WO1993022418A1

WO1993022418A1 - Bioreactor system

Info

Publication number: WO1993022418A1
Application number: PCT/GB1993/000875
Authority: WO
Inventors: Stanley John Pirt
Original assignee: Pirtferm Limited
Priority date: 1992-04-28
Filing date: 1993-04-27
Publication date: 1993-11-11
Also published as: AU4266893A; CA2134545A1; GB9209175D0; EP0640123A1; AU661598B2

Abstract

The invention provides fermentation vessels (10) comprising at least one fermentation zone, each of said zones being divided into two or more compartments by transverse baffles (16) each containing bypass means (17, 18) to permit passage of medium from the inlet (19) of the zone to an outlet (20) longitudinally spaced therefrom, a substantially vertical partition (11) being provided which divides the said zone from the region of said inlet to the region of said outlet into two channels connectd by gaps above and below said partition along its length and gas inlet means (25) being situated at the bottom of the said zone and on one side of the partition so that, in operation, influx of gas causes fermentation medium to flow transversely around the partition in the flow path defined by the two channels and the gaps above and below the partition while also moving longitudinally through the compartments of the said zone, means being provided to permit recycling of at least part of the effluent. These vessels are suitable for scale up. The invention further provides fermentation methods using such vessels wherein stirring is effected by cyclical movement of the medium therein.

Description

BIOREACTOR SYSTEM

This invention r._-u-.T-.es tc bioreactors (fermentation vessels) and to as--. r..---lies of such vessels.

In general, biorea.r rs are vessels designed to permit growth of microorπ.nnisms or other cells therein and to digest the substrate medium in which the cells are suspended. In continuous fermentation, such vessels permit passage of medium containing the cells through the vessel and continuous collection of the digested medium. In one type of continuous fermentation, the vessel is elongated and the r.edium passes in plug flow or an approximation thereto, so that back and forward mixing are minimized and at any point along the length of the vessel, the medium is essentially under batch fermentation conditions and undergoes the required fermentation in the time taken to traverse the length of the tank.

However, it is highly beneficial for the suspended cells to maintain constant contact with the medium and to avoid settling out under gravity. Furthermore, it has been found that traditional cylindrical fermentation vessels, such as tanks cr vats are difficult to scale up to capacities of several thousand cubic metres since the change in geometry rfectε the biological environment within the vessel. Also it is frequently necessary to aerate the system with a stream of air bubbles which generates turbulence and causes mixing that are in conflict with ideal plug flow. These problems can be overcome by use cr a long split channel bioreactor, for example of the typ- -.escribed in our International patent application No. PCT-ΕP 1, 01322 , published under number WO92/01779, in which a tank is longitudinally divided by a vertical panel in which medium moving longitudinally through the tank is also caused to move cyclically around approximation of plu flow.

We have now surprisingly found that the incorporation of transverse baffles into such a vessel such that by pass means --re provided tc permit the medium to pass longit dinally through the vessel whilst also cyclically movincr tround the panel not only improves the plug flow but permits efficient scale up.

According tc the present invention we provide a fermentation vessel comprising at least one fermentation zone, each cf said zones being divided into two or more compai'tments by transverse baffles each containing bypass means to permit passage of medium from the inlet of the zone tc an outlet longitudinally spaced therefrom, a substantially vertical partition being provided which divides the said zone from the region of said inlet to the region of said outlet into two channels connected by gaps above and below said partition along its length and gas inlet means being situated at the bottom of the said zone and on one side of the partition so that, in operation, influx of gas causes fermentation medium to flow transversely around the partition in the flow path defined by the two channels and the gaps above and below the partition while also moving longitudinally through the compartments of the said zone, means being provided to permit recycling cf at least part of the effluent.

