Conceptual Design of Methyl Ethyl Ketone Production via 2,3

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Conceptual Design of Methyl Ethyl Ketone Production via 2,3-Butanediol for Fuels and Chemicals Daniel Penner, Christian Redepenning, Alexander Mitsos, and Joern Viell Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03678 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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Industrial & Engineering Chemistry Research

Conceptual Design of Methyl Ethyl Ketone Production via 2,3-Butanediol for Fuels and Chemicals †

Daniel Penner, Christian Redepenning, Alexander Mitsos, Jörn Viell* Aachener Verfahrenstechnik - Process Systems Engineering, RWTH Aachen University, Turmstr. 46, 52064 Aachen, Germany

Keywords: methyl ethyl ketone; 2-butanone; 2,3-butanediol; acetoin; isobutyraldehyde; process design;

Abstract: Methyl ethyl ketone (MEK, also 2-butanone) is both a common chemical and potentially a promising fuel. It can be produced biotechnologically via 2,3-butanediol (2,3-BD) followed by dehydration with the by-products acetoin and isobutyraldehyde (IBA). This article presents a process development including thermodynamic modeling of the separation steps. Alternatives are analyzed for energy efficiency including heat integration as a first measure to assess viability but also the effect of capacity. The efficiency largely depends on the separation technology for 2,3-BD and its efficiency. Hybrid separation strategies show a total product yield of nearly 38 wt-% at a specific primary energy demand of 0.25 MJ/MJMEK. The competitiveness to bioethanol and the low degree of heat integration make the concept promising for small-scale

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production with acetoin and IBA as high-value products. Efficient production mandates further research on simplification and realization of 2,3-BD separation integrated with fermentation, dehydration and management of dilute and concentrated ions.

1.

INTRODUCTION

The context of bioeconomy prompts the design of biorefinery processes for sustainable supply of biofuels and chemicals. Methyl ethyl ketone (MEK), also termed 2-butanone, is currently a common industrial solvent that is so far synthesized from fossil C4-raffinates.1 Nevertheless, MEK was recently identified to be a promising fuel for spark ignition engines. Achieving the same engine efficiency as ethanol, MEK further offers a higher heat of combustion, lower hydrocarbon emissions, less oil dilution and better cold-start properties.2 Hence, MEK is a promising candidate in the supply for chemicals or biofuels. While the chemical application is well established, a future biofuel application should be competitive to existing biofuels regarding the energy balance. As biobased MEK production is not yet established, this open issue could be clarified only by process design of MEK production from a biorenewable source. The differences in availability in comparison to fossil raw materials will however favor smaller production capacities than established large-scale processes.3 Biobased MEK production will have to compete against these established production routes thus urgently demanding for a careful conceptual process design to benchmark MEK production efficiency. All suggested biotechnological MEK conversions in literature start from sugars as raw material. The conversion of purified sugar will clearly be a best-case assumption for a lignocellulosebased biorefinery to demonstrate the yield as an upper limit. Nevertheless, the reported yields


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differ largely. Chemo-catalytic routes via decarboxylation of levulinic acid have not yet reached relevant yields of MEK but produce acetic acid and acetone as by-products.4 The direct fermentation of sugar to MEK would be very appealing but disappoints with very low yield of only 0.004 gMEK/gglucose.5 The industrially-established production of MEK via dehydrogenation of 2-butanol points to the biotechnological production of 2-butanol. Although few patents disclose such a route,6,7 the same low yield as with the direct fermentation disqualifies this route as well. A more promising intermediate is 2,3-butanediol (2,3-BD). Its fermentation is known for almost a century,8,9,10,11 which was evolved to titers as high as 150 g2,3-BD/L and yields close to the theoretical limit of 0.5 g2,3-BD/gglucose. In principle, the 2,3-BD conversion into MEK can be realized by hydrogenation to 2-butanol12 and dehydrogenation to MEK as in the previous pathway or via direct dehydrogenation. While the former route via 2-butanol requires additional hydrogen and gives low yields of 0.115 gMEK/gglucose, the direct dehydration of 2,3-BD into to MEK is reported with yields up to 95 mol-%.8,13 2,3-BD is also an interesting intermediate in biorefinery concepts as it can also be dehydrated into butadiene.14 The two separate steps of fermentation and dehydration are known very well.8,13,14 However, the integrated conversion process has not been addressed in literature. Despite the promising yields for the 2,3-BD fermentation, it is known to be limited by the separation of 2,3-BD from the fermentation broth. Numerous laboratory concepts have been developed over the years15 but the particular characteristics of these improvements have not been assessed on process level. Additionally, by-products like acetoin and isobutyraldehyde (IBA) are valuable products,16,17 which can increase valorization in a biorefinery based on 2,3-BD. The prospect of biobased MEK and its by-products thus depends on the assessment of these production routes, which is not available in literature in terms of process performance.


