JPS60171377A - Method of recovering argon - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] The present invention relates to a method for recovering argon, and in particular, when separating and recovering argon contained in ammonia synthesis purge gas by a cryogenic separation method, it is possible to omit the high-pressure circulating nitrogen compressor that was conventionally considered indispensable. This invention relates to a method for economically recovering argon by reducing equipment costs and power consumption.

In the ammonia synthesis reaction using steam-reformed naphtha gas as raw material, undecomposed methane and argon contained in the feed air accumulate in the synthesis loop and inhibit the reaction, so they are looped together with unreacted hydrogen. It is necessary to release it outside the system, and this is called purge gas, which contains hydrogen, nitrogen,
Contains argon and methane. The present invention provides a particularly useful technique for economically recovering argon from such ammonia synthesis purge gas in a high yield.

Currently, the methods for recovering argon from the ammonia synthesis purge gas described above are as follows:
This is a cryogenic separation method as shown in Figure 1 (flow diagram). In this figure, a raw material gas containing hydrogen, nitrogen, argon, and methane is sequentially introduced into a first heat exchanger 1 and a second heat exchanger 2, through which it passes in the opposite direction, and hydrogen is taken out of the equipment. The temperature is lowered by undergoing heat exchange with the gas and off-gas. Next, flash distillation is carried out in the first flash drum 3 to separate hydrogen with the lowest boiling point, and the remainder is liquefied.Furthermore, after removing a small amount of low boiling point components in the second flash drum 4, methane is extracted from the line LX. It is supplied to the middle part of tower 5. A boiler 6 is installed at the bottom of the methane column 5, and the boiler 6 is powered by high pressure liquid nitrogen at a temperature of about -160°C, which is sent from the high pressure circulating nitrogen compressor a7 through the line L2, as will be described later. Distillation is carried out in the methane tower 5 by the heat of the liquid nitrogen, and high-boiling point methane enters the first heat exchanger 1 from the bottom of the tower through line L3, exchanges heat with the raw material gas, and then exits the system as an off-gas. is discharged to. On the other hand, a part of the mixed gas of nitrogen and argon that has risen in the methane tower 5 is cooled by a condenser 8 installed at the top of the tower (the refrigerant is liquid nitrogen supplied from a liquid nitrogen reservoir 9), becomes liquid, and is transferred to the methane tower. It returns to the top of 5 and becomes a reflux liquid. The remaining nitrogen and argon mixed gas is sequentially extracted from line L5 and sent to the argon column 10.
Here, argon is recovered. That is, argon tower 1
A boiler 11 is provided at the bottom of the reboiler 11, and the reboiler 11 is heated to a temperature of approximately -173°C by medium pressure sent through line L6 from a medium pressure circulating nitrogen compressor 12 provided behind.
It is heated with liquid nitrogen at ℃, and distillation is performed using this as a heat source.

The high boiling point argon is then extracted as a product from the bottom of the column. In addition, the nitrogen gas containing a trace amount of argon is cooled by the condenser 1B installed at the top of the argon column 10 (the refrigerant is liquid nitrogen supplied from the liquid nitrogen reservoir 9), and some of it becomes liquid and circulates, but it does not liquefy. High purity nitrogen with a low boiling point is sequentially withdrawn from the top of the column through line L7. In addition, a part of the low-purity liquid nitrogen (containing a trace amount of argon) circulating in liquid form is sequentially extracted from line L8 and
After being combined with the off-gas from the flash drum 4, it exchanges heat with the raw material gas in the second heat exchanger 2 and first heat exchanger 1, and is then discharged to the outside of the system.

On the other hand, the high-purity nitrogen gas extracted from the top of the methane tower 5 through the line L4 is combined with the high-purity nitrogen gas extracted from the argon tower 1O in the line L7, and then passed from the line L to the supercooler 14, Medium pressure circulating nitrogen compressor 1 via second circulating nitrogen heat exchanger 15 and first circulating nitrogen heat exchanger 16
2 and compressed, a portion of which is passed through each heat exchanger 16 and 15 in reverse to adjust the temperature to an appropriate temperature (approximately -173°C), and is circulated and supplied as a heat source to the argon column reboiler 11 through lines ① and 6. The liquid nitrogen condensed by radiating heat here is further cooled in the subcooler 14 and then sent to the liquid nitrogen reservoir 9 through the line L1o. In addition, the remaining intermediate pressure nitrogen discharged from the intermediate pressure circulating nitrogen compressor 12 is further pressurized and heated by the high pressure circulating nitrogen compressor 7, and then passes through each heat exchanger 16, 15 in reverse to reach an appropriate temperature ( Line L after being adjusted to approximately -160℃)
2 as a heat source for the methane column reboiler 6, and the liquid nitrogen condensed by dissipating heat here is sent to the liquid nitrogen reservoir 9 from the line L□□. 17 in the figure is an expansion turbine,
It is used for cooling high-purity nitrogen and, in turn, for cooling the entire equipment. Also, in the figure, the part surrounded by a dashed line is placed in an insulated cold box.

