WO1988002276A1

WO1988002276A1 - Gas/liquid exchange device and method

Info

Publication number: WO1988002276A1
Application number: PCT/FR1987/000363
Authority: WO
Inventors: Yves Aurelle; Henri Roques; Michel Roustan
Original assignee: Institut National Des Sciences Appliquees De Toulo
Priority date: 1986-09-30
Filing date: 1987-09-23
Publication date: 1988-04-07
Also published as: FR2604371A1; FR2604371B1

Abstract

Gas/liquid exchange device and method wherein the gas is dispersed into the liquid in the form of bubbles. The method comprises a circulation of the liquid and the gas in a closed loop (1) and the tapping of a fraction of the liquid flow circulating in the loop through a partition wall (3) which is permeable to the liquid and capable of stopping the gas bubbles. The method and the device according to the invention may be applied particularly to perform the dissolving of a gas in a liquid, or a reaction between a gas and a liquid, particularly in the presence of a catalyst, or for the degassing of a liquid containing a dissolved gas.

Description

METHOD AND DEVICE FOR EXCHANGING BETWEEN A GAS AND A LIQUID

The invention relates to a method for carrying out an exchange between a gas and a liquid with a view to obtaining a transfer between these two phases such as dissolution of the gas in the liquid or reaction between gas and liquid; it extends to an implementation device.

Gas / liquid exchange devices can be classified into two main categories: devices in which the liquid is dispersed in the form of films or droplets in the gas phase (packed columns, spray columns ...) and devices in which the gas is dispersed in the form of bubbles in the liquid phase.

The invention relates to a device of the second type.

The major drawback of known devices of this type comes from the limited residence time of the gas bubbles in the liquid phase, which leads to limited transfers and to gas losses. For example, in a free-rise bubble aeration tank 4 meters high, the bubbles only stay on average for 16 seconds in the water and, therefore, the oxygen transfer efficiency is around only 15 to 20%.

An improved device designated by

"bicone" is currently manufactured by the company "L'AIR

LIQUID ". This device comprises an enclosure formed by two cones joined by their base, which is crossed by the liquid in the descending direction, the bubbles being carried in the same direction by the liquid circulation; the shape of the enclosure leads to a decrease in the downward circulation speed, so that only very small gas bubbles are entrained at the base of the enclosure, which considerably limits gas losses. The transfer yields of such devices can be of the order from 80 to

90%. However, in these devices, the efficiency is directly linked to the volume of the enclosure and a major defect in these lies in their size. In addition, these devices do not lend themselves to three-phase operation and are in particular ill-suited to gas / liquid transfer reactions requiring the presence of a solid catalyst (or reagent). Furthermore, patent GB 2,072,027 describes a device for dissolving oxygen in an aqueous phase, in which an aerator ensures bubbling of the aqueous phase, part of the aqueous phase being recycled in an open loop to improve the yield of transfer; However, the latter, although superior to that of traditional aeration tanks, remains mediocre due in particular to gas losses, the impossibility of working under pressure ... In addition, it is very difficult to obtain with this device a given precise concentration of dissolved gas or in the event of dissolution of a gas mixture, to obtain a given precise composition of the dissolved gas.

The present invention proposes to indicate an improved method for ensuring an exchange between a gas and a liquid, and to provide a new gas / liquid exchange device, which benefits from improved transfer yields, not limited by volume and l size of the device.

An object of the invention is thus to provide a device of reduced size compared to existing bicones, capable of achieving transfer yields close to 100%.

Another objective is to make it possible to dissolve a gas in a liquid by very precisely obtaining a desired concentration of dissolved gas, or even to dissolve a mixture of gases in a liquid with the possibility of precisely regulating the concentration and the composition dissolved gases.

Another objective is to provide a device benefiting from a great flexibility making it possible to adapt the conditions of implementation to the envisaged applications.