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The performance of a process concept in general is closely related to its efficiency. The material efficiency is clearly the most important parameter in this regard. It is determined by the reaction itself and the separation technology. The energy demand as the second performance parameter could be eventually tackled by heat integration in an industrial process, but it may conflict smallscale realization of biorefineries.3 As heat exchange equipment is expensive, such concepts lack economy of scale at the advantage of flexible integration for fuels and energy in local value chains. Clearly, the diversity of processing routes with novel engineered materials complicates comparable costing at this early stage. Alternatively, a precise analysis of the conceptual designs for low primary energy demand thus seems to be a viable measure to screen for promising pathways of biobased MEK production via 2,3-BD. This way, it can be assessed whether the produced MEK can be envisioned as a chemical or even as a fuel, in case it shows a net energy balance comparable to established biofuels. In this contribution, we present a systematic investigation of process alternatives for efficient bio-based MEK production. As the available concepts are at an early stage of development, we chose sugars as representative intermediate from lignocellulosic biomass to evaluate feasibility. Section 3 presents several developed conceptual processes based on literature data to screen for the most efficient processing pathway. These alternative process concepts are then evaluated in Section 4 according to yield and primary energy demand. Necessary and feasible improvements of biotechnological MEK production in a biorefinery are discussed in view of the most promising processing options to reach competitiveness in bioeconomy.


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2.

METHODS

A systematic process development identifies four steps of MEK processing from sugars (Figure 1). The block flow diagram shows first the fermentation of glucose to 2,3-BD followed by separation of 2,3-BD from the microorganisms, water and acetoin. After dehydration to MEK in the third step, the MEK is separated from IBA and water, and finally purified. Any unconverted substrate in the conversion steps has to be recycled for efficient processing.

water acetoin microorganisms

air, CO2

water IBA

water, glucose microorganisms Fermentation

Separation of 2,3-BD

Separation of MEK

Dehydration

air

MEK

2,3-BD

Figure 1. Block flow diagram of MEK production by fermentation and dehydration with separation steps and the considered streams.

The fermentation step is the first process step. Microorganisms ferment glucose into 2,3-BD, CO2 and acetoin. The concentrations are chosen according to the work of Ma et al.18 with 150 g2,3-BD/L after fed-batch fermentation with K. pneumoniae at a yield of 0.40 g2,3-BD/gglucose. Near theoretical yields close to 0.5 g2,3-BD/gglucose can be achieved at a lower titer (i.e., 95.5 g/L8). The fermentation also produces acetoin at a concentration of 10 gacetoin/L.18 The growth of microorganisms and substances 90% in a single extraction.52

S1_hphilSOEDI ethanol K2HPO4

ethanol/water - azeotrope water, ethanol, 2,3-BD, (acetoin) water

SOE water, 2,3-BD, acetoin

back to fermentation

DI1

water, K2HPO4, ethanol (2,3-BD, acetoin)

DI2

acetoin DI3

2,3-BD


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Figure 6. Salting-out extraction process concept with the hydrophilic ethanol/K2HPO4 system (hphilSOE) and separation of the extract phase by distillation (DI) for 2,3-BD separation.

