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Chemical plant design & operations FAQ

Plant Modeling

How do I calculate Theoretical Stages using Plant Data? by sshep
Posted: 25 May 07 (Edited 4 Mar 11)

Use this proven approach with any simulator to calculate the number of theoretical stages in an operating distillation column when the product flowrates and compositions are known.

1) Using plant data, blend the distillate and bottoms stream flowrate and composition to synthesize a feed stream (see pfd below)
2) Bring the feed to plant enthalpy and pressure conditions with heater blocks (example shows combination of heating and flashing across valve)
3) Specify the distillation column block using plant values for: tower pressure, distillate flowrate, reflux flowrate and temperature. NOTE: this is the same distillate flowrate as used in step#1 above
4) Run sensitivity on number of theoretical stages keeping feed location in proportion to actual tower. NOTE: if you believe section efficiencies can vary you can run a 2 dimensional search on total stages and feed stage.
5) Plot the sensitivity results and find the closest match to your distillate composition- see example. This is your number of theoretical stages. NOTE: bottoms will also be close due to material balance constraint.
6) The last step is a reasonability check. Consider the example. When the intersections fall to the far right, we could be using excess reflux. In such a case we should collect data at a lower reflux rate.

Power Point Example
My procedure is designed to address these stumbling blocks which I have found common:

using plant feed analysis and expecting to match product flows and compositions- step#1 insures the material balance is consistent between feed and products;

not realizing that reflux and distillate flow should be set to plant values- step#3 addresses this.
Once you are familiar with this approach, you can make various refinements to suit your needs including: sidedraws, pumparounds, and heaters. These are all set at plant values.

Multi-component seperations may require some quantification of "best match" in step#6. Using reasonably choosen aggregations will allow for the same solution plotting as is possible with binaries.

For a trayed tower, overall efficiency can be calculated as theoretical/actual stages. For a packed tower, HETP can be calculated as actual bed depth/theoretical stages

best wishes,

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