Cleaning of the hottest air separation plant

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Cleaning of air separation equipment

1, air separation equipment and its performance characteristics

1.1 air separation equipment

air separation equipment is a device that takes air as raw material, turns air into liquid through compression, circulation and deep freezing, and then gradually separates and produces inert gases such as oxygen, nitrogen and argon from liquid air through rectification

at present, there are many forms and types of air separation equipment produced in China. There are devices for producing gaseous oxygen and nitrogen, as well as devices for producing liquid oxygen and nitrogen. But in terms of basic processes, there are mainly four kinds, namely, high-pressure, medium pressure, high-low pressure and full low-pressure processes. The production scale of China's air separation equipment has developed from an oxygen generator that can only produce 20m3/h (oxygen) in the early stage to a super large air separation equipment that can produce 20000 m3/h, 30000 m3/h and 50000 m3/h (oxygen)

1.2 basic system of air separation equipment:

from the perspective of process flow, the air separation equipment can be divided into five basic systems:

1.2.1 impurity purification system: it mainly purifies the mechanical impurities, moisture, carbon dioxide, acetylene, etc. mixed in the air through air filters, molecular sieve absorbers and other devices

1.2.2 air cooling and liquefaction system: mainly composed of air compressor, heat exchanger, expander and air throttle valve, it plays the role of deep freezing of air

1.2.3 air rectification system: the main components are rectification tower (upper tower, lower tower), condensation evaporator, subcooler, liquid air and liquid nitrogen throttle valves. Play the role of separating various components in the air

1.2.4 heating and purging system: regenerate the purification system by heating and purging

1.2.5 instrument control system: control the whole process through various instruments

2. Air separation equipment surface cleanliness and its inspection methods

2.1 reasons why air separation equipment is prone to combustion

three conditions must be met for combustion and even explosion: the use of a certain amount of combustible iron tools is also the reason for the increase of iron content in molten aluminum. 1. Quality, the existence of a corresponding amount of oxidants, and the minimum energy guarantee. The greatest feature of the working environment of air separation equipment is that medium oxygen flows under low or normal temperature conditions. Pure oxygen is a strong oxidant. Even in the liquefied low temperature state of -183 ℃, as long as the quantity or concentration of inflammables and explosives exceeds the explosion limit, and the energy accumulated by medium oxygen due to high-speed flow friction reaches a certain value, explosion will still occur, causing casualties and equipment damage. Therefore, the surface cleanliness requirements of all parts of the air separation equipment that can contact the medium oxygen are very high, and no mechanical impurities, grease and other organic substances are allowed. These substances must be removed

degreasing cleaning of oil prohibited parts of air separation equipment is to select appropriate cleaning agents to treat their surfaces through specific cleaning processes through physical or chemical methods, so as to ensure that the concentration of organic substances on their surfaces is controlled below the explosion limit. This is a necessary measure for the safe operation of air separation equipment. After surface treatment, it can only be put into use after strict inspection and inspection

2.2 types of dirt on the surface of air separation equipment

the indicators for checking the surface cleanliness of air separation equipment should include the following four types of substances:

(1) solid substances: organic substances such as organic rust inhibitor, wood, paper, fiber, paint, etc; Welding slag and splash, metal chips, welding wires and other metals; Sand and similar granular materials, as well as other substances that may dissolve under working conditions

(2) cleaning fluid and water

(3) floating rust and oxide skin

(4) mineral oil and grease

2.3 inspection method of surface cleanliness

solids, and focus on the tiling-/trim-in-place technology used in the production of cup products. The quality, cleaning fluid, water and rust scar can be directly visually inspected with eyes. Under the bright light, observe the surface of the tested equipment with your eyes to see if there are any residual solid substances. Solid particles with a diameter (or diagonal) of more than 0.5mm are not allowed. The sum of solid particles with a diameter (or diagonal) of 0.25 ~ 0.5mm should be less than 100 particles/m2, and there should be no fiber, dust and fabric. The length of individual residual fibers shall not exceed 2mm; Residual cleaning fluid and water are not allowed, and the surface should be completely dry

the determination of mineral oil and grease can be divided into direct inspection method and quantitative determination method. Direct inspection methods include filter paper wiping method, ultraviolet fluorescence method, water coating test method and dripping diffusion method, etc. quantitative analysis methods can be divided into gravimetric method and oil concentration determination method

at present, the measurement of residual oil on the surface of air separation equipment in China is basically based on the oil concentration measurement method, which is also the most widely used method

3. Determination of oil residue standard on the surface of air separation equipment

degreasing and cleaning of air separation equipment is to remove oil on the surface of components to meet the requirements of surface oil residue specified in the standard. The lower the amount of residual oil specified in the standard, the higher the requirements for cleaning equipment, the more cumbersome the cleaning process, and the labor intensity and cleaning cost will be greatly increased. How to determine the measurement standard of surface residue of oil prohibited parts of air separation equipment? I think the principle should be to implement ISO9000, ensure product quality, ensure the absolute safety of system operation, and set the quantity value of residual oil reasonably and appropriately

I think the essence of this reasonable value is the behavior, enrichment and explosion limit concentration of oil in oxygen medium. Some literature review articles [1] and mckinley C[2] and others have pointed out that the lower explosion limit of the mixture of hydrocarbons dissolved in liquid oxygen can be taken as 5% (mole) methane equivalent. When the hydrocarbon concentration in the mixture exceeds the above limit, it will become an explosive mixture with special danger

this limit concentration is also suitable for the uniform distribution of hydrocarbons in liquid oxygen in the form of suspension or emulsion. After conversion, 5% mole methane equivalent is equivalent to 28kgcnh2n/m3 liquid oxygen. The grease in liquid oxygen comes from the residual oil dirt on the surface in contact with medium oxygen in the air separation unit. The oil enrichment of parts with different specific surfaces is different. The larger the surface per unit volume, the greater the enrichment. We assume that when all the residual oil on the surface enters liquid oxygen, the limit value of the residual oil on the surface of parts with different specific surfaces can be calculated through conversion (see Table 1)

