Efficient Ethanol Recovery from Yeast Fermentation Broth with

Article pubs.acs.org/IECR

Efficient Ethanol Recovery from Yeast Fermentation Broth with Integrated Distillation−Membrane Process Leland M. Vane,*,† Franklin R. Alvarez,† Laura Rosenblum,‡ and Shekar Govindaswamy§ †

U.S. Environmental Protection Agency, 26 W. Martin Luther King Drive, Cincinnati, Ohio 45268, United States Shaw Environmental & Infrastructure, Inc., 26 W. Martin Luther King Drive, Cincinnati, Ohio 45268, United States § Lakeshore Engineering Services, Inc., U.S. EPA Test & Evaluation Facility, 1600 Gest Street, Cincinnati, Ohio 45204, United States ‡

S Supporting Information *

ABSTRACT: A hybrid process integrating vapor stripping with vapor compression and vapor permeation membrane separation, termed Membrane Assisted Vapor Stripping (MAVS), was evaluated for recovery and dehydration of ethanol from aqueous solution as an alternative to conventional distillation−molecular sieve processes. Ethanol removal/drying performance of the MAVS system with binary ethanol−water mixtures and a yeast fermentation broth were evaluated and the fate of secondary fermentation products in the system was assessed. Simple alcohols, esters, and organic acids displayed varying degrees of recovery in the vapor stripping based on the relative vapor−liquid partitioning of the compounds. All volatilized organic compounds were concentrated to the same degree in the membrane step. Membrane permeance, permselectivity, and overall energy usage of the hybrid process were the same with the fermentation broth as with binary ethanol−water solutions. The MAVS system required less than half the energy of a distillation−molecular sieve system.

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INTRODUCTION In the conventional process of producing ethanol biofuel from corn starch, the recovery of ethanol from the fermentation broth is accomplished using a multicolumn distillation system which yields an ethanol-rich stream near the ethanol−water azeotrope of 95.6 wt % ethanol.1 Molecular sieve adsorbents are then used to remove water to meet the specification for use in fuel mixtures: 1 vol% (1.3 wt %), 0.3 wt %, and 0.4 vol% (0.5 wt %) water in the United States (ASTM D4806-11a), European Union (EN 15376:2011), and for fuel ethanol imports to Brazil (Resoluçaõ ANP 7/2011), respectively.2−4 Ethanol concentrations in fermentation broths generated from corn starch are as high as 15 wt %. At this concentration, the energy required to recover and dry the ethanol is estimated to be 4−5 MJ-fuel/kg5− relatively modest compared to the 27.0 MJ/kg Lower Heating Value (LHV) of the recovered ethanol.6 However, distillation energy rises rapidly as the concentration of ethanol decreases, particularly below 5 wt % ethanol. 5,7−11 Low ethanol concentrations are likely to be encountered as ethanol is biologically produced from a synthesis gas or sugar substrate derived from lignocellulosic biomass or directly by algae.12−14 Energy spent recovering and drying ethanol contributes to the upstream greenhouse gas (GHG) emissions and energy-related environmental impacts of any ethanol biofuel. Such emissions will determine under which renewable fuel category the product ethanol is categorized for the purposes of the U.S. Energy Independence and Security Act of 2007 (EISA).15 Thus, reducing energy in this part of the process can significantly impact the sustainability and marketability of an ethanol biofuel. Previously, we presented a hybrid distillation−membrane process which reduces the recovery and dehydration energy by over 50% relative to conventional distillation−adsorption systems.16−18 As depicted in Figure 1, the Membrane Assisted © 2012 American Chemical Society

Vapor Stripping (MAVS) hybrid process consists of a vapor stripping column which recovers ethanol from the feed liquid ①, creating an ethanol-rich overhead vapor ③ and an ethanoldepleted bottoms liquid stream ②. The overhead vapor stream is compressed and fed to a vapor permeation membrane system in which a water-selective membrane creates an ethanol-rich retentate product stream ④ and a water-rich permeate vapor ⑤. The permeate vapor from at least the first membrane stage is directly returned to the stripping column to form a significant portion of the stripping vapor in the column. Steam or reboiler heat is added to generate the balance of the necessary stripping vapor. The ethanol−water separation performance and energy savings were previously demonstrated with binary ethanol− water mixtures and an actual yeast fermentation broth.16,18 Whereas ethanol and water are by far the highest concentration components of an ethanologen yeast fermentation broth, the microorganisms create a variety of secondary fermentation products such as organic acids, alcohols, esters, ketones, and aldehydes. Each of these compounds will have different vapor liquid equilibrium (VLE) and membrane transport behaviors. The movement of these compounds through the separation process is of interest in understanding system performance and product quality. Here, we report on the fate of secondary fermentation products in the hybrid ethanol−water separation process and the effect of an actual fermentation broth on overall process performance. Special Issue: Baker Festschrift Received: Revised: Accepted: Published: 1033

