Modeling and simulation of Bioethanol production from Sila sorghum stalks and maize cobs

Abstract

Commercialization of bioethanol production from lignocellulosic biomass is hindered by low yield of fermentable sugars as well as insufficient techno-economic data on large-scale production. Simulation of the production process is a feasible way to obtain the necessary techno-economic data while the yield of fermentable sugars can be improved through optimization of the hydrolysis process. The main objective of this research was to model and simulate a large-scale bioethanol production process from Sila sorghum stalks and maize cobs (substrates) found in Kenya. The specific objectives were to select the most suitable pretreatment, hydrolysis and fermentation technologies in terms of bioethanol production rate, energy demand and energy intensity; determine the effect of varying cost and process parameters on the minimum bioethanol selling price (MBSP); hydrolyse the substrates using concentrated acid and establish conditions for optimal yield of fermentable sugars; establish kinetic parameters for glucose production and degradation during hydrolysis of substrates. Dilute acid, steam explosion and alkaline pretreatment, separate hydrolysis and co-fermentation (SHCF) and simultaneous saccharification and co-fermentation (SSCF) bioethanol production technologies were separately modeled and simulated using Aspen Plus software. The MBSP was calculated from the discounted cash flow rate of return (DCFROR) model. Hydrolysis of substrates was done by varying temperature (40 o C– 80 o C), time (30- 90 min) and concentration of acid (30 - 70%, w/w). Optimization of hydrolysis parameters was done using Central Composite Rotatable Design (CCRD). Kinetic study was done by varying reaction temperature (30 o C – 80 o C) and time (0 - 60 min). From the simulation results, the bioethanol production rate from dilute sulphuric acid, steam explosion, alkaline pretreatment and SSCF technologies was 21664.5, 18698.6, 12032.7 and 31074.4, 24749.4 and 13266.6 kg/h from sorghum stalks and maize cobs respectively. The energy demand for pretreatment and SSCF was 169787.23, 200053.08 and 93411 MJ/h for sorghum stalks and 225707.51, 242852.04 and 104211 MJ/h for maize cobs when using dilute sulphuric acid, steam explosion and alkaline pretreatment. The energy intensity for pretreatment, SSCF and product purification was 12.39, 16.50 and 19.79 MJ/L of bioethanol from sorghum stalks and 11.96, 13.53 and 15.34 MJ/L of bioethanol from maize cobs when using dilute sulphuric acid, steam explosion and alkaline pretreatment technologies. The MBSP increased from $0.81/L and $0.68 /L to $1.11/L and $0.89/L using sorghum stalks and maize cobs respectively when the cost of substrate increased from $20/ton to $100/ton. From experimental results, glucose yield reached a maximum of 87.54 and 90.02% (w/w) using sorghum stalks and maize cobs respectively. Optimum hydrolysis conditions were established as 60°C, 60 min and 50 % (w/w) acid concentration. The activation energy for glucose formation and degradation was 25.41 kJ/mol, 75.69 kJ/mol and 26.80 kJ/mol, 52.02 kJ/mol for sorghum stalks and maize cobs respectively. In conclusion, dilute acid pretreatment and SSCF is the most suitable technology. The main factors that impact the MBSP are cost of substrate, conversion of cellulose to glucose in the SSCF reactor and the flow rate of substrate. Concentrated acid hydrolysis results in high yield of fermentable sugars due to high activation energy of glucose degradation during hydrolysis of substrates. The findings herein provide insight on techno-economic feasibility of large-scale bioethanol production from sorghum stalks and maize cobs. In order to develop a single model that can handle alternative substrates, further research is recommended to update the models used in this study so as to handle other types of substrates.