US1914073A - Method for the digestion of materials containing cellulose - Google Patents

Description

June 13, 1933. A. BROBECK 1,914,073

METHOD FOR THE DIGESTION OF MATERIALS CONTAINING CELLULOSE Filed Feb. 3, 1932 3 Sheets-Sheet 1 B7: MW

A. BROBECK 1,914,073

METHOD FOR THE DIGESTION OF MATERIALS CONTAINING CELLULOSE June 13, 1933.

Fild Feb. :5, 1952 3 Sheets-Sheet 2 Bmbeck veN 1W8 June 13, 1933, A. BROBEQK N 4 73 METHOD FOR THE DIGESTION OF MATERIALS CONTAINING CELLULOSE Filed Feb. 5,-1932 3 Sheets-Sheet 3 H- Ember/R Patented June 13, 1933 UNITED STATES PATENT OFFICE ARNOLD IBBOBEOK, OF 38m GAVLE, SWEDEN mirnon ron THE monsrrou or IA'IEBIALS oon'rnn'me cnnLULo'sa Application filed February 8, 1932, Serial Elo. 590,871, and in Sweden February 6, 1881.

The inventor has previously invented improved methods of producing cellulose by means of direct or indirect heating of the digesting liquor. by the circulation method.

As a result of these earlier inventions the important problem of the prevention of the formation of scale or incrustations on the heating surfaces has been satisfactorily solved, since the said earlier inventions have proved in practice that it is possible almost entirely to avoid the formation of incrusta tions by passing the liquor through the heat ing apparatus with a relatively high speed along the heatin surfaces. By using polished heating sur aces the speed of the liquor can to some extent be reduced.

However, further investigations have shown that considerable improvements in the method of digesting may also be arrived at by improving the conditions in the digester itself. Owing to the fact that little attention has hitherto been given to these conditions the pulp produced by the methods known up to the present has been uneven in several respects. This is primarily to be ascribed to the circumstance that the conditions of temperature in the digester. have hitherto been unfavourable as a result of which the degree of di estion of the ulp has been different in di erent parts 0% the digester.

The investigations of the inventor have proved that the conditions which are required for the production of as uniform 'a pulp as possible'are firstly that the temperature within each horizontal zone of the digester is practically uniform, and, secondly, that the average temperature throughout-the digestion is highest at the top of the boiler and falls successively towards the base.

The investigations of the inventor have further proved that the conditions necessary for arriving at these results are, firstly, that the suction of thecirculating liquor from the digester is carried out uniformly either round the entire inner circumference of the digester or from points on the same which are equldistant from each other and, secondly, that the circulation of the liquor through the "digester, especially during the actual process f of digestion, takes place either uniformly upward from and down towards the strainer through which the liquor is removed or, if the strainer is located at some distance froni the bottom, that circulation is effected both from above downwards and from below up: wards towards the strainer, the temperature of the li uor which is introduced below the strainer eing kept lower than the temperature of that part of the liquor which is introduced higher up, the object of this being that suitable conditionsof temperature in the digester should he arrived at.

It is also possible by means of the invention described above to make improvements when using systems of circulation in which regard has not been paid to the desirability of maintaining practically uniform temperatures in the various horizontal zones.

The annexed drawings show two examples of a form of construction carrying out the invention described above.

Fig. 1 shows the digester with its preheater and pipelines, constructed according to the present invention.

Figs. 2, '3, 4 and 5 show details of the strainer on a larger scale. Fig. 6 shows a modified form of construction of the strainer and Figs. 7 and 8 show details.

' The digester 21 is fitted at the lower end of its cylindrical part with r a strainer 22, through which the liquor is sucked by the pump 24 through the pipe 23 and is passed through the pipe 25, which goes out from the pressure side of the pump, to-the preheater 26, where it is preferably heated indirectly. From thelatter, reheated liquoris carried by the pipe 27 into the upper end of the digester.

Hitherto, the liquor which has eased the reheater, 26 has been introduce into the ower end of the digester through the pipe indicated by a dotted line, as a result of which liquor of the same temperature has been introduced into both the upper and the lower end with the disadvantages described above.

