Integration of Enzymatic Catalysts in a Continuous Reactive

Article pubs.acs.org/IECR

Integration of Enzymatic Catalysts in a Continuous Reactive Distillation Column: Reaction Kinetics and Process Simulation Rene Heils,*,† Alexander Niesbach,‡ Matthias Wierschem,‡ Dierk Claus,† Sebastian Soboll,‡ Philip Lutze,‡ and Irina Smirnova† †

Institute of Thermal Separation Processes, Hamburg University of Technology, Eissendorfer Straße 38, D-21073 Hamburg, Germany ‡ Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, TU Dortmund University, Emil-Figge-Straße 70, D-44227 Dortmund, Germany S Supporting Information *

ABSTRACT: This work presents a feasibility study for an enzymatic reaction in a continuously operated reactive distillation column. As a model reaction, the transesterification of ethyl butyrate with n-butanol in the presence of lipase CALB was considered. For use in the distillation column, lipase CALB was immobilized by entrapment in a hydrophobic silica xerogel and introduced as granulate into the catalytic packing Katapak-SP-11. The reaction kinetics was experimentally determined for different concentration and temperature ranges and described by means of the Michaelis−Menten double-substrate kinetic model in combination with the Arrhenius model. With these kinetic data, process simulations were carried out with an Aspen Custom Modeler nonequilibrium-stage model validated for a DN50 pilot-scale column. The concentration of n-butanol in the reactive section was maintained low to decrease the inhibiting effects on the enzyme. For an optimized setup and operating conditions, conversion rates of more than 90% were achieved for n-butanol and 26% for ethyl butyrate. These results clearly demonstrate that lipase CALB can be applied in a continuously operated reactive distillation column. transesterification of ethyl butyrate with n-butanol.7 This was the first completely integrated setup on the laboratory scale of an enzymatic reaction in a reactive distillation column. An alternative method for providing catalyst in a reactive distillation column is to coat the column internals with a catalytically active coating. This has been demonstrated for several chemical catalysts, such as magnesium oxide,8 zeolite crystals,9,10 and cation-exchange resins,11 but not for biological catalysts. In our previous work, the first enzymatic coating for structured packings was developed to immobilize lipase CALB for reactive distillation processes.12 The coatings were successfully applied for a transesterification reaction in a reactive distillation setup and showed good stability under process conditions. After a washing step, the loss of silica coating was approximately 2 wt % per run in the reactive distillation column (substrates = ethyl butyrate, n-butanol; Tbottom = 333 K; duration = 6−9 h).13 More recently, a horizontal reactive distillation setup was presented for the production of four enantiopure compounds by means of an enzyme-catalyzed reaction.5 Here, the heterogeneous catalyst (lipase CALB on polyacrylate resin known as Novozyme 435) was not placed directly within the column but, rather, was placed in an external fixed-bed reactor connected to a circulation loop. It was concluded that the rate of enzymatic reactions is too low to be applied in vertical

1. INTRODUCTION For the production of fine chemicals, enzymes have become an attractive alternative to traditional chemical catalysts because of their increased regio- and enantioselectivities. The use of enzymes can be economically and ecologically advantageous, because enzymes work under mild reaction conditions and can be produced by fermentation from renewable resources.1 However, the yields of enzymatic reactions are often limited by inhibition phenomena, product toxicity, and/or unfavorable positions of reaction equilibria. One possibility for overcoming these limitations is the separation of the product from the reactive section in integrated reaction−separation setups. An overview of integrated processes for different enzyme classes was published by Bechtold and Panke.2 The increased temperatures in distillation columns constrain the application of thermosensitive biocatalysts. Therefore, the minimum requirement for the application of enzymes in distillation columns is the stabilization of the biocatalyst by means of immobilization on a solid carrier. The classical approach would be to introduce such a heterogeneous catalyst into a distillation column by means of reactive packing structures. To use a lower temperature for biocatalytic reaction compared to the temperature in the distillation column, the catalyst can also be moved into an external side reactor. Different setups have been presented with an external fixed bed containing the enzyme connected to a distillation column, either to the bottom of the column3−5 or, for more volatile substrates, to the condenser.6 Only one example is known in which enzymes were implemented within a distillation column. In the work of Paiva et al., beads with immobilized lipase were placed into the inverted pear bulbs of a distillation column to catalyze the © 2014 American Chemical Society

