With the aim of obtaining valuable information on the technoeconomic and envi­ronmental performance of ethanol production from sugarcane in a stand-alone facility under Colombian conditions, a characteristic technological configuration for bioethanol production was simulated in previous works (Cardona et al., 2005b; Quintero et al., 2008). For this, the process was analyzed considering five main pro­cessing steps: raw material conditioning, fermentation, separation and dehydration, effluent treatment, and co-generation. In the simulated process that is depicted in Figure 11.2, the feedstock is washed, crushed, and milled to extract the sugarcane juice and produce bagasse. The cane juice is sent to a clarification process, where pH is adjusted, some impurities are removed, and the press mud is generated. This material is the filter cake obtained during the removal of suspended solids in the rotary drum filter employed for juice clarification. The press mud is commercial­ized as a component of animal feed or for composting. The cane juice is sterilized and directed to the fermentation stage. Using the yeast S. cerevisiae, which is con­tinuously separated by centrifugation and recycled back to the fermenter, performs the fermentation. Fermentation gases, mostly CO2, are washed in an absorption column to recover more than 98% of the volatilized ethanol from the fermenter,

and sent to the first distillation column. The culture broth containing 8 to 11%

(by weight) ethanol is recovered in a separation step consisting of two distillation columns. In the first (concentration) column, aqueous solutions of ethanol are con­centrated up to 63%. In the second (rectification) column, the concentration of the ethanolic stream reaches a composition near the azeotrope (95.6%). The dehydra­tion of this ethanol is achieved through adsorption in vapor phase with molecular sieves by the PSA technology (see Chapter 8, Section 8.2.5). The stream obtained during the regeneration of molecular sieves containing 70% ethanol is recycled to the rectification column.

The stillage treatment consists of an evaporation step allowing the generation of a marketable by-product employed as a fertilizer of cane plantations. If the stillage is not concentrated or evaporated at a low degree, it can be used for both irriga­tion and fertilization of sugarcane plantations surrounding the ethanol production facility. Hence, the environmental impact of the whole process is reduced since the most important liquid effluent is converted into a value-added product. Condensed water from evaporators and bottoms from the rectification column are collected and sent to the wastewater treatment step. Part of this water can be used as feed water for the co-generation system. Currently, the bagasse obtained is employed in sugar
mills and cane-based distilleries for combined generation of the steam and power required by the process. For this, co-generation units have to be installed. These units basically comprise a burner (combustor) for combustion of solid bagasse, a boiler where the feed water is converted into steam, and a turbogenerator (steam turbine), where exhausted steam for the process is obtained along with power. The electricity surplus not consumed by the plant can be sold to the energy network.

The simulation of this process was carried out employing Aspen Plus®. Main input data employed for process simulation are shown in Table 11.1. The simula­tion considered a production capacity of about 17,830 kg/h anhydrous ethanol. The simulation approach described in Chapter 8, Case Study 8.1 and others, was also applied for this case study. The economic analysis was performed using the Aspen Icarus Process Evaluator® (Aspen Technology, Inc., Burlington, MA, USA) pack­age. This analysis was estimated in US dollars for a 10-year period at an annual interest rate of 16.02% (typical for the Colombian economy), using the straight­line depreciation method and a 33% income tax. The above mentioned software estimates the capital costs of process units as well as the operating costs, among other valuable data, employing the mass and energy balance information provided by Aspen Plus. In addition, specific information regarding the local conditions was used for the economic analysis in the framework of the package utilized. In this way, the net present value (NPV) of the process was determined.

Some simulation results of main streams for the process studied are shown in Table 11.2. The compositions of the streams calculated by simulation, agree very well with those reported for commercial processes. The moisture and fiber con­tents of bagasse and press mud are close to the contents of moisture (bagasse: 50%, press mud: 75%) and fiber (bagasse: 46%, press mud: 13%) previously reported for these co-products (ETPI, 2003; Moreira, 2000). The value of generated cane stillage per liter of ethanol obtained from simulation (11.01 L/L EtOH) is within the range reported by Wilkie et al. (2000) from experimental data (10 to 20 L/L EtOH). The stillage composition calculated by simulation is close to the stillage composition of Brazilian distilleries, as cited by Sheehan and Greenfield (1980). For instance, the content of organic matter in nonconcentrated stillage is calculated at 26 g/L, while the corresponding average values in Brazilian distilleries using cane juice and cane molasses are 19.5 g/L and 63.4 g/L, respectively. In general, streams data determined through simulation for this processes were compared to available data of existing production facilities taken from literature and personal communications. Hence, the simulation results were satisfactorily validated.

The results obtained for ethanol yield in the process analyzed, along with total operating and capital costs are shown in Table 11.3. For sugarcane in the case of the most productive zone in Colombia (the Cauca River valley), this value is 123 ton/ha for a harvesting time of 13 months (CENICANA, 2003). The average yield for all the country, including nontechnified cane crops, reaches 92.7 ton/ha which can be compared to the average yield of sugarcane in Brazil (73.91 ton/ha) and India (59.05 ton/ha; FAO, 2007), the major sugar producers in the world. The calculated ethanol production cost (Table 11.4) is higher than the average production cost of Brazilian ethanol (US$0.198/L in 2007; Xavier, 2007). The price of Brazilian hydrous etha­nol could be even lower—about US$0.150/L (Macedo and Nogueira, 2005). This could be explained by the lower cost of the sugarcane in Brazil (about US$0.010/kg in some producing states). As with Brazil, the high productivity of sugarcane, the advantageous output/input energy ratio of the cane-to-ethanol process compared to

