FR2623105A1 - Device for measuring or controlling at least one physical characteristic of a fluid contained in a receptacle, such as its mass - Google Patents

Abstract

Device for measuring or controlling at least one physical characteristic of a fluid contained in a receptacle, such as its mass, its density or other parameters, involved in a process to which the fluid is subjected between its entry and its draining. The fluid enters a vessel 1 provided with stirring means 2, 3 and its mass is determined from the pressure measured within the fluid by a differential sensor 9. Measurement signals are applied to a programmable digital processing system 15 which, under the control of an operator, is capable of performing particularly calculations of averages so as to determine precisely the mass of the contents and/or to deliver control signals to an abstraction instrument 8 so as to keep precisely constant the mass of the fluid stirred in the receptacle and optionally other control signals. Application to the control of, for example, a fermentation process.

Description

The subject of the present invention is a device for measuring or regulating at least one physical characteristic of a fluid contained in a container, such as its mass for example.

The device according to the invention is particularly well suited for regulating the mass of a liquid subjected to a certain agitation. This is the case when the liquid is contained in a tank or a tank equipped with a stirring system or even pipes for the injection at a greater or lesser pressure of a gas necessary for the progress of a process. processing this liquid. This is also the case when an ongoing biological, chemical or physicochemical process generates gases within the liquid, which causes turbulence. It may, for example, be a continuous or discontinuous fermentation process producing biomass, fermentation products and / or enzymes, etc.

The most conventional method of determining the mass of a liquid by weighing the container that contains it, is not always convenient to use or particularly precise. The weighing means must often be included from the origin when the tank or tank is installed. If the progress of a process requires the addition of devices or equipment not originally provided for, we must take their mass into account and modify the taring device associated with these weighing means.

A weighing method is preferably used to measure the mass of the contents alone. To regulate the mass of a fluid contained in a tank, it is possible to envisage combining a means of measuring said mass with an electronic regulation assembly wired to control the measured value to a set value set by the operator, by instant or delayed corrections.

However, such a relatively fixed system does not lend itself to fine regulation which takes account of the changes that may occur in the operating conditions or even of the agitation of the fluid during certain processes.

o
By French Patent No. 2,104,926, a process and a device are known for determining the mass and other quantities relating to a liquid contained in a tank, by measuring the hydrostatic pressure prevailing at a precise depth of the tank which contains it. The pressure measured is that of a gas substantially at the pressure of the liquid at the bottom of the tank. Equi-pressure is obtained by increasing the pressure of the gas enough to fill a pipeline which brings it to the level where the measurement is to be made.

A convenient way to control the pressure is to increase the pressure until gas bubbles escape through the bottom port of the pipeline. The gas pressure causes the liquid to rise in a barometric column and an opto-electronic system placed along it determines its height. A computer is used to deduce in particular the mass of the contents of the tank. This device is particularly suitable for monitoring a relatively calm liquid mass contained in a reservoir.

The device according to the invention allows the precise measurement and / or regulation of at least one physical characteristic of a fluid and mainly of its mass, the fluid being contained in a container comprising means for introducing said fluid and means for collect it. The device comprises means for measuring at least one pressure prevailing within the fluid contained in the container. It is characterized in that it comprises a processing system comprising an input-output assembly provided with acquisition means for digitizing values of said pressure measured at determined time intervals, programmable calculation means for performing a combination of several pressure measurements, successively digitized and determining at least said physical characteristic content of the fluid in the container. The treatment system can comprise an interface assembly controlled by the calculation means, for controlling the sampling means, so to measure or regulate the physical characteristic of the fluid contained in the container (and in particular its mass).

The device comprises for example a differential sensor for measuring the pressure difference prevailing between at least two different locations of the container, at least one of these locations being within the fluid, the sampling means comprise a valve with adjustable flow and the calculation means are adapted to generate for this valve, a variable control signal allowing the regulation of tadite physical characteristic of the fluid in the container. If the physical characteristic sought is the mass of the fluid, the second measurement location is outside the fluid.

