Index

Abstract

This paper presents an effective way to control the temperature of bio-reactors (fermenters) used in ethanol production and to reduce the volume of cooling water required per square meter of ethanol produced.  This paper specifically focuses on the fermentation of cassava starch using Saccharomyces cerevisiae at 32ºC. The flow across tube banks model is employed as the cooling mechanism of the bio-reactor. Cooling water at 28 enters a shell containing five rows of fifteen (15) bio-reactors in a square in-line arrangement and exits the shell at . The total working volume of all fifteen (15) bio-reactors in the bank equals . Each bio-reactor in the bank is designed to have an effective heat transfer area to volume ratio of two (2) to enhance heat transfer. The total quantity of cooling water required per cubic meter of ethanol produced is found to be . A total amount of 1.303kW is required to power anchor impellers placed in each bio-reactor to provide mixing. The rotation speed of the impeller in each bio-reactor is. A total of 3.453*10-6W is required to move cooling water through the bio-reactor bank at a speed of. An overall heat transfer coefficient of  was found for the bio-reactor cooling system. Employing flow across tube banks model in cooling ethanol bio-reactors required significantly less amount of cooling water per cubic meter of ethanol produced compared to using internal cooling coils.

Keywords: Cooling water economy, Shell-and-tube heat exchanger, Bio-ethanol production, Heat transfer, Mass transfer, Fermentation.

Received: 16 June 2022 / Revised: 2 August 2022 / Accepted: 19 August 2022 / Published: 9 September 2022

Contribution/ Originality

This study shows how cooling water can be economized when flow across tubes in bank model is employed instead of using internal cooling coils as a cooling mechanism.

1. INTRODUCTION

This paper presents an effective way to cool industrial-scale bio-reactors (fermenters) to maintain optimum operating temperature and to reduce the volume of cooling water required per square meter of ethanol produced. 

The yeast Saccharomyces cerevisiae remains the preferred organism for ethanol production due to its high ethanol, inhibitor, and osmo-tolerance in industrial processes, but it lacks starch degrading enzymes required for the efficient utilization of starch [1], therefore starch has to be first reduced to simple sugar before fermentation can occur. This process involves the conversion of cooked starch to maltodextrin using an α-amylase enzyme (Liquification Process) and the conversion of maltodextrin to glucose and fructose using glucoamylase enzyme (Saccharification Process) [2]. The simple sugar formed is then fermented anaerobically with yeast to produce ethanol and carbon dioxide (fermentation). Equation 1 describes the fermentation of glucose to produce ethanol.

2. METHOD

Flow across tube banks is a model of the shell-and-tube exchanger where heat exchange occurs between a fluid flowing through a bank of tubes and another fluid moving through a tube in a perpendicular direction. The tubes are usually placed in a shell, especially when the fluid flowing through the bank is a liquid. Fluid flows through the space between the tubes and the shell for heat exchange to occur. The tubes are either arranged in-line or in a staggered manner [9].

Excess heat generated within the bio-reactor is removed by water flowing through reactor banks in a square in-line arrangement.

3. THEORY/ CALCULATIONS

3.1. Thermal Properties of Cassava Starch Solution

The thermal properties and density of cassava starch solutions studied as a function of temperature (30-50°C) and concentration (20-50%w/w) is reported by Cansee, et al. [10] and presented in Table 1.

Table 1 . Thermal properties of Cassava starch [10].

The intrinsic viscosity of Cassava Starch that has undergone hydrolysis is reported by Rocha, et al. [11] as 2.20. Equation 2 is used to determine the dynamic viscosity from intrinsic viscosity.


Table 2. Dimensions and properties of proposed fermenter/bio-reactor

Table 2 above contains dimension and physical properties of proposed new fermenter or bio-reactor to enable effective heat transfer.

3.2. Rate of Heat Generation Within Bio-Reactor

3.3.1. Calculation of Reynolds Number, Prandtl’s Number, and Friction Factor

Table 3. Properties of water at arithmetic mean temperature [18].

