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Mass Transfer and Bioethanol Production in an External-Loop Liquid-Lift Bioreactor Gerald D. Stang, Douglas G. Macdonald, and Gordon A. Hill* Department of Chemical Engineering, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada
A novel, spinning-sparger, external-loop, liquid-lift bioreactor (ELLB) has been studied for the purpose of enhancing the production of bioethanol by simultaneous mass transfer and fermentation. Oleic acid was used to produce circulatory fluid flow in the ELLB and also to absorb ethanol from the aqueous, fermentation media. The overall ethanol mass-transfer coefficient (KCa) was found to vary between 7.0 × 10-7 and 3.0 × 10-4 s-1. KCa increased with the sparger spinning speed, oleic acid flow rate, and ethanol concentration in the aqueous phase. Oleic acid holdup (φL) in the bioreactor increased in a similar manner for each of these independent variables. Both KCa and φL were empirically fit to extensive experimental data sets using power-law models. Two batch fermentation experiments demonstrated improved production of bioethanol when oleic acid was used to remove high concentrations of ethanol from the fermentation broth. Introduction The use of bioethanol as an alternate liquid fuel source is favored by increasing costs for nonrenewable crude oil and the continuing decline in the World agricultural economy. Bioethanol is cleaner burning than most hydrocarbon fuels because of the additional OH group. Cleaner exhaust and renewable raw materials have made bioethanol manufacturing an attractive alternative to fossil fuels, but the raw material and processing costs for bioethanol have historically been much higher than those for gasoline. Batch fermentation produces bioethanol at a low volumetric efficiency. Although continuous operation could greatly improve productivity, problems with contamination have prevented the wide-scale adoption of this technology. A second major problem with bioethanol production is the toxicity of ethanol. Very few microorganisms can continue to produce ethanol at concentrations above 100 g/L, and those that do produce ethanol very slowly.1 A third negative factor involves using distillation to purify ethanol from the dilute fermentation broth because of the large energy demand.2 A variety of new processes have been studied to try and overcome these problems, one of which is the use of an organic solvent to remove ethanol during the fermentation reaction. This can lead to process improvements by removing the ethanol toxicity problem3 and improving the ease at which ethanol can be purified from water.4 Minier and Goma5 first applied this strategy in a packed column bioreactor. Several subsequent extraction studies have taken place in well-mixed fermentors6-8 and in membrane reactors.9 Even so, there can be difficult operating problems encountered when using in situ liquid extraction in a fermentor. Crabbe et al.10 observed a tenfold reduction in the overall mass-transfer coefficient when yeast was present in the bioreactor as compared to extraction from pure water. They attribute this problem to the formation of * To whom correspondence should be sent. E-mail: hill@engr. usask.ca. Fax: 306-966-4777.
layers of yeast on the outside of the oleic acid droplets. Gyamerah and Glover7 reported on the formation of an emulsion which resulted in the termination of their continuous fermentation process. Emulsion formation was enhanced by the excreted metabolites of the microorganisms. Previously, Zhang and Hill11 and Jassal et al.12 demonstrated that oleic acid was a good solvent for ethanol extraction from fermentation broth. Oleic acid has a high selectivity for ethanol over water (ratio of the number of ethanol to water molecules in the oleic acid phase >100 at 20 °C), is nontoxic to Saccharomyces cerevisiae, and is not miscible with water. Modaressi et al.13 investigated the use of a new extractive fermentation vessel: a spinning-sparger, external-loop, liquidlift bioreactor (ELLB). They studied the hydrodynamic properties of an oleic acid-water system including drop size, gas holdup, and circulation velocity. The objective of this research is to study mass transfer of ethanol from an aqueous phase to oleic acid in the spinning-sparger, ELLB. A predictive model and simultaneous mass-transfer and ethanol fermentation data are also presented. Experimental Section Chemicals and Microorganisms. Oleic acid (Emersol 213) was provided by Henkel Canada (Mississauga, Ontario, Canada). Its physical properties (as found by Zhang and Hill11) are listed in Table 1. The equilibrium distribution of ethanol in water and oleic acid was taken from the work of Zhang and Hill (also verified in this study). The ethanol used in the experiments was a technical grade produced by Commercial Alcohols Inc. (Brampton, Ontario, Canada). Fermentation broth (aqueous phase used in this study) was composed of distilled water with 20 g/L of corn steep powder provided by the Marcor Development Corp. (Hackensack, NJ) and 1.71 g/L of technical grade diammonium phosphate (96% purity) supplied by Van Waters & Rogers (Saskatoon, Saskatchewan, Canada). During the two fermentation runs, 240 g/L of glucose
10.1021/ie000990x CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5075 Table 1. Physical Properties of Oleic Acid11
mol wt
density (kg/m3 at 20 °C)
viscosity (cP at 20 °C)
boiling point (°C at 100 mmHg)
freezing point (°C)
282
893
34.8
286
16.3
