EP0567360B1 - Process and installation for the transfer of a liquid - Google Patents

Description

The present invention relates to a method for transferring a liquid, via a riser fitted with an expansion valve, from a first distillation column operating at a first pressure, typically relatively high, to equipment, in particular to a second distillation column, operating at a second pressure, typically relatively low, lower than the first pressure.

It applies in particular to the ascent of liquids from the medium pressure column of a double air distillation column, in the direction of the low pressure column, overcoming this medium pressure column, of the double column, and / or the head condenser of an oxygen / argon separation column coupled to the low pressure column.

Apparatuses for the separation of gases from air by cryogenic distillation most often use the conventional double column scheme. The liquids produced in the tank (oxygen-rich liquid), in the intermediate part (lean liquid) and at the top (liquid nitrogen) of the medium pressure column (or MP column), are sent to an intermediate point or at the top of the bottom column pressure (or BP column). Most often, for economic reasons, the low pressure column is placed above the medium pressure column. Liquids must therefore be sent to a point higher than the point where they are taken. Conventionally, the pressure difference between the medium pressure column and the low pressure column is greater than the hydrostatic pressure of the liquid column between the sampling point in the MP column and the arrival point of the LP column.

Recent developments in air distillation column technology have seen packing columns with low pressure drop on the one hand and vaporizers-condensers with a small temperature difference between the two fluids on the other. linked to heat exchange (nitrogen gas and liquid oxygen). These two improvements go in the direction of a reduction in the operating pressure of the medium pressure column, with a view to reducing the expenditure of energy, and also of an increase in height. columns, the economic optimum moving towards further distillation.

On the other hand, an unfavorable consequence of this development lies in an increased difficulty of raising the liquids by simple hydrostatic effect. In fact, in certain cases, the pressure available in the column MP is no longer sufficient to bring the liquids up to the column BP, particularly when the apparatus must be able to operate also in reduced mode, that is to say with column pressure MP lower than at nominal speed.

The state of the art makes it possible to solve this problem by using pumps which deliver liquids at sufficient pressures. The disadvantages are obvious: energy cost, investment cost, degraded reliability of the device, greater operating complexity, etc.

The invention aims to allow, in a simple and effective manner, a safe transfer of liquids, without the use of a pump.

To this end, the process according to the invention is characterized in that a lightening gas available at a pressure higher than the pressure created by a column of said liquid is injected into the riser, downstream of the expansion valve. between the point of injection of the gas and the point of introduction of the liquid into said equipment.

• le gaz d'allègement est disponible à la pression de ladite première colonne de distillation et est injecté dans le liquide dans la colonne montante, au-dessus du point de soutirage de ce liquide ;
• on utilise comme gaz d'allègement un gaz soutiré en un point de la première colonne et choisi de façon à ne pas modifier substantiellement la composition du liquide transféré ;
• pour le transfert du liquide de la cuve de la colonne moyenne pression d'une double colonne de distillation d'air à un point intermédiaire de la colonne basse pression, surmontant cette colonne moyenne pression, de la double colonne, et/ou au condenseur de tête d'une colonne de séparation oxygène/argon couplée à la colonne basse pression, on utilise un faible débit d'air entrant comme gaz d'allègement.

According to particular embodiments of the invention:

• the lightening gas is available at the pressure of said first distillation column and is injected into the liquid in the riser, above the point of withdrawal of this liquid;
• a gas withdrawn at a point in the first column and chosen so as not to substantially modify the composition of the transferred liquid is used as the lightening gas;
• for transferring the liquid from the tank of the medium pressure column from a double air distillation column to an intermediate point of the low pressure column, surmounting this medium pressure column, from the double column, and / or to the condenser of head of an oxygen / argon separation column coupled to the low pressure column, a low flow of incoming air is used as the lightening gas.

