JP7746320B2 - Nitrogen production method and nitrogen production device - Google Patents

Description

The present invention relates to a nitrogen production method and apparatus, and more specifically, to a method and apparatus that separates and purifies feed air using a cryogenic liquefaction separation method to extract product nitrogen (product nitrogen gas/product liquefied nitrogen). This nitrogen production method and apparatus achieves a high product yield by efficiently extracting product nitrogen at higher air pressures than conventional methods.

Conventionally, the most widely used method for separating and purifying feed air using cryogenic liquefaction separation to obtain product nitrogen is to use a single fractionator to separate the feed air into product nitrogen and oxygen-enriched waste gas. Meanwhile, various processes have been proposed to improve product yield and power consumption (for example, Patent Documents 1 and 2). The processes proposed in Patent Documents 1 and 2 use two fractionators, and the waste gas from the first fractionator, which would be discarded in nitrogen production methods using a single fractionator, is used as feed for the second fractionator to produce further product nitrogen, thereby significantly improving product yield and power consumption.

Patent No. 4515225 Japanese Patent Application Publication No. 5-71870

However, the processes proposed in Patent Documents 1 and 2 inherently suffer from the problem that as the operating pressure of the device is increased, the proportion of fluid pressure that is reduced without being effectively utilized increases, resulting in energy loss and reduced efficiency. This is described in more detail below.

Figure 3 is a system diagram equivalent to that of the nitrogen production apparatus disclosed in Figure 1 of Patent Document 1. In the process of this nitrogen production apparatus, the waste gas discharged from the main heat exchanger 7 (waste gas discharge path 31) is used to regenerate the purifier 13, which purifies the feed air, and is then discharged into the atmosphere. At this time, the pressure of the outlet fluid of the upstream expansion turbine 6 is higher than atmospheric pressure by at least the amount of the pressure loss in the waste gas discharge path 31 until it is discharged into the atmosphere.

Here, the expansion turbine 6 generates the refrigeration required for the device by utilizing the pressure difference between its inlet and outlet; the smaller this pressure difference, the greater the flow rate required to generate the same amount of refrigeration. Therefore, as the pressure of the second oxygen-enriched liquefied fluid, which is the source of the expansion turbine fluid supplied to the expansion turbine 6, decreases, the flow rate of the expansion turbine 6 increases. Therefore, the pressure of the second oxygen-enriched liquefied fluid reaches its lower limit when the entire amount of the second oxygen-enriched liquefied fluid is introduced into the expansion turbine 6.

In order to evaporate and vaporize the second oxygen-enriched liquefied fluid, the condensation and liquefaction temperature of the second nitrogen gas that is heat exchanged in the second condenser 4 must be higher than the temperature of the second oxygen-enriched liquefied fluid, which results in the pressure of the second nitrogen gas being higher than the pressure of the second oxygen-enriched liquefied fluid.

Therefore, the pressure of the second nitrogen gas and the first oxygen-enriched liquefied fluid upstream of it is limited to the value at which heat exchange in the second condenser 4 is possible.

Furthermore, since the same relationship as that of the heat exchange in the second condenser 4 described above also applies to the first condenser 2, there is a lower limit to the pressure of the first nitrogen gas and the upstream feed air.

On the other hand, the pressure of the feed air is determined retroactively from the pressure of the first product nitrogen gas required by the user. If this pressure is higher than the lower limit of the feed air pressure mentioned above, there will be excess pressure in one of the fluids in the process, such as the second oxygen-enriched liquefied fluid, which will result in energy loss due to the unnecessary pressure reduction to the lower limit, resulting in reduced efficiency.

