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10 neW or improved proCesses Category High-purity isobutene adds higher value Using methanol to convert isobutene to MTBE is an equilibrium reaction and, therefore, can also run in the opposite direction. This is where the team set to work: what kind of process could recover the chemically bound isobutene from the MTBE? How would the catalyst required for this process have to be designed? How can the byproducts of the products be removed cleanly to produce highpurity isobutene? These were crucial considerations because isobutene is a high valueadded substance with a growing market. Among other uses, it is a key raw material for methyl methacrylate, or MMA. MMA, for its part, is a highly soughtafter product used to make such products as PLEXIGLAS® for highly advanced LED flatscreen monitors, ecofriendly coatings, or even lightweight and therefore fuelsaving plastic parts in automobile production. The global market for MMA is growing by 5 percent per year. In Asia, it is growing considerably faster. Isobutene is a starting material for other chemical products such as butyl rubber (tires), adhesives, and pharmaceutical products, too. The key to success: the catalyst MTBE conversion is a heterogeneously catalyzed reaction that occurs, for best results, in the gas phase at high temperatures. The experts spent much time designing the optimal heat supply and gas flow in the reactor to ensure the highest possible conversion rate. However, the key to the success of the process is a customdesigned and highly selective catalyst. So the team Figure 1 Typical composition of C4 crack, a C4-hydrocarbon mixture produced in steam crackers. Because isobutene and 1-butene show nearly identical physical properties, they cannot be separated through distillation or extraction Component Isobutane Isobutene 1-Butene 1,3-Butadiene n-Butane trans-2-Butene cis-2-Butene elements34 Issue 1|2011 Boiling point [°C] -11.7 -6.9 -6.3 -4.6 -0.5 0.9 3.6 Chemical structure Content in C4 crack [Mass-%] 1–3 20–28 14–20 40–45 4–8 4–6 2–5 also had to find a new catalyst for the new isobutene production process, because conventional formulations were less selective and produced too many byproducts. They also aged too quickly. In the search for this catalyst, <strong>Evonik</strong> profited from its wealth of expertise in catalyst development and kinetic screening. The screening process involved producing a large number of catalytically active substances and then testing them in miniaturized automated laboratory reactors operated in parallel. Within only nine months, <strong>Evonik</strong> specialists screened 90 catalysts with various substrates, promoter quantities, and preparation methods. One thousand five hundred experiments were necessary to vary the parameters, which included temperature, pressure, dwell time, and MTBE composition. At the end of this highthroughput screening, the team had a catalyst composed of different inorganic oxides that splits the MTBE very selectively, is longlived, highly active, and generates few byproducts. Reaching the target fast by applying simulations However, experiments will get you only part of the way when you are developing new processes. Simulation, in fact, monitors and supports the entire development process of a new product— from the idea, through the catalyst search and process synthesis, to the design of commercialscale plants. Simulations have a variety of advantages. They uncover significant issues which require additional laboratory work, e.g. whether the reaction can be carried out under a pressure that considerably simplifies downstream processing. For example, if a catalyst generates byproducts that cause problems during separation, researchers can use simulation calculations to take account of this early on in process development. Figure 2 Splitting MTBE into isobutene and methanol—equilibrium conversions in the liquid and gas phases. Conversion in the gas phase at high temperatures can shift the equilibrium further in the direction of isobutene and methanol Liquid phase Gas phase O MeOH MTBE MTBE conversion [%] Isobutene Methanol 100 80 60 40 20 0 50 100 150 200 250 300 + Temperature [°C]
The simulation, on the other hand, is refined by new findings from the experiments. Not least, advanced software is indispensible for continuously reviewing the costeffectiveness of new processes and thereby minimizing the risk of investment errors. This iterative process of simulation, experiment, and evaluation also formed the framework of the development of the new isobutene process. Permanent exchange between theory and practice allowed the realization of a complex production process resulting in isobutene with a purity of over 99.9 percent. The high purity is crucial for success. Since only pure starting products allow efficient and economical downstream processes, customers‘ raw materials standards are constantly rising. For the process synthesis, experts from the Process Technology & Engineering Service Unit first evaluated available substance physical property data and used molecular simulation to assess missing information. On the basis of the initial results from the catalyst screening, they created several possible process variations and simulated them on the computer. The results of these calculations were then verified on the laboratory scale. Iterative testing and evaluation ultimately yielded the optimal process, which was tested in a pilot reactor. To simplify subsequent scaleup, the engineers made this pilot reactor a single tube with measurements similar to the reactor pipes in the intended commercial plant. Test runs provided additional key information to the process engineers about byproducts and catalyst aging, which they then integrated back into the simulation. Feedback is important because catalysts often change their activity and selectivity—through the later addition of adhesives and additives, for instance—when a process is scaled up. Feedback also yielded information on the optimal temperature and pressure range, and the steps required to discharge byproducts. 333 Figure 3 Potential secondary reactions in the splitting of MTBE to isobutene and methanol. The marked components are either specified within extremely narrow limits in the isobutene product or they can enrich themselves in the process loop, as well as the corresponding starting materials 2 2 MeOH + H 2 O O O O evonik innovation aWard 2010 11 High-throughput screening In their quest for the optimal catalyst for splitting MTBE, researchers conducted about 1,500 experiments on 90 catalysts in miniaturized laboratory reactors operated at the same time OH + H 2O + MeOH + MeOH = Sharp specification in isobutene product = Accumulation in process loop elements34 Issue 1|2011
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