Technical Report on the Energy-Saving New Process of Fully Radial Tower Series Connection

Thread Content

1 General Overview: Shandong Ruixing Chemical Co., Ltd. (hereinafter referred to as Ruixing Company) originally had three ammonia synthesis systems with diameters of φ1800mm, φ1400mm, and φ800mm, resulting in an ammonia production capacity of 300,000 tons. The highest stable operating load of the original φ1800mm ammonia synthesis system was 520 tons per day. In order to unlock the production potential of the existing system, achieve further energy savings and reduction in consumption through increased output, and reach a total annual capacity of 360,000 tons of synthetic ammonia, a comparative analysis led to the decision to adopt the \"Energy-Saving Full-Radial Stacked Tower Ammonia Synthesis New Process\" developed by Nanjing Guochang Chemical Technology Co., Ltd. (hereinafter referred to as Guochang Company) for upgrading the existing φ1800 ammonia synthesis system. This involves installing a GC-type φ1800 full-radial ammonia synthesis tower and another waste heat boiler between the existing waste heat boiler and the pre-tower heat exchanger; the process conditions of the existing equipment remain largely unchanged. Guochang Company was entrusted with the engineering design for the upgrade works, as well as the design and manufacturing of the internal components for the GC-type full-radial ammonia synthesis tower. After the completion of the renovation work, it was integrated into the system and put into operation on April 8, 2006. The actual operational results showed that the first industrial application of the \"energy-saving full-radial stacked-tower ammonia synthesis new process\" was successful. This technological upgrade not only required less investment, had a shorter construction period, and yielded quick results, but also resulted in significant increases in production and reductions in consumption, bringing obvious economic benefits. It exceeded the expected goals for increased production and reduced consumption, with the renovation completed in the same year, operation starting in the same year, and benefits being realized in the same year. 2 Design Conditions According to the design requirements set by Ruixing Company: assuming 330 days of production per year, the daily ammonia output should be greater than 165 tons, and the resistance in the modified section should not exceed 0.6 MPa. The newly added #2 waste heat boiler has roughly the same dimensions as the existing #1 waste heat boiler; two additional sets of water cooling fins have been added to the existing water cooler. 2.1 Composition of fresh gas (%): H2N2CH4 + Ar – 74–75; 24–25–1. Circulation volume: Nm3/h – ~300,000.
2.2 Composition of gas entering Tower No. 1 (%): H2N2CH4, NH3, Ar – 53.2%, 20.46%, 17.42%, 2.56%, 6.38%.
2.3 Composition of gas entering Tower No. 2 (%): H2N2CH4, NH3, Ar – 42.9%, 17.46%, 19.23%, 13.36%, 7.05%. Production capacity: t/d – ≥165. Pressure of gas entering the towers: MPa – 25.08. Gas flow rate: Nm3/h – ~271,400. Tower resistance: MPa – ≤0.35. System resistance increase: MPa – ≤0.6. Temperature of the catalyst layer: °C – ≤15.

