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Chapter 2 Literature review of SSF design (Packed bed)

Chapter-2 Literature Review of SSF Design

Ph.D. Thesis, UICT, North Maharashtra University, Jalgaon 29

CHAPTER – 2

LITERATURE REVIEW OF SSF DESIGN

(PACKED BED)

The Chapter 1 deliberates the need of more research in SSF and hence in the same

tune the objectives were set for the research work. The Chapter 2 states the literature

review of SSF design and highlights the merits and demerits of each design,

challenges of SSF applicability and range of fermented products produced by SSF

using various cultures.

2.1 Introduction to SSF design

SSF is a multiphase system consisting of the solid phase, the thin liquid film, and the

continuous gas phase. The different phases create many problems associated with

mass and heat transfers, which are not still understood fully.SSF is a cost effective

and environment friendly process. Hence, SSF is a thrust area for the technologists

and scientists. However, precise control of the process variable is difficult in SSF and

warrants a thorough literature survey. In view of new product development

researchers have articulated newer SSF designs to impart better efficiency. However,

still the majority of work has been stagnated to lab-sacle and its transformation to

pilot scale and large scale is still awaited. Although, some work has been done on this

level; more works is the need of the time. SSF is an ancient technique to be

transformed and modified for new tasks, using collective approaches of microbiology,

biochemistry, biochemical engineering as well as advancement in instrumentation.

Food fermentation and enzyme production are the main areas where SSF technology

has immense application. Scientific research was concentrated on SSF after 1940’s in

view of developing and improving the production of drug penicillin. Penicillin was

developed using SmF as well as by SSF, however; with the advent of ease in

operation and precise control over the process SmF became the method of choice.

After 1970’s SSF technology was used for the production of protein enriched cattle

feed and for the production of enzymes [1]. C.W. Hesseltine was the one, who first

time published the scientific information on SSF in 1977. Kumar in 1987[2] has

reported medium scale production of enzymes, especially pectinases, in a scientific

manner for the first time in India. SSF has proved to be an attractive technique to

produce cellulase economically because of its lower capital investment and operating

cost [3]. The ease in operations and cost effective production of metabolites can act as

a magnet that can pull the investors and people from agroindustries to use SSF;

however it needs concerted efforts upon its interdisciplinary approaches. . In order to

take full advantage of SSF potential, it is desirable to obtain a deep understanding of

the fermentation process.

2.2 Pros and cons of SSF

One of the most important and critical aspects of any commercially viable

fermentation technology is scaling up. Scaling up is more difficult in SSF, since, there

is no free flowing water present in SSF which creates difficulties for removing

metabolic heat. In SSF these heat can be removed by agitation or by forced aeration

while agitation brings undesirable effects leading to disruption of fungal mycelia. The

scale – up methods for SmF are well developed, these SmF scale-up methods could

not be applied directly to SSF [4-5] due to variation in physical structure of the systems

and inherent difference of availability of free flowing water. Hence, no assertive and

quantitative scale-up criteria are currently available for SSF. Cannel and Moo-

Young[12] found that moisture variation poses a great difficulty in SSF design which is

quite different from SmF. In order to address such problems many researchers have

developed new models to address heat and mass transfer processes in packed bed

solid state fermenter (PBSSF) [6,7], tray bioreactors[8,9] and rocking drum

bioreactors[10], which are commonly used in SSF. These models can be used to guide

scale up processes in SSF[11]. During the microbial growth, heat is generated by the

metabolic activities of microorganisms. The heat generated during the process must

be dissipated quickly since; microbial growth is very sensitive to a rise in temperature,

affecting vegetative growth of fungal cells and that leads to formation of spores

adversely affecting the product formation.

