Abstrict
A "pressure-cycle" fermentation process and fermenter
wherein an oxygen-enriched gas containing at least 30% by volume
oxygen is supplied to the fermenter downcomer and the nutrient supply
to the fermenter is such as to allow growth of the culture to continue
in the downcomer. The process and fermenter may be used in the production
of single cell protein using bacteria such as Methylophilus methylotrophus
or yeasts such as Fusarium graminearum Schwabe.
Claims
We claim:
1. A process for the aerobic fermentation of a culture containing
microorganisms in which the culture is continuously circulated around
a system having a riser and a downcomer communicating with each
other at their upper and lower ends, oxygen-containing gas being
supplied to culture in the lower part of the riser and in the downcomer
in amounts sufficient for the culture and to circulate it around
the system and nutrients including a carbon source being supplied
to culture in the riser wherein the oxygen-containing gas supplied
to the downcomer contains at least 30% by volume of oxygen and is
supplied in an amount within the range 0.2% to 20% by volume of
the amount of oxygen-containing gas which is supplied to the riser
and sufficient nutrients including the carbon source are supplied
to culture in the downcomer or are carried by the circulating culture
into the downcomer to permit growth of culture to occur in the downcomer.
2. A process according to claim 1 wherein the oxygen-containing
gas supplied to the downcomer is supplied to the upper half thereof.
3. A process according to claim 2 wherein the oxygen-containing
gas is supplied in a region between 10% and 40% of the length of
the downcomer from the top thereof.
4. A process according to claim 1 which forms a step in a process
for the production of single cell protein.
5. A process according to claim 4 wherein the microorganism is
a bacterium belonging to the species Methylophilus methylotrophus
and the culture contains methanol as a carbon source.
6. A process according to claim 4 wherein the microorganism is
a yeast belonging to the species Fusarium graminearum Schwabe and
the culture contains glucose as a carbon source.
7. A process according to claim 1 wherein the amount of oxygen
in the gas added to the downcomer is sufficient to maintain the
dissolved oxygen tension (DOT) in the culture at the bottom of the
downcomer at a value above 10 mm Hg.
8. A fermenter which comprises a riser and a downcomer communicating
with each other at their upper and lower ends, riser-gas supplying
means for supplying oxygen-containing gas to the lower part of the
riser sufficient to aerate and circulate a culture contained in
the fermenter, downcomer-gas supplying means for supplying gas to
the downcomer, and nutrient-supplying means for supplying nutrients
including a carbon source to culture in the riser characterised
in that the downcomer-gas supplying means is such that it can supply
a gas containing at least 30% by volume of oxygen in an amount within
the range 0.2% to 20% by volume of the gas supplied to the riser
by the riser-gas supplying means and the nutrient-supplying means
is such that it can supply nutrients including the carbon source
to culture in the riser or in the riser and the downcomer in a manner
such that nutrients are available to culture in the downcomer in
an amount sufficient to permit growth of the culture to occur in
the downcomer.
Description This invention relates to a fermentation process using a fermenter
of the "Pressure Cycle" type and to an improved "Pressure
Cycle" fermenter.
A "Pressure Cycle" fermenter is essentially a system
for aerobic fermentations which comprises a chamber of ascending
flow (riser) and a chamber of descending flow (downcomer) connected
at their upper and lower ends to permit a culture to be circulated
around the system. An oxygen-containing gas can be injected into
the system at or near the lower end of the riser and this serves
to aerate the culture and to cause it to circulate upwardly in the
riser and downwardly in the downcomer. Most growth takes place in
the riser and nutrients including the carbon source are usually
added to culture in the riser. "Pressure Cycle" fermenters
are very suitable for processes for the production of single cell
protein (SCP) such as the processes disclosed in UK Pat. Nos. 1346061
and 1370892. In the "Pressure Cycle" fermenters disclosed
in UK Pat. Nos. 1353008 and 1417487 there is provision for injecting
an oxygen-containing gas into culture in the downcomer.
In conventional processes for the production of SCP using "Pressure
Cycle" fermenters the productivity of the process can sometimes
be reduced due to oxygen-deficient growth of culture taking place
in the downcomer. Oxygen-deficient growth occurs more often when
the microorganism being cultured is a yeast rather than a bacterium
due mainly to viscosity effects and in the case of a yeast can lead
to the production of ethanol.
According to the present invention we provide a process for the
aerobic fermentation of a culture containing microorganisms in which
the culture is continuously circulated around a system having a
riser and a downcomer communicating with each other at their upper
and lower ends, oxygen-containing gas being supplied to culture
in the lower part of the riser and in the downcomer in amounts sufficient
for the culture, i.e. the amounts being matched to the needs of
microorganisms in the culture, and to circulate it around the system
and nutrients including a carbon source being supplied to culture
in the riser characterised in that the oxygen-containing gas supplied
to the downcomer contains at least 30% by volume of oxygen and is
supplied in an amount within the range 0.2% to 20% by volume of
the amount of oxygen-containing gas which is supplied to the riser
and sufficient nutrients including the carbon source are supplied
to culture in the downcomer or are carried by the circulating culture
into the downcomer to permit growth of culture to occur in the downcomer.
