Reactor runaway chemical

Chemical Reactor Runaway Chemical reactions may create too much pressure for the container holding the material. Inadequate cooling, insufficient stirring, too much catalyst and other factors may cause the reaction to go out of control. In a sealed container, pressure increases may result from the reaction itself and from temperature increases following the Boyle-Charles law (see Chapter 19). 

Figure 9-9 Nomograph for sizing two-phase reactor reliefs. Source H. K. Fauske, Generalized Vent Sizing Nomogram for Runaway Chemical Reactions, Plant/Operations Progress (1984), 3(4). Used by permission of the American Institute of Chemical Engineers.

McIntosh, R.D., Nolan, P.F., Rogers, R.L. and Lindsay, D. (1995) The design of disposal systems for runaway chemical reactor relief. Journal of Loss Prevention in the Process Industries, 8 (3), 169-83. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

H. Wu, M. Morbidelli, and A. Varma. An approximate criterion for reactor thermal runaway. Chemical Engineering Science, 53 3341-3344, 1998. 

The key element in temperature control of chemical reactors is to provide sufficient heat transfer surface area or some other heat removal mechanism so that dynamic disturbances can be safely handled without reactor runaways. 

Boyle [15] and Huff [16] first accounted for two-phase flow with relief system design for runaway chemical reactions. A computer simulation approach to vent sizing involves extensive thermokinetic and thermophysical characterization of the reaction system. Fisher [17] has provided an excellent review of emergency relief system design involving runaway reactions in reactors and vessels. Fauske [18] has developed a simplified chart to the two-phase calculation. He expressed the relief area as ... 

In addition to appropriate mass transfer rates, sufficiently rapid heat transfer is essential to control the behavior of chemical reactors. For example, if the local rate of heat removal does not match the rate of heat produced by the chemical reaction, hot spots may form. Because reaction rates depend exponentially on temperature, reactor performance, product yield, and selectivity are strongly influenced by non-isothermicity. In the case of exothermic reactions, a steep local temperature increase may lead to reactor runaway. 

An analysis of 189 industrial incidents in the UK involving thermal runaway chemical reactions in batch/semi-batch reactors classified the incidents in terms... 

Protection of a reactor against runaway chemical reaction by emergency relief venting poses the additional problem of the safe disposal of the resulting relief stream. If the reactor contents or products formed during the runaway reaction are toxic, corrosive, flammable or foul smelling, it is unlikely that venting directly to atmosphere will be acceptable. 

Specialized microwave reactors for chemical synthesis are commercially available from such companies as CEM [20], Lambda Technologies [21], Microwave Materials Technologies (MMT), Milestone [22], PersonalChemistry [23], and Plazmatronika [24] which are mostly adjusted from microwave systems for digestion and ashing of analytical samples [25]. They are equipped with built-in magnetic stirrers and direct temperature control by means of an IR pyrometer, shielded thermocouple or fiber-optical temperature sensor, and continuous power feedback control, which enable one to heat reaction mixture to a desired temperature without thermal runaways. In some cases, it is possible to work under reduced pressure or in pressurized conditions within cavity or reaction vessels. 

While continuous processing is standard for most products manufactured on a large scale, batch production tends to be the norm in the fine chemical and pharmaceutical industries. Although batch manufacture may be superficially flexible, it runs counter to the whole intensification ethos. This is because the heat and mass transfer duty for the entire batch is concentrated in a restricted period and the heat/ mass transfer equipment must be sized to accept the peak batch load. If this is not done, then reactor runaway can ensue, with disastrous consequences. On the other hand, with continuous production, the respective steady state heat and mass transfer rates are much less and automatically involve smaller equipment. 

As the specific surface area (Equation 2.24) and the heat transfer coefficient (Equation 2.21) increase with decreasing diameters, it follows that microstructured channel reactors are characterized by very short cooling times, thus improving temperature control and reducing the risk of reactor runaway. As a consequence, microstructured reactors can be operated under harsh reaction conditions such as high temperature and pressure. The chemical kinetics is speeded up drastically reducing the characteristic reaction time. This concept is often called novel process windows [20]. 

Once a decision to use QRA has been made, you must decide whether frequency and/or consequence information is required (Steps 6 and 7). In some cases you may simply need frequency information to make your decision. For example, suppose you wish to evaluate the adequacy of operating procedures and safety systems associated with a chemical reactor. The main hazard of concern is that the reactor could experience a violent runaway exothermic reaction. You believe that you know enough about the severe consequences of a runaway and nothing more will be gained by quantifying the consequences of potential run-... 

Adequate heat removal facilities are generally important when controlling the progress of exothermic chemical reactions. Common causes of thermal runaway in reactors or storage tanks are shown in Figure 7.4. A runaway reaction is most likely to occur if all the reactants are initially mixed together with any catalyst in a batch reactor where heat is supplied to start the reaction. 

Thermal runaway reactions are the results of chemical reactions in batch or semi-batch reactors. A thermal runaway commences when the heat generated by a chemical reaction exceeds the heat that can be removed to the surroundings as shown in Figure 12-5. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase, and subsequently accelerates the rate of heat production. Thermal runaway occurs as follows as the temperature rises, the rate of heat loss to the surroundings increases approximately linearly with temperature. However, the rate of reaction, and thus the... 

The RSST can rapidly and safely determine the potential for runaway reactions. It also measures the temperature rates and, in the case of gassy reactions, pressure increases to allow reliable determinations of the energy and gas release rates. This information is combined with analytical tools to evaluate reactor vent size requirements. This is extremely useful when screening a large number of different chemicals and processes. 

Similar approaches are applicable in the chemical industry. For example, maleic anhydride is manufactured by partial oxidation of benzene in a fixed catalyst bed tubular reactor. There is a potential for extremely high temperatures due to thermal runaway if feed ratios are not maintained within safe limits. Catalyst geometry, heat capacity, and partial catalyst deactivation have been used to create a self-regulatory mechanism to prevent excessive temperature (Raghaven, 1992). 

A chemical reactor, being started, was filled with the reaction mixture from another reactor which was alic.uly on line. The panel operator increased the flow of fresh feed while watching an eye level temperature recorder He intended to start cooling water flow to the reactor when the temperature began to rise, but did not because the tempe.r.j, ure recorder was faulty, thus a runaway reaction. 

Safety. The MR is much safer than the MASR. (1) The reaction zone contains a much smaller amount of the reaction mixture (hazardous material), which always enhances process safety. (2) In case of pump failure, the reaction automatically stops since the liquid falls down from the reaction zone. (3) There is no need to filter the monolithic catalyst after the reaction has been completed. Filtration of the fine catalysts particles used in slurry reactors is a troublesome and time-consuming operation. Moreover, metallic catalysts used in fine chemicals manufacture are pyrophoric, which makes this operation risky. In a slurry reactor there is a risk of thermal runaways. (4) If the cooling capacity is insufficient (e.g. by a mechanical failure) a temperature increase can lead to an increase in reaction, and thus heat generation rate. 

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