3 Characteristics of the energy-saving full-radial stacked-tower process
3.1 Process flow and description of the stacked-tower process: The process flow of the energy-saving full-radial stacked-tower process is shown in Figure 1. Figure 1: Schematic diagram of the stacked-tower process. The recycled gas is pressurized to 25–26 MPa by a recycle compressor and then fed into the existing Tower No. 1 for ammonia synthesis. The gas emerging from this reaction (at ~304.5°C) enters the existing Waste Heat Boiler No. 1, where medium-pressure saturated steam at 1.1–1.3 MPa is produced. The gas exiting Boiler No. 1, at around 195°C, enters the newly added Tower No. 2. After being heated by the heat exchanger at the bottom, it enters the catalyst bed for further reaction. After heat exchange, the gas exits the tower at around 245°C and enters the newly added Waste Heat Boiler No. 2, where more saturated steam at 1.1–1.3 MPa is produced. The temperature of the gas then drops to around 195°C, after which it enters the heat exchangers and water coolers in the original synthesis system for cooling and separation. After the product ammonia is separated, the recycled gas returns to the recycle compressor to start another cycle. 3.2 Key technologies of the stacked tower process In an ammonia synthesis plant, the ammonia synthesis tower is the core of the entire facility; once the operating conditions are determined, the purity and yield of ammonia are determined by the structure of the ammonia synthesis tower, as well as by the amount and performance of the catalyst used therein. 3.2.1 The following key technical problems need to be solved: ⑴ Solving the problem of how to promote the ammonia synthesis reaction at high ammonia concentrations. In the ammonia synthesis reaction, ammonia is one of the reaction products. As can be seen from equation (3-1): (3-1), the concentration of ammonia has a significant impact on the equilibrium constant of this reaction. Based on the process conditions provided by Rising Sun Company, the gas composition entering Reactors #1 and #2 for ammonia synthesis is as follows: Table 1 shows the gas composition and pressure at the inlets of Reactors #1 and #2. Parameter, Location, Gas Composition (%), Total Pressure, MPa: H2, N2, NH3, CH4, Ar. At the inlet of Reactor 1: Yi = 53.20%, 20.46%, 2.56%, 17.40%, 6.38%; Pi = 13.69, 95.26, 80.65, 94.48, 11.64, 325.75. At the inlet of Reactor 2: Yi = 42.90%, 17.46%, 13.36%, 19.23%, 7.05%; Pi = 10.75, 94.37, 93.35, 14.82, 31.76, 825.08. Using Equation (3-1), it can be calculated that the equilibrium constant KP for Reactor #1 is 0.0056449, while that for Reactor #2 is 0.04503773 – a difference of nearly one order of magnitude. Therefore, how to ensure that the net increase in ammonia in Tower 2 meets the design requirements is a key technical issue that must be addressed first. ⑵ Solving the problem of resistance: With the addition of a new ammonia synthesis tower and the corresponding waste heat boiler, the resistance in the system inevitably increases. If this increase is too significant, it will be limited by the existing capacity of the circulator, which in turn affects the increase in the volume of circulating gas and thus impacts the effectiveness of the capacity expansion upgrade. How to optimize the design and control the increase in system resistance in order to minimize the resistance of the synthesis tower is another key technique in the cascade tower process. This is directly related to the selection of the internal components of the synthesis tower; generally, the resistance of a synthesis tower with an axial or axially-radial multi-layer structure is 0.5–0.8 MPa, whereas the resistance of a synthesis tower with a purely radial structure is only 0.2–0.3 MPa. ⑶ The heat balance and distribution issues in the series of towers: With the addition of a new synthesis tower and a waste heat boiler, the heat balance of the original synthesis system is disrupted as a result of this modification. Moreover, a portion of the reaction heat from Tower No. 2 is carried by the cooling gas from the annular gap (the outlet gas) to the circuit of Tower No. 1. Therefore, the design for this modification must take into account the heat balance and distribution within the new system, especially the heat balance of Tower No. 2. ⑷ To resolve operational issues under various operating conditions: After the tandem operation of two ammonia synthesis towers is implemented, since their operating conditions differ, the service life of the catalysts used in them also varies. It is possible that the catalyst in one ammonia synthesis tower has lost its activity and must be replaced, while the catalyst in the other tower still performs excellently. In such a scenario, when the catalyst in the tower undergoing replacement is reduced, it may have an adverse effect on the catalyst in the other tower. At one plant utilizing the C•F•Braun process, replacing the catalyst in the first synthesis tower (R5) caused water vapor generated during the reduction process to enter the two subsequent towers (R6 and R7), thereby disrupting production for three consecutive months. Therefore, ensuring that the system can operate properly under different operating conditions is a key issue in the tower-in-tower process. 