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Process Systems & Pilot Plants

Distillation Column FAQ’s

Common Inquiries and FAQ’s for Distillation Systems

We get many questions surrounding distillation and the design, build, integration process for custom distillation columns. For more specific questions or further information regarding distillation towers and equipment, please contact EPIC 314-272-4149.

A distillation column is the major piece of equipment use to distill or separate liquid mixtures into the individual components comprising the total mixture. It is enclosed cylindrical structure, usually made of metal of glass, in which liquids are added and removed in at specifically engineered points.   Differences in relative boiling points (a.k.a. volatilities) allow certain substances to be evaporated off the mixture and removed from the column. Fractional distillation towers are a common unit operation of chemical engineering used to separate large quantities of liquids in a single column, such as in petroleum and natural gas processing.

A simple column separates two components into two product streams. In multi-component systems, the two main components to be separated are designated as the light and heavy keys. The light key is the more volatile component in greater purity in the top product stream, and the heavy key is the less volatile component in greater purity in the bottom product stream.

There are many types of distillation systems, including fractional, vacuum, steam, azeotropic, and extractive columns depending on what you are trying to separate and under what operating conditions. Columns come in lab-scale, pilot, demonstration, and full production scale.

We provide custom distillation column design and fabrication services that you can learn more about on our distillation systems page.

The cost of distillation equipment design and integration depends on many factors. Depending on your specific application, the operating conditions, and the desired level of separation the price of a distillation tower can vary from tens-of-thousands to millions of dollars. Some specific factors that can increase custom distillation column cost include:

  • Feed composition – mixtures with similar boiling points or that contain azeotropes or chlorides are harder to separate and may result in a more expensive system
  • Organics and other sensitive materials may require operating under vacuum to avoid breaking down too quickly. While this reduces the required heat input, creating a vacuum or taking other precautions to prevent feed breakdown can add expense
  • Required operating pressure – while this is something we work on with you, the higher the required operating pressure in the column the more expensive the column will be. See our blog on how to determine operating pressures for more information in how these are set
  • Height – the taller your column, generally, the more expensive. Materials of construction also play a major role in adding column costs. Final height will be determined based off of several design considerations, including operating pressure.

To get a well-defined price for custom distillation equipment design, integration, and fabrication, you will need to start by clearly laying out your project specifications. Try our distillation project planning sheet for guidelines on what you need to get accurate RFQ pricing from distillation design/build vendors.

You simply need to contact us and request one. It’s helpful if you at least have a basic mass and energy balance, a licensed or finalized technology with clear processing steps, and have a simple process flow diagram. But even if you don’t have all those things, we can help you develop them. Call EPIC at 314-845-0077 or fill out a contact form to get started.

The starting point upon which all column design is based is the relative volatility of the key components to be separated. Using a mass and energy balance simulation program, such as Aspen Plus or Hysys®, the user must set up the basis of the simulation by selecting an appropriate fluid package and the components present in the feed. Activity coefficients, estimated by the program or provided by the user, are used to relate non-ideal component interactions.

From this starting point, we walk your column through the following distillation column design steps:

  1. Determine feed vapor-liquid equilibrium
  2. Specify column operating objectives
  3. Define operating pressure range
  4. Find the minimum reflux ratio (R/Dmin) and the minimum number of ideal separation stages (Nmin)
  5. Set the diameter and height of the column
  6. Simulate the proposed separation strategy in Aspen Hysys
  7. Generate final mass & energy balances
  8. Review proposed designed for feasibility and process tradeoffs
  9. Complete plant physical design using 3D CAD software
  10. Develop P&ID’s and detailed general arrangements, PFD’s, equipment and instrumentation lists
  11. Peer review designs and automation strategy to ensure no major design assumptions are missed

EPIC has over 25 years of proven industrial column design & fabrication experience. Our engineers are experts in sizing, mechanical design, system modeling, and practical design for manufacturing environments. You can read more about our step-wise procedure to distillation equipment design here, or check out these additional resources for more on column design:

Column fabrication methods will depend on the specific design of a column. There are companies that specialize in advanced distillation vessels that we occasional purchase equipment from, but we more commonly build your pilot scale distillation column or distillation tower, and complete distillation equipment integration in-house.

When we build distillation equipment for integration, the following advantages of our off-site production facility apply:

  • Column fabrication happens inside EPIC’s state-of-the-art machine shop by our skilled crewed of full-time craftsmen
  • System programming and electrical wiring is completed in parallel with equipment assembly and fabrication
  • Quality assurance checks are performed throughout the fabrication and assembly process
  • The complete column is fully tested per ASME requirements and undergoes thorough Factory Acceptance Testing (FAT) testing before leaving our facility
  • EPIC oversees and actively manages site civil and facilities engineering
  • Site civil and facility modifications occur in parallel with distillation tower fabrication in the EPIC fabrication plant
  • Completed modules are insured, packaged, and shipped to our client’s manufacturing sites where installation is supervised by EPIC’s experienced project managers

You can learn more about our production-style fabrication shop here, or contact EPIC to start discussing your custom distillation project.

Distillation is a separation process that takes advantage of differences in boiling points and relative volatilities of different substances. By heating a solution at the bottom of a column, lighter particles that boil off faster rise to the top of the column and can be separated from heavier particles.

Industrial distillation towers are more complicated, but the basic idea applies whether you are separating two or a hundred substances. You can learn more about basic principals in our “Basics of Distillation Design” infographic, or by watching this helpful video.

To calculate the number of stages in a distillation tower you have to first identify the substances being separated and then determine the their relative volatility. Most chemical engineers designing distillation columns use a software package such as Aspen Plus or Hysys to select an appropriate fluid package and feed components and automatically layer in hundreds of data points about how these chemicals will behave inside a column.

Once vapor-liquid equilibrium is determined, the number of required stages is determined by taking the simulation through the following steps:

  1. Specify column operating objectives
  2. Define operating pressure range
  3. Find the minimum reflux ratio (R/Dmin) and the minimum number of ideal separation stages (Nmin)

The simulation software allows you to calculate the number of stages based upon total reflux operation to find the number of stages that allow the minimum reflux ratio (R/Dmin) and minimum number of ideal separation stages (Nmin) to be determined for systems as small as a pilot scale distillation column to a lage fractionating column.

From there, rigorous simulation of the distillation at a given feed rate and composition may now be accomplished by specifying the following: top and bottom product compositions, number of stages, feed stage, and top and bottom pressure.

Next, use constrained iterations of this simulation to verify the estimated number of stages and feed location. Add and subtract stages from both the stripping and rectifying section of the column. Do this until the required reflux ratio becomes approximately 1.2 times the minimum reflux ratio, or the tradeoff between utility usage and the number of stages appears optimal for the specific column.

For more details on how chemical engineers specifically determine distillation tower stages, required heights, and other technical information, see our post on our technical distillation column design procedure or contact us to start work with one of distillation experts.

Chances are good that we have worked in your industry or one that is similar. Discuss your project with a distillation equipment integration specialist (314) 845-0077. We have a wide-range of experience designing and building custom distillation columns across many industries and applications.

A distillation tower is installed at a refinery

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EPIC Modular Process Systems
4134 Meramec Bottom Rd
St. Louis, MO 63129, US
(314) 845-0077

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