Design specifications for the DN1200 ammonia synthesis system and relevant information on internal components
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Design Specifications for DN1200 Ammonia Synthesis System I. Design of DN1200 Ammonia Synthesis System 1. Overview In a standard DN1200 ammonia synthesis system, the clear height of the DN1200 ammonia synthesis tower is 16,000 mm; the other equipment (such as heat exchangers, waste heat boilers, etc.) and pipelines are designed in accordance with standard specifications. The production capacity can reach 360 tons per day. With an operating flexibility of 60–100%, and assuming 330 days of operation per year, the annual production of ammonia will be between 71,000 and 120,000 tons. When the system operates at a capacity of 100,000 tons of ammonia per year, the system pressure remains ≤28 MPa. The synthetic system flow adopts a two-in, two-out configuration with a post-stage waste boiler. 2. Brief introduction to the synthesis system process: The syngas coming from the outlet of the circulating oil is introduced into the synthesis tower from its top. After passing through the gap between the inner and outer cylinders, it exits the tower and goes outside for heat exchange to reach a temperature of around 160°C before being reintroduced into the tower. After reacting inside the tower, it exits again and is directed to a subsequent waste boiler for steam recovery. The gas exiting the waste boiler goes to a heat exchanger for further heat exchange; after that, it enters a water heater, where it is cooled by water. An ammonia separation unit is installed at the outlet of the heat exchanger, with the aim of separating some of the liquid ammonia present in the gas exiting the heat exchanger. This helps to reduce the consumption of liquid ammonia used for cooling as well as the loss of liquid ammonia in the vented gas. The gas after ammonia separation undergoes venting and recharging before entering another ammonia cooling unit for further cooling, followed by another pass through the heat exchanger to separate liquid ammonia. Finally, this gas mixes with the gas that has been cooled by water, before reaching the inlet of the circulation pump, thus completing this cycle repeatedly. Additional notes on the process flow: (1) In the ammonia synthesis process, the air release point is located after the ammonia is separated at the outlet of the heat exchanger; here, the ammonia concentration is low and the pressure loss is also small. (2) The ammonia separator is generally placed after the water cooler; in some plants, it happens that liquid ammonia cannot be separated or can only be separated in very small amounts during the production process. Adjust according to this process: Place the ammonia after the cold exchanger; the syngas is condensed in the cold exchanger, with the gas temperature being reduced to ≤20°C. This allows for the large-scale separation of liquid ammonia, thereby reducing the load on the cold exchanger and minimizing losses of ammonia through venting. (3) If conditions permit, the system can be equipped with a fresh gas ammonia cooler and an oil-water separator; the fresh gas passes through these devices before entering the system, where the large amount of saturated water and oil contaminants present in the fresh gas (at ~30°C) are initially condensed and separated. It is then added to the system; the location for adding ammonia can be before the main ammonia cooling stage (recommended, as this yields better purification of fresh gas) or after the main ammonia cooling stage (for manufacturers facing shortages of liquid ammonia). The temperature during this ammonia cooling process should be maintained between 5 and 10°C – it should not be too low to prevent freezing and blockages. (4) It should be noted that there should be a certain distance between the venting position and the air supply position. (5) The main pipelines in the process can be determined based on actual production conditions; if configured according to the standard system (120,000 tons per year), Dn150 (Φ219×35) pipes are required. For the detailed process, see Figure 3. Process calculations for the synthesis system: (1) Calculation basis – Ammonia production: ~100,000 tons per year (i.e., 13 t/h). Composition of the feed gas: H2 (74.25%), N2 (24.75%), CH4+Ar (1.0%). Concentrations in