The SOE process concept with ethanol/K2HPO4 as suggested by Jiang et al.48 is shown in Figure 6. The fermentation broth is mixed with ethanol (24 wt-%) and K2HPO4 (25 wt-%), which improves the distribution coefficient K2,3-BD = 34.6 and enables almost quantitative 2,3-BD recovery from the top phase48 modeled here with 98% recovery. The top phase is separated in three distillation columns, whereby the ethanol/water azeotrope is recycled back to the SOE unit. Replacing the hydrophilic solvent with a hydrophobic solvent results in a decrease of the distribution coefficient but increases the separation factor (i.e., K2,3-BD = 2-7 and S2,3-BD/water = 458 with dipotassium hydrogen phosphate (K2HPO4) and n-butanol53). Yields above 80% in single-stage extraction are reported.49,54 The process concept is similar to the previous one with ethanol but enables recycling of n-butanol in a decanter (S1_hphobSOEDI in Figure 7). As in the S1_ROEXDI process concept, n-butanol extraction takes place in a multiple stage extraction column but adding 10 wt-% of K2HPO4 to the feed increases the distribution coefficient to K2,3-BD = 2.26. The yield for 2,3-BD and acetoin recovery is 99%.50 The saturated nbutanol/water stream is recycled back to the extraction column.


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n-butanol, water, (acetoin)

S1_hphobSOEDI n-butanol, water, 2,3BD, acetoin

water, 2,3-BD, acetoin

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acetoin DI1

SOE

DI2

K2HPO4

n-butanol 2,3-BD, acetoin

back to fermentation

water, K2HPO4, (2,3-BD, acetoin, n-butanol)

2,3-BD

water, (n-butanol, acetoin)

Figure 7. Salting-out extraction process concept with the n-butanol/ K2HPO4 system (S1_hphobSOE) and separation of the extract phase by distillation (DI) for 2,3-BD separation.

Alternative approaches that might become feasible but are not yet established shall be mentioned very briefly. ATPE with PEG/dextran55,56 shows only minor increases in the distribution coefficient (K2,3-BD = 1.15) and even a decrease of the separation factor (S2,3-BD/water = 1.03) compared to solvent extraction. Salting-out without a solvent, also referred to as repulsive extraction, is carried out with similar salts as in SOE. Recoveries up to 97% have been reported in the presence of high salt contents of approximately 50 wt-%.57,58 The large amounts of salt however require a concentrated 2,3-BD stream.45,15 Furthermore, a yet unexplored issue is salt management at such high concentrations (removal of fermentation by-products and loss of salt via the top phase).45 A rather novel approach is the separation of 2,3-


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BD by adsorption in a modified ZSM-5 zeolite.59 Desorption was realized by diethyl ether at ambient temperature. A high selectivity was found, but this concept lacks too much data for a conceptual design at this point.

Conceptual design of MEK separation

After dehydration, the product consists of MEK, water and IBA, which have to be separated to yield the pure components (Figure 1). The product stream at the exit of the dehydration reactor is 65 wt-% MEK, 18 wt-% water, 10 wt-% 2,3-BD and 7 wt-% IBA. 2,3-BD has to be recycled to avoid carbon loss. MEK and IBA are evaporated with similar boiling temperatures (Tb,MEK = 79.3°C, Tb,IBA = 64.3°C). The ternary diagram of MEK/water/IBA (Figure 8) shows two temperature-minimum azeotropes in the binary mixtures of MEK/water and IBA/water at atmospheric pressure. The distillation boundary (shown as a dashed line in Figure 8) complicates the separation of pure MEK from a dilute solution. The distillation boundary can be crossed by the liquid-liquid equilibrium upon addition of IBA to the binary mixture of water/MEK. Hence, the miscibility gap of the organic products with water can be utilized for separation in a sequence of distillation columns and liquid-liquid phase separation units.


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Water (100 °C)

DI1

Feed

distillation boundary

Water/MEK - azeotrope (73.7 °C) Water/IBA - azeotrope (60 °C) DI3

DI4 IBA (64.3 °C)

MEK (79.3 °C) mixture mass balance

Figure 8. Ternary diagram of water, methyl ethyl ketone (MEK), isobutyraldehyde (IBA) with boiling points of the pure components and the azeotropes, the miscibility gap with tie-lines at dew point and the distillation boundary at p = 1.013 bar (dashed line). The colored lines show the mass balance lines of distillation sequence S2_DS3 after 2,3BD has been separated.

Four promising sequences in terms of efficiency are shown in Figure 9. In all of these sequences, the high boiling 2,3-BD leaves the first distillation column as the bottom product with a second column (DI2) to separate surplus water. Except for sequence S2_DS2, the distillate of the first column is split in a decanter, with a top phase of MEK that also contains IBA and some water. Without a sharp split between water and IBA/water, the sequences S2_DS1 and S2_DS4 require five columns to separate MEK in a pure state, whereas the other two concepts are based on four columns. In sequence S2_DS3 lacks one column because a pure IBA stream is mixed with the distillate of the first column to break the distillation boundary). Except for concept S2_DS3, all other concepts show a small loss of MEK in the aqueous phase of the liquid-liquid separation.