2.3 SSF process methodology:

Pretreatment of the lignocellulosic materials is very essential. Lignocellulose is a

complex polymeric material containing cellulose, hemicelluloses and lignin. The

pretreatment of such substrate involves generally heat treatment and chopping or

cutting to a uniform size. The absence of pretreatment leads to poor growth and

subsequently low product yield. Pore size of the substrate increases due to

pretreatment and reduce the crystallinity of cellulose. The SSF process involves the

following basic steps as shown in Fig. 2.1.

Fig.2.1 SSF process flow chart

The design of SSFr is the key issue on which the success of the process relies. This

design should be such that it shall maintain the optimum conditions favorable for

microbial growth and product formation inside the reactor. Hence, common design of

packed bed SSFr is discussed in the next subsection.

2.4 Common theoretical designs of packed bed

The objective of the fermenter design is to ensure that the desired activity of the

micro-organisms shall not be limited by the characteristics of the equipment [34]. The

most significant aspect to be considered during the construction of a packed column

of SSF is the effective distribution of air and heat removal. The effects of various

designs and operating parameters on the performance of the PBSSF are discussed in

Chapter 3 and 4. Heat transfer, channeling, flooding concepts have been theoretically

discussed in this chapter.

2.4.1 PBSSF

For competent use of the process, larger fermentation volumes are needed. For large

capacity, most commonly used vessel is the packed bed. PBSSF is a preferred design

over most of the other designs because of better control and process management. The

major design and operating parameters of the PBSSF include the height of the column

and main parameters to be controlled are moisture of the bed and the temperature of

the inlet air. PBSSF is a cylindrical vessel with perforated base and hemispherical

head as shown in Fig.2.2. This vessel is filled with biomass as substrate where

biomass itself is the source energy for micro-organism. The moisture to be maintained

inside the column is 40% to 60%. PBSSF is cooled with water jacket from outside.

Evaporation induced by forced aeration is a frequently used cooling method in large

scale PBSSF. Such evaporation is beneficial for SSF since it helps for cooling, but on

the other side it is limiting the microbial growth by decreasing the moisture content

from the bed. For large scale operation, the effectiveness of the water jacket reduces

due to the decrease in the surface area for heat transfer to the volume of the substrate

bed [26]. PBSSF is suitable for aerobic micro-organism. For aerobic fermentation

generally air is forced from the bottom of column. PBSSF can be aerated from any

point and for a vertical column the air may enter from either bottom or top. PBSSF is

very easy for fabrication compared to another SSFr, but process parameters are very

difficult to control. Perforated plate is situated at the bottom of the packed bed so that

air can force through it. This kind of reactor can be used in two ways [6, 35]

i) To evaluate the overall process empirically and determine the process

parameters

ii) To study O2 diffusion along with mass and heat transfer phenomena

Fig.2.2 Schematic representation of PBSSF

Heat accumulation is the major technical problem in almost all SSFr while it is more

prevalent with large capacity of PBSSF. Microbial growth in SSF generates

significant amount of metabolic heat. It has been reported that 100-300 KJ of heat per

kg of cell mass is generated in a SSF process [36]. Several reasons are responsible for

heat accumulation in PBSSF that include low thermal conductivity of substrate,

absence of free water, channeling etc. High temperatures must be avoided inside a

column, as they adversely affect on microbial activity. With the increase in bed height

of the column the PBSSF temperature gradient gradually increases. When the

temperature crosses the critical value, the growth of microbes is adversely affected.

Hence, critical temperature is the main design parameter which determines the bed

height of the fermenter. Heat generated inside a column is directly proportional to the

metabolic activities of the micro-organism. For laboratory scale packed bed column,

temperature can be controlled by wall cooling, however; in large scale packed bed

column, conductive heat transfer is insufficient and hence convective mode of heat

transfer needs to be used. Convective heat transfer is taking place due to evaporative

cooling. Due to the absence of free flowing water and the poor conductivity of the

substrate material, removal of heat is troublesome which finally generates temperature

gradients inside the column.

Evaporation taking place inside a column also reduces water content of the bed.