Further according to the invention we provide a fermenter which
comprises a riser and a downcomer communicating with each other
at their upper and lower ends, riser-gas supplying means for supplying
oxygen-containing gas to the lower part of the riser sufficient
to aerate and circulate a culture contained in the fermenter, downcomer-gas
supplying means for supplying gas to the downcomer, and nutrient-supplying
means for supplying nutrients including a carbon source to culture
in the riser characterised in that the downcomer-gas supplying means
is such that it can supply a gas containing at least 30% by volume
of oxygen in an amount within the range 0.2% to 20% by volume of
the gas supplied to the riser by the riser-gas supplying means and
the nutrient-supplying means is such that it can supply nutrients
including the carbon source to culture in the riser or in the riser
and the downcomer in a manner such that nutrients are available
to culture in the downcomer in an amount sufficient to permit growth
of the culture to occur in the downcomer.
The gas supplied to the downcomer in the process of the invention
may be oxygen or any mixture of a gas or gases inert in terms of
the fermentation and oxygen which contains at least 30% oxygen by
volume. Preferably the downcomer gas is oxygen-enriched air containing
at least 30% oxygen by volume, the additional oxygen being provided
from any suitable source. The gas supplied to the riser may be oxygen,
air or any mixture of a gas or gases inert to the fermentation and
oxygen which can provide sufficient oxygen adequately to aerate
the culture in the riser. Preferably the riser gas is air.
Oxygen-containing gas may be supplied to the riser and the downcomer
by any suitable means. Conveniently gas is supplied to the risrr
through a perforated pipe located at its bottom end which provides
a plurality of nozzles. A perforated pipe may also be used to supply
gas to the downcomer. However it is also possible to supply gas
to the downcomer using more complicated spargers such as venturi
nozzles, porous media, open ended pipes with associated downstream
bubble break up meshes, grids, vanes or moving parts, side stream
addition using some additional motive forces which would be wholly
or in part the downcomer gas itself or any combination of these
techniques.
The main supply of gas to the downcomer is preferably made to the
upper half thereof. In some situations it may be desirable to make
a further addition of gas to the lower half of the downcomer but
this will usually be an auxilliary addition of a lesser amount of
gas than is added to the upper half. In particular it is preferred
that the main addition of downcomer gas is made in the region between
10% to 40% of the length of the downcomer from the top thereof.
In a tall fermenter it may be desirable to make more than two additions
of gas to the downcomer, for example in a downcomer of height 40
meters five additions may be made.
The process of the invention is suitably used to produce single
cell protein (SCP), microorganisms being grown in a culture containing
an appropriate carbon source. Any suitable microorganisms may be
grown, including yeasts and bacteria, and a wide range of carbon
sources may be used for example carbohydrates, hydrocarbons and
oxygenated hydrocarbons such as alcohols. An example of a process
in which bacteria are grown is the process disclosed in UK Specification
No. 1370892. In this process strains of the species Methylophilus
methylotrophus (formerly known as Pseudomonas methylotropha) are
grown in a culture containing methanol as a carbon source. Cultures
of suitable strains of Methylophilus methylotrophus have been deposited
at three cuIture collections with the following corresponding accession
numbers at the different collections:
1. National Collection of Industrial Bacteria (NCIB), Torrey Research
Station, Aberdeen, Scotland, UK - NCIB Nos. 10508 to 10515 and 10592
to 10596;
2. Agricultural Research Culture Collection (NRRL), Peoria, Ill.,
USA - NRRL Nos. B 5352 to 5364;
3. Fermentation Research Institute, Agency of Industrial Science
and Technology, Japan - FERM 1215 to FERM 1227. An example of a
process in which yeasts are grown is the process disclosed in UK
Specification No. 1346061. In this process strains of Fusarium graminearum
Schwabe are grown in a culture containing glucose as a carbon source.
A culture of a suitable strain of Fusarium graminearum Schwabe has
been deposited at the Commonwealth Mycological Institute, Kew, Surrey,
England with the accession number IMI 145425. A number of variants
of IMI 145425 have also been deposited at the Commonwealth Mycological
Institute and have the accession numbers IMI 154209 to IMI 154213.
