US3798917A

US3798917A - Fractionation of air to obtain oxygen of about seventy percent purity

Info

Publication number: US3798917A
Application number: US00137887A
Authority: US
Inventors: F Juncker
Original assignee: Messer Griesheim GmbH
Current assignee: Messer Griesheim GmbH
Priority date: 1970-05-12
Filing date: 1971-04-27
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26
Also published as: SU403206A3; ZA713053B; FR2092141A1; DE2022953C3; JPS5146078B1; AU2838771A; DE2022953A1; NL7106072A; ES390769A1; BE766516A; DE2022953B2; GB1288173A; LU63113A1

Abstract

Oxygen of approximately 70% purity is obtained by a two-stage rectification process in which air is cooled against the fractionation products, part of the cooled air is liquefied against evaporating oxygen product, thus liquefied air is subcooled, expanded and evaporated against vapor of the first rectification column to provide reflux therefor, and the evaporated air is fed to the second column. Bottom liquid of the first column is also used to produce reflux at the top thereof and then discharged into the second column. A process gas stream is expanded to produce refrigeration.

Description

0 United States Patent 1191 111 3,798,917 Juncker Mar. 26, 1974 [54] FRACTIONATION OF AIR TO OBTAIN 2,822,675 2/1958 Grenier 62/38 OXYGEN 0 ABOUT SEVENTY PERCENT 3,563,046 2/1971 Van Bausch... 62/29 PURITY 2,873,583 2/1959 Potts 62/38 3,260,056 7/1966 Becker.... 62/38 [75] Inventor: Friedrich Juncker, 3,113,854 12/1963 Bernstein.... 62/38 Bergen Enkheim Germany Grunberg.... 2,584,985 2/1952 Cicacese..... 62/29 1 gn Messer Griesheim GmbH, 2,918,802 12/1959 Grunberg 62/38 Frankfurt, Germany [22] Filed: Apr. 27, 1971 Primary Examiner-Norman Yudkoff Appl. No.1 137,887

Assistant Examiner-Arthur F. Purcell Attorney, Agent, or Firm-Paul W. Garbo [57] ABSTRACT Oxygen of approximately 70% purity is obtained by a two-stage rectification process in which air is cooled against the fractionation products, part of the cooled air is liquefied against evaporating oxygen product,

8 Claims, 3 Drawing Figures VIII/11,1}-

MTENTEB MAR 26 I974 -SHEET 10F 3 INVENTOR. FRIEDRICH JUNCKER mmgnmzemm 3.798317 SHEET 2 0F 3 AIR INVENTOR. FRIEDRICH JUNCKER BY Wm AGENT mgmgumzs i974 3.798.917

SHEET 3 or 3 AIR INVENTOR. FRIEDRICH JUNCKER AGENT FRACTIONATION OF AIR TO OBTAIN OXYGEN OF ABOUT SEVENTY PERCENT PURITY BACKGROUND OF THE INVENTION This invention relates to a process of obtaining oxygen of approximately 70 percent purity by two-stage rectification of air in a medium-pressure column and a low-pressure column and by work-performing expansion of a process stream to the pressure of the lowpressure column, in which process the incoming air is cooled against fractionation products and is divided into at least two streams, one of which is conducted directly into the lower part of the medium-pressure column and another into the low-pressure column.

It is known that the separation of gas mixtures into their individual components requires a greater expenditure of energy the greater the desired purity of the products is. It is furthermore known that, referred to the nitrogen separated from the air, a minimum consumption of energy results when oxygen of a purity of about 60 to 80 percent is produced (Handbuch der Kaltetechnik, Vol. 8, published by Springer-Verlag, 1957, pages 196-197).

Since the metallurgical industry in particular uses large quantities of oxygen-enriched air, it has been endeavored for a long time to develop methods in which the minimum energy consumption is economically utilized. A prerequisite for this is that themethods operate with the lowest possible initial pressure of the air which is to be fractionated. In distillative separation it is difficult in this connection to make available sufficient liquid nitrogen as wash liquid at the head of the rectification column, since the temperature of condensation of the nitrogen decreases with a decrease in the pressure.

Refrigeration for the condensation of the nitrogen is available in particular as a result of the evaporating oxygen product. Since the pressure of the evaporating oxygen product cannot be lowered without limit unless it is withdrawn from the plant by a vacuum pump, the evaporation temperature of the oxygen product is also fixed.

