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Title: The mechanism of formation and subsequent behaviour of deposits in the storage of bitumen
Author: Davie, F. M.
ISNI:       0000 0001 3408 9718
Awarding Body: Polytechnic of the South Bank
Current Institution: London South Bank University
Date of Award: 1991
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Following the investigation of a number of incidents involving bitumen storage tanks a number of suggestionsw ere made concerning the possible presence of the components of the fire triangle. A thorough investigation of the coke-like deposits formed on the underside of the roof of heated bitumen storage tanks was made and it has been shown that these deposits are the result of the condensation of vapours given off from the surface of the heated bitumen. The deposit was found to build up at a rate of 0.31 g/m2 h at the currently recommended storage temperatures. Thermal analytical techniques have been used to determine the exothermic nature of the deposits and to support the idea of deposits acting as a "sponge" to light hydrocarbons. The deposit was found to undergo an oxidation/ smouldering reaction at temperatures found at the underside of the roof of storage tanks, onset temperatures for exothermic behaviour ranged from 337 to 406 K. The deposit taken from refinery tanks was found to have an activation energy in the range of 8-9 kJ/mol. Electron microscopy and elemental analysis have indicated the possible presence of pyrophoric material within a sample of bitumen deposit. High levels of sulphur were found in areas rich in iron and part of this sulphur was in the form of a sulphate, a product of the weathering of pyrite. The sulphur found in the iron-free areas of the deposit was in the form of either a sulphide or elemental sulphur. Flammability readings of the vapour space were taken for different grades of oxidised bitumens, all of which gave readings in excess of the lower explosive limit at the recommended storage temperatures. This confirms the presence of a flammable atmosphere within tanks containing oxidised grade bitumens and so confirms the presence of the components of the fire triangle in non-inerted tanks. A model was developed to simulate the processes occurring within the deposit layer and a critical thickness of the deposit determined using this model. A critical thickness for 105/35 deposit was found to be approximately 5.5 mm. From this the frequency of cleaning of the tanks necessary to prevent the selfheating of deposits can be determined. The deposit was found to undergo a low temperature smouldering, reaching maximum temperatures of around 700-780 K. The rate of reaction was found to be dependent on the rate of diffusion of oxygen into the deposit, the highest temperatures being reached when the heat and mass transfer coefficients had a high value.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemical engineering