Avoiding Secondary Reactions

The condensation of an aromatic nitro compound with a second reactant should have been performed in an aqueous solution with DMSO in the semi-batch mode. The nitro-compound is initially charged into the reactor with water and DMSO as solvent. Before the progressive addition of the second reactant had been started, the initial mixture was heated to the process temperatures of 60-70 °C. Then a failure of the cooling water system of the plant occurred. It was decided to interrupt the process at this stage and to maintain the mixture under stirring until the failure had been repaired. The feed of the second reactant was postponed and the jacket of the reactor had been emptied. 

Thermal Safety of Chemical Processes Risk Assessment and Process Design. Francis Stoessel Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31712-7 

Secondary decomposition reactions may have serious consequences when they get out of control. In this case history, the thermal stability of the reaction mass was not known before the incident. If only the energy released by this decomposition had been known, the production staff would not have decided to maintain this reaction mass without active temperature control and monitoring. Thus, assessing the consequences and the triggering conditions of secondary decomposition reactions and predicting their behavior requires a specific knowledge and a systematic approach. 

This chapter describes a runaway scenario. The first section presents a general review of the decomposition reaction characteristics. The second section is devoted to the energy release that defines the consequences of a runaway. The third section deals with triggering conditions of undesired reactions, based on the concept of TMRld. The next section reviews some important aspects for the experimental characterization of decomposition reactions. Finally, the last section gives some examples stemming from industrial practice. 

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This procedure is useful for avoiding secondary reactions of the aldehydes leading to high-boihng products. It is particularly advisable when linear aliphatic aldehydes are synthesized using cobalt catalysts. ... 

Tb avoid secondary reactions, the flow is cooled by cold ethylene injected by a proprietary device (8). Such quenching controls acetic acid at negligible values and corrosions are undetectable. 

Bromine Addition to Alkenes. Alumina can advantageously replace protic solvents thus avoiding secondary reactions due to their nucleophdicity. This situation is evidenced in the bromation of alkenes [14]. When performed in methanol, bromine addition leads to a mixture of a frans-dibromo adduct and a trans-bromo ether compound. The latter results from competitive attack by pro-tic solvent on the bromonium ion intermediate. This byproduct can be suppressed using Br2/alumina, as the support behaves as a non-nucleophilic polar medium (Scheme 3). 

Leftin and Chmeil (139) considered the effects of solute structure on the rate of formation of the 3000 A band in concentrated sulfuric acid solutions. In order to avoid secondary reactions, due to the presence of unreacted or undissolved compounds, measurements were restricted to the concentration range between 10-4 and 2 x 10-3 M. Figure 47... 

These results underline the efficiency of the vacuum pjT olysis process in avoiding secondary reactions of acids since oil 5 was produced at 415 C in the vacuum pyrolysis demonstration unit and the reactions in the precoker were observed at 350 -450 C. It is therefore clear that the vacuum pyrolysis has an important asset due to its ability to preserve intermediate products of thermal degradation which may include highly priced fine chemicals. 

Open systems consist of vessels from which the volatile pyro-products can leave the heated reactor thus avoiding secondary reactions. Closed systems are sealed reactors that confine all reactants and products to a restricted volume. The removal of gaseous products from the open system reactor can be enhanced by purging with inert or chemically active... 

The selective oxidation of ethylene to ethylene oxide (EO) is performed on supported silver catalysts at temperatures of250—280 °C, and a pressure of roughly 20 bar. In this process, it is necessary to avoid secondary reactions of EO. Typical industrial catalysts may contain 8—15 wt% silver dispersed on low surface area (X-AI2O3 (0.5-1.3 m g ) with a porosity of about 0.2—0.7 cm g In addition, the catalyst may contain several promoters in varying amounts (ppm by weight) 500—1200 ppm alkali metal (mostly cesium), 5-300 ppm of sulfur as cesium or ammonium sulfate, 10-300 ppm offluorine as ammonium fluoride, or alkaft metal fluoride (427). 

Cyclization of 2 in concentrated sulphuric acid [14-16] predominantly leads to p-ionone (17). The reaction proceeds rapidly even below room temperature and, to avoid secondary reactions, is carried out continuously. The precooled streams of sulphuric acid and the solution of 2 in petroleum ether or liquid CO2 are mixed in a reactor and then quenched with cold water. Small amounts of a-ionone (18) can be separated off by distillation during isolation of the product. In the cyclization step large amounts of approximately 40% aqueous sulphuric acid are produced. Treatment to deal with this is expensive but is essential for environmental reasons. Organic impurities are broken down to carbon dioxide in a cracking furnace with heavy oil burners. In the course of this process, sulphuric acid is thermally converted into sulphur dioxide, which is reoxidized in the contact plant. 

Microporous membranes in general (pore diameter < 2 nm), and zeolite membranes in particular, have pores whose dimensions are similar to those of many molecules. This means that often molecules cannot pass each other in a restrictive pore medium, and single file diffusion occurs. Such a molecular queuing (see Figure 11.24) may provide a new scenario for avoiding secondary reactions, that is, to increase selectivity in consecutive reaction networks with a valuable intermediate... 

Dehydration of hexoses to hydroxymethylfurfural (5-HMF) H-ZSM-5, H-Y, H-P, H-mordenite Different acid zeolites have been studied to replace industrial homogeneous systems based on the use of sulfuric acid as catalysts. The use of biphasic systems is desirable to extract formed 5-HMF and avoid secondary reactions [61]... 

The reaction products formed after the reaction of COf with the substituted benzenes were determined either by GC-MS or HPLC under the described experimental conditions. Products were analysed for different irradiation times up to 10 min and for <10% reactant consumption in order to avoid secondary reactions of the reaction products. The products formed from the substrates investigated and properly identified are listed in Table 1. Aliphatic compounds of low molecular weight, i.e. containing four carbon atoms or less, are eluted with the solvent and could not be detected with our chromatographic set-up. 

While the first reaction is easily obtained from a thermal point of view the formation of the second NH3 molecule through isocyanic add hydrolysis represents a key step in the process [45]. The latter can be accelerated by using a catalyst, with the added advantage of avoiding secondary reactions (see below) that lead to the formation of larger molecules and possibly to the deactivation of the system ... 

HR modification with MA is accompanied by secondary reactions. Branches and crosslinks can appear by intermolecular grafting or by direct radicalic processes between species. Another possible reaction allowed by the presence of some impurities (Si, HjO, Na", K", Ca, Mg ) is MA decomposition, which can evolve toward explosion. To avoid secondary reactions, additives are introduced into the system (phenols, di- and trisubstituted phenols, Cu-naphthenate and acetylace-tone, halogenated triazine, triazols, phenylenediamine, phenylenecatechol). 

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