According to the present invention that part of the liquor which is to be introduced into the lower end of the digester is now taken from the pipe 25 on the pressure side of the pump through pipe 28 and is forced through the latter into the lower end of the digester without having passed through the preheater 26. By leaving the pipes 23 and 28 Without insulation the liquor is cooled to some extend while it passes through them, so that it is introduced into the lower end of the digester with a somewhat lower temperature than it has when it leaves the strainer.

As has been mentioned above the form of construction of the strainer through which the liquor is sucked out is of importance since it must be constructed in such a manner that the liquor is sucked out as uniformly as possible all the wayround the inner circumference of the digester.

The efi'ective straining area must be many times greater than the area of the out-suck pipe in order that a sufficient quantity of liqnor can be sucked out through the straining area when the fibres have been set free when digestion has taken place for some time. WVith the forms of strainer which have been usedup to the present this relatively large effective straining area has had as a result that at the beginning of the digestion, before the woodchips have been broken up, suction has only occurred through the parts of the strainer nearest the out-suck'pipe and it is only when the fibres have been successively set free that the suction has gradually extended to the entire straining area. V

By using the form of strainer which is the subject of this invention these disadvantages are eliminated by means of dividing the strainer aggregate into a primary strainer provided with a large number of small holes through which the liquor is sucked, and in con]unction with the primary strainer and working together with it, a secondary strainer through which the liquor is sucked into an outlet channel, the secondary strainer having a smaller number of comparatively large openings, the combined area of which bears a certain proportion to the area of cross-section of the out-suck pipe. This proportion should be so adapted with regard to the existing resistance to the flow that the liquor is sucked out uniformly over the whole length of the strainer. In passing, it may be explained that if the area of the cross-section of the out-suck pipe be represented by 1,

then the combined area of the above-mentioned openings should amount to between 1 and 2. This means therefore that the total area of the holes in question must not be less than the area of the out-suck pipe and should not be more than double the area of the outsuck pipe.

A suitable form of construction of the strainer is shown in the annexed drawings in Figs. 2-5, of which Fig. 2 is a horizontal section of part of a pulp digester with its strainer. Fig. 3 shows on a larger scale a cross-section through the strainer along the line AB in Fig. 2. Fig. 4 shows a longitudinal section along the line CD in Fig. 3. Fig. 5 shows the same in perspective.

The steel-plate mantle of the digester is indicated by 1 and the lining by 2. The strainer which forms the object of the present invention is placed round the inside of the digester lining and at the lower part thereof. The strainer consists of two parts X and Y in order to permit of expansion and contraction, the two parts joining on to the out-suck pipe Z. The strainer consists of two straining elements, an outer one 3, which is provided with a large number of small holes, and an inner one 4, which is provided with a small number of larger openings 5. According to the drawings the strainer elements are built up from a number of strainer plates provided with angle flanges, which run across the strainer and are riveted to each other. The flanges on the plates in the upper strainer element are indicated by 6 and 7 and those in the lower strainer element by 8 and 9. A plate 10 is attached to the digester lining and forms the bottom of the channel 11 in the ring-shaped strainer. At one point in its length the channel joins on to the out-suck pipe Z.

The flanges 8 and 9 on the plates forming the inner strainer element support the lower side of the outer strainer element 3 and divide the space between the strainer elements into a number of separate spaces. The flanges serve further to give the strainer elements stability. The flanges 6, 7 8, are however not essential and the plates may overlap each other and be riveted together or they may be placed edge to edge without fastening.

