Ultrasonic Sonochemistry Assisted Production of Biodiesel
Petroleum diesel is currently the main form of fuel in the transportation and logistics industry, although its use consumes limited crude oil resources and harms human health and the environment.
Biodiesel, as a renewable, biodegradable, and non-toxic biofuel, is an attractive alternative fuel that is environmentally friendly and can be used in existing diesel engines. It provides the same power and torque as petroleum diesel, while also having superior lubricity.
Unfortunately, due to the low efficiency of its manufacturing process, the cost of biodiesel remains high, hindering its widespread implementation.
The production of biodiesel is achieved through a relatively slow and energy consuming chemical process, namely triglyceride exchange - vegetable oil or animal fat.
This process is usually carried out in a batch reactor, where the raw triglycerides are mixed with alcohol (methanol or ethanol) in the presence of a catalyst (base, acid, or enzyme), heated to about 65 ℃, and mechanically stirred for 1-4 hours. Then, these products are allowed to phase separate for 5-10 hours (glycerol containing excess alcohol and catalyst forms the bottom layer, biodiesel forms the top layer), after which they are collected separately and subjected to post-treatment, typically producing approximately 85-87% biodiesel.
There is a lot of room for improvement in several aspects of this process:
1. Low reaction rate.
The ester exchange reaction is a reaction that limits mass transfer - it occurs at the interface between the raw oil and alcohol, which are only partially miscible. Mechanical stirring can help improve the contact between reagents to some extent, but the reaction is still very slow because stirring cannot fully emulsify the two liquids (forming a stable colloidal mixture) to increase the interfacial surface area between them.
2. Long term separation.
Excessive alcohol and catalyst are typically used to drive the ester exchange reaction for the balanced synthesis of biodiesel. However, in the following steps, excessive alcohol tends to dissolve glycerol in biodiesel, which slows down their separation.
3. Post treatment of complications.
Biodiesel must be washed with water to remove any residual catalysts and soap by-products. The remaining alcohol needs to be purified and recycled. Therefore, excessive use of alcohol and catalysts can complicate the post-treatment process.
Ultrasonic assisted production of biodiesel
Ultrasonic assisted ester exchange reactions are highly advantageous for each process step:
1. High reaction rate.
High intensity ultrasound generates acoustic cavitation, resulting in strong and asymmetric implosion of vacuum bubbles, causing shock waves, microjets, strong shear forces, and extreme local temperatures and pressures. This leads to highly effective mixing, resulting in immiscible or partially miscible liquids of ultrafine, kinetically stable emulsions (nanoemulsions). Therefore, the interface area between liquids increases by an order of magnitude, which greatly promotes reactions that limit mass transfer, such as ester exchange.
Research has shown that reaction time can be reduced from hours to minutes, and alkaline catalyzed rapid ester exchange of soybean oil utilizes ultrasound, energy, and fuel.
2. Short separation time.
Due to the better contact between the raw oil and alcohol caused by ultrasonic exposure, a much lower excess amount of alcohol is required. This reduces the amount of alcohol remaining after the reaction. The solubility of glycerol in biodiesel mediated by ethanol decreases, and the separation time decreases from 5-10 hours to around 30 minutes.
3. Simplified post-processing.
Using a lower excess of alcohol can reduce the effort required for subsequent recycling. In addition, due to higher mixing efficiency, the amount of catalyst used can be significantly reduced (50-60%), simplifying the washing steps of biodiesel.
4. Additional benefits.
a) Exposure to high-intensity ultrasound can provide the activation energy required for the reaction. Therefore, heating the reaction mixture becomes less important as it can save energy and effort.
b) The process has shifted from batch to continuous, greatly increasing the production scale.
c) The production of biodiesel can be increased to around 95-99%.
The Importance of High Sound Amplitude
The ultrasonic enhancement of commercial scale biodiesel production requires the use of industrial scale flow ultrasonic processors that can maintain high vibration amplitudes. The amplitude is directly related to the strength of the shear force generated by ultrasonic cavitation and must be maintained at a sufficiently high level for effective mixing.
Similar amplitude values have been found to be necessary for producing high-quality nanoemulsions - a prerequisite for limited mass transfer ester exchange reactions.
