Ethanol Recovery from Stripping Gas Mixtures by Gas Absorption

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Ethanol Recovery from Stripping Gas Mixtures by Gas Absorption: Experimental and Modeling Kaio C. S. Rodrigues, Ivan I. K. Veloso, Antonio J. G. Cruz, Andre ́ Bernardo, and Alberto C. Badino* Graduate Program of Chemical Engineering, Federal University of São Carlos, P.O. Box 676, São Carlos, São Paulo 13565-905, Brazil

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S Supporting Information *

ABSTRACT: Ethanol removal by CO2 stripping during alcoholic fermentation is one way of overcoming the problem of inhibition by the product. However, the lack of efficient methods to recover ethanol from the gas phase still makes the use of stripping unviable. In this work, gas absorption was evaluated as a method for the recovery of ethanol from the gas mixture generated by CO2 stripping. First, the solvents water, monoethylene glycol (MEG), and diethylene glycol were evaluated in terms of their performance in ethanol absorption. MEG was selected as the most appropriate absorbent because it provided a satisfactory ethanol recovery percentage, in addition to the fact that it is already used in distilleries to obtain anhydrous ethanol. Subsequent assays using MEG were conducted to investigate the influence of the initial MEG volume in the absorber, the recirculation volumetric flow rate of solvent, and the use of two absorbers connected in series. A modeling procedure was developed based on mass balance equations for the species involved (ethanol, water, CO2, and MEG), stripping and absorption kinetics, and vapor−liquid equilibrium concepts and was able to accurately describe the process behavior. The use of two absorbers, each with 0.80 L of MEG, enabled recovery of up to 93.1% of the ethanol from the stripping gas mixture. The results showed that gas absorption with MEG is a highly promising strategy for ethanol recovery, with potential for applications in industrial ethanol fermentation processes. tions.19 Sonego et al.18 employed a sucrose concentration of 300 g L−1 in the feed in fed-batch fermentations and achieved a productivity of 8.6 g L−1 h−1 and a total ethanol concentration of 136.9 g L−1 (17.2 °GL), which was 65% higher than that for fermentation without stripping. Rodrigues et al.19 simulated continuous fermentation and showed that it was possible to use feed musts containing up to 400 g L−1 of sucrose and obtain high substrate conversion, ethanol productivity of 10.8 g L−1 h−1, and total ethanol production of 174.8 g L−1 (22.2% v v−1). Other studies have also reported the successful use of CO2 stripping for the removal of butanol in acetone−butanol− ethanol fermentations.20,21 Although CO2 stripping can be advantageous in the ethanol fermentation process, it is necessary to find efficient and costeffective methods for recovery of the ethanol removed, in order to make the technique economically feasible. However, there have been few studies concerning this issue. In evaluation of the performance of different adsorbents for ethanol recovery, Hashi et al.22 reported that activated carbon (WV-B 1500) exhibited a high ethanol adsorption capacity (∼0.5 gethanol gadsorbent−1) and preferentially adsorbed ethanol, relative to water and CO2. In other studies, ethanol has been recovered by condensation.23,24 However, high costs are associated with adsorbents and the energy consumption of condensers operating at sub-zero temperatures, which hinders large-scale application. The aim of the present work was to evaluate the feasibility of using gas absorption as a method for recovery of

1. INTRODUCTION Ethanol is one of the options for replacing fossil fuels1 and is currently the biofuel with the greatest global consumption.2 In 2017, its worldwide production reached 102 billion liters, with the United States and Brazil being the main producers (85% of the total).3 Ethanol can be produced from sugarcane4 or starchy materials5 (first generation), raw lignocellulosic biomass (second generation),6 and more recently from algal biomass (third generation).7 Production of first generation ethanol is limited by product inhibition of the fermenting microorganism, which is usually the Saccharomyces cerevisiae yeast. According to Maiorella et al.,8 ethanol completely inhibits yeast growth at concentrations of around 95 g L−1 (12 °GL or % v v−1). As a result, it is necessary to start the fermentation with a dilute sugar solution (usually up to 200 g L−1 of total fermentable sugars), in order to achieve complete conversion within a reasonable time.2,9 Consequently, there are high costs associated with large fermenters and energy consumption for product recovery by distillation, as well as the generation of large volumes of vinasse.10,11 One way to overcome the ethanol inhibition is to use CO2 stripping to remove ethanol during fermentation. Stripping enables the selective removal of volatile compounds and is a simple and inexpensive technique.12 Studies investigating the use of CO2 stripping during ethanol production from corn starch, in continuous fermentations performed at different scales, have reported high productivities (up to 17 g L−1 h−1) and complete conversion of highly concentrated substrate feed.5,11,13−15 Our research group has evaluated the feasibility of applying CO2 stripping during ethanol production from sugarcane in batch,16 fed-batch,17,18 and continuous fermenta© XXXX American Chemical Society

