GB2108151A - Fermenter - Google Patents

Abstract

A fermenter for continuous circulation and aeration of a culture wherein an enclosed system is formed by a riser and a downcomer which communicate with each other and with a compartment above their upper ends, the enclosed system comprising a combination of features which together make it suitable for large scale commercial operation, eg baffles in the riser.

Description

SPECIFICATION Fermenter This invention relates to a fermenter for the continuous circulation and aeration of a growing culture.

The successful commercial exploitation of a number of the fermentation processes developed within the past two decades and in particular of processes for the production of single cell protein necessitates large-scale operation and the use of fermenters of increased capacity. In the production of suitably large fermenters it has not proved to be possible simply to "scale-up" fermenters of conventional design since the development of the large fermenters has presented many unexpected problems. The solution of these problems necessitates significant changes in fermenter design. In order to develop a successful process for the production of single cell protein it is of crucial importance to develop a large fermenter having a suitable combination of features to enable the fermentation to be satisfactorily controlled.

According to the present invention we provide a fermenter for the continuous circulation and aeration of a growing culture wherein an enclosed system is formed by a riser and a downcomer which communicate with each other and with a compartment above their upper ends characterised in that the system comprises in combination the following features:: (a) pipe means for introducing a liquid into the system, provided with means for sterilizing said liquid; (b) pipe means for removing a liquid from the system designed according to velocity criteria to produce a velocity of outward liquid flow sufficient to prevent ingress of microorganisms to the system along the pipe; (c) means for injecting a gas into the riser; (d) means, containing or associated with a cyclone, for removing gas from the compartment; (e) distribution means within the riser for supplying a limiting nutrient to culture in the riser at an average of at least one position per cubic metre of the volume of the riser occupied by the culture; and (f) a plurality of baffles positioned within the riser and spaced apart at intervals vertically above one another, the solid part or parts of each baffle occupying from 30% to 80% of the internal cross-sectional area of the riser.

The gas injected into the riser can be used to cause the circulation of the culture around the system. In this case the gas is preferably injected into the lower part of the riser or, depending upon the geometry of the system, below the lower end of the riser or into a pipe connecting the lower ends of the riser and the downcomer. When the fermenter is used in an aerobic fermentation process, the gas injected into the system is an oxygen-containing gas such as air. In addition to the gas injected into or near the lower part of the riser as described above, in an aerobic fermentation process it is preferred to inject some gas into the upper part of the downcomer.

The design and dimensions of the enclosed system of the fermenter have many possible variations, including the devices described in our UK Specifications Nos. 1353008, 1417486 and 1417487. The invention is particularly useful in connection with the large scale production of single cell protein, e.g. by the process of UK Specification No. 1 370892 and particularly by growing bacteria of the species Methylophilus methylotrophus (formerly named Pseudomonas methylotropha) on a culture medium containing methanol as the carbon source.

The fermenter of the invention comprises a combination of six major features which together render it very suitable for use in the operation of a commerical process for the production of single cell protein. These features (a) to (f) which are listed above are described in more detail below.

(a) Pipe means for introducing a liquid into the system The fermenter is provided with one or more pipes along which solutions containing nutrients can be supplied continuously to a culture within it, one or preferably more of these pipes being provided with means for sterilizing the nutrient solutions passing along them.

Sterilization, which can be for example heat or filter sterilization, is carried out to prevent unwanted extraneous microorganisms from being carried into the fermenter. Preferably steam sterilization is used, a suitable arrangement being a valve, steam supply, condensate removal followed by a second valve located in sequence in the pipe. The valves are preferably fitted with a steam gland arrangement to prevent microorganisms ingress down the valve step. The pipes are used to supply inorganic nutrients such as sources of nitrogen, magnesium, phosphorus and other appropriate nutrients. A separate distribution system will usually be used to supply the carbon source which will generally be the limiting nutrient.

(b) Pipe means for removing a liquid from the system It is possible for microorganisms to travel along a pipe against the bulk flow of a fluid therein. Therefore the pipes carrying liquids (particularly product culture) and preferably also gases out of the fermenter are designed according to velocity criteria to ensure that the velocity of the fluid at at least one part of the pipe exceeds a minimum value and prevents microorganisms travelling towards the enclosed system against the fluid flow in the pipe. This can be done in a variety of ways, e.g. by suitably narrowing a portion of the pipe, by use of a restriction orifice or by inclusion of a flow restriction device such as a contoured piece of material in the pipe.This prevents the ingress of unwanted microorganisms to the enclosed system along the internal walls of exit pipes including product exit pipes, gas exit pipes and pipes for sampling devices.

(c) Means for injecting a gas into the riser The fermenter is provided with means, suitably a sparger having a plurality of holes, for injecting a gas into the riser as described above. In an aerobic fermentation the gas is suitably air. Suitably the gas is injected in amounts and at pressures sufficient to cause circulation of the culture, by the differential voidage that occurs between the riser and the downcomer, in addition to aerating the culture.

