Other Fermentation Vessels

Some of the other forms of fermentation vesels will now be considered. These vessels have more limited applications and have been developed for specific purposes or closely related processes. Some are historical developments, such as the earliest forms of packed tower, others were being developed in parallel with the standard mechanically stirred fermenter during the 1940s, while other approaches are more recent. Aspects of this topic have been reviewed by Prokop and Votruba (1976), Katinger (1977), Hamer (1979), Levi et al. (1979), Solomons (1980), Schugerl (1982, 1985), Sittig (1982), Winkler (1990) and Atkinson and Mavituna (1991a). i

The Waldhof-type fermenter

The investigations on yeast growth in sulphite waste liquor in Germany, Japan and the United States of America led to the development of the Waldhof-type fermenter (Inskeep et al., 1951; Watanabe, 1976). Inskeep et al. (1951) have given a description of a production vessel based on a modification of the original design of Zellstofffabrik Waldhof. The fermenter was of carbon steel, clad in stainless steel, 7.9 m in diameter and 4.3-m high with a centre draught tube 1.2 m in diameter. A draught tube was held by tie rods attached to the fermenter walls. The operating volume was 225,000 dm3 of emulsion (broth and air) or 100,000 dm3 of broth without air. Non-sterile air was introduced into the fermenter through a rotating pin-wheel type of aerator, composed of open-ended tubes rotating at 300 rpm (Fig. 7.43). The broth passed down the draught tube from the outer compartment and reduced the foaming.

Acetators and cavitators

Fundamental studies by Hromatka and Ebner (1949) on vinegar production showed that if Acetobacter cells were to remain active in a stirred aerated fermenter, the distribution of air had to be almost perfect within the entire contents of the vessel. They solved the full-scale problem by the use of a self-aspirating rotor (Ebner et al, 1967). In this design (Fig. 7.44), the turning rotor sucked in air and broth and dispersed the mixture through the rotating stator (d). The aerator also worked without a compressor and was self-priming.

Micro Compressor Rotor
Fig. 7.43. Top view and section of a Waldhof aeration wheel (Inskeep et at., 1951).

Vinegar fermentations often foam and chemical an-tifoams were not thought feasible because they would decrease aeration efficiency (Chapter 9) and additives were not desirable in vinegar. A mechanical defoamer therefore had to be incorporated into the vessel and as foam builds up it is forced into a chamber in which a rotor turns at 1,000 to 1,450 rpm. The centrifugal force breaks the foam and separates it into gas and liquid. The liquid is pumped back into the fermenter and the gas escapes by a venting mechanism. Descriptions of the design and various sizes of model have been given by Ebner et al. (1967). Fermenters of this design are manufactured by Heindrich Frings, Bonn, Germany. An illustration of the basic components is given in Fig. 7. 45. In 1981, 440 acetators were in operation all over the world with a total production of 767 X 106 dm3 year™1. The major vinegar producers were the U.S.A. (152 X 106 dm3), France (90 X 106 dm3) and Japan (46 X 106 dm3) while the remainder was produced by over 50 countries (Ebner and Follmann, 1983).

Chemap AG of Switzerland manufacture the Vinegator. A self-aspirating stirrer and a central suction tube aerates a good recirculation of liquid. Additional air is provided by a compressor. Foam is broken down by a mechanical defoamer (Ebner and Follmann, 1983).

Antifoam Baffle Fermented Design

as a hollow body (a) with openings which are arranged radially and open against the direction of rotation (b). The openings are shielded by vertical sheets (c). The turbine sucks liquid from above and below and mixes it with air sucked in through the openings. The suspension is thrown through the stator (d) towards the circumference of the tank. An upper and lower ring on the turbine (e,f) helps to direct and regulate the air-liquid suspension. The stator (d) consists of an upper and lower ring (g,h) which are connected by vertical sheets (i) inclined at about 30° towards the radius.

as a hollow body (a) with openings which are arranged radially and open against the direction of rotation (b). The openings are shielded by vertical sheets (c). The turbine sucks liquid from above and below and mixes it with air sucked in through the openings. The suspension is thrown through the stator (d) towards the circumference of the tank. An upper and lower ring on the turbine (e,f) helps to direct and regulate the air-liquid suspension. The stator (d) consists of an upper and lower ring (g,h) which are connected by vertical sheets (i) inclined at about 30° towards the radius.

