Liquid Nitrogen Generator and Process

This application claims the benefit of U.S. Provisional 63/640,708, filed Apr. 30, 2024, which is herein incorporated by reference in its entirety.

The present invention generally relates to a method and apparatus for efficiently operating an air separation plant that produces a nitrogen product stream. More specifically, the present invention relates to a method and apparatus for producing liquid nitrogen without the use of a booster air and/or nitrogen compressor.

Air separation plants are industrial facilities designed to separate atmospheric air into its primary components, mainly nitrogen and oxygen. Other gases like argon, xenon, and krypton, often referred to collectively as air gases, can also be recovered depending on the process design and market needs.

Cryogenic air separation is a common method that relies on cooling the air to very low temperatures to liquefy it, followed by distillation. A typical cryogenic process generally involves several key stages:

Various methods for generating nitrogen using cryogenic techniques are known. A common configuration, particularly for producing liquid nitrogen, utilizes a double distillation column arrangement with a double condenser, as exemplified in U.S. Pat. No. 9,726,427 ('427), which is incorporated herein by reference. A notable feature of such systems is the compression scheme, which typically involves both a Main Air Compressor (MAC) and a secondary compressor stage, such as a Booster Air Compressor (BAC) or an air/nitrogen recycle compressor. In the '427 design, the MAC discharge pressure and the BAC inlet pressure are roughly aligned with the operating pressure of the high-pressure (HP) distillation column (around 6-7 bar(a)). Example stream parameters for this configuration are provided in Table I below (reproduced from Table 2 of '427).

Other prior art designs also exist. For instance, U.S. Pat. No. 4,715,873 ('873) shows a nitrogen generator where warm and cold turbines discharge at different pressures, both linked to recycle compressors. U.S. Pat. No. 10,488,106 discloses a double column system where the air feed pressure is boosted using nitrogen turbine drives, but this boost is limited relative to the higher-pressure column pressure, and a warm nitrogen turbine expands from the pressure of the HP column to the pressure of the lower-pressure (LP) column, while a cold nitrogen turbine expands from the pressure of the LP column to the pressure of the adsorber regeneration. Furthermore, DE 10339224 describes a gaseous nitrogen generator with limited liquid production potential, featuring a double column/condenser and a turbine expanding from approximately HP to LP column pressure.

A common thread among these known processes for generating liquid nitrogen is the reliance on two distinct compression duties: the MAC provides the air feed for separation, while the BAC or Nitrogen Recycle compressor supplies the additional energy required for significant liquefaction. While potentially combinable into a single complex machine casing, this typically necessitates a highly customized and costly compressor assembly. Consequently, there is a need for a liquid nitrogen generation process that can operate effectively using only a single, relatively simple MAC, avoiding complex side-stream draws or injections.

Additionally, many conventional cryogenic plants require pre-cooling the air (e.g., using a mechanical refrigeration unit) before it enters the front-end purification system. This pre-cooling step helps reduce the size and cost of the purification unit but adds equipment complexity, additional capital cost, reduced reliability and increased operating expenses. Therefore, eliminating the need for this upstream pre-cooling unit is also a desirable goal for improving overall plant cost-effectiveness and reliability.

The present invention is directed to a method and apparatus that satisfies at least one of these needs.

In one embodiment, the invention can include a method and apparatus to produce liquid nitrogen without the use of a booster air compressor or nitrogen recycle compressor. In certain embodiments, the method can include a lost air turbine that is configured to provide additional refrigeration capacity. In certain embodiments, the higher-pressure column can receive input streams from a cold turbine, as well as liquid air via a JT expansion valve. As used herein, a “lost air turbine” refers to a turbine that expands air (or a stream having a similar composition to air) for refrigeration purposes without sending the expanded air to the distillation column system for recovery of air gases of the expanded air.

Certain embodiments of the present invention may have some or all of the following features: pre-determined pressure ratios of lost air turbine and cold turbine, ratios for the flow rates of lost air turbine and cold turbine, liquid nitrogen efficiency yields similar to the amount of liquid air sent to the column. In another embodiment, condensing liquid air flow yields high efficiency heat exchange profile and resulting high efficiency and unexpected/unforeseen liquid nitrogen specific power.

