Model nitrobenzene hydrogenation

Some recent models have also appeared discussing the operation of three phase catalytic membrane reactors by Torres et al. [82]. The models which represent extension of prior models by Akyurtlu et al. [79] and Cini and Harold [80] are numerically analyzed and appear to simulate well the experimental results of the nitrobenzene hydrogenation reaction in a three phase catalytic membrane reactor. 

M. Torres, J. Sanchez, J.-A. Dalmon, B. Bemauer and J. Lieto, Modeling and simulation of a three-phase membrane reactor for nitrobenzene hydrogenation. Ind. Eng. Chem. 

Torres et al. (1994) studied nitrobenzene hydrogenation on an asymmetric catalytic porous tubular membrane where a thin catalytic layer (ca. 3 pm) is supported by a thick (ca 1.1 mm) macroporous inert tube and they developed a model using experimental kinetics. They showed that by using an inorganic membrane the most beneficial configuration to the overall reaction kinetics is when the gas phase flows directly on the catalytic layer while the liquid phase flows along the inert support. It should be noted that the support was impregnated by the liquid phase. 

Torres, M., Sanchez, J., Dalmon, J.A., Bernauer, B., Lieto, J., 1994. Modeling and simulation of aThree-phase catalytic membrane reactor for nitrobenzene hydrogenation.and Engineering Chemistry Research 33,2421-2425. 

The challenge of monitoring batch processes for deviations was addressed by Gabrielsson et aP" (also taken up by Wong et al. °) with the hydrogenation of nitrobenzene to aniline as a model reaction. The study followed a factorial design of experiments. Process deviations could only be detected when the UV spectroscopic data set and the process data set were used in a combined manner, not seperately. 

We decided to use as a very sensitive model reaction the hydrogenation of 1-chloro-2,4-di-nitrobenzene in order to find new and efficient dehalogenation inhibitors. For comparison, dicyandiamide was used as standard inhibitor. We concentrated our search of new modifiers on nitrogen containing compounds and found the most interesting effects with compounds of the following general structure... 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

Hydrogenations of nitrobenzene and p-nitrotoluene over supported noble metal catalysts are often investigated as model reactions, as they consist of various elemental reactions with different intermediates that can react with each other as depicted in the scheme below for p-nitrotoluene [321], At short reaction times, the intermediates are predominantly formed, whereas complete conversion to p-methyl aniline is achieved at long reaction times. Aniline itself can react further to form side products such as cyclohexanol, cyclohexyl amine and other species. 

The hydrogenation of nitrobenzene has been chosen as model reaction in a detailed exploration of the CMR... 

It is worth mentioning that the rate constant for the reaction of OH radical with H2, which is one of the most important reactions in water chemistry, has also been studied up to 350°C by competition kinetics using nitrobenzene as a competing OH scavengerd At higher temperatures, the rate constant undershoots an extrapolation of the Arrhenius plot and actually decreases in value above 275°C. At 350°C, the measured rate constant is more than a factor of five below the Arrhenius extrapolationd This implies that the amount of hydrogen injection calculated by the current model of water radiolysis in nuclear reactor should be reconsidered. 

Case Study 11.5 Comparison of Selected Fluid-Bed Models for the Catalytic Reduction (by Hydrogen) of Nitrobenzene to Aniline... 

Hydrogenation of nitrobenzene over Pd-Al203 catalyst was successfully used as a model reaction for testing a continuous-flow MW reactor under pressure for both... 

Non-hydrogen-bonded liquids of high dielectric constant are often referred to as dipolar aprotic solvents. Nitrobenzene, V,V-dimethyl-formamide, acetonitrile, and nitromethane have dielectric constants in the range 35 to 38, close to that of methanol. The dielectric constants of sulpholane, dimethylsulphoxide and propylene carbonate are higher. Any peculiarities of acid-base behaviour in dipolar aprotic solvents as compared with methanol cannot be accounted for on the basis of the Born sphere-in-continuum model. 

There are not many measurements of elementary reactions with free radicals in supercritical water. Two examples are the addition of OH radical to nitrobenzene [114] and the reaction of the OH radicals with methanol [115]. The experimental rate constants for hydrogen abstraction from methanol by OH radicals are significantly higher than the values used by a variety of researchers for modeling oxidation of methanol in supercritical water using detailed chemical kinetics models [115]. 

Figure 9.6. Calculated variation of the initial rate of hydrogenation of nitrobenzene, NB, for various models. (— - ) simple Eley-Rideal varying [NB] ( ) full Eley-Rideal varying [NB] (—— —) full Eley-Rideal...

Example 8.1 Catalytic hydrogenation of nitrobenzene to aniline Application of the basic model... 

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