was added to this aqueous phase to support the growth of yeast cells. Instant dried yeast (Fermipan, Delft, Holland) was grown for 18 h in shake flask cultures (containing the same media) to obtain 300 mL of active inoculum, which was then added to 12 L of fresh, sterile media in the ELLB to begin batch fermentations. Analysis. The dispersed phase holdup was measured by comparing the liquid interface level in the bioreactor during extraction runs to the level in the reactor when no oleic acid was passing through the system.14 Ethanol concentrations in the aqueous and organic phases were determined using a Hewlett-Packard 5890 gas chromatograph (GC) with a flame ionization detector (FID) and split flow injection. Helium carrier gas passed through a 5 m × 0.53 mm × 2.65 µm film thickness methyl silicone gum, capillary column in an oven held at 38 °C. The injector temperature was 180 °C, and the detector temperature was 200 °C. An internal standard of 1-butanol was added to each sample before injection into the GC using an autoinjector. Samples of fermentor broth were first filtered through 0.2 µm membranes to remove the yeast (present only during fermentation runs) and large particulates before being mixed 50-50 by volume with the internal standard. The oleic acid samples could not be analyzed for ethanol concentration directly in the GC because of column fouling. The oleic acid samples were, therefore, backextracted into water using the procedure of Zhang and Hill.11 The original ethanol concentration of the oleic acid was calculated using the known equilibrium relationship of the ethanol-water-oleic acid system. Yeast concentrations were determined by optical density measurements on a Milton Roy spectrophotometer (model 1011 Plus) at 620 nm. Glucose was measured using a Hewlett-Packard model 1100 HPLC, a Waters Sugar Pak I column, and a refractive index detector. The carrier fluid was 0.005 g/L of Ca EDTA in water flowing at 0.5 mL/min and 80 °C. Apparatus and Procedures. The ELLB shown in Figure 1 was used for all experiments. The oleic acid was pumped into the spinning sparger, located at the base of the riser just above the lower connection of the downcomer, using a Masterflex peristaltic pump (Cole Parmer, Chicago, IL) and C-Flex tubing. The spinning sparger was powered by a variable-speed motor. After coalescence at the top of the riser, oleic acid was drawn off and sent to a flash tank, where the ethanol was removed. The regenerated oleic acid was then returned to the feed tank and reinjected into the ELLB. The oleic acid in the feed tank was continually mixed, and samples were taken from time to time to determine residual ethanol concentrations not removed by the flash process. This residual value was less than 5% of the amounts absorbed into the oleic acid during masstransfer experiments. Experiments to measure holdup and mass-transfer rates were completed at sparger speeds from 150 to 800 rpm, 5-23% ethanol concentrations, and oleic acid flow rates ranging from 3 to 60 L/h. High ethanol concentrations were studied to analyze the benefit of coupling a solvent extraction process with the high-gravity fer-
Figure 1. Schematic of the spinning sparger, ELLB.
mentation process developed by Ingledew and Casey.15 The independent variable values for performing experiments were determined using the Central Composite Rotatable Design statistical methodology.16 Before commencement of an extraction run, a sample of the initial aqueous phase was taken from a sample port near the bottom of the loop. The sparger was then turned on to the appropriate spinning speed, and the oleic acid was pumped in at the desired flow rate. After operation for several minutes (to allow steady state to be achieved), a sample of the oleic acid flowing out of the top of the column was taken. For the next run, a new sample of the aqueous ethanol/water mixture was taken and the sparger was set to a new speed or the oleic acid flow rate was adjusted to a new rate. After a sufficiently long time, a new oleic acid sample was taken. Each run produced one data point and required about 15 min for the system to reach steady state and the samples to be taken. The relatively low rate of mass transfer meant that the ethanol concentration in the aqueous phase could be assumed to be constant during each sampling period. Initial attempts to perform fermentation runs in the ELLB failed because of high concentrations of CO2 bubbles and drastically altered hydrodynamic conditions in the vessel. The CO2 bubbles also resulted in significant quantities of the aqueous phase and yeast cells being carried out of the top of the ELLB with the exiting oleic acid droplets. To overcome this problem, a gravity settler (a 4 L Erlenmeyer flask) was installed after the ELLB and before the flash apparatus. The aqueous phase and yeast cells readily settled to the bottom of this settler and were pumped back into the middle of the riser column of the ELLB. The trapped CO2 bubbles were released out of the top of the Erlenmeyer flask. To demonstrate the feasibility of carrying out simultaneous, in situ liquid-liquid extraction and fermentation, two yeast runs were then successfully completed. In the first run, no oleic acid was used and the cells simply grew on glucose and produced ethanol. Nitrogen gas at 100 mL/min was used as the driving force for circulation in the bioreactor, while air was slowly bubbled into the downcomer (10 mL/min) in order to
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provide a small amount of oxygen to support growth. The sparger was operated at 300 rpm (Ut ) 0.69 m/s). The second run was operated in the same fashion except that the nitrogen gas was switched to oleic acid (flowing at 200 mL/min, JDR ) 6.3 × 10-4 m/s). Model Equations Empirical power law equations are commonly used to predict holdup and mass transfer from air bubbles in an external loop bioreactor. In this work, both the holdup of oleic acid droplets and the mass-transfer coefficent in the ELLB (φT) were modeled using an empirical equation similar to that used by Fraser and Hill17 for air bubbles in an External Loop Airlift Bioreactor (ELAB). For the case of holdup, this equation can be written as:
