Process and Installation for the Production of Oxygen and/or Nitrogen Under Pressure at Variable Flow Rate

The present invention relates to the production of gaseous oxygen and/or nitrogen under pressure at a variable flow rate. It relates in the first instance to a process for the production of a variable flow of at least one principal constituent of air under pressure, of the type in which the constituent is withdrawn in liquid phase from an air distillation apparatus, this liquid is brought to a vaporization pressure, and the liquid is vaporized under the vaporization pressure by heat exchange with a calorific fluid under high pressure.

The principal application of the invention is to the production of gaseous oxygen under pressure at a variable flow rate, and this is why the invention will be explained hereinafter with reference to this use.

The pressures in question hereinafter are absolute pressures.

Air distillation apparatus is generally of the double column type and comprises a medium pressure column and a low pressure column coupled by a vaporizer-condenser. In the so-called "pump" apparatuses, liquid oxygen withdrawn from the base of the low pressure column is pumped to a relatively high pressure, then is vaporized under this pressure, generally in the heat exchange line associated with the double column and by heat exchange with air in the course of liquefaction.

This technique, which very advantageously permits avoiding the use of a compressor for gaseous oxygen, which is difficult to use, is however limited by the fact that the pressure of the calorific air increases rapidly with the vaporization pressure of the oxygen. Thus, a vaporization pressure of 12 bars corresponds to an air pressure of about 25 bars. An air pressure near the critical pressure (about 38 bars) is rapidly reached, for which the stage of air condensation disappears. It is thus necessary to compress to high pressure a very large flow of air, and the energy consumption becomes prohibitory.

This is why, to produce oxygen under high pressure, typically of the order of 40 to 50 bars, it is useful to vaporize oxygen under an intermediate pressure, typically of the order of 12 bars, and to compress the gaseous oxygen under this pressure leaving the warm end of the heat exchange line. It is in this context that the invention has its principal interest, which will be explained in this application.

When the demand for oxygen under pressure varies, there are the following phenomena, which will be explained with respect to FIGS. 2 and 3 of the accompanying drawings.

There exists for each component of the installation a relation between the operating pressure and the flow rate, the so-called characteristic curve. The elements can be classified in two categories according to the appearance of the characteristic curves:

(1) Compressors: For a centrifugal compressor, as a first approximation, the characteristic curve 1 connects the compressor load TC to the actual inlet flow rate D (FIG. 2).

When the flow rate decreases, the compressor load increases. Below a certain flow rate a pumping phenomenon takes place, which is an unstable and dangerous manner of operation for the machine. It is therefore not possible to decrease the flow rate below a limit of 2, the locus of these limits forming a curve 3 called an anti-pumping curve. For a given speed of rotation and a given compressor geometry, the characteristic curve is unique. The characteristic curve can be changed, either by changing the speed of rotation, or by acting on particular members called blading or variable blades (or movable blades).

Moreover, according to the place in which the operating point is located on the characteristic curve, the efficiency of the compressor is affected. The equal efficiency curves are shown at 4 in FIG. 2. The central curves correspond to the best efficiency for the operating points relatively close to the anti-pumping curve.

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

The characteristic curve 5 is much simpler (FIG. 3). It is a single curve of pressure P/flow rate D, rising from the origin.

When the flow rate varies, the operating points of the different components are displaced according to characteristics which are not necessarily mutually compatible. It therefore is necessary to add adjustment means, which are valves or blading.

When the product oxygen flow rate decreases, the oxygen compressor follows its characteristic curve, and the compression load increases. With a conventional in-line compressor, of constant speed and without variable blading, it is usual to install an input valve for the compressor to decrease the input pressure and thereby to permit the increase of the compression load and the obtention of the required production pressure. The operating point then displaces from A to B (FIG. 2). This pressure drop, however, represents a loss of energy at low flow rate.

This loss can be limited by using a compressor provided at its inlet with variable blades, which permits changing its characteristic. There is thus no need to throttle the intake, and the operating point displaces from A to C upon a reduction of flow rate. However, the use of variable blades on an oxygen compressor is delicate and uncommon.

