Optimization and improvement of the method for determining trace methanol content in 1-butene
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Optimization and improvement of the method for determining trace methanol content in 1-butene [Abstract] Based on the shortcomings in the analytical methods used to determine trace methanol in 1-butene, the sampling method was improved, a sample vaporization and enrichment device was added, and the processing conditions were optimized. The improved method is easy to operate, has a low detection limit, and good reproducibility. [Keywords] Methanol, vaporization, enrichment, repeatability. 1-Butene is one of the important chemical raw materials; it is primarily used as a comonomer for ethylene in the production of linear low-density polyethylene, and it can also be used in the manufacture of 1-butene resins. Methanol may remain in the industrial production of 1-butene products, and this substance can affect downstream polymerization. Due to the excessive methanol content in 1-butene, it can affect the high-load operation of the Zhongyuan ethylene plant and even pose a risk of shutting down the plant; therefore, accurate detection of trace amounts of methanol in 1-butene is of crucial importance for production and quality control. At present, the polyethylene, instrumentation, and 1-butene analysis sections in the laboratory of the Zhongyuan Ethylene Center all offer this analysis service. The standard adopted is SH/T1547 \"Determination of trace amounts of methanol and tert-butyl ether in 1-butene – Gas chromatography method.\" In practice, it has been found that this method has poor operability and a high detection limit. Through repeated discussions and experimental improvements, we found that by referring to GB/T12701-90 \"Gas chromatographic method for the determination of trace methanol in industrial ethylene and propylene\", and by adding a sample vaporization device to enrich the methanol in the sample using an absorbent solution before analysis, good results were achieved in terms of simplifying the procedures, reducing the detection limit, and eliminating errors. 1 Experimental Section 1.1 Materials and Reagents Carrier gas: Hydrogen, with a purity of ≥99.99% (by volume). Auxiliary gas: Nitrogen, with a purity of ≥99.99% (by volume). Standard reagent: Methanol, used for preparing external standard samples; its purity should be at least 99% (by mass). Distilled water. Pyridine (analytical grade). 1.2 Instruments 1.2.1 Gas chromatograph (equipped with a FID detector and a data processing workstation). The peak height generated by impurities at the lowest concentration required by the standards must be greater than twice the noise level. 1.2.2 Chromatography column: Capillary column Innowax 30m×0.25mm. Operating conditions: Injection port temperature 250°C, detector temperature 300°C, column temperature 70°C, carrier gas flow rate 0.9 ml/min, split ratio 50:1, injection volume 1 μL. 1.2.3 Injection device: Microsyringe: 1 μL. 1.2.4 Absorption and enrichment device: Gas flow meter 5–100 L/h; absorption bottle (with a glass sand core for the gas distribution plate), flow meter. 1.2.5 Vaporization device: Water bath at 50–70°C; heating coil (made of stainless steel capillaries with a length of 2–4 m and an inner diameter of 0.2 mm)
1.3.3 Sample enrichment: Connect the absorption device, immerse the absorption flasks (A, B, C) in a water bath, and add 10 ml of absorption solution to each flask； Connect the absorption device to the vaporization device, and ventilate the gaseous sample at an appropriate rate for a certain period of time; use a gas flow meter to accurately measure the volume of the sample taken ； Flush the pipeline with nitrogen at a rate of 5-10 l/h ； Chromatographic analysis was performed on the absorbents in bottles C and B to confirm that they contained no methanol ； Transfer the absorbent solution from bottle C to a 25 ml volumetric flask, dilute it to the mark with distilled water, and mix thoroughly. 1.4 Determination steps 1.4.1 Calibration: Inject exactly 1 μL of a liquid standard with a known concentration, and record the peak area of methanol. 1.4.2 Sample determination: Accurately inject the absorbent solution obtained in 1.3.3, and record the peak area of methanol. The analysis results are expressed as the arithmetic mean of two repeated measurements. 1.4.3 Calculation The methanol content, expressed in mg/kg, is given by the following formula: X = E2 × 25/1000 × A1/AE × 1/Vρ × (273 + t) × 101325/p = 9.3 × [E2 × A1 × (273 + t) / AE × V × ρ × p] Where E2 represents the methanol content in the liquid standard sample, in mg/l; A1 represents the peak area of methanol in the sample ； AE---Peak area of methanol in the standard sample ； V---Volume of the sample absorbed, L ； P---Atmospheric pressure at the time of measuring the sample volume, Pa ； t---Temperature at which the sample volume is measured,℃ ； ρ---the density of 1-butene at 0°C and a pressure of 101325 Pa