METHOD FOR OPTIMIZING THE ENERGY CONSUMPTION OF A FAN IN WATER TREATMENT

Description

[0001] The present invention relates to the field of water treatment, and is particularly concerned with devices for injecting oxygen into purification basins. This oxygen is used by the biomass present in the basins to consume the pollution present in the water.

[0002] The following documents illustrate examples of such devices US 4,280,910 A, US 2015 / 265979 A1, WO2012160300A1,

[0003] EP-995 485, FR-2594112A1.

[0004] But more generally the invention concerns the management of the operation of devices positioned on the surface of the pool, equipped with a system allowing it to be kept above the water (floating), and comprising: an injection system for an oxygen-rich gas (typically between 22% and 100%, preferably between 80 and 100% oxygen). The injection system can be a simple crown pierced with orifices, or a porous candle; these systems can be accompanied by complementary systems such as a bladed rotor, straight or inclined, a venturi with a pump can also be suitable as an injection system for such an oxygen-rich gas; a gas mixing and dispersion rotor such as a three-bladed propeller or a Rushton turbine, etc.

[0005] The essential features of the present invention are found in method claim 1. Another embodiment is described in dependent claim 2.

[0006] The present invention is particularly concerned with control cabinets for the operation of such devices.

[0007] These cabinets allow in particular the control of the motors of these devices, and the management of the injection of oxygen-rich gas.

[0008] Tests carried out by the Applicant on a device such as that described in document WO2012160300A1 have shown that for low flow rates of oxygen injected into such a device, it is possible to reduce the power consumed while maintaining an efficiency of stirring / mixing of the medium and a transfer of the gas in the medium equivalent to the nominal power used, i.e. the power consumed by the motor when the motor is powered at the network frequency without modification (most often 50 Hz).

[0009] This change in power consumed by the device was achieved by reducing the size of the main moving part, the pumping propeller.

[0010] In such floating aeration devices, the performance (flow rates of gas / oxygen actually dissolved and therefore available for treatment and therefore bacteria), apart from the design of the agitation system, depends on both the agitation speed and the gas flow rate.

[0011] Also, to avoid observing poor yields, we generally tend to keep the agitation flow rate constant (at nominal therefore) and this, whatever the flow rate of injected gas (over the entire performance range in general, that is to say between zero and the maximum gas flow rate).

[0012] However, it turns out that the need for oxygen can be low or even zero for a good part of the day, up to 80% of the time over a day.

[0013] Maintaining the devices at their nominal speed then implies unnecessary continuity of electricity consumption.

[0014] The present invention then proposes to adapt the operation of the device, according to the oxygen requirement in the environment and / or according to the flow rate of gas to be injected, this will be understood with the objective of optimizing the energy consumption of the equipment according to the real requirement for pure oxygen.

[0015] To this end, it proposes to implement a frequency converter in the control cabinet to control the motor of the device used for injecting the oxygen-rich gas. The converter acts on the power supply frequency of the motor and therefore on the rotation speed of the shaft on which the stirring / dispersion / injection mobile(s) are mounted.

[0016] For illustration purposes, the following operating methods can be implemented: Nominal: the device operates (in terms of agitation and injection) for a given range at nominal, i.e. as intended in its design. In other words, the device operates at the network frequency, at this frequency it implements a nominal rotation speed which results in a nominal gas injection and transfer capacity. Maximum: the device can be accelerated by increasing the motor supply frequency (within the acceptable limits of mechanical design tolerances, in particular for the motor), which has the effect of increasing both the agitation and (consequently) the dispersion of the gas being injected and therefore pushing the gas injection beyond the design value.In the case of devices for which a high gas flow rate would threaten to clog the equipment, equipment equipped for example with a self-priming turbine, this supply at a frequency that is permissible but higher than the design value makes it possible to push back the limit of the clogging flow rate. Indeed, depending on the design of the engine, to reduce the electrical consumption in the case where the oxygen requirement is low (typically < 50% of the maximum injection flow rate) it is generally possible to reduce the frequency by 10 to 20 Hz. In the case where it is desired to increase the injection capacity (injection flow rate higher than the maximum flow rate at the frequency of 50 Hz), it is possible to increase the motor supply frequency by approximately 10 Hz, often between 2 and 5 Hz. Minimum / reduced operation: the rotation speed of the moving parts of the device (self-priming turbine, shear blades, mixing propeller, etc.) is then slowed down (frequency lower than the nominal).At the same time, the oxygen flow rate is reduced so as to maintain, for this reduced rotation speed, a gas transfer efficiency equivalent to the nominal operation of the equipment. The nominal speed is in fact not required, for this reduced gas flow rate, in order to maintain good dispersion and therefore good gas absorption efficiency.

