GB1498031A - Level controller - Google Patents

Abstract

1498031 Controlling centrifugal strainer IMPERIAL CHEMICAL INDUSTRIES Ltd 6 Aug 1975 [22 Aug 1974] 36935/74 Heading BID The feeding of each successive batch of slurry to the drum of a centrifugal strainer is controlled by a valve 15 which is closed automatically when a pressure-sensitive electric switch or pneumatic relay 9 detects a rise in pressure resulting from the closure by the slurry level 1 of open tip 6 of a pipe 5 through which liquid flows continuously into the drum. The drum rotates continuously, and liquid is fed to pipe 5 by a low rate meter 11 and a higher rate meter 14 in parallel. Valve 12 is closed automatically 3-30 seconds after valve 15 cuts off the slurry feed, while meter 11 continues to feed liquid to keep tip 6 clean The liquid may be water or organic solvent and may be identical to the liquid content of the slurry. Tip 6 may be slightly inclined to the slurry surface and located at least half a revolution forward of pipe 4 which delivers the slurry. The filter cake is removed by a knife (not shown) after each centrifuging, but residual cake progressively clogs the drum screen which results in a progressive reduction in the filling time of the drum to level 7 as less liquid is removed from successive batches during the filling steps. In Fig. 4 (not shown) a timer incorporated in unit 17 sends a first signal to discriminator (21) a fixed time after slurry feed valve 15 opens and discriminator (21) also receives a second signal from relay 9 when the slurry closes tip 6. The drum is sequentially filled with batches of slurry until discriminator (21) receives the second signal before the first, and when this happens it signals back to control unit 17 causing it to initiate a screen-washing step after the present batch of slurry has been centrifuged.. Wash timer (25) then opens valve (27) to deliver washing solvent to the drum via pipe 28 in place of the next batch of slurry. The discriminator (21) may be a double air-operated shuttle valve or two electric pressure switches coupled to latching relays. Slurries of dimethyl terephthalate in methanol and terephthalic acid in acetic acid may be centrifuged.