Title of Invention

A PROCESS FOR VAPORIZING LIQUID OXYGEN

Abstract

The present invention relates to a process for vaporizing liquid oxygen in which, in standard operation, liquid oxygen is introduced into a main evaporator (3) and partially vaporized, a first bleed stream (5) is removed in the liquid state from the main evaporator (3), the first bleed stream (5) is partially vaporized in an additional evaporator (6) and a second bleed stream (7) is withdrawn in the liquid state from the additional evaporator (6) characterized in that in the process, the standard operation is interrupted by a heating operation and in the heating operation no liquid (5) is passed from the main evaporator (3) into the additional evaporator (6) and the additional evaporator (6) is brought to a temperature which is markedly higher than its temperature in the standard operation.

Full Text

Description Process and apparatus for evaporating liquid overran The invention relates to a process for evaporating liquid oxygen and to its use in a process for producing oxygen by low-temperature fractionation of air. Oxygen, in the present application, is taken to mean any mixture which has an oxygen content elevated with respect to air, for example at least 70%, preferably at least 98%. (In this application, all percentages denote molar amounts, unless explicitly stated otherwise.) This includes, in particular, impure oxygen, and also Industrial-grade pure oxygen and high-purity oxygen having a purity of 99.99% or above. For a host of applications, it is necessary to convert liquid oxygen present, before its use, into the gas form by evaporating it in a main evaporator by indirect heat exchange with a heat carrier. An evaporation of this type occurs in particular in the production of gaseous oxygen by low-temperature rectification, in which the oxygen product occurs in the liquid state at the bottom of a rectification column, since it is less volatile than nitrogen and argon. To obtain the product in the gas form and to generate ascending vapour for the rectification column, the oxygen occurring in the liquid state must likewise be evaporated in a main evaporator. The most widespread here is the classic Lined double-column process in which the main evaporator is disposed in the bottom of a low-pressure column and is operated by condensing nitrogen from the top of the pressure column (see Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering] , 2nd Edition, Section 4.1.2 on page 284). The main evaporator in this case is operated as a condenser-evaporator and is frequently termed main condenser. It is also implemented by one or more heat-exchange blocks which are operate as concreting or railing-film evaporators. The invention also relates to other double-column processes in which the main evaporator is operated with air, for example, and also processes having three or more columns for nitrogen-oxygen separation. Downstream of the rectification column or columns for the nitrogen-oxygen separation, apparatuses for producing other air components, in particular noble gases, can be connected, for example for argon production. If liquid oxygen is evaporated completely or essentially completely, less volatile impurities, such as CO2 or N2O, for example, can accumulate in the evaporator, even if these impurities are only present in very low concentrations in the oxygen (or the air to be fractionated) to be evaporated. (However, the acetylene which was feared earlier is no longer a problem in air-fractionation plants having preliminary purification by adsorption) . Some of these less volatile substances, CO2 and N2O, for example, can precipitate as solids and must be removed from time to time so that blockage of the heat-exchange passages in the main evaporator is avoided. To remove these solids which have separated out, the entire plant must be shut down. In a large air-fractionation plant, this can mean a works shutdown of from two to five days, for example. To reduce the accumulation of less volatile components, it is customary to take off continuously, or from time to time, some liquid in the form of a flushing stream from the main evaporator and to discard this stream. Together with this flushing volume, the less volatile impurities accumulated in the oxygen which has remained in the liquid state are also removed, so that their concentration can be limited in the main evaporator. In an air-fractionation plant having preliminary purification by adsorption, the flushing volume is customarily from 0,02 to 0.04% of the total amount of liquid oxygen introduced into the evaporator. Since, for air purification stream of the rectification, molecular sieve ad sorters have been used instead of the reversible heat exchangers (Revex) or regenerators previously used, the problems due to the accumulation of combustible less volatile components in an oxygen evaporator of this type (main evaporator) have decreased to the extent that a flushing stream of this type is sufficient Jo prevent hazardous concentrations of hydrocarbons without requiring additional measures (see Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2nd Edition, Section 4,5.1.5 on pages 312 and 313). US 2664719 discloses a process of the type mentioned in the introduction in which liquid oxygen is introduced from a main evaporator into an auxiliary evaporator which has a flushing line. The object underlying the invention is to increase the availability of a main evaporator for evaporating liquid oxygen and, in particular, to prevent interruptions to operations as far as possible. This object is achieved by the features of Patent Claim 1. In this, the (first) flushing stream which is taken off from the main evaporator is passed into an auxiliary evaporator which is disposed separately