According to a second aspect, the subject of the invention is a process for transferring a liquid, via a rising pipe fitted with an expansion valve, from a first distillation column operating at a first pressure to equipment, in particular a second distillation column, operating at a second pressure, lower than the first pressure, characterized in that the liquid is sub-cooled before its expansion, with the exception of a minority fraction of this liquid, so as to produce a controlled quantity of flash gas, this flash gas is injected downstream or upstream of the expansion valve and serves as liquid lightening gas.

The invention also relates to a distillation installation intended for the implementation of such a process. According to a first aspect, this installation, of the type comprising a first distillation column operating at a first pressure, equipment, in particular a second distillation column, operating at a second pressure, lower than the first pressure, and a riser pipe equipped with '' an expansion valve and connecting a point of withdrawal of liquid from the first column to a point of introduction of liquid into said equipment, is characterized in that it comprises means for injecting into the riser, downstream of the expansion valve, a lightening gas available at a pressure higher than the pressure created by a column of said liquid between the point of injection of the gas and the point of introduction of the liquid into said equipment.

According to a second aspect, the installation according to the invention, of the type comprising a first distillation column operating at a relatively high pressure, equipment, in particular a second distillation column, operating at a relatively low pressure, and a riser fitted an expansion valve and connecting a point of withdrawal of liquid from the first column to a point of introduction of liquid from said equipment, is characterized in that the riser pipe passes through a sub-cooler upstream of the expansion valve and is provided with a bypass of this subcooler.

• la figure 1 représente schématiquement une installation de distillation d'air conforme à l'invention ; et
• les figures 2 et 3 illustrent deux variantes de l'invention.

Examples of implementation of the invention will now be described with reference to the attached drawing, in which:

• Figure 1 schematically shows an installation of air distillation according to the invention; and
• Figures 2 and 3 illustrate two variants of the invention.

The air distillation installation represented in FIG. 1 essentially comprises a double distillation column 1. This comprises a medium pressure column 2 surmounted by a low pressure column 3. A vaporizer-condenser 4 puts in relation to heat exchange the top vapor of column 2, consisting of practically pure nitrogen, and the bottom liquid of column 3, consisting of oxygen at a determined purity.

In operation, air at a pressure typically of 5 x 10⁵ to 6 x 10⁵ Pa is introduced into the tank of column 2 via a supply line 5. "Rich liquid" (air enriched in oxygen) is withdrawn in tank of this column 2 via a pipe 6 equipped with an expansion valve 7, sub-cooled in a sub-cooler 8 upstream of this expansion valve, expanded in the latter at a pressure slightly higher than atmospheric pressure, and introduced at an intermediate point of the LP column 3. Between the subcooler 8 and the expansion valve 7 is a rising pipe 9 fitted with an expansion valve 10 and leading to the head condenser of an oxygen separation column / argon (not shown) coupled to the BP 3 column in a conventional manner.

"Lean liquid" (impure nitrogen) is drawn off at an intermediate point in column 2 via a line 11 fitted with an expansion valve (not shown) and, after sub-cooling and expansion, is introduced at an intermediate point of column 3. Almost pure liquid nitrogen is drawn off at the top of column 3 via a line 12 fitted with an expansion valve 13, sub-cooled in a sub-cooler 14 upstream of this expansion valve, expanded in the latter and introduced at the top of column 3.

FIG. 1 also shows pipes 15 for the production of gaseous oxygen leaving the tank of column 3, 16 for the production of pure nitrogen, starting from the top of this column 3, 17 for blowing air in. an intermediate point in column 3, and 18 for discharging residual gas (impure nitrogen) from the upper part of this column.

It can be seen that three different liquids must be reassembled from the lower column 2 to the upper column 3 and that one liquid must be reassembled at the top of the oxygen / argon separation column. If these columns are of the packed type, in particular structured, and / or have many theoretical plates, and / or if the vaporizer / condenser 4 is of a type with small temperature difference, it may happen that the pressure difference between the two columns 2 and 3 is barely sufficient to ensure these liquid rises.

