Removal of Acidic Impurities from Corn Stover ... - ACS Publications

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Removal of Acidic Impurities from Corn Stover Hydrolysate Liquor by Resin Wafer Based Electrodeionization Saurav Datta,† Yupo J. Lin,† Daniel J. Schell,‡ C. S. Millard,† Sabeen F. Ahmad,† Michael P. Henry,† P. Gillenwater,† Anthony T. Fracaro,† A. Moradia,†,§ Zofia P. Gwarnicki,†,⊥ and Seth W. Snyder*,† †

Process Technology Research, Energy Systems Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, USA ‡ National Renewable Energy Laboratory, 15013 Dever West Parkway, Golden, Colorado 80401, USA ⊥ Master of Biotechnology Program, Northwestern University, 2145 Sheridan Road, Chicago, Illinois 60611, USA ABSTRACT: Dilute acid (sulfuric acid) pretreatment of lignocellulosic biomass releases monomeric xylose, xylo-oligomers, and acetic acid by degradation of hemicellulose. Acids inhibit both the enzymatic hydrolysis of cellulose to monomeric sugars and downstream fermentation of sugars to biofuels. Removal of acetic acid and sulfuric acid, as well as the nonionic toxic impurities, from the hydrolysate liquor prior to enzymatic hydrolysis may improve biofuel yields. Development of an efficient acid separation technique is essential for enhanced process performance and cost-effective biofuel production. We evaluated the use of an electrically driven membrane separation technique, resin wafer based electrodeionization (RW-EDI), for the removal of ionic impurities (acetic acid and sulfuric acid) from corn stover hydrolysate liquor. RW-EDI provides the capability to control solution pH in situ by voltage adjustment. In situ pH control enables pKa-selective recovery of acids (or bases). The results indicate that RW-EDI is capable of removing ionic impurities using fewer unit operations and less chemicals and water than the existing process using overliming as a conditioning step to dilute acid pretreatment of corn stover. We report greater than 99 and 95% removal of sulfuric and acetic acids, respectively, from dilute sulfuric acid pretreated corn stover hydrolysate liquor. Sugar retention was greater than 98%. We also report strategies to selectively separate sulfuric acid and acetic acid into two individual acid enriched streams from the hydrolysate liquor by manipulating the operating conditions. These results point toward a deployment strategy with sequential (mineral then organic) acid-removal steps.

1. INTRODUCTION The Energy Independence and Security Act of 2007 mandates annual production of 36 billion gallons of biofuels by 2022, of which 16 billion gallons must be cellulosic. A simplified schematic of the lignocellulosic biomass derived biofuel production pathway (biochemical) is presented in Figure 1. The biomass pretreatment step contributes ∼20% of the total production cost.1 In the pretreatment step, the rigid crystalline structure of plant cell wall matrix is disrupted by chemical treatment (acid, alkali, steam, and ammonia, etc.) at elevated temperatures and pressures to expose the cellulosic moiety.2 The slurry is passed through a solid−liquid separations process to recover the solubilized hemicellulose sugars prior to enzymatic hydrolysis. In the enzymatic hydrolysis step, cellulose is hydrolyzed into soluble monomeric sugars (primarily glucose) by cellulase cocktails. The monomeric sugars are fermented to biofuels. Alternatively, the monomeric sugars can be converted to other biobased products by biochemical or catalytic methods. Dilute acid treatment with sulfuric acid is the most common lignocellulosic biomass pretreatment technique.3,4 Pretreatment produces fermentation inhibitors (organic acids, furans, and phenolics, etc.) along with the solubilized five carbon sugars, primarily xylose from hemicellulose, in the hydrolysate liquor. The inhibitors are detrimental to downstream processing.5 Acidity of hydrolysate liquor (e.g., by sulfuric acid and other organic acids) inhibits enzymatic hydrolysis and fermentation. Hence, pH adjustment and reducing the salt content by adequate removal of these © 2013 American Chemical Society