October 31, 2011 December 30, 2011 January 4, 2012 January 4, 2012 dx.doi.org/10.1021/ie2024917 | Ind. Eng.Chem. Res. 2013, 52, 1033−1041

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of simplified Membrane Assisted Vapor Stripping process (MAVS) with single membrane stage and permeate vapor return to stripping column, which is the phase 3 experimental configuration.

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EXPERIMENTAL SECTION The experimental MAVS apparatus, located at the U.S. Environmental Protection Agency (USEPA) Test & Evaluation Facility in Cincinnati, OH, consisted of a feed liquid conditioning loop in which the feed liquid was recirculated from either a 1000-gal epoxy tank or a 55-gal stainless steel drum through a 30-kW heater and 1-μm bag filter before returning to the source tank or drum. A slip stream from this loop was metered into the top of the vapor stripping column using a peristaltic pump. For all experiments reported here, the temperature of the feed liquid was controlled to 55 °C. The column was a shell-and-tube unit originally designed to study the fractional condensation or “dephlegmation” of ethanol− water vapor mixtures generated from pervaporation membranes.19,20 As a result, it was not designed to deliver a specific stripping efficiency. The column consisted of a central 2.44-m long, 7.62-cm (3-in.) inner diameter stainless steel pipe that contained a high surface area structured packing (BX wire gauze, Koch-Glitsch, KY). The height of the packed section of the stripping column was 2.4 m. Based on earlier tests, this column delivered six theoretical stages of VLE.16 The outside of the stripping column was insulated to reduce heat losses to the room. Heat losses from the column to the ambient atmosphere were estimated to be 7.7 kJ/min (equivalent to 3.4 g/min steam) by measuring the steady state temperature drop of liquid passing through the stripping column without vapor stripping at various feed temperatures and flow rates. All vapor transfer pipes and hoses were heat traced and insulated to prevent condensation of vapors. The bottoms liquid effluent from the column was continuously collected in a reservoir that was automatically drained via a gear pump activated by a level sensor. Steam was added directly into the bottom of the stripping column using a peristaltic pump, which metered deionized (DI) water into an evaporator unit plumbed to the stripping column below the packing. The mass flow rates of both the feed liquid and steam generator peristaltic pumps were checked daily. The column operated under a partial vacuum with all experiments reported here performed at an absolute pressure of 150 Torr (20.0 kPa). The overhead vapor from the stripping

column was directed to the suction side of a set of four oil-free scroll vacuum pumps (model IDP-3, Varian, MA) plumbed in parallel. A vacuum pressure regulator (model 329, LJ Engineering, CA) located between the suction inlet of the vacuum pumps and the top of the stripping column maintained the desired column pressure. The compressed vapor leaving the scroll pumps was filtered, transferred through a heat transfer coil, and directed to the feed port of the vapor permeation membrane module. The filter, coil, and membrane module were housed in a temperature-controlled chamber with a set point of 110 °C. The hydrophilic membrane module was a 2.5-in. diameter spiral wound unit fabricated by Membrane Technology and Research, Inc. (Menlo Park, CA) and was estimated to contain 0.60 m2 of active membrane area. The membrane was a multilayer composite in which the selective layer consisted of a hydrophilic cellulose ester layer with a silicone rubber overcoat. The selective layer was coated on a porous support layer for mechanical strength.21,22 To monitor the fate of broth compounds throughout the system, several analytical methods were enlisted to analyze samples obtained at the five locations identified in Figure 1. Feed and bottoms liquids to/from the stripping column were sampled to determine the effectiveness of the stripping column. The three vapor streams to/from the membrane unit (feed vapor, retentate vapor, and permeate vapor) were also sampled. In the cases of the feed vapor and permeate vapor, a small portion of the respective vapor stream was withdrawn from the system using a heated head diaphragm vacuum pump (R254, Air Dimensions, FL). Each compressed vapor stream was then directed to a separate condenser coil of 1/8-in. diameter stainless steel tubing in a chilled water bath and the resulting condensate was collected in a glass vial. The retentate vapor from the module, which was at atmospheric pressure, was condensed using a glass condenser chilled to 1 °C and connected to a 4-L flask. The retentate condensate was then transferred by peristaltic pump (activated by an optical level sensor) either back to the feed liquid source (drum or tank) or to a small container during collection events. The retentate condensate collection reservoir was vented to the room through a second 1 °C condenser coil. 1034

