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Distillation Technology and Design
Distillation is one of the most reliable, cost-effective methods of chemical separation and is a crucial step or “unit operation” in a wide range of industrial processes. Thermal Kinetics provides both “Modular” and “Field-erected” Distillation systems that support many industries, including:
- General Chemical Production
- Renewable Fuels
- Food Processing
Distillation is a separation processes involving both Heat and Mass transfer. While there are many methods for separating chemical compounds, distillation has been proven to be the most reliable and economical design for many applications.
Single Stage & Multiple Stage Separation Processes
Before delving into the specifics of the distillation process and design considerations, it is important to first establish the difference between single and multiple stage separation processes in general.
One single stage of separation is defined as evaporation or more specifically but not commonly termed “Flash Distillation.” In this process one volatile component is removed in a solution by heating and consequently boiling the solution. The more volatile component in the solution is vaporized and removed from the process.
Multiple effect evaporation applies several stages of evaporation in the separation process. In multiple effect evaporation the process is limited by the sequential process design. Distillation employs the same multi-stage principle in countercurrent stages to allow for a more efficient process minimizing waste from the system.
Basics of Distillation
At its most basic definition, distillation is the separation of components in a solution based on their relative volatility. To accomplish the Distillation process, the liquid and vapor interact in a counterflow arrangement within a vertical column
Compounds separated by distillation are characterized by their volatility, which is related to the chemical’s vapor pressure. Chemicals exhibit different boiling temperatures at different pressures. For example, water boils at 212 °F at atmospheric pressure, or 14.7 psia. At 1.94 psia, it boils at 125 °F. In this example, water has a vapor pressure of 14.7 psia at 212 °F and 1.94 psia at 125 °F. Ethanol, as another example, has a vapor pressure of 32.5 psia at 212 °F and 4.55 psia at 125 °F. Since the vapor pressure (VP) of ethanol is higher than water, it is more volatile.
Relative volatility and activity coefficients are key parameters used in the design of a distillation process. Since most distillation systems contain multiple components the matrix of equations becomes quite large. Relative volatility is a measure comparing the vapor pressures of the components in a system. The activity coefficient is a factor used in the calculations to account for deviation from ideal behavior.
Distillation Design Procedures
Distillation design is a highly complex task made simpler through the use of models, simulations, and equations. Design engineers select these tools carefully to help them account for all variables involved in a system’s design.
Graphical Method for Continuous Distillation: The McCabe-Thiele Diagram
Developed in 1925 and still used today, the McCabe-Thiele diagram evaluates the separation of components in a binary system. This x-y diagram simplifies some variables to provide a quick, high-level evaluation of a simple distillation based on established Vapor/Liquid Equilibrium (VLE) data. While useful in providing a visual representation of the distillation process, this graphical method is limited to analyzing two variables.
For more complicated distillation systems with three or more components, computer-based simulation software is used in the design process. Two popular brands of software are Aspen and ChemCad. The software programs allow designers to select an appropriate VLE model for the mixture and generate accurate predictions and distillation-stage calculations.
The more complex systems also may require pilot testing for final design validation. Thermal Kinetics can help develop Pilot Testing Equipment unique to a given test system. These pilot runs provide valuable data for statistical analysis and development of a more robust simulation model. These customized solutions are invaluable for more complex systems that can otherwise be difficult to validate.
The Distillation Design Process
Regardless of the specific models used, the overall design process follows a standard practice. The general steps are as follows:
- Evaluate the vapor-liquid equilibrium data
- Calculate the necessary equilibrium stages
- Determine tray hydraulics
- Select the appropriate tray or packing efficiencies
The use of hand calculation procedures is justified in cases when an engineer is tasked with: rough system cost analysis, general valuation of operating variables, separations with coarse purity requirements, designs for ideal and close-to-ideal systems.
Conversely, a rigorous design approach is used for several situations:
- Designing a system that deviates significantly from ideal behavior or the lack of good VLE data
- High product purity requirements
- Highly precise cost estimate requirement
- Multicomponent distillation with a tight range of component boiling points
Thermal Kinetics will often advise the use of Pilot Testing in a variety of situations, especially when a design carries a performance risk.
Selection of Design and Operating Conditions
Once the engineer has settled on a basic system design, they must analyze the critical operating parameters of the equipment, including:
In general, decreasing a column’s operating pressure facilitates separation by improving relative volatility. However, other factors must be considered when reducing column pressure, such as reboiler and condenser temperatures.
For design purposes, it is frequently assumed that pressure at the bottom of a still is the same as pressure at the top. That is not the case, as there can be no vapor flow unless a pressure gradient exists. To be concise, changes in VLE would have to be evaluated for each equilibrium stage. Using today’s simulation software avoids this issue as the rigorous modeling is quite accurate and easily evaluates the pressure gradient in the column design.
