Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 1455 – 1459
The 6th International Conference on Applied Energy – ICAE2014
Effect of mass transfer on performance of substrate degradation within annular fiber-illuminating biofilm reactor for continuous hydrogen production Chuan Zhanga,b,c*, Rong Chenc, Yi Wangb, Yanjin Wangb , Quanguo Zhangb a Institute of Electric Power, North China University of Water Resources and Electric Power, Zhengzhou 450011, China Key Laboratory of New Materials and Facilities for Rural Renewable Energy, Ministry of Agriculture, Henan Argicultural University, Zhengzhou 450002, China c Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education of China, Chongqing University, Chongqing 400044, China b
Abstract To promote hydrogen production of immobilized photosynthetic bacteria, an annular fiber-illuminating bioreactor (AFIBR) for biofilm formation on the surface of side-glowing optical fiber was developed. Mathematical model on coupled mass transport processes of substrate transport and biodegradation within biofilm with substrate diffusion and convection within bulk flow region during continuous hydrogen production was proposed. Effects of mass transfer on performance of substrate bio-degradation were investigated. 10 g/L inlet substrate concentration and 100 mL/h flow rate were identified to be Optimum conditions for maximal substrate degradation in the bioreator. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: Photosynthetic Bacteria; Cell-immobilized bioreactor; Substrate transport; Photo-biological reaction; Side-glowing optical fiber (SOF)
1. Introduction Increasing application of cell-immobilized systems has been found in the field of the biological H2 production. Compared with gel entrapment, biofilm is considered to be more suitable because of the advantages of sufficient light supply, preferential retention of active microbial mass, and high substrate conversion efficiency [1]. But, overall performance of biofilm reactor was determined to not only biological reaction within biofilm but also mass transfer within bioreactor. The system needs to be
* Corresponding author. Dr. Chuan Zhang; Tel.: +0086-0371-65790043; fax: +0086-0371-65790043. E-mail address:
[email protected] (C.Zhang)
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.146
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properly operated to enhance both mass transfer and photo-biological reaction. Mathematical models for various immobilized bioreactors have been reported. So far, few models have considered on mass transfer and photo-biological reaction of PSB biofilm reactor. In this study, an annular fiber-illuminating biofilm reactor (AFIBR) for enhancement on continuous H2 production is proposed [2]. A model describes coupled processes of substrate convection and diffusion in fluid bulk zone with substrate diffusion and biodegradation in biofilm was proposed. Operational conditions for optimum substrate degradation were then investigated. 2. Model description The scheme of the annular fiber-illuminating bioreactor (AFIBR) for hydrogen production is shown in Fig.1. Stable biofilm with a thickness of Lf covers on the optical fiber surface. An axial flow through the AFIBR in velocity u, and the produced H2 flows out with the solution for separation. Thus, the AFIBR can be generically described as a system with two separated zone: the biofilm zone and the bulk fluid zone. Substrate in the solution transfers into the biofilm along the radial direction to satisfy the degradation of PSB, leading to axial variation of the substrate concentration in the bulk fluid zone. Therefore, a two– dimensional steady-state mass transfer model is proposed based on above understanding.