Furthermore, the nitrogen production process disclosed in Patent Document 2 differs from the process disclosed in Patent Document 1 in the method of extracting nitrogen from the second fractionator. In other words, in the nitrogen production process disclosed in Patent Document 1, nitrogen is extracted as a gas from the second fractionator, whereas in the nitrogen production process disclosed in Patent Document 2, nitrogen is extracted as a liquid (reference numeral 27 in Figure 1 of Patent Document 2) from the second fractionator (reference numeral 200 in Figure 1 of Patent Document 2) and introduced into the first fractionator (reference numeral 200 in Figure 1 of Patent Document 2) via a liquefied nitrogen pump (reference numeral 60 in Figure 1 of Patent Document 2). Despite these differences, the fluid relationships in the two condensers (reference numerals 101 and 201 in Figure 1 of Patent Document 2) are the same as in the process disclosed in Patent Document 1, and similarly, there are issues such as energy loss and reduced efficiency when the pressure of the feed air is higher than the lower limit.

The present invention therefore aims to provide a nitrogen production method and nitrogen production apparatus that can be operated more efficiently than conventional methods when the pressure of the feed air is relatively high.

In order to achieve the above object, the nitrogen production method of the present invention, in which product nitrogen is extracted from feed air, comprises a first separation step in which compressed, purified, and cooled feed air is subjected to cryogenic distillation to separate it into first nitrogen gas and a first oxygen-enriched liquefied fluid; a first indirect heat exchange step in which the first nitrogen gas is indirectly heat exchanged with the depressurized first oxygen-enriched liquefied fluid to condense and liquefy the first nitrogen gas to obtain first liquefied nitrogen, and at the same time, the first oxygen-enriched liquefied fluid is evaporated and gasified to obtain a first oxygen-enriched gas fluid; and a second indirect heat exchange step in which the first oxygen-enriched gas fluid is adiabatically expanded to generate the refrigeration required for operation. The method is characterized by including a refrigeration generation step, a second separation step in which the first oxygen-enriched gas fluid after adiabatic expansion is subjected to low-temperature distillation to separate it into a second nitrogen gas and a second oxygen-enriched liquefied fluid, a second indirect heat exchange step in which the second nitrogen gas is indirectly heat exchanged with the depressurized second oxygen-enriched liquefied fluid to condense and liquefy the second nitrogen gas to obtain second liquefied nitrogen and simultaneously evaporate and gasify the second oxygen-enriched liquefied fluid to obtain the second oxygen-enriched gas fluid, and a first product recovery step in which a portion of the first nitrogen gas separated in the first separation step is recovered from heat and then extracted as a first product nitrogen gas.

The nitrogen production method of the present invention is also characterized by including a second liquefied nitrogen pressurization process in which a portion of the second liquefied nitrogen is pressurized and introduced into the first separation process.

The nitrogen production method of the present invention is also characterized by including a second product recovery step in which a portion of the second nitrogen gas is extracted as second product nitrogen gas after heat recovery.

The nitrogen production method of the present invention is also characterized by including a product liquefied nitrogen extraction step in which at least a portion of the first liquefied nitrogen or a portion of the second liquefied nitrogen is extracted as product liquefied nitrogen.

The nitrogen production apparatus of the present invention extracts product nitrogen from feed air, and includes a first fractionator that performs cryogenic distillation of compressed, purified, and cooled feed air to separate it into a first nitrogen gas at the top of the column and a first oxygen-enriched liquefied fluid at the bottom of the column; a first condenser that performs indirect heat exchange between the first nitrogen gas and the depressurized first oxygen-enriched liquefied fluid to condense and liquefy the first nitrogen gas to obtain first liquefied nitrogen and simultaneously evaporate and gasify the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; and a first condenser that adiabatically expands the first oxygen-enriched gas fluid to generate the refrigeration required for operation. The system is characterized by comprising an expansion turbine, a second fractionator that performs cryogenic distillation of the first oxygen-enriched gas fluid after adiabatic expansion to separate it into a second nitrogen gas at the top of the column and a second oxygen-enriched liquefied fluid at the bottom of the column, a second condenser that performs indirect heat exchange between the second nitrogen gas and the depressurized second oxygen-enriched liquefied fluid to condense and liquefy the second nitrogen gas to obtain second liquefied nitrogen and simultaneously evaporate and gasify the second oxygen-enriched liquefied fluid to obtain the second oxygen-enriched gas fluid, and a first product recovery path that recovers heat from a portion of the first nitrogen gas and then discharges it as a first product nitrogen gas.

