For the production of synthetic ammonia from natural gas, what kind of system is the Prisen system?

In 2019, the separator renovation project for the Praxair unit in Sinopec Hubei Fertilizer Company’s ammonia synthesis system was put into operation. Under the same load conditions, the hydrogen recovery rate increased by 17%, allowing an additional 600 cubic meters of hydrogen to be recovered per hour; at the same time, liquid ammonia production increased by 0.3 tons, resulting in annual cost savings of 3.64 million yuan. The Prisen unit is a hydrogen recovery system introduced by the company in 1983; it uses membrane separators to recover most of the hydrogen from the off-gases generated by the ammonia synthesis system, and sends this hydrogen back to the inlet ports of the high and low pressure cylinders of the feed gas compressor. After 36 years of operation under high load, the Prisons unit has aged and poses safety risks; the hydrogen recovery rate of the polysulfone membrane has dropped from the designed value of 92.5% to around 70%. A large amount of hydrogen is sent to the fuel gas network along with the vent gas, resulting in significant waste of feed gas. It is urgent to upgrade and renovate the unit. Following the approval of this research project, the company took advantage of the minor maintenance shutdown at the end of October this year to carry out technical upgrades to the separator in the Prisen unit. The system, which previously used two sets of membrane separators operating in series – consisting of six cylinders and six membrane modules – will now have two cylinders and five membrane modules. After being put into operation, the device runs stably; it not only meets the process requirements and eliminates safety hazards but also reduces costs, achieving energy savings and consumption reduction – truly killing several birds with one stone.

Background technology: The overall process for synthesizing ammonia in nitrogen fertilizer plants involves the following steps: Natural gas from long-distance pipelines first enters a gas distribution station, where it is buffered and pressure-regulated before proceeding to the normal-temperature desulfurization system of the ammonia synthesis unit. Thereafter, the gas undergoes compression, high-temperature desulfurization, heat-exchange-based first-stage steam reforming and second-stage oxygen-enriched air reforming, high- and low-temperature carbon monoxide conversion, decarburization using the modified thermal potassium-alkali method, and further purification through methanation to remove residual CO and CO2. The syngas is then compressed, and ammonia is synthesized at 14.0 MPa; finally, cryogenic separation is carried out to obtain liquid ammonia as the product. The main purpose of the synthesis system is to react hydrogen and nitrogen in an ammonia synthesis tower, under high temperature and pressure as well as in the presence of a catalyst, to produce gaseous ammonia. This gaseous ammonia is cooled through various stages of coolers to turn into liquid ammonia, which is then separated out. The liquid ammonia is transported via pipelines to storage tanks for storage. The gas components entering the synthesis system are mainly hydrogen and nitrogen, along with some inert gases such as methane and argon. During the production process of the synthesis system, inert gases such as methane and argon do not participate in the synthesis reaction; these inert gases continue to circulate and accumulate within the system. Once they reach a certain amount, they consume a large amount of energy in terms of compression power by the compressors. At the same time, inert gases also reduce the synthesis efficiency of hydrogen and nitrogen in the synthesis tower. During production, when a certain amount of inert gas accumulates, we partially discharge the synthesis cycle gas through the vent line located behind the ammonia separator, thereby reducing the content of inert gas in the cycle gas of the synthesis system. The air released contains mainly hydrogen, ammonia, nitrogen, methane, and argon. If it is discharged directly without recycling, it will result in a large amount of waste of raw gas as well as environmental pollution.

