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1 Preface
Molecular sieve purification system is the key equipment for air separation unit to purify air and ensure safe operation. It is of great value for the actual operation management of the air separation unit to understand its working principle and be familiar with various factors influencing its normal operation. 2 Process introduction and important role 2.1.1 process introduction In the air separation plant, the purification system of molecular sieve is set behind the air precooling system to remove impurities such as water, carbon dioxide and acetylene in the air. It consists of molecular sieve adsorber, regeneration heating device, Valve, pipeline and instrument electric control system. Purifying the adsorptive impurities in the air is done by the molecular sieve adsorber. The adsorber is filled with molecular sieve, and other adsorbents (active alumina is mainly used to adsorb water, and molecular sieve is mainly used to adsorb hydrocarbon impurities such as carbon dioxide and acetylene ). Molecular sieve purification system generally uses two adsorbers to switch to use, one is saturated after absorption, and the other is regenerated and put into operation. After the absorption is saturated, the adsorbents lose the ability of continuous absorption and can only be used after regeneration. Regeneration is the inverse process of adsorption -- because the adsorbents lose the adsorption capacity after being absorbed and divided, certain measures must be taken to drive away the absorbed components from the surface of the adsorbents, restore the adsorption capacity of the absorbents, which is regeneration. Regeneration process is divided into four steps: depressurization, heating, chilling and boosting. 2.1.2 absorption characteristics Modern large air separation unit, molecular sieve adsorber often uses molecular sieve and activated alumina to form a double bed. Mainly introduce the characteristics of these two kinds of absorbents. Molecular sieve: 1) it has strong adsorption capacity and selective adsorption performance; 2) it has extremely high dryness and good drying capacity for high temperature and high speed airflow. The lower the content of water vapor is, the smaller the relative humidity is, the more significant the adsorption capacity is. But when the relative humidity is relatively high, the adsorption capacity is relatively small; 3) good stability, and the normal adsorption capacity can still be maintained under 200℃. The service life is also relatively long; 4) The adsorption on water is extremely strong, followed by acetylene and carbon dioxide. Activated aluminium oxide: 1) high compressive strength, low abrasion rate, no pulverization and no burst; 2) strong cold and heat resistance; 3) Good water absorption, low price and low resistance. Generally, it is arranged in the lower layer to preliminarily dry the air entering the adsorber. 2.2 Important role Air is the raw material of air separation production, which contains a small amount of gas such as water, carbon dioxide, acetylene and other hydrocarbon, as well as solid impurities such as dust. Although the content of these impurities is not large in the air per cubic meter, because the processing air volume of the large air separation unit per hour is tens of thousands or even hundreds of thousands of cubic meters, therefore, the total amount of the air separation unit brought in per hour is still very considerable. These impurities do great harm to the safe operation of the air separation installation. With the cooling of the air, the frozen water and carbon dioxide deposit in the low temperature heat exchanger, turbine expansion machine or distillation column, it will block channels, pipelines and valves; acetylene gathering in liquid oxygen may cause the danger of explosion. Removing and purifying these impurities and ensuring the long-term safe operation of the air separation unit are the important functions of the molecular sieve purification system. 3 common problems of molecular sieve purification system and corresponding measures 1) the air intake temperature is too high: the processed air enters the molecular sieve after being cooled down by the Air precooling system. The higher the temperature is, the worse it will be to the adsorption of the molecular sieve, the normal and effective operation of the air precooling system must be ensured (the air volume of nitrogen and polluted nitrogen feeding water cooling tower can be appropriately increased to strengthen the cooling effect) generally control the temperature of the air outlet air cooling system not higher than 15℃. 2) the molecular sieve bed is impacted by airflow: maintain the uniformity of the molecular sieve bed, keep the pressure difference of the molecular sieve system stable (less than 10kPa), and cooperate closely with the personnel in the air pressure position, prevent the drastic fluctuation of air volume. 3) insufficient quantity of cold air blown by heating: This problem is mostly caused by the failure of the valve switching of the molecular sieve system. During the operation, each valve should be strictly monitored, and the air volume should be adjusted and increased timely, in severe cases, the operation process of the molecular sieve system can be suspended to prolong the regeneration time. 4) electric heater failure: it belongs to equipment problem, and it should be checked and maintained by cooperative maintenance personnel. At ordinary times, you must pay attention to ensure that there is a complete spare electric heater. 5) molecular sieve with water: this is a very serious problem, once with water, it will directly cause the whole set of equipment shut down, seriously affect the production. The exchange of air and water is carried out in the air cooling tower, so as to keep the pressure in the air cooling tower stable and prevent the accidents with water caused by dramatic changes (it is easy to happen when stopping or driving ); the liquid level of the water cooling tower should also be strictly prevented from being too high, and overflow pipes can be set to prevent water flow through the sewage Nitrogen regeneration pipe. 6) poor regeneration effect: in actual production, the highest peak value during cooling of the sub-sieve is mainly used as the judgment standard. Ensure that every regeneration cycle is not lower than 90℃, and the peak value does not need to be too high, which can only cause the waste of renewable gas. 7) poor adsorption effect: from the characteristics of molecular sieve, it can be seen that its adsorption effect on carbon dioxide is the lowest. As long as it is qualified, it means that the adsorption effect of other impurities is also guaranteed. The adsorption effect (generally less than 1.5PPm) is supervised by the parameter "CO2 content of air into the cold box". When it is found to be abnormal, the authenticity of the data should be confirmed first, and the content is indeed too high, find out the reason and deal with it. If necessary, report to the supervisor and stop the car. 4-the conclusion From the above analysis, we can know that the molecular sieve purification system is the basis of the safe production of the air separation plant and must ensure normal and reliable operation. |
