First, the technical characteristics of the four-in-one unit of nitric acid
The four-in-one nitric acid unit is a typical energy self-sufficient unit. In the selection of each single machine, the principle of high efficiency and energy conservation must be followed to ensure the energy balance of the process system. The compressor should have a wide flow adjustment range to adapt to the changing conditions of the process system. When the flow is adjusted, the pressure fluctuation of the compressor is small, which satisfies the pressure requirement of the nitric acid production, and ensures that the emission of nitrogen oxides in the exhaust gas meets environmental protection requirements.
1. Technical characteristics of each single machine in the four-in-one unit of nitric acid
(1) Technical characteristics of air compressor The raw materials for nitric acid production are air and ammonia. With the expansion of nitric acid production, the use of axial compressors as air compressors in four-in-one units has become a criterion. Under the same conditions, the variable efficiency of the axial compressor is 3% to 8% higher than that of the centrifugal compressor, which reduces the compression power consumption.
The axial flow compressor adopts a fully static vane adjustable structure for variable working conditions, which can adapt to 70% to 110% flow change requirements. The vane adjustment is matched to the turbine's variable speed regulation and can be adapted to the specific operating conditions of the process, such as start-up or extreme partial load operation.
(2) Technical characteristics of nitrogen oxide compressors Nitric oxide compressors generally use centrifugal compressors. When the intake air volume is greater than 50000 mh, axial flow compressors can also be used. The internal structure of the nitrogen oxide compressor has no dead zone and effectively prevents the ammonium salt from crystallizing.
The compressor shaft seal adopts a labyrinth air-filled structure to ensure that the process medium does not leak and does not pollute the environment.
The nitrogen oxide compressor adopts the design of ternary flow, high efficiency and wide flow range. It matches the static vane adjustment of the air compressor to meet the needs of operation under partial load of the process.
(3) Technical characteristics of the exhaust gas expander The exhaust gas expander is the key equipment in the four-in-one unit. The pressure of the process system is realized by its throttling. It recovers the pressure energy and heat energy of the exhaust gas to drive the compressor. The exhaust gas expander is axial flow type, the first stage static vane is adjustable, the vane adjustment is matched with the axial flow compressor vane adjustment, and the working efficiency under the partial load of the exhaust expander is improved to meet the operation requirements of the process system.
(4) Technical characteristics of the steam turbine The steam turbine is used to supplement the loss between the recovery work and the compression work of the exhaust gas expander. The steam is derived from the superheated steam produced by the ammonia oxidation reaction. An external supply of steam is required during the start-up phase of the unit. The design of the steam turbine must have a large overload capacity for the different stages of the process operation, especially in the start-up phase, the exhaust expander output is less.
2. Overall design characteristics of four-in-one nitric acid unit
(1) Optimized design to match the speed of each single machine to ensure that the air compressor, nitrogen oxide compressor and exhaust gas expander operate at higher efficiency.
(2) Each single machine is calculated according to kinetics, so that the critical speed avoids the working speed; the torsional vibration analysis of the unit is carried out to make the unit run safely for a long period.
(3) Select the appropriate coupling to solve the problem of thermal expansion and axial thrust between the individual machines.
(4) Calculate the power balance between the individual units of the unit to meet the operational requirements under different conditions of the unit.
(5) The matching design between the air compressor vane adjustment and the first stage static vane adjustment of the nitrogen oxide compressor and the exhaust gas expander meets the operation of the nitric acid plant under variable working conditions.
(6) The labyrinth that is reliable and easy to maintain is filled with air to ensure that the nitrogen oxide medium and exhaust gas are not leaked.
(7) Using steam cleaning, the ammonium salt formed in the nitrogen oxide compressor is cleaned on-line to ensure safe operation of the unit.
3. Automatic control safety system of nitric acid four-in-one unit The four-in-one unit is the core equipment for nitric acid production. Any machine failure will affect the process production. It is especially important to monitor the operation status of the unit. The automatic control system is especially important. The nerves of the crew are an integral part of the crew's operation. In addition to the normal inspection items, the unit's automatic control system has the following characteristics.
(1) Axis operation monitoring system This system selects the 3300 series vibration instrument and fault diagnosis system of BENTLY Company of the United States to monitor the shafting of the unit, correctly reflect the operation of the unit, and provide various data charts for the operator to judge whether the unit is running normally.
(2) Flow cascade control system The flow adjustment of the four-in-one unit is realized by the vane of the axial compressor. According to the flow requirements of the process system, the opening of the vane is changed to realize the flow adjustment function. When the flow adjustment is completed, the vane opening degree of the exhaust expander can be changed according to the flow value, so that the expander operates in the high efficiency zone. The axial flow compressor vane adjustment flow is automatically operated according to requirements, and the vane expansion machine's vane operation is operated by the hand-held device.