Received: October 10, 2018 Revised: December 4, 2018

A

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Schematic illustration of the experimental apparatus and the streams (circled numbers) and CVs (dotted lines) used in the process modeling. (1) Pure CO2 stream entering the bioreactor; (2) stream leaving the bioreactor; (3) stream entering the first absorber (A) as gas at 25 °C; (4) stream entering the first absorber as condensed liquid, due to the temperature reduction from 34 to 25 °C; (5) stream leaving the first absorber; and (6) stream leaving the second absorber (B), when applicable. 2.2.2. Absorbent Selection. Absorption experiments were carried out to evaluate different solvents in terms of their capacity to absorb ethanol. Water (used as a reference solvent), DEG (99.9% purity, Synth), and MEG (99.0% purity, Synth) were tested (assays A1, A2, and A3, respectively). An absorber containing 0.60 L of solvent was used. In order to choose the best absorbent, recovery performance was evaluated using the ethanol recovery percentage (ER), according to eq 1

ethanol from stripping gas mixtures. First, water, monoethylene glycol (MEG), and diethylene glycol (DEG) were evaluated in terms of their capacities for ethanol absorption. Subsequently, experiments using the best absorbent were conducted to evaluate the influence of the solvent volume in the absorber, the recirculation volumetric flow rate of solvent, and the use of two absorbers connected in series. An in-depth modeling procedure was developed, based on stripping and absorption kinetics, mass balance equations, and vapor−liquid equilibrium concepts, considering all of the species involved in the process (ethanol, water, CO2, and absorbent).

ER (%) =

mE,Recov mE,Strip

100 (1)

where mE,Recov is the mass of ethanol (g) recovered in the absorber and mE,Strip is the mass of ethanol (g) removed from the bioreactor by CO2 stripping. 2.2.3. Assays with MEG. After selection of the absorbent, ethanol absorption using MEG was evaluated under different conditions, considering the initial MEG volume in the absorber (VMEG), the recirculation volumetric flow rate of MEG (QMEG), and using one absorber or two absorbers connected in series. First, three assays were performed using one absorber, with VMEG of 0.40, 0.60, and 0.80 L (assays A4, A5, and A6), without MEG recirculation. These volumes represented 20, 30, and 40% of the bioreactor working volume, respectively, in which ethanol was removed by the CO2 stripping. Afterward, three assays were conducted using QMEG of 0.30, 0.60, and 0.90 L min−1 (assays A7, A8, and A9), with one absorber and a fixed initial MEG volume of 0.8 L. Finally, three assays were conducted using two absorbers in series, with MEG volumes of 0.40, 0.60, and 0.80 L (assays A10, A11, and A12) in each absorber, without MEG recirculation. The recovery performance was evaluated using the ethanol recovery percentage (ER, eq 1) and the overall volumetric coefficients of absorption for ethanol (KyaE) and water (KyaW), obtained from process modeling. 2.2.4. Analytical Methods. During the absorption experiments, the concentrations of ethanol, MEG, and DEG were determined by highperformance liquid chromatography, using a chromatograph equipped with a refractive index detector and an Aminex HPX-87H column (300 × 7.8 mm, Bio-Rad) kept at 80 °C. The eluent was ultrapure water, at a flow rate of 0.5 mL min−1. The standards used were ethanol, MEG, and DEG solutions at concentrations between 0.1 and 8.0 g L−1. The water concentration (CW) was obtained using the following equation

2. MATERIALS AND METHODS 2.1. Ethanol Stripping by CO2. Figure 1 illustrates the system used in the experiments. The gas stream with ethanol and water was generated by CO2 stripping in a bubble column pneumatic bioreactor (2 L working volume, 47.7 cm total height, and 9.2 cm internal diameter) containing a solution of ethanol at a concentration of 80 g L−1 (∼10% v v−1). Commercial CO2 (99.5% purity) stored in a cylinder (25 kg and 60 atm when full) was injected into the bioreactor through a perforated cross-sparger (36 holes with a diameter of 0.5 mm, spaced 5.0 mm apart) located at the bottom of the bioreactor. A thermal mass flow controller (GFC 37, Aalborg) was used to control the CO2 flow rate at 5.0 L min−1, corresponding to a specific flow rate of 2.5 vvm (the ratio between the volumetric CO2 flow rate and the bioreactor working volume). The temperature was maintained at 34 °C by recirculating water from a thermostatic bath to the bioreactor jacket. Sample aliquots (2 mL) were removed every hour for analysis of ethanol, and the liquid phase volume was determined by measuring the height of the liquid column after switching off the gas system to avoid bubble interference. 2.2. Absorption Assays. 2.2.1. Absorption Procedure. The gas mixture stream leaving the bioreactor was injected into a bubble column absorber (37.0 cm total height, 6.0 cm internal diameter) through a sintered glass sparger at the base of the column. The temperature was maintained at 25 °C by immersing the absorber in a thermostatic bath. The experiments were conducted in duplicate and lasted 6 h. Samples (2 mL) were removed every hour for analysis of ethanol and water, and the liquid phase volume was also determined by measuring the height of the liquid column after switching off the gas system. In the assays used for evaluating the influence of QMEG, recirculation of the liquid phase was provided using a peristaltic pump (323Dz, Watson-Marlow). In the assays with two absorbers, the stream leaving the first absorber was directly injected into the second absorber (Figure 1).

