Heat and Settle Tank
By Rick Da Tech
The drier your oil is before making biodiesel, the better your biodiesel is. One of the oldest techniques used to dry oil for making biodiesel is the heat and settle method. In this article, I repurposed an old Appleseed water heater for this task. It is a 53-gallon AOSmith water heater I purchased a few years back and has served me wonderfully as an Appleseed. I am using a steel water heater stand as a base because the Best Practices Manual suggests minimizing both wood and plastic in our designs.
I am using stainless steel and galvanized plumbing where possible, because around here steel turns to rust quickly when exposed to the weather. There probably is some coating that can stand up to the occasional splashing with oil and biodiesel as well as harsh UV exposure; I just haven't found it yet. So, since I already have most of what I need for this project in stainless and I am tired of working with rust, I am going high tech here.
Since I am mounting the water heater upside down for better draining, I had to drill holes through the stand to clear the drain pipe. I used a hole saw to cut clearance holes over all the plumbing connections, so they can be accessed from below if I need to change or tighten anything. I have drilled a dozen clearance holes for sheet metal screws near the outside edge for securely attaching the stand to the water heater.
The Site Tube
We need a site tube to show us how much oil we have loaded. If we do not have enough oil to cover the element completely, it burns out when we turn on the heat.
I am using a polyethelyene tubing temporarily for my site tube. I plan to upgrade it to a PTFE site tube. So far I have only found PTFE tubing in 25 ft minimum lengths. At a couple of dollars a foot, that makes it an expensive addition. I have plans for some more equipment with site tubes, and I plan to upgrade them all at the same time.
I put a valve at the bottom of the site tube to isolate it. That way if the site tube is damaged the tank does not drain its oil on the floor. I only need the site tube while pumping oil into the tank to indicate how much oil is there.
The bottom of this water heater is not insulated. In fact, I have not seen any insulated on the bottom. When a water heater is right side up, it does not need any insulation on the bottom since heat rises and there is almost no heat loss through the bottom. However, I am turning mine upside down, and if I do not insulate, it loses heat too fast.
I decided to insulate the easy way, with spray foam insulation. I drilled holes in the sheet metal cover and filled the void with foam. As the foam cured, it expanded and came back out the same holes it went in. After a day of curing, a razor blade and some silicone cleans and seals it all up. After filling the void on top, I added a 1/2" sheet of styrofoam insulation. Now, I can heat it to 150°F, and it takes more than two days to get back down below 100°F.
I installed a wireless BBQ grill thermometer. It has a probe that I slipped between the insulation and the tank. The sender is sitting on top of the tank. Now I can check the temperature from anywhere on the property.
The two main areas of concern are electrical safety and fire safety.
Electrical safety is all about not getting electrocuted. We do that by following good electrical wiring procedures. Make sure our electrical connections are all covered with no bare wire exposed. Make sure the tank is grounded properly, and use GFCI fault protection on 110V circuits. If you are uncomfortable with doing the wiring yourself, get an Electrician friend to look over your work.
Fire safety has three main areas here, the first is to make sure we never overheat our oil, the second is to make sure we have no leaks or drips that could cause spontaneous combustion, and the third is good housekeeping. Kitchen fires are the most common type of fires in homes in the US, and they almost always are caused by overheating cooking oil. The intrinsically safe way to do this job is to heat the oil indirectly. Indirect heating is typically heating water elsewhere and piping the hot water or steam over to the settling tank. That is how commercial plants do it. Unfortunately, indirect heating is too complicated and too expensive for most home brewers.
This project uses the less expensive direct heating with multiple safety shutdowns. We use two snap-discs that shut off the electricity when it reaches a certain temperature and a timer. The idea is to use various sensors and switches to shut off the flow of electricity. This method is not foolproof. It is possible for the controller to malfunction allowing the heating element to overheat the oil. A routine check of the electrical system should help prevent a malfunction from happening.
Spontaneous combustion. Oil soaking into a fibrous insulation of a heated oil tank can cause spontaneous combustion. It is simple to prevent with this type of tank, watch for leaks and fix them promptly.
Oil expands when heated. With a closed vent, pressure can build up. To keep the pressure low enough to prevent the plastic site tube from rupturing, I have added a pressure relief valve set to 30 psi. I have also added a pressure gauge that lets me periodically test the relief valve.
Good housekeeping or keeping our workspace neat, clean, and organized goes a long way towards preventing accidents and fires. Make sure you have the materials you need to clean up oil spills right away. Make sure you have a fire extinguisher nearby. Don't leave hoses stretched across walkways. These all sound like common sense. They are, but ‘common’ does not mean 'without thought.' Pay attention to the details of your work area with safety in mind.
The One Shot Element Controller
This Element Controller lets the element heat until it reaches temperature, then shuts off until it is turned back on manually. It also has a failsafe timer that cuts off all power when it times out. Finally, if there is an interruption in power, it stays powered down until manually restarted. The wiring diagram below is for 120V wiring.
To operate it, set the timer for a little longer than it takes to heat the oil up to temperature, then push the green button. To turn it off, push the red button. The light comes on when the element is heating.
