WO1983002293A1

WO1983002293A1 - Method and plant for deaerating fibre suspensions

Info

Publication number: WO1983002293A1
Application number: PCT/SE1982/000426
Authority: WO
Inventors: Nils Anders Lennart Wikdahl
Original assignee: Nils Anders Lennart Wikdahl
Priority date: 1981-12-30
Filing date: 1982-12-14
Publication date: 1983-07-07
Also published as: PL239799A1; GR77817B; IT1153689B; IT8224912A0; IT8224912A1; PT76007A; PT76007B; SE425808B

Abstract

Eliminating air in gaseous form from a diluted fibre suspension supplied to a paper machine (8), said elimination taking place by deaerating in a deaerating means (25) the water used for dilution, and coming from the paper machine (8) as white water, so thoroughly that not only gaseous air but also air dissolved in the water is substantially removed. The diluted water (18) can thereby be caused to take up in solution the gaseous air which is present in the undiluted fibre suspension supplied.

Description

Method and plant for deaerating fibre suspensions

In paper manufacture and also in manufacturing paper pulp, a fibre suspension is caused to be dewatered into a web of substantially dewatered fibres kept together by hydrogen bonds. In the most usual case prevailing today, the suspensio 5 is allowed to flow out onto an endless web (wire) for de- watering, first by self-drainage, afterwards over suction boxes and finally, after liberation from the wire, by pressin and usually further drying.

It is often a situation today that more than one layer 10 of the suspension is laid on the wire, e.g. for obtaining a- laminated structure, where the layers have different proper¬ ties. It is also known to use double wires instead of a singl wire, the suspension being injected between the wires for sub sequent dewatering by pressing. 15 Development is towards ever increasing speeds in manu¬ facture, to the accompaniment of a number of different con¬ trol and quality problems, for which solutions are subsequent¬ ly provided as milestones in the gradual course of develop¬ ment. 20 One of these problems is the requirement that the pulp suspension may not contain too great quantities of air. Air bubbles, fed out with the suspension onto a wire, may lead to small holes in the finished paper ("pinholes") as well as control of the plant being made more difficult. The intention .25 is that suspensions are to be sprayed out from. a head box on¬ to a wire (or between two wires, etc) with a certain amount of guidance along the transverse direction of the web and suitable rate of deposition in relation to the receiving wire. Air in the suspension results in that the latter becomes com- -30 pressable, and it will be seen that this must lead to dyna¬ mic disturbances in controlling the process.

There are means in the prior art intended for libera¬ ting fibre suspensions from air. Examples or such are the "Perivac", manufactured by Noss AB and the "Deculator" , manu- 35 factured by Clark & Vicario, their basic principle being that the liquid is caused to boil at low pressure during fine divi¬ sion or as thin and turbulent flow layers. The low pressure is obtained by barometric effect, obtained by the "boiling space" being situated sufficiently high (pressure drop about 1 bar/10 m) and the air thus liberated is pumped away with a vaceuum pump.

In paper manufacture it is customary to recirculate the major portion of the water of which the fibre suspension con¬ sists, and use the white water coming from the paper machine to provide a suitable degree of dilution. On being supplied to the wire the suspension should normally have a fibre con¬ tent in the order of magnitude of 0,3 - 1,0 % . A plant usual, and typical of the prior art, is illustr¬ ated in Figure 1, which is a principle sketch of a paper- making machine with associated cleaning apparatus. In this block diagram the flow of the major portion of fibres is de¬ picted with somewhat thicker lines. Coming from a source 1, which is unnecessary to describe in the present context, a suspension, which may have a fibre content of 3 - 10 % or more, is diluted with about 10 parts of water per part of suspension from a conduit 18 and then passes a pump 2. This main flow subsequently passes a screen 3 (where coarser screened material is taken to a further screen 3' , from which screened-off material leaves the system via a conduit 17 and from which accepted fibres return to the conduit 18). The major portion of the diluted suspension which is the accept from the screen 3 continues to a hydrocyclone separator 4, from which the accept (which is the main portion)^* continues to a deaerator 5 which may be one of the mentioned, known structures. In parenthesis it may be noted that the reject from the hydrocyclone separator 4 is taken via a pump 14 to a further hydrocyclone separator 4¹ , the accept of which is returned to the conduit 18 and the reject of which is further cleaned or separated in a third hydrocyclone separator 4" after having passed a pump 15. The refuse or discard from all three hydrocyclone installations leaves the system via the conduit 16. The suspension going to the deaerator 5 has now been cleaned from impurities (contraries) and is suitable for the production of paper or pulp semi-finished products after de- aeration has taken place. A pump 6 conveys the suspension to the head box 7 from which it is laid out on the wire in the schematically shown paper machine 8.

