The control of foam build up in a fermenter

Abstract: 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.

Description: The present invention relates to a foam control system for the control of foam build up in a fermenter, 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 US Patent 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 comprises 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 US Patent 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 US Patent No. 4,390,793, which discloses a multifunction electronic probe circuit for water level control. 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 relates to a foam control system a foam control system for use with a fermentation vessel (2) comprising an antifoam supply regulator (9 and 10) and a sensor, which sensor comprises a probe (7) comprising 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, means for independently connecting each of the electrodes to a power source (18), and vessel mounting means (16) for positioning the detecting electrode at the predetermined level with the guard electrode insulated from the vessel by the second insulator; and an electric circuit (8) 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 (31), and means for providing the antifoam supply regulator with an electric signal when foam causes a change in potential at the detecting electrode. 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 sterilization 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. The electric circuit may optionally 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. 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 the 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. The invention will now be described in more detail by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of a fermenter which includes a probe for use in accordance with the present invention; Figure 2 shows a side view of the probe shown in Figure 1; Figure 3 shows a view of one end of the probe shown in Figure 2; Figure 4 shows a view of the other end of the probe shown in Figure 2; and Figure 5 shows a partial longitudinal section along the line X-X of the probe as shown in Figure 3. Figure 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 Figure 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 Figure 1. Two wires (18a and 18b) connect the socket to the electrodes. As is apparent from Figures 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 Figure 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 (not shown). 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 Figure 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.

Claims: A foam control system for use with a fermentation vessel (2) comprising an antifoam supply regulator (9 and 10) and a sensor, which sensor comprises a probe (7) comprising 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, means for independently connecting each of the electrodes to a power source (18), and vessel mounting means (16) for positioning the detecting electrode at the predetermined level with the guard electrode insulated from the vessel by the second insulator; and an electric circuit (8) 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 (31), and means for providing the antifoam supply regulator with an electric signal when foam causes a change in potential at the detecting electrode. A foam control system as claimed in claim 1, in which the vessel mounting means comprises a flange or neck on the second insulator. A foam control system as claimed in claim 1, in which the electrodes are composed of stainless steel. A foam control system as claimed in claim 1, in which the insulators are composed of fibreglass loaded polytetrafluoroethylene, glass, polyetheretherketone or polyoxymethylene. A foam control system as claimed in claim 4, in which the insulators are composed of polyetheretherketone. A foam control system as claimed in claim 1 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. A fermenter comprising a foam control system as claimed in any of claims 1 to 6. A method of controlling the build up of foam in a fermentation vessel using the foam control system as defined in any one of claims 1 to 6, which comprises supplying power to the detecting electrode positioned at a predetermined level in the fermentation vessel; controlling the potential of the 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.

Inventors: GALLAGHER STEPHEN FRANCIS, GALLAGHER, STEPHEN FRANCIS
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1989
Main classification: C12M1
IPC codes: B01D19, C12M1, C12Q3, G01N27