Minimum moisture level to be maintained in SSF is approximately 12% since all

biological activities are ceased below this level [14]. In a static packed bed, the relative

humidity of the inlet air is not a more useful controlling parameter because it is

impossible to circulate humidified air equally through the packed bed. Hence water

activity values vary within the bed. Evaporative water loss promotes bed drying and

need to be minimized in packed column. Gowthaman et al., (1993) [28-29] and Ghildyal

et al., (1994) [27] showed that the rate at which bed drying occurs is dependent on the

location within the bioreactor and the air flow rate. Water evaporation promotes the

bed shrinkage in the column, although; bed shrinkage is a function of growth of

hyphae on the surface of the substrate in majority. Spraying of humidified air does not

stop the evaporation from the bed but can reduce evaporation loses from the bed

compared to the dehumidified air. Heat transfer within the bed due to conduction is

negligible. Majority of heat is transferred due to convection and evaporation [32,13].

Generally, bed temperature increases with increasing height of the bed [31]. The

extreme temperature always posses greater problems than the oxygen availability to

the microorganisms within PBSSF[33].

Metabolic heat is generated inside the packed column due to the action of microbes on

substrate towards energy generation by respiration process. This is transferred to

surrounding by two means (i) forced aeration and (ii) water jacket cooling. If packed

bed column is divided vertically into various parts (A,B,C) as shown in Fig. 2.3 then

it can be observed that part A is in vicinity of water jacket, where temperature

gradient effect is less. As the substrate is bad conductor of heat water jacket effect in

part B and C could be very minor. Hence, forced aeration plays an important role in

part B and C. Again it can be said that part C may show high temperature gradient

effect compared to part A and B.

Fig.2.3 Packed column divided vertically

If packed column is divided horizontally as shown in Fig.2.4 then it can be observed

that part C may face the problem of flooding (high moisture content) while part A and

B of the bed may face the problems of dryness. In addition, temperature rise is more

in part C followed by part B and A.

Fig.2.4 Packed column divided horizontally

Channeling basically promotes the heat accumulation in a PBSSF. Channeling occurs

when the gas is flowing through the packed bed and finds a “chosen/ ideal or fixed

path” through the bed (Fig. 2.5).

Fig. 2.5 : Channeling effects in PBSSF

Bed shrinkage occurs due to three reasons, viz. (i) evaporation (ii) fungal growth and

(iii) substrate consumption. Most of the biomass material gets shrink due to

evaporation of water and fungal growth, this shrinkage again increase channeling

problem in packed column. In large capacity column, it is hard to check practically

whether channeling problem increases or not. Intensity of channeling problem can be

decided from the moisture content of the upper layer of the bed. PBSSF tends to be

very simple in design and even easy for loading and unloading but always critical to

maintain desirable parameters. Forced aeration can be used in PBSSF for heat and

mass transfer. But, it results in high temperature near the air outlet hence cooling

system must be designed according to actual temperature gradient formed inside the

PBSSF. High pressure drop is generated in a large capacity PBSSF which increase the

operating costs of the process. Generated pressure drop in PBSSF is mainly depends

on three components viz. (i) height of the column (ii) substrate size and the way of

packing components inside the column and (iii) the growth of micro-organisms.

However, it further needs in-depth understanding the pressure gradient effect that has

been discussed below.

2.4.2 Pressure gradients in PBSSF

In static operation, hyphae that grow into the inter particle spaces increase the

pressure drop. Hypha tend to bind the substrate particles and form large agglomerates

(Fig.2.6). The increase in pressure drop during the fermentation depends on the

growth of microorganisms within the bed while, the pressure drop is also a function of

the superficial velocity and the height of the PBSSF. Pressure drop parameter of the

column is directly affecting the operating cost of the process especially the cost of

generation of pressurized air through blower or compressor. [13].

Fig.2.6 Substrate particles (a) distantly located with more gaps before fermentation

(b) compactly located with each other after fermentation.