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The last column is a simple flash leaving pure IBA as bottom product while the azeotropic mixture of IBA/water is recycled back to the decanter.

S2_DS1

S2_DS2

DI3 DI3

DI1

DI4

DI5

water, (IBA, MEK)

water, (IBA)

DI1

IBA

DI2

MEK

water

IBA water, (IBA)

2,3-BD DI4

DI2

water, (MEK) MEK

2,3-BD

S2_DS3

S2_DS4 M

DI5 DI3

DI1

water

DI1

DI4

IBA

DI3

water water, (IBA)

DI2

MEK 2,3-BD

IBA

water, (IBA, MEK)

DI2

DI4

water, (IBA) MEK

2,3-BD

Figure 9. Four distillation sequences for the separation step S2 for MEK and IBA separation from water and for the recycling of 2,3-BD. The distillation columns are numbered (DI1-DI4), while decanters are identified by the output streams (M = mixer).


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4.

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RESULTS & DISCUSSION

MEK separation The process yield is the result of both conversion steps in combination with the alternative separation sequences. As purified 2,3-BD is utilized as feed for the second reactor in the concept in Figure 1, the first step is to identify the best distillation sequence to purify MEK and IBA. The sequences show small differences in MEK yield in the order S2_DS3 > S2_DS1 > S2_DS4 > S2_DS2 (72.0 wt-%, 71.4 wt-%, 69 wt-%, 66 wt-%). These differences are due to the loss of MEK with the aqueous phase after phase splitting, which is circumvented only in sequence S2_DS3 by recycling of IBA to cross the distillation boundary (cf. Figure 8 and Figure 9). However, the yield of IBA is 8 wt-% in sequences S2_DS1, S2_DS2 and S2_DS4 but drops to 6.9 in S2_DS3. The recycling of IBA thus causes a much higher loss with the aqueous phase from the decanter. Hence, S2_DS2 is good at separating both MEK and IBA while S2_DS3 is preferably efficient in the separation of MEK. The SPED for MEK separation in the distillation sequences without heat integration are listed in Table 1. It is clearly lower in the distillation sequences S2_DS2 and S2_DS3 with four columns as opposed to the more complex sequences S2_DS1 and S2_DS4. In fact, the sequence S2_DS3 shows the lowest SPED of all designs with a reduction of the primary energy demand by 65% compared to distillation sequence S2_DS4. This demonstrates the advantage of only one azeotropic distillation (IBA/water azeotrope) in contrast to the distillation at both azeotropes in the other designs. Considering the rather small decrease in IBA yield in absolute numbers in comparison to this reduction in energy demand, S2_DS3 is selected as the most efficient sequence for MEK separation.


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Table 1. Specific primary energy demand (SPED) for MEK separation concepts per separated MEK mass flow (without heat integration).

Distillation sequence

SPED [MJ/kgMEK]

Distillation sequence 1 (S2_DS1)

16.6


10.7


4.8


17.0

2,3-BD separation and process efficiency With a chosen MEK separation sequence, the process yield is then determined by the 2,3-BD separation (Table 2). The simple distillation process concept S1_DI-S2_DS3 without any 2,3-BD loss benchmarks the highest possible yield of 0.288 gMEK/gglucose in this design. Replacing the distillation concept by reverse osmosis and extraction (S1_ROEXDI) results in a slightly reduced process yield of 0.283 gMEK/gglucose due to small 2,3-BD losses in the permeate of the reverse osmosis. This effect becomes much more dominant in case of 2,3-BD separation by extraction and pervaporation (S1_EXPVDI-S2_DS3) because the permeate of the second pervaporation unit cannot be recycled completely to maintain phase separation in the extraction column. The yield thus decreases to only 0.22 gMEK/gglucose, which is clearly less efficient than S1_DIS2_DS3. Reactive extraction with distillation (S1_RERDDI) and the salting out concepts (S1_hphilSOEDI-S2_DS3 and S1_hphobSOEDI-S2_DS3) perform similar to the concept using reverse osmosis and extraction (S1_ROEXDI-S2_DS3).