3.2.2 Design strategies for technical problems: To address the aforementioned technical issues, optimization is primarily pursued through considerations related to the structure of the components inside the ammonia synthesis tower, the selection of catalysts, and the special design of the process flow. Based on the operational data of the original No. 1 ammonia synthesis tower provided by Rising Company, the design parameters for the No. 2 synthesis tower can be calculated, as shown in Table 2: Table 2 Design Parameters of No. 2 Synthesis Tower Pressure, MPa Circulating gas volume, Nm3/h Temperature, °C Gas composition, % H2 N2 NH3 CH4 Ar Inlet of No. 2 tower 25.0 827 1400 1924 2.91 7.46 13.36 19.23 7.05 Outlet of No. 2 tower 24.73 2621 5024 538.97 16.44 17.38 19.91 7.30 Production capacity of No. 2 tower, t/d ≥165 t/d (an increase of ~30%) (1) The internal components of the No. 2 ammonia synthesis tower are designed specially. The internal components for the ammonia synthesis tower of model φ1800mm GC-R002Y from Guochang Company are used; these components consist of a fully radial catalyst basket and a lower heat exchanger. The catalyst basket is divided into upper and lower sections, while the lower heat exchanger is of the tube-type design. A radial structure with upper and lower bed layers is adopted to enable multiple deflections of the gas, thereby facilitating better contact between the gas and solid phases and supporting the ammonia synthesis reaction. Additionally, a low-temperature, highly active catalyst with small particle size is used, which increases the activity coefficient of the catalyst and greatly contributes to the ammonia synthesis reaction, thus helping to raise the yield of ammonia. By adopting a fully radial structure, it is possible to **reduce the resistance in the catalyst bed, thereby effectively controlling the resistance in the synthesis tower. ⑵ Optimize catalyst selection. Increasing the catalyst amount can further deepen the synthetic reaction. How can the ammonia net value be increased by another 3% to 5%? The selection of a catalyst is an important issue; different catalysts possess varying properties, and their kinetic data have a significant impact on catalytic reactions. In particular, the frequency factor and activation energy of a catalyst play a crucial role in determining both the operating temperature of the catalyst and the progress of the reaction. Based on the authoritative \"REACTOR\" ammonia synthesis reactor calculation program used domestically and internationally, and by taking into account the kinetic equations of various catalysts available in China, Guochang Company utilizes its extensive experience to select the most suitable types of catalysts, determines the optimal catalyst models, and calculates the amount of catalyst required for the reactions within the reactor. After thorough research and repeated calculations, we selected the low-temperature ammonia synthesis catalyst with small-particle pre-reduced catalysts; its reduction rate is 1.6 times that of the A110-1 type catalyst, and its activity is higher than that of the A110-1 type catalyst. Moreover, the use of pre-reduced catalysts ensures high-quality reduction overall, thereby providing the most favorable conditions for ammonia synthesis in Tower 2. ⑶ Due to the high ammonia concentration entering Tower 2, the ammonia synthesis reaction is relatively at equilibrium, resulting in less heat generation during this reaction. To maintain the temperature of the gas entering the catalyst layer, a careful design was applied to the heat exchangers within the tower; large heat transfer areas and cold bypass circuits were incorporated to ensure the ability to adjust the temperature at zero degrees. At the same time, a bypass line from the outlet of Tower 1 to the inlet of Tower 2, equipped with a flow-limiting orifice plate, has been installed to meet the need to increase the inlet temperature of the catalyst bed in Tower 2 during its later stages of use, thereby maintaining thermal balance in the series circulation circuit. ⑷ The service life of the catalysts in Reactor 1# and Reactor 2# is different. To facilitate the replacement of catalysts in these two reactors as well as the heating and reduction processes, two process bypass lines have been designed: a bypass line for the inlet and outlet of Reactor 1# (to adjust the inlet temperature of Reactor 2#) and another bypass line from Reactor 1# to the inlet of the heat exchanger, in order to meet the specific operational requirements under different conditions. 