EP0629828B1 - Process and apparatus for the production of oxygen and/or nitrogen under pressure in variable quantities - Google Patents

Description

The present invention relates to the production of oxygen and / or nitrogen gas under pressure with variable flow. It primarily concerns a variable gas flow production process of at least one main constituent of pressurized air, of the type in which the component is drawn off in liquid form of an air distillation apparatus, we bring this liquid at spray pressure above the withdrawal pressure, and the liquid is vaporized under vaporization pressure by heat exchange with circulating fluid under high pressure. Such a process is known from document EP-A-0 422 974.

The main application of the invention is production of gaseous oxygen under pressure at flow rate variable, and this is why we will explain below the invention with reference to this application.

The pressures discussed below are absolute pressures.

Air distillation devices are usually of the double column type and include a medium pressure column and a low pressure column coupled by a vaporizer-condenser. In devices so-called "pump", liquid oxygen withdrawn from a tank the low pressure column is pumped up to a pressure relatively high, then is vaporized under this pressure, generally in the heat exchange line associated with the double column and by heat exchange with air being liquefied.

This technique, which very advantageously allows to save a gaseous oxygen compressor, delicate to implement, is however limited by the causes the circulating air pressure to increase rapidly with the vaporization pressure of oxygen. Thus, a vaporization pressure of 12 bars corresponds at an air pressure of around 25 bars. So we are coming quickly at air pressures close to the pressure critical (around 38 bars), for which the level of air condensation disappears. You must then compress at high pressure a very high air flow, and the energy consumption becomes prohibitive.

This is why, to produce oxygen under high pressure, typically of the order of 40 to 50 bars, it is common to vaporize oxygen under pressure intermediate, typically of the order of 12 bars, and compress the gaseous oxygen under this outgoing pressure the hot end of the heat exchange line. It's that context in which the invention is mainly interested, which will be explained in this application.

When the oxygen demand under pressure varies, the following phenomena occur, which be explained with reference to Figures 2 and 3 of the drawings attached.

There are for each component of the installation a relationship between the operating pressure and the flow, called the characteristic curve. We can classify elements in two categories according to the appearance of characteristic curves:

(1) The compressors : For a centrifugal compressor, as a first approximation, the characteristic curve 1 links the compression ratio TC to the actual flow D aspirated (Figure 2).

When the flow decreases, the compression ratio increases. Below a certain flow appears pumping phenomenon, which is a mode of operation unstable and dangerous for the machine. It is therefore not possible to reduce the flow below a limit 2, the place of these limits forming a curve 3 called curve anti-pumping. For a given rotation speed and a given compressor geometry, the characteristic curve is unique. We can change the characteristic curve, either by changing the speed of rotation, or by acting on particular organs called blading or variable (or movable) blades.

Furthermore, depending on where you are the operating point on the characteristic curve, the compressor performance is affected. Squaring curves are shown in 4 in Figure 2. The central curves correspond to the best yields for operating points relatively close to the anti-pumping curve.

(2) Static elements (adsorption purification device and heat exchange line):

The characteristic curve 5 is much more simple (Figure 3). It is a pressure P / flow D curve monotonous, increasing, passing through the origin.

When the flow varies, the operating points different components move on characteristics which are not necessarily compatible between them. We must therefore add means of adjustment, which are valves or vanes.

When the flow of oxygen produced decreases, the oxygen compressor follows its characteristic curve, and the compression ratio increases. On a compressor classic line, at constant speed and without blading variable, it is usual to install a suction valve the compressor to decrease the suction pressure and thus allow the increase in the rate of compression and obtaining production pressure requested. The operating point then moves from A to B (Figure 2). This rolling however represents a loss low flow energy.

We can limit this loss by using a compressor equipped at its inlet with variable blades, which allows you to change characteristics. There is then no need to laminate on suction, and the point changes from A to C when moving to reduced flow. However, the use of variable blades on an oxygen compressor is delicate and little widespread.

