Techno-Economic Assessment of Peruvian Stipa Ichu Microfibres by Steam Explosion

ABSTRACT Steam explosion as a pre-treatment for lignocellulosic fibers is an eco-friendly technology for obtaining cellulose microfibres. This study used a sustainable process design to produce Stipa Ichu (endemic fiber) cellulose microfibres on a pilot scale. Pseudolignin models and experimental data from recent studies were used to validate and simulate the steam explosion in a range of 150 to 220°C using acetic acid concentrations of 0.1 to 0.3 g/g of fiber. A peak yield of 92% was obtained for lignin recovery. The proposed process, which would produce 5 tons of dried cellulose microfibres per year, is economically feasible with an ROI of 48% and a PBP of 2.45 years. Additionally, the process is environmentally viable, with an E-factor of 9.42.


Introduction
Microfibres has had a significant influence on the development of the industrial sector worldwide, and during the last few years, it has been used in different applications (Louis, Venkatachalam, and Gupta 2022;Mori et al. 2020).Currently, the production on a pilot scale is possible with capacities up to 1000 kg/day (dry equivalent) (Miller 2018).Consequently, there is a great interest in deepening the study of pre-treatment for delignification with solvents, degumming, or steam explosion (Da Costa Lopes 2021; Huang et al. 2022;Jiang, Zhu, and Jiang 2021;Li, Meng, and Yu 2015;Lin et al. 2022;Wang et al. 2022).Several authors (Chaker et al. 2013;Deepa et al. 2011;Kaushik and Singh 2011;Liu et al. 2017;Manhas et al. 2015;Tanpichai, Boonmahitthisud, and Witayakran 2019;Yang et al. 2018) mostly use the steam explosion process as a pre-treatment for its selectivity in non-cellulosic components and recovery of lignin.Steam explosion has been reported to be a promising alternative process to extract cellulose fibers with a rapid process.It induces autohydrolysis, reducing molecular mass and subsequent lignin condensation (Lam et al. 2009).The autohydrolysis can be represented by the pseudolignin kinetic model.This model can predict the amount of condensed lignin and hydrolyzed hemicellulose that subsequently degrade into other compounds (e.g., pseudolignin) during the steam explosion.This hydrolysis is catalyzed by acids and alkalis (Tabil, Adapa, and Kashaninej 2011).The steam explosion can be assisted with oxalic acid or citric acid, with a total process time of about two hours (Deepa et al. 2011;Manhas et al. 2015).Likewise, it can be assisted with bleaching using hydrogen peroxide and high-speed stirring followed by an ultrasound to obtain around 20 nm in diameter fibers with a crystallinity index of 62.3% (Yang et al. 2018).This methodology can be complemented with microwave-assisted hydrolysis post-processing, with results demonstrating an increase in cellulose content from 44% to 94% (Liu et al. 2017).Also, steam explosion, as the primary process, has been developed with a solid methodology (Cherian et al. 2010;Manhas et al. 2015;Tanpichai, Boonmahitthisud, and Witayakran 2019).Cherian et al. report steam explosion with a second steam treatment with oxalic acid.Particles with a crystallinity index (CI) of 35.97% were obtained after pretreatment, more than degumming process (Huang et al. 2022).Oxalic acid makes no harm to the 50 nm individualized particles as more CI represents better properties (Huang et al. 2022;Li, Meng, and Yu 2015;Lin et al. 2022) and AFM analysis shows (Cherian et al. 2010).
In addition, many researchers developed sustainable materials from forest residues, for its high availability and cellulose content, while revaluing them and reducing process costs (Lin et al. 2020;Moraes et al. 2014;Shen et al. 2022;Silveira et al. 2015).Stipa Ichu is an endemic fiber of Peru with an availability of 70 000 t/year, high cellulose content but flammable (Gemmer et al. 2021;Mori et al. 2020).Therefore, developing cellulose microfibres from Stipa Ichu fibers is proposed in this study.Technicaleconomic feasibility and environmental impact are assessed in a South America context.Starting with adjusting the kinetics of the steam explosion of lignin hydrolysis for wood (first instance) as part of the pre-treatment of an Andean crop like Stipa Ichu, through simulations in MATLAB, the scaling of the process, economic pre-viability and E-factor are determined as part of an environmental impact analysis.

