On-Line Octane Measurement and its Influence on Blending Plant Design and Operation

The aim of a refinery is to obtain the desired quality and quantity of product at the minimum of cost. To realise this aim, a great deal of money and effort is being devoted to the design and development of instruments which measure the quality of the products of a process as they are produced, namely 'online'. A vital quality of a finished gasoline is the measure of its resistance to detonation indicated by the octane rating, and to monitor and control this quality the 'Octel' comparator has been developed. This instrument continuou~\'j m()ni\.o,"~anu c.an \)e \l~eu \.0 c.on\xo\ the octane rating of a finished gasoline leaving an in-line blender. This enables a refinery to blend, with confidence, directly into sea-going tankers and other vessels thus reducing the need for finished gasoline storage tanks. On existing refineries the storage tanks saved may be diverted to other uses and on new ones there is a considerable saving in the cost of storage. There is also a potential saving in total inventory. The present method of starting a blend is to check the quality of bottom, middle and top samples from component storage tanks and to assume that the average of these gives a representative sample of the tank contents. These average samples are then used as the basis for determining the blend. However, owing to stratification the quality may vary from layer to layer in the tank so that the average of the three samples may not be a truly representative sample, particularly if the whole of the tank content does not go into the blend. This method also means that unless the quality of the product flowing into the tank is known to be the same as the quality of the material in the tank, blending cannot be carried out from the tank while it is being used as a run-down tank. However, with the Comparator monitoring the blend the variation of anti-knock rating of the components are taken into account so that the final blend will be on specification. Blending on a volumetric basis can only take into account flow rates and cannot of itself compensate for changes in the quality of the components. The Octel Comparator transforms the standard ASTM-CFR knock-test engine into an on-line stream analyser which gives a signal representing the difference between the octane rating of the blended gasoline and a reference gasoline of known octane rating. This signal can then be used to trim the controller of an in-line blending system to adjust the rate of addition of anti-knock compound to the blend so as to give a final product which meets the required specification within very close litpits. When properly calibrated and maintained the comparator is capable of producing a more accurate quality control than