Mixing Eductors for Biodiesel
Improving Mixing With Eductors
The HF or NT pumps make good mixing pumps up to about 20 gallons. After that the lack of "inside the tank" mixing starts to effect quality. To compensate we need to increase our methanol ratio from 20% to 22% to be able to make high quality fuel. We also have to add our methoxide slowly to get an even distribution of methoxide inside the tank. There are a number of solutions to improving mixing. Adding a bigger pump works, but then we start having to add special wiring to our shop to handle the high amperage draw from these big pump. We could add a propeller inside the tank, I have not seen this done on an Appleseed yet. You could do it, but it would not be easy. The solution as it turns out has been around for a long time in the chemical industry. A mixing eductor lets us keep our little energy efficient pumps and have the mixing we get from monster pumps.
One of the first places mixing eductors became popular in industry was in the electroplating industry. A tank of electroplating solution has metal chemicals and powders suspended in the solution and it takes vigorous mixing to keep them suspended. They had constant problems with the chemicals falling out of solution in the corners of the tanks. They couldn't add propeller mixing because it would reduce the useful size of the tank, not to mention end up getting heavily plated in the process. Mixing eductors let them continue to mix with a pump and get mixing in the tank at the same time. They ended the issue of chemicals falling out of solution in the corners of the tanks.
Mixing eductors work on the same principal as a venturi. We narrow down the plumbing to increase the pressure and the velocity of the stream exiting the eductor nozzle. This high pressure stream will then literally suck in the liquids next to the stream and violently mix them. It will pull as much as three times as much liquid into the stream as was in the original stream. This gives is actual mixing inside the tank. The efficiency can be improved through the addition of a diffuser. A diffuser helps control the initial mixing and can increase the mixing from a 3X mixing to a 5X mixing. They look like a horn hanging out over the end of the nozzle.
We wanted to use the NT Clear Water Pump to power our mixing eductor, so we had to run some experiments in a drum of hot biodiesel to find a starting place for nozzle size. We tried some home made nozzles first and a custom made copper eductor to get a rough idea of the nozzle size we needed. What we found was that we really needed an nozzle diameter under 0.3" to get any mixing in the tank. Then we started looking at a variety of nozzles. We had one made from a black pipe nipple and several more custom made eductors.
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Advice from Chemical Engineers
The next step was to interview a few chemical engineers that used mixing eductors every day in their work. We got some basic "rules of thumb" to use in our application.
- What we want is a high velocity stream. The higher the velocity of the stream exiting the nozzle the better the mixing.
- As we reduce the nozzle size we increase the velocity and increase pressure on the pump.
- A smooth transition from large diameter to nozzle diameter will give us more velocity with less pressure build up than an abrupt change in diameters.
- Placing the nozzle under the surface of the liquid will promote maximum mixing with minimum aeration.
- Placing the nozzle parallel to the centerline of the tank offset by about half the distance from the center to the edge gives maximum mixing.
- Short fat tanks are easier to mix with an eductor than tall skinny tanks
- A diffuser can improve mixing by over 100%.
The Theory
What happens in a biodiesel processor is the glycerin likes to stick together forming balls of glycerin. These balls of glycerin will also attract methanol and lye to them, removing the methanol and lye from the reaction. When the balls of glycerin get big enough they fall rapidly to the bottom of the tank. I've measured a 6" drop in a second with a BB sized glob. When they fall to the bottom we get a layer of glycerin on the bottom that contains a great deal of the methanol and lye, but almost no oil. If our pump is small we can not pull out the glycerin fast enough to overcome this concentrating effect. Our pump does a good job of mixing, but if there is not very much oil going to the pump then we are just mixing the glycerin with itself.
Now if we can keep the glycerin suspended in the biodiesel/oil mix, then we are better able to mix the glycerin with oil and biodiesel inside the pump. This helps to reduce the size of the glycerin beads and helps slow the rate of fall of the glycerin. It also puts the little glycerin balls back into the reaction where they can give up their methanol and lye as needed to the reaction. This is where the eductor comes into play. If our high velocity stream is strong enough reach the bottom of the tank and wash it out with fresh biodiesel, then we draw a better mix into the pump. For our purposes we need a bit of mixing, but also some rolling of the fluids in the tank. You can see what I mean by rolling by throwing some grass clippings in a bucket of water and then using a garden hose with a spray nozzle on it to mimic a mixing eductor. The more the grass clippings roll around in the bucket, the better our processor mixing will be.