The present system, termed a baffled split channel bioreactor (BSCB) , makes possible scale-up of the process while retaining unaltered conditions for fermentation- On the iarge scale tne vessels used in accordance with the invention will be about 3m in height and 0.3m in breadth with the length td'justed to accommodate tne desired volume, for example 20m long for a tank with a volume of about 18m³. These dimensions can be scaled down for smaller vessels wnich may have a volume less than 201. The efficiency :-■; tne plug flow and the flexibility of the reactor design .-re greatly increased by the introduction of the transverse baffles, which divide the reactor into compartments. In order to increase the scale of operation, it . -: possible tc connect an assembly of such vessels in parallel. It will be seen hereinafter, that by appropriate siting of the baffles and bypass means it is possible to arranc-ie the flow of the liquid medium so that the inlet and outlet of the bioreactor are side by side or one above the other, which facilitates both recycling of the biomass (cells; and scale up in parallel assemblies.

In one convenient embodiment, a dividing wall separates the vessel into two fermentation zones one on each side of the wall so that . longitudinal flow is through the compartments on one side, then after passage through a port in the dividing wall the horizontal liquid flow is reversed in the compartments on the other side of the dividing wall and inlet and outlet ends of the reactor are brought into juxtaposition.

In cases where flov; reversal occurs, by pass means may be provided such that medium may pass between compartments separated by ividing walls arranged substantially parallel to the vertical partition. However, as further explained hereinafter, it may be advantageous to join two or more tanks in series; a number of such series- combinations may ΛIΞO be connected in parallel.

Any suitable by-pass means may be used to permit passage of medium in a direction along the longitudinal axis of the vessel, however apertures within the baffles, for example ports, -.rr- preferred. In such a case, appropriate siting of said ports may be used to control the movement of medium. For example, positioning of ports alternately towards the top or bottom of adjacent baffles ensures the medium cvcies vertically.

Accord r. ■^• tc - further feature of the invention, ^' we provide - method of fermentation in which a fermentation medium :r introduced into the inlet of a fermentation vessel \'^~ πerein described and passes to the outlet therecr

gas fro the gas inlet means of said ferment.-1.on vessel causes the medium to move cyclically in a substantially perpendicular plane with respect to the base of the fer entor.

It is further desirable that the bioreactors are provided with means for controlling their temperature, for example, by mean? of .-. heat exchanger installed in each compartment, either heating or cooling may be required.

The design of BSCB modules is illustrated in Figures 1-13. A key to the figures is given below:

Figure 1 shov/s in perspective a straight BSCB module with four compartments.

Figure 2 shov/s in perspective two baffles and the partition in one compartment.

Figure 3 shows an elevation of a transverse cross section (entry end) of a BSCB.

Figure 4 shov/s a longitudinal elevation of a straight

BSCP with tour compartments.

Figure 5 shews a plan (top view) of a rectangular

BSCB with rour compartments.

Figure 6 shov/s a BSCB with four compartments arranged for reversal of horizontal liquid flow.

Figure 7 shov/s an elevation of the inlet and outlet end of a BSCB with reversal of liquid flow.

Figure 8 shows a longitudinal elevation of a BSCB with tour compartments arranged for reversal of liquid flow.

Figure 9 shov/s in perspective a BSCB with four compartments arranged for reversal of liquid flow. External dimensions are approximately to scale for a total capacity of about 18.81 with length 30cm,

--rreadth 25cm and height, 25cm.

Figure 10 shows in perspective a BSCB module with

:our compartments arranged for reversal of liquid flew drawn approximately to scale for a module with scout 18m³ capacity with length 10m, breadth 0.6m and

"eight 3m.

Figure 11 shows a plan of the top of a BSCB module with four compartments arranged for reversal of liquid flow, drawn approximately to scale for an 18m³ todule with breadth 0.6m and length 10m.

Figure 12 shows in perspective a tv/o tier BSCB module with four compartments, two above and two below, and reversal of liquid flow.

Figure 13 shows in perspective a multimodule BSCB consisting of four two-tier modules each with the first two compartments in the upper tier and the second two compartments (with reversal of liquid flow) in the lower tier.

