Discussion on Water Treatment Technologies for Ammonia Nitrogen Emissions in Ammonia Synthesis Plants

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Discussion on Water Treatment Technologies for Ammonia Nitrogen Emissions in Ammonia Synthesis Plants I. Introduction In ammonia synthesis plants, ammonia is lost in significant quantities alongside the process of artificial nitrogen fixation. This ammonia accumulates in natural environments such as soil and water bodies; when its amount exceeds the natural capacity for reduction, it causes nitrogen pollution in nature. Most ammonia is lost through wastewater discharge, which is also a waste of resources and energy. How to reduce the ammonia emission concentration in wastewater and control the total amount of ammonia emitted has always been one of the problems plaguing ammonia synthesis plants. This paper focuses on the approach of selective removal to discuss how to reduce the ammonia concentration in water bodies, either by recovering ammonia or removing nitrogen, with the aim of minimizing ammonia emissions. 　 II. Different Methods of Denitrification in Water Bodies and Their Effects 2.1 Overview of Methods Reducing the ammonia concentration in wastewater is essentially a process of denitrifying the water (with the recovery of ammonia being a special case of this). Nitrogen removal technologies for wastewater can be classified into three main categories: membrane separation methods, selective removal methods, and microbial assimilation methods. As shown in the figure below. 　 Figure 1 Branches of nitrogen removal methods. Although membrane separation is promising for treating industrial ammonia-nitrogen wastewater, the technology is not yet mature, resulting in limited application at present. The air stripping method typically involves blowing ammonia from water into the atmosphere using air, which causes secondary pollution. The ion exchange resin method and the chlorination method are not cost-effective. The truly mature treatment methods are steam stripping and biological methods. The steam stripping method is primarily used for treatment and control aimed at recovering ammonia and urea, and it is particularly suitable for treating wastewater with high concentrations of ammonia nitrogen. Biological methods are suitable for treating wastewater with low concentrations of ammonia nitrogen. 2.2 Steam stripping method: Nitrogen removal by steam stripping involves bringing steam into direct contact with wastewater, thereby causing the volatile ammonia in the wastewater to diffuse into the gas phase in a certain proportion, and this process serves to separate ammonia from the wastewater. The treatment of the condensate from the low-temperature shift process in ammonia synthesis is a successful example of the application of this method. This stripping process is carried out in a packed tower using a portion of medium-pressure superheated steam. The process condensate enters the tower from the top after being preheated by the effluent from the stripping tower, while the stripping steam is fed in from the bottom of the tower. Steam and process condensate come into counterflow contact in the packing layer. The medium-pressure steam is used to heat the condensate, thereby reducing the partial pressures of various impurity components in the vapor phase of the stripping tower; this allows the impurity components present in the condensate to be stripped out, primarily NH3 and CO2. The stripping gas, after exiting from the top of the tower, is sent as process steam to the first-stage furnace for gas conversion. The stripping condensate coming from the bottom of the tower is cooled in a heat exchanger and a cooler, and then sent to the desalination unit as feed water for the high-pressure boiler. The synthetic ammonia off-gas scrubber has a structure similar to that of a stripping tower. High-pressure water is used inside the tower to wash away NH3 from the vent gas, producing industrial ammonia water with a concentration of 20%, thereby avoiding the discharge of dilute ammonia water. The steam used for stripping can be medium-pressure steam or low-pressure steam. Compared to low-pressure steam stripping, the medium-pressure steam stripping process is more advanced. Medium-pressure stripping increases the pressure inside the tower, thereby increasing the density of the steam; this allows for a reduction in the diameter of the stripping tower ； As pressure increases and temperature rises, less reflux is required from the cooling system to condense NH3, resulting in NH3 of high purity that is sufficient for reuse in the system. However, medium-pressure stripping places high pressure resistance requirements on the stripping tower, heat exchangers, and transfer pumps ； Medium-pressure operation also increases energy consumption. Issues to note when recovering NH3 via steam stripping: 1. The NH3—H2O evaporation system is prone to foaming, and foaming reduces the efficiency of the stripping tower. Adding a water-soluble defoamer can solve this problem. 2. When the temperature is low under pressurized operating conditions, NH3 and CO2 react to form ammonium carbamate (abbreviated as AMMONIUM CARBAMATE). The reaction equation is as follows: NH3 (gas) + CO2 (gas) = NH4COONH2 (solid) + heat. This reaction is reversible. Ammonium hydroxide deposits can contaminate the stripping tower after crystallization. As long as an appropriate operating pressure is selected to ensure that the temperature inside the tower remains above the highest temperature at which methylammonium is formed, its decomposition can be achieved. Medium-pressure steam stripping can prevent the formation of methylammonium. In short, when recovering ammonia from water using the stripping method, choosing the operating pressure is the most fundamental aspect. The optimal operating pressure ensures the economic efficiency of tower operation. 　 Table 1: Design parameters of the medium-pressure stripping tower for the condensate from Kellogg’s low-temperature conversion process
Parameter | Process specification | Unit | Design value (range)
---|---|---|---
Condensate flow rate to the stripping tower | T/h | 27.48
Temperature | °C | 232
Condensate flow rate leaving the stripping tower | T/h | 10.99
Temperature | °C | 251
Temperature of medium-pressure steam entering the stripping tower | °C | 380
Pressure | KPa(G) | 4219
Impurities in the condensate entering the stripping tower: NH3 | mg/l | 1000; CO2 | mg/l | 3000; Methanol | mg/l | 1000
Impurities in the condensate leaving the stripping tower: NH3 | mg/l | 50; CO2 | mg/l | 0; Methanol | mg/l | 50