Usually, air pressure drop across the inlet and outlet of the column increases with

respect to the column height and time. It was found that pressure drop decreases at the

end because a bed shrinkage forms a gaps between bed and wall of the column [30,31].

Hence air can pass easily through this gap as shown in Fig.2.7 however; it does not

contribute in a positive way as it is not judicious. Further, design considerations need

to be understood the merits and demerits of each design which are discussed in the

next section.

Fig.2.7: Gap formed between the wall and bed of the substrate in PBSSF.

Inset: Substrate particles with hyphae

2.4.3 Basic design for the head of PBSSF

In the old head design of PBSSF shown in Fig. 2.8, head surface is comparatively

cooler due to condensation as a result of pressure drop at the head space of reactor.

This is problematic, since the condensed water comes down and overflows or floods

the top portion of the bed [13]. To overcome this problem, the conceptual design

shown in Fig. 2.9 is suggested where hemispherical head is replaced by conical head.

Fig.2.8: Old head design for PBSSF

Fig.2.9 : Improved head with weir and plate in PBSSF

Hemispherical head shown in Fig. 2.8 can withstand high pressure. But, PBSSF is not

generating very high pressure; hence this head can be replaced by conical head shown

in Fig. 2.9. Conical head can minimize flooding problem generated at the top of

column. The high slope of conical head allows the condensate collected in side tray

to be removed by side pipe minimizing the high moisture content or flooding problem

at the upper layer of substrate in the bed.

Generally large capacity reactors have water jacket along the wall of column for

temperature control, but it is not enough. Hence, small (one or more) perforated tube

called as draft tube as shown in Fig. 2.10 is inserted along central axis of the column.

This perforated tube provides better aeration and can remove more heat from central

position [13] by convection.

Fig.2.10 : PBSSF with draft tube at central axis

2.4.4 Conical packed bed solid state fermentation system

The disadvantages of traditional packed bed solid state fermentor can be overcome by

a novel conceptual design of conical packed bed solid state fermentor (CPBSSF) as

shown in Fig. 2.11. Two extra air inlets are provided for upper region in CPBSSF.

CPBSSF gives more surface area at the top. This high surface area increases the rate

of heat transfer at the top portion of the column. Water evaporation rate at the top

portion is also high. Even loading and unloading is comparatively easy in CPBSSF.

CPBSSF provides such design that can reduce problems of pressure drop to certain

level.

Fig.2.11: Conical packed bed solid state fermenter (CPBSSF)

2.5 Applications of PBSSF

Many products produced by PBSSF are summarized here (Table-2.1) along with

substrates and culture used. Continuous research is going on packed bed column due

to low product value at present date but is projected as future technology with high

efficiency.

Table 2.1 Various products formed by PBSSF.

SSF Type

Specifications Substrate Culture used Product Refe-rence

Packed bed column

34.5 cm height & 15 cm diameter

steamed wheat bran (1.3 Kg)

Aspergillus niger

Gluco-amylase

[15,16]

Zymotic packed bed column

Rectangular shaped bioreactor.

sugarcane bagasse & wheat bran (13-40kg)

Trichoderma harziamum

Cellulase

3 Packed bed column

Column aeration only for the first 10 hr of operation.

Agrowaste 4.1 kg bed

Schwanniomyces castelli

Ethanol production

Packed bed column

Size 14.7cm Dia. & 50cm Height

Sweet potato residue

Streptomyces rimosus

Oxytetracycline production .

5 Packed bed column

Two steel cylindrical units. Each unit was of 2m length and 0.5m diameter

Chaff and wheat bran mixture

Trichderma viride

Produc-tion of cellulase enzyme

Packed bed column

Two cylin-drical reactors containing baskets of 40cm diameter and 50cm height.

