A Six Sigma Approach to Evaluating Vacuum Filtration Technologies
Abstract: This article compares three designs of vacuum belt filters (moving tray, rubber carrier belt and the BHS continuous-indexing technology). The evaluation had several stages from initial investigation, bench-top testing and finally pilot-scale testing. The BHS Continuous-Indexing Vacuum belt filter was the selected technology to replace a manually-operated filter press. The return-on-investment (ROI) calculation based upon the BHS process, operating and maintenance benefits allowed the project to be funded with a payback of less than 6 months.
Solid-liquid separation can be accomplished by pressure or vacuum filtration in a batch or continuous mode. Generally, following filtration, the cake must be washed and subsequently dried to specifications. The technology choice process depends on a wide range of parameters, including the nature the solids (crystal size and distribution, shape and type), the liquids (temperature, viscosity and nature) and the process itself This article examines die use of a continuous-indexing vacuum belt filter (CI-VBF) as a replacement for manually operated filter presses and offers insight into one plant's technology-selection process.
A specialty chemical plant was using several filter presses to produce various high-valued products in the form of thin filter cakes. Each unit contained 136 plates with a total filter area of 4,400 ft2 (440 m2). Unfortunately, the plant was experiencing a wide range of operating and process problems:
- excessive use of wash liquid and requirement of high wash ratios to meet a conductivity specification
- high moisture content in the discharged cake required longer drying times
- frequent filter-media blinding resulted in high maintenance costs for changcouts
- manual processes, such as discharge of cake from the filter press, raised operating costs
- difficult)' in handling the wet-cake discharge required frequent operator intervention
- mixing of the mother liquor and wash filtrates increased the costs of distillation and liquid recovery.