C W = ρ − C E − CS

(2)

where ρ is the specific mass of the solution (g L−1), determined by weighing a 25 mL volume of sample contained in a volumetric flask at B

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels constant temperature, CE is the ethanol concentration (g L−1) and CS is the solvent concentration (MEG or DEG, g L−1). 2.3. Process Modeling. The mathematical model developed to describe the process was based on mass balance equations for the species involved (ethanol, water, CO2, and MEG), incorporating stripping and absorption kinetics, together with vapor−liquid equilibrium concepts. The streams and control volumes (CVs) considered in the modeling are shown in Figure 1. As the compositions in the gas streams and in the liquid phase in the bioreactor and the absorber changed over time during the process, all of the variables of the modeling were determined for each instant “t”. Control Volume 1 (CV1)Bioreactor. The gas phase composition in stream 2, leaving the bioreactor, was obtained using CV1. The molar fractions of ethanol (yE), water (yW), and CO2 (yC) were determined according to the following equations

reduction in temperature, from 34 to 25 °C, when it entered the absorber. As a result, two phenomena occurred at the same time, namely, condensation and absorption. It was assumed that condensation occurred instantaneously when the gas contacted the solvent at the lower temperature. In order to successfully model the process, it was convenient to consider these phenomena separately. A hypothetical situation was proposed in which a cooler was placed prior to the first absorber (CV2 in Figure 1). Under this condition, stream 2, at 34 °C, would pass through the cooler, which would reduce the temperature to 25 °C. This would result in a stream entering the first absorber as gas at 25 °C (stream 3), together with a separate stream entering as condensed liquid (stream 4). Seven equations were used to describe this situation. Equations 12−14 are based on mass balances for ethanol (E), water (W), and CO2 (C). Equations 15 and 16 are the sums of the molar fractions for the species in the gas and liquid phases, respectively. Equations 17 and 18 represent the gas−liquid equilibrium for the system consisting of ethanol, water, and CO2, using the γ−φ approximation26 and neglecting the CO2 solubility.

yE2 =

nE2 ̇ nE2 ̇ + nW2 ̇ + nC2 ̇

(3)

yW2 =

nW2 ̇ nE2 ̇ + nW2 ̇ + nC2 ̇

(4)

n T2 ̇ yE2 = n T3 ̇ yE3 + n T4 ̇ x E4

(12)

(5)

n T2 ̇ yW2 = n T3 ̇ yW3 + n T4 ̇ x W4

(13)

n T2 ̇ yC2 = n T3 ̇ yC3

(14)

yE3 + yW3 + yC3 = 1

(15)

x E4 + x W4 = 1

(16)

yE3 P = γE4x E4PSE

(17)

yC2 = 1 − yE2 − yW2

where ṅE, ṅW, and ṅC are the molar flow rates of ethanol, water, and CO2 (mol h−1), respectively. The subscript “2” represents the stream number. The values for ṅE2, ṅW2, and ṅC2 were determined using eqs 6−8, respectively. Equations 6 and 7 were based on the kinetic model of ethanol and water removal proposed by Rodrigues et al.19 Equation 8 was obtained from mass balance in the gas phase for CO2, assuming an ideal gas and neglecting the CO2 solubility in the ethanol solution in the bioreactor. nE2 ̇

1 kESC ESVS = MME

nW2 ̇ =

1 k WSC WSVS MMW

nC2 ̇ = nC1 ̇ =

yW3 P = γW4x W4PSW

(6)

where ṅT is the total molar flow rate (mol h ), γE and γW are the activity coefficients for ethanol and water, respectively, xE and xW are the molar fractions of ethanol and water in the liquid phase, respectively, and PSE and PSW are the vapor pressures for ethanol and water (atm), respectively. The subscripts “3” and “4” represent the stream numbers. The system composed of eqs 12−18 has seven unknown variables (ṅT3, ṅT4, xE4, xW4, yE3, yW3, and yC3) and could therefore be solved (zero degree of freedom). The value for ṅT2 came from the sum of the molar flow rates of ethanol, water, and CO2 in stream 2 (ṅT2 = ṅE2 + ṅW2 + ṅC2), shown in eqs 6−8. The activity coefficients for ethanol (E) and water (W) were calculated using the UNIQUAC equation (rE = 2.1055, qE = 1.9720, rW = 0.9200, qW = 1.4000, aEW = 2.0046, aWE = −2.4936, bEW = −728.9705 K, bWE = 756.9477 K; parameters obtained using Aspen Plus v. 8.2 software). The vapor pressure for the component “i” (PSi, atm) was calculated using the Antoine equation

(7)

PQ C RT1

(8)

where MME and MMW are the molecular weights of ethanol and water (g mol−1), respectively, kES and kWS are the rate constants of ethanol and water removal by stripping (h−1), respectively, CES and CWS are the ethanol and water concentrations in the stripping solution (g L−1), respectively, VS is the volume of the stripping solution (L), P is the absolute pressure (atm), QC is the CO2 volumetric flow rate (L h−1), T is the absolute temperature (K), and R is the gas constant (L atm K−1 mol−1). The subscript “1” represents the stream number. The model proposed by Rodrigues et al.19 considered ethanol and water removal, as well as changes in the solution volume. It can be represented by eqs 9−11, derived from the mass balances in the liquid phase for ethanol (E) and water (W), and the total mass.

