A spray tower (or spray column or spray chamber) is gas-liquid contactor used to achieve

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

and

heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

between a

continuous Continuity or continuous may refer to: Mathematics * Continuity (mathematics), the opposing concept to discreteness; common examples include ** Continuous probability distribution or random variable in probability and statistics ** Continuous ...

gas

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

(that can contain dispersed solid particles) and a dispersed liquid phase. It consists of an empty cylindrical vessel made of steel or plastic, and nozzles that spray liquid into the vessel. The inlet gas stream usually enters at the bottom of the tower and moves upward, while the liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in opposite directions is called

countercurrent flow Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...

Overview

This type of technology can be used for example as a

wet scrubber The term wet scrubber describes a variety of devices that remove pollutants from a furnace flue gas or from other gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the l ...

for air pollution control. Countercurrent flow exposes the outlet gas with the lowest

pollutant concentration Measures of pollutant concentration are used to determine Risk assessment#Risk assessment in public health, risk assessment in public health. Secondary sector of the economy, Industry is continually Chemical synthesis, synthesizing new chemicals, t ...

to the freshest scrubbing liquid. Many nozzles are placed across the tower at different heights to spray all of the gas as it moves up through the tower. The reason for using many nozzles is to maximize the number of fine droplets impacting the pollutant particles and to provide a large surface area for absorbing gas. Theoretically, the smaller the droplets formed, the higher the collection efficiency achieved for both gaseous and particulate

pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like oi ...

s. However, the liquid droplets must be large enough to not be carried out of the

scrubber Scrubber systems (e.g. chemical scrubbers, gas scrubbers) are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. An early application of a carbon dioxide scr ...

by the scrubbed outlet gas stream. Therefore, spray towers use nozzles that produce droplets that are usually 500–1000 µm in diameter. Although small in size, these droplets are large compared to those created in

venturi scrubber A venturi scrubber is designed to effectively use the energy from a high-velocity inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively refe ...

s that are 10–50 µm in size. The gas velocity is kept low, from 0.3 to 1.2 m/s (1–4 ft/s), to prevent excess droplets from being carried out of the tower. In order to maintain low gas velocities, spray towers must be larger than other

s that handle similar gas stream flow rates. Another problem occurring in spray towers is that after the droplets have fallen a short distance, they tend to agglomerate or hit the walls of the tower. Consequently, the total liquid surface area for contact is reduced, reducing the collection efficiency of the scrubber. In addition to a countercurrent-flow configuration, the flow in spray towers can be either a cocurrent or crosscurrent in configuration. In cocurrent-flow spray towers, the inlet gas and liquid flow in the same direction. Because the gas stream does not "push" against the liquid sprays, the gas velocities through the vessels are higher than in countercurrent-flow spray towers. Consequently, cocurrent-flow spray towers are smaller than countercurrent-flow spray towers treating the same amount of exhaust flow. In crosscurrent-flow spray towers, also called horizontal-spray scrubbers, the gas and liquid flow in directions perpendicular to each other. In this vessel, the gas flows horizontally through a number of spray sections. The amount and quality of liquid sprayed in each section can be varied, usually with the cleanest liquid (if recycled liquid is used) sprayed in the last set of sprays.

Particle collection

Spray towers are low energy

s. Contacting power is much lower than in

s, and the

pressure drop Pressure drop is defined as the difference in total pressure between two points of a fluid carrying network. A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. The main det ...

s across such systems are generally less than 2.5 cm (1 in) of water. The collection efficiency for small particles is correspondingly lower than in more energy-intensive devices. They are adequate for the collection of coarse particles larger than 10–25 µm in diameter, although with increased liquid inlet nozzle pressures, particles with diameters of 2.0 µm can be collected. Smaller droplets can be formed by higher liquid pressures at the nozzle. The highest collection efficiencies are achieved when small droplets are produced and the difference between the velocity of the droplet and the velocity of the upward-moving particles is high. Small droplets, however, have small settling velocities, so there is an optimum range of droplet sizes for scrubbers that work by this mechanism. This range of droplet sizes is between 500 and 1,000 µm for gravity-spray (counter current) towers. The injection of water at very high pressures – 2070–3100 kPa (300–450 psi) – creates a fog of very fine droplets. Higher particle-collection efficiencies can be achieved in such cases since collection mechanisms other than inertial impaction occur. However, these

spray nozzle A spray nozzle is a device that facilitates the dispersion of a liquid by the formation of a spray. The production of a spray requires the fragmentation of liquid structures, such as liquid sheets or ligaments, into droplets, often by using k ...

s may use more power to form droplets than would a venturi operating at the same collection efficiency.