from the main evaporator. In this auxiliary evaporator a large part of the first flushing stream is evaporated and can thus be produced as oxygen product or as intermediate oxygen product. In turn, a second flushing stream is taken off from the auxiliary evaporator and discarded- (In the special case that krypton and/or xenon are to be produced from the liquid oxygen, further work-up is necessary) Whereas the first flushing stream is continuously passed from the main evaporator to the auxiliary evaporator, the second flushing stream can be , taken off continuously or batchwise. In the invention, a relatively large amount of liquid can be taken off from the main evaporator as first flushing stream so that all of the less volatile components can be ejected and their concentration can be kept low in the main evaporator. In particular, no solids deposits occur either in the main evaporator. However, this large volume of flushing liquid is not completely lost, since some of the first flushing stream is evaporated in the auxiliary evaporator and taken off in .the gas form. From the auxiliary evaporator, merely a customary flushing volume is taken off as second flushing stream, for example from 0.02 to 0.5%, preferably from 0.02 to 0.2%, of the amount of liquid oxygen introduced into the main evaporator. (In the case of batchwise taking off of the second flushing stream, the percentages refer to the time average•) The remainder of the first flushing stream is evaporated in the auxiliary evaporator and can be utilized as gaseous oxygen product. Using the invention it is possible to flush the main evaporator so intensively that the content of less, volatile components which could lead to solids deposits is kept extremely low. The less volatile components are passed completely to the auxiliary evaporator and there removed via the second flushing stream and the heating operation performed from time to time. Solids deposits can therefore occur only in the auxiliary evaporator, but not in the main evaporator. However, the auxiliary evaporator can be freed from solids considerably more simply than the main evaporator by heating. For this purpose, the normal operation is occasionally interrupted by a heating operation, in the heating operation the auxiliary evaporator being separated from the main evaporator with no liquid being passed from the main evaporator into the auxiliary evaporator. Simultaneously, the auxiliary evaporator is brought to a temperature which is markedly higher than its temperature in the normal operation, for example by at least 20 K, preferably from 20 to 50 K. The operation of the main evaporator and the plant in which it is installed does not need to be interrupted in this process. Due to the intensified flushing of the main evaporator, this no longer needs to be heated to remove solids. It is expedient if the amount of the first flushing stream which is taken off from the main evaporator in the normal operation is at least 1%, preferably at least 3%, and/or at most 10%, preferably at most 5%, of the amount of liquid oxygen introduced into the main evaporator. The invention further relates to the use of the process according to Claim 1 or 2 in a process for the low-temperature fractionation of air according to Patent Claim 3 and, in a corresponding apparatus according to Patent Claim 6, in particular air-fractionation processes and plants having air reunification by adsorption, for example on a molecular sieve. Processes and plants of this type serve for the production of oxygen, nitrogen and/or other gases present in atmospheric air. In addition, the invention relates to an apparatus for evaporating liquid oxygen according to Patent Claims 4 and 5, The invention and other details of the invention are explained in more detail below with reference to exemplary embodiments shown in the drawings. In the drawings: Figure 1 shows a first exemplary embodiment having a main evaporator consisting of a block and Figure 2 shows a second exemplary embodiment having a main evaporator consisting of a plurality of blocks. Figure 1 shows a section of a double column for the low-temperature fractionation of air, namely the upper part of pressure column 1 and the lower section of the low-pressure column 2. A main evaporator 3 serves to evaporate liquid oxygen which flows off from the lowest mass transfer section of the low-pressure column 2. (The lowest mass transfer section is shown as plate 4 in the drawing, but this could also be an arranged packing.) Gaseous oxygen product is taken off from the low-pressure column via line 9. The main evaporator can - as shown in Figure 1 - be disposed within the double column, in particular in the bottom of the low-pressure column. Alternatively, it can be implemented as a separate component outside the double column or be integrated into another component separate from the double column, for example into a methane ejection column, as shown in DE 4332S70 Al or DE 2055099 A. Via a line 5 disposed in the lower region of the main evaporator 3, a first flushing stream is continuously taken off and introduced into an auxiliary evaporator 6. From the lower area of the auxiliary evaporator 6, a second flushing stream 7 is taken off continuously or batchwise, while evaporated oxygen 8 is returned to the low-pressure column. Alternatively thereto, the vapour 8 can be passed from the low-pressure column into the oxygen product line 9 or into another apparatus, for example into the lower area of a methane ejection column according to DE 4332870 Al or DE 2055099 A. The heat carrier 10 used for the indirect heating of the main evaporator is nitrogen from the top of the pressure column 1. The nitrogen 11 condensed in the main evaporator is used as reflux to both columns. The auxiliary