 If it is its density, the second location is also within the fluid.

The use of a processing system controlled by programmable calculation means to which the signals from a pressure sensor are applied directly, allows a selection of the measured values and averaging calculations over a more or less long time, from or it results in a very precise determination of the mass of fluid in particular and this even when this fluid is agitated by stirring or gas evolution, and very fine regulation. The treatment system used is further adapted to a rapid and precise adaptation of the device to any change in operating conditions or possibly to a new process. With the device according to the invention, the use of a relatively high cost opto-electronic pressure measurement means is avoided. It may also be noted that the processing system makes it possible to possibly manage several different regulation loops of the fact that it includes an acquisition set and an input-output set capable of receiving or emitting several measurement or control signals.

Other characteristics and advantages of the device according to the invention will appear better on reading the following description of an embodiment given by way of nonlimiting example, with reference to the accompanying drawings in which - Figure 1 shows the device associated with a fermentation tank
for example, - Figure 2 shows a block diagram of the regulatory system, and - Figure 3 shows an example of an algorithm followed to regulate the
mass contained in the tank.

The device is associated for example with a fermentation tank 1 provided with stirring means comprising blades 2 driven in rotation by an electric motor. The tank 1 is connected by a pipe 4 to a tank 5 containing a liquid mixture to be transformed by fermentation. It may especially be a question of producing a biomass by a continuous or discontinuous process in order to extract different substances from it such as enzymes or even for example of producing alcohols, ethanol in particular from sugar plants or cereals. An injection control element 6 allows the transfer of the liquid mixture into the tank. Sampling means comprising a pipe 7 connected to the base of the tank and closed by a sampling member 8, a withdrawal valve for example, make it possible to extract the liquid after a determined fermentation time. This residence time, which it is necessary to know for example to optimize the growth of microorganisms in the tank by fermentation, is determined in particular by the ratio of the mass of the liquid mixture in the tank to the flow rate of liquid which is injected there. .

Associated with the fermenter tank is a device for determining precisely the mass of the mixture being fermented, by the method known per se for measuring the excess pressure prevailing within said mixture at a determined location in the lower part of the tank. .

The device comprises a differential pressure sensor assembly 9 (a metal membrane sensor for example) connected to a first pipe 10 immersed in the liquid mixture. Its open lower end is located substantially at the bottom of the tank. The sensor 9 is also connected to a second pipe opening into the upper part of the tank well above the liquid level. The differential pressure sensor 9 therefore measures the pressure exerted by a column of liquid of height H (distance between l the lower end of the pipe 10 and the surface of the liquid) and of density d and translates it into the form of an electrical signal. The mass of the mixture is directly proportional to the pressure difference measured by the sensor, the proportionality coefficient depending on the section of the tank 1.

The pressure difference is measured by the so-called "bubbling" method. A source of gas under variable pressure 12 is connected to the pipe 10 by a pipe 13 carrying a flow meter 14.

Gas is injected into the line 13 and its injection pressure is increased enough for bubbles to escape into the liquid mixture through the lower end of the line 10.

The low gas flow is measured by the flow meter 14. The differential pressure sensor therefore measures the pressure of a gas substantially in equi-pressure with the liquid at the bottom of the tank. Line 11 is optional if the pressure above the liquid is constantly equal to atmospheric pressure, for example because the upper part of the tank communicates with the outside. The measurement signals delivered by the sensor are either an electric voltage or an electric current proportional to the measured overpressure.

The device also includes a processing system 15 ensuring in particular the control of the total mass of the liquid mixture in the tank from the signals delivered by the differential pressure sensor 9 and by the flow meter 14.

