Pump Washing Biodiesel
Written by Biotom
Pump washing is a harsh but very effective way to wash biodiesel. In this process the oil and water are broken into much finer particles than in other wash processes which results in a greater interface between the water and oil, and because of this the soap is quickly transferred from the biodiesel to the water. By following these simple steps, emulsions will be a thing of the past even with this aggressive wash method!
One word of caution: be sure your biodiesel is well made; i.e. pass 3/27 without any dropout. Under-reacted oil will contain an abundance of partially processed oil, monoglycerides and diglycerides. These are emulsifiers and will only cause problems, such as emulsions and poor or no water-oil separation.
In preparation for pump washing you must do two or three coarse but gentle static spray washes; think rain drops with a force similar to what you would get from a watering can. Larger droplets are used in this static wash process to speed up the settling time and reduce the chance of an emulsion that a finer spray with a closer oil/water interface could cause. The water can be drained in 30 to 45 minutes and the next static wash started immediately. When draining the wash water, stop if the consistency begins to look like milk; this is a concentrated soap layer or possibly even a minor emulsion. Just leave it in for the next wash, it will disappear. At this stage an emulsion is a very real possibility if you use garden hose pressure! Remember there is always byproduct in the biodiesel even after you take pains to try and drain it all out. Byproduct is soluble in methanol, and so is biodiesel so you can never get it all out! I don’t even try, I drain a volume equal to 75% of the methanol I use for the base process and rinse the remainder out in the static wash process.
All washes, both pump and static, should be done with water having an electrical conductivity of 20uS/cm, think deionized or rain water. I don't have an osmosis filtering unit so I use warm/hot 45°C softened water, if you use cold hard you may have to increase the number of washes to achieve these results. water.
After the first static spray wash, the drain water will look like this, note how brown the water is indicating the byproduct is being removed. The white rod stuck into the drain is an Electrical Conductivity probe.
After the second static spray wash the water is more white, and after the third static spray wash the drain water looks like this.
Don’t be fooled by the color of the water, there is still lots of soap in the biodiesel, but this wash procedure has removed as much as is practical.
In pump washing the water and biodiesel are pumped from the bottom of the tank and allowed to fall harshly on top of the liquid in the tank. For a 160 L batch it only takes a few seconds with the blue pump (NT) to turn the entire tank into a butterscotch colored liquid.
For the first pump wash, add 5% warm water and run the pump on for 15 seconds, then let sit for 1 hour, drain the water, recover at least 80% of the starting water volume. If you can't recover this amount, repeat this step using another 5% warm water. If you reach a point where the drainage has the consistency of milk, stop draining. This is a minor emulsion layer and it will disappear with further washing.
For the second wash add 5% warm water and pump mix for 30 seconds, allow to settle for 1 hour then drain as above.
For the third pump wash add 5% warm water and pump mix for 15 minutes settle and drain as above. For most batches of biodiesel I make this is all it takes. I take a 100mL sample of biodiesel, add 100mL of warm water and shake well, after the liquids separate I use a syringe and draw water from the bottom of the jar and check the pH. It should be pH 7, or the same pH as the water before the wash. At this point you will be guaranteed to pass the soap test. If more pump washing is required, as determined by pH, just repeat step 3 above, but now you can pump for 30 minutes without fear of an emulsion.
This picture shows the color of the liquid (biodiesel + water) as it is being pump mixed
The first column is after 3 seconds, the second column is after 1 minute, the third column is after 15 minutes, and the fourth column is after sitting for one hour. You will notice that the biodiesel is not clear as you may have come to expect with the bubble wash process; this is the result of better mixing, and not a problem with the biodiesel reaction. The biodiesel will clear but don't expect that to happen quickly without heating the sample. Once the drying process is started the water will evaporate leaving the biodiesel clear
This picture shows the color of the drainage and resulting pH for each of four pump washes,the resulting biodiesel once dry and two conversion tests,3/27 then 6/27.
Sample 1) 15 second pump wash; pH 8.5
Sample 2) 30 second Pump wash; pH 7.9
Sample 3) 15 minute pump wash; pH 7.4
Sample 4) 30 minute pump wash; pH 7.1 (starting pH of water)
Sample 5 dry biodiesel soap tested at 21ppm soap.
This is a picture showing the pH meter in use
1) volume of your pump, a higher volume pump would require less pump 'on' time. I use the NT pump for 160 L batch size.
2) thirty minutes is the maximum pump on time!
3) rain water although it doesn't contain any minerals, it contains atmospheric contaminants, checking the pH many times in the past few weeks (rain rain rain) pH of the water varies form 7.9 down to 5.5. The acidic or base contaminants can be transferred to your biodiesel.
Advantage of pump washing
1) It is fast and efficient.
2) This process uses much less water than other wash techniques
3) The biodiesel is not exposed to oxygen as it is in bubble washing and therefore oxidization is avoided.
--there are none.