To this end, the gas / liquid exchange process targeted by the invention with a view to producing the dissolution of a gas in a liquid or a reaction between gas and liquid is of the type in which the gas is dispersed in the liquid under bubble shape; in accordance with the invention, this method consists in admitting the liquid and the gas into a sealed closed loop with a determined inlet flow rate, in circulating the liquid in said sealed closed loop at a speed greater than the limit ascending speed gas bubbles in the liquid and to be derived from the continuous circulation a fraction of the flow of liquid flowing in the loop through a separating wall permeable to liquid and capable of stopping the gas bubbles, so as to reach a permanent regime where the flow d he entry of liquid into the loop is equal to the flow evacuated by the bypass.

The implementation device essentially comprises a closed loop for the circulation of the liquid and the gas, an arrival of the liquid phase in said sealed closed loop, an arrival of the gas phase in said sealed closed loop, means for dispersing the phase gas in the form of bubbles in the liquid phase, means for circulating the liquid in the loop, an extractor provided with a separating wall permeable to liquid and capable of stopping the gas bubbles, said extractor being mounted on the loop so in diverting through the separating wall a fraction of the flow of liquid circulating in the loop, and means for discharging the liquid flowing in the extractor through the separating wall.

By "gas" or "liquid" is meant both pure gases or liquids as mixtures of gases or liquids.

By "limit ascent speed" is meant the highest rate of rise of the bubbles present in a liquid supposed to be immobile.

Thus, in the device of the invention, the gas bubbles contained in the closed circulation loop are permanently trapped in it by the separating wall of the extractor, which derives a fraction of the liquid flow circulating in the loop and allows its evacuation in the almost total absence of gas bubbles. In steady state, the flow of liquid thus discharged is equal to the inlet flow, the gas remaining in the loop until a total transfer is obtained. Means for dispersing the gas phase can be constituted by specific means (rotary agitator ...) or by pumping means also fulfilling the role of means for circulating the liquid or also by hydro-injection means also fulfilling the role of means of introduction of the gas phase.

In the case of dissolution, the transfer efficiency achieved by using the device can reach values very close to 100%: it is thus possible to obtain, at the outlet of the extractor, a liquid containing a concentration of predetermined dissolved gas, simply by providing a flow rate of inlet gas corresponding to this concentration (insofar as this concentration is below the saturation limit of the liquid).

In the case of a gas / liquid reaction, a catalyst or a solid reagent can if necessary be placed in the closed loop either in suspension in the liquid, or in a fixed bed. It should be noted that, if the reaction leads to a precipitate, the latter remains trapped in the closed loop and can be purged sequentially or continuously.

Furthermore, to allow a total or almost total stop of the gas bubbles, the separator wall of the extractor is chosen from a material preferably wetted by the liquid and is adapted to introduce a pressure drop below the capillary pressure of the bubbles. gas with respect to said wall in the presence of the liquid. In addition, a wall will be used (constituted in particular by a porous membrane or a grid) whose pores or meshes are of diameter less than 100 micrometers, much less than the average diameter of the gas bubbles (generally of the order of 1 to 3 millimeters).

(By "material preferably wetted by the liquid" is meant a material preferably wetted by the liquid with respect to the gas bubbles placed in said liquid, that is to say, according to the current definition, a material with which, in the presence liquid, a gas bubble forms a contact angle greater than 90 °).

The stéiique characteristics, of yes yes Ability and pressure drop defined above "make the separating wall capable of ensuring, at the level of the extractor, an extremely effective separation of the gaseous phase with respect to the fraction derived from the liquid phase: the percentage of gas entrained in the derived liquid flow is thus negligible, the transfer efficiency becoming substantially equal to 100%.

In addition, the separating wall of the extractor is advantageously arranged in a tangential position relative to the loop so that the main flow of said loop flows in a general direction substantially parallel to said separating wall and that the flow diverted through the latter flows in a general direction substantially orthogonal to the former. The gas bubbles are thus entrained parallel to the separating wall with a low probability of contact with the latter, a probability the lower the higher the circulation speed in the loop.