the gas entering the reactor: NH3: 2.5%, CH4: 18%, Ar: 18% ； NH3 at the outlet of the tower: 15.5%. The inlet pressure is 28.0 MPa; the pressure of the steam generated by the waste heat boiler (gauge pressure) is 1.27 MPa. (2) Material balance (calculated based on the nodes in the flow diagram on the next page, without considering the water heater equipment): Results of the material balance: Node | Item | H2 | N2 | CH4+Ar | NH3 (gas) | Total | Liquid ammonia (kg) | 1 | Inlet gas volume, Nm3/h | 90735.05 | 30245.02 | 27391.32 | 3804.35 | 152174 | 0 | Composition % | 59.63 | 19.88 | 18 | 2.50 | 100 | 2 | Outlet gas volume, Nm3/h | 65042.03 | 21680.68 | 27391.32 | 20932.16 | 135046.19 | 0 | Composition % | 48.17 | 16.06 | 20.28 | 15.50 | 100 | 3 | Gas volume before ammonia separation, Nm3/h | 65042.03 | 21680.68 | 27391.32 | 8011.4 | 122125.46 | 9805.91 | Composition % | 53.26 | 17.75 | 22.43 | 6.56 | 100 | 4 | Gas volume after ammonia separation, Nm3/h | 65042.03 | 21680.68 | 27391.32 | 8011.4 | 122125.46 | 0 | Composition % | 53.26 | 17.75 | 22.43 | 6.56 | 100 | 5 | Vent gas volume, Nm3/h | 848.64 | 282.88 | 357.46 | 104.55 | 1593.69 | 0 | Composition % | 53.26 | 17.75 | 22.43 | 6.56 | 100 | 6 | Supplementary refined gas volume, Nm3/h | 26541.66 | 8847.22 | 357.46 | 0 | 35746.35 | 0 | Composition % | 74.25 | 24.75 | 1.0 | 0 | 100 | 7 | Gas volume before main ammonia cooling, Nm3/h | 90735.05 | 30245.02 | 27391.32 | 7906.85 | 156278.12 | 0 | Composition % | 58.06 | 19.35 | 17.53 | 5.06 | 100 | 8 | Gas volume after main ammonia cooling, Nm3/h | 90735.05 | 30245.02 | 27391.32 | 3804.35 | 152174 | 3114.74 | Composition % | 59.63 | 19.88 | 18 | 2.50 | 100 | 9 | Gas volume after heat exchange, Nm3/h | 90735.05 | 30245.02 | 27391.32 | 3804.35 | 152174 | 0 | Composition % | 59.63 | 19.88 | 18 | 2.50 | 100 | Amount of ammonia produced in the reaction (kg/h): 13000 ； Ammonia production rate (Kg/h): 9805.91 ； Ammonia release rate during cold separation (Kg/h): 3114.74; Ammonia release rate in vent gas (Kg/h): 79.35; Nm3 of refined ammonia gas consumed per ton: 2750 ； Air release volume (Nm3/h): 1593.69; Air supply volume (Nm3/h): 35746.35 m3/h. Heat balance: For cooling processes, the inlet temperature of the hot gas is 35.0℃ and the outlet temperature is 18.0℃; the inlet temperature of the cold gas is -11.5℃ and the outlet temperature is 20℃. For ammonia cooling, the inlet temperature of the gas is 20.0℃ and the outlet temperature is -10℃; the evaporation temperature of liquid ammonia
DN1200 ammonia synthesis tower internals – Related information 1: Technical specifications. The DN1200 internally cooled–insulated shaft-type ammonia synthesis tower internals are a mature product developed by the Machinery Factory of Zhejiang University of Technology, based on over 30 years of experience in designing and manufacturing such components. It takes advantage of the strengths of various ammonia synthesis tower internals available in China today. It features advanced technology, a mature and reliable structure, and great operational flexibility, enabling it to adapt to production under different gas flow rates and operating conditions. The axial and radial temperature differences are small, resulting in low tower resistance. The catalyst can be reduced easily and completely, with high values of ammonia purity and ammonia yield. There is a significant effect in saving energy and reducing consumption, achieving an advanced level among similar components in China. The internally cooled–insulated axial radial internal component consists of an upper catalyst basket and a lower heat exchanger; the upper catalyst basket is a continuously bedded structure that is axially connected without any partitions in between, allowing for continuous installation and removal of the catalyst. In the axial section, there are components designed for continuous heat transfer—single-tube baffle-type cooling tubes—while the radial section is composed of insulating layers. The gas flow transitions gradually from axial to radial, with the gas flowing from the outside inward in the radial layer. This internal component retains the advantages of the traditional fully axial internally cooled–insulated design, requires no additional work during installation, and can reduce the bed resistance by approximately 30–35%. The internal components of the DN1200 internally