Air separation plant is one of the important production devices in petrochemical, metallurgy and other industries. Due to its special structure and physical and chemical properties of media, the risk of explosion is relatively high. In recent years, due to manufacturing defects and poor management of air separation plant, there have been many explosion accidents of air separation plant. According to incomplete statistics, there have been more than 100 explosion accidents of small air separation plant all over the country, there are more than 30 accidents of large and medium-sized air separation plant, especially the explosion caused by excessive hydrocarbon substances in the evaporator of air separation main condensing is the main reason for frequent accidents in recent years, which not only affects the smooth operation of production unit, and cause great economic losses to enterprises and the country. The following is a brief discussion on the explosion mechanism and explosion-proof measures of the main cold evaporator of the air separation unit based on the practical operation experience of our unit.
1 Chemical Explosion mechanism of air separation
1.1 Main condensing evaporator explosion mechanism
There are many reasons for the explosion of the air separation tower, which are also complicated, but they can be basically divided into physical explosion and chemical explosion. From the analysis of most explosion cases, chemical explosion is the main one. There are three main factors for the formation of chemical explosion: The first is combustibles, the second is combustion-supporting substances, and the third is the detonating source. In the evaporator of the main condensing of the air separation plant, the combustibles are mainly the explosive dangerous impurities such as acetylene, hydrocarbon or oil, etc.; the combustion engines are gas oxygen and liquid oxygen; the main detonators are: (1) explosive impurity solid particles friction with each other or with the vessel wall; (2) electrostatic discharge. When liquid oxygen contains a small amount of ice particles and solid carbon dioxide, electrostatic load will be generated. If the content of carbon dioxide increases to 200 ~ 300 × 10-4%, the generated electrostatic level can reach 3000 V; (3) pressure pulse caused by gas wave shock, fluid shock or cavitation phenomenon, cause high local pressure and increase temperature; (4) substances with particularly strong chemical activity (ozone, nitrogen oxide, etc.) exist, which increases the explosion sensitivity of the mixture of combustible substances in liquid oxygen.
1.2 Formation Conditions of explosion source
In addition to oxygen and nitrogen, there is also a small amount of gas such as water vapor, carbon dioxide, acetylene and other hydrocarbon and a small amount of solid substances such as dust in the air, domestic large and medium-sized molecular sieve purification process most of the methods to remove impurities such as moisture, carbon disixide and acetylene in the air adopt the absorption method, that is, using molecular sieve or silica gel as absorbents to remove air (liquid empty, liquid oxygen) the moisture, carbon dioxide, acetylene and other impurities contained in it are separated and concentrated on the surface of the adsorbents, and removed when heated and regenerated, so as to achieve the purpose of purification.
However, due to the concentration of chemical equipment, if the leakage of the equipment is too high or the hydrocarbon products are emptied directly, the content of hydrocarbon absorbed by the air separation plant will exceed the standard, and the air separation unit for the process of separation sieve, 13X molecular sieve has the feature of strong adsorbability of polar molecules with close aperture, and basically it can be removed from the molecular sieve adsorber by moisture, carbon dioxy and acetylene, most of other hydrocarbon, such as methane and ethylene, enter into the air separation tower along with the air. Most of these substances are dissolved in the liquid, and a small amount is taken away along with the evaporation of oxygen. When the concentration of hydrocarbon in the liquid increases continuously and exceeds its solubility, it will be precipitated and gathered in the form of solid, mixing with oxygen to form an explosive source under certain conditions, when the detonating factor exists, chemical explosion will occur.
A large number of facts prove that the explosion sensitivity of acetylene in liquid oxygen is the highest. Because the partial pressure of acetylene in the air is very low, that is to say, if the air is cooled to-173 ℃, acetylene will not be precipitated in solid form, and it will be carried into the air separation tower along with the air, while the solubility of acetylene in liquid is relatively high, which is about 20 cm 3/dm3. Therefore, generally, it will not be precipitated in the liquid air, while entering the upper tower with the liquid air, the solubility of acetylene in liquid oxygen is extremely low, which is about 5.5 cm 3/dm3. When liquid oxygen evaporates in the evaporator of the main condensing, the amount of acetylene along with the gas-oxygen zone is only about 1/24 of the total amount of acetylene in the liquid oxygen. Thus, with the evaporation of liquid oxygen, the concentration of acetylene in the liquid oxygen increases continuously, when acetylene exceeds its solubility, excess acetylene will float in liquid oxygen with white solid particles, and acetylene is an unsaturated hydrocarbon compound with high chemical activity, the property is extremely unstable, these solid acetylene or other hydrocarbon particles have friction with the tower wall and the passage wall or liquid oxygen boiling generates pressure pulse, and the energy generated by the promotion of ozone and NOx will probably explode the air separation tower. But in the actual production, sometimes the acetylene and other hydrocarbon in liquid oxygen do not exceed the standard and explosion occurs, which is mainly due to the unreasonable structure of condensation evaporator, there are some manufacturing defects. If some channels are blocked and operated improperly, the local mobility of liquid oxygen is not good, and acetylene is concentrated and exploded.
Other unsaturated hydrocarbon can also have explosive decomposition reaction. Although their solubility in liquid oxygen is higher than acetylene, the adsorptive capacity of adsorber to these hydrocarbon is extremely small, therefore, there is also the possibility of explosion caused by accumulation in liquid oxygen. A large number of studies show that due to the different solubility and chemical activity of each component in liquid oxygen, the explosion sensitivity of hydrocarbon is also different, and the explosion sensitivity increases in the following order: it can be seen that acetylene has the highest sensitivity.