The processing system 15 comprises (FIG. 2) a computer 16 with a microprocessor communicating with a bundle of conductors 17 which is usually designated by bus. By means of this, it communicates with a storage block 18, with a control keyboard 19 by means of a card or interface 20 for managing the keyboard and with an input-output assembly 21. This assembly comprises a multiplexer 22 at the inputs of which the signals delivered by the sensors are applied. These are electric currents and / or voltages translating each pressure difference measured by the differential sensor assembly 9. The analog signal from the multiplexer 22 is applied to an analog-digital converter 23 connected to the bus 17. assembly 21 also includes an interface card 24 allowing the interconnection of the bus 17 to an alpha-digital display member 25. An output card (or port) 26 communicating with the bus 17, includes output channels for the regulation signals generated by the computer 16 in response to the pressure measurement signals. A first output 1 communicates with an amplification block 27. The latter delivers an electric current for controlling the withdrawal valve 8 (FIG. 1) as a function of the signal present at the output 01. A timer element 28, such that a monostable, is interposed between the output 01 and the amplification block 27, to limit the maximum duration of the control signal applied to the draw-off valve.

This prevents it from blocking in the open position (possibly leading to emptying of the tank 1 (Fig.1), following any abnormality in the operation of the regulation system).

A second output 02 delivers an electric current for an indicator light 29 indicating the opening of the sampling member 8.

The assembly 21 also includes a digital / analog converter 30 connected to the bus 17. The digital words translating for example the mass data calculated by the computer 16, are converted into analog signals for a graphic recording device 31.

The microprocessor is programmed to - control the digitization of the analog signals from the sensor means 9 with a determined sampling period and store the values of the samples of signals successively digitized; and - calculate an average value over a time interval fixed by the operator.

It develops regulation signals and transmits them by the bus 17 to the output card 26 and the amplification block 27 so as to control the opening of the sampling member 8. We choose for example a valve of the all type -or-nothing, the average flow rate of which only depends on its duration of opening. The microprocessor regulates the liquid mass contained in the tank 1 by adapting the duration of withdrawal of the solenoid valve, both to the characteristics of the latter to the measurement rate imposed by the operator, and to the rate of introduction of the mixture. liquid in the tank. It applies a time slot signal of appropriate duration and constant repetition period, or of constant duration but with a variable repetition period, or else a combined signal or duration and repetition period are adjustable. The minimum duration is fixed for example at 0.5 seconds.

The regulation signals depend on an average value of the overpressure measured over a time interval which can be chosen to be shorter or longer depending on the conditions of the process to be controlled. It is found in fact that the agitation to which the liquid mixture is subjected by the means of stirring the tank, creates instantaneous overpressures that are not significant of an increase in mass.

The computer 16 therefore performs a sorting among the samples of signals delivered by the converter 23 (C.A.N), to eliminate those whose deviation from the average voltage is too large. Via the control keyboard 19, the operator sets the parameters to be followed in order to perform the sorting, for example it imposes - the setpoint at which it is desired to maintain the measured pressure difference; - the regulation range set as a percentage relative to the setpoint between the limits of which a variation in the pressure difference is tolerated; - the high and low clipping thresholds above or below which the pressure samples are rejected as not significant; and - the time interval during which the average of the values of significant samples must be taken.

In addition, the operator determines the pressure measurement scale according to the differential sensor used and its sensitivity which can vary depending on the case in the range from 0 to 1 mV for fine variations or in a much wider range ( from O to several bars if necessary) for very large tanks. It controls the multiplexer 22 so as to choose the nature of the signal translating the pressure difference either a voltage (from 0 to 10 v) or an electric current (from 4 to 20 mA for example). The algorithm according to which the control operations are carried out, for example, is shown in FIG. 3.

With the device according to the invention and despite the agitation to which the liquid is subjected, it is possible to regulate its mass with an accuracy of up to 0.1% due to the sorting and possible various digital processing carried out on the samples. pressure signals.

The use of a treatment system controlled by a programmable computer allows rapid and precise adaptation of the device to any change in operating conditions: modification of the liquid mass (and therefore of the set values) or of the regulation tolerances, injection of gas at a more or less high pressure intervening in certain types of process and which makes it necessary to modify the parameters etc. The device according to the invention can therefore be easily adapted to any existing installation.

