Device enabling gas bubbles contained in a liquid composition to be eliminated

Abstract: The invention concerns a device enabling the gas bubbles present in a liquid composition to be dissolved. The device comprises a chamber (10) provided with an inlet orifice (11) through which the composition to be debubbled is introduced, and an outlet orifice (12) through which the debubbled composition is discharged, an ultrasonic transducer (13, 14, 15, 16, 17, 18, 19, 20), a power supply (21) for supplying the said transducer, the said power supply (21) being regulated in frequency and power at the same time. Application to photographic compositions.

Description: The present invention relates to the elimination of gas bubbles contained in liquid compositions and more particularly concerns a device adapting automatically to any changes in characteristics of the liquid composition to be debubbled. Many products in the chemical industry, the pharmaceutical industry, the food products industry and related industries, in particular emulsions, suspensions, pastes and high viscosity liquids or similar contain air or gases which are dissolved or in the form of small bubbles which, during manufacture, inevitably come to be incorporated in the liquid but must not occur in the final product. Thus, for example, in the case of photographic emulsions, the gas bubbles greatly impair the quality of the films or photographic papers produced with these emulsions since the bubbles or small gas bubbles disturb the volume flow in the coating devices, thus giving rise to the formation of streaks which make the photographic materials unusable. It has already been contemplated to eliminate bubbles in liquid compositions. For example FR-A-2,221,165 describes an ultrasonic device for eliminating bubbles from a liquid composition and more particularly from photographic composition. Although effectivity of the device with respect to various parameters of the composition is hinted at, no proposal to improve said effectivity is mentioned. WO-A-92/12790 describes a tubular ultrasonic device which could be used for degassing and/or dissolving of gas in liquid composition. The natural resonance of the reactor is used to improve propagation of ultrasonic waves of a plurality of ultrasonic transducer. No regulation of the energy provided to each transducer is mentioned. EP-A-246,528 describes the use of an ultrasonic transducer to agitate a liquid medium with high-frequency sound energy. Said ultrasonic transducer is used for cleaning. The various circuits described permits regulation of the power applied on the transducer by defining the amount of time during each cycle the power signal is on. Fig 1, to which reference is now made, shows diagrammatically a conventional photographic emulsion downfeed. According to such a conventional arrangement, the emulsion downfeed includes a vat 1, maintained under agitation, into which the emulsion to be processed is introduced. The emulsion is then conveyed to a preliminary processing device 2, in which a first processing is applied, by means of ultrasonics, in order to allow a rudimentary debubbling of the said emulsion, the term "debubbling" meaning elimination of gas bubbles from the composition to be processed. The composition is then carried, by means of a pump 3, to a bubble eliminator 4, which will be designated hereinafter by the initials ECR and in which an ultrasonic processing is also applied for the purpose of reincorporating in the photographic composition any gas bubbles remaining at the end of the preliminary processing. The ECR will be the subject of a more detailed description later. The ECR is powered by means of a power supply 7. The processed solution is then conveyed to a utilisation station 8 such as, for example, a photographic coating station. Generally other devices, for example of the partial vacuum type, not shown, are incorporated upstream of the ECR. Likewise the vat can itself be subjected to ultrasonic vibration in order to eliminate some of the gas bubbles at this stage. Fig 2, to which reference is now made, shows in detail an ECR of the type used conventionally for this type of application. These devices, well known in the art, comprise principally a processing chamber 10, for example made from stainless steel, provided with an inlet orifice 11, through which the solution is introduced, and an outlet orifice 12, through which the processed solution is discharged. The ECR also comprises an ultrasonic transducer fitted into a chamber (not shown), which transducer transmits vibrations to a titanium rod 13, disposed in the processing chamber 10, through a diaphragm 14, generally made from titanium. The transducer is in fact formed by an assembly of crystals and piezoelectric ceramics 16, 17, disposed in a so-called "Langevin triplet" arrangement and capable of expanding and contracting at the same rate as the frequency which is fed to them through the connections 15. The so-called "Langevin triplet" arrangement consists of two piezoelectric discs separated by an intermediate ring. Each of the ceramics 16, 17 has one of its faces connected to earth, the other being connected to the power supply point 21. The two ceramics are insulated by an aluminium ring 18. The transducer also comprises a rear counterweight 19 enabling most of the ultrasonic wave to be reflected back to the titanium rod 13 in contact with the solution to be processed, the whole being prestressed by means of a bolt 20 which enables the point of repose of the ceramics to be moved, thus allowing the application of stronger electric fields without any risk of having the ceramic rupture under the effect of excessively large tensile stresses, the compressive strength of the ceramic being in fact greater than its tensile strength. Generally the power supply frequency varies between 38 and 43 kHz. Such an ultrasonic device can, in reality, be likened to a circuit of the RLC type in which the term R corresponds to the electrical resistance related to a mechanical damping due to the diaphragm 14, to the fluid and to the pressure inside the processing chamber 10; the term L corresponds to the mass of the vibrating assembly; the term C corresponds to the interelectrode capacitance, that is to say between the two ceramics 16, 17. In consequence, such a device will function in an optimum manner