Paper making and fiber liberation – Apparatus – Digester
Patent
1997-12-22
2000-08-01
Alvo, Steven
Paper making and fiber liberation
Apparatus
Digester
422226, 422271, D21C 708
Patent
active
060961708
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for pressurized bleaching of pulp and more specifically to a method for securely carrying out pressurized peroxide bleaching.
Our own patent SE-C-500 616 previously disclosed a method for carrying out pressurized peroxide bleaching of pulp with a consistency exceeding 8% in a bleaching vessel dimensioned for positive pressure, in which the pulp is fed to the vessel by means of a pump, is heated to a temperature exceeding 90.degree. C., and is bleached with peroxide in an amount exceeding 5 kg/ADMT.
Oxygen gas is formed in conjunction with the decomposition of peroxide. If the output from the above-described bleaching vessel is suddenly stopped, the pressure in the reactor will successively increase as a result of the decomposition of peroxide and the formation of oxygen gas. The problem is not limited to peroxide, but rather of course also applies to other bleaching chemicals with this tendency toward "post-fermentation" at high temperature, such as, for example, chlorine dioxide.
In our own patent application SE-9404299-1 is shown a security system that eliminates the risk that the above-mentioned impermissible pressure limit is reached within the vessel or any part of its peripheral equipment. Among other things is shown that the output scraper, which is arranged in the top of the reactor and cleans in front of the outlet valve as well as the emergency valve, so that plugging up of the pulp in and in front of these valves is eliminated.
The upper part of the reactor as shown in the above-mentioned application, with an output scraper and valves, works well but is relatively expensive to manufacture. According to the known principle, the output valve and the emergency valve are provided on a separate unit, normally a pipe, above and on the reactor housing itself. A result of this construction is that the length of the axle of the output scraper, which is driven by a motor that sits arranged farthest at the top, becomes relatively long, which leads to problems both of dimensioning and in the bearings. Furthermore, the amount of material used is relatively large.
The object of the present invention is to provide a construction that solves the above-mentioned problems yet still offers the same advantages when it comes to security.
The object is achieved with a pressure vessel having an output scraper for use with pulp bleaching primarily using peroxide, at high temperature and high pressure, comprising a pressure vessel housing with a first end, and an output arrangement at a second end, whereby the output arrangement comprises an output scraper with at least one scraper arm as well as at least one cleaning member, a driving arrangement, a driving axle, a bearing, a supporting structure for the driving arrangement, and at least one normally opened outlet, as well as, preferably, a second normally closed outlet arrangement, whereby the said normally opened outlet is arranged in the wall of the pressure vessel at the curved gable at the second end.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained as follows in greater detail with reference to the attached figures in which:
FIG. 1 shows a known embodiment for providing a security system in conjunction with a pressurized peroxide bleaching vessel;
FIG. 2 shows a preferred detailed solution according to the invention of the output end of such a vessel.
DETAILED DESCRIPTION
A known pressurized peroxide bleaching vessel is shown in FIG. 1. The vessel comprises a housing 1 with an input arrangement 2 provided at its lower end, as well as an output arrangement 3 arranged at its upper end. The pulp is output normally with the aid of an output scraper through a first outlet opening 11 that is connected with a first outlet conduit 14. The pulp is led via the outlet conduit 14 to a so-called standpipe 26, where gas is removed from the pulp. The standpipe 26 is further provided with a spillway 27 that opens into a region that is at least partially surrounded by a wall 28 that is impermeable to fluids.
The upp
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The Bleaching Of Pulp, by R.P. Singh, Tappi Press, Third Edition, Revised; pp. 203, 657; undated.
Derwent's abstract, No. 63838 D/35, week 8135, Abstract of SU, 787-520 (Chipizubov M V), Dec. 18, 1980; 1 page.
Olsson Hans
Schildt Yrjo
Alvo Steven
Fasth Rolf
Kvaerner Pulping Aktiebolag
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