The acquisition set can include inputs for other signals. One can use for example a flowmeter 14 (Fig.1) delivering a signal proportional to its flow and sample this signal. The operator is also able to impose a value of cpnsigne and regulation parameters on the computer 16 which also generates a control signal, suitably shaped by the interface assembly 21, to control the pressure delivered by the pressurized gas source 12. The pressure at the lower end of the pipe 10 can thus be maintained substantially equal to the pressure of the liquid mixture at the bottom of the tank.

We would not go beyond the scope of the invention by applying to the acquisition set other signals translating other variables. One of them may be, for example, the pressure difference measured between two submerged locations located at two different depths, the difference h (Fig. 1) is known with precision, so as to calculate the density of the liquid mixture and its variations during the current process, using the weights and averages already described.

Other signals can in particular translate a variation in the level of the substance in the tank and serve, by means of the programmable regulation system already described, to regulate various parameters of the process in progress.

We would also remain within the scope of the invention by using a sampling device with adjustable continuous flow rate such as a pneumatic valve, a pump, etc. The microprocessor is adapted, depending on the case, to generate an analog signal or a pulse signal at variable rate to control the flow rate of the sampling device.

Claims (10)

1 - Device for measuring or regulating at least one physical characteristic of a fluid contained in a container (1), such as its mass, the container being provided with means (4-6) for introducing said fluid and means (7- 8) for sampling, the device comprising sensor means (9) for measuring at least one pressure prevailing in the fluid contained in the container and being characterized in that it comprises a treatment system (1-5) comprising a set of input-output (21) provided with acquisition means (22-23) for digitizing values of said pressure measured at determined time intervals, programmable calculation means (16-17) for performing a combination of several measurements of said pressure successively digitized and determine said physical characteristic of the fluid contained in the container. 2 - Device according to claim 1, characterized in that the processing system (15) comprises an interface assembly (26-27) controlled by the calculation means for controlling the sampling means (7-8) so as to regulate said physical characteristic of the fluid contained in the container. 3 - Device according to claim 2, characterized in that the sensor means (9) comprise at least one differential sensor for measuring the pressure difference prevailing between at least two different locations of the container, at least one of these locations being within the fluid, the withdrawal means (7-8) comprise a valve with adjustable flow and the calculation means (16) are adapted to generate for said valve, a variable control signal, allowing the regulation of said physical characteristic of the fluid in the container. 4 - Device according to claim 2 or 3, comprising a gas source (12) delivering gas within the fluid at a pressure sufficient to ensure a low flow and a measuring element (14) of the gas flow, characterized in that that the flow measurement element comprises a flow meter producing a signal dependent on the measured flow and means for applying said signal to the acquisition means (22-23), the programmable calculation means (16-17) being adapted to generate a signal for regulating the pressure delivered by the gas source to a value substantially equal to the value measured within the fluid in the container. 5 - Device according to one of claims 1 to 3, characterized in that the sensor means (9) comprise two differential sensors for measuring at least two pressures at two separate locations of the container (1) within said fluid, the signals translating the difference between the two pressures being applied to said acquisition means (22-23), so as to measure or regulate the density of said fluid. 6 - Device according to claim 1, characterized in that the sensor means (9) comprise at least one differential sensor for measuring the pressure difference prevailing between at least two different locations of the container, the acquisition means (22-23) comprising several inputs, at least one of them being connected to the sensor means (9), the input-output assembly (21) also including a digital-analog converter (30) for the conversion into analog signals of values determined by the calculation means (16-17), said digital-analog converter being provided with at least one output for a recording element (31) and an interface element (24) for display means (25). 7 - Device according to claims 2 and 6, characterized in that the regulation system (15), further comprises means (19-20) for the introduction of instructions by an operator. 8 - Device according to claim 2, characterized in that The interface assembly (26-27) comprises a security element (28) for limiting the maximum duration of application to the sampling means, control signals generated by the calculation means (16-17). 9 - Device according to claim 1 or 2 characterized in that the acquisition means (22-23) comprise several inputs, at least one of these being connected to a measurement element of another parameter. 10- Device according to claim 9, characterized in that said measuring element is a flow meter (14).