On the other hand, when the oxygen flow rate decreases, the flow rate of the air supercharger must also decrease to maintain the thermal balance, and the flow rate of entering air must itself also, at least if the installation produces no liquid, be reduced to maintain the material balance. The curve of FIG. 3, applicable to the heat exchange line, shows that the pressure of the distillation apparatus, and in particular the medium pressure, falls. The high pressure being constant, the compression load of the supercharger therefore increases, and the operating point follows its characteristic curve, which is again of the type shown in FIG. 2. For this air supercharger, it is easier to use compressors, with a so-called integrated multiplier, with variable blades, and the adaptation of the characteristic of the compressor to that of the double column is easily effected. However, the required flexibility affects the output in the following way: when it is not possible that the decreased flow rate (for example the point B in FIG. 2) be less than that of the pump, the normal operating point A is displaced toward the right, toward the low equal output curves. It is moreover to be noted that the oxygen compressor is penalized in the same manner when operating at normal flow rate.

In short, it will be seen that the flexibility required for the oxygen flow rate under pressure has unfavorable consequences on energy consumption, on the one hand because of the pressure drop of the gaseous oxygen, on the other hand because of the requirement to operate the oxygen compressors and the air supercharger with relatively mediocre output.

The invention has for its object to improve the overall performances of the installation, both at reduced flow rates and at nominal flow rate, all the while without having recourse to variable blades, which are delicate to use, for the final compressor.

To this end, the invention has for its object a process of the type described, characterized in that the flow rate of said constituent product is adjusted by modifying the flow rate of the liquid to be vaporized and said vaporization pressure.

The process can comprise one or several of the following characteristics:

the vaporization pressure is intermediate the withdrawal pressure and the production pressure, and the gas resulting from vaporization is compressed to the production pressure;

this modification is effected in a manner such as to permit the resulting gas compressor to follow its characteristic curve;

to effect said modification, the liquid to be vaporized is throttled in a variable manner;

to effect said modification, the liquid sent to the vaporization heat exchanger is pumped at variable speed.

to effect said modification, the liquid is pumped at a constant flow rate, and a variable flow of it is sent to the distillation apparatus, the rest of the liquid being vaporized.

The invention also has for its object an installation for practicing such a process. This installation, of the type comprising an air distillation apparatus, means to withdraw a liquid from this apparatus, means to bring the withdrawn liquid to a vaporization pressure, a compressor for calorigenic fluid, and a heat exchanger to vaporize the liquid under said vaporization pressure by heat exchange with the calorigenic fluid under high pressure, is characterized in that it comprises means for adjusting the flow rate of the liquid to be vaporized and for adjusting said vaporization pressure.

An example of an embodiment of the invention will now be described with regard to the accompanying drawings, in which:

FIG. 1 shows schematically a gaseous oxygen production installation according to the invention;

FIG. 2 is a characteristic curve of the operation of the compressors of this installation;

FIG. 3 is a characteristic curve of the operation of the passive components of the installation;

FIG. 4 shows the advantages achieved by the invention; and

FIG. 5 is a fragmentary schematic view of a modification.

The installation shown in FIG. 1 is adapted to supply a variable flow rate of gaseous oxygen under high pressure, for example about 40 bars, via a product outlet conduit 6. It comprises essentially: an atmospheric air compressor 7; an apparatus 8 for purification from water and carbon dioxide by adsorption; a heat exchange line 9; an air supercharger 10 with variable blades; an expansion turbine 11; a double distillation column 12 comprising itself a medium pressure column 13 surmounted by a low pressure column 14, the head of the column 13 being coupled to the base of the column 14 by a vaporizer-condenser 15; a subcooler 16; a liquid oxygen pump 17 with constant speed of rotation; a throttle valve 18 mounted in the output conduit 19 of this pump; and an oxygen compressor 20 having no variable blades.

The double column is provided with conventional conduits 21 for raising "rich liquid" (air enriched in oxygen), 22 for raising "poor liquid" (nearly pure nitrogen), these two conduits connecting the medium pressure column 2 to the low pressure column and being provided with respective expansion valves, and conduit 23 for the evacuation of residual gas W (impure nitrogen) from the summit of column 14, the residual gas subcooling the rich liquid and the poor liquid in the subcooler 16.