The key to the Figures 1-14 is as follows:

10, tank; 11, partition; 12, gas supply manifold; 13, liquid upflow stream (riser) ; 14, liquid downflow stream (downcomer) ; 15, liquid level; 16, ^•;affle; 17, low port; 18, high pert; 19, inlet for substrate; 20, effluent culture stream; 21, flange tor lid seal; 22, vent for gas; 23, recycled gas; 24, effluent gas; 25, gas sparger; 26, sedi entor; 27, recycled bio ass; 28, excess sludge (biomass) discharge; 29, effluent supernatant; 30, direction of horizontal liquid flow; 31_/ '0' ring or other type of seal; 32, lid; 33, heat exchanger; 34, i let for air or other gas; 35, dividing wall; 36, culture teed from upper to lower tier; 37, base of upper tier of compartments. The straight BSCB module shown in Figure 1 consists of a tank ^"--0; divided into four compartments by three transverse baffles (16) and the compartments are split into two channels by the partition (11) either central or offset, which has a gap both above and below it. The gaps allow tr.e liquid contents of the module to be cycled vertic.--.--ly around the partition by means of an air or gas lift pr-vided by means of gas sparger (25) situated on the riser ,-ιpflow) side (13) of the partition. The riser liquid stream (13) and the downflow liquid stream (downccmer) (14) are shown in Figure 3. The size of the gap below the partition is about half the breadth of the vessel. The transverse baffles (16) extend from the base of the tank to above the liquid level (15) but leave a gap below tr.e lid (32) to unify the gas space. A perspective view of the baffles and partition in one compartment is shown in Figure 2. The baffles prevent horizontal flow of the liquid except for passage through at least one port in each baffle, alternately low (17) and high (18) . The alternating levels of these ports prevents the liquid from by passing the vertical liquid cycling. The diameter of the ports should be sufficiently large to permit turbulent flow through it and offer no obstruction to solids suspended in the medium.

A sice elevation and a plan of the top of a straight BSCB are s own in Figures 4 and 5 respectively.

Air cr other gas is supplied through the inlet (34) and mixec --r necessary with the recycled gas (23) , the gas mixture passes via the manifold (12) to the spargers situate! in each compartment. The tank is normally sealed with ^~ι - - (32) which rests on a flange (21) . As shown in Figure ^" the lid is sealed on the flange by an ^rG' ring or other -■ύitable sealing device. A gas vent (22) is provided in the lid. Means may be provided for withdrawing effluent gas; a part of the effluent gas can e r e n s eam an e excess s e s a (24) .

Temperature control is achieved by means of a heat exchanger (33) shown located on the partition in Figure 4 but other sites could be used.

The substrate stream (19) is fed into the riser side of the first compartment. After passage through the compartments the digested medium exits in effluent stream (20) . An overflow weir or other means is provided to keep the liquid level (15) constant. If required, the effluent stream is passed to a sedimentor (26) or other device to concentrate the biomass (cells) . Part of the biomass concentrate is recycled to the module in stream (27) , excess biomass exits from the system in stream (28) , the supernatant liquor leaves the sedimentor in stream (29) .

The BSCB is constructed of stainless steel or other non toxic metal or plastic including glass fibre plastic or other resistant and non-toxic material. The structure must be reinforced with ribs and possibly transverse struts between the walls to maintain the rigidity of the walls and the partition and baffles. The bottom of each longitudinal channel is preferably dished and corners are rounded to avoid angles where suspended solids could accumulate.

The number of compartments and their volume and length can be varied in order to isolate the various stages in the metabolic process. Usually the number of compartments will be in the range of 2-6.

