Patent Application
20190242646
The invention relates to a method for obtaining compressed nitrogen by cryogenic separation of air according to the preamble of Claim 1.
The method relates in particular to systems involving the withdrawal of nitrogen product from the high-pressure column. The nitrogen product can come from both columns, for example by gaseous nitrogen (GAN) being passed both directly out of the low-pressure column and out of the high-pressure column. Alternatively, at least a part of the low-pressure-column nitrogen can be withdrawn in liquid form (LIN—liquid nitrogen), fed into the high-pressure column and drawn off therefrom as a GAN product. Such methods involving low-pressure-column LIN being “pumped back” into the high-pressure column are known from US 2004244417 A1,
The invention is based on the object of improving the method mentioned at the beginning and a corresponding apparatus in terms of energy consumption and at the same time to allow safe operation of the system.
This object is achieved by all of the features of Claim 1.
The use of a forced-flow evaporator as low-pressure-column top condenser allows a particularly lower pressure difference between the evaporating and the condensing stream with the same average temperature difference as in a bath evaporator. This noticeably reduces the energy consumption of the plant, for example by 3.2% at a product output pressure in the nitrogen of 10 bar, which corresponds to the high-pressure-column pressure; if a further compression from 10 to 60 bar is also figured in, the energy saving is 2.2% of the total energy consumption.
However, the loss of the liquid bath above the low-pressure column is also accompanied by the loss of the possibility of withdrawing a purge stream and discharging high-boiling components, in particular propane. In the invention, this is compensated in that a purge stream is drawn off from the bottom of the high-pressure column. Above this withdrawal (and the infeed of feed air), a barrier-plate section is provided, which retains the high-boiling components, in particular propane, in the bottom of the high-pressure column. The oxygen-enriched liquid stream for the low-pressure column is withdrawn above the barrier-plate section and contains fewer high-boiling components and in particular virtually no propane any more. Even with two theoretical plates in the barrier-plate section, given a propane content of 0.0075 ppm in the air downstream of the air cleaner (with an exemplary assumption for propane retention in the molecular sieve of the air cleaner of about 85%), 99.8% of the propane is removed with the purge stream. In the process, 84% of the N2O is also separated out (relative to the N2O quantity which passes through the air cleaner). The degrees of separation of other components are 69% for C2H6, 15% for C2H4 and about 2.5% for methane, which is less critical. “High-boiling components” are understood here to be substances which have a higher evaporation temperature than oxygen.
In principle, the abovementioned measures can be used to ensure safe operation of the plant. These measures are known per se from WO 2016131545 A1, but are applied therein at a relatively high process pressure, which has the result that there is no pre-liquefaction, i.e. no liquefaction of the feed air upstream of the distillation; rather all the air is introduced into the high-pressure column in gas form.
Overall, there are the following differences between the method mentioned at the beginning according to US 2004244417 A1,
The two methods have such different natures that there would be no question of combining them for an impartial person skilled in the art.
In US 2004244417 A1, on account of the relatively low pressure in the process (or relatively small pressure difference with the streams emerging from the rectification system), the feed air also contains a small liquid content during the feed into the high-pressure column - this would be the case even with very little liquid product being obtained or purely gas operation. Therefore, a relatively large quantity of liquid would end up in the bottom of the high-pressure column, if the abovementioned measures (see also WO 2016131545 A1) were applied to one of these methods. This quantity would be drawn off as a whole with the purge stream and noticeably reduce the product yield or have a negative effect on the energy consumption of the plant.
For this reason, Claim 1 also contains a further feature, according to which the gaseous nitrogen stream from the high-pressure column, before being warmed in the main heat exchanger, is warmed in a counter-current subcooler in indirect heat exchange with the oxygen-enriched liquid stream from the high-pressure column. At first look, it appears unclear what this measure is supposed to have to do with the discharging of the high-boiling components. At any rate, it results in an increase in the enthalpy of the gaseous nitrogen stream at the inlet into the main heat exchanger. Since the difference in enthalpy of a balancing group remains unchanged around the distillation column system (with unchanged product quantities and constant heat input from the environment), this causes a temperature increase at the cold end of the main heat exchanger. This is experienced by the cooling feed air stream; therefore, it likewise has higher enthalpy and a higher temperature than in the absence of warming of the nitrogen in the counter-current subcooler. This increase in enthalpy prevents or reduces pre-liquefaction of the air and in many cases even has the result that the air stream is slightly superheated at the inlet into the high-pressure column, i.e. its temperature is slightly above the dew point temperature; the temperature difference with respect to the dew point in the case of superheating is for example 1.4 K (in the method in which low-pressure-column LIN is “pumped back” into the high-pressure column and the nitrogen product is withdrawn primarily from the high-pressure column). Thus, at the inlet into the high-pressure column, the feed air no longer contains any liquid and the purge stream consists only of the reflux liquid, which exits the barrier-plate section at the bottom.