The off-gases from the synthesis system (whose main components are NH3, N2, H2, and CH4) are transported via pipes to the hydrogen-ammonia membrane recovery system. These off-gases first enter the system at the bottom of the high-pressure ammonia scrubbing tower, which is equipped with structured packing. After being pressurized by a high-pressure ammonia washing pump, the desalinated water is conveyed through pipes and enters from the upper part of the high-pressure ammonia washing tower. The exhaust gas comes into countercurrent contact with the desalinated water inside the tower; the ammonia in the exhaust gas is absorbed by the water, resulting in an ammonia solution with a concentration of about 13%, which is then discharged from the bottom of the tower. The gas after ammonia removal exits from the top of the tower and enters a cooler; once its temperature drops to around 20°C, it goes into a separator where the moisture in the gas is removed. After that, the gas is heated to around 40°C in a heater before entering a high-pressure membrane separation unit. The high-pressure hydrogen produced as a result is sent back through pipes to the second stage of the synthesis compressor for reuse. The low-pressure hydrogen separated in the low-pressure membrane separator is returned through pipes to a section of the synthesis compressor for reuse. The H2 released into the air is essentially completely separated and recovered, while the remaining CH4 and N2 in the gas are sent via pipes to a combustion furnace to be used as fuel. The ammonia solution coming out of the bottom of the high-pressure ammonia washing tower is sent to the inlet pipeline of the circulating cooler through a level control valve. The ammonia solution from the high-pressure ammonia washing tower mixes with that from the low-pressure ammonia washing tower before entering the circulating cooler. After being cooled by the circulating cooler, the ammonia solution enters the tower from the top of the low-pressure ammonia washing tower. There, it comes into countercurrent contact with the vapor (whose main component is NH3) that enters from the bottom of the tower; the ammonia in the vapor is absorbed as a result of this contact. The resulting ammonia then exits from the top of the low-pressure ammonia washing tower and returns to the fuel pipeline network to be used as fuel gas. A portion of the ammonia water in the low-pressure ammonia washing tower is pressurized by a low-pressure ammonia washing pump and reused, while another portion is sent to an ammonia water storage tank through pipes at the bottom of the tower for storage.