Process of obtaining cellulose microfibres from Stipa Ichu
Figure 1 presents the flowchart of the preliminary process proposed (some by previous authors) to obtain cellulose microfibres.The diagram represents the the fiber downsizing stage (Tabil, Adapa, and Kashaninej 2011), pre-treatment-bleaching, lignin-free fiber is treated in a stirred tank with sodium chlorite and acetic acid, 8.4 g and 3.4 mL per 10 grams of fiber, respectively (Benini et al. 2018).The solid/liquid ratio used was 1:3, and the residence time was 1 hour; second steam explosion, was performed with steam at a pressure of 20 lbs. and oxalic acid as a catalyst (8% weight) for 1 hour (Cherian et al. 2010); liquid/solid separation and drying stages (Bondancia et al. 2020).The chemical constituents were determined following the standard outlined in the TAPPI tests methods T203 cm-99, T222 om-02, T204 cm-97, T211 om-02, T421 om-02 and T257 as described in (Tenazoa et al. 2021).

Modelling and validation of the kinetics of autohydrolysis
For the initial simulation, the models and kinetic parameters were those proposed by (Lam et al. 2009;Relvas, Morais, and Bogel-Lukasik 2015).Since the models originally contemplated wood as a raw material, experimental information about rod grass fibers from the same Stipa Ichu family was used to validate the model.The proximity of the results was assessed using the residue square difference (SSE) method.Iterative adjustments were made heuristically until the appropriate parameters were found to describe the trend regarding the apparent lignin concentration and hemicellulose concentration.The steam explosion pre-treatment was simulated with MATLAB R2021-a version with ODE 45 function.

Design of the main reactor
In the simulation of the operation in the main reactor the temperature and concentration of acetic acid were varied from 150°C − 225°C and 0.1 and 0.3 g/g of fiber respectively.Performance was defined by ec. 1.
Likewise, the degree of progress of the reaction was measured by the amount of lignin condensed in the steam explosion at a time (t max), where "t max" is defined as the time when the yield percentage reaches its maximum value.As part of the kinetic model, where lignin or pseudolignin is generated from hemicellulose, the condensed amount is compared to the lignin apparent at each time (t).It is considered a batch-type reactor with two independent variables: temperature and acetic acid concentration, as well as two response variables: maximum time and performance.
Once the results were obtained, the operating conditions that allowed a higher yield were selected based on the unwanted compounds (lignin and hemicellulose).Non-comparable variables: relevant temperature range (°C), relevant pressure value, and time (min) are considered (ec.2).

Scale up
The mass balance was performed at each stage according to the calculated and theoretical yields of each process, considering the following: Stage 1: Size reduction: A mass loss of 3% was assumed, suggested value by (Tramper 2004).Stage 2: Steam explosion according to the simulation of the process.The overall mass yield was determined according to the results found at the laboratory scale.
In the size reduction, the specific amount of energy was found with the correlation described by (Bitra et al. 2010).For the estimation of the agitation power, the rheological variables were considered, following the methodology proposed by (McCabe and Smith 2016).

Economic assessment
The technical-economic analysis was developed for the pre-treatment of the Stipa Ichu fiber and for the synthesis of cellulose in the form of microfibres, for which the parameters of the pseudolignin kinetics model for a "grass" fiber were adjusted, as well as the steam explosion pre-treatment reactor.
According to (Sinnott and Towler 2019) and with an expected annual production of 5 tons of microfibres, CAPEX was calculated using the cost of the leading equipment.These values were updated in the 2021 price using average annual inflation (5%).Additionally, other costs suggested as installation (Peters, Timmerhaus, and West 2003).OPEX was calculated based on fixed and variable costs.Variable manufacturing costs included raw material and additives.Fixed manufacturing costs included resources and utilities from the material and energy balance, 10% linear depreciation of equipment, taxes, maintenance costs, insurance, and labor as proposed by (Peters, Timmerhaus, and West 2003).Non-rigorous indicators were used, as described by (Seider et al. 2017), to evaluate the economic feasibility with a tax of 29.5%, discount rate of 13% and a price of 620 US/kg.A sensitivity analysis was also carried out following (Sinnott and Towler 2019).Core equipment costs are shown in Table 4.They were calculated based on information from catalogs and research for similar processes and were updated to 2021 in South America context.

Environmental impact
For the environmental impact of the process, a type of quantitative Environmental Impact Assessment (EIA) was used, which relates the metric of waste generated with the amount of product obtained, known as the E-factor as ec.
(3) presents.To that end, an acceptable range of 1 to 5 and 5-100 was used, depending on the design consideration (IISD 2016).

