Process and apparatus for producing pressurized gaseous nitrogen by cryogenic separation of air

Process and apparatus for producing pressurized gaseous nitrogen by cryogenic separation of air. The distillation column system includes a high pressure column, a medium pressure column, a main condenser and top condenser both being condenser-evaporators. Compressed and purified feed air is cooled in a heat exchanger and introduced to the distillation system. A gaseous nitrogen stream from the high pressure column is condensed in the main condenser. Bottom liquid of the medium pressure column is evaporated and gaseous nitrogen from the medium pressure column is condensed in the top condenser. Liquid nitrogen from the medium pressure column is pressurized and introduced to the high pressure column. A second gaseous nitrogen stream from the high pressure column is recovered as pressurized gaseous nitrogen product. A portion of the compressed and purified feed air is work-expanded and then warmed in the main heat exchanger.

The invention regards a process for producing pressurized gaseous nitrogen by cryogenic separation of air. It further concerns an apparatus for producing pressurized gaseous nitrogen by cryogenic separation of air.

“Condenser-evaporator” means a heat exchanger, in which a first, condensing fluid stream is brought into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator comprises a liquefaction space and an evaporation space which, respectively, consist of liquefaction passages and evaporation passages. In the liquefaction space, the condensation (liquefaction) of the first fluid stream is performed; in the evaporation space the evaporation of the second fluid stream is conducted. Evaporation and liquefaction spaces are formed by groups of passages, which are in heat transfer relationship. The evaporation space of a condenser-evaporator can be realized as a bath evaporator, a falling film evaporator or a forced-flow evaporator.

The above kind of process and an apparatus are known from U.S. Pat. No. 6,868,207 [P16C012-EPR3, L'AL2003]. The refrigeration is provided either by liquid assist or by a turbine exhausting into the medium pressure column or by both. The first variant consumes cold and thereby energy from the outside, the second variant does not, but incorporates operational problems.

The problem to be solved by the invention is to minimize influences of the cold production on the distillation, thereby ensuring a particularly smooth and flexible operation of the system as a whole.

Such problem is solved by the features of the invention. By this special turbine configuration expanding a portion of the feed air from about high pressure column pressure to normally somewhat above atmospheric pressure, turbine expansion is completely decoupled from distillation, as no fluid from the distillation is sent to turbine. There is also no additional compressor needed to produce the cold.

The work-expanded air can be, e.g., sent to the medium pressure column, in particular to its bottom, or by-passed around the distillation, e.g. by a separate main heat exchanger passage warming the work-expanded air to a temperature up to that of the warm end of the main heat exchanger and rejecting it to the atmosphere.

In a preferred embodiment of the invention, however, the work-expanded turbine stream is mixed with a waste stream upstream of the main heat exchanger, such waste stream being taken from the vapor produced in the evaporation space of the medium pressure column top condenser. As a consequence, also no fluid to the distillation goes through the turbine, i.e., there is a full decoupling of refrigeration production and distillation. Simultaneously, the main heat exchanger configuration is nearly as simple and compact as in the liquid assist variant, as there is no separate group of passages needed for the work-expanded air; just an intermediate withdrawal for the turbine air must be provided.

A portion of the refrigeration requirements can be provided by liquid assist, i.e., by introducing a cryogenic liquid from an external source and/or by using a cryogenic liquid that has been internally produced at another point of time into the distillation column system. In the first alternative, the cryogenic liquid comes from another air separation or nitrogen liquefaction plant, or from a tank filled by such other plant. In the second alternative, at least a portion of the cryogenic liquid is produced by the process itself, e.g. during periods of low energy cost and/or low product demand, and re-introduced into to the plant during periods of higher energy cost and/or higher product demand. By this method, there can be, e.g., a constant production of gaseous nitrogen with varying energy consumption.

The cryogenic liquid is preferably liquid nitrogen, but any other mixture or pure fraction of liquefied air gases may be used as well. In principle, the plant may also be operated by liquid assist only, i.e. without a turbine.

The introduction of the liquid is performed at one or more of the following places:the medium pressure column,the high pressure column,the pressurized liquid nitrogen line upstream or downstream the pressurising step,the evaporation space of the medium pressure column top condenser,the evaporation space of the main condenser.

Preferably, no gaseous nitrogen from the top of the medium pressure column is fed to the main heat exchanger and recovered as product. Even more preferably, the complete gaseous nitrogen produced at the top the medium pressure column is condensed in the liquefaction space of the medium pressure column top condenser and then pumped to at least high pressure column pressure and finally withdrawn as pressurized gaseous nitrogen under at least high pressure column pressure. Thereby, all the nitrogen produced is naturally recovered under the higher distillation pressure. The high pressure column gaseous nitrogen can of course be further compressed in one or more nitrogen compressors.

It is advantageous, if the compressed and purified feed air stream that is introduced into the main heat exchanger under the first pressure comprises the total feed air for the distillation column system. As a consequence, only a single group of passages for cooling air in the main heat exchanger and only a single air compressor is required.

Preferably, the expansion machine that expands the turbine stream is the single expansion machine in the process. There is no other cold production in the system except, optionally, liquid assist, i.e., introducing liquid produced at other places or at different times into the distillation system. This makes the respective plant compact and cheap.

The operating pressure at the top of the high pressure column is preferably chosen in the invention to be between 7.4 and 9.2 bars, in particular between 7.6 and 8.5 bars.

Preferably, the second pressure the turbine stream is expanded to, is lower than 1.6 bar, and lies in particular in the range of 1.2 to 1.4 bar.