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Table 2. Process yield differentiated for the 2,3-BD separation concepts with regard to glucose (glc). Concept

Yield

Yield

[kgMEK/kgglc]

[kgacetoin/kgglc]

Distillation: S1_DI-S2_DS3

0.288

0.027

Reverse osmosis - extraction - distillation: S1_ROEXDI-S2_DS3

0.283

0.025

Extraction - pervaporation – distillation: S1_EXPVDI-S2_DS3

0.223

0.017

Reactive extraction and distillation: S1_RERDDI-S2_DS3

0.280

n/a

SOE with hydrophilic solvent: S1_hphilSOEDI-S2_DS3

0.287

0.027

SOE with hydrophobic solvent: S1_hphobSOEDI-S2_DS3

0.287

0.026

The acetoin yield from 2,3-BD separation concepts shows similar trends as the performance of MEK separation. While the SOE concepts show maximum yields as in the distillation concept, the losses in the permeate of the RO in S1_ROEXDI become noticeable. The extraction and pervaporation concept (S1_EXPVDI) is least efficient because acetoin also permeates the second pervaporation unit, which is then lost with the purge (cf. Figure 4). Furthermore, the retentate of the second membrane unit still contains n-butanol together with acetoin and 2,3-BD, which cannot be separated into pure components using a single distillation column. When providing pure 2,3-BD at the bottom, the acetoin condensed at the top contains ~16% of n-butanol in the acetoin stream. The concept would require an additional distillation unit for separation of acetoin and complete recovery of n-butanol.


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Table 3. Specific primary energy demand (SPED) of the different 2,3-BD separation concepts (without heat integration).

Process concept

SPED [MJ/kg2,3-BD]

Distillation (S1_DI)

20.2

Reverse osmosis - extraction - distillation (S1_ROEXDI)

17.7

Extraction - pervaporation - distillation (S1_EXPVDI)

21.6

Reactive extraction and distillation (S1_RERDDI)

15.7

SOE with hydrophilic solvent - distillation (S1_hphilSOEDI)

24.5

SOE with hydrophobic solvent - distillation (S1_hphobSOEDI)

14.1

The variance of the process energy demand with a chosen MEK separation is determined by the efficiency of the 2,3-BD separation (Table 3). The demand of the distillation concept (S1_DI) is moderate in comparison to the other concepts. While the pervaporation after extraction (S1_ROEXD) and the hydrophilic salting out (S1_hphilSOEDI) are less efficient per kg of produced 2,3-BD, the other concepts show a considerably lower energy demand. The extraction of 2,3-BD from a dewatered stream by n-butanol (S1_ROEXDI) decreases the SPED despite the low 2,3-BD concentration (9 wt-%) and the high water content (34 wt-%) in the extract phase. The solvent mixture still has a lower heat of evaporation in comparison to water. A selective removal of solvent and water was also the objective of the pervaporation in combination with extraction in concept “S1_ROEXPVDI”. Water and 8% of n-butanol are removed by the vapor of the first pervaporation and the second leaves a purified retentate with a residual n-butanol content of 1 wt-%. Nevertheless, 25% of the 2,3-BD and acetoin pass the membranes. The necessary purge stream is the reason for low yield and eventually the high


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energy demand. In fact, accounting the electrical energy for vacuum generation will further decrease the efficiency of pervaporation in this case. Hence, pervaporation might be good to remove volatiles from fermentation broth selectively but does not give promising options for 2,3BD separation so far. Clearly, solvent extraction of 2,3-BD is preferably combined with concentrated streams of low water content. Reactive extraction with subsequent reactive distillation (S1_RERDDI) shows a lower SPED than the process concepts S1_DI and S1_ROEXDI. The decrease is due to an even lower enthalpy evaporation. This evaporation energy is also the reason why the process concept S1_hphilSOEDI shows a higher SPED than the distillation process concept. Despite the high distribution coefficients. there is an even lower 2,3-BD concentration compared to the feed in the extract phase of ethanol and water. The heat demand for separating 2,3-BD and acetoin from water and ethanol is thus higher. Substantial water in the top phase therefore has to be avoided for energy efficiency in the subsequent distillation. This is realized with the hydrophobic solvent (S1_hphobSOEDI) showing the lowest energy consumption. As in the hybrid process concept S1_ROEXDI, the extraction achieves almost mass transfer in the extraction column but the added salt reduces the water solubility in the nbutanol phase which in turn decreases the energy consumption in the recovery column. A very low salt transfer to the top phase is of course a prerequisite, which would otherwise increase the separation effort require separation from the 2,3-BD (e.g., crystallization integrated in the distillation process48,60). Altogether, the hybrid process concept S1_ROEXDI-S2_DS3, the reactive extraction (S1_RERDDI-S2_DS3) and the salting out concept (S1_hphobSOEDI-S2_DS3) are most