According to another feature of the invention, the device comprises means for measuring and adjusting the pressure inside the circulation loop. These means make it possible to fix the pressure of the loop at the optimal value depending on the application concerned; for example, in the case of dissolution, an increase in the pressure of the loop increases the kinetics of dissolution and makes it possible to reduce the risks of gas accumulation in the loop.

Other characteristics, objects and advantages of the invention will emerge from the description which follows with reference to the appended drawings, which present by way of nonlimiting example an embodiment; on these drawings which form an integral part of this description:

FIG. 1 is a view in axial section through a vertical plane of a gas / liquid exchange device, with a view to dissolving a gas in a liquid,

- Figure 2 is a schematic detail section of this device, illustrating its operation.

The exchange device shown by way of example in FIGS. 1 and 2 is intended for dissolving a gas in a liquid and comprises a sealed tight loop 1, formed by a conduit of circular cross section; this loop is composed in the example by two vertical sections, 1a and 1b, connected by two sections, upper and lower, 1e and 1d.

At the vertical section 1a and upwards, an extractor 2 is arranged formed by a separating wall 3 and by an annular enclosure 4. The separating wall 3 is in the example constituted by a cylindrical grid which replaces on a predetermined height the sealed wall of the loop. In the examples given below, said grid was made of stainless steel and its square-shaped meshes had an equivalent diameter of the order of 64 micrometers (diameter of the circular section with the same perimeter).

In the tests carried out, the liquid consisted of water and the gas consisted of CO ₂ or air. Stainless steel is preferably wetted with water, that is to say that the contact angle Θ of the gas bubbles with the grid in the presence of water is greater than 90 °, as illustrated in FIG. 2 (this angle is 165 ° for CO ₂ ): in this way, the grid develops tension forces which tend to oppose the passage of bubbles. The annular enclosure 4 has a cylindrical shape, with a diameter greater than that of the loop 1 and of the grid 3. It surrounds the latter so as to form a sealed annular chamber for collecting the fraction of liquid having passed through the grid in the centrifugal direction. An evacuation duct 5 makes it possible to evacuate the flow of liquid arriving in the enclosure 4.

This conduit 5 is provided with a valve 6 which makes it possible to adjust the liquid outlet pressure. When the other parameters of the device are fixed, this valve thus makes it possible to adjust the pressure prevailing inside the loop; a pressure gauge 7 provides a measurement of this pressure and can if necessary control the valve 6 (in this case constituted by a pneumatic valve or a solenoid valve) according to a set pressure. In addition, a differential pressure gauge 8 mounted between loop 1 and enclosure 2 of the extractor provides a measurement of the pressure difference between said loop and said enclosure. It is thus possible to control the value of the pressure drop across the grid, so as to verify that it remains below the capillary pressure Pc of the gas bubbles with respect to the grid. In the tests carried out, this pressure drop remained very low and, in all cases, much lower than the capillary pressure Pc. This condition prevents certain bubbles from passing through the grid, deforming and refracting by the "SNAP OFF" effect (despite the small mesh size compared to that of the bubbles and despite the preferable wettability of stainless steel over water regard).

It should be recalled that the capillary pressure Pc of the bubbles with respect to the grid in the presence of the liquid is expressed by the relation:

Pc =

where ɣ is the gas / liquid surface tension, θ is the contact angle of the gas bubbles with the grid in the presence of the liquid,

D is the average equivalent diameter of the grid meshes.

Furthermore, in its lower section Id, the closed loop 1 is equipped with a pump 9 which has the dual role, on the one hand, of generating a flow of circulation of the liquid in the loop, on the other hand, d '' ensure a dispersion of the gas in the form of bubbles (diameter of the order of 1 to 3 mm). In the tests, the pump used was a centrifugal pump. Of course, it can be replaced by any equivalent means.