dx.doi.org/10.1021/ie2024917 | Ind. Eng.Chem. Res. 2013, 52, 1033−1041


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Temperature and pressure gauges were located throughout the system, including gauges to measure the pressure and temperature of the feed vapor and retentate vapor as well as conditions at the top and bottom of the stripping column. The system was operated in two modes, referred to as phase 2 and phase 3. In phase 3, depicted in Figure 1, the permeate vapor was returned to the bottom of the stripping column in the same pipe as the added steam. In this case, the permeate vacuum was generated ultimately by the scroll pumps operating on the overhead vapor stream from the column. In phase 2, depicted in Figure 2, the permeate vapor was not returned to the

Article

ANALYTICAL METHODS

Ethanol concentrations in the feed liquid, bottoms liquid, and permeate vapor condensate were determined using a gas chromatograph (GC, Agilent 6890) equipped with a Flame Ionization Detector (FID). Ethanol and water were quantitated in the feed vapor and retentate vapor condensates using a thermal conductivity detector (TCD). For experiments with the fermentation broth, all five samples were subjected to an additional set of analytical procedures to measure the concentration of the target analytes. Based on earlier scoping experiments and analyses with a similar broth as well as commonly observed yeast fermentation byproducts, the compounds listed in Table 1 were identified as target analytes. Table 1. Compounds Monitored in MAVS Samples from Experiments with S. cerevisiae Fermentation Broth organic acids

Figure 2. Schematic diagram of phase 2 experimental configuration in which permeate vapor is condensed in vacuum system and not returned to stripping column.

alcohols

esters

ketones and aldehydes

other

acetic acid

ethanol

ethyl acetate

acetone

glucose

succinic acid

1-butanol

ethyl octanoate

butyraldehyde

water

butyric acid

amyl alcoholsa

butyl acetate

furfural

1-octanoic acid

2-propanol

ethyl butyrate

L-malic

1-propanol

amyl acetatesb

acid

propanoic acid

1-hexanol

1-decanoic acid

furfuryl alcohol glycerol tert-butanol

a

Isoamyl alcohol and active amyl alcohol isomers coeluted and were quantitated together. bIsoamyl acetate and active amyl acetate isomers expected to coelute.

stripping column. Instead, the permeate vacuum was generated using a condenser/liquid ring vacuum pump system chilled to 2 °C. The phase 2 permeate pressure was controlled using an automatic upstream pressure controller and vacuum gauge (Barocel Manometer 658 AB, Edwards, USA). During an “experiment”, the process conditions were held constant and the liquid reservoirs collecting process streams were drained at specific time intervals to determine the rate of accumulation and composition of the process streams. Each experiment consisted of three 1−2 h successive collections. Between experiments, the system operated continuously and all the streams from the system were recycled back to the feed source. The feed composition changed little during an experiment and only slightly between experiments as steam addition slowly added water to the solution. Dilution in phase 2 experiments was larger due to the higher steam addition rates. To maintain relatively constant ethanol concentrations in the feed stream, fresh ethanol was added or a portion of the stripped bottoms stream was discarded between experiments, except during broth experiments. The unit was allowed to operate overnight after changing operating conditions, usually the steam addition rate, to ensure steady state had been reached, although only 2−3 h was necessary. The collected liquid samples were weighed using electronic balances with 6- or 50-kg capacities and 0.1-g precision. A feed sample was acquired each time the liquid reservoirs were emptied. Results for each experiment represent the averages of the three collection periods and duplicate analyses of each of the five process samples per collection. Error bars, when shown, represent 95% confidence intervals. Duplicate experiments were performed periodically to ensure the same steady state conditions and performance could be achieved after other experiments had been performed.