dC ES 1 dVS zyz ji zzC ES = − jjjkES + j z dt V S dt { k

PSi =

1 A i − Bi / (T + Ci) e 760

(19)

where Ai, Bi, and Ci are the Antoine equation constants. The values used for ethanol were AE = 18.91, BE = 3803.98, and CE = −41.68, and for water were AW = 18.30, BW = 3816.44, and CW = −46.13.27 Control Volume 3 (CV3)First Absorber (A). Considering both phenomena (condensation and absorption) for ethanol and water, as well as the changes in the solution volume, and neglecting volume changes due to mixing, the modeling of the system can be represented by eqs 20−22, derived from the mass balances in the liquid phase for ethanol (E) and water (W), and the total mass.

(9)

ij ij dC WS 1 dVS yzz 1 dVS yzz zzC WS = − jjjk WS + zz(ρ − C ES) = − jjjk WS + j z j dt VS dt { VS dt z{ S k k (10)

dVS V = − S (kESC ES + k WSC WS) dt ρS

(18) −1

(11)

ṁ C dV dC EA * ) − EA A , = E4 + MMEK yaE(ylnEA − yEA VA VA dt dt yE3 − yE5 ylnEA = ln yE3 − ln yE5

where ρS is the specific mass of the stripping solution (g L−1), obtained as the arithmetic mean of the specific mass values for the ethanol solution at each instant.25 Control Volume 2 (CV2)Hypothetical Cooler. The gas in the stream leaving the bioreactor (stream 2) underwent an abrupt C

(20)

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels ṁ C dV dC WA * ) − WA A , = W4 + MMW K yaW (ylnWA − yWA VA VA dt dt yW3 − yW5 ylnWA = ln yW3 − ln yW5 (21)

the masses recovered over time. The yE5 and yW5 values were then determined using eqs 30 and 31. The molar flow rates of ethanol (ṅE5) and water (ṅW5) were obtained by dividing the values of ṁ E5 and ṁ W5 by the corresponding molecular weights (MME and MMW). The CO2 molar flow rate in stream 5 (ṅC5) was equal to the molar flow rate in stream 2 (ṅC2, eq 8) because it was considered insoluble.

dVA V 1 *) = (ṁ E4 + ṁ W4 ) + A [MMEK yaE(ylnEA − yEA dt ρA ρA * )] + MMW K yaW (ylnWA − yWA

ṁ E5 = ṁ E2 − (22)

ṁ W5 = ṁ W2 −

where CEA and CWA are the ethanol and water concentrations in solution for the first absorber (g L−1), respectively, ṁ E and ṁ W are the mass flow rates of ethanol and water (g h−1), respectively, KyaE and KyaW are the overall volumetric coefficients of absorption for ethanol and water based on the gas film (mol L−1 h−1), respectively, ylnEA and ylnWA are the logarithmic mean molar fractions of ethanol and water in * and yWA * are the gas phase for the first absorber,28,29 respectively, yEA the molar fractions of ethanol and water in the gas phase in equilibrium with the liquid phase composition for the first absorber, respectively, VA is the volume of solution in the first absorber (L), and ρA is the specific mass of solution in the first absorber (g L−1). The subscript “5” represents the stream number. The second term on the right side of eqs 20 and 21 is based on the two-film theory of gas absorption that describes the mass transfer of a solute from the gas to the liquid phase.30,31 The absorption rate is conveniently described in terms of an overall coefficient based on either the gas film (Ky) or the liquid film (Kx), which can be related to the individual film coefficients (ky and kx).32 The relationship between Ky and the coefficients ky and kx is given as follows

Hxy 1 1 = + Ky ky kx

d d (mEA ) = n T2 ̇ yE2 MME − (mEA ) dt dt

where Hxy is the effective Henry’s law constant, in mole fraction units (mol mol−1). Note that in the modeling procedure, the absorption rate was described in terms of the overall volumetric mass transfer coefficient (Kya), incorporating to Ky the interfacial area (a) per unit volume. The gas film was adopted as a reference because the solutes (water and ethanol) have very high solubility in the solvent (MEG).32,33 The values of ṁ E4 and ṁ W4, which represent the mass flow rates of ethanol and water entering the absorber as condensed liquid, were calculated * and yWA * were determined using using eqs 24 and 25. The values of yEA eqs 26 and 27, which describe the gas−liquid equilibrium for the system consisting of ethanol, water, MEG, and CO2, using the γ−φ approximation26 and considering that CO2 is insoluble. (24)

ṁ W4 = n T4 ̇ x W4 MMW

(25)

* P = γ x EAPSE yEA EA

(26)

* P = γ x WAPSW yWA WA

(27)

(29)

yE5 =

nE5 ̇ nE5 ̇ + nW5 ̇ + nC5 ̇

(30)

yW5 =

nW5 ̇ nE5 ̇ + nW5 ̇ + nC5 ̇

(31)

where mEA and mWA are the masses of ethanol and water in the first absorber. The derivatives were determined based on polynomial fitting to the experimental mEA and mWA data. Control Volume 5 (CV5)second absorber (B). The modeling for the absorption process in the second absorber (B) was represented by eqs 32−34, derived from the mass balances for ethanol (E) and water (W) and the total mass, considering the absorption of ethanol and water, together with the changes in the solution volume, and neglecting volume changes due to the mixing. Note that the difference of this model, relative to the model for the first absorber, concerns the term for ethanol and water condensation. In the second absorber, there was no condensation because the gas entering the absorber was at the same temperature as the absorption liquid phase (25 °C).

