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Automatic control renovation plan for technical upgrades of large-scale ammonia synthesis plants Author: Tong Qiujie [Abstract] Automatic control renovation plan for technical upgrades of large-scale ammonia synthesis plants 0 Introduction A batch of ammonia synthesis plants that use natural gas as raw material and have an annual production capacity of 300,000 tons were introduced to China in the mid-1970s; these were the first large-scale ammonia synthesis plants built in the country. These ammonia synthesis plants have been in operation for over 20 years, making significant contributions to improving the backward state of ammonia production in our country. However, from the perspective of the development of ammonia synthesis production technology, these current ammonia synthesis plants face numerous difficulties in terms of energy savings and increased production, and the aging of some equipment also poses a threat to safe operation. Therefore, many manufacturers have put the technical upgrading of such devices on the agenda. The technical renovation of Yuntianhua’s ammonia synthesis plant has completed its engineering design phase, and it is now in the equipment procurement stage ； Lutianhua has completed the negotiations to introduce patented technologies for technological upgrades ； Companies such as Sichuan Chemical and Daqing Fertilizer Plant have completed the evaluation of their technical reform feasibility studies, and are actively carrying out the preparatory work for construction. When introduced, these ammonia synthesis units had a daily production capacity of 1,000 tons, with an energy consumption of 9 Gcal/MT. After several years of operation, they have undergone technical upgrades to varying degrees; currently, the average daily production volume is over 1,100 tons, with energy consumption amounting to 8.3 Gcal/MT. At present, the goal of these energy-saving and production-increasing technical upgrades for ammonia synthesis plants is mostly to achieve a daily output of 1,500 tons, with an energy consumption of 7.8 Gcal/MT. The technical upgrades of ammonia synthesis plants involve modifications to the production processes, process pipelines, chemical vessels, rotating equipment, electrical systems, instrumentation, and more. Since all of these large-scale ammonia synthesis plants have now completed the DCS transformation, the instrumentation upgrades mainly involve the installation of new instruments on site as well as the modification of various control systems; the DCS transformation itself consists only of hardware expansion and the configuration of application software. Taking the renovation of on-site instruments and control systems during a technical upgrade of an ammonia synthesis plant as an example, this article discusses the automation renovation plans for large-scale ammonia synthesis plants undergoing technical upgrades. 1. Implementation plan for on-site instrument modification: During the process and equipment upgrades, larger devices such as a process condensate stripping tower, molecular sieve dryer, and electric air compressor were added ； Some of the structures and internal components of equipment such as a converter, ammonia synthesis tower, and syngas compressor were modified and replaced ； Some process pipelines were added and replaced. These modifications to the processes and equipment necessarily require the addition of new and the replacement of some on-site instruments. The instruments that are most frequently added or replaced on-site are flowmeters and control valves. Since the project example in this paper involves the purchase of foreign process improvement design packages and basic engineering design packages, the foreign engineering company conducted calculations and made recommendations for the selection of flow meters and control valves in accordance with the requirements of the process improvements. Based on these calculations and selections, two characteristics can be observed: (1) Diversified selection of flowmeters and consideration of the permanent pressure loss in the throttling elements. This technical renovation project utilized various types of flowmeters; in addition to differential pressure flowmeters, vortex flowmeters and mass flowmeters were also used. In differential pressure flowmeters, various types of throttling elements are used. Under normal circumstances, standard orifice plates are used as throttling elements; in applications where a lower permanent pressure loss is acceptable and large pipe diameters are involved, other types of throttling elements are employed. In the case of measuring the medium-pressure steam flow rate in a conversion furnace, orifice plates were previously used. After the technical upgrades, the steam flow rate increased while the pipe diameter remained unchanged; if orifice plates continued to be used, the permanent pressure loss would exceed the allowable values. This technical upgrade uses a Venturi tube, which not only reduces the permanent pressure loss but also ensures sufficient measurement accuracy (<±1%) ； For applications where the permanent pressure loss is low, the pipes are large (φ200–300), and the measurement accuracy requirement is not very high (±1%), Alubra is used. Previously, the measurement of the syngas flow rate between the stages of the syngas compressor (for the anti-surge control system) also used orifice plates; after the technical upgrade, the flow rate increased while the pipe diameter remained unchanged, and this time Aruba meters were adopted instead. (2) Use of V-type ball valves and selection of two-way flow control valves: Due to the addition of new pipelines and the enlargement of certain pipelines, this technical renovation project involved the installation of more than 30 new and replacement control valves, accounting for approximately one-third of the total number of control valves in the entire facility. Among the more than 30 control valves, approximately half are