Figure 1 The scheme of the annular fiber-illuminating bioreactor (AFIBR)
2.1 Governing Equations 2.1.1 Mass transfer in bulk fluid zone Considering the convective diffusion in the bulk fluid zone, the substrate transport is described as: u
•C 1 •C • 2 C • 2 C ? D( ) •x r •r •r 2 •x 2
x ? 0,
x ? L,
r ? ri - L f , r ? ro ,
(1)
C ? Ci ;
(2)
•C ? 0 •x
D
•Cb •C ? De •r •r
(3)
•C ?0 •r
The velocity distribution in the bulk fluid zone is governed by the momentum equation: du dp u d (r )? dr dx r dr
(4)
r ? ri - L f , u ? 0
r ? r0 ,
(5) (6)
u?0
2.1.2 Diffusion and degradation in biofilm In the biofilm zone, the mass transport and consumption of substrate can be described using Fick’s law and Monod kinetics as: • 2 Cb • 2 Cb 1 •Cb 1 (7) )? oC - mC D ( e
r •r
•r 2
•x 2
Yx / s
x
x
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•Cb ?0 •r •Cb •C r ? ri - L f , De ?D •r •r
r ? ri ,
x ? 0,
x ? L,
(8)
Cb ? Ci ;
(9)
•Cb ?0 •x
The effective diffusion coefficient of glucose in the biofilm is determined by De ? gD where g can be expressed as g ? 1/
(10)
0.43t x0.9 2 11.19 - 0.27 t x0.9 9
,
(11)
Furthermore, specific growth rate Ê, maximum specific growth rate, Êmax and maintenance coefficient, m of the PSB can be determined by batch experiments and are expressed as [3,4]: (12) C s 2 2 o max C s o ? exp [ /(1 / (
o max ? 0.23413 exp [ /0.3713 ( I / I opt / 1)
2
/ 8.564
9.9
) ) ]
k s - Cs
( pH / pH opt / 1) 2 / 3.324
(T / Topt / 1) 2 ]
(13)
m=0.76 (14) 2.2 Numerical simulation scheme A finite-difference scheme is implemented in the numerical solution equations described in section 2.1. The parameters used are shown in Table 1. Table 1 Parameters used in the model [3,4] Parameters
tx (kg/m3)
Values
Parameters
Values
150.1
Iopt (lx)
6000
D (m2/s)
7.94·10-10
Topt (@)
30
Yx/s
0.85
pHopt
7.0
ks (kg/cm3)
5.204
The performance of substrate degradation in the AFIBR can be evaluated by substrate consumption rate (SCR) and substrate degradation efficiency (SDE) with their definitions as following: (15) Amount of substrate consumed (mmol ) SCR ( mmol / g dry cell / h) ? SDE (%) ?
Substrate consumption time (h) · dry cell weight(g )
Amount of substrate consumed (mol ) ·100% Amount of substrate provided (mol )
(16)
3. Results and discussion Prior to the calculation, basic experiments were performed. Results of model calculation were compared with the experimental data. Effects of mass transfer within AFIBR, such as, inlet concentration and flow rate on performance of substrate biodegradation were investigated. 3.1 Influence of inlet substrate concentration Results on the substrate consumption rate (SCR) and substrate degradation efficiency (SDE) of the AFIBR under various inlet substrate concentrations (8, 9, 10, 11 and 12 g/L) are shown in Fig. 2. Compared with the experimental data, both the results indicate that the SCR and SDE increase with increasing inlet substrate concentration to reach their peak values at inlet concentration of about 10 g/L and then decrease with further increase in the inlet concentration. This implies that the PSB attain the best microbial activity at substrate concentration of about 10 g/L. The simulation results show good agreement with the experimental data.