evaporator 6 is heated in the normal operation either likewise with nitrogen from the pressure column or with air as heat carrier 12. The condensed heat carrier is taken off via line 13 and fed into one or more of the rectification columns. At certain time intervals of, for example, from three to twelve months, preferably about six months, there is a switchover from the normal operation to the heating operation by closing valve 14 in the first flushing line 5. The feed of the heat carrier 12 is also closed. Instead, hot air at about 300 K is conducted via line 15 into the liquefaction compartment of the auxiliary evaporator 6 and removed again via line 16. A heating phase comprises shutting off, emptying, heating, recoiling and startup and lasts, for example, from 10 to 24 hours, preferably about 20 hours. It is advantageous, but in the context of the invention not absolutely necessary, if the first flushing stream 5, prior to its introduction into the auxiliary evaporator 6, is passed through a device 19 for removing less volatile components, for example by adsorption. The exemplary embodiment of Figure 2 differs from Figure 1 by the main evaporator being formed by a multiplicity of blocks 3a, 3b. The blocks 3a, 3b are disposed, for example, concentrically about a central tube which serves as the feed 10 of gaseous nitrogen from the pressure column 1. Obviously, this exemplary embodiment can also be equipped with a device for removing less volatile components (19 in Figure 1) . Patent claims 1. Process for evaporating liquid oxygen in which in the normal operation • liquid oxygen is introduced into a main evaporator (3) and there partially evaporated, • a first flushing stream (5) is removed in the liquid state from the main evaporator (3), • the first flushing stream (5) is partially evaporated in an auxiliary evaporator (6) and • a second flushing stream (7) is taken off in the liquid state from the auxiliary evaporator (6), where, in the process, the normal operation is interrupted by a heating operation and in the heating operation • no liquid (5) is passed from the main evaporator (3) into the auxiliary evaporator (6) and • the auxiliary evaporator (6) is brought to a temperature which is markedly higher than its temperature in the normal operation. 2. Process according to Claim 1, in which in the normal operation the amount of the first flushing stream (5) which is taken off from the main evaporator (3) is at least 1%, preferably at least 3%, and/or at most 10%, preferably at most 5%, of the amount of liquid oxygen introduced into the main evaporator (3). 3. Use of the process according to Claim 1 or 2 in a process for producing oxygen by low-temperature fractionation of air in a rectification system which has a pressure column (1) and a low-pressure column (2) for evaporating liquid oxygen from the low-pressure column (2), at least some of the vapour (8) generated in the main evaporator (3) and in the auxiliary evaporator (6) being introduced into the low-pressure column (2) and/or being taken off as gaseous oxygen product (9)- 4. Apparatus for evaporating liquid oxygen having • a main evaporator (3), • an auxiliary evaporator (6), • cleans .for introducing liquid oxygen into the main evaporator, • a first flushing line (5) for taking off a first liquid flushing stream from the main evaporator (3) and for introducing the first liquid flushing stream into the auxiliary evaporator (6), • a second flushing line (7) for taking off a second liquid flushing stream from the auxiliary evaporator (6), • a gas product line (8) for taking off vapour from the auxiliary evaporator, a heating apparatus (15, 16) for heating the auxiliary evaporator (6) to a temperature which is considerably higher than its temperature in normal operation, and • means (14) for interrupting the introduction of the first liquid flushing stream into the auxiliary evaporator (6). 5. Apparatus according to Claim 4 having a control device for setting the amount of the first flushing stream (5) in the normal operation to at least 1%, preferably at least 3%, and/or at most 10%, preferably at most 5%, of the amount of liquid oxygen introduced into the main evaporator (3). 6, Use of the apparatus according to Claim 4 or 5 in an apparatus for producing oxygen by low-temperature fractionation of air having a rectification system which has a pressure column (1) and a low-pressure column (2) , the means for introducing liquid oxygen into the main evaporator (3) being connected to the low-pressure column (2) and the apparatus having an oxygen product line (9, 8.) for taking off gaseous oxygen product from the main evaporator (3) and/or from the auxiliary evaporator (6). 1, Apparatus according to Claim 6 in which the apparatus for producing oxygen by low-temperature fractionation of air has a gas product line (8) for taking off vapour from the auxiliary evaporator (6) which is connected to the low-pressure column (2) or to a column- product line (9) connected to th« low-pressure A process for evaporating liquid oxygen substantially as herein described with reference to the accompanying drawings. An apparatus for evaporating liquid oxygen substantially as herein described with reference to the accompanying drawings.

Documents:

in-pct-2000-302-che-abstract.pdf

in-pct-2000-302-che-claims filed.pdf

in-pct-2000-302-che-claims grant.pdf

in-pct-2000-302-che-correspondnece-others.pdf

in-pct-2000-302-che-correspondnece-po.pdf

in-pct-2000-302-che-description(complete)filed.pdf

in-pct-2000-302-che-description(complete)grant.pdf

in-pct-2000-302-che-drawings.pdf

in-pct-2000-302-che-form 1.pdf

in-pct-2000-302-che-form 26.pdf

in-pct-2000-302-che-form 3.pdf

in-pct-2000-302-che-form 5.pdf

in-pct-2000-302-che-other document.pdf

in-pct-2000-302-che-pct.pdf