At least three other vinegar fermenters are no longer manufactured. The Bourgeois process was sold in Europe between 1955 and 1980 and the Fardon process between 1960 and 1975. The Yeomans cavitator was sold in the U.S.A. between 1959 and 1970 (Cohee and Steffen, 1959; Mayer, 1961; Ebner and Follmann, 1983). The fermenter had an agitator of different design, but similar operating principles to the acetator. Uniform distribution of air bubbles was obtained by means of the circulation pattern created by the centrally located draught (draft) tube. The agitator withdrew liquid from the draught tube and pushed liquid into the main part of the vessel. The outer level rose and overflow occurred back into the top of the draught tube.

The tower fermenter

It is difficult to formulate a single definition which encompasses all the types of tower fermenter. Their main common feature appears to be their height:diam-eter ratio or aspect ratio. Such a definition has been

Frings Acetator Diagram
Fig. 7.45. Diagram of a section through a Frings generator fermenter used for the manufacture of vinegar. The fermenter, which can be used semicontinuously or continuously, employs vortex stirring (Greenshields, 1978).

given by Greenshields et al. (1971) who described a tower fermenter as an elongated non-mechanically stirred fermenter with an aspect ratio of at least 6:1 for the tubular section or 10:1 overall, through which there is a unidirectional flow of gases. Several different types of tower fermenter exist and these will be examined in broad groups based on their design.

The simplest types of fermenter are those that consist of a tube which is air sparged at the base (bubble columns). This type of fermenter was first described for citric acid production on a laboratory scale (Snell and Schweiger, 1949). This batch fermenter was in the form of a glass column having a height:diameter ratio of 16:1 with a volume of 3 dm3. Humid sterile air was supplied through a sinter at the base. Steel et al. (1955) reported an increase in scale to 36 dm3 for a fermenter of this type. Pfizer Ltd has always used non-agitated tower vessels for a range of mycelial fermentation processes including citric acid and tetracyclines (Solomons, 1980; Carrington et al., 1992). Recently Pfizer Ltd sold their citric acid interests to Arthur Daniels Midland who are operating such vessels up to 23 m high (Burnett, 1993).

\,.]unies of between 200 m3 and 950 m3 have been reported elsewhere (Rohr et al., 1983).

In 1965 the brewing industry began to use tower fermenters which were more complex in design and could be operated continuously. Hall and Howard (1965) described small-scale fermenters that consisted of water jacketed tubes of various dimensions which were inclined at angles of 9 to 90° to the horizontal. Air and mash were passed in at the base and effluent beer was removed at the top.

A vertical-tower beer fermenter design (Chapter 2) was patented by Shore et al. (1964). Perforated plates were positioned at intervals in the tower to maintain maximum yeast production. The settling zone which could be of various designs, was to provide a zone free of rising gas so that the cells could settle and return to the main body of the tower and the clear beer could be removed. This design must be considered as an intermediate between single- and multistage systems. Towers of up to 20,000 dm3 capacity and capable of producing up to 90,000 dm3 day-1 have been installed. Green-shields and Smith (1971) commented that it was difficult to predict the upper operating limits for these fermenters. Experiments with particular yeast strains in pilot-size towers were essential to establish optimum full-scale operating conditions.

The next group of tower fermenters are the multistage systems, first described by Owen (1948) and Vic-torero (1948) for brewing beer, although these systems were not used on an industrial scale. Later work reported using these systems includes continuous cultures of E. coli (Kitai et al., 1969), bakers' yeast (Prokop et al., 1969) and activated sludge (Lee et al., 1971; Besik, 1973). The fermenters used by all these workers were basically similar. Each consisted of a column forming the body of the vessel and a number of perforated plates which were positioned across the fermenter, dividing it into compartments. Approximately 10% of the horizontal plate area was perforated. The possibility of introducing media into individual stages independently was discussed by Lee et al. (1971). Besik (1973) decribed a down-flow tower in which substrate was fed in at the top and overflowed through down spouts to the next section while air was supplied from the base. Schugerl, Lucke and Oels (1977) have written a comprehensive general review.

Cylindro-conical vessels

The use of cylindro-conical vessels in the brewing of lager was first proposed by Nathan (1930), but his ideas were not adopted for the brewing of lagers and beers until the 1960s (Hoggan, 1977). Breweries throughout the world have now adopted this method of brewing. The vessel (Fig. 7.46) consists of a stainless-steel vertical tube with a hemispherical top and a conical base with an included angle of approximately 70° (Boulton, 1991).

