Distillation Principals Introduction Distillation is a commonly used separation technique in chemical processing facilities. The chemical industry relies heavily on distillation to create many of the products we use every day, such as the gasoline in our cars, the materials in plastic bottles, some alcoholic beverages, and distilled water. Distillation operates on the principle that light and heavy molecules have different boiling points. Distillation separates the lighter and heavier molecules from a liquid mixture by exploiting the differences in the molecules boiling points. Here, substance A has a lower boiling point than substance B in the mixture. As the mixture is heated to the boiling point for substance A, it changes into a gas and evaporates from the mixture. This gas is transferred to a different location and cooled back to a liquid form. The condensed purified substance A is called the distillate. Let’s look at an example. What happens when we boil salt water in our distillation process? As the water boils, the lighter, hot vapor of the water molecules travels to the top of the container. The water vapor travels out the top of the vessel and returns to the liquid phase as it is cooled in the condenser. Much of the salts, minerals, and impurities present remain in the mixture and are not transferred to the distilled water product. Let’s take a closer look to understand what is happening in this process. Remember that matter can change from one state to another. Changing from one state to another is called a phase change. The distillation process uses the phase changes of vaporization and condensation. Distillation processes require a lot of energy to produce these phase changes. The composition of the liquid mixture determines how much energy is needed to cause a phase change. Our salt water is a mixture of two compounds: water and table salt (NaCl). You should recall that ionic compounds such as the table salt have much higher boiling points than covalent compounds such as the water. This is a relatively simple distillation process because the two substances in the liquid have very different boiling points. As the water molecules change to a gas, the salts, minerals, and impurities that have higher boiling points remain and are concentrated in the solution. Heat Distillation operations use a variety of concepts and terminology. These key concepts will help you understand the variety of distillation processes and issues you will encounter in this course. Heat input that causes a change in temperature and that can be measured is called sensible heat. Heat input that causes a phase change without changing the temperature is called latent heat. For example, heat is required to keep the mixture boiling to create vapor, but the temperature does not change. The term “initial boiling point” is the temperature at which a liquid turns to a vapor state. More specifically, it is the temperature when the first bubble of vapor is formed from the liquid mixture. The initial boiling point of a liquid can also be defined as the temperature at which the vapor pressure of the liquid equals the pressure of the atmosphere surrounding the liquid. This initial boiling point for a given process is a function of pressure and composition of the mixture. The term “final boiling point” refers to the temperature where a natural material or fraction finishes boiling. This temperature is also called the “end point”. The “boiling range” is defined by the span between the initial and the final boiling temperatures of a multi-component solvent. Pressure Remember that in addition to temperature, pressure can affect phase changes. Lowering the surrounding pressure decreases the boiling point. Raising the surrounding pressure increases the boiling point. Several types of pressure are important to the distillation process. Vapor pressure is the pressure exerted by a vapor above the liquid at a given temperature. The higher the vapor pressure is at a given temperature, the lower the normal boiling point of the liquid. In our mixture of salt and water, each component has a vapor pressure. The vapor pressure contributed by the water alone is called a “partial pressure”. The salt also has a partial vapor pressure contributing to the vapor pressure for the mixture. The total pressure of a system is the sum of the vapor pressures for all the components in the mixture. Vapor pressure is related to the boiling point, temperature, and total pressure of the distillation system. Differential pressure is the difference between two pressure readings inside a column, usually between the base pressure and the top pressure. Sometimes differential pressure readings measure the pressure drop from one area on a column to the next. Some distillation processes operate with added pressure to enhance the separation. This is referred to a process being “under pressure”. Remember that liquids under pressure can be heated and stored at temperatures higher than their standard boiling point. Some compounds are distilled “under vacuum”. This means the pressure has been reduced in the system. Running the distillation process at lower than atmospheric pressure lowers the boiling point of a substance. Volatilization is a term used to describe the tendency of a substance to vaporize. If the liquid boils easily, the liquid is said to have high volatility. Liquids with high vapor pressure, and therefore high volatility, boil at lower temperatures. Relative volatility is the difference in volatility of two components. It describes how difficult it would be to separate the components by distillation. A mixture with high volatility can be easily separated. A mixture with low volatility is more difficult to separate. Process Types Chemical industry distillation processes can be single-batch, successive-batch, or continuous distillation systems. Batch distillation systems take in one batch of feed mixture at a time and separate it into two products by separating the lighter, more volatile component, which is called the overhead product. The heavier compounds that were not vaporized are called the bottoms product. For single-batch systems, the batch of feed is placed in the process and then run. When the distillation is complete, the process is repeated with a new batch of feed. Successive batch distillation systems selectively remove the more volatile components over time using several single batch systems chained together. Each stage removes the most volatile component as the overhead product and passes the bottoms product to the next stage where another compound is separated as its overhead product. The most volatile components are also called “low boilers” because they boil at a lower temperature. “High boilers” are the less volatile components that make up bottoms product. Batch distillation processes are not widely used in the chemical industry. However, these processes may be used in limited situations. For example, batch processes might be used to recover high value or special solvents. Continuous distillation systems take in a continuous feed mixture and separate it into two or more products. The feed liquid is added at the same rate the top and bottom products are removed from the system. Most chemical industry distillation processes use fractional distillation columns, also called towers, for continuous distillation processes. Sometimes distillation processes can require too many trays and result in extremely tall and expensive columns. Therefore, some distillation processes use two or more distillation columns to separate the mixture into the desired products. These joined columns are called distillation trains. Fractional distillation columns are necessary because many mixtures have very close boiling points. Each level of the column has different temperature profiles. You will learn more about different distillation column configurations in the next module. Heat introduced to a distillation column is precisely controlled. A preheater is sometimes used to heat the feed before it enters the column. Heat is also added by a reboiler, which heats part of the bottom product and adds it back to the column. The heat input from the reboiler keeps the process at the latent heat of vaporization required to vaporize the liquid. In some special cases, steam may also be sprayed below the surface of the liquid. Heat is removed from the column by the hot vapors exiting in the overhead product. A condenser removes the latent heat of condensation as the vapor changes to liquid. The hot bottom product leaving the process also removes heat from the column. The ambient temperature outside of the column also provides heat, or removes heat, from the process. Applications The distillation process is used widely in chemical manufacturing. For example, distillation is used to purify chemicals and create products from oil and gas feeds. In this example, Acetic Anhydride and Acetic Acid are produced using four distillation columns in a continuous process. Column A separates the anhydride and acid from EDA, ethylidene diacetate, which is a waste product in this product stream. The anhydride and acid are the low boilers and are taken off the top of the column and sent to Column B. In this column, the acid is the low boiler and it is taken off the top of the column as Acetic Acid product. The anhydride is the high boiler and is underflowed to Column C that cleans the product up a little more, removing any residual tars. The anhydride is now the low boiler and is taken off the top of column C and sent to a tank as Acetic Anhydride product. The residuals are underflowed back to the feed tank. Column D receives the EDA from the Column A and separates any residual anhydride from the EDA and takes it off the top of the column and sends it back to a feed tank where it will be fed back through the first column. The EDA waste underflows to a waste tank for disposal. Distillation is also used in other process industries such as: • oil refining • water treatment • pap