(d) Means for removing gas from the compartment The off-gas from the fermentation process is removed from the compartment along a pipe.

For the purpose of foam control and droplet removal one or more cyclones are located in the compartment and/or in the pipe along which off-gas is removed.

(e) Distribution means for supplying a limiting nutrient The limiting nutrient is usually the carbon source and is supplied to a culture in quantities such as to be limiting to the growth of the culture with respect to the other constituents of the culture medium. The fermenter of the invention contains a distribution system located within its working fluid volume, particularly in the riser but also elsewhere, which enables the limiting nutrient to be supplied to the culture at an average of at least one position per cubic metre of the volume occupied by the culture. Preferably the limiting nutrient is supplied at a greater number of positions than one per cubic metre, particularly 10 to 10,000 positions.The purpose of the distribution system is to achieve good distribution of the limiting nutrient throughout the culture in the fermenter and to ensure that the localized excessive concentrations of the limiting nutrient do not occur causing an alternative nutrient, e.g. oxygen, to become the limiting nutrient. In the production of single cell protein the distribution system enables an increased yield of product to be achieved.

(f) A plurality of baffles positioned within the riser In order to promote good mixing of gas and liquid a plurality of baffles are positioned within the riser and spaced apart vertically at intervals above one another. The solid part or parts of each baffle occupy from 30% to 80% of the internal cross-sectional area of the riser, preferably from 50% to 60% of of the crosssectional area. Preferably the baffles are positioned within the riser at regular intervals along its height. Suitably the baffles are positioned within the riser at intervals ranging from D/4 to D, where D is the diameter of the riser. Thus for example in a riser of diameter 4 metres and height 50 metres the baffles are suitably positioned at intervals from 1.2 metres upwards. The baffles may be formed in a wide variety of ways.They may be plates having one or a plurality of orifices, single straight members or a plurality of straight members forming grids which may or may not comprise intersecting straight members. When the grids are formed from straight members it is preferred, for reasons of stiffness and ease of mounting, that these should be profiled beams, e.g. beams having T or I cross-section. A baffle may be formed from a number of groups of T or I beams with narrow slots between individual beams in each group.

The beams themselves may contain holes in their horizontal parts. The baffles may be attached to the wall of the riser by conventional means e.g. bolting or welding.

In addition to the features (a) to (f) described above it is preferred that the fermenter is fitted with other appropriate devices to facilitate the control of the fermentation. Such other appropriate devices include pressure measuring devices, devices for measuring oxygen concentration and instruments for measuring and controlling pH. These other devices and instruments may be located in the riser and/or in the downcomer or in the case of instruments for controlling and measuring pH in pipes leading to or from the enclosed system of the fermenter.

The invention is illustrated by the accompanying drawings wherein: Figure 1 is a side elevation of a fermenter according to the invention.

Figure 2 is a cross-section along the line A A of Fig. 1.

Figure 3 is a diagram showing the details of pipe means for introducing nutrients into the system provided with steam sterilization means.

Figure 4 is a diagram showing the details of pipe means for introducing nutrients into the system provided with filter sterilization means.

Figure 5 shows a detail of a limiting nutri ent distribution system.

The fermenter shown in the drawings has an outer shell which comprises two cylindrical sections of different cross-sectional area sur mounted by a dome. The cross-sectional area of the upper of the two sections exceeds that of the lower. The lower section is divided by a pair of partitions parallel to its axis into a riser 2 and a downcomer 1, the downcomer being effectively divided into two zones. The upper section and the dome enclose a compartment 3 into which gas produced during fermentation is disengaged from a culture contained in the fermenter. Air is injected into the lower part of riser 2 through sparger 4 and gas is disengaged from the culture in the fermenter in the upper parts of riser 2 and downcomer 1, passing through liquid surface B... B into gas-filled part of compartment 3 from whence it leaves the fermenter through gas exit pipe 5.Foaming in compartment 3 is controlled by cyclone 6 with gas inlet slots 28 located therein.

The majority of the inorganic nutrients are supplied to the fermenter along nutrient feed pipes 7 which are provided with steam sterilization means. A typical pipe 7 is shown in detail in Fig. 3 of the drawings. The steam sterilization means comprises a sterilizer 8 (which can be steam - or hot water - heated), a retention vessel 9, main pipe valves 10 and 11, steam inlet 12 and condensate outlet 1 3 together with system sterilizing valve for steam 14 and system sterilizing valve for condensate 1 5. Nutrients are supplied to nutrient feed pipe 7 at end 1 6 thereof and pass into the fermenter at end 1 7 of the pipe.

Combinations of nutrients whose physical and chemical properties are compatible can be fed to the fermenter through the same sterilized pipe 7. Steam sterilization of nutrient feed pipes such as 7 is effected by the following sequence of operations: Firstly the part of pipe 7 adjacent to the fermenter is steam sterilized. To do this valves 10 and 11 are closed whilst the valves in steam inlet 1 2 and condensate outlet 1 3 are opened, thus ensuring that the portion of pipe 7 between valves 10 and 11 is sterilized.