The nitrogen production apparatus of the present invention is also characterized by including a liquefied nitrogen pump that pressurizes a portion of the second liquefied nitrogen and introduces it into the first rectification column.

The nitrogen production apparatus of the present invention is also characterized by having a second product recovery path that discharges a portion of the second nitrogen gas as second product nitrogen gas after heat recovery.

The nitrogen production apparatus of the present invention is also characterized by having a product liquefied nitrogen discharge path that discharges at least a portion of the first liquefied nitrogen or a portion of the second liquefied nitrogen as product liquefied nitrogen.

The present invention provides a nitrogen production method and nitrogen production apparatus that can be operated with power consumption equivalent to or less than conventional methods, even when the pressure of the feed air is relatively high.

1 is a system diagram showing a first embodiment of a nitrogen production apparatus to which the nitrogen production method of the present invention is applied. FIG. 2 is a system diagram showing a second embodiment of a nitrogen production apparatus to which the nitrogen production method of the present invention is applied. FIG. 1 is a system diagram showing an example of a conventional nitrogen production apparatus. 1 is a graph showing the results of comparing power consumption when the feed air pressure is changed in an example.

Figure 1 is a system diagram showing a first embodiment of a nitrogen production system to which the nitrogen production method of the present invention is applied. In the following description, components that are the same as those in the nitrogen production system shown in Figure 3 and have the same functions are designated by the same reference numerals, and detailed descriptions will be omitted.

First, this nitrogen production system 100 is a double-column type with two rectification columns. The air is compressed by a feed air compressor 12 via a filter 11, purified by a purifier 13 to remove impurities such as moisture and carbon dioxide, and then cooled by a main heat exchanger 7 in a refrigerated outer tank 14, and subjected to cryogenic liquefaction separation to extract product nitrogen. This nitrogen production system 100 comprises a first rectification column 1 that performs cryogenic distillation of the compressed, purified, and cooled feed air to separate it into a first nitrogen gas at the top of the column and a first oxygen-enriched liquefied fluid at the bottom of the column; a first condenser 2 that performs indirect heat exchange between the first nitrogen gas and the depressurized first oxygen-enriched liquefied fluid to condense and liquefy the first nitrogen gas to obtain first liquefied nitrogen and simultaneously evaporate and gasify the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid; and an expansion turbine 6 that adiabatically expands the first oxygen-enriched gas fluid to generate the refrigeration required for operation. The system is equipped with a second fractionator 3 that performs cryogenic distillation of the first oxygen-enriched gas fluid after adiabatic expansion to separate it into a second nitrogen gas at the top of the column and a second oxygen-enriched liquefied fluid at the bottom of the column; a second condenser 4 that performs indirect heat exchange between the second nitrogen gas and the depressurized second oxygen-enriched liquefied fluid to condense and liquefy the second nitrogen gas to obtain second liquefied nitrogen and simultaneously evaporate and gasify the second oxygen-enriched liquefied fluid to obtain the second oxygen-enriched gas fluid; and a first product recovery line 32 that recovers heat from a portion of the first nitrogen gas and then discharges it as a first product nitrogen gas.

In the first fractionator 1, the compressed, purified, and cooled feed air is introduced from the main heat exchanger 7 through the feed air inlet line 33 into the bottom of the column, where it is separated by low-temperature distillation using a cryogenic liquefaction separation method into a first nitrogen gas at the top of the column and a first oxygen-enriched liquefied fluid at the bottom. A portion of the first nitrogen gas extracted from the top of the column through line 34 exchanges heat with the feed air in the main heat exchanger 7, and after heat recovery, is discharged as first product nitrogen gas through the first product recovery line 32. The remaining first nitrogen gas is introduced into the first condenser 2 through line 35.

