Biodiesel Synthesis from Animal Fats Using Solid Catalysts

By Annel K. Greene, PhD, Center Director, Clemson University
Animal Co-Products Research and Education Center

In the United States, approximately 11.5 billion pounds of animal fats and greases are generated annually, which, if used for biofuels, could generate 1.5 billion gallons of biodiesel. Recognizing the great potential of using animal co-products for biodiesel production, Clemson University Animal Co-Products Research and Education Center (ACREC) researcher Dr. James G. Goodwin Jr. has conducted studies on improving catalysts for optimizing biodiesel production.

Biodiesel can be produced from a variety of feedstocks, including highly refined vegetable oils, animal fats, and waste greases, via the chemical process known as transesterification of triglycerides and esterification of free fatty acids (FFAs). Catalysts affect chemical reaction rates by changing the activation energy needed for the reaction to proceed. As a result, catalysts can increase or decrease a reaction rate. In the reaction, the catalyst is not consumed by the reaction; however, generated reaction products and/or contaminants such as water can poison the catalyst, resulting in the need to purchase more catalyst for additional reactions.

In the production of biodiesel, either alkaline or acid catalysts may be used. Conventional biodiesel production techniques use a liquid catalyst. However, the liquid catalyst can only be used once and, upon completion of the reaction, must be separated from the product by neutralization and removal of the resulting salt. All of this greatly adds to the costs of biodiesel production.

In conventional biodiesel production, refined vegetable oils (triglycerides) are reacted with low molecular weight alcohols such as methanol or ethanol in the presence of a catalyst. Most of the earlier biodiesel reactors were batch reactors as opposed to continuous reactors. The catalyst used in this batch reactor system is known as a “homogeneous catalyst” and typically is a single alkaline chemical such as sodium methylate, sodium hydroxide, or potassium hydroxide. The use of these refined vegetable oils is expensive – often costing 60 to 75 percent of the biodiesel value per gallon. Use of lower cost raw materials such as high FFA greases and animal fats could reduce the cost of biodiesel production and improve its competitiveness with petroleum diesel. However, lower cost waste grease and animal fat feedstocks are not readily amenable to biodiesel production methods using strong alkaline catalysts. High levels of FFAs and water in the starting materials can undergo undesirable reactions with alkaline catalysts resulting in contaminants that are difficult and expensive to remove or which can poison the catalyst.

Goodwin realized that use of a different kind of catalyst and a different kind of reactor system could have significant advantages for biodiesel producers. A group of catalysts known as “heterogeneous” or “solid catalysts” could result in less cleanup and separation steps to remove the catalyst from the finished product and by-products. Because the heterogeneous catalyst is a solid material, removal of the catalyst is accomplished by simple separation methods. In comparison to the liquid alkaline catalysts typically used, the cost of separation is much lower when using solid catalysts, allowing a more efficient and economical way of manufacturing biodiesel. In addition, solid catalysts are less corrosive to processing equipment than the homogeneous catalysts.

In general, any type of batch chemical reactor results in more expensive processing costs as compared with a continuous processing system. Goodwin recognized that the biodiesel industry needs a continuous reactor system and that conversion to continuous biodiesel production methods has been prevented by the limitations of homogeneous catalysts. Choice of a solid catalyst opens new possibilities for continuous reaction systems that could greatly reduce processing costs while increasing product yields.

A by-product of biodiesel production is glycerin. The common batch type biodiesel production system using homogeneous catalysts yields glycerin that contains a variety of contaminants, including catalyst residues, methanol, and other reaction by-products. As a result, this glycerin can only be used as cheap heating fuel at one- to two-cents per pound as opposed to clean glycerin that has greater potential use and sells for upwards of 98-cents per pound. Also, in the batch reactor system using homogeneous catalysts, high FFAs are detrimental to biodiesel production as soaps can form. Water can cause the formation of FFAs from triglycerides and any residual water in animal fats could impede biodiesel production with homogeneous catalysts and thus increase processing costs.

Cleaner glycerin by-products that could be used in more applications and command a higher price are generated using solid heterogeneous catalysts. Goodwin also noted that use of fats that contain a high level of FFAs and some water might actually be beneficial for biodiesel production with solid heterogeneous catalysts and would allow lower quality and lower cost feedstocks to be converted into value-added biodiesel.

Goodwin conducted a study on the use of solid acid catalysts for generating biodiesel from high FFA oils and fats using reaction temperatures of 110 to 150 degrees Celsius (230 to 302 degrees Fahrenheit) at atmospheric pressure. Since FFAs react faster than the intact triglycerides on acid catalysts, he proposed the idea of a two-step reaction using solid acid catalysts rather than the use of liquid acid catalysts that involves four steps of pre-esterification with a homogeneous (liquid) acid catalyst, separation of residual catalyst and generated water, transesterification with a homogeneous (liquid) alkaline catalyst, and removal of the alkaline catalyst. Use of a two-step solid acid catalyst system would allow use of feedstock containing greater than five to 15 percent FFAs and high concentrations of water.

Goodwin proposed in his project to study efficient heterogeneous catalyst systems for producing biodiesel that is economically competitive with petroleum-derived biodiesel. He also proposed to investigate the performance of catalysts in high reaction temperatures higher than 110 degrees Celsius (212 degrees Fahrenheit) for using cheaper feedstocks to create biodiesel.

Goodwin conducted his work using tungstated zirconia and found promising results that could lead to improved systems for economically generating biodiesel from high FFA greases and animal fats. He has completed his study and submitted his final report. He generated six publications on his work that were published in Applied Catalysis A: General, Journal of Catalysis, and Industrial and Engineering Chemistry Research. His work was partially supported by funding from the Fats and Proteins Research Foundation through ACREC and the U.S. Poultry and Egg Association Poultry Protein and Fat Council, and from an $894,000 U.S. Department of Agriculture grant. Goodwin has sought additional funding of $2 million through the National Science Foundation for further work in this area.

ACREC Solutions – February 2010 RENDER | back