Main Process Data for Simulation of Fuel Ethanol Production

TABLE 11.1 from Sugarcane feature Value feature Value Feedstock Sugarcane Product Fuel ethanol Composition Sugars 14%a, fiber Composition Ethanol 99.5%, water Feed flow rate 13.5%, protein 0.4%, ash 1.5%, acids and fats 0.6%, moisture 70% 292,619 kg/h Flow rate 0.5% 17,822 kg/h Co-product 1 Pretreatment Cachaza Co-product 2 Ethanol dehydration Concentrated stillage Milling Technology PSA with molecular sieves Number of mills 2 Number of units 2 Water flow rate 75,640 kg/h Temperature 116°C pH conditioning and sucrose hydrolysis Pressure 1.7 atm (adsorption) 0.14 atm (desorption) Agent Dilute H2SO4 Cycle time 10 min Temperature 65°C Co-generation system Residence time 5 min Solid fuel Cane bagasse Number of units 1 Solid fuel flow rate 77,623 kg/h Sucrose conversion 90% Flue gases temperature 176°C Fermentation Temperature of steam from boiler 510°C Bioagent Saccharomyces cerevisiae Pressure of the exhausted steam from turbines Temperature 31°C High 13 atm Residence time 48 h Low 4.42 atm Number of units 16 Very low 1.68 atm Ethanol content 6% Stillage concentration Conventional distillation Number of evaporators 5 Number of columns 2 Average area of each evaporation unit 2458 m2 Pressure of columns 1 atm Involved components 21 Ethanol content at distillate (1st column) 58% Blocks 34

Main Process Data for Simulation of Fuel Ethanol Production

TABLE 11.1 (Continued) from sugarcane feature Value feature Value Ethanol content at 90% Streams 137 distillate (2nd column) Substreams in streams 3

Source: Quintero, J. A., M. I. Montoya, O. J. Sanchez, O. H. Giraldo, and C. A. Cardona. 2008. Energy 33 (3):385-399. Elsevier Ltd. With permission. a All the percentages are expressed by weight.

table 11.2

FIow Rates and Composition of some streams for sugarcane-Based Ethanol Process

streams Press Concentrated sugarcane Bagasse mud Purge Ethanol stillage Compounds wt.% wt.% wt.% wt.% wt.% wt.% Ethanol — — — 0.02 99.62 — Sugars 14.00 1.02 — — — 28.84 Fiber 13.50 47.33 16.28 — — — CO2 — — — 98.25 — — Protein 0.40 0.12 1.94 — — — Water 70.00 49.90 67.80 1.67 0.38 44.93 Ash 1.50 1.53 7.30 — — 10.38 Others 0.60 0.10 6.68 0.06 — 15.85 Total flow rate (kg/h) 292,618.77 77,623.30 20,369.78 17,143.62 17,821.67 24,702.60

Source: Quintero, J. A., M. I. Montoya, O. J. Sanchez, O. H. Giraldo, and C. A. Cardona. 2008. Energy 33 (3):385-399. Elsevier Ltd. With permission.

TABLE 11.3

Ethanol Yields and Total Capital and Operating Costs for Fuel Ethanol Production from Two Feedstocks

Item sugarcane Corn

Ethanol yield (L/ton of feedstock) 77.19 446.51

Ethanol yield (L/[ha*year]) 8,764.00 6,698.00

Total capital costs (thous. US$) 75,613.00a 36,447.50

Total operating costs (thous. US$/year) 36,255.20 70,670.30

Source: Quintero, J. A., M. I. Montoya, O. J. Sanchez, O. H. Giraldo, and C. A. Cardona. 2008. Energy 33 (3):385-399. Elsevier Ltd. With permission.

a Includes the cost of the co-generation unit.

table 11.4

unit Costs of fuel Ethanol (us$/L of Anhydrous Ethanol)

	Corn-Based		Cane-Based
item	Process	share/%	Process	share/%
Raw materials	0.2911	70.84	0.1611	66.45
Utilities	0.0604	14.70	0.0033	1.35
Operating labor	0.0017	0.41	0.0028	1.14
Maintenance and operating charges	0.0053	1.30	0.0117	4.83
Plant overhead and general and	0.0322	7.84	0.0218	8.97
administrative costs
Depreciation of capital a	0.0202	4.91	0.0418	17.26
Co-products credit	-0.0728	—	0.0272	—
Total	0.3381	100.00	0.2153	100.00

Source: Quintero, J. A., M. I. Montoya, O. J. Sanchez, O. H. Giraldo, and C. A. Cardona. 2008. Energy 33 (3):385-399. Elsevier Ltd. With permission. a Calculated by straight line method.

corn or lignocellulosic biomass, and the low cost of labor force, among other fac­tors, makes this feedstock the more viable option for new ethanol production facili­ties. The commercialization of the co-products (e. g., press mud and concentrated stillage) allows a substantial economic balance improvement. The data presented in Table 11.5 shows a confirmation of the economic viability of this process.

One of the features of the simulation presented above is the inclusion of the co­generation unit. The simulation of this unit allows performing a more complete envi­ronmental evaluation of the overall technological configuration. The co-generation step employs the combustion of cane bagasse to cover the needs of both thermal and electric energy required by the whole ethanol production facility. In the following case study, the specific aspects of the co-generation simulation are presented.

TABLE 11.5

Some Economic Indicators of Two Processes for Fuel Ethanol Production using Different feedstocks

Source: Quintero, J. A., M. I. Montoya, O. J. Sanchez, O. H. Giraldo, and C. A. Cardona. 2008. Energy 33 (3):385-399. Elsevier Ltd. With permission.