Aniline gasifies and undergoes a hydrogenation reaction with preheated hydrogen gas under the action of a catalyst to produce crude cyclohexylamine, which is purified by rectification to obtain cyclohexylamine. Cyclohexylamine is mainly used in the production of food additives cyclamate, synthetic rubber accelerators, desulfurizers, corrosion inhibitors, water treatment, synthetic pesticides, petroleum product additives, gel coagulants and high-grade dyes. Since cyclohexylamine has strong basicity and can react with acid to form salt quantitatively, its purity can be analyzed by acid-base titration, but the results of this method are not obvious for the impurity content of cyclohexylamine, while cyclohexylamine industry standard HG / T2816-1999 has strict control indexes for aniline. We use gas chromatography, because HP5 quartz capillary column is used, and program temperature is introduced, so some operating conditions (such as carrier gas flow rate, column temperature, gasification temperature and sample injection volume, etc.) should be based on specific instrument performance during operation. Adjust appropriately to find a better combination value. To this end, we conducted experiments.
l. Main instruments and reagents
1.1 Instrument
(a) HP4890D gas chromatograph;
(b) HP5 capillary column (non-polar);
(c) Detector: flame ionization detector (FID);
(d) Recorder: HP3955 integrator;
(e) Sample injector: 10 μL micro syringe.
1.2 Reagent
(a) cyclohexane;
(b) cyclohexylamine;
(c) Aniline;
(a) -cyclohexylamine;
(e) Absolute ethanol. The above reagents use analytical grade.
1.3 Air source
(a) High-purity nitrogen volume fraction> 99.999%;
(b) The volume fraction of high purity hydrogen is> 99.999%;
(c) Dry compressed air.
2. Operating conditions of chromatographic instruments
(1) Detection temperature 280 ℃;
(2) Gasification temperature 250 ℃;
(3) The column temperature is 50 ℃;
(4) The initial time is 1min;
(5) Heating rate 10 ℃ / min;
(6) End temperature 100 ℃;
(7) End time 3min;
(8) Carrier gas (nitrogen) flow rate 35mL / min;
(9) Combustion gas (hydrogen) flow rate 43mL / min;
(10) The fuel gas (compressed air) flow rate is 400mL / min;
(11) The split ratio is 12: 1. Determination of the correction factor in absolute weight
In order to pay more attention to the peak area, we adopt the split ratio parameter
"Mountain climbing method" condition test, from 10: 1 to 11: 1, 12: 1, 15: 1, 20:
1, 25: 1, 30: 1, 35: 1, 40: 1, respectively, to compare the recurrence of peak area
Sex. Step by step to find a better split ratio of 12: 1.
(12) Injection volume: When the normalization method is used, the peak height (or after attenuation) of the subject should account for more than 70% of the full scale on the recorder. To meet this condition, the sample is diluted with ethanol before processing, because the normalization method does not require an accurate concentration. Our method is to use a dropper to drop 2 drops of cyclohexylamine product, dilute with absolute ethanol to about 2mL, and the injection volume is 1.0mL, so that the simulated input signal of the peak of each component measured is less than 1000mV to achieve quantification .
(13) Column temperature: The capillary column generally adopts temperature program. According to the requirements of the cyclohexylamine industry standard HG / T2816-1996 required to monitor the aniline, dicyclohexylamine two impurity components relative to the main body cyclohexylamine retention time and the boiling point and polarity of each component, empirical tests . For example, the most polar dicyclohexylamine can peak before 8.5min under various test conditions, and the retention time is not far apart, so we decided to use a first-order program temperature increase, and the initial temperature, initial time, heating rate, end The temperature and end time should be determined by the physicochemical properties of the solvent and each component. Empirical tests should also be conducted to evaluate the chromatograms. Two temperature-programming schemes (a) and (b) are used. There is no significant difference in quality score. From the chromatogram, the separation effect between the components is better. The peak time of all samples is about 8.5min, which is suitable for normal sample analysis. It can be seen that the feasible program of temperature programming is not the only one, and the program of temperature programming (a) is adopted in the accuracy and precision of the measurement and evaluation methods of absolute weight correction factors.
3. Parameters of the accumulator
4. Selection of quantitative methods
4.1 Determination of absolute mass correction factor
Since the analytically pure cyclohexylamine we used detected cyclohexylamine and aniline under this test condition, and analytically pure aniline did not detect cyclohexylamine, cyclohexane, and dicyclohexylamine under this test condition, so we prepared When using a single calibration solution of cyclohexylamine, cyclohexane and dicyclohexylamine, use aniline as the solvent. For the single standard solution of aniline, analytically pure cyclohexylamine solvent was used, and the peak area of ​​aniline brought into the cyclohexylamine solvent was deducted accordingly. To avoid overlapping contamination, no calibration mix solution was prepared. Using weighing method (weighing to 0.0002g), prepare three standards that are similar to the calibration component index, and determine according to the measurement conditions of the sample.
4.2 Whether to use relative quality correction factor
The industry standard control project has strict requirements on the content of aniline in cyclohexylamine. When analyzing the purity of the main body, the content of impurity aniline must be accurately analyzed. Through experiments, the operating conditions such as the concentration and split ratio of the aniline calibration solution were changed, and its correction factor relative to cyclohexylamine was relatively stable and close. After obtaining this conclusion, we did not correct the peak area with a relative correction factor, but directly used the area fraction to find the mass fraction, calculated the area with an integrator, and normalized the measured aniline content to the single standard method. For comparison, the measured mass fraction of cyclohexylamine is compared with the mass fraction of cyclohexylamine measured by the acid-base titration method to determine whether the results obtained by the two methods are significantly different. The mass fraction of cyclohexylamine measured by the area normalization method in the table has deducted moisture. Under the same conditions, different samples are analyzed in parallel, and then the results of each pair of parallel analysis are tested by the T test method. After comparison, there was no significant difference in the measurement results. For several other impurity components, because there is no requirement in the industry standard and the content is very small, no quality correction factor is used. Therefore, the quantitative method uses the integrator to calculate the area normalization method, avoiding the preparation of standard samples.
5. Method error
In order to verify the accuracy of the method, a recovery test is usually carried out. However, because the time-programming and [INTG] functions provided by the integrator are used in the area normalization method, the solvent peak is not integrated, so the injection concentration of the cyclohexylamine product does not affect the groups calculated by the area normalization method The quality score of the score is not applicable to the recycling test of this method. We use this method to repeat the analysis of chromatographically pure cyclohexylamine with a known purity (mass fraction greater than 99.8%) 12 times, and the analysis results (measured values) are consistent with the standard values. Precision of analysis results. The data can be regarded as high precision and good repeatability. In addition, through the analysis test, it is proved that this method is also applicable to the analysis of cyclohexylamine hydrogenation products, pre-fractions, and residual liquid, and it is effective for the follow-up analysis in the production process of cyclohexylamine.
6. Application
Various operating condition parameters are fixed, and a method is established through the functions and commands provided by the integrator and stored in the integrator memory. Before analysis, first load the method into the active work area of ​​the integrator, and analyze it automatically according to the method, making the operation easier and faster.
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