JPH04130001A - Method of material use of product flash gas - Google Patents

Abstract

PURPOSE: To effectively utilize the contained material of product flash gas by feeding the product flash gas of a pressure lower than the pressure of a reforming process to an ejector, compressing it by using the gas flow of the pressure higher than the pressure of the reforming process and returning it to the upstream of a synthetic gas generation part.

CONSTITUTION: Raw material natural gas 20 is successively supplied to a compression process 1, a desulfurization process 2, the reforming processes 3 and 4, a conversion process 5 and a CO₂ removal process 6, etc., and synthetic gas whose main component is hydrogen is generated. Then, the synthetic gas is compressed in the compression process 8, ammonia is generated by the contact conversion of the hydrogen and nitrogen in an ammonia reactor 10 and liquid ammonia is separated in a separator 11. Then, the liquid ammonia is fed to a flash drum 15 and evacuated and dissolved gas is discharged as the product flash gas and fed to an ejector 14. Then, it is compressed to the pressure of the reforming process by the gas of the pressure higher than the pressure of the reforming process and returned to the synthetic gas generation part on the upstream of the reforming process.

COPYRIGHT: (C)1992,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method of material utilization of product flash gas in an ammonia plant. This method involves producing hydrogen for synthesis from hydrocarbon-rich feedstock by the steam lafoming process, in which a certain amount of synthesis recycle gas is dissolved in liquid ammonia, and product flash gas is generated at low pressure. Applicable to plants.

[Prior Art] Most of the ammonia currently produced is based on natural gas. Currently, plants developed in the 1960s and 1970s dominate.

Ammonia production in this period is divided into conventional process steps.

1. Compression of the natural gas used; 2. Desulfurization of the natural gas; 3. Converting most of the hydrocarbons in the natural gas into hydrogen, carbon oxide and carbon dioxide by blowing steam into the second reformer; 4. Next, add process air to the reforming equipment,
5. Converting methane to hydrogen, carbon oxide and carbon dioxide; 5. Converting carbon monoxide to carbon dioxide in a two-stage carbon monoxide conversion unit; 6. Chemical and/or physical washing liquid in a special absorption tower. 7. Convert the carbon monoxide and carbon dioxide remaining in the gas into methane using a methanizer. 8. Compress the new synthesis gas to the required synthesis pressure. 9. Inject the synthesis gas into the synthesis loop. Partial conversion of hydrogen and nitrogen into ammonia, 10. Separation of product ammonia from synthesis recycle gas, 11.
Removal of inert methane and rare gases (separation of synthetic recycle gas, 12, recovery of hydrogen and rare gas from separated synthetic recycle gas, 13)
, recompression of synthesis recycle gas, and generation of synthesis gas are usually carried out at pressures of 2.5 to 5 MPa. Therefore, depending on the pressure of the raw material used, for example natural gas, if the pressure is high, it is necessary to lower it within the battery limit, and if the pressure is low, it is necessary to raise it to the required pressure for gas generation using, for example, a centrifugal compressor. . Most of the natural gas used contains small amounts of sulfur.
This acts as a catalyst poison in downstream process steps and must be removed by a desulfurizer. For this purpose,
For low sulfur contents, for example in the range up to loOmg/Nm'', it is customary to use zinc oxide, which forms zinc sulfide and scavenges hydrogen sulfide.

However, all the sulfur may exist as hydrogen sulfide (
Since it is partially bound as organic sulfur, for example in the form of mercaptans, it must first be converted to hydrogen sulfide with hydrogen. The hydrogen required for this purpose is
A substream of syngas is usually recycled from an intermediate pressure stage of the syngas compressor. This substream contains approximately 75% by volume hydrogen and typically has a pressure of 5 MPa to 10 MPa. Since this gas is returned to the synthesis gas generator, it must be reduced to the same pressure, and this reduction takes place without making energetic use of the existing pressure difference. Depending on the sulfur content in the natural gas, the required hydrogen concentration is from 10 to 10% by volume, ie the natural gas and hydrogen-rich gas are mixed in a ratio of 14:1 to 6.5:1. Conversion of organic sulfur to hydrogen sulfide is performed at 350 °C in the presence of cobalt/molybdenum or nickel/molybdenum catalysts.
The total sulfur content in the gas is generally less than 0.5 mg/Nm3 when passed through a downstream reactor filled with zinc oxide. The conversion down to 10 to 12% by volume is carried out by injecting excess process steam into a fuel-heated tube furnace, a so-called secondary reformer, at a temperature of about 800°C.The so-called secondary reformer is then fed with ammonia synthesis. Upon blowing process air to introduce the necessary nitrogen component, conversion of unconverted hydrocarbons occurs at about 1.000° C. and the residual methane content is reduced to about 0.5% by volume.

