Safe batch reaction

These points are explained in detail in this chapter. In a first section, the general aspects of reaction engineering for batch reactors are briefly presented. The mass and heat balances are analysed and it is shown that a reliable temperature control is central to the safety of batch reactors. The different strategies of temperature control and their consequences on reactor safety are explained in the following sections. For each strategy, the design criteria and the safety assessment procedure are introduced. The chapter is closed by recommendations for the design of thermally safe batch reactions. 

Thus, the following Golden Rules for Safe Batch Reaction can be formulated ... 

The Center for Chemical Process Safety (CCPS) has identified the need for a publication dealing with process safety issues unique to batch reaction systems. This book, Guidelines for Process Safety in Batch Reaction Systems, attempts to aid in the safe design, operation and maintenance of batch and semi-batch reaction systems. In this book the terms batch and semi-batch are used interchangeably for simplicity. The objectives of the book are to ... 

Frequently a piece of equipment is used in different processes during its lifecycle. This could result in process conditions that exceed the safe operating limits of the equipment. Equipment inspection may provide a poor prediction of the equipment s useful life and reliability, due to the change of material handled or change in process chemistry over the life of equipment. Batch operations are also characterized by frequent start-up and shut-down of equipment. This can lead to accelerated equipment aging and may lead to equipment failure. This chapter presents issues and concerns related to the safe design, operation, and maintenance of various pieces of equipment in batch reaction systems, and provides potential solutions. 

Tisted below are practices that should be considered in the design and safe operation of equipment in batch reaction systems. 

Laboratory studies are very important for providing basic knowledge to scale-up of batch reactions. Modeling a batch system is very important as well. When the batch reaction and system are well understood, a large scale-up factor may be applied while still maintaining safe operations. 

Designing and Operating Safe Chemical Reaction Processes (HSE 2000). Published by the U.K. Health and Safety Executive and directed to small to medium-sized chemical manufacturing companies using batch and semi-batch processes. It addresses chemical reaction hazards and inherently safer processes, hazards assessment, preventive and protective measures, and management practices. 

Analysis and Safeguards Lees, Loss Prevention in the Process Industries ISBN 0750615478 Guidelines for Safe Storage and Handling of Reactive Materials Guidelines for Process Safety in Batch Reaction Systems Guidelines for Hazard Evaluation Procedures, Second Edition with Worked Examples... 

Following these mles ensures a safe semi-batch reaction, even in cases of technical deviations or deviations from the normal operating conditions. 

Different criteria have been introduced in the past decades to individuate runaway boundaries in batchwise operated reactors. Most of them can be used to ensure a safe batch operation only when the reaction kinetics is fully known and the hypothesis of perfect mixing is satisfied. These criteria also strongly depend on the mode of operation with respect to heat exchange. Excluding isothermal conditions, the following modes of operation can be considered ... 

Ducry and Roberge reported controlled nitration of phenol in a glass microreactor with a channel width of 500 pm and an internal volume of 2.0 ml [2]. Nitration was most efficient and controlled under nearly solvent-free conditions at 20 °C without the addition of sulfuric acid or acetic acid (Scheme 4.2). Under these concentrated conditions, autocatalysis spontaneously started in the mixing zone, allowing safe control of the reaction. Undesirable polymer formation, which is significant in batch reactions, was effectively suppressed by a factor of 10. 

Temperature control of a batch reactor In a particular batch reaction process, temperature control is critical to the safe operation of the process, as excess heat resulting from control failure could cause a mnaway reaction. The reaction kinetics are such that insufficient time is available for an operator to respond to a high-temperature alarm. It was also determined that any actions initiated by a high-temperature SIF would be inadequate to prevent an overpressure demand on the mpture disk due to the response time of the sensor. 

When failure occurs, the simplest action is to stop or hold at the current operating state in response to any abnormal condition, and let the process operator determine the cause of the problem. However, some failures lead to hazardous conditions that require immediate action waiting for the operator to decide what to do is not acceptable. The appropriate response to such situations is best determined in conjunction with process hazards and operability (HAZOP) studies. For example, guidelines for safe operation of batch reaction systems have been published (Center for Chemical Process Safety, 1999). 

For exothermic batch reactions, the heat release is often controlled within the reactor cooling capacity by limiting the addition rate, or weight added, of one of the reactants. A prime objective of commissioning is to confimi the process design and to establish a reliable, safe operating procedure for routine production. 

Other recent industrial examples include the nitration and bromination of heterocycles, where the reaction not only could be conducted more safely in flow, but also it was reported that less impurities were observed within the flow synthesis. Various examples of the Swem oxidation have been reported recently, where the products were obtained in higher selectivity than in batch reactions, even when the reactions were conducted at higher temperatmes. Furthermore, the scientific community has frequently expressed concerns about conducting reactions involving heterogeneous catalysts within flow reactors, but a recent pubhcation has shown that it is possible to handle catalyst slurry within reactors when performing hydrogenation... 

The transformation from batch to continuous processing, the safe operation with bromine at temperatures over 170°C and the decrease of reaction time, i.e. increase of space-time yields, were drivers for the development here. 

Most accidents in the chemical and related industries occur in batch processing. Therefore, in Chapter 5 much attention is paid to theoretical analysis and experimental techniques for assessing hazards when scaling up a process. Reaction calorimetry, which has become a routine technique to scale up chemical reactors safely, is discussed in much detail. This technique has been proven to be very successful also in the identification of kinetic models suitable for reactor optimization and scale-up. 

Runaway criteria developed for plug-flow tubular reactors, which are mathematically isomorphic with batch reactors with a constant coolant temperature, are also included in the tables. They can be considered conservative criteria for batch reactors, which can be operated safer due to manipulation of the coolant temperature. Balakotaiah et al. (1995) showed that in practice safe and runaway regions overlap for the three types of reactors for homogeneous reactions (1) batch reactor (BR), and, equivalently, plug-flow reactor (PFR), (2) CSTR, and (3) continuously operated bubble column reactor (BCR). 

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