Solution_126341 - Troubleshooting Distillation Columns in Hysys

AspenTech Support Center Solution 126341

1/14/2014

Troubleshooting distillation columns in Aspen HYSYS Solution ID:

126341

Product(s):

Aspen HYSYS

Version(s):

2004.2, 2006, 2006.5, V7.0, V7.1

Primary subject: Unit Operations, Column Operation, Troubleshooting Convergence Last Modified:

02-Jul-2009

Associated Solutions: ID: 110069

McCabe-Thiele Plot Unit Operation Extension

ID: 109971

My column will not solve because the "Equilibrium Error" is oscillating and not converging. What should I check?

ID: 109406

My column wont converge, complaining of finding two liquid phases on one of the stages. What should I check?

Problem Statement What steps should be considered if there are difficulties in converging a distillation column or obtaining the required performance from the column.

Solution If the column is not converging the diagnostic output generated in the trace window during the column iterations provides helpful clues on how the solver is progressing. If the equilibrium errors are approaching zero, but the heat and spec errors are staying relatively large, the specifications are likely difficult to meet. If both the equilibrium errors and the heat and spec errors do not converge, then the column inputs for example, the initial estimates, the specifications, and the column configuration should be examined. Before starting the troubleshooting steps listed below, first reset the column and then run it again. The Reset button will purge all estimates from the previous solution. Column Configuration Check the following list for column configuration: ÿ

Feed Stage Location: If the feed stage is too high or low from the optimal then the column might be pinched. Pinching can be best detected by using a x-y diagram by using the McCabe Thiele extension for Aspen HYSYS, available from solution 110069.

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Feed Temperature: Too hot or too cold feed temperature might affect some trays and cause either pinching or a portion of the column to run dry.

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Reflux ratio: Usually distillation columns operate close to the minimum relfux. However numerically this can be difficult to converge at minimum reflux condition.You may first start with a higher reflux ratio an and if the column solves with this then lower this progressively to the actual value.

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Impossible specifations: Some specifications might not be feasible. If the simulation has problems meeting an active specification, check the Design | Monitor or Spec page of the column: ÿ

Extreme purity: The purity specification might not be feasible with the current current setup of the column.

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Specification of more than one purity or recovery for the same component may not be possible to meet.

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Avoid specifying all the products from the column.

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AspenTech Support Center Solution 126341

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ÿ

If any product rate is specified check that this can be achieved with the given feed rate.

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There is no vapour or liquid phase exist at the specified temperature.

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Bad Estimates: Estimates can help converging the column. A bad estimate might take longer to solve or may not converge at all.

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Two liquid phases: If there is two liquid phases you might need to add water draws for the water to exit or use Sparse Continuation solver (refer to solution 109406) .

Once a column is solved the results can be viewed from the Performance tab within the column flowsheet. In the Performance tab | Plots Page of the column, you can display temperature, flow composition and Key ratio plots or tables (LK/HK - Light Key / Heavy Key components). Initial Estimates If the column specifications and feeds are setup correctly the problem might be due to poor initial estimates. Difficult separations require temperature and or flow estimates. Temperature and flow estimates can be entered in the Parameters tab | Profiles page of the column. Composition estimates are available from the Parameters tab | Estimates Page. Estimates can also be entered as non active specifications,  by ticking the estimate radio button. However make sure the estimates are reasonable otherwise the column might not converge at all. To generate estimates you can activate the "Dynamic Integrator for IEG" option from Parameters settings. This will generate estimates by running the column in dynamics (relaxation method). Solver If the solver oscillates it might be useful to decrease the dumping factor or to enable the adaptive damping from the Parameters tab | Solver page. Solver methods: HYSIM Inside-Out

General purpose method, which is good for most problems.

Modified HYSIM Inside-Out

General purpose method, which allows mixer, tee, and heat exchangers inside the column subflowsheet. Only a simple Heat Exchanger Model (Calculated from Column) is available in the Column subflowsheet. The Simple Rating, End-Point, and Weighted models are not available.

Newton Raphson Inside-Out

General purpose method, which allows liquid-phase kinetic reactions inside the Column subflowsheet.

Sparse Continuation Solver

An equation based solver. It supports two liquid phases on the trays, and its main use is for solving highly nonideal chemical systems and reactive distillation.

Simultaneous Correction

Simultaneous method using dogleg methods. Good for chemical systems. This method also supports reactive distillation.

For more details on the various solver methods and further troubleshooting advice consult the Aspen HYSYS Operations Guide document supplied with the installation disks or from the support website.

Keywords Column Convergence, Solver method

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