Reactors for solid substrate fermentation

Solid substrate fennentation reactors aim to provide appropriate environmental conditions for the fennentation. Although there are numerous designs of reactors found in literature, development on an industrial scale is still limited. Mudgett ( 1 986) has provided an introduction to some industrial scale reactors used in Japan, but there is lack of sufficient description. Pandey ( 199 1) classified the reactors as: fennentation without agitation, and with occasional or continuous agitation, while other researchers (Mitchell et aI, 1 992; Lonsane et aI, 1985) divided them into five groups, as shown in Table 2. 1 . Table 2.2 gives a comparison of some basic characteristic of these reactors. The following gives a brief description of the operating process and mass transfer analysis of each type of reactor.

The tray reactor is the most popular for current commercial production, particularly in oriental countries for Koji or food fennentations (Aidoo et aI, 1982; Mudgett, 1986). Its commercial scale is often in multi-tray. These trays, mounted one above another, are incubated in a room with air circulation. The oxygen supply, moisture content and temperature of the fennentation are controlled by the air flow. As the air is blown in a horizontal direction, the aeration and temperature of each tray are dependent on diffusion and/or water evaporation. Thus the mass transfer limits the thickness of substrate to just a few centimetres (Rathbun and Shuler, 1983). In this way, a large scale operation of this process may consist of a large room and a number of trays, where the temperature control and air distribution could be uneven, and the operation is probably labour­ intensive.

The packed bed reactor has attracted much attention (Lonsane et aI, 1985, Pandey, 1 99 1), and has been used in several commercial applications (Mudgett, 1986), due to its potential advantages, as shown in Table 2.2. The typical design is a column with a

Table 2. 1 Application of reactors in solid substrate fermentation

Type Substrate Capacity Process Organism

Tray wheatbran 2 kg fungal rennet Mucor miehei

Tray bagasse+sucrose 1 2inx 1 5inx2in citric acid A. niger

Multi-tray raw cassava 1 00 g protein enrichment Rhizopus oligosporus

Packed bed citrus peel l OO g protein enrichment A. niger

Packed bed cassava meal 450 g protein enrichment A.niger

Packed bed sago-beads up to 350 g protein enrichment Rhizopus oligosporus

Packed bed apple pomace 50 g citric acid A. niger

Packed bed sugar beet pulp 1 000 kg protein enrichment Trichoderma viride

Rotating drum wheatbran 1 50 g hydrolase A. niger

Rotating drum soya beans NA foods Rhizopus oligosporus

G-S fluidized bed wheatbran 400 g enzymes A. niger

G-S fluidized bed sucrose 40 pound ethanol yeast

L-S-G fluidized bed corncob NA mycelium Trichoderma reesi

L-S-G fluidized bed liquid medium 1 .21 Antibiotics Penicillin Unticae

Note:NA, not available; G-S, gas-solid; L-S-G; liquid-solid-gas.

Reference Thakur et ai, 1 990 Lakshminarayana et ai, 1 975 Soccol et ai, 1 994 Rodriguez et ai, 1 985 Sauced-Castaneda et aI, 1 990 Gumbira-Sa'id et ai, 1 993 Hang, 1 987

Durand and Chereau, 1 988

Nishio et aI, 1 979 Reu et ai, 1 993

Tanaka et ai, 1 986

Rottenbacher et ai, 1 987

Adisasmito et aI, 1 987 Behie and Gaucher, 1 988

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Table 2.2 Characteristics of reactors for solid substrate fermentation

Type Moisture Temperature Aeration Abrasion or Capacity Mechanical

control control control Shear stress complexity

Tray natural good poor no limited simple

Packed bed good fair good little excellent simple

Rotating drum good good good large limited complicated

G-S Fluidized bed poor excellent excellent large limited simple

L-G-S Fluidized bed excellent excellent limited medium limited simple

Note:G-S, gas-solid; L-G-S, liquid-gas-solid.

