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Introduction

The Python Simulator allows you to virtually operate the distillation column to evaluate distillation performance. You will also be able to design your own column and test its performance based on the simulation algorithm. There are two main simulation modules available here:

  1. Distillation simulation: used essentially to assess the performance of a separation based on specified operation variables and column parameters. There are three types of simulations available:
    1. Shortcut distillation simulation: simplest simulation but very useful for initial evaluation of the distillation column.
    2. Steady-state rigorous distillation simulation: a more accurate simulation than the shortcut distillation simulation. Tray column hydraulics are also calculated here.
    3. Unsteady-state rigorous distillation simulation: allows to investigate the transient behaviour of the column when perturbated by a step change.
  2. Distillation column design calculation: this model allows the user to design a distillation column based on given specifications, such as desired distillate concentration and feed flow rate. Those specifications can be very different from those in the actual distillation column investigated in the VR Tour.

A simple Vapour Liquid Equilibrium (VLE) calculation tool is also available to allow users to explore and compute the VLE data and diagrams of multiple binary systems that are needed for distillation performance analysis and distillation column design.

Shortcut Distillation Simulation

Since there may be several unknown operational and design variables associated with the distillation column, shortcut distillation simulation is a good starting point for evaluating the column performance. The shortcut distillation simulation uses the McCabe-Thiele shortcut method. It simplifies the operation and design problem by solving only the mass balance equations and the vapour-liquid equilibrium because of the assumption of Constant Molal Overflow (CMO). This assumption implies constant vapour and liquid molar flow in each section. Vapour and liquid molar flow rates in the enriching section are represented using $L$ and $V$, respectively. Correspondingly, $\bar{L}$ and $\bar{V}$ (or $L_b$ and $V_b$ in the shortcut distillation simulation results) are used for the stripping section. The molar flow rates in the two sections are related through the feed quality value, $q$, as shown in the Distillation Column component page. The shortcut distillation simulation also assumes a uniform pressure in the column at 760 mm Hg.

The inputs for the shortcut distillation simulation are:

  • simulation mode
  • binary system for distillation
  • feed settings:
    • feed flow rate
    • feed composition
    • feed temperature
  • column settings:
    • reflux ratio
    • reboiler power
    • reboiler efficiency
    • actual number of stages
  • simulation settings:
    • simulation file name (default: "1")

The outputs of the shortcut distillation simulation are:

  • performance characteristics at steady-state:
    • tray liquid molar composition ($x$)
    • tray vapour molar composition ($y$)
    • tray temperature ($T$)
    • distillate molar composition ($x_D$) and molar flow rate ($D$)
    • bottom product molar composition ($x_B$) and molar flow rate ($B$)
    • vapour molar flow rate ($V$) in enriching section
    • vapour molar flow rate ($\bar{V}$) in stripping section
  • column design variables:
    • condenser duty
    • reboiler duty
    • optimum feed location

Steady-State Rigorous Distillation Simulation

A more accurate evaluation of the distillation column performance can be achieved using the steady-state rigorous distillation simulation. Compared to the shortcut method, the rigorous method solves a set of equations containing mass balances, equilibrium relationships, composition summations, and heat balance, also known as the MESH equations, as shown in the section of Design and Optimization. Tray column hydraulics are also calculated in the rigorous method by considering tray geometric parameters, such as tray diameter, active area, hole/slot area, and weir height. The tray geometric parameters used are based on the PIGNAT Distillation Column used for the actual experiments.

