Thanks to their large specific surfaces, microprocessing plants are usually significantly more efficient than conventional technology in terms of the transport of materials and heat. In concrete terms this means: greater selectivity, larger yields and increased safety for fast exothermic reactions. Enough of the theory. How this translates into practice was shown at conference entitled “Industrial use of microprocessing technology” staged by Dechema.
The name microprocessing technology almost inevitably leads one to assume that the components involved are on the small side. And many of the early developments do indeed look cute. However, this is a misconception, as Dr. Steffen Schirrmeister from Uhde explained: He established that micro-effects are only required in the relevant areas critical to reactions; otherwise, it is perfectly possible on the whole for the dimensions of a reactor to be in the order of a meter. The incoming and outgoing feeders are then also of the usual size. The throughput also reaches the normal proportions: Dr. Hieng Kim presented a Vortex mixer developed at Clariant by way of example. The mixing takes place as desired due to the micro-effects, but the throughput remains extremely high at 15,000 kilograms/per hour.
Vinyl acetate plant
However, implementation of micro-reaction technology in practical industrial applications is still a major challenge. The trick is to produce the microstructures cost-effectively on a large scale and to add them to reactors with the capacities typical of industrial applications. The report on the joint project between Degussa and Uhde on the development of a production microstructure reactor for gas phase syntheses aroused considerable interest. This is designed to significantly reduce the investment cost of a complete process for manufacturing vinyl acetate and the corresponding operating costs.
Vinyl acetate, a base chemical for plastics, colors and adhesives, has been manufactured for decades by means of oxidative connection of acetic acid and ethylene in tubular reactors. The annual volume of the vinyl acetate market is roughly five million tons and is growing by an average of three per cent a year. The distinctive feature of the process is the significant exothermic reaction. The limited surface-volume ratio of the tubular reactor, particularly for the desired high space-time yields, results in local temperature increases of up to 100 Kelvin and in turn to dips in selectivity and premature ageing of the catalyst.
Another disadvantage of the tubular reactor is the enormous cost of adding the catalyst. If a change of catalyst is required, production can be suspended for up to four weeks. This is set to change as a result of the joint project. The aim is to develop a microstructure reactor that can be used for industrial production and which is intended to reduce the investment cost for a whole process by more than a ten per cent and operating costs by several percentage points. According to those involved in the project, this will offer considerable potential for savings: In a production plant with an annual capacity of 150,000 tons for example, the ongoing costs in terms of depreciation and operating costs are reduced by more than three million euros a year.
Advantages for selectivity
The basis of micro-reactor technology and the intensification of processes is the continuous running of processes. This is always cheaper than production at intervals, as Dr. Thomas Schwalbe explained in relation to micro-reactor systems. Not only that: changing from batch to continuous production also offers quality and environmental benefits. Schwalbe was particularly impressed by just how much the processing time from a raw material through to the end product is reduced; many non value-adding steps are eliminated. Schwalbe: “Micro-reaction technology quite simply speeds up the chemistry.” The impression gained was that all large chemical and pharmaceutical companies are now starting to use microprocessing technology, and some have already been using it for quite some time. It has been ten years since Merck, for example, began to explore the possibilities and opportunities for using micro-reaction technology, as Dr. Michael Häberl reported: “A production plant equipped with micro-reactors was in operation from 1999.” In the meantime, as he noted, micro-reaction technology has become a standard tool in the development of processes.
This was contingent on the development of modular and highly versatile units to be used, adapted for their respective task, in research and development laboratories with minimum preparation time. In order to arrive at an economic assessment of the use of micro-reaction technology in development projects, the potential savings that could feasibly be achieved by using micro-reaction technology can be assessed on the basis of the potential theory, it was reported. Häberl: “If we take one reaction as an example, it shows that the development costs can be reduced to no more than a third of those of conventional batch development if micro-reaction technology is used.“ Another advantage cited by Häberl is that significantly less material is required. While some 50 kilograms are required for a reaction in a conventional batch reactor, one kilogram is usually sufficient for microprocessing technology. This is naturally a major advantage for extremely expensive starting materials.
Dr. Dominique Roberge from Lonza believes that pharmaceuticals production offers interesting scope for using continuously operated micro-reactors. The background to this, as he explained, is that five to ten per cent of production regularly fails to conform to specifications and has to either undergo costly processing or be rejected. According to Roberge, micro-reaction technology could be the answer to this problem: more targeted control of the temperature and flow and improved mixing could enable closer compliance with product specifications. Dr. Kai Lovis from Schering confirmed these expectations by presenting micro-processing technology projects already completed. The company has gathered practical experience of this through the following:
- Reactions under extreme conditions (high-/low-temperature reactions);
- Gas/liquid reactions (reactions with reactive gassing such as ozonolysis; reactions with release of large volumes of gas);
- Reactions close to decomposition conditions;
- Reactions with toxic substances.
In the case of a low-temperature ozonolysis, for example, the small-volume production process for a steroid from batch operation first had to be converted for continuous running of the reaction under mild conditions with the same level of quality using a falling film micro-reactor. A scale-up was possible using a larger falling film reactor. A continuously operated stirrer is available for reactions which release large volumes of gas, posing safety problems. The event can be summarized as follows: Microprocessing technology has proved above and beyond its effective use as a tool for developing and optimizing processes that it can also offer an effective alternative to conventional processes in the industrial production environment.
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