Then, in a usually two-stage carbon monoxide conversion unit, the carbon monoxide in the process gas is converted to hydrogen and carbon dioxide by steam at temperatures of about 200°C to 450°C, leaving a residual amount of 0.
．． 2 to 0.5% by volume.

The removal of carbon dioxide is carried out in special columns by washing with chemical and/or physical washing liquids,
The remaining amount decreases to about 0.1% by volume. Since residual amounts of carbon monoxide and carbon dioxide in the process gas are catalyst poisons in ammonia synthesis, the minimum amount, usually 10
+ng/Nm”. This removal is most often carried out by converting these carbon oxides to methane with hydrogen at temperatures of about 300°C to 350°C in a methanator. .

After this process step, the synthesis gas is in its final form used for ammonia synthesis. That is, synthesis gas contains hydrogen and nitrogen in a ratio of 3=1, as well as a small amount of methane, usually 0.5 to 1.5% by volume, and a rare gas, mainly argon, in the range of 0.3 to 0.7% by volume. Contains. Here, methane is caused by residual methane from the primary forming and/or secondary rifting as mentioned above, - residual from the carbon oxide conversion process - residual carbon dioxide from the carbon oxide and carbon dioxide removal process, and the rare gas is It is mainly introduced with the process air supplied to the secondary reforming equipment, and also partly with the natural gas used.

Methane and rare gases accumulate in the synthesis loop since they are inert components with respect to the ammonia component. Therefore, it is important to continuously remove from the loop the same amount of inert gas that enters the synthesis loop. Depending on the technological situation currently in use, this removal can take place, for example in the case of separation of ammonia from the synthesis recycle gas by partial condensation, on the one hand by dissolving it in the product ammonia; Depending on the synthesis pressure and the permissible concentration of inert gas in the synthesis recycle gas, up to 50% of the inert gas can be removed. Furthermore, this removal is carried out by diverting a portion of the synthesis recycle gas, the so-called recycle flash gas, to the other side. This amount of recycled flash gas is generally less than 5% of the synthesis gas sent to the synthesis loop.

When product ammonia is separated from the synthesis recycle gas by fractional condensation, the liquid ammonia is then normally depressurized stepwise from the synthesis pressure to the downstream working pressure or to near atmospheric pressure. Most of the dissolved synthesis recycle gas is then released again. The amount of this gas mixture, called the product flash gas, depends on the synthesis loop pressure and the final pressure of the product ammonia after flashing, but ranges from 92 ON m3 to 7 Nm3 per ton of ammonia.
It is between ONm3. The components are always hydrogen, nitrogen, ammonia, methane and rare gases, primarily argon.

It is known that this product flash gas can be used as energy for heating purposes (e.g. US-PS
3.705.009, US-PS 3.432.2
65, US-PS3, 598.527, DE-O5
2,129,307, DE-O51,767,2141
It is also known that the hydrogen, nitrogen and ammonia of the product flash gas can be recycled to the gas generator for material utilization (DE-PS 235.385). Simultaneous circulation of the rare gas, consisting primarily of argon, increases the efficiency and recovery of the downstream rare gas recovery equipment, since the loss of rare gas associated with the product ammonia is prevented. When flashing liquid ammonia in stages, there are different ways of joining these product flash gases into the synthesis gas generation section, depending on the selected flash pressure.

In the above patent, ammonia is flashed in a flash drum to a pressure of about 4 MPa. After cooling, this product flash gas is converted into natural gas upstream of the next reformer.
Combine with process steam mixture. The remaining product liquid ammonia is further heated to 2.2 MPa in another flash drum.
The pressure is reduced to The generated product flash gas is briefly purified (refrigerated) and then sent to the suction side of the natural gas compressor.

A disadvantage of this method is that it requires additional compression energy of the natural gas compressor to compress the product flash gas, which is generated at low pressure and is returned to the suction side of the natural gas compressor.

OBJECT OF THE INVENTION It is an object of the invention to utilize a product flash gas consisting of hydrogen, nitrogen, methane and rare gases in the synthesis gas generation part of an ammonia plant without additional compression costs.

[Means for solving the problem] The basic problem of the present invention is preferably 2.5 to 4.5 M
The product flash gas at a pressure below the reforming process pressure, which is Pa, without the need for additional compression energy.
It is recycled to the synthesis gas generation section of the ammonia plant. This problem is solved by the present invention as follows. i.e. recycled flash gas extracted from a high pressure ammonia plant (preferably at a pressure of 5 to 30 MPa)
After cooling and removing the condensed product ammonia, the product flash gas released at a pressure of preferably 1.0 to 2.5 MPa lower than the discharge pressure of the natural gas compressor is subjected to injection compression. The internal energies of these two gas streams are balanced and combined in an ejector, and this recycle-product flash gas stream is then transferred to the process gas stream upstream of the desulfurization step at a pressure above the discharge pressure of the natural gas compressor. It will be sent back to.

