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Particle suspension in a circulating bed fermenter
The Chemical
Engineering
Journal,
32 (1986)
B39
- B41
B39
Short Communication
Particle suspension fermenter
V. Y. LOH,
S. R. RICHARDS*
AFRC Food Britain) (Received
in a circulating
Research
November
Institute,
bed
and P. RICHMOND Norwich
(Gt.
p = PDJ,
12,1985)
The internal circulation loop or circulating bed reactor has been developed for use with immobilized microbial cultures [l] and has been successfully employed for the full-scale treatment of waste waters [ 2, 31. An essential criterion for the efficient utilization of this kind of fermenter is that the immobilized cells are fully suspended. We have investigated the suspension characteristics of a circulating bed fermenter and the results are reported here. A glass vessel 0.216 m in diameter was used for the reactor. The operating volume was varied from 1.0 X 10e2 to 2.8 X lo-* m3 which produced aspect ratios (height to diameter) of between 1.20 and 3.40. An H-shaped sparger with 12 1.0 mm orifices was located off centre on the fermenter base for distribution of air over half the fermenter crosssectional area. Reticulated polyurethane foam cubes 8 mm X 8 mm X 8 mm in size (FiltrenT obtained from Recticel, Wetteren, Belgium) were used as support particles. The particles have a voidage of 0.97 and a porosity of approximately 60 pores per linear inch. The particles were filled with 2% w/w agar to simulate immobilized biomass [ 41 and thus displaced a volume of 0.512 cm3. Using this value the foam volume or particle concentration within the fermenter was calculated. Complete particle suspension, which was defined as the state attained when no particles remain stationary on the base or at the top of the fermenter for more than 3 s, was *Present Britain.
address:
0300-9467/86/$3.50
BioTechnica
Ltd.,
Cardiff,
determined visually and the corresponding minimum suspension gas flow rate u, was noted. The minimum specific power P required for particle suspension was then evaluated from
Gt.
(1)
by assuming that the air expanded isothermally [ 51. Visual observations of the movement of the foam particles at low particle concentrations indicated that the flow characteristics of the circulating bed fermenter were very similar to those of a conventional bubble column [6]. In addition to an overall upflow and downflow circulation over the whole height of the column, there existed small circulation loops which appeared in height to be of the order of the column diameter. However, as the particle concentration increased to about 30%, no small circulation loops were observed. This suggested that the liquid circulation was restricted. Figure 1 illustrates the effects of particle concentration on the minimum specific
I 5
15
25
Particle concentration
Fig. 1. Effect mum specific l, C = lo%;4 c = 30%.
35
(%)
of particle concentration C on the minipower required for particle suspension: C = 15%; n, C = 20%;4 C = 25%; 0,
@ Elsevier
Sequoia/Printed
in The Netherlands
B40
power required for suspending the particles. At particle concentrations less than 30%, the minimum specific power increases slowly with concentration. Above 30%, a slight increase in particle concentration necessitates a large increase in power input to maintain complete particle suspension. The point at which this occurs therefore represents the critical particle concentration and is an indication of the maximum amount of particles that can be efficiently suspended in the fermenter. Provided that this critical value is not exceeded, the energy required for particle suspension is small and the corresponding superficial gas velocity is low (less than 0.01 m s-l). Walker and Adams [ 31 have reported that in a waste water treatment plant (160 m3) containing approximately 31% biomass support particles the minimum specific power required for acceptable levels of particle mixing was between 20 and 30 W m-3. The aspect ratio of the plant was approximately 1.0. From Fig. 1 the minimum specific power required to suspend the foams at a particle concentration of 31% in a fermenter with an aspect ratio of 1.2 is 20 W me3 which is similar to that obtained by Walker and Adams. The work of Black et al. [ 11, using a laboratory-scale vessel (2.8 X 10e3 m3) with an aspect ratio of 3.5 and containing 34% particles, suggested that adequate particle movement could be achieved using a lower specific power (approximately 17 W m-3). Black et al. used 6 mm cube foam particles, whilst Walker and Adams used much larger particles (25 mm X 25 mm X 12.5 mm) compared with. 8 mm cubes in our study. The choice of particle size used by the various researchers was governed by the type of biomass immobilized and the operational constraints in the fermenter. This result suggests that the particle size is an important parameter that has to be considered when investigating the mixing characteristics of a circulating bed fermenter. The effect of the aspect ratio on particle suspension is shown in Fig. 2. At a constant particle concentration the minimum suspension power increases with aspect ratio. In general, the rate of increase is higher when the aspect ratio is above 2.0. This result suggests that particle suspension is more efficient in a short circulating bed fermenter than in a tall one and is in good agreement with the observation made by Atkinson and Lewis [ 71.