At nominal operation, atmospheric air compressed in 7 to the medium pressure of the column 13 and purified in 8, is divided into two flows: a first flow which is cooled in 9 to about its dew point and introduced into the base of the column 13; and a second flow which is supercharged in 10 to a high pressure adapted to the vaporization pressure of the liquid oxygen. The supercharged air is cooled in 9 to an intermediate temperature T, at which is divided into two fractions: the first fraction which continues its cooling and is liquified, and if desired subcooled, to the cold end of the heat exchange line, then is divided between the columns 13 and 14 after expansion in corresponding expansion valves; and a second fraction which left the heat exchange line, was expanded in 11 to the low pressure and introduced into the column 14, this expansion ensuring the cold supply of the installation. As a modification, the turbine could expand air to the medium pressure, the expanded air being then introduced into the column 13.

Liquid oxygen is withdrawn from the base of the column 14 and brought by the pump 17 to an intermediate pressure. The valve 18 is in its fully opened position, such that this intermediate pressure is substantially the vaporization pressure of the liquid oxygen in the heat exchange line. The vaporized oxygen leaves, at about ambient temperature, the cold end of the heat exchange line and is then compressed to the production pressure by the compressor 20.

When the demand for oxygen decreases, the flow of liquid oxygen at intermediate pressure leaving the pump 17 is throttled by means of the valve 18. The vaporization pressure of the oxygen falls at the same time as the flow rate of liquid oxygen, and the throttling is adjusted so as to permit compressor 20 to follow its characteristic curve. At the same time, the flow rate of air treated is decreased, to maintain the material balance, and the high pressure of the air is also reduced, to maintain the same temperature difference between the air to be liquified and the oxygen to be vaporized. Thus, the compression load of the supercharger 10 increases substantially less, when passing from the nominal flow rate to the reduced flow rate, than in the prior art, recited above, in which the flow of gaseous oxygen which supplies the compressor 20 is throttled, which corresponds to an energy gain.

With reference to FIG. 4, the comparison can be made in the following manner: in the prior art, acting upon variable blades of the supercharger 10, the operative point passes from A, for nominal flow rate, to B, for reduced flow rate. Upon throttling the liquid, the operative point with reduced flow rate passes to C.

Consequently, the compressor can be so designed as to shift to the right the anti-pumping curve, which passes from 3 to 3A. The equal output curves shift correspondingly to the right, from 4 to 4A, and the operation at nominal flow rate then takes place with improved output.

Thus, it will be seen that the simple installation of a throttle valve in the output conduit of the pump 17 permits obtaining both a gain in energy at low flow rates and a gain in output, and hence of energy, at the nominal flow rate.

The same principal of varying the vaporization pressure of the liquid oxygen as a function of the flow rate of the gaseous oxygen to be produced can be practiced by other means than the valve 18, all these means being adapted to be used alone or in combination with each other: by driving the pump 17 by means of a variable speed motor, or else as shown in FIG. 5, by returning a variable flow rate of liquid oxygen, controlled by a valve 24, from the output of the pump to the base of the column 14. It is to be noted that in FIG. 5, the other portions of the installation, which are identical to those of FIG. 1, have been omitted for clarity.

According to another variation, the pressure of the liquid oxygen withdrawn from the double column can be increased without the use of a pump, by a hydrostatic head created in a descending conduit.

The invention is applicable also to apparatus for the distillation of air having its own medium pressure air compressor, as described above, as well as to an apparatus integrating a gas turbine.

Moreover, the invention is also applicable to the production of nitrogen under high pressure at variable flow rate. It brings the same advantage relative to the air supercharger (or, more generally, to the compressor of calorific cycle fluid assuring its vaporization), and permits using a final nitrogen compressor without variable blades, which is therefore more economical.

As will be understood, the invention is applicable also to the case in which the installation does not comprise a final compressor 20. The pressure of the oxygen product is thus a function of the flow rate of vaporized oxygen and is defined by the characteristic curve of the consumer equipment.