A Novel Sample Preparation Method for the Determination of Chloride in Bitumen Samples

Accurate salt content determination in petrochemical products such as crude oil and bitumen is of utmost importance to petrochemical companies. The presence of salt (present as sodium chloride) causes several problems during transportation and the downstream refining processes, such as corrosion to transportation lines, fouling, and deactivation of catalysts. For these reasons refineries employ various desalination processes and analytical tests for the measurement of salt and water content present in crude oil and related petrochemical products.

Standard lab-scale test methods, such as solvent extraction under atmospheric conditions combined with titration or potentiometric measurement, are limited and time consuming. Applying conventional extraction methodologies to high-viscosity petrochemical products that contain high amounts of water or solvent can lead to variable and erroneous chloride results.

This article presents a new microwave-assisted sample preparation technique using the Multiwave PRO microwave system (Anton Paar, Ashland, VA). The applicability of the microwave sample preparation technique was determined by processing a variety of bitumen samples over several days, with subsequent chloride determination.

Challenges of conventional chloride extraction methods

Conventional and standardized tests for the determination of salt in crude oils and related petrochemical products are well established, but are known to be complicated and very labor intensive. Standard methods, for example, ASTM D-6470 and D-3230 as well as IP 77/72, are inefficient and are not applicable to a wide variety of petrochemical samples.

Standard methods ASTM D-6470 and IP 77/72 involve the extraction of water-soluble chlorides under atmospheric conditions. Typically an aliquot of the petrochemical sample is dissolved in xylene at 65 °C, and is subsequently extracted with specified volumes of toluene, acetone, and water in an electrically heated extraction apparatus. A portion of the aqueous extract is then analyzed for total halides by potentiometric titration. The process involves the use of more than 200 mL of organic solvents and can take several hours of heating. Moreover, proper mixing and homogenization of the petrochemical sample with the solvent mixture has a key influence on the accuracy of the extraction.

Table 1 – Average chemical compositions of diluted bitumen samples

The major limitations of the conventional tests can be summarized as follows:

  • Very high solvent consumption
  • Extraction temperature limited to boiling point of solvent mixture
  • Laborious setup of conventional glassware (round-bottom flasks, reflux condensers, etc.)
  • Limited applicability of the test method to petrochemical products with varying viscosities and water contents
  • Lack of reaction parameters documentation.

Samples

To determine the applicability of the microwave assisted extraction technique, two diluted bitumen samples with varying matrix compositions were processed daily over a period of 10 days. These samples were supplied by a large multinational petrochemical manufacturer. The known chloride contents of the two bitumen samples were previously determined via a conventional extraction method based on the methodology outlined in the standardized test method IP 77/72.

The typical compositions of these bitumen samples are indicated in Table 1.

Instrumentation

Figure 1 – Multiwave PRO microwave reaction system.

The Multiwave PRO microwave reaction system (Figure 1) equipped with the Rotor 8NXQ80 was used for the extraction or sample preparation step. The rotor is supplied with eight quartz glass vessels and has simultaneous pressure measurement capability on all digestion vessels, which enables the rotor to proactively detect exothermic reactions and take suitable action (such as reducing the microwave power) to keep it within safe temperature and pressure limits. With maximum operation conditions of 300 °C and 80 bar simultaneously, this configuration is suitable for processing the widest range of sample types. Analysis of extracted aqueous phases for chloride content was performed with a 761 Compact Ion Chromatography system (Metrohm, Riverview, FL).

Analytical procedures

Conventional extraction technique

The conventional chloride extraction technique is based on standard method IP 77/72. A sample mass of ca. 40 g of the diluted bitumen sample was weighed directly into a 1-L Erlenmeyer flask. This was followed by the addition of 120 mL of toluene and 100 mL of deionized water. The reaction mixture was then continuously refluxed for a period of 2 hr. Once cooled down to ambient temperature, a fraction from the aqueous layer was collected, filtered, and analyzed via ion chromatography for chloride content.

Microwave-assisted extraction technique

A mass of ca. 2.0–3.0 g of diluted bitumen sample was weighed directly into each NXQ80 quartz glass reaction vessel of the Multiwave PRO microwave system. This was followed by addition of 10.0 g of toluene and 12.2 g of deionized water to each reaction vessel. A PTFE-coated stir bar was added to each reaction vessel to facilitate efficient phase mixture.

Reaction vessels were closed and placed into the Multiwave PRO microwave system. A power-based heating profile was applied for approx. 90 min. Under closed-vessel conditions, maximum reaction parameters of 40 bar and 210 °C were maintained, as can be seen in Figure 2. After cooling, a fraction from the aqueous layer of each reaction vessel was collected for subsequent chloride analysis via ion chromatography.

Figure 2 – Reaction parameters of microwave-assisted extraction technique.

Evaluation of the microwave-assisted extraction technique

The efficiency of the microwave-assisted extraction technique was evaluated by comparing the results with chloride values obtained from the conventional extraction technique. As shown in Table 2, the chloride results obtained via the microwave-assisted extraction compared very favorably with the results obtained by conventional extraction. The microwave-assisted extraction technique also exhibited good repeatability, with all results being within 2 standard deviations of the mean result for each sample (sample 1: 2.8σ = 2.41 mg/L, 95% confidence; sample 2: 2.8σ = 3.73 mg/L, 95% confidence).

Table 2 – Recovery data for chloride extraction using the microwave-assisted technique

Conclusion

The microwave-assisted chloride extraction technique offers a viable alternative to conventional standardized extraction methodologies. The key advantage of the microwave-assisted technique is the ability to perform extractions under closed-vessel conditions, thereby allowing extractions to be performed at temperatures much higher than can be achieved with conventional extraction techniques. An increase in extraction temperature ofFers an exponential improvement in the efficiency of the extraction.

Additional benefts that can be realized by using a microwave-assisted technique for chloride extraction include:

  • Substantial reduction in organic solvent usage
  • High-throughput, simultaneous processing of up to 16 samples possible with the Multiwave PRO system
  • Efficient heating and sample agitation capability of the microwave-assisted method allows for the processing of a wide variety of petrochemical sample matrices
  • Full control and documentation of reaction parameters (temperature, pressure) lead to high repeatability and reproducibility.

Reynhardt Klopper is Product Specialist, Analytical and Synthetic Chemistry, Anton Paar USA, Inc., 10215 Timber Ridge Dr., Ashland, VA 23005, U.S.A.; tel.: 804-550-1051, ext. 147; fax: 804-550-1057; e-mail: reynhardt.klopper@antonpaar.com; www.anton-paar.com

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