Aromatic recovery from distillate oil of oily sludge through catalytic pyrolysis over Zn modified HZSM-5 zeolites

As a potential energy resource with high content of crude oil, hydrocarbon recovery from oily sludge has attracted intensive interests. In this paper, a catalytic pyrolysis experiment of oily sludge has been conducted using HZSM-5 and Zn loaded HZSM-5 zeolites in a two-stage tubular fixed-bed reactor to recover aromatics. The physicochemical properties of HZSM-5 zeolites before and after Zn loading were characterized by XRD, FTIR, N2-adsorption and NH3-TPD. The detailed compositions of oil product samples obtained from pyrolysis at differene residence time and over modified HZSM-5 were analyzed by gas chromatography-mass spectrometry (GC-MS). Results showed that the total aromatic hydrocarbons (TAH) yield increased from 48.7% to 92.2% as residence time changed from 1.0 s to 7.6 s, indicating longer contact time led to a much higher aromatics production. However, under 7.6 s, aromatics were more concentrated on polyaromatics with three or more benzene rings. By contrast, 3.8 s was regarded as the optimum residence time for a higher aromatic yield (84.8%) and the highest selectivity (67.4%) of naphthalenes yield. The catalytic effects of Zn modified HZSM-5 were investigated under 1.9 s. The incorporation of 3% Zn enhanced the TAH yield from 58.7% to 81.0%, as well as the naphthalenes yield (from 31.5% to 67.5%). The promotion of dehydrogenation and deoxygenation over Zn modified zeolite was confirmed by a remarkable increase of the H2 and CO2 yield. Coke depositon on the catalysts was further studied by thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) analysis. The coke was determined as polyaromatics coke due to the polycondensation reaction over HZSM-5. Moreover, longer residence time promoted the formation of coke while the loading of Zn slightly reduced it.