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Don’t Be a Primitive Pete » Biodiesel is Good

Don’t Be a Primitive Pete

August 26th, 2010 by galen Leave a reply »

Galen Bowen – Senior Mechanical Engineer

I remember back in high school shop class we were shown a film on featuring a very intrepid, creative, and foolish animated fellow named Primitive Pete.  He always figured out wrong ways to use tools, ignored safe practices, and generally created an entertaining and deadly environment in his immediate vicinity.

There’s a fine line between foolishly using something in a wrong and dangerous manner (the Primitive Pete method) and ingeniously using something for a new, outside-the-box purpose (Brilliant Bob?).  Making biodiesel is one endeavor which provides many an opportunity to wind up on either side of this tremendously important fine line.

Of course, the reason that we make BioPro processors is to provide a third option.  Using a well-constructed, safely designed processor allows people who don’t have the time, interest, or inclination of a Brilliant Bob to still make biodiesel without being a Primitive Pete.   To those people, I offer a heartfelt thanks, as they’re the ones that allow our company to exist.

For those those Brilliant Bobs out there, though, who want to make their own system, I want to take a couple paragraphs to discuss the pitfalls of one of the most potentially hazardous parts of the system, where being a Primitive Pete could result in you becoming a Deceased Dan.

I’m talking about heating.  You’ve gotta have it for making biodiesel.  I’ll leave all the chemical reasons for this to my brother in the Chemistry blog, but suffice to say, you need to heat up your stuff.

In most home-built systems, the target temperature in the reactor vessel is around 130-140F (as hot as possible while having a margin of safety below the point at which methanol’s vapor pressure exceeds that of the atmosphere.)  Since many homebuilt systems make use of a water heater as a reactor vessel (while certainly not ideal, it’s a pretty darn good idea, considering that you’ve got a nice sealed steel vessel with insulation already in place), the next logical ‘brilliant’ idea is to make use of a water heater element to heat your mixture.  After all, the water heater is built to use one, they put out lots of heat, they are cheap and readily available.  Sadly, this idea falls on the Primitive Pete side of the line, and has some real potential to make you a Deceased Dan.

The primary problem is the heat density of the element.  This is a measure of how much heat is being put out divided by the amount of surface area.  Think of a seat warmer in a luxury car vs. a soldering iron.  They might each put out a similar amount of heat, but which would you rather sit on?  The soldering iron has a much higher heat density; because of its small surface area it has to achieve a much higher temperature in order put out a given amount of heat.

The maximum heat density that can be used for heating a fluid is a function of a number of factors including the following:

Thermal conductivity of the fluid. Can it take away heat from the heater element as fast as the heater can put it out?  If not, the heater element will keep getting hotter and hotter.

Movement of the fluid. Is the fluid moving over the element?  This will increase convection, the movement of heat away from the element as cool fluid moves in to replace the hot fluid right next to the element.

Combustibility of the fluid. Can the fluid burn?  (Or can particles in the fluid burn?).  If yes, and the element gets hot enough, then a layer of charred fluid or particles will begin to build up on the element.  This has a snowball effect, as it becomes increasingly difficult for the fluid to cool off the element through this growing layer of charred gunk.  The element gets hotter and hotter, which not only will cause it to burn out, but more importantly creates a genuinely dangerous situation.

Flammability/volatility of the fluid. If the element is ever not submerged, will it get hot enough to ignite vapors from the fluid? (resulting in fire or explosion)

With these above factors considered the maximum heat density that can typically be used for heating in biodiesel reactions is around 23-30 Watts/sq inch. By comparison, water heater elements are typically in the range of 75-150 Watts/sq inch. Water heater elements can deliver such dense heat because water has high thermal conductivity, and is non volatile, non flammable, and non combustible.  When making biodiesel, on the other hand, you’ve got oil, which has low thermal conductivity and is combustible, and you usually have some tiny combustible particles if you’re using recycled restaurant oil.  Right there you’ve got potential for burned out elements and possible fire hazard.  Add methanol to the mix, which is flammable and quite volatile, you’ve got a recipe for disaster.  (Not to mention the fact that these elements usually come with a natural rubber gasket that will not stand up to biodiesel and will begin to leak in time.)

Only watching your house burn down somehow isn’t as funny as watching Primitive Pete, blackened by an explosion, with chirping birds circling his head.

If you must use an immersion heater, spring for one with an appropriate watt density.  They’re a couple orders of magnitude more expensive than a water heater element that you get from the hardware store, but they’re also a couple orders of magnitude safer.  Cough up the extra dough to get one with Incoloy or stainless elements so that it doesn’t corrode and short out in your tank.

Better yet, use a blanket heater.  These have an even lower watt density (typically they are available in 5-10 watts/sq inch).  They adhere to the outside of your vessel, and conduct heat through the vessel wall to heat your fluid.  Of course, you need a vessel with a conductive wall, such as a metal drum or metal tank to use these.  It’s a bit complicated if you’re using a water heater tank, since these have insulation you have to clear away on the outside, and some have a liner on the inside which is not very conductive.  The picture below shows the blanket heaters on the bottom of a BioPro 190.

A blanket heater is intrinsically safer, not only because of the lower watt density, but also because you’re not making any direct contact between your heater and your flammable fluids.   And if you do happen to overheat your blanket heaters, (say, by running them with an empty tank), they won’t get glowing hot.  They burn out.  While burnt out heaters might not make you a Happy Hank, it beats being a Primitive Pete.

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