Category Archive: Distillation

Distillation Design Procedures

Separating chemical mixtures is an essential step in many chemical production and related industrial processes. Distillation—one of the most common methods for separating chemicals—uses heat to separate chemicals based on their relative volatilities. When the mixture is heated the countercurrent flow of the liquid and vapor establishes a concentration gradient in the distillation column. Depending on the number of chemical compounds in the mixture the process of distillation will often require multiple units to achieve the process goals.

The basic graphical design methodology for a 2 component distillation system was developed in 1925 by McCabe & Thiele. This process is taught to all chemical engineering students and provides a foundational understanding of distillation design and the challenges an engineer will have to overcome to meet the process specifications of a distillation design. Today, the McCabe Thiele diagram has been supplanted by computer simulation programs that can solve large matrices of equations related to many components in the feed stream to a distillation system.

The McCabe-Thiele Diagram

The McCabe-Thiele Diagram is a graphical design procedure used to calculate a distillation system and its operating parameters. A plot of the equilibrium curve of a binary mixture—which displays the mole fraction of a chemical mixture in the liquid phase that is in an equilibrium condition with that chemical mixture in the vapor phase—is the basis of the graphical method.

The output from the diagram will display the number of theoretical stages, or trays, needed to distill a binary mixture. While the analysis involves several assumptions, these assumptions are generally not significant to the calculation. To determine the number of distillation stages, users plot three lines on top of the equilibrium curve:

• The rectifying section operating line
• The feed line
• The stripping section operating line

Once the operating lines are in place, the stages required are then drawn as right triangles in the space between the equilibrium curve and the lines. The number of triangles equals the number of necessary distillation stages.

Advantages

The McCabe-Thiele method is an excellent educational tool but is rarely used in designing a system in today’s world. The real value of the method is the simplicity it lends to the understanding of the distillation design process.

Computational (Simulation) Methods for Distillation Design

Process-simulation programs have become the main design method for chemical engineers since the development of the PC. Two of the most popular software solutions—Aspen and ChemCad—are great tools that produce accurate solutions. An accurate solution does require that the user of the software has a full understanding of the thermodynamics and component interaction parameters used in the mathematics producing a converged solution in the software package. The notes below detail the limitations and risks of relying on a simulation package as a design basis to build a system.

Advantages

Programs can predict non-ideal behaviors. A process simulator uses different mathematical models to predict both physical and chemical interactions. For example, different models can be used for highly dilute systems, systems with polar components, and systems with non-polar components. An engineer using a process simulator need only choose the correct model to fit different interactions best. That model selection process is a critical step in properly setting up a process simulation.

Combined with comparisons to empirical vapor-liquid equilibrium data, these simulations can closely predict behaviors and identify the ideal distillation process.

Disadvantages

Unfortunately, even process-simulation programs can’t capture every interaction accurately. Some distillation processes will require laboratory tests to determine the right design. This is typically experienced in designs with multiple components that produce conflicting results in a simulation due to the lack of information in the software for chemical interactions.

A Case for Laboratory and Pilot Testing

Mixtures with multiple components, including interacting solids, and newly developed chemicals will need to undergo laboratory tests.

If your company is processing chemical mixtures that have complex interactions or include components that cannot be accurately simulated, Thermal Kinetics can help assist with the design and execution of a pilot testing campaign. The output dataset of a pilot campaign will include the required operating points and sizing information to properly design a full scale operating distillation system that meets the separation process specifications.

Following a pilot test campaign, the results can be added to a process simulation program to regress the data into vapor/liquid equilibrium data. Future simulations can then be used with confidence to predict the full-plant performance. This is a great tool used by many plants when slight changes in operating conditions or composition need evaluation.

The engineers at Thermal Kinetics can help scale up your distillation process or optimize your current process and equipment. Contact Thermal Kinetics today to receive a quote for our process design and pilot plant testing services.

What You Need to Know About Distillation

Distillation is an essential separation process used across a broad range of sectors including the food and beverage, petrochemical, specialty chemicals, pharmaceutical, water treatment, and agriculture industries. Distillation involves separating two or more components or substances from a liquid mixture based on the variance in their boiling points. Depending on the industry, the distillation process can be used to perform several operations including concentration enhancement, purification, devolatilization, and resource recovery.

Below are some of the distillation equipment, technologies, and processes that are currently being leveraged across industries.

Single Stage & Multiple Stage Technologies

Distillation processes are typically done in single or multiple stages. Single-stage thermal distillation is a continuous operation where a liquid mixture is fully or partially vaporized in one single phase. The solution is heated to its boiling point, causing the more volatile components to evaporate, where it is subsequently cooled and condensed. In multi-stage distillation, portions of the liquid are vaporized and cooled/condensed in successive stages, usually with decreasing pressure and temperature.

In another type of separation technique, known as scrubbing, volatile components in a gaseous phase contact and combine with a counterflowing liquid via absorption. Molecular sieve dehydration is another well-known separation method and uses a system of sieves that allows the molecules of a particular substance to fit into its pores, where it adheres to the sieve; this process is called adsorption. Molecular sieve dehydration is typically used after alcohol/ethanol distillation.

Distillation Basics

All distillation processes, regardless of the technologies employed, operate by exploiting the differences in the volatility of a mixture’s various components. This volatility is related to a compound’s vapor pressure or boiling point. The boiling point of a liquid is directly proportional to the applied external pressure. For instance, at 14.7 psia, water will boil at 212^oF; however, at 1.94 psia, the boiling point changes to 125^oF. The variance in vapor pressures and boiling points between liquids are referred to as relative volatility. The distribution coefficient is another critical component of distillation and is related to the composition and concentration of the vapor in equilibrium with the liquid mixture.

Design Procedures

The distillation process, though relatively simple in concept, requires careful analysis and meticulous calculations to ensure its success and efficiency at an industrial scale. Over the years, several calculation procedures, equations, diagrams, and mathematical models have been developed to represent the various stages of the process.

One of several design methods presented in our eBook is the McCabe-Thiele Diagram. This graphical method is commonly used to determine the ideal number of stages and the location of feed trays for a particular distillation process. To ensure the efficiency of the distillation process, all design procedures must consider a number of factors including:

• Calculation of the number of equilibrium stages
• Determination of tray hydraulics
• Selection of tray or packing efficiencies

Equipment Components of the Distillation System

Distillation systems consist of numerous components; with each playing a different role in the process. In our eBook, we look at the various elements of several distillation systems and describe the factors that influence their design. Some of the components we cover include:

• Contacting equipment
  • Sieve tray
  • Valve tray
  • Bubble cap tray
  • Dual-flow tray
  • Packing
• Downcomer design
• Columns
• Feed distribution and redistribution systems
• Column intervals

Conclusion

The team at Thermal Kinetics has recently released their new eBook, “Distillation 101 a Guide to Distillation and Separation Technologies”. This publication gives an in-depth look at the distillation process and its various elements, including:

• Distillation design procedures
• Distillation equipment and essential components
• Industrial applications

If you would like to dive deeper into the distillation process, click here to download our free eBook.