Monday, December 13, 2010

Why is "The Green Machine" the Way to Go?

The Green Machine project is novel because it sets out to eliminate the time-consuming, expensive steps involved with traditional batch testing to make biodiesel. Batch testing involves homogeneous catalysis in which a catalyst is dumped into a tank of the reactants and agitated to make the reaction go. In batch testing, the catalyst does not go away, and it is sometimes poisoned, meaning it cannot be reused. Formerly, sodium hydroxide (or NaOH) was used as the catalyst. However, this formed an unfavorable byproduct, which was soap. This leads to numerous time-consuming steps needed to obtain the actual biodiesel. After the reaction occurs, the soap must be strained out, then the catalyst must be strained out, and finally the product must be purified to separate out the biodiesel.

Biodiesel has never before been produced using microreactor technology and a solid NiO catalyst. There are several benefits to producing biodiesel in this way, which will now be addressed. The microreactor testing involves heterogeneous catalysis, meaning that the catalyst is fixed. In this case, the NiO is coated on the walls of the microreactor channel and is not consumed in the reaction. Consequently, the microreactor can be consistently re-used to produce more biodiesel. Also, because the catalyst is fixed, there is no need to separate it out after the reaction has carried through. Lastly, the NiO does not produce an unfavorable byproduct when used to catalyze soy oil and methanol. Hence, another separation step is eliminated. Unfortunately, however, there are a few drawbacks to the use of this fixed catalyst. For example, the molecules of the liquid reactants must come in contact with the catalyst at the walls in order for them to react. Sometimes this can be difficult to do. However, that problem is combated by making the dimensions of the channel really small—hence, “micro”. The fact that the dimensions are so small for the microreactor channel enables all the reactants to reach the surface of the catalyst. To give an idea of the actual dimensions of the channel, the reactor in Figure 3 has a channel depth of 50µm and a width of 500µm. Additionally, there are some drawbacks to the use of microreactors themselves. First, microreactors will produce only one drop of product at a time. One would argue that this would take far too long to produce a substantial amount of biodiesel. However, to counter that, the plan is to eventually scale up the Green Machine design and possibly put a thousand microreactors in parallel to produce a couple of orders of magnitude more fuel at a time. But, one may argue that using a thousand of these reactors would be far too expensive. Currently, a microreactor is about $200. Therefore, it is true that this idea would be expensive, but, if people learn the technology better, they can learn how to make these reactors efficiently and cheap. This is outside the scope of this project though so that issue cannot be addressed in any more detail at the current time. As one final point on microreactors, it is important to note that they are continuous flow reactors. The continuous nature of microreactor flow avoids the typical workup delays of isolating and purifying the biodiesel that are associated with batch testing. Therefore, microreactors are generally regarded as more efficient and easier to control, which can save time and money.

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