new and half are replaced. For the phenanthroline solution used for CO2 removal, since it is a medium that crystallizes easily, V-ball valves (Fisher V300 valves) are used as control valves for both flow control and level control. Previously, dual-seat V-shaped port plunger valves were used in such applications, and their performance was not very good; improvements were made in the valve selection as part of this technical upgrade. In the molecular sieve dryer unit, the pressure filling valve installed between the two molecular sieve dryers is a control valve that regulates the bidirectional flow of the medium. That is, once the regeneration process of the first dryer is complete, it is necessary for the second dryer, which is currently in operation, to pressurize it via a pressure control valve (with the medium flowing from end A of the valve to end B). Once the second drying and regeneration process is completed, the first dryer that is currently in operation is used to pressurize it via a pressure control valve (with the medium flowing from port B to port A) (see PV-50 in the schematic diagram of the program-controlled process for the molecular sieve dryer in this document). Such valves for controlling the reverse-flowing medium have not been used in ammonia synthesis plants before. In the technical discussions with the suppliers of FISHER control valves, they also did not have much practical experience. Later, by drawing on a set of imported units for the selection of such valves, it was decided to use a single-seat plunger-type control valve. It remains to be seen what the effects will be after it is put into operation by vehicle. Other important instruments added on-site are: to monitor the combustion status of the auxiliary boiler burners, new type of flame detectors have been installed next to each of the five auxiliary boiler burners. This type of flame detector integrates detection, transmission, and control functions, with the entire unit installed on-site. The programmable valve of the molecular sieve dryer unit is a track-type programmable ball valve. When opening and closing this valve, the valve stem guide groove moves along the guide pin as a track, causing the ball to move away from the valve seat before rotating, thereby eliminating wear on the valve seat and effectively addressing the leakage issue that occurs over time with prolonged use of the valve. 2 Implementation plans for the renovation of automatic control systems and programmable control systems (1) Renovation of automatic control systems: The automatic control systems in ammonia synthesis plants are mainly single-parameter control systems, with few complex control systems. This technical upgrade project adds 12 single-parameter automatic control systems (including 2 range-control systems) and 1 remote control system. Since the operation of the newly added electrically driven air compressor is associated with the H/N control system, this complex H/N control system was modified during the technical upgrade. This ammonia synthesis plant did not have an H/N control system when it was first introduced. Since this system is related to the energy-saving and productivity-enhancing benefits of the plant, the management initially intended to install it; however, due to issues with the analyzers for H2 and N2, automatic control of H/N has not been implemented to date. Before this technical upgrade, the factory purchased a single-channel gas chromatograph, which created the conditions for adding an H/N control system. This H/N control system is a air flow cascade control system with selective air/feedstock or H/N ratio control, designed by a foreign engineering company; see the control system schematic diagram. The H/N unit has 2 control systems: one is installed at the inlet of the syngas compressor, and the other is installed at the inlet of the synthesis tower. Since the gas chromatograph has only one flow path, only one controller is active when the system is operating; this active controller performs fine-tuning of the H/N control. The air/raw gas controller installed in the gas generation section serves to perform rough adjustment of H/N control. The outputs of the H/N controller and the air/raw material controller are used as remote setpoint signals for the air flow controller via a low-value selector, which connects only the controller from the system with a larger ratio deviation between H/N and air/raw material gases to the air flow controller. The system is well-designed, and relevant limiting measures are taken to ensure its security. Since the H/N control system is a large-time-delay system, lag compensation issue A is not taken into account in this system, and it remains to be seen whether effective control can be achieved in the future. (2) Addition of a programmable control system This technical upgrade installation includes a molecular sieve dryer unit, which was designed by our company. This unit is installed between the second-stage outlet and the third-stage inlet of the syngas compressor 103-J. The molecular sieve dryer dries the syngas by removing trace amounts of CO2 from it, thereby increasing the ammonia synthesis rate and protecting the synthesis catalyst. This unit consists of 2 molecular sieve dryers, 1 molecular sieve regeneration gas heater, 1 molecular sieve regeneration gas dryer, and other equipment; see the simplified flow diagram of the molecular sieve dryer control process. The online operation and regeneration of the 2 molecular sieve dryers alternate with each other, controlled by a programmable unit according to a predetermined procedure. Program-controlled operations are primarily reflected in the regeneration process of the dryer. This regeneration process consists of steps such as depressurizing the dryer, introducing regenerating gas, heating the regenerating gas, raising the temperature of the dryer, cooling the dryer, and pressurizing the dryer; see the logic diagram of the molecular sieve dryer’s program control system. The programming unit for the dryer is configured within the DCS of the ammonia synthesis plant; therefore, the DCS handles all the programming functions related to the operation and regeneration of the dryer. The feature of this programming system is that, in steps such as depressurizing the dryer, heating the regeneration gas, cooling the dryer, and pressurizing the dryer, the settings for the controllers that regulate pressure and temperature are determined using a programmed approach; for example, when depressurizing or pressurizing the dryer, it is done at a rate of 300 kPa per minute ； The regenerated gas is heated at a rate of 5°C per minute ； The dryer cools down at a rate of 48°C per minute; these set values, which vary in terms of rate, are all determined by the respective program control units. Such programming method can ensure stable operation of the process and equipment safety. 