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promising. Heat integration can further reduce the energy demand. The analysis of the other combinations of mentioned separation concepts with heat integration shows no change to the ranking depicted above (not shown for brevity). However, the energy demand of S1_ROEXDIS2_DS3 is reduced by 25% (32.7 to 24.2 MJ/kgMEK), mainly by integration of the heat provided in the condenser of solvent recovery column (DI1 in S1_ROEXDI). The relatively small savings are due to the high energy demand of that particular column. The 2,3-BD separation by salting-out extraction with the n-butanol/K2HPO4 (S1_hphobSOEDIS2_DS3) calculates a heat integrated SPED of 27.4 MJ/kgMEK, which can be decreased to 17.3 MJ/kgMEK by heat integration. Again, the moderate reduction of 37% is due to the solvent recovery column DI1 as the largest energy consumer. The heat integration of the reactive extraction process concept (S1_RERDDI-S2_DS3) is estimated based in available data of the reactive extraction22 to ~26 MJ/kgMEK. Although further potential in reactive extraction could be exploited by using IBA from MEK dehydration also in extraction of 2,3-BD,61 the relatively high energy demand in this work and the fate of residual aldehydes15 compromise the concept’s applicability. Hence, the hybrid process concept (S1_ROEXDI-S2_DS3) and the salting-out extraction with the n-butanol/K2HPO4 (S1_hphobSOEDI-S2_DS3) are the most promising candidates for a biobased MEK production in a multi-product environment to date. Nevertheless, the competitiveness with regard to a biofuel application is still to be addressed.

The path forward for efficient biotechnological MEK fuel production The benchmark in simplicity and economic performance is clearly ethanol fermentation. A stateof-the-art bioethanol production from sugars requires a heat-integrated SPED of 0.19


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MJ/MJethanol.62 The SPED of the MEK production concepts is 0.55–0.77 MJ/MJMEK (S1_hphobSOEDI-S2_DS3 and S1_ROEXDI-S2_DS3, both with heat integration) is in fact more than two times higher. Exploiting the by-products acetoin and IBA will increase economic viability, but a sustainable production of MEK from sugars for chemicals and fuels thus requires further improvement in energy demand to reach competitiveness. This is particularly important as the utilization of real biomass includes the unpredictable performance of pretreatment and its many by-products that will likely not enhance efficiency of bio-based MEK production. Analysis of the individual contributions to the total energy demand of the process concept S1_ROEXDI-S2_DS3 reveals that the biggest leverage to reduce the energy demand is the separations. The energy consumption for fermentation and dehydration is small. MEK separation is approximately one quarter. In contrast, 2,3-BD separation constitutes the largest part of the energy demand. More precisely, the evaporation of the n-butanol/water mixture in the solvent recovery distillation column is the largest energy consumer. Therefore, viability and energetic competitiveness of the process are determined by the 2,3-BD separation. The best separation would be clearly no 2,3-BD separation which seems inviable due to catalyst deactivation and tremendous yield losses.19,63 Hence, further efficiency potential of the MEK process can only be rendered using an energy-neutral separation from the fermentation broth. In this view, the losses caused in the pervaporation units should be avoided at all costs. Furthermore, water should be preferably removed before the extraction step. It becomes particularly important in high-yield fermentation at lower titers (i.e., 95.5 g/l at 0.49 g/g with K. oxytoca8). The method of choice is RO, showing only small losses in the permeate.


