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Name: Justin
Status: student
Grade: 9-12
Location: UT 
Country: N/A
Date: 12/21/2005

How does water work as a purifying agent? This speaker from an industrial plant explained to my class that before the "gaseous by-products", or smoke, reaches the filters in the smoke-stack, the smoke is forced through water to remove particulates. He said this was a low cost solution. How does the water molecule accomplish this? Can other water-based solutions accomplish the same thing depending on what molecule you want to extract?

The use of water "scrubbers" for furnace effluents is old technology that has been used for decades. There are numerous configurations depending upon the nature of the effluents. The water just wets the particles and suspends them in the water. The issue is -- what is done with water/smoke particle slurry after it is collected? If it is disposed of properly is one thing, if it is dumped into a river, lake, or storm drain is another.

Vince Calder

Hi Justin,

Wet scrubbing is used extensively in many industries as one type of air emissions control device. Many industrial processes produce gaseous emissions. Most often these emissions contain significant amounts of particulate matter which must be removed in order to meet clean air standards. Sometimes other contaminants such as SO2 or NO2 must also be treated before the emission can be discharged through the stack.

Many companies have developed wet scrubbing technologies, each with a little different twist on the physics of how they work. There are low energy units and high energy units and each have specific uses. For particulate removal the basic idea is to impact a water droplet with a particulate. Particulate can be very small say 10 micron or less and they do not readily settle out of the gas stream since they are so light. So, how to make them heavier?

With a high energy venturi scrubber, the gas stream is passed through a venturi throat which increases the gas velocity. Water sprays are also introduced before and within the throat causing particulate in the gas to impact and be captured by water droplets. The gas stream now exits to a large diameter vessel where the air velocity is reduced and the droplet, now being very heavy, drops to the bottom taking the particulate with it. The water is collected, the solids removed, and the water reused.

The type of solution you want to use depends on what you are trying to remove. For fine particulate, you would use water. To remove SO2 you would use a sodium based alkaline solution in water. This type of scrubber would be a low pressure spray tower as opposed to the high energy venturi. Each application is different and must be looked at separately.

I hope this helps.

Bob Hartwell

Three ways to "force gas through water":

1) bubbling gas up in a water bath

2) spraying water down in an upwards-flowing gas-chimney.

3) turbulently blowing the gas over water (doesn't work real fast)

Spraying is usually the easiest way of the three to get that job done in an industrial smoke-stack.

Notice in all three cases you have not water vapor, but water surface facing the gas. they must be extensively divided and/or stirred so all parts of the gas "touch" some water surface sometime, causing the suspended solids and oily liquids to get stuck on the water surface by ordinary wetting.

Even "water-repellent" oils and DuPont's Teflon (TM) have some degree of wetting-attraction to water, once they are touching.

Each impurity molecule in air has a diffusion length: how far its molecules migrate around in a given number of seconds, and this defines how close it must be to be "touching". Electrostatic effects can attract solid particles to the water surface from some short distance too.

It is conceivable to force condensation of water in air:

- heat water so it evaporates into the gas/air

- cool the air so the water must precipitate, and forms a fog. (Droplets will form first on nucleation sites: dust and alien molecules.)

- vortex/centrifuge the air so the fog settles out like rain, taking crud with it.

This is slightly more like "a molecule doing the job". But cooling air is slow compared with industrial stream velocities, so this is a rather expensive way to do it, and is not often done yet.

Suppose your tiny sprayed water droplets fall steadily through all the gas in a chimney, from top to bottom. by the time they get to the bottom they are coated with oil and some solid dust.

At the bottom there is probably a bath of such water. The droplet may break up so its water sinks to the bottom and its oil stays at the top of the bath. There are anti-emulsifier chemicals which can help this process. The dust may do anything: settle to the bottom, stay mixed in water, or stay mixed in the oil. You can usually do something to filter it out. Then perhaps the water could be sipped out of the middle and re-sprayed. Oil on top and dust-sludge on bottom would be concentrated waste, able to be disposed of some relatively helpful way.

It is conceivable to have soap or other emulsifier in the water to prevent this stratification. If one wanted to take in water, use it once as thickly as possible, then spit it out somewhere, perhaps that is what one would do. But I think that is not usually the best way these days. Other than that, there is little you can do to water to improve water or change its affinity much. Even 100% pure liquids completely different than water would probably have the same effect, because all liquids do wetting. You could say that halfway through its fall the water droplet is no longer water but oil, because its surface completely covered with oil. If you wanted selectivity, you would probably take the concentrated waste streams and refine apart the special components you wanted to recover.

Gasses have differing solubilities in water and other liquids. Some mild selectivity occurs there, because the gas can dissolve deep into the water, and re-emit from the water if it is not quite so happy there. Selectivity often involves such reversible behaviors. Oxygen has ~2x stronger solubility than nitrogen in water at room temperature, so you could probably make an oxygen-separation plant using counterflows of air and water, and make changes in the water's temperature to crank those solubilities up and down at will. I suppose that very concentrated (~10%?) soap in water might dissolve more butane than pure water, because the soap nacelles are tiny droplets of a similar hydrocarbon phase. (Assuming butane is what you wanted to catch. Not saying there is any in smoke.)

Chromatography is a fairly large science of selective-adsorption of various molecules onto specially-tailored solid surfaces. Solids are much easier to control, because a mono-layer-thick addition to the surface changes their whole affinity behavior and is easy to make. Solids also do not try to evaporate or otherwise mix themselves into the materials stream. Chromatography is not just analytical; it really does get done in small industrial sizes in some fields. So perhaps looking into that would feed your imagination.

Jim Swenson

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