The fermenter of the invention is suitably a modification of the
basic fermenters disclosed in UK Specification Nos. 1353008, 1417486,
1417487 and 2002417 having appropriate arrangements for admitting
gas to the downcomer and nutrients including the carbon source to
the riser or to the riser and the downcomer. Nutrients are supplied
to culture in the fermenter in a manner such as to permit growth
of the culture to occur in the downcomer.
An important advantage of the invention when used in a process
for the production of SCP is that it enables a significant improvement
in the productivity of the SCP production process to be achieved.
The reasons for this improvement in productivity include:
1. The addition of oxygen to the downcomer enables the dissolved
oxygen tension (DOT) profile therein to be improved so that supplies
of a carbon source added to culture in the downcomer or carried
over into the downcomer can be used in a more uniform manner by
microorganisms in the culture for growth; and
2. The oxygen added to the downcomer facilitates the maintenance
of minimum DOT requirements throughout the fermenter cycle. Preferably
the DOT at the bottom of the downcomer is maintained above 10 mm
Hg.
The invention is illustrated by the accompanying drawings wherein:
FIG. 1 is a side elevation of one embodiment of the fermenter according
to the invention.
FIG. 2 is a cross-section along the line A--A of FIG. 1.
FIG. 3 is a side elevation of a second embodiment of the fermenter
according to the invention.
The fermenter shown in FIGS. 1 and 2 has an outer shell which comprises
two cylindrical sections of different cross-sectional area surmounted
by a dome, the cross-sectional area of the upper of the two sections
being greater than 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. Air
is injected into the lower part of riser 2 through sparger 4 and
oxygen or oxygen-enriched air is injected into the two zones of
downcomer 1 through upper pipes 6 and lower pipes 7. Gas is disengaged
from the liquid in the fermenter in the upper parts of riser 2 and
downcomer 1, passing through liquid surface B . . . B into the gas-filled
part of compartment 3 from whence it leaves the fermenter through
port 5. Nutrients are added to the culture in the fermenter through
pipes not shown in FIGS. 1 and 2. Nutrients including the carbon
source are either added to the culture in both the riser and the
downcomer or into culture in the riser only in amounts sufficient
for nutrients to be carried over into the downcomer to enable growth
of the culture to continue in the downcomer. Preferably the carbon
source is added to culture in the riser through a large number of
holes in a network of pipes as described in UK Patent Specification
No. 1523583. Riser 2 also contains a series of baffles such as 24.
The two zones of downcomer 2 may contain heat exchangers (not shown
in the drawings).
The fermenter shown in FIG. 3 has a riser comprising cylindrical
upper and lower sections 8 and 9 respectively connected through
a reducing piece 10, upper section 8 having a smaller diameter than
lower section 9. The upper section 8 of the riser is connected through
upper connecting piece 11 to the upper end of cylindrical downcomer
12. Lower section 9 of the riser is connected at 13 with lower connecting
piece 14 which connects it with the base of downcomer 12. Air is
sparged into the lower section 9 of the riser through sparge pipe
15 causing continuous circulation of a culture which occupies the
fermenter up to the level C--C. Upper connecting piece 11 is not
allowed to run full of liquid in order to allow a free surface from
which gases such as air and carbon dioxide escape from the medium
and pass out through port 16. Oxygen or oxygen-enriched air is sparged
into downcomer 12 through upper pipe 17 and lower pipe 18, upper
pipe 17 being the main supply with lower pipe 18 being an auxilliary
supply source.
Nutrients including the carbon source are added to culture in the
riser and the downcomer or to culture in the riser alone in which
case the nutrients are added in amounts such that they are carried
over into the downcomer so that growth of the culture can continue
there. Nutrients may be added through pipes 19 and 21. Preferably
the carbon source is added to the culture in the riser through a
large number of holes in a network of pipes as described in UK Patent
Specification No. 1523583. Product is removed from the fermenter
through pipe 20. The upper section 8 of the riser contains a series
of baffles such as 22. Downcomer 12 contains a heat exchanger 23.
When the process of the invention is operated to produce single
cell protein by growing cells of a Methylophilus methylotrophus
strain in a culture containing methanol as a carbon source, operation
of the process to produce a well designed DOT profile around the
fermenter results in polysaccharide production being considerably
reduced. When the process is used to grow Fusarium graminearum Schwabe
on a glucose-containing culture a well designed DOT profile leads
to a reduction in ethanol production.
For example a culture of Fusarium graminearum Schwabe growing in
the fermenter of FIG. 3 having a nominal volume of 50 m.sup.3 which
is not supplied with oxygen-enriched gas in the downcomer will produce
approximately 1 g/l of ethanol when the cell concentration is approximately
10 g/l. When the process is operated in accordance with the invention
and oxygen is injected into the culture in the upper half of the
downcomer the ethanol content falls to approximately 0.01 g/l for
a similar concentration. The optimum position for injecting the
oxygen in any particular case can be determined by computer calculation. |