4 Main technical parameters of the GC-type ammonia synthesis tower 4.1 Structure of Tower No. 2 Tower No. 2 for ammonia synthesis is of a cold-wall, fully radial design, consisting of a high-pressure outer cylinder and internal components. The internal components of the fully radial ammonia synthesis tower were designed and manufactured by Nanjing Guochang Chemical Technology Co., Ltd. They consist of a catalyst basket and a lower heat exchanger. The catalyst basket is designed as a single radial layer; however, structurally it is divided into two sections. This arrangement causes the reaction gases to undergo multiple directional changes, thereby coming into contact with the entire catalyst layer—a setup that further facilitates the ammonia synthesis reaction while keeping pressure drop at a low level ； The lower heat exchanger is of the tube-type design, featuring a simple and reliable structure. Thanks to the one-bed structure and the self-discharging technology for the catalyst in the internal components, the equipment features a simple and reliable design, easy operation, and reduced operating costs. 4.2 Gas flow in Tower 2# The cold cycle gas (~45°C) from the outlet of the partial recycler enters the tower through an inlet at the upper part of the tower; it cools the tower walls from top to bottom along the gap between the outer cylinder and the internal components, before exiting the tower through an outlet at the bottom and going to the pre-tower heat exchanger. The syngas at ~192°C, coming from Unit 1, enters Tower 2 via the two lower inlets. It is heated to 380–395°C in the shell side of the lower heat exchanger. From the central tube up to the zero-meter layer of the catalyst, it flows radially from the outside inward through the first radial layer, where the ammonia synthesis reaction takes place. Afterwards, the gas gathers in the central gas collection sleeve and flows downward to the distribution pipes of the second radial layer; from there it flows radially from the inside outward through the second radial catalyst layer. The temperature of the gas after the reaction rises to 430–450°C℃ ； After leaving the catalyst bed, the reaction gas is cooled within the tubes of the lower heat exchanger, its temperature dropping to around 245°C before it exits the tower to be sent to Waste Boiler No. 2 for recovering the residual heat from the process. 4.3 Technical parameters of the full-radial ammonia synthesis tower Table 3 Technical parameters of the full-radial ammonia synthesis tower Parameter Production capacity, t/d ≥165 Tower diameter, mm φ1800 Tower height, mm 18000 Structural design: two-layer radial structure with a heat exchanger in the lower part Catalyst loading volume, m³ 24 Design pressure, MPa 31.4 Resistance of the synthesis tower, MPa ≤0.35 4.4 List of new equipment Table 4 List of new equipment Serial number Name Specifications Quantity Remarks 1 Ammonia synthesis tower shell φ1800 H, net height = 18000 1 2 Ammonia synthesis tower internals φ1800 V, catalyst volume = 24 m³ 1 3 Waste heat boiler φ2200/φ2600 F, surface area = 218 m² 1 Same as the existing waste heat boiler! 4 Water coolers (spraying); F = 240 m2. 2 5 Catalyst reduction technology: Due to the short reduction time required for the catalyst, the theoretical total water output is only around 1.5 tons, which does not affect the use of liquid ammonia. In order to minimize any impact on the normal operation of the system, after thorough research and evaluation, the heating and reduction process for Tower No. 2 was carried out using the technical approach of \"Tower No. 1 operating at normal pressure for production, while Tower No. 2 undergoes reduction simultaneously,\" with the operating temperature of Tower No. 1 remaining normal during the reduction process. At a pressure of approximately 17 MPa, power was supplied to Tower No. 2. Eight hours later, when the surface temperature of the catalyst in Tower No. 2 reached around 