Cake & precoat filtration – Evaluating influences of increased backwash efficiency on cloth lifetime.

Abstract

Raw juice, thin juice, thick juice, standard liquor and molasses are examples of liquid streams for which state of the art candle type cake/precoat filters are currently being considered as a technology upgrade to pressure leaf filters by sugar mills, particularly beet sugar ones. Despite the fully automated design of candle filters, especially in cake building applications such as 1^st carbonation, cloth lifetime of the candle filter systems that have been on the market for decades has typically been extremely short. It is not uncommon for plants operating these older candle filter system designs to require cloth changes as often as thirteen times per campaign (120 days). Significant labor requirements as well as cost demands for replacement cloths have been experienced by sugar producers and processors using old design candle filters.  Manufacturers of improved/upgraded designs of candle filters have allocated significant time and money resources to validating and optimizing backwash efficiency. In the chemical industry, for example, it has long been believed that design improvements in media backwash efficiency would directly contribute to improved filter performance. This hypothesis was recently validated on a pilot plant scale when a 3 w% CaCo₃ slurry was used to in a candle backwashing pilot test aimed at comparing the total weight of retained particles trapped in filter cloths following the backwashing of filter candles of three different designs.  In this specific candle backwashing test, a sugar industry known candle design, one without a dip tube, was pilot tested. Another candle design commonly found in the chemical industry having a built-in dip tube was also tested. Thirdly, a new “channeled” patent-pending candle design also having a built-in dip tube was also tested.  The outcome of the pilot test illustrated that the weight of the residual CaCO₃ particles per m² of filter cloth installed on the candle without a dip tube was 96 mg/m². That of the candle with dip tube commonly used in the chemical industry was 67 mg/m². The new “channeled” candle having a dip tube was only 22 mg/m². While these the total particle weights are interesting in absolute terms, especially interesting is the ratio of the total residual particle weights of the different candle designs since better backwash efficiency equates to longer filter cloth lifetime.  In addition to closer look on above mentioned trials, the results of a five-month trial (September 2022 to January 2023) during the 2022/2023 campaign at a sugar factory in Germany will be summarized at the upcoming ASSBT conference in Savannah.