[0017] By having the possibility of varying the rotation speed of the equipment / stirring speed, using a frequency converter (within a certain range acceptable to the device, in particular to the motor), the total energy consumption will be reduced over the day (80% of the time at reduced operation for example) without harming the efficiency of the equipment.

[0018] Those skilled in the art of aeration of liquid media are familiar with this notion of “nominal” regime.

[0019] Indeed, such aerators are assemblies of standardized parts, performing the two functions necessary for maximized aeration, i.e. the mixing of the liquor and the dispersion of the gas.

[0020] The parts are offered by the manufacturers of such aerators who have designed and optimized them for this: a motor: its design imposes a speed range, number of poles, characteristics of its power supply (including the nominal frequency imposed by national electricity suppliers: 50 Hz in France). The most common motors rotate at approximately 1200 rpm for 50 Hz power supply. The number of poles (winding) allows the alternating current (at its frequency) to rotate the shaft contained in the center of the poles at the nominal speed, therefore. at least one agitation-mixing system such as a propeller, a turbine... which will transform the movement of the shaft / power transmitted by the shaft into movement of the mixed fluid / liquor and ensure the dispersion of the gas. These systems are designed and manufactured by suppliers who again give the correct operating or use range, rotation speed range around a nominal point.

[0021] A speed of 200-500 rpm corresponds to a range often recommended by suppliers for moving fluids such as water (therefore not too viscous, rather Newtonian type). On the other hand, for very viscous fluids (pasty, non-Newtonian, etc.), different rotors are traditionally used (Archimedes screw for example, which presents the maximum surface area to the fluid) and which rotate very slowly (< 100 rpm).

[0022] In other words, those skilled in the art of aerators know that for each type of equipment, the manufacturer gives a range of rotation speeds recommended "by construction".

[0023] It is therefore the merit of the present invention to have proposed to vary the operation of the aerator without significant modification and without damage to any part, noting that there is no need to permanently maintain a fixed operating point, and that there is a real gain in making it move a little, very little around this point: a little more to aerate more when the pool needs it (peak load), without the need to invest more. a little less in a major part of the day of a pool since the pollution does not arrive continuously, thus minimizing the power consumed.....

[0024] Nevertheless, and in order to clarify the operating conditions recommended according to the present invention, the preferred ranges for this rotation speed are indicated here: between 50 and 1000 rpm: We can indeed consider that above this range, we are aiming at very particular agitators which are little or not used in the field of the present invention and likewise below this range we will find agitators which rotate slowly to agitate pasty fluids. and more preferably between 50 to 500 rpm: because in fact we can consider that above 500 rpm, we find small mobiles or particular shapes which are little or less applicable to the field targeted by the present invention. and even more preferably between 150 and 350 rpm.

[0025] An example of walking on a floating apparatus, pushing the gas injected from the bottom using agitators / propellers mounted on an agitation shaft, apparatus such as that described in document WO2012 / 160300A1, is described below.

[0026] The curves provided in figure 1attached (one for a system at a depth of 3.4m, the other at 5.4m) show that the transfer efficiency or yield increases, for a given stirring speed, when the flow rate of injected gas (pure oxygen here) is reduced.

[0027] We therefore take advantage of this gain to reduce the stirring speed and maintain a high transfer efficiency (>80% in general).

[0028] The transfer efficiency mentioned here is determined using the standard method described in the literature which consists of, starting from an initial dissolved oxygen value close to 0, injecting the gas (O 2 ) at a fixed flow rate and monitoring the evolution of the oxygen concentration in the water over time. From this curve (asymptotic, final value corresponding to saturation), we deduce the transfer coefficient (called kL.a) or the quantity actually dissolved (therefore transferred). Finally, the efficiency corresponds to the ratio between the dissolved quantity and the injected quantity.

[0029] An example of implementation in an activated sludge basin used in a water treatment plant is described below.

[0030] This basin is aerated and agitated by floating oxygen injection devices such as those described in document WO2012160300A1.

[0031] In normal ("nominal") operation, P°=16 kW is consumed for a stirring speed of N°=283 rpm and a stirrer diameter D=680 mm (the diameter of the propeller at the end of the shaft in the document mentioned above, which creates the overall movement, therefore both the stirring / mixing and therefore the dispersion of the gas in the basin).

[0032] The gas flow (oxygen) injected for this operation at nominal is of the order of 56 Nm 3 < / h with a transfer rate of the order of 80% in real conditions.

