Fundamentals of chemical separationsSeth Price | March 15, 2023
Chemical engineering can be simplified into two pieces: putting things together and taking things apart. Chemicals are either reacting to produce something new or need to be separated from other chemicals to purify or clean them to make them saleable.
This article focuses on the latter, separating chemicals to remove contaminants. It is not intended to be a complete guide that lists every technique, but more as a way to highlight common methodologies and demonstrate how they work. While new techniques and optimizations will be created in the years to come, they will likely be based upon these foundations.
Motives for separations
The separation of materials is one of the most important concepts in everyday life. Whether it is the extraction of flavor and caffeine from coffee beans in the coffee pot, the human body extracting nutrients from foodstuffs, the separation of hydrocarbons to produce gasoline or many other examples, human life is only made possible through separation techniques. Regardless of the separation technique used, they all have a similar purpose: remove the undesirable from the desirable. This could mean filtering out contaminants, unreacted chemicals, undesirable phases, byproducts or any other substance.
While there are lots of separation techniques, most of them fit into several categories. Any difference in desirable and undesirable products has the potential to be a separation mechanism, though some are harder to use than others.
The simplest separation technique is to take advantage of differences in density. Density differences are used in settling and centrifugal operations. They work for separating immiscible liquids, and solid-liquid mixtures.
Settling is the most common form of density separation, with settling ponds as the most common method. Settling ponds are used to remove solid materials from liquids or to separate liquids of different densities. In settling, desirable products can be extracted from any depth in the pond. These are commonly used by the mining industry to separate out contaminants from wastewater streams.
Closely related to settling is flotation. At first glance, this looks like settling, only extracting from the top, but it is a little different. Flotation takes advantage of the fact that some materials are attracted and stick to air bubbles. Whereas settling is performed in a still pond, flotation is when air pumps are used to intentionally agitate the mixture. The small particles attach to the air bubbles and float to the top to be removed as a froth and processed further using other techniques. Flotation is commonly used in the mining industry.
Good engineering practice as well as local, state and federal environmental laws dictate the construction of settling and flotation tanks and ponds, often including liner requirements to prevent materials from contaminating groundwater. With settling, the key variable is time; the longer the material is allowed to settle undisturbed, the more thorough the separation, though it often reaches an asymptote where more time produces diminishing returns.
Besides settling, centrifugal forces can be used to speed up density separations. Materials are spun at high speeds so that solids or liquids of higher density are forced to the end farthest from the center of rotation. This is commonly performed during blood laboratory operations, such as donating platelets, though it can be performed at the industrial scale as well.
Another very common method for separating materials is to force them into a phase change. Distillation uses the differences in boiling to remove one material from the other. The mixture is heated between their respective boiling points so that one material is released as a vapor while the other remains as a heated liquid. Often, the vapor is collected and condensed through cooling, rather than being released into the atmosphere. Liquors are made from capturing the vapor and allowing it to condense for the final product. Many products of crude oil are made from tapping different parts of a distillation column.
Some separations are performed by taking advantage of freezing temperatures or by cooling one vapor to its condensation point. The additional complexity, especially when using industrial coolants, such as liquid ammonia, can make this operation more expensive.
Filters are also a common method of removing materials. Dry filters remove solid particles from fluid streams, allowing the fluid to pass but holding back the solids. As the filters become loaded, the pressure drop across them becomes greater, and limits the flow. Eventually, the filter will need to be removed and replaced. Because the pressure drop increases, pressure can be used to determine when filters should be changed.
One area of constant research in separation techniques is the development of new membranes for separating chemicals. Membranes that allow one material to pass and not another rely on particle size separations, electrostatic attraction, or some other such mechanism. Environmental engineers are hoping to find new ways to clean polluted water simply by passing it through a membrane to filter out chemical contaminants.
In chemical engineering, there are a number of ways to separate materials through solubility differences across various chemical families and all phases of materials.
In some cases, such as liquid-liquid extraction (LLE), the trick is to put one mixture in contact with another liquid, where one part of the mixture is more soluble in the second liquid. This purifies the mixture. Gas-liquid adsorption uses a similar trick, only absorbing some component from a gas into the liquid. This technique can be used to clean up a gas stream before releasing it into the atmosphere.
Another common solubility separation is to crystallize out a solid. This can be performed by driving off a solvent, slowly increasing the concentration until the solubility limit is reached. Consider the natural salt lakes — rock salt has dissolved the water, but then as the water evaporates, salt crystals are left behind.
While solids sorting is its own field entirely, it is worth mentioning that electrical properties and magnetic properties can be used to sort metals. The two most common methods of sorting metals are through strong magnets and eddy currents.
Perhaps the best example is in scrap steel processing. Shredded cars pass along a conveyor, and the steel is removed by magnet. However, cars also have glass, foam, plastic, copper, aluminum, magnesium and other materials. Eddy currents can be used to separate the other metals and remove them from the line, leaving behind the other waste for recycling or placing in a landfill.
There are many techniques for separating chemicals, and more will be developed in the near future. Each of them will be some new approach to optimizing the above techniques or based on the physical and chemical phenomena listed above.