Organic Chloride in Crude per ASTM D4929 Part C

Scope

The analysis of organic chloride in crude is demonstrated per ASTM D4929 Part C - determination of organic chloride in the washed naphtha fraction of crude oil by x-ray fluorescence spectrometry - using monochromatic energy dispersive x-ray fluorescence (MEDXRF).

Background

Chlorides in crude contribute to corrosion in the piping at refineries during cracking as well as mid-stream in pipelines. Organic chlorides do not naturally occur in crude, however inorganic chlorides in the form of salts as well as residual organic chlorides from various natural sources as well as adulteration can contaminate crude. Contracts at the pipelines may contain clauses limiting the amount of organic chloride allowed in the crude. And at the refinery after desalting and desulfurization, crude needs to be analysed for any residual organic chlorides possibly still entrained in the feedstock to avoid potential damage during the refining process.

Instrumentation

1. Type: Monochromatic EDXRF (MEDXRF)
2. 90â—¦ Cartesian Geometry Polarization
3. Model: Rigaku NEX CG

Sample Preparation

ASTM D4929 is designed for the measurement of residual organic chlorides in crude. The crude sample is first prepared by distillation and wash to remove H2S and inorganic chlorides. After distillation and wash processes the resulting naphtha fraction is analysed for Cl content by XRF using Part C methodology. The naphtha fraction is typically stable containing less than 1000 mg/g S. XRF calibration is made using mineral oil calibration standards, as mineral oil models naphtha for X-ray response.

To prepare a sample for XRF analysis simply place 4.0g of the naphtha sample in a standard 32mm XRF sample cup and measure directly.

ASTM D4929 Calibration - Part C XRF

Empirical calibration was made using commercially available mineral oil standards. For optimum calibration the standards contain both S and Cl, with S content ranging from 100 to 1000 mg/kg. Alpha corrections are automatically employed to compensate for sulfur's effect on chlorine X-rays.

 ^{* See Figure 1 - D4929 Part C XRF Calibration}

Precision & Repeatability

To demonstrate recovery, precision and repeatability of the measurement, a few calibration standards were selected with results shown here.

Detection Limits

The empirical method is used to determine detection limits using 300 sec measurement time. Ten repeat analyses of a blank mineral sample are taken with the sample in static position and the standard deviation (σ) is determined. The Lower Limit of Detection (LLD) is then defined as three times the standard deviation (3σ).

^{* See Figure 2 - Precision & Repeatability; Detection Limits}

Conclusions

NEX CG using Cartesian Geometry monochromatic EDXRF gives superior performance for the measurement of organic chloride in crude as per ASTM D4929 Part C. NEX CG also complies with many other ASTM methods and EPA requirements for critical measurements in the petroleum industry, making it an excellent choice as an all-in-one analyser for measuring sulfur and ultra-low sulfur, metals in crude and residual oil, and lube oils.