Summary
To recap, our little pumps are good mixers, but the flow rate on them is too low to prevent the glycerin from concentrating in the bottom of the tank. The chemical industry has long used mixing eductors to improve in tank mixing while using small energy efficient pumps to do the job. The experiments I performed indicated we needed a nozzle inside diameter of under 0.300" to get any real mixing. We continue this thread with a post discussing a beta test conducted by a dozen volunteers using a specific mixing eductor. Stay tuned for the results.
Use ourforum to make comments and suggestions.
Beta Testing a Mixing Eductor
We used this mixing eductor in a test with 12 people to see how well it would work in an Appleseed Processor. I put together a little kit to install it in a standard appleseed. There were a couple of testers that used an upside down water heater or AppleTurnover. These testers had to fabricate their own install kit since they were not using the same ports we used on the standard Appleseed.
The kit consisted of the eductor in the picture above along with a length of 1/2" black pipe and an adapter nipple. The adapter nipple was a 3/4" x 4" black pipe nipple with 1/2" threads cut on the inside on one end. Also since the eductor was going to have to be inserted through the upper element hole, we added some twine to fish the eductor through to upper element hole and out through the anode rod hole.
Before going too far with this, I tried it out on a drum of oil. The video is using the largest nozzle from the eductor above. I also tested a nozzle made from pipe. As you can see in the video the eductor really gets the oil moving around in the drum. The pipe fitting version was less effective, but even it offered a major improvement over no nozzle at all.
Using the plastic eductor:
The pump used for testing had a maximum pressure of about 50 psi. |
Here you can see how deep into the Appleseed we wil be installing the eductor. The hot and cold water ports on the water heater are fitted with ledges to hold a dip tube in place, so we had to use the anode rod hole for our return line.
We dropped a string in through the anode rod hole and fished it out through the upper element port with a wire coat hanger. Then fed the string through the inside of the 1/2" pipe and eductor and tied a washer to the end. Then fed the eductor in the element port then pull the string coming out of the anode rod port until the 1/2" pipe was sticking through. Then we screwed the 1/2" pipe into the adapter pipe and screwed the assembly into the water heater.
Once it was installed we replumbed our return line and put the spare element back in the upper element port to plug it up. Then it's time to run a test batch.
On my first batch I wanted to test out the appleseed for both reducing the methanol and fast loading of the methoxide. Normally I use 22% methanol add the methoxide over a 20 minute period and mix for two hours before passing the 3/27 test. On this test I was able to urn 18% methanol, add the methoxide as fast at the pump would pump it in (under 2 minutes), and still pass the 3/27 test with 1.5 hours of mixing. I was sold.
The beta testers had similar experiences. One tester said that before, he had to use the base/base process to pass the 3/27 test, and after he was able to use the single stage base method. Another tester reported passing the 3/27 test in under 30 minutes using his old recipes. Before it was taking him several hours to pass.
The beta test showed unquestionably that using an eductor with our little blue pumps make huge improvements. Some of the testers were worried about the pressure building up on the pump and drilled out the nozzle to 1/4" and still saw the major improvements.
Now for the down side. This little plastic eductor did not instill confidence in the beta testers for it's ability to survive long term inside a processor. The general consensus was a desire for a metal eductor. There was also a bit of concern over pressures involved. The plastic tubing many of us are using on the return line is normally not heavily pressurized. With an eductor it becomes pressurized and is more likely to fail sooner. The solution is to go with an all steel return line.
One additional issue I noticed after the beta test was that the amperage on the pump rises from about three amps up to about six amps when using an eductor. The higher the pressure the more amperage it will draw. At max pressure it will draw about 8 amps. This can be an issue if you are heating and running the pump at the same time on the same breaker. Use ourforum to make comments and suggestions.
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