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Stripping (chemistry)

physical science Maturity 9-11 Next article →

Stripping is a vital physical separation process used in many industrial settings. It works by removing one or more specific components from a liquid stream. To do this, a vapor stream is passed through the liquid. This process relies on a principle called mass transfer. Mass transfer occurs when a component, often called a solute, moves from the liquid phase to the vapor phase. For this to happen, the substance must cross a gas-liquid interface. The total amount of the substance that moves across this boundary is known as the flux of A.

Bubble Cap Trays
Bubble Cap Trays

Engineers design stripping columns to maximize this mass transfer. The liquid and vapor streams can flow in two different ways. They might move in a co-current flow, where both travel in the same direction. Alternatively, they can move in a countercurrent flow, where they travel in opposite directions. Most industrial stripping is performed inside tall vertical columns. These columns are typically either trayed towers or packed columns. They can also occasionally be done in spray towers, bubble columns, or centrifugal contactors.

Trayed towers, also known as plate columns, use internal structures to work. In these towers, the liquid flows in at the top and exits out the bottom. The vapor phase enters at the bottom and exits through the top. Inside the column, there are several trays or plates. These trays force the liquid to flow back and forth horizontally. At the same time, the vapor bubbles up through holes in the trays. This specific movement increases the contact area between the liquid and the vapor.

Bubble Cap Trays
Bubble Cap Trays

Trays can be designed as sieve trays, valve trays, or bubble cap trays.

Packed columns offer a different method for achieving separation. Like trayed towers, the liquid and vapor enter and exit in the same way. However, packed columns do not contain trays. Instead, they are filled with various types of packing material. This packing is used to increase the contact area between the two phases. Packed columns are often preferred for smaller operations. This usually applies to columns with a diameter of less than 2 feet and a height of no more than 20 feet. They are also useful for handling corrosive or high-foaming fluids.

Bialecki rings
Bialecki rings

There are many different types of packing materials available. Engineers choose them based on surface area, flow area, and pressure drop. Older generations of packing included ceramic Raschig rings and Berl saddles. Newer, more common materials include metal and plastic Pall rings. There are also ceramic Intalox saddles. One notable example is the Białecki ring, which was described in Polish Patent 101135 in 1976.

Bialecki rings
Bialecki rings

Modern packing is designed to prevent materials from stacking on top of each other. If packing stacks, it drastically reduces the available surface area for mass transfer.

Designing a stripper requires managing many different variables. Engineers must consider the entering conditions and the required degree of recovery. They must also choose the right stripping agent, such as steam, air, inert gases, or hydrocarbon gases. The choice of agent depends on solubility, stability, cost, and corrosiveness. To minimize flow, operators often work at high temperatures and low pressures. It is also important to manage the size of the equipment. If the column is too large, flow channeling can occur. This is when the liquid does not spread out evenly. If this happens, a redistribution plate is used to reestablish the contact area.

Stripping plays a massive role in environmental protection and remediation. It is frequently used to remove harmful contaminants from waste streams. For example, steam stripping can be used to clean harbor soils. Contaminated soils are dredged and mixed with water to create a slurry. Steam is then used to strip contaminants like TBT and PAH from the mixture. This process can decontaminate soils almost completely.

Bialecki rings
Bialecki rings

Additionally, steam is used in water treatment. It removes volatile organic compounds from groundwater and wastewater. This is essential because these compounds are often present due to industrial or agricultural activities.

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