US2745712A - Process for countercurrent washing of cooking liquor out of pulp - Google Patents

Description

May 15, 1 56 E. R. BURLING EFAL PROCESS FOR COUNTERCURRENT WASHING OF COOKING LIQUOR OUT OF PULP 4 Sheets-Sheet 1 Filed Oct. 22, 1953 INVENTORS 4M Q. BURL/N6 3 JOHN f? lQ/CH ATTORNEY May 15, 1956 E. R. BURLING ETAL PROCESS FOR COUNTERCURRENT WASHING OF COOKING LIQUOR OUT OF PULP 4 Sheets-Sheet 2 Filed Oct. 22, 1953 /oRAM/Nou$ SHELL INVENTORS ELMER A. BURL/N6 @7- JOHN P. lQ/CH ATTORNEY May 15. 1956 E. R. BURLING ETAL 2,745,712

PROCESS FOR COUNTERCURRENT WASHING OF cooxmc LIQUOR OUT OF PULP 4 Sheets-Sheet 3 Filed Oct. 22, 1953 LIg.

INVENTORS [ZMER 1?. BURL/N6 c JOHN P. RICH BY gm W TTORNEY May 15, 1956 E. R. BURLING EI'AL 2,745,712

PROCESS FOR COUNTERCURRENT WASHING 0F COOKING LIQUOR OUT OF PULP Filed Oct. 22, 1953 4 Sheets-Sheet 4 INVENTORS ELMER R. BURL/N6 JOHN P. ,Q/cH

TTORNEY United States Patent PROCESS FOR COUNTERCURRENT WASHING F COOKING LIQUOR OUT OF PULP Elmer R. Burling and John P. Rich, Nashua, N. H., as-

siguors to Improved Machinery, Inc, Nashua, N. H., a corporation of Maine Application October 22, 1953, Serial No. 387,694

4 Claims. (Cl. 8156) The present invention relates to a new use of pulp thickeners generally of the type disclosed in Patent Nos. 1,816,132 and 2,537,414, and others of a similar nature.

Filters of the type above mentioned operate under a low vacuum head and the vacuum is internally generated by the rotation of the filter drum. Such filters have been in use for many years but only as thickeners or dewaterers, or as in individual washers.

Within the pulp and paper industry heretofore countercurrent washing of pulp for the removal and recovery of, say, cooking liquors has been done on countercurrent washing principles by numerous methods but most frequently by a series of open cylinders or high vacuum filters. Wash liquor, applied by means of showers, has been drawn through the pulp as it travels around the drum. The liquor drawn through each drum is collected in a tank and used as a shower on the next preceding drum and as repulping liquor to prepare a slurry from the thickened pulp leaving the preceding drum to make it ready for the washing action of the next drum. In these vacuum installations for best operation the level of liquor in the tanks receiving the liquor from each drum must be from to 38 feet below the center line of the drum from which the tank receives the liquor in order that efficient vacuum may be maintained or else vacuum pumps must be used. In this relatively great fall the liquor tends to be thoroughly mixed with entrained air to form a foam. One compensation for this to deaerate the liquor is greatly to increase the s ze of the liquor receiving tank. Moreover since the liquor received in the tank must be pumped again to at least the center line level of the drum, pumping costs are very heavy indeed. This foaming tendency is more pronounced where alkaline cooking liquors are involved since such liquors saponify the fatty acid content of the wood.

The present invention, using the low head internally generated vacuum permits the tanks and their liquor level to be placed a minimum distance below the center line of the drum. There is less agitation of the liquor and consequently less foam so that the tanks, in order to handle the same amount of liquor, need be only about /3 the size of the tanks in conventional high vacuum washing. Moreover the pumping head is very greatly reduced. This latter factor will be appreciated when it is realized that for every 100 tons of pulp, at least 5000 tons of liquor must be pumped for each stage of the washing. Moreover, the internally generated vacuum permits the use of much larger discharge pipes than are tolerable in a high vacuum system. Obviously the use of larger diameter discharge pipes reduce the velocity and therefore the turbulence within the pipes.

It is an object of this invention to provide a process for the countercurrent washing of pulp which may be carried out in far less plant cubic footage than conventionally now used, and/or at greatly reduced capital investment and/ or at greatly reduced operating cost.

. The above and other objects will be made clear from the following detailed description taken in connection with the annexed drawings, in which:

Figure l is a schematic diagram of a four-stage washing installation;

Figure 2 is a plan view showing a somewhat difierent relationship between the washers and the receiving tanks;

Figure 3 is a view similar to Figures 1 and 2 but showing an arrangement for the elimination of repulpers between the washers;

Figure 4 is a radial section through one of the washers showing the means by which rotation of the drum generates the vacuum;

Figure 5 is a longitudinal section through a washer and its associated tank and illustrating control of foaming conditions; and

Figure 5A schematically illustrates the connections between the unit of Figure 5 and a foam breaker system;

Figure 6 is a view similar to Figure 5 but showing a somewhat different relationship between the tank and the washer.

Figure 6A schematically illustrates the connection between the arrangement of Figure 6 and a foam breaker system.

Referring now to Figure 1 there are shown drums A, B,

C and D rotating, respectively, in vats A, B, C and D.

Liquor extracted by drum A passes through a line 2 into a tank A". Liquor from drum B goes through a line 4 to a tank B". Liquor from drum C goes through a line 6 to a tank C" and liquor from drum D goes through a line 8 to a tank D". Pulp from the drum A is removed by a doctor 10 to a repulper i2 whence it goes to the vat B and is drawn onto