EP0165161B1 - Method and plant for freezing soils by means of a cryogenic liquid - Google Patents

Abstract

The injection of liquid nitrogen into the probe (2) is regulated in an on-off manner so as to maintain the temperature of the ground at the coldest point above a value within the range of between -140 DEG C. and -160 DEG C. so as to cause calefaction of the liquid nitrogen. The liquid nitrogen is alternately injected into the central passage (9) and into the annular passage (11) of the probe, these passages being thermally insulated from each other.

Description

The present invention relates to the technique of freezing soils by injecting a cryogenic liquid, in particular liquid nitrogen, into at least one freezing probe embedded in the soil and comprising a central passage and an annular passage.

We know that consolidation of soils by freezing allows the opening of public works sites in damp and unstable soils. It is performed by injecting a coolant into probes introduced from place to place in the ground. This cooling freezes the soil step by step, until forming a continuous wall when the freezing zones of each probe have joined their neighbors.

It is known to inject a cryogenic liquid such as liquid nitrogen into the probes (see for example FR-A-2 113 980 or FR-A-2 041 356). This direct injection of liquid nitrogen has several drawbacks, in particular the difficulty of controlling the heat exchange coefficients with the soil. Indeed, by yielding from the cold, the nitrogen vaporizes, and the exchange coefficients between the probe and the pure liquid nitrogen first, then the liquid and gas mixtures in variable proportions, then the cooled gas alone are very different. This results in a large heterogeneity in the thickness of the frozen soil around the probe and a loss of time and energy so that the less frozen areas join to form the consolidated wall, while the most frozen areas are unnecessarily sub-cooled and oversized. In addition, the soils being generally heterogeneous, certain parts freeze faster than others; this heterogeneity of the soil further increases the extent of unnecessarily frozen and sub-cooled areas.

The invention aims to make the process more economical by ensuring more homogeneous cooling of the soil.

To this end, the subject of the invention is a process for freezing soil by injecting a cryogenic liquid, in particular liquid nitrogen, into at least one freezing probe embedded in the soil and comprising a central passage and an annular passage , characterized in that the injection of the cryogenic liquid is regulated so as to maintain the temperature of the soil in the vicinity of the probe, over the entire length of the latter, above a predetermined limit value, this limit value being at least 35 ° C above the boiling point of the cryogenic liquid, and alternating between periods of injection of the cryogenic liquid into the central passage and periods of injection of the cryogenic liquid into the annular passage.

Thus, it is guaranteed that the temperature difference between the wall of the probe and the cryogenic liquid is sufficient to permanently cause the curing of the cryogenic liquid in contact with this wall. As a result, heat exchange between the liquid and the wall of the probe always takes place via a gaseous heat-transfer layer. This avoids the excessive transfer of cold to the wall that would cause the frank liquid-wall contact if the temperature of the wall of the probe dropped too close to the boiling point of the liquid. These conditions, which are usually encountered in the processes of the prior art, allow a very rapid cold transfer but which can only be ensured over a very limited probe height. It also guarantees the absence of strong temperature gradients between the bottom and the end of the probes.

In particular, when the cryogenic liquid is liquid nitrogen, said limit value is preferably between -140 ° C and -160 ^Q C approximately.

The invention also relates to a ground freezing installation intended for the implementation of a method as defined above. This installation, of the type comprising at least one freezing probe comprising a central passage and an annular passage, and means for injecting into this probe a cryogenic liquid such as liquid nitrogen, is characterized in that the probe comprises at least one temperature sensor on its outer wall, in the vicinity of each of its ends, and in that said injection means are suitable for injecting the cryogenic liquid either in the central passage or in the annular passage of the probe.

• - la figure 1 est un schéma d'une partie d'une installation de congélation conforme à l'invention; et
• - la figure 2 est un diagramme qui illustre le fonctionnement de cette installation.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:

• - Figure 1 is a diagram of part of a freezing installation according to the invention; and
• - Figure 2 is a diagram which illustrates the operation of this installation.