With regard to a feed air quantity of 100 000 Nm3/h, this feed-air superheating, brought about by the warming of the pressurized nitrogen in the counter-current subcooler, is substantial and corresponds to a liquid production of about 1000 Nm3/h of liquid nitrogen. It is thus possible for example for about 1% of the air quantity to be obtained as liquid product, without pre-liquefaction occurring; rather, the overall air quantity can be introduced into the high-pressure column in gas form. However, even at higher quantities of liquid nitrogen production (up to about 2% of the air quantity), there is still a certain amount of superheating in the air stream, since with increasing liquid product, the feed air pressure is raised.
In a specific numerical example for a plant with 100 000 Nm3/h of feed air and a liquid production of less than 0.1% of the feed air quantity, in the following text, the invention is compared with an operating mode in which the pressurized nitrogen is not passed through the counter-current subcooler. If these measures are dispensed with, 96 600 Nm3/h of air at 8.50 bar and a vapour content of 0.9966864 flow into the high-pressure column, that is to say 320 Nm3/h of air enter the high-pressure column in liquid form (pre-liquefaction). If, by contrast, the method is run in accordance with the invention, 96 105 Nm3/h are fed into the high-pressure column at 8.55 bar with superheating of 1.405 K (with a similar size of the main heat exchanger or with the same average temperature in the main heat exchanger compared with the case with warming of the pressurized nitrogen in the counter-current subcooler). Although this temperature difference with respect to the dew point seems slight at first look, it has a very great effect on the process, because it relates of course to the entire air quantity flowing into the high-pressure column.
With the aid of the warming, according to the invention, of the pressurized nitrogen in the counter-current subcooler, the fraction of air which is passed into the high-pressure column in liquid form is therefore reduced in a method in which more pre-liquefaction would otherwise occur. This “reduction” can go as far as zero or furthermore result in superheating of the air fed into the high-pressure column, i.e. in heating beyond the dew point. The invention does not relate to methods in which pre-liquefaction already does not occur without introduction of the pressurized nitrogen into the counter-current subcooler.
The described measure is relatively simple in terms of apparatus, but very effective. It uses equipment that is required anyway, the counter-current subcooler, and allows stable setting of the purge stream quantity which is withdrawn from the high-pressure-column bottom, with good product yield and relatively low energy consumption. This results overall in a particularly efficient method for obtaining pressurized nitrogen.
The operating pressures in the method according to the invention are:
Low-pressure column (at the top): for example 4.0 to 7.0 bar, preferably 4.5 to 6.5 bar
High-pressure column (at the top): for example 7 to 12 bar, preferably 8 to 11 bar
Low-pressure-column top condenser on the evaporation side: for example 1.5 to 3.5 bar, preferably 1.9 to 3.2 bar
With the aid of the invention, pre-liquefaction can be reduced. In individual cases, decreased pre-liquefaction will still occur. Preferably, the pre-liquefaction is completely eliminated by the invention, however; in other words, the feed air flows into the high-pressure column in a fully gaseous state under the dew point or with slight superheating. “Slight superheating” is understood here to mean a temperature difference of at least 0.1 K, for example (depending on liquid production) 0.1 K to 2.0 K, preferably 0.2 K to 1.8 K.
Preferably, the evaporation space operated as a forced-flow evaporator is operated with an oxygen-rich liquid from the low-pressure column; this can come in particular from the bottom of the low-pressure column. The gas generated in the evaporation space of the low-pressure-column top condenser is preferably warmed as residual gas to an intermediate temperature in the main heat exchanger and subsequently expanded in a work-performing manner in a residual-gas turbine, and then reintroduced into the main heat exchanger and warmed to around ambient temperature. As a result, cold for the method can be obtained economically.
The residual-gas turbine can be decelerated by an electric generator or by a compressor. The latter can compress for example the warmed expanded residual gas or a part thereof.
The efficiency of the method can be increased further when the evaporation space of the main condenser is also in the form of a forced-flow evaporator.
The invention also relates to an apparatus according to Claim 10. The apparatus according to the invention may be supplemented by apparatus features which correspond to the features of individual, multiple or all dependent method claims.
