(341f) Simultaneous Optimization and Heat Integration for the Production of Algae Based Biodiesel Using Bioethanol | AIChE

(341f) Simultaneous Optimization and Heat Integration for the Production of Algae Based Biodiesel Using Bioethanol


Severson, K. - Presenter, Carnegie Mellon University
Martín, M., University of Salamanca
Grossmann, I. E., Carnegie Mellon University

Biodiesel, along with bioethanol, are the two most promising biofuels in today’s market. Vegetable oils hold promise as alternative fuels for diesel engines. 1,2 However, using raw vegetable oils for diesel engines can cause numerous engine-related problems. 3 Thus, research has focused on developing transformation processes like pyrolysis, micro emulsion and transesterification. The process of transesterification has been most widely used. It removes glycerol from the triglycerides, and replaces it with radicals from the alcohol used for the conversion process. 4As the source of oil we consider algae for several reasons such as the fact that microalgae convert sunlight, nutrients and CO2 into proteins carbohydrates and lipids with a growth rate that doubles their biomass up to five times a day. Algae can grow not only on normal carbon sources, such as glucose, fructose, etc, but on waste from agriculture and food industries. It is also possible to use saline water or wastewater, eliminating the problem of water usage and lowering the cost of microalgae oil, algae growth can also be used to treat water in order to remove pollutants such as NH4+, NO3-, PO43- . 5 Finally, the yield of biofuel for algae per area is between 10 and 100 times higher than conventional raw materials for biodiesel, and between 3 and 10 times higher than conventional raw materials for ethanol. Thus, it is expected that algae oil will be capable of meeting the US diesel need with 2-5% of the current US cropland. 6-8

For quite some time the use of methanol in the transesterification process has been based on its lower cost and quicker reaction times compared to ethanol. 9,10 However, methanol is currently produced mainly from non renewable sources which implies that the production of biodiesel using methanol is highly dependent on fossil fuels. Current biorefineries are becoming petrochemical complexes where a number of different products including bioethanol and biodiesel are produced. Furthermore, lately it has been reported that similar yields of biodiesel can be obtained using either ethanol or methanol as the transesterification agent. Therefore, in order to avoid the dependence on fossil fuels, we investigate the integration of the use of bioethanol in the production of biodiesel because the use of bioethanol can result in a large economical benefit in the actual operation of the complex.1,11

In this paper we optimize the production of biodiesel applying simultaneous optimization and heat integration12 followed by minimization of freshwater consumption. The proposed superstructure produces biodiesel from algae oil using bioethanol by defining the best transesterification technology and the operating conditions at the reactor including the products separation stages. We first grow the algae in ponds, dehydrate them and extract the oil. Next, we considered four different transesterification paths: alkali, enzymatic and heterogeneous catalysts and under supercritical conditions. The reactors are modelled using response surface methodology based on experimental results from the literature13-18 to account for the effect of different variables such as operating temperature, excess of ethanol, catalyst load, time and pressure. These reactor models are implemented together with short-cut methods, experimental and rules of thumb based models for the recovery of ethanol, the separation of the polar and non polar phases and the final purification of the biodiesel and glycerol produced to formulate the problem as a superstructure of alternatives. The aim of this paper is to simultaneously optimize and heat17 and water integrate the production of biodiesel using ethanol in terms of the reaction technology and the operating conditions.

Due to the simultaneous optimization and heat integration, the optimal conditions in the reactors differ from the ones traditionally used because our results take the separation stages into account. In terms of the best process, the alkali catalyzed process is the most profitable, while the enzymatic based one is even more promising due to the lower consumption of energy and water. However, it requires that the enzyme cost to be reduced. Finally, only for production costs of bioethanol below $0.5/gal will this process become economically competitive with that using methanol. On the one hand, his value is in the range of some of the values reported in the recent literature for bioethanol production.19 Furthermore, this process does not depend on fossil fuels nor on fossil fuel based intermediates.

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19 Martin, M.; Grossmann, I. E. 2011 AICHE J 57 3408-3028

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