Received: Revised: Accepted: Published: 19612

July 30, 2014 November 13, 2014 November 20, 2014 November 20, 2014 dx.doi.org/10.1021/ie502827f | Ind. Eng. Chem. Res. 2014, 53, 19612−19619

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repeated immersion into the silica sol solution and dried at room temperature until the sol solution solidified. The coatings were dried until the weight was constant. For granulate, the gel containing the enzyme was produced as a monolith. After being aged at room temperature for at least 12 h, the xerogel was comminuted in a mortar to a particle size of 0.71−1.4 mm. Enzyme Stability Test. For the determination of the longterm stability of the enzyme, the coated packings were kept at different temperatures in a reactor containing the substrate, ethyl butyrate. The liquid was stirred with a top-mounted magnetic stirrer at 120 rpm. At different times, the enzyme activity was determined for the transesterification reaction of ethyl butyrate with n-butanol. For the activity tests, the coated packings were transferred to 30 mL of a preheated solution of substrates (xEtBu = 0.7, xBuOH = 0.3), and time measurement was started. At certain time points, samples (100 μL) were taken and diluted with acetonitrile in a dilution ratio of 1:10. Possibly entrained enzyme was mechanically deactivated by being mixed for 10 s in a vortex mixer. The initial reaction rate (or enzyme activity) was determined, which corresponds to the timedependent formation of the product butyl butyrate in the first 20 min of the reaction. Determination of the Enzyme Kinetics. All kinetic measurements were carried out with enzyme granulate in 1.5 mL plastic tubes with a total reaction volume of 1 mL. The loading with immobilized enzyme was between 15 and 50 mg per milliliter of reaction mixture. The time measurement was started when the second substrate was added to the substrate solution. Samples of 10−20 μL were withdrawn at different time points and diluted with acetonitrile in a dilution ratio of 1:4 or 1:10. Deactivation of entrained enzyme was ensured by mixing the samples in a vortex mixer for 10 s. Again, the initial reaction rate was determined by analyzing the formation of the product, butyl butyrate, in the first 20 min of the reaction. Gas Chromatography Analysis. All reaction components were analyzed in a gas chromatograph equipped with a CW20M-CB (CS Chromatographie, Langerwehe, Germany) column containing a PEG-based stationary phase. The oven temperature was held at 373 K for 11 min and then increased to 393 K (4 K/min) and held constant for 3 min. Samples were detected by flame ionization detector (FID) (Tdetector = 523 K, Tinjector = 453 K). The gas flow rate was equal to 30 mL/min for H2 and N2 and 400 mL/min for synthetic air. Samples were injected at a split ratio of 1:30. Isoamyl alcohol was used as an internal standard. 2.2. Enzyme Stability. The maximum operating temperature in the reactive distillation process is mainly determined by the thermal stability of the enzyme. Therefore, the long-term stability of the immobilized lipase was monitored during storage in substrate solution at a certain temperature. The activity of the immobilized CALB could be preserved for at least 78 days at 333 K. As in our previous work,13 it was observed that, after the first activity test, the enzyme activity rose by 28% compared to the initial activity (results not shown). One of the reasons for this behavior is the increased mass loss after the first tests (∼20%). In addition, structural changes of the gel during the aging process might facilitate access of the substrate to the catalytic sites of the enzyme. At 353 K, the relative enzyme activity decreased to 88% after 3 days (see Figure 1). The enzyme was completely deactivated after 6 days.