It will be noted that the pump 6 is controlled e.g. by the pressure in the head box 7, via the indicated control line 22 and that with the aid of this control a minor, vari¬ able percentage of the flow to the pump 6 will be mixed, via the conduit 13, with the recirculating water intended for mixture in the conduit 18.

In parenthesis it should be noted that the water col- lected from the paper machine in a white water collector or vat 10 maintains a given level 11 via overflow means, the in¬ put sides of all pumps in the system being related to said level 11 via the conduits 12, 13, 19 and 18, respectively, in a manner describerd in the Swedish Patent Specification No. 224271, resulting in the advantages described there.

In accordance with the invention it has now been under¬ stood that this described structure, although functioning well, can be simplified and improved with respect to deaera- tion. As far as the systems mentioned in the introduction are concerned, where several fibre flows are to be combined in one and the same paper machine, it is clear, for example, that in the structure just described a deaeration apparatus must be used for each fibre flow.

The initial object of the invention is therefore to de- crease the number of deaerators in paper machines or the like, where the suspension is simultaneously supplied from two or several systems. A starting point of the invention i.s thereby the realization that it is not necessary to deaerate the sus¬ pension, which is purified, cleaned and ready for paper manu- facture, if it is instead ensured that the constituents there¬ of do not have too high an air content before dilution.

The inventive solution is further built on the under¬ standing of the importance of the air content in a fibre sus¬ pension being present in three forms, namely as dissolved in liquid, as free air (mostly as small gas bubbles) and as gas adhering to the fibres. The gas dissolved in the liquid is hereby without importance for the suspension compressability properties, and previously there has not appeared to be any reason for eliminating the dissolved portion of the air con¬ tent in the suspension.

The inventive concept is based on the circumstance that the three forms in which air may be present form "comir-unica- ting vessels" to a certain extent. This makes it possible to mix a smaller amount of air-containing suspension with a larg¬ er amount of extremely air-impoverished liquid, where a major portion of the dissolved air has also been removed. If these two are subsequently mixed well, the air which is free and that which is tied to the fibres in the minor portion of the suspension will substantially completely go over into dis¬ solved form in a state of equilibrium. In accordance with the invention it is then possible to eliminate deaeration of the suspension, if the dilution water is deaerated to a suffi- ciently high degree instead.

At 20°C and atmospheric pressure, water is capable of dissolving a volume of 1,9 % air and at the temperatures generally used in modern paper making machines, solubility attains a full one percent. If dilution is therefore carried out on an unaerated suspension, e.g. one part of suspension is diluted with 10 parts of water, it should be possible to bring gaseous air into solution, this air corresponding to as much as 20 % in the unaerated suspension, which is obviously seldom required. It is therefore already sufficient to use a dilution water where deaeration is not entirely complete in relation to dissolved air. However, it is generally desired to provide quick action, and a state of equilibrium which only occurs after a long time lacks real value. A more or less complete elimination of air in the dilution water is therefore of in- terest, and it is also advantageous to increase by agitaion the absorption (entrainment) rate of free air.

The above-mentioned advantages as well as other advan¬ tages and properties apparent from the continued description are achieved more specifically and in accordance with the in- vention by a method as claimed in claim 1 and by a plant having the distinguishing features disclosed in claim 3. Advan tageous embodiments are apparent from the dependent claims.

The invention will now be described in detail with the aid of an embodiment, which is not to be regarded as limiting, and in conjunction with the drawings. Figure 1 is a flow diagram of a plant as in the prior art, and which has been discribed above. Figure 2 illustrates an embodiment in accordance with the invention. Figure 3 illu trates another embodiment in accordance with the invention. Similar or analogous means in Figure 2 have been given the same reference numerals as those in Figure 1, and what is applicable for these means is to be sought in the previous section of the description. A comparison with Figure 1 shows that the deaerator 5 situated in the main fibre flow is elimi nated in Figure 2.