On the other hand, when the oxygen flow decreases, the air blower flow must decrease also to balance the heat balance, and the flow incoming air must also, at least if the installation does not produce liquid, be reduced to balance the material balance. The curve of Figure 3, applicable at the heat exchange line, shows that the pressure of the distillation apparatus, and in particular the medium pressure, drop. The high pressure being constant, the compression ratio of the booster increases therefore, and the operating point follows its characteristic curve, which is again of the type shown on the Figure 2. For this air blower, it is easier to use compressors, known as multipliers integrated, with variable blades, and the adaptation of the characteristic of the compressor to that of double column is easily done. However, flexibility requested affects performance as follows since it is not possible that the reduced flow (by example point B in Figure 2) is less than that of pumping, the normal operating point A is found rejected to the right, towards the equi-yield curves low yield. It should also be noted that the oxygen compressor is penalized in the same way in normal flow operation.

In summary, we see that the flexibility requested on the flow of oxygen under pressure has consequences unfavorable on energy consumption, on the one hand due to the rolling of gaseous oxygen, on the other hand made it necessary to operate the compressors oxygen and air blower with yields relatively poor.

US-A-3,214,925 also describes a system in which oxygen pumped liquid is vaporized against a flow of pressurized air, the air flow being thus liquefied.

The invention aims to improve the overall performance of installation, both at reduced and nominal flow rates, without having use of variable blades, delicate to implement, for the final compressor.

To this end, the invention relates to a process of the aforementioned type, characterized in that, when the demand for said gaseous component decreases, its flow rate is adjusted by modifying the flow rate of the liquid to be vaporized and said vaporization pressure and the flow of circulating fluid is reduced so balance the material balance and reduce the high pressure of circulating fluid to maintain the same temperature difference between the circulating fluid and the liquid that vaporizes.

• la pression de vaporisation est intermédiaire entre la pression de soutirage et la pression de production, et on comprime jusqu'à la pression de production le gaz résultant de la vaporisation;
• on effectue ladite modification de manière à permettre au compresseur du gaz résultant de suivre sa courbe caractéristique;
• pour effectuer ladite modification, on lamine de manière variable le liquide à vaporiser;
• pour effectuer ladite modification, on pompe à vitesse variable le liquide envoyé dans l'échangeur de chaleur de vaporisation;
• pour effectuer ladite modification, on pompe un débit constant du liquide, et on en renvoie un débit variable vers l'appareil de distillation, le reste du liquide étant vaporisé.

The method may include one or more of the particular embodiments of the invention:

• the vaporization pressure is intermediate between the withdrawal pressure and the production pressure, and the gas resulting from the vaporization is compressed to the production pressure;
• said modification is carried out so as to allow the compressor of the resulting gas to follow its characteristic curve;
• to effect said modification, the liquid to be vaporized is variably laminated;
• to effect said modification, the liquid sent to the vaporization heat exchanger is pumped at variable speed;
• to effect said modification, a constant flow rate of the liquid is pumped, and a variable flow rate is returned to the distillation apparatus, the remainder of the liquid being vaporized.

The invention also relates to an installation for the implementation work of such a process. This installation, of the type known from document EP-A-0 422 974, comprising an apparatus for air distillation, means for withdrawing liquid from this apparatus, means for increasing the pressure of the liquid withdrawn to a pressure of vaporization, circulating fluid compressor, and heat exchanger for vaporizing the liquid under said vaporization pressure by exchange of heat with circulating fluid under high pressure, is characterized in that that it includes means for adjusting the flow rate of the liquid to be vaporized and for said vaporization pressure and means for reducing the flow rate of the fluid circulating in order to balance the material balance and to reduce the high pressure of the circulating fluid to maintain the same temperature difference between the fluid circulating and the fluid which vaporizes, when the demand of said constituent decreases.

• la figure 1 représente schématiquement une installation de production d'oxygène gazeux conforme à l'invention;
• la figure 2 est une courbe caractéristique du fonctionnement des compresseurs de cette installation;
• la figure 3 est une courbe caractéristique du fonctionnement des composants passifs de l'installation;
• la figure 4 illustre les avantages apportés par l'invention; et
• la figure 5 est une vue schématique partielle d'une variante.