E factor ¼ annual waste generation annual productivity
(3)

Process of obtaining cellulose microfibres from Stipa Ichu
Initially, the fibers must be washed to remove dirt and impurities and reduce the difficulty of subsequent cutting.The size reduction of Stipa Ichu fibers was carried out by employing a blade mill.This grinding process sought to reduce the length of the fibers to approximately 10 mm, following what was indicated by (Oyeoka et al. 2021).Subsequently, the steam explosion conducted the pre-treatment of the Stipa Ichu fiber.In autoclave number 1, the first steam explosion was carried out during pre-treatment with pressure for the pulping, together with the fiber (fiber ratio, solution 1:20).Then washing with water was carried out, followed by filtration.The filtrate has a high content of recoverable lignin (obtained by the amount of condensed lignin).Subsequently, the fiber was bleached at 70°C to obtain the cellulose pulp of Stipa Ichu (partially free of hemicellulose and lignin).In the second steam explosion, the remains of lignin and hemicellulose are solubilized, which affects the fibers and causes them to reduce their size (De Corato, Sharma, and Zimbardi 2011).At each stage of washing, a filtering operation occurs; in this case, the filtrate has a high content of oxalic acid that can be recovered by the treatment proposed by (Cheng et al. 2018).In centrifugation, the precipitate corresponds to the cellulose microfibres, while the supernatant has the pulp with unreacted particles.The fibers obtained in this stage go through ultrafiltration stages by dialysis to separate trace contaminants from previous stages (Bondancia et al. 2020).Finally, the fibers are dispersed by an ultrasonicator for homogenization; and drying process in an aerosol-type dryer.

Simulation of hydrolysis kinetics
The simulation is based on the initial composition of samples of Stipa Ichu fibers from Cusco, Peru, shown in Table 1.The presence of acids catalyzes hydrolysis (Relvas, Morais, and Bogel-Lukasik 2015).However, the use of acid in the steam explosion for the untreated fibers was counterproductive since organic acids, acetic acid, are generated due to the hemicellulose content in the Stipa Ichu fibers (Lam et al. 2009).The generation of the acid can be considered constant throughout the process at 0.1 g/g of wood fiber.The amount of cellulose and other components this author exposes is like that in other research papers on grass-type fibers (Hu et al. 2010).The same is true for the values of pre-exponential factors, activation energies, and orders of magnitude of acid catalysis, which are also used in other wood species and even in pastures, corn, and poplars.The condensation of lignin in the first stages is mostly pure lignin and not pseudolignin, degradation product of the sugar-based reactions in hydrolysis (Relvas, Morais, and Bogel-Lukasik 2015).In general, pseudolignin is found in conditions of steam explosion and hydrolysis (Carvalho, Queiroz, and Colodette 2017;De Carvalho et al. 2015;Garrote and Parajó 2002;Li, Henriksson, and Gellerstedt 2007;Relvas, Morais, and Bogel-Lukasik 2015).As the reaction continues, acid insertion (carbonic and acetic) and higher temperatures (180-224°C); by-products can be promoted (Garrote and Parajó 2002).Figure 2 shows the results obtained from the literature and the experimental values for Stipa Ichu from Cusco, Peru.
The results shown in Table 2 (initial model) were obtained for both apparent lignin and hemicellulose for each experimental point of grass and the points obtained from the simulation (Bonfiglio, Mussatto, and Menéndez 2019).
Table 2 presents the kinetic parameters that were changed and the initial ones.The kinetic parameters were iteratively adjusted until the trends were approached and the error was significantly reduced.It is essential to highlight that the lignin condensation parameter is the mechanism that most influences the apparent lignin concentration adjustment.
As seen in the table above, the representativeness of the data decreased by one-tenth or one on a logarithmic scale for apparent lignin and pseudo-lignin generation.The final trends can be seen in Figure 3, which shows the nonlinear representation of the data and the results of the model.On the other hand, the increase in the concentration of hemicellulose did not suffer more significant variation and was maintained with moderate errors of 0.00254 and 0.00649.This can be explained by the fact that the hydrolysis process of hemicellulose is more intrinsic than that of lignin and does not depend on the type of fiber (wood or grass) (Lam et al. 2009).
In addition, with the experimental sample from Cusco, a second validation was made similarly using a working temperature of 130°C.The second validation could indicate the robustness of the model because, as shown in Figure 4, the data predicted by the model also conform to the experimental results at a lower temperature (130°C) and with a different fiber, showing representativeness through the SSE of 0.00182, which is on the same logarithmic scale as the first error.

