Feasibility of Microwave-Induced Combustion for Digestion of Crude Oil Vacuum Distillation Residue for Chlorine Determination

The microwave-induced combustion (MIC) method was applied for digestion of crude oil vacuum distillation residue for further Chlorine determination. Ignition wits performed by microwave radiation using a microwave oven designed for conventional pressurized wet digestion. Combustion was carried out using 20 bar of oxygen and NH4NO3 as the igniter. Water or H2O2, Na2CO3, and (NH4)(2)CO3 Solutions were investigated for analyte absorption as well as the necessity of a reflux step. Digests were suitable to be analyzed by inductively Coupled plasma optical emission spectrometry and ion chromatography. Accuracy was evaluated using spiked samples and certified reference materials of coal (BCR 181) and fuel oil (NIST 1634c). Recoveries for Cl were in the range of 98.4-100.2% for all investigated Solutions using the reflux step. The agreement with certified values was better than 95% using 25 mmol L-1 (NH4)(2)CO3 as the absorbing Solution and reflux step. Poor recoveries for Cl were observed using microwave-assisted wet digestion and water extraction procedures. Residual carbon content for MIC was below 1%, whereas it was about 20% using microwave-assisted wet digestion. Using MIC, up to 400 mg of sample Could be decomposed and digestion was complete in less little (5 min for combustion and 20 min for cooling) in comparison to other conventional procedures (more than 1 h). Up to eight samples could be digested simultaneously, making the proposed procedure suitable for further routine Cl determination in oil refineries.