When operating this type of equipment, compressors and expanders are used to pressurize or depressurize the processing fluid (gas or liquid) depending on the operating conditions, but especially the power required to operate the compressor. The reality is that these costs account for most of the power costs for the entire facility. In particular, this type of equipment is often quite large-scale and requires high power costs, so reducing these power costs is an unavoidable problem if the cost of recovered argon is to be reduced.

The present invention takes into account these points, and by discontinuing the use of a high-pressure circulating nitrogen compressor in the argon recovery equipment shown in Figure 1 and organically combining new technical means, it is possible to at least improve the quality of the product. The purpose is to reduce the power cost required for conventional argon recovery equipment and increase economic efficiency without compromising argon recovery efficiency and smoothness. However, the structure of the present invention that has achieved this purpose is that a part of the liquid methane taken out from the bottom of the methane tower is combined with the raw material gas using the argon recovery method shown in Figure 1. After being subjected to heat exchange to raise the temperature, it is circulated to the bottom of the methane tower and used as a heat source for the methane tower, and the remaining liquid methane taken out from the bottom of the methane tower is mixed with liquid nitrogen recovered from the top of the argon tower. The gist is that medium-pressure nitrogen introduced from outside the system for heating the argon column is used for heat exchange with the raw material gas.

The structure and effects of the present invention will be explained below with reference to the drawings, but the drawings are representative examples and do not limit the present invention. Of course, it is also possible to appropriately change the number and position of exchangers, piping lines, etc., and all of these are included in the technical scope of the present invention. FIG. 2 is a flow diagram showing an embodiment of the present invention, and the basic configuration is the same as the conventional example shown in FIG. Therefore, it is omitted. The unique feature of the present invention is that the installation of the high-pressure circulating nitrogen compressor 7 in Fig. 1 is actively omitted, and the line shown in bold line in Fig. 2 is newly installed to enable medium-pressure operation. Since they exist in many places, I will mainly explain them.

First, in the present invention, a branch line L12 is connected to the methane extraction line L3 provided at the bottom of the methane tower 5, and a part of the liquefied methane is guided from the branch line L1□ to the first heat exchanger 1 to be exchanged with the raw material gas. After the gasified methane was heated and gasified by exchanging heat between the two, the gasified methane was reintroduced into the bottom of the methane tower 5 and used as a heat source for distillation in the methane tower, making it possible to omit reboiler heating using high-pressure circulating nitrogen. Herein lies the first feature. That is, conventionally, in order to heat the bottom of the methane tower 5 to around -160°C, which is the boiling point of methane, high-pressure circulating nitrogen of about 80 Kf 7 cm2G is used, and for this purpose, the high-pressure circulating nitrogen compressor 7 is used. was considered essential. This supplies circulating nitrogen for heating to -160
This is because latent heat heating is possible by condensing at a temperature level of °C, and high pressure is essential to maintain the heat exchange balance of circulating nitrogen. On the other hand, for heating the methane tower 5,
It is possible to use medium/low pressure nitrogen below K7/cIn2G, but since methane is heated and evaporated at a temperature of about 1160°C at the bottom of the methane tower 5, medium/low pressure nitrogen that does not liquefy at this temperature level is considered. When reboiler 6 is heated, latent heat (evaporation of raw material gas) and sensible heat are exchanged, so the nitrogen temperature on the inlet side of circulating nitrogen rises extremely, and the heat exchange balance of circulating nitrogen is disrupted, making it impossible to operate. There is a risk of falling.

Therefore, in order to avoid such a loss of heat exchange balance, it is necessary to perform heating in the reboiler by heat exchange between latent heats, and for this purpose, it is necessary to use circulating nitrogen at high pressure and -i
The state must be such that liquefaction is possible at a Breaking away from the common wisdom of "heating the methane tower over the boiler" and adopting a unique heating method that heats the liquefied methane extracted from the methane tower and returns it to the bottom of the methane tower, we are able to eliminate any negative effects on operational efficiency, etc. The high-pressure circulating nitrogen compressor can now be omitted without any negative impact.

By the way, if the high pressure circulating nitrogen compression 837 is omitted,
Methane from high pressure circulating nitrogen? 55 is no longer heated, and the heat of the raw material gas is used to heat the methane tower, so when considering the heat balance of the first heat exchanger 1 and the second heat exchanger 2, the cooling effect of the raw material gas in this part becomes excessive. The feed temperature to the methane tower 5 decreases, the negative β of heating at the bottom of the methane tower 5 increases, and as a result, from the line L1□ to the first heat exchanger 1
It is conceivable that the heating by the liquid methane that is circulated and supplied through the process may become insufficient and the process may not be viable. Therefore, in the present invention, in order to prevent excessive temperature drop of the raw material gas, medium pressure circulating nitrogen (generally about 7 KF!/cm2G) is guided from the line L6 to the second heat exchanger 2 through the line L13. By condensing the medium-pressure circulating nitrogen at this point, the temperature of the feedstock to the methane tower 5 is increased in this part, thereby preventing an increase in the heating load at the bottom of the methane tower. The medium-pressure circulating nitrogen after heat exchange in the second heat exchanger 2 is sent to the liquid nitrogen reservoir 9 through the line L14.