To guarantee a good rise of the liquids, in a regular and controlled manner, in all the operating modes of the installation, an air line 19 is pricked on the supply line 5 and is divided into two branches 20, 21. Each of these branches is equipped with an expansion valve 22, 23 and joins the lines 6 and 9 respectively just downstream of their expansion valves 7 and 10. Similarly, a gas line 24 equipped with an expansion valve 25 starts from the top of column 2 and joins line 12 just downstream of the expansion valve 13. Another gas line 26, equipped with an expansion valve (not shown), starts from a location in the column 2 next to the point of withdrawal of the lean liquid (line 11) and joins this line 11 just downstream of the expansion valve thereof.

In operation, a low flow of air conveyed, at the supply pressure of column 2, through line 19, 20, is expanded in the expansion valve 22 and injected into the rich liquid which has just been expanded in the expansion valve 7. The air bubbles lighten the rich liquid and reduce the pressure necessary to make it rise up to column 2.

For the same purpose, a low air flow conveyed by the pipe 19, 21 is expanded in the expansion valve 23 and injected into the rich liquid which has just been expanded in the expansion valve 10. The air flow total deflected by line 19 is low, typically less than 1% of the air flow entering the installation.

Likewise, practically pure nitrogen conveyed by the pipe 24 is expanded in the expansion valve 25 and injected into the liquid nitrogen which has just been expanded in the expansion valve 13, and impure nitrogen conveyed by line 26 is, after expansion, injected into the lean liquid conveyed by line 11 and expanded.

It should be noted that, due to the purity of the liquid nitrogen conveyed through line 12, the corresponding lightening gas (in line 24) must be practically pure nitrogen. On the other hand, the compositions of the rich liquid and of the lean liquid are not critical, so that the corresponding lightening gases may have compositions slightly different from these liquids, provided that they do not pollute them, all the more that the flow rate of these gases is very low.

In practice, the main expansion valves 7, 10 and 13 are placed as low as possible to guarantee their supply with clean liquid, and gas bubbles are introduced just downstream of these expansion valves to assist propulsion towards the top of the liquids in question. More precisely, the pressure of the lightening gases must be sufficient to overcome the height of liquid which overcomes the point of injection of the gas, and this pressure is obtained, in the example shown, thanks to the fact that each gas, which is available at the pressure of column 3, is injected above the point of withdrawal of the associated liquid.

Figures 2 and 3 illustrate, in the case of the rise of liquid nitrogen via line 12, two variants for obtaining the lightening gas. In these two variants, the pipe 24 and the expansion valve 25 are eliminated.

In the variant of FIG. 2, a controlled minority flow of liquid nitrogen conveyed by the pipe 12 by-passes the sub-cooler 14 via a by-pass pipe 24A equipped, preferably at its lowest point, with an expansion valve 25A and terminating downstream of the expansion valve 13.

The liquid thus derived, not being sub-cooled, produces by expansion, a relatively large and adjustable quantity of flash gas, which serves as lightening gas.

In the variant of FIG. 3, the expansion valve 25A is omitted, and there is provided, in line 12, a three-way valve 27 having an inlet connected to line 12 upstream of the subcooler 14, an outlet connected at the inlet of this subcooler and another outlet connected to the 24A bypass line.

In addition, this pipe 24A ends upstream of the expansion valve 13.

Thus, a controlled minority flow of liquid nitrogen is not sub-cooled, so that an adjustable amount of flash gas is produced upon expansion in the expansion valve 13 and serves as a lightening gas.

Of course, the variants according to Figures 2 and 3 also apply to the rise of other liquids.

It is understood that the variants of FIGS. 2 and 3, although based on the same idea as that of FIG. 1, are less effective in the sense that they make it possible to lighten the rising liquids while limiting the production of gases to a minimum. of flash, which is unfavorable to distillation, but that they do not allow the restarting of the installation in the event of accidental blockage of the risers.

