Optimising bagasse diffuser performance using percolation velocities

Invicta Mill at Giru, north Queensland, is a dual-train sugar mill that crushes about 3 Mt of cane each season. The factory has a traditional milling train on the A-side and a bagasse diffuser on the B-side installed in 1995. During the 2019 crushing season, Invicta Mill had several reliability issues with the bagasse diffuser. The root cause of these issues was difficult to identify and prompted an investigation to better understand how this unit operated. The objectives were to eliminate the diffuser-reliability issues experienced and improve the B-side bagasse loss for which the diffuser was a key contributor. The general principle of a bagasse diffuser relies on counter-current maceration/juice flow being passed through a slowly moving bed of fibre. This creates a washing action that extracts the sucrose from the fibre. Depending on the size of the diffuser, there are several intermediate juice cells that are situated below the diffuser bed and capture the intermediate juice. This juice is then pumped forward and applied upstream to continue with the washing process. The velocity at which this applied juice passes through the fibre bed is termed the percolation velocity and is a key parameter in optimizing diffuser performance. It is important to ensure that the application point of each intermediate juice stream is positioned correctly for the minimizing of by-passing and recycling of the juice as well as prevent flooding of the diffuser whilst allowing sufficient time for the washing mechanism to occur. Work at Invicta Mill during the 2020 crushing season was to determine the percolation velocities of the intermediate juice streams in order to optimise juice application positions and, hence, overall diffuser performance. The work comprised trials in which lime was injected into the juice stream and the juice pH was measured in each intermediate cell to identify where the juice was percolating and the velocity at which the percolation occurred. Using this data, recommendations and modifications were made to the intermediate juice-application points to optimise the diffuser performance. The outcome from this trial work was a final bagasse loss on B-side of 1.4–2.5% each week for the entire 2020 season. Key words Diffuser, percolation, bagasse, optimisation, bagasse loss, velocity