Both the hydrogen and nitrogen components in the recycled flash gas and product flash gas can be used as feedstock for ammonia synthesis without the required natural gas and process air compression and energy or additional feedstock for methane cracking. It will be done. The returned ammonia fraction is decomposed into basic components hydrogen and nitrogen in the primary foaming and/or secondary reforming process, and these hydrogen and nitrogen are also used as raw materials for ammonia synthesis. Unlike the rare gases that are returned with the recycled flash gas, remain unchanged and are sent back to the synthesis loop along with the new synthesis gas, methane, which acts as an inert gas in the synthesis loop, is Since it is converted into hydrogen and carbon monoxide by a steam reforming reaction in the secondary reforming step, it does not return to the synthesis loop along with new synthesis gas.

By applying the method of the invention, only the rare gases returned with the recycle flash gas (and not the methane entrained in the recycle flash gas) are sent to the synthesis loop.
The inert gas proportion in the recycled synthesis gas changes clearly in favor of rare gases. This increases and improves rare gas recovery in downstream rare gas equipment.

Since the recycled flash gas returned to the gas generator according to the present invention usually has a main component of 55 to 665% hydrogen, if this gas is recycled upstream of the desulfurizer, it will be organically fixed. It can be used as a hydrogen source to convert sulfur into hydrogen sulfide. Depending on the sulfur content of the natural gas, the circulation of the synthesis gas from the intermediate pressure stage of the synthesis gas compressor can be completely or partially stopped in this way.

In the present invention, instead of a hydrogen-containing gas such as the above-mentioned recycled flash gas, a hydrocarbon-containing gas, preferably at a pressure of 5 to 10 MPa, which is a raw material for the re-former unit, is used as the ejector driver gas.

[Examples] The present invention will be explained in detail based on Examples. Figure 1 shows a process diagram of an ammonia plant.

Approximately 38.00 ONm3/h of raw natural gas is supplied from line 20 to natural gas compression step 1, where it is compressed to approximately 4 MPa. This gas passes through line 21 to desulfurization step 2, where sulfur is removed in two stages until the remaining amount is 0.
．． It becomes 5 mg/Nm3 or less. Crawl into line 21 5
6,000 Nn+3/h of circulating flash gas and 200 Nm3/h of product flash gas supplied from No. 2 are added. - Before entering the next forming device 3, the natural gas in line 22 is supplied from line 23 with approximately 100%
t/h process stream and flash drum 1
5 and mixed with the product flash gas supplied via line 45 and heated at 500° in the convection section of the next forming process.
heated to C. Catalytic decomposition of methane has a residual amount of approximately 11.5
This is done until the volume reaches %. Here, the crude synthesis gas is about 80
Heated to a temperature of 0°C. The crude synthesis gas is sent via line 24 to the secondary reformer 4 where the methane is further converted until approximately 0.3% by volume remains. The heat required for this is approximately 55,000
Oxygen introduced with the process air required to supply ONm3/h of nitrogen is supplied by burning hydrogen. - To convert the carbon monoxide generated in the secondary and secondary reforming devices to a carbon monoxide residual amount of about 0.3% by volume, the crude synthesis gas is sent through line 26 to the two-stage conversion step 5; Here, it is converted into hydrogen and carbon dioxide. The crude synthesis gas rich in carbon dioxide is sent via line 27 to carbon dioxide removal unit 6 . Here, the carbon dioxide is washed away with activated hot potash solution, and the remaining amount is reduced to about 0.1% by volume. Removing the carbon monoxide and carbon dioxide remaining in the crude synthesis gas to reduce the remaining amount to 10 ppm or less is
This is accomplished by sending the crude synthesis gas through line 28 to a methanation step 7 where it is converted to methane using hydrogen. Approximately 3,900 Nm3/h of hydrogen fraction is sent from the outlet of the hydrogen recovery device 12 via line 38 and the rare gas device 1
Approximately 350 Nm sent via line 39 from the outlet of 3
After adding 3/h of hydrogen-nitrogen fraction, the fresh synthesis gas is sent via line 29 to the synthesis gas compressor 8, where about 30
Compressed to MPa. Line 3 is used for mixing with synthetic recycle gas.
This is done by 0 and 31. The mixed synthesis recycle gas is then sent to the ammonia reactor 10 via line 32. Ammonia is produced here by catalytic conversion of hydrogen and nitrogen. After cooling the synthesis recycle gas and condensing the liquid ammonia, the liquid-gas mixture enters separator 11 via line 33 where the liquid ammonia is separated. Before returning the synthetic recycle gas to the synthetic recycle gas compression stage 9 via line 34,
Approximately 6.00 ONm3/h of circulating flash gas is extracted from line 35 and sent to the hydrogen recovery device 12, and approximately 6.00 Nm3/h of recycled flash gas is
00 Nm/h of circulating flash gas is sent to the ejector 4 via line 36. Hydrogen recovery bag = 12
The recycled flash gas is separated into the above-mentioned hydrogen fraction and a residual gas fraction of about 2.000 Nm3/h by low-temperature separation, and the hydrogen fraction joins the new synthesis gas through line 38 to form the residual gas fraction. is sent to rare gas unit 13 via line 37.

