Rotary vacuum filter scale-up calculations — and the use of computer spreadsheets

Abstract

In laboratory studies of the filterability of a slurry it is usual to determine the cake permeability (or specific resistance) and cloth resistance. This data is then used to design the commercial filter. The data obtained in the laboratory can, however, provide a considerable amount of information about the performance of a filter, with operating conditions (ie design variables), which are not usually thoroughly investigated. This is due to the complex computations or computer program which would be needed for such a numerical investigation. This can easily be performed on a computer spreadsheet, and the result can be visually presented for immediate selection of preferred operating conditions. This has been achieved for a slurry formed by wet scrubbing a flue gas from a metallurgical smelter as an example. Rotary vacuum filter (RVF) performance was calculated incorporating the resistance due to the filter medium, by solving the resulting quadratic equation. It is usual to simplify the performance calculations by neglecting medium resistance, the error in making this assumption is shown. The performance of a pilot scale RVF on this slurry was experimentally investigated and compared with the spreadsheet prediction. The resulting scale-up calculations show the options open to the design engineer; to scale-up on dry solids throughput, filter area, cake depth or slurry throughput. Clearly this varies from one filter application to another, but the use of the spreadsheet makes these scale-up calculations easy and visible. It is also possible to test the affect on filter throughput of concentrating the feed slurry. This being one option to increase the capacity of an existing filter installation.