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Blog series: Using Simulation Models to Support Plant Operations

February 09, 2017 by Tami Okano / 0 Comment
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Tami Okano

Searching for Tips on How to Accelerate Column Design? Look No Further!

Searching for Tips on How to Accelerate Column Design?

Distillation columns are the most common pieces of equipment used in the chemicals and energy industries to separate components. The separation in a column is based upon differences in volatilities. Though columns are prevalent in the industry, designing and rating columns is no easy feat.

 

Steps for Designing and Rating Columns

The first step in designing a new process is to validate phase equilibrium predictions. With a tool like Aspen Properties®, property analysis can compare predictions against actual data, such as from the National Institute of Standards and Technology (NIST) or Gesellschaft für Chemische Technik und Biotechnologie (DECHEMA). The next step is to verify that the desired separation is feasible by using a product like Aspen Distillation Synthesis. Using this tool, conceptual models can be used to find azeotropes and phase boundaries in order to determine the minimum number of theoretical stages and minimum reflux for separation. This will be the foundation for the process model you will build.

Once a conceptual model derives the appropriate theoretical parameters for a feasible separation, a more rigorous model can be used. In Aspen Plus®, RADFRAC is a rigorous column model that can be used to model the thermodynamics and hydraulics of a distillation or fractionation tower. Theoretical stages are a good basis for modeling the thermodynamics in an equilibrium system. When moving to an equipment-based approach that considers hydraulics, the actual number of stages will be needed with either an equilibrium or rate-based model. The actual number of trays can be derived by dividing the theoretical stages by an efficiency (specified or calculated). This will be the basis for hydraulic calculations which can consider tray pressure drop and other hydraulic constraints, such as jet flooding, downcomer backup flooding, tray weeping, etc.

For the internals design, the biggest question is whether packing or trays should be used. When deciding what is best for your application, it’s important to consider vessel diameter, operating pressure, pressure drops, liquid/vapor ratios, operating flexibility, foaming and more. There are some great articles out there that provide helpful guidelines on internals selection.

Rating columns is performed after the design parameters have been set for running what-if scenarios around the desired or possible future operating conditions. Examples include evaluating turn-up and turn-down scenarios or multiple feedstocks.


Tips on Designing Columns

1.      Start with a simple model and use that as a basis for more complex models

2.      Use process simulation software that has built-in tray and packing options to save time

3.      Determine whether packing or trays are right for your application with these guidelines


Tips on Rating Columns

Use an integrated design and rating model to:

1.      Bring the manufacturer’s expertise in-house

2.      Check their recommendations against your column model

For existing columns, visualize your column hydraulics and clearly see your operating envelope.

Lastly, if available, always validate your column model against plant data, tuning appropriately to improve accuracy. Good luck!

LEAVE A COMMENT

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