(23)

ṁ E4 = n T4 ̇ x E4 MME

d d (mWA ) = n T2 ̇ yW2 MMW − (mWA ) dt dt

(28)

dC EB C dV * ) − EB B , = MMEK yaE(ylnEB − yEB dt VB dt yE5 − yE6 ylnEB = ln yE5 − ln yE6

(32)

dC WB C dV * ) − WB B , = MMW K yaW (ylnWB − yWB dt VB dt yW5 − yW6 ylnWB = ln yW5 − ln yW6

(33)

dVB V * ) + MMW K yaW (y * )] = B [MMEK yaE(ylnEB − yEB − yWB lnWB ρB dt (34) where CEB and CWB are the ethanol and water concentrations in solution for the second absorber (g L−1), respectively, ylnEB and ylnWB are the logarithmic mean molar fractions of ethanol and water in the * and yWB * are the gas phase for the second absorber,28 respectively, yEB molar fractions of ethanol and water in the gas phase in equilibrium with the liquid phase composition of the second absorber, respectively, VB is the volume of solution in the second absorber (L), and ρB is the specific mass of solution in the second absorber (g L−1). The subscript “6” represents the stream number. * and yWB * ) and the gas phase The equilibrium mass fractions (yEB composition leaving the second absorber (yE6 and yW6, stream 6) were determined similarly to the procedure used for the first absorber (eqs 26−31). 2.4. Numerical Procedures. The ordinary differential equations (eqs 9−11, 20−22, and 32−34) were integrated using the fourthorder Runge−Kutta method, implemented in an Excel spreadsheet (Microsoft Office 2010). The generalized reduced gradient (GRG) nonlinear solving method was used to find the rate constants for ethanol and water removal by stripping (kES and kWS), in eqs 9−11, and the absorption mass transfer coefficients for ethanol and water (KyaE and KyaW), in eqs 20−22 (first absorber) and eqs 32−34 (second absorber). The criterion used for finding the best fit was the

where γEA and γWA are the activity coefficients for ethanol (E) and water (W) in the first absorber liquid phase, respectively, and xEA and xWA are the molar fractions of ethanol and water in the liquid phase of the first absorber, respectively. The γEA and γWA values were calculated using the UNIQUAC equation, considering the contribution of MEG (M) in the composition (rM = 2.4087, qM = 2.2480, aEM = −8.2308, aME = 2.6876, aWM = −0.6018, aMW = 0.6018, bEM = 2632.9260 K, bME = −959.565 K, bWM = 120.7787 K, and bMW = −18.6714 K; parameters obtained using Aspen Plus v. 8.2 software). Control Volume 4 (CV4). The composition of the gas phase leaving the first absorber (yE5 and yW5, stream 5) was determined using CV4. First, the mass flow rates of ethanol and water (ṁ E5 and ṁ W5) were calculated from the mass balance for ethanol and water, according to eqs 28 and 29, which represent the difference between the mass flow rates of ethanol and water leaving the bioreactor (ṁ E2 and ṁ W2) and D

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels minimization of the sum of squared residuals between the calculated and experimental data. The unknown variables in CV2 (ṅT3, ṅT4, xE4, xW4, yE3, yW3, and yC3) were obtained using the nonlinear GRG method to solve eqs 12−18, for each instant “t”.