Bill of Materials
|1||Plastic Junction Box|
|1||Spring Wound Timer 20A SPST 6hr without hold|
|1||120V LED Indicator Lamp|
|1||Watertight conduit and terminations|
|1||On button, NO momentary contact, green|
|1||Off button, NC momentary contact, red|
|1||30A relay SPST
Cutler-Hammer D8PR6TFA or equilivent
|1||Pigtail from 12/3 15A extension cord|
|.||Assorted 12ga (yellow) crimp on wire terminations|
|10||Wire Nuts for 2-5 12ga single or multi strand wires|
|1||Spool of 12ga multistrand wire|
|1||Steel cover plate for timer|
The Timer Math
From the article: Time to Heat Oil
Time in hours = 0.5 X Number of Liters X Temperature rise in °C ÷ by element wattage
T = 0.5VΔT/W
Where T = time in hours, V= volume in Liters, ΔT = temperature rise in ºC, W = the wattage of the element doing the heating.
Calculating our timer
To determine the setting for our fail-safe timer, we need to make a few calculations.
Since we are using a steel tank water heater, the hottest we want to take our oils is the smoke point. The smoke point is the temperature at which the oil starts to break down and become a fire hazard. For new oil that is usually above 300°C (572°F). However, used oil has a lower smoke point. At the cheap food joints, they change out their oil when its smoke point drops to their normal cooking temperatures. To be on the safe side, I am assuming the smoke point of the oil I collect could be as low as 150°C (300°F). So, the maximum temperature I think is safe would be 150°C (300°F). On a hot day, my WVO could be as hot as 40°C (104°F). I am rounding to make the math easier. So the ΔT is 110°C (230°F).
My 53 Gallon AOSmith water heater can only hold about 45 gallons, and I still need to leave some room for the oil to expand, so I am making my full mark at 40 gallons. That converts to about 150 liters.
My water heater has a 4500W 240V element wired for 110V. That means the actual power consumption is 1125W.
Plugging it all in I have:
T = 0.5 x 150 x 110 / 1125 = 7.3 hours
Meaning it would take 7.3 hours to heat up my oil to 150°C (300°F). So I would always set the cutoff timer at less than 7.3 hours.
Another approach is to look at how long we need to heat the oil to reach 50°C (120°F). Since it takes longer on cold days than on hot days, I am going to use a cold starting temperature of 10°C (50°F) for that frigid fall day. Since I am heating to 50°C (120°F), that would put my delta T at 40°C (104°F).
T = 0.5 x 150 x 40 / 1125 = 2.7 hours
So, I would set my timer between 2.7 and 7.3 hours which is long enough to get the job done, but not long enough to overheat the oil.
A 1125W heating element uses 1.125KW. Electrical Power is measured in KW-hours. We will operate our heater for about 2 hours and consume 2.25kw-hr of power. According to the US Government, the average American is paying 11.6 cents per kilowatt-hour. That would place our total cost to heat our WVO at $0.26. You can look on your electric bill for your electric rate.
After loading up the tank with well-settled oil, I heated it up to about 160°F. Two days later I drained a couple of gallons out at 108°F.
Using the Weigh-Heat-Weigh method of determining water content, I came up with 0.5% water before and undetectable after drying. In the photo, Before drying is on the left and after drying is on the right.
Notes to Self
I have added some wooden braces to prevent the plumbing from moving when operating the valve.
The One-Shot controller works like a dream.
The temperature gauge is all wrong and does not read correctly. It is probably because the stem is too short to reach into the tank. I have added a $25 digital wireless BBQ grill thermometer from a big box hardware store. It had a long stainless-steel probe that I slipped between the insulation and the tank. Since it is wireless, I can carry the remote with me and monitor the temperature from anywhere on the property. I do not need the bimetallic thermometer just yet, and it does not leak, so it is staying where it is until I need it for another project.
I added a type B cam-lock with a plug to the lower plumbing. The fill hose from the pump has a type E cam-lock on the end of it. The cam-locks make it easy to change the plumbing configuration. For safety sake, I would prefer to hard plumb everything with steel rather than using transfer hoses, but while I am testing everything, I need the flexibility hoses offer.
Need to add pipe insulation on the upper plumbing.
Need to add an insect screen on the vent.
I need to fix the site tube so it can be drained when the tank is full
Put a note on tank down low to open vent before opening drain or risk stirring everything up.
Alternative Heat and Settle Tanks
The system described above is sweet, but it is not for everyone. The key is to heat up the oil to at least 130˚F then let it cool slowly in a heavily insulated tank. The tank could be as simple as a steel drum with thick insulation. As far as I can tell, a well-insulated steel drum is the most common form of Heat n Settle Tank in use. Try to avoid fiberglass insulation because of the chance of spontaneous combustion if oil spills on it. Bubble wrap and styrofoam both work, although the styrofoam melts when biodiesel gets on it.
There are a bunch of neat ideas out there for heating the oil. Two I like are using a heat tape, designed to prevent pipes from freezing, and the spear heater. You can even use the sun to warm smaller volumes on the cheap. Just be sure to move the oil to an insulated container out of the sun to settle, or heat from the sun can set up convection currents that keep the water and oil mixed.