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Tips & Tricks - Column Builder Tutorial

Building, sizing, initializing, and finally running a distillation column in dynamics can be very challenging. Even if all of the sizes are known, it can be tedious to assemble and configure all of the unit ops, and doing so in such a way that the column will run stably is even more difficult. To help simplify this process, there is a new Column Builder feature which will create a dynamic column model from an existing steady-state column. This tutorial will show you how to use this feature.

The Steady-State Model

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We will start with a converged, steady-state model of a Naptha Stabilizer. This 40-stage model includes a kettle reboiler and a partial condenser. To use this model as the blueprint for a dynamics model, start by adding a flowsheet and converting it from steady-state to dynamics. The conversion option is in the flowsheet form, which can be opened by right-clicking on the flowsheet icon and then selecting “Open Form.”

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The flowsheet should look like this when you are done. Next, open the simulation tree. This can be done by pressing Ctrl+T or through the View Menu. 

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Click on the dots to the right of FS1, and then select “Create Tower Flowsheet from SS Tower…” This will bring up a box to select which SS tower to use. In this case, there is only one tower.  

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Click OK, and the model will be created. If it asks you if you want to initialize values from Steady State, click “Yes”. 

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The program will now generate a new dynamics flowsheet and column model. 

The Dynamic Model

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Let’s examine the model to see how it works. Open the distillation section, and then select the geometry tab. The first things to note are the stage diameters. At the feed stage (15) and below, the diameter is about 11 feet, and above it is about 5.5 feet. This is to accommodate the extra liquid flow from the feed. Also note that the program automatically calculates the k value for the holes and the downcomer. 

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The reboiler is modeled as a separator with a fixed duty (i.e. a kettle reboiler):

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This is a simple approach, but it can be easily modified if necessary.  

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The model for the overhead condenser is also simple. The condenser and overhead accumulator are modeled as a single separator with a temperature specification (the duty back-calculated). Pressure control is achieved through a pressure spec in the vapor product stream.    

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Behind the Scenes - What the Builder Does for You
Distillation Section

The distillation is sized based on actual vapor and liquid traffic derived from the steady state model, and heuristics for downcomer and dry-hole pressure loss. In theory, this would make the geometry for each tray unique, but trays with similar geometry are grouped together with one size. The geometry tab on the distillation section form makes it easy to review and/or modify the tray geometry data.

Condenser / Overhead Accumulator

The accumulator is sized based on a 30 minute liquid residence time and a level control point of 50%. Sizing for the water boot uses the same criteria.

Reflux Pump

A pump may or may not be needed for overhead product. The column builder opts for a conservative design, using a single pump for both reflux and product. The pump head is calculated so that there is sufficient pressure drop to achieve good control, and to keep the size of the control valves reasonable.

Note that by default, static head arising from differences in elevation between pieces of equipment is not modeled in VMG dynamics (It can be enabled in the flowsheet options tab). This means that in the real world, the pump will need extra head to overcome the elevation difference between the pump/valve and the nozzle on the column for the reflux return.

Reflux/Product Valves

The initial design is based on an assumed opening of 50% at the design point, and a pressure drop set by the difference between the pump discharge pressure and the pressure downstream of the valve. The valve is assumed to be a globe valve, and the valve size is then adjusted to give a valve trim in inches.

Water Product

Again, a pump may or may not be needed, so one is added by default. Often, water rates are very small. In order to be able to use a standard type valve, an equal percentage trim is used.

Sump/Reboiler

The sump diameter is sized to match the column diameter. The length is based on liquid residence time.

The heat input is configured as a duty specification. In general, column control works better with one duty type control and one temperature type control. Controlling the temperature at both ends can lead to very large swings in the reboiler and condenser duty.

Control Loops

The column as configured needs three level control loops and a flow control loop. The assistant currently assumes that the reflux ratio is greater than one, and hence pairs overhead level control with the reflux flow.

Reflux ratio control can be achieved with either a selector block or a process calculator. Use the measured flow from the level control loop and the reflux ratio as the setpoint for the flow controller. If you convert the level control loop to a cascade loop, i.e. LC->FC, then the reflux ratio can be implemented using the setpoint rather than the actual flow.

Composition control can be layered on top of the basic control loops using cascade control.

Conclusion

Building a dynamic model for a distillation column can be a difficult and time-consuming process. The column builder provides a quick and convenient way to convert a steady-state tower model to a dynamic model, reducing engineering time. The column builder will size the equipment (trays, separators, valves, etc.), and configure basic control loops. It will also configure the duty on the reboiler and set the temperature on the condenser.

This model can easily be made more detailed and realistic if desired. A few examples of how to extend the model are:

  • The temperature in the condenser can be controlled by adding a heat exchanger between the top of the tower and the condenser, instead of setting the temperature directly.
  • Use of a hot-gas bypass system to control the pressure in the condenser.
  • Modeling a thermosiphon reboiler using a separate heat exchanger and a circulation flow from the column sump.

For more details on these advanced models, see the “Modeling Distillation Columns” section in the VMG manual

Mark Beyleveld, B.Sc.(Eng), B.Com.

Please contact your local VMG office for more information.

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