Runaway’ reaction equipment safety

The first major objective for the inherent safety review is the development of a good understanding of the hazards involved in the process. Early understanding of these hazards provides time for the development team to implement recommendations of the inherent safety effort. Hazards associated with flammability, pressure, and temperature are relatively easy to identify. Reactive chemistry hazards are not. They are frequently difficult to identify and understand in the lab and pilot plant. Special calorimetry equipment and expertise are often necessary to fully characterize the hazards of runaway reactions and decompositions. Similarly, industrial hygiene and toxicology expertise is desirable to help define and understand health hazards associated with the chemicals employed. 

The information to be compiled about the chemicals, including process intermediates, needs to be comprehensive enough for an accurate assessment of the fire and explosion characteristics, reactivity hazards, the safety and health hazards to workers, and the corrosion and erosion effects on the process equipment and monitoring tools. Current material safety data sheet (MSDS) information can be used to help meet this requirement but must be supplemented with process chemistry information, including runaway reaction and over-pressure hazards, if applicable. 

Noronha, J. A., R. J. Seyler, and A. J. Torres, "Simplified Chemical and Equipment Screening for Emergency Venting Safety Reviews Based on the DIERS Technology," in Proceedings of the International Symposium on Runaway Reactions, Center for Chemical Process Safety/AIChE, New York, NY (1989). 

Figure 1.4 shows a simplified flow chart of the process. An intermediate in the processes is methyl isocyanate (MIC), which was stored in a series of tanks partially underground and equipped with cooling systems and a series of safety control devices (Figure 1.5). In fact, MIC is a highly toxic (maximum exposure TLV-TWA, during an 8-hour period is 20 parts per billion), flammable gas that has a boiling point near to ambient temperature and gives a runaway reaction vdth water unless chilled below 11 °C. Table 1.4 gives the list of MIC safeguards.

For process equipment, designers need to specify necessary safety features and the tests for meeting requirements. For process equipment, there should be fail-safe features. Fire protection, overpressure, excess heat, runaway reactions, dust control, exhaust ventilation, dangers of flammable liquids, leaks, sensing devices to report status are all examples of important safety features. Designers and purchasers need to consider access for setup, maintenance and cleaning. There may be a need for access by stairs, fixed ladders or platforms as part of large equipment. 

For this reaction system, the plant safety office believes that an upper temperature limit of 180 should not be exceeded in the tank. This temperature is the practical stability liinit. The practical stability limit represenis a temperature above which it is undesirable to operate because of unwanted side reactions, safety considerations. secondary runaway reactions, or damage to equipment. CoiLsequently, we see that if we started at initial temperature of 7) - ]60°F and an initial concoi-tration of 0.14 mol/dm- the practical stability limit of I80°F would be exceeded as the reactor af oached its steady-state temperature of 138 F. See the concentra-tion-temperaiure trajectory in figure El3-4.4. 

The pilot plant must also be carefully designed so that its control and safety systems are, fail-safe, and any unexpected equipment or utility failure brings the unit into a safe and de-eneigized condition. Unexpected or rapid process changes, if they can herald or lead to dangerous conditions (eg, runaway exothermic reaction), should be continuously monitored by appropriate instrumentation and suitable automatic action provided (1,55—67). 

Initially a few particularly sensitive tubes of the bundle will run away, i.e., the reaction changes, for example, from a selective partial oxidation to a total combustion, and the temperature rises rapidly. In a multitubular reactor with thousands of tubes every tube cannot be equipped with temperature profile measurements it is therefore likely that this runaway will remain undetected, especially if it involves only a few tubes. Although temperatures above 1000 °C can often be reached in the catalyst during such runaways, there is no safety risk, provided the tube is surrounded by a liquid heat-transfer medium. Because of the good heat transfer to the fluid the tube temperature remains close to that of the heat-transfer medium, and melting of the tube does not occur. 

As mentioned earlier, the most commonly employed alkylene oxides for producing block nonionic surfactants are the very reactive three-membered cyclic ethers such as EO and PO. Particularly, EO is highly flammable, explosive in some condition, and also toxic and an irritant for skin and eyes. For these reasons, the reactor employed in the synthesis, described earlier, must be equipped with sophisticated safety devices, process monitoring systems, and distributed control systems (DCS) to keep the reaction in safety condition and avoid gas-phase decomposition and the so-called runaway... 

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