In the course of expanding production, the BHS customer needed a solution for solid-liquid separation in the chemical pre-stage for a particular polymer. At the start of the process, chemical reactions provide a material that has to be cleaned of by-products. The filtration of solid particles is particularly important here as impurities have a hugely detrimental effect on the tear resistance of the plastic. Technical construction requirements at the customer’s site posed additional challenges for the BHS process technology experts. A solution that was originally planned, comprising one indexing belt filter with a 12-stage counterflow wash, could not be realized.
Following process technology advice provided to the customer, the project team opted for two indexing belt filters (BF) – one with four and one with eight wash stations. An indexing belt filter is a horizontal, continuously operating vacuum filter, which enables uniform suspension feeding and stands out thanks to its versatility in terms of process technology. The filter cake that forms on the filter cloth can be further processed in a variety of ways by means of various process steps, such as washing out, dry suction or pressing. In the present case, only a dilution wash for the filtration was possible because of the large inner surfaces of the particles. With a continuously operating filter, however, the retention time of the liquid in each wash zone is relatively short. Regular construction of the filter would therefore cause the wash liquid to pass through too quickly.
Multi-stage cascade wash optimizes freshwater consumption
Drawing on experience from other applications, such as cellulose derivatives, BHS therefore built the wash stations in the form of a cascade. Using sieve plates, the wash liquid is distributed uniformly over the entire surface of the filter cake. What’s special here is that the liquid is repeatedly applied at the same point – a so-called re-flux washing. The cascading design allows a self-regulation of the entire washing stage-package. As soon as a certain level is reached in one station, the additional volume is self-fed into the next, lower-level station. This ensures that every particle is washed eight or four times respectively.
“This combination of reflux and counterflow wash on the indexing belt filter enables us to double or triple the retention time of the liquid. Or in other words, for the retention time required here, the consumption of fresh water is reduced considerably compared to centrifuges. The result is a particularly high degree of washing out with efficient use of washing agent,” explains Detlef Steidl, Managing Director, BHS-Sonthofen Process Technology.
BHS-Sonthofen designed and delivered the system, which comprises the two indexing belt filters including separators, as well as measuring and control technology. Delivery was preceded by extensive lab and pilot testing in the U.S. test center, where BHS has expanded its testing capacities for filtration, mixing and drying over the last two years. “In the lab, our work included determining the required fresh water to consumables ratio, which was then reproduced on the pilot filter with multiple wash stages. This enabled us to demonstrate to the customer that the high-quality standards were fully met with this filter solution – making considerable savings on consumables compared to other solutions,” says Steidl. The system was put into service in late 2020.