Methyl acetate, process design

Case E Eastman Chemical Methyl Acetate Process (Siirola, 1995, 1998). In the commercial operation of the Eastman methyl acetate process, many reaction and separation functions are combined in one single large column (a few meters in diameter and 80 meters high (38). The number of pieces of major equipment compared to a conventional design is reduced by a factor of 10 (36). The primary energy consumption and capital expenditure is reduced by a factor of 5 (38). 

Reactive distillation is one of the classic techniques of process intensification. This combination of reaction and distillation was first developed by Eastman Kodak under the 1984 patent in which methyl acetate was produced from methanol and acetic acid. One of the key elements of the design is to use the acetic acid as both a reactant and an extraction solvent within the system, thereby breaking the azeotrope that exists within the system. Likewise, the addition of the catalyst to the system allowed sufficient residence time such that high yields could be obtained, making the process commercially viable. Other examples in which reactive distillation may enhance selectivity include those of serial reactions, in which the intermediate is the desired product, and the reaction and separation rates can be systematically controlled to optimize the yield of the desired intermediate. ... 

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. 

Consider the following industrial example of a process for the production of methyl acetate, first synthesized in the conventional conceptual process design manner, then modified using evolutionary approaches, and finally resynthesized using the lessons from the hierarchical process synthesis procedure experience. For simplicity here, only identity, amount, and composition differences are resolved (that is, temperature and pressure changers will be ignored). 

The classic flowsheet for the production of acetates was unsuitable for adoption for methyl acetate. However, the flowsheet generated by the conventional conceptual process design approach using literature schemes and standard patterns seems especially complicated for such simple chemistry involving so few components. This first flowsheet is a candidate for evolutionary modification. 

On the basis of this development afforded by Eastman and Halcon, in 1983 the Eastman Chemical Company (Kingsport, TN) started the commercial process for the manufacture of acetic anhydride (Figure 5). Methyl acetate, the feedstock for the carbonylation reaction, was produced in a separate esterification step from acetic acid and methanol. The process was designed to produce 225 000 tons of acetic anhydride and 75 000 tons of acetic acid/year. The overall yield of acetic anhydride based on methanol is approximately 96 % [2, 47]. 

The term reactive distillation (RD) refers to both catalyzed and uncatalyzed reaction systems. Catalytic distillation systems may use a homogenous or heterogenous catalyst to accelerate the reaction. Reactive distillation is a well-known example of reactive separation process, and is used commercially. The first patent and early journal articles deal mainly with homogenously catalyzed reactions such as esterifications, transesterifications, and hydrolysis.f Heterogenous catalysis with RD is a more recent development. The key advantages for a properly designed RD colunm are complete conversion of reactants and attainment of high selectivity. An example of the benefits of RD is the acid catalyzed production of methyl acetate by... 

Today the main process application for bubble cap trays is in reactive distillation columns or in chemical absorption columns in either case, it may be necessary to control very carefully the residence time of the liquid to complete a reaction step. For example, bubble-cap trays are used for the methyl acetate column described earlier and published by Agreda et al. An abridged version of the Bolles treatment of bubble-cap tray design is given in the fifth edition of Perry s handbook." ... 

Agreda, V.H., L. R, Partin, W.H. Heise, 1990, High-purity methyl acetate via Reactive Distillation, Chem. Eng. Progr., 86, Febmary, 40-46 Allen, D.T., K., S. Rosselot, 1996, Pollution Prevention for Chemical Processes, Willey Bildea, C. S., A. C. Dimian, 1998, Stability and Multiplicity Approach to the Design of Heat-Integrated PFR, AIChE J, vol. 44,2703-2712... 

BP Chemicals Low Pressure Process Design. A process flow diagram for the BP Chemicals carbonylation process is shown in Figure 3 [9]. The reactor contains acetic acid, water, hydrogen iodide, methyl iodide, and the rhodium-based catalyst. Methanol is pumped to the reactor and carbon monoxide is compressed to approximately 36 bars (525 psig) and sparged into the bottom of the liquid filled reactor. 

Flowsheets for processes are sometimes generated without following the hierarchy of properties described previously. As an example, Siirola [20] proposed a reactive-distiUation solution to make methyl acetate. Unit operations that combine the property differences present abrupt departures from common methodologies. With the advent of various pieces of equipment, such as differential side-stream feed reactors (i.e., semicontinuously fed batch reactors), continuous evaporator-reactors (e.g., wiped-film evaporators), and reactive distillation columns, one can consider these unit operations in the development of conceptual designs. As an example, Doherty and Malone [21] have presented systematic methods for reactive distillation design. 

In this section, design and operation of an industrial column for acetic acid dehydration via heterogeneous azeotropic distillation is investigated. The entrainer used for this industrial column to aid the acetic acid and water separation is also isobutyl acetate. This entrainer is circulating inside the column through OR stream from a decanter. Multiple column feed streams from various parts of the upstream process are fed into this column. The feed components besides acetic acid and water also include small amount of methyl acetate and m-xylene. These components are intermediate boilers and tend to accumulate inside the column. They cannot leave the column system through either the top decanter AO stream or... 

Figure 7.2. The gradual change in the process configuration is the direct consequences of two factors increased immiscibility (Fig. 7.3) and the shift of the boiling point ranking of the two products (water and acetate). Table 7.5 gives the optimized design for the production of methyl acetate (MeAc, type I), ethyl acetate (EtAc, type II), isopropyl acetate (IPAc, type II), butyl acetate (BuAc, type IH), and amyl acetate (AmAc, type IE) systems.

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