(3) Compressor anti-surge regulation Surge is an inherent characteristic of the compressor. In order to avoid the hazard to the unit and the device caused by the surge, it is necessary to set the anti-surge adjustment system of the compressor. Due to the particularity of nitric acid production, the anti-surge regulation system is different from the anti-surge adjustment of other compressors. In order to ensure the safety of the device, when the compressor operating point is close to the anti-surge line, the surge pre-alarm is issued. When the working point enters the anti-surge area, the anti-surge valve opens, and an interlocking stop signal is issued to cut off the process ammonia. The intake valve, unit and unit are shut down at the same time. Identify the cause and restart the unit to resume production.
Second, the first set of domestic nitric acid four-in-one unit operation situation and discussion of problems in application
The first domestic four-in-one nitric acid unit was successfully put into operation in the 350td nitric acid plant, ending the history of domestic double-pressurized nitric acid production by introducing foreign units. The unit basically meets the requirements of process operation and meets the production needs.
1. Matching and adjustment between compressors In the four-in-one unit of nitric acid, the axial compressor and the nitrogen oxide compressor are the most consumed. The use of high-efficiency and energy-saving compressors is essential for power balance, otherwise the device cannot be maintained. Self-sufficient energy. It can be seen from Fig. 1 that the exhaust pressure of the axial compressor should satisfy the ammonia oxidation pressure of the oxidation furnace, and the discharge pressure of the nitrogen oxide compressor should meet the absorption requirements of the absorption tower. These pressures are all dependent on the exhaust gas expander. Throttling to achieve. In nitric acid plants, the matching between the compressors and between the compressor and the exhaust gas expansion is critical to ensure unit performance as the flow rate of the chemical reaction compressor varies.
In the first set of four-in-one nitric acid unit, the system's nitric acid absorption pressure is low, and the ammonia oxidation pressure does not meet the process requirements. The reason for this phenomenon is that the compressor design is mature and the performance has been tested in the factory. The design requirements, while running in the process system, can not achieve satisfactory operating parameters, the reason is through the first-hand data analysis of the actual recorded on-site, the problem is in the matching design between the air compressor and the nitrogen oxide compressor. Only the exhaust gas expander intake shut-off valve on the process line can be used for system pressure regulation, and the pressure of the system is realized by the throttling of the exhaust gas expander during normal operation. There is no pressure regulating valve on the exhaust line of the air compressor, and its pressure is established by the resistance of the nitrogen oxide compressor, the exhaust gas expander and the pipe network. The reason why the operating pressure of the four-in-one unit of nitric acid is lower than the process requirement is that the reaction of the air compressor is too small when the process unit is regulated, and the function of the air compressor is improved, which becomes the key to the design of the four-in-one unit.
The flow regulation of the four-in-one nitric acid unit is realized by the vane of the air compressor. In the design of the nitrogen oxide compressor, it must be considered that it has sufficient flow adjustment range, which is compatible with the design of the air compressor.
2. The design of the tail gas expander is different from the design of the blast furnace gas expander and the FCCU flue gas expander. The exhaust gas expander has a high medium pressure, a small flow rate, no solid particles, and the gas is corrosive; In contrast, the low flow rate and the large solid particles wear, so the exhaust expander is generally multi-stage expansion, and the selected materials are resistant to nitric acid corrosion.
In the nitric acid plant, the system pressure is established by the throttling of the exhaust expander. Therefore, the flow area of ​​the exhaust expander must not only meet the needs of normal operation, but also take into account the section when the unit is partially loaded. Flow requirements. The first stage of the exhaust expander is adjustable, and its adjustment range is adapted to the adjustment of the air compressor, and the expander is operated in a higher efficiency zone.
3. Selection of special valves for four-in-one unit of nitric acid The special valves in the four-in-one unit of nitric acid mainly include the anti-surge valve of the compressor, the check valve, and the intake shut-off valve and bypass quick-open valve of the exhaust expander.
The compressor check valve and anti-surge valve have mature experience, and the use of stainless steel resistant to nitric acid corrosion in the material selection can meet the requirements of process operation. The intake shut-off valve and the bypass quick-opening valve of the exhaust expander not only meet the corrosion resistance requirements, but also satisfy the fast closing and quick opening functions. Due to the pressure of the exhaust gas and the high temperature, in addition to the fast closing and quick opening requirements of the valve, the following requirements must be fulfilled: the intake shut-off valve should pay attention to the stuck problem after the quick closing, and the bypass quick-opening valve is sealed at high pressure and high temperature. It is better to reduce the amount of exhaust gas leakage.
Third, the conclusion
Most domestic nitric acid production facilities were built in the 1950s and 1960s. The production technology level is low, and the concentration of nitrogen oxides in the exhaust gas exceeds the standard, which pollutes the environment and causes waste. The double-pressurization nitric acid production process can make the concentration of nitrogen oxides in the exhaust gas less than 200ppm, and can meet environmental protection requirements without tail gas treatment. The production process is at the leading level in the world. Due to the high price of the foreign four-in-one unit, the domestic enterprises purchase foreign used equipment during the technical transformation, resulting in high maintenance costs and high production costs, making it difficult to promote the double-pressurization nitric acid production technology.
The successful commissioning of the first domestic four-in-one nitric acid unit will speed up the pace of technological transformation of domestic nitric acid plants. It will provide a new space for technological transformation of nitric acid enterprises and contribute to the revitalization of national industries.
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