Energy Developing

requirement expectation

little large scale

fair large scale

high large scale

high medium scale

medium small scale

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perforated bottom, associated with forced aeration. The adjustable forced air flow makes it very flexible to control the temperature, oxygen supply and moisture content of the substrate. However, due to the complexity of bioreactions, and the nature of the substrate, there is insufficient understanding of mass transfer in the packed bed reactor

(Mudgett, 1980). Andre et ai ( 198 1) have studied the measurement of mass transfer

coefficient in a packed bed reactor, while Ghildyal et ai ( 1994) and Gowthaman ( 1 993)

reported that steep gas and temperature gradients existed in this type of reactor.

Gowthaman et ai ( 1 995) attempted to calculate the mass transfer coefficient, but

unfortunately, they used an inappropriate calculation. Recently, more and more

modifications have been made on packed bed reactors. Durand et ai (1993) developed

a large scale ( 1 .6 m3) packed bed reactor with agitation, while Xue et ai ( 1 992) used a

25 tons capacity packed bed reactor equipped with a movable agitation device. The addition of an agitation device to a packed bed reactor may be a great improvement for heat removal and mass transfer, however, it may also break the filamentous organism. Nonetheless, a packed bed reactor with forced aeration, an agitation device and even a heat exchange device may be the developing way for large scale solid substrate

fermentation. The rotating drum reactor has many types (Murthy et ai, 1993), and has

been used for many solid substrate fermentations (Mudgett, 1986, Mitchell et ai, 1992).

In general, this reactor consists of a rotating drum and aeration system. The rotation provides agitation to the substrate, associated with aeration, so that the mass transfer and

heat exchange are much improved compared to the packed bed reactor (Reu et ai, 1993).

Recently, many modifications have been made to this type of reactor, including the

addition of baffles (Murthy et ai, 1993), a heat exchange and water spray system

(Mudgett, 1986; Ryoo et ai, 199 1), and a computer-controlled rocking motion (Ryoo et

ai, 1 991). However, these modifications, although they may improve the reactor's

performance, also complicate the reactor and increase the manufacturing cost.

With solid particles in suspension in an appropriate air flow, excellent mass and heat transfer can be provided by a gas-solid fluidized bed reactor. However, the operating procedure may be complicated, and the parameters may not be stable. In particular, the low moisture content of the air flow may restrict growth of the organism (Shuged,

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1989). Adisasmito ( 1987) reported that an air flow with low relative humidity made growth of Trichoderma reesei _{on corncob impossible, while it has also been reported}

that high humidity caused agglomeration of a particle substrate (Shtigerl, 1989). Although, several reports are available using gas-solid fluidized bed reactors, most of them involve yeast, which is able to grow in low moisture conditions, and some additional equipment e.g. an impeller (Moebus and Teuber, 1982, 1985; Tanaka et ai,

1986), was incorporated, or uneven fluidization was obtained (Hong et ai, _{1989). The}

only report of filamentous fungal growth in this reactor type was by Tanaka et ai

( 1 986), but the growth was limited. Due to the necessity for low humidity, and the high energy requirement, the application on a large scale of this type of reactor may be limited to a few organisms, and only for production of a few valuable enzymes (Shtigerl,

1989; Bauer, 1986).

A liquid-gas-solid fluidized bed reactor has been widely used for waste water treatment (Cooper, 1983; Rao Bhamidimarri and Greenfield, 1990) and also for production of some valuable materials using, e.g. plant cells of coffee arabica _(Bramble et ai, _{1 990),}

and mammalian cells of mouse hybridoma (Dean et aI, 1988), because of its excellent mass and heat transfer properties, and low shear stress. In _{solid substrate fermentation,}

Adisasmito et ai _{( 1987) used a liquid-gas-solid fluidized bed reactor for} 'Trichoderma! reesei growth on corncob, and improved biomass production and activity were claimed. However, because of its high operating cost and the high risk of contamination, this type of reactor has only limited application in solid substrate fermentation.

In addition to the types of reactors described above, other types, e.g. stirred tank reactors (Levonen-Munoz et ai, 1983; Baldensperger et ai, 1 985), and liquid-solid fluidized bed reactors (Tzeng et ai, _{1 99 1), have also been used in solid substrate fermentation.}

However, they are unlikely to be used on a large scale with solid substrates because of their high operating costs.

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