Solid State Fermentation (SSF) is defined as the growth of microorganisms on moist solid substrates without free flowing water. SSF gained numerous attentions lately as it can be used to produce various products such as ethanol from cassava roots and sugar beets, enzymes, organic acids, biogas, antibiotics, mushrooms, surfactants, biocides etc. (Raghavarao et al., 2003). There are many factors affecting the SSF process such as the particle characteristic, the contact between the substrate and the microbes, removal of metabolic heat, diffusion of air through the substrate, removal of byproduct gases, maintenance of desired moisture in the substrate (Krishnaiah et al. 2005). Most of these problems can be efficiently handled in fluidization system in which the particles move independently like a fluid and the heat and mass transfer coefficients are very high between particle to gas, gas to particle, or bed to surface and surface to bed (Kunii and Levenspiel, 1999). Some applications of fluidized bed as solid state bioreactor have been reported in the literature (Doelle et al., 1992; Bellon-Maurel et al., 2003). Moebus and Teuber (1982 and 1985) reported the production of ethanol using fluidized bed. Cell biomass and ethyl alcohol were produced in a fluidized bed reactor using yeast (Mishra, 1982). Tanaka et al. (1986) reported the production of enzymes, amylase and protease using yeast in an air solid fluidized bed fermenter. Bakers yeast was produced in an air fluidized bed fermenter using potato substrate (Hong et al., 1987). Gas solid fluidized bed fermenter proved to be suitable for producing fungal starter culture (Tengerdy et al., 1987). The above applications indicate that fluidization system can be applied in solid state fermentation process.
Several advantages of fluidized bed fermenter such as (Krishnaiah et al.,
||Efficient removal of metabolic heat by the gas phase fluidizing medium.
||Healthy and good growth of aerobic microorganisms due to very good aeration
by the fluidizing air. However, this is restricted to microbe that able
to survive the shear stress due to substrate movement.
||Quick scavenging of gaseous and volatile metabolic products which inhibit
the fermentation process.
||In situ drying of the product if required.
||Absence of temperature and moisture gradients due to good mixing of solid
substrate which also enable superior control of process parameters.
||Small substrate particles provide higher surface area for better transfer
of heat and mass and microbial growth.
||Higher productivities compared to traditional solid state fermentation
process resulting in savings of plant space and operating costs.
Minimum fluidization velocity is one of the important parameters used to characterize
a fluidized bed. In the past, although many works had been done to determine
the experimental and/or predicted minimum fluidization velocity using different
bed material, particle size, density, temperature, but there was no study on
PKC particle. In this study, minimum fluidization velocity of PKC particle sizes
was studied. PKC, a byproduct from palm oil industry, was chosen because it
is a potential substrate in the solid state fermentation. Improved PKC quality
through fermentation can be used poultry feed formulation (Rhule, 1996; Abu
et al., 1997; Abu and Yeong, 1999).
MATERIALS AND METHODS
Experimental set up: The experiments in this study have been performed in a 4.6 cm I.D. and 2 mm-thick acrylic fluidized bed reactor, which equipped with a perforated plate (2 mm thick, diameter = 50 mm, hole pitch = 5 mm, clearance = 3 mm, hole diameter = 2 mm). The height of the bed column was 100 cm. Fluidized air (RH<18%) was supplied by the air compressor and the fluidizing gas flow rate was regulated by a rotameter (Fig. 1). The pressure drop across the bed was measured by a digital manometer (TPI, 625).
Initially, 450 μm average river sand particles were used as bed material which had nearly the same density for all sizes (ρp = 2.6 g cm3) to evaluate the efficiency and effectiveness of the fluidized bed reactor. Then, PKC particles were used to determine its Umf. The height of static bed for all the experiments in this study were in the range of H/D = 1 to 2. If the range is more than 5, slugging is encountered in the bed, resulting poor contact between the fluidizing medium and the solids (Krishnaiah et al., 2005). The operating conditions for each experiment were shown in Table 1. All the experiments throughout the study were operated at room temperature (30±2°C) and atmospheric pressure.
