Biodiesel is made by reacting a triglyceride with an alcohol traditionally in the presence of a base catalyst, and most recently without catalyst using supercritical fluid process conditions. The reaction is known in general terms as transesterification. The resulting desired product is a fatty acid methyl ester (FAME) when using methanol as the reacting alcohol. The FAME is the biodiesel. Acid catalysts can also be used, but the transesterification reaction is much slower. The supercritical system has also been successful in creating fatty acid ethyl ester using ethanol as the reacting alcohol. Traditionally, base catalyzed biodiesel systems need low free fatty acid feedstock to avoid the formation of soap. The supercritical system uses various levels of free fatty acid feedstock as the competing soap reaction is not a factor. Other methods of converting high free fatty acid feedstocks have been successful by first esterifying all the FFA then transesterifying the remaining triglycerides in the feedstock. The two key process chemistry concerns is the moisture in the reaction system and the level of free fatty acid in the feedstock. The published water effect on these reactions is conflicting at the moment, but it has been found that if water in excess of 5 wt% is allowed to build in the reactors, then the reverse reaction will overtake the desired fatty acid methyl ester reaction and the conversion of the feedstock will become severely affected. Since water is a factor in the alcohol separation as well, and water is created in the conversion of the free fatty acid in the feedstocks, the amount of water in each stream must be managed carefully to insure that the conversion is not affected beyond economic practicality. In these systems, feedstock moisture was managed below 0.5 wt%, and the separation of the FFA reaction created water from the methanol stream was controlled to no more than 2 wt% in the recycled methanol to control the buildup of moisture in the downstream processes.
Biodiesel production systems technology application is dependent upon the feedstock available for conversion. Cold pressed seed oil systems are simple and can be operated at reasonable temperatures and make use of more straightforward extraction washing systems. As the feedstock becomes lower quality as defined by higher free fatty acid and increased moisture, the unit operations needed for conversion become more complex. Usually as feedstock FFA is higher so is the sulfur. Sulfur is usually not a problem in cold pressd seed oil feedstocks, but it becomes a significant factor in the use of brown grease and especially in the use of waste oil feedstocks. Considerations must be made as to feedstock issues and reaction preparation, analytical methods, conversion efficiencies and finishing system effectiveness.
Traditional Batch Base Transesterification
- Feedstock less than 1.0 wt% FFA, and 0.5 wt% water.
Continuous Using Intensification
- Feedstock less than 4.0 wt% FFA and 0.5 wt% water.
- Feedstock up to 90% FFA and less than 1.0 wt% water.
Key Biodiesel Considerations:
- Water affects all reactions.
- Less catalyst is required when using intensification.
- Supercritical conversion is higher with higher FFA feedstock
- Glycerol dehydration is a concern in the supercritical reactor
- Distillation affects analytical methods
- Finishing systems are critical to meet ASTM 6751 specifications
- Sulfur levels are significant in higher FFA feedstocks
Bio feedstock derived diesel is produced by esterification, transesterification and hydrogenation reactions and sold as either biodiesel which meets the ASTM 6751 fuel specification or as green diesel which meets the petroleum feedstock derived diesel ASTM 975 fuel specification. Both specifications have a limit of no greater than 15 ppm sulfur be present in the fuel. Repurposed waste bio derived feedstocks have higher levels of sulfur than the more common and higher cost virgin bio based feedstocks.
Sulfur is present in many forms.
Large petrochemical refinery operations use hydrodesulfurization as the primary unit operation. This has been traditionally economical at a large scale. The 15 ppm sulfur limit in diesel fuel has presented even the long established refinery operations a challenge as with any separation as the permitted levels reduce, the amount of effort for further reductions increases.
Many of the unit operations in the process of converting feedstocks to fuel have equipment and conditions that can be modified to progressively remove sulfur in each of the steps. Typically the repurposed waste has additional process steps to make the material suitable for the downstream unit operations. The most useful removal steps that might be present in the current systems are the physical and chemical separations and the conversion reactions themselves. These could include filtration, centrifugation, adsorption, and distillation. In the case of green diesel production, the hydrogen reaction step not only produces the fuel components, but also reacts with the sulfur to allow it to be separated in downstream unit operations.