This list covers a range of volatile, semivolatile, and nonvolatile species generated by the yeast or which might be present in the fermentation media. Acetic acid, propanoic acid, butyric acid, 1-octanoic acid, 1-decanoic acid, and furfuryl alcohol were analyzed with a GC mass spectrometer (GC/MS) (Agilent 6890/5973). The sum of the acid and acetate species of an organic acid was determined by acidifying the samples with dilute aqueous formic acid. Acetic acid analytical results for membrane vapor streams were limited due to interference from analyte carryover from previous samples. Only acetic acid values from vapor stream samples run after a blank have been included. Acetic acid analyses for stripping column feed and bottoms liquids were not affected. Headspace GC/MS was used to quantitate the following volatile alcohols, esters, and aldehydes in the fermentation broth samples: acetone, 1-butanol, tert-butanol, butyl acetate, butyraldehyde, ethyl acetate, ethyl butyrate, ethyl octanoate, furfural (furan-2-carbaldehyde), 1-hexanol, isoamyl and active amyl acetates (3-methyl-1-butyl acetate and 2-methyl-1-butyl acetate), isoamyl and active amyl alcohols (3-methyl-1-butanol and 2-methyl-1-butanol), isobutanol (2-methyl-propan-1-ol), 1-propanol, and 2-propanol. Samples were diluted into salted water to ensure uniform and significant volatilization of the analytes into the headspace. Detection limits were 10 μg/L or lower for most target analytes. Active amyl alcohol and isoamyl alcohol isomers were not chromatographically resolved. In addition, active amyl acetate and isoamyl acetate isomers would not be chromatographically resolved with the methods employed. 1035



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liters of medium was inoculated with 600 mL of active budding yeast suspension (1% inoculum, corresponding to 5 mg/mL of dry weight cells). Samples were taken aseptically at regular time intervals into clean, heat-sterilized, 20-mL sample vials. Analytical samples were filter-sterilized using 0.45-μm PTFE syringe filters (Gelman Acrodisc, Pall Corporation, USA) and 5-mL sterile syringes (BD Diagnostics, USA). Fermentation was terminated after 36 h and ethanol was analyzed by GC/FID prior to downstream processing of the spent medium. Before removal from the fermentor, the broth was chilled to 2 °C to suspend biological activity. Upon removal from the fermentor, the broth was subjected to cell separation using a high-speed disk centrifuge (model TA-05, GEA Westfalia Separator GmbH, Germany) at 12 000 rpm at a flow rate of 0.5 L/min. Centrifugation was employed here to make the broth stable during storage and because the experimental vapor stripping column was not designed to handle solids. In an actual MAVS installation, the vapor stripping column would be very similar to the beer column in a conventional distillation system and, thus, designed to handle the solids present in the fermentation product. Cell-free broth was collected in 5-gal carboys and stored at 4 °C. Four 60-L batches of broth were required to carry out the vapor stripping broth experiments. More detailed fermentation broth procedures are provided in Supporting Information.

Glucose was analyzed by ion chromatography (IC) (Dionex 2500/PA-1 column, Sunnyvale, CA) with pulsed amperometric detection using a gold working electrode. Malic and succinic acids were quantitated by enzymatic assay (Megazyme, Ireland) with UV−vis spectrophotometric measurement (Hach DR4000, Loveland, CO). More detailed descriptions of the analytical methods are provided in Supporting Information. Membrane performance is reported in terms of molar gas permeance (ΠiG) and molar permselectivity of compound i relative to compound j (αij), which were calculated based on the flux of species through the membrane and the partial pressure driving forces as follows:

ΠG i =

αij =

Ji MW( i pi̅

feed

− pi̅

perm

)

[ = ]kmol/m 2· s· kPa (1)

ΠG i ΠGj

(2)

where Ji is the mass flux of species i, MWi is molecular weight, p̅ifeed is the log-mean average feed-side partial pressure of i (based on feed and retentate concentrations), and pi̅ perm is the average partial pressure of i on the permeate side of the membrane. Common units for permeance are gas permeation units (GPU) where 1 kmol/m2·s·kPa = 2.99 × 109 GPU and 1 GPU = 10−6 cm3(STP)/cm2·s·cmHg.