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As the extraction with organic solvents has been identified favorable from an energetic point of view, the concentration of extracted 2,3-BD should be as high as possible. Nanofiltration could be a better alternative than pervaporation for concentration of 2,3-BD in n-butanol.64 Commercial membranes also showed high rejection of 2,3-BD, while other alcohols passed the membrane.65 This way, the recycling of the top phase after extraction might work even without distillation. Further improvement can be expected by omitting the organic solvent at best. A recent report presents sugaring-out of 2,3-BD from a fermentation broth with addition of only small amounts of phosphates and n-butanol.66 Although the concentrated sugars have been utilized as feed to the fermenter, the top phase contained considerable amounts of sugar (~20%), which need to be recovered before dehydration for high yield processing. Just recently, salting out has been demonstrated without additional solvent reaching a distribution coefficient and a selectivity in the range of several thousand.67 If the management of the kosmotropic substance (sugar, salt, etc.) can be achieved without evaporation, it is very much close to the direct dehydration. The lower bound of such a process concept without energy-intensive 2,3-BD separation step can be estimated using the developed process designs at maximum yield in the dehydration without water evaporation (Figure S2 in SI). Such a full conversion has been reported in case of precise reaction monitoring.13 The resulting SPED for this direct-dehydration concept is 0.30 MJ/MJMEK, which is more than 50% reduction in comparison to the base case (S1_ROEXDI-S1_DS3). Considering that acetoin and IBA and are valuable by-products, the allocated energy demand for MEK production is 0.25 MJ/MJMEK. This efficiency is comparable to bioethanol production for the benefit of better fuel properties and the co-production of multiple valuable chemicals. Further investigation of concentrating kosmotropic substances in biorefinery processes is thus


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recommended. Such concepts without flammable solvents, distillation columns in the first step and intensive heat transfer between the unit operations further envision simple processes at relatively low complexity and smaller scale.

5.

CONCLUSIONS

The challenge was to assess the prospect of bio-based MEK (or 2-butanone) production. Several conceptual process designs for the production of MEK from glucose were developed in this work to identify the most promising synthesis and separation concept. The favorable conversion pathway is via 2,3-BD fermentation and subsequent as the direct fermentation of sugars into MEK require an improvement by two orders of magnitude. Decrease of catalytic yield by fermentation broth and excess water require an intermediate separation step of 2,3-BD. The MEK separation from water, isobutyraldehyde and residual 2,3-BD is complicated by a distillation boundary. The sequence of distillation columns and decanters designed in this work crosses the distillation boundary by internal recycling of IBA to enable very efficient separation. The largest contribution to the primary energy demand is then however the separation of 2,3-BD. Energy-efficient processing at high yields favors membrane preconcentration but losses of product and auxiliary chemicals have to be avoided. The extraction improved by kosmotropic substances is clearly favored with a lower bound of a specific primary energy demand of 0.25 MJ/MJMEK. This envisions competitiveness to established biofuels and promising biorefinery applications due to coproduction of chemicals at low complexity.


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ASSOCIATED CONTENT SUPPORTING INFORMATION Physical properties calculation details, assumptions made during process modeling, details on the fictive process concept to estimate the lower-bound of 2,3-BD separation in MEK production Corresponding Author *Corresponding author. Tel.: +49 241 80 97010. E-mail address: [email protected] Present Addresses †Dohm Pharmaceutical Engineering, Simonsstraße 90, 42117 Wuppertal, Germany Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. Daniel Penner wrote the manuscript, Christian Redepenning and Daniel Penner worked on the conceptual process design. Alexander Mitsos coordinated and guided the work. Jörn Viell edited the manuscript and supervised the process design. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, and was funded partly by the Excellence Initiative by the German federal and state governments to promote science and research at German universities and partly by the German


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department for nutrition, agriculture and consumer protection (BMELV) as part of the project “AUFWIND”. The authors like to thank Kirsten Skiborowski for the fruitful discussions and Moll Glass and Dominique Dechambre for their help in NRTL parameter estimation.

ABBREVIATIONS 2,3-BD, 2,3-butanediol; ATPE, Aqueous-two-phase-extraction; CO2, carbon dioxide; IBA, isobutyraldehyde; MEK, methyl ethyl ketone; RO, reverse osmosis; SOE, salting-out extraction; SPED, specific primary energy demand.

References

(1)

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Table of Contents / Abstract Graphics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

glucose

fermentation & dehydration

water

acetoin

IBA 2,3-BD


MEK

Conceptual Design of Methyl Ethyl Ketone Production via 2,3

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