Waste vegetable oil disposal has often been a problem in most households. However, with the high cost of fuels, whether automobile fuel or home heating fuel, the disposal of such waste oil has been systematically accomplished by using the oil for the production of Biodiesel fuel or direct combustion as home heating fuel. Basic reaction chemistry for the production of biodiesel from such waste vegetable oil has also been developed; and on the basis of that a reactor design effectively embodying the underlying sciences has been elicited for algal biodiesel production, that is applicable for the transesterification reaction that is supported when the oil has less than 2.5 percent free fatty acids, and the algae oil generally meets the specification. The specialization of the transesterification reactor to support the recycle of domestic waste vegetable oil transesterification reaction is fairly readily implemented.

The specialization of the concept reactor design to support the transesterification reaction for waste vegetable oil likely to have more than 2.5 percent of Free Fatty Acids simply must derive from the differences of the chemistry of the two reactions. The transesterification reaction which is preferred when the free fatty acid content of the oil is less than 2.5 percent, has the reaction catalyzed by Sodium hydroxide, which also simultaneously reacts with the free fatty acids to form soap. However, the esterification reaction which constitutes a pretreatment of the Free Fatty Acids prior to the initiation of the transesterification reactions though optional is  preferred when the free fatty acid content of the oil is more than 2.5 percent, and is catalyzed by sulphuric acid, which then must be subsequently neutralized with sodium hydroxide.

The adaptation of the Algal  Transesterification Reactor design for the simultaneous support of an esterification reaction when the oil has more than 2.5 percent free fatty acids - a case that is likely to prevail with respect to domestic waste waste vegetable oil - entails the implementation of  the actual modification of reactor configuration.

The starting configuration of the reactor is the CSTR as the designated Transesterification Reactor, that obtained from the direct map of the process steps. Further, the design is such as to prevent the almost instantaneous separation of the glycerine from the bio-diesel fuel or solution of bio-diesel and soap depending on whether the vegetable oil contained free fatty acids. In addition to this intrinsic specification, must now be included the additional specification derivable from the underpinning science. The esterification reaction essentially entailed the removal of the Free Fatty Acids. The reactor design specification therefore is to enable such removal and still support the  transesterification reaction. So now in addition to

 the inlet ports for the Sodium Hydroxide and the Oil as was in the case of the reference reactor, there now also have to be implemented the inlet ports for feeding the acid catalysts and methanol to convert the Free Fatty Acids into esters. 

The Reactor Design Rationale
The selection of reactor as a CSTR configuration was rather for ease of mapping from the batch reactors that are being used currently. However, as a result of that choice, an implicit specification of the separation of the glycerine and the bio-diesel or of soap solution not to be allowed to occur in the reactor, was supported. The first consideration of the modification therefore is the support of the same condition, somewhat, while still enabling the support of the additional specifications. 

The first modification then is the  partitioning of the reactor into two virtual zones: the reference transesterification reaction zone, and the esterification reaction zone, and to have the active reactant-mixture flow from the esterification reaction zone into the transesterification reaction zone. Further, the active reactant-mixture must be free of the water produced as a result of the esterification reaction.

Beginning with the last requirement and keeping in mind that the water has higher specific gravity than the vegetable oils, the oils inlet will now be switched from the top as of the algal transesterification reactor and be placed towards the bottom of the reactor. In effect then, by default, the esterification reaction zone must now be at the lower section of the reactor while the transesterification reaction zone will be at the upper section. The two zones will now be partitioned practically by the emplacement of a flow baffle as in heat exchangers or a sort of flow dampers such that the stirring of the fluid in the transesterification zone is not transmitted wholly into the esterification reaction zone. Further whatever the form of implementation, the oils should be able to flow through into the transesterification zone.

The esterification reaction zone design now has the oil and the methanol for the esterification reaction pumped in through several distributed tubes opening upwards. Interspersed between these tubular openings are placed other sets of tubes also opening upwards through which the acid is sparged into the oil. The level of the water that is formed must not rise above the oil inlet feed-ports. So the water outlet flowrate that was previously simply flowed out with the outlet of the reactor, must be controlled to keep the water level below the oil inlet tubes rims.

The transesterification zone design now must have the only the transesterification methanol and hydroxide mixture to be pumped in from the top of

the reactor. The stirrer at the upper zone now has a length that falls within the height of the upper zone. A lateral outlet is also now added to the upper zone and the reaction-products must be pumped out of the reactor through this lateral outlet. The hydroxide of this reactor however must be increased to allow for the neutralization of the esterification catalyst acid.

Further the residence time of any reaction fluid particle must also be the same as the reaction time for the completion of the transesterification reaction. If the residence time and the reaction time do not match then one of two situations will develop: The reaction will be virtually complete within the reactor and the glycerol and biodiesel separation will occur within the reactor or The reaction will be incomplete at the point of exiting the reactor - only to keep reacting in the transport pipe.  The latter case is better though not desired, while the former clearly will have the glycerol form droplets that will fall down towards the water while also coalescing along the way. The matching of the residence and reaction times very strongly impacts the upper zone of the reactor. Every other consideration related to the residence time and reaction time relation are still in effect but only restricted to the upper zone or transesterification zone. The requirement of matching the residence time and reaction time, to ensure the prevalence of the production-specified extent of reaction is better enabled as the transesterification reaction should be planned to be conducted with feed-oil that is characterized with properties-specificity as the process design will have been designed to support properties-specificity of the biodiesel product.

The design rationale developed here is clearly consistent with the operation of the reactor as a CSTR as per asserted in the analysis of the bio-diesel production process; there are in fact other concept designs, but those are more complex and not readily amenable for the use of the public. Ultimately however, the reactor design engineer will take the intrinsic specification imposed by the reaction to develop an efficacious reactor. However for high performance operations, the reactor need to be tightly integrated with an electronic control systems. This is necessary in order to make real-time continual determination of the quantity of free-fatty acid in the feed so that the inlet quantities of the hydroxide and methanol are also be adjusted to  satisfies the specified operating conditions.

  Company