The experimental procedures were started with 50 g of 855 μm PKC diameter placed inside the fluidized bed reactor. The digital manometer was zero before running the experiment to confirm the accuracy and the initial bed height was measured. At the beginning, the compressed air flow rate was adjusted to 1 LPM, then the total pressure drop across the bed was read and recorded after 5 min and the bed height was measured. The compressed air flow rate was gradually increased until the total pressure drop across the bed was constant for at least 3 air flow rates. Reversely, the de-fluidization, which the compressed air flow rate was gradually decreased until 0 LPM and the total pressure drop across the bed was read and recorded and the bed height was measured for every compressed air flow rate. Graph of bed pressure drop versus air velocity and graph of bed height versus air velocity were plotted. The experimental procedures were repeated using average PKC particle sizes of 655 and 363 μm.
fluidized bed solid state fermenter|
operating conditions for each experiment|
Determination of distributor plate pressure drop and bed pressure drop:
The distributor plate pressure drop was determined using a digital manometer.
In this study, the distributor plate pressure drop was defined as the total
pressure drop for the distributor plate and a layer of 37 μm stainless
stain mesh. Then, the bed pressure drop was calculated using the following equation:
RESULTS AND DISCUSSION
The fluidization experiment using river sand as fluidized material was carried
out and the results was illustrated in Fig. 2. The intersection
of diagonal line with horizontal line was defined as Umf, where Umf
for increasing gas velocity and deceasing gas velocity were almost identical.
It indicated that the fluidized bed reactor was properly working. Figure
3,5 and 7 show the bed pressure
drop was increasing with the increase of air velocity at the beginning. At this
air flow range, the bed is considered as fixed bed. Further air velocity increase
did not change the bed pressure drop. At this time the bed is considered as
fluidized bed. The intersection of diagonal line with horizontal line is defined
Umf was also measured from the graph H versus Ug as illustrated
in Fig. 4, 6 and 8.
The bed expanded into a new bed height after fluidization for each experiment.
plot of bed pressure drop versus air velocity for river sand particles|
plot of bed pressure drop versus air velocity for 855 μm PKC particles|
plot of bed height versus air velocity for 855 μm PKC particles
plot of bed pressure drop versus air velocity for 655 μm PKC
plot of bed height versus air velocity for 655 μm PKC particles|
The summary results of the experimental Umf measured from graph
(ΔP)b versus Ug and graph H versus Ug are
given in Table 2. Umf for both measurements were
almost the same (±7%). From the obtained results, Umf increases
as the PKC particle size increases. Thus, this indicates that Umf
is a function of particle size. However, more particle sizes of PKC especially
the bigger one should be tried to examine the generalization of this findings.
Besides the experimental Umf, there are many equations to predict
minimum fluidization velocity such as Wen and Yu (1966) equation as given in
plot of bed pressure drop versus air velocity for 363 μm PKC|
plot of bed height versus air velocity for 363 μm PKC particles|
of the experimental and calculated Umf in this study|
Umf measured from graph (ÄP)b versus Ug.
H : Umf measured from graph H versus Ug |
Umf was predicted using Equation (1) with an assumption that PKC
density equals to 1000 kg/m3, a maximum possible value. The calculated
and experimental Umf are compared in Table 2. The
calculated and experimental value of Umf differed between 30 to
50%. The difference could be higher if the true density of PKC could be used
because the value of calculated Umf is proportional to PKC density
(true density of PKC should be <1000 kg m3).
The difference of experimental Umf for increasing fluidization and
de-fluidization shown in Fig. 3, 5 and 7.
The big difference between calculated and experimental Umf might
be caused by several reasons: The non-spherical PKC particle shape and the physical
characteristic of PKC particle is not well known, Shell content in PKC might
have a different density and create a binary mixture and Breaking down of PKC
particle during fluidization might also create a binary mixture. Thus, it is
normally recommended to measure the Umf experimentally to minimize
the error level.
Minimum fluidization of PKC for three particle sizes; 855, 655 and 363 μm were found to be 0.340, 0.205 and 0.080 m sec1, respectively. However, actual minimum fluidization velocities for these particles during solid state fermentation should be measured again to take into consideration the effects of moisture content of PKC and also the presence of microbes.
This study was funded by Universiti Malaysia Sabah fundamental research grant (Project No.: B-0101-13/PRU037.
||Column cross section area (m2)
||Diameter of the column (cm)
||Average diameter of the particle (μm)
||Acceleration due to gravity, (m sec2)
||Height of fluidized bed (cm)
||Bed pressure drop (mmH2O)
||Pressure drop across the distributor (mmH2O)
||Total pressure drop across the bed (mmH2O)
||Particle Reynolds number at minimum fluidization velocity
||Air velocity (m sec1)
||Minimum fluidization velocity (m sec1)
||Weight of the particles (kg)
||Viscosity of fluidizing medium (kg m1 sec1)
||Density of particle (kg m-3)
||Density of fluidizing medium (kg m-3)