In the aitern t -^•••• torm of the BSCB shown in Figure 6 the compartments are arranged to achieve reversal of the horizontal liquid flow and so bring the entry and outlet ends of the reactor into juxtaposition as shown in Figure 7. The plan of the top of a BSCB with flow reversal given in Figure <-. shows that the dividing wall (35) and the baffles (16) divide the BSCB into four compartments each of which is split into channels by the partitions (11) . The dividing wall (35) rises from the base to above the liquid level a.d leaves a gap below the lid, as shown in the transverse section, Figure 7 and the longitudinal elevation, Figure 3. Substrate and recycled biomass entering the BSCB at (19) , pass into the riser fl3) of the first compartment where vertical cycling around the partition (11) is induced by the gas sparger (25) . The riser (13) and downcomer (14) on each side of the dividing wall (35) are depicted in the cross section shown in Figure 7. The liquid passes into the second compartment through the low port (17) , cycles vertically around the central partition there, then crosses the dividing wall through the high port (18), into the third compartment, cycles vertically, then passes into the fourth compartment through low port (17) , and finally, after vertical cycling, exits from the BSCB into the effluent stream (20) .

It can be seen that the compartmented reactor facilitates reversal of liquid flow with consequent juxtaposition of inlet and outlet streams. A heat exchanger (33) may be either attache-- to the partition or inserted below it as shown in Figures 7 and S . There is a common gas space above the baffles and dividing wall and gas leaves through vent (22) .

Figure 9 show?: in perspective a BSCB -with four compartments and reversal of liquid flow. This figure is drawn with externax dimensions of the reactor approximately tr --caie for a 18.81 total volume and a length of 30cm. Tne direction of the horizontal liquid flow on each side of the dividing wall (35) is shown by the horizontal arrows (30) . Biomass is concentrated in the sedimentor (26) and partially recycled in stream (27) . Supernatant leaves in stream (29) and excess biomass sludge is removed in stream (28) .

A larger scale BSCB with four compartments and reversal of flow is snown in perspective in Figure 10 and a plan of the top in Figure 11. These figures are drawn approxir.-tely to scale for a reactor of total capacity 18m³ and a l ngth of 10m. The narrow cross section favours high rate vertical cycling and oxygen transfer. The arrows (30) indicate the direction of horizontal flow.

For flow reversal, the compartments may be arranged in a number ot tiers. In a two-tier system, medium enters at one end cf the top tier and flews longitudinally cycling vertically around the vertical panel and upon reaching the opposite end of the tier, the medium overflows, for example via a weir, into the compartment vertically below whereupon longitudinal flow is reversed.

Figure 12 shows in perspective a BSCB with four compartments constructed in tv/o tiers. The first two compartments form the top tier. Horizontal flow directions are indicated by the arrows (30) . The culture medium after passing through the top two compartments overflows through a v/eir and passes via stream (36) to the lower t.-:.r where the flow of the medium is reversed and the culture emerges in stream (20) . In Figure 12 the base (37) of the upper tier is the lid of the bottom tier, however it may be convenient to have a gap between the two tiers. .las vents (22) are provided for each tier. This facility for vertical stacking of the compartments reduces the her: .-ontal area required, brings the entry and exit ends ~ : the reactor into juxtaposition, also it facilit es recycling of the biomass and scale up of the plant. Scale-up of the bioreaction process is achieved by means of a multimodule assembly of BSCB. Figure 13 shows in perspective a multimodule consisting of four modules -of the two tier type with reversal of liquid flow. The dividing walls (35) between the modules do not extend to the lid of the module so that there is a common gas space above the liquid level. Each tier has its common gas vent (22) . The liquid level in each module is set by a weir at the outlet. A common sedimentor (26) provides recycled biomass for the entire multimodule.

The multimodule concept (advanced in our patent application No PCT/EP91/01323) permits scale-up without disturbance of the conditions in the bioreaction and provides economies of scale. Thus it is possible to adapt the BSCB both to large centralized plants and down to single modules.

For multistage processes which require different conditions or different organisms in each stage modules are connected in series with appropriate changes in the conditions, for example cf substrate and temperature from module to module in the series.