The freezing installation shown in FIG. 1 essentially comprises a tank 1 for storing liquid nitrogen and a series of freezing probes 2, only one of which has been shown and which are all identical.

The probe 2, assumed to be driven vertically into the ground, comprises three concentric tubes 3 to 5. The outer tube 3 is closed at its lower end by a bottom 6 and defines with this latter the heat exchange surface of the probe with the ground surrounding 7. The intermediate 4 and inner 5 tubes extend from the upper orifice of the probe to a short distance from the bottom 6, and they are connected at this location by a horizontal annular wall 8.

• L'installation comporte également des moyens pour injecter de l'azote liquide dans les passages 9 et 11. Ces moyens d'injection comprennent deux conduites 12 et 13 débouchant respectivement dans ces deux passages, reliées à la partie inférieure du réservoir 1 et équipées de vannes d'arrêt respectives 14 et 15. Une vanne commune 16 d'arrêt et de réglage de débit est également prévue à la sortie du réservoir.

L'extrémité supérieure des passages 9 et 11 est munie par ailleurs de moyens d'évacuation de gaz, schématisés par des conduites respectives 17, 18 équipées de vannes d'arrêt respectives 19, 20.Thus are delimited in the probe 2: a central passage 9 defined by the inner tube 5 and opening onto the bottom 6; an intermediate annular space 10, delimited by the tubes 4 and 5 and the wall 8, which can be filled with a thermally insulating material such as perlite; and an outer annular passage 11 delimited between the tubes 3 and 4 and opening onto the bottom 6.

• The installation also includes means for injecting liquid nitrogen into the passages 9 and 11. These injection means comprise two pipes 12 and 13 opening respectively into these two passages, connected to the part bottom of the tank 1 and equipped with respective shut-off valves 14 and 15. A common shut-off and flow control valve 16 is also provided at the outlet of the tank.

The upper end of the passages 9 and 11 is also provided with gas discharge means, shown diagrammatically by respective pipes 17, 18 equipped with respective stop valves 19, 20.

The valves 14, 15, 19 and 20 are provided with a simultaneous actuation device with two positions (not shown). In one position, the valves 14 and 20 are open while the valves 15 and 19 are closed; the other position reverses the state of the four valves. The valve 16 makes it possible to interrupt and restore the flow of liquid nitrogen.

On the outer face of the outer tube 3, the probe 2 carries three temperature sensors 21, constituted for example by thermocouples, which are adapted to measure the temperature of the ground in the immediate vicinity of the probe, at depths of 2 m, 10 m and 18 m respectively (temperatures T ₂ , T ₁₀ and T ₁₈ respectively). There is also a temperature sensor connected to each of the lines 17 and 18 and adapted to measure the temperature T _G of the nitrogen gas leaving the probe.

In fact, the installation comprises a series of probes 2 arranged along a line which defines the frozen wall to be produced. All the probes which are connected in parallel to the tank 1 in the manner described above, each probe comprising its own set of valves 14, 15, 16, 19 and 20.

• - d'une part, l'admission d'azote liquide est régulée en tout ou rien, au moyen de la vanne 16, de manière que la température du sol au point le plus froid reste en permanence comprise entre deux valeurs limites prédéterminées, à partir des indications fournies par les capteurs de température 21 de la sonde considérée. Comme expliqué plus haut, la limite inférieure est au moins égale à
• -160°C afin d'assurer la présence permanente d'une couche gazeuse de caléfaction entre l'azote liquide et le tube 3, de sorte que la totalité de l'échange thermique entre l'azote et la sonde est un échange gaz-métal.
• - d'autre part, pendant les périodes d'injection de l'azote liquide, une fois terminée la phase transitoire de mise en froid, le débit de liquide est réglé, au moyen de la vanne 16, de manière que le gaz résultant de la vaporisation de l'azote liquide ait à sa sortie de la sonde une température voisine d'une température de consigne prédéterminée choisie de manière à optimiser l'utilisation de frigories. Une température de consigne de l'ordre de
• -70 °C s'est révélée satisfaisante.