In general, in the invention, the preferred ranges of the operating pressures of the columns at their tops are:high pressure column4: 7.4 bar to 9.2 bar, in particular 7.6 bar to 8.5 barmedium pressure column5: 3.7 bar to 4.6 bar, in particular 3.9 bar to 4.3 bar.

Moreover, the invention regards an apparatus for producing pressurized gaseous nitrogen. The apparatus according the invention may be supplemented by apparatus features described herein.

The total feed air1is compressed in a main air compressor50to a first pressure of e.g. 8.2 bars. The compressed air stream51is purified in a molecular sieve station52, The compressed and purified air53is introduced at the first pressure to a main heat exchanger2at its warm end. A first portion of the air (non-turbine air)3is cooled to the cold end of the main heat exchanger2and introduced into a high pressure column4. The high pressure column4is operated at a pressure of e.g. 7.9 bar at the top. It is a part of a distillation column system which further comprises a medium pressure column5, a main condenser6and a medium pressure column top condenser7. Both condensers6,7are constructed as condenser-evaporators.

A first gaseous nitrogen stream from the top the high pressure column is totally condensed in the liquefaction space of the main condenser6. The liquid nitrogen9produced in the main condenser6is introduced into the top of the high pressure column4as reflux. Bottom liquid of the high pressure column (crude liquid oxygen)10is cooled in a first subcooler11and expanded to medium pressure column pressure in a valve12. The expanded crude oxygen13is sent to an intermediate section of the medium pressure column5.

A first stream14of oxygen-enriched bottom liquid of the medium pressure column5is sent to the evaporation space of the main condenser6and at least partially evaporated. The evaporated first stream15is fed back to the medium pressure column bottom and serves as rising vapour inside the medium pressure column5.

A second stream16of oxygen-enriched bottom liquid of the medium pressure column5is cooled in a second subcooler17and in a third subcooler18. Controlled by valve20, the subcooled liquid19,21,22,23is sent to the evaporation space of the medium pressure column top condenser7. A small portion may be withdrawn as purge stream24. Controlled by valve27, the vapour25,26from the evaporation space of the medium pressure column top condenser7is sent as waste gas to subcoolers18,11. The prewarmed waste gas28is fully warmed in the main heat exchanger2. The warm waste gas29is vented and/or used in the molecular sieve station as regenerating gas.

Gaseous nitrogen30from the top the medium pressure column5is condensed in the liquefaction space of the medium pressure column top condenser7. Liquid nitrogen31produced thereby is fed back to a cup32in the top of the medium pressure column4. A first portion of such liquid nitrogen is used as reflux in the medium pressure column5. A second portion53of such liquid nitrogen is withdrawn from the medium pressure column4, pressurized in a pump33to a pressure which is at least equal, preferably higher than the high pressure column pressure. At least a first portion34,36of the pressurized liquid nitrogen flows through pump pressure control valve35and subcooler17into the high pressure column4. If necessary, a second portion37of the pumped liquid nitrogen may flow through re-circulation path38,39back to the medium pressure column5.

A second gaseous nitrogen stream40from the top the high pressure column4is warmed in the main heat exchanger2. The warmed second gaseous nitrogen stream41is recovered as pressurized gaseous nitrogen product.

In the embodiment, the primary source of refrigeration is an air turbine42. The compressed and purified feed air stream1is split at an intermediate temperature of the main heat exchanger2into a turbine stream43and the non-turbine stream3. The turbine stream is work-expanded in the air turbine42from the first pressure to a second pressure. The work-expanded turbine stream44is mixed with the waste stream28upstream the main heat exchanger2. The mixed stream is warmed in main heat exchanger2. The air turbine can be braked by any known brake mechanism, preferably by an oil brake, an air brake, oil bearing, gas bearing or foil bearing. Preferably no booster compressor is coupled to the air turbine.

As additional source of refrigeration by “liquid assist”, a cryogenic liquid45from an external source, e.g., liquid nitrogen can be introduced into the medium pressure column5(as shown in the drawing) or into the high pressure column4(not shown). The plant as shown can be operated differently at different points of time: air turbine running, no liquid assist air turbine running combined with liquid assist air turbine not running-liquid assist only.

In a particular embodiment of the invention, in a first operating mode, a portion of the pumped liquid nitrogen34,37is recovered under pressure and stored in a pressurized liquid nitrogen tank (not shown in the drawing). In a second operating mode, the air turbine is shut off or operated with reduced throughput, and the stored liquid is taken for liquid assist (line45).

Coming back to the drawing, the dashed line around the large rectangle indicates the outer wall of a first cold box46surrounding all cryogenic parts except the nitrogen pump33. The space between the apparatus and the outer wall is filled with pulverised insulation material like perlite. There is a separate cold box section47enclosing the nitrogen pump33only.

In another plant, the air turbine is omitted and the plant is steadily run with liquid assist as the single source of refrigeration.

In yet another plant, the nitrogen pump is omitted and a gaseous nitrogen stream from the top of the medium pressure column is warmed in the main heat exchanger and withdrawn as gaseous pressurized product. It can separately warmed from the high pressure column gaseous nitrogen product, so that two pressurized gaseous nitrogen products are recovered under different pressures, or the high pressure column gaseous nitrogen product is expanded to medium pressure column pressure and then mixed with the medium pressure column gaseous nitrogen product.

In yet another plant, the turbine expansion42is replaced by another type of cold production like a cryocooler, piston or sterling etc.