Notably, embodiments of the invention can also include the following features:

A process for production of liquid nitrogen from a system of columns comprised of a higher-pressure column, a lower-pressure column, a first condenser disposed in a bottom portion of the lower-pressure column, and a second condenser is provided. The process can include the steps of: purifying a feed air in a purification unit at a pressure greater than 15 bar(a) to yield a compressed dry air feed; cooling the compressed dry air feed in a heat exchanger; expanding a first portion of the compressed dry air feed in a cold turbine to form an expanded fluid; sending at least a portion of the expanded fluid to the higher-pressure column for separation therein; liquefying a second portion of the compressed dry air feed within the heat exchanger before sending at least a portion of the second portion of the compressed dry air feed to the higher-pressure column; sending an oxygen-rich liquid from a bottom portion of the higher-pressure column to an intermediate portion of the lower-pressure column; and sending a second oxygen-rich liquid from a bottom portion of the lower-pressure column to the second condenser, wherein at least a portion of the second oxygen-rich liquid is vaporized at less than 2 bar(a) to form a top gas, and wherein at least a portion of the top gas is warmed in the heat exchanger against the compressed dry air feed, mixing a first liquid nitrogen stream that was condensed by the first condenser with a second liquid nitrogen that was condensed by the second condenser to yield a liquid nitrogen product, wherein a lost air stream is warmed and expanded to less than 2 bar(a) in a warm turbine, wherein the lost air stream is selected from the group consisting of a portion of the expanded fluid from the cold turbine, a stream withdrawn from the higher-pressure column, and combinations thereof.

In optional embodiments of the method for producing liquid nitrogen:

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

The terms “nitrogen-rich” and “oxygen-rich” will be understood by those skilled in the art to be in reference to the composition of air. As such, nitrogen-rich encompasses a fluid having a nitrogen content greater than that of air. Similarly, oxygen-rich encompasses a fluid having an oxygen content greater than that of air. Further, “substantially similar to air” is meant to encompass a stream having a similar composition as air (e.g., the ratio of nitrogen to oxygen is between 3:1 to 17:3, more preferably between 7:2 to 9:2) As used herein, “substantially similar” in pressure is a pressure that is similar to another pressure taking into account pressure losses in the system. In certain embodiments, this can be within 0.5 bar(a), more preferably within 0.3 bar(a), more preferably within 0.25 bar(a).

illustrates a first embodiment of the cryogenic air separation process according to the present invention.

The process begins with atmospheric airbeing drawn into the Main Air Compressor (MAC)where it is compressed. The compressed air is then cooled in compressor after-coolerbefore entering a front-end purification unit. This unit removes impurities such as water vapor and carbon dioxide that would freeze in the cryogenic temperatures of the main heat exchanger.

Following purification, the air stream is further compressed in boosters,and cooled in associated after-coolers, resulting in a high-pressure air stream. This streamenters the main heat exchangerto be cryogenically cooled against returning process streams.

Inside the main heat exchanger, the cooling air streamis divided. A first portioncontinues through the main heat exchangeruntil it is fully cooled near its cold end. This cold, high-pressure streamis then expanded across an expansion valveto generate Joule-Thomson cooling and potentially liquefaction, before being fed into the distillation column system. In certain embodiments, expansion valvemay be replaced by a liquid turbine. This system comprises a higher-pressure column, a lower-pressure column, and a top condenser.

A second portionof the main air stream is withdrawn from an intermediate point within the main heat exchangerat a warmer temperature than the first portion. This intermediate streamis directed to the cold turbine, where it expands to generate refrigeration. The expanded, colder air exiting the cold turbineis then fed via lineinto the higher-pressure columnfor separation therein. The outlet pressure of this cold turbineis typically set to be approximately equal to the operating pressure of the higher-pressure column(accounting for pressure losses within the system).

To provide additional refrigeration for the process, a warm turbine, functioning as a “lost air” or waste expansion turbine, is utilized. In the primary configuration shown, a stream, typically having a composition similar to air, is withdrawn from the higher-pressure column. This streamis partially warmed by passing through the main heat exchangerbefore being extracted from an intermediate point (or from the warm end of the main heat exchanger). It is then expanded across the warm turbine. The resulting cold, expanded fluidis reintroduced into the main heat exchangerto recover its remaining refrigeration by warming against incoming air, before being withdrawn from the warm end of the heat exchanger and potentially vented to the atmosphere.

