Modeling a column reboiler as a detailed heat exchanger
Note: this article applies to HYSYS 2006.5 and earlier - in v7 Aspentech introduced reboiler options in the Input Expert. Nevertheless, our conclusions are essentially the same.
When modeling a column in HYSYS, it is convenient to use the standard templates that are available from the object palette, which allow you to add columns with or without condensers and reboilers.
The standard condenser and reboiler models use a duty stream to represent the cooling or heating applied. If you wanted to model the condenser and reboiler more rigorously, you could replace it with a heat exchanger.
Here we review several options in HYSYS for modeling the reboiler as a heat exchanger:
- Model the reboiler in the column subflowsheet
- Model the reboiler in the main flowsheet as part of the column
- Model the reboiler in the main flowsheet using an “Internal Stream”
Process Ecology’s experience indicates that while we have used all of these strategies successfully, option 3 is usually best for convergence. All three of these options are discussed below.
1. Column Subflowsheet
To model the detailed reboiler in the column subflowsheet, one needs to enter the column environment, delete the reboiler, and add specific equipment as shown here:
On the surface, this is an appealing method – as the column internals are dealt with in the column subflowsheet and there is no need to add additional equipment in the main flowsheet. However, it can be much more difficult to converge a column with additional equipment in the column subflowsheet and it is not recommended for novice users. In addition, there are limitations; for example, the steam rate cannot be directly calculated since all feeds to a column – which include “Steam In” – must be fully defined. Also, in the column environment the heat exchanger is modeled as an “End Point” exchanger, with no apparent way to change the method – meaning that the heat curves are linear and may not properly model the phase change occurring in the exchanger.
2. Reboiler in Main Flowsheet
To get around these problems, one could simulate the reboiler in the main flowsheet – modeling the column as a simple absorber, and simulating the reboiler equipment outside of the column.
3. Reboiler in Main Flowsheet – using an Internal Stream
Perhaps the simplest method is to use an “Internal Stream” to create a duplicate of a column subflowsheet stream in the main subflowsheet, then model the heat exchanger in the main subflowsheet.
The details of adding the internal stream are discussed later in this article. The biggest advantage of this method is the simplicity – it is easy to set up, the column subflowsheet is left as is, and no recycle operations are required. It is also noted that every time the column converges, the internal stream will also be updated. The disadvantage is that it makes the flowsheet somewhat more difficult to interpret – to another user it may not be clear how stream “To Reb” is being solved.
Summary
The methods and their advantages/disadvantages are summarized below.
We believe that the simplest and most effective method is to model the heat exchanger in the main flowsheet using an Internal Stream. The model can be documented to help ensure that the source of the Internal Stream is clear.
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