The problem with this system is as follows: during normal operation, the operating pressure in the high-pressure ammonia washing tower is around 11.8 MPa, while that in the low-pressure ammonia washing tower is around 1.2 MPa. The pressure of the high-pressure hydrogen separated by the high-pressure membrane separator is around 6.5 MPa, and the pressure of the low-pressure hydrogen separated by the low-pressure membrane separator is around 3.0 MPa. The membranes used in our high-pressure and low-pressure membrane separators are Prisen membranes. The function of these membrane tubes is to separate H2 from the exhaust gas for reuse, while the remaining gas is sent back to the fuel system for combustion. The Prisen membrane is sensitive to ammonia and water; if the ammonia content in the gas exceeds 10 ppm, or if water gets into the membrane tubes, it will cause severe damage to them. Therefore, it is strictly prohibited to allow gases containing ammonia or water to enter the membrane tubes during operation. After years of use in production, we have gradually identified several serious problems with the hydrogen-ammonia membrane recovery system: 1. When the high-pressure ammonia washing tower is under load, it is observed that ammonia-containing liquid often reaches the subsequent processing stages. In severe cases, the separators installed in these subsequent stages are unable to completely remove the moisture present in the gas, allowing ammonia-containing liquid to reach the membrane separator and thus causing damage to it and reducing the service life of the membrane tubes. 2. When there is a malfunction in the synthesis system, no air is allowed to enter the high-pressure ammonia washing tower, while the low-pressure ammonia washing tower often continues to operate normally. Once the pressure in the high-pressure ammonia washing tower drops below that in the low-pressure one, ammonia solution may flow back into the high-pressure ammonia washing tower through the pipes. In severe cases, once the high-pressure ammonia washing tower is filled with ammonia solution, this solution will overflow into the membrane tubes in the subsequent processing stages, thereby causing serious damage to those membrane tubes. 3. All the high-pressure and low-pressure hydrogen separated by the high-pressure membrane separator and low-pressure membrane separator in the original design is sent back to the synthesis compressor for reuse; there are no pipelines designed to send the separated hydrogen back to the desulfurization unit. If there are problems with the hydrogen return pipeline leading to the desulfurization unit in the processes prior to ammonia synthesis, then no hydrogen will be available to feed into the desulfurization system, resulting in the organic sulfur in natural gas not being converted into inorganic sulfur for removal, and this in turn causes the catalysts in subsequent processes to become poisoned and ineffective. (Hydrogen and organic sulfur can react in a desulfurization unit to produce inorganic sulfur, which is then removed by the desulfurizing agent. ) 4. In the original design, after the vent gas is separated into hydrogen using a high-pressure membrane separator and a low-pressure membrane separator, the remaining gas (with N2 and CH4 as its main components) is recovered and used as fuel gas. Since this remaining gas contains a large amount of N2, its combustion in the heating furnace results in very poor heat release; moreover, N2 carries away the heat from within the heating furnace, leading to heat loss. Technical Solution: To address the aforementioned technical problems, the objective of this utility model is to provide a hydrogen-ammonia membrane recovery system for ammonia synthesis off-gas, thereby preventing the membrane tubes in high-pressure and low-pressure membrane separators from being damaged by ammonia and water and extending their service life. To achieve the above objectives, the technical solution of this utility model is: a hydrogen-ammonia membrane recovery system for ammonia off-gas in ammonia synthesis, which includes a high-pressure ammonia washing tower, a low-pressure ammonia washing tower, a cooler, a separator, a heater, a high-pressure membrane separator, and a low-pressure membrane separator. The feature of this system is that the high-pressure ammonia washing tower is equipped with packing; a demister is placed above said packing. The feed pipeline for deionized water into the high-pressure ammonia washing tower is located above the demister, while the inlet pipeline for the ammonia synthesis off-gas is situated below the packing. The upper end of the deionized water nozzle is connected to the deionized water feed pipeline, and its lower end extends through the demister to lie between the demister and the packing. A water-blocking ring is fitted around the upper part of the deionized water nozzle; this ring is attached to the upper surface of the demister and is fixed above it via a fixing support frame ； The gas outlet at the top of the high-pressure ammonia washing tower is connected to a cooler; after being cooled, the gas enters a separator to have the moisture removed. The gas that comes out of this separator is then heated by a heater before entering a high-pressure membrane separator. The hydrogen gas separated from the high-pressure membrane separator is sent to a synthesis compressor. The gas remaining after separation in the high-pressure membrane separator enters a low-pressure membrane separator, and the gas obtained after hydrogen has been separated from this low-pressure membrane separator goes into a methane membrane separator. An exhaust pipeline is provided at the top of the methane membrane separator ； The ammonia wastewater discharge pipeline at the bottom of the high-pressure ammonia washing tower is connected to the circulating cooler, and the outlet pipeline of the circulating cooler is connected to the inlet pipeline at the upper part of the low-pressure ammonia washing tower. A check valve is installed on the ammonia wastewater discharge pipeline at the bottom of the high-pressure ammonia washing tower. With the above approach, a check valve is first installed on the ammonia discharge pipeline at the bottom of the high-pressure ammonia washing tower, to prevent ammonia from flowing back into the high-pressure ammonia washing tower when its pressure drops below that of the low-pressure ammonia washing tower. A demister and a water baffle are installed at the top of the high-pressure ammonia washing tower to prevent large amounts of ammonia solution from entering subsequent processes, ensuring that the subsequent separators can remove as much water as possible. The water barrier ring is secured by a fixed support frame to prevent ammonia from escaping through the gaps in the ring and entering subsequent processes along with the gas. The newly added methane membrane separator completely separates the methane from the off-gas, which is then returned to the process gas system for further