distillation columns. In this work, we discuss this problem more closely and show that the right choice of operating conditions helps to overcome this problem. To the best of our knowledge, this work presents the first feasibility study for the application of enzymes in a vertical distillation column under continuous operation. As a model reaction, the transesterification reaction of ethyl butyrate with n-butanol was investigated, as in our previous study.13 For this theoretical investigation, the setup of the pilot-scale reactive distillation unit available at the Laboratory of Fluid Separations at TU Dortmund University was used. This pilot-scale unit has already been used for several experimental investigations, for example, for the syntheses of nbutyl acrylate14,15 and dimethyl carbonate.16 In the first part of this study, the kinetics of the lipasecatalyzed transesterification reaction was studied for the concentration and temperature ranges expected in the distillation column. In the second part, simulations were performed in Aspen Plus to determine the feasibility of a continuously operated biocatalytic reactive distillation process by investigating achievable conversions and operating ranges. Therefore, the thermodynamic and physical properties of the reaction system were collected from the literature and the Aspen Plus database. In the simulation study, several design parameters, including the size of the reactive section, the position of the feed, and the reflux ratio, were varied, and a setup optimized for maximum conversions is presented.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Materials. Lipase CALB was kindly provided by Novozyme (Bagsvaerd, Denmark) as CALB L solution. Ethyl butyrate (EtBu, CAS No. 105-54-4), butyl butyrate (BuBu, CAS No. 109-21-7), and polyethylene glycol (PEG, MW = 400, CAS No. 25322-68-3) were purchased from Merck (Darmstadt, Germany); n-butanol (BuOH, CAS No. 71-36-3), ethanol (EtOH, CAS No. 64-175), methanol (MeOH, CAS No. 67-56-1), acetonitrile (CAS No. 75-05-8), and isoamylalcohol (CAS No. 123-51-3) were obtained from Carl Roth (Karlsruhe, Germany); tetramethyl orthosilicate (TMOS, CAS No. 681-84-5) and methyl trimethoxysilane (MTMS, CAS No. 1185-55-3) were obtained from Fluka (Buchs, Switzerland); and sodium fluoride (CAS No. 7681-49-4) and acetone were purchased from Prolabo (East Grinstead, U.K.). All chemicals were pro analysis grade and were used without further purification. Enzyme Immobilization. Lipase CALB was immobilized by entrapment in a hydrophobic silica-gel matrix as described in our previous work.13 For the preparation of the silica sol, 8.0 wt % TMOS was mixed with 21.8 wt % MTMS and 33.9 wt % methanol and stirred in an ice bath. All water-soluble components, namely, 4.9 wt % sodium fluoride (1 M), 1.4 wt % polyethylene glycol (average Mn = 400), 10.5 wt % enzyme solution CALB L, and 12.6 wt % water, were mixed in a separate vessel and then added to the solution containing the silica precursors to start the sol−gel reaction (all mass fractions correspond to the total mass of the sol). The sol solution was cooled in an ice bath for the first 3 min of the reaction to avoid thermal deactivation of the enzyme during the exothermal sol− gel reaction and was then stirred at room temperature. Depending on the cooling, the gelation time was between 6 and 30 min. The immobilized enzyme was produced as either a granulate or a coating on structured packings. For the production of catalytic coatings, the structured packings were dip-coated by 19613

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decreased, indicating a substrate surplus inhibition for nbutanol (see Figure 3). For variations in ethyl butyrate, a linear

Figure 1. Long-term stability of lipase CALB immobilized within biocatalytic coating in ethyl butyrate at 353 K. The activity was determined for the transesterification reaction of ethyl butyrate with nbutanol.

As a trade-off among stability of the enzyme, column pressure, and reaction rate, the maximum temperature in the reactive section was restricted to 343 K. 2.3. Kinetics of CALB-Catalyzed Transesterification of Ethyl Butyrate with n-Butanol. For simulation studies in a reactive distillation column, a description of the reaction kinetics is required. The reaction kinetics of the transesterification reaction of ethyl butyrate with n-butanol was investigated for different substrate concentrations and the temperature range expected in the distillation column. (For the reaction equation, see Figure 2.) The influence of the products on the reaction rate was neglected because the volatilities of the products differed such that both products were immediately removed from the reactive section. For the description of the reaction kinetics, the Michaelis− Menten double-substrate model was used. The proposed substrate surplus inhibition by n-butanol on the lipase CALB activity was taken into account through an additional inhibition term vBuBu =

⎛ ⎜K + 1+ ⎝ m,BuOH

(

Figure 3. Initial rate measurements for the lipase CALB-catalyzed transesterfication reaction of ethyl butyrate with n-butanol in nheptane at 333 K. The concentration of one of the substrates was varied while the other substrate concentration was kept at 2.69 M.

