Spray tower - AbsoluteAstronomy.com

Spray towers or spray chambers are a form of pollution control technology. They consist of empty cylindrical vessels made of steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

or plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite direction is called countercurrent flow. Figure 1 shows a typical countercurrent-flow spray tower. This type of technology is a part of the group of air pollution

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

controls collectively referred to as wet scrubber

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 liquid, by forcing it through a pool of liquid, or by...

s.

Countercurrent flow exposes the outlet gas with the lowest pollutant

Pollutant

A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

concentration 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 reasons 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

Pollutant

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

Scrubber

Scrubber systems are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream...

by the scrubbed outlet gas stream. Therefore, spray towers use nozzles to produce droplets that are usually 500 to 1,000 µm in diameter. Although small in size, these droplets are large compared to those created in the venturi scrubber

Venturi scrubber

A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...

s that are 10 to 50 µm in size. The gas velocity is kept low, from 0.3 to 1.2 m/s (1 to 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 scrubber

Scrubber

s that handle similar gas stream flow rates. Another problem occurring in spray towers is that after the droplets fall short distances, 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

Crosscurrent

Crosscurrent is Jake Shimabukuro's second solo album. It was released in August 2003.-Track listing:# "Crosscurrent"# "Toastman's Dilemma"# "Fragile"# "I've Been Thinking"# "Wish On My Star"# "Spain"# "Mrs. Robinson"# "Skyline"# "Orange World"...

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

Crosscurrent

-flow spray towers, also called horizontal-spray scrubbers, the gas and liquid flow in directions perpendicular to each other (Figure 2).

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 scrubber

Scrubber

s. Contacting power is much lower than in venturi scrubber

Venturi scrubber

A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...

s, and the pressure drop

Pressure drop

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...

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 to 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 to 1,000 µm for gravity-spray (counter current) towers. The injection of water at very high pressures – 2,070 to 3,100 kPa (300 to 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 nozzles may use more power to form droplets than would a venturi

Venturi scrubber

A venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...

operating at the same collection efficiency.

Gas collection

Spray towers can be used for gas absorption, but they are not as effective as packed

Packed bed

In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing...

or plate

Theoretical plate

A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as an equilibrium stage, ideal stage or a theoretical...

towers. Spray towers can be very effective in removing pollutant

Pollutant

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

HCL or HCl can stand for:* Hairy cell leukemia, an uncommon and slowly progressing B cell leukemia* Hardware compatibility list...

gas from the tail-gas exhaust in manufacturing hydrochloric acid

Hydrochloric acid

Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

. In the production of superphosphate used in manufacturing fertilizer

Fertilizer

Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...

, 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

Bone meal is a mixture of crushed and coarsely ground bones that is used as an organic fertilizer for plants and formerly in animal feed. As a slow-release fertilizer, bone meal is primarily used as a source of phosphorus....

and tallow

Tallow

Tallow is a rendered form of beef or mutton fat, processed from suet. It is solid at room temperature. Unlike suet, tallow can be stored for extended periods without the need for refrigeration to prevent decomposition, provided it is kept in an airtight container to prevent oxidation.In industry,...

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

Sulfur dioxide is one of the elements forming acid rain. Tall flue-gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions....

systems as the first or second stage in the pollutant

Pollutant

removal process.

In a spray tower, absorption can be increased by decreasing the size of the liquid droplets and/or increasing the liquid-to-gas ratio

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...

(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 scrubber

Scrubber

s is their completely open design; they have no internal parts except for the spray nozzles. 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 being used in many flue gas desulfurization

Flue gas desulfurization

Sulfur dioxide is one of the elements forming acid rain. Tall flue-gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions....

systems to reduce plugging and scale buildup by pollutant

Pollutant

s.

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 pressure drop

Pressure drop

s across the towers are very low - usually less than 2.5 cm (1.0 in) of water; thus, the 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 Table 1.

Table 1. Operating characteristics of spray towers
Pollutant	Pressure drop (Δp)	Liquid-to-gas ratio (L/G)	Liquid-inlet pressure (p_L)	Removal efficiency	Applications
Gases	1.3-7.6 cm of water	0.07-2.70 l/m³ (0.5-20 gal/1,000 ft³	70-2,800 kPa	50-90⁺% (high efficiency only when the gas is very soluble)	Mining industries Chemical process industry Boilers and incinerators Iron and steel industry
Particles	0.5-3.0 in of water	5 gal/1,000 ft³ is normal; >10 when using pressure sprays	10-400 psig	2-8 µm diameter

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