The Components of a Fractional Distillation Column By Jesse Sharifi Introduction Fractional distillation is the process in which mixtures of liquid components with similar boiling points are separated by utilizing phase changes. Fractional distillation is one of the most common separation techniques used in manufacturing. The process can be found in the pharmaceutical, chemical, cosmetic, and fuel industries. The process is so common because of how lucrative it is. Requiring nothing more than a glass column equipped with heating and cooling elements, fractional distillation can account for up to 70% of the revenue for a given product in the chemical industry. At its essence, fractional distillation can be boiled down to the separation of components by heating them; a process that most people perform daily. However, several key components and parameters need to be considered for fractional distillation to be efficient. The purpose of this document is to inform a general audience about the fractional distillation process. Since fractional distillation is such a common process, this document shows how common products are made utilizing this technique. This process is rather simple in theory, but can be quite cumbersome for a large-scale system. Having a general understanding about phase equilibria, the stable coexistence of liquid and gas, will help the reader understand this document better, but is not entirely necessary. Terms that seem foreign to the audience will be properly defined after they are introduced within the text. This document will focus on the main components that make the distillation process possible. These components are the feed, reboiler, column, condenser, reflux drum and the distillate. To facilitate the understanding of this process, with the exception of the feed stream, this document has been organized spatially. This allows the reader to understand how crude (unpurified) product travels through the apparatus to undergo purification. Below is a diagram of a fractional distillation column and its components that will be covered in this document. Feed As the name suggests, the feed is comprised of the crude liquid components which enter the fractional distillation column to be separated. This feed stream typically enters through the middle of the column and cools down what is known as “the stripping region” of the column. The stripping region is comprised of the plates, boilup and liquid in the lower half of the column. These components will be discussed in the concurrent sections of this document. The feed entering the column will vary from industry to industry. In college laboratory classes, the feed is typically a 15% ethanol : 85% water mixture. This is a common feed as ethanol is a component which is volatile, or susceptible to evaporation, and has a boiling point that is approximately 20º C lower than water. This allows for a complete and simple separation for first time users. This is in contrast to a more complex feed, such as a common fuels’ industry feed, comprised of crude oil and fuel. Up to six different products can be separated from this feed including kerosene, diesel fuel, and oil. The Reboiler At process startup, all of the liquid feed falls to the reboiler, the heating element that initiates the separation process. The reboiler continuously heats the crude feed until it reaches the desired temperature. This temperature is typically the boiling point of the more volatile component. When the feed has reached this temperature in the stripping region of the column it is referred to as the boilup. The boilup will continue to feed from the bottom of the column through the reboiler until the separation process is complete. When the column has reached the desired temperature and the final product has fulfilled all specifications, a valve is opened that allows the product to flow out of the process. This product is designated as the Xb stream on the graphic. This liquid is also referred to as the “bottoms”, as it is the components left at the bottom of the column. The term “bottoms” and “heavies” are used interchangeably, as more often than not; the heavier component has a higher boiling point than the “distillate” or “lights”. The Column With heating, we find that the boilup begins to travel up through the column in the vapor phase. The distillation column itself is a glass cylinder wrapped in insulation. Its purpose is to allow the feed, distillate, and boilup to flow simultaneously through the column during multiphase equilibrium. Inside the column are several metal slats called “trays”. The spacing and design of these trays determine the overall efficiency of the column. Each tray has several intricately designed holes called “downcomers”. These holes are what allow vapor and liquid to freely flow through the column without blockage. The Condenser At the top of the column is the condenser, the heat exchanger where the desired product is collected. The condenser is a heat exchanger comprised of an inner and outer shell. In the fractional distillation process, the inner shell is where the vapor product can be found. Water is constantly run through the outer shell to cool the unit. This allows the vapor product to return to the liquid phase. A picture of a typical heat exchanger is found below: A common product from the vapor stream is artificial flavoring. This is because most aromatic compounds are very volatile and have low boiling points. Separating an artificial flavor from its crude components using fractional distillation will allow for a higher purity to be achieved. This will rid the product of any off taste that is caused by impurities. The Reflux Drum The reflux drum, also known as the accumulator or decanter, is where condensed vapor is collected. From the reflux drum, liquid product, or distillate, can either flow back into the column or out of the system. Typically the distillate will be fed back into the top of the column until the desired operating temperature is reached. When no distillate is being collected and all of the product is being fed back into the column the system is running at “total reflux”. Total reflux is most common during equipment startup. This allows the column to reach equilibrium and achieve the desired purity. When this purity is reached, the column then functions as normal. The Distillate The distillate is the cooled vapor that has traveled through the column, passed through the condenser, and has been collected in the reflux drum. If all has worked correctly, the distillate will have the desired purity that was calculated using a t-xy diagram, also known as an equilibrium plot. The purity of the compound is often determined by specific gravity, or density. A device called a hygrometer measures the density as a function of buoyancy. If the hygrometer’s reading does not match the calculated value, the column is put back in total reflux and the temperature is adjusted. When the column reaches equilibrium once again, the column can be set to collect distillate. Conclusion Fractional distillation is one of the most practical, lucrative, and effective way to separate a crude feed into its individual components. This allows for a multitude of products to be made for the cosmetic, pharmaceutical, fuel, and chemical industries. This process is achieved by passing a feed through the reboiler, through the column and its trays, a condenser, and then the decanter at desired temperatures.
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