EP0567360B2 - Process and installation for the transfer of a liquid - Google Patents

Description

The present invention relates to a method of transfer of a liquid, via a riser fitted an expansion valve, a first column of distillation operating at a first press, typically relatively high, to equipment, especially to a second distillation column, operating at a second press, typically relatively low, less than the first press.

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

Apparatuses for the separation of gases from air by distillation cryogenic most often use the scheme classic double column. Liquids produced in tank (liquid rich in oxygen), in the middle (lean liquid) and at the top (liquid nitrogen) of the column medium pressure (or MP column), are sent at an intermediate point or at the top of the column low pressure (or BP column). Most of the time, for economic reasons, the low pressure column is placed above the medium pressure column. It is therefore necessary to send liquids to a point located higher than the point where they are taken. Conventionally, the pressure difference between the average column pressure and the low pressure column is higher at hydrostatic pressure of the liquid column between the sampling point in the column MP and the end point of the BP column.

Recent developments in column technology air distillation has appeared on the one hand columns with packings, low pressure drop, on the other hand, vaporizers-condensers with small temperature difference between the two fluids put in exchange relation thermal (nitrogen gas and liquid oxygen). These two improvements go in the direction of a reduction of the operating pressure of the medium pressure column, to reduce energy expenditure, and also an increase in the height of the columns, the optimum economical moving towards more distillation thrust.

On the other hand, an unfavorable consequence of this evolution lies in an increased difficulty to go up liquids by simple hydrostatic effect. Indeed, in some cases the pressure available in the column MP is no longer sufficient to raise liquids to the BP column, especially when the device must be able to operate also in reduced mode, that is to say with a pressure of the MP column more lower than at rated speed.

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

The invention aims to allow, in a simple way and efficient, safe transfer of liquids, without use of a pump.

To this end, the method according to the invention is characterized by injecting it 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 injection point 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 invention relates to a process for transferring a liquid, via a pipe rising fitted with an expansion valve, a first distillation column operating at a first pressure on equipment, especially one second distillation column, operating at a second pressure, less than first pressure, characterized in that the liquid is sub-cooled before its expansion, at except for a minority fraction of this liquid, so as to produce a controlled amount of gas from flash, this flash gas is injected downstream of the expansion valve and serves as a lightening gas for the liquid.

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

According to a second aspect, the invention relates to an installation according to claim 10.

• la figure 1 représente schématiquement une installation de distillation d'air conforme à l'invention ; et
• la figures 2 illustre une variante de l'invention.
• la figure 3 illustre une autre variante d'obtention de gaz d'allègement qui n'est pas conforme à 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 air distillation installation according to the invention; and
• Figures 2 illustrates a variant of the invention.
• FIG. 3 illustrates another variant for obtaining lightening gas which is not in accordance with the invention.

The air distillation installation shown in the figure 1 essentially comprises a double column distillation 1. This includes a medium column pressure 2 surmounted by a low pressure column 3. A vaporizer-condenser 4 puts in exchange relation thermal steam at the top of column 2, consisting practically pure nitrogen, and the tank liquid column 3, consisting of oxygen at a determined purity.

In operation, air at a pressure typically of 5 × 10 ⁵ to 6 × 10 ⁵ Pa is introduced into the tank of column 2 via a supply line 5. "Rich liquid" (air enriched in oxygen) is withdrawn from the 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 sub-cooler 8 and the expansion valve 7 is pricked a rising pipe 9 equipped with an expansion valve 10 and leading to the head condenser of a column of oxygen / argon separation (not shown) coupled to the BP 3 column in a conventional manner.

"Poor liquid" (impure nitrogen) is drawn off in one intermediate point of column 2 via line 11 equipped with an expansion valve (not shown) and, after sub-cooling and expansion, is introduced in an intermediate point in column 3. Liquid nitrogen practically pure is withdrawn at the head of column 3 via a pipe 12 fitted with an expansion valve 13, sub-cooled in a sub-cooler 14 upstream of this expansion valve, relaxed in the latter and introduced at the top of column 3.

