Distillation and other Separation Techniques

One of the most important unit processes in chemical engineering is the separation of chemical compounds. This is where the products from a chemical reaction are split into

usable, sellable products. In a chemical process, after the reactor, the products and any unreacted reactants are all mixed together in one process stream. The goal of the subsequent separation steps is to separate the desired or valuable chemicals from the waste products. The separation apparatus will often also separate the unused reactants to be fed back to the reactor as recycle. Recall that recycle helps to reduce raw material costs as well as keep feed ratios constant. Separation processes and be uni-step or multi-step depending on the degree of difficulty of the separation. There are many different techniques that can be used for separation depending on the type of chemicals involved and their properties.

One of the most important types of separation is distillation. Recall that

distillation is involved in the production of alcoholic spirits. It is the post-fermentation step used to increase the alcohol content. This is done by distilling the alcohol from the water to create a purer concentration. Distillation is also used in the production of gasoline and other oil products. A series of distillation columns are used to separate the heavy hydrocarbons from the lighter ones. The lighter hydrocarbons are used for the production of gasoline, kerosene, motor oil, and liquefied natural gas while the heavier hydrocarbons are either cracked down to create lighter hydrocarbons or used for the production of diesel. Distillation columns are used in a variety of other applications as well, but these are the most common and relate to products that are used daily. Some other examples of distilled products are pharmaceuticals and liquefied air.

Recall that distillation operates on the principle of different boiling points. The feed is fed into the column as a liquid and is heated to the boiling point of the more volatile component. The more volatile component will evaporate more quickly. This is called the light key and is found in abundance at the top of the tower. The less volatile key will remain in the liquid phase at the bottom of the tower. This is called the heavy key. Selectivity plays a large role here because if the two components are very close in boiling point, their selectivity will be low and a large separation will not be achieved via distillation. This is when other separation tactics may be needed.

During distillation the inlet is typically a liquid and is fed in at a specific height on the column. This feed tray location is determined via a series of complex optimization equations that determine which tray is the optimal feed tray. There are a unique number of trays in each distillation column. This is also determined via rigorous equations that decide the number of equilibrium stages needed for proper separation. Vapor liquid equilibrium is achieved at each

stage on the distillation column. Recall the process of VLE from lesson 10. McCabe-Thiele diagrams are used to determine the optimum reflux ratio, compositions, and feed stage. At the top of the distillation column, the vapor is fed into a condenser in order to condense it back into liquid that can be refed into the column or pumped out as product. After the condenser, the reflux is fed into a reflux drum where it is held. It is then pumped back into the tower or pumped out as product. The amount released as product vs returned to the tower is called the reflux ratio.

𝑟 = 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑟𝑒𝑙𝑒𝑎𝑠𝑒𝑑 𝑜𝑢𝑡

𝑑𝑖𝑠𝑡𝑖𝑙𝑙𝑎𝑡𝑒 𝑟𝑒𝑡𝑢𝑟𝑛𝑒𝑑 𝑡𝑜 𝑐𝑜𝑙𝑢𝑚𝑛

Where r is the reflux ratio. At the bottom of the tower, the liquid is fed into a reboiler. This will heat the liquid into a vapor-liquid mix that can either be refed into the column or released as a finished product.

Typical distillation columns, such as those found in oil refineries are run continuously. This means that feed is constantly being added and products removed from the system. The only time these columns are shut down is for cleaning, regular maintenance, or in the case of a disaster. Continuous distillation is economically beneficial and safer for large scale operations as the most dangerous periods for columns are in the transient phase (either during start up or shut down). This is the most likely time for a disaster. For smaller scale operations, however, batch distillation can be used. This is typically done for laboratory scale production or in the pharmaceutical industry. This is also used in many small scale breweries. Similarly, batch distillation is how moonshine is created and other “home distilleries”. One of the labs in this course is an experiment that deals with batch distillation. In batch distillation, a set amount of feed stock is used and no more feed is fed in after the initial step. A pot containing a mixture of 2

liquids is boiled. The vapors are collected and cooled, resulting in a higher concentration product with more of the lower-boiling point (more volatile) component.

When the boiling points of the mixture components are too similar, as stated before, it is necessary to explore other forms of separation. These include:

Adsorption: Adsorption relies on the principle of adhesion of certain molecules to a surface, the adsorbent. There are specific surfaces used in this process due to their large void fractions, allowing many molecules to adsorb to it. Only certain molecules will adsorb to specific surfaces, thus allowing the rest of the fluid to pass though. Adsorption is a surface based process where a film of adsorbate is created on the surface of the adsorbent. It is an exothermic process that’s rate increases steadily until equilibrium. Equilibrium is achieved when the surface is completely saturated with the adsorbed molecules. Adsorption is used in synthetic resin, water purification, the pharmaceutical industry, and biomedical devices.

Absorption: Absorption is a process where a fluid is dissolved by a liquid or a solid. This is the absorbent. There is a transfer of one of more components from the gas phase to the liquid phase. Absorption occurs in the bulk of the fluid. It is an endothermic process that occurs at a uniform rate. This is used to separate gas mixtures, remove impurities, and/or recover valuable chemicals. Absorption is used in space cooling, ice production, and the carbonation of beverages.

Stripping: Stripping is a physical separation process where a liquid or vapor stream removes one of more components from a mixture. It is basically the opposite of an absorption operation. Stripping removes an absorbed solute from a solvent. Typically, the stripping fluid flows counter current to the mixture. This typically takes place in a packed bed or tray column.

Extraction: Extraction is when a mixture is brought into contact with a solvent in which only the desired substance is soluble, but the other components are insoluble. The principle of extraction is that 2 phases that do not mix are used to separate a substance from one phase to another. This is typically done using an aqueous and an organic phase, which can then be separated via a separatory funnel.

Membrane: A membrane separation is one in which fluid is flowed through a semi porous membrane where only certain particles are able to pass through the membrane wall. This is usually dependent on pore size of the membrane and the size of the molecules trying to pass through. The smaller molecules will be able to move through the walls of the membrane while the larger ones will not. This type of separation relies on pressure drop through the walls as the driving force for the separation.