FR2826296A1 - DEVICE FOR DISSOLVING OXYGEN IN A LIQUID CONTAINED IN A REACTOR, METHOD FOR DISSOLVING OXYGEN IN A LIQUID, AND METHOD FOR IMPROVING A DEVICE FOR DISSOLVING OXYGEN IN A LIQUID - Google Patents

Abstract

The invention relates to a device for dissolving oxygen in a liquid contained within a reactor, provided with a vertical shaft (2) with a principal agitator (3), an air injection means (4), an oxygenated-gas injection means (5), placed at a distance from the air injection means (4), an additional agitator (6), placed below the principal agitator (3) and below the air injection means (4) and wherein the oxygenated-gas injection means (5) is placed below the additional agitator (6).

Description

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 The invention relates to a method for dissolving oxygen in a liquid, a device for dissolving oxygen in a liquid contained in a reactor and a method for improving this type of device. The invention is particularly useful in the field of fermentation.

v fermenteur. Production by fermentation consists in using microorganisms to obtain specific organic molecules from a food based on sugars. The conventionally prepared products are organic acids, various and varied proteins (amino acids, pharmaceutical products), antibiotics, enzymes, yeasts. These fermentations are often aerobic: oxygen is generally provided by air circulation and agitation of the fermentation medium. However for certain strains of microorganisms or at certain periods, the supply of oxygen by the air is insufficient. Thus the efficiency of use of the oxygen in the air (that is to say the ratio of the quantity of dissolved and consumed oxygen to the quantity of oxygen introduced into the reactor) is generally of the order of from 10 to 20%. It is possible to supplement the demand of living cells with a supplement of an oxygen-rich gas. Two main classes of methods for introducing an oxygen-rich gas into a fermenter are commonly used: - oxygen can be added directly to the air circuit, which has the advantage of avoiding modifying the fermenter, but it is necessary to check the compatibility of the materials and the method with the oxygen-enriched gas, - the separate addition of the oxygen-rich gas which is often used when there is incompatibility of the oxygen-rich gas with a mixture air and additives; it is then necessary to add a separate device for introducing the oxygen-rich gas into the

v fermenter.

 It has been noted that the above devices make it possible to increase the quantity of oxygen transferred into the fermentation medium without, however, increasing the yield, so that a large quantity of the oxygen introduced is not used for the fermentation. Devices using both air and an oxygen-rich gas and making it possible to improve this yield have been proposed: - EP 477 818 describes the introduction into the stirred reactor of air and gas rich in oxygen distant places from each other, air being introduced at the bottom of the reactor and oxygen-rich gas in the upper part of the reactor, so as to minimize the coalescence of the air bubbles with the bubbles oxygen,

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 - EP 829 534 describes a bubble column where convection movements are created by large air bubbles injected at the periphery of the reactor. The oxygen-rich gas is injected in small bubbles in the central part of the reactor and in the downward flow of the fermentation liquid, - EP 901 812 describes a stirred reactor where the oxygen-rich gas is injected into a hollow, a turbulence or a stationary vortex.

 For all these solutions, the increase in yield has not really been proven? In addition, all these solutions cannot always be implemented in existing fermenters without complete modification of the internal agitation system, that is to say without high cost.

 An object of the present invention is to provide a new device for dissolving oxygen in a liquid.

 Another object is to propose a new device for dissolving oxygen in a liquid having a high oxygen transfer efficiency in the liquid.

 Another object is to propose a method for improving the oxygen transfer efficiency of the devices for dissolving oxygen in a liquid, in particular fermenters, existing without significant modification of these devices.

 For these purposes, the invention firstly relates to a device for dissolving oxygen in a liquid contained in a reactor comprising: - a vertical shaft equipped with at least one main stirring means immersed in the liquid, - at at least one means for injecting air into the liquid, - at least one means for injecting an oxygenated gas placed at a distance from the air injection means, - said device comprises at least one additional stirring means placed below the main stirring means and below the air injection means, and in which the means for injecting oxygenated gas is placed below (in the vicinity?) of the additional stirring means.

