In previous works (Cardona and Sanchez, 2004, 2006), the analysis of several inte­grated process flowsheets for production of fuel ethanol from lignocellulosic bio­mass was performed. The flowsheets were compared with a base case representing a nonintegrated configuration. The comparison criterion was the energy consump­tion defined as the thermal and electric energy demanded during the production of ethanol from biomass.

The different flowsheet configurations were simulated using Aspen Plus. Short­cut methods based on the principles of the topological thermodynamics (analysis of the statics, see Chapter 2) were employed for the synthesis of the distillation train as highlighted in Chapter 8, Case Study 8.1. The amount of feedstock (lignocellulosic biomass) was the same for every combination of process configurations (160,950 kg/h). Wood chips were analyzed as feedstock during the simulations. The analysis was made taking into account the best variants of each configuration assuming that no technological limitations were present for the proposed technologies. For example, it was assumed that the cellulases used for hydrolysis were purchased from commercial suppliers, which ensured their availability and efficiency. It was also assumed that SSF and SSCF processes were fully developed.

The considered overall process included all the steps required for ethanol pro­duction from pretreatment until effluent treatment. The defined nonintegrated base case is shown in Figure 11.12. This configuration comprised

• The pretreatment step using dilute sulfuric acid

• Detoxification step for the liquid fraction of pretreated biomass (hemicel — lulose hydrolyzate) through ionic exchange followed by alkali neutralization (not shown in Figure 11.12)

• Pentose fermentation using the xylose-assimilating yeast C. shehatae

• Enzymatic hydrolysis of cellulose contained in the solid fraction of pre­treated biomass

• Hexose (glucose) fermentation using S. cerevisiae

• Ethanol separation by distillation

• Ethanol dehydration by azeotropic distillation

• Effluent treatment step by evaporation of stillage with recovery of lignin

The alternative integrated configurations were synthesized through the combi­nation improvements in some of the steps making up the overall process. Thus, two types of pretreatment and hydrolysis schemes (with deviation of the liquid fraction

of hemicellulose hydrolyzate or without it); three types of fermentation processes (separate hexose and pentose fermentation, SSF, or SSCF); two types of separation technologies (azeotropic distillation or pervaporation); and three types of effluent treatment schemes (without recycling of water or with two alternatives for recy­cling water) were selected for the subsequent simulation. The selection procedure included the technologies that are more perspective considering the use of qualita­tive improvements of the process and the viability of their implementation. For example, all the analyzed configurations included the use of dilute sulfuric acid for the pretreatment of biomass. In this way, six alternative configurations were synthesized and analyzed (Table 11.11).

Data for the comparison of energy consumption for each configuration were obtained from the simulation results (Table 11.12). Considering the results shown, it was evident that those alternatives involving a higher degree of process integra­tion (SSCF, recirculation of water streams, coupling of distillation with pervapora- tion) presented lower energy costs. In particular, configuration 6 that included the SSCF process and ethanol dehydration by pervaporation had a 23% reduction in the energy consumption related to the nonintegrated base case. The second best

Process Configurations Considered during Process Simulation and Energy Analysis

Flowsheet Variant

da

dlf

Det

eh

hf

PF

ssf

SSCF

Dist

Az

Perv

Ev

RW,

RW.

Base case

V

V

V

V

V

V

—

—

V

V

—

V

—

—

Configuration 1

V

V

V

—

—

V

V

—

V

V

—

V

—

—

Configuration 2

V

—

V

—

—

—

—

V

V

V

—

V

—

—

Configuration 3

V

—

V

—

—

—

—

V

V

—

V

V

—

—

Configuration 4

V

—

V

—

—

—

—

V

V

V

—

V

V

—

Configuration 5

V

—

V

—

—

—

—

V

V

V

—

V

V

V

Configuration 6

V

—

V

—

—

—

V

V

—

V

V

V

V

TABLE 11.11

Source: Cardona, C. A., and O. J. Sanchez. 2006. Energy 31:2447-2459. Elsevier Ltd. With permission.

Note: DA = dilute acid pretreatment; DLF = deviation of liquid fraction of hemicellulose hydrolyzate for pentose fermentation; Det = ion exchange detoxification; EH = enzymatic hydrolysis; HF = hexose fermentation; PF = pentose fermentation; SSF = simultaneous saccharification and fermentation; SSCF = simultaneous saccharification and co-fermentation; Dist = conventional distillation; Az = azeotropic distillation; Perv =- pervaporation; Ev = stillage evaporation; RW1 = recycling of water for washing hemicellulose hydrolyzate; RW2 = recycling of water for washing hemicellulose hydrolyzate and for pretreatment reactor. The symbol “V” indicates that a given step is included in the configuration.

TABLE 11.12

Comparison of Simulated Configurations according to Their Energy Consumption

Flowsheet Variant

Base Case Configuration 1 Configuration 2 Configuration 3 Configuration 4 Configuration 5 Configuration 6

Source:

unit energy Costs
(MJ/L ЕЮН)

34.84

33.12

28.56

27.83
28.37

27.84

26.84

energy Costs (% of the base case) 100.00 95.06 81.98 79.87 81.43 79.92

77.05

scheme corresponded to configuration 5, which had a higher degree of integration as well (Figure 11.13) and offered a 20% reduction in energy costs.

The effect of water recycling on the energy costs of the entire process should be noted. From the multiple recycling configurations, two basic schemes were selected. In the first case, the bottoms of the rectification column were mixed with a fraction of the liquid stream from centrifuge to utilize this combined stream for washing the hemicellulose hydrolyzate (configuration 4). This stream contains water and very small amounts of soluble compounds such as glucose, xylose, and acetic acid. The second case considered, besides the above-mentioned recycled water, the additional use of the evaporated water obtained in the effluent treatment step as process water for the pretreatment reactor (configurations 5 and 6), as sug­gested by Wooley et al. (1999).

The recycling of water has two main goals: (1) reduction of the amount of fresh water utilized in the process and (2) the increase of ethanol yield through more complete utilization of remaining fermentable sugars contained in the recycled wastewater. Increased yields lead to reduced energy consumption for producing the same amount of final product. In addition, the main effluent, the stillage from the first distillation column, resulted in more concentrates (10.1% solids) than the stillage corresponding to the base case (5.8% solids) as a consequence of the higher amounts of fresh water utilized throughout the latter process. Therefore, the recy­cling of water reduces the amount of water to be evaporated in the effluent and the cost of treatment of wastewater by subsequent treatment processes like anaero­bic digestion. Thus, the simulation shows that the energy consumption during the partial evaporation of wastewater can be reduced from 11.60 MJ/L EtOH for the base case to 7.58 MJ/L EtOH for configuration 5 (34.72% reduction) (Cardona and Sanchez, 2006).

This case study illustrates the advantages and possibilities that process simula­tion offers during the synthesis of technological schemes with a high energy perfor­mance. In addition, the information obtained through simulation can be the base for life-cycle analysis of processes for bioethanol production as exemplified below.