Column Not Trying? (Hints for columns that will not execute)

Craig Spears

Feb 25, 2011

Sometimes a column will not solve when the execute button is pressed, and occasionally the warnings or messages are missing, hiding or not helpful. The following are some of the most common things to check to try and get your column to begin iterations:

• Specify the Column Pressure – check the stage data, summary grouping to make certain that the column has a known pressure profile. This can be specified in several manners: setting the top and bottom pressure, setting any stage pressure and a pressure drop, or just the pressure drop and letting ProMax use the inlet stage pressures. Regardless of the method, ProMax must know the column pressure.
• Check that the Degrees of Freedom are Zero – this must be exactly zero. If the number shown is positive, then you must add additional specifications. If the number is negative, then the column has managed to be over-specified, and one or more specifications must be removed. The specifications may be found on the specifications tab in the tower, or in streams and blocks connected to the tower (such as condensers, reboilers, side towers, or side draws).
• Verify that any Condenser/Reboiler are attached – If the column is placed on the flowsheet with the condenser and/or reboiler already attached, ProMax groups them automatically. If you draw them separately, or utilize a thermosyphon reboiler, then you must manually attach them to the column. This option is found at the bottom of the Project Data tab of the distillation column.
• Don’t have any Conflicting Specifications – this condition should be noted in the message log as an over-specification. An example of conflicting specifications would be setting both boil-up ratio and reboiler heat duty.
• Supply Pressures for any Attached Equipment – any attached equipment (condensers, reboilers, side towers) should have pressure drops given. ProMax will automatically give a zero pressure drop for grouped condensers and reboilers, but these can be overwritten by the user, and side columns are not given a default pressure drop.
• Attach any Missing Equipment Energy Streams – Condensers and Reboilers should always have energy entering or leaving in some method. If a single-sided exchanger is being used, verify that an energy stream is attached.
• Fully Specify all Pump-around Blocks – If the distillation column has a pump-around with a heat exchanger, pump or other block, check that the pressure drop and any required specifications have been made. Also check that an energy stream is attached to the heat exchanger if it is single-sided and to any pumps or compressors.
• Add any Required Pump-Around Estimates – Verify that any required estimates have been made, especially if the column has a pump-around or side column. Duty or flow ratio estimates might be required. The most common occurrence of a required pump-around duty estimate is the presence of a reflux pump. A short-cut in this case is to temporarily bypass the reflux pump. Once the column has a solution, then the pump-around estimate is no longer required, and the reflux pump may be added back to the loop.

In the very rare case that a column still refuses to execute, even after checking the list above, please send it in to our support engineers to take a look at it. If the column attempts to execute but fails, look forward to a separate blog entry covering steps to take.