Up to now in principle two methods have been known which make it possible to-get along with the lowest possible initial pressure of the air. These are, on the one hand, the recycle methods, under which also the 'so-called two-pressure methods must be included, As examples mention may be made here of German Pat. No. 1,187,248 and British Pat. No. 1,033,931.

In these recycle methods a part of the wash nitrogen is more easily liquefied under a somewhat higher pressure. Such methods are disadvantageous as they are relatively expensive from the standpoint of the apparatus. They require a second compressor and additional recycle heat exchangers, whereby additional loss of flow and cold result. In the twopressure methods, a more complicated air compressor as well as expensive heat exchangers are required.

In the other method, so-called parallel-flow evaporators or columns are used for fractional condensation. As examples mention may be made ofGerman Pat. No. 1,177,658 and German Application No. 1,934,755. In these methods, the impure oxygen does not evaporate at constant temperature but rather over a temperature range. In a normal rectification column, the cooling power is made available at the head of the column at the temperature prevailing there. This industrially is the simplest solution but it is not optimum from a thermodynamic viewpoint since more ability to perform work (energy) is expended than is actually necessary. These methods attempt to make the cooling power available overv the sliding temperature range of the rectification column. For these methods, special rectification columns are necessary for which there has not been available a fully satisfactory industrial solution. Since, for reasons of safety, the oxygen product must not experience dry evaporation, the effect of the sliding evaporation temperature cannot be fully utilized. Hence, for reasons of safety, the design possibilities and the operation of such special columns are limited.

Theobject of the present invention is to provide a method which is at least as favorable with respect to the consumption of energy as the known methods but operates with well-proven, ordinary apparatus components and without expenditure for special apparatus. In particular, the advantages of sliding evaporation of the oxygen product without use of a special column are to be utilized, and the method is to require compression of the inlet air to a single pressure level and no recycling.

SUMMARY OF THE INVENTION .According to this invention, oxygen of approximately percent purity is obtained by a two-stage rectification process in which the incoming air is cooled against the fractionation products and divided into at least two streams, one of which is conducted directly into the lower part of the first or medium-pressure column and the other stream is condensed againstevaporating liquid impure oxygen withdrawn from the second or lowpressure column. The condensed air stream is further cooled to a low temperature, expanded, evaporated against the vapor of the top of the medium-pressure column and thereupon conducted into the low-pressure column. Furthermore, the top of the medium-pressure column is cooled by the crude oxygen from the bottom of that column and a process gas stream is expanded with the performance of work to the pressure of the low-pressure column.

It is advantageous if a part of the stream ofair, which has been condensed by the evaporation of liquid impure oxygen from the low-pressure column, is conducted as additional liquid feed into the mediumpressure column. Similarly, a part of the condensed air stream, after being subcooled, can be branched off and expanded directly into the low-pressure column. In this way energy is saved, since with this method the rectification operates with only slight disturbances in equilibrium.

As the process stream which is to be expanded with the performance of work, there is advantageously employed a part of the incoming air or a gas fraction obtained from the first column.

BRIEF DESCRIPTION OF THE DRAWINGS The invention willnow be further described with ref- I pressure column is expanded with the performance of work; and

FIG. 3 is a flow sheet of still another embodiment in Y which a gaseous intermediate fraction is withdrawn DESCRIPTION OF THE PREFERRED EMBODIMENTS:

In the process shown in FIG. 1, air compressed to about 3.5 ata (atmospheres absolute) passes at room temperature through line 5 into

heat exchangers

6 and 7 and into gas-phase filter 8 in which hydrocarbons contained in the air and traces of carbon dioxide which were not frozen-out in

heat exchangers

6 and 7 are retained. The air is cooledin

heat exchangers

6 and 7 to close to the dew point.

After gas-phase filter 8 the air is divided into two streams. About 20 percent of the incoming air flows through line 10 into heat exchanger 7 and is warmed somewhat therein. If desired, this air can in part also bypass heat exchanger 7 via valve 1 lb. Via bypass line 11a, a part of the reheated air can be fed through regulating heat exchanger 12 so that the air finally enters turbine 13 with a temperature of about l68C and is there expanded to 1.32 ata, the pressure of lowpressure column 2. The air cooled by the workperforming expansion then, flows through line 14 into low-pressure column 2.

The other stream, constituting about 80 percent of the incoming air, flowsthrough air precondenser 16 and is then divided into second and third streams. The second stream, constituting about 50 percent of the total air, passes through line 9 directly into mediumpressure column 1. This air is fed above the bottom liquid pool in column 1 and provides vapor upflow therein.