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3.2 Influence of flow rate Results on the SCR and SDE under various inlet flow rates (40, 60, 80, 100 and 120 mL/h) are shown in Fig. 3. The inlet substrate concentration is 10 g/L. The SCR increases with increasing influent flow rate to obtain the maximal value of 2.5 mmol/g dry cell/h at flow rate 100 mL/h, as shown in Fig. 3a, while it turns to decrease with further increase in the flow rate. For a given inlet substrate concentration, high flow rate results in high total substrate load. The substrate transported into the biofilm under low flow rate is insufficient to service the metabolic needs of PSB, leading to low SCR. As the flow rate increases, increased load and speed simultaneously enhance both concentration-diffusion and convectiondiffusion of the substrate into the biofilm to supply sufficient substrate for the degradation of PSB, hence increasing SCR. However, the dominated process shifts from mass transfer process at low flow rate to biologic reaction at high flow rate. The activity of the PSB is depressed at high substrate load, hence leading to drop in consumption rate. It is noted from Fig. 3b that the SDE monotonously decreases with increasing flow rate. This can be understood that the increment in the substrate degradation is lower than that in the load resulted from increase in the flow rate. 3.5 Experimental data Simulation results
3.0
a
2.5 2.0 1.5
SCR(mmol/g dry cell/h)
SCR(mmol/g dry cell/h)
3.5
Experimental data Simulation results
3.0
a
2.5 2.0 1.5 1.0 60
1.0 45
55
b
b
SDE (%)
50
SDE (%)
40 35
45 40 35
30 25
30 25
8
9
10
11
12
Substrate concentration (g/L)
Figure.2 Effect of substrate concentration on (a) substrate consumption rate, (b) substrate degradation efficiency
40
60
80
100
120
Flow rate (mL/h)
Figure.3 The effect of flow rate on (a) substrate consumption rate, (b) substrate degradation efficiency
4. Conclusion Processes of substrate transport and biodegradation in AFIBR with immobilized PSB were investigated. Effect of mass transfer on performance of substrate degradation showed that optimum operational parameters for substrate degradation are: inlet substrate concentration of 10 g/L and flow rate of 100 mL/h. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No.51376056), National High Technology Research and Development Program(863 Program)(No.2012AA051502),China Postdoctoral Science Foundation(No.2012M521395) and LLEUTS(No.201402). Reference [1] Zhang C, Zhu X, Liao Q, Wang YZ, Li J, Ding YD, Wang H. Performance of a groove-type photobioreactor for hydrogen production by immobilized photosynthetic bacteria. Int J Hydrogen Energ 2010; 35:5284-92. [2] Zhang C, Liao Q, Zhu X, Wang YZ. Performance of continuous hydrogen production in annular fiber-illuminating biofilm reactor. CIESC J 2011; 62:3248-55. [3] Wang YZ, Liao Q, Zhu X, Li J, Lee DJ. Effect of culture conditions on kinetics of hydrogen production by photosynthetic bacteria in batch culture. Int J Hydrogen Energ 2011; 36:14004-13. [4] Liao Q, Wang YZ, Zhu X, Tian X, Ba SL, Zhang C. Effect of initial substrate concentration on kinetics of hydrogen production by photosynthetic bacteria in batch culture. China Biotech 2008; 27:51-6.
Chuan Zhang et al. / Energy Procedia 61 (2014) 1455 – 1459
Biography Chuan Zhang, MS in Engineering from Chongqing University, Chongqing, P.R China in 2003; PhD in Engineering from Chongqing University in 2010; An assistant professor of North China University of Water Resources and Electric Power; Post doctoral fellow in Key Laboratory of New Materials and Facilities for Rural Renewable Energy, Ministry of Agriculture, Henan Argicultural University; Principal fields of interest including thermal engineering, heat and mass transfer, bio-energy conversion; Current research focusing on thermal energy ultilization and energy conversion, biological hydrogen production and cell-immobilized technology.
Nomenclature D
the effective diffusion coefficient of glucose in the bulk fluid, m2/s
C
glucose concentration in the bulk fluid, g/L
Ci
glucose concentration at inlet of bioreactor, g/L
u
velocity in the bulk fluid zone, m/s
L
length of the bioreactor, m
ri
radius of the optical fiber, m
Lf
biofilm thickness, om
ro
inner radius of the bioreactor, m
uav
average velocity in the bulk fluid zone, m/s
De
effective diffusion coefficient of glucose in the biofilm, m2/s
Cb
glucose concentration in the biofilm, g/L
Cx
cell concentration in the biofilm, g/L
o
the specific growth rate, 1/s
Yx/s
the biomass yield
m
the maintenance coefficient of the PSB, 1/s
ks
the Monod half-saturation constant, g/L
I
the light intensity at the surface of the optical fiber, W/m2
tx
the density of the biofilm, kg/m3
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