Shutdown, large continuous processes

While startup and shutdown occur relatively infrequently in large continuous plants, they are inherent in batch plant operation. Most startup and shutdown procedures, whether devised empirically or theoretically, are designed to follow a recipe of actions with no feedback. Thus, if upsets occur, there is often no way to change the startup or shutdown in time to avoid imwanted process exclusions. Procedures are needed that incorporate feedback and adaptive techniques to the problem of plant startup and shutdown. 

First of all, before we compare flow reactors, let us mention the batch reactor briefly. The batch reactor has the advantage of small instrumentation cost and flexibility of operation (may be shut down easily and quickly). It has the disadvantage of high labor and handling cost, often considerable shutdown time to empty, clean out, and refill, and poorer quality control of the product. Hence we may generalize to state that the batch reactor is well suited to produce small amounts of material and to produce many different products from one piece of equipment. On the other hand, for the chemical treatment of materials in large amounts the continuous process is nearly always found to be more economical. 

The field of process systems engineering refers to the various techniques to design, simulate, optimize, and operate chemical processes. A majority of chemical processes operate in a continuous fashion that is, raw material is continuously fed to the manufacturing process and product is continuously produced. Large manufacturing processes often run for 18 months or more without any major shutdown. For this reason, most process systems engineering techniques have been applied to continuous processes, which are the focus of this article. Batch processes are presented elsewhere. 

When no hazard exists, continued operation of the process is generally preferable to initiating an unnecessary shutdown. Shutdown can cause additional hazards, such as large flaring events or cascade shutdowns of other process units, and does result in the need to restart the process unit with all of its potential hazards. The decision to shut down versus continued operation is made by balancing the risk posed by the potential process hazard if a process demand occurs and the risk associated with shutdown and start-up. This decision is greatly influenced by the relative robustness of the SIS architecture and system utilities. The path can be selected at the channel, sub card, card, or CPU level of the system. The device safety manual may contain specific requirements that should be followed, unless a detailed analysis of the final system architecture demonstrates that deviation from the safety manual is acceptable. 

Aerobic fermentation processes also require a continuous supply of large quantities of air, typically on the order of one volume of air per volume of liquid per minute, VVM. Sterilization of this air is mandatory in almost all fermentations. Absolute filter cartridges of polymeric membranes are now used almost exclusively in the fermentation industry. Relatively small units have replaced the large depth filters used in the past. Still, water and particulates pose a major problem for filters thus requiring the use of prefilters and traps to remove these contaminants before they reach the absolute filter. Parallel installation of the filters prevents a total shutdown of the fermentation process in the event of filter clogging. 

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