if, at any time, the frequency of the power supply coincides with the natural resonant frequency of the RLC circuit. A disadvantage of existing ECRs lies in the fact that the frequency adjustment of the ultrasonic transducer power supply is carried out manually by an operator. This adjustment is in reality carried out once and for all for each batch to be processed and consequently often becomes inappropriate as the term R varies, in particular because of the wear on the diaphragm 14 or the change in pressure inside the processing chamber 10. Moreover, in certain cases, the adjustment by the operator is carried out by varying the frequency not continuously but discretely, that is to say in steps (of the order of a few hundred hertz). Such a system does not therefore allow precise adjustment of the ultrasonic transducer power supply frequency. The consequence of this is obviously that the yield of the electrical energy/mechanical energy conversion afforded to the titanium rod 13 is not optimum, thus making the debubbling produced in the liquid composition unsatisfactory. Another problem lies in the power adaptation of the transducer power supply. It is in fact desirable to have an immediate adaptation of the energy transferred to the transducer according to the operating conditions, namely the flow rate, temperature, pressure or viscosity of the composition, without any intervention on the part of the operator. This is necessary when the device is not always used for the same compositions, but for compositions in which certain parameters, in particular the viscosity, change. It is in fact very disadvantageous from the point of view of efficiency to have to repeat the adjustments each time that the composition to be processed is changed. Thus one object of the present invention is to provide a device making it possible to eliminate the gas bubbles present in an aqueous composition by means of an ultrasonic transducer whose power supply is automatically adapted to the operating parameters and notably to the characteristics of the composition to be processed. Another object of the present invention is to be able to dispense with the preliminary processing devices existing in conventional installations. Other objects will become clear in more detail in the following description. These objects are achieved by producing a device enabling the gas bubbles contained in a liquid composition to be eliminated, comprising as provided in claim 1: a chamber provided with an inlet orifice through which the composition to be debubbled is introduced, and an outlet orifice through which the debubbled composition is discharged; an ultrasonic transducer inducing an alternating pressure field inside the said chamber; a power supply for supplying the said transducer; said power supply being regulated in frequency and power at the same time to match the natural resonant frequency of said transducer; and, according to the invention, the frequency regulation is based on a measurement of the phase difference between the current and voltage at the ultrasonic transducer terminals. According to another advantageous characteristic, the device also comprises means enabling an operator to carry out a preliminary adjustment of the frequency, means being provided to indicate to the operator when the preliminary adjustment has been carried out correctly. Advantageously again, the ultrasonic transducer has a structure of the Langevin triplet type. During the following description, reference will be made to the drawing in which: Fig 1 shows diagrammatically a conventional photographic emulsion downfeed; Fig 2 shows in detail the ultrasonic debubbling device (ECR); Fig 3 is a graph showing the current at the terminals of the ECR (the curve passing through the points Δ) and the phase difference between the current and voltage (the curve passing through the points +) as a function of the frequency; Fig 4 shows, in the form of blocks, an outline diagram of one embodiment of the circuit for regulating the power supply to the device according to the present invention. According to the present invention, the intention is that the ECR power supply frequency should at all times coincide with the natural resonant frequency of the RLC circuit, corresponding to the ultrasonic transducer, the resonant frequency corresponding to the frequency for which the phase difference between the current and voltage at the terminals of the ECR is zero. From the graph shown in Fig 3, it is clear that there are two frequencies for which the phase difference is zero: a series resonant frequency F s for which the current is maximum; a parallel resonant frequency F p for which the current is minimum. For reasons of yield, the aim will naturally be to opt for the series resonant frequency, that is to say under the conditions where the internal resistance of the system is minimum. The ECR used according to the present invention is of the same type as the one described with reference to Fig 2 and consequently does not require any additional description. Only the control of the ECR power supply will be the subject of a detailed description. Fig 4, to which reference is now made, shows, in the form of functional blocks, one embodiment of the circuit for frequency and power regulation of the power supply 20 to the ECR 21. The frequency regulation is achieved by means of a phase locking loop whose input stage 22 is a circuit in which the signals representing the voltage and current at the terminals of the ECR are shaped. In this stage the said current and voltage signals are shaped as a square signal. These signals are then transmitted to a phase comparator 23 which produces a voltage proportional to the phase difference between the voltage and current at the terminals of the ECR. The phase signal coming from the comparator 23 is then integrated by means of an integrator 24. When the system is started up, the operator enters a preliminary adjustment frequency 25. During this preliminary adjustment, the phase signal coming from the integrator is transmitted to a window comparator 26, which compares the signal which is sent to it with two predetermined thresholds, corresponding to the upper and lower limits of the preliminary adjustment desired. If the value of the input signal is between these two thresholds, an indicator, for example a visual indicator