Sugar recovery from lignin slurries by using Larox® PF pressure filter

Larox® PF is a tower press filter where the chambers are aligned horizontally. The chamber alignment ensures even cake washing results throughout the whole filtration area as water is pressed through the cake evenly in all chambers. This technology has shown great savings in use of cake wash liquid compared to conventional filter presses with vertical plate alignment while achieving excellent cake wash results. Lower wash liquid consumption also ensures higher sugar concentration in the liquid phase thus saving costs in the downstream processes.

Testwork

Various lignin slurries have been tested with both conventional filter presses and Larox® PF filters. The testing has been done with Larox® PF 0.1 model which has a filtration area of 0.1 m², and with a conventional filter press with filtration area of 0.27 m². The results have also been verified by using larger container test units on pilot trials on sites.

Filtration cycles included slurry feed, 1^st pressing, cake washing, 2^nd pressing and air drying before the cake discharge. The washing efficiency has been evaluated by using different wash ratios. Furthermore, the washing efficiency can be followed by measuring the sugar content of the filtrate in different points of the filtration cycle.

When looking at the sugar recovered from the cake, higher recovery rates with lower wash liquid consumption has been seen when using Larox® PF filtration technology. The chart below illustrates the sugar recovery in percentages with different wash ratios by Larox® PF filter and a conventional filter press.

As seen in the chart above, the sugar recovery without cake washing is around 80 % depending on the residual moisture content of the cake. When cake washing is included, the sugar recovery increases significantly with the Larox® PF technology already with lower wash liquid consumption. When comparing lignin B, a wash ratio of around 1 m³/t D.S. is sufficient for >90 % sugar recovery with Larox® PF. With the conventional filter press technology, on the other hand, a wash ratio of around 3.5 m³/t D.S. is needed to achieve as good results. With a wash ratio of 3.5 m³/t D.S. results in a sugar recovery of 95 % with the Larox® PF filter.

Another way to estimate the washing efficiency between filtration technologies is to analyze the sugar content of the filtrate during filtration cycle. This can be done with Brix measurement or by using chromatography measurement. It has been seen that with the Larox® PF filtration technology, the sugar content in the filtrate stays relatively constant all through the washing step and then drops rapidly indicating that the wash has been completed. This indicates that wash liquid has evenly replaced the sugar solution in the cake over time.

With the conventional filter press technology, the sugar content value in filtrate drops quickly after the washing step is started. This indicates that the wash liquid is not washing the cake evenly and is channeling through the cake.

Moving into full scale plants

It must be kept in mind that the presented results are achieved with test units with small surface area. When moving into full scale plant unit sizes, the differences will be even bigger.

The horizontal plate alignment ensures that the Larox® PF filter has even cake washing regardless of the plate size. With conventional filter presses, as the plate area increases, the channeling effect will remain present and impact the washing result over the whole area of the cake and  diluting the sugar concentration in the liquid phase.