Performance targets defined by retro-techno-economic analysis for the use of soybean protein as saccharification additive in an integrated biorefinery

Abstract

The use of additives in the enzymatic saccharification of lignocellulosic biomass can have positive effects, decreasing the unproductive adsorption of cellulases on lignin and reducing the loss of enzyme activity. Soybean protein stands out as a potential lignin-blocking additive, but the economic impact of its use has not previously been investigated. Here, a systematic evaluation was performed of the process conditions, together with a techno-economic analysis, for the use of soybean protein in the saccharification of hydrothermally pretreated sugarcane bagasse in the context of an integrated 1G-2G ethanol biorefinery. Statistical experimental design methodology was firstly applied as a tool to select the process variable solids loading at 15% (w/w) and soybean protein concentration at 12% (w/w), followed by determination of enzyme dosage at 10 FPU/g and hydrolysis time of 24 h. The saccharification of sugarcane bagasse under these conditions enabled an increase of 26% in the amount of glucose released, compared to the control without additive. The retro-techno-economic analysis (RTEA) technique showed that to make the biorefinery economically feasible, some performance targets should be reached experimentally such as increasing biomass conversion to ideally 80% and reducing enzyme loading to 5.6 FPU/g in the presence of low-cost soybean protein.

Figure 1

Response surface for each dependent variable analyzed: (a) glucose release (g/L), (b) cellulose conversion (%), and (c) process gain (%). The experiments were carried out at 50 °C, with a fixed enzyme loading of 5 FPU/g dry bagasse and a hydrolysis time of 24 h.

Figure 2

Response surface (a) and contour plot (b) for the desirability function, showing the values of the solids and soybean protein loadings that simultaneously ensured high glucose release, cellulose conversion, and process gain.

Figure 3

Effect of soybean protein (12% w/w) over time (24, 48, and 72 h), using different enzyme loadings (5, 10, 15, and 20 FPU/g dry bagasse) and a fixed sugarcane bagasse loading of 15% (w/w). (a) Glucose release (g/L), where the bars with solid colors show the control (hydrolysis without additive), while the hatched bars show the glucose released after the addition of soybean protein. (b) Process gain (%) provided by the addition of soybean protein.

Figure 4

Glucose released during the enzymatic hydrolysis in the bench-scale reactor compared to the 5 mL tube. The hydrolysis was carried out for 24 h and 50 °C, using a sugarcane bagasse loading of 15% (w/w), an enzyme dosage of 10 FPU/g dry bagasse, and 12% (w/w) of soybean protein. The control (solid colors) represents the hydrolysis without additive.

Figure 5

Techno-economic analysis for a hydrolysis time of 24 h. (a) Effect of enzyme price on the process NPV, considering a fixed enzyme loading of 18 FPU/g cellulose, 12% (w/w) of soybean protein, and 15% (w/w) of solids in the hydrolysis reactor. Cellulose conversions of 70 and 80% were evaluated. (b) Evaluation of the effect of soybean protein cost (0, 1, and 3 US$/kg) on the NPV of the biorefinery. For this analysis, solids loading, hydrolysis time, enzyme dosage, and cellulose conversion were fixed at 15% (w/w), 24 h, 18 FPU/g cellulose, and 80%, respectively. The black dashed line represents NPV = 0. (c) Feasibility curves obtained by the RTEA evaluating the effects of the variables solids loading in the hydrolysis reactor and soybean protein concentration on the maximum enzyme dosage required for the biorefinery to have a null net present value (NPV = 0). This analysis was carried out assuming that the soybean protein costed 1.00 US$/kg for the biorefinery. Conversion was set at 80% and solids loading (SL) of 5, 10, 15 and 20% (w/w) were analyzed. The regions of feasibility are below the curves.