Aspect ratios are usually 3:1 and fermenter heights are 10 to 20 m. Operating volumes are chosen to suit the individual brewery requirements, but are often 150,000 to 200,000 dm3. Vessels are not normally agitated unless a particularly flocculant yeast is used, but small impellers may be used to ensure homogeneity when filling with wort (Boulton, 1991). In the vessel, the wort is pitched (inoculated) with yeast and the fermentation proceeds for 40 to 48 hours. Mixing is achieved by the generation of carbon dioxide bubbles that rise rapidly in the vessel. Temperature control is monitored by probes positioned at suitable points within the vessel. A number of cooling jackets are fitted to the vessel wall to regulate and cause flocculation and settling of the yeast (Ulenberg et al, 1972; Maule, 1986;

Conical nozzle with sightglass

Conical nozzle with sightglass

Conical jacket outlet

Fermentation Vessels Conical

Pipe for C02 entry and pressure cleaning

Vessel cleaning and pressure delivery pipe

C02 washing lantern

Thermometer^ C02 injection cock

Fig. 7.46. Cylindro-conical fermentation vessel (Hough et al., 1971).

Conical jacket outlet

Thermometer^ C02 injection cock

Pipe for C02 entry and pressure cleaning

Vessel cleaning and pressure delivery pipe

C02 washing lantern

Conical jacket inlet

Outlet / cock Yeast cock with sightglass ^

Fig. 7.46. Cylindro-conical fermentation vessel (Hough et al., 1971).

Boulton, 1991). The fermentation is terminated by the circulation of chilled water via the cooling jackets which results in yeast flocculation. Thus, it is necessary to select a yeast strain which will flocculate readily in the period of chilling. Part of this yeast may be withdrawn and used for repitching another vessel. The partially cleared beer may be left to allow a secondary fermentation and conditioning. Some of the adantages of this vessel in brewing are:

1. Reduced process times may be achieved due to increased movement within the vessel.

2. Primary fermentation and conditioning may be carried out in the same vessel.

3. The sedimented yeast may be easily removed since yeast separation is good.

4. The maturing time may be reduced by gas washing with carbon dioxide.

Effluent gas exit

Liquid level Air sparger

Downcomer

Heat exchanger

Sterile medium inlet

Effluent gas exit

Liquid level Air sparger

Downcomer

Heat exchanger

Sterile medium inlet

Air Lift Fermentor

Culture exit

Bubble breakup device

Riser

Direction of flow

Air/ammonia sparge pipes

Air-lift fermenter with outer loop (Taylor and Senior,

Culture exit

Bubble breakup device

Riser

Direction of flow

Air/ammonia sparge pipes

Air-lift fermenter with outer loop (Taylor and Senior,

Air-lift fermenters

An air-lift fermenter (Fig. 7.47) is essentially a gas-tight baffled riser tube (liquid ascending) connected to a downcomer tube (liquid descending). Figure 7.47a shows an external riser and Fig. 7.47b an internal riser. Air or gas mixtures are introduced into the base of the riser by a sparger during normal operating conditions. The driving force for circulation of medium in the vessel is produced by the difference in density between the liquid column in the riser (excess air bubbles in the medium) and the liquid column in the downcomer (depleted in air bubbles after release at the top of the loop). Circulation times in loops of 45-m height may be 120 seconds. More details on liquid circulation and mixing characteristics are discussed by Chen (1990). This type of vessel can be used for continuous culture. The first patent for this vessel was obtained by Scholler and Seidel (1940).

It would be uneconomical to use a mechanically stirred fermenter to produce SCP (single-cell protein) from methanol as a carbon substrate, as heat removal would be needed in external cooling loops because of the high rate of aeration and agitation required to operate the process. To overcome these problems, particularly that of cooling the medium when mechanical agitation is used, air-lift fermenters with outer or inner loops (Fig. 7.47) were chosen. Development work for operational processes for SCP has been done by ICI pic in Great Britain (Taylor and Senior, 1978; Smith, 1980), Hoechst AG-Uhde GmbH in Germany (Faust et

Exhaust gas

Exhaust gas

Quorn Airlift Fermenter

al, 1977) and Mitsubishi Gas Chemical Co. Inc. in Japan (Kuraishi et al., 1978). Although ICI pic initially used an outer-loop system in their pilot plant, all three companies preferred an inner-loop design for large-scale operation. Hamer (1979) and Sharp (1989) have reviewed these fermenters. In the ICI pic continuous process, air and gaseous ammonia were introduced at the base of the fermenter. Sterilized methanol, other nutrients and recycled spent medium were also introduced into the downcomer. Heat from this exothermic fermentation was removed by surrounding part of the downcomer with a cooling jacket in the pilot plant, while at full scale (2.3 X 106 dm3) it was found necessary to insert cooling coils at the base of the riser.

