Process For Separation Of Ternary Gaseous Mixtures By Rectification

Abstract

The present invention concerns a process in which a gaseous fraction is withdrawn from an intermediate level of a main rectification and subjected to a rectification in an auxiliary zone, the liquid fraction obtained in this auxiliary zone being returned to the said intermediate level of the main rectification. The improvement according to the invention concerns the condensation of the said withdrawn gaseous fraction which is effected at the bottom and possibly simultaneously at the top of the said auxiliary zone by the vaporization of the liquid fractions originating from the main rectification. The process is applicable to the separation by rectification of a mixture formed of at least three constituents, two of which are in major proportions, the boiling point of the constituent in small proportions being between the boiling points of the two constituents in major proportions and being closer to that of the less volatile of these two constituents. For example, the invention is applicable to a two-stage air-separation plant with an argon side column.

Description

The present invention relates to an improved process by which a mixture composed of at least three constituents is separated by rectification.

The present invention is more specifically concerned with a process which makes possible an increase in the extraction yield of a constituent which is in small proportions in a mixture of at least three constituents, of which two are in major proportions, the boiling point of this constituent present in small proportions being between the boiling points of the two constituents in major proportions and being closer to that of the less volatile of these two constituents.

The present invention is particularly applicable to the separation of air by rectification and has for its object a process permitting the rate of extraction of argon to be increased.

The present invention is also concerned with any installation for carrying the above processes into effect.

It is known to be possible to separate air into its constituent elements by means of two successive rectification zones, for example, at medium pressure and low pressure. Thus, the air, purified beforehand from traces of CO.sub.2 and water which it contains, is brought to its dew point for the pressure in question in a first rectification zone, called the medium pressure zone. A liquid fraction rich in oxygen is collected at the bottom of this zone and a liquid fraction poor in oxygen is collected at the top thereof. At least a part of each of these two collected liquid fractions is sent into a second zone, called the low pressure zone. The fraction with low oxygen content is introduced into the top of this second zone, while the rich fraction is introduced approximately at the middle of this second zone.

In the conventional processes, some of the coldness which is necessary for keeping the installation cold can be obtained by expanding a part of the supply air stream and by blowing this into the second rectification zone at an intermediate level of the said zone.

The amount blown in is generally between 5 and 15 percent by volume, relatively to the volume of supply air. If the amount blown in is too high, however, this causes firstly an appreciable decrease in yield of oxygen in the separating operation and secondly an almost total impossibility of separating the argon.

Different processes for improving the extraction yield have been proposed.

It is thus possible to utilize the cold units liberated by the vaporization of a part of the liquid oxygen collected at the bottom of the said second zone for condensing, in a condenser at the top of an auxiliary column, a part of a fraction withdrawn from an intermediate level of the said second zone, this condensed part being reintroduced into the said second zone at the same intermediate level. This process is relatively satisfactory but nevertheless it involves a high energy expenditure. In effect, to condense a mixture containing essentially argon in the auxiliary condenser, it is necessary for the vaporization pressure of the oxygen to be lower than that of the mixture undergoing condensation, and this has the disadvantage of requiring, for example, the presence of an extraction blower, with the dangers of leakages and of introducing air, due to working under vacuum, or of noticeably increasing all the pressures in the said two zones of the main rectification.

The present invention consists in an improvement in existing processes and installations, which permits the aforesaid inconveniences to be obviated and the cost price and also the energy consumption to be reduced to the greatest possible degree. The improvement forming the subject of the present invention makes possible an improvement in the extraction yield of a constituent which is in small proportions in a mixture; it is possible to operate without any special installation being required.

The present invention has for its object an improved separation process by rectification, making possible an increase in the extraction yield of a constituent which is in small proportions in a mixture of at least three constituents, two of which are in major proportions, the boiling point of this constituent in small proportions being between the boiling points of the two constituents in major proportions and being closer to that of the less volatile of these two constituents, in which a gaseous fraction is withdrawn from an intermediate level of a main rectification and subjected to a rectification in an auxiliary zone, the liquid fraction obtained in this auxiliary zone being returned to the said intermediate level of the main rectification, and in which the condensation of the said withdrawn gaseous fraction is effected at the bottom and simultaneously at the top of the said auxiliary zone by the vaporization of liquid fractions originating from the main rectification.

According to one embodiment of the present invention, the said withdrawn gaseous fraction is at least partly condensed at the bottom of the said auxiliary zone by the vaporization of at least a part of the liquid fraction collected at the bottom of the second zone of the said main rectification.

According to another embodiment of the present invention, the said withdrawn gaseous fraction is at least partly condensed at the bottom of the said auxiliary zone by the vaporization of at least a part of the liquid fraction collected at the top of the first zone of the said main rectification.

According to another embodiment of the present invention, the withdrawn gaseous fraction is at least partly condensed at the top of the said auxiliary zone by the vaporization of at least a part of the liquid fraction collected at the top of the first zone of the said main rectification.

According to yet another embodiment of the present invention, the said withdrawn gaseous fraction is at least partly condensed at the top of the said auxiliary zone by the vaporization of at least a part of the liquid fraction collected at the top of the first zone of the said main rectification, and simultaneously by the vaporization of at least a part of the liquid fraction collected at the bottom of the first zone of the said rectification.

According to one embodiment of the present invention, the concentration in the withdrawn gaseous fraction of the said constituent in small proportions, relatively to the least volatile constituent, is the highest possible, and the concentration of the most volatile constituent is the lowest possible.

According to one embodiment of the present invention, the liquid fractions originating from the said zones of the main rectification are, after at least partial vaporization in the auxiliary rectification zone, preferably returned into the second zone of the said main rectification.