cooled–insulated axial ammonia synthesis tower have the following features: (1) The cooling tubes in the axial section adopt a single-tube baffle design with co-current flow as the primary mode and counter-current flow as a supplementary mode; moreover, a double-ring tube structure is used to arrange the cooling tubes more reasonably, enhancing their structural strength and resulting in a more uniform gas distribution. By adjusting the heat transfer area of the cold tubes and the ratio of upper to lower cold tubes, the temperature distribution within the bed is brought close to the optimal temperature distribution curve, thereby achieving a high net value. The co-current cooling tubes are the main components for removing the heat of reaction, while the presence of upward-flowing cooling tubes raises the inlet temperature of the bed (usually around 410–420°C) and reduces the axial temperature gradient (typically around 35°C), ensuring that the entire bed remains within an appropriate catalyst-activity temperature range. Due to the high inlet temperature of the bed layer, it is not necessary to install an upper insulation layer, which also simplifies the structure. (2) An adiabatic layer with radial flow is arranged at the lower part of the bed layer. This radial basket structure is advanced and rational; back pressure is used to control the airflow distribution in the radial layer, resulting in a relatively uniform gas distribution. Moreover, this radial basket is not prone to damage and is highly reliable. Practice has shown that when the temperature difference across the radial baskets does not exceed 10°C, the use of such radial baskets yields excellent results. It not only helps to reduce the resistance in the tower but also increases the gas processing capacity of the entire tower, raising its daily production volume. It is also easier to raise the temperature at the bottom during the reduction process; moreover, the use of small-particle catalysts improves the activity of the catalysts at the lower levels, resulting in a higher ammonia yield. (3) The gas enters the center tube first, and the cold gas enters later. During temperature-raising reduction, the cold tube acted as a heat pipe, enabling thorough reduction of the catalyst and effectively mitigating the impact of the \"cold tube effect\". (4) The cold tube, the central tube, and the catalyst basket are independent of one another and can expand and contract freely, thereby effectively solving the problem of thermal compensation. (5) This internal component is easy to install and operate. During operation, the bed temperature can be controlled using either the main loop flow rate or the tower side stream. (6) The structure of the cold tube shell has been improved, effectively enhancing the reliability and service life of the internal components. (7) The insulation shell structure has been redesigned to be more efficient and reliable. (8) Install a catalyst discharge device as needed. At present, this type of internally cooled and insulated shaft-type radial ammonia synthesis tower internals, models DN1000 and DN1200ZF, have been put into use by more than a hundred manufacturers in China, all achieving good performance. Among them, the internal components of the DN1200 internally cooled–insulated shaft-type ammonia synthesis tower passed the technical appraisal organized by the China Nitrogen Fertilizer Industry Association in June 2000. The technical evaluation indicates that this type of internal component features a unique design: it ensures a uniform temperature distribution across the bed layer, results in small temperature differences, operates stably, offers considerable operational flexibility, and facilitates easy catalyst loading/unloading as well as maintenance and replacement of the cold tubes. 2 Names of process parameters for the internal components design: Internal components of DN1200 ammonia synthesis tower. Net height of the surrounding casing (mm): 16000. Designed gas flow rate (Nm3/h): ~1.75×105. Designed production capacity (t/d): ~360. Space velocity inside the tower (h-1): ~16000. Pressure at the inlet to the tower (Mpa): 31.4. Pressure drop across the tower (Mpa): 0.8. Percentage of CH4+Ar: 18. Percentage of ammonia at the inlet to the tower: 2.5. Percentage of ammonia at the outlet from the tower: ~15.5. Catalyst volume loaded (m3): ~11.4