Unfortunately the production of SCP for animal feed has not proved an economic proposition because of the price of methanol and the competition from animal feeds based on arable protein crops. ICI pic's vessel at Billingham, U.K. has now been dismantled.

In 1964, Rank Hovis McDougall decided to develop a protein-rich food primarily for human consumption

(Trinci, 1992). They have grown Fusarium graminareum on a wheat starch based medium using a modified ICI pic 40 m3 air-lift fermenter (Fig. 7.48) to produce the myco-protein Quorn. The use of an air-lift fermenter for culture of a mycelial fungus would seem unusual as lower rates of oxygen transfer occur in a viscous culture which give rise to lower biomass yields. Because low shear conditions are present in the vessel, long fungal hyphae can be cultured (the preferred product

C02 produced by fungal respiration is continously extracted

The 'downcomer'-as 02 is consumed and C02 disengaged, the culture becomes denser and descends the fermenter loop

The 'riser' -rising bubbles cause circulation of the culture up the fermenter loop

Glucose, biotin and mineral salts pumped in at a constant rate to give a dilution to rate of 0.19 h_1

C02 produced by fungal respiration is continously extracted

The 'riser' -rising bubbles cause circulation of the culture up the fermenter loop

The 'downcomer'-as 02 is consumed and C02 disengaged, the culture becomes denser and descends the fermenter loop

Glucose, biotin and mineral salts pumped in at a constant rate to give a dilution to rate of 0.19 h_1

Mycoprotein Fermenter

Myco-protein harvested

Steam to increase temperature to 64° for RNA reduction

Myco-protein harvested

Steam to increase temperature to 64° for RNA reduction

Culture is harvested at the same rate as Heat exchanger- fresh medium fed the culture generates into the fermenter heat but the exchange ensures a constant temperature of 30°

Fig. 7.48. Schematic diagram of the air-lift fermenter used by Marlow Foods at Billingham, England, for the production of myco-protein in continuous flow culture (Trinci, 1992).

form) even though production yields are only 20 g dm~3. At the present time a fermenter to produce 10,000 tonnes per annum of myco-protein is considered economically feasible.

Okabe et al, (1993) modified a 3-dm3 air-lift fermenter by putting stainless steel four-mesh sieves at the top and bottom of the draught tube to manipulate the morphology of Aspergillus terreus for optimum production of itaconic acid as the culture circulates in the vessel flow path. The fungal morphology was an intermediate state between pellets and pulp. Using this vessel, the itaconic acid production rate (g dnr1 h 1 ) was double that obtained with a stirred fermenter or an air-lift fermenter with a conventional draught tube.

Work has also begun to examine oxygen transfer rates with modified draught tubes. Carrington et al. (1992) used a 20 m3 pilot-scale bubble column fermenter fitted with an internal helical cooling coil (Fig. 7.49) or a solid draught tube. The fermentation studied was a commercial Streptomyces antibiotic fermentation in a complex medium which produced a viscous non-Newtonian broth. Tracer studies indicated that the vessel fitted with only the cooling coil behaved like an air-lift fermenter with a region of good mixing in the zone above the cooling coil. The coil acted as a leaky draught tube with back mixing taking place between the coils into the riser section. No poorly oxygenated zones were observed. Liquid velocities of 1 m sec were measured giving circulation times of 9 to 12 seconds and mixing times of 14 to 18 seconds. The K, a at different power inputs and viscosities was found to increase almost linearly with increasing power input and decreased exponentially with increasing viscosity. When a solid draught tube was installed inside the cooling coil the circulation time was similar, but the mixing time increased to 18 to 24 seconds. The KLa was also determined at different power inputs and viscosities. This gave a reduction of 5 to 25% in K, a with the biggest reduction at a high viscosity.

Wu and Wu (1990) compared KLas in a range of mesh draught tubes and a solid draught tube in an air-lift vessel of 15 dm3 working volume. When a 24-mesh tube was used at high superficial gas velocities the KLa was double that of the same vessel with a solid draught tube.

Bakker et al. (1993) have developed a multiple air-lift fermenter in which three air-lift fermenters with internal loops are incorporated into one vessel (Fig. 7.50). Fresh medium is fed into the central compartment, depleted medium overflows into the middle compartment, from here to the outer compartment where medium is eventually discharged. The hydrodynamics

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