The inputs for the steady-state rigorous distillation simulation are:

  • same as the shortcut distillation simulation; and
  • feed tray location

The outputs of the steady-state rigorous distillation simulation are:

  • performance characteristics at steady-state:
    • tray liquid molar composition ($x$) and molar flow rate ($L$)
    • tray vapour molar composition ($y$) and molar flow rate ($V$)
    • distillate molar composition ($x_D$) and molar flow rate ($D$)
    • bottom product molar composition ($x_B$) and molar flow rate ($B$)
    • tray temperature ($T$)
    • Murphree vapour tray efficiency ($E_{MV}$)
  • tray hydraulics at steady-state:
    • tray pressure drop ($P$)
    • tray clear liquid height ($h_{cl}$)
    • tray froth height ($h_{Fe}$)
    • tray froth density $(ρ_E$ or ${phi}_E)$

Unsteady-state Rigorous Distillation Simulation:

The operation and control of a distillation column is a dynamic process. The unsteady-state rigorous distillation simulation can be used to simulate such processes where a set of transient MESH Equations as shown in the section of Design and Optimization, are solved for column performance characteristics and tray hydraulics at each time step within the simulation time.

The inputs for the unsteady-state rigorous distillation simulation are:

  • all the inputs for the steady-state rigorous distillation simulation; and
  • mode*
  • simulation time
  • intervention option

The outputs of the unsteady-state rigorous distillation simulation are:

  • same as the steady-state rigorous distillation simulation, except recorded at the end of simulation time; and
  • tray liquid molar holdup ($M$)

*“Mode” refers to different initial conditions for dynamic distillation simulations and three options are available: Startup Mode, Transient Mode, and Transient from Default Mode. Please refer to Simulation Settings → Mode dropdown menu for detail. The intervention option allows the user to mimic the situation where feed and column settings are changed during a transient process.

To achieve a specified separation in distillate and bottom products with known feed conditions, the distillation column process design needs to be implemented. As shown in the section of Design and Optimization, the design is typically iterative, and the first step involves distillation column sizing with the determined operation conditions from simulation such as vapour flow rates. Afterwards, the distillation column process design can be optimized by conducting column performance evaluation using distillation simulations.

The inputs for the distillation column design calculation are:

  • binary system for distillation
  • feed settings:
    • feed rate and unit
    • feed composition and composition type
    • feed temperature
  • design specification settings:
    • desired distillate composition and composition type
    • desired bottom product composition and composition type
    • reflux ratio and reflux ratio range type
  • column design settings:
    • $f_1$: fraction of flooding velocity $(U_{NF})$
    • $f_2$: fraction of column net area $(A_{net})$
    • tray spacing
  • calculation and result settings:
    • result flow rate unit
    • result composition type
    • design calculation file name (default: "1")

The outputs of the distillation column design calculation are:

  • performance characteristics at steady-state:
    • tray liquid and vapour composition
    • tray liquid and vapour flow rate
    • tray temperature ($T$)
    • distillate composition and flow rate
    • bottom product composition and flow rate
  • column design variables:
    • minimum reflux ratio
    • condenser duty
    • reboiler duty
    • overall column efficiency
    • number of equilibrium stages
    • actual number of stages
    • optimum feed location
    • plate diameter

Vapour-liquid equilibrium (VLE) is essential for distillation performance analysis and distillation column design. The VLE calculation computes the VLE data of binary systems on a point-by-point base by simulating an ebulliometer or boiling experiment with similar experimental settings and outputs. Moreover, the VLE calculation can generate graphical representations of the VLE data in form of T-x-y diagram and x-y diagram based on as many data points as desired. These diagrams are essential to the preliminary design of the distillation column as they can provide important information such as boiling point, dew point, binary system ideality, and azeotrope, etc.

The inputs for the vapour liquid equilibrium calculation are:

  • VLE settings:
    • type of binary system
    • two components in the binary system
    • system pressure and unit
    • VLE calculation file name (default: “1”)
  • VLE point estimation:
    • initial composition and composition type
  • VLE plots:
    • number of data points

The outputs of the vapour liquid equilibrium calculation are:

  • from VLE point estimation:
    • boiling point
    • liquid and vapour molar composition
    • more volatile component (MVC)
    • less volatile component (LVC)
  • from generating VLE plots:
    • T-x-y diagram
    • x-y diagram
    • VLE data points: liquid and vapour molar compositions and boiling points