Continuous ethanol fermentation using membrane bioreactors (MBR)

Transcription

1 Continuous ethanol fermentation using membrane bioreactors (MBR) Johan Thuvander Department of Chemical Engineering, LTH, Lund University Abstract Continuous ethanol production using membrane bioreactors (MBR) for internal cell recycling was studied in this work. Diluted beet molasses was fermented by baker s yeast, Saccharomyces cerevisiae. Two MBR systems were studied, one system with a membrane module integrated in the fermentor and one system where the fermentor was equipped with an external membrane module. The membrane used was a microfiltration membrane normally used in waste water treatment applications. The ethanol productivity in the system with internal membranes was 2.7 g/l h. This shows the potential of an MBR system, even though far from optimized in this prestudy. Introduction Bioethanol has received a lot of attention as a new liquid fuel for vehicles. The production of bioethanol is today done by fermenting starch or sugars [1]. The fermentation method mainly used is batch fermentation with the yeast Saccharomyces cerevisiae [2]. reactor [3, 4]. An MBR can be used to recycle yeast during ethanol fermentation, in very much the same way. In this study, the possibility to use a MBR module usually used for waste water treatment for ethanol fermentation was investigated. Materials and Methods Equipment A jacketed fermentor with the working volume of 14 liters was used for the fermentations. An MBR module from Alfa Laval (Denmark) was used in the experiment with an internal membrane module, Fig. 1. The module consisted of 9 cartridges that each was equipped with two 3x3 cm MFP 2 membranes (Alfa Laval, Denmark) with the pore size of 2 μm. The total membrane area of the module was 1.62 m 2. A diagram of the fermentation system with internal membranes can be seen in Fig. 2. The ethanol productivity during batch fermentation without cell recycle is g/l h [1].The productivity could be increased by making the process continuous and increasing the yeast concentration which would lower the cost of the produced ethanol. However, continuous processes require a yeast recycling system to reach high productivities and existing methods for achieving this are expensive [1]. MBR s have been used in waste water treatment applications to achieve long residence time of active sludge in a small Figure 1. Picture of the internal membrane module.

2 Figure 2. Equipment setup for continuous fermentation with internal membranes. An Alfa Laval PilotUnit Combi M39 L (Alfa Laval, Denmark) was used for the experiment with an external membrane module. The external module was equipped with.8 m 2 of flat sheet membranes of the same type as in the internal membrane module. A diagram of the fermentation system with external membranes can be seen in Fig. 3. Figure 3. Equipment setup for continuous fermentation with external membranes. Microorganism and fermentation medium The yeast was ordinary baker s yeast (S. cerevisiae) from Jästbolaget (Sweden) in the form of active dry yeast. Beet molasses from Nordic sugar (Sweden) was used as fermentation substrate. The molasses was diluted to the sucrose concentration 12 g/l and ph adjusted to ph 5.5 with sulfuric acid to minimize the risk and severity of infections [6]. The fermentation medium was sterilized through boiling for 3 minutes. Experimental procedure To ensure high cell viability, the active dry yeast was added to sterilized tempered water for slow rehydration for minutes before being gently stirred and added to the fermentor [5]. For each fermentation 28 g of active dry yeast was used, giving the initial yeast concentration 2 g/l in the fermentor.

3 The continuous fermentations were started as batch fermentations. The continuous fermentation was started after hours by extracting filtration permeate and adding an equal amount of feed solution to the fermentor. Permeate was extracted at the rate 9 l/h (5.6 l/m 2 h) in the continuous fermentation experiment with internal membranes. The continuous fermentation with external membranes had a constant transmembrane pressure of.8 bar and a cross flow velocity of.16 m/s. The temperature in the fermentors was kept constant at 3 C by feeding cooling water to the cooling jacket of the fermentor. Analysis The yeast concentration was measured by filtering 5 ml of the samples through.2 µm ME 24 filters (Schleicher & Schuell, Germany).The sample was washed with an equal volume of water and dried at 5 C for 24 hours. The yeast concentration was determined by weighing the filters before filtration and after drying. The concentration of sugar, ethanol, glycerol and organic acids in the samples was determined using high-performance liquid chromatography (HPLC). Two HPLC setups (Shimadzu, Japan) were used, both with a refractive index detector (Shimadzu, Japan). The column used in the HPLC setup for analysis of ethanol, lactic acid, acetic acid and glycerol was an Aminex HPX-87H column (Bio-Rad, USA). The eluent used was 5 mm sulfuric acid at 5 C with a flow rate of.5 ml/min. The HPLC setup for analysis of sucrose, glucose and fructose used an Aminex HPX- 87P column (Bio-Rad, USA) and.5 ml/min deionized water at 85 C as eluent. Results & Discussion Fermentation medium Besides sucrose the molasses mainly contained glucose, galactose, lactic and acetic acid, see Tab. 1. Yeast growth inhibition was expected as lactic and acetic acid start to cause yeast inhibitions at 8 and.5 g/l respectively [7]. The molasses was therefore diluted but had still a reasonable sugar concentration. Table 1. Concentration of sugars and organic acids in untreated and diluted molasses. Molasses Sucrose (g/l) Glucose (g/l) 46 9 Galactose (g/l) 16 3 Lactic acid (g/l) Acetic acid (g/l) Diluted molasses The maximal theoretical amount of ethanol that can be produced from this medium can be calculated using the stochiometrical equations of sugars converted to ethanol. Eq.1 is the conversion from glucose and galactose into ethanol and Eq. 2 is the conversion of sucrose into ethanol. 1 kg of glucose and galactose can produce.51 kg of ethanol and 1 kg of sucrose can produce.54 kg of ethanol, according to Eqs.1 and 2. The theoretical maximal ethanol concentration that can be reached with the used fermentation medium is thus 7 g/l. 2 2 (1) 4 4 (2) Batch fermentation Batch fermentation resulted in a final ethanol concentration of 5 g/l, 7 % the theoretical maximal ethanol concentration. Continuous fermentation with internal membranes The average ethanol concentration was 42 g/l, between 2 and 4 hours of fermentation, as shown in Fig. 4. This corresponds to 6 % of the theoretical maximum ethanol yield and the ethanol productivity rate 2.7 g/l h. The yeast quickly hydrolyze sucrose to glucose and fructose [8,9] which is why only the glucose concentration is shown in Fig 4.