I 143
2.0
30
4-o
Aspect ratio
Fig. 2. Effect of aspect ratio H/D on the minimum specific power required for particle suspension: 0,
H/D = 1.2;4 HID = 1.9;A, HID = 2.6; 0, H/D = 3.0; n, HID = 3.4.
In a circulating bed fermenter, optimization of the volumetric particle concentration is the simplest method of maximizing its volumetric productivity. Increasing the particle concentration would lead to an increase in volumetric productivity but this increase may be nullified if the associated increase in power requirements for mixing is too large. Under the conditions in this study using 8 mm foam cubes, the maximum particle concentration that may be efficiently suspended is approximately 30% foam volume. The use of short reactors (with an aspect ratio of less than 2.0) would minimize the power needed to keep the particles in suspension. The design of a vessel within these limits would therefore minimize the energy required to operate a circulating bed fermenter.
References 1 G. M. Black, C. Webb, T. M. Matthews and B. Atkinson, Biotechnol. Bioeng., 26 (1984) 134. 2 I. Walker and E. P. Austin, in P. F. Cooper and B. Atkinson (eds.), Biological Fluidised Bed
B41
of Water and Wastewaters, Ellis Horwood, Chichester, West Sussex, 1981, p. 272. 3 I. Walker and P. Adams, Water Pollut. Control, to be published. 4 H. Tanaka, Biotechnol. Bioeng., 24 (1982) 425. Treatment
5 J. J. Heijnen and K. Van? Riet, Chem. Eng. J., 28 (1984) B21. 6 J. B. Joshi, Trans. Inst. Chem. Eng., 58 (1980) 155. 7 B. Atkinson and P. J. S. Lewis, in J. E. Smith, D. R. Berry and B. Kristiansen (eds.), Fungal Bio-
technology, British Mycological Ser. 3, Academic Press, London,
Society 1980,
Symp. p. 153.
APPENDIX
C D g H P u,
A: NOMENCLATURE
volume concentration of particles (5%) diameter of fermenter (m) gravitational constant (m SC*) height of fermenter (m) specific power for aeration (W me3) gas superficial velocity (m s-l)
Greek symbol p liquid density
(kg mP3)
Engineering
Journal,
32 (1986)
B39
- B41
B39
Short Communication
Particle suspension fermenter
V. Y. LOH,
S. R. RICHARDS*
AFRC Food Britain) (Received
in a circulating
Research
November
Institute,
bed
and P. RICHMOND Norwich
(Gt.
p = PDJ,
12,1985)
The internal circulation loop or circulating bed reactor has been developed for use with immobilized microbial cultures [l] and has been successfully employed for the full-scale treatment of waste waters [ 2, 31. An essential criterion for the efficient utilization of this kind of fermenter is that the immobilized cells are fully suspended. We have investigated the suspension characteristics of a circulating bed fermenter and the results are reported here. A glass vessel 0.216 m in diameter was used for the reactor. The operating volume was varied from 1.0 X 10e2 to 2.8 X lo-* m3 which produced aspect ratios (height to diameter) of between 1.20 and 3.40. An H-shaped sparger with 12 1.0 mm orifices was located off centre on the fermenter base for distribution of air over half the fermenter crosssectional area. Reticulated polyurethane foam cubes 8 mm X 8 mm X 8 mm in size (FiltrenT obtained from Recticel, Wetteren, Belgium) were used as support particles. The particles have a voidage of 0.97 and a porosity of approximately 60 pores per linear inch. The particles were filled with 2% w/w agar to simulate immobilized biomass [ 41 and thus displaced a volume of 0.512 cm3. Using this value the foam volume or particle concentration within the fermenter was calculated. Complete particle suspension, which was defined as the state attained when no particles remain stationary on the base or at the top of the fermenter for more than 3 s, was *Present Britain.
address:
0300-9467/86/$3.50
BioTechnica
Ltd.,
Cardiff,
determined visually and the corresponding minimum suspension gas flow rate u, was noted. The minimum specific power P required for particle suspension was then evaluated from
Gt.
(1)
by assuming that the air expanded isothermally [ 51. Visual observations of the movement of the foam particles at low particle concentrations indicated that the flow characteristics of the circulating bed fermenter were very similar to those of a conventional bubble column [6]. In addition to an overall upflow and downflow circulation over the whole height of the column, there existed small circulation loops which appeared in height to be of the order of the column diameter. However, as the particle concentration increased to about 30%, no small circulation loops were observed. This suggested that the liquid circulation was restricted. Figure 1 illustrates the effects of particle concentration on the minimum specific
I 5
15
25
Particle concentration
Fig. 1. Effect mum specific l, C = lo%;4 c = 30%.
