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Automatic control improvement plan for technical upgrades of large-scale ammonia synthesis plants 0 Introduction: The batch of ammonia synthesis plants that were introduced to China in the mid-1970s, using natural gas as raw material and with an annual production capacity of 300,000 tons of ammonia, were among 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 synthesis production in our country. However, from the perspective of the development of ammonia synthesis production technology, these existing 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 their agenda. The technical renovation work for 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 at 8.3 Gcal/MT. At present, the goal of these energy-saving and productivity-enhancing upgrades for ammonia synthesis plants is mostly to achieve a daily production 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 process, process pipelines, chemical vessels, rotating equipment, electrical systems, instrumentation, and more. Since these large-scale ammonia synthesis plants have all completed the DCS transformation, the instrument 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 reform plans for such upgrades in large-scale ammonia synthesis plants. 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 inevitably require the addition of new and the replacement of some on-site instruments. The flow meters and control valves are those with the most additions and replacements at the site. Since the project example in this paper is one that purchased foreign process improvement design packages and basic engineering design packages, the foreign engineering company carried out calculations and made recommendations for the selection of flow meters and control valves in accordance with the requirements of the process improvement. 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; whereas in applications where lower permanent pressure losses are 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 requirements are 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 upgrade 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 ones. 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, double-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 counterflow media had not been used in ammonia synthesis plants before. In the technical discussions with the FISHER control valve suppliers, 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 condition 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 in 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 solving the leakage problem 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 split-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 equipment, the factory 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 sets of controllers, one located at the inlet of the syngas compressor and the other 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/feed gas controller installed in the gas generation section serves to perform coarse 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 A is not considered in this system, and it remains to be verified whether effective control can be achieved in the future. (2) Addition of a program-controlled system This technical upgrade installation includes a molecular sieve dryer unit, which was designed by our company. This unit is installed between the 2nd stage outlet and the 3rd 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. Automatic control improvement plan for technical upgrades of large-scale ammonia synthesis plants. Last edited by Warm Home on 2009-1-31 19:52.]