Abstrict
The use of a probe for the control of foam build up in a fermenter,
which probe comprises a guard electrode (13), first and second insulators
(14 and 15) separated by the guard electrode, a detecting electrode
(12) separated from the guard electrode by the first insulator,
and a plug (18) and socket (17) for connecting the probe to a power
source, the second insulator being provided with fermenter mounting
means in the form of a flange (16), a sensor comprising the aforementioned
probe for use with an antifoam supply regulator, a foam control
system and a method of controlling the build up of foam in a fermentation
vessel.
Claims
What is claimed:
1. A method of controlling the build up of foam in a fermentation
vessel, which comprises
supplying power to a detecting electrode positioned at a predetermined
level in the fermentation vessel;
controlling the potential of a guard electrode positioned between
the detecting electrode and the fermentation vessel such that a
zero potential difference is maintained between the two electrodes;
and
supplying a controlled amount of an antifoam substance into the
fermentation vessel whenever foam causes a change in potential at
the detecting electrode.
2. A foam control system for use with a fermentation vessel comprising
an antifoam supply regulator and a sensor, which sensor comprises
a probe comprising a guard electrode, first and second insulators
separated by the guard electrode, a detecting electrode separated
from the guard electrode by the first insulator, means for independently
connecting each of the electrodes to a power source, and vessel
mounting means for positioning the detecting electrode at a predetermined
level within the fermentation vessel with the guard electrode insulated
from the vessel by the second insulator; and an electric circuit
which comprises means for providing the detecting electrode with
alternating potential, means for controlling the potential at the
guard electrode so as to maintain a zero potential difference between
the two electrodes, and means for providing the anitfoam supply
regulator with an electric signal when foam causes a change in potential
at the detecting electrode.
3. A foam control system as claimed in claim 2, in which the vessel
mounting means comprises a flange or neck on the second insulator.
4. A foam control system as claimed in claim 2, in which the electrodes
are composed of stainless steel.
5. A foam control system as claimed in claim 2, in which the insulators
are composed of fiberglass loaded polytetrafluoroethylene, glass,
polyetheretherketone or polyoxymethylene.
6. A foam control system as claimed in claim 5, in which the insulators
are composed of polyetheretherketone.
7. A foam control system as claimed in claim 2 in which the electric
circuit further comprises a timer which enables the electric circuit
to provide the antifoam supply regulator with the electric signal
for a preset time.
8. A fermenter comprising a foam control system as claimed in claim
2.
9. A foam control system, comprising:
a fermentation vessel;
an antifoam supply reservoir;
means including an antifoam supply regulator for supplying an antifoam
substance to said vessel;
foam level sensor means in said vessel, said sensor means including
a probe incorporating a guard electrode, first and second insulators
separated by the guard electrode, a detecting electrode separated
from the guard electrode by said first insulator, and vessel mounting
means for positioning the detecting electrode of said probe at a
predetermined level within the fermentation vessel with the guard
electrode insulated from the vessel by the second insulator; and
an electrical circuit including a source of alternating potential,
means connecting said alternating potential to said detecting electrode
and to said guard electrode and including means for controlling
the potential at the guard electrode so as to maintain a zero potential
difference between said guard and detecting electrodes, and circuit
means responsive to a change in potential at said detecting electrode
caused by foam reaching said predetermined level for supplying an
electric signal to said antifoam supply regulator for operably controlling
said regulator.
Description BACKGROUND OF THE INVENTION
The present invention relates to the use of a probe for the control
of foam build up in a fermenter, to a sensor comprising the probe
for use with an antifoam supply regulator in the control of foam
build up in a fermenter, to such a foam control system, to a fermenter
comprising the foam control system, and to a method of controlling
foam build up in a fermenter.
During many microbiological fermentation processes foam is produced.
Its presence is undesirable since it reduces the amount of oxygen
reaching the broth during the fermentation and impedes the processing
of the broth after the fermentation has been completed. Furthermore,
if uncontrolled, it can block exit gas lines and eventually spill
out of the fermenter.
Much attention has been devoted to the development of automatic
control systems for the prevention of foam build up in fermenters.
Such systems typically comprise a sensor which detects when foam
has risen up to a predetermined level, and an antifoam dispenser
which supplies an antifoam substance to the fermenter when the sensor
detects the foam. Suitable antifoam substances include polypropylene
glycol 2,000 and silicone based products, for example Dow Corning
silicone RID emulsion.
One known and commercially available type of foam sensor comprises
a probe having an electrode which, in use, is inserted through a
wall or closure in the fermenter to the predetermined level and
powered with a low voltage. When foam has risen up to touch the
electrode, an electric current begins to flow through the foam and
fermentation broth to earth. Thus the potential at the electrode
drops, and this change in potential is exploited to activate an
antifoam supply. However, it is found that such probes are unreliable
over a long period of time. Thus foam control systems comprising
this known type of probe tend to deliver increasingly excessive
amounts of antifoam substance to the fermentation broth as the fermentation
progresses.