35
(%)
of particle concentration C on the minipower required for particle suspension: C = 15%; n, C = 20%;4 C = 25%; 0,
@ Elsevier
Sequoia/Printed
in The Netherlands
B40
power required for suspending the particles. At particle concentrations less than 30%, the minimum specific power increases slowly with concentration. Above 30%, a slight increase in particle concentration necessitates a large increase in power input to maintain complete particle suspension. The point at which this occurs therefore represents the critical particle concentration and is an indication of the maximum amount of particles that can be efficiently suspended in the fermenter. Provided that this critical value is not exceeded, the energy required for particle suspension is small and the corresponding superficial gas velocity is low (less than 0.01 m s-l). Walker and Adams [ 31 have reported that in a waste water treatment plant (160 m3) containing approximately 31% biomass support particles the minimum specific power required for acceptable levels of particle mixing was between 20 and 30 W m-3. The aspect ratio of the plant was approximately 1.0. From Fig. 1 the minimum specific power required to suspend the foams at a particle concentration of 31% in a fermenter with an aspect ratio of 1.2 is 20 W me3 which is similar to that obtained by Walker and Adams. The work of Black et al. [ 11, using a laboratory-scale vessel (2.8 X 10e3 m3) with an aspect ratio of 3.5 and containing 34% particles, suggested that adequate particle movement could be achieved using a lower specific power (approximately 17 W m-3). Black et al. used 6 mm cube foam particles, whilst Walker and Adams used much larger particles (25 mm X 25 mm X 12.5 mm) compared with. 8 mm cubes in our study. The choice of particle size used by the various researchers was governed by the type of biomass immobilized and the operational constraints in the fermenter. This result suggests that the particle size is an important parameter that has to be considered when investigating the mixing characteristics of a circulating bed fermenter. The effect of the aspect ratio on particle suspension is shown in Fig. 2. At a constant particle concentration the minimum suspension power increases with aspect ratio. In general, the rate of increase is higher when the aspect ratio is above 2.0. This result suggests that particle suspension is more efficient in a short circulating bed fermenter than in a tall one and is in good agreement with the observation made by Atkinson and Lewis [ 71.
I 143
2.0
30
4-o
Aspect ratio
Fig. 2. Effect of aspect ratio H/D on the minimum specific power required for particle suspension: 0,
H/D = 1.2;4 HID = 1.9;A, HID = 2.6; 0, H/D = 3.0; n, HID = 3.4.
In a circulating bed fermenter, optimization of the volumetric particle concentration is the simplest method of maximizing its volumetric productivity. Increasing the particle concentration would lead to an increase in volumetric productivity but this increase may be nullified if the associated increase in power requirements for mixing is too large. Under the conditions in this study using 8 mm foam cubes, the maximum particle concentration that may be efficiently suspended is approximately 30% foam volume. The use of short reactors (with an aspect ratio of less than 2.0) would minimize the power needed to keep the particles in suspension. The design of a vessel within these limits would therefore minimize the energy required to operate a circulating bed fermenter.
References 1 G. M. Black, C. Webb, T. M. Matthews and B. Atkinson, Biotechnol. Bioeng., 26 (1984) 134. 2 I. Walker and E. P. Austin, in P. F. Cooper and B. Atkinson (eds.), Biological Fluidised Bed
B41
of Water and Wastewaters, Ellis Horwood, Chichester, West Sussex, 1981, p. 272. 3 I. Walker and P. Adams, Water Pollut. Control, to be published. 4 H. Tanaka, Biotechnol. Bioeng., 24 (1982) 425. Treatment
5 J. J. Heijnen and K. Van? Riet, Chem. Eng. J., 28 (1984) B21. 6 J. B. Joshi, Trans. Inst. Chem. Eng., 58 (1980) 155. 7 B. Atkinson and P. J. S. Lewis, in J. E. Smith, D. R. Berry and B. Kristiansen (eds.), Fungal Bio-
technology, British Mycological Ser. 3, Academic Press, London,
Society 1980,
Symp. p. 153.
APPENDIX
C D g H P u,
A: NOMENCLATURE
volume concentration of particles (5%) diameter of fermenter (m) gravitational constant (m SC*) height of fermenter (m) specific power for aeration (W me3) gas superficial velocity (m s-l)
Greek symbol p liquid density
(kg mP3)