US2140218A - Cellulose cooking process - Google Patents

Description

Dec. 13,l 193s. Q BUNE 2,140,218

OELLULOSE COOKING PROCESS Filed Aug. 19 1936 www:

WML-z5@ Qzfmf/Ow Patented Dec. 13, 1938 2,140,218 cELLULosE cooKlNG PROCESS Otto Brune, Wangen/Allgau,

to Zellstoiffabrik Waldhof,

Germany, assigner Mannheim-Waldhof, Germany, a German company l Application August 19 1936, Serial No. 96,881

In Germany October 1, 1934 1 Claim.

There are a number of processes, the object of l which is to improve the circulation of the liquor in cellulose digesters. In these known processes, the cooking liquid is caused to circulate by means of a pump or by means of an injector with various arrangements of the suction and pressure conduits, while the heating of the cooked material is effected either indirectly by means of heaters or directly by addition of hot steam in the 1o circulation conduit or in the digester itself.

Indirect heating it is true aords the advantage that the cooking liquor is not diluted by the 'condensate of the steam, thereby obviating, in the sulte process, a detrimental reduction of the 1.3 very important lime content of the cooking liquor, and in addition it is possible to cook with steam which has a lower pressure than that temporarily prevailing in the digester. On the other hand, these processes employing indirect heaters l are accompanied by considerable disadvantages. Unless in fact very large quantities of liquid are passed through the heating pipes at a high speed, the pipes very readily become encrusted. Furthermore, in accordance with the varying 15 drop in temperature, the absorption of cooking steam at the commencement of the cooking operation is very great, but is very small towards the end thereof. In addition, the installation and operation of these processes are very expensive :lo and therefore in many cases uneconomical on account of the large dimensions of the acid-proof pumps, pipe lines and heating apparatus.

The processes employing direct heating of the circulated liquorhave heretofore been accom- 3.3 panied by the disadvantage that it has only been possible to employ steam possessing a pressure greater than the digester pressure. Low-pressure steam which is generally available in large quantities in cellulose factories and 'for which 4U there is often no possible use available, could not be employed by the known processes. Further disadvantages of this process employing direct heating are the danger of encrustation ,of the pipe lines and the contamination of the cooked masiderably uctuating steam consumption.

It has now been found that, while obviating thesedisadvantages, it is possible to supply heat directly to the circulated liquor by `employing a ,50 jet apparatus built into the pressure conduit of the pump for the purpose of supplyingfsteam and by operating so that steam supplied to the jet apparatus disposed immediately behind the pump has a pressure which is lower than the pres- 55 sure prevailing in the digester.

. .i5 terial with calcium sulphate, as well as the con- According to the new process, it is possible te employ low-pressure steam with a very cheap installation and economical operation, whereby in the back-pressureor tapped power installation, the power for the circulation pump is obtained 5 more or less free of charge. .It is furthermore possible to make thesteam consumption practically uniform through the entire heating period and thus to reduce the load on the steam and power plant and to simplify said plant. In addition, in the sulflte cooking process it is possible to employ liquors or" a high lime content which means a higher pulp yield and in addition compensates for the dilution by condensate. In addition to a lower coal consumption and smaller l5 production of waste substances, the very uniform hydrolysis of the cooked material and the improved quality and better colour are further advantages of the process.

It is well known that the arrangement of the I strainer is also essential for a good'circulation of the cooking liquor. In the new process, the suction strainer in the lowest part of the digester may be so arranged and constructed that even the lowermost parts of the .cooked material are wholly within the range of the circulation, thereby rendering it unnecessary to draw off or resupply the liquor at several points.

In the case of the direct heating of the circulated liquor, it has indeed already been proposed. 80 to connect the steam supply conduit directly to the liquor circulating system, namely' to the pressure conduit. In this case, however, the connection was effected without the medium of a jet apparatus, so that contrary to the present invention, the pressure ofthe steam to be supplied had necessarily to be not only, as usual, a definite amount above the digester pressure, but the pressure had also to be increased beyond that in accordance with the lift of the pump.

