Method of Operation

Two methods of contacting the gas and liquid are possible: counter-current operation and co-current operation. See the Figure below. We will focus principally on the counter-current gas absorption, as it was widely used in the industry. The main differences between the two configurations will be highlighted. Note that for counter-current operation, the gas enters the column or tower from below as leaves at the top, while liquid enters from the top and flows in opposite direction and exits from the bottom.

[ once again, recall the similarity with heat exchanger operation ]

Method of Operatop

 

We will be concerned primarily with counter-current gas absorption. As shown in the Figure below, the gas flows upwards while the liquid flow downwards. Inside the column where there is vapour-liquid contact, mass transfer by absorption occurs, i.e. there is a transfer of solute(s) from the gas phase to the liquid phase.

Schematic of Solute Transfer in Gas Absorption

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Absorption Equipment

Absorption and stripping are conducted in tray (or plate or stage) columns, packed columns, spray towers, bubble columns, and centrifugal contactors. In general, operating pressure should be high and temperature low for an absorber, to minimize stage requirements and/or absorbent flow rate to lower the equipment volume required to accommodate the gas flow. The reverse is true for stripping. However, the operating pressure should not be too high and the operating temperature should not be too low as to condense the feed gas.

Tray Column

The types of trays used in absorption include: sieve tray, valve tray and bubble-cap trays. These internals are the same as those covered in "Distillation".

Packed Column

Both random and structured packings had been used. Again, please refer to the topics covered in "Distillation".

For more information on trays and packing, check out the PLANT OPERATION & MONITORING Section.

Spray Column

The gas flows upward continuously through an open chamber in which scrubbing liquid droplets falls from spray nozzles through the gas. The gas pressure drop is small, but separation is not as good as the bubble column. This column is widely used for its simplicity, low pressure drop, and resistance to scale deposition and plugging.

Spray column for gas absorption

Bubble Column

The gas is forced under pressure through perforated pipes submerged in the scrubbing liquid. As such the gas phase is dispersed and the liquid phase is continuous. As the bubbles rise through the liquid, absorption of the gas occurs. This type of device suffers from the high pressure drop due to the liquid hydrostatic head.

Bubble column for gas absorption

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