increase of the initial reaction rate was observed for increasing concentrations of ethyl butyrate (see Figure 3). In the work of Strompen et al.,18 it was proposed that this behavior indicates a low affinity of the ester for the lipase. The Michaelis−Menten kinetic model was fitted to the experimental data from both sets of experiments by the leastsquares method using Microsoft Excel solver. The parameters are summarized in Table 1.

vmax [BuOH][EtBu] ⎞ [BuOH] [BuOH]⎟(K m,EtBu + [EtBu]) K i,BuOH ⎠

)

(1)

The inhibition of primary alcohols on the lipase activity is a well-known phenomenon and is described elsewhere.17 To study the influence of one of the substrates, the enzyme activity was first measured in a solvent system to be able to vary one of the substrates while the other substrate was kept constant. nHeptane was chosen as a solvent because the enzyme activity measured in this solvent was higher compared to other solvents. In addition, the solvent should have a relatively high boiling point to reduce evaporation losses. Results for enzyme activity in different solvents are shown in the Supporting Information (Figure S1). For varying n-butanol concentrations, it was observed that the initial reaction rate increased up to concentration of 0.27 M. For higher n-butanol concentrations, the initial reaction rate

Table 1. Kinetic Parameters for the Michaelis−Menten Double-Substrate Model kinetic parameter

value

units

Km,BuOH Ki,BuOH Km,EtBu Ea A

0.05 8.79 14.5 63344 3.69 × 1011

mol L−1 mol L−1 mol L−1 J mol−1 mmol h−1 gcat−1

The temperature dependency of the reaction rate was integrated within the parameter vmax and can be described by the Arrhenius equation

Figure 2. Transesterification of ethyl butyrate with n-butanol catalyzed by lipase CALB. 19614


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was chosen based on simulation results investigating the effects of different column setups on the reactant conversions. The decision on the column setup was made by comparing the achievable conversions and the resulting concentration profiles (e.g., avoiding sections with high n-butanol excesses in the reactive section due to the reactant-inhibited reaction rate). Furthermore, the temperatures in the reactive section should be below 343 K to limit catalyst deactivation. For these simulations, configurations with one or two feeds were taken into account, and three different feed positions were investigated: above, in the middle of, and below the reactive section. The setup shown in Figure 6 was chosen for the simulation study presented in this article.

(2)

The initial rate measurements for the temperature variation were carried out in a solvent-free system with 0.3 mol/mol of nbutanol at temperatures between 303 and 343 K. The plot of ln(vmax) versus 1/T (see Figure 4) yielded the respective parameters for the Arrhenius equation that are listed in Table 1.

Figure 4. Arrhenius plot of vmax for the transesterification reaction of ethyl butyrate with n-butanol in the presence of lipase CALB.

With the parameters determined for the Michaelis−Menten model, the initial reaction rates in the solvent-free system could be properly described (Figure 5).

Figure 6. Reactive distillation column setup chosen for the simulation study.

3.2. Model Description. Process simulations of the reactive distillation process presented in this study were carried out in Aspen Custom Modeler. Details on the model used for the calculations were published by Klöker et al.19 The reactive distillation model, which was developed by the Laboratory of Fluid Separations of TU Dortmund University, has already been successfully used for the analysis of various processes, such as the synthesis of n-butyl acrylate,14,20 the transesterification of dimethyl carbonate,21 the production of npropyl propionate,22 and the synthesis of propylene glycol and dimethyl carbonate.23 In this study, this model was used for the first time for the simulation of an enzyme-catalyzed process. In general, two basic approaches, nonequilibrium stage (NEQ) and equilibrium stage (EQ), can be used for the description of reactive distillation processes and are available in the presented model. A description of these approaches, including the relevant modeling equations, was published by Taylor and Krishna.24 For the simulations performed in this work, an NEQ modeling approach was used. NEQ approaches calculate energy- and mass-transfer coefficients and, thereby, take the energy- and mass-transfer rates between different phases into account.24 The two-film theory was implemented for the description of the heat- and mass-transfer25 and effective

Figure 5. Initial rate measurements for CALB immobilized within silica xerogel at different n-butanol/ethyl butyrate (BuOH/EtBu) compositions (333 K, solvent-free). Modeling of the initial rate measurements by means of the Michaelis−Menten model (eq 1).

