Case Study Methyl Acetate

Here we briefly discuss results from a case study on methyl acetate RD, in which, based on detailed investigations of thermodynamics [2] and fluid dynamics 3], different process models were not only compared with each other but also with experimental data. Full details are reported by Moritz [4]. 

Methyl acetate production is often used as an example to demonstrate the advantages of RD processes [5]. The basic set-up of the heterogeneously catalyzed methyl 

In the fluid dynamic design with Models 1-3, information on the density and viscosity of the mixtures is needed, which usually has to be estimated as no experimental data is available. This can, however, be done with an accuracy sufficient for most cases using standard methods [8]. 

The amount of additional information needed to be able directly to take into account heat and mass transfer in Model 4 is high. Using the two-film theory, information on the film thickness is needed, which is usually condensed into correlations for the Sherwood number. That information was not available for Katapak-S so that correlations for similar non-reactive packing had to be adopted for that purpose. Furthermore, information on diffusion coefficients is usually a bottleneck. Experimental data is lacking in most cases. Whereas diffusion coefficients can generally be estimated for gas phases with acceptable accuracy, this does unfortunately not hold for liquid multicomponent systems. For a discussion, see Reid et al. [8] and Taylor and Krishna [9]. These drawbacks, which are commonly encountered in applications of rate-based models to reactive separations, limit our ability to judge their value as deviations between model predictions and experimen- 

Differences between Model 2 and Model 3 would only have been expected for residence time sensitive reaction systems, such as, for instance, ethylene glycol production from ethylene oxide and water, but not for the esterification studied here. The very simple Model 1 still gives reasonable predictions, which would, for instance, be sufficient for conceptual design studies. The fact that the rate-based Model 4 gives entirely wrong predictions in the present case should not be overemphasized and may be due to the very limited amount of available thermophysical and fluid dynamic input data. 

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Initial theoretical studies focused on steps (1) and (2). Several model systems were examined with ab initio calculations.1191 For the reaction of methyl amine with methyl acetate, it was shown that the addition/elimi-nation (through a neutral tetrahedral intermediate) and the direct displacement (through a transition state similar to that shown in Figure 5a) mechanisms for aminolysis had comparable activation barriers. However, in the case of methyl amine addition to phenyl acetate, it was shown that the direct displacement pathway is favored by approximately 5 kcal/mol.1201 Noncovalent stabilization of the direct displacement transition state was therefore the focus of the subsequent catalyst design process. 

Additional acylation studies were also reported (24), (26). In the first case it is claimed that acylation of thiophene is achieved by means of HC104 and acetic anhydride affording a 65 % yield of 2-acetylthiophene. In the second paper Levine and coworkers reported that while 2,5-dimethylthiophene could be readily acetylated, 2,5-dichlorothiophene acetylated sluggishly. This is, however, readily explained, since the presence of chlorine atoms on the thiophene ring decreased its reactivity in electrophilic substitution reactions. In the case of methyl substitution, however, the 3 and 4 positions of the ring are activated toward electrophilic substitution by the inductive and hyperconjugative effects. Thus 2,5-dimethylthiophene was successfully acylated by the boron fluoride etherate method in high yield with three aliphatic anhydrides. 

The synthesis of methyl acetate from methanol and acetic acid analyzed in this case study is the same as described by Reaction (R17) ... 

Case Study of Tetrahydro-1,4-Oxazine (Morpholine). As in the case of aromatic pollutants, the oxidation of the N atom and the C atoms was observed with or without the destruction of the morpholine cycle (92). Also, the abstraction of H atoms to form methyl groups was illustrated by the formation of A-hydroxyamino-2-ethenyl ethyl ether, /V-formyl-formamine, and acetate ions. When the starting morpholine concentration was increased, intermediate products with mass peaks much greater than the molecular mass of morpholine were detected, showing that coupling reactions occurred the concentrations of these intermediates were very low. 

Hydrolysis of methyl acetate for producing methanol and acetic acid using SMB and Vaiicol Process Two cases (a) maximization of purity of both raffinate and extract streams, and (b) maximization of yield of both raffirrate and extract streams. NSGA The study found that the reactive Varicol performs better than SMB reactor for the apphcation studied. Yvetal. (2005)... 

For the other fuel oxygenates detailed studies on the reaction pathways are not as abundantly available in literature. However, the similarity in chemical structure implies similar reaction by-products. In the case of TAME, tert-amyl formate and tert-amyl alcohol were observed instead of fBF and fBA, and their subsequent degradation products however, acetone and methyl acetate were observed as well [117]. The attack on the methoxy group was observed to be the major pathway, corresponding to the MTBE ehmination. During the elimination of ETBE the same reaction by-products were observed as with MTBE, with the exception of fert-butyl acetate which was formed instead of fBF [30]. 

Fig. 4.2 RD process model types used in the methyl acetate case study...

For the methyl acetate synthesis, dynamic modeling effects are investigated, whereas for other systems, the focus is on different steady-state issues, for example the influence of liquid-liquid separation, operational conditions and different column internals (ethyl acetate) or selectivity effect (dimethyl carbonate transesterification). The comparison between the simulation and experimental data made for all RD case studies proves that the rate-based approach is capable of predicting correct process behavior, both steady state and dynamic. 

It is not our aim to give a detailed discussion of experimental results obtained in the thermodynamical study of polymer solutions. We will do no more than consider a few examples. Among the earlier measurements we may mention the calorimetric determinations of the heat of dilution in solutions of cellulose nitrate in various solvents and the heat of solution of cellulose acetate in methyl acetate In both these cases heat was generated on mixing < 0). Later... 

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