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RESULTS AND DISCUSSION The system was operated continuously with the same membrane module for 74 days. For the first 57 days, the feed liquid to the stripper was an ethanol−water system. During most of that time, a laboratory-prepared ∼5 wt % ethanol solution contained in the 1000-gal tank was the feed solution to the stripping column. The yeast ethanol broth was the feed for 5 days during that time. Stored broth was transferred to the 55-gal feed drum on the 22nd day of operation to initiate the broth tests. Phase 3 experiments were performed with the broth on days 23 and 24, and phase 2 experiments were performed on days 25 and 26. A picture of the different samples from a phase 3 experiment is provided in Figure 3. The feed source was switched back to the 1000-gal tank after the second phase 2 broth experiment. With the laboratory prepared ethanol−water feed solution, a handful of experiments were performed at a

FERMENTATION BROTH PREPARATION Saccharomyces cerevisiae ATCC 4126 (American Type Culture Collection, USA) was revived in sterile YPD (yeast extract, peptone, dextrose, from Sigma Chemical Company) medium and periodically subcultured to maintain strain viability and purity. Overnight (O/N) culture was prepared by growth of cells at 30 °C. Growth was initiated by aseptically transferring 1 mL of O/N culture to 100 mL of the medium (1% inoculum). Seed culture for pilot-scale fermentation was prepared by transferring 2 mL of active cell suspension to 200 mL of YPD medium. Pilot-scale fermentations were carried out in an 80 L fermentor (Bioflo 5000, New Brunswick Scientific, USA) using commercial chemicals: bulk dextrose monohydrate (Ferro Pfanstiehl, USA), yeast extract (Biospringer, France), and hydrolyzed soy peptone (Marcor Corporation, USA). Sixty

Figure 3. Samples obtained during first phase 3 experiment with fermentation broth. 1036



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exceeded 90 wt % for most experiments while the permeate vapor generally contained below 5 wt % ethanol. Unlike the feed vapor concentration, both retentate and permeate vapor ethanol concentrations decreased with increasing feed flow rate to the membrane module. For a fixed feed concentration, a higher flow rate translates into a shorter residence time in the module and less water removal relative to the amount of ethanol moving through the membrane module and leaving in the retentate. Thus, the water concentration leaving the module in the retentate is higher. The higher retentate water concentration (and lower retentate ethanol concentration) increases the feed-side water partial pressure, which drives higher water flux and lower ethanol flux through the membrane, resulting in a lower permeate ethanol concentration. The retentate and permeate concentrations for the broth experiments are shown in Figure 4 as open symbols and followed the same trends as those for the binary solution. The ethanol and water molar gas permeances and ethanol− water molar permselectivities were calculated using eqs 1 and 2. The results are shown in Figures 5 and 6 for both the binary

higher feed liquid flow rate to achieve a higher overhead vapor flow rate without reducing the overhead vapor concentration. Several experiments were performed at 50 and 90 Torr permeate pressure to assess the effect of this parameter on module performance. A butanol−water system was studied for the last 17 days of operation. Experiments were halted with this particular membrane module when a system shutdown for facility maintenance resulted in a step change decrease in membrane selectivity. One measure of stripping column performance is the extent of removal of a compound as calculated by comparing the mass of a compound entering in the feed liquid to that leaving in the bottoms liquid effluent, indicating transfer to the overhead vapor stream and on to the membrane module. This stripping efficiency is dictated primarily by the VLE behavior of the compound in the aqueous system. For a given steam stripping column and for a specific volatile compound, removal depends most significantly on steam addition rate, feed liquid flow rate, feed concentration, and feed temperature. When the feed stream conditions are also fixed, the only variable is steam addition rate. In phase 2 operation, with a ∼5 wt % ethanol feed stream and a stripper feed rate of 145 g/min, ethanol removal in the stripping column ranged from 63.5 to 98.5% as steam addition rate increased from 12.25 to 20.65 g/min. Up to about 90% recovery of ethanol in the overhead vapor, the ethanol concentration in the overhead vapor was relatively unaffected by recovery or the flow rate of the overhead vapor. However, as recovery was pushed beyond 90%, the concentration of ethanol in the overhead vapor did decrease slightly, indicating the stripping column lost efficiency above 90% ethanol removal. For the two phase 2 yeast broth experiments, ethanol removal was 91.3 and 96.4% for 15.34 and 17.23 g/min steam rate, respectively. The ethanol concentration in the feed vapor to the membrane module is shown in Figure 4 as a function of feed

Figure 5. Effect of feed-side water partial pressure (log-mean average) on the permeance of ethanol and water through the membrane for all experiments performed with 150 Torr permeate pressure (1 Torr = 0.13 kPa). Closed symbols represent results with binary ethanol−water feed solution, and open symbols represent fermentation broth feed to stripping column.