The presence of excess antifoam substance in a fermentation broth
is disadvantageous because it reduces the rate of inward diffusion
of oxygen and makes downstream processing, particularly product
recovery, difficult. Clearly, therefore, there is a need for a probe
capable of being used in the control of foam produced during fermentation
without resulting in unacceptably high levels of antifoam substance
in the fermentation broth.
The specification of U.S. Pat. No. 3,119,266 (US-A-3,119,266) discloses
a probe for detecting or measuring the level or quantity of liquid,
powder, granular or aggregate materials in bins, tanks or other
containers, which comprise first and second, or measuring conductive
electrodes and a guard electrode which is driven in phase with the
voltage derived between the measuring electrodes with a voltage
of substantially the same magnitude as the measuring voltage.
The specification of U.S. Pat. No. 4,027,172 (US-A-4,027,172) discloses
a modified version of the probe described in US-A-3,119,266, with
particular emphasis placed on the use of the modified probe in detecting
the presence or absence of boiler water in heating plant boilers.
The features of the probes of both US-A-3,119,266 and US-A-4,027,172
are combined in the specification of U.S. Pat. No. 4,390,793, which
discloses a multifunction electronic probe circuit for water level
control.
SUMMARY OF THE INVENTION
Most surprisingly, it has now been found that the use of a probe
comprising a guard electrode in the control of foam build up in
microbial fermentation processes provides a foam sensor having a
significantly greater reliability over prolonged periods of fermentation
compared with the reliability of the known types of probe available
commercially.
Accordingly, the present invention provides the use of a probe
for the control of foam build up in a fermenter, which probe comprises
a guard electrode, first and second insulators separated by the
guard electrode, a detecting electrode separated from the guard
electrode by the first insulator, means for independently connecting
each of the electrodes to a power source, and vessel mounting means
for positioning the detecting electrode at the predetermined level
with the guard electrode insulated from the vessel by the second
insulator. It has been found that, unlike known probes, the performance
of a probe comprising a guard electrode, when used to control foam
build up in a fermenter does not decline as the fermentation progresses.
The vessel mounting means preferably comprises a flange or neck
on the second insulator. This flange or neck may be an integral
part of the second insulator or may be a removable part such as
a sleeve.
Since the probe is primarily intended for use in a fermenter, it
is preferably composed of materials that are capable of withstanding
serialization in an autoclave. Thus, the electrodes may, for example,
be composed of stainless steel. The insulators may, for example,
be composed of polyetheretherketone, fibreglass loaded polytetrafluoroethylene,
glass or polyoxymethylene, with polyetheretherketone being preferred.
Conveniently the means for independently connecting each of the
electrodes to a power source comprises a plug, a socket and wires
connecting the electrodes to the socket.
According to another aspect the invention provides a sensor for
use with an antifoam supply regulator in the control of foam build
up in a fermenter, which comprises a probe as described above and
an electric circuit which comprises means for providing the detecting
electrode with alternating potential, means for controlling the
potential at the guard electrode so as to maintain a zero potential
difference between the two electrodes, and means for providing the
antifoam supply regulator with an electric signal when foam causes
a change in potential at the detecting electrode. The electric circuit
may optionally also comprise a timer which enables the electric
signal to be provided for a preset time. This is particularly advantageous
since the antifoam substance does not take effect immediately and
it is important to supply only the minimum amount of antifoam substance
necessary to control the foam.
According to a further aspect, the invention provides a foam control
system for use with a fermenter, which comprises an apparatus as
defined above and an antifoam supply regulator.
In the foam control system the antifoam supply regulator preferably
comprises a pump or valve.
The invention also provides a fermenter comprising a foam control
system as defined above.
According to another aspect the invention provides a method of
controlling the build up of foam in a fermentation vessel, which
comprises
supplying power to a detecting electrode positioned at a predetermined
level in the fermentation vessel;
controlling the potential of a guard electrode positioned between
the detecting electrode and the fermentation vessel such that a
zero potential difference is maintained between the two electrodes;
and
supplying a controlled amount of an antifoam substance into the
fermentation vessel whenever foam causes a change in potential at
the detecting electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail by way of example
with reference to the accompanying drawings, in which:
FIG. 1 shows a schematic representation of a fermenter which includes
a probe for use in accordance with the present invention;
FIG. 2 shows a side view of the probe shown in FIG. 1;
FIG. 3 shows a view of one end of the probe shown in FIG. 2;
FIG. 4 shows a view of the other end of the probe shown in FIG.
2; and
FIG. 5 shows a partial longitudinal section along the line X--X
of the probe as shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a fermenter (1) comprising a stainless steel fermentation
vessel (2) containing a fermentation broth (3). The broth has a
covering layer of foam (4). The fermentation vessel is equipped
with a paddle stirrer (5) and has a stainless steel headplate (6).
A probe (7) is mounted on the headplate and projects into the fermentation
vessel towards the layer of foam (4). The probe is connected via
an electric circuit (8) to a pump (9) which is capable of pumping
an antifoam substance through supply line (10) into the fermentation
vessel (2). The fermentation vessel is earthed as shown at (11).