highest ER value was obtained for water (65.9%), followed by MEG (61.2%) and DEG (59.7%). Comparison of the means using the Tukey test indicated that the ER values for A2 (DEG) and A3 (MEG) were statistically equal, but different from the value for A1, at a 90% confidence level. When a 95% confidence level was adopted, the values for A1 and A3 were statistically equal. In absorption, it is desirable that a solute “i” should present a high molar fraction in the liquid phase (xi), at a low partial pressure in the vapor phase (Pi). Because xi values are usually low, activity coefficients at infinite dilution for the solute in the solvent (γ∞) provide a good guide in comparing absorbents.34 The lower the γ∞ value, the more appropriate the solvent should be for absorbing the solute. However, here the results revealed the opposite behavior. For instance, the values of γ∞ for ethanol in water and MEG were 4.60 and 0.55, respectively (determined using the UNIQUAC method). The superior recovery performance for water could be attributed to more effective mass transfer, compared to the other solvents, because of differences between the solvents in terms of physical properties such as viscosity, surface tension, and specific mass. At 25 °C (the temperature of the experiments), the dynamic viscosity for water is approximately 20-fold lower than that for MEG and 35-fold lower than that for DEG.34 Similar behavior was reported by Chiang et al.35 for the absorption of ethanol in aqueous glycerol solutions, using a rotating packed bed, and by Gomez-Diaz et al.36 for CO2 absorption in aqueous solutions of κ-carrageenan (nonNewtonian fluids), using a bubble column. Figure 2 also shows the water concentrations (CWA) during the assays using DEG (A2) and MEG (A3). The CWA values were similar. The amount of water in these assays was considerably lower than that for the control (A1), in which water itself was used as an absorbent. For subsequent recovery of the ethanol, this is an important aspect to be considered in selection of the solvent.34 Because of the smaller amount of water, the energy consumption for ethanol recovery by distillation should be lower for DEG and MEG. Furthermore, water and ethanol form an azeotrope.34 Therefore, although a higher amount of ethanol was recovered in assay A1 (Figure 2 and Table 1), water would not (in economic terms) be the most suitable choice for use as the absorbent. Considering MEG and DEG, the former should be of greater interest from an industrial perspective because it would be readily available for use in the process because it is already used in distilleries for extractive distillation to obtain anhydrous ethanol.37,38 Furthermore, the solution resulting from absorption (containing ethanol, water, and MEG) could be used in the extractive distillation itself. Therefore, MEG was chosen for use in the subsequent tests. 3.3. Absorption Assays using MEG as an Absorbent. In the experiments with MEG as an absorbent, evaluation was made of the effects on absorption performance of the initial MEG volume in the absorber (VMEG), the recirculation volumetric flow rate of MEG (QMEG), and the use of two absorbers connected in series. The ethanol recovery percentages (ER) and the overall volumetric coefficients of absorption for ethanol (KyaE) and water (KyaW) are shown in Table 1. The values of KyaE and KyaW ranged from 2.9 to 51.4 mol L−1 h−1 and from 22.6 to 56.8 mol L−1 h−1, respectively. Mass transfer data for ethanol and water absorption are scarce in the literature. However, there are studies that have evaluated other operating conditions (different solutes, solvents, contactors, or liquid and gas flow

3. RESULTS AND DISCUSSION 3.1. Ethanol and Water Removal by CO2 Stripping. In the absorption assays, the stream of ethanol and water was generated by CO2 stripping in a bubble column bioreactor. The conditions of temperature (34 °C) and initial ethanol concentration (80 g L−1) represented typical values for industrial ethanol production processes,2 whereas the specific CO2 flow rate (2.5 vvm) was based on the literature and was considered sufficient to reduce the ethanol inhibition and provide satisfactory results.17,23 As found in our earlier study,19 the reduction of the ethanol concentration (CES) was accompanied by a slight increase of the water concentration (CWS), showing that CO2 preferentially entrained ethanol (data not shown). This behavior was a result of the higher vapor pressure of ethanol, compared to water.25 The total mass of water (67.6 g) removed from the bioreactor was larger than the mass of ethanol (53.9 g). In terms of percentage, these values represented 3.8 and 34.6% of the initial masses of water and ethanol in the bioreactor solution, respectively, which confirms that the ethanol was preferentially removed. The values obtained for the removal rate constants for ethanol (kES) and water (kWS) were 0.0707 ± 0.0023 h−1 and 0.0069 ± 0.0003 h−1, respectively. The model was suitable for describing the removal of ethanol and water by CO2 stripping. The comparison between the experimental and simulated data of CES and CWS (see Figure S1 in the Supporting Information) showed a good correlation between the data (R2 = 0.9992). Besides, the residual standard deviation (RSD) values were lower than 1% for both, CES and CWS, indicating an excellent fit to the experimental data. 3.2. Absorbent Selection. Absorption assays were conducted in order to evaluate the ethanol absorption capacities of water (A1, reference run), DEG (A2), and MEG (A3). The experimental data for the ethanol concentration in the absorber (CEA) are shown in Figure 2. Table 1 provides the ethanol recovery (ER) percentages. The

Figure 2. Experimental data for the concentrations of ethanol (CEA, filled symbols) and water (CWA, empty symbols) in the absorption assays with water (A1, squares), DEG (A2, triangles), and MEG (A3, circles). E

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

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Table 1. Experimental Conditions and Results for the Ethanol Recovery Percentages (ER) and Overall Volumetric Coefficients of Absorption for Ethanol (KyaE) and Water (KyaW) in the Different Assays assay A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A10 A11 A11 A12 A12

(1st)c (2nd)d (1st) (2nd) (1st) (2nd)

VA/VSa(%) 30 30 30 20 30 40 40 40 40 20 20 30 30 40 40

QMEGb (L min−1)

ER (%) 65.9 ± 1.3 59.7 ± 1.4 61.2 ± 1.2 48.3 ± 0.9 61.2 ± 1.2 66.5 ± 0.6 67.3 ± 1.3 68.1 ± 0.9 70.3 ± 1.8 76.1e± 2.0

0.00 0.00 0.00 0.00 0.30 0.60 0.90 0.00 0.00 0.00 0.00 0.00 0.00

84.9e± 2.2 93.1e± 4.2

KyaE (mol L−1 h−1)

KyaW (mol L−1 h−1)

6.5−49.3 2.9−38.4 6.0−28.5 6.3−28.6 5.9−31.3 7.5−30.9 5.7−47.1 15.1−51.4 5.6−37.1 16.5−33.8 8.2−31.5 18.6−27.4