Claims (10)

1. Method for transferring a liquid, via an ascending pipe (6, 9, 11, 12) equipped with an expansion valve (7, 10, 13), from a first distillation column (2), operating at a first pressure, to an item of equipment (3), notably a second distillation column, operating at a pressure lower than the first pressure, characterised in that, into the ascending pipe, downstream of the expansion valve, there is admitted a lifting gas available at a pressure higher than the pressure created by a column of the liquid between the gas admission point and the point of introduction of the liquid into the said item of equipment (3).
2. Method according to Claim 1, characterised in that the lifting gas is available at the pressure of the said first distillation column (2) and is admitted into the liquid in the ascending pipe above the point at which this liquid is drawn off.
3. Method according to Claim 2, characterised in that a gas drawn off at a point in the first column (2) and chosen so as not to substantially modify the composition of the transferred liquid is used as the lifting gas.
4. Method according to Claim 2, for the transfer of the kettle liquid from a medium-pressure column (2) in a double air distillation column (1) to an intermediate point on a low-pressure column (3) installed above this medium-pressure column, and/or to the head condenser of an oxygen/argon separation column connected to the low-pressure column (3), characterised in that a small flow of air for feeding the double column (1) is used as the lifting gas.
5. Method for transferring a liquid, via an ascending pipe (6, 9, 11, 12) equipped with an expansion valve (7, 10, 13), from a first distillation column (2), operating at a first pressure, to an item of equipment (3), notably a second distillation column, operating at a second pressure lower than the first pressure, characterised in that the liquid is supercooled before it is expanded, with the exception of a minor part of this liquid, so as to produce a controlled quantity of flash gas, this flash gas is admitted downstream or upstream of the expansion valve (7, 10, 13) and serves as a lifting gas for the liquid.
6. Distillation installation of the type comprising a first distillation column (2) operating at a first pressure, an item of equipment (3), notably a second distillation column, operating at a second pressure, lower than the first pressure, and an ascending pipe (6, 9, 11, 12) equipped with an expansion valve (7, 10, 13) and connecting a point at which the liquid is drawn off from the first column (2) to a point at which the liquid is introduced into the said item of equipment (3), situated above the drawing-off point, characterised in that it comprises a means (19 to 26) of admitting into the ascending pipe, downstream of the expansion valve, a lifting gas available at a pressure higher than the pressure created by a column of the said liquid between the gas admission point and the point at which the liquid is introduced into the said item of equipment.
7. Installation according to Claim 6, characterised in that the lifting gas is taken off in the first column (2) or at the inlet thereof, and in that the said admission means (19 to 26) opens out into the ascending pipe (6, 9, 11, 12) at a level above the starting point of this pipe.
8. Installation according to Claim 7, characterised in that the said admission means comprises a gas pipe (11, 24) equipped with an expansion valve (25) and starting from a point on the first column (2) adjacent to the point at which the said liquid is drawn off.
9. Installation according to Claim 7, in which the first column (2) is the medium-pressure column (2) of a double air distillation column (1) and the said item of equipment is the low-pressure column (3), mounted on top of this medium-pressure column, of the double column, and/or the head condenser of an oxygen/argon separation column connected to the low-pressure column (3), characterised in that the said admission means comprises a pipe (19 to 21) equipped with an expansion valve (22, 23) and starting from the pipe (5) admitting the air to be distilled or from the kettle of the medium-pressure column (2).
10. Distillation installation of the type comprising a first distillation column (2) operating at a relatively high pressure, an item of equipment (3), notably a second distillation column, operating at a relatively low pressure, and an ascending pipe (6, 9, 11, 12) equipped with an expansion valve (7, 10, 13) and connecting a point at which liquid is drawn off from the first column (2) to a point at which liquid is introduced into the said item of equipment (3), characterised in that the ascending pipe (6, 12) passes through a supercooler (8, 14) upstream of the expansion valve (7, 13) and is provided with a bypass (24A) for this supercooler.

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