inhibitors and residual sulfuric acid is essential for further processing of the solids (cellulose and lignin). Acetic acid (deacetylation of hemicellulose), furfural (degradation of five carbon sugars), and hydroxylmethyl furfural (HMF, degradation of six carbon sugars) are the primary fermentation inhibitors produced by dilute acid pretreatment. Acetic acid and sulfuric acid are ionic impurities that could be removed using their ionization properties. Nonionic impurities, such as, furfural and HMF could be removed using adsorption methods.4 In existing pilot-scale facilities, sulfuric acid is removed by treating with lime (overliming) and precipitating sulfate ions as CaSO4 (gypsum),6 as shown in Figure 1. Although overliming removes sulfate ions from hydrolysate liquor, it increases the number of unit operations and residence time (overliming tank + pH adjustment tank/settlement tank + separation devices), requires addition of chemicals, and generates a low-value and potentially toxic gypsum byproduct. Acetic acid is not removed in the overliming process. The flash vaporization prior to solid−liquid separation removes only 8% of the total acetic acid. All these factors contribute to the cost and complexity of cellulosic biofuel production.7 To avoid the above-mentioned challenges associated with overliming, it is substituted with ammoniation in the latest proposed technolReceived: Revised: Accepted: Published: 13777

June 5, 2013 August 19, 2013 August 26, 2013 August 26, 2013 dx.doi.org/10.1021/ie4017754 | Ind. Eng. Chem. Res. 2013, 52, 13777−13784

Industrial & Engineering Chemistry Research

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Figure 1. Schematic of lignocellulosic biomass based biofuel production route (biochemical) and the role of intermediate separation steps. EDI could be used as an alternative to conventional overliming for removing sulfuric acid as shown by the dotted pathway. EDI enables a pathway with fewer unit operations, reduced use of chemicals (lime and others) and water, and the potential of recycling the acid.

ogy by the National Renewable Energy Laboratory (NREL).8 Ammoniation reduces the number of unit operations, including the solid−liquid separation step. However, acetate ions are still present in the ammoniated biomass slurry. Also, the added ammonia and sulfate are expensive, and a technique to recover them could improve the process economics. This study deals with only the acid pretreated corn stover hydrolysate liquor conditioned with overliming and proposes a technology for removal and possible recovery of the acid impurities. Several investigators are still using overliming to avoid the cost of additional consumables. In addition, the reported process will decrease costs for wastewater treatment. Several methods have been explored to study the detoxification of lignocellulosic biomass hydrolysate liquor. Removal of fermentation inhibitory compounds (both ionic and nonionic) by both polymeric ion exchange resin beads9−11 and polymeric adsorptive beads12,13 have been reported. Other methods include alkali treatment,14 sulfite addition,15 activated carbon treatment,15,16 and bioabatement by microorganisms.17 Use of ion exclusion chromatography for separation of sulfuric acid and sugar has also been reported.18,19 Researchers have also demonstrated the recovery of sugar from hydrolysate liquor by batch elution chromatography process using polymeric adsorbents.20 Han and co-workers21 have conducted a comparative study between an anion exchange membrane and an anion exchange resin bead for the removal of acetic acid from hydrolysate liquor and indicated the superiority of the membrane over the resin bead. Grzenia et al. have studied the removal of acetic acid using hollow fiber membrane extraction22 with amine based solvents. They have obtained up to 60% removal of acetic acid from corn stover hydrolysate liquor. Huang and Juang23 have studied the recovery of sulfuric acid from a mixture containing glucose and xylose using multicomponent electrodialysis (ED). The effects of current density and mass transfer were discussed in that report. These methods offer some advantages in removing fermentation inhibitory compounds; however, increased production costs and lack of efficient pathway to scale up have prevented them from being deployed at a commercial scale. To overcome these challenges, we use resin wafer electrodeionization (RW-EDI) as a conditioning method for removal