A deaerator 25 in the white water stream 12 coming from the paper machine has been inserted in the embodiment instead. A pump 26 urges water therethrough, and to retain the advan¬ tage, e.g. described in the Swedish Patent Specification No. 224 271 with a definite pressure reference 11 to all parts of the plant there is a bypass conduit 27.

The 'deaerator 25 must provid somewhat more effective deaeration than the deaerator 5 according to Figure 1, since it must eliminate not only free air present, but also a portio of the air dissolved in the water. The increased demands have here been compensated, at least partially, by deaeration takin place in substantially pure water instead of in a suspension.

In order for the plant in accordance with the invention^' to function it is necessary that air is not brought into the different flows from any other place in the plant. The use of hydrocyclone separators or vortex cleaners of the type where the reject output opens out into air is thus practically ex¬ cluded here. Hydrocyclone separators according to the Swedish Patent Specification No. 215 471 are thus recommended as the hydrocyklone separators 4, 4' and 4".

It may be noted that the screen means 3 and particular¬ ly the hydrocyclone separator 4 have the effect that the attainment of a state of equilibrium is accelerated, so that gaseous air is transferred to the dissolved substantially innocuous condition.

In a special embodiment, schematically illustrated in Figure 3, there is a head box 37 for a paper machine fed by a pluralty, in this case three, of different flows of diluted fibre suspension. All the flows are diluted with white water from the paper machine, this water first passing through the deaerating unit 25. A particular advantage is thereby achieved in relation to previously known technology, i.e. only a single deaerating unit is needed instead of a unit for each suspensio flow.

It will also be noted from the figure that each of the screens 33, 33' ,33" with the respective associated hydrocyc¬ lone separator 4, 4¹ 4" replaces the combination illustrated in Figure 2, of the series-coupled screens 3, 3¹ and hydro¬ cyclone separators 4, 4* 4", the latter being coupled in series and with a pump (14,15) between each hydrocyclone separator. It should also be contemplated that in the descrip¬ tion and the annexed claims hydrocyclone separator^'also and preferably designates a hydrocyclone separator unit having several hydrocyclone separators coupled in parallel.

OMFI

Claims

1. A method of eliminating air present in gaseous form in a fibre suspension taken to a paper machine (8), a suspension coming from a fibre source (1) being diluted with white water coming from a collection means (10) incorporated in the paper machine (8), c h a r a c t e r i z e d in that the white water is taken through a deaerating means (25) wherein air is removed from the white water, said air being in gaseous form as well as being a substantial portion of that present in dissolved form, subsequent to which the thus-treated white ^• water is mixed with the unaerated suspension coming from the fibre source (1), whereby gaseous air in the latter suspensio is dissolved, after dilution, in the supplied white water which was impoverished of dissolved air.

2. Method as claimed in claim 1, c h a r a c t e r i z e in that the diluted suspension is taken through a cleaning plant containing at least one closed hydrocyclone separator installation (4) operating without air, in which the gaseous air is rapidly dissolved by mechanical agitation.

3. ^' Plant for preparing fibre suspension supplied to a paper machine (8), the presence of gaseous air being substantially eliminated in said fibre suspension, said plant including a fibre suspension source (1), a pump means (2, 6), a deaerating means and a white water collector (10) associated with a paper machine, a flow of water (12) being taken from said collector for dilution of a flow of more concentrated fibre suspension supplied from the fibre suspension source (1), c h a r a c ¬ t e r i z e d in that the deaerating means (25) is coupled into the water flow (12) from the white water collector (10) and is adapted for removing air, not only the gaseous air in the white water but also, at least to a major extent, air which is dissolved in said white water, whereby gaseous air in the supplied, more concentrated fibre suspension is soluble in the water of the diluted suspension.

4. Plant as claimed in claim 3, c h a r a c t e r i z e d in that it includes a cleaning plant coupled into the flow of diluted suspension, said -cleaning plant including.^' at least one hydrocyclone separator installation (4) operating without the admission of air.

5. Plant as claimed in claim 3 or 4, in which the paper machine is fed with at least two fibre suspensions to provide a multilaminated paper or carton, and whereby at least two flows of white water from the paper machine are each coupled for diluting its own fibre suspension, c h a r a c t e r ¬ i z e d in that all of the white water intended for dilution is connected via a common deaerating means (25), and division into separate white water flows for dilution is first arranged after the white water has passed through the deaerating means (25).