METHOD AND DEVICE FOR DETERMINING THE QUANTITY OF DISPERSED SOLID MATERIAL IN A LIQUID

Abstract: To determine the quantity of solid material in a liquid, a beam of light is passed into the liquid through a gas-liquid interface window; the meniscus of said window is periodically or continuously renewed by a current of gas and the scattering or transmission of the light in the liquid is determined by a detector. The device for carry out this determination comprises a sensor (1) to be immersed in the liquid said sensor having an internal chamber (5) into which there debouch at least a gas inlet channel (6), a gas removal channel (7), a means (8) for supplying light and a channel (9) for forming a meniscus (10). This channel is disposed directly opposite said means (8). The sensor is biologically and chemically inert and is capable of being sterilized at approximately 120°C. Contamination and build-up of solid constituents on interface surfaces can be avoided or corrected. The cost price is low and a reasonable measurement result even with a relatively high density of the dispersed solid constituents is achieved. The sensor is able to function in coloured and turbulent solutions.

Description: The invention relates to a method and device for determining the quantity of dispersed solid material in a liquid. In various chemical and biochemical processes solid material is formed which is dispersed in a reaction liquid. Thus, in biotechnological fermentation processes, micro-­ organisms are cultivated. It is of great importance that such processes are optimally regulated, for which purpose measurements are necessary. In addition, inter alia, the oxygen content, the degree of acidity, the carbon dioxide content, the temperature and the content of other sub­ stances, it is often of importance to determine the quan­ tity of solid substances, which are, in the case of a fermentation process, micro-organisms and/or substances excreted by micro-organisms, and which are contained in the reactor vessel. It is usual to take samples from the vessel and to determine the quantity of solid substance by chemical, electrical, microscopic or photometric met­ hods, or by weighing. Taking samples is beset with a number of drawbacks. A relatively large amount of time is lost between taking a sample and the measurement so hat the information obtained may be unreliable or is no longer relevant. Moreover, taking a sample is risky in certain cases because infectious microbes might get into the reactor vessel. There is therefore a strong prefer­ ence for carrying out measurements not on samples but in the reactor vessel itself. In order to obtain satisfactory results for measure­ ments in the vessel, the method of measurement or measuring device will have to satisfy a number of conditions. The sensor must be biologically and/or chemically inert and must be capable of being sterilized at approximately 120°C. It must be possible to avoid or correct contamination and build-up of solid constituents on interface surfaces which play a part in the measurement. The cost price has to be low. It must be possible to achieve a reasonable measurement result even with a relatively high density of the dispersed solid constituents. The sensor must be able to function in coloured and turbulent solutions. In the first instance the object of the invention is to provde a method for determining the quantity of dispersed solid material in a liquid which satisfies the said conditions. The method according to the invention is characterized in that a beam of light is passed into the liquid through a gas-liquid interface window, the meniscus of said window is periodically or continuously renewed by a current of gas, and the scattering or trans­ mission of the light in the liquid is determined by a detector. The possibility is provided of always removing the gas via the meniscus so that said meniscus is always kept clean. However, said method has the disadvantage that the gas bubbles may have a disturbing effect. It is therefore preferable that the current of gas is passed continuously across the meniscus and that periodically the gas pressure is increased so that gas is forced through the meniscus into the liquid. The device to be used for determining the quantity of dispersed solid material in a liquid is characterized in that it comprises a sensor to be immersed in the liquid having an internal chamber into which there debouch at least a gas inlet channel, a gas removal channel, a means for supplying light and a channel, disposed directly opposite said means, for forming a meniscus. If the debouchments of the gas inlet channel, the gas removal channel, and the meniscus channel are below the level of said means and said means is disposed in a gastight manner, the liquid will not be able to come into contact with the means even when the device is not in operation since a gas bubble will always remain trapped in the chamber. The possibility is not excluded that the detector is not connected to said device and is set up, for example, as a separate unit remotely from the device. However, for simplicity there is a strong preference that the sensor is also provided with a second internal chamber into which there debouch at least a gas inlet channel, a gas removal channel, a detector or a means for guiding light to a detector, and a channel, disposed directly opposite the detector or the means, for forming a meniscus. In oroer to convey light from the light source to the liquid and to remove light from the liquid to the detec­ tor, use is made of a light guide, preferably a glass fibre. The glass fibres and the gas inlet channels are placed in a tube. If the pressure of the gas has to be increased periodically in order to renew the meniscuses this can be carried out completely automatically or manually in a prac­ tical manner if there debouches into each gas inlet channel a gas inlet system provided with two parallel gas pipes each having its own valve. Lasers form an ideal light source: they provide a non-dispersed coherent beam and are obtainable with various powers and wavelengths. In the case of a population of cells in the liquid, the intensity of the light scattered by the micro-organisms is then essentially proportional to the den­ sity of the population of cells. The invention will now be explained in more detail by reference to the figures in which an exemplary embodiment is shown. Figure 1 shows a section through a device, having a gas inlet system, for determining the quantity of dispersed solid material in a liquid. Figure 2 shows the device in Figure 1 in the dis­ mantled state. The device shown is intended for determining, in a liquid present in a reactor and containing a quantity of dispersed solid material, how large said quantity of solid material is. The principle of the method of measurement is that a beam of light is passed into the liquid, that the light is scattered by the dispersed particles, and that the transmission of the light through the liquid or the scat­ tering of the light by the liquid is determined by means of a detector, said transmission or scattering being a measure of the quantity of solid material in the liquid. The device is, in particular, suitable for determining the density of bacteria in biological processes. Use in normal chemical reactions is, however, not ruled out. Measurement can be carried out continuously or with short breaks. The device is suitable for use in the reactor itself and there is no need for sampling. The device comprises a sensor 1 which is immersed in the liquid and consists of a measuring head 2, an adapter 3, and a stainless-steel tube 4. The measuring head 2 has two chambers 5 into which there debouch a gas inlet channel 6, a gas removal channel 7, a channel containing a light-guiding glass fibre 8 and a channel for forming a meniscus 10 which is indicated by dotted lines. The one glass fibre 8 is connected to a laser device (not shown), while the other glass fibre is connected to a detector (not shown) which can convert the light signal into an electrical signal. The gas inlet pipes are connected to a system 11 for supplying gas, for example nitrogen, which system comprises two parallel pipes, connected to a gas bottle 16, each having a needle valve 14 or 15 respectively. The pipe 13 also incorporates a magnetic valve 17 with which the ratio of the quantities of gases flowing through the pipes 12, 13 can be regulated. A bacteria filter 18 is incorporated between the system 11 and each of the gas supply pipes 6. The device functions as follows: Gas is conveyed via pipe 13 to the gas inlet channel 6. The valve 14 is closed so that line 12 is inoperative. Said gas flows into the chamber 5 and leaves the latter through channel 7. At the interface between gas and liquid a meniscus 10 is formed beneath each channel 5. A laser beam is passed through a glass fibre 8, a chamber 7, and a meniscus 10 into the liquid in which the quantity of dis­ persed solid material has to be measured. The light incident in the liquid is scattered by the particles and a portion of the scattered light falls, through the other meniscus 10, the other chamber 7 and the other glass fibre 8, onto a detector which delivers an electrical signal as a function of the quantity of reflected light which may be a measure of solid particles, such as cells, dispersed in the liquid. After suitable calibration of the detector it is possible to determine immediately how many solid particles are contained in the liquid at any instant. The meniscuses are subject to contamination by the build-up of solid particles. They must therefore be cleaned or renewed at intervals. For this purpose the valve 14 is opened periodically, as a result of which the gas flows into the pipes 6 also through the pipe 12 in addition to flowing through the pipe 13. The pressure in the chambers 5 becomes so great that gas escapes also through the channels 9 in addition to escaping through the channels 7, so that the mensicuses disappear. As soon as the valve 14 is closed, the meniscuses re-establish themselves at the desired point in the channels 9. It is essential to the invention that a quantity of gas is contained between the end of the glass fibres 8 directed towards the chamber 5 and the liquid to be inves­ tigated so that there is liquid-gas interface. A variant in which the gas formed in the chambers 5 always leaves the measuring head completely through the channel 9 falls within the scope of the invention. However, the gas bub­ bles produced under these conditions form a disturbing factor because they are situated in the beam of incident light. The embodiment shown, in which gas is only perio­ dically forced through the channels 9, is therefore to be preferred. Of course, separate sensors could be used for passing a light beam into the liquid and passing the scattered light to the detector. Important advantages are, inter alia, that the sensor can be sterilized and is insensitive to the build-up of solid particles. Even very large particle densities can be deter­ mined. A satisfactory sensivity can be achieved. The cost price is low. It can be established by calculation that, under certain circumstances, there is a linear relationship between the particle concentration in the liquid and the scattering intensity of the light. Various embodiments are possible within the scope of the invention. There is the possibility that, in the case of a fermentation process in which methane is liberated, said methane is used instead of nitrogen to fill the chamber 5. In said case the methane will be pumped round by a pump.