Alternatively, the feed to the first turbinecan be sourced differently. Instead of using streamfrom the column, a side streamderived from the flow path after expansion in the cold turbinecan be directed to the warm turbineto perform a similar refrigeration duty.

With respect to the system of columns, an oxygen-rich liquidis withdrawn from a bottom portion of the higher-pressure column, subcooled, and expanded to the pressure of the lower-pressure column before being introduced into said lower-pressure column. The lower-pressure columnis thermally linked to the higher-pressure columnvia a first condenser/reboiler, which is configured to condense gaseous nitrogenagainst liquid oxygen collected in the bottom of the lower-pressure columninto liquid nitrogen. The liquid nitrogenthen flows back into the higher-pressure columnas a reflux stream.

A second oxygen-rich liquidis withdrawn from the bottom section of the lower-pressure columnand then expanded to a pressure that is above atmospheric before being transferred to the top condenser/reboiler. This liquid oxygen-rich fluid vaporizes while condensing low-pressure gaseous nitrogeninto lower-pressure liquid nitrogen. A waste oxygen streamis removed and then heated in the subcooler and then main heat exchanger. The streamcan be vented, or used as a regeneration gas for the front-end purification unit(not shown).

Finally, the desired liquid nitrogen productis collected from the process, potentially being drawn from one or both the lower-pressure columnand the higher-pressure column, via linesand, respectively.

While the present invention shares some visual similarities with prior art systems, such as the one depicted in FIG. 2 of U.S. Pat. No. 9,726,427 ('427), several key operational differences lead to distinct advantages, particularly in refrigeration capacity. Notably, unlike the '427 patent, certain embodiments of the present invention do not require a booster air compressor ().

A more significant distinction lies in the operating conditions of the turbine system. Although the turbo-booster arrangement in '427 (turbines/and compressors/) appears similar, the present invention operates its cold turbine and warm turbine under conditions optimized for enhanced refrigeration.

Designing cryogenic liquefaction units involves navigating practical equipment limitations. For instance, commonly used brazed-aluminum heat exchangers generally have a maximum operating pressure of about 70 bar(a). Turbines also face constraints, typically limited to a pressure expansion ratio of around 12. In prior art systems like '427, these limits dictate that the maximum inlet pressure to the cold turbine is 70 bar(a), resulting in a minimum outlet pressure of approximately 5.83 bar(a) when operating at maximum capacity (70 bar(a), pressure ratio of 12). This pressure constraint inherently limits the refrigeration potential achievable through turbine expansion.

Furthermore, conventional cryogenic plant design often sets column pressures relative to atmospheric pressure. Typically, the lowest pressure section operates slightly above atmospheric, the medium-pressure column operates 2-4 bar(a) higher, and the high-pressure column operates another 2-4 bar(a) above that. The '427 patent exemplifies this, with a top condenser at 1.28 bar(a), a low-pressure column at 3.2 bar(a), and a high-pressure column around 5.2 bar(a).

In the '427 design, both turbines discharge at essentially the same pressure (approximately 6.05 bar(a)). This is because their outlet streams () are combined and fed to the booster air compressor (). This configuration restricts the operational flexibility of the warm turbine (), largely fixing its inlet and outlet pressures based on the overall plant requirements. Consequently, the pressure ratio across this warm turbine in '427 is limited to 5.0, yielding a temperature drop of approximately 84° C.

Conversely, embodiments of the present invention optimize the system differently. The warm turbineis configured to discharge at a significantly lower pressure, typically just above atmospheric (e.g., 1.25 bar(a) as shown in Table II). This allows for a considerably higher pressure ratio across this turbine compared to the prior art. For example, an illustrative embodiment achieves a pressure ratio of 5.8. While the numerical difference between 5.8 and 5.0 may seem modest, it results in a substantially larger temperature drop across the turbine −97° C. in this example, compared to 84° C. in '427. This increased temperature drop translates directly to greater refrigeration capacity generated by this turbine stage.

Table II below presents comparative data for three prior art cryogenic facilities alongside an illustrative embodiment of the present invention ():

As highlighted in Table II, prior art examples typically maximize the pressure drop (and thus temperature difference) across the cold turbine. In contrast, the present invention shifts a larger portion of the refrigeration duty to the warm (or lower-pressure) turbine by maximizing its pressure ratio and temperature drop.

