Tower classification

Tower equipment is one of the most important devices in chemical and petroleum refining processes. A tower is a device used in chemical production processes to facilitate close contact between gas-liquid or liquid-liquid phases, thereby achieving mass and heat transfer between these phases. There are many methods for classifying tower equipment. Based on the operating pressure, they are classified into pressurized towers, atmospheric towers, and vacuum towers ； They are classified into towers with fixed phase interfaces and those in which phase interfaces are formed during the flow process, based on the manner in which the interfaces that cause repulsion are created ； Based on the internal components of the tower, they are divided into plate towers and packed towers ； Based on unit operations, they can be classified as follows: 1. Distillation tower. Distillation relies primarily on the difference in volatility of various components within a mixture to achieve separation. Substances with higher volatility have a higher concentration in the gas phase than in the liquid phase; therefore, separation of light and heavy components is achieved through multiple processes of partial vaporization and partial condensation. 2. Absorption tower: Absorption primarily involves the process of one or more gases dissolving in a liquid. 3. Desorption tower: Desorption is the reverse process of absorption, namely the transfer of a volatile component in a liquid mixture into the gas phase. 4. Extraction tower: The extraction tower is one of the important unit operations for separating and purifying substances. In the liquid phase (the first phase), the relationships between the various components can be separated based on the interphase transfer processes. 5. Reaction tower: Reaction is the process by which new substances are formed from a mixture under certain conditions such as temperature and pressure. 6. Regeneration Tower: The regeneration process involves the desorption and renewal of the solution through steam mass transfer and stripping. 7. Drying Tower: The drying of solid materials involves two basic processes. First is the heat transfer process, in which the solid is heated to vaporize the moisture. Second is the mass transfer process, during which the vaporized moisture, having a high vapor pressure, diffuses into the gas phase. Meanwhile, the moisture is continuously transported from within the solid material to its surface through mechanisms such as diffusion; this is a mass transfer process occurring inside the material. Therefore, the drying process is characterized by the simultaneous occurrence of mass transfer and heat transfer processes.

According to the relevant regulations, classification should be based on the characteristics of the unit operations: catalytic lift columns should be classified as reaction vessels, distillation columns and stabilization columns should be classified as separation vessels, while drying towers and refrigeration towers belong to heat exchange vessels. These are my personal opinions; please feel free to correct me.

A cooling tower is a widely used thermal equipment whose function is to dissipate the heat from hot cooling water into the atmosphere through heat and mass exchange, thereby reducing the temperature of the cooling water. Cooling towers are classified into different types based on various criteria: (1) Based on the way air comes into contact with water, they can be divided into wet cooling towers and dry cooling towers, as well as hybrid dry-wet cooling towers that combine both approaches. In wet cooling towers, air and water come into direct contact to facilitate heat and mass exchange, resulting in high efficiency in such exchange. The limit temperature of the cooling water is determined by the wet-bulb temperature of the air. The disadvantages include evaporation losses and drift losses of the cooling water, as well as an increase in salinity due to water evaporation, which necessitates the addition of more water ； In dry cooling towers, water or vapor exchanges heat with air through indirect contact, with no mass transfer occurring. They are primarily used in water-scarce areas and for special applications; their heat exchange efficiency is generally low, and they require significant investment as well as high energy consumption. (2) Based on the ventilation method, they are divided into naturally ventilated cooling towers and mechanically ventilated cooling towers. Natural ventilation cooling towers, also known as ducted or hyperbolic towers, utilize the ventilation draft generated by the difference in air density inside and outside the tower to facilitate air circulation (natural ventilation). They offer stable cooling performance, low operating costs, few failures, and are easy to maintain. The high elevation of the air ducts results in minimal environmental impact from droplets and mist. However, their drawback is the small difference in air density between the inside and outside, which leads to a weak ventilation draft; as a result, they are not suitable for use in areas with high temperatures and high humidity ； Mechanical ventilation cooling towers are further divided into exhaust-type and forced-air type cooling towers, which utilize exhaust fans or blowers to force air flow. They offer high cooling efficiency and stability, require less space, and have lower capital investment costs; however, their operating expenses are high. The exhaust-type cooling tower creates a negative pressure inside the tower, which facilitates water evaporation, while the forced-air type does the opposite. Forced-air type cooling towers are mainly used in small cooling towers or those where water is corrosive to the fans. (3) Based on the flow direction of water and air, they can be divided into counterflow cooling towers and crossflow cooling towers. In counterflow cooling towers, water flows from top to bottom while air flows from bottom to top; in crossflow cooling towers, water flows from top to bottom and air enters from a horizontal direction. A cooling tower system generally consists of five components: wetting fill, water distribution system, water collector (dehumidifier), ventilation equipment, and air distribution device. The function of the wetting filler is to transform the hot water entering the cooling tower into as many small water droplets or a thin water film as possible, thereby increasing the contact area and contact time between water and air, which facilitates heat and mass exchange between them. There are three common types: drip irrigation systems, film-type irrigation systems, and grid-pattern template irrigation systems. The function of the water distribution system is to distribute hot water evenly across the surface of the entire sprinkler device, so as to maximize its effectiveness. It is further divided into: tubular water distribution systems (fixed and rotary), trough-type water distribution systems, and pool-type water distribution systems. The advantages and disadvantages of these three types are shown in the table below:

| Water Distribution System Type | Advantages | Disadvantages |
|------------------------------|------------|---------------|
| Tubular | Even water distribution, fine water droplets, good cooling effect; easy to ensure installation quality; algae does not easily grow inside the pipes. | Requires higher water pressure at the nozzles; pipes may get clogged if the water quality is poor. |
| Trough-type | Lower water pressure required; easier to clean. | Sediment and algae easily accumulate in the troughs; complex structure; high air resistance. |
| Pool-type | Even water distribution; easy to clean; lower water pressure; simple structure. | Sediment and algae easily accumulate in the pool. |

The function of a water collector (dehumidifier) is to reduce the moisture content in the air exiting the cooling tower. After passing through the sprinkling devices and water distribution system, the air contains many tiny water droplets. It is necessary to use a water collector to capture some of these droplets before the air leaves the cooling tower, thereby reducing losses of cooling water. Ventilation equipment is used to generate high air flow speeds and a steady air flow rate, thereby improving cooling efficiency and ensuring effective cooling. Mechanical ventilation cooling towers mainly use axial flow fans, which are characterized by high ventilation volume, low air pressure, low energy consumption, and resistance to erosion by water droplets and mist. Typically, to ensure even distribution of air across the entire cross-section of the cooling tower, air distribution devices such as air inlets, louvered panels, and guide vanes are also required

Static equipment: liquefied gas storage tanks, various distillation columns, various vertical or horizontal storage tanks, various heat exchangers. Rotating equipment: water pumps, gearboxes, mixers, fans.