(142e) Plantwide Design and Control of Biodiesel Production Processes Via Two-Step Syntheses or by Simultaneous Esterification/Transesterification
Biodiesel made from vegetable oil, animal fat or waste food oil is an alternative diesel fuel. It has attracted increasing interest because of decreasing petroleum reserves and concerns about environmental protection. At present, there are several ways to produce biodiesel, e.g., pyrolysis, micro-emulsification and transesterification [1-2]. Among of the options for biodiesel production, transesterification with methanol as the other reactant is the most common and profitable method [1-2]. Transesterification can be alkali-, acid- or enzyme-catalyzed , where alkali-catalyzed transesterification is most often used commercially because of its faster reaction. Recently, however, a main challenge for biodiesel production is the cost of feedstock and its impact on food price. Lower-cost alternative feedstocks, such as waste cooking oil or yellow grease, are becoming more attractive even though they usually contain high free fatty acid (FFA) content which causes problems with soap formation in the presence of alkaline catalyst. If a feedstock with FFA is considered, because of the restriction of FFA content in the alkali-catalyzed process, two-step process with esterification to use up FFA followed by transesterification is required. An additional cost for this esterification pretreatment should be included in the alkali-catalyzed process. Therefore, the acid-catalyzed process with simultaneous esterification and transesterification reaction may be more attractive. In this work, we investigate the design and control of biodiesel production processes with oil feedstock containing FFA. Two processes are considered: one is alkali-catalyzed (sodium hydroxide as catalyst) and the other is acid-catalyzed (oleophilic acid catalyst). The economics of the two processes are evaluated, and the effect of FFA content in the oil feed is also explored. To simplify the analysis of kinetics and thermodynamics, soybean oil is regards as pure triolein, a triglyceride in which all three fatty acid chains are oleic acid. Parameters of the kinetics for the sodium hydroxide catalyst are taken from Noureddini et al , and those for oleophilic acid catalyst are regressed from data given by Lien et al .
For the transesterification reactor system, because the reactor effluent exhibits phase separation (two-liquid phase), a reaction/separation system is devised to improve the conversion. The system has a decanter after each reactor and with an internally recycled glycerol phase rich in methanol which can dramatically reduce the feed ratio of methanol to triglyceride. Because there is a strong interaction between the reactor cost and the separation cost (recycle cost) in the biodiesel process, we design the reactor system and separation system simultaneously. We start with an alkali-catalyzed plant-wide process without FFA content in the feedstock to demonstrate the benefit of this reaction system. Comparison is made between the conventional (without recycling glycerol phase) and the proposed (including recycling glycerol phase) biodiesel production processes. The results show that a 16% reduction in the total annual cost (TAC) can be achieved using simultaneous reaction with internally recycled glycerol phase. This also corresponds to a 22% reduction in the energy cost.
Next, we look at the effect of FFA content on the economic comparison between the alkali-catalyzed (two-step process) and the acid-catalyzed process (simultaneous esterification and transesterification reaction), in which a reaction system with internally recycled glycerol phase is used. In this case, when the FFA content increases, it can be expected that the acid-catalyzed process will be more economically competitive compared to the alkali-catalyzed process.
Finally, plantwide operability of the entire biodiesel production plant is evaluated. The results indicate that the proposed process can handle 20% production rate changes with reasonably good dynamic performance.
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