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

• FIG. 1 schematically represents an installation for producing gaseous oxygen in accordance with the invention;
• Figure 2 is a characteristic curve of the operation of the compressors of this installation;
• Figure 3 is a characteristic curve of the operation of the passive components of the installation;
• FIG. 4 illustrates the advantages provided by the invention; and
• Figure 5 is a partial schematic view of a variant.

The installation shown in Figure 1 is intended to provide a flow variable gaseous oxygen under high pressure, for example under about 40 bars, via a product outlet pipe 6. It essentially comprises: an atmospheric air compressor 7; an apparatus 8 for purifying water and carbon dioxide by adsorption; a heat exchange line 9; a air blower 10 with variable blades; an expansion turbine 11; a double distillation column 12 itself comprising a medium column pressure 13 surmounted by a low pressure column 14, the head of the column 13 being coupled to the tank of column 14 by a vaporizer-condenser 15; a sub-cooler 16 a liquid oxygen pump 17 at rotational speed constant; a rolling valve 18 mounted in the discharge line 19 of this pump; and one oxygen compressor 20 without variable blades.

The double column is equipped with pipes usual 21 "rich liquid" rise (air enriched in oxygen), 22 for the rise of "poor liquid" (almost pure nitrogen), these two pipes connecting the medium pressure column to the low pressure column and being equipped with respective expansion valves, and 23 exhaust gas W (impure nitrogen) from the top of column 14, the sub-cooling waste gas the rich liquid and the poor liquid in the subcooler 16.

In nominal operation, atmospheric air, compressed in 7 at medium column pressure 13 and refined in 8, is divided into two streams: one first current which is cooled in 9 to the neighborhood from its dew point and introduced into the column tank 13; and a second current which is boosted in 10 to one high pressure adapted to the vaporization pressure of liquid oxygen. The compressed air is cooled in 9 up to an intermediate temperature T, at which it is divided into two fractions: a first fraction which continues to cool and is liquefied, and possibly sub-cooled, until the cold end of the heat exchange line and then is distributed among the columns 13 and 14 after expansion in gates of corresponding triggers; and a second fraction which is outlet of the heat exchange line, expanded at 11 at low pressure and introduced into column 14, this expansion ensuring that the installation is kept cold. Alternatively, the turbine could expand by air at medium pressure, the relaxed air then being introduced in column 13.

Liquid oxygen is drawn off in a tank column 14 and brought by pump 17 to a pressure intermediate. The valve 18 is in the open position maximum, so this intermediate pressure is substantially the vaporization pressure of oxygen liquid in the heat exchange line. Oxygen vaporized leaving, near room temperature, the hot end of the heat exchange line is then compressed to production pressure by the compressor 20.

When the oxygen demand decreases, we throttles the flow of liquid oxygen at intermediate pressure leaving the pump 17, by means of the valve 18. The oxygen vaporization pressure then drops in same time as the flow of liquid oxygen, and the throttle is set to allow compressor 20 to follow its characteristic curve. We reduce at the same time time the treated air flow, to balance the balance material, and we also reduce the high pressure by air, to maintain the same temperature difference between the air to be liquefied and the oxygen to be vaporized. So the compression ratio of booster 10 increases markedly less, when going from nominal flow to reduced flow, than in the prior art, recalled above, where the gaseous oxygen stream which supplies the compressor 20, which corresponds to an energy gain.

Considering Figure 4, the comparison can be done as follows: in the technique anterior, by playing on the variable blades of the booster 10, the operating point changes from A, for the nominal flow, at B, for reduced flow. By laminating the liquid, the operating point at reduced flow changes to C.

As a result, we can design the booster so as to shift the anti-pumping curve to the right, which goes from 3 to 3A. Equi-yield curves shift all the way to the right, 4 in 4A, and operation at nominal flow takes place with better yield.

So we see that the simple installation a throttle valve on the discharge line pump 17 provides both a gain in energy at low flow rates and a gain in efficiency, and therefore in energy, at nominal flow.

The same principle of pressure variation vaporization of liquid oxygen as a function of flow gaseous oxygen to be produced can be implemented by other means than the valve 18, all these means can be used alone or in combination with each other others: by driving the pump 17 by means of a motor variable speed, or as shown in the Figure 5, returning a variable oxygen flow liquid, controlled by a valve 24, of the discharge of the pump to the tank in column 14. Note that in Figure 5, the other parts of the installation, which are identical to those in Figure 1, have been omitted for clarity.

