Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1995-11-22
1997-09-02
McCarthy, Neil
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210739, 210744, 210 961, 210 97, 210104, 210108, 210143, 73 6164, C02F 152, B01D 2130
Patent
active
056628056
DESCRIPTION:
BRIEF SUMMARY
This invention relates generally to processes and apparatus for automatic on-line control of the dewatering of suspensions such as sewage sludge, cellulosic paper-making sludge and mineral suspensions. In particular it relates to apparatus and methods for controlled addition of polymeric flocculant (or other chemical that influences dewatering) to aid dewatering, to control apparatus for automatic control of the addition of the flocculant or other chemical, and to components of this control apparatus.
It is standard practice to flow a suspension through a service line towards a dewatering plant where the suspension is dewatered and to flocculate the suspension prior to dewatering by adding polymeric flocculant to the suspension by flocculant dosing equipment. Optimum dewatering performance depends on the nature of the suspension, but this tends to be variable. As a result, various methods have been developed for determining the suitability of any particular suspension for dewatering.
Conventional methods involve taking a sample from the suspension and subjecting it to an appropriate test in a laboratory to ascertain a test parameter (eg. turbidity, viscosity or filtration rate), and then adjusting the overall process manually in response to the observed value. The laboratory test may be entirely manual, for instance filtration through a Buchner funnel, or the laboratory test may be automated to some extent, for instance as described in "An Automated Method for Filterability Determination" by de Moor and Gregory, World Filtration Congress III, 1982, pages 253 to 261. When the suspension is a sewage suspension, capillary suction time tests are common. When the suspension is a cellulosic suspension, drainage and freeness tests are common.
In particular, in the manufacture of paper it is conventional to adjust the level of beating and/or dilution of the cellulosic suspension in accordance with the measured freeness of the thick stock, and various batchwise laboratory methods are known for determining freeness in order to assist in optimising the process.
An automated method conducted on the suspension in the mill has been proposed by the Thompson Equipment Company (Teco) in their advertising brochure "Drainac (trade mark) Continuous On-Line Freeness Measurement". This brochure states that the Teco Drainac device operates on the filtration principle similar to the standard laboratory freeness tester. The Teco Drainac detector is said to consist of a vertical riser containing a perforated plate. A pressure differential between the suspension service line and the distant side of the filter plate causes filtration through the plate and the accumulation of a pad of fibres on the plate. The time required to draw a known volume of filtrate through the pad is used to calculate freeness. Once freeness has been determined, air pressure is increased to return filtrate and fibre to the stock line in preparation for the next cycle and at the same time flush water is introduced into the chamber to clean the screen. The flushing is said to force the collected fibre mat back into the service line.
Presumably it is intended that this should be done at the thick stock stage so that the fibre mat has an opportunity of becoming redispersed into the suspension. If it was done at the thin stock stage, the fibre mat would be likely to cause breaks or imperfections in the final paper sheet. The brochure states that the Teco Drainac system is suitable for use at stock line pressures of around 1.4 to 6.5 bar (20 to 90 psig) with air pressure and flush water each being at a pressure of not more than 0.7 bar (10 psig) above the line pressure. Accordingly the maximum water spray pressure is about 0.7 bar, and is often less. The Teco Drainac system is described solely in the context of monitoring freeness and other stock properties so as to give information about the stock, but the brochure also proposes that it can include a microprocessor, the output from which can be input to a refiner control system, i.e., for adjusting the freeness of
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World Filtration Congress III, 1982, pp. 253-261.
Teco Advertising Brochure "Drainac Continuous On-Line Freeness Measurement" and accompanying document Drainac Stock Line System.
Cameron Timothy Ian
Field John Rodney
Glachan Mark Stephen
Tovey Peter James
Allied Colloids Ltd.
McCarthy Neil
Morrison Betsey J.
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