φT ) C1JDRR(1 + Ut)β
(1)
where JDR is the dispersed phase superficial velocity in the riser and Ut is the tangential velocity of the spinning orifices. Both JDR and Ut are independent variables under the control of the investigator. The tangential velocity of the orifice (Ut) can be controlled both by changing the rotation speed of the sparger and by changing the location of the orifice holes on the flat plate. It must be pointed out, however, that the uniqueness of the best-fit correlation parameters (C1, R, and β) reported in this study only applies to a flat-plate, rotating sparger. Fraser and Hill17 found that these coefficients were not affected by the presence of salt or algae media at concentrations of 1 wt %. On the other hand, in this study, ethanol concentrations from 5 to 23 wt % were present in the aqueous solution in the ELLB. Ethanol affected the holdup by causing the formation of smaller droplets of oleic acid to emanate from the spinning sparger orifices. There was strong interaction between the amount of ethanol present in the aqueous system and the rotation speed of the sparger. This was empirically modeled by allowing β to vary exponentially between an upper and lower limit:
β ) C2 + C3[1 - exp(-P/C4)]
(2)
where P is the ethanol concentration, C2 is the lower limit, C3 is the difference between the lower and upper limit, and C4 is the concentration of ethanol at which the change between the lower and upper limits has reached 63.2% of its ultimate value. Equations 1 and 2 predict that small quantities of ethanol at low concentrations create large changes in holdup compared to low changes of holdup caused by similar additions of ethanol at high concentrations. To measure the mass-transfer coefficient, the ELLB was operated at pseudo steady state. Pure oleic acid was pumped through the spinning sparger and flowed up the riser column. As the oleic acid traveled up the riser, ethanol was extracted out of the aqueous phase and into the oleic acid phase. The concentration of ethanol in the water and in the effluent oleic acid was measured and used to evaluate the overall mass-transfer coefficient based on the dispersed phase volume (KDa) from the equation:
KDa ) NA/∆YLM
(3)
where the experimental log mean ethanol concentration
driving force (∆YLM) and the ethanol flux rate (NA) can be calculated from:
(Y1 - Y*) - (Y0 - Y*) Y0 - Y* ln Y1 - Y*
]
(4)
NA ) FD(Y1 - Y0)/V
(5)
∆YLM )
[
where FD is the volumetric flow rate of the dispersed phase into the riser column, Y is the ethanol concentration in oleic acid, and Y* is the concentration of ethanol in oleic acid in equilibrium with the ethanol concentration in the aqueous phase (P). The dispersed phase volumetric mass-transfer coefficient was converted to the continuous phase volumetric mass-transfer coefficient (KCa) using experimental knowledge of the equilibrium distribution (m) of ethanol between water and oleic acid:11
KCa ) mKDa
(6)
The overall mass-transfer coefficient (KCa) in the ELLB was predicted using eqs 1 and 2 except using different best-fit values for the coefficients. With the hydrodynamic conditions known,13 masstransfer characteristics inside the ELLB studied in this work, and knowledge of the growth characteristics of S. cerevisiae fermenting glucose to ethanol,18 it was possible to develop a model incorporating both mass transfer and fermentation. For the purpose of this model, it was assumed that the liquid phase was well mixed because mass transfer and the growth of the cells were slow (measured in minutes or hours) relative to the rates at which the aqueous phase circulated about the column (circulation time of less than 10 s). Therefore, the model is:
(
XS dX P ) µmax 1dt KS + S PM
( )
)
µmax XS P dS 1)dt YX/S KS + S PM
(
(7)
)
(8)
dP YP/S XS P ) µ 1- KCa(P* - P) (9) dt YX/S maxKS + S PM dY dY 1-φ ) UD + KCa(P* - P) dt dz φ
(10)
where t is time, z is the axial distance in the riser column (assumed to be radially homogeneous), UC is the interstitial velocity of the aqueous phase, UD is the interstitial velocity of the oleic acid, X is the biomass concentration, and S is the glucose concentration. The microbial growth kinetics are based on linear product inhibition, similar to the model used earlier.18 Boundary conditions included known concentrations at time zero and the known concentration of ethanol in the influent oleic acid phase. Results and Discussion Visual observations of oleic acid droplets in the column were first undertaken. The amount of ethanol absorbed by each oleic acid drop is dependent on the amount of time each drop is in contact with the
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Figure 2. Comparison of empirical power law model (solid lines, eqs 1 and 2) to experimental oleic acid holdup data (symbols) at an ethanol concentration of 4.7%.
surrounding fluid. This, in turn, is dependent on the drop size, oleic acid holdup, and broth circulation velocity. Under most operating conditions, essentially 100% of the droplets rise directly from the sparger to the coalescing area at the top of the riser (see Figure 1) with minimal interaction with other drops or the wall of the riser column. No coalescence of droplets in the riser or downcomer was observed, except at the collection area at the very top of the riser. This lack of coalescence means that there is no change in the drop size distribution for different locations in the riser column. At low sparger speeds (