The third stream, about 30 percent of the total air, passes through line into air condenser 17 in which it condenses. A small part of the condensed air passes via line 18 into medium-pressure column 1 and there fortifies the reflux. The main quantity of the condensed air which continues through line I5 is cooled to about -1 89C in subcooler 4 and split into streams of approximately equal size in

lines

19 and 20.

The stream flowing through line 19 is expanded in throttle valve 21a into the upper part of low-pressure I column 2. The stream in line 20 is expanded in throttle valve 21b to the pressure of low-pressure column 2 and y is evaporated in condenser 3,. whereby it assists in cooling vapor from the top of medium-pressure column 1. The evaporated air combines with the expanded air from turbine 13 in line 14 and is introduced together with the latter into the lower third of low-pressure column 2.

Medium-pressure column 1 fractionates the air fed thereto into nitrogen and crude oxygen which is obtained in the bottom of column 1 in liquid form containing about 41 percent oxygen. The nitrogen is withdrawn in gaseous form by line 22 from the top of medium-pressure column 1 and is condensed in condenser 3. A part of the condensed nitrogen passes through line 23 as reflux back into medium-pressure column 1. The balance of the condensed nitrogen passes through line 24 into nitrogen subcooler 25 and then is introduced via throttle valve 26 as reflux into low-pressure column Liquid crude oxygen is withdrawn from the bottom of medium-pressure column 1 through line 27 and subcooled in cooler 4; it is then expanded in throttle valve 28 and evaporated in condenser 3, whereby nitrogen vapor from the top of medium-pressure column 1 is condensed. In gaseous form,'the crude oxygen then flows through line'27 into low-pressure column 2 above the liquid pool in the bottom and produces a gaseous upflow' in column 2.

Low-pressure column 2 effects the final fractionation. From its top, through line 29 there escapes gaseous nitrogen which after flowing through

subcoolers

25 and 4, air precondenser l6, regulating heat exchanger 12 and

heat exchangers

7 and 6, leaves the plant at room temperature. Subcooler 25 is provided with bypass line 30 and regulating valve 31 for the gaseous nitrogen flowing from low-pressure column 2 through line 29.

From the bottom of low-pressure column 2, liquid oxygen of about percent purity is withdrawn through line 32 and expanded to nearly atmospheric pressure in throttle valve 33. Thereupon, it passes into condenser 3 where it partially evaporates, thus taking up about one-third of the heat necessary for its evaporation. This heat is withdrawn from the vapor from the top of medium-pressure column 1. Separation of this oxygen stream into phases then takes place in separator 34. The liquid phase is withdrawn through line 35 and is pumped by circulating pump 36 through oxygen filter 37 where any-hydrocarbons still present in the liquid are retained by adsorption.

About two-thirds of the circulated liquid phase then evaporates in air condenser 17, while air stream 15 flowing in the opposite direction is completely condensed. The remainder of the liquid oxygen which has been about two-thirds evaporated passes back into separator 34. Gaseous oxygen of about 70 percent purity leaves separator 34 through line 39. This gaseous oxygen gradually gives off its remaining cold in air precondenser 16, regulating heat exchanger 12 and

heat exchangers

7 and 6. Thus, the plant yields as product, gaseous oxygen of about 70 percent purity, at room temperature and atmospheric pressure.

By the method of this invention, it is possible to use a large amount of the refrigeration in the impure liquid oxygen from low-pressure column 2 to cool vapor from the top of medium-pressure column 1.

The impure liquid oxygen can, to be sure, be evaporated only to the extent of about one-third in condenser 3 since with evaporation it becomes warmer and warmer due to the enrichment of oxygen in the remaining liquid, but the unevaporated portion can' be evapo rated against the incoming air in line 15. The air in line 15 is thus condensed and refrigeration originally in the impure liquid oxygen can now be given off near the top of medium-pressure column 1 by re-evaporation of condensed air introduced through line 18. The impure liquid oxygen could be evaporated completely in condenser 3 only if the pressure of medium-pressure column 1 were increased. The turbo-compressor (not shown) for the air flowing into the plant would then require more energy.

In the process of FIG. 2, the incoming air is divided into only two streams. The stream which in the process of FIG. 1 was expanded with the performance of work in turbine 13 is eliminated. In its place, a partial stream of the nitrogen withdrawn in gaseous form by line 22 from the top of medium-pressure column 1 is passed into line 40, heated in air precondenser

l6 andheat exchangers

12 and 7, and expanded with the performance of work in turbine 41. The expanded, cooled nitrogen flows through line 42 and is combined with the nitrogen withdrawn through line 29 from the top of low-pressure column 2. As in the process of FIG. 1, bypassline l la and valve 1112 are also provided up-stream of turbine 41.

