Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
9816
1 ~ 29721/29722
Gas-liquid Circulatin~ and Contactin~ Apparatus
This invention relates to a method and apparatus for oiro-
ulating a liquid and contacting it with a gas, particularly to a
fermentation method and a fermenter. The invention also relates to
an ener~y recovery system for use in combination with a fermentation
process (hereinafter referred to as a fermentation prooess of the
kind described) of the kind in which a feed ga9, having passed
through a compre~sor, is supplied under pressure to the process and
an off-gas under pressure is released from the process. More part-
icularly the process of the kind described i9 an aerobic fermentationprocess, the feed gas being an ox~gen-containing ga~ such as air and
the off-gas COmprising the feed gas depleted in oxygen to~ether with
carbon dioxide produced in the process.
For successful operation, aerobic fermentation processes
require intimate contact between a liquid and a gas. Recent develop-
ments, including the introductlon of new processes for the production `
of single cell protein and other fermentation products, have neces-
sitated the use of fermenters of greatly increased capacity. ~hese
developments have in turn led to the development of improved fer-
menters designed to enable large amounts of air to be introduced
into and intimately mixed with growing cultures efficiently.
Examples of such improved fermenters include those described and
claimed in our ~K specifications ~os. 1353008, 1417486 and 1417487
and in our Canadian Patent Specification No 1067395
There is a need for fermenters of even larger capacity than
those de~igned to date. For the successful com~ercial operation of
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~1~9816
2 B 29721/29722
these very large fermenters it is important that intimate mixing Or
gas and liquid is achieved as efficiently as possible.
In operatlng the fer~enters of our UK specifications
Nos. 1353008, 1~174~6 .~nd 1417487 ~nd of ourCanadian Patent Specifica-
5 tion No.1067395 we have not used a significant overpressure inorder to avoid increasing the concentration of ~issolved carbon
dioxide in the culture to a high level which might prove harmful to
microorganisms present in the culture.
Surprisingly we have now found that, in processes for
growing bacteria of the species ethylophilus methylotro~phus
(formerly named Pseudomonas met~ylotropha) in culture media contain-
ing methanol as the carbon source for growth, e.g. processes des-
cribed in our UK specification No. 1370892, the tolerance to carbon
dioxide is sufficient to allow significant overpressures to be used.
According to the present invention we provide a method for
contacting a liquid and a gas in an enclosed system comprising a
riser and a downcomer which communicate with each other and with a
compartment above their upper ends wherein the liquid îs continu-
ously circulated around the system, a ~as is continuously in~ected
into the liquid and a gas is continuously disengaged from the liquid
and, after passing through the compartment, is removed from the
system, the ga~ pressure above the liquid in the compartment being
at least 2 bars gauge.
~urther according to the invention we provide an apparatus
for continuous gas/liquid contact and circulation which comprises an
enclosed system formed by a riser and a downcomer which communicate
with each other and with a compartment above their upper ends, means
for introducing a liquid into the system, means for removing a
liquid from the system, means for injecting a gas into the system and
means for removing a gas from the compartment, the system being
! designed or having means to maintain a gas pressure of at least 2
bars gauge in the compartment.
The ~as injected into the system can be used to cause the
circulation of the gas/liquid mixture. In this case the gas is pre-
ferably injected into the lower part of the riser or, depending upon
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~ B 29721/29722
the geometry of the system, below the lower end o~ the riser orinto a pipe connecting the lower ends o~ the riser and the down-
comer. When the invention is used in an aerobic fer~entation
process, the ~as injected in-to the system is an o~gen-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 ferment-
ation process it is preferred to inject some gas into the upper
part of the downcomer.
~he design and dimensions of the enclosed system of the
gas/liquid contact method and apparatus of the invention have many
possible variations, including all the devices described and claimea
in our UK specifications Nos. 1353008, 1417486 and 1417487 and in
our Canadian Patent Specificaticn ~c:. 1067395. Most suitably
the method i9 an aerobic fermentation method and the apparatus is a
fermenter. The invention is particularly useful in connection with
large scale fermentation proce~ses e.g. in the large scale production
of single cell protein.
The pressure in the space above the liquid in the compart-
ment at the upper end of the enclosed system may be attained in any
suitable manner, Suitably the pressure is caused by restricting -the
escape of gas from the compartment, e.g. using a control valre. In
an aerobic fermentation process conducted according to the invention,
air is injected into the lower part of the riser and has two iunc-
tions. It serve~ to supply the oxygen requirements of microorgan-
isms in a culture and, as mentioned above and as described in detailin our ~K specification No. 1353008, it provides the driring force
for circulation. In the fermenter, oxygen is transferred frGm the
gas into solution and used by the culture whilst carbon dioxide
produced br the culture is transferred from solution into the gas
and is erentually removed from the system when the gas is disengaged
from the liquid at the top of the riser~ ~he oxygen transfer occurs
principally in the lower part of the riser and the carbon dioxide
transfer principally in the upper part.
