OPTIMISING ETHANOL GASOLINE BLENDSTOCKS THROUGH ADDITION OF ETHANOL TO ON-LINE BLENDING ANALYSER SAMPLE LINES

摘要

This paper provides an overview of the problems associated with controlling and certifying the final product quality of gasoline when ethanol is added to meet biofuels mandates. Ethanol is currently, nominally added at 5%, 7%, 10% or 15% depending on country, location or marketing preference (1,2). This paper describes a method and equipment to allow the continued optimum use of process analysers to control and, where applicable, certify, the gasoline product at the refinery although the ethanol is added remotely at the marketing terminals. The ethanol is added at the marketing terminals to avoid water pick-up due to the highly hygroscopic nature of ethanol during transport to the marketing terminals. The ethanol has an impact (property boost or depression - the rest of this paper will use the term boost to generically mean either) on a number of the critical gasoline product quality specifications; most importantly octane, vapour pressure and distillation. Therefore, refineries need to produce what is commonly known as a BOB (Blendstock for Oxygenates Blending) with product quality properties such that it produces a final on-specification gasoline, when the requisite amount of ethanol is added at the marketing terminal (3). This boost effect is sample matrix specific i.e. depends on the specific component compositions and their proportions used to blend the BOB. The boost also depends on the base property value of the BOB and can be non-linear. This means the BOB's have to be conservative in respect to final product quality property give-away, to cope with the worst case boost, to ensure the final gasoline is on specification after the addition of the ethanol. For conventional gasoline blending, there are large incentives to use on-line analysers for on-line control and certification of the blend. Due to compositional dependency and the fact that the composition is not known, the property prediction contains significant error. For BOB's the incentives for using on-line analysers for BOB blending are more limited as they only measure the neat BOB. When blending BOB's, the analysers can only measure and control to the conservative BOB property targets, not the final gasoline product quality after the ethanol is added. They also can no longer be used to certify the final product with the loss of the significant benefits for sites that use on-line certification for conventional gasoline.