SMPO process

Buijink, J.K.F., van Vlaanderen, J.J.M., Crocker, M., and Niele, F.G.M. (2004) Propylene epoxidation over titanium-on-silica catalyst-the heart of the SMPO process. Catal. Today, 93-95,199-204. 

There are several alternatives to the polluting chlorohydrin route. One is the styrene monomer propene oxide (SMPO) process, used by Shell and Lyondell (Figure 1.6a) [14]. It is less polluting, but couples the epoxide production to that of styrene, a huge-volume product. Thus, this route depends heavily on the styrene market price. Another alternative, the ARCO/Oxirane process, uses a molybdenum... 

Figure 1.6 a SMPO process b the catalytic oxidation of propene with tert-butyl hydroperoxide in the presence of a molybdenum-oxo complex. 

About 10 million tons of styrene and 3.5 million tons of propylene oxide are produced annually, worldwide. Most of the styrene is still produced by dehydrogenation of ethylbenzene and it is estimated that ca. 800,000 tons of styrene and 400,000 tons of propylene oxide are produced by the SMPO process. 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

Propylene Epoxidation via Shell s SMPO Process 30 Years of Research and Operation... 

Shell has coproduced propylene oxide (PO) and styrene using its proprietary styrene monomer propylene oxide (SMPO) process for three decades. Research, development, and plant trials have been performed on a continuous basis in order to improve its efficiency and cost competitiveness. We report here some of the key fundamental and technological learnings gathered over various parts of the process. 

We will describe here the SMPO process, with particular attention to the propylene epoxidation step, and exemplify numerous process improvements that stem from a deep understanding of all aspects of the process. We will start with a general process description to provide the necessary context. 

The SMPO process comprises four main reaction steps that are schematically drawn in Fig. 13.1. A simplified process flow sheet is found in Fig. 13.2 ... 

FIGURE 13.1 Main reaction steps of the SMPO process. 

FIGURE 13.2 SimplifiecI process flow scheme for the SMPO process. 

The catalytic epoxidation step may be considered to be the heart of the SMPO process. Tlie catalyst is prepared in a multistep gas-phase process by treatment of a silica carrier wifh fifanium tetrachloride, heating the obtained material, followed by steaming and silylation. Possible improvements to this process and research on the support material have already been reviewed [1]. Here, we discuss the nature of the active site, the mechanism of epoxidation, and the various catalyst deactivation mechanisms that exist for fhis unique cafalysf. 

We believe that the mechanism for the Sharpless epoxidation can be generalized, and that the heterogeneously catalyzed propylene epoxidation step in the SMPO process proceeds in a similar fashion, as depicted in the simplified mechanism shown in Fig. 13.6. 

Similar to the heterogeneously catalyzed propylene epoxidation step discussed above, a detailed imderstanding of all conversion steps has been obtained in 30 years of operation of the SMPO process. This has led to numerous process improvements that helped optimize the overall conversion of propylene and EB to PO and SM. In turn, this has led to much reduced specific capital employed for the production of these bulk chemicals, as is shown for PO in Fig. 13.8. 

FIGURE 13.8 Historic overview of effect of SMPO process development on specific capital employed for SMPO plants. 

The authors would like to acknowledge all Shell employees who have contributed over the past decades to the build-up of knowledge on the SMPO process. 

Perfumery interest in this process is four-fold. Firstly, one of the major products, styrene, is a starting material for perfume ingredients, as described above. Secondly, the other major product, propylene oxide, is also a precursor for a number of fragrance materials and for dipropylene glycol, one of the major solvents used in perfumes. Thirdly, the intermediate, styrallyl alcohol, is the starting material for a number of esters used in perfumery, the acetate in particular. Fourthly, but by no means least important, the crude styrallyl alcohol contains a small trace of 2-phenylethanol. Since the SMPO process is run on a scale far beyond that of the perfume industry, what is a small... 

It is extremely difficult to purify this by-product 2-phenylethanol to the odour quality of that produced by either of the above routes. However, most of the companies, such as ARCO in the USA and Sumitomo in Japan, who run the SMPO process can produce 2-phenylethanol of a quality which can be used in perfumery (often in collaboration with a perfumery company). The amount of 2-phenylethanol available from this route is dictated by the demand for styrene and propylene oxide, the market value is dictated largely by material from the other two routes and all three run in economic balance. 

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