Patent GB-838666-A (GB)
Title: Process and apparatus for automatic control of filtration
IPC: B01D37/02
Filed: 1957-10-25
Abstract: A rotary leaf filter of the Vallez type is provided with a conduit system controlled by air-operated valves and with a time-cycle controller which governs actuation of auxiliary valves in the air supply of the air-operated valves and which also governs the operation of pumps in the conduit system and of motors driving the filter leaves and a cake-removing scroll respectively. The primary application of the apparatus is to the removal of catalysts containing chromium oxide from hydrocarbon solutions of olefin polymers produced, e.g. as in Specifications 790,195 and 804,641 (see Group IV(a)).  Precoating.  When the controller is started, precoat slurry is pumped from a mixing tank through the filter, most of the liquid passing through the filter leaves and thereafter through an outlet valve, arranged to permit a high rate of flow, back to the mixing tank.  A small proportion of the slurry is bled off from the filter casing to vent vapours therefrom and returns to the tank.  On completion of the precoat stage the rate of flow of the filtered precoat-carrier liquid is substantially reduced by a low-flow-rate valve and (where sufficient precoat for a single filter cycle has been mixed) the liquid is continuously recirculated from the tank through the filter for an indefinite period constituting a "precoat-holding" stage to prevent precoat from falling off the filter leaves.  (Where larger batches of precoat mix are prepared, the liquid for this stage is circulated from a separate tank).  The time-cycle controller stops itself after having set the valves for the "holding" stage.  Filtration.  When it is desired to start the filtration stage proper, the controller is again started, whereupon suspension to be filtered instead of precoat slurry is pumped to the filter, and the resulting filtrate is passed through the low-flow-rate valve to a product recovery zone.  After a predetermined time the suspension feed is stopped, and a drain valve is opened, and the suspension content of the filter tank is displaced therefrom by wash solvent from another conduit.  The displaced suspension is wasted or returned to supply.  Washing.  Next, the filter cake is washed by a quantity of the same solvent which now passes through the filter leaves and is returned for re-cycling.  The wash solvent is then drained from the apparatus and replaced by vapour passing in from a wash-vapour receiver.  Sluicing.  Finally the filter cake is washed off the filter leaves by a sluicing solvent and is removed from the filter casing by the scroll, now operating, to a solvent recovery zone.  The motors driving the filter leaves, scroll, sluicing solvent pump and the time-cycle controller then stop, leaving the apparatus in condition for the next series of cycles.  Warning devices.  A turbidity-meter-annunciator is provided in a bleed circuit in parallel with the filtrate conduit to warn of filter medium breakthrough, and a pressure-differential transmitter, linked to a pressure-differential recorder, switch and annunciator, is connected between the inlet and filtrate conduits of the casing to warn of excessive pressure drop.  The pressure in the casing is continuously recorded.