scale, aiming at industrial fine-chemical production [48, 108]. This included fast mixing, efficient heat transfer in context with a fast exothermic reaction, prevention offouling and scale-/numbering-up considerations. By this means, an industrial semi-batch process was transferred to continuous processing. 

The numerical solutions indicate that under conditions where rj 1, there is a large temperature gradient at the periphery of the pellet, but the gradient flattens out considerably as one approaches the center of the pellet. For fast exothermic reactions (large cf)s, / > 0), the interior of the pellet will be at a relatively uniform high temperature, but there will be a fairly sharp decrease in temperature as one nears the exterior surface. In this regime r becomes inversely proportional to s, as in the isothermal case, and the bulk of the reaction takes place... 

Commonly, when chloroplatinic acid is the source of the catalyst for hydrosilation and sometimes when salts of other metals are used, no effect is observed for some time after it has been added to a mixture of reagents. Then, rather suddenly a rapid reaction can ensue. This induction period can in some cases last for a relatively long time, even hours. An unpredictable induction period, followed by a very fast exothermic reaction, leads to difficult, sometimes dangerous situations when large amounts of reagents are employed. 

The destruction of chlorine in a sodium hydroxide solution is a fast exothermic reaction. Hypochlorite, hypobromite and bromate are formed ... 

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. 

Schneider, M.-A., Ryser, T., Maeder, P. and Stoessel, F. (2004) A microreactor-based system for the study of fast exothermic reactions in liquid phase characterization of the system. Chemical Engineering Journal, 101 (1-3), 241-50. 

Schneider, M.A. and Stoessel, F. (2005) Determination of the kinetic parameters of fast exothermal reactions using a novel microreactor-based calorimeter. Chemical Engineering Journal, 115, 73-83. 

A fast exothermal reaction is to be performed in a semi-batch reactor. In order to control the temperature course of the reaction, one of the reactants is added at a constant rate, producing a constant heat flow. The reactor is cooled with water from a river (at 15 °C in winter). The cooling water should not be rejected at a temperature higher than 30 °C. 

At bHr Group (Phillips et al. (78)), tests performed on fast exothermic reactions have shown that energy savings up to 40% could be achieved and that the amount of by-product was significantly reduced. Extension of this work to commercial compact heat exchangers is currently being considered the first results indicate that chemical heat exchangers (CHEs) could be suitable as continuous chemical reactors. 

Surface reaction with diffusion and heat transfer resistance In fast exothermic reactions, in addition to grad c, also grad T (TG Ts) is present in the boundary layer between the gas bulk phase and the catalyst surface. For the outer effectiveness factor qext this means that... 

Even the HO2 radical in this system, although it can account for termination of branching, is nevertheless an active radical for the oxidation of CO by the probably fast, exothermic reaction... 

The modeling procedure explained above is not valid for all types of homogeneous reactions, for example, a very fast exothermic reaction such as the combustion of gaseous or vaporized hydrocarbon vith oxygen in air or the reaction of chloride and hydrogen in inert nitrogen, etc. We vill develop an example of such reactions belo v ... 

Figure 3.58 Description of a plug flow reactor with a constant wall temperature where a fast exothermic reaction occurs.

The set of Eqs. (3.166)-(3.173) represents the general mathematical model of the described fast exothermic reactions taking place in an externally cooled plug flow reactor. If we use the dimensionless expressions of time T = Y3px/r, coordinates X = x/rj,Y = y/tj, Z = z/1, R = r/ri, temperature Tp = t/tg and conversion X as dimensionless concentration of the limitative reactant, then the process model can be described by the relations contained in Table 3.14. 

Table 3.14 Dimensionless mathematical model forthe heat and mass transfer in a plug flow reactor for a fast exothermic reaction.

Figure 3.59 FlexPDE software for a fast exothermic reaction in a PF reactor.

Figure 3.61 Evolution of the conversion of the reactant in the PF reactor A/hen a fast exothermic reaction takes place.

Early transition states are frequently typical of fast, exothermic reactions in which there is no substantial bond making or breaking. Hence, early transition states resemble starting materials in structure. Late transition states are frequently typical of relatively slow, endothermic transformations. There is substantial bond formation in late transition states consequently, late transition states resemble products. 

Shell has developed a catalyst system for the RIM polymerization of DCPD which is the reaction product of 2 mol of 2,6-diisopropylphenol and 1 mol of WClg the co-catalyst is a trialkyltin hydride. Both components are soluble in DCPD and inherently storage-stable. In addition, this catalyst system has the advantage of being able to polymerize DCPD of technical quality. In a very fast exothermic reaction a complete conversion of the DCPD monomer takes place into the crosslinked polymer [78]. 

A general characteristic feature of the H2PtCl6-solvent catalytic system is the induction period which is followed by a fast exothermic reaction. Recent studies... 

For fast exothermic reactions, temperature control can be a problem. This is often solved by external circulation of part of Ihe reactor content through a heat exchanger, or by adding an internal heat exchange area. Alternatively, semi-batch operation can be applied, i.e., part of a reactant can be fed steadily over time or at certain intervals. Ihis also minimizes the occurrence of unwanted side reactions. 

A chemical reaction might blow up, or explode. This happens when it releases a lot of energy all at once. Explosions are very fast exothermic reactions. These reactions can happen in many different ways. 

Ability to cope with very fast exothermic reactions (corresponding to heat fluxes ofupto 100kW/m2. 

Elementary processes (5-164)-(5-170) are responsible for the formation of primary ammonia dissociation products NH2, NH, and H. The formation of molecular hydrogen and nitrogen, which are major stable products of ammonia dissociation (5-163), proceeds according to Slovetsky (1980) in the following fast exothermic reactions ... 

Active radical HS generated in reactions (10-63) and (10-64) react with each other and form molecular hydrogen and sulfur in the following fast exothermic reaction ... 

Environments of Fast Exothermic Reactions Shock Waves in Tubes Physical Effects in Shock Tubes... 

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