Mixing Eductors for Biodiesel
By Rick Da Tech
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 a 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.
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%.
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.
Eductor or NOT?
You can test your processor for mixing ability by drawing a sample after the pump just before you turn off the mixing pump for settling. Let the sample settle with your batch. Once both the large batch and the sample have settled, measure the glycerin portion of each. On the sample divide that number by the total sample size. Example if you took a 500 ml sample and 200ml of glycerin settled out you would divide and get 0.4. Multiply that by 100 to get the percent glycerin in the sample. Next add the total volume of oil to the volume of methanol you added. example 100 liters of oil + 20 liters of methoxide = 120 liters. Divide that into the volume of glycerin drained. Continuing the example: 120 liters divided into 22 liters drained = 0.18. Multiply by 100 to get a percentage.
If your sample has a significantly higher ratio of glycerin than the total batch you have poor mixing. In our example we have a sample that is 40% glycerin and batch with 18% glycerin. In this case we have poor mixing and might benifit from adding an eductor.
Dultmeier sales - Small plastic eductor
Dultmeier sales - Large polyproplene Eductors
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.
More on Mixing Eductors