Studies On Catalytic Cracking Of Fatty Acid Esters

Petroleum is an unrenewable resource, and the quality of crude oils is poorer and poorer, however, the demands of gasoline, diesel oil, propylene and ethylene are growing. Currently, because of their suitable properties (negligible sulfur, nitrogen, and metal content), increasing attention has been focused on vegetable oils and animal fats (mainly fatty acid esters) as a renewable source of fuels and chemicals. Catalytic cracking reaction of fatty acid esters represents an alternative route to replace partially petroleum resources.In the paper, thermal cracking and catalytic cracking reactions of fatty acid esters are investigated in the laboratory-scale fixed-bed micro-reactor and riser fluid catalytic cracking units. The results show that the cracking of fatty acid esters increases with the increase of reaction temperature properly, and the yields of liquefied petroleum gas (LPG) and propylene improve notably. At the same time, the yields of by-products such as dry gas, CO and CO2 increase a little. Generally speaking, the conversion of thermal cracking is lower than catalytic cracking, and there are a lot of oxygenates in the liquid products, moreover, the mass ratio of CO/CO2 in the gas is less than 1. However, the oxygen element is removed in the form of H2O, CO and CO2 primarily over zeolites catalysts, and the mass ratio of CO/CO2 in the gas is more than 1. Furthermore, USY catalyst exhibits high selectivity of gasoline and diesel oil, while the formation of LPG and propylene is enhanced over ZSM-5 catalyst.GC-MS and FT-IR analysis indicates that decarboxylation and decarbonylation reactions of fatty acid esters, especially the cleavage of C-O bond, take place readily before C-C bond cleavage of the resulting hydrocarbon radicals, which are subjected to thermal and catalytic cracking. And then subsequent cracking, deoxygenation, polymerization, cyclization, hydrogen-transfer, isomerization and aromarization of the resulting heavy oxygenates on acidic sites of catalysts into light molecules (such as LPG and gasoline) appears to be greatly enhanced over strong acidic sites. However, weak acidic catalyst permits a mild secondary cracking resulting in lower conversion. On the other hand, secondary cracking has been occurred a little over non-acidic catalyst, so there are a lot of heavy oxygenates in the liquid products. Additionally, in-situ IR proves that the cleavage of C-O bond is the initial decomposition step and fatty acid is one of the intermediates.The optimal reaction conditions and catalysts for different fatty acid esters are basically the same. The yields of LPG, propylene, butylenes increase with the content of saturated fatty acid (mainly stearic acid and palmitic acid), but the yields of gasoline and diesel oil decrease. This rule is not only applied to the single cracking of fatty acid esters, but also to the cracking of the mixtures of fatty acid esters and conventional catalytic stocks.Compared with conventional catalytic stocks, the cracking of fatty acid esters singly can give high yield of LPG, light olefins and good qualities of gasoline and diesel oil. At the same time, the high concentration of aromatics in gasoline fraction can be obtained. Such as 45% LPG and 23% propylene yields are produced with fatty acid esters cracking singly by using the two-stage riser fluid catalytic cracking (TSRFCC) technology and matching catalysts. At the same time, the aromatic concentration (C7～C9 aromatics predominantly) in gasoline fraction can reach 83%.Industrial test of fatty acid esters is also investigated in a 180 kt/a commercial FCC unit in the form of blending over Shenghua equilibrium catalysts, at a reaction temperature of 508°C～525°C. The results demonstrate that fatty acid esters can be used as one of alternative feeds of FCC. The product distribution and product quality improve after blending fatty acid esters in conventional catalytic stocks, especially the contents of sulphur and nitrogen in gasoline, diesel oil and slurry drop notablely.In a word, various fatty acid esters can be used for the production of LPG, propylene, and clean fuels by the application of FCC technology with optimum reaction conditions and catalysts.