Process maloperations

Additionally, consequences of possible process maloperations, such as incorrect charging sequence, contamination of reactants, agitation failure and poor temperature control, adding reactants too quickly. 

The full range of process maloperations, including system failures that might lead to process runaway will first have to be considered by a systematic evaluation of the plant and process concerned141. These may, for examplel be due to human error, hardware failure, or due to failure of a computerised sequence controller. To assess the likely/ credible maloperations accurately, it is recommended that personnel who will be operating the plant are involved in the hazard assessment. 

Process definition is considered further in this chapter and the general hazards of plant operation are considered in Chapter 7. Chapters 3,4 and 5 cover the evaluation of chemical reaction hazards and the effect of process maloperations. The selection of safety measures is the subject of Chapter 6. Chapter 8 covers the implementation and maintenance of safety measures. 

This process definition covers faults which, though not common, are known to occur in chemical processing. Examples are agitator failure, loss of plant cooling, leaks of cooling liquid into the batch, and process maloperation. Malopera-tion covers over- and under-charging of reactants, solvents or catalysts. Non-specific faults are so called because they are not specific to individual processes and the effect of them can be included in the hazard assessment without additional process description. 

It is important to investigate the effects on the reaction kinetics and rates of heat generation and gas evolution of factors such as scale-up, agitator configuration, materials of construction, variations in addition rate, reactant concentration and hold times. The effects of process maloperations should also be established. 

Many processes require equipment designed to rigid specifications together with automatic control and safety devices. Consideration should be given to the control, and limitation of the effects, of equipment malfunction or maloperation including ... 

Irregular Plant maloperation Process plant Dust and fume extraction plant... 

These tests demonstrated that the Lurgi Rectisol process provides an extremely pure synthesis gas which can be charged directly to the metha-nation plant without problems of sulfur poisoning of the nickel catalyst. However, in order to cope with a sudden sulfur breakthrough from Rectisol as a result of maloperation, a commercial methanation plant should be operated with a ZnO emergency catchpot on line. 

Irregular Plant maloperation Process plant Dust and fume extraction plant Flare stack Emergency/occasional flaring Plant failure Process plant — emergency venting Extraction/collection plant (cyclones, precipitators, filters, scrubbers)... 

From Table 6 it can be seen how the selected parameters have a connection to the basic principles of inherent safety. For instance the subindices of equipment safety and safe process structure contain several characteristics of inherent safety such as limitation of effects or tolerance to maloperation. It is practical to include several characteristics into few parameters, since the inherent safety principles are both very broad and overlapping. The philosophy behind them cannot be described just by one process parameter. The selected parameters are discussed in more detail on the following pages. 

Tolerance - resistant to maloperation corrosiveness equipment safety safe process structure... 

Unlike relief system sizing for non-reacting systems, a considerable amount of experimental information is normally required for the design of chemical reactor relief systems. It is necessary to assess all the credible maloperations and system failures that may occur on the process/ plant to determine the reaction runaway that requires the largest relief system. The Workbook also summarises the main steps necessary to do this. 

A list of typical maloperations is given, in Figure 3.2. However, it should be noted that these will be specific to the process and plant concerned and this list should not be regarded as comprehensive. ... 

When chlorine is reacted with a fuel on a catalyst bed, maloperation will result in catalyst burn- out and/or gas phase explosion before or after the catalyst. Here the determination of the fuel gas flammable limits in chlorine are of interest if the feed gas is not in the flammable range in normal process conditions. 

This is reflected also by discriminating between individual and societal risk. In the case of individual risk, the risk to a single person, who is either directly involved in the process performance or affected by damages caused by maloperation of the process, is assessed. In contrast, societal risk indicates the possible number of fatalities in their dependence on event probability. The difference of the two risk values shall be demonstrated in a short example. 

It is the task of development to optimize the route of synthesis with respect to envi-rorunental sustainability, economy, and quality. When a certain maturity of the process has been achieved the first larger scale-up step is undertaken. The damage which can possibly be caused by a maloperation of this significantly larger process has to be assessed quite differently. Nonetheless, it still remains controllable as, in the case of the chemical process, the ratio of heat transfer area to reaction volume is still very favour-... 

The decisive characteristic of an adiabatic temperature/time-course of a batch process is the enormous self-acceleration of the temperature rise over time. This self-acceleration process is responsible for the fact that corrective action is possible only in the initial maloperation phase. Therefore it is of great importance to have a measure at hand which indicates how long it takes for the adiabatic process to reach a phase of un-... 

If the Damkoehler number of isothermal semibatch processes is smaller than 10 the degree of accumulation becomes high. In such cases the reaction rate and thereby the heat production rate is not proportional to the feed rate alone. For such processes a synthesis optimization is recommended which in the last consequence will also benefit the overall process safety. As will be demonstrated in the context of the safety technical assessment of maloperations (c.f. Section 4.4), the reduction of the maximum accumulation is a very decisive factor. 

The systematic analysis of possibly occurring maloperations and process deviations as well as the assessment of their consequences is quite a demanding task, which must be solved in process safety engineering. The problem arises because of the close... 

Independent of the mode of reactor operation it is of tremendous importance to stop any further addition of fijesh reactants to the system, once a maloperation has occurred. This should be ensured technically, whenever possible. In elderly plants, organizational measures can be sufficient, provided they are of very good quality. The interruption in supplies eitsures that the hazard potential related to the maloperation remains restricted to the substance amoimt present in the reactor at the time of process disturbance. All further elaborations assume this effective interruption in supplies. 

The coolii failure shall be taken as a representative example for the numerous possible maloperations with an influence on the thermal stability of the process. For the moment, the cause of this failure remains more or less unimportant. A breakdown of the coolant supply pump and a stirrer defect have comparable effects, hi consequence the reactor behaviour will be close to adiabatic, which means that there will be no heat removal from the system to the environment anymore. 

It is recommendable in general that the MTSR should be as low as possible, because if higher values are reached the risk of triggering decomposition reactions increases. But Equ.(4-234) also shows that this requirement cannot easily be fiilfilled. The MTSR takes low values if either the desired process temperature is low itself or the accumulation potential at the time of maloperation is small. The accumulation potential, however, depends on the extent of reaction just achieved and on the mode of operation. If the process temperature is chosen very low, then the achievable conversion is also small and the accumulation potential will be very high. This indicates an optimization problem. 

The effect of any maloperations — agitation failure, rapid charging, loss of cooling, extended reaction times and so on — which have not been considered previously should be evaluated. The integration of the process with other manufactures may also need to be considered. 

A hazard assessment based on this level of process definition should adequately protect a process when it is operating normally. It does not, however, look at the consequences of common maloperations. 

Where necessary this assessment should be expanded to cover any maloperations that could realistically occur in the process. 

The assessment needs to cover not just the intended reaction but also unwanted or unexpected side-reactions and the possible decomposition of individual compounds in the reaction mixture. When designing a plant it is necessary to consider all foreseeable operating conditions, maloperations and future changes to the plant or process. 

The tests are normally run in a ramped temperature mode which ensures that the samples are subjected to temperatures in excess of the normal process temperature. This is es.sential in order to identify potential energetic decomposition which may be accessed by a maloperation of the desired process. 

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