Olive stone is an important by-product generated in the olive oil extraction and pitted table olive production

Olive stone is an important by-product generated in the olive oil extraction and pitted table olive production. It represents 10-30 % wt of the fruit (Garrido-Fernández, Fernández-Diez et al. 1997), implying an annual production of three million tones, approximately. Currently, the main use of this by-product is the direct combustion to produce energy as electricity or heat. There are however, other possibilities for taking full advantage of this renewable source. In this context, the olive stone valorization would be an economic improvement in farms, since it has lower handling and transportation costs due to its small size, high density and concentration in the same manufacturing plants. In this work, the raw material was previously characterized and a biorefinery scheme to take full advantage of this reside is presented.
Olive stones were supplied by the olive-oil mill factory â??S. C. A. Unión Oleícola Cambilâ? located in Jaén, Spain. The stones were separately removed from the olive pomace with an industrial pitting machine, with a 6 mm sieve separator, which is the standard size in this industrial process, soaked in water, washed to free them from any adhering flesh, air-dried and then dried for 24 h at 50 °C. Olive stone was characterized in terms of ash (0.5%), extract (5.5%), acetyl groups (4.6%), moisture content (7.5%) and lignocellulosic composition: 19.2% cellulose; 28.6% hemicelluloses (representing xylose more than 80%) and 37.2% lignin. In addition olive stone contains pulp (1.6%) and skin (1.4%) residues. The composition of the raw material was determined according to NREL (National Renewable Energy Laboratory, Golden, CO, USA) analytical methods for biomass. In the same way, the acid pretreatment have been experimentally assessed as well as the ethanol production from the resulting pentose-rich liquor using the strain Escherichia coli MM160 (kindly donated by Dr. Lonnie Ingram from University of Florida). All the experimental stages have been developed in the Chemical Engineering Laboratory at the Jaen University.
Based on the results obtained for the characterization, a biorefinery where xylitol, ethanol, furfural, poly-3-hydroxybutyrate and bioenergy are produced was proposed. Then an optimization based on technologies and raw material distribution was carried out according to the Figure 1.

Figure 1. Distribution of the proposed biorefinery

Technologies for Figure 1 are as follows:
(1) Acid pretreatment using sulfuric acid at 2% w/v (Experimental stage)
(2) Dilute acid hydrolysis using sulfuric acid at 0.4% (Rinaldi and Schüth 2009). (3) Enzymatic hydrolysis (Morales-Rodriguez, Gernaey et al. 2011)
(4) Simultaneous saccharification and fermentation using E. coli (Morales-Rodriguez, Gernaey et al. 2011)
(5) Furfural production via xylose cyclodehydration (Agirrezabal-Telleria, Gandarias et al. 2013)
(6) Xylitol production by Candida moggi (Tochampa, Sirisansaneeyakul et al. 2005)
(7) Ethanol production from xylose using E. coli MM160 (Experimental stage)
(8) Ethanol production from xylose and glucose using E. coli (Morales-Rodriguez, Gernaey et al. 2011)
(9) Ethanol production from glucose using recombinant Zymmomonas mobilis
(10) Ethanol production from glucose using recombinant Saccharomyces serevisiae
(11) PHB production from glucose using Ralstonia eutropha (Shahhosseini 2004) (12) Electricity production by gasification (Rincón, Becerra et al. 2013)
Then and based on the yields and energy requirements for each technology, an optimization problem which target is to maximize the profit margin of the biorefinery was proposed. The profit margin is defined as the difference between the product sales and the production costs. After solving the optimization problem and having into account some restrictions related to productivity, the best technologies and raw material distribution were both found. For the obtained scheme, the techno-economic and environmental assessments have been
carried out using simulation tools. For the techno-economic analysis, the main simulation tools used were the commercial package Aspen Plus v8.0 and Aspen Economic Analyzer V8.0 (both from Aspen Technology, Inc., USA). Specialized package for performing mathematical calculations especially for kinetic analysis such as Matlab was also used. In the case of the environmental analysis, the Waste Reduction Algorithm WAR, developed by the National Risk Management Research Laboratory from the U.S was used. Finally, this resulting scenario was compared with the current used given to the raw material, the direct combustion. The comparison was made based on the economic and environmental results obtained through simulation for the both cases.
The approach used in this work allowed finding the best scheme for producing value-added products from a feedstock which currently does not have and industrial application. Besides, the economic and environmental comparison with the base case shows that the proposed biorefinery not only provides a good profit margin but also is environmental friendly.

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