Distillation is one of the most important operations in chemical engineering and also one of the most challenging operations for modeling and control. Sometimes, calculating a column using a process simulation package can be a time-consuming task. To help you get the results of the simulation quickly and every time - follow the easy instructions below. 

The basis of distillation is phase equilibrium, specifically, vapor–liquid phase equilibrium. Distillation is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation. Understanding of vapor–liquid equilibrium is essential for the analysis, design, and control of distillation columns. 

The design of a distillation column involves many parameters: product compositions, product flowrates, operating pressure, total number of trays, feed tray location, reflux ratio, reboiler heat input, condenser heat removal, column diameter, and column height.

Not all of these variables are independent, so a “degrees of freedom” analysis is useful in pinning down exactly how many independent variables can be specified to completely define the system.

A rigorous degrees-of-freedom analysis involves counting the number of variables in the system and subtracting the number of equations that describe the system. 

A typical situation in distillation design is that the feed conditions are given: flowrate, composition, temperature, and pressure. The desired compositions of the product streams are also typically known. 

A Step-by-Step Guide

1. Configuring a new simulation case

Whatever process simulator software you are using (Hysys, Aspen Plus, ChemCad, Pro II), the first step is to open a blank worksheet and add a pieces of process equipment: column, condenser, reboiler (as part of the column or separate), pumps and valves and add a streams between them. Optionally, you can also define accumulation vessel for the condensate.

2. Specifying chemical components

Next, we must define the chemical components involved in the separation. 

3. Specifying thermodynamics properties 

Most of the software packages will suggest the thermodynamic property package that suits defined components. If you do not have any specific knowledge or requirements, it is best to use the suggested thermodynamics package.

4. Specifying stream properties

The input streams to the process must be specified. In a simple case, as the basic column described above, there is only one input stream, the column feed stream. The flowrate, composition, temperature, and pressure of this stream must be specified. If you are using any additional input streams such as steam for heating or additional feed, then those streams need to be specified as well.

5. Specifying equipment parameters

The parameters for all the equipment included must be specified by opening equipment icons and defining necessary data. 

6. Specifying pumps

If you do not have any specific data or requirements, it is appropriate to make the assumptions that both pumps will generate a pressure difference of 5 bars between suction and discharge. So, make the specification for both pumps in accordance.

7. Specifying valves

The pressure at the exit of the feed valve must be equal to the pressure on the feed tray. We do not know exactly what this is at this point, but we will make an assumption and define it as slightly higher than expected to be on a safe side for the calculation purposes. At this point, it is also possible to avoid putting the valve into flowsheet and add it later when the column has converge and pressure is known. After you have converged the column, come back and adjust it. For the other two control valves use the assumption of pressure drop of about 3 bars, if not defined differently with available data. 

8. Specifying column parameters

The following parameters need to be defined:

• Number of stages - if you are calculating a known column and are familiar with the column data, then use the parameter as defined in the documentation. If not, make the assumptions to get the first results. Acceptable assumptions can be to define 30 trays + 2 stages for reboiler and condenser. 
• Feed tray – Again, if you have known data, use them, if not, make an assumption for the moment and set this in the middle of the column, on stage 16. Later, feed tray option can be analyzed further and optimized by finding the tray that minimizes reboiler heat input.
• Pressure – The column top pressure and the pressure drop should be defined. If you have known data, use them, if not, then you should look at the equilibrium data for the defined system of components. Use the literature data to define approximate pressure to calculate the first results. Once the column has converged, the pressure should be adjusted in a way to enable the separation while at the same time using the minimum energy. A reasonable tray pressure drop is about 0.007 bar per tray.
• Condenser – define it as total or partial if the distillate were removed as a vapor.
• Reboiler - the vapor from the reboiler is in equilibrium with the liquid bottoms product withdrawn and is a major energy source for the column. 

9. Convergence

A distillation column has 2 degrees of freedom once the feed, pressure, number of trays, and feed tray location have been fixed.

At this stage of the simulation, the usual approach is to fix the distillate flowrate in accordance with the composition of feed, and the reflux ratio. Later, once you obtain a converged solution, you can change the specified variable so that the product specifications are met.

Again, if you have an approximate data, use them to get the results and then optimize the calculation to meet the requirements. If you don’t have the specific data, specify an approximate distillate flowrate and assume a reflux ratio. The reflux ratio is the ratio between the amount of reflux that goes back down the distillation column and the amount of reflux that is collected in the receiver (distillate). The range of reflux ratio is very wide. In practical experience, it can range from 0.3 up to about 20. It also varies with the amount of the overhead component(s) in the feed stream. For example, separation of iC4 and nC4 can have reflux ratios from about 10 to about 20 with the only change being the amount of iC4 in the feed stream. Usually, the good start is assuming reflux ration of 10, getting the column to converge and then decrease accordingly, if possible. The flowsheet is fully specified at this point and we are ready to run the simulation. From the given descriptions, it should converge easily.

10. Obtaining the final results

Having the column to converge is not enough to call the distillation column model finished. The parameters should now be adjusted to receive the required product quality and optimize operational conditions and energy consumption.

Therefore, once you obtain a converged solution, you should change the specified variables so that the product specifications are met. It is always a better way to adjust one variable after another, so there is not too many disturbances in the calculation and converged solution isn’t lost.

Other general considerations and adjustments to be done are:

• The easier the separation, the fewer trays are required and the lower is the required reflux ratio (lower energy consumption), so those parameters need to be explored and adjusted further.
• The higher the desired product purities, the more trays are required. 
• The lower the pressure, less energy is needed for separation, so also adjust pressure accordingly.

Have in mind that once you have the converged column to save your work and keep it aside as a backup. Because any of the following explorations and calculation could disturb the stability of calculation so you will lose your work if you did not keep it safe.