Preferably the gas pressure in the upper p~rt o~ the system
is within the range 2 to 15 bars gauge, particularly 3 to 10 bars
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4 B 2g721/29722
gauge. The effective length of the apparatus, i.e. the length of
the riser, is preferably at least 20 metres, especially 40 to 80
metres. Particularly suitable arrangements are fermenters of
effective length 60 or 70 metres with gas pressures in their upper
parts of 6 or 5 bars respectively.
The gas/liquid contact method of the present invention
enables ox,ygen to be supplied efficiently to a culture in a large
scale fermentation process, for example a process for producing
sin~le cell protein by growing bacteria of the species ~ethyloPhilus
meth,ylotrophus* in culture media containing methanol. It also has
other advantages relevant to single cell protein production, namely:-
1. The product per unit volume of the fermenter can be in-
creased leading to a smaller and less expensive fermenter for a
given output.
2. The cell mass at a given dilution rate can be increased
thereby reducing the quantity of liquid which needs to be separated
from the solid with obvious reduction in costs.
3. A higher proportion of the ox,ygen in the air supplied to
the culture can be used since the effective height of the fermenter
and hence the residence time of the gas oan be increased. ~his i~
because a higher pressure at the top of the riser prevents unmanage-
able voidages developing in the risers of taller fermenters, since
the effective expansion ratio of the ga~ bubbles during transit is
reduced.
* OTE: ~he characteristics of the species
Meth,ylophilus meth,ylotrophus (Pseudomonas met~vlotropha) are des-
cribed in our UK specification No. 1370892. Representative strains
include strains NCIB Nos. 10508-15 and 10592-6.
Barge scale aerobic fermentation processes such as proces-
ses for the production of single cell protein, require large amounts
of energy and in -the commercial operation of such processes the
efficient use of energ,y is important.
Also according to the present invention we provide a
combination of a fermentation process of the kind described and an
ener~y recover,y s~stem wherein the off-gas at above atmospheric
~6~9816
~ ~91~1/297~2
pressure i9 passed to the ener~y recovery syatem in which it i~
heated and thereafter expanded in a gas expander and the resulting
power produced by the gas expander is transmitted to the compre~sor
and supplies all or part of the power requirements thereof in com-
pressing the f0ed gas.
Al~o according to the invention we provide a combination
of a fermenter and ~n energy recovery system which comprises a
fermenter for the operation of a fermentation process of the kind
described together with an energy recovery system wherein the energy
recovery system comprises a gas expander which provides all or part
of the power requirements of the compressor or compressor~ in com-
pressing the feed gas, gas conducting means for conducting the off-
gas to the expander, heating means for heating the off-gas before
it enters the expander and power transmitting means for transmitting
power from the expander to the compressor.
Preferably in the combination proces~ and apparatuæ the
gaa expander can supply power in excess of that required by the
compre~or(s) thus allowing an export of power for other uses.
We have found that as the presaure above the culture i~
increased the energy which can be recovered uaing the expander
rises more rapidly than the energy required by the compressor(s),
i.e. the net power requirement fall~ and additionally the operation
of the fermenter ia improved. Therefore off-ga~ auitably leaves the
fermenter at a pressure of at least 2 bar3 gauge, preferably between
2 and 15 bars gauge. Most suitably the pres~ure of the off-gas is
between 3 and lO bars gauge.
On leaving the fermenter the off-gas will typically be at
a temperature of approximately 40C.
If this ga~ is supplied directly to a gas expander then
the work energy available i8 substantially les~ than the energy
required by the compres~or. Also, the vapour present in the gas is
llkely to condense causing damage to the expander~ Thus the off-gas
is preferably heated to a temperature limited at the lower value by
the need to prevent condensation in the expander and at the upper
value by the ~uitability of the expander. ~or preferred operation
98~6
6 B 29721/29722
the two extremes are thus:-
(A) To heat the gas to a temperature as high as possible
consistent with a ~eliable expander. In this case it is likely that
the expander will provide the power required by the process air
compressor or compressors and additionally supply power for other
purposes, i.e. "export" power.
(~) To heat the gas to a temperature which i~ sufficiently
high to avoid harmful condensation in the expander, i.e. the gas is
heated to a temperature such that at no time whilst it is in the
expander does its temperature fall below the dew point. In this
oase the expander will probably only provide part of the power
necessary to drive the process air compressor or compressors.