3. SIMULATION STUDY OF BIOCATALYTIC REACTIVE DISTILLATION 3.1. Column Setup. The pilot-scale reactive distillation column consisted of approximately 6 m of packing and had a diameter of 51 mm. It was divided into six sections that were defined as either a reactive section with the reactive packing Sulzer Katapak-SP11 or a separation section with the nonreactive packing Sulzer BX. The two reactants could be fed separately at two different feed positions or together at a single feed position. The feed positions could be attached between every section. In general, for reactive distillation processes, the heavy-boiling feed component is fed above the reactive section, and the lowboiling feed component is fed below the reactive section. Because of the very similar boiling behaviors of the reactants, ethyl butyrate and n-butanol, the feed position for this column 19615



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diffusion coefficients26,27 were used to calculate the masstransfer rates. The Chilton−Colburn analogy28 was implemented for the calculation of the heat transfer between the phases. Furthermore, in NEQ models, the characteristics of the used column internals are considered.24 In this model, the packing-specific calculations used were developed by Brunazzi and Viva29 for the reactive packing Sulzer Katapak SP-11 and by Rocha et al.30 and Bravo et al.31 for the separation packing Sulzer BX. Based on the height equivalent to a theoretical plate (HETP) values of the packings, an axial discretization was performed in the column, and the height of each discrete section was fixed to one-fourth of the HETP value of the particular packing (according to Niesbach et al.14). For the thermodynamic description of the phases, nonideal behavior in the liquid phase was described using the UNIQUAC model,32 and the required parameters were estimated using the Dortmund modified UNIFAC group contribution method.33,34 3.3. Thermodynamic and Physical Property Data. Accurate property data for the involved substances are a prerequisite for reliable simulation results. To gather these data, experimental measurements published in the literature and in the Aspen Plus databases and the Aspen Plus property estimation feature were used. The investigated properties were the densities, viscosities, heat capacities, and thermal conductivities of the liquids and gases, vapor pressures and surface tension of the liquids for the pure components (section 3.3.1), and vapor−liquid equilibria (section 3.3.2). The components considered in this study, their chemical formulas, CAS numbers, and boiling points at atmospheric pressure are summarized in Table 2.

cannot be determined a priori, every method has to be tested to make an appropriate choice. For instance, Figure 7 presents the

Figure 7. Comparison of estimated surface tension data for butyl butyrate with experimental data from Vogel.36

results of three different methods for the calculation of the surface tension of butyl butyrate in comparison with literature data from ref 36 (lines are estimation results; symbols represent experimental data). For the determination of the most suitable estimation method for the surface tension, experimental data were compared to the estimation methods of Brock and Bird,37 McLeod and Sugden,38 and Li et al.39 In this case, the Brock and Bird method37 fit best and was chosen for further simulations. The methods used for the determination of the other required thermodynamic and physical properties of butyl butyrate were determined in the same manner and are summarized in Table 3. A detailed description of the individual methods used for the estimations can be found in Aspen Plus.

Table 2. Relevant Components and Their Chemical Formulas, CAS Numbers, and Boiling Points at Atmospheric Pressure

a

component

chemical formula

CAS no.

Tba (K)

ethanol n-butanol ethyl butyrate butyl butyrate

C2H6O C4H10O C6H12O2 C8H16O2

64-17-5 71-36-3 105-54-4 109-21-7

351 391 393 438

Table 3. Butyl Butyrate Temperature-Dependent Parameters and Their Integration into Aspen Plus

At p = 1013 mbar.