The characteristics of the pump are chosen so that it is capable of ensuring a flow rate much higher than the limit upward speed of bubbles in the liquid (approximately 0.3 m / s for CO ₂ and water), so that said bubbles are entrained by the liquid along the descending section 1b.

The liquid and gas are introduced upstream of the pump 9 (by means of the direction of circulation in the loop) by separate liquid 10 and gas 11 conduits. Flow meters 12 and 13 are mounted on these conduits.

The dissolution examples described below were used in a dissolution device of the type shown in FIGS. 1 and 2.

EXAMPLE 1

In this example, it is proposed to dissolve CO ₂ in water from the network (hydrotimetric degree: 15 ° TH).

The characteristics of the device were as follows:

. stainless steel grid with cross weave of 50 microns per side (equivalent diameter: 64 microns),

. total volume of the loop: 1.1 liters,. diameter of the loop section: 50 mm,

. grid diameter: 50 mm,. grid height: 140 mm,

. diameter of the annular extractor chamber: 160 mm,

. pump power: 80 watts. The implementation parameters were as follows:. water input flow: 10 liters / minute (equal to the output flow in steady state),

. circulation flow downstream of the pump: 50 liters / minute,. speed of circulation in the loop:

0.5 m / s,

. pressure in the loop: 2.5 bars,. differential pressure (pressure drop across the grid): negligible (less than 0.05 bar),. capillary pressure Pc in absolute value (calculated): 0.45 bar.

Several tests have been carried out by varying the gas inlet flow rate, which remains below the solubility limit of CO ₂ in water at 2.5 bars (4.6 g / l at 20 ° C) : 1st measurement:

Gas inlet flow: 4.7 g / min.,

Concentration of the liquid at the outlet of the extractor in steady state: 0.46 g / l. 2nd measurement:

Inlet flow: 2.4 g / min.,

Concentration of the liquid at the outlet: 0.24 g / l. 3rd measurement: Inlet flow: 7 g / min.,

Concentration of the liquid at the outlet: 0.68 g / l.

In all cases, once the steady state is reached, the liquid at the outlet contains in dissolved form almost all of the gas injected at the inlet. The dissolution yield is therefore substantially 100%. It should be noted that the steady state is reached all the more quickly as the desired gas concentration is further from the solubility limit. By increasing the operating pressure inside the loop, the solubility limit and therefore the difference between it and the concentration is increased, which conditions increased transfer kinetics.

It turned out that the stainless steel grid formed a remarkable barrier opposing the passage of gas bubbles mainly due to the respect:

. steric condition (equivalent mesh diameter -64 microns- much less than the average bubble diameter -1 to 3 mm-),

. the wettability condition (preferable wettability of the liquid: contact angle of the gas bubbles much greater than 90 ° - of the order of 150 ° -),

. the pressure drop condition (pressure drop less than 0.05 bar- much lower than the capillary pressure Pc -0.45 bar-). It should be noted that the effectiveness of this barrier effect with respect to the bubbles also comes from the displacement of the bubbles which takes place tangentially with respect to the grill, which reduces the probabilities of bubble / grid meeting, and this d 'especially as the speed of circulation is higher.

EXAMPLE 2 This example illustrates one of the advantages of the device which is to allow, under steady conditions, to dissolve in the liquid a gaseous mixture of composition strictly identical to that of the gaseous mixture introduced at the inlet (since of course the limit solubility of one of the gaseous components is not reached). On the contrary, in all known devices, the composition of the dissolved gas differs from that of the injected gases (due to the different value of the Henry's constants of these gases). The device of the invention therefore makes it possible to very precisely adjust the composition and the concentration of the dissolved gas by a simple adjustment of the composition and of the concentration of the inlet gases.

The inlet gas in this example was air containing 21% oxygen and 78% nitrogen, the liquid being as before water.

