1. Air separation equipment and its performance characteristics

1.1 Air separation equipment

Air separation equipment is an equipment that uses air as raw material, turns the air into a liquid state through the compression cycle deep freezing method, and then gradually separates it from the liquid air through distillation to produce inert gases such as oxygen, nitrogen, and argon.

At present, the air separation equipment produced in my country comes in various forms and types. There are equipment to produce gaseous oxygen and nitrogen, and there are also equipment to produce liquid oxygen and nitrogen. But in terms of basic processes, there are four main types, namely high pressure, medium pressure, high and low pressure and full low pressure processes. The production scale of my country's air separation equipment has grown from producing only 20 cubic meters/hour (oxygen) oxygen generators in the early days to currently producing 20,000 cubic meters/hour, 30,000 cubic meters/hour and 50,000 cubic meters/hour ( Oxygen) very large air separation equipment.

1.2 Basic system of air separation equipment.

Air separation equipment can be divided into five basic systems in terms of process flow.

1.2.1 Impurity purification system: It mainly uses equipment such as air filters and molecular sieve absorbers to purify mechanical impurities, water, carbon dioxide, acetylene, etc. mixed in the air.

1.2.2 Air cooling liquefaction system: It is mainly composed of air compressor, heat exchanger, expander and air throttle valve, etc., which plays the role of deep freezing of air.

1.2.3 Air distillation system: It mainly consists of a distillation tower (upper tower, lower tower), condensing evaporator, subcooler, liquid air and liquid nitrogen throttle valve, etc. Plays the role of separating various components in the air

1.2.4 Heated air blowing system: Regenerate the purification system through heated air blowing.

1.2.5 Instrument control system: The entire process is controlled through various instruments.

2. Surface cleanliness of air separation equipment and its inspection methods

2.1 Reasons why air separation equipment is prone to combustion

Three conditions must be met for combustion or even explosion to occur: a certain amount of combustibles, the presence of a corresponding amount of oxidizer, and minimum energy guarantee. The most important feature of the working environment of air separation equipment is the medium oxygen circulation at low temperature or room temperature. Pure oxygen is a strong oxidant. Even in the low-temperature state of liquefaction of -183°C, as long as the quantity or concentration of flammable and explosive substances exceeds the explosion limit and the energy generated by the medium oxygen due to high-speed transportation friction accumulates to a certain value, an explosion will still occur. Explosion, causing casualties and equipment damage. Therefore, the surface cleanliness requirements of all parts of the air separation equipment that come into contact with the oxygen in the medium are very high, and mechanical impurities and organic matter such as grease are not allowed to exist. These substances must be removed.

The degreasing and cleaning of oil-free parts of air separation equipment is based on physical or chemical methods, selecting appropriate cleaning agents and cleaning their surfaces through a specific cleaning process to ensure that the concentration of organic matter on the surface is controlled below the explosion limit. This is a necessary condition for the safe operation of air separation equipment. After surface treatment, it must undergo strict inspection and inspection before it can be put into use.

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 categories of substances.

(1) Solid substances: such as organic anti-rust agents, wood, paper, fiber, paint and other organic substances; welding slag and spatter, metal shavings, welding wire and other metal substances; sand and similar particulate matter, and other substances that may dissolve under working conditions substance.

(2) Cleaning fluid and water

(3) Floating rust and scale

(4) Mineral oil and grease

2.3 Surface cleanliness inspection method

Solid materials, cleaning fluids, water and rust marks can be visually inspected directly with the eyes. Under bright light, visually observe whether there is any residual solid matter on the surface of the equipment being inspected. Solid particles with a diameter (or diagonal) greater than 0.5mm are not allowed to exist. The sum of solid particles with a diameter (or diagonal) between 0.25-0.5mm should be less than 100 particles/m2. There should be no fibers, dust and Fabric. The length of a single residual fiber 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 drop diffusion method, etc. Quantitative analysis methods can be divided into gravimetric methods and oil concentration determination methods.

At present, the oil concentration measurement method is basically used to measure the residual oil on the surface of air separation equipment in my country, and this method is the most widely used.

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

The degreasing cleaning of air separation equipment is to remove the grease on the surface of components to meet the standard requirements for the amount of grease residue on the surface. The lower the amount of residual oil specified in the standard, the higher the requirements for equipment cleaning. The cleaning process will be more cumbersome and labor-intensive, and the cleaning cost will also increase greatly. How should the standard for prohibiting residual oil measurement on the surface of air separation equipment components be determined? I think the principle should be to implement ISO9000, ensure product quality, and ensure the absolute safety of system operation. At the same time, the value of the residual oil volume should be set reasonably and appropriately.

I think the essence of this reasonable value is the behavior, enrichment and explosion limit concentration of grease in oxygen media. Some literature review articles [1] and Mckinley.c [2] and others pointed out that the lower explosion limit of hydrocarbon mixtures dissolved in liquid oxygen can be taken as 5% (molecular) methane equivalent. When the concentration of hydrocarbons in a mixture exceeds these limits, it becomes an explosive mixture with special hazards.

This limiting concentration also applies to the uniform distribution of hydrocarbons in suspended or emulsified form in liquid oxygen. After conversion, 5% molar methane equivalent is equivalent to 28kgCnH2n/m3 liquid oxygen. The grease in liquid oxygen comes from the residual grease on the surface in contact with the medium oxygen in the air separation equipment. The oil-rich content of parts with different specific surfaces is different. The larger the surface per unit volume, the greater the oil-rich content. We assume that all surface residual oil enters liquid oxygen. After conversion, the limit values of surface residual oil for parts with different specific surfaces can be obtained (see Table 1).

Table 1 Calculated values of oil residue limits that are different from those on the product surface

Product specific surface area residual oil limit calculation value

Name Specification g/m2μm

Adjusting packing 750#350# 750350 3580 4194

Oxygen cooler 160 175 200

Liquid oxygen cryogenic storage tank 50m3 100m3 42 700014000

It can be seen from Table 1 that if the residual grease on the surface in contact with the medium oxygen in the air separation system is absolutely uniformly dispersed in the liquid oxygen (this is the ideal state in the laboratory), then the explosion limit grease residual amount is equivalent high.

What is the actual form of oil on the instrument wall at low temperatures? The article "Performance of Oil Films on Structural Packing at Low Temperatures" [3] introduces the research on the performance of oil films in oxygen and liquid oxygen by Linde, Anton Kissinger and others. Kerhart used n-hexadecane as a typical substance in combustion experiments in liquid oxygen. At low temperatures, the n-hexadecane oil film on the stainless steel pipe wall will solidify. When the oil film thickness is >5g/m2, part of the oil layer is observed to peel off. , the solid n-cetane oil film formed floats on the surface of liquid oxygen. Although n-hexadecane is transportable at ambient temperatures and flakes off at low temperatures, Kehat was unable to ignite oil, so he proposed that oil contamination of about 5 g/m2 could be tolerated in a system using oxygen.