A packed tower consists of three main components: the tower shell, the packing, and the tower internals. The shell is typically a vertical cylindrical vessel containing a bed of packing of a specific height. Internals include packing support devices, liquid distributors, liquid redistributors, mist eliminators, and packing hold-down plates. Liquid distributors ensure uniform distribution of the liquid entering from the top, thereby maximizing the wetted surface area and contact area; common types include shower-head, perforated pipe or spray, notched-trough, and perforated-tube distributors. Liquid redistributors are often conical devices designed to prevent liquid maldistribution or wall flow.
The packing is the core component of a packed tower. Key packing characteristics include specific surface area, void fraction, and packing factor. Packings are classified by material (e.g., ceramic, metal, plastic), installation method (structured/ordered stacking vs. random dumping), and shape (e.g., rings, saddles, corrugated sheets, or mesh structures). Broadly, packings fall into two categories: random packing and structured packing.
Gas-liquid flow within the tower involves critical parameters such as spray density, tower pressure drop, superficial gas velocity, wetting rate, and liquid holdup. When plotted on a log-log scale, the relationship between pressure drop across the packing bed and superficial gas velocity reveals three distinct regions-constant liquid holdup, loading, and flooding-marked by specific loading and flooding points. The flooding gas velocity depends on packing characteristics, fluid properties, and operating conditions; the Eckert correlation chart is commonly used for this determination. To calculate the tower diameter, the liquid-to-gas ratio is determined based on gas throughput, the flooding gas velocity is calculated, a design superficial gas velocity (typically 0.5 to 0.8 times the flooding velocity) is selected, and the diameter is then computed.
A packed tower is a mass-transfer device that utilizes packing as the contact medium between gas and liquid phases. Liquid is sprayed onto the packing from the top via a distributor and flows down along the packing surfaces, while gas enters from the bottom, passes through a distribution device, and flows upward-counter-current to the liquid-through the voids in the packing bed; mass transfer occurs through intimate contact between the gas and liquid phases on the packing surfaces. Packed towers are continuous-contact gas-liquid mass transfer devices in which the compositions of the two phases vary continuously along the height of the tower; under normal operating conditions, the gas phase acts as the continuous phase, while the liquid phase acts as the dispersed phase.
Absorption towers should meet the following basic requirements: provide sufficient contact area and adequate contact time between the gas and liquid phases; ensure vigorous turbulence to minimize absorption resistance and maximize absorption efficiency; offer a wide operating range, stable performance, and appropriate operational flexibility; minimize resistance to gas flow to ensure low energy consumption; possess sufficient mechanical strength and corrosion resistance; and feature a simple structure that facilitates manufacturing and maintenance.