42.6−56.8 27.7−48.0 22.6−43.5 22.8−44.7 25.5−45.9 24.3−45.0 39.6−52.0 25.1−28.5 27.0−47.5 24.2−29.5 26.0−42.4 26.8−30.2

VA/VS: percentage ratio of the absorbent volume to the liquid phase volume of the bioreactor. bQMEG: recirculation volumetric flow rate of MEG. 1st: first absorber. d2nd: second absorber. eAccumulated ethanol recovery percentage (total amount of ethanol, considering the two absorbers).

a c

rates), obtaining coefficients on the same order of magnitude as the KyaE and KyaW values found here, which supports the results obtained in the present study.29,32,39,40 The ER values for the absorption assays using VMEG of 0.40 (A4), 0.60 (A5), and 0.80 L (A6) (20, 30, and 40% of the bioreactor working volume), without MEG recirculation, were 48.3, 61.2, and 66.5%, respectively. Figure 3 shows a comparison of the experimental and simulated data for the concentrations of ethanol (CEA) and water (CWA), over time, for these assays. The proposed modeling (eqs 20−22), considering condensation and absorption phenomena, provided an excellent fit and was suitable for describing the dynamics of the process. In contrast to the behavior of ER, the KyaE and KyaW values decreased with the increase of VMEG (Table 1). The ranges obtained in assays A4, A5, and A6 were 6.5−49.3, 2.9−38.4, and 6.0−28.5 mol L−1 h−1 (KyaE) and 42.6−56.8, 27.7−48.0, and 22.6−43.5 mol L−1 h−1 (KyaW), respectively. Note that KyaE and KyaW are volumetric mass transfer coefficients, defined as the product of the convective mass transfer coefficient (Ky) and the interfacial area (a) per unit volume.31 The CO2 specific flow rate values were higher for lower MEG volumes (12.50, 8.30, and 6.25 vvm for assays A4, A5, and A6, respectively), which resulted in higher gas holdup (εG) and specific area values. Gas holdup is defined as the ratio between the volumes of the gas (bubbles) and the gas−liquid dispersion in the absorber. Therefore, a greater quantity of bubbles in the absorber results in a larger mass transfer area. The εG values were 9.9, 6.9, and 5.1% for assays A4, A5, and A6, respectively, which could explain the behavior observed for KyaE and KyaW. It can also be seen from Figure 3 that for the three assays, the amount of ethanol recovered (Figure 3a) was lower than the amount of water (Figure 3b). This could be explained by several factors. In addition to the higher values of KyaW, compared to KyaE (Table 1), the driving * ) than force for mass transfer was higher for water (ylnWA − yWA that for ethanol (ylnEA − y*EA), which also contributed to increasing the absorption mass transfer rate. In addition, the fact that the mass of water removed by CO2 was larger than the mass of ethanol contributed for the high values of water concentration in the absorber. Last, it is important to mention that condensation and absorption occurred simultaneously in

Figure 3. Experimental (symbols) and simulated (lines) data for the concentrations of (a) ethanol (CEA) and (b) water (CWA) in the absorption assays with MEG volumes of 0.40 (A4, squares), 0.60 (A5, circles), and 0.80 L (A6, triangles), with no MEG recirculation. These volumes represented 20, 30, and 40% of the bioreactor working volume.

F

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels the assays because of the temperature reduction from 34 °C (bioreactor) to 25 °C (absorber). Under these conditions, condensation resulted in retention of a much higher amount of water, compared to ethanol. Considering the total amounts of ethanol and water removed by CO2 stripping in the bioreactor, 6.6% of ethanol and 35.9% of water were retained by condensation. For the absorption assays with recirculation of the liquid phase (MEG), namely, assays A7 (0.30 L min−1), A8 (0.60 L min−1), and A9 (0.90 L min−1), the values for CEA and CWA (data not shown) and the performance parameters (ER and ranges of KyaE and KyaW, see Table 1) were similar to those for assay A6, without recirculation of MEG, showing that QMEG did not affect the mass transfer. According to Higbie’s penetration theory, kx is inversely proportional to the time of exposure of the liquid to the gas bubbles containing solutes. Therefore, a higher liquid velocity should have increased the values of KyaE and KyaW because Ky is proportional to kx (eq 23). It can be argued that the CO2 flow rate was sufficiently high to maximize the values of KyaE and KyaW, which then became dependent only on the interfacial area. The latter parameter could be considered the same in these assays because the CO2 specific flow rate and the gas holdup were equal, for the same MEG volume (0.80 L). Cerri et al.41 showed that in pneumatic reactors, the volumetric mass transfer coefficient for oxygen (kLa) was a function of the interfacial area but did not depend on the kL coefficient, in agreement with the results obtained in the present work. Figure 4 shows the experimental and simulated data for the concentrations of ethanol (CEA and CEB) and water (CWA and CWB), over time, in the absorption using two absorbers in series (A and B, Figure 1), with VMEG of 0.40 (A10), 0.60 (A11), and 0.80 L (A12). The proposed models for the first absorber (eqs 20−22) and the second absorber (eqs 32−34) provided excellent fits and were suitable for describing the process. The ranges of KyaE and KyaW values for the first absorber were similar to those obtained in assays A4, A5, and A6 with only one absorber (see Table 1). The values obtained for the second absorber differed from those for the first absorber, with ranges of 15.1−51.5, 16.5−33.8, and 18.6−27.4 mol L−1 h−1 (KyaE) and 25.1−28.5, 24.2−29.5, and 26.8−30.2 mol L−1 h−1 (KyaW) for assays A10, A11, and A12, respectively. For KyaE, the trend was the same as that observed for the first absorber, with the values decreasing with the increase of the MEG volume, because of the reduction of the interfacial area. For KyaW, the values were similar among the assays. The total ethanol recovery percentages, considering the two absorbers, were 76.1 (A10), 84.9 (A11), and 93.1% (A12). For assay A10, this value was 14% higher than that for assay A6, which was also performed using an MEG volume of 0.80 L, showing that the use of an additional absorber enhanced the absorption efficiency. The profiles of KyaE and KyaW for the different assays are shown in Figures 5 and 6, respectively. The values generally decreased over time during the assays, with the exception of the KyaW values for the second absorber (see Figure 6c), which presented little variation. As the ethanol and water concentrations for both the liquid phase and the gas phase changed over time, the physical properties (viscosity, specific mass, and surface tension) and the ethanol and water diffusivities also changed, which influenced the KyaE and KyaW values. This could also explain the differences between the values for the first and second absorbers in assays A10,

