The Role of Sustainability Analysis in the Revalorization of Tequila Residues and Wastes Using Biorefineries

Abstract

This work shows the role that sustainability analysis may play in the revalorization of agroindustrial residues and wastes. The tequila industry in western Mexico is taken as case study, since residues and wastes from this industry are the source of serious environmental problems in the tequila-producing regions. The proposed solution uses these residues and wastes as feedstock of an integrated multi-feedstock biorefinery for the co-production of second generation bioethanol and electricity. The sustainability analysis employed in this work considers the environmental and economic domains and monetizes their indicators on the same basis, thus highlighting the contribution of environmental aspects that might otherwise be overlooked using standard techno-economic analysis approaches. This solution is compared against alternative designs using separate facilities handling these residues and wastes. Results show that the proposed solution may be attractive since the monetized environmental impact of treating the tequila residues and wastes is only one third of the economic cost of operating the proposed biorefinery, besides solving the environmental problem.

Graphic Abstract

Annex 1

Process Description

Pretreatment and Neutralization

The AB feedstock stream (111A) is first shredded, then cleaned in a vibrating screen and size reduced in a grinder coupled with a vibrating screen. The conditioning system reduces the bagasse to stems of approximately 5 cm [in length]. Larger stems are recycled for re-milling. The process stream is then fed into a tank and a diluted solution (0.75%, v.v.) of sulfuric acid at 90 °C soaks solids for 20 min, assuming that no reaction takes place [37]. Acid pretreatment is then carried out in a continuous tubular reactor with a diluted solution stream (0.75% v/v) of sulfuric acid at 90 °C and a 20-minute residence time, achieving a conversion of 80.10% to xylose and 11.90% to arabinose. Only 3.15% of cellulose is converted to glucose [31]. Wet solids with high cellulose concentration are separated from liquid hydrolysate using vacuum filtration. The solids stream (mainstream) is stored while liquid hydrolysate (311) is sent to overliming.

Overliming

Process stream 311, mainly liquid hydrolysate (xylose, arabinose and inhibitors), is mixed with a calcium hydroxide solution in order to remove inhibitors. Hydrolysates pH is adjusted and the precipitated solids are separated using drum vacuum filters. Liquid stream is sent to tank containing mainstream from pretreatment giving the final product of the stage (stream 411C). Solids are disposed as gypsum (321B).

Saccharification

The remaining polysaccharides (mostly cellulose) from stream 411C enter the enzymatic saccharification stage, carried out in a reactor battery using the necessary dosage of commercial cellulase cocktails, as determined by the cellulose content in the feedstock suggested by the enzyme provider (residence time 24 h, temperature 45 °C). At the end of saccharification 92% conversion of hemicellulose and 44% conversion of cellulose to glucose is obtained. These conversion values were experimentally corroborated at 250 mL scale [38]. Stream 911B is sent to fermentation stage.

Fermentation

The monosaccharide mixture is fed into a battery of batch fermenters (residence time 24 h, temperature 31 °C) that employ Saccharomyces sp. for alcoholic fermentation. Conversion at this step reaches 95% of glucose and 90% of xylose and arabinose to ethanol and carbon dioxide. The liquid stream (511A) with 31.15 g/L ethanol concentration is sent to the separation stage.

Separation

In this stage of the process, the fermentation broth is fed into a beer distillation column (P = 1 atm, T = 78 °C). The top stream, with 25.8% w/w ethanol, is fed into a rectification column, and bottom stream (521 A) is sent to cogeneration stage. From rectification column (P = 1 atm, T = 80 °C), it is obtained a minimum of 87.36% w/w ethanol concentration. A final purification step is carried out in a molecular sieve, for a final concentration of 99.7% w/w (stream 514). Bottom stream from rectification column is sent to the waste water treatment stage (stream 541 A) [39]. Organic components from rectification in the separation stage are mostly liquor, acetic acid, ethanol, water, and diammonium phosphate (DAP).

Waste Water Treatment

Sludge from the separation stage is fed into the WWT stage. For the MFB and VTP schemes, an additional input (TV stream) is considered. Water is treated sequentially, first in an anaerobic reactor (with a residence time of 7 days), where the organic components react to produce methane and CO2 as main products, and then in an aerobic reactor (with a residence time of 7 days). The biogas output stream (621) contains 63.66% w/w methane and 36.34% w/w carbon dioxide. Solids are separated with a clarifier and then sent to a vacuum filter; from there, they are taken to the co-generation stage (661).

Co-generation

Biogas, lignin and biomass residues from all bioethanol production stages are fed to the co-generation stage. The turbine supplies high-pressure (13 bar) and low pressure (4.5 bar) steam streams, as well as a condensate stream. The amount of electricity co-generated depends on the sugar content in the feedstock and plant capacity; for a 500 MT DB/day MFB, the total was 3539.42 kWh/h, and for 2000 MT DB/day, it was 15,746.07 kWh/h.