use in production; the remaining N2 is simply vented. This approach prevents large amounts of N2 present in the off-gas from entering the furnace, thereby avoiding issues related to impaired combustion efficiency and heat loss. In the above scheme: a level control valve is also installed on the ammonia aqueous solution discharge pipeline at the bottom of the high-pressure ammonia washing tower. In the above scheme: the low-pressure hydrogen exhaust pipeline of the low-pressure membrane separator is divided into two branches; one branch leads to the synthesis compressor, while the other branch leads to the desulfurization system. A new hydrogen return pipeline from the membrane separator to the desulfurization system has been added. When there are problems with the hydrogen return pipeline for desulfurization in the processes prior to ammonia synthesis, hydrogen can be supplied to the desulfurization tank through this new pipeline, thereby ensuring that the organic sulfur in natural gas reacts completely with hydrogen in the desulfurization tank and is removed, thus guaranteeing the safe use of catalysts in subsequent processes. Beneficial effects: The synthesis system of this utility model effectively prevents ammonia water from damaging the membrane tubes, thereby extending their service life. It solves the problem of preventing large amounts of N2 in the air from entering the heating furnace and affecting combustion efficiency as well as carrying away heat. Meanwhile, a hydrogen return pipeline is provided for the desulfurization unit to ensure that the organic sulfur in natural gas reacts completely with hydrogen in the desulfurization tank before it is removed, thus ensuring the safe use of catalysts in subsequent processes. Well-designed with low renovation costs. Figure Description: Figure 1 is a flowchart illustrating this utility model. Specific Implementation Modes: The present utility model is further described below through examples in conjunction with the accompanying drawings: In Example 1, as shown in Figure 1, the hydrogen-ammonia membrane recovery system for ammonia off-gas consists of a high-pressure ammonia washing tower 1, a cooler 2, a separator 3, a heater 4, a high-pressure membrane separator 5, a low-pressure membrane separator 6, a methane membrane separator 7, a low-pressure ammonia washing tower 8, a circulating cooler 9, a low-pressure ammonia washing pump 10, a check valve 11, a liquid level control valve 12, as well as connecting pipelines and other valves. Filler 101 is installed inside high-pressure ammonia washing tower 1; a demister 102 is placed above the filler 101. The feed pipeline for deionized water into the high-pressure ammonia washing tower 1 is located above the demister 102, while the inlet pipeline for the synthetic gas released from the tower is situated below the filler 101. The upper end of the deionized water nozzle 103 is connected to the deionized water feed pipeline, and its lower end passes through the demister 102 to extend between the demister 102 and the filler 101. A water-blocking ring 104 surrounds the upper part of the deionized water nozzle 103; this ring is attached to the upper surface of the demister 102 and is fixed above it via a fixing support frame 105. The gas outlet at the top of the high-pressure ammonia washing tower 1 is connected to the cooler 2; after being cooled, the gas enters the separator 3 to have its moisture removed. The gas emerging from the separator 3 is heated by the heater 4 before entering the high-pressure membrane separator 5. The hydrogen separated from the high-pressure membrane separator 5 is sent to the synthesis compressor. The gas separated by the high-pressure membrane separator 5 enters the low-pressure membrane separator 6; the gas resulting from the separation of hydrogen in the low-pressure membrane separator 6 then goes to the methane membrane separator 7. The low-pressure hydrogen exhaust pipeline from the low-pressure membrane separator 6 splits into two branches: one branch leads to the synthesis compressor, while the other branch leads to the desulfurization system. The top of the methane membrane separator 7 is equipped with a vent line; the methane separated by the methane membrane separator 7 returns to the process gas system. The ammonia wastewater discharge pipeline at the bottom of high-pressure ammonia washing tower 1 is connected to the circulating cooler 9; the outlet pipeline of the circulating cooler 9 is connected to the inlet pipeline at the upper part of low-pressure ammonia washing tower 8. A check valve 11 and a level control valve 12 are installed on the ammonia wastewater discharge pipeline at the bottom of high-pressure ammonia washing tower 1. The top of the low-pressure ammonia washing tower is equipped with a desalinated water inlet pipeline, while the bottom is equipped with a flash vapor pipeline. At the bottom of the low-pressure ammonia washing tower 8, there is also an ammonia solution return pipeline connected to the circulating cooler 9; a low-pressure ammonia washing pump 10 is installed on this ammonia solution return pipeline. The ammonia water from high-pressure ammonia washing tower 1 is mixed with the ammonia water coming from low-pressure ammonia washing tower 8, and then the mixture enters circulation cooler 9. After being cooled by circulation cooler 9, the ammonia water enters the tower from the top of low-pressure ammonia washing tower 8. There, it comes into countercurrent contact with the flash vapor (whose main component is NH3) that enters from the bottom of the tower; the ammonia in the flash vapor is washed away and absorbed, after which it exits from the top of low-pressure ammonia washing tower 8 and returns to the fuel pipeline network to be used as fuel gas. A portion of the ammonia water in the low-pressure ammonia washing tower 8 is pressurized by the low-pressure ammonia washing pump 10 and reused, while another portion is sent to the ammonia water storage tank through pipes at the bottom of the tower for storage.

This post was last edited by Dedication on 2021-2-24 at 17:05. It looks really familiar; entering the chemical industry starts with the production of synthetic ammonia from natural gas. Judging from the equipment codes and the content of the article, it’s very likely one of the 13 large-scale chemical plants introduced with Premier Zhou’s approval. The Prisen plant covers an area of over 20 square meters, and it’s equipped with numerous shiny pipes and valves used to recover hydrogen from off-gases. After all, the hydrogen obtained through the conversion of natural gas cannot be wasted carelessly

You can guess all of this; it seems to be aged fertilizer indeed

Where is this hydrogen then used? ? ?

This post was last edited by Dedication on 2021-2-26 09:02: the high-pressure fluid was sent back to the high-pressure section, and the low-pressure fluid was sent back to the low-pressure section, thanks to the compressor

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