The operation of this installation, illustrated in FIG. 2, is based on a double regulation of the injection of liquid nitrogen into the probes, each probe being regulated independently of the others to take account of the heterogeneity of the terrain:

• on the one hand, the admission of liquid nitrogen is regulated in all or nothing, by means of the valve 16, so that the temperature of the soil at the coldest point remains permanently between two predetermined limit values, at from the indications provided by the temperature sensors 21 of the probe in question. As explained above, the lower limit is at least equal to
• -160 ° C in order to ensure the permanent presence of a gaseous caulking layer between the liquid nitrogen and the tube 3, so that the entire heat exchange between the nitrogen and the probe is a gas exchange- metal.
• - on the other hand, during the periods of injection of liquid nitrogen, once the transient cooling phase has ended, the liquid flow rate is adjusted, by means of the valve 16, so that the gas resulting from the vaporization of liquid nitrogen has at its exit from the probe a temperature close to a predetermined set temperature chosen so as to optimize the use of frigories. A set temperature of around
• -70 ° C has been found satisfactory.

This way of proceeding clearly improves the homogeneity of the cooling of the soil compared to the case where, according to the prior art, we are content to regulate the flow of liquid nitrogen as a function of the temperature of the gas leaving the probe. However, it cannot be avoided by the above regulations alone to obtain a strong temperature gradient between the bottom and the top of the probes. To largely eliminate this heterogeneity, the valves 14, 15, 19 and 20 of all the probes are periodically reversed.

Thus, assuming that we started by injecting liquid nitrogen for a few hours through the central passage 9 while evacuating the gaseous nitrogen through the peripheral passage 11, the inversion results in that liquid nitrogen is injected into the passage 11 while the nitrogen gas is evacuated by the central passage 9.

In this way, a new inverted equilibrium is reached after a few hours in which the coldest point, regulated between the two aforementioned limit temperatures, is located at the top of the probe and the least cold point at the bottom. It is understood that by periodically reversing the nitrogen streams, good thermal uniformity is obtained in the soil.

The following numerical example will illustrate the process described above: We wish to consolidate as quickly as possible by freezing a wall 1 m thick in moist sandy soil to a depth of 20 m and a width of 50 m. To do this, fifty probes 2 are inserted into the ground 2 spaced 1 m apart.

Each probe has an outside diameter of 150 mm and is composed of three concentric tubes 3 to 5 with respective diameters 150, 128 and 68 mm. The central annular space between the 128 mm tube 4 and the 68 mm tube 5 is filled with perlite. The diameters are chosen so that the free cross section of the central passage 9 and that of the outer annular passage 11 are equal.

Freezing begins with the injection of liquid nitrogen into the central tubes 5. The temperatures in the vicinity of the probes are all initially of the order of 14 ° C. In each probe, the nitrogen vaporizes, yielding cold to the ground and rising through the outer annular passage. The liquid nitrogen flow rate D per probe is the maximum flow rate of 15 l / min (ie 750 l / min for the fifty probes), adjusted by means of the valves 16.

In the following, a numerical example will be described, with reference to FIG. 2, the behavior of a single probe, it being understood that each of the probes is regulated independently of the others. The limit values for the coldest soil temperature are chosen at -145 ° C and -138 ° C, and the nitrogen gas output setpoint at -70 ° C.

The temperature of the nitrogen gas leaving the probe drops during the first hour from -10 ° C to -70 ° C. Meanwhile, the outside temperatures reach -140 ° C at 18 m, -100 ° C at 10 m and -62 ° C at 2 m deep. As the transient cooling phenomena are still felt, the injection of liquid nitrogen is continued with the same flow rate for an additional 10 mm, during which the temperature of nitrogen gas outlet reaches -78 ° C and the outside temperature at 18 m reaches -145 ° C.