Design of the main reactor
Several conditions were raised within the reactor simulation to simulate the performance of the pretreatment reactor.It aimed to determine the degree of progress of the reaction.The independent variables were temperature (also linked to working pressure) and acid concentration as a catalyst.The results are shown in Table 3, where "t" is the time in minutes, "R" is the performance, and "F.S." is the severity factor for each case, which is a measurement of the combined conditions of temperature and residence time proposed in (Bonfiglio, Mussatto, and Menéndez 2019;Naveda Rengifo et al. 2019;  Stelte 2013).Figure 5 shows that at greater times and under more severe conditions, lignin condensation will be affected, generating a higher content of pseudolignin (which is present in the fiber) or furfural, which are unwanted by-products in the pre-treatment of this type (Garrote and Parajó 2002).
Then, in all conditions, a trend can be observed where the yield decreases from a specific time (for example, 15 minutes) due to the generation of other subspecies (pseudolignin).Finally, by focusing the  analysis on a temperature of 150°C, it is found that increasing the concentration of acid shows a reduction in performance (Garrote and Parajó 2002).A decrease in acid concentration to this value would not be possible because this concentration is linked to the ash content in lignocellulosic fibers (Lam et al. 2009).
According to these results, the optimal reaction conditions that were considered were 14 minutes of operation, 150°C of temperature, and no addition of acid or basic catalyst (only a concentration of 0.1 g/g of fiber product of the generation is assumed according to the percentage of ash in the fiber).These conditions meet a severity factor of 2.6, which means it operates in moderate conditions.The main objective was to increase the yield or, in this case, increase the hydrolysis of lignin and hemicellulose.The remains partial and still needs a bleaching process after pretreatment (Garrote and Parajó 2002).

Scale up
A Latin American share of 10,635 tons is estimated (Miller 2018), where the plant with the highest capacity is 25 tons, so the processing capacity is set at 5 tons per year.The pre-treatment consisted of a first steam explosion followed by bleaching; the first presented a yield of 92% concerning the recovery of lignin or condensation of lignin and a final cellulose concentration of 53% in this step.Mass balance takes a global yield of 22.27%, so as to, 60 kg of raw fibers must be processed each day to feed the process to a 5 tons/year production of microfibres.
Since the fibres/water weight ratio in the main reactor is 1:20, approximately 550 kilograms per day are processed in this reactor, or its equivalent in volume (600 liters).Typically, the volume of work in reactors is 80% due to the formation of vapors; therefore, it was determined that a reactor with 1000 liters of capacity would be included.

Economic assessment
Core equipment costs are shown in Table 4.They were calculated based on information from catalogs and research for similar processes and were updated to 2021 in South America.The "spray" dryer is one of the pieces of equipment with the highest cost as in similar processes and is decisive for the feasibility of the project.According to (Bondancia et al. 2020), the dryer also represents a significantly high cost for the purity and specification of your product.The same author says that not having this dryer at its minimum sale price could be reduced by 10%.Then, if the dryer were not included, the microfibres would have to be marketed in the form of a solution (about 80% by weight); this would have a direct influence on transport and storage, as well as on the sale price (Bondancia et al. 2020;De Assis et al. 2017).Other equipment represents the cost very well (prices less than 100,000 USD) according to the scale of the process.
The total calculation of CAPEX was made using equipment costs and other direct costs (such as unlisted equipment and installation, among others), indirect costs, and other costs considering the scale of the process.The total CAPEX of the project was estimated at 5.60 million USD.For the calculation of OPEX, a division was made between variable costs and fixed costs.All batch costs are shown in Table 5 (assuming 4 batch per day).
Twenty operators were considered for the cost of labor calculation, with six-hour shifts distributed according to the demand of each process step for 288 (operation factor) days annually, resulting in an annual demand of 34,560 person-hours.As the base cost of labor was stipulated at 6 USD/h on average between specialized and non-specialized, a total cost of labor of 207 360 USD was calculated.The percentages of the electrical network, raw materials, and reagents are comparable with those obtained (De Assis et al. 2018).The authors start with cellulose pulp, hence the high price for the raw material.Nevertheless, their energy costs represent the second highest operating cost.This situation is observed similarly in the processes involved with cellulose microfibres due to the need to apply mechanical energy to the fibers.
An economic cash flow for 10 years of life was developed.It has an initial investment of USD 2.60 million and a loan contemplating five years for the remaining capital of USD 3 million.The production cost in this economic cash flow amounts to 290.7 USD/kg.70% of the price published by the company Nanografi Nano Technology, was used as a reference, and considering a reduction of this price for commercialization on a larger scale (620 USD/kg) (Nanografi Nano Technology 2022).NVP was 2.28 USD million, and IRR was 27%.A ROI of 48% and a PBP of 2.45 years determine the prefeasibility of the project for 10 years and for a pilot plant that produces 5 tons of dry cellulose microfibres per year.Greater production of cellulose microfibres would not necessarily suggest feasibility prices of equipment such as autoclaves tend to present a logarithmic increase according to an increase in capacity and mode of operation (Lindstrom et al. 2021).Moreover, the feasibility of this kind of processes is related to the cost of production, biomass, and investment (equipment), which were considerably reduced in this study, whereas is an option to develop industry in rural areas and no creating a utopic process (Lopes and Lukasik 2020).

