Absorption and stripping are important mass-transfer operations within the chemical process industries (CPI). This one-page reference provides some basic information on the principles and applications of stripping and absorption in industrial chemistry settings.
General operation
In absorption, a solute from a gas stream is removed via uptake into a nonvolatile liquid (the solvent or absorbent). In the reverse process (desorption or stripping), a component from a liquid stream is removed via vaporization and uptake into an insoluble gas stream. Because these unit operations are complementary, they can be used together as a cycle. Both absorption and stripping can be operated as equilibrium-stage processes using trayed columns or, more commonly, using packed columns.
Solubility of the molecule of interest in the liquid phase is a key factor in absorption. The more soluble the solute, the more readily it is transferred from the gas to the liquid phase. Solubility of gases in liquids is affected by temperature, pressure and the properties of the gas and liquid. Liquid solvents are chosen such that the solute of interest in the gas mixture is preferentially dissolved in the solvent rather than the gas mixture. Common solvents include water, alkanolamines (MEA, DEA), and physical solvents (Selexol™, refrigerated methanol).
Absorption is driven by the concentration gradient between the gas and liquid phases (the difference in the concentration of the solute between the gas and liquid phases). Solute transfer from gas to liquid phase occurs until equilibrium is reached. Stripping is driven by the concentration gradient in the opposite direction, with the solute transferring from the liquid phase to the gas phase.
Absorption and stripping processes use counter-current gas flow, in which gas enters the bottom of a column and exits out the top. Meanwhile, liquid enters at the top of the column and flows downward opposite the flow of gas (Figure 1).
FIGURE 1. Counter-current flow involves gases rising through the column, while liquid falls downward against the flow of gas
Common applications
Absorption processes can be used to separate gas mixtures, remove impurities, contaminants or pollutants, or catalyst poisons from a gas stream. In some cases, they can be used to recover valuable chemicals. One application example is the removal of H2S from hydrocarbons using amines as solvent. Others include: washing ammonia from ammonia-air mixtures using water as a solvent; gas purification by removal of CO2; removal of acidic gases from natural gas; recovery of valuable components from process gas streams (ammonia, hydrocarbons); removal of contaminants or moisture.
Stripping applications include the removal of volatile organic compounds (VOCs, such as benzene or toluene) from wastewater; regeneration of solvents by removing dissolved gases or other impurities; and deaeration of liquids (removal of dissolved O2 and CO2).
Absorption and stripping columns
Absorption and stripping columns are typically designed as packed towers or tray columns. The choice of column type depends on factors such as the gas and liquid flowrates, the required separation efficiency and the pressure drop. In packed towers, the mass transfer between the gas and liquid phase occurs on the surface of packing material, which provides a large interfacial area for mass transfer. Common packing materials include Raschig rings, Pall rings or structured packing.
In tray columns, a series of horizontal trays create stages for gas-liquid contact. The liquid flows across each tray and then down to the next tray through downcomers. The gas rises through openings in the trays, bubbling through the liquid on each tray.
Design considerations
The design of absorption and stripping columns involves determining the column diameter, height and type of internals (either packing or trays) based on the required separation efficiency, gas and liquid flowrates, and the properties of the gas and liquid.
•Key design parameters include the gas and liquid flowrates, the inlet and outlet concentrations of the substance of interest, the equilibrium curve (relating the concentration of the solute in the gas and liquid phases at equilibrium) and the mass-transfer coefficients (quantifying the rate of mass transfer between the gas and liquid phases).
•The column diameter is determined by the gas and liquid flowrates and the allowable pressure drop.
•Column height is determined by the number of transfer units (NTU) and the height of a transfer unit (HTU).
References
1. Naik, S. and others, University of Michigan College of Engineering. Absorbers, Visual Encyclopedia of Chemical Engineering Equipment, https://encyclopedia.che.engin.umich.edu/absorbers/.
2. Falconer, J., Introduction to Absorption Columns, University of Colorado, presentation, youtube.com.
3. Falconer, J., Introduction to Stripping Columns, University of Colorado, presentation, youtube.com.
4. Rajasekhar, M., Absorption and Stripping in a Packed Column, Presentation, www.slideshare.net.