Figure 4. Effect of membrane feed vapor flow rate on the concentration of ethanol in the feed vapor, retentate vapor, and permeate vapor streams from membrane unit for all experiments performed with 150 Torr permeate pressure. Closed symbols represent results with binary ethanol−water feed solution, and open symbols represent fermentation broth feed to stripping column.

vapor flow rate. The feed vapor ethanol concentration was fairly constant over all experiments with the laboratory-prepared ethanol solution, averaging 35.9 wt %. As the ethanol−water vapor passed through the membrane module, water preferentially permeated the membrane, yielding a retentate vapor concentrated in ethanol and a permeate vapor depleted in ethanol relative to the feed vapor. As shown in Figure 4, the concentration of ethanol in the retentate vapor

Figure 6. Water−ethanol molar permselectivity (αw/e) calculated from permeance results shown in Figure 5.

ethanol−water mixture and broth experiments as a function of the log-mean average feed-side partial pressure of water. In earlier experiments with a similar module, the log-mean feedside water partial pressure was determined to be a better 1037



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extent of removal error prone. Nevertheless, it can be concluded the esters and simple alcohols were removed at least as well as ethanol (at least about 90% removal) as would be expected from VLE behavior. In addition, organic acid removal was about an order of magnitude lower than that of ethanol, again, in keeping with VLE behavior and partial deprotonation of the acids at the broth pH of 4.3 (acetic and butyric acid pKa values are 4.76 and 4.83, respectively). The overhead vapor from the stripping column (stream 3) was enriched in esters, ethanol, and other simple alcohols, as shown in Figure 8. Whereas the top three secondary compounds

predictor of water permeance than other process parameters. This earlier observation agreed with the reported large effect of water concentration on the ethanol and water permeances and selectivity of cellulose ester membranes.22 However, due to the high stage cut (∼80 mol % of feed was permeated) and relatively constant overhead vapor concentration, the log-mean average feed-side water partial pressure in the membrane module was found to be a strong function of the feed vapor flow rate (linear fit R2 =0.94). As a result, the feed vapor flow rate would have given a similar trend if used as the independent variable in Figures 5 and 6. Water permeance rose with log-mean average feed-side partial pressure of water, likely the result of a number of factors, including swelling of the selective layer due to the higher average feed-side water concentration, less feed-side concentration polarization due to higher flow rate, and less membrane area that has flux dictated by the feed/permeate pressure ratio limitation. By contrast, ethanol permeance was unaffected by the log-mean average feed-side partial pressure of water. As a result, the ratio of these two permeances, the molar permselectivity, increased with increasing log-mean average feed-side partial pressure of water, from 60 up to 140, as shown in Figure 6. For all of these parameters, the values obtained with the fermentation broth were within the range of those calculated from the binary ethanol−water solution data. The removal of broth components other than ethanol in the stripper was higher or lower than ethanol, depending on the relative VLE behavior. Feed liquid concentrations and MDL values for those broth compounds measured above MDL, about half of the analytes listed in Table 1, are shown in Figure 7 for

Figure 8. Concentration of analytes detected in the membrane vapor feed stream during the four broth experiments. Amyl acetates method detection limit was 1.1 × 10−5 wt%. Butyric acid was not observed above MDL in the feed vapor, but is included here because it was quantified in the retentate vapor (Figure 9).

in the broth were acetic acid, succinic acid, and amyl alcohols, the top three secondary compounds in the overhead vapor were amyl alcohols, isobutanol, and ethyl acetate, which was also true for the retentate vapor (Figure 9). Comparison of Figures 8 and 9

Figure 7. Concentration of analytes in the feed liquid to the stripping column for experiments with S. cerevisiae fermentation broth. Experiments 1 and 2 were in phase 3 mode while experiments 3 and 4 were in phase 2 mode. Detection limit for glycerol was 3 × 10−3 mg/L.

each of the four broth experiments. The MDLs are shown in Figure 7. About half of those shown in Figure 7 were at or near their MDL. After passing through the stripping column, the broth organic compounds, except ethanol, fell into two categories: (1) esters and simple alcohols were removed to below their MDL and only the minimum degrees of removal could be calculated and (2) organic acids, glycerol, and glucose were poorly removed. For those compounds which did not strip well, the concentrations in the bottoms stream were similar to those in the feed liquid, making calculation of an

