In batch flow, some physical mass of materials is processed together as a unit. Recipe management is essential. Predetermined quantities of raw materials are added to the vessel and blended and reacted with the entire batch being completed at one time. A plant can keep records by batch and, if necessary, troubleshoot by batch.
In a continuous process, the product is comingled continuously. The process is either on or off; material flow never stops while the process is running. Raw materials enter at the beginning of the process, and the final product comes out continuously.
In a batch process, 5-min averages of process values are seldom meaningful. Chemicals can be added to the batch at various times, at which point the temperature may drop. Then, the batch can be heated for a period of time, held at a particular temperature for a while, and then allowed to cool. In this process, the average temperature for the whole cycle offers no real information about the batch properties. A different type of data gathering and archiving is needed that is more event-oriented.
Continuous processes rely heavily on regulatory controls. In these processes, the control system can be set to read a temperature data point every 5 sec, average it for 5 min, and present the resulting data as a representative sample of the process temperature.
In batch systems, linkage to product specifications and the daily shipping schedule are commonly needed and can be provided by DCS. In continuous processes, recipe management is less of a concern, and shipments are typically in bulk.
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