1. Introduction

The typical gas-liquid contacting operations include distillation, absorption, stripping, leaching and humidification. Distillation and absorption are two most widely used mass transfer processes in chemical industries. Design of plate column for absorption and distillation involves many common steps of calculation such as determination of number of theoretical plates, column diameter, plate hydraulic design, etc. In absorption process, a soluble component is absorbed in a liquid (called solvent) from a gaseous mixture. The gas and liquid streams leaving the tray are in equilibrium under the ideal condition. The separation in distillation is based on the relative volatility of the components. Additional vapor phase is generated by the vaporization of more volatile components (called stripping) and by condensation of relatively less volatile components(called absorption) adds to the liquid phase.

Selection of column type: Plate or Packed: Packed towers (columns) are also used as the contacting devices for gas absorption, liquid-liquid extraction and distillation. The gaseous mixture is allowed to contact continuously with the liquid counter-currently in a packed column. The liquid flows downward over the packing surface, and the gaseous mixture flows upward through the space in the packing. The performance of the column strongly depends on the arrangement of the packings to provide good liquid and gas contact throughout the packed bed. The solute gas is absorbed by the fresh solvent (liquid) entering at the top of the tower where the lean gas leaves system. The liquid enriched with absorbed solute gas, leaves the column bottom through the exit port.

In a plate tower, the liquid and gas are contacted in stage-wise manner on the trays; while gas-liquid contact is continuous in a packed column. There are always some uncertainly to maintain good liquid distribution in a packed tower. For this reason, it is difficult to accurately estimate the packed tower efficiency. The course content is limited to design of plate column only and some typical criterions for the selection of column type are discussed below.

 

 Plate towers exhibit larger pressure drops and liquid holdup at higher gas flow rate. While, packed towers are not appropriate for very low liquid flow rates. Packed column is the preferred choice than a plate column to handle toxic and flammable liquids due to lower liquid holdup to keep the unit as small as possible for the sake of safety.

  Plate columns are normally suitable for fouling liquids or laden with solids. They are easier to clean and could handle substantial temperature variation during operation.

  Packed towers are more suitable for foaming and corrosive services.

  It is easier to make the provision for the installation of internal cooling coils or withdrawal of side streams from a plate column.