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Understanding physics of this ozone injector

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victorpbr

Mechanical
Oct 13, 2014
74
Hello,

I visited a sugar plant last week and found myself intrigued with an ozone injector there present and did not fully understand the physics behind it. I made an sketch and attached it. They use it to kill bacteria on the sugar cane juice without using chemicals.

So basically it works like a venturi, where there is an area reduction of about 25, after the reduction of area there is a pressure reduction that creates negative gage pressure which then causes a gas (Mixture of Ozone and Oxygen) that comes from a pipe at ambient pressure to be injected.


So far so good, the thing that really intrigued me is that this piece of equipment is situated at 15 meters above the line of water at the tank, and the people there said that this was the only way to get a propper vaccum and that if the height was lower it would not work (they said they tested it), another interest fact is that the fluid inside the is kept risen 4-6 meters above the water line at the tank I also do not quite understand that, initially I thought that this was because of the negative pressure generated at the injector, but this low pressure is very localized, and the pressure a few centimeters after the area growth should already be equal to ambient pressure, what causes this?

Clearly I'm missing something, could anyone help me firgure this?

Just to provide more data, here are some parameters I could measure there

The pressure and the head of the injector is of 3,5 kgf/cm2
The sugar cane juice flow is of 100 m3/h
The gage pressure measured at the gas inlet at the injector is 8" mmHg (-0,26 bar)
The gas production is 3Nm3/min

getfile.aspx

 
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what a strange device.

I think what is happening is that the restriction is simply feeding liquid into a partial vacuum caused by the fall in liquid inside the tube.

How it got to that state from a standing start is a query, but I can only assume that over time any air left in the vertical tube was swept out as fine bubbles into the tank and gradually a low pressure zone emerged which is partially fed by the gas coming in through what I imagine is quite a small nozzle??

The restriction is there to simply allow a back pressure at the top of the piping of > 1 bara to keep the liquid from creating a vacuum in the top piping.

As it then falls into the big pipe it cannot maintain pressure > 1bara as the vertical drop and increase in size means that the friction drop is less than the head of the liquid.

The level in the pipe is an indication of the amount of vacuum at the top of the pipe - 4-6 m indicates a pressure of 0.6 to 0.4 bara respectively.

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Also: If you get a response it's polite to respond to it.
 
Ozone is generated by an electrical discharge through dry air or pure oxygen and is generated onsite because it decomposes to elemental oxygen in a short amount of time. After generation, ozone is fed into a down-flow contact chamber containing the wastewater to be disinfected. From the bottom of the contact chamber, ozone is diffused into fine bubbles that mix with the downward flowing wastewater.

Like oxygen, ozone has limited solubility and decomposes more rapidly in water than in air. This factor, along with ozone’s reactivity, requires that the ozone contactor be well covered and have multiple diffusers to maximize ozone contact with the wastewater.

 
 http://files.engineering.com/getfile.aspx?folder=90460939-4835-4d2e-8708-e9d2e5d843ae&file=ozone.jpg
Thanks for the insights, some valuable information were given,

However, the physics behind it is still unclear, why the need of such height? And how would one quantify the vaccum generated by the fall of the liquid?

 
It does seem to be a bit higher than it needs to be but maybe this helps with the mixing of the water and ozone. The fluid as it comes through that nozzle will essentially vapourise and spray out creating a large surface area for the ozone to work over.

They could have done it via a contacting tower, but this is a very "agricultural" version.

You quantify it by measuring the pressure.

At 10m high above water level the pressure at the top of a column of water which is submerged below the water level should be equal to the vapour pressure of the liquid - for cool water probably about 0.1 bara.

The pressure in your device is less as there is gas coming in and it is also a dynamic system with liquid constantly being added and vapourising caused by the spraying action.

If it works for them then it's ok. It's probably a trial and error design but there's nothing wrong in that sometimes.

They could have pressurized the ozone and injected it into a flowing pipe or small contact chamber - many different ways.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
The device does not appear to be a venturi at all. It is a contact chamber between gas and water. The water jet hitting the surface of the water will entrain bubbles. The smaller bubbles get carried out the bottom of the chamber with the water flow and this is what creates a negative pressure.
 