The injection of liquid nitrogen is then cut off. In 10 minutes, the outside temperature at 18 m rises to -138 ° C. A flow rate of 10 l / min of liquid nitrogen is restored. This flow rate is lower than previously since the cold temperature of -145 ° C had been reached only for a too low temperature of the nitrogen gas leaving the probe.

In 20 minutes, the outside temperature at 18 m drops again to -145 ° C and the gas outlet temperature at -75 ° C. The nitrogen flow is again cut off, then it is restored to 8 I / min (for the same reason as above) when the temperature at 18 m rises to -138 ° C.

This alternation of injections and stops is continued by reducing the flow rate as long as the temperature of the gases drops below -70 ° C. The flow rate is kept constant when this temperature stabilizes in a range of -68 ° C to -72 ° C it is increased if this temperature rises above -68 ° C, and decreased if it drops below -72 ° C.

After 5 hours of freezing, the flow rates of all the probes are reversed by injecting liquid nitrogen through the external annular passage 11, the gas emerging through the central passage 9.

The flow rate is then fixed at 8 I / min. The gas initially comes out very cold at -120 ° C; it is a transient regime which comes from the passage of nitrogen against the current of the soil temperature. After 10 min, the gas outlet temperature rises to -70 ° C and the external temperatures are: -100 ° C to 2m, -100 ° C to 10m, -65 ° C to 18m; 20 min later, the external temperature at 2 m is at -145 ° C and the gas comes out at -75 ° C.

The nitrogen flow is cut off, and it is restored to 7 l / min when the temperature at 2 m rises to -138 ° C., which occurs after about 5 min.

This succession of injections and stops is continued for 5 hours, possibly modifying the flow of liquid nitrogen as indicated above. The flows are again reversed at the end of this period.

The regulation thus defined is continued by reversing the flow rates every 5 hours. The soil is consolidated by freezing to a thickness of 1 m in around 50 hours, with very satisfactory temperature uniformity.

Claims (8)

1. Method for freezing a soil by injection of a cryogenic liquid, especially liquid nitrogen, in at least one freezing probe (2) driven in the soil and comprising a central passage and an annular passage, characterized in that one controls the injection of the cryogenic liquid so that the temperature of the soil in vicinity of the probe along the entire length thereof is maintained above a predetermined limiting value, said limiting value being about at least 35 °C above the boilina temperature of the cryogenic liquid, and that one alternates periods of injection of the cryogenic liquid in the central passage (9) and periods of injection of the cryogenic liquid in the annular passage (11).

2.. Method according to claim 1 in which the cryogenic liquid is liquid nitrogen, characterized in that the said limiting value is between about -140 °C and -160 °C.

3. Method according to one of the claims 1 and 2, characterized in that one interrupts the injection of the cryogenic liquid, when the said limiting value is reached, and that one reestablishes it, when the temperature of the soil at the coldest point is risen until a second limiting value.

4. Method according to one of the claims 1 to 3, characterized in that one controls the throughput of cryogenic liquide during the periods of injection so that a temperature of the gas (T_G) at the exit of the probe near the predetermined value is obtained.

5. Method according to claim 4 in which the cryogenic liquid is liquid nitrogen, characterized in that the said predetermined value is about -70 °C.

6. Apparatus for freezing a soil for carrying out the method according to claim 1, of the type comprising at least one freezing probe (2) having a central passage and an annular passage and means (12 to 15) for injection of a cryogenic liquid, such as liquid nitrogen, into the said probe, characterized in that the probe (2) comprises at least one temperature detector (21 ) on its exterior wall (3) in vicinity of each of its extremities, and that the means for injection (12 to 15) are adapted to inject the cryogenic liquid either in the central passage (9) or in the annular passage (11) of the probe.

7. Apparatus according to claim 6, characterized in that the probe (2) comprises also a detector of the temperature (T_G) of the gas leaving the probe.

8. Apparatus according to one of the claims 6 and 7, characterized in that the probe (2) comprises means of thermal insulation (10) interposed between the central passage (9) and the annular passage (11).

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