By the way, when the raw material gas is cooled by a heat exchanger, a certain temperature, for example, the temperature of liquid methane (approximately -1
Assume that the cold source for cooling the raw material gas is concentrated at 60°C. When the concentration of the two cold sources increases in this area and the amount of heat exchange in this area increases, the raw material gas is cooled to around the temperature of liquid methane. However, the cooling side is 1160℃
If so, it is impossible for the raw material gas to drop to a temperature lower than 1160°C, and this means that the amount of heat that can be exchanged at each temperature level has a maximum value that corresponds to the amount of heat in the raw material gas. (A pinch is when the amount of heat exchanged at a certain temperature level exceeds the maximum amount of heat that can be exchanged at that temperature level).

When considering these matters by applying them to Fig. 1 (conventional example) and Fig. 2 (inventive example), in the conventional example, the raw material gas and liquid methane are exchanged in the first heat exchanger 1 as in the present invention. (Methane tower 5
Since heat exchange (for heating the bottom part) is not performed, liquid methane from the bottom part of methane tower 5 is directly transferred to the first heat exchanger l.
Even if it is supplied to the liquid methane temperature (-160° C.), the amount of heat exchanged does not concentrate at the level of the liquid methane temperature (-160° C.), the amount of heat exchanged does not exceed the maximum amount of heat that can be exchanged, and the above-mentioned “pinch” problem does not occur. However, in the present invention, the remainder of the liquid methane extracted from the bottom of the methane tower 5 (excluding the liquefied methane that is circulated as a heating source) is discharged out of the system through the first heat exchanger l, as in the conventional example. Then, in the first heat exchanger 1, heat exchange with the raw material gas is used for both (1) heating the liquefied methane for heating the methane tower 5 and (2) heating and evaporating the remaining liquefied methane, but the liquefied methane temperature level The amount of heat exchanged above (■) and (■) in
occurs. Therefore, in the present invention, in order to reliably prevent such adverse effects, liquid methane extracted from the bottom of the methane column is replaced with low-purity nitrogen (line L8) extracted from the top of the argon column.
) to lower the evaporation temperature of methane, and from the line L15 to the second heat exchanger 2 and the first heat exchanger 1.
By discharging the heat to the outside of the system through the heat exchanger 112, the temperature of the heat exchanger 112 at each temperature level is prevented from exceeding the maximum exchangeable heat exchange claw, and the occurrence of a pinch is reliably avoided.

The present invention is constructed as described above, but the key points are: 1) Use liquid methane extracted from the methane tower and heated and circulated as a heating source for the methane tower, and 2) Prevent an increase in heating load at the bottom of the methane tower. Therefore, by guiding medium-pressure circulating nitrogen to a heat exchanger for cooling the raw material gas and cooling it to liquefy it, the temperature at which the raw material is fed to the methane tower is increased. Furthermore, the liquefied methane extracted from the bottom of the methane tower is transferred to the argon tower. By combining with the low-purity nitrogen extracted from the top and then discharging it through the heat exchanger, the pinches that can occur in the heat exchanger are avoided, and the high-pressure circulating nitrogen is maintained without causing any problems in operational efficiency. It became possible to positively omit the use of a compressor. As a result, the equipment cost was significantly reduced, the power required to operate the compressor became unnecessary, the power required for the entire equipment was significantly reduced, and the operability of the process itself was improved.

[Brief explanation of drawings]

Figure 1 is a flow diagram showing conventional argon recovery equipment, Figure 2
The figure is a flow diagram showing an embodiment of the present invention. l...first heat exchanger, 2...second heat exchanger, 3...
...First flash drum, 4...Second flash drum, 5...Methane tower, 7...High pressure circulating nitrogen compressor, 6.11...Reboiler, 8.13...Condenser , 9...Liquid nitrogen reservoir, lO...Argon column, 12.
... Medium-pressure circulating nitrogen compressor, 15... Second circulating nitrogen heat exchanger, 16... First circulating heat exchanger, 17... Expansion turbine. Hisashi Ueki, patent attorney representing applicant Kobe Steel, Ltd.)

Claims (1)

[Claims] A mixed gas containing hydrogen, nitrogen, methane, and argon is cooled by heat exchange and hydrogen is separated by partial condensation, then led to a methane tower, where methane is separated from the bottom of the tower, and liquid nitrogen and liquid argon taken out from the top of the tower are separated. This method recovers high-purity argon by guiding the mixed liquid into an argon column and taking out argon from the bottom of the column. A part of the liquid methane taken out from the bottom of the methane column is heated by subjecting it to heat exchange with the raw material gas. After that, the remaining liquid methane taken out from the bottom of the methane tower is mixed with the liquid nitrogen recovered from the top of the argon tower to generate heat with the raw material gas. A method for recovering argon, characterized in that a part of medium-pressure nitrogen for heating the argon column introduced from outside the system is subjected to heat exchange with a raw material gas.