Biocatalysis has attracted significant attention recently, mainly due to its high selectivity and potential benefits for sustainability. Applications can be found in biorefineries, turning biomass into energy and chemicals, and also for products in the food and pharmaceutical industries. However, most applications remain in the production of high-value fine chemicals, primarily because of the expense of introducing new technology. In particular lipasecatalyzed synthesis has already achieved efficient operations for high-value products and more interesting now is to establish opportunities for low-value products. In order to guide the industrial implementation of immobilized-lipase catalyzed reactions, especially for highvolume low-value products, a methodological framework for dealing with the technical and scientific challenges and establishing an efficient process via targeted scale-down experimental work is described in this thesis. The methodology uses economic targets to test options characterized via a set of tools. In order to validate the methodology, two processes based on immobilized lipase-catalysis have been studied: transesterification and esterification of vegetable oils for the production of biodiesel. The two processes are focused on the conversion of the two main components of vegetable oil materials, glyceride esters and free fatty acids respectively, into fatty acid alkyl esters. Although biodiesel is conventionally prepared via chemical-catalyzed transesterification of vegetable oils with methanol to produce fatty acid methyl esters (FAME), this work has been focused on the production of fatty acid ethyl esters (FAEE) with bioethanol due to the expected improved sustainability of this type of biodiesel. A key reaction characteristic of the immobilized lipase-catalyzed transesterification is that it is multi-phasic system. The by-product glycerol can potentially impose inhibitory effects on immobilized lipases and likewise the un-dissolved ethanol can inhibit the lipase. The options for addressing these issues can be used as the basis for selecting the biocatalyst and the reactor (e.g. a hydrophobic carrier for the immobilized lipase and the capabilities to provide sufficient mixing as well as stepwise/continuous feeding of ethanol to the reactor). An STR is efficient for batch operation while a PBR is efficient for a continuous production. An STR can more easily provide sufficient external mass transfer for a reaction, but will lead to more mechanical damage of the biocatalyst particles, than a PBR. A reactor combination of CSTR with PBR can couple the advantages of both, delivering an efficient continuous process. The second case study (esterification) shares some similar process characteristics to the first case (e.g. the multi-phasic nature). However, instead of glycerol, water shows a great impact on the extent of reaction. The removal of water should therefore be feasible during the operation of the reactor, either intermittently or preferably in situ. Highly anhydrous reaction conditions and the smaller substrates for this reaction place particular requirements on the lipase. In order to validate the established processes at a larger scale, both lipase-catalyzed transesterification and esterification developed in the lab-scale STRs have been carried out in pilot-scale STRs. Results in both scale STRs correlate well with respect to the biocatalyst performance and mechanical stability. Once the technical and scientific challenges of the process have been addressed, it is of course important to evaluate its economic and environmental feasibility. To that end, process evaluation has been performed for six processes composed of transesterification and product purification for making ‘in-spec’ biodiesel and the conventional chemical process is taken as a bench mark for comparison. The optimal process is a process composed of lipase-catalyzed transesterification with ‘in-spec’ biodiesel product as output with less feedstock input and waste production and much saved energy from the absence of product purification.
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Nordblad, Mathias, Woodley, John
Technical University of Denmark, Department of Chemical Engineering, 2012