The harm of tar in semi-water gas to the ammonia synthesis system
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The generated semi-water gas contains tar, phenols, and dust; after carbon contact dust removal and washing and cooling, desulfurization is required. However, there is still oil and water in the gas – how can these be removed? What other harms can oil and water cause to the system?
The hazards of oil and water are roughly as follows: 1. They tend to accumulate within pipes, reducing the effective flow area of these pipes; sometimes a water seal is also formed inside the pipes, which increases the resistance in the pipes； 2. When oil and water enter the desulfurization system, they tend to adhere to the surface of the packing, affecting gas-liquid mass transfer ； 3. Once oil and water enter the heat exchange equipment, it affects the equipment’s heat transfer efficiency ； 4. Oil and water entering the compressor can easily clog its air valves, reducing the compressor’s air delivery capacity ； 5. The introduction of oil and water into the catalyst bed can lead to carbon deposition on the surface of the catalyst, blocking its micropores and affecting its activity ； 6. Saturated water enters the compressor along with the gas, reducing the effective gas delivery volume and affecting the ammonia synthesis output. Common methods for removing oil and water include: 1. Installing cyclone separators and drain devices at the pipeline ； 2. Install a coke filter to use coke to absorb oil and moisture from the gas ； 3. A static electrostatic defoaming tower can remove the vast majority of oil and water from the gas ； 4. Effectively reduces the inlet temperature of the first stage of the compressor, thereby minimizing the water content in the air entering that stage.
The separation of oil and water from the feed gas is primarily achieved through gravity separation; some oil-water separation tanks are equipped with packing that serves a filtering function. Nowadays, some ammonia synthesis plants use molecular sieves in the synthesis process, taking advantage of the difference in molecular sizes to separate water. The main hazards associated with oil and water in ammonia production include: 1) Due to the incompressibility of liquids, their entry into compressors can cause damage to valves and cylinders; 2) Oil and water can contaminate solutions, such as copper solutions; 3) Oil deposits on catalysts reduce their activity. Water can cause catalysts to break down, form clumps, or become powdered, and may even lead to catalyst poisoning.
High tar levels can also prevent sulfur foam from being generated during desulfurization
A coke filter is installed at the gas holder outlet, as well as electrostatic coke removers at the desulfurization inlet and outlet, to cool the semi-water gas entering the converter and enable efficient separation. The most significant hazards: 1. Clogging of the desulfurization tower, preventing foaming during desulfurization regeneration ； 2. The compressor valve is blocked, which affects the air inflation volume and the operating cycle ； 3. The total solids content in the water of the saturated tower is difficult to keep within acceptable levels; the amount of waste discharged is high, and this can even lead to clogging of the packing, resulting in liquid carrying over into the saturated tower ； 4. Transformation of catalyst carbon deposition and surface scarring, blockage of the catalyst’s surface micropores, reduced catalyst activity, increased system resistance, and impact on production ； 5. The coolers in the first and second stages of the compressor are prone to carbon buildup, which affects heat exchange and other functions.
Harm: 1. Clogging the compressor’s air valves and water-cooled compressor, affecting its compression capacity； II. Blockage of the desulfurization tower packing, affecting the absorption efficiency of the tower ； III. The quality of the gas undergoing transformation is poor, which can easily lead to the deactivation of low-temperature catalysts and affect long-term stable operation ； IV. The hot water circulation system in transformation facilities has poor water quality, high discharge volumes, and causes environmental pollution. Precautionary measures: 1. Install electrostatic coking eliminators at the inlet and outlet of the desulfurization unit, and ensure their efficient operation to remove coal tar ； II. Increase coke filters or wire mesh demisters by changing imports ； III. Strengthen the emission control from various scrubbers, oil-water separators, etc., to prevent gas entrainment.