The two products, butyl butyrate and ethanol, constitute the highest- and lowest-boiling components, respectively. This fact indicates a simple separation of the coproduct ethanol from the desired product butyl butyrate. The boiling temperatures of the components ethyl butyrate and n-butanol are nearly the same, at 391 K for n-butanol and 393 K for ethyl butyrate at atmospheric pressure. 3.3.1. Pure-Component Property Data. The property data for ethanol, n-butanol, and ethyl butyrate were obtained from the Aspen Plus database and could be validated with experimental data using the connected NIST ThermoData Engine. Butyl butyrate was the only component not found in any of the databases; its properties had to be inserted manually. The scalar parameters boiling temperature, critical temperature, critical pressure, and critical volume were available in the literature.35 Missing scalar parameters were estimated using the methods implemented in Aspen Plus and compared to literature data. Aspen Plus often provides multiple methods for the estimation of one property. Because the best method

parameter

source

method or ref

liquid density vapor density liquid heat capacity vapor heat capacity liquid viscosity vapor viscosity liquid thermal conductivity vapor thermal conductivity vapor pressure surface tension

estimation estimation estimation estimation estimation estimation estimation regression estimation estimation

Le Bas Redlich−Kwong−Soave Ruzicka Redlich−Kwong−Soave Orrick Reichenberg Sato−Riedel Yaws et al.40 Riedel Brock and Bird

The data from Aspen Plus exhibit entirely satisfactory accordance with the literature data. Figure 8 illustrates these results, showing a comparison of experimental and estimated data for the vapor pressures of all components. The estimations were performed using the Riedel method. For ethanol, n-butanol, and butyl butyrate, the estimation results fit the literature data precisely. For ethyl butyrate, the slight divergence occurring at temperatures above approximately 370 K was negligible, because the process is operated in a lower temperature range. 19616



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For a reflux ratio (RR) of 1.5, a distillate-to-feed (DF) ratio of 0.4, a total feed mass flow rate of 4 kg h−1, and a molar reactant ratio of 1, a conversion of 29% was achieved for both reactants in the described setup. For these operating parameters, the maximum top pressure in the column was determined to be 0.19 bar, to fulfill the temperature requirements in the reactive section. Raising the reflux ratio led to slightly increased conversion rates. Up to DF = 0.55, the conversion rose with increasing distillate-to-feed ratios, whereas a further increase in the DF ratio led to a decrease of the conversion and, hence, to reduced butyl butyrate production. This result was attributed to the n-butanol inhibition of the reaction, as an increasing distillate-to-feed ratio led to higher concentrations of n-butanol in the reactive section. At DF < 0.55, the increase of the reaction rate due to the increased reactant content in the reactive section overcompensated for the inhibiting effect. For DF > 0.55, the inhibiting effect of nbutanol dominated, and the conversion in the column decreased. Figure 10 illustrates the conversion rates of both reactants as functions of the n-butanol molar fraction in the feed. It is

Figure 8. Comparison of estimated vapor pressures and literature data for all components. The experimental data were taken from Yaws41 for ethanol, n-butanol, and ethyl butyrate and from Gmehling et al.42 for butyl butyrate.

3.3.2. Binary Data. In this work, UNIQUAC was used as the gE model for the calculation of VLE. UNIQUAC parameters for the VLE were estimated using UNIFAC DMD (see Figure 9) and validated using available literature data. For all binary combinations except butyl butyrate/ethyl butyrate, literature data were found. All other VLE and the available experimental data can be found in the Supporting Information (Figure S2). 3.4. Simulation Results. To investigate the feasibility of the enzymatic production of butyl butyrate in a reactive distillation process, simulation studies were performed in this work using the Aspen Custom Modeler model described in section 3.2. During all of the presented simulations, the maximum operating temperature in the reactive section (343 K) was not exceeded, as this would result in a significantly increased enzyme deactivation. Simulation studies investigating different column setups revealed that placing the n-butanol feed below the reactive section leads to a lack of n-butanol at the top of the reactive section and, thereby, to reduced conversions of the reactants in comparison to the setup chosen for the simulation studies. Thus, as shown in Figure 6, a column setup was chosen in which ethyl butyrate was fed above and n-butanol was fed in the middle of the reactive section.

