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Research Article
 

Minimum Fluidization Velocity of Palm Kernel Cake Particles in Fluidized Bed Fermenter



Foong Chee Woh, Jidon Janaun, Auti Prabhakar and Kamatam Krishnaiah
 
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ABSTRACT

This study measured the minimum fluidization velocity of Palm Kernel Cake (PKC) particles. PKC was used as it is a potential substrate in solid state fermentation. A laboratory scale fluidized bed reactor has been fabricated for this investigation. Minimum fluidization velocity (Umf) for sand particles was initially tested to ensure the workability of the set up. Subsequently, Umf for 855, 655 and 363 μm particle size of PKC were measured and found to be 0.340, 0.205 and 0.080 m sec-1. This study showed that Umf increased with the increased of PKC particle size. This indicated that PKC particle can be fluidized and investigation of its fermentation in fluidized bed fermenter is feasible.

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  How to cite this article:

Foong Chee Woh, Jidon Janaun, Auti Prabhakar and Kamatam Krishnaiah, 2007. Minimum Fluidization Velocity of Palm Kernel Cake Particles in Fluidized Bed Fermenter. Journal of Applied Sciences, 7: 2183-2187.

DOI: 10.3923/jas.2007.2183.2187

URL: https://scialert.net/abstract/?doi=jas.2007.2183.2187

INTRODUCTION

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. Baker’s 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., 2005):

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 cm–3) 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.

Fig. 1: A fluidized bed solid state fermenter

Table 1: The 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:

(ΔP)b = (ΔP)t - (ΔP)DP

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 as Umf.

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.

Fig. 2: Typical plot of bed pressure drop versus air velocity for river sand particles

Fig. 3: Typical plot of bed pressure drop versus air velocity for 855 μm PKC particles

Fig. 4: Typical plot of bed height versus air velocity for 855 μm PKC particles

Fig. 5: Typical plot of bed pressure drop versus air velocity for 655 μm PKC

Fig. 6: Typical 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 Eq. 1:

(1)

Fig. 7: Typical plot of bed pressure drop versus air velocity for 363 μm PKC

Fig. 8: Typical plot of bed height versus air velocity for 363 μm PKC particles

Table 2: Summary of the experimental and calculated Umf in this study
(ΔP)b: Umf measured from graph (ÄP)b versus Ug. H : Umf measured from graph H versus Ug

where,


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 m–3).

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.

CONCLUSION

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 sec–1, 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.

ACKNOWLEDGMENTS

This study was funded by Universiti Malaysia Sabah fundamental research grant (Project No.: B-0101-13/PRU037.

ABBREVIATIONS

A : Column cross section area (m2)
Ar : Archemedies number
D : Diameter of the column (cm)
dp : Average diameter of the particle (μm)
g : Acceleration due to gravity, (m sec–2)
H : Height of fluidized bed (cm)
(ΔP)b : Bed pressure drop (mmH2O)
(ΔP)DP : Pressure drop across the distributor (mmH2O)
(ΔP)t : Total pressure drop across the bed (mmH2O)
Remf : Particle Reynold’s number at minimum fluidization velocity
Ug : Air velocity (m sec–1)
Umf : Minimum fluidization velocity (m sec–1)
W : Weight of the particles (kg)
ε : Bed voidage
μ : Viscosity of fluidizing medium (kg m–1 sec–1)
ρs : Density of particle (kg m-3)
ρf: : Density of fluidizing medium (kg m-3)
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