The characteristics of the device were the same as before, with the exception of the pressure in the loop adjusted to 4 bars (limit of solubility of oxygen at this pressure: 37 mg / 1 and of nitrogen: 60 mg / l at 2.0 ° C).

All the tests carried out (well below the solubility limit) made it possible, after obtaining the steady state, to find in dissolved form, the composition of the air admitted to the inlet. The analyzes of the outlet liquid were carried out for oxygen by the "Winkler" method and, for nitrogen, by measurement of the volume of degassed air.

Claims

1/ - Process for carrying out an exchange between a liquid and a gas, such as dissolution of the gas in the liquid or reaction between the gas and the liquid, in which the gas is dispersed in the liquid in the form of bubbles, characterized in that the liquid and the gas are admitted into a sealed closed loop with a determined inlet flow rate, the liquid is circulated in said sealed closed loop at a speed greater than the limit upward speed of the gas bubbles in the liquid and a fraction of the flow rate of liquid circulating in the loop is derived from the continuous circulation through a separating wall permeable to the liquid and capable of stopping the gas bubbles, so as to reach a permanent regime where the flow rate of liquid entry into the loop is equal to the flow discharged by the diversion.

2/ - Gas/liquid exchange device making it possible to implement the method according to claim 1, characterized in that it comprises in combination a sealed closed loop (1) for circulating the liquid and the gas, an inlet ( 10) of the liquid phase in said sealed closed loop, an arrival (11) of the gas phase in said sealed closed loop, means of dispersing (9) the gas phase in the form of bubbles in the liquid phase, means of circulating the liquid in the loop (9), an extractor (2) provided with a separating wall (3) permeable to the liquid and capable of stopping the gas bubbles, said extractor being mounted on the loop (1) so as to diverting through the separating wall (3) a fraction of the flow rate of liquid circulating in the loop, and means (5) for evacuating the liquid flowing into the extractor through the separating wall.

3/ - Exchange device according to claim 2 with a view to carrying out a reaction between a gas and a liquid, characterized in that a catalyst is placed in solid form in the circulation loop (1).

4/ - Device according to claim 2, characterized in that the extractor (2) comprises a separating wall (3) made of a material preferably wetted by the liquid, adapted to introduce a pressure drop lower than the capillary pressure of the gas bubbles with respect to said separating wall in the presence of the liquid.

5/ - Device according to claim 4, characterized in that the separating wall (3) of the extractor is constituted by a porous membrane or a grid, having pores or meshes of dimensions less than 100 micrometers.

6/ - Device according to claim 2, characterized in that it comprises means (8) for measuring the pressure difference between the circulation loop and the extractor (downstream of the separating wall).

7/ - Device according to claim 2, characterized in that it comprises means (6, 7) for measuring and adjusting the pressure inside the circulation loop (1).

8/ - Device according to claim 7, characterized in that the means for measuring and adjusting the pressure inside the loop comprise a pressure gauge

(7) associated with said loop and a valve (6) mounted on the means (5) for evacuating the liquid from the extractor.

9/ - Device according to claim 2, characterized in that the separating wall (3) of the extractor is arranged in a tangential position relative to the loop (1) so that the main flow of said loop flows in a general direction substantially parallel to said dividing wall and that the flow derived through the latter flows in a general direction substantially orthogonal to the first.

10/ - Device according to claim 9, characterized in that the extractor (2) comprises an annular enclosure (4) surrounding a section (la) of the circulation loop, said section being at least partially delimited by the dividing wall ( 3).

11/ - Device according to claim 10, characterized in that:

. the circulation loop (1) has a circular cross section,. the dividing wall (3) is cylindrical, the annular enclosure (4) of the extractor is cylindrical in shape, with a diameter greater than that of the circulation loop. 12/ - Device according to claim 2, characterized in that the circulation loop (1) comprises two vertical sections (1a, 1b), connected by upper (1c) and lower (1d) sections, the extractor (2) being located at one of the vertical sections (1a).