Table 3 above presents properties of water at arithmetic mean temperature

Substituting values into Equation 22,

Where;  is the number of rows, f is friction factor, x is a correction factor,  maximum velocity as calculated in Equation 24 above, x is equal to 1 for a square arrangement of tubes in a bank [9]. Friction factor(f), a function of Reynolds number and the ratio between the distances between any two tubes in a column to the outer diameter of the tube. The value of f is read from a graph [9].
Substituting value into Equation 50,

3.6. Overall Heat Transfer Coefficient (U)

4. RESULTS

Table 4 presents a summary of mechanical and chemical engineering design parameters of newly designed fermenter/bio-reactor.

Table 4. Summary of chemical/mechanical engineering design parameters

5. DISCUSSIONS

Khatiwada and Silveira in a case study of an Indian distillery reported that, fermentation of 400 wort using S. cerevisiae completes in 32h with 8% (v/v) ethanol concentration, which requires 550cooling water per cubic meter of ethanol produced to maintain the bio-reactor temperature at 32ºC using internal cooling coils as a heat exchange medium [20] 97.833m3 cooling water per cubic meter of ethanol produced presented by this paper is five times lower and can significantly reduce the unit cost of ethanol production.

A benefit of using a flow across tube banks to using internal cooling coils to control the temperature of ethanol bio-reactors is a lower risk of fouling. When internal cooling coils are used in ethanol bio-reactor typically in the fermentation of viscous fluids such as cassava starch, the content of the reactor can easily coat the outside surface of the cooling coils thereby decreasing effective heat transfer. In this design, the whole curved surface area of the cylindrical bio-reactor is used as a heat transfer area given an effective heat transfer surface area to volume ratio of 2. A sudden temperature change within the bio-reactor system is unlikely due to the large volume of water (1696.5) flowing through the shell. This is because an amount of about  of energy is required to cause a one-degree temperature change in the mass of cooling water in the shell of the bio-reactor system compared to a total heat generation rate of . This design assumes no heat is lost to the environment from the bio-reactor but all heat is transferred to cooling water flowing through the bio-reactor bank.
The Anchor impeller type used is designed to sweep through the whole volume of the bio-reactor to enhance effective heat transfer [19].

The close temperature difference between the cooling water inlet and outlet of 3.84 saves energy required in the cooling tower to restore the cooling water to 28 for reuse to cool the bio-reactor.

One major disadvantage of this design is, it required high initial capital investment to build many small bio-reactors instead of building a single large bio-reactor.

6. CONCLUSION

Employing flow across tube banks model in cooling ethanol bio-reactors can save the amount of cooling water required to produce a cubic meter of ethanol by about a fifth compared to using internal cooling coils.

Funding: This study received no specific financial support.  

Competing Interests: The authors declare that they have no competing interests.

Authors’ Contributions: All authors contributed equally to the conception and design of the study.

REFERENCES

[1]         L. Long-Liu, Y.-J. Ma, H. R. Chien, and W.-H. Hsu, "Construction of an amylolytic yeast by multiple integration of the Aspergillus awamori glucoamylase gene into a Saccharomyces cerevisiae chromosome," Enzyme and Microbial Technology, vol. 23, pp. 360-365, 1998.Available at: https://doi.org/10.1016/s0141-0229(98)00058-1.

[2]         S. Sumardiono, G. Budiarti, and Kusmiyati, "Conversion of cassava starch to produce glucose and fructose by Enzymatic Process Using Microwave Heating," presented at the MATEC Web of Conferences (p. 156). Les Ulis: EDP Sciences, 2017.

[3]         C. Wyman, Handbook on bioethanol production and utilisation. Oxfordshire: Routledge, 1996.

[4]         S. Kumar, P. Dheeran, S. P. Singh, I. M. Mishra, and D. K. Adhikari, "Cooling system economy in ethanol production using thermotolerant yeast Kluyveromyces sp. IIPE453," American Journal of Microbiological Research, vol. 1, pp. 39-44, 2013.Available at: https://doi.org/10.12691/ajmr-1-3-1.