As shown more clearly in FIG. 2, the probe comprises a detecting
electrode (12), a guard electrode (13), a first insulator (14) separating
the detecting electrode from the guard electrode, and a second insulator
(15), provided with a fermenter mounting flange (16), which is separated
from the first insulator by the guard electrode. The second insulator
is also provided with a socket (17) into which fits a plug (not
shown). In use of the probe the plug is connected to the electric
circuit (8), as shown in FIG 1. Two wires (18a and 18b) connect
the socket to the electrodes.
As is apparent from FIGS. 2 to 5, the probe is of circular cross
section, the electrodes and insulators being arranged along a common
axis. It is assembled from five circularly cylindrical members which
correspond to the two electrodes, the two insulators and the plug.
The four members corresponding to the electrodes and insulators
each have cooperating screw threads which enable them to be screwed
together during assembly of the probe.
Thus the first insulator (14) is a circularly cylindrical body
having two opposed, axially projecting screw threads (21 and 22)
and an axial bore. It may be made of glass filled polytetrafluoroethylene.
Preferably the first insulator (14) is made of polyetheretherketone.
The detecting electrode (12) is a circularly cylindrical body having
a threaded recess which is adapted to receive the screw thread (22)
of the first insulator. Centrally the recess continues as a small,
axially-aligned cavity (20) which is the electric contact point
for wire (18a) emanating from the socket (17). The detecting electrode
is suitably made of stainless steel.
The guard electrode (13) is a circularly cylindrical body, suitably
made of stainless steel. It has two opposed, threaded recesses (23
and 24) which communicate through a relatively narrow axial bore
(19a) and a smaller bore (19b) adjacent to the axial bore (19a).
The smaller bore is the electric contact point for wire (18b) emanating
from the socket (17). The axial bore accomodates the wire connecting
the detecting electrode to the socket. The first of the threaded
recesses (23) is adapted to receive the screw thread (21) of the
first insulator.
The second insulator (15) may be made from a single piece of glass-filled
polytetrafluoroethylene, but is preferably made from a single piece
of polyetheretherketone. The second insulator (15) has an axial
bore and comprises three portions. The first portion is relatively
long and narrow and has at one end an axially projecting screw thread
(25) which is adapted to be received by the second of the threaded
recesses (24) of the guard electrode. The second portion, which
is relatively short and wide, constitutes the mounting flange (16).
The third portion is relatively long and narrow and incorporates,
at its free end, the socket (17).
In the assembled probe, the axial bores of the insulators and the
guard electrode form a single cavity. Through this cavity run the
two wires (18a and 18b) which connect the socket to the electric
contact points (19a and 20) on the electrodes. The wires are suitably
connected to the electric contact points by means of solder or silver
loaded epoxy resin. `O` rings (32) may be incorporated in the assembled
probe to form a seal between the first insulator (14) and each of
the detecting electrode (12) and the guard electrode (13), and between
the second insulator (15) and the guard electrode (13). The `O`
rings, which may be made from any suitable sealing material, for
example silicone, prevent the ingress of fluid into the probe.
The dimensions of the probe will depend upon the size of the fermentation
vessel with which it is to be used. Typically a probe for use in
a standard 3 litre fermenter will be about 12 cm long and 1.2 cm
in diameter. The electrodes will typically be about 1 cm long and
spaced about 1 cm apart.
Referring again to FIG. 1, the electric circuit (8) comprises a
sub-circuit (26) for providing low alternating potential to the
detecting electrode. This sub-circuit includes a resistor (27).
The electric circuit further comprises a sub-circuit (28) for maintaining
the guard electrode at the same potential as the detecting electrode.
This sub-circuit includes a buffer amplifier (29). The electric
circuit also comprises sub-circuit (30) for providing an electric
signal to the antifoam supply regulator when rising foam causes
a change in potential at the detecting electrode (12). This sub-circuit
includes a comparator (31). Optionally the electric circuit also
comprises a timer (32).
The probe together with the electric circuit constitute a sensor
which is suitable for use with an antifoam regulator, such as the
pump (9), in the control of foam build up in a fermenter. Together
with an antifoam supply regulator, the apparatus constitutes a complete
foam control system.
The method of using the probe will now be described with reference
to FIG. 1.
As the level of the foam (4) in the fermentation vessel (2) rises,
it will eventually touch the detecting electrode of the probe (7).
When the foam touches the detecting electrode a current path is
formed between the detecting electrode and earth (11). The resultant
drop in potential at the detecting electrode is sensed by comparator
(31). The comparator then activates a timer (not shown) which in
turn activates a pump (9) for a preset time. A small quantity of
antifoam is then supplied to the fermenter. This causes the foam
to collapse.
The guard electrode of the probe is powered by the buffer amplifier
(29) at all times at exactly the same potential as the detecting
electrode. Consequently no electric field exists between the two
electrodes, and hence no current may flow between them. The performance
of the probe is therefore insensitive to an accumulation of material
on the probe from the fermentation broth. |