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Abstract:The design and building of new alternative fuel plants is an increasing necessity to replace old technology and non-renewable fossil fuels. To optimize the performance of these plants and to obtain an economically feasible production of these types of fuels, it is necessary to have a total control of each variable involved in the process of production and how these factors affect the yield of fuel production. In this paper it is proposed a model of a digester to generate gas using a Vensim software designed to generate simulations in dynamic state. This simulation was developed using differential equations to model the behavior at each stage of the process and auxiliary conditions to complement the mathematical description of the model. The main factors in the biogas production are the retention time and the methanogen mortality ratio. For retention time lower than 10 h the process loses effectiveness due to bacterial growth is not completed efficiently, but a high retention time involves a bigger reactor and the yield of production decreases considerably for retention time higher than 40 h. The best yields were obtained for a mortality ratio in methanogen and acidogenic bacteria lower than 0.2 and a retention time of 30 h with a final production of 3.33 L by each kilogram of biomass.

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The scarcity of fossil oils in medium and long term has led to propose different alternatives to replace it, and this is the reason why biodiesel has been proposed as a suitable replacement of conventional diesel oil. One of the most widely inquired heterogeneous catalysts in biodiesel production is calcium oxide (CaO) due to some advantages such as low price and high activity. Unfortunately, this compound is leached by methanol, and in this case, compounds such as CaO and ZnO have been suggested as an alternative to avoid this unwanted phenomenon. According to this, in the current work mixed oxides of calcium-zinc catalysts were synthesized using the co-precipitation method, and later, they were characterized and their behaviors were studied identifying transesterification yields and catalysts life cycle varying some of its operational conditions. Methanol/oil ratio over catalyst amount and Zn/Ca atomic ratio were identified as the main factors that affect the transesterification reaction yield. The maximum yield was 86.99%, obtained with 7.5 wt% catalyst, Zn/Ca atomic ratio of 3.0, methanol/oil molar ratio of 30:1 and a reaction time of 2 h at 56.9 °C. To test its reactivation capacity, a reactivated catalyst with the best behavior was used again obtaining a yield of 83.87% which indicates an insignificant decrease in its catalytic activity. However, the leaching process was detected which does not allow a decrease in purification costs due to residual calcium oxide.