|An example of inline ultrasonic processing for biodiesel production. Click to enlarge. Source: Hielscher.|
Researchers at Mississippi State University report that ultrasonic processing used in biodiesel production delivers a biodiesel yield in excess of 99% in five minutes or less, compared to one hour or more using conventional batch reactor systems. The work is described in the current issue of the journal Energy & Fuels.
Ultrasonication can also help to reduce the separation time from 5 to 10 hours required with conventional agitation, to less than 15 minutes, according to Hielscher, a small German company providing ultrasonic processing equipment for a variety of sonochemical applications, biodiesel production being one.
The ultrasonication also helps to decrease to amount of catalyst required by 50 to 60% due to the increased chemical activity in the presence of cavitation. Another benefit is the increase in purity of the glycerol.
Sonochemistry—the application of ultrasound to chemical reactions and processes—is based on the phenomenon of cavitation: the formation, growth and implosive collapse of bubbles in a liquid.
Cavitation can be produced through different means, including high pressure nozzles, high velocity rotation, or ultrasonic transducers. The input energy is transformed into friction, turbulences, waves and cavitation.
Cavitation bubbles are vacuum bubbles, created by a fast moving surface on one side and an inert liquid on the other. The resulting pressure differences serve to overcome the cohesion and adhesion forces within the liquid. Cavitational collapse produces intense local heating (~5,000 K), high pressures (~1,000 atm), and enormous heating and cooling rates (>109 K/sec and liquid jet streams (~400 km/h).
The fraction of the input energy that is transformed into cavitation depends on several factors describing the movement of the cavitation generating equipment in the liquid, according to Hielscher.
The intensity of acceleration is one of the most important factors influencing the efficient transformation of energy into cavitation. Higher acceleration creates higher pressure differences. This in turn increases the probability of the creation of vacuum bubbles instead of the creation of waves propagating through the liquid. Thus, the higher the acceleration the higher is the fraction of the energy that is transformed into cavitation.
In case of an ultrasonic transducer, the intensity of acceleration is described by the amplitude of oscillation. Higher amplitudes result in a more effective creation of cavitation. In addition to the intensity, the liquid should be accelerated in a way to create minimal losses in terms of turbulences, friction and wave generation. For this, the optimal way is a unilateral direction of movement.—Hielscher backgrounder on Sonochemistry
Biodiesel conventionally is produced by the base-catalyzed transesterification of fatty acids from vegetable oils or animal fats with methanol in a batch reactor using heat and mechanical mixing energy as energy inputs. Glycerol is a byproduct of the reaction, and must be separated from the biodiesel product.
The use of ultrasonic processing not only can provide energy for the reaction, but can achieve better mixing and more rapid separation.
Hielscher estimates that costs for ultrasonication in biodiesel processing will vary between €0.002 and €0.015 per liter (€0.008 to €0.06/gallon) when used in commercial scale, depending on the flow rate.
Other researchers and companies are using ultrasonic processing for other bio- and alternative-fuel products:
Researchers at Iowa State University are exploring the use of ultrasonics to boost ethanol production from corn. (Earlier post.)
Advanced Plant Pharmaceuticals, through its merger with World Health Energy, is adopting ultrasonication as its biodiesel process to provide itself with production cost advantages. (Earlier post.)
GreenShift Corporation formed General Ultrasonics Corporation, a development stage company that owns patented technologies that use ultrasonic energies to enhance physical and chemical reactions including more efficient steam reformation to produce hydrogen. (Earlier post.)
“Base-Catalyzed Fast Transesterification of Soybean Oil Using Ultrasonication”; Alok Kumar Singh, Sandun D. Fernando, and Rafael Hernandez; Energy Fuels, ASAP Article 10.1021/ef060507g S0887-0624(06)00507-X