 The invention also relates to a method for dissolving oxygen in a liquid contained in a reactor, in which:

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 - the liquid is stirred by at least one main stirring means placed on a vertical shaft immersed in the liquid, - air and an oxygenated gas are introduced separately into the liquid, - the liquid is stirred by at least one means additional stirring placed below the main stirring means and below the place of air injection, and - the oxygenated gas is introduced into the liquid at a place placed below the additional stirring means.

 The invention finally relates to a method for improving the oxygen dissolution efficiency of an oxygen dissolution device in a liquid contained in a reactor comprising a vertical shaft equipped with at least one main / mobile stirring means. (with radial flow) immersed in the liquid, and at least one means for injecting air into the liquid, in which is added to said device: - at least one additional stirring means placed below the main stirring means and below the air injection means, and - at least one means for injecting an oxygenated gas placed below the additional stirring means.

 Other characteristics and advantages of the invention will appear on reading the description which follows. Forms and embodiments of the invention are given by way of nonlimiting examples, illustrated by FIG. 1 which is a schematic view of a device according to the invention.

 The invention therefore relates first of all to a device for dissolving oxygen in a liquid contained in a reactor comprising: - a vertical shaft equipped with at least one main stirring means immersed in the liquid, - at least one means for injecting air into the liquid, - at least one means for injecting an oxygenated gas placed at a distance from the air injection means, - at least one additional stirring means placed below the means d main agitation and below the air injection means, and in which the oxygen gas injection means is placed below the additional agitation means.

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 The device according to the invention is equipped with a main means for agitating the liquid to be treated. In general, this main stirring means consists of one or more mobiles fixed on the vertical shaft of the reactor. These mobiles can be: a shearing type agitator, such as for example a Rushton type propeller comprising 4 to 6 blades, and / or a non-shearing type agitator such as a marine propeller. In general, several mobiles are fixed on the shaft so as to uniformly agitate all the liquid to be treated. Their number varies according to the ratio of the diameter of the reactor to the height of the reactor, in general four main stirring means are used. The main stirring means are then advantageously identical and placed at equal distance from each other. In general, the diameter of a main stirrer is between a third and a half of the internal diameter of the reactor. Preferably, if only one main stirring mobile is used, a stirring mobile of the shearing type is chosen which creates a flow of radial liquid; it may for example be a Rushton type propeller. Similarly, if several main stirring mobiles are used, then among these the stirring mobile located in the lower part of the reactor is preferably a stirring mobile of the shearing type which creates a flow of radial liquid such than a Rushton type propeller.

According to a particular implementation of the invention, the reactor can be equipped on its internal wall with counterpales, for example 3 counterpales. These improve the mixing of the liquid to be treated. Preferably, these counterpales are fixed to the wall of the reactor so as to have an angle of at most 60 with the vertical axis and in a direction such that they ensure the displacement against the wall of the reactor of the flow of liquid from the main stirring means from the top of the reactor to the bottom of the reactor.

 According to the invention, the device comprises means for injecting air and oxygenated gas which are separated. They are placed at a distance from each other; preferably at a distance corresponding to at least half the diameter of the lower main agitator. In general, as is the case in conventionally used fermentation reactors, the air injection means injects air into the lower part of the reactor and under the main stirring means. The means for injecting air and oxygenated gas can be of any kind. They can be, for example, toroids, drilled tubes, porous tubes or nozzles.

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 According to one of the essential characteristics of the invention, the device also comprises an additional stirring means placed below the main stirring means and below the air injection means. This additional stirring means is generally a mobile of the shearing type which can be for example: an agitator with vertical blades such as a propeller of the Rushton type having 4 to 8 blades. This propeller can be placed between two discs thus forming a turbine. Preferably, the additional stirring means is a Rushton type propeller having at least 6 blades. A preferred device of the invention comprises, as additional and main stirring mobiles, Rushton type propellers, the additional mobile having a number of blades greater than the main stirring mobile. Advantageously, the additional agitation means is also placed on the vertical shaft carrying the main agitation means. It can then be driven by the drive device of the shaft. This shaft drive device is generally placed above the reactor. According to another essential characteristic of the invention, the means for injecting the oxygenated gas is placed below this additional stirring means, preferably as close as possible to the additional stirring means.

 According to a particular implementation of the invention, the reactor shaft is held in its lower part by a tripod and the means for injecting oxygenated gas and the additional stirring means are both placed inside the space formed by this tripod. This implementation is particularly suitable in the case where a conventional device supplied only with air is improved in accordance with the present invention and where the means for injecting oxygenated gas and the additional stirring means are added to this device.

