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Five Critical Factors to Vapor-Liquid Equilibrium During Distillation

The first, and most critical, step in successful distillation system design is modeling vapor-liquid equilibrium. Understanding the factors that affect vapor-liquid equilibrium is essential for successful modeling of this key interaction. Qualified engineers should review the following five factors before moving forward to column operating objectives.

1. Relative Volatility:

The boiling point of a pure liquid is defined as: “the temperature at which the vapor pressure of a liquid is equal to the pressure exerted on it by the atmosphere.” The basic principal behind any type of distillation is separating two or more substances based on their boiling points.

The boiling point of a mixture is a function of the vapor pressures of the various substances within the mixture. The total pressure of a solution is the sum of the partial vapor pressures of individual components. Each substance has its own dedicated boiling point at which it evaporates, leaving behind substances with higher boiling points. The boiling points and vapor pressures of each individual substance indicate the relative volatility of a chemical to the rest of the mixture. Determining the vapor pressures and, in turn, the relative volatility, will indicate how difficult it will be to separate the substances in the mixture via distillation.

During distillation, substances separate based on their boiling points. The boiling point of the mixture varies as vapor rises in a distillation column, due to changes in temperature and pressure. This process divides distillation into stages.

The liquid and vapor present at each “stage” will reach equilibrium and cannot be further separated. With each progressive stage, the ratio of substances present in the rising vapor and condensed liquid changes to more or less concentrated per the process needs. Distillate (vapor reaching the top of the column) or bottoms (liquid falling to the bottom of the column) can be further purified either through further distillation or other filtering processes, if desired.

Distillation is a little more complicated than just boiling off individual substances. When a mixture of chemicals is considered, the boiling point of the entire mixture is somewhere between the lowest and highest boiling point of the pure substances contained in the mixture, depending on the relative concentrations of the parts of the mixture. That’s when the next factor, activity coefficient, becomes important.

2. Activity Coefficient – Surface Tension Example:

The attraction of individual molecules at the surface of a solid or a liquid creates a force that is directed solely inward. This is called surface tension. It requires more energy to vaporize a liquid because individual molecules must overcome surface tension. Substances with lower surface tension vaporize with less difficulty. More vapor molecules create increased vapor pressure on a substance. If the surrounding vapor pressure is increased, the boiling point will be lowered. If the surrounding vapor pressure is decreased, the boiling point will rise. The substance with the highest vapor pressure will contribute more molecules to the vapor boiling off, because less energy is required to release molecules from the surface tension.

The lighter components (lower-boiling point) of a mixture concentrate in the vapor phase. The heavier components (higher-boiling point) remain in the liquid phase. Liquid runs down the column while vapor rises, contacting as they go. Vapor becomes richer in light components as it rises and liquid becomes richer in heavy components as it falls. Liquid reaching the bottom of the column is re-boiled and partially vaporizes back up the column. The remainder of the bottom liquid is withdrawn as bottom product (bottoms). Vapor reaching the top of the column is cooled and condensed to liquid and removed as distillate or overhead product.

Surface tension must be considered because it can cause deviations in expected behavior in a mixture of chemical substances. This is where activity coefficient comes in. An activity coefficient is a factor used in thermodynamics to account for non-ideal behaviors. Surface tension is one factor that can cause deviations. The overall separation of chemicals achieved, during distillation, depends on the following: the relative volatilities of the components, their activity coefficients, the ratio of the liquid-phase flow rate to the vapor-phase flow rate and the ratio of surface area to liquid within the distillation column.

 3. Solubility:

The tendency of specific substances to dissolve in a solvent (another solid, liquid, or gaseous substance) to form a homogeneous solution affects how well a substance can be separated via distillation. The more alike two substances are, in boiling point especially, the harder they will be to separate. Solubility changes with temperature and pressure, which means it may become harder to separate two substances as distillation progresses.

What solubility usually boils down to (for distillation) is this: if you have a non-soluble, non- heterogeneous azeotrope, you will have to separate it in two different stages.

4. Maximum achievable concentration:

There is always a limit to the concentration of a certain substance that is achievable through distillation. Repeated distillation can refine a mixture only to the maximum achievable concentration, after which point distillation cannot further separate the substances. It is important to know the maximum achievable concentration of any substance through distillation so that parameters can be set for the distillation process and further refining methods, such as filtering, can be planned for, if needed.

An example of this is rubbing alcohol. If you look at a bottle of rubbing alcohol it will say that it is 91% alcohol, the remaining 9% is water. During distillation, when the mixture reaches this 91% alcohol ratio, the remaining water and alcohol form an azeotrope, in which the two components boiling points are so close together they cannot be further separated through distillation.

5. Surface Area:

Achieving the optimum amount of surface area for the liquid and vapor present in a distillation column maximizes the efficiency of a distillation column. As liquid spreads out, the amount of surface area increases, making it easier to boil the liquid and allowing more contact between liquid and vapor molecules. The more times a set of molecules cycles between the liquid-vapor state, the faster two substances can be separated. Only the molecules with the highest boiling point will reach the top of the column as vapor.

As mentioned earlier, each “stage” of a distillation column is an area in which the vapor and liquid molecules present have reached equilibrium. Equilibrium is a condition where the rate of evaporation equals the rate of condensation. At equilibrium no further vapor can be boiled off to rise higher in the column.

Tray_Distillation_Tower

The number of stages required to separate the substances into the desired ratio will determine the height of the column. Reflux must also be considered in conjunction with stage requirements. Reflux is the practice of returning condensed vapors back into the column to further increase the amount of liquid and vapor running through the column. The amount of liquid refluxed backed into the column can lower the number of stages required.

 

The traditional way of designing a distillation column will have a tray at each stage level which collects condensed liquid and allows it to remain in constant contact with vapor in the column. Having a tray at each level allows liquid to spread out across the trays in their natural stages.

 

packed-column-distillation

A more recent way to achieve the maximum amount of surface area is to use a packed-column design, where the distillation column is filled with packing material, in either random dumped formation or a structured formation within the column. The packing material provides large amounts of surface area for the liquids to spread out on as the mixture separates into stages.

MODULAR DISTILLATION SYSTEMS BY EPIC SYSTEMS, INC.

EPIC Modular Process is experienced in taking your distillation technology and applying our expertise in column sizing, mechanical design and process manufacturing. The process technology for each unique project is most often non-linear, and must be designed and implemented by qualified engineers. Contact EPIC to speak with an engineer, or learn more about modular distillation systems.

SOURCES:

  • Perry, Robert H., Don W. Green, and James O. Maloney. Perr’ys Chemical Engineers’ Handbook. 6th. New York: McGraw-Hill chemical engineering series, 1984. Print.
  • Nixon, Mike. “Disillation: How It Works.” JourneyToForever.Org N.p., n.d. Web. 13 Aug 2012.
  • “Fractionating Column.” Wikipedia. 16 08 2012: 4. Print.
  • “Theoretical plate.” Wikipedia. 15 04 2012: 6. Print.

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