Economic scenarios
Figure 6 shows the results of the analysis.The process is more sensitive to the variation in the cost of technological equipment and fixed costs without making it negative.It is not feasible (they modify the ROI by a maximum of approximately 30%).In other words, equipment with better instrumentation and control can be purchased to improve operation, increase process safety, and prevent failures in each production batch.In addition, it can be predicted that the process will remain profitable for the following engineering steps, even if these costs are determined more closely.
For variable costs and profits, direct contact (negotiation) with distributors of services and fine chemicals would make the process less affected by these factors, especially in periods of economic volatility.On the other hand, the market price is a determining factor for the profitability of the process.A deeper analysis of this factor and its impact on profitability can be seen in Figure 7.
The process would become unprofitable at a selling price of 450 USD/kg (7-8% reduction), whereas a price of 800 USD/kg might render highly lucrative but uncompetitive prices.The sale price was also decisive for the profitability of the processes proposed by (De Assis et al. 2017; Vanhatalo, Parviainen,  and Dahl 2014).The market is still very volatile, and new processes could reduce the selling price in the coming years.Therefore, new raw materials from biomass are alternatives with enormous potential.According to other authors, in 2017, it was estimated that nanofibres pre-treated with enzymes had a production cost of 11-16.3USD/kg (Serra et al. 2017).

Environmental impact
From the mass balance, the amount of waste produced per kg of the product was obtained; this calculation was made on an annual production basis.Table 6 shows the results obtained and the relationship between residues and products.If we analyze this process as a fine chemical, the accepted range is 5-50 (Sheldon 2007).As shown in Table 6, the process meets the environmental range.In other words, the chosen process does represent a cleaner alternative to other synthesis routes.However, it is recognized that there is a considerable generation of waste per unit of synthesized product (almost 10 times more waste than product); this occurs in the washing and separation processes since the product sought is of high purity.

Conclusions
The best conditions in the steam explosion reactor for Stipa Ichu pre-treatment are at 150°C with a concentration of 0.1 g/gram of fiber lasting 14 minutes.Under these conditions, there is a 92% yield for lignin recovery.The equipment with the highest acquisition cost was the atomizer dryer with 831,000 USD, in second place, the autoclave 2 with 309,000, and the autoclave 1 with 274,000 USD.The process of obtaining cellulose microfibres from Stipa Ichu is cost-effective according to nonrigorous indicators.The ROI of the process is 48%, the investment payback period is 2.44 years.The sensitivity analysis yields a minimum viable price of 450 USD/kg.The selling price affects the most the profitability of the process.E-factor parameter of 9.42 was obtained.

Highlights
• Pretreatment of Peruvian Stipa Ichu was performed by steam explosion to highly hydrolyzate lignin avoiding Pseudolignin formation to isolate cellulose to produce microfibers.• Better results were obtained in the steam explosion reactor at 150°C and 14 minutes by simulation of modified Pseudolignin kinetic.• Isolation and synthesis of cellulose microfibres is economically feasible in a pilot scale overview with production of 5 tons per year.Method proposed also indicates that is sustainable using organic acid as only catalyst and by E-factor.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This paper was written in the context of the Project funded by CONCYTEC, under the contract number N° 141-2020 -PROCIENCIA.The authors of this paper appreciate the financial support from the Peruvian Government.

Figure 3 .
Figure 3.Comparison of the adjusted kinetic model with the polynomial regression of data from the literature.

Figure 4 .
Figure 4. Comparison of the adjusted kinetic model with experimental fiber data from Stipa Ichu from Cusco, Peru.

Figure 5 .
Figure 5. Performance results of lignin condensation in a steam explosion reactor for different conditions.

Figure 6 .
Figure 6.Sensitivity analysis according to the most critical components of the process.

Figure 7 .
Figure 7. Sensitivity analysis for the selling price of cellulose microfibres.
Figure 2. Results of concentration profiles obtained from three bibliographic sources.

Table 2 .
Kinetic parameters and the associated error were adjusted according to the apparent lignin concentration.

Table 3 .
Results of the batch reactor simulation for different initial conditions.

Table 4 .
Cost of purchase of the main equipment.

Table 5 .
Demand and cost of raw materials per production batch.

Table 6 .
Data and results of the calculation of the E-Factor.