Speciation identification of trace sulfides in liquefied gas

In this paper, the capillary column and pulsed flame photometric detector were used to identify the sulfide in the liquefied gas produced by Sinopec Jinan Branch. Compared with the sulfur determination method, this method has the advantages of simple operation and high sensitivity, and is suitable for the analysis and identification of trace sulfides in liquefied gas and other gases in refineries.

The sulfide in liquefied gas in refineries is usually the poison that poisons the catalyst during its deep processing, which will affect the quality of subsequent products and needs to be removed [1]. To this end, the sulfide must first be analyzed and identified, so as to select or optimize the desulfurization process in a targeted manner. For sulfides in liquefied gas, gas chromatography is mainly used to separate and detect various sulfides in liquefied gas. Previous researchers used chromatographic-flame photometric detector (GC-FPD) [2], chromatography-double Flame photometric detector (GC-DFPD) [3], chromatography-flame ionization and flame photometric detection technology (GC-FI-FPD) [4] and chromatography-mass spectrometry (GC-MS) [5] were measured Although some progress has been made in qualitative aspects, the operation is cumbersome, hydrocarbon peaks and sulfide peaks interfere with each other, identification is difficult, and higher requirements are placed on the separation performance of the column [6].

Gas chromatography-pulse flame photometric detection technology (GC-PFPD) is a new technology developed for the analysis and detection of sulfides in recent years [7]. Compared with the traditional GC-FPD, due to the use of pulse flame combustion technology, sulfur filter filtering hydrocarbon luminescence technology, and the use of different delay time gate amplifiers to receive S and C luminescence, respectively, it has high sensitivity and S / C selection Good performance (up to 107), no hydrocarbon quenching and other advantages [8]. In this paper, the method of morphology identification and content analysis of sulfide in liquefied gas was established by GC-PFPD. This method was used to identify sulfide in liquefied gas of Sinopec Jinan Branch Company. A total of 9 kinds of sulfides were analyzed and the subsequent impact was identified. Sulphide form and content of propylene polymerization unit.

1 Experimental instruments and samples

The samples used in this research are the liquefied gas produced by Sinopec Jinan Branch, where C3 components account for more than 40%, C4 components account for more than 45%, and C2 components account for about 10%.

1. 1 Instruments and reagents

CP-3800 gas chromatograph, pulse flame photometric detector (PF2PD) (Varian, USA), Gaspro quartz capillary chromatography column: 30m × 0.32mm (J & W, USA); HP-6890 gas chromatograph, atomic emission spectrum Detector (AED) (US HP company); WK-2B type oxidation microcoulomb sulfur determination instrument (Jiangsu Jianghuan Analytical Instrument Co., Ltd.).

The sulfide standards are: carbonyl sulfide, hydrogen sulfide, methyl mercaptan, ethyl mercaptan, dimethyl sulfide, and dimethyl disulfide standard gases produced by Beijing Hebei Pufen Gas Industry Co., Ltd .; produced by Fluka Chemi AG, Germany N-propyl mercaptan (analytical purity); diethylene disulfide (purity> 99%) produced by ACROS Organics, USA, etc.

1. 2 Working conditions of the instrument

The volume of the quantitative sampling tube is 500 × 10-3cm3, the temperature of the gas inlet is 150 ℃, the carrier gas is high-purity hydrogen (99.999%), the column flow rate is 3.5cm3 / min; the combustion air is: the flow rate of air 1 is 17. 0 cm3 / min, the flow rate of air 2 is 10. 0 cm3 / min, the flow rate of hydrogen is 14.0 cm3 / min; the ignition voltage is 200 mV, and the detector temperature is 200 ° C.

During the analysis, a temperature program was used. The initial temperature of the capillary column was 40 ° C, and the temperature was raised to 100 ° C at a rate of 5 ° C / min, then to 220 ° C at a rate of 20 ° C / min, and the temperature was kept constant for 7 min.

1. 3 Experimental methods

Collect liquefied gas samples with a metal sampling bag, automatically inject samples, quantify sulfide using an external standard method, and use a chromatographic workstation equipped to perform atlas data processing.

2 Experimental results

2. 1 GC-PFPD minimum detection limit

According to the definition of the minimum detection limit, usually 2 times the noise signal is regarded as the theoretical minimum detection limit. Because the PFPD detector responds proportionally to the square of the sulfide concentration, the peak height is calibrated using a sulfide standard with a known concentration, and the peak height of the noise response to the standard is doubled, squared, and multiplied The standard sample concentration is the calculated minimum detection limit. The lowest theoretical detection limit of sulfide by the PFPD detector under certain chromatographic conditions was 0.037 μg / g using a carbonyl sulfur standard sample with a concentration of 5.05 μg / g.

2. 2 GC-PFPD instrument repeatability

Repeated detection of a representative carbonyl sulfide standard sample to investigate the repeatability of the instrument. The results of 5 parallel detections of carbonyl sulfide standard samples with a concentration of 5.05 μg / g were 5.052, 5.048, 5.039, 5.029, and 5.050 μg / g, respectively, and the average value was 5.043 μg / g g, equivalent to 99.86% of the standard sample concentration, indicating that the accuracy of the test results is good. The relative standard deviation of 5 tests is 0.20%, which shows that the method has good repeatability.

2. 3 Morphological identification of sulfide in liquefied gas

Temperature programmed separation of hydrocarbons and various forms of sulfide in liquefied gas was carried out. The retention time of the sulfide in the test sample was compared with the retention time of the sulfide standard sample for qualitative identification of the sulfide form. The temperature and heating rate are verified, and combined with the Jinan branch liquefied gas desulfurization process to analyze and verify.

Since the sulfur-carbon selectivity of PFPD can reach 107, and hydrocarbons have basically no response peaks, each chromatographic peak in Fig. 2 is a response peak of sulfide. Compare the retention time of each sulfide standard substance in Table 1 with the retention time of each chromatographic peak in Figure 2, and determine the sulfide form of each corresponding peak as follows: ① carbonyl sulfide (2.50 min); ② methyl sulfide Alcohol (12.92min); ③ ethyl mercaptan (15.51 min); ④ dimethyl sulfide (16. 78min); ⑤ dimethyl disulfide (18. 32 min); ⑥ unknown peak (20. 16min); ⑦ diethyl disulfide (23. 28 min).

In summary, a total of 9 forms of sulfides were detected in the liquefied gas produced by the Jinan branch, namely: ① carbonyl sulfide; ② hydrogen sulfide; ③ methyl mercaptan; ④ ethyl mercaptan; ⑤ dimethyl sulfide ; ⑥ Propyl mercaptan; ⑦ dimethyl disulfide; ⑧ methyl ethyl disulfide; ⑨ diethyl disulfide, and various sulfides can be well separated.

2. 4 carbonyl sulfur and hydrogen sulfide peak position discussion

The boiling point of hydrogen sulfide is -60. 4 ℃, and the boiling point of carbonyl sulfide is -0.33 ℃, according to the outflow characteristics of the weak polar column, the peak position of hydrogen sulfide should be before carbonyl sulfide. However, in actual detection, it was found that the peak position of hydrogen sulfide was after carbonyl sulfide. The reason for this anomaly may be that the dipole distance of hydrogen sulfide is 0.9 debyes, and the dipole distance of carbonyl sulfide is 0.7 debyes, the polarity of hydrogen sulfide is greater than the polarity of carbonyl sulfur, so that hydrogen sulfide The strong interaction with the weakly polar Gas-pro column makes the hydrogen sulfide flow out slowly.

2. 5 GC-PFPD compared with other sulfur determination methods

GC-AED is an existing trace sulfide detection instrument with high sensitivity, but its S / C selectivity is low, only 104, which puts higher requirements on the separation performance of the chromatographic column; Due to the long time required for GC-AED detection, it usually takes 80 minutes, and its high price and high operating cost limit its promotion and application.

For liquefied gas samples, the carbonyl sulfur detected by GC-AED was 30.65μg / g, and the carbonyl sulfur detected by GC-PFPD was 30.95μg / g; thus it can be seen that the analysis results of the two are very close The relative error is only 0.98%. Oxidized microcoulometric sulfur is a commonly used method for determining total sulfur, which is widely used in the determination of total sulfur in liquefied gas in refineries and is listed as an industry standard. However, due to its low sensitivity and the inability to identify the form of sulfide and analyze the content of monomer sulfide, it cannot provide effective guidance for production.

When analyzing the liquefied gas samples by GC-PFPD method, a total of 4 kinds of sulfides were detected, and their morphologies and contents were carbonyl sulfide 5.6 μg / g, methyl mercaptan 140.8 μg / g, and ethyl mercaptan 13. 2 μg. / g, dimethyl disulfide 43.6 μg / g, methyl ethyl disulfide 3.9 μg / g, the sum of various forms of sulfur content is 207.1 μg / g; The total sulfur detection result of the sample was 201μg / g, the measurement results of the two were relatively close, and the relative error was 3.0%. Therefore, it proves the reliability of the GC-PFPD test results.

2. 6 Identification of sulfide in refined acrylic gas

During a certain period of time, the customer reported that the non-woven fabric produced by the Jinan branch using polypropylene has an odor. For this reason, we conducted an investigation into the causes of the odor. Using GC-PFPD to analyze and detect the gas in the non-woven bag, it was found that hydrogen sulfide was present, and the content was 0.046μg / g, which has exceeded the low limit of human olfaction.

3 Conclusion

Using the GC-PFPD method established in this paper, the sulfide in the liquefied gas of Sinopec Jinan Branch was identified, and a total of 9 forms of sulfide were identified, namely: ①carbonyl sulfur; ②hydrogen sulfide; ; ④ ethyl mercaptan; ⑤ dimethyl sulfide; ⑥ propyl mercaptan; ⑦ dimethyl disulfide; ⑧ methyl ethyl disulfide; ⑨ diethyl disulfide; identified the final entry into the polypropylene plant The form and content of sulfide remaining in refined propylene. Compared with the existing microcoulometric sulfur determination method and gas chromatography sulfur determination method equipped with atomic emission spectrometry, this method has the advantages of simple operation and high sensitivity, and is suitable for the morphological identification of trace sulfides in liquefied gas in refineries. It can also be used for the analysis and identification of trace or trace sulfides in other gases.

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