 The invention also relates to a method for dissolving oxygen in a liquid contained in a reactor, in which: - the liquid is stirred by at least one main stirring means placed on a vertical shaft immersed in the liquid, - the separately from air and an oxygenated gas in the liquid, - the liquid is stirred by at least one additional stirring means placed below the main stirring means and below the place of injection of the air, and - The oxygenated gas is introduced into the liquid at a place placed below the additional stirring means.

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 This method relates to the use of a reactor comprising at least one main stirring means placed on a vertical shaft immersed in the liquid and at least one additional stirring means placed below this main stirring means. The liquid is usually agitated by the main agitation means so as to create a radial flow.

 According to the process of the invention, the air and the oxygenated gas are introduced separately into the liquid to be treated. Generally, air is introduced into the lower part of the reactor and under the main stirring means. One of the essential characteristics of the process lies in the place of introduction of the air which must take place above the additional stirring means. Another essential characteristic of the process relates to the place of introduction of the oxygenated gas which must take place in a place placed below the additional stirring means.

 The ratio of the flows created by the main and additional agitation means are generally of the same order.

 The process is particularly suitable for the fermentation of liquids, in particular for liquids having a viscosity of between 0.1 and 1 Pa. S, and for liquids having a salinity of at most 2 g / i.

 In the present description, the term “oxygenated gas” means a gas having a greater oxygen content than air. In general, it is an oxygenated gas with an oxygen content of at least 70% by volume.

 Finally, the invention relates to a method for improving the oxygen dissolution yield of a device for dissolving oxygen in a liquid contained in a reactor comprising a vertical shaft equipped with at least one main submerged stirring means. in the liquid, and at least one means for injecting air into the liquid, in which is added to said device: - at least one additional stirring means placed below the main stirring means and below the d means air injection, and - at least one means for injecting an oxygenated gas placed below the additional stirring means.

 This process generally makes it possible to improve by at least 30% the efficiency of dissolution of oxygen in the liquid of an existing device.

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 Figure 1 illustrates the device according to the invention. This device comprises a vertical reactor (1) provided with a vertical shaft (2) held by a tripod (7) and used for driving four Rushton turbines, corresponding to the main stirring mobiles (3), and d 'Another Rushton turbine corresponding to the additional agitation mobile (6). The air is injected by a torus (4) and the oxygenated gas by a torus (5) each connected to a source of the corresponding gas.

EXAMPLES
Example 1 according to the invention
A device similar to that of FIG. 1 is used in the form of a 2 liter laboratory reactor. The main agitator is a Rushton turbine with 4 blades of 60 mm in diameter and the additional agitator is a Rushton turbine with 8 blades of 50 mm in diameter.

 The liquid treated is water at 25 ° C. and its volume is 1.8 1. The speed of the shaft is 500 rpm. The oxygenated gas is 100% pure oxygen by volume and its introduction rate is 12.3 l / h. The air introduction rate is 95.7 hours.

 The KLa transfer coefficient of oxygenated gas in the liquid was measured by the method of Humphrey ("Degassing Gassing Techique" S. Aiba, A. E. Humphrey, N. F. Millis, Biochemical Engeneering Acadamic Press New York, 1973,332-335).

In the examples of the present invention, this Humphrey method has been implemented in the following manner: before introduction of the oxygenated gas, the liquid to be treated is deaerated by bubbling nitrogen until an oxygen content is obtained dissolved close to zero. This content is measured by an electrochemical probe. The liquid is then subjected to the introduction of oxygenated gas and, during this oxygenation, the content of dissolved or C is measured using the electrochemical probe. When equilibrium is reached, the dissolved oxygen content no longer changes and reaches the asymptotic value Cs, concentration at saturation. From the values measured over time until equilibrium is obtained, the curve In (Cs-C) is plotted as a function of time. For this plot, values of C that are outside the range of 10 to 90% of the value of Cs are not used.

The curve obtained is a straight line with a slope equal to: -KLa. The coefficient KLa is the opposite of the value of the slope of the straight line obtained.

