The influence of substituted quinolines, pyrroles, indoles, and pyridines on deposit formation in a diesel fuel is evaluated. Significant increases in deposition rate are found which are dependent upon the basicity of the nitrogen compound within a given compound class. These effects correspond closely with those produced in a Jet A fuel. Removal of highly polar fuel components renders the nitrogen compound influence inoperative. These components are therefore participants in deposit-forming reactions.
After World War II, when demand for both gasoline and middle-distillate fuels increased, petroleum marketing firms were forced to blend straight-run, middle-distillate with catalytically-cracked fuel. This procedure resulted in difficulties with respect to deposit and sludge formation, especially in blends containing components derived from high-sulphur crudes. Two explanations have been offered for the formation of deposits in diesel fuels during storage. Clinkenbeard' attributes the instability to formation of autoxidation products of hydrocarbon components of the fuel and their subsequent reaction with sulphur-, oxygen-, and nitrogencontaining organic compounds. Elmquist' has stated that instability arises from the presence of easily oxidizable aromatic thiols, certain hydrocarbons, and oxygen. The small difference between these viewpoints becomes significant when production of fuels from non-petroleum sources is considered. High heteroatom content in some synfuels may lead to considerable instability if, as suggested by Elmquist', the trace compounds are incorporated into the deposit.
Thompson et aL3 found that addition of thiophenol to a mixture of catalytically-cracked and virgin fuel oil produced large increases in the amount of soluble gum in the fuel and little change in the rate of formation of insoluble deposits. Primary mercaptans were found to stabilize the fuel slightly, while tert-butyl mercaptan caused no change in the amount of deposit. Thiophenes and alkyl sulphides were also shown to have little effect upon gum formation. However, disulphides, polysulphides, and free sulphur tended to cause increased deposition.
Thompson et aI4 also investigated the effect of nitrogen compounds on fuel oil storage stability. Pyrroles caused lhe largest amounts of insoluble deposits; pyridines, while detrimental, were less effective in promoting deposit formation.
This is in agreement with the findings of Mapstone for nitrogen-spiked gasoline. However, Thompson used nitrogen concentrations greater than 500 ppm and may have been observing the formation of 'pyrrole black' rather than promotion of deposit formation from fuel components5.