Biodiesel has become an attractive diesel fuel substitute due to its environmental benefits since it can be made from renewable resource. However, the high costs surrounding biodiesel production remains the main problem in making it competitive in the fuel market either as a blend or as a neat fuel. More than 80% of the production cost is associated with the feedstock itself and consequently, efforts are focused on developing technologies capable of using lower-cost feedstocks, such as recycled cooking oils and wastes from animal or vegetable oil processing operations. The main issue with spent oils is the high level of free fatty acids found in the recycled materials. The conventional technology employs sodium methoxide as a homogeneous base catalyst for the transesterification reaction and illustrates the drawbacks in working with feedstocks that contain high levels of free fatty acids. on the other hand, homogeneous acidic catalysts are being used for exactly such feedstocks. Both acid and basic homogeneous catalyzed processes require downstream purification equipment to neutralize the catalyst and to purify the biodiesel as well as the glycerol. Recent studies have been conducted to employ heterogeneous catalysts, such acidic or basic solid resins, or immobilized lipases. These catalysts will allow the use of different feedstocks that will permit operation at lower investment costs and will require less downstream process equipment. A conceptual design of these alternative production plants has been done with a techno-economic analysis in order to compare these alternatives. A process simulator was employed to carry out the conceptual design and simulation of each technology. Using these models it was possible to analyze different scenarios and to evaluate productivity, raw material consumption, economic competitiveness, and environmental impacts of each process. (C) 2008 Elsevier B.V. All rights reserved.