4 Concentration (g/l) Figure 4. Ethanol and glucose concentration during the continuous fermentation experiment with internal membranes. As the glucose concentration decreased, the yeast growth rate decreased and leveled out at 9 g/l, as shown in Fig 5. Concentration (g/l) Figure 5. Yeast concentration during the continuous fermentation experiment with internal membranes. TMP increased when the yeast concentration increased during the experiment but was still below,15 bar after 4 hours of fermentation. Continuous fermentation with external membranes Ethanol Glucose The flux during the continuous fermentation experiment with external membranes was initially higher than for the experiment with internal membranes as a result of a higher TMP, but the flux decreased fast due to cake formation. The permeate flow was between 5 and 3.7 l/h during the main part of the experiment, as shown in Fig 6, which corresponds to a flux between 6.3 and 4.6 l/m 2 h. Permeate flow (l/h) Figure 6. Flux during the continuous fermentation experiment with external membranes. The average ethanol concentration was 55 g/l, 78 % of the theoretical maximal yield, during 34 to 58 hours of fermentation (see Fig 7). The average permeate flow rate was 4 l/h and the ethanol production rate was 1.6 g/l h. Concentration (g/l) Figure 7. Ethanol and glucose concentration during the continuous fermentation experiment with external membranes. The yeast concentration increased continuously during the experiment and reached almost 15 g/l at the end of the fermentation, as shown in Fig 8. Concentration (g/l) 15 Figure 8. Yeast concentration during the continuous fermentation experiment with internal membranes- Conclusions Ethanol Glucose Yeast tends to form filter cakes and high TMP shall be avoided because of that. It was observed that when using membranes integrated in the fermentor and the flux 5.6 l/m 2 h was used, the agitation in the fermentor

5 was sufficient to counteract most of the filter cake formation. The ethanol productivity is directly connected to the throughput of fermentation medium in the fermentor. Because of this, higher productivities would be possible if the amount of membrane area in the fermentor would be increased. The ethanol yield was low during the experiments which was due to unfavorable fermentation conditions such as high concentration of organic acids. If the fermentation conditions are optimized, higher yields would be reached, as well as significantly higher ethanol productivities. Future work The ethanol productivity can be increased if: More membrane area is used in the fermentor, which will enable higher throughput in the fermentor without increasing the flux. Higher yeast concentration is used which can ensure complete utilization of the added sugars. Fermentation conditions are made more favorable so higher ethanol yield is achieved. Higher flux and longer operation times can be reached if a cleaning cycle is integrated into the continuous operation. If the filtration operation is stopped periodically the accumulated material on the membranes can be swept away by the agitation in the fermentor. Acknowledgements The author would like to thank Holger Krawczyk and Anders Arkell for making this thesis possible and for their guidance and assistance through the work. Thanks go to Alfa Laval for providing the membranes and for adapting the internal membrane module for this work. Special thanks to Frank Lipnizki and Franck Hansen from Alfa Laval for their interest in this thesis and assistance. The author also like to thank Nordic Sugar and Jästbolaget that generously donated the beet molasses and yeast that were used in this thesis. Finally, thanks to the wonderful people at the Department of Chemical Engineering at Lund University for their support. References [1] Kosaric N., Duvnjak Z., Farkas A., Sahm H., Bringer-Meyer S., Goebel O., Mayer D., 211, Ethanol, Ullmann's Encyc. Ind. Chem.. [2] Logsdon J. E., 24, Ethanol, Kirk- Othmer Encyc. Chem. Tech.. [3] Le-Clech P., Chen V., Fane T. A. G., 26, Fouling in membrane bioreactors used in wastewater treatment, J. Membrane Sci., 284 (1-2), [4] Sutherland K., 2, The rise of membrane bioreactors, Filtr. & Separat.., 47 (5), [5] Jenkins D. M., Powell C. D., Fischborn T., Smart K. A., 211, Rehydration of Active Dry Brewing Yeast and its Effect on Cell Viability, Ins. Brew. Disti., 117( 3), [6] Narendranath NV., Power R., 25, Relationship between ph and medium dissolved solids in terms of growth and metabolism of lactobacilli and Saccharomyces cerevisiae during ethanol production, Appl. Environ. Microbiol., 71(5), [7] Jacques K., Lyons T.P., Kelsall D.R., 21, The alcohol textbook, Nottingham U. Press. [8] Anderson S. B., Luiz C. M., Boris U. S., 24, Sucrose fermentation by Saccharomyces Cerevisiae lacking hexose transport, J. Mol. Microbiol. Biotechnol., 8( 1), [9] Fuente G., Sols A., 1961, Transport of sugar in yeast: II. Mechanisms of utilization of disaccharides and related glycosides, Biochim. Biophys. Acta, 56(1962),