of acetic acid and sulfuric acid from corn stover hydrolysate liquor. Electrodeionization (EDI) is a modified version of electrodialysis (ED) that incorporates ion exchange (IX) resin beads within an ED stack. The IX beads enhance ionic conductivity of a dilute solution by absorbing and concentrating the ions on the resin beads. Furthermore, the “electrically induced water splitting” reaction on the bead surface enables in situ regeneration of the IX beads and aids in the transport of ionic species across the IX membranes under an applied electric field.24 Water splitting reaction produces protons and hydroxyl ions at the resin−resin and resin−membrane interfaces. Protons and hydroxyl ions continuously replace the adsorbed ions on the IX bead surface, thereby; promoting the transfer of ions through the more conductive IX resin pathway rather than the resistive dilute solution pathway. Due to this, EDI is considered as an IX resin column with continuous regeneration and is superior to ED for complete removal of ionic species from dilute solution.25 It has been employed for production of ultrapure water,26−28 concentration of radioactive waste,29 recovery of organic acids,24,30 and recovery of heavy metal ions.31 We have made significant advancements over conventional EDI technology. Instead of using loose IX resin beads as a conventional EDI device, we mold the IX resins into a porous resin wafer (RW)32 and insert it into the ED stack. Compared to conventional EDI, the RW-EDI provides simpler assembly and efficient operation for EDI device optimization using different arrangements of IX membranes and resin compositions. Due to the rigidity of the porous wafer, RW-EDI also demonstrated very consistent process performance. It broadens the scope of applications compared to the conventional EDI, such as, esterification,33 enzyme based conversion and recovery of organic acids,34 and capture of carbon dioxide from flue gas.35 We previously reported on RW-EDI based detoxification of dilute acid pretreated ponderosa pine slurry in combination with polyelectrolyte polymer adsorbents. It significantly improved the conversion and the rates of both hydrolysis and fermentation.36 However, the details of the RW-EDI treatment were omitted from that publication, and the focus was primarily on the effect of the removal of inhibitory compounds on the 13778

dx.doi.org/10.1021/ie4017754 | Ind. Eng. Chem. Res. 2013, 52, 13777−13784


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Figure 2. Schematic of different components inside a resin wafer based electrodeionization (RW-EDI) stack. Acids get transferred from the hydrolysate liquor in the diluate compartment to the recovery solution in the concentrate compartment under an applied electric field. The diluate compartment contains a porous ion exchange resin wafer. C and A are cation and anion exchange membranes, respectively.

Western States Machine Co., Hamilton, OH, USA) operated at 1400 rpm (900 g). Filtration was performed with a polypropylene cloth lining the basket with an approximate 100 μm pore size. 2.2. Experimental Methods. All experiments were conducted in a RW-EDI stack consisting of six cell pairs. Each cell pair consists of a diluate compartment (ions depleted) and a concentrate compartment (ions accumulated) separated by cation and anion exchange membranes as shown in Figure 2. The diluate compartment contains a porous ion exchange resin wafer (195 cm2 cross-section surface area and 0.25 cm thickness). The membranes are arranged to facilitate unidirectonal ion flow under an applied electric field. Ions can only move out from the diluate compartments and can only move in to the concentrate compartments. Therefore, when the hydrolysate liquor passes through the diluate chamber, ionic impurities (sulfate and acetate ions) are transported from the diluate compartment, across the ion exchange membranes into the concentrate compartment. As a result, ionic impurities are removed from the hydrolysate liquor (diluate) and accumulate in the recovery solution (concentrate). Initially, a set of experiments was conducted to study the effect of flow rate on the removal of sulfuric acid and acetic acid. For that, a mixture of 20 g/L sulfuric acid and 10 g/L acetic acid was used. These concentrations represent typical pretreatment stream titers. Flow rate was varied between 30 and 500 mL/min, whereas, the applied electric potential was kept constant. The results of the effect of feed flow rate on sulfate and acetate ions removal were utilized to optimize selective separation of sulfuric and acetic acid from hydrolysate liquor. Synthetic hydrolysate liquor was used first to study the effectiveness of RW-EDI for selective separation of sulfuric and acetic acids. In all experiments with batch operation of RWEDI, 2 L of hydrolysate liquor (synthetic or corn stover) was processed. The synthetic hydrolysate liquor contained 20 g/L sulfuric acid, 8 g/L acetic acid, 50 g/L xylose, 1.5 g/L furfural, and 0.5 g/L HMF. 1 g/L of sulfuric acid was used as the