of the light emitting diode type 27, informs the operator that the preliminary adjustment has been carried out correctly. Advantageously, this preliminary adjustement is replaced by an automatic and continuous adjustment process. To this end, the sign of the phase difference between the current and the voltage at the terminals of the ECR is measured. Depending on the sign of said phase difference, a counter is incremented or decremented. Said counter controls a digital-to-analog converter (DAC), which in turn provides an adjustment voltage. Said voltage which is continuously self-adjusted, replaces the preliminary adjustment voltage, entered by the operator in the above mentioned embodiment, said counter being incremented or decremented until the phase difference be within a given range defined by the said two predetermined thresholds. Such a correction system, of the integral type, allows to correct at any time for any resonant frequency drift, whatever the origin of said drift is (T°, wear of the ECR horn). Furthermore, said counter can be reset if the amplitude difference between the current and voltage signals is greater than a given value. A difference greater than said value would in fact imply that said regulation loop is locked on a frequency for which the efficiency is not maximal. As an example, a sharp variation of the frequency in the processing chamber could cause the locking of the regulation loop on the parallel resonant frequency for which the efficiency is minimal. The reset of said counter allows to lock again the regulation loop on the series resonant frequency for which the efficiency is maximal. According to the embodiment described here, the voltage coming from the integrator 24 varies in fact between 0 volts for x degrees of negative phase difference and 15 volts for x degrees of positive phase difference. This signal is transmitted to a phase shifter 28 to be realigned on 0 volts. The signal then varies between -7.5 V and +7.5 V. This signal is then added to the preliminary adjustment voltage supplied by the operator or to the continuously self adjusted voltage provided by the DAC, by means of an adder 29. The resulting voltage feeds a voltage controlled oscillator (VCO) 30 which in response produces a frequency of between 38 and 43 kHz. This frequency, through an output stage 31, feeds the power part of the power supply 20. Thus, after carrying out the required preliminary adjustment, the power supply adapts automatically in frequency according to the operating parameters of the system, and this in a continuous fashion. After this description of the frequency regulation stage, the power regulation stage will now be described. The operator enters a power reference input 32 and this reference input is compared 33 with the power actually supplied to the ECR by the power supply 20. The power actually supplied by the power supply is measured, for example, by means of a wattmeter board. The resulting error voltage supplies a power variator 34 of the dimmer type, which itself feeds the power stage of the power supply 20 so as to cancel out continuously the said error voltage. This regulation loop enables the power supply to be adapted in respect of the power whatever the characteristics (viscosity, temperature) of the composition to be processed. Such a regulation, both in frequency and power at the same time, makes it possible to avoid the use of auxiliary debubbling devices as mentioned previously, thus limiting the cost of the equipment and its maintenance. Such a simplification also results in a reduction in head losses.

Claims: Gas bubble eliminator for a liquid composition, comprising: a chamber (10) provided with an inlet orifice (11) through which the composition to be debubbled is introduced, and an outlet orifice (12) through which the debubbled composition is discharged; an ultrasonic transducer (13, 14, 15, 16, 17, 18, 19, 20) inducing an alternating pressure field inside the said chamber; a power supply (21) for supplying the said transducer; said power supply (21) being regulated in both frequency and power to match the natural resonant frequency of said transducer within the liquid composition; said gas bubble eliminator being characterised in that means are provided for frequency regulation based on a measurement of the phase difference between the current and voltage at the terminals of the ultrasonic transducer. Gas bubble eliminator according to Claim 1, characterised in that it also comprises means (25) enabling an operator to carry out a preliminary adjustment of the frequency, means (27) being provided to indicate to the operator when the preliminary adjustment has been carried out correctly. Gas bubble eliminator according to Claim 2, characterised in that the said indicator means comprises a light emitting diode (27) controlled by a window comparator (26). Gas bubble eliminator according to Claim 1 further comprising : means for measuring the sign of the phase difference between the current and voltage at the terminals of the ultrasonic transducer ; means for, depending on said sign, incrementing or decrementing a counter until the phase difference be within a given range, said counter controlling a digital-to-analog converter which, in turn, provides a corresponding adjustment voltage which is added to a voltage representative of said phase difference. Gas bubble eliminator according to Claim 4, further comprising means for resetting said counter each time the amplitude difference between the current and voltage signals is greater than a given value. Gas bubble eliminator according to any one of Claims 1 to 5, characterised in that the power supply frequency varies between 38 and 43 kHz. Gas bubble eliminator according to any one of Claims 1 to 6, characterised in that the said chamber (10) is made from stainless steel. Gas bubble eliminator according to any one of Claims 1 to 7, characterised in that the said liquid composition is a photographic composition. Gas bubble eliminator according to any one of Claims 1 to 8, characterised in that the ultrasonic transducer has a structure of the Langevin triplet type.

Inventors: BEAU LAURENT, BEAU, LAURENT, Beau, Laurent, c/o KODAK-PATHE
Applicants: EASTMAN KODAK CO, KODAK PATHE, EASTMAN KODAK COMPANY, KODAK-PATHE
Application year: 1993
Main classification: B01F1
IPC codes: B01F3, B06B1