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Learning Outcomes

By the end of this section, you should be able to:

  • Describe the uniqueness of the distillation operation procedure.
  • Apply the hazard assessment knowledge to distillation operation procedure.
  • Operate distillation column safely and consistently.
  • Apply process variables and material balances to determine operation steady state.
  • Assess the hazards and risks involved with distillation process and operations.
  • Apply the underlying theories and simulation to approaching and solving safety problems.

Introduction

In this section we will cover three main topics: significance of distillation column operation, policy and procedures, and safety.

A screenshot of a control panel interface set beside an image of the corresponding setup in a lab.

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Distillation Operation: Operation/Control Interface to Distillation Column in Action

Site Author(s) and University of Waterloo

Read: Overview of Distillation Operation

As distillation column is a relatively tall structure loaded with materials at their boiling temperatures when in operation, the necessity of operating a distillation column in the safest possible manner is paramount not only for the overall operation, safety, and sustainability, but also for expected column performance under different operation conditions. In industrial settings, a proper operation procedure means reduced start-up time, decreased operation disturbance, and consistent column performance and product quality. In distillation column design, there are many design and control mechanisms such as level control, as shown on the Overhead and Bottom component pages and cascade pressure control that also define the unique operation procedure for distillation operations.

As distillation column operation heavily involves the boiling and condensation of the materials in the process, the operation procedure needs to reflect on the effect of the change in process pressure and temperature due to the phase change. As well, the fact that an overall material balance for the entire column operation must be maintained all the time with all the process input and output streams calls for level controllers, as shown on the Overhead and Bottom component pages, instead of commonly used independent flow controllers to ensure operation continuity, stability, and safety. These requirements result in some general operation control measures for typical distillation operation as follows.

  1. The vent temperature or pressure is typically used as an emergency shutdown setting.
  2. As the local (overhead drum) and overall material balances must be satisfied, level controllers are commonly used for overhead drum and reboiler liquid level control.
  3. Regardless of distillation column operation mode, column operation always starts from total reflux operation, and similarly, the first step for column shutdown is to transition from continuous operation to total reflux.
  4. The heating load must be increased or decreased incrementally to avoid drastic change of column pressure.
  5. Reflux stream should be heated to saturation temperature to make the most of top tray. This can be done through heat integration with vapour stream coming into the condenser. This is shown in Distillation Column component page.

Although specific standard operation procedures (SOP) for individual distillation columns exist because of the unique safety and operation requirements of individual chemical systems, the following general start-up SOP is consistent with the procedure recommended by the American Institute of Chemical Engineers (AIChE) in order to ensure the safety, stability, and performance of distillation column.

Start-up SOP for Distillation Column
  1. Start cooling water first, and normally start the cooling flow rate to the condenser at maximum allowable flow rate.
  2. Start heating the reboiler at minimum heat loading, until reboiler is boiling.
  3. When vapour reaches the overhead drum, start reflux control for total reflux operation.
  4. Gradually ramp up the heating load until the desired operation load is reached.
  5. Switch to continuous distillation operation by setting feed, bottom product, and reflux flow.
  6. Use process histograms for all the process variables and material balances to determine the operation steady state.
Shut-down SOP for Distillation Column

The shut-down procedure is the reverse of the above procedure, that is, switch back to total reflux by shutting off controllers for distillate, bottom, and feed flow rates, and gradually reduce the heating load. This last step is necessary because a sudden shutoff of the heating load can cause the column pressure to drop below the atmospheric pressure and this can allow air to get into the column with chemicals still at relative high temperature.

Distillation process and operation safety is utterly important for two major reasons:

  1. There are significant amount of materials (liquid and vapour) loaded along with the column at boiling points, and most of the materials are flammable or toxic; and
  2. there are complex and interacting distillation equipment components that involve various control mechanisms for operation safety and stability as well as and emergency measures.

Because of the complexity in chemical systems and process controls, disturbance during operation such as the malfunction or failure of individual components can lead to hazardous situations and undesirable consequences. As such, it is imperative to evaluate process and operation hazards and have process safety control measures in place.