Claims: 1. Method for determining the quantity of solid material in a liquid, characterized in that a beam of light is passed into the liquid through a gas-liquid interface window, that the meniscus of said window is periodically or continuously renewed by a current of gas, and that the scat­ tering or transmission of the light in the liquid is deter­ mined by a detector. 2. Method according to Claim 1, characterized in that a current of gas is passed continuously across the men­ iscus and that periodically the gas pressure is increased so that gas is forced through the meniscus into the liquid. 3. Device for determining the quantity of dispersed solid material in a liquid, characterized by a sensor (1) to be immersed in the liquid having an internal chamber (5) into which there debouch at least a gas inlet channel (6), a gas removal channel (7), a means (8) for supplying light and a channel (9), disposed directly opposite said means, for forming a meniscus (10). 4. Device according to Claim 3, characterized in that the sensor is also provided with a second internal chamber (5) into which there debouch at least a gas inlet channel (6), a gas removal channel (7) a detector or means (8) for guiding light to a detector, and a channel (9), disposed directly opposite said means, for forming a menis­ cus. 5. Device according to Claim 4, characterized in that the debouchments of the gas inlet channel (6), the gas removal channel (7) and the meniscus channel (9) are below the lebel of said means (8) and that said means (8) is disposed in a gastight manner. 6. Device according to Claim 4 or 5, characterized in that the means (8) for supplying light to the liquid and the means for guiding the light to a detector consist of a light guide. 7. Device according to Claim 6, characterized in that the light guides consist of a glass fibre (8) and the glass fibres (8) and the gas inlet channels (6) are placed in a tube (4). 8. Device according to one of the Claims 3 to 7 incl., characterized in that there debouches into each gas inlet channel (6) a gas inlet system (11) provided with two parallel gas pipes (12, 13) each having its own valve (14, 15). 9. Device according to one of the preceding claims, characterized in that the light source is a laser source.

Inventors: HOUPT, PIETER MARCUS
Applicants: NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO
Application year: 1987
Main classification: G01N21
IPC codes: G01N21