Provided the overpressure is sufficient it is found that
there is a particularly advantageous case between these two
extremes at which the available energy from the expander is ju~t
sufficient to supply all the energy for the compressor i.e. self
sufficiency. Typical combinations of temperature and the expander
to inlet pressure required to achieve self sufficiency are shown
in Figure 5 which is constructed for a 60 metre high fermenter
operating under a pressure of 6 bar gauge. Points above and to the
ri~ht of the curve show that excess energy is available and it is
therefore possible to supply power for other purpose~ i.e. export
power. Points below and to the left indicate that some additional
power is required.
The temperature required for self sufficiency is a strong
function of expander inlet pressure. Specifically the practical
temperature limitations imposes the need to u~e substantiAl expander
inlet pressures.
In (A) the gas is suitably preheated to a temperature
30 within the range 400C to 1200 C before entering the expander, for
example to 650 C. In (~) the pre-heating temperature is suitably in
the range 100 to 400C for example 260C.
The gas leaving the fermenter may contain liquid/solid
matter carried over from the culture which could impair the effect-
iveness of the expander. To minimise the amount of liquid/solid
~l~g8i6
7 B 29721/29722
matter in the gas entering the expander a suitable separation
device may be included in the system between the fermenter and the
expander.
~efore entsring the expander the gas may be pre-heated
by any suitable method. Suitable me-thodn include:-
(i) A heat exchanger using the gas expander or compressor
exhaust. This method may be combined with eithex of methods (il)
or (iii) below.
(ii) Adding hot flue gas, from a separate heat generator, to
the gas from the fermenter.
(iii) ~urning a fuel in the gas from the fermenter. This is
possible ~ince the ga~ leaving the fermenter will contain su~fic-
ient oxygen. In addition this apparatus may be designed to inoin-
erate any combustible material in the off-gas. The apparatus may
also include means for removing harmful chemicals such as sulphur
oxides which could damage the expander. This may be done in the
combustion apparatus or at a later stage in the system.
~ rom the gas expander the fermenter off-gas, (which
normally will be at a pres~ure of about l bar absolute,) may be
di~oharged into the atmosphere~ However this gas may be at a high
temperature and it can be used to provide heat for other stages of
an overall fermentation prooess. In the production of single cell
protein for example it may be used to generate steam for feed
sterilisation and/or other process duties and/or to provide heat for
drying the protein product directly or indirectly using a gas/air
heat exchanger. ~he gas is particularly suitable for the direct
drying of the protein product as the oxygen content has been reduced
well below the value at which mixtures containing protein dust can
give rise to explosions.
The advantage of the energy recovery system of the inven-
tion i9 that it makes use of the super atmospheric pressure of the
fe~mentation proce~s off-gas to enable cheap heat energy to be used
- efficiently to provide some or all of the work energy required to
drive the prooes~ gas compressor.
The invention is illustrated by the accompanying drawing~
~. , , ~ .
: ~ . : ' '
316
wherein:-
Figure 1 is a side elevation of a fermenteraccording to the invention.
Figure 2 is a cross-section along the line A-A of
Figure 1.
Figures 3 and 4 are schematic diagrams of energy
recovery systems used in conjection with fermenters.
Figure 5 is a graph showing the relationship ~etween
temperature self-sufficiency and fermenter over pressure.
The fermenter shown in Figures 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 in which a gas pressure of at least 2 bars
gauge is maintained. Air is injected into the lower part
of riser 2 through sparger 4 and 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, The flow of gas leaving the
fermenter i5 controlled by a valve (not shown in the drawing)
in such a manner as to maintain the gas pressure in
compartment 3 at a required value. Nutrients ara added to
the culture in the fermenter and culture is withdrawn from
the fermenter through pipes not shown in the drawing,
391316
8A
The effect of a gas pressure of at least 2 bars
gauge on the operation of the fermenter is illustrated in `
the following Example:-
EXAMPLE 1
Taking that 2 kg 2 is required to produce 1 kg of
micro-organisms.
Then if the gas pressure applied to the culture in
the upper part of the fermenter, i.e. the overpressure,
is 0.5 bars gauge and the effective fermenter height is
40 metres, the oxygen disolution rate will typically be
8 kgs 02/hr m
,
11~9B~6
~ ~ 29721/2972
Production = 4,0 kg microorganisms/hr m3
~ermenter volume required to produce 4000 k~/hr = 1000 m3.
However, if the overpressure is increased to 4 bar~ gauge,
the dis~olution rate in a fermenter of the ~ame height can be in-
creased to 16 kg 02/hr m3.