downstream processes. Here we provide a much more detailed description and analysis of RW-EDI based deacidification. In this study, RW-EDI was employed to remove ionic impurities (acetic and sulfuric acids) from dilute acid pretreated corn stover hydrolysate liquor. We hypothesized that RW-EDI based deacidification of hydrolysate liquor will provide a pathway with fewer unit operations, lower operation time, and reduced use of chemicals and water (Figure 1). A strategy was also designed for selective separation of sulfuric acid and acetic acid that will lead to the recovery of nearly pure acid streams. This selective separation process would enable reuse of sulfuric acid in the acid pretreatment step. The acetic acid rich stream could be recovered as a valued-added co-product. The fate of the nonionic impurities and sugars was also observed.

2. MATERIALS AND METHODS 2.1. Equipment and Materials. RW-EDI experiments were conducted in a EUR2B-10 ED stack purchased from Ameridia Corp. (Somerset, NJ, USA). The Neosepta cation exchange (CMX), anion exchange (AMX), and bipolar membranes (BP) were also purchased from Ameridia). The resin wafers (RWs) were fabricated using a proprietary method discussed in detail elsewhere.31 The wafer fabricating mixture consisted of ion exchange resin beads (mixture of cation and anion exchange resin beads), a binder polymer, and a porosigen in specific ratio. A mixture of Purolite PFA444 anion exchange resin beads and Purolite PFC100E cation exchange resin beads was used. The mixture was then heated to a specific temperature and casted in a mold to form a porous resin wafer. The chemicals were purchased from Sigma (Saint Louis, MO, USA). The corn stover hydrolysate liquor provided by NREL was produced in a pilot-scale 900 kg/day vertical pretreatment reactor operated at conditions of 190 °C, an approximate 1 min residence time, 55 mg/g dry biomass sulfuric acid loading, and a 25% (w/w) total solids loading.37 The hydrolysate liquor was separated from the pretreated slurry by a perforated basket centrifuge (Quadramatic Q-120, 13779



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Figure 3. Effect of flow rate (residence time) on removal efficiency and productivity (inset) of separation of sulfuric acid and acetic acid from a mixture at 1 V applied electrical potential in a six-cells-pair RW-EDI stack. Each data point indicates the percent removal (productivity for the inset) of sulfuric acid or acetic acid at a fixed flow rate under steady-state operation.

negligible. Percent removal (fraction of ions removed) in the diluate compartment is influenced by the reaction time for ion exchange and is decreased with the increase in feed flow rate, i.e., with the decrease in residence time. The inset plot of Figure 3 presents the effect of flow rate on productivity (mass flux) of removal of sulfate and acetate ions. For sulfate removal, productivity increases with flow rate, reaches a maximum at 500 mL/min (290 g/m2/h), and then decreases at 800 mL/min (205 g/m2/h). A similar trend is observed for acetic acid with maximum removal rate at 150 mL/min (27 g/m2/h) and then reduction to negligible productivity at 300 mL/min and above. Due to the constant electric potential, current was a function of the resistance (or conductivity) of the solution. An increase in feed flow rate decreases the resistance of the solution (higher ionic conductivity) and results in higher current. Higher current ensures a higher ion transport rate. Therefore, the removal productivity increases with flow rate. Additionally, higher flow rate ensures better flow distribution throughout the porous RW matrix, which enables enhanced mass transfer (better utilization of available surface area). Hence, ionic flux (productivity) increases with flow rate up to ∼500 mL/min for sulfuric acid and 150 mL/min for acetic acid (Figure 3). After that the disadvantage of lower residence time at higher flow rate offsets the advantage of conductivity and mass transfer, and the productivity decreases after going through a maximum. These observations indicate that a flow rate of 500 mL/min at an applied potential of 1 V across six cell pairs offers the most favorable condition for selective separation of sulfuric acid over acetic acid. Ion transport is a strong function of the degree of dissociation of ionic species. At a working pH < 2, acetic acid primarily remains undissociated (pKa = 4.76), while sulfuric acid is almost completely ionized (pKa = 1.9). Therefore, ion transport across the IX membranes under an applied electric field is dominated by the sulfate ions. Acetate transport across the IX membranes could be enhanced by applying a higher electric potential (ion flux is proportional to applied electric potential), which in turn would generate more acetate ions by dissociation to maintain the equilibrium. For selective