Gas collection

Spray towers can be used for gas

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

, but they are not as effective as

packed Data structure alignment is the way data is arranged and accessed in computer memory. It consists of three separate but related issues: data alignment, data structure padding, and packing. The CPU in modern computer hardware performs reads and ...

plate Plate may refer to: Cooking * Plate (dishware), a broad, mainly flat vessel commonly used to serve food * Plates, tableware, dishes or dishware used for setting a table, serving food and dining * Plate, the content of such a plate (for example: ...

towers. Spray towers can be very effective in removing

s if the pollutants are highly soluble or if a chemical reagent is added to the liquid. For example, spray towers are used to remove

HCl HCL may refer to: Science and medicine * Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia * Harvard Cyclotron Laboratory, from 1961 to 2002, a proton accelerator used for research and development * Hollow-cathode lamp, a spe ...

gas from the tail-gas exhaust in manufacturing

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

. In the production of

superphosphate Triple superphosphate is a component of fertilizer that primarily consists of monocalcium phosphate, Ca(H2PO4)2. Triple superphosphate is obtained by treating phosphate rock with phosphoric acid. Traditional routes for extraction of phosphate rock ...

used in manufacturing

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

, SiF₄ and HF gases are vented from various points in the processes. Spray towers have been used to remove these highly soluble compounds. Spray towers are also used for odor removal in

bone meal Bone meal is a mixture of finely and coarsely ground animal bones and slaughter-house waste products. It is used as a dietary supplement to supply calcium (Ca) and phosphorus (P) to monogastric livestock in the form of hydroxiapathite. As a slow-r ...

and

tallow Tallow is a rendering (industrial), rendered form of beef or mutton fat, primarily made up of triglycerides. In industry, tallow is not strictly defined as beef or mutton fat. In this context, tallow is animal fat that conforms to certain techn ...

manufacturing industries by scrubbing the exhaust gases with a solution of KMnO₄. Because of their ability to handle large gas volumes in corrosive atmospheres, spray towers are also used in a number of

flue-gas desulfurization Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide () from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration. Methods ...

systems as the first or second stage in the

removal process. In a spray tower,

can be increased by decreasing the size of the liquid droplets and/or increasing the

liquid-to-gas ratio An important parameter in wet scrubbing systems is the rate of liquid flow. It is common in wet scrubber terminology to express the liquid flow as a function of the gas flow rate that is being treated. This is commonly called the liquid-to-gas ratio ...

(L/G). However, to accomplish either of these, an increase in both power consumed and operating cost is required. In addition, the physical size of the spray tower will limit the amount of liquid and the size of droplets that can be used.

Maintenance problems

The main advantage of spray towers over other

s is their completely open design; they have no internal parts except for the

spray nozzles Spray or spraying commonly refer to: * Spray (liquid drop) ** Aerosol spray ** Blood spray ** Hair spray ** Nasal spray ** Pepper spray ** PAVA spray ** Road spray or tire spray, road debris kicked up from a vehicle tire ** Sea spray, refers to ...

. This feature eliminates many of the scale buildup and plugging problems associated with other scrubbers. The primary maintenance problems are spray-nozzle plugging or eroding, especially when using recycled scrubber liquid. To reduce these problems, a settling or filtration system is used to remove abrasive particles from the recycled scrubbing liquid before pumping it back into the nozzles.

Summary

Spray towers are inexpensive control devices primarily used for gas conditioning (cooling or humidifying) or for first-stage particle or gas removal. They are also used in many

systems to reduce plugging and scale buildup by pollutants. Many scrubbing systems use sprays either prior to or in the bottom of the primary scrubber to remove large particles that could plug it. Spray towers have been used effectively to remove large particles and highly soluble gases. The

across the towers is very low – usually less than 2.5 cm (1.0 in) of water; thus, scrubber operating costs are relatively low. However, the liquid pumping costs can be very high. Spray towers are constructed in various sizes – small ones to handle small gas flows of 0.05 m³/s (106 ft³/min) or less, and large ones to handle large exhaust flows of 50 m³/s (106,000 m³/min) or greater. Because of the low gas velocity required, units handling large gas flow rates tend to be large in size. Operating characteristics of spray towers are presented in the following table.US EPA Air Pollution Training Institute
developed in collaboration with North Carolina State University, College of Engineering (NCSU)

References

Bibliography

*Gilbert, J. W. 1977. Jet venturi fume scrubbing. In P. N. Cheremisinoff and R. A. Young (Eds.), Air Pollution Control and Design Handbook. Part 2. New York: Marcel Dekker. *McIlvaine Company. 1974. The Wet Scrubber Handbook. Northbrook, IL: McIlvaine Company. *Richards, J. R. 1995. Control of Particulate Emissions (APTI Course 413). U.S. Environmental Protection Agency. *Richards, J. R. 1995. Control of Gaseous Emissions. (APTI Course 415). U.S. Environmental Protection Agency. {{Authority control Pollution control technologies Air pollution control systems Scrubbers Wet scrubbers Liquid-phase contacting scrubbers