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Advantages of on-Site Microreactors from Safety Viewpoint

Fatemeh Ebrahimi, Eero Kolehmainen, Terhi Virkki-Hatakka, Benguang Rong, and Ilkka Turunen. Chemical TECHNOLOGY, LAPPEENRANTA UNIVERSITY OF TECHNOLOGY, Lappeenranta, Finland

On-site production of chemicals is sometimes feasible even though the production in large scale, for the needs of several custumers, is usually more economical. Often the main advantage is connected with safety issues. By producing on-site, the Transportation of dangerous chemicals can be avoided. Moreover, smaller on-site production processes also mean a step towards inherently safer technology. Microreactors represent a technology, where the safety advantages resulting from small scale are maximized. In some cases the advantages of this new technology can also make the on-site production economically competitive to larger production units.

Safety advantages of microreactors in on-site production are studied in this paper. Production of peracetic acid is used as a test case. This unstable and explosive chemical is used e.g. in treatment of municipal waste water.

The study is based on comparison of a traditional production process with the capacity of 170 kg/h and an on-site microprocess producing 10 kg/h peracetic acid. Preliminary design of these processes was carried out. Then four different methods were used to analyze the safety of the processes: Reaction matrix [1], Dow fire and explosion index [2], Inherent safety index [3] and the worst case consequence analysis [4].

The reaction matrix is a systematic method to study the interactions among the various materials used in a chemical process. It is used to reveal the possible incompatibility or hazardous behavior of substances in a process. Since e only the chemicals and construction materials are considered in the method, it does not reveal any safety differences between the processes being compared here. Of course the on-site installation might bring along new unexpected contacts of substances, which should be revealed by the method. In this sense, the selection of on-site production might sometimes increase the dangers of production. On the other hand, on-site production makes the transportation unnecessary, which often brings along a very important safety advantage.

The second method, Dow fire and explosion index, indicated that the on-site microreactor was clearly a safer alternative. This was mostly based on smaller inventory of dangerous materials. However, it seems that Dow index does not take fully into account the dramatic size difference between the microreactors and conventional ones.

According to the third method, the inherent safety index, the traditional production technology would be safer than the on-site microscale production. This conclusion is probably false. It seems that the index is not well applicable to evaluate the safety of very novel technology, such as microreactors, that differ significantly from traditional solutions.

The fourth method, so called “worst consequence analysis” indicated that the on-site microprocess technology was clearly safer. This method seemed to be most suitable for the evaluation of safety of novel technology. In addition, the method was able, more than the other ones, to take into account the specific features of the processes under study.

Two general conclusions can be made from the study. Firstly, microreactors have plenty of advantages from safety viewpoint, when compared to traditional production technologies. These advantages are mostly based on radically smaller reaction volumes and on effective heat transfer which allow a fast temperature control. The safety advantages of microreactors can be effectively utilized in on-site production.

Secondly, the conventional methods for analysis of process safety seem not to be reliable and adequate for radically novel technology, such as microprocesses. This is understandable because the methods are more or less based on experience, which is very limited in the connection of totally novel technology.

References

1.Mosley, D.W., Ness, N, Hendershot, D.C, Tools for understanding reactive chemical hazards early in process development, 2000

2.Dow's fire & explosion index hazard classification guide, AIChE technical manual, 7th edition, American institute of chemical engineers, New York 1994

3. Heikkilä, A-M., Inherent safety in process plant design, PhD Thesis, Technical Research Centre of Finland, VTT publications 385, Espoo, 1999

4. Kleindorfer P.R., Belke J.C., Elliott M.R., Lee Kiwan, Lowe R.A., and Feldman H.I., Accident epidemiology and the U.S. chemical industry: Accident history and worst-case data from RMP info, Risk Analysis, Vol.23, No.5, 865-881, 2003.