In another variant, the oxygen pressure liquid withdrawn from the double column can be increased without the use of a pump, by a hydrostatic height created in a down pipe.

The invention applies equally well to air distillers having their own medium pressure air compressor, as described more high, than devices integrated into a gas turbine.

Furthermore, the invention also applies for the production of nitrogen under high pressure at flow rate variable. It brings the same advantage vis-à-vis the air blower (or, more generally, compressor cycle of circulating fluid ensuring vaporization), and allows the use of a final nitrogen compressor without variable blades, and therefore more economical.

As can be understood, the invention applies also in case the installation does not include final compressor 20. The pressure of the oxygen produced is then a function of the flow rate of vaporized oxygen and is defined by the characteristic curve of the equipment consumer.

Claims (12)

1. Process for producing at least one main constituent of the air under pressure at a variable gaseous flow rate, of the type in which the constituent is withdrawn in liquid form from an air distillation apparatus (12), this liquid is brought to a vaporization pressure above the pressure at which it is withdrawn, and the liquid is vaporized at the vaporization pressure by heat exchange (in 9) with a heating fluid at a high pressure, characterized in that, when the demand for the said gaseous constituent falls, its flow rate is adjusted by modifying the flow rate of the liquid to be vaporized and the said vaporization pressure and the flow rate of the heating fluid is reduced so as to establish the materials balance and the high pressure of the heating fluid is reduced to maintain the same temperature difference between the heating fluid and the liquid which is vaporizing.
2. Process according to claim 1, characterized in that the gas resulting from vaporization is compressed to the production pressure (in 20).
3. Process according to claim 2, characterized in that the said modification is carried out so as to enable the compressor (20) of the resulting gas to follow its characteristic curve (1).
4. Process according to any one of claims 1 to 3, characterized in that, in order to carry out the said modification, the liquid to be vaporized is throttled in a variable manner (in 18).
5. Process according to any one of claims 1 to 4, characterized in that, in order to carry out the said modification, the liquid conveyed through the vaporizing heat exchanger (9) is pumped at a variable speed (in 17).
6. Process according to any one of claims 1 to 4, characterized in that, in order to carry out the said modification, a constant flow of the liquid is pumped and is returned at a variable flow rate (in 24) to the distillation apparatus (12), the remainder of the liquid being vaporized.
7. Plant for producing at least one main constituent of the air under pressure at a variable flow rate, of the type comprising an air distillation apparatus (12), means for withdrawing a liquid from this apparatus, means (17) for increasing the pressure of the liquid withdrawn to a vaporization pressure, a compressor (10) for heating fluid, and a heat exchanger (9) for vaporizing the liquid at the said vaporization pressure by heat exchange with the heating fluid at high pressure, characterized in that it includes means (18;24) for adjusting the flow rate of the liquid to be vaporized and the said vaporization pressure and means (10) for reducing the flow rate of the heating fluid so as to establish the materials balance and for reducing the high pressure of the heating fluid to maintain the same temperature difference between the heating fluid and the fluid which is vaporizing, when the demand for the said constituent falls.
8. Plant according to claim 7, characterized in that it includes a compressor (20) for bringing the gas resulting from the said vaporization to the production pressure.
9. Plant according to claim 8, characterized in that the compressor (20) has no variable turbine blades at its inlet and/or is driven by a constant speed motor.
10. Plant according to any one of claims 7 to 9, characterized in that it includes a constant speed pump (17) connected upstream from the distillation apparatus (12) and downstream from the passages for vaporizing the liquid of the heat exchanger (9), and in that the means of adjustment include a throttle valve (18) mounted in the delivery pipe from this pump.
11. Plant according to any one of claims 7 to 9, characterized in that it includes a pump driven by a variable speed motor, connected upstream from the distillation apparatus (12) and downstream from the passages for vaporizing the liquid of the heat exchanger (9) .
12. Plant according to claim 10 or 11, characterized in that it includes a return pipe, equipped with a flow rate adjusting valve (24), connecting the delivery from the pump (17) to the distillation apparatus (12).

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