A portion of the first oxygen-enriched liquefied fluid extracted from the bottom of the first rectification column 1 passes through line 36, is depressurized by pressure reducing valve 9d, and then merges with the second oxygen-enriched liquefied fluid described below, serving as a refrigeration makeup fluid to compensate for heat losses in the second rectification column 3 and second condenser 4. The remainder of the first oxygen-enriched liquefied fluid is depressurized to a predetermined pressure by pressure reducing valve 9a and introduced into the first condenser 2 through line 37, where it undergoes indirect heat exchange with the first nitrogen gas. The first nitrogen gas condenses and liquefies to form the first liquefied nitrogen, and at the same time, the first oxygen-enriched liquefied fluid evaporates and gasifies to form the first oxygen-enriched gas fluid. The first liquefied nitrogen is introduced into the top of the first rectification column 1 through line 38 and serves as reflux.

Meanwhile, a portion of the first oxygen-enriched gas fluid discharged from the first condenser 2 through path 39 is branched off into path 40 toward the second fractionator 3 as an adjustment for the operation of the unit. The majority of the first oxygen-enriched gas fluid is introduced into the main heat exchanger 7 through path 41, heated to an intermediate temperature, and withdrawn. It then flows into the expansion turbine 6 and undergoes adiabatic expansion to generate the refrigeration necessary for the operation of the unit. It then passes through path 42 and merges with the first oxygen-enriched gas fluid that has been depressurized by the pressure reducing valve 9c in the branched path 40, before being introduced into the second fractionator 3.

The first oxygen-enriched gas fluid introduced into the bottom of the second fractionator 3 via the expansion turbine 6 is separated by cryogenic distillation into a second nitrogen gas at the top of the column and a second oxygen-enriched liquefied fluid at the bottom. The second nitrogen gas 16 extracted from the top of the column via line 43 is introduced into the second condenser 4. The second oxygen-enriched liquefied fluid 17, which is extracted from the bottom of the second fractionator 3, merges with a portion of the first oxygen-enriched liquefied fluid, and is then reduced in pressure to a predetermined level via pressure reducing valve 9b, is introduced into the second condenser 4 via line 44. The second oxygen-enriched liquefied fluid and the second nitrogen gas exchange heat indirectly, causing the second nitrogen gas to condense and liquefy to form second liquefied nitrogen, while the second oxygen-enriched liquefied fluid evaporates and gasifies to form the second oxygen-enriched gas fluid.

A portion of the second liquefied nitrogen is introduced into the top of the second rectification column 3 via line 45 and becomes reflux liquid, while the remainder passes through line 46 branching off from line 45 and is pressurized by liquefied nitrogen pump 5. A portion of this is discharged as product liquefied nitrogen from product liquefied nitrogen discharge line 50, and the remainder is introduced into the top of the first rectification column 1 via line 47 as pressurized reflux second liquefied nitrogen and becomes reflux liquid together with the first liquefied nitrogen.

The second oxygen-enriched gas fluid discharged from the second condenser 4 undergoes heat recovery through heat exchange with the feed air in the main heat exchanger 7, and is then discharged as waste gas through the waste gas discharge path 31.

The process thus created has two rectification columns, just like the conventional technology shown in Figure 3, but the position of the expansion turbine 6 is different, allowing for more effective use of the pressure surplus that occurs when the feed air pressure is higher than the lower limit, as will be described later.

Figure 2 is a system diagram showing a second embodiment of a nitrogen production system employing the nitrogen production method of the present invention. In the first embodiment, the second liquefied nitrogen discharged from the second rectification column 3 is sent to the first rectification column 1. In contrast, in the nitrogen production system 200 of the second embodiment, a portion of the second nitrogen gas discharged from the top of the second rectification column is introduced into the second condenser 4 via line 43, and the remainder is extracted as second product nitrogen gas via the second product recovery line 48. The second product nitrogen gas can also be compressed by the nitrogen compressor 8 provided in the second product recovery line 48 and merged with the first product nitrogen gas. Note that a portion of the second liquefied nitrogen is introduced into the top of the second rectification column 3 via line 45 and is introduced together with the reflux liquid, while the remainder is extracted as product liquefied nitrogen from the product liquefied nitrogen discharge line 51.

Even in a process configured as in the second embodiment, the functions of the rectification column and condenser and the advantages obtained compared to conventional processes are the same as in the first embodiment.

In both the first and second embodiments, a portion of the second liquefied nitrogen is discharged as product liquefied nitrogen, but a portion of the first liquefied nitrogen may also be discharged as product liquefied nitrogen.