Figure 4. Experimental (symbols) and simulated (lines) data for the concentrations of (a) ethanol (CEA and CEB) and (b) water (CWA and CWB) in the absorption assays using two absorbers with MEG volumes of 0.40 (A10, squares), 0.60 (A11, circles), and 0.80 L (A12, triangles), without MEG recirculation. Filled and empty symbols represent the first (A) and second (B) absorbers, respectively.

A11, and A12 (see Figures 5c and 6c). In previous absorption studies using a packed tower42 and a wet spray scrubber,32 it was shown that the mass transfer coefficient, based on the gas film, varied with the solute concentration in the gas phase. This phenomenon was attributed to the changes in surface tension, rather than dependence of the mass transfer coefficient on the solute concentration.42 Jia et al.43 showed that the presence of ethanol in the gas phase could lead to an imbalance in the surface tension and alteration of the mass transfer coefficient for CO2 absorption. Surosky and Dodge44 showed that the value of the convective coefficient increased when the diffusivity increased. Furthermore, studies of oxygen mass transfer in bioreactors45,46 have reported dependence of the mass transfer coefficients on the gas and liquid physical properties. Despite being related to different applications, these earlier findings corroborate the results of the present study. The results showed that excellent absorption performance was achieved using MEG as the absorbent in an alternative technique for the recovery of ethanol removed by stripping with CO2. This is an important finding because MEG is already used in distilleries for extractive distillation to obtain anhydrous ethanol. Furthermore, the proposed modeling was G

DOI: 10.1021/acs.energyfuels.8b03556 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 5. Overall volumetric coefficients of absorption for ethanol (KyaE), according to time, during the assays: (a) without MEG recirculation, for VMEG of 0.40 (A4), 0.60 (A5), and 0.80 L (A6); (b) with MEG recirculation, for QMEG of 0.30 (A7), 0.60 (A8), and 0.90 L min−1 (A9) (A6, without MEG recirculation, is also shown for comparison); (c) using two absorbers connected in series, for VMEG of 0.40 (A10), 0.60 (A11), and 0.80 L (A12) (filled and empty symbols represent the first and second absorbers, respectively).

Figure 6. Overall volumetric coefficients of absorption for water (KyaW), according to time, during the assays: (a) without MEG recirculation, for VMEG of 0.40 (A4), 0.60 (A5), and 0.80 L (A6); (b) with MEG recirculation, for QMEG of 0.30 (A7), 0.60 (A8), and 0.90 L min−1 (A9) (A6, without MEG recirculation, is also shown for comparison); and (c) using two absorbers connected in series, for VMEG of 0.40 (A10), 0.60 (A11), and 0.80 L (A12) (filled and empty symbols represent the first and second absorbers, respectively).

able to accurately describe the behavior of the process and could be used to simulate other process conditions. In order to show the quality of the model fitting, the RSD values were calculated. The values were lower than 5 and 22% for ethanol and water concentrations in the absorbers, respectively. In addition, the comparison between the experimental and simulated data showed a good correlation between the data (see Figure S2 in the Supporting Information). Last, a plot of the residuals (difference between experimental and simulated data) showed that the values are randomly distributed (see Figure S3 in the Supporting Information). Although it has not been addressed in this study, it is important to mention that our research team has used simulations to investigate the feasibility (in terms of energy requirements) of feeding the solution resulting from the ethanol absorption process (consisting of MEG, ethanol, and water) directly in the extractive distillation. We have also undertaken experimental work to validate the use of the technique proposed here for the recovery of ethanol in

extractive ethanol fermentations with CO2 stripping. The values for the ethanol molar fraction (