~ hen Production = 8 kg microor~anism~/hr m3
Fermenter volume required to produce 4000 kg/hr - 500 m3
Figure 3 ~hows ener~y recovery system alternative (1)
which i9 a self-sufficient system. Off-gas leaving a fermenter 6
passes as shown via separator 7 and pre-heater 8 to expander 9 from
whence it i~ exhausted along line 11. Air entering fermenter 6
pa3ses as shown along line 12 and is compre~sed by oompressor 10.
All the power for compressor 10 is supplied by expander 9.
Figure 4 can be used to illustrate energ~ recovery system
altem atives (2) and (3). Re~arding their basic components these
are the same a~ alternative (1), the essential differences being as
follows:-
In alternative (2), the power supplied by expander 9 isinsufficient to drive of itself compressor 10 and an auxiliary power
source is required. This is shown as 13.
In alternative (3), the power supplied by expander 9 is in
exce~s of that required by compressor 10. ~here is an export of
power which can be supplied to an alternator. In this alternative
therefore 13 is an alternator.
A useful refinement is ~hown in broken lines in Figure 4.
In this air is supplied from point 15 of compressor 10, which is
enlarged as shown by 16, along line 14 to pre-heater 8. This gives
the sy~tem a degree of flexibility in operation for the following
reason~ :-
(a) It becomes pos~ible to adjust the temperature in expander
9 either by directly cooling air leaving the pre-heater or by enabling
more gas to be burnt in the pre-heater thus raisin~ the temperature of
the air.
(b) It enables more gas to be burnt in the pre-heater giving a
temperature increase, in a situation in which there is insufficie~t
~109816
~ 2g721/29722
oxygen in the gas leaving the fermenter.
When this refinement is included it is nece~sary to have
a larger compressor. I.ine 14 takes gas from a position 15 which is
part-way along the compressor sinc0 the gas pres~ure required i~
that of the fermenter exit gas rather than of the inlet gas. As an
alternative to an enlarged compressor, two compressors could be used,
air line 14 leading off from the first of these. The refinement
could also, although less conveniently, be effected using gas from a
source other than compressor 10.
The operation of alternative (1) to (3) is illustrated in
the following examples.
EXAMPLE 2
Consider the energy recovery system shown in Figure 3.
With a fermenter 60m high and with an expander limited to 660C, it
is necessary to operate with a fermenter overpressure of 5 bar gauge
in order to achieve self sufficiency. For larger overpressures,
self sufficiency can be achieved with expander inlet temperatures
lower than 660C (see Figure 5) but there is an upper limit on over-
pres~ure determined by consideration of C02 toxicity on the micro-
organism. With the fermenter 6 operating with an overpressure of5 bar gauge ~d with the compressor 10 delivering 90 kg/sec air, the
compressor requires approximately 27 MW power. Off-¢as leaves the
fermenter at a rate of 85 kg/sec and at a temperature of 40C.
Natural gas is burnt in preheater 8 at a rate of 1.1 kg/sec to raise
the temperature of the off-gas from 40C to 660C. The power recov-
ered in expander 9 iB 27 MW which is just sufficient to drive the
compre~sor. The expander exhaust which is a temperature of 430C is
passed along line 11 to another heating system and produoes an
energy saving of 0.5 kg/sec of natural gas or the equivalent thereof
of another fuel being used there.
Ea~L~ ~
Fox the alternative (2) energy recovery system shown in
Figure 4, consider a fermenter 60 metres high and operating with an
overpressure of 6 bar gauge. If the compressor is delivering 90
seo air, the power required is 28 MW. Off-gas leaves the fermenter
.
98~6
11 ~ 29721/29722
at a rate of 85 kg/sec and at a temperature of 40 C. Natural gas
is burnt 1~ preheater 8 at a rate of 0,4 kg/sec to rai~e the temp-
erature of the off-~as from 40C to 260C. The power recovered in
the expander ~ i9 16 MW. Thus, whilst an additional power source
providing 12 MW is required, 16 MW of the necessary power iq being
generated by an extremely efficient method.
EXAMPLE 4
For the ener~y recovery system, alternative (3), shown
in Figure 4 consider a fermenter 60 metres high and operating at an
overpressure of5 bar gauge. If the compressor is delivering 90kg/
sec air, the power required is 27MW. Off-gas leaves the fermenter
at a rate of 85kg/sec and at a temperature of 40 C. ~atural gas is
burnt in preheater 8 at a rate of 1.2 kg/seo to raise the temperature
of the off-gas to 725 C. The power recovered in expander 9 is 27.8 ~W.
Thus there is an export of 0.8 MW of power to alternator 13.
In the above examples 2 to 4 simplified system~ have been
considered to i~ Ustrate the efficacy of the invention clearly. The
effect of introducing the refinement shown in broken lines on Figure
4 has been ignored.
PA/JNA ~
21 July 1978