System sterilizing valves 14 and 1 5 are opened to sterilize the pipework and retention vessel 9 which lie between them. A nutrient solution is then fed through end 1 6 of pipe 7 into sterilizer 8 and system sterilizing valves 14 and 1 5 are closed. After an interval during which a suitable pressure develops in the system, the valves in steam inlet 1 2 and condensate outlet 1 3 are closed and main pipe valves 10 and 11 are opened thus admitting sterilized nutrient to the fermenter through end 1 7. Preferably the valves in this system are fitted with steam glanded arrangements to prevent ingress of unwanted microorganisms down the valve stems.

Other nutrient feed pipes 1 8 are used to supply to the fermenter nutrients such as methanol, ammonia and oxygen which can be filter sterilized. A typical pipe 1 8 is shown in detail in Fig. 4 of the drawings. Nutrients pass along pipe 18 to the fermenter from end 1 9 to end 20 through prefilters 21 and sterile filters 22. Prefilters 21 typically filter particles of sizes down to 1-2y and are included in the system to prolong the useful lives of sterile filters 22. Sterile filters 22 are equipped with steam sterilization means (not shown in Fig.

4).

Product leaves the fermenter along product exit pipe 23 which contains a flow restricting device at 24 which produces a velocity of outward liquid flow at that position which is sufficiently large to prevent the ingress to the system of microorganisms beyond 24. Further along pipe 23 in a direction away from the fermenter there is located means for sterilizing the pipe (not shown in Fig. 1). The extend of the narrowing at 24 produced by the restricting device depends upon velocity criteria for the system under consideration, the velocity criteria depending on whether the flow is single phase, two phase, or three phase.

Riser 2 contains a series of baffles 25 formed from perforated profiled beams arranged in groups. The baffles 25 are spaced apart at intervals vertically above one another as shown. Riser 2 also contains a distribution system for supplying a limiting nutrient to the culture throughout the riser. The limiting nutrient is supplied through a large number of pipe grids such as 26 each having many orifices 27 in it. A typical pipe grid 26 is shown in the detailed drawing Fig. 5. The limiting nutrient is supplied to pipes 26 through manifold pipes (not shown in the drawing) and the whole distribution system is arranged to fit in with the series of baffles 25.

The fermenter shown in the drawings is very suitable for the commercial production of single cell protein in particular by a process such as that described in UK Specification No. 1 370892 in which bacteria of the species Methylophilus methylotrophus are cultured in a medium containing methanol as the limiting carbon source which is supplied to the culture through pipes 1 6 of the distribtion system.

Culture is continuously withdrawn from the fermenter along exit pipe 1 3 and bacterial cells continuously separated from the withdrawn culture and dried to form single cell protein product which is a useful component of animal feeds. When used in such a process for the production of single cell protein, the fermenter of the invention has the following advantages: 1. High oxygen transfer rates into cultures can be achieved and therefore with the associated fine distribution of methanol, the limiting nutrient, a high productivity of microorganisms and high utilization of substrate can be obtained.

2. The absence of moving parts in the fermenter results in a lower energy input per unit mass of production than with conventional fermenters and also results in easier maintenance of sterility.

3. The fermentation system can readily be scaled up.

4. The fermenter enables a product having a highly uniform quality to be produced.

Claims (5)

1. A fermenter for the continuous circulation and aeration of a growing culture wherein an enclosed system is formed by a riser and a downcomer which communicate with each other and with a compartment above their upper ends characterised in that the system comprises in combination the following features:: (a) pipe means for introducing a liquid into the system, provided with means for sterilizing said liquid; (b) pipe means for removing a liquid from the system designed according to velocity criteria to produce a velocity of outward liquid flow sufficient to prevent ingress of microrganisms to the system along the pipe; (c) means for injecting a gas into the riser; (d) means, containing or associated with a cyclone, for removing gas from the compartment; (e) distribution means within the riser for supplying a limiting nutrient to culture in the riser at an average of at least one position per cubic metre of the volume of the riser occupied by the culture; and (f) a plurality of baffles positioned within the riser and spaced apart at intervals vertically above one another, the solid part or parts of each baffle occupying from 30% to 80% of the internal cross-sectional area of the riser.

2. A fermenter according to claim 1 characterised in that the distribution means within the riser supplied the limiting nutrient to the culture at an average of at least 10 positions per cubic metre of the volume of the riser occupied by the culture.

3. A fermenter according to claim 1 or claim 2 characterised in that the solid part or parts of each baffle in the riser occupies from 50% to 60% of the internal cross-sectional area of the riser.

4. A fermenter according to any one of claims 1 to 3 characterised in that the baffles in the riser are formed from perforated profiled beams.

5. A fermenter for the continuous circulation and aeration of a growing culture substantially as described and as shown in the drawings.