Microbial utilization of crude glycerol for the production of value-added products.

Energy fuels for transportation and electricity generation are mainly derived from finite and declining reserves of fossil hydrocarbons. Fossil hydrocarbons are also used to produce a wide range of organic carbon-based chemical products. The current global dependency on fossil hydrocarbons will not be environmentally or economically sustainable in the long term. Given the future pessimistic prospects regarding the complete dependency on fossil fuels, political and economic incentives to develop carbon neutral and sustainable alternatives to fossil fuels have been increasing throughout the world. For example, interest in biodiesel has undergone a revival in recent times. However, the disposal of crude glycerol contaminated with methanol, salts, and free fatty acids as a by-product of biodiesel production presents an environmental and economic challenge. Although pure glycerol can be utilized in the cosmetics, tobacco, pharmaceutical, and food industries (among others), the industrial purification of crude glycerol is not economically viable. However, crude glycerol could be used as an organic carbon substrate for the production of high-value chemicals such as 1,3-propanediol, organic acids, or polyols. Microorganisms have been employed to produce such high-value chemicals and the objective of this article is to provide an overview of studies on the utilization of crude glycerol by microorganisms for the production of economically valuable products. Glycerol as a by-product of biodiesel production could be used a feedstock for the manufacture of many products that are currently produced by the petroleum-based chemical industry.

Wetting at high capillary numbers.

The coating of liquids onto solids is an important industrial process. A prerequisite for successful coating is that the liquid dynamically wet the surface of the solid. One of the limits to high-speed coating is the onset of dynamic wetting failure, which leads to air entrainment. In simple experiments in which a tape or fibre plunges vertically into a pool of liquid, air entrainment usually occurs at capillary numbers Ca < 1. However, this limit is not immutable. Indeed, the term "hydrodynamic assist" has been coined to emphasise the fact that coating flows may be manipulated to promote wetting and so postpone air entrainment. Commercial curtain coating typically operates in the range 0.5 < Ca < 10. Flow visualisation of this process has shown that hydrodynamic assist leads to a reduction in the dynamic contact angle for a given wetting speed and it is this that permits the higher coating speeds. Methods of coating optical fibres have evolved to the point where comparatively viscous liquids can be coated successfully at very high speeds with Ca of order 1000. In a recent paper Jacqmin, (D. Jacqmin, J. Fluid Mech. 455 (2002) 347) has suggested that in this case air entrained into the coating dissolves under the high fluid pressures found in the coating die, which are of order 1 MPa. Here we report successful curtain coating over the interval 0.5 < Ca < 50. The new study supports an alternative hypothesis that the postponement of air entrainment to very high capillary numbers is the result of intense hydrodynamic assist.