Also shown in Figure 1 pipes 15 for the production of gaseous oxygen from the tank in column 3, 16 for the production of pure nitrogen, leaving from the top of this column 3, 17 of air insufflation at an intermediate point of column 3, and 18 of evacuation waste gas (impure nitrogen) from the top of this column.

We see that three different liquids must be refitted from lower column 2 to upper column 3 and that a liquid must be raised to the top of the column of oxygen / argon separation. If these columns are of the upholstery type, in particular structured, and / or have many theoretical platforms, and / or if the vaporizer / condenser 4 is of a low type temperature difference it may happen that the difference in pressure between the two columns 2 and 3 is barely sufficient to ensure these liquid rises.

To guarantee a good flow of liquids, in a regular and controlled manner, in all modes of operation of the installation, an air line 19 is tapped into supply line 5 and splits into two branches 20, 21. Each of these branches is equipped with an expansion valve 22, 23 and joined respectively lines 6 and 9 just downstream of their expansion valves 7 and 10. Similarly, a pipe 24 gas equipped with a relief valve 25 starts from the top from column 2 and joins line 12 just by downstream of the expansion valve 13. Another line for gas 26, fitted with an expansion valve (not shown), starts from a location in column 2 next to the point of withdrawal of the poor liquid (line 11) and joined this pipe 11 just downstream of the expansion valve of it.

In operation, a low flow of conveyed air, at the supply pressure of column 2, through the pipe 19, 20, is expanded in the expansion valve 22 and injected into the newly expanded rich liquid in the expansion valve 7. The air bubbles lighten the rich liquid and reduce the pressure required for bring up to column 2.

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

Similarly, practically pure nitrogen carried by the line 24 is expanded in the expansion valve 25 and injected into the newly expanded liquid nitrogen in the expansion valve 13, and impure nitrogen carried via line 26 is, after expansion, injected into the lean liquid carried by line 11 and expanded.

It should be noted that, due to the purity of the liquid nitrogen carried by line 12, the lightening gas corresponding (in line 24) must be nitrogen practically pure. In contrast, the compositions of the liquid rich and poor liquid are not critical, so that the corresponding lightening gases can have somewhat different compositions from these liquids, provided that they do not pollute them, all the more more than the flow of these gases is very low.

In practice, the main expansion valves 7, 10 and 13 are placed as low as possible to guarantee feeding them with frank liquid, and we introduce gas bubbles just downstream of these valves trigger to assist the upward propulsion of liquids in question. More specifically, the pressure of lightening gas must be sufficient to overcome the height of liquid above the injection point of the gas, and this pressure is obtained, in the example shown, thanks to the fact that every 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 ascent liquid nitrogen via line 12, two variants obtaining lightening gas. In these two variants, the pipe 24 and the expansion valve 25 are eliminated.

In the variant of Figure 2, a minority flow control of liquid nitrogen conveyed by line 12 bypass the sub-cooler 14 via a bypass pipe 24A fitted, preferably at its lowest point, an expansion valve 25A and ending downstream of the expansion valve 13.

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

In the variant of FIG. 3, the expansion valve 25A is deleted, and there is provision, in line 12, a three-way valve 27 having an input connected to the line 12 upstream of the sub-cooler 14, an outlet connected to the input of this subcooler and another output 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 quantity flash gas is produced when relaxing 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.

We understand that the variant of Figure 2, although based on the same idea as that of the figure 1, is less effective in that it allows well to lighten the rising liquids by limiting to a minimum flash gas production, which is unfavorable distillation, but that it does not allow restart of the installation in the event of congestion rising pipes accidental.

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 injected a lifting gas available at a pressure higher than the pressure created by a column of the liquid between the gas injection 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 injected 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 fraction of this liquid, so as to produce a controlled quantity of flash gas, this flash gas is injected downstream 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) for injecting 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 injection 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 from the first column (2) or at the inlet thereof, and in that the said injection 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 injection 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 injection 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 and the valve (13), this by-pass being equipped with an expansion valve (25A).

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