240°C, the analyzed water vapor concentration was 0.4 g/Nm³. At that time, the pressure was about 13.4 MPa, the flow rate was approximately 155,000 Nm³/h, and the H₂ concentration was around 64%. This indicates that water output from the catalyst was quite significant at this point. Under the aforementioned conditions, after the temperature of the catalyst bed was successfully raised to ~410°C, the pressure was adjusted to reduce the gas flow rate into the tower and to increase the power of the electric furnace, thereby raising the catalyst bed temperature to ~450°C. Further measures such as reducing pressure, adjusting the H2/N2 ratio, controlling the outlet temperature of Tower 2, and increasing the steam pressure in Waste Boiler 1 were taken to raise the temperature of the upper layer to ~491°C. The temperature in the lower layer also gradually increased, with the bottom temperature eventually reaching over 480°C for 6 hours, after which the reduction process was complete. Throughout the reduction process, the tower internals and the entire system functioned perfectly. 6 Operational Results: After the implementation of the new tandem tower process, very good economic results were achieved, as evidenced in the following aspects (the design and operational parameters of Tower 2 are shown in Table 5, while the actual operational data after the modification are shown in Table 7): 6.1 Significant increase in production by 37.8%: With the adoption of this new tandem tower process, ammonia production increased substantially. According to the data recorded at 0:00 on May 25, 2006, with an inlet gas flow rate of 356,900 Nm3/h, ammonia production rose to 751 tNH3/d. Of this amount, Tower 1 produced 545 t NH3/d, while Tower 2 produced 206 t NH3/d; thus, Tower 2 contributed to a 37.8% increase in total production, indicating a considerable rise in output. Table 5 Comparison of Design and Operational Technical Parameters Parameter Design Value Actual Operational Value Production capacity of Tower 2, t/d ≥165 206 Gas flow rate into Tower 2, Nm3/h 271,400 325,589 Operating pressure of Tower 2, MPa 25.0 829.5 Resistance in Tower 2, MPa ≤0.35 0.2 Additional resistance in the system, MPa ≤0.6 0.3 (total resistance reduced by ~0.2) Temperature difference across the catalyst column, °C ≤15 9.62 An increase of 4.02% in ammonia yield. Based on an inlet temperature of -3°C and a pressure of 28.9 MPa in the cold exchanger, it can be calculated that y_in = 2.92%. The heat balance data of the pre-tower heat exchanger calculated based on actual operating data are shown in Table 6. Table 6: Heat balance table for heat exchange. Flow rate, Nm3/h; Temperature, °C; Pressure, MPa. Heat in: 31597419129.2; Heat out: 315974108; Cold air in: 26660078 (calculated value); Cold air out: 26660018130. According to the operation report, the temperature at the outlet of the circulating oil is 48°C. The rise in the temperature of the cold air entering the heat exchanger to 78°C is due to the heat carried away by the gas from the gap of Tower No. 2 (the outlet gas). Calculations show that this amount of heat can raise the temperature of the gas exiting Tower No. 2 by about 20°C ； Furthermore, given that the inlet operating temperature of Tower 2# is 197°C and the outlet operating temperature is 238°C, it can be calculated that the heat released during the actual ammonia synthesis reaction in Tower 2# results in an adiabatic temperature rise of 61°C within the tower. The net increase in ammonia concentration exceeds 4%. When the ammonia content at the outlet of Tower 1# is 12.33%, the ammonia content at the outlet of Tower 2# reaches 16.35%. Table 7: Typical operating data – Location of the synthesis tower, gas flow rate in Nm3/h, temperature in °C, pressure in MPa, inert gas content in %, ammonia content in %. First layer, second layer, third layer: Inlet of Tower 1: 356,900; 413,424; 415,300; 0.019; 0.082; 2.92. Outlet of Tower 1: 327,284; 426,472; 460,290; 29.72; 0.91; 2.33. Inlet of Tower 2: 327,284; 381,416; -----; 29.52; 0.91; 2.33. Outlet of Tower 2: 315,974; 416,431; -----; 29.32; 16.35; 21.69. The system resistance decreased by more than 0.2 MPa. The cascade tower process involves adding one more ammonia synthesis tower and one waste heat boiler; there were concerns that the addition of these devices would increase the system resistance, but the user specified that the increase in the overall system resistance should be less than 0.6 MPa. During actual operation, when the gas flow rate in Tower 2 for ammonia synthesis reached 325,589 Nm3/h, the pressure drop across Tower 2 was only 0.2 