In other cases, steam has also been supplied by means of two jet apparatuses. In this case, however, it has not been the intention to employ lowpressure steam, since the arrangement of the jet apparatuses in the total apparatus only permits 456 of functioning by a method employing a pressure above the digester pressure, since the fact that, with this process, two jet apparatuses, situated in very diierent' pressure ranges, are allocated to one pump makes it impossible to use low-pressureA steam. One of the injectors, being situated in a considerably lower pressure range than the other, would in fact receive much more liquid from the pump than the other. InV order to equalize this difference, it would be necessary to throttle the .'rst injector at the inlet or outlet connection, in

prevailing in the digester.

which case however it would work snasmodically andl would not draw in any more steam. The two injectors therefore can only function as heating apparatus if they receive steam at a higher pressure than the digester pressure.

Finally,v attempts have already been made to introduce steam into the circulation pump conduit by a combination of several injectors. For

theoretical reasons, such a device would only be able to work at all if the total pressure of the injector system were higher than the pressure In practice, however, such a complicated system would scarcely ever function.

The process according to the invention may be carried out for example by means of the device shown in the accompanying drawing.

Thel cooking liquor is drawn oi from the digester l through the suction strainer 2, arranged at the lowest point thereof, by means of the centrifugal pump 3 situated outside the digester, and is re-introduced through the jet apparatus 4 into the upper part of the digester. 'I'he pressure pipe 5 may also run partly inside the digester (as.

pipe 6) as is shownl in dotted lines. Connected to the suction flange of the jet apparatus 4 is the steam pipe I in which a regulator 8 and ashutoff check member 9 are interposed. Preferably,

the pipe lines for the spent liquor, wash water,

fresh liquor'and fresh water are connected to the suction conduit of the pump, so that there is no necessity to provide further connecting flanges on the digester, and the pump may also be em- -ployed for filling theV digester with liquor and for riod, and the check member shuts -oi'f automatically-when the pump 3 is to stand at the digester pressure exceeding the steam pressure.

'I'he capacity of the pump may be small by reason of the direct heating and of the physical factors occurring in the circulation from above downwardly, for example it may be about 3 cubic in the digester and in the steam pipe. Normally,

it will be approximately 20 meters. vUnder 4these conditions, there is a power consumption of vabout 20 H. P. whereas for the same digester with in-v direct heating, a capacity of 10 cubic meters and a lift of 10 meters, approximately 35 H. P. must be employed.

Thesteam pressure in the pipe is 4 atm, in the new process according to the example selected. In the case of heating by means of indirect heaters it can scarcely beless than this on account of the heat content of the steam.

'I'he quantity of steam to be added in the case of the ten ton digester duringl the entire heating period lasting for example 7 hours until the attainment of the maximum temperature will amount to about 3000 kilograms. After this period of '7 h0urs,ythe steam will be turned off and then cooking will be completed only with continued circulation by pumping. The duration and the intensity of the heating period also partly depend upon the number of digesters available, the aim being to keep the total. consumption of steam always as uniform as possible. In the case of indirect heating about 8000 kilograms of steam per hour are employed at the commencement of cooking. This consumption falls gradually to about 1000 kilograms or less per hour towards the end of cooking,

The increase in temperature of the cookingy liquor in the jet apparatus is very slight when the steam is added in the manner described. Since in addition, the liquor at the point at which 'the steam is added has a very high velocity, in-

crustation or deposition of calcium sulphate cannot occur anywhere in the entire system, so that the lime content of the cooking liquor may be made higher from the commencement in order to allow'for the dilution by condensate.

I claim: y A process of cooking cellulose in which the cooking liquor is circulated through a digester and conduit, comprising the steps of withdraw-- ing thecooking liquor from the lower part of the digester by means of a pump, passing said liquor through a jet apparatus arranged adjacent the =outlet side vof said pump, supplying steam to the suction'side of said jet'apparatus, at a pressure lower than that prevailing in the digester, for

heating said liquor, and leading the heated liquor into the upper part of the digester. OITO BRUNE.

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