Effect of reactor temperature and residence time on physiochemical performance of cellulose hydrothermal products

Abstract: The characteristics of cellulose's products under hydrothermal treatments in an autoclave were investigated at various temperature (200~400℃) and residence time (5~120 min). With the temperature increasing from 200℃ to 250℃, heavy oil yield increased sharply from 5% to 14.75%. The temperature of 250℃ was suitable for the liquefaction to obtain higher yield of liquid products, with a maximum of 14.75%. With the increase in residence time, heavy oil yield increased in the first 10 minutes, and thereafter decreased with further increase in residence time because of the cracking reaction occurring in the primary hydrothermal product of heavy oil. The hydrochar yield decreased in the first 5 minutes, and then increased. At the same temperature with residence time of 5 min, the hydrochar yield reached a maximum of 50% and heavy oil yield was 16.25%. The yields of light oil and gas increased with the time increasing. The chemical compounds of heavy oil identified at 200, 250, 300, 350, and 400℃ in residence time of 30 min were mainly furans, phenols, carboxylic acids, aldehydes and ketones, and high molecular compounds were determined by gas chromatography-mass spectrometry (GC-MS). Aldehydes, phenols, ketones, acid groups and sugars in the light oil were determined by GC-MS as well as Fourier transform infrared (FTIR). The hydrochar was analyzed by elemental analyzers, scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis, x-ray photoelectron spectroscopy (XPS) and FTIR. The elemental composition of the hydrochar from hydrothermal degradation of cellulose at 200~400℃ for 30 min showed the relationship between the hydrogen/carbon (H/C) and oxygen/carbon (O/C) ratios of the hydrochar. The ratios of H/C and O/C decreased continuously with increasing temperature, however, after the temperature increased to 250℃, the ratios became constant at higher temperatures. These results were consistent with the yield of the gases and hydrochars at the same temperature. Through the SEM and TEM, it was found that the hydrochar had a core-shell structure. From the FTIR spectra, the hydrochar from cellulose hydrothermally treated at 250℃ with residence time of 30 min and 2 h exhibit was completely different from the cellulose FTIR traces. There was no O=C=O adsorption peak under the conditions of 250℃ and residue time of 2 h. The adsorption peak of the C=O vibration was wider, which indicated the existence of ketones and aldehydes. The results showed that the hydrochar was composed of aromatics and polymeric products, which revealed that aromatization processes took place during hydrothermal treatment. The XPS spectra of cellulose and hydrochar produced from hydrothermal treatment at 250℃ for 2 h showed that there was a large amount of hydroxyl groups attached to carbon atom in the cellulose, and the hydrochar contained additional aliphatic/aromatic carbon groups (C-C/C=C) and carboxylic groups, esters, or lactones (-COOR). Compared with the results from the FTIR spectra, the basic behavior of the hydrochar's functional groups could be investigated by XPS. The CH, C-O, and C=O groups decreased with the increase in residence time, similar to the results obtained with FTIR. From the chemical point of view, oxygen groups of the hydrochar differed in the core and shell, inferred from the XPS and FTIR measurements. Indeed, the core contained ketone and ether groups and the shell contained carboxylic and carbonyl groups. In conclusion, the formation of the hydrochar is through hydrolysis and dehydration of cellulose, and the hydrochar has a core-shell structure and better physicochemical characterization at lower temperature and longer residence time.