[5]         N. Kosaric, "Ethanol–potential source of energy and chemical products," Biotechnology: Products of Primary Metabolism, pp. 121-203, 1996.Available at: https://doi.org/10.1002/9783527620883.ch4.

[6]         K. Theerarattananoon, Y.-H. Lin, and D.-Y. Peng, "Metabolic heat evolution of Saccharomyces cerevisiae grown under very-high-gravity conditions," Process Biochemistry, vol. 43, pp. 1253-1258, 2008.Available at: https://doi.org/10.1016/j.procbio.2008.07.006.

[7]         M. Madigan, J. Martinko, K. Bender, D. Buckley, D. Stahl, and T. Brock, Brock biology of microorganisms. London: Pearson, 2014.

[8]         J. Nielsen, J. Villadsen, and G. Liden, Bioreaction principles. New York: Springer, 2002.

[9]         Y. Cengel, Heat transfer a practical approach. New Jork: Mcgraw-Hill, 2002.

[10]       S. Cansee, C. Watyotha, T. Thivavarnvongs, J. Uriyapongson, and J. Varith, "Effects of temperature and concentration on thermal properties of cassava starch solutions," Songklanakarin Journal of Science & Technology, vol. 30, pp. 405-411, 2008.

[11]       T. d. S. Rocha, A. P. d. A. Carneiro, and C. M. L. Franco, "Effect of enzymatic hydrolysis on some physicochemical properties of root and tuber granular starches," Food Science and Technology, vol. 30, pp. 544-551, 2010.Available at: https://doi.org/10.1590/s0101-20612010000200039.

[12]       C. Tanford, "Intrinsic viscosity and kinematic viscosity," The Journal of Physical Chemistry, vol. 59, pp. 798-799, 1955.

[13]       B. F. Blackwell, W. Gill, K. J. Dowding, and T. E. Volth, "Determination of the thermal conductivity of 304 stainless steel using parameter estimation techniques," in Proceedings of NHTC '00 34th National Heat Transfer Conference, (pp. 20-22). Pittsburgh, Pennsylvania, 2000.

[14]       G.-W. Choi, H.-J. Um, Y. Kim, H.-W. Kang, M. Kim, B.-W. Chung, and Y.-H. Kim, "Isolation and characterization of two soil derived yeasts for bioethanol production on Cassava starch," Biomass and Bioenergy, vol. 34, pp. 1223-1231, 2010.Available at: https://doi.org/10.1016/j.biombioe.2010.03.019.

[15]       S. H. M. Azhar, R. Abdulla, S. A. Jambo, H. Marbawi, J. A. Gansau, A. A. M. Faik, and K. F. Rodrigues, "Yeasts in sustainable bioethanol production: A review," Biochemistry and Biophysics Reports, vol. 10, pp. 52-61, 2017.Available at: https://doi.org/10.1016/j.bbrep.2017.03.003.

[16]       V. Gnielinski, "New equations for heat and mass transfer in turbulent pipe and channel flow," International Chemical Engineering, vol. 16, pp. 359-368, 1976.

[17]       C. J. Geankoplis, Transport processes and unit operations. City, State, Country: PTR Prentice Hall, 1993.

[18]       E. Eckert and R. Drake, Analysis of heat and mass transfer. McGraw-Hill: New York, 1972.

[19]       K. Raju, Fluid mechanics, heat transfer and mass transfer chemical engineering practice. Hoboken, New Jersey: John Wiley and Sons, Inc, 2011.

[20]       D. Khatiwada and S. Silveira, "Net energy balance of molasses based ethanol: The case of Nepal," Renewable and Sustainable Energy Reviews, vol. 13, pp. 2515-2524, 2009.Available at: https://doi.org/10.1016/j.rser.2009.06.028.

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