When the circulating pump is working the liquor is first sucked in through the openings in the outer strainer element and enters the spaces 13, from which it proceeds through the openings 5 into the channel 11 and from there to the out-suck pipe. It is a suitable arrangement to make the openings 5 of increasing size the farther they lie from the out-suck pipe, so that the largest openings are farthest from the out-suck pipe. In this manner the quantity of liquor sucked out through each of the openings per unit of time will be practically the same in spite. of the varying pressures. By correctly adapting the areas of the openings 5 in the inner strainer element with relation to each other and the combined areas thereof with relation to the area of'cross-section of the out-suck With the form of construction just described the sucking out of the liquor takes place uniformly along the entire inner circumference of the digester. As has been indicated above, however, the sucking out need not necessarily take place along the entire circumference but may also take place at a number of points thereon which are equidistant or practically equidistant from each other.

Fig. 6 shows arr example of a strainer in which the sucking out takes place in the manner last described. The fi ure shows a horizontal section of a pulp digester taken just above the strainer. The plate mantle of the digester is indicated here also by 1 and the lining by 2. A pipe 30 is fitted round the inner circumference of the digester, the ends of the pipe being at ca little distance from each other for purposes of expansion and contraction. A number of perforated suction pipes 31 are joined at regular intervals to the pipe 30 on both sides of the out-suck pipe 32 which leads from the pipe 30. The pipes 31 extend down wards towards the bottom 33 of the digester along the tapering lower part of the same. The pipes 31 may either be all of the same length or they may be alternately longer or shorter as is "shown by dotted lines.

Fig. 7 shows a vertical section of a part of the digester, illustrating how the pipes 31 are fitted. Immediately below the junction between each of the pipes 31 and the pipe 30 there is a flange 35 in which is fitted a disc 34. In this disc there is an opening 36 corresponding to the openings 5 shown in Figs. 3 and 4. What has been said above regarding the size of the openings 5 applies also to the openings 36, i. e. that the total area of the openings should be in the proportion [;:l to the areaof the out-suck pipe 32. In this second form of construction the discs 34 thus form the above mentioned secondary straining elements.

Since the pipes 31 suck out the liquor from the whole of the lower part of the digester, it is not necessary with this form of construction tointroduce liquor into the part of;

the digester below the straineras was indicated in Fig. 1 when a strainer according to Figs. 25 was used.

I lit is evident that the invention forming the object of this applicat'on is not restricted to the form of embodiment shown in the drawings but thatthe elements may be modified in several ways without departing from the principle of the invention.

I claim: o 1. Method of digesting materials contaning cellulose with positive circulation of the liquor through the'digester and through a heating apparatus and by sucking out the liquor through a strainer, located in the primary strainer element provided with a H great number of relatively small perforations strainer elements provided with a small number of relatively large openings distributed at equal distances from one another, said openings communicating with a common chitnnel, and an out-suck tube from said channe on its surface and one or several secondary 3. Strainer for effecting a uniform suck- 3 ing out of digesting liquor from cellulose digesters, comprising a primary strainer element provided with a greater number of relatively small perforations, a secondary strainer element provided with a small number of relatively large openings distributed at equal distances from one another, said openings communicating with a common channel, and an out-suck tube from said channel, the relation between the sum of the areas of said openings and the area ofthe cross section of the outlet tube being 1.

4. Strainer for effecting a uniform sucking out of liquor from cellulose digesters, throu h which the liquor is circulated, consisting o a ringshaped strainer body adapted to be fitted round the inner circumference of the cellulose digester, said strainer body comprising in combination a primary strainer element having the form of a curved plate body with a great number of small perforations and a secondary strainer element consisting of a curved plate body with a relatively small number of equidistant larger openings, and located at. some distance from the primary strainer element, a bottom plate so as to form a channel between said secondary strainer element and said bottom, an out-suck pipe communicating with said channel, a pump in said out-suck pipe, the relation between the sum of the areas of said openings in the secondary strainer element and the area of cross section of the out-suck pipe being as 1.

5. Strainer as claimed in claim 4 characterized by the fact thatthe space between the primary and secondary strainer elements is divided into a number of compartments of substantially the same volume, each compartment communicating with the channel throu h one opening.

6. traineraccording to claim 4, characterized by the fact that the openings in the secondary strainer elements are the larger the farther they are situated from the out-suck pipe.

In testimony whereof I have hereunto affixed my signature.

ARNOLD BROBEGK.

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