The process of FlG.'3 is substantially similar to that of FIG. 2. However, gaseous nitrogen from the top of medium-pressure column 1 is not expanded, but rather an intermediate gaseous fraction with about percent oxygen is removed by line 45 from medium-pressure column 1 at the level where liquid air is charged through line 18. The intermediate fraction then flows through air precondenser l6 and heat exchangers l2 and7. After expansion with performance of work in turbine 43, the intermediate fraction passes through line 44 into low-pressure column 2. It enters there at the level where liquid air is introduced through line 19.

What is claimed is:

1. The process for obtaining oxygen of approximately 70 percent purity by the two-stage rectification of air and by the expansion of a process gas stream with the performance of work, which comprises cooling the incoming air by indirect heat exchange with the products of rectification, dividing the cooled air into at least two unfractionated streams, discharging one of said unfractionated streams into the lower part of the first medium-pressure column, condensing another of said unfractionated streams by indirect heat exchange with evaporating liquid oxygen of approximately 70 percent purity withdrawn from the second low-pressure column, sub-cooling and then expanding thus condensed unfractionated air which is then evaporated by indirect heat exchange with the top vapor of said first column, discharging thus evaporated unfractionated air into the lower part of said second column, and evaporating the bottom liquid of said first column by indirect heat ex- 4. The process of claim 1 wherein the liquid oxygen of approximately percent purity withdrawn from the second column is partially evaporated by indirect heat exchange with top vapor of the first column prior to completion of evaporation by indirect heat exchange with the air stream condensed thereby.

5. The process of claim 1 wherein the stream discharged into the lower part of the first column is at least 50 percent of the cooled air at a pressure of about 3.5 ata and part of the condensed air stream, after being subcooled, is expanded and discharged directly into an'upper level of the second column as additional reflux liquid.

6. The process of claim 1 wherein the condensation of the air stream by indirect heat exchange with evaporating liquid oxygen of approximately 70 percent purity is carried out by pumping said liquid oxygen through said heat exchange to effect only partial evaporation thereof, the resulting vapor and liquid are separated, and the separated liquid is combined with said liquid oxygen being pumped through said heat exchange.

7. The process of claim 6 wherein the liquid oxygen of approximately 70 percent purity withdrawn from the second column is partially evaporated by indirect heat exchange with top vapor of the first column prior to completion of evaporation by indirect heat exchange with the air stream condensed thereby.

8. The process of claim 7 wherein the stream discharged into the lower part of the first column is at least 50 percent of the cooled air at a pressure of about 3.5 ata and part of the condensed air stream, after being subcooled, is expanded and discharged directly into an upper level of the second column as additional reflux liquid.

Claims

2. The process of claim 1 wherein part of the condensed air stream is discharged into an upper level of the first column as additional reflux liquid.
3. The process of claim 1 wherein part of the condensed air stream, after being subcooled, is expanded and discharged directly into an upper level of the second column as additional reflux liquid.
4. The process of claim 1 wherein the liquid oxygen of approximately 70 percent purity withdrawn from the second column is partially evaporated by indirect heat exchange with top vapor of the first column prior to completion of evaporation by indirect heat exchange with the air stream condensed thereby.
5. The process of claim 1 wherein the stream discharged into the lower part of the first column is at least 50 percent of the cooled air at a pressure of about 3.5 ata and part of the condensed air stream, after being subcooled, is expanded and discharged directly into an upper level of the second column as additional reflux liquid.
6. The process of claim 1 wherein the condensation of the air stream by indirect heat exchange with evaporating liquid oxygen of approximately 70 percent purity is carried out by pumping said liquid oxygen through said heat exchange to effect only partial evaporation thereof, the resulting vapor and liquid are separated, and the separated liquid is combined with said liquid oxygen being pumped through said heat exchange.
7. The process of claim 6 wherein the liquid oxygen of approximately 70 percent purity withdrawn from the second column is partially evaporated by indirect heat exchange with top vapor of the first column prior to completion of evaporation by indirect heat exchange with the air stream condensed thereby.
8. The process of claim 7 wherein the stream discharged into the lower part of the first column is at least 50 percent of the cooled air at a pressure of about 3.5 ata and part of the condensed air stream, after being subcooled, is expanded and discharged directly into an upper level of the second column as additional reflux liquid.