The invention and further details of the invention are explained in more detail in the following text by way of exemplary embodiments illustrated schematically in the drawings, in which:
In
According to the invention, the high-pressure column 4 has a barrier-plate section 8, which is arranged immediately above the point at which the feed air 3 is introduced. It consists for example of one to five, preferably of two to three conventional rectifier plates. Alternatively, a section with structured packing of for example one to five, preferably two to three theoretical plates can also be used. This section retains high-boiling constituents of the air, in particular propane, which are withdrawn with a purge stream 9A (Purge) from the bottom of the high-pressure column 4 and are removed therewith from the distillation column system. To this end, the purge stream 9B can, as illustrated, be introduced in a warm waste stream 10.
Above the barrier-plate section 8, an oxygen-enriched liquid stream 11 is withdrawn from the high-pressure column 4, cooled in a counter-current subcooler 12 and fed to the low-pressure column 6 at an intermediate point via line 13. This stream is virtually free of propane and other high-boiling components. This then also goes for all other oxygen-rich fractions in the low-pressure column, in particular for the bottoms liquid, which can be evaporated without risk both in the main condenser 5 (via line 14) and in the low-pressure-column top condenser 7 (via the lines 15 and 16). Complete evaporation can be carried out without problems in the low-pressure-column top condenser 7. With two theoretical plates in the barrier-plate section, given a propane content of 0.0075 ppm in the air downstream of the air cleaner (with an exemplary assumption for propane retention in the molecular sieve of the air cleaner of about 85%), 99.8% of the propane is removed with the purge stream. In the process, 84% of the N2O is also separated out (relative to the N2O quantity which passes through the air cleaner). The degrees of separation of other components are 69% for C2H6, 15% for C2H4 and about 2.5% for methane, which is less critical.
In the main condenser 5, a part 18 of the nitrogen tops gas 17 from the high-pressure column 4 is condensed. The liquid nitrogen 19 obtained in the process is returned to the high-pressure column 4 as a recirculation flow. The low-pressure-column top condenser liquefies tops gas 20 from the low-pressure column 6. Liquid nitrogen 21 generated in the process is returned to the low-pressure column 6. A part thereof is immediately drawn off from the low-pressure column 6 again as a liquid nitrogen stream 22. (Alternatively, this stream could also be withdrawn directly from the liquefaction space of the low-pressure-column top condenser 7). A pump 23 brings the liquid nitrogen stream 22 to approximately high-pressure-column pressure. The pressure liquid 24 is supplied to the top of the high-pressure column 4 via the counter-current subcooler 12 and line 25A/25B.
A gaseous nitrogen stream from the top of the high-pressure column 4 is withdrawn via line 17/26A/26B and initially warmed according to the invention in the counter-current subcooler 12. Subsequently, the nitrogen 27 is warmed in the main heat exchanger to around ambient temperature and can be drawn off at 28 as gaseous pressurized nitrogen product under high-pressure-column pressure. In this example, however, it is compressed even further by one or for example two nitrogen compressors 29, 30 in each case with intermediate cooling or postcooling, such that the final pressurized nitrogen product 31 (PGAN) exhibits a pressure of for example 120 or 150 bar here.
As a result of the evaporation of the low-pressure-column bottoms liquid 16 in the low-pressure-column top condenser 7, a residual gas 32 is generated, which is initially warmed in the counter-current subcooler 12. Subsequently, it flows via line 33 to the main heat exchanger 2, in which it is warmed to an intermediate temperature. Subsequently, it is expanded in a work-performing manner in a residual-gas turbine 35 with a bypass 37. The expanded residual gas is reintroduced in two parts into the main heat exchanger and warmed to around ambient temperature. A first part 38 is fed as regeneration gas to the air cleaner via line 39. The rest 40 is discharged into the atmosphere (ATM) via line 10.
A part 41 of the tops gas of the low-pressure column 6 is discharged via the lines 42 and 43 and through the counter-current subcooler 12 and the main heat exchanger 2 as sealing gas (Seal).
The line 44 shows the balancing group around the distillation column system. It intersects the purge gas line 9A, the residual gas line 33 and the sealing gas line 41 and especially the feed air line 3 and the pressurized nitrogen line 27 (illustrated in bold here). H_Luft means the enthalpy of the air stream, H_Prod the enthalpy of the product streams, WPump the heat introduced by the pump 23.
In
In the method in
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 102018000842.9, filed Feb. 2, 2018, are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102018000842.9 | Feb 2018 | DE | national |