concentrate solution at the beginning of an experiment to provide sufficient conductivity to the solution. After removing sulfuric acid from the liquor (diluate solution), the run was paused for 30 min, the RW-EDI stack was washed with copious amounts of water, and the sulfuric acid enriched recovery solution (concentrate solution) tank was replaced with a new tank containing 1 g/L acetic acid solution. The experiment was resumed for removal of acetic acid. Conductivity of both hydrolysate liquor and recovery solution was monitored throughout the experiment. The run was terminated once the conductivity of the ion-depleted hydrolysate liquor dropped below 0.2 mS/cm. Experiments with corn stover hydrolysate liquor were conducted in a way similar to that of the synthetic hydrolysate liquor except the corn stover hydrolysate liquor was permeated through a 65 KDa hollow fiber membrane prior to RW-EDI. Typically, in a RW-EDI stack, the life expectancy of the membranes and wafers vary between 12 to 18 months with periodical cleaning. 2.3. Analytical Methods. Xylose, acetic acid, HMF, and furfural were analyzed using a Waters high-performance liquid chromatography (HPLC) system equipped with refractive index (RI) and photodiode array (PDA) detectors, a Bio-Rad (Richmond, CA) Aminex HPX-87H column and a guard column at a controlled temperature of 60 °C. A 5 mM sulfuric acid solution was used as the mobile phase at a flow rate of 0.5 mL/min. Sulfate was determined using ion chromatograph (Metrohm IC Plus system) with a Grace (Deerfield, IL) Allsep anion column and a conductivity detector. A flow rate of 1 mL/ min with a mobile phase of 4 mM hydroxylbenzoic acid and 100 mM sulfuric acid as the suppressor liquid was used.

3. RESULTS AND DISCUSSION 3.1. Effect of Flow Rate. Figure 3 illustrates the effect of feed flow rate on selective removal of sulfate and acetate ions at applied electric potential of 1 V across the six-cell-pair RW-EDI stack. With increase in feed flow rate (decrease in residence time), the percent removal of both ions decreases, and at a flow rate of 300 mL/min and above, removal of acetate becomes 13780



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Figure 4. Removal of sulfuric acid and acetic acid from synthetic hydrolysate liquor using batch mode RW-EDI. The operating parameters are manipulated to obtain selective removal of sulfuric acid over acetic acid until 210 min of operation time. Then, the sulfuric acid enriched solution in the concentrate tank was replaced with 1 g/L of acetic acid solution, and the experiment was resumed after 45 min (total 255 min) for removal of residual acid from synthetic hydrolysate liquor (diluate solution).

Figure 5. Removal of sulfuric acid and acetic acid from corn stover hydrolysate liquor using batch mode RW-EDI. The operating parameters are manipulated to obtain selective removal of sulfuric acid over acetic acid until 240 min of operation time. Then, the sulfuric acid enriched solution in the concentrate tank was replaced with 1 g/L of acetic acid solution, and the experiment was resumed after 30 min (total 270 min) for removal of residual acid from corn stover hydrolysate liquor (diluate solution).