Dry sugar beet (3kg)

Non fermentative

Sugar beet pulp

SSF Type

Specifications Substrate Culture used Product Refe-rence

Packed bed column

Size 9.5cm Dia. &25cm Height

Amberlite Imbibed with nutrients (1.5Kg)

Aspergillus niger

Citric acid

Packed bed glass- bioreactor

60 cm height and 5cm diameter

Wheat bran

Aspergillus niger

Biomass production

9 Packed bed column

20cm height and 4 cm diameter

Buck-wheat seed

Aspergillus niger

Production of spores [24]

10 PBSSF

cylindrical unit with a capacity of 4.4 litres having 25 cm long vertical cylinder with internal diameter of 15 cm

Aspergillus oryzae

Cellulase [25]

Thus, it can be concluded that PBSSF is widely used for the production of large

number of products with a wide range of substrate and therefore the research initiative

needs further direction, analysis and inferences from newer experimentations,

modeling and comparison. Some of the aspects have been attempted and deliberated

in next chapters.

References:

[1] Pandey A., Soccol C.R., Leo J., Nigam P. (2008). Solid state fermentation in biotechnology. Asiatech publishers, Inc., New Delhi.3-4.

[2] Kumar P.K.R. (1987). Microbial production of gibberellic acid. Ph.D. Thesis, Mysore University, Mysore, India.

[3] Cen P.L., Xia L.M. (1999). Production of cellulase by solid state

fermentation.Adv.Biochem.Engin.Biotechnol.65:68-92.

[4] Pandey A., Soccol C.R., Leo J., Nigam P. (2008). Solid state fermentation in biotechnology. Asiatech publishers, Inc., New Delhi.125-137.

[5] Singhania R. R., Patel A.K, Soccol C.R., Pandey A. (2009). Recent advances in solid state fermentation. Biochemical engg. j. 44:13-18.

[6] Saucedo C.G., Gutierrez R.M., Bacquet G., Raimbault M., Viniegra-Gonzalez G. (1990). Heat transfer simulation in solid substrate fermentation. Biotechnol Bioeng.35:802-808.

[7] Sangsurasak P., Mitchell D.A. (1995). Incorporation of death kinetics into a 2-D dynamic heat transfer model for solid state fermentation. J.Chem. Technol. Biotechnol.64:253-260.

[8] Ragheva Rao KSMS., Gowthaman M.K., Ghildyal N.P., Karanth N.G. (1993). A mathematical model for solid state fermentation in tray bioreactors. Bioprocess Engg.8:255-262.

[9] Rajagopalan S.,Modak J.M. (1994). Heat and mass transfer simulation studies for solid state fermentation processes. Chem. Eng. Sci.49:2187-2193.

[10] Sargatanis J., Karim M.N., Murphy V.G., Ryoo D., Tengerdy R.P. (1993). Effect of operating conditions on solid substrate fermentation. Biotechol. Bioeng.42:149-158.

[11] Mitchell D.A., Pandey A., Sangsurasak Penjit, Krieger N. (1999). Scale-up strategies for packed bed bioreactors for solid state fermentation. Process Biochemistry.35:167-178.

[12] Cannel E., and Moo-Young M. (1980). Solid-state fermentation systems. Procss. Biochem, 4:2-7.

[13] David M., Nadia K., Marien B. (2006). Solid state fermentation bioreactors, fundamentals of design and operation. Springer 38-100.

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[15] Ghildyal N.P., Gowthaman M.K., Raghava Rao KSMS., Karanth N.G. (1994). Interaction between transport resistances with biochemical reaction in packed bed solid state fermentors: Effect of temperature gradients. Enzyme Microb.Technol.16:253-257.

[16] Gowthaman M.K., Ghildyal N.P., Raghava Rao KSMS, Karanth N.G.(1993). Interaction of transport resistances with biochemical reaction in packed bed solid state fermenters: The effect of gaseous concentration gradients. J. Chem. Technol. Biotechnol.56:233-239.

[17] Roussos S., Raimbault M., Prebois J.P., Lonsane B.K. (1993). Zymotis a large scale solid state fermenter. Appl. Biochem Biotechnol.42:37-52.