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In the present example, a saturation concentration, Cs, of
11.4 mg / l and a KL transfer coefficient. from 95 hour-1. From the transfer coefficient KLa, we deduce the initial speed of transfer of oxygen in the liquid, when the concentration of 02 in the solution is zero: KLa Cs = 95 x 11.4 = 1083 mg / i of solution and per hour, with Cs concentration at saturation, or 1949 mg / h for the 1, 8 1 of liquid treated. By comparison with the 46.3 g / h of oxygen introduced (by air and by pure oxygen), it is deduced therefrom that the oxygen dissolution yield is 4.2%.

Comparative Example 2
The same device and the same operating conditions are used as in Example 1 except that no oxygenated gas is introduced under the additional mobile and that the rate of introduction of air under the main agitation mobile is 108 itch.

 The transfer coefficient KLa of the oxygenated gas in the liquid is 71 hours-1, measured by the Humphrey method. It is deduced therefrom that the oxygen dissolution yield is 3.1%.

Example 3
The same device and the same operating conditions are used as in Example 1 except the stirring speed which is 1000 rpm.

 The transfer coefficient KLa of the oxygenated gas in the liquid is 392 hours-1, measured by the Humphrey method. It is deduced therefrom that the oxygen dissolution yield is 17.4%.

Comparative example 4
The same device and the same operating conditions are used as in Example 3 except that the places of introduction of the air and the oxygenated gas are reversed. Thus the air is introduced under the additional agitation mobile and the oxygenated gas is introduced between the additional agitation mobile and the main agitation mobile.

 The transfer coefficient KLa of the oxygenated gas in the liquid is 342 hours-1, measured by the method of Humphrey. It is deduced therefrom that the oxygen dissolution yield is 15.2%.

Claims (13)

1. Device for dissolving oxygen in a liquid contained in a reactor (1) comprising: - a vertical shaft (2) equipped with at least one main stirring means (3) immersed in the liquid, - at least one means for injecting air into the liquid (4), - at least one means for injecting an oxygenated gas (5) placed at a distance from the air injection means, characterized: - in that said device comprises at least one additional stirring means (6) placed below the main stirring means and below the air injection means, and - in that the oxygenated gas injection means is placed below additional stirring means.  2. Device according to claim 1, characterized in that the air injection means injects air into the lower part of the reactor and under the main stirring means.  3. Device according to claim 1 or 2, characterized in that the main stirring means creates a radial flow.  4. Device according to one of the preceding claims, characterized in that the main stirring means is a Rushton type propeller.  5. Device according to one of the preceding claims, characterized in that the additional stirring means is placed on the vertical shaft carrying the main stirring means.  6. Device according to one of the preceding claims, characterized in that the additional stirring means is a Rushton type propeller. <Desc / Clms Page number 10>  7. Device according to one of the preceding claims, characterized in that it comprises, as additional and main agitation mobiles Rushton type propellers, the additional mobile having a number of blades greater than the main agitation mobile 8. Device according to one of the preceding claims, characterized in that the shaft is held in its lower part by a tripod (7) and in that the means for injecting oxygenated gas and the additional stirring means are placed inside the space formed by said tripod.  9. Method for dissolving oxygen in a liquid contained in a reactor, in which: - the liquid is stirred by at least one main stirring means placed on a vertical shaft immersed in the liquid, - it is introduced separately from the air and an oxygenated gas in the liquid, and characterized in that: - the liquid is stirred by at least one additional stirring means placed below the main stirring means and below the place of injection of the air , and - the oxygenated gas is introduced into the liquid at a place below the additional stirring means.  10. Method according to the preceding claim, characterized in that the air is introduced into the lower part of the reactor and under the main stirring means.  11. Method according to one of claims 9 or 10, characterized in that the liquid is stirred by the main stirring means so as to create a radial flow.  12. Method according to one of claims 9 to 11, characterized in that it is used for a fermentation reaction.  13. Method for improving the oxygen dissolution yield of a device for dissolving oxygen in a liquid contained in a reactor comprising a vertical shaft equipped with at least one main stirring means immersed in the liquid, and at least one means for injecting air into the liquid, <Desc / Clms Page number 11>  characterized in that said device is added: - at least one additional stirring means placed below the main stirring means and below the air injection means, and - at least one injection means d 'An oxygenated gas placed below the additional stirring means.