flow rate (500 mL/min) at the beginning to remove sulfate preferentially over acetate ions. Once the concentration of sulfate was reduced to a targeted concentration, conditions were adjusted to remove acetate (flow rate, 60 mL/min; voltage, 30 V). To evaluate the performance of this strategy, experiments were conducted with synthetic hydrolysate liquor. A typical ion removal profile is presented in Figure 4 that depicts the concentration of sulfuric and acetic acids in synthetic hydrolysate liquor as a function of operating time. During the lower voltage−higher flow rate condition (up to

separation, we deliberately kept both the applied electric potential and residence time at lower values to ensure inefficient acetate transport, as described in the next section. With the removal of sulfuric acid, pH of the solution increases and acetic acid starts dissociating. Once sulfate ions are removed, under higher applied electric potential acetate is induced to transport across the IX membranes. 3.2. Separation Performance of Synthetic Hydrolysate Liquor. From the above observations, a strategy was adopted that is comprised of applying lower voltage (1 V) and higher 13781



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Figure 6. Concentration profile of the acids in the recovery solution (concentrate solution) during the removal of sulfuric acid and acetic acid from corn stover hydrolysate liquor using batch mode RW-EDI. For the first 240 min, sulfuric acid was preferentially transferred to the concentrate side in comparison to acetic acid due to favorable operating conditions. After that, the experiment was paused for 30 min to replace the sulfuric acid enriched solution in the concentrate tank with 1 g/L of acetic acid solution, and the experiment was resumed (270 min) to transfer residual acid.

sulfuric acid enriched solution from the concentrate side with fresh 1 g/L acetic acid. The experiment was resumed at 270 min with high voltage−low flow rate condition, which enabled enhanced ion transport (predominantly acetate and remaining sulfate ions) due to favorable residence time and electrical driving force. After 390 min of operating time the experiment was terminated with nearly 100 and 95% removal of sulfuric acid and acetic acid, respectively. The final concentrations of sulfuric acid and acetic acid in the hydrolysate liquor were 0 and 0.5 g/L, respectively. Five experiments have been conducted with corn stover hydrolysate liquor, and all of them demonstrated very similar separation performance. After each experiment, almost 100% of sulfuric acid and at least 95% of acetic acid were removed. Figure 5 also demonstrates that more than 98% of xylose was retained in the hydrolysate liquor at the end of the run. Xylose was not detected in the recovery solution (concentrate). Furfural (40%) and HMF (25%) concentrations were significantly reduced (not shown in the figure), but not detected in the concentrate. These results suggest that xylose, furfural and HMF are partially adsorbed on the diluate compartment IX resin wafers. This observation leads to a potential option of removing nonionic impurities using suitable (adsorptive) resin beads inside the wafers in the diluate compartment. The time profile for sulfuric acid and acetic acid accumulation in the recovery solution (concentrate) is presented in Figure 6. During the first phase (0−240 min), sulfate was preferentially transported and with negligible acetate transport. The recovery solution had a 22:1 ratio of sulfuric acid to acetic acid (19 g/L sulfuric acid and 0.8 g/L of acetic acid). After flushing with copious amount of water in the absence of current (to remove residual ionic species) the sulfuric acid enriched concentrate solution was replaced with a fresh 1 g/L acetic acid solution (270 min). During the second phase, enhanced transport of the rest of the sulfuric acid (0.5 g/L) and acetic acid occurred due to the favorable voltage and residence time conditions. After 120 min (390 min total time), the