Figure 2: Schematic diagram of the medium-pressure stripping process for the condensate from the low-temperature conversion process
2.2 An example of modifying the process flow to recover ammonia: During the crystallization production process, the secondary steam generated in the neutralizer, first-stage evaporation, second-stage evaporation, and vacuum crystallization units contains varying concentrations of NH3 and NH4NO3. Taking the original process flow of Sichuan Chemical One as an example, if a air cooler is used to cool the discharged wastewater with straight-flow water, 647 tons of wastewater will be discharged per hour, with an ammonia nitrogen concentration in that wastewater of around 155 mg/l, which exceeds the specified discharge standards. 　 After the technical upgrade, the system was equipped with a secondary steam condensate recovery unit, which uses circulating water to cool the secondary steam. The secondary steam generated by the evaporation in the neutralizer is first used as a heating source for a stage evaporator, while the cooled condensate is discharged into the surface cooling liquid tank. The secondary steam from the first and second stages of evaporation is pumped to the surface condensers of those stages using steam jet pumps; it is cooled indirectly by circulating water, and the condensed liquid is collected in the surface cooling fluid tank. The uncondensed steam is then pumped to the intermediate cooler using secondary jet pumps for condensation, with the resulting condensed liquid also being sent to the surface cooling fluid tank. Air and uncondensed steam that enter during negative pressure conditions are vented through the jet pumps. The condensate from the surface cooling liquid tank is pumped out by a condensate pump, cooled in a cooler, and then sent to the dilute nitric acid workshop as water for the nitric acid absorption tower. 15.66 tons of condensate can be recovered per hour, which reduces the amount of water required for direct cooling in the original coolers by 402 tons per hour; at the same time, NH3 and NH4NO3 present in the condensate are returned to the system. The only direct emission comes from the crystallization process. The amount of secondary steam used in the crystallizer accounts for 7.6% of the total secondary steam volume; this secondary steam is cooled directly with tap water in an air cooler before being discharged, and the ammonia nitrogen concentration in the discharged water is ≤29.5 mg/l, which is below the **discharge standards. Compared to the original process, the reduction in ammonia nitrogen in the discharged water is very significant. 2.3 Biological method Biological nitrogen removal: A method that simulates the nitrogen cycle in natural environments, and utilizes the combined action of specialized aerobic nitrifying bacteria and facultative denitrifying bacteria present in sludge to convert nitrogen-containing compounds in water into nitrogen gas. 　 The reactions are as follows: Nitrification: 2NH4+ + 4O2 → 2NO3- + 4H+ + 2H2O; Denitrification: NO3- + organic carbon source → CO2 + N2 + H2O + OH-. In ammonia synthesis plants, in addition to wastewater containing high concentrations of ammonia nitrogen, there is also wastewater with lower concentrations of ammonia nitrogen, around 500 mg/l. This portion of the wastewater is suitable for treatment using biological methods. The nitrogen removed through biological denitrification can be recovered or released into the atmosphere, without causing secondary pollution. It is easy to operate and can be integrated with existing treatment systems. 　 The currently more common biological treatment method is the suspended biological process. This method is only suitable for denitrification of domestic wastewater (with an ammonia nitrogen concentration of 100 mg/l or less), due to its low volume load. The lower the volumetric load, the larger the volume of reactor required to treat the same amount of ammonia-nitrogen wastewater (with an ammonia-nitrogen concentration of 500 mg/l), and the capacity of the supporting facilities also needs to be increased accordingly. The large land area required and high infrastructure costs limit the application of the suspended biological method in the treatment of industrial ammonia nitrogen wastewater. There are examples of this method being used abroad; the DSM Green Industrial Complex in the Netherlands uses the activated sludge process to treat industrial wastewater with an ammonia nitrogen concentration of 100–500 mg/l. The volumetric load for total nitrogen is 0.122 kg-N/m3, and the effective volume of the nitrification tank is 110,000 cubic meters, which is extremely large. 　 Low volumetric load is an inherent flaw of the suspended biological method. To overcome this deficiency and cultivate a high concentration of microbial communities, it is necessary to change the way microorganisms grow. Experiments conducted jointly by Sichuan Chemical Research Institute and the Environmental Safety Department have shown that using the fixed-microorganism method for treating industrial ammonia-nitrogen wastewater is effective. In the experiment, a lift-type tower-type two-stage three-phase nitrification reactor designed by Sichuan Chemical Research Institute and a USB high-efficiency denitrification system were used. When connected in series, the two form an efficient nitrogen removal process, with a system load of 1.5 kg-N/m3. The entire reactor has a tower-like structure, can extend underground, and requires little land area. Devices similar to this system have been officially put into use in Japan, achieving significant treatment results. Therefore, the use of fixed microbial methods to treat industrial ammonia-nitrogen wastewater in China should have great potential for development. 　 III. Conclusion: Handling the discharge of ammonia-nitrogen wastewater properly in ammonia synthesis plants not only contributes to environmental protection for the sake of ** and society as a whole, but also fulfills their own environmental responsibilities. It is important to choose a good, proven method for treating ammonia nitrogen wastewater. At present, it is better to use medium-pressure steam for the stripping of wastewater with high concentrations of ammonia nitrogen, and this method is widely used in practice. Fixed microbial treatment is suitable for low-concentration ammonia nitrogen wastewater, but it is used less frequently. To fundamentally address the issue of ammonia nitrogen emissions in ammonia synthesis plants, it is essential not only to carry out technical upgrades, innovate processes, and adopt clean production methods, but also to continuously develop new treatment technologies and improve existing ones.