[0033] Also, in continuous operation (gas flow and stirring speed), we will consume: P totale ∼ 1 , 38 10 6 kJ / jour = 16000 × 24 × 3600

[0034] It can be noted that in turbulent conditions, the adimensional number called "power" Np is constant (whatever the operating parameters N and D), expressed according to the following formula: Np = P / (ρ.N 3< .D 5< ) with ρ the density of the stirred medium (water, taken at 1000 kg / m 3< in the present example). (N: stirring speed in rev / s, and D diameter of the main rotor in m)

[0035] In the case presented, Np ~ 1.05 = 16000 / [1000 x (283 / 60) 3< x 0.68 5< ]

[0036] In the context of the present invention, considering that in 80% of the time, that is to say outside the period when a pollution peak arrives at the wastewater treatment plant, the oxygen requirement is much lower, this much lower requirement only requires one injection per device of between 0 and 30 Nm 3 < / h while maintaining a transfer efficiency of the order of 80%. The device will therefore operate at reduced speed for 80% of the day (i.e. 19.2 h out of 24 h) when the oxygen requirement (pure oxygen or oxygen-rich gas) is between 0 and here 30 Nm 3 < / h. This operation at a reduced speed makes it possible to maintain a sufficient stirring / mixing capacity to keep the sludge (biological flocs) in suspension in the basin and a gas transfer capacity equivalent (to + or - 5%) to that determined for the nominal operation of the equipment.

[0037] In our example, the speed is reduced by only 8%, which leads to N*=263 rpm.

[0038] And so the new power consumed P*-12.8 kW = 1.05x1000*(263 / 60) 3< x0.68 5<

[0039] The total new energy consumed during the day will be: P tot * = 1 , 16 10 6 kJ / jour = 16000 × 4 , 8 + 12800 × 19 , 2 × 3600

[0040] It corresponds in fact to the contribution of each step during the day.

[0041] The energy gain achieved thanks to the invention by adding a speed variator is therefore approximately 16%, simply by varying the stirring speed by 8% and of course maintaining sufficient stirring / mixing efficiency to keep the sludge in suspension and gas transfer equivalent to the nominal conditions.

[0042] The present invention then relates to a method for managing the operation of an oxygen injection device in a purification basin, oxygen used in particular by the biomass present in the basin to consume the pollution present in an effluent to be treated contained in this basin, device characterized by the following components and functionality: the device is positioned on the surface of the basin, and equipped with a system allowing it to be kept floating above the liquid; it comprises a drive device, intended to be placed above the liquid, provided with a vertical or inclined output shaft, shaft equipped at its end with at least one mobile for mixing and dispersing the injected gas such as a three-bladed propeller; it comprises a system for injecting a gas containing oxygen (typically between 20% and 100%, preferably between 80 and 100% oxygen); method characterized in that the rotational speed of the shaft is varied using a frequency variator, the speed variation applied being between plus 15% and minus 15% of the nominal speed of the equipment, and more preferably between plus 10% and minus 10% of the nominal speed of the equipment, i.e. in conditions where the motor is supplied at the network frequency without modification, in order to optimize the electrical consumption according to the need injection of gas containing oxygen and the mixing capacity required for the purification basin where the equipment is installed

Claims

1. Method for managing the operation of an apparatus for injecting oxygen into a purification basin, the oxygen notably being used by the biomass present in the basin to consume the pollution present in an effluent feedstock contained in this basin, the apparatus being <b>characterized by the following components and functionalities: - the apparatus is positioned at the surface of the basin and equipped with a system enabling it to be kept floating above the liquid; - it comprises a drive device, intended to be positioned above the liquid, and provided with a vertical or inclined output shaft, the shaft being equipped at its end with at least one moving part for mixing and for dispersing the injected gas, such as a three-bladed impeller; - it comprises a system for injecting a gas containing oxygen; the method being characterized in that a frequency variator is used to vary the rotational speed of the shaft, the applied variation in speed being comprised between plus 15% and minus 15% of the nominal speed of the equipment, and more preferably between plus 10% and minus 10% of the nominal speed of the equipment, i.e. the speed under conditions in which the motor is powered at the mains frequency without modification, this being so as to optimize the electrical power consumption according to the need for injection of the oxygen-containing gas and the necessary agitation capacity for the purification basin in which the equipment is installed.

2. Method according to Claim 1, characterized in that the nominal rotational speed is situated in the range between 50 and 1000 rpm, more preferably between 50 and 500 rpm, and more preferably between 150 and 350 rpm.