Figure 10. Conversions of the two reactants n-butanol and ethyl butyrate as functions of the n-butanol mole fraction in the feed.

shown that decreasing the n-butanol fraction in the feed led to a significant increase in n-butanol conversion because if the higher excess of ethyl butyrate as well as the reduced inhibiting effect of n-butanol. However, from xBuOH,feed = 0.3 downward, a

Figure 9. Comparison of UNIFAC DMD data estimated by Aspen Plus and experimental data taken from Ortega et al.43 and González and Ortega44 for the VLE of butyl butyrate and n-butanol (left) and butyl butyrate and ethanol (right). 19617



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zone with low (almost negligible) n-butanol concentrations at the top of the reactive section existed . Therefore, the whole reactive section was no longer being used, and the total conversion of the reactants dropped. This is apparent from the slightly intensified decrease of the ethyl butyrate conversion rate. After investigating the impact of the operating parameters separately, the maximum achievable conversion in the pilotscale reactive distillation column was determined. At a pressure of 0.2 bar, a reflux ratio of RR = 2, a distillate-to-feed-ratio of DF = 0.64, an ethyl butyrate feed mass flow rate of 3.4 kg h−1, and an n-butanol feed mass flow rate of 0.6 kg h−1, conversions of 93% for n-butanol and 26% for ethyl butyrate were achieved in the pilot-scale setup. In Figure 11, the column profiles for the mole fractions of the components and the temperature over the column height are shown.

above-described biocatalytic coatings for structured packings will be tested for application in the pilot plant.

■

ASSOCIATED CONTENT

S Supporting Information *

Solvent screening for lipase activity tests and VLE data of all reaction components including experimental data and UNIFAC_DMD correlations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research leading to these results received funding from the German Research Association (DFG SM 82/9-1). Abbreviations

BuBu = butyl butyrate BuOH = n-butanol CALB = Candida antarctica lipase B DF = distillate-to-feed mass ratio EQ = equilibrium stage EtBu = ethyl butyrate EtOH = ethanol HETP = height equivalent to a theoretical plate MeOH = methanol MTMS = trimethoxymethylsilane NEQ = nonequilibrium stage NH4OH = ammonium hydroxide RD = reactive distillation RR = reflux ratio TMOS = tetramethoxysilane UNIFAC DMD = universal quasichemical functional group activity coefficients Dortmund extension UNIQUAC = universal quasichemical VLE = vapor−liquid equilibria

Figure 11. Column profiles with mole fractions of the components (left) and vapor temperature (right). The reactive section, the reboiling section, and the positions of the feed streams are indicated.

Both substrates, ethyl butyrate and n-butanol, were converted into the products, ethanol and butyl butyrate, which were separated in situ. The low-boiling product ethanol simply left the column at the top, and the high-boiler butyl butyrate was mainly withdrawn at the bottom. In the reactive section, a high concentration of ethyl butyrate was present to limit the inhibition of the enzyme by n-butanol. Because of its high excess, ethyl butyrate was withdrawn with the distillate and the bottom stream. The temperature in the reactive section was maintained below 343 K at the applied pressure of 0.2 bar to avoid extensive denaturation of the enzyme.

Symbols

4. CONCLUSIONS Reduced enzyme stability usually involves low reaction rates and thus requires long residence times in reactive distillation columns. However, by operating at a reduced pressure and using an enzyme with a high temperature tolerance (lipase CALB), this simulation study showed that enzymatic reactions are fast enough to be carried out in a continuously operated reactive distillation column. For an optimized column setup, conversions of more than 90% for n-butanol can be achieved in the lipase-catalyzed transesterification reaction with ethyl butyrate. Because of inhibition caused by the reactant nbutanol, operating conditions and a column setup were chosen, achieving a small n-butanol concentration in the reactive section to avoid a significant reduction in the reaction rate. Because of the positive outcome of this simulation study, the validation of the simulation results with experiments in the pilot plant is the focus of our current research. Furthermore, the

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Tb = boiling point temperature (K) vBuBu = formation rate of butyl butyrate (mmol h−1 gcat−1) vmax = maximum reaction rate (mmol h−1 gcat−1) Ki,BuOH = inhibition constant for n-butanol (mol L−1) Km,BuOH = Michaelis−Menten constant for n-butanol (mol L−1) Km,EtBu = Michaelis−Menten constant for ethyl butyrate (mol L−1) Ea = activation energy (J mol−1) A = prefactor (mmol h−1 gcat−1)

REFERENCES

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Integration of Enzymatic Catalysts in a Continuous Reactive

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