210 min) sulfate ions are preferentially removed over the acetate ions. After 180 min of operation, 96% of the sulfate was removed and the sulfate concentration in the diluate was reduced to 0.7 g/L, whereas no acetate was detected in the concentrate solution. After 210 min, with more than 97% sulfuric acid and 4% acetic acid removed, the experiment was paused and the sulfuric acid enriched solution was removed and replaced with 1 g/L acetic acid on the concentrate side. The concentrate compartment of the RW-EDI stack was flushed with water (no electricity) to remove residual sulfuric acid. The experiment was resumed at 255 min with high voltage−low flow rate ((36 V)−(60 mL/min)) condition to take advantage of the higher driving force and higher residence time. Since the sulfate concentration was reduced by 97%, acetate transport was facilitated. At the end of the experiment (360 min), nearly 100% of sulfuric acid and 93% of acetic acid was removed from the synthetic hydrolysate liquor with final concentrations of 0 and 0.6 g/L sulfuric acid and acetic acid, respectively. These results were repeated three times with very similar results 100% removal of sulfuric acid and 93−95% removal of acetic acid. This strategy sequentially separated the two acids into separate recovery solutions; one with almost pure sulfuric acid (19.5 g/L sulfuric acid and 0.3 g/L acetic acid) and the other with high ratio of acetic acid to sulfuric acid (8 g/L acetic acid and 0.55 g/L sulfuric acid). 3.3. Separation Performance of Corn Stover Hydrolysate Liquor. The same strategy was applied to corn stover hydrolysate liquor. The composition of different batches of corn stover hydrolysate liquor (NREL sample nos. P090626CS and P101214CS) were in the ranges of 15−20 g/L sulfuric acid, 8−10 g/L acetic acid, 40−50 g/L xylose, 1−1.5 g/L furfural, and 0.5 g/L HMF. Figure 5 is a typical time profile of concentration of sulfuric acid and acetic acid in corn stover hydrolysate liquor processed by RW-EDI. After 210 min of operation 95% of sulfate and 6% of acetate ions were removed from the hydrolysate. The experiment was paused after 240 min (with 97% sulfate and 8% acetate removal) to replace the 13782



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in removing the ionic impurities from synthetic as well as corn stover hydrolysate liquors. RW-EDI provides an efficient separation route of inhibitors that could lead to an economically viable cellulosic biofuel and biobased chemical production process. The electricity consumption for deacidification using RW-EDI was around 4−6% ethanol LHV based on corn stover to ethanol production ratio estimated by NREL’s technoeconomic analysis. More detailed technoeconomic analysis of the process will be discussed in a future publication.

concentrate solution had 0.45 g/L sulfuric acid and 9.2 g/L acetic acid (20:1 ratio of acetic to sulfuric acid). Removal of sulfuric acid by RW-EDI eliminates the need for overliming, and therefore generation of low value solid waste gypsum. In the overliming process, the residence time for liming, settling of gypsum crystals, and pH adjustment is around 4.5 h.6 RW-EDI significantly reduces this conditioning time. Chemical pH adjustment is not required for RW-EDI treatment. Also, in the conventional conditioning process, about 20 wt % additional water6 is typically added while recombining the hydrolysate liquor and solid biomass to reduce concentration of the impurities prior to enzymatic hydrolysis. RW-EDI reduces the concentration of impurities sufficiently to avoid the need for dilution (reduced water usage) and, therefore, enables higher sugar titers. The recovered sulfuric acid enriched stream (∼2%) could be recycled back as-it-is for the dilute acid pretreatment step, thereby further reducing water demand. The concentration of captured acetic acid could be 20−35 wt % depending on the type of ion exchange membrane used. It will need to be further concentrated to be a viable coproduct. For process economics, based on 1 year lifetime of consumable wafer and membranes, the process operations cost was found to be in a range of 10−30% total processing cost. We will refine these estimates after we complete extended pilot-scale runs. The electricity consumption of deacidification using RW-EDI was around 4−6% of ethanol “lower heating value (LHV)” based on corn stover to ethanol production ratio estimated by NREL’s technoeconomic analysis.6 More detailed technoeconomic analysis of the process will be discussed in a future publication. These results point toward a strategy for deployment. Removal of sulfuric acid and acetic acid in sequential RW-EDI units, each operating under optimized conditions for the targeted acid, will enable efficient recovery and recycle. We did not have sequential RW-EDI units available, so we were required to conduct the experiment in a batch recirculating mode. Our results suggest that sequential continuous operation could be deployed to produce a deacidified hydrolysate stream and separated sulfuric acid and acetic acid.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 1-630-252-7939. Present Address §

Department of Chemical Engineering, University of Illinois, Urbana−Champaign, 600 S. Mathews, Urbana, IL 61801. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the DOE EERE Bioenergy Technologies Office (primary sponsor) and the EERE Assistant Secretary Technology Commercialization Fund for financial support of this research.

