Determination of hydrocarbon concentration in liquid oxygen by gas chromatography

After a long-term operation of the air separation plant, trace acetylene and other hydrocarbons remaining in the tower air will inevitably be gradually concentrated in liquid oxygen. When the content exceeds its solubility, solid particles will precipitate out, which constitutes Greatly harmful. In order to ensure the long-term safe operation of the air separation system, the content of acetylene and other hydrocarbons in liquid oxygen is analyzed and monitored in order to provide a reliable analysis basis for air separation production. Our factory purchased HZT-02 gas chromatograph. Using hydrogen flame ionization gas chromatography to solve the qualitative and quantitative analysis of hydrocarbons in liquid oxygen.

1 Instruments for analysis

HZT-02 type trace total hydrocarbon chromatography analyzer. Computer Data Processing Hydrogen Generator High Purity Nitrogen A bottle of instrument air.

Gas source requirements: The hydrogen must be ultra-pure and dry. The air is oil-free and dry air, using bottled nitrogen as the carrier gas, with a purity of more than 99.995%.

2 Analysis method

2.1 Column selection

To successfully analyze the sample, the type of column, column length, and inner diameter must be correctly selected for the analysis object. In order to analyze samples quickly and efficiently.

Chromatography column selection: choose 1 meter stainless steel debugging column and 1 # concentration column according to the manufacturer's provision and daily analysis

In short, the chromatographic column is the core to determine the quality of the separation. A good quality column should have high efficiency, good selectivity, inert inner wall and a wide temperature range.

2.2 Selection of carrier gas and its flow rate

For a certain column and sample, there is an optimal carrier gas flow rate, at which time the column efficiency is the highest. In addition, the type of detector must also be considered. Different types of detectors have different requirements for carrier gas. Especially when using a hydrogen flame ion detector (FID), the maximum sensitivity requires a certain concentration of a standard sample containing the desired compound to optimize the flow rate. The standard sample is used to experiment with different carrier gas, air and hydrogen flow rates to determine the maximum response. The flow rate of which the decisive variable is the ratio of hydrogen to carrier gas.

According to the manufacturer's supply and experiment, our factory selected the pressure of the following sampling system:

Carrier gas: 0.08MPa Hydrogen: 0.04MPa Air: 0.055MPa

2.3 Selection of column temperature

Column temperature is an important operating parameter that directly affects resolution and analysis speed. First, the column temperature must not exceed the maximum temperature of the column. Generally, increasing the column temperature makes the components volatilize close, which is not conducive to separation. If the column temperature is too low, the peak shape becomes wider, the column efficiency decreases, and the analysis time is extended. The general principle of selection is to use the lowest column temperature as possible under the premise of making the most difficult components to be separated as well as possible, but the peak shape does not trail the tail and the retention time is appropriate. If the boiling point range of the sample is wide, the temperature program should be adopted. The choice of specific operating conditions should be based on different actual conditions.

What we want to analyze is the gaseous hydrocarbons within C4. Although the boiling point range is not too wide, it is also quite sensitive to the change of column temperature. Generally, if the constant temperature can meet the analysis requirements, try not to use the program temperature increase. After the experiment, we set the column temperature to 50 degrees and the detector temperature to 100 degrees.

2.4 Choice of analysis method

In theory, there are three analysis methods: normalization, internal standard and external standard. However, the content of hydrocarbon components in liquid oxygen cannot be determined, and the concentration range of each component does not change much. It is a gas sample analysis of homogeneous trace components. According to these characteristics, the external standard method is more suitable, and it is convenient and fast. So we use the external standard method as a quantitative analysis. The specific method is to find the correction factor of the corresponding component through standard sample analysis. Recently, the content of the corresponding component was determined. The determination of the correction factor is the key, because if you want to get the accurate content of each component, you must ensure that the correction factor is accurate and without deviation, the amount of injection should be moderate, generally 1ml.

3 Conclusion

The hydrogen flame ionization gas chromatograph can successfully solve the analysis of hydrocarbons in liquid oxygen, and can also analyze the liquid air and hydrocarbons in the oxygen tank. Moreover, the stability is good and the operation is convenient, which ensures the safe production of air separation.

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