Apr-2020

Detailed how to measure chlorides in crude oil by ASTM D4929C and water extraction

Organic chlorides do not occur naturally in crude oil; however, they can make their way into crude oil as a result of cleaning operations in pipelines and other refining equipment.

Leslie McHenry and Holly Dunn
XOS

Article Summary

Although very small concentrations of organic chloride can typically be handled by refineries with no deleterious effects to the equipment, once they reach a certain threshold, severe damage can ensue1. The dangers of chloride-contaminated crude oil are well-known to those in the oil industry. Among these dangers is the creation of hydrochloric acid, which can lead to the corrosion and damage of pipelines and refining equipment.

Solution
To avoid the pitfalls of corrosion and other chloride-related concerns, countries throughout the world have implemented standard methods and procedures to ensure that the organic chloride content in their crude is kept below a certain level. Such methods include ASTM D4929 (Russia’s GOST R 52247; a modified translation of ASTM D4929-2004, and GOST 33342; a modified translation of ASTM D4929-2007), as well as procedures like water extraction, each to be discussed in further detail throughout this paper.

Since its launch in 2007, Clora® Benchtop Analyzer has been widely adopted by refineries and test labs to analyse chlorine. With over 200 systems in the field, XOS customers know they can trust Clora for testing products from crude oil to naphtha cuts and VGO. In this paper, we will discuss two popular Clora benchtop procedures utilised by petroleum laboratories around the world to measure total chlorine:

1. Organic Chlorides by ASTM D4929C
2. Organic and Inorganic Chlorides Using Water Extraction

1  Organic chlorides by Astm D4929 – first there was A & B
ASTM D4929 Standard Test Method for Determination of Organic Chloride Content in Crude Oil was first published with two procedures:

“1.3 Procedure A covers the determination of organic chloride in the washed naphtha fraction of crude oil by sodium biphenyl reduction followed by potentiometric titration.

1.4 Procedure B covers the determination of organic chloride in the washed naphtha fraction of crude oil by oxidative combustion followed by microcoulometric titration.”1

Both Procedures A and B of the original ASTM D4929 method require users to first distill a crude oil sample to 204°C (400°F), then wash the resulting naphtha fraction with caustic to remove H2S, and finally, wash with water to remove inorganic chlorides. Afterwards, either Procedure A or B is used to determine chloride content using back calculation.

The need for a faster & easier alternative – and then there was C
While the distillation of the sample during preparation wasn’t something that could be eliminated, the chloride analysis portion of both Procedures A and B were difficult and time-intensive processes. This left room for a simplified process to be created. On October 15th, 2017, ASTM approved the addition of Procedure C to D4929:

“1.5 Procedure C covers the determination of organic chloride in the washed naphtha fraction of crude oil by X-ray fluorescence spectrometry.”
This is an important step forward, as users will be able to officially use XOS’ Clora, Clora 2XP, and Sindie +Cl analysers for organic chloride analysis in crude oil without ASTM method modification.

In order to include Procedure C with Method D4929, an interlaboratory study (ILS) involving eight participants was performed. Each of the participants distilled ten crude oil blends of varying nominal organic chloride concentrations in blind duplicate using the D4929 procedure which is based on the ASTM D86 method for distillation of petroleum products. The D4929 procedure differs in that the 500 ml crude sample was weighed prior to distillation so that the naphtha mass fraction could be calculated. Using a distillation rate of approximately 5 ml/min, the crude oil sample was distilled until 204°C (400°F), and then the resulting naphtha fraction was weighed. The naphtha mass fraction was calculated by dividing the mass of the naphtha fraction by the mass of the crude oil sample prior to distillation.

Resulting naphtha cuts were triple washed with caustic and water per the method before analysis using XOS’ Clora Monochromatic Wavelength Dispersive X-ray Fluorescence (MWDXRF) analyser, among other instruments with varying types of XRF techniques. Because organic chlorides concentrate in the naphtha fraction, in order to obtain the organic chloride in the original crude sample, the measured naphtha chlorine concentration was multiplied by the naphtha mass fraction to obtain the final D4929 result. (Note: this last back-calculation step would not be performed if the desired result is chlorine in the naphtha cut, as in some pipeline specifications.)

The ILS resulted in separate precision statements for differing XRF techniques. In the following figures, the ILS precision is compared with the original method precision statements for Procedures A and B (Figures 1 and 2). The ILS found the following:
•    Clora (MWDXRF) has better reproducibility than the other Procedure C XRF techniques and exhibits equivalent or better reproducibility than Procedure B (Figure 1)
•    Clora (MWDXRF) consistently exhibited better repeatability than Procedure B (Figure 2)

2  Organic and inorganic chlorides using water extraction
Although all XRF techniques are capable of only total elemental analysis, with some sample preparation, Clora, Clora 2XP, and Sindie +Cl can also be used to characterise inorganic and organic chlorides in crude oil. Using a hot water extraction, crude oil may be separated into its organic chloride and inorganic chloride constituents with the organic chlorides staying in the crude oil layer and the inorganic chlorides precipitating into the water layer. Clora can then be used to measure each layer to determine organic and inorganic chlorides. While many laboratories have successfully used this technique, unfortunately, this technique has crude-dependent limitations. Not all crude oils are easily extracted, and this sample preparation technique may have variable within-lab repeatability and poor between-lab reproducibility. Procedures 1-3 will assist the user in separating chlorine in crude oil into its organic and inorganic counterparts using a water extraction.

Procedure Notes:
• Note 1: Be sure to prepare the crude/solvent mixture by weight, not volume.
• Note 2: Deionised (DI) water and toluene or xylene should be analysed by Clora prior to use to check for chlorine contamination. Any chlorine contamination in the DI water should be subtracted from the inorganic chlorine result, and any chlorine contamination from the solvent should be subtracted from the organic chlorine result.
• Note 3: Assuming the density of water = 1 g/ml, then weight equals volume. Therefore, 75 ml water = 75 g.
• Note 4: Some customers using paper filters, such as Whatman 125 mm grade 1 filter paper circles, have reported that the filter paper can add to the chlorine content. This can be verified by testing chlorine levels in DI water before and after filtration. Subtract any interferences from the inorganic chlorine result.
• Note 5: When pipetting the water sample, expel air from the pipette when initially putting it into the water sample to keep oil (often present as a thin film on the surface of the water) out of the measured sample cup.
• Note 6: When calculating inorganic chlorine, use the weight of the crude in the calculation. Do not use the combined weight of the solvent and crude.

Procedure 1: Crude Oil Sample Preparation
Depending on crude type and composition, it may be hard to obtain a stable homogenous sample. Viscous or asphaltene and bitumen containing crudes are particularly difficult to extract. The addition of toluene or xylene to these types of crudes will increase the solubility of heavier crude components leading to a more homogenous, less viscous sample that is more efficiently water extracted. Prepare approximately 25 ml of 50/50 wt% crude to solvent (toluene or xylene) mixture per Note 1. Shake or homogenise the crude oil sample prior to obtaining a specimen for dilution. A 12.5 g crude to 12.5 g solvent mixture will work well for most crudes. See Note 2 regarding chlorine contamination in solvents. When analysing chlorine content with Clora, remember to multiply the measured results by 2 to account for this dilution.