Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-03-19
2002-11-12
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S656000
Reexamination Certificate
active
06477860
ABSTRACT:
CROSS-REFERENCE OF RELATED APPLICATION
This application is related to our concurrently filed application entitled “Process for Obtaining Gaseous Nitrogen,” application Ser. No. 09/810,708 based on German Priority Application No. 10013074.7, filed Mar. 17, 2001.
This invention relates to a process for producing gaseous and liquid nitrogen with a variable proportion of liquid product by low-temperature separation of air in a distillation column system, said system being based on a single column rather than a conventional double column.
Single-column processes are known for the production of nitrogen. In contrast to the double-column process, single-column processes have only a high pressure column (the single column) and no conventional low-pressure column, the latter being normally operated with a reflux of a liquid nitrogen containing stream and a feed of oxygen-enriched air, both the reflux and the feed being obtained from the high pressure column under a lower pressure than the high pressure column. Nevertheless, the distillation column system of this invention may have additional columns beyond the single column, for example for obtaining ultra pure nitrogen or oxygen. Such additional columns are distinguished from the single column insofar as a stream having at least as much oxygen as air is not passed into the ultra pure nitrogen column and a liquid nitrogen stream is not passed into the ultra pure oxygen column.
The “distillation column system” comprises distillation columns that are connected to one another, but not the heat exchangers or machines such as compressors or expansion engines. In the simplest case, the distillation column system is formed exclusively by the single column.
“Oxygen-enriched” is defined here as a mixture of producer gases that has a higher oxygen concentration than air up to virtually pure oxygen. For example, oxygen-enriched fractions have an oxygen content of 25 to 90%, preferably 30 to 80%. (All percentages related here and below are molar percents, unless otherwise indicated.)
The process is used for simultaneously obtaining gaseous and liquid product nitrogen, whereby the proportion of liquid (molar ratio between liquid and gaseous product nitrogen) can be variable. At various times, different stationary operating conditions can thus prevail, in which a varying proportion of nitrogen product in liquid form is obtained. In the extreme case, this proportion can be zero. The operation of the process can then be varied between two boundary cases (1), the maximum gas production (MaxGAN case) with minimum proportion of liquid and (2) the maximum liquid production (maxLIN case) with maximum proportion of liquid and minimum proportion of gas (optionally only liquid production of nitrogen). Furthermore, any value of the liquid portion that lies between the two boundary values for minimum and maximum liquid proportions can also be adjusted.
Known from U.S. Pat. No. 4,400,188 is a process with a nitrogen circuit which comprises cooling compressed feed air in a main heat exchanger and introducing the resultant cooled feed air to said single column operating under pressure; withdrawing a nitrogen-rich fraction from the distillation column system and compressing said nitrogen-rich fraction, at least in a part in a circulation compressor; passing a part of said nitrogen-rich fraction downstream from said circulation compressor to a liquefaction chamber of a condenser-evaporator and condensing said first part of said nitrogen-rich fraction under a pressure higher than the operating pressure of said single column so as to form a nitrogen-rich liquid; passing a liquid, oxygen-enriched fraction from the distillation column system to an evaporation chamber of the condenser-evaporator so as to at least partially evaporate said liquid, oxygen-enriched fraction; passing a first oxygen-enriched gas (
234
,
533
) formed in the evaporation chamber, as ascending vapor into the single column; and withdrawing a second portion of nitrogen-rich fraction at least at times as gaseous nitrogen product, according to the introductory clause of claim 1 is known from U.S. Pat. No. 4,400,188.
A condenser-evaporator, which represents the bottom heating of the single column, is heated with nitrogen, which was brought to a level above column pressure in a circulation compressor. Process cold is produced by an ordinary residual-gas turbine, which is operated with gas from another condenser-evaporator, a top condenser. Such processes with a nitrogen circuit are more advantageous in terms of energy than single-column processes without bottom heating. Because of the circulation, it is believed that a liquid nitrogen product in a variable amount can also be produced in this process, even if this is not described in the publication itself. In such a process, however, difficulties would be encountered if it were desired to vary the proportion of liquid product. If, for example, the liquid proportion is increased, the oxygen concentration and thus the evaporation temperature at the bottom would be decreased with a uniform amount of air. The pressure in the nitrogen circuit must be correspondingly lower, and the circulation compressor thus must be readjusted accordingly. Without changing the circulation pressure, the pressure in the column would increase; in this case, the exhaust pressure of the air compressor must be adjusted accordingly.
SUMMARY OF THE INVENTION
The object of the invention is to provide a process of the above-mentioned type and corresponding apparatus, in which in addition to the gaseous nitrogen product, a variable amount of liquid product can be obtained at relatively low cost.
Upon further study, other objects and advantages of the invention will become apparent.
These objects are achieved by withdrawing a portion of the nitrogen-rich liquid from the condenser-evaporator at least at times as a liquid product, operating the evaporation chamber of the condenser-evaporator under a pressure higher than the operating pressure of the single column, and removing a second oxygen-enriched gas from one of the columns of the distillation column system and/or from the evaporation chamber of the condenser-evaporator, machine expanding same and heating the resultant cold gas in the main heat exchanger.
The liquid product can be removed directly in the liquefaction chamber of the condenser-evaporator. It is preferably first depressurized, however, and in this case the flash gas that is produced is separated. The phase separation can be performed, for example, in the single column or in a separate separator.
The operating pressures of the condenser-evaporator and the single column are decoupled by the increased pressure on the evaporation side of the condenser-evaporator. In the case of increasing liquid production, the pressure on the liquefaction side of the condenser-evaporator (nitrogen circuit) does not need to be altered. The pressure on the evaporation side can rather be adjusted—regardless of the operating pressure of the single column—with uniform evaporation temperature on the lower oxygen concentration without any compression machines having to be readjusted.
The second oxygen-enriched gas, which is provided for active pressure reduction, is preferably produced from the vapor formed in the condenser-evaporator like the first oxygen-enriched gas. The two oxygen-enriched gases have, for example, the same composition. The inlet pressure of the active pressure reduction is not—as is otherwise common in residual-gas turbines—bonded to the single column- or top condenser pressure, but rather preferably to the evaporation pressure in the condenser-evaporator. The inlet pressure of the turbines within the framework of an increase of the proportion of liquid product can therefore increase analogously to the evaporation pressure. By the correspondingly increased enthalpy difference in the machine expansion of the second oxygen-enriched gas, additional cold is produced, which is necessary for the increased product liquefaction. The increase of the residual-gas amount also increase
Kunz Christian
Rottmann Dietrich
Doerrler William
Drake Malik N.
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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