Low-temperature separation is performed in the rare gas device 13, and the hydrogen/nitrogen fraction is separated into the above-mentioned hydrogen/nitrogen fraction, nitrogen fraction, methane fraction, and argon fraction, and the hydrogen/nitrogen fraction is combined with the new synthesis gas via line 39. , the nitrogen fraction is sent to various uses through line 40, the methane fraction is added to the fuel gas of the ammonia plant via line 41, and the argon fraction is sent through line 42 to various uses. By utilizing the present invention, the argon production amount of 18 t/d when not applying the present invention can be reduced to 19 t/d.
．． It becomes possible to multiply to 2t/d.

Liquid ammonia from separator 11 is sent via line 43 to flash drum 15 where it is reduced in pressure to about 4 MPa. The product flash gas containing ammonia is sent to the refrigeration separator 16 and cooled to -20°C. Condensed ammonia is separated and sent from flash tram 15 to line 4.
Similarly, the liquid sent via line 46 is sent to another flash drum 17. Approximately 150 ONm<3>/h of treated product flash gas is sent via line 45 to the primary forming step 3 where it joins the natural gas-process gas mixture. The product liquid ammonia sent to the flash drum 17 is reduced in pressure to about 2.2 MPa. The released product flash gas is sent downstream from line 48 to refrigeration separator 18 .

The concentrated ammonia is sent via line 49 to ammonia discharge line 50. Product ammonia is sent out of the system through line 5o at 1520 t/d. The remaining product flash gas is drawn from the line 51 by the circulating flash gas passing through the ejector 14 to 4MP
a and then joins the compressed natural gas stream upstream of desulfurization step 2 via line 52.

[Effects of the Invention] 4゜The present invention relates to a method of utilizing product flash gas. By means of the method of the invention, the product flash gas is recycled to the synthesis gas generation section of the ammonia plant without additional compression energy.

The necessary compression is provided by an injection compressor (ejector). A portion of the recycled flash gas acts as a driving gas.

[Brief explanation of drawings]

Figure 1 shows a process diagram of an ammonia plant. In the figure, ■・・・Natural gas compression process 2...Desulfurization process 3...-Next reforming process 4...Second reforming process 5... Conversion process 6... Carbon dioxide removal device 7...Methanation process 8...New synthesis gas compression process 9...Synthetic recycled gas compression process 0...Ammonia reactor 11. Separator 12...Hydrogen recovery equipment 13...Rare gas equipment 14... Ejector 15...Flash drum 16... Refrigeration separator 17...Flash drum 18... Refrigeration separator 20-52... line respectively.

Claims (1)

[Claims] 1. Hydrogen for synthesis is produced from hydrocarbon-rich raw materials by a steam reforming process, the synthesis gas contains inert gases, especially methane and rare gases at the inlet of the synthesis loop, and the product ammonia is separated from the recycle gas by fractionation, dissolving hydrogen, nitrogen, rare gases and methane;
The product ammonia is rapidly depressurized in stages or in one stage to a pressure lower than that of the reforming section of the ammonia plant, and the dissolved gas is partially released as product flash gas, which is higher than that of the reforming stage. In an ammonia plant where one or more gas streams at pressure are sent as feed to the reforming process, the product flash gas at a pressure lower than that of the reforming process is sent to the ejector where it is used as the driving medium for the reforming process. in an ammonia plant, characterized in that one or several gas streams having a pressure higher than the pressure required for Methods of material utilization of product flash gas. 2. The driving medium of the ejector is 5MPa to 30MPa.
a hydrogen-rich gas with a pressure of up to The method according to claim 1, wherein: 3. The driving medium of the ejector is 5MPa to 10MPa.
2. Process according to claim 1, characterized in that a hydrocarbon-rich gas having a pressure of up to .a.