Table 1 calculation value of residual oil limit of products with different specific surface

calculation value of residual oil limit of specific surface area of products

name specification g/m2 μ M

structured packing 750 #350 # 750350 3580 4194

oxygen cooler 160 175 200

liquid oxygen low temperature storage tank 50m3 100m3 42

it can be seen from table 1 that if the residual oil on the surface in contact with medium oxygen in the air separation system is absolutely evenly dispersed in liquid oxygen (this is an ideal state in the laboratory), the limit oil residue for explosion is quite high

what is the actual shape of the oil on the wall at low temperature? "Performance of oil film on structural fillers at low temperature" [3] introduces the performance research of oil film in oxygen and liquid oxygen by Linde, Anton Kirzinger, etc. Kehat uses n-hexadecane as a typical substance and conducts combustion experiments in liquid oxygen. At low temperature, the n-hexadecane film on the stainless steel pipe wall will solidify. When the oil film thickness is more than 5g/m2, it is observed that part of the oil layer peels off, and the solid n-hexadecane film formed floats on the surface of liquid oxygen. Although n-hexadecane has fluidity at ambient temperature and will peel off at low temperature, kehat still cannot ignite the oil, so he proposed that in the oxygen system, oil pollution of about 5 g/m2 is tolerable

Ball reached the same conclusion in another article. He suggested that the allowable limit of oil pollution is 1 g/m2 in oxygen enriched environment. McKinley, mentioned above C after the discussion that the lower explosive limit of the mixture of carbon and oxygen compounds in liquid oxygen can be taken as 5% mole methane, it is also mentioned that when explosive impurities exist in liquid oxygen in the form of a film on the metal surface, under the explosion limit, even if the layer thickness is very thin, the combustion will expand along the film, which has a critical thickness. See Table 2 for explosion parameters of some hydrocarbons in liquid oxygen

h (see the original text) [4] et al. Determined the minimum hydrocarbon layer thickness causing explosion through experiments: the layer thickness of industrial oil-12 is 40 ~ 50 m, and the explosion effect occurs when the layer thickness of other hydrocarbons is m, which will cause Ф 10mm × 1mm、 Ф 12.5mm × 1.5mm copper and aluminum tubes were damaged. This experimental result is consistent with the values listed in Table 2

Table 2 explosion parameters of hydrocarbons in liquid oxygen [4]

name explosion velocity m/s critical thickness δ Critical, μ M

acetylene pentane ethane industrial oil --12 --121035

now the above data and the relevant surface residual oil volume standards are unified. The density of industrial oil-12, n-hexadecane and grease are converted to 0.85g/cm3, which is listed in Table 3

Table 3 maximum allowable residual oil quantity on the surface of air separation equipment

author's allowable large residual oil quantity g/m2 μ M

ehat 3025.75 * 35*30*5.9

balljb/t 1*0.125* 1.20.147

from table 3, it can be seen that the provisions of residual oil in the standard jb/t 6896-93 surface cleanliness of air separation equipment are far below the explosion limit of grease, which takes into account the different specific surface areas of various components of air separation equipment and the uneven distribution of grease

China's jb/t 6896-93 industry standard stipulates that the residual amount of grease on the surface of parts in contact with oxygen and the surface of parts that may be brought into oxygen by residual oil during operation shall not exceed 125mg/m2. The air separation equipment manufactured according to this standard has not exploded due to "oil" nationwide. Can this explain that the 125 mg/m2 residual oil volume is also safe for the components with the most stringent requirements for cleanliness. Of course, with the development of science and technology, the sixth generation air separation equipment uses structured packing. The concept of residual oil on the surface of fillers originally used in the chemical industry does not exist. That is to say, the grease on the surface of chemical fillers will not affect their use. However, those transplanted into the air separation plant are different. Because of their large specific surface area and high oil concentration, they must be carefully cleaned before they can be used. At this time, the cleanliness of the packing as an important issue has attracted widespread attention:

how to determine the standard of residual oil on the surface of the structured packing? So far, there is no unified statement. In 1995, Wuxi Xuelang chemical packing plant produced SW-1 structured packing for Hangyang "1000" air separation argon plant. We stipulate that the residual oil standard is 100mg/m2, which is taken as the inspection and acceptance standard. It has been operating normally since it was put into operation. It is proved that it is appropriate to stipulate the standard of residual oil on the surface of structured packing in this way

4 cleaning agent commonly used for degreasing and cleaning of air separation equipment

cleaning the metal surface to make it clean is one of the important factors to prevent products from being corroded by the atmosphere. In the air separation system, in addition to corrosion prevention, explosion prevention is more important, and it is an important part that cannot be ignored in the air separation system. There are various materials used in the manufacture of air separation equipment, including stainless steel products, aluminum and its alloys, copper and copper alloy products. The overall dimensions vary greatly, and the requirements for cleaning accuracy (aperture tolerance) are also different. Therefore, we have developed different cleaning processes with different cleaning agents for different working conditions, but the final cleaning of products generally adopts carbon tetrachloride immersion or scrubbing to meet the industry standard of surface grease residue ≤ 125 mg/m2

then residual oil ≤ 125 mg/m2

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