The nitrogen production apparatus 100 with the configuration shown in Figure 1 was used as Example 1, and the nitrogen production apparatus disclosed in Patent Document 1 shown in Figure 3 was used as a comparative example. The results of simulations for both were shown in Table 1.

In Table 1, the flow rate of each part is shown as a relative value, with the flow rate of the feed air in the comparative example set at 100, and the pressure (MPa) indicates absolute pressure. In these two simulations, product nitrogen gas of the same flow rate, pressure, and purity (oxygen concentration) is produced, but product liquefied nitrogen is not produced. In addition, to clearly demonstrate the effects of the invention, the pressure of the feed air and the pressure loss of the route are assumed to be the same, and the power consumption for each is shown in Table 2.

The power consumption of each device in Table 2 is expressed as a relative value, with the total power consumption of the comparative examples set at 100.

In the comparative example, a product nitrogen gas with a flow rate of 56 (the sum of the first product nitrogen gas (C) and the second product nitrogen gas (K)) is produced when the feed air flow rate is 100 and the air pressure is 1.3 MPa. For comparison, the second product nitrogen gas generated from the second fractionator 3 is compressed to the same pressure as the first product nitrogen gas by nitrogen compressor 8, and Table 2 includes the power consumption for this. In contrast, in Example 1, a product nitrogen gas with a flow rate of 56 (only the first product nitrogen gas (C)) is produced when the feed air flow rate is 102 and the air pressure is 1.3 MPa. The second liquefied nitrogen generated from the second fractionator 3 is sent to the first fractionator 1 by liquefied nitrogen pump 5, and Table 2 includes the power consumption for this.

Looking more specifically, the pressures of the feed air, first oxygen-enriched gas fluid, and waste gas are the same in both the comparative example and example 1. In the comparative example, the second oxygen-enriched gas fluid vaporized in the second condenser 4 is introduced into the expansion turbine 6, and the pressures of the second oxygen-enriched gas fluid and second nitrogen gas are increased as much as possible to increase the expansion ratio. In contrast, in example 1, the first oxygen-enriched gas fluid vaporized in the first condenser 2 is introduced into the expansion turbine 6 and then into the second fractionator 3, and the pressures of the second oxygen-enriched gas fluid and second nitrogen gas are reduced as much as possible to increase the expansion ratio.

In this way, in both examples, the expansion ratio of the expansion turbine 6 was set to its maximum. As a result, in the comparative example, 26% of the flow rate of the second oxygen-enriched gas fluid was introduced into the expansion turbine 6, and the remainder was depressurized without being effectively used, whereas in Example 1, 56% of the flow rate of the first oxygen-enriched gas fluid was introduced into the expansion turbine, resulting in a greater proportion being effectively used. Looking at Table 2, it can be seen that although Example 1 has a lower nitrogen yield and therefore consumes more power in the air compressor, the overall power consumption, including the amount used to compress the product of the second fractionator 3, is lower in Example 1, and nitrogen is produced more efficiently.

The above comparison was made at a feed air pressure of 1.3 MPa, but Figure 4 shows the results of a similar comparison of power consumption when the feed air pressure was changed. This comparison also includes Example 2, which used the nitrogen production system 200 configured as shown in Figure 2.

As a result, in the low pressure range, there are ranges in which the power consumption of Example 1 is greater than that of the comparative example, but at high pressures, the situation reverses, with Example 1 being advantageous, and Example 2's power consumption is equal to or less than that of the conventional method. From the above, it can be seen that the present invention can provide a nitrogen production method and apparatus that supplies nitrogen at relatively high pressures of 1.1 MPa or higher, particularly 1.2 MPa or higher, with power consumption equal to or less than that of conventional methods.