A key consequence of this approach is that the cold turbine (e.g., higher pressure turbine) in the present invention operates with a lower pressure ratio (e.g., 8.7 compared to 10.4-12 in the prior art). This reduced expansion allows the cold turbine's outlet pressure to be significantly higher (e.g., 7.4 bar(a) vs. ˜6.1 bar(a)). This higher pressure stream feeds the distillation columns, enabling operation of both the higher-pressure and lower-pressure columns at elevated pressures compared to traditional designs.

The operating pressures of the columns of a double column, double condenser process as described herein and in FIG. 2 of prior art '427 are the result of several parameters. For example, the pressure of the waste stream vaporized in top condenser/vaporizer is determined by the pressure drop to atmosphere, and the pressures of the low pressure and high pressure columns are the result of the compositions of streams being condensed and vaporized in both of the condenser/vaporizers Finally, by increasing the temperature difference between inlet and outlet of the warm turbine and reducing the temperature difference between inlet and outlet of the cold turbine embodiments of the present invention can also experience reduced heat transfer losses in the main heat exchanger, further contributing to overall efficiency. This is because the narrower temperature range of the cold turbine allows its refrigeration to be placed in the section of the air condensing phase change where refrigeration is needed. Whereas, the wider temperature range of the warm turbine can be spread over the warm section where the heat transfer curves are more linear because there is no phase change.

Furthermore, by strategically adjusting the distribution of refrigeration load, as well as warm and cold turbine temperatures and pressures, embodiments of the present invention achieve reduced overall heat transfer losses within the main heat exchanger. This benefit is particularly impactful in the cold section of the exchanger. Visual representations, such as the temperature approach curves shown inand, illustrate this effect; the area between the heating and cooling curves in the cold section is smaller for the present invention, signifying reduced thermodynamic losses.

While the prior art system ('427) may exhibit a slightly smaller temperature difference (and thus higher efficiency) in the warm section of the heat exchanger, optimizing performance at the cold end is more critical for maximizing the overall efficiency of cryogenic processes, since colder refrigeration is more expensive to produce.

It is known that the areas between the heating and cooling curves represent irreversible losses (inefficiency) of the heat exchange process. At a high level, comparing the max dT in each section can give an indication of this efficiency loss. Therefore, effective heat transfer aims to minimize the temperature difference (dT) between the exchanging streams throughout the exchanger.

Table III provides a comparison of the maximum temperature differences (Max dT) within key sections of the main heat exchanger for an embodiment of the present invention versus the '427 design:

As indicated in Table III, the present invention achieves smaller maximum temperature differences in the cold section (9.5° C. vs 10.1° C.) and the middle section (10.5° C. vs 15° C.) compared to the '427 configuration. Although the '427 design shows a tighter temperature approach in the warm section (4° C. vs 6.8° C.), the enhanced performance of the present invention in the colder, thermodynamically more sensitive sections contributes significantly to its improved overall efficiency compared to the prior art.

Conventional Main Air Compressor (MAC) comprise intercoolers and an after cooler where cooling water supply (e.g. from evaporative tower and/or closed loop cooling system) cools the air to near atmospheric temperature. Some water is condensed in the intercoolers and after cooler and removed from the system. The quantity of remaining entrained moisture content in the saturated compressed air exiting the after cooler increases with increasing temperature and reduces with increasing pressure. For example, saturated air at 30° C. and 6.4 bar(a) contains 5.47 kg water/KNmair.

Sizing a purification system for such water content would yield very large and possibly unfeasible adsorbers for large flows. The conventional solution is adding a precooling system to cool the compressed air. For example 5.8° C. and 6.4 bar(a) yields 1.18 kg water/KNmair. However, a high pressure air system at 30° C., 32 bar(a) also yields 1.18 kg water/KNmair (same water content in compressed air but no precooling). In addition, the volumetric flowrate is inversely proportional to pressure. Thus, equipment size and piping at 32 bar is five times smaller than a purification system at 6.4 bar and passing the same mass flowrate.

Certain embodiments of the present invention have 3 to 4% lower power consumption for same net production (i.e., 3 to 4% improved efficiency) compared to prior art '427. In addition, certain embodiments of the present invention have significantly lower equipment cost due to (1) one compressor (1×HPMAC) rather than two compressors (1×MAC+1×BAC), (2) smaller adsorbers, and (3) optionally no precooling.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.