What does it mean by cycling in distillation columns?
see
https://www.aiche.org/resources/publications/cep/2015/december/revive-your-columns-cyclic-distillation
one time liquid streams flow in the column and vapors stop. in other times, vapor stream flow upward while liquid lines are stagnat.
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In a Batch / Continuous distillation column there is a term that is Reflux Ratio
( Rd) is the ratio of flow of distillate vs bottom products . This Rd value will guide
the recycling of Distillate portion .
Cyclic distillation involved segregating the fluid traffic in the distillation column, with fast (i.e. in the order of seconds) switching between (downward) liquid flow and (upward) vapor flow. Segregation reduces unnecessary mixing between the vapor and liquid phase, and between the liquid material on adjacent trays.
Columns intended for cyclic operation use customized/dedicated internal components (including special trays that have no downcomers). The concept of cyclic operation was also applied to extraction columns. In general, adding such technology to a chemical plant is likely acceptable for new investments, but retrofits of existing distillation or extraction towers may prove costly and problematic.
Recycling mean returning part of already distilled fluid back into the distillation column for better separation of the components. This can be achieved in two ways. 1 By reflux ratio process 2. By returning part of the distilled fluid through the inlet fluid stream and introducing a mixer to blend inlet and the recycling stream together.
Cycling might mean:
1. Internal rhythmic rapid vaporization followed by condensation, especially where strong subcooling in the condenser allows the condensate to condense excessive internal vapors within the column.
2. Process cycling: A recycle cut from a batch distillation is fed into the column for additional purification to reach product purity specification.
Rowland's continuous recycle to enrich the feed will only make sense in a column that is very large relative to the feed throughput.
When I hear the term "cycling" associated with a distillation column I think of a control issue that causes the operation to continuously adjust one or more variables (usually reflux and/or flash zone temperature) in a relatively narrow range but never actually settles on a steady state operation.
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