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:
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.
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.
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 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.
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.
Regardless of the specific models used, the overall design process follows a standard practice. The general steps are as follows:
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:
Thermal Kinetics will often advise the use of Pilot Testing in a variety of situations, especially when a design carries a performance risk.
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: