Determination of Trace Water in Polymer‑Grade Propylene

Overview

Propylene is an important polymer‑grade olefin raw material, widely used for synthesizing polymer materials such as polypropylene. Water is a critical impurity in feedstock. Excessive water content will directly impair catalyst activity, trigger abnormal polymerization reactions and eventually degrade product quality. Therefore, accurate determination of trace water in propylene is essential.

The test is performed in accordance with GB/T 3727‑2003 Determination of Trace Water in Industrial Ethylene and Propylene. Direct vaporization from ordinary gas cylinders leads to higher or lower test values with significant data fluctuation. The SH201 Flash Vaporization Sampler is specially developed for sample pre‑treatment of liquefiable gaseous samples like liquid hydrocarbons. It realizes constant‑composition vaporization and ensures authentic and reliable trace‑water test results.

Test Samples and Instruments

Test sample: Polymer‑grade propylene

Test instruments:

1. SH201 Flash Vaporization Sampler
2. Karl Fischer Trace Moisture Tester
3. Sampling cylinder, stainless‑steel pipelines, gas filter and other accessories

Operating Procedures

1. Place the complete set of instruments inside a fume hood and connect vent pipelines for safe exhaust of waste gas. Connect the sampling cylinder with its outlet downward to ensure liquid propylene flows out.
2. Power on the SH201 Flash Vaporization Sampler and enter the pre‑heating interface. Set vaporization temperature to 60 ℃ and activate the heater. Wait until the instrument finishes pre‑heating and reaches constant temperature.
3. Connect the sampler outlet port to the inlet of the Karl‑Fischer moisture tester. Use short stainless‑steel or PTFE pipelines to minimize water interference caused by pipeline adsorption.
4. Set instantaneous flow rate and cumulative sampling volume according to test requirements. The conventional flow rate is 1.5‑2 L/min; increase sampling volume appropriately for low‑moisture samples.
5. After the moisture tester is ready, open the valve of the sampling cylinder and start sampling. The mass flow meter precisely controls sample flow. Samples pass through the metal bath for constant‑composition flash vaporization before entering the moisture analyzer. Upon reaching the preset cumulative volume, the instrument automatically switches to vent mode.
6. Read values from the moisture tester. Apply the gas conversion factor for correction to obtain the true trace‑water content of propylene samples, and print the analysis report.
7. Upon completion of the test, close the cylinder valve first, power off instruments and disconnect the sampling cylinder. For test reliability, generally run 5 parallel tests.

Data Analysis and Result Evaluation

Five parallel tests were carried out on one batch of polymer‑grade propylene sample at a flow rate of 1.8 L/min and cumulative sampling volume of 10 L per run. Corrected with the propylene conversion factor of 0.398, the five trace‑water results are 22.6 ppm, 23.1 ppm, 22.4 ppm, 23.3 ppm and 22.8 ppm respectively.

Calculation gives an average value of 22.84 ppm. The maximum reading is 23.3 ppm and the minimum is 22.4 ppm, with a range of 0.9 ppm and relative standard deviation RSD = 1.52%.

For the identical sample tested via conventional direct cylinder vaporization, data fluctuates from 14.1 ppm to 34.7 ppm with a range up to 20.6 ppm. Severe data distortion occurs, failing the control requirements for polymer‑grade feedstock.

The SH201 Flash Vaporization Sampler achieves constant‑composition vaporization of liquid propylene and avoids result drift caused by water condensation‑adsorption and component fractionation in conventional vaporization modes. The basic error of built‑in flow measurement is ≤±5%. The RSD of parallel samples in this test is only 1.52 %, showing excellent repeatability. Test results satisfy the pre‑treatment technical requirements specified in GB/T 3727‑2003 for trace‑water analysis of industrial propylene. This system is applicable for incoming inspection and on‑site process analysis of polymer‑grade propylene feedstock.

Note: This equipment is also suitable for sample pre‑treatment of ethylene, butene and LPG for analysis items including trace oxygen, trace sulfur, CO and CO₂.