MPa, while that in Tower 2 for waste heat recovery was 0.1 MPa. The original system resistance was 1.2–1.5 MPa; after the tower connection, the reduced flow rate of gas entering the separation system lowered the system resistance to 1.0–1.2 MPa. 6.4 Energy consumption reduced by 30%: Due to a decrease in system resistance of around 0.2 MPa and an increase in the net ammonia concentration, the volume of gas exiting Tower No. 2 decreased. Meanwhile, the increased amount of ammonia was condensed in the water cooler; as a result, the total electrical power consumption of the circulator and chiller remained roughly the same before and after the renovation. In practical production, the same number of circulators and chillers were in use before and after the renovation, yet the production volume increased by over 30%. Therefore, the power consumption per ton of ammonia for these devices decreased by about 30%. 6.5 30% lower investment costs and half a shorter construction period: By adopting the tower-in-series process, it offers advantages such as reduced investment costs, shorter construction times, and faster time to productivity gains compared to building new facilities. Taking the tower-in-series project carried out by Shandong Dongping Ruixing Company as an example, by utilizing existing equipment and the framework of the synthesis towers, the impact on the existing production system is minimal, and the existing facilities are made full use of; as a result, the investment cost is lower than that of other options for expanding production capacity, with savings of over 30%. Due to the small volume of work and limited land area required, it is possible to install the new equipment in the spaces left vacant by the existing production equipment, thus avoiding the complications associated with acquiring land. Thanks to careful planning, the new equipment fits neatly alongside the existing one, which reduces the amount of work needed on site. As a result, the construction period is **shortened; it takes less than a year from design and construction to commissioning, which is less than half the time required to build a new set of facilities. Another reason for the rapid results is the meticulous design, which includes the internals of the ammonia synthesis tower, the overall process configuration, the selection of catalysts, and the temperature-raising reduction scheme. Thanks to this design, production can be started immediately after the catalyst reduction process is completed. In the case of Ruixing Company’s sequential-tower process, the load was significantly increased on the third day following the completion of catalyst reduction; the supplementary gas flow rate rose from 63,500 Nm³/h to 88,000 Nm³/h. 7 Conclusions 7.1 Practice has shown that the new tower-connection process meets the policy requirements set forth currently, which aim to reduce energy consumption and increase production capacity; it is of great significance for the development of the ammonia synthesis industry, and represents a new energy-saving process worth promoting and applying. 7.2 The net ammonia output of the cascade tower process increases by 3%–5%; production can be raised by over 30% without increasing the circulation volume; the resistance of the fully radial synthesis tower is only 0.2 MPa; and energy consumption is reduced by about 30%, resulting in significant economic benefits. 7.3 The tower-in-tower process can reduce the system resistance; as a result, the power consumption of the circulator does not increase significantly, which facilitates an increase in the circulation volume to boost ammonia production. 7.4 The tower-series process is easy to adjust, and its operation and running are stable. 7.5 The use of fully radial structural internals is the core of the tower-in-series process. Guochang Company’s GC-R002Y type ammonia synthesis tower internals, which are covered by independent intellectual property rights, can meet the special requirements of this process. Not only is the system resistance increased only slightly, but thanks to the unique patented technologies – such as the unevenly perforated porous plates and the fish-scale pattern secondary distributor structure – gas distribution is even, with a temperature difference across the column surface of only 9°C. This value is significantly lower than that of other types of radial sections used in China; it is also very close to the 8°C temperature difference observed in large ammonia towers abroad. As a result, the catalyst’s activity can be fully utilized, thereby ensuring an increase in ammonia yield of 3% to 5%.