3. Alternative self-control improvement solutions: The instrument modification work for large-scale ammonia synthesis plants may not be perfectly formulated due to various constraints, such as funding issues and technical infrastructure problems. However, from the perspective of continuously improving the process control technology in ammonia synthesis plants in order to better achieve energy savings, increased production, and improved economic efficiency for enterprises, in addition to the aforementioned and other necessary renovation plans, there are several additional options that can be considered. This article proposes two major plans for selection when carrying out instrument-related renovations. These two options were taken into consideration in some technical improvement projects. (1) Integrated control of turbines and compressors: The compressor control systems used in a number of large-scale ammonia synthesis plants that were built in the 1970s and utilized natural gas as a feedstock were relatively outdated, featuring low reliability and a high failure rate. Later, some units underwent some minor technical modifications to their control systems, and speed control devices were installed on individual compressors; however, the control effects in most cases were not satisfactory. In the current international market for control equipment, a combined turbine-compressor control system has been introduced. It is an integrated device that combines steam turbine speed control and extraction control, compressor anti-surge control, performance control, decoupling control, separator level control, as well as self-protection interlock logic control. The system features a triple-redundancy fault-tolerant design, offering a long average time between failures and high reliability. Furthermore, this system features flexible configuration, making it easy to modify control programs and expand the system. To improve the safety and reliability of compressor operation and ensure the long-term, continuous operation of the ammonia synthesis plant, choosing this integrated control system for the technical upgrades is an excellent option. The ITCC integrated control system TS3000 from the American company TRICONEX has been adopted in the 200,000-ton/year ammonia synthesis unit of the newly built Zepu Fertilizer Plant in Xinjiang, and it has performed well since its commissioning. (2) Advanced control of ammonia synthesis plants: The first batch of ammonia synthesis plants introduced suffered from a lower level of control due to their early introduction (although at that time it was considered that the control level was quite high). Later, various improvements were made to the instrumentation in these plants; for instance, upgrading conventional control instruments to DCS represented a significant advancement. However, after adopting DCS, not enough has been done to utilize its advanced functions to improve the level of process control. Currently, there is an increasing understanding of Advanced Process Control (APC) technologies for ammonia synthesis plants; therefore, taking advantage of this favorable opportunity for technological upgrades to such plants, considering the use of APC in the instrumentation upgrades would be a good approach. This is also an important way to ensure the long-term, continuous safe operation of the device, save energy, reduce consumption, and improve economic efficiency. There are several types of APCs for ammonia synthesis plants. Since most of the first ammonia synthesis plants introduced used technology from the American company KBR (formerly Kellogg Company), this article only provides a brief overview of KBR’s APC technology for ammonia synthesis plants. KBR’s APC control strategy is “multivariable predictive control,” and the principle behind developing this APC control strategy is to reduce variations in key process parameters ； Reduce the impact of external disturbances on process operation ； Carry out better operations and management. The APC control item is: H/N control ； Temperature balance control of a converter riser tube ； Excess O2 control in a converter ； Synthesis loop control ； Conversion rate control ； Synthetic ammonia production control (raw material limits/production constraint control), etc. The company’s APC technology has been adopted in more than a dozen ammonia synthesis plants in countries such as the United States, Canada, Turkey, and the Netherlands. The economic benefits resulting from its use are an increase in ammonia production of 1–2%, along with an improvement in energy efficiency of 1–2%. Within half a year of APC operation, the economic benefits generated are sufficient to recover its investment cost. In the technical upgrades of ammonia synthesis plants, the use of APC technology not only involves introducing foreign technologies but also allows for collaboration with universities and research institutions to jointly develop this technology. Some domestic universities and research institutions have carried out extensive work in this area, gained considerable experience, and laid a solid foundation for technical cooperation.