Figure 9. Concentration of analytes detected in the membrane retentate vapor stream during the four broth experiments. Amyl acetates method detection limit was 1.3 × 10−5 wt%.

indicates that all of the organic compounds were enriched by about a 3-fold ratio in the retentate vapor relative to the feed vapor. This enrichment was accomplished by the preferential and robust permeation of water through the membrane, which 1038



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resulted in a molar stage cut approaching 80%. Such high stage cuts are generally undesirable for assessing membrane performance because of the large change in water partial pressures experienced by the membrane and potential to change from a permselectivity controlled regime to a pressure-ratio-limited regime.23 Nevertheless, performance of the membrane module can be assessed based on the rejection of species by the membrane and/or on the permeation of species through the membrane, particularly with respect to ethanol. Due to rejection of organic compounds by the membrane, the permeate vapor was dilute in most organic species, with only a few above the MDL, making calculation of membrane permeance impossible for most species. The fraction of secondary organic compounds in the feed vapor retained in the retentate vapor, relative to the fraction of ethanol retained, averaged over the four broth experiments, is shown in Figure 10. Relative retention averaged 0.99 for the Figure 11. Ratio of the permeances of compounds quantitated in all membrane streams of fermentation broth experiments to the permeance of ethanol. *Data sets for acetic acid and 1-propanol were limited. Water not shown for clarity of scale.

supported by the observation from Figure 5 that ethanol permeance was independent of feed-side water partial pressure while that of water trended upward by a factor of 2 as feed-side water partial pressure increased. In this case, water was also transported through the nonselective pathways, but that partial flux would be swamped by water transport through the hydrophilic cellulose ester material. Such nonselective pathways could be pinholes in the cellulose ester layer, which were covered over by the silicone rubber coating. Silicone rubber, while often viewed as a hydrophobic material, is relatively unselective for polar organic compounds relative to water. For example, the molar permselectivity of silicone rubber for ethanol relative to water has been shown to be less than 1, with 0.6 and 0.8 reported for ethanol,24,25 while that for acetone and 1-butanol relative to water were 0.6 and 1.8,25 respectively, when removing these compounds by pervaporation from dilute aqueous solution. The ethanol−water permselectivity of silicone rubber was 1.1 when the pervaporation feed was an 80/20 w/w ethanol/water mixture.26 An unprotected pinhole or other unobstructed defect would likely lead to a much higher ethanol permeance than was observed. In our previous papers on this technology, we reported on the fuel equivalent energy required to recover and dehydrate ethanol from dilute aqueous solution using the MAVS process and compared it to literature values for a distillation−molecular sieve system.16,18 In Figure 12, this energy, per unit ethanol recovered, is plotted as a function of ethanol concentration in the feed solution to the separation system. The most important parameters in the calculation of fuel equivalents are the efficiencies assumed for converting fuel to heat, fuel to electricity, and electrical energy to compressor work, 0.9, 0.33, and 0.75, respectively. Following the same protocol, the fuel equivalent energy required for the binary ethanol−water mixtures and the four broth experiments was calculated. Details for the MAVS calculations are provided in Supporting Information while those for the MAVS simulations are provided in our earlier publication.18 Phase 3 results were used to calculate the sum of the thermal- and compressor-related energy for the MAVS process. Recovery of energy from retentate condensation in heat exchange with the reboiler was credited for MAVS systems. Steam required for both steam stripping and MAVS systems

Figure 10. Fraction of each compound in the feed vapor that exits module in retentate vapor relative to the fraction of ethanol retained. Average and 95% confidence limits for all fermentation broth experiments are shown.

organic compounds and only ranged from 0.84 to 1.23, indicating these species were retained to the same relative extent as ethanol. There was no apparent difference between types of organic compoundsesters, ketones, alcohols, and acids all behaved similarly. For reference, the ratio of water retained relative to ethanol retained was 0.0445. The ratio of the permeances of secondary organic compounds measured in the permeate vapor to the permeance of ethanol, averaged over the four broth experiments, is shown in Figure 11. The species in Figure 11 represent a subset of those in Figure 10. As with the relative retention shown in Figure 10, the organic species exhibited the same permeance as that of ethanol, with an average permeance ratio of 0.97 and a range of 0.85−1.2. The data bar for water is not shown in Figure 11 for reasons of scalethe water/ethanol permeance ratio (the permselectivity) averaged 106 for these same experiments. The fact that all the organic compounds were rejected by and/or permeated the membrane similarly to ethanol leads to one of two conclusions: (1) the selective layer of the membrane had the same permselectivity for all organic species or (2) transport of organic species was predominantly via nonselective pathways while transport of water was predominantly through the selective material of the membrane. This latter conclusion is 1039



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typically have higher values than the primary ethanol product, although they could be retained in the fuel ethanol.