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REFERENCES

(1) Aden, A.; Foust, T. Technoeconomic analysis of the dilute sulfuric acid and enzymatic hydrolysis process for the conversion of corn stover to ethanol. Cellulose 2009, 16, 535−545. (2) Kumar, P.; Barrett, D. M.; Delwiche, M. J.; Stroeve, P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind. Eng. Chem. Res. 2009, 48, 3713−3729. (3) Torget, R.; Walter, P.; Himmel, M.; Grohmann, K. Dilute-acid pretreatment of corn residues and short-rotation woody crops. Appl. Biochem. Biotechnol. 1991, 28/29, 75−86. (4) Pienkos, P. T.; Zhang, M. Role of pretreatment and conditioning processes on toxicity of lignocellulosic biomass hydrolysates. Cellulose 2009, 16, 534−545. (5) Tucker, M. P.; Kim, K. H.; Newman, M. M.; Nguyen, Q. A. Effects of temperature and moisture on dilute-acid steam explosion pretreatment of corn stover and cellulose enzyme digestibility. Appl. Biochem. Biotechnol. 2003, 105−108, 165−177. (6) Aden, A.; Ruth, M.; Ibsen, K.; Jechura, J.; Neeves, K.; Sheehan, J.; Wallace, B. Lignocellulosic biomass to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and enzymatic hydrolysis for corn stover. Technical Report of National Renewable Energy Laboratory (NREL/TP-510-32438), 2002. (7) Biomass Multi Year Program Plan; Office of the Biomass Program, Energy Efficiency and Renewable Energy, U.S. Department of Energy: Washington, DC, 2008. (8) Humbird, D.; Davis, R.; Tao, L., Kinchin, C.; Hsu, D.; Aden, A.; Schoen, P.; Lukas, J.; Olthof, B.; Worley, M.; Sexton, D.; Dudgeon, D. Process design and economics for biochemical conversion of lignocellulosic biomass to ethanolDilute-acid pretreatment and enzymatic hydrolysis of corn stover. Technical Report of National Renewable Energy Laboratory and Harris Group Inc. (NREL/TP5100-47764), 2011. (9) Nilvebrant, N.-O.; Anders, R.; Larsson, S.; Jonsson, L. J. Detoxification of lignocellulose hydrolysates with ion-exchange resins. Appl. Biochem. Biotechnol. 2001, 91−93, 35−49. (10) de Mancilha, I. M.; Karim, M. N. Evaluation of ion exchange resins for removal of inhibitory compounds from corn stover hydrolyzate for xylitol fermentation. Biotechnol. Prog. 2003, 19, 1837−1841. (11) Horváth, I. S.; Sjöde, A.; Nilvebrant, N.-O.; Zagorodni, A.; Jönsson, L. J. Selection of anion exchangers for detoxification of dilute-

4. CONCLUSION Resin wafer electrodeionization (RW-EDI) was employed to remove ionic impurities from dilute acid treated corn stover hydrolysate liquor. The reported results reveal that RW-EDI is an effective platform for removal of ionic impurities from hydrolysate liquor. More than 99% of sulfuric acid and 95% of acetic acid were removed from the corn stover hydrolysate liquor. The selectivity of sulfuric acid to acetic acid can be tuned by properly adjusting the feed flow rate and applied voltage. RW-EDI successfully separated sulfuric and acetic acid from the hydrolysate liquor into separate purified streams. This was achieved first by selectively removing sulfuric acid and then acetic acid under specified operating conditions. The sulfuric acid enriched stream contained 19 g/L sulfuric acid and 0.8 g/L acetic acid, whereas, the acidic acid enriched stream contained 0.45 g/L sulfuric acid and 9.2 g/L acetic acid. The sulfuric acid enriched stream could potentially be recycled to the pretreatment reactor, and the acetic acid enriched stream could be used as a valuable co-product. This technique was also able to remove up to 40 and 25% of furfural and HMF, respectively, whereas, >98% of xylose was retained in the treated hydrolysate liquor. Overall, this study demonstrated the efficacy of RW-EDI 13783



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