Perhaps this tank was left over and reused for this application.

Don't think there is any physics involved. When working with a slightly soluble gas, you need a tank to dissolve the gas into solution.
 
It is an interest point Compositepro. I think that this may be a mixture of both. The way I see it, if the water jet hitting the surface of water would be the only thing creating vaccum, the water level at the pipe would be below the water line as the jet is pushing it down and hence sucking the gas the "compensate". However since the water level inside the pipe is up to 4-6 meters above the tank water level, my guess is that the negative pressure is due to some venturi effect, what do you think?
 
Removing any gas from the vapor space begins to produce a negative pressure. It is not the momentum of the jet hitting the water that that causes anything except splashing and entrainment of air bubbles. Large bubbles will rise, small bubbles will be carried with the water flow and will dissolve. The height of the water column in the chamber is determine by the equilibrium gas leaving the chamber and gas entering the chamber. A venturi requires a divergent nozzle after the air injection point to prevent back flow of gas. Even if this were a proper venturi it could not create a vacuum in the chamber because it would be discharging into the same chamber that it is drawing gas from.
 
I agree with your view of the water level. However, still not conviced that the device also works as a venturi, we can see a convergent (intant area reduction at the head), and a divergent (instant area growth at the other end). I do not understand when you say that it is discharginf into the same chamber that it is drawing gas from. The gas come from ozone generators piping , only the fluid comes from the tank to be recirculated.
 
Your drawing shows the gas inlet going into the chamber near the jet. It does not show the gas going into the fluid jet then entering the chamber. If this were the case, the jet would be pumping the gas into the chamber and pressurizing it, not creating a vacuum.
 
This arrangement seems to be an attempt to replicate the operation of an eductor nozzle, with the motive medium being the recirculating sugar solution. The ozone mix is pulled in at the vena contracta region where pressure is lowest.

The higher the level of liquid in this chamber, the greater is the dp produced by the centrifugal pump for a fixed flow (since imposed differential level is less). This increase in motive medium pressure creates a lower vena contracta region pressure. This is possibly why the vacuum pressure at the ozone nozzle is less when liquid height is reduced.

Not sure why a regular off the shelf eductor is not used here in place of this arrangement.
 
Ozone generators have glass internals and hence a very low discharge pressure. Ozone decomposes by self-reaction and this self-reaction increases with increasing pressure and temperature, hence the yield of ozone increases with decreasing discharge pressure from the machine as well as increased pressure and temperature from any compressor which you might try to use on the discharge of the ozone generator. All this means that you want to use ozone at or near atmospheric pressure.

A venturi eductor can generate a pressure lower than atmospheric pressure- ideal for the introduction of ozone into a liquid stream to be ozonated- but only if the discharge pressure of the eductor is low enough. As you increase the discharge pressure, say by immersing the gas/liquid discharge of the venturi eductor deeper and deeper into a tank (increasing the liquid head on the discharge), you will gradually raise the suction pressure of the venturi until the point that the ozone generator can't handle it.
 
Ok,to add to my previous post.

1) This thing is NOT a venturi as others have said.
2) the restrictor at the top would appear to be there to provide back pressure to the pump so that the section from the restriction to the pump is > 1bara at all times and hence remains a liquid
3) If the inlet where the O3 goes in was fully open, the bottom part of the tube would simply become a fall pipe with the liquid level at or very close to the tank level.
4) As you restrict the air flow entering the fall pipe, over time air will escape from the tube due to absorption in the water / small bubbles etc and hence the pressure in the tube slowly decreases. As it does so atmospheric pressure on the liquid in the tank forces it up the tube until you reach an equilibrium.
5) The benefit of being so tall is that at worst you cannot achieve a total liquid column such that it covers the inlet or reaches the restriction as 10m is the max liquid column in the tube
6) I would say someone has worked this out by trial and error as if the water flow is too high or the air flow is too high it won't work as well or at all.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
OK, I opened the sketch- the thing is basically a spray tower, providing contact between the gas phase ozone and liquid phase sugar solution with both going down-flow. It's not a venturi, but does ensure that the ozone is ejected into a stream with minimal back pressure, accomplishing the same goal as noted in my message above by another means.
 
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