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CONCLUSIONS The large changes in feed-side water partial pressure and molar flow rate along the length of the membrane module and the availability of only the average permeate concentration limited the potential to tease out finer details in module performance. Nevertheless, general conclusions can be made regarding ethanol− water separation performance and the fate of secondary broth compounds. First, water permeance increased with increasing feed-side water partial pressure while ethanol permeance was unaffected by process variables. As a result, water−ethanol permselectivity increased with feed-side water partial pressure. Second, since the ethanol and water permeances for the fermentation broth experiments overlapped the ranges observed with the binary ethanol−water solutions, there was no noticeable effect of the broth on membrane performance. Third, all of the secondary organic compounds measured in the feed and retentate vapors were retained to the same relative extent as ethanol (Figure 10). Fourth, all of the secondary organic compounds measured in the permeate vapor had the same permeance as ethanol (Figure 11). If nonselective pathways control organic compound permeation, then similar membranes, which lack such pathways, would have similar water permeances, but significantly lower organic compound permeances. In that case, one might also observe different permeances for the different organic species.

Figure 12. Comparison of energy required to recover and dehydrate ethanol from dilute aqueous solutions as a function of ethanol concentration in the solution. Distillation + molecular sieve and MAVS simulation values from literature references.5,7−11 Energy usage for representative MAVS experiments with binary ethanol−water solutions and the four broth experiments also shown. Energy is reported in fuel equivalents, accounting for conversion of fuel into heat and electricity energy.

were corrected for heat losses from the column. The calculated energy values are shown in Figure 12. Results obtained with the binary ethanol−water mixtures with ethanol recoveries of 90% were averaged together and are shown as single data points, one each for phase 2 and phase 3 experiments. Each of the four broth experiments are shown as individual points in Figure 12 because of the changing ethanol concentration in the broth during the course of the broth experiments. The energy calculated for the phase 2 steam stripping experiments, whether binary or broth feeds, fall within the predicted band for distillation-molecular sieve systemswhich is also the expected range for simple fractional evaporation of ethanol from the solution.18 The energy usage calculated for the phase 3 MAVS experiments was slightly above that predicted for a MAVS system from chemical process simulations. Energy usage per kg ethanol recovered in the broth tests was slightly higher than that for the binary ethanol−water mixture experiments. The higher energy usage for the broth experiments is reasonable considering the rapid rise in energy usage predicted for both MAVS systems and distillation systems below 5 wt % ethanol and the lower ethanol concentrations in the broth, particularly for the later phase 2 experiments. It can be concluded there was no significant difference between the predicted energy values and those based on experimental observations. With the MAVS configuration evaluated in this work, organic species stripped from the fermentation broth were concentrated by the vapor permeation membrane, exiting with ethanol in the retentate vapor. In the conventional two-column distillation system, the first column would act in the same manner as the MAVS vapor stripping column. However, the second column, the rectifier, can be designed and operated to separate some of the secondary fermentation products from ethanol, if desired or necessary for column performance.27,28 In particular, a side draw from the rectification column can be used to withdraw a minor stream concentrated in higher alcohols (i.e., propanols, butanols, amyl alcohols), sometimes termed “fusel oil”, which

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ASSOCIATED CONTENT

S Supporting Information *

Additional details of the analytical and fermentation procedures as well as of the energy usage calculations for MAVS experiments. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 1-513-569-7799. E-mail: [email protected].

ACKNOWLEDGMENTS We thank Don Schupp and Jill Webster of Shaw Environmental & Infrastructure, Inc. for logistical and analytical support. This work was part of a Cooperative Research and Development Agreement (CRADA) between the USEPA and Membrane Technology and Research, Inc. Mention of trade names or commercial products does not constitute an endorsement or recommendation for use. L.M.V. thanks Richard Baker for fostering a collegial collaborative atmosphere, for promoting sound, practical approaches to problems, and for reminding us of why we were attracted to the research and development enterprise in the first place.

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REFERENCES

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