[18] Saucedo-Castaneda G., Lonsane B.K., Krishnaiah M.M., Navarro J.M., Roussos S., Raimbault M. (1992). Maintenance of heat and water balances as a scale-up criterion for the production of ethanol by Schwanniomyces castelli in a solid state fermentation system. Proc. Biochem.27:97-107.

[19] Yang S.S., Chiu L., Yunan S.S. (1994). Oxytetracycline production by Streptomycyces rimosus:gas and temperature patterns in a solid state column bioreactor. World J. Microbiol. Biotechnol.10:215-220.

[20] Tao S., Zuohu L., Deming L. (1996). A novel design of solid state fermenter and its evaluation for cellulase production by Trichoderma viride SL-1. Biotechnol. Tech. 10: 889-895.

[21] Durand A., Renaud R., Maratray J., Almanza S., Diez M. (1996). INRA-Dijion reactors for solid state fermentation: Designs and applications. J.Sci. Ind Res.55:317-332.

[22] Gutierrez-Rojas M., Hosan S., Auria R., Revah S., Favela-Torres E. (1996). Heat transfer in citric acid production by solid state fermentation. Proc.Biochem.31:363-369.

[23] Seyed A., Shojaosadati, Zohreh H.E.,Parisa H.,Ebrhim V.F., Arian R. (2007). Evaluatio of strategies for temperature and moisture control in solid state packed bed bioreactors. Iranian J. of Biotech.5:219-225.

[24] Ramachandran S., Fontanille P., Pandey A., Larroche C. (2008). Stability of glucose oxidase activity of A.Niger spores produced by solid state fermentation and its role as biocatalyst in bioconversion reaction. Food Technology and Biotechnology.46(2):190-194

[25] Bathe G.A., Patil V.S., Chaurasia A.S. (2012). Study on temperature gradients and protein enrichment by Aspergillus oryzae in solid state fermentation on packed bed bioreactor using jowar (sorghum) straw as substrate. J. of sustainable bioenergy system.2:33-36.

[26] Couto S.R., Barreiro M., Rivela I., Longo M.A., Sanroman A. (2002). Performance of a semi-solid-state immersion bioreactor for ligninolytic

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[27] Ghildyal N.P., Gowthaman M.K., Raghava Rao KSMS, Karanth N.G.

(1994). Interaction between transport resistances with biochemical reaction in packed bed solid state fermentors: Effect of temperature gradients. Enzyme Microb Technol.16:253-257.

[28] Gowthaman M.K., Raghava Rao KSMS., Karanth N.G. (1993). Interaction of transport resistances with biochemical reaction in packed bed solid state fermenters:The effect of gaseous concentration gradients. J. Chem. Technol. Biotech.56:233-239.

[29] Gowthaman M.K., Raghava rao, KSMS, Ghildyal N.P.,Karanth N.G. (1993). Gas concentration and temperature gradients in a packed bed solid state fermentor. Biotech. Adv.11:611-620.

[30] Gumbira-Sa’id E.,Greenfield P.F., Mitchell D.A., Doelle H.W. (1993).

Operational parameters for packed beds in solid state cultivation. Biotech Adv.11:599-610.

[31] Weber F.J., Oostra J., Tramper J., Rinzema A.(2002).Validation of a

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[32] Gutierrez-Rojas M., Hosn A.A. S, Auria R., Revah S, Favela T. E.(1996).

Heat transfer in citric acid production by solid state fermentation. Process. Biochem.31:363-369.

[33] Ali H.Kh.Q., Zulkali M.M.D. (2011). Design aspects of bioreactors for

solid state fermentation: A review. Chem. Biochem. Eng.Q.25(2):255-266. [34] Blakebrough N. (1973). Fundamentals of fermenter design. Pure Appl.

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M.D., Rodrigues M.I., Treichel H., Maugeri F. (2009). Optimization of inulinase production by SSF in a packed bed bioreactor. Society of Chemical Industry. 85: 109-114.

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