1...First rectification column, 2...First condenser, 3...Second rectification column, 4...Second condenser, 5...Liquefied nitrogen pump, 6...Expansion turbine, 7...Main heat exchanger, 8...Nitrogen compressor, 9a, b, c, d...Pressure reducing valve, 11...Filter, 12...Feed air compressor, 13...Purifier, 14...Cold insulation outer tank, 31...Waste gas discharge path, 32...First product recovery path, 33...Feed air inlet path, 34-47...Path, 48...Second product recovery path, 50, 51...Product liquefied nitrogen discharge path, 100, 200...Nitrogen production device

Claims (8)

In a nitrogen production method in which product nitrogen is extracted from raw air,
a first separation step of cryogenically distilling the compressed, purified, and cooled feed air into a first nitrogen gas and a first oxygen-enriched liquefied fluid;
a first indirect heat exchange step of indirectly exchanging heat between the first nitrogen gas and the depressurized first oxygen-enriched liquefied fluid to condense and liquefy the first nitrogen gas to obtain first liquefied nitrogen, and simultaneously evaporating and gasifying the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid;
a refrigeration generating step of adiabatically expanding the first oxygen-enriched gas fluid to generate refrigeration necessary for operation;
a second separation step of cryogenically distilling the first oxygen-enriched gaseous fluid after adiabatic expansion into a second nitrogen gas and a second oxygen-enriched liquefied fluid;
a second indirect heat exchange step in which the second nitrogen gas is indirectly heat exchanged with the depressurized second oxygen-enriched liquefied fluid to condense and liquefy the second nitrogen gas to obtain second liquefied nitrogen, and simultaneously evaporate and gasify the second oxygen-enriched liquefied fluid to obtain a second oxygen-enriched gas fluid;
a first product recovery step of recovering heat from a portion of the first nitrogen gas separated in the first separation step and then outputting the portion as a first product nitrogen gas. The nitrogen production method described in claim 1, characterized in that it includes a second liquefied nitrogen pressurization process in which a portion of the second liquefied nitrogen is pressurized and introduced into the first separation process. The nitrogen production method described in claim 1, further comprising a second product recovery step in which a portion of the second nitrogen gas is discharged as second product nitrogen gas after heat recovery. The nitrogen production method described in any one of claims 1 to 3, characterized in that it includes a product liquefied nitrogen extraction step in which at least a portion of the first liquefied nitrogen or a portion of the second liquefied nitrogen is extracted as product liquefied nitrogen. In a nitrogen production system that separates product nitrogen from raw air,
a first fractionator for cryogenically distilling the compressed, purified, and cooled feed air into a first nitrogen gas at an upper portion of the first fractionator and a first oxygen-enriched liquefied fluid at a bottom portion of the first fractionator;
a first condenser that condenses and liquefies the first nitrogen gas by indirect heat exchange with the depressurized first oxygen-enriched liquefied fluid to obtain first liquefied nitrogen and simultaneously evaporates and gasifies the first oxygen-enriched liquefied fluid to obtain a first oxygen-enriched gas fluid;
an expansion turbine that adiabatically expands the first oxygen-enriched gaseous fluid to generate the refrigeration required for operation;
a second fractionator for cryogenically distilling the first oxygen-enriched gaseous fluid after adiabatic expansion into a second nitrogen gas at the top of the column and a second oxygen-enriched liquefied fluid at the bottom of the column;
a second condenser that condenses and liquefies the second nitrogen gas by indirect heat exchange with the depressurized second oxygen-enriched liquefied fluid to obtain second liquefied nitrogen and simultaneously evaporates and gasifies the second oxygen-enriched liquefied fluid to obtain second oxygen-enriched gas fluid;
a first product recovery line for recovering heat from a portion of the first nitrogen gas and then discharging the portion as a first product nitrogen gas. The nitrogen production apparatus according to claim 5, further comprising a liquefied nitrogen pump that pressurizes a portion of the second liquefied nitrogen and introduces it into the first rectification column. The nitrogen production apparatus according to claim 5, characterized in that it is equipped with a second product recovery path that discharges a portion of the second nitrogen gas as second product nitrogen gas after heat recovery. The nitrogen production device described in any one of claims 5 to 7, characterized in that it is equipped with a product liquefied nitrogen discharge path that discharges at least a portion of the first liquefied nitrogen or a portion of the second liquefied nitrogen as product liquefied nitrogen.