According to yet another embodiment of the present invention, at least one of the liquid fractions originating from the said zones of the main rectification is, after at least partial vaporization in the auxiliary rectification zone, preferably expanded in a turbine so as to assure a supply of cold to the installation.

Preferably, the said mixture of at least three constituents is air, from which it is desired to extract the argon (which is found therein in small proportions), of which the boiling point is between that of the nitrogen and that of the oxygen and is closer to that of oxygen, which is the least volatile.

However, it must be understood that the present invention is applicable to any mixture, whether it is gaseous or liquid at ambient conditions as regards temperature and pressure.

The present invention is also concerned with any installation for carrying out the process according to the present invention.

The invention will be best understood from reading the following description and the accompanying drawings, which show different embodiments of the present invention as non-limiting examples, applicable to the case of the production of argon.

FIGS. 1 and 2 represent different embodiments of the present invention. For the purpose of simplification the similar parts of these figures bear the same references.

FIG. 1 represents an embodiment in which the low pressure oxygen is at least partly vaporized at the bottom of the auxiliary column and the liquid with high oxygen content from the medium pressure column is possibly partly vaporized at the top of the auxiliary column.

FIG. 2 shows an embodiment in which the medium pressure liquid nitrogen is at least partly vaporized at the bottom of the auxiliary column.

These embodiments are obtained by means of a double rectification column comprising two zones, which are a medium pressure zone 1 and a low pressure zone 2, and an auxiliary rectifier column 4. The air, previously purified from traces of CO.sub.2 and water which it contains, is brought in the vicinity of its dew point temperature for the medium pressure in question into this medium pressure column 1 through the pipe 5. The liquid fraction rich in oxygen which is obtained at the bottom leaves this said column 1 through the pipe 12, while the liquid fraction rich in nitrogen obtained at the top of the column 1 leaves the latter through the pipe 13. All or part of the liquid fractions drawn off from the column 1 are then introduced into the low pressure column 2.

The auxiliary rectifier column 4 comprises a condenser 6 at the top and a condenser 7 at the bottom.

The liquid oxygen is drawn off from the double rectifier column through the pipe 14 and the gaseous nitrogen is drawn off through the pipe 10. At an intermediate level of the column 2, a gaseous fraction is withdrawn through the pipe 8; this fraction is supplied to the auxiliary rectifier column 4, in which the oxygen which it contains is condensed, and then the fraction thus condensed is returned through the pipe 9 to the low pressure zone.

The level at which the withdrawal takes place is a function of the operating conditions: pressure, nature of the constituents of the mixture, etc. In the case of low pressure rectification of air, the withdrawal takes place at the level where the argon concentration is the highest possible and the nitrogen concentration is the lowest possible. In a conventional installation comprising approximately 60 plates, the withdrawal is generally effected at the level of the 15th plate.

A much larger proportion of the argon in the air is withdrawn through the pipe 11 than is the case with the known processes. Instead of recovering 70 percent of the argon present in the air, as is generally the case in the known processes, it is found, for example, in the case of FIG. 1, that more than 78 percent of the argon present in the air are recovered. A much better separation between oxygen and nitrogen is also obtained.

In FIG. 1, the liquid fraction rich in oxygen and leaving the column 1 through the pipe 12 is introduced by way of the pipe 112 into the condenser 6, in which at least a fraction of the fraction rich in oxygen is vaporized, whereafter this fraction rich in oxygen is conveyed through the pipe 212 into the low pressure column 2. Another part of the cold supply necessary for the rectification is obtained in the condenser 7 by vaporizing a part of the liquid oxygen fraction collected in the column 2.

In FIG. 2, the fraction rich in oxygen and leaving the column 1 by way of the pipe 12 is brought directly into the column 2, another part being supplied through the pipe 112 to the condenser 6, in which it is at least partly vaporized, and then by way of the pipe 212 to the column 2.

A part of the fraction rich in nitrogen, which is obtained in the column 1, is conducted through a pipe 113 into the condenser 7, which it leaves, ac least partly vaporized, by way of the pipe 213.

It is obvious that the invention is not limited to the embodiment which has been illustrated and that it is capable of numerous variations available to the person skilled in the art, depending on the proposed applications, and without thereby departing from the scope of the invention.

Claims

What I claim is :

1. In a method for the separation of a mixture of gases by rectification in first and second vertically elongated rectification zones, said mixture comprising at least three components two of which are present in relatively major proportions and a third of which is present in a relatively minor proportion, said first zone being at a higher pressure than said second zone, in which liquid withdrawn from said first zone is expanded and introduced into said second zone, and in which fluid is withdrawn from an intermediate level of said second zone and is introduced into an auxiliary vertically elongated rectification zone and liquid from the bottom of said auxiliary zone is returned to an intermediate level of said second zone, and in which the top of said auxiliary zone is cooled by indirect heat exchange with liquid from one of said first and second zones while said minor component is withdrawn in gaseous phase from the top of said auxiliary zone; the improvement comprising cooling the bottom of said auxiliary zone by passing a cooling liquid from one of said first and second zones through the bottom of said auxiliary zone and in indirect heat exchange with the bottom of said auxiliary zone independently of liquid in heat exchange with the top.

2. A method as claimed in claim 1, in which said liquid for cooling the bottom of said auxiliary zone is withdrawn from the bottom of said second zone.

3. A method as claimed in claim 1, in which said liquid for cooling the bottom of said auxiliary zone is withdrawn from an upper portion of said first zone.

4. A method as claimed in claim 1, in which said mixture of gases is air and said gaseous phase withdrawn from the top of said auxiliary zone is enriched in argon.