Reaction, coupled endothermic

Energy coupling between the reactor and regenerator are crucial in designing the FCC reactor, because the heat liberated from burning off the coke from the catalyst suppHes the heat to maintain the temperature in the reactor where reactions are endothermic. Therefore, the energy balance equations and the description of flow of fluid and solid phases must be considered carefully in this reactor. 

Carbon Suboxide Photoiysis. In principle, carbon suboxide (1) can be used as a precursor to atomic carbon and two molecules of carbon monoxide as shown in Eq. 2. However, this reaction is endothermic by 141 kcal/mol and can only be realized in the vacuum ultraviolet (UV) at wavelengths that destroy most organic substrates. However, photolysis of 1 at 1470 A produces C atoms in a low-temperature matrix. The short wavelength flash photolysis of 1 coupled with atomic absorption has been used to measure the rate constants for various spin states of carbon with simple substrates. 

We now turn to the n = 5 case, 1,2-dimethylcyclopentene, for which there are literature enthalpy-of-hydrogenation measurements in an innocuous solvent coupled with a stereochemical analysis of the products67. Of all the dialky Icy cloalkenes that are mentioned in this section, it is with dimethylcyclopentene that equation 14, now with n = 5 and R = Me, is most expected to be essentially thermoneutral. We find the reaction is endothermic by ca 11 kJmol-1 and fail to understand the discrepancy75. 

Integrated reactors One type of integrated reactor is micro structured heat exchanger/reactor concepts, which may work as cross- or counter-flow reactors. Another type couples endothermic and exothermic reactions in two separate flow paths normally operated in the co-current mode. Both reactor types are designed as prototype components of future fuel processors for mobile applications. 

Fig. 1.2. Schematic flow configurations of heat-integrated processes for coupling endothermic and exothermic reactions, (a) Countercurrent flow of process streams, (b) Cocurrent flow of the process streams in the reactor stages and heat recovery in separate circuits.

The above considerations indicate that, independent of implementation details, the space-time yield of endothermic reactions could be significantly enhanced by shifting the reaction site to the heat-exchanging surfaces. This intention has led to the production of a large variety of multifunctional reactor concepts for coupling endothermic and exothermic reactions. In the following section the state of the art in this area will be discussed for selected examples. 

M. van Sint Annaland, A novel reverse flow reactor coupling endothermic and exothermic reactions. PhD thesis, Twente... 

At higher energies, several additional reaction pathways open. These are shown in reactions (6) to (11). These reactions are endothermic in all cases, and their cross sections can be analyzed to provide thermodynamic information regarding the products. A particularly interesting aspect of reactions (6) to (8) is that both ionic and radical silicon hydrides are formed such that coupled information about the cations and neutrals can be obtained from these data. This is discussed in more detail in Section III.E. 

If a batch reactor is completely insulated from the surroundings and there is only one chemical reaction, then the mass and thermal energy balances can be combined analytically to yield the maximum temperature rise for exothermic reactions. The same procedure provides an estimate of the maximum temperature drop if the reaction is endothermic. If pressure effects are negligible, in accord with the previous analyses, coupled heat and mass transfer yield (see equation 6-15) ... 

Itoh N, Wu T-H (1997) An adiabatic type of palladium membrane reactor for coupling endothermic and exothermic reactions. J Membr Sci 124 213-222... 

While the construction of PB MRs is simple, their operation is complicated, especially when coupling endothermic and exothermic reactions. The main disadvantage in this case is the unavoidable presence of hotspots (Balakotaiah, Christoforatou, West, 1999 Friedemann Balakotaiah, 1992), which results in catalyst deactivation and loss of membrane stability and selectivity. 

Limestone (CaCOs) can be calcined in a kiln to produce solid CaO and CO2 gas. The reaction is endothermic, and so, in every piece of limestone, heat must be supplied to the reaction site. This heat must be transported through the laminar flowing boundary layer in the gas phase, and through the CaO product layer. Simultaneously, the CO2 which is produced must be transported away in the opposite direction. The overall process involves a coupled transport of heat and mass. The relationship between the CO2 partial pressure and the temperature at the reaction site can be determined from thermodynamic data (unless thermodynamic equilibrium is not achieved, in which case additional kinetic data will also be required to determine the relationship). Fuel is burned in the kiln to supply the heat necessary to maintain the reaction. 

Gas-solid exothermic and endothermic reaction coupling (Ramaswamy et al., 2006)... 

Systems far from equilibrium exhibit unusual kinetic behavior. Feedback may lead to instability and ultimately explosion or to undamped (or weakly damped) oscillations about an inaccessible steady state. Here we treat another example of chemical instability— the abrupt switching between steady states with an attendant chemical hysteresis.The coupled variables are temperature and the progress variable for a chemical reaction, the same coupling that creates thermal explosion (Section 7.2) and thermal oscillation (Section 7.6.4). The difference is that here the chemical reaction is endothermic so explosion is ruled out. Instability is induced by heat flow into the system. Unlike the examples considered previously, the mathematical description of this coupled system is simple. The resulting equations may be solved and detailed theoretical predictions verified. 

As an example consider a second-order reaction Here the suitable coupling between pex and V, over and above the couphng due to the compressibility of the system (the equation of state triangle in Fig. 15.3), comes from the fact that on decreasing V the number of reactions per unit time increases as V for fixed total number of particles. For an exothermic (endothermic) reaction this effect leads to increased (decreased) production of heat as V decreases, and consequently to temperature and pressure changes, in addition to those due to compression. For first-order reactions coupling can be achieved through the temperature dependence of rate coefficients. 

The majority of ethylbenzene (EB) processes produce EB for internal consumption within a coupled process that produces styrene monomer. The facility described here produces 80,000 tonne/yr of 99.8 mol% ethylbenzene that is totally consumed by an on-site styrene facility. As with most EB/styrene facilities, there is significant heat integration between the two plants. In order to decouple the operation of the two plants, the energy integration is achieved by the generation and consunption of steam within the two processes. The EB reaction is exothermic, so steam is produced, and the styrene reaction is endothermic, so energy is transferred in the form of steam. 

Ergun, S. Fluid flow through packed columns , Chem. Eng. Prog., 48,89-94 (1952). Itoh, N. A membrane reactor using palladium , AIChE J., 33,1576-1578 (1987). Itoh, N. and T. Wu An adiabatic type of palladium membrane reactor for coupling endothermic and exothermic reactions , J. Membrane ScL, 124,213-222 (1997). 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

Prior to the chemical reaction of the silane with the silanol-groups on the sUica surface, the silane molecule has to make contact with the sUica surface by adsorption. Then the chemical reaction of silica with an alkoxy-silyl moiety of the coupling agent takes place in a two-step, endothermic reaction. The primary step is the reaction of alkoxy-groups with silanol-groups on the silica filler surface [4]. Two possible mechanisms are reported ... 

The coimter-current coupling of an endothermic reaction to a heating gas stream in a multi-layer architecture was studied by Hardt et al. [120], The 2-D geometry their model was based on is displayed in Figure 2.52. 

The oxidative coupling of methane has been studied by several authors. The most elusive transformation has been the oxidative coupling of methane into C2 hydrocarbons (ethene, ethane), because the reaction is more endothermic than other transformations [2]. The application of fast and efficient microwave heating to endothermic reactions is particular interest. 

Since the direct carbonylation of C-H bonds with CO leading to aldehydes is endothermic, the reaction is conducted under photochemical conditions.109,109a 109e On the other hand, the direct coupling of a C-H bond, CO, and an olefin leading to a ketone is exothermic and can proceed under thermal reaction conditions. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

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