In a vacuum column, the pressure drop may be a large fraction of the absolute pressure. In such cases, relative volatility can vary appreciably from the condenser to the reboiler.
All Tray and Packing vendors can provide firm pressure drop figures as part of a calculation package.
Energy conservation is an important consideration when designing distillation equipment. Some energy-saving measures to consider include:
- Implementing recuperative heat exchangers
- Cascading columns such that the vapor from one condenses in the reboiler of the next
- Using mechanical vapor recompression (MVR)
- Generating low-pressure steam in condensers for reuse in the system
- Operating at the highest possible pressure to decrease refrigeration levels in low-temperature separations
- Recompressing overhead vapor for use as a reboiler heat source
- Employing additional unit operations to add heat into the system
Equipment Components of a Distillation System
The other primary design consideration for distillation systems is the type of equipment components that should be used. Some of the equipment components in a standard distillation system include:
- Trayed columns
- Packed columns
- Scrubber systems
There are two main column options for a distillation system: trayed columns and packed columns, which differ in their shape, reliability, and ability to withstand pressure.
Trayed columns are primarily used in services that process a liquid feed with solids present or in a service that has the propensity to foam while processing. Trays are available in a wide range of configurations with a variety of flow patterns. Common tray designs include valve trays, bubble cap trays, and sieve trays.
Regardless of the tray design used, each stage of a trayed column operates in a similar fashion. Regardless of the style of tray used each tray is designed to approach the concept of a true equilibrium stage. All mass transfer takes place within the frothy mixture maintained on the tray. To eliminate recycling or bypassing, care is taken to separate the flow path of the liquid and vapor between adjacent trays..
The diverse range of trayed column designs enables trayed columns to be used for many types of applications. Some common factors that call for a trayed design include:
- Variable liquid or vapor loads
- Low liquid rates with many (20-30) stages
- Large (>30 in.) diameter column
- High liquid residence times
- Need for easy cleaning
- Need to withstand high pressure or high levels of thermal or mechanical stress
Packed columns are filled with loose, randomly oriented packing materials or structured sections which are kept in place by a support plate and irrigated by a liquid distribution header. Packing is designed to provide a large area of contact between the vapor and liquid phases as they pass countercurrently through the bed of packing.
Some conditions that might call for a packed column design include:
- Small (<24 in.) diameter column
- Designs calling for specialty materials
- Vacuum distillation applications with lower pressure drops
Liquid Feed Inlet Design
Both trayed and packed columns facilitate contact between liquid and vapor, or in the case of a Scrubber, a gas stream. A well-designed liquid feed system ensures even distribution at an appropriate velocity. Typically, a liquid feed should not exceed 3 ft/s. In some cases, such as a trayed column with multiple phases, an additional structure (e.g., a plate or baffle) is necessary to break momentum and provide sufficient space for expansion and a full and even distribution profile.
Scrubbers are air pollution control devices that use liquid to remove particulate matter or gases from an industrial exhaust or flue gas stream. This atomized liquid (typically water) entrains particles and pollutant gases in order to effectively wash them out of the gas flow.
One of the most basic of the various industrial scrubbers is the wet industrial scrubber. In these units, water (or another fluid) is cascaded countercurrent to a contaminated gas stream in a packed column. The water absorbs the contaminate and is purged at the base of the column. Other liquids can be used to effectively remove varied contaminates.
Advantages of Scrubbers:
- Can handle flammable and explosive dusts with little risk
- Provides gas absorption and dust collection in a single unit
- Provides cooling of hot gases
- Compact; can often be retrofitted into existing collection systems
- Corrosive gases and dusts can be neutralized
Integrated Distillation Systems
A final consideration to discuss with your design engineer is integration.
Process integration is growing in popularity as a means of improving performance while reducing cost in traditional distillation columns. The concept is straightforward: capturing latent heat and using it elsewhere in the distillation process decreases energy expenditures, given that this heat would otherwise be wasted. However, the implementation of this sort of energy-recapture system can be difficult for any design more complex than a single distillation column. Design engineers also draw on technologies like multi-effect evaporation and mechanical vapor recompression (MVR) to maximize the efficiency and cost-effectiveness of a system.
A Comprehensive Guide to Distillation
Solvent recovery is a complex field with many variables affecting even the simplest distillation setups. For a more in-depth presentation of the design considerations and equipment components of a distillation system, request a copy of Thermal Kinetics’ Distillation 101 guide today, or contact us to get answers straight from our team of experts.