Heavy crude oil sample preparation by pyrohydrolysis for further chlorine determination

Abstract

In this work, a method for chlorine determination in heavy crude oil using pyrohydrolysis for sample decomposition was developed. Chlorine was determined using ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry. In this case an ICP-MS spectrometer equipped with a dynamic reaction cell (DRC-ICP-MS) was used. Heavy crude oil samples were homogenized and placed on a quartz holder, which was then introduced into the pyrohydrolysis apparatus. Nitric acid (0.2 mL) was used to assist the decomposition of the sample. Solutions with different concentrations of ammonium hydroxide were evaluated to collect the volatile chloride produced using pyrohydrolysis. A 0.75 mol L⁻¹ ammonium hydroxide solution was chosen in view of the better accuracy achieved. Best precision and accuracy were obtained by heating the sample for 10 min up to 1000 °C. The limits of quantification (LOQs, 10σ) of chlorine measured by IC, ICP OES and DRC-ICP-MS were 4.5, 48 and 3.6 µg g⁻¹, respectively. A sample amount of 300 mg and final volume of 20 mL were taken into account to calculate the LOQs. The relative standard deviation (RSD) was lower than 5% (4 replicates). Accuracy was evaluated by analysis of certified reference material (NIST 1634c) and comparison with the results obtained using the ASTM D6470 method. In both cases the results obtained using pyrohydrolysis were in agreement. The proposed method was suitable for heavy crude oil decomposition and chlorine determination by IC, ICP OES and DRC-ICP-MS. Three samples per hour can be processed using a simple and low-cost pyrohydrolysis apparatus. The pyrohydrolysis procedure is easy to perform, which is very attractive for routine analysis of crude oil, a very complex matrix. According to the authors’ knowledge this is the first application of pyrohydrolysis for heavy crude oil sample preparation.