3.1. Safety Assessment of Distillation Column

Safety assessment is the most effective way to ensure distillation process and operation safety. Safety assessment mainly involves the identification of hazards and risks as well as the estimation of frequency and consequences. These assessments usually consist of:

  • Hazards associated with operation pressure and high temperature.
  • Chemical hazards involving hazardous chemicals and their properties under different operation pressure and temperature.
  • Risks associated with potential run away of control variables and excessive heat or pressure.
  • Toxic or flammable chemicals due to leaks and control failure scenarios.
  • Instability of process controls such as level controls.

The safety assessment can lead to specific equipment design and operation control measures to ensure chemical safety, operation safety, and long term stability of the system.

3.2. Distillation Safety Analysis Based on Dynamic Simulation

Dynamic simulation is a systematic approach to assessing the safety hazards and risks associated with distillation. Dynamic simulation is achieved by solving all the material, energy, and hydrodynamic process equations to predict the dynamic behaviours of the distillation column under operation disturbances such as different feed conditions and the failure of individual components, and it is capable of providing information on the effects of operation disturbances on operation hazards and risks and suggest the safe operation condition to achieve hazard–free operation. Combined with the safety assessment method, dynamic simulation has become a powerful tool for safe design and operation of distillation column.

Explore

Explore the pilot distillation column through the interactive VR Tour.

Perform

Use the Python unsteady-state simulator to simulate the unsteady-state behaviours of the distillation column, and visualize the column dynamics in terms of temperature and concentration as well as associated safety hazards.

Activities and Learning Strategies

Below are listed some ideas for activities and/or learning strategies the student can expect to encounter on each component page. They are labelled based on the year and semester of study [e.g., 1B means first year (1), second semester (B)].

  1. (1B) From the VR Tour and process diagram, describe how process flow rates (F, L, D, B) and temperatures are measured.
  2. (2B) Use the VR Tour and information on this site to answer this question: Which of the following statements is true about total reflux?
    1. It is an essential step for distillation startup and shutdown for any distillation mode.
    2. It is often used to evaluate distillation efficiency.
    3. It exhibits lowest efficiency.
    4. All the above.
  3. (2B) Using the distillation operation diagram and the VR Tour, describe how to use overhead flow and reflux flow controller to set up a total reflux operation or a desired reflux ratio.
  4. (2B) Based on the VR Tour, what is the absolute first step for distillation column startup? What is the first step for distillation column shutdown?
  5. (4A) Using the distillation operation diagram and the VR Tour, describe how the bottom flow is controlled. Should you expect more fluctuation of the bottom flow rate as opposed to feed flow rate?
  6. (2B) In the VR Tour or the pilot-scale column, if the steam pressure is set to 1.2 bar, what would be the inner wall temperature of the condensation tubes submerged in the reboiler? When the ethanol-water mixture in the reboiler is boiling, the ethanol fraction in the liquid is 0.095, what would be the liquid temperature? What are some of the hazards associated with the steam and liquid temperatures?
  7. (2B, 4A) Run the unsteady-state simulation of ethanol-water and methanol-isopropanol systems along with the VR Tour and use the outputs of the simulation to explore and make sense of the following operation outcomes.
    1. Identify potential safety hazards under different operation conditions and disturbances.
    2. Predict time-dependent safety hazards related to temperature and concentration.
    3. Predict the relative time to reach operation steady-state, and
    4. Predict pressure and temperature change along the column and associated hazards.

References

Green, D. W., Southard, M. Z. (2019). Perry’s Chemical Engineers’ Handbook, 9th ed., McGraw-Hill, electronic version is available at: https://www-accessengineeringlibrary-com.proxy.lib.uwaterloo.ca/content/book/9780071834087

Sinnott, R. K. (1988). Coulson and Richardson's Chemical Engineering Volume 6 - Chemical Engineering Design, 4th ed., Elsevier

Wankat, P. C. (1988). Separations in Chemical Engineering: Equilibrium Staged Separations, Prentice Hall