In Tank Blending
In-tank blending is the most traditional method of blending a number of products. However, due to the complexity of recipe calculations and the variability of bunker residual feedstock, it has been found to have a number of shortfalls, in addition to being time consuming. The process involves sequentially or simultaneously adding measured volumes (normally using a flow meter or level gauge) of the components to be blended into a dedicated blending tank with mixing facilities. The components are then mechanically mixed to homogenise the contents of the tank. A sample is extracted and then analysed to verify the blend quality. Any adjustments required are then made to the blend by adding additional components after which it is re-mixed and re-analysed to validate the batch. This is clearly a time consuming process requiring the availability of blending tanks, and possibly pre-blended feedstock, as well as the capability to predict, plan demand and store blended products.
Mixing is a vital component of in-tank blending and there are two main methods used; mechanical in-tank mixers and sparge mixing. In both cases the mixing process can be lengthy and in larger tanks can take a number of hours to ensure the product is fully homogenous before samples can be taken to confirm the batch quality. In-tank mixers are typically side or bottom mounted paddles/stirrers or jet type mixers inserted around the base of the tank. These physically agitate the fluids for sufficient time for the contents of the whole tank to be homogenised. Mechanical mixing systems generally tend to be large and are most often used on larger tanks, due to the cost of installation and maintenance. For smaller capacity tanks (and often those on barges) sparge type mixing is used. Sparge mixing re-circulates the contents of the tank generally returning the flow through a sloping tube with holes cut in it to enhance the dispersion effect. Most often used on barges, sparging generally means a barge can only carry one grade of fuel that is sparge mixed on route to delivery. In this case any errors in the blend cannot be corrected, often resulting in ‘over specified' and hence costly blends. Both methods of mixing for in-tank blending are lengthy and unless operated within the design constraints of the mixing system supported by accurate sampling and analysis, there are risks that the whole tank and hence blend may not be homogenous. In addition in-tank blending is only as good as the recipe and the consistency of the feedstock.
The fact is that any blend recipe for bunker fuels is an approximation and residual feedstock is often layered, meaning the quality changes during the batch. These are some of the reasons that in-tank blending is being replaced by in-line blending as it is able to resolve many of these issues and deliver significant returns on investment for bunker suppliers. For in-tank blending the accuracy of the blend is totally reliant on the accuracy to specification of both base oils, and the equipment performing the sequential metering. Batch blending, however, is still a viable method where market demand is small and / or intermittent.