Two kinds of startup and shutdown are commonly encountered, depending on whether the column base is full ("wet") or empty ("dry") when the column is shut down. As will be seen, wet column startups and shutdowns are much faster. The same basic override system permits automatic startups regardless of whether they are wet or dry. It should be noted that no formal program or sequence control is necessary. Instead, as soon as steam and feed are available, the column comes on line at the maximum speed permitted by the constraints. Depending on circumstances, a column may or may not follow the same sequence of constraints from startup to startup.
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