The BSCB lends itself well to installation off shore, which is of great interest for the activated sludge treatment of sewage in coastal areas where convenient land sites are often not available. For this purpose the BSCB in multi odular form can be installed in a floating or shipboard plant or on a platform anchored offshore. In many cases such offshore plants may be coupled up to existing sewage outfalls ending in the. sea. The BSCB can also be installed in mobile plants mounted on lorries or trailers, which is of interest where the substrate availability is seasonal cr temporary. T e may e u se or e ermen a on o a var e y of organisms, for example microorganisms, plant or animal cells.

The BSCB may additionally be provided with means to enable a vacuum or partial vacuum, to be applied to the tank.

As specific embodiments of a fermenter module two continuous fermentation processes are described below by way of example and with reference to the above description of the module. The first process is the aerobic activated sludge process for sewage purification. The second process is the anaerobic fermentation of sugar to produce ethanol .

In each case the process makes use of a BSCB module of the • type shown in Figure 10. The module is divided into four compartments with reversal of liquid flow. The module has a height of 3m and breadth of 0.6m so that the distance between the partition and the wall is 0.15m. The gap between the partition and the base of the module is 0.15m. The length of the module -is 10m. The total volume of the module is about 18m³ and the working culture volume is 15m³.

ACTIVATED SLUDGE TREATMENT OF SEWAGE

In the single stage activated sludge process the module is filled with sewage with a B.O.D. (biochemical oxygen demand) of about 250 mg l^*1. The temperature of the sewage is set as high as possible in the range 15-30°. The contents of the module are inoculated with activated sludge then aerated by means of the sparger. The gas flow rate through the sparger is fixed between about 1.5 to 3.0m³ in to generate a liquid velocity in the downcomer and riser streams of about 5m in^"1. The vertical cycling keeps in suspension particles of matter present in the medium. The aeration gas is either air or part recycled gas with air. The dissolved oxygen concentration as measured by an oxygen electrode placed near the base-of the downcomer channel is maintained at about 2mg I^"1 by control of the rate of air flow from the manifold.

After the initial batch culture during which the activated sludge is propagated, sewage together with recycled biomass sludge is continuously fed into the module through the inlet (19) . The temperature of this feed should be adjusted to the reaction temperature before it enters into the module. The feed rate of the sewage stream (19) is 72m³ d^"1.

A sedimentor or other type of separator concentrates the biomass solids in the biomass recycle stream (27) to 50kg dry matter m^"3 and the flow rate of this stream (27) is adjusted to be 2.23m³ d^'1.

During passage of the sewage through the module the volatile suspended solids (VSS) in the liquid increase by about 0.175 kg m^"3. The plug flow of the culture along the module facilitates the digestion of those substrates which are used in sequence. This process produces a liquid effluent (29) with a B.O.D. of 20 mg I^"1 and a volatile suspended solids (VSS) content of 30 mg l^"1.

Modules in series are added if additional stages of purification such as anaerobic digestion, nitrification, denitrification and phosphate removal are required.

The process is scaled up by the use of a multimodule for example of the type shown in Figure 13. For instance, a sewage flow rate of 1000m³ d^"1 would require 14 modules, each of 15m³ working capacity, in the multimodule f rmenter. In the ethanol fermentation, the 18m³ rectangular module after cleaning and disinfecting is charged with disinfected culture medium, 13.5m³. The culture contains: glucose, 185g l^"1 together with sources of 5 vitamins, nitrogen, such as ammonia or urea, phosphate, sulphate, magnesium, iron and trace elements as required to produce a yeast concentration of 20.9g dry weight 1^"". The pH value of the medium is adjusted to pH 4.5. Before use the medium is pasteurized if necessary. The temperature of the medium is adjusted to 30-35⁰C. The inoculum of Saccharomyces uvarum is grown in the complete medium described above with 185g glucose l^"1. The module is inoculated with 1.5m³ of the inoculum culture which has just about reached its maximum gas production rate. Gas generation in the module is allowed to reach its peak then the effluent gas is recycled to the sparger at a rate of about 1.0m³ min^'1. Complete culture medium is fed into the module through stream (19) at a flow rate of 1.0m³ h^"1. A bleed of air from (12) at about 1.0m³ min^"1 into the spargers is required to avoid a sterol deficiency in the yeast.

After passage through the sedimentor, biomass concentrated to 150g dry weight 1^"; in the recycle stream .27) is fed back to the module at a flow rate of 0.5m-⁵ h^"1. The liquid effluent from the culture contains about 80g ethanol l^"1 and the productivity of the culture is about 5.4 kg ethanol m^"3 h^"1.

Application of a vacuum to the culture in the fermenter module through the gas vent (22) causes the culture to boil at 35"C when the gas pressure is 50 mm Hg. Evaporation of the culture by vacuum fermentation with the temperature at the boiling point in the upcomer stream provides a means of cooling the contents of the module, removing ethanol from the culture, decreasing the carbon dioxide partial pressure and concentrating the biomass, the effects of v/hich permit increase in the substrate feed rate and the medium strength, in particular the glucose concentration, and increase the ethanol productivity of the module (kg m^"3 h^" up to four fold.

Claims

1. A fermentation vessel comprising at least one fermentation zone, each of said zones being divided into two or more compartments by transverse baffles each containing bypass means to permit passage of medium from the inlet of the zone to an outlet longitudinally spaced therefrom, a substantially vertical partition being provided which divides the said zone from the region of said inlet to the region of said outlet into two channels connected by gaps above and below said partition along its length and gas inlet means being situated at the bottom of the said zone and on one side of the partition so that, in operation, influx of gas causes fermentation medium to flow transversely around the partition in the flow path defined by the two channels and the gaps above and below the partition while also moving longitudinally through the compartments of the said zone, means being provided to permit recycling of at least part of the effluent.

2. A vessel as claimed in claim 1 wherein the by-pass means comprise apertures within the baffles.

3. A vessel as claimed in claim 2 wherein the apertures are alternately positioned in adjacent baffles either towards the top of the baffle, or towards the bottom of the baffle.

4. A vessel as claimed in any one of the previous claims wherein the ratio of the height to breadth is in the range 3:1 to 10:1.

5. A vessel as claimed in any one of the preceding claims in which the gas inlet means is a sparger in each compartment located on one side of the longitudinal partition.

6. A vessel as claimed in any one of the preceding claims v/hich is sealed and in which means are provided to withdraw any effluent gas.

7. A vessel as claimed in any one of the preceding claims in which means are provided to apply at least a partial vacuum.

8. A vessel as claimed in any one of the preceding claims provided with heating and/or cooling means.

9. A vessel as claimed in any one of the preceding claims in v/hich the outlet is connected to separation means to concentrate solids in the effluent and/or clarify the effluent.

10. A vessel as claimed in any one of the preceding claims wherein a dividing wall having by-pass means separates the compartments into two groups, one on each side of the wall so that longitudinal flow of medium is through the compartments on one side of said wall followed by passage through the by-pass means in said dividing wall and longitudinal flow in the reverse direction through the compartments on the other side of the dividing wall, and wherein inlet and outlet are brought into juxtaposition.

11. A vessel as claimed in any one of claims 1 to 9 wherein the compartments are arranged in tiers wherein longitudinal flow of medium in an upper tier is followed by overflow to the tier below in which longitudinal flow is in the reverse direction.

12. An assembly of vessels as claimed in claim 1 connected in parallel.

13. An assembly as claimed in claim 12 wherein the vessels share dividing walls and having a common gas space between the liquid level and the lid.

14. An assembly of vessels as claimed in claim 1 connected in series.

15. A method of fermentation in which a fermentation medium is introduced into the inlet of a fermentation vessel as defined in claim 1 and passes to the outlet thereof while gas from the gas inlet means of said fermentation vessel causes the medium to move cyclically in a substantially perpendicular plane with respect to the base of the fermentor.

16. A method as claimed in claim 15 in which the medium comprises solid particles maintained in suspension by transverse cycling around the substantially vertical partition.

17. A method as ^■ claimed in claim 15 or claim 16 wherein the medium comprises microorganisms or plant or animal cells.