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Converging a Crude Column

Solution ID: 114173

Product(s):  Aspen HYSYS

Version(s): Multiple

Primary subject: Unit Operations, Column Operation, Troubleshooting Convergence

Last Modified: 09-Dec-2005

Applicable Version(s)

 All

Problem Statement

 Are there any tips and tricks for converging refinery crude columns?

Solution

Below are a few tips for converging refinery crude columns:

1. Observe the oil distribution chart in the Oil Manager. If the oil characterization for the stream was not done

within the HYSYS case, you can re-engineer the TBP curve based on the Boiling Point Curves utility so that

you can then view the oil distribution chart.

The oil distribution chart (in terms of volume) allows you to see the production limit for each product stream. With the

oil distribution chart and the feed flowrate, you can roughly work out the proportion of flow for each product stream;

these values can then be used as initial column specifications.

2. Converge the side operations in a few steps. E.g. Start with Gas Oil pumparounds & Gas Oil side strippers,

run and converge the column; then add Diesel pumparounds & Diesel side strippers, run and converge the

column; then add Kerosene pumparounds & Kerosene side strippers, run and converge the column.

3. Do not use the 'Reset' button, because the history of solutions helps you get to the next solution. It may or 

may not converge again after pressing 'Reset'.

4. Start by using variables that make for easier column convergence when used as active specifications. You

should have few problems getting a converged column completed with all the side operations if you start with

"easy" specifications. Flow specs for product streams and duty specs for pumparounds are easy specs; the

reflux ratio is another possibility.

Once you have a converged column, slowly "trade" the easy specification variables for "difficult" specification

variables:

e.g. Start with duty specs, then "trade" for temperature specs. To guarantee convergence, it is important to trade the

'corresponding' specs E.g. Duty for Diesel_Cooler replaced with Diesel_return_temperature spec.

e.g. Start with product flow specs, then "trade" for cut point specs. To guarantee convergence, it is important to trade

the 'corresponding' product flow and product quality specs. E.g. Kerosene product rate replaced with Kerosene cut

point spec.

Before making a problematic specification active it is a good idea to first observe its current calculated value. Observe

the calculated values for a few specified values of the corresponding easy spec variable, so that you can get a feel for 

the upper and lower bounds for your difficult spec. If the spec value you intend to use differs a great deal from this

bound, it gives you some indication that your intended specification may not be achievable with the current column

conditions.

5. If you find that a new specification is not achievable, consider adjusting other specifications to achieve the

desired target spec. For example if you want to specify a higher flow rate for the diesel product, you may want

to specify a lower flow rate for the kerosene product. You can extend this idea to the more difficult specs, e.g.

you can adjust cut point & RVP spec values for one stream relative to other specifications.

6. Save your HYSYS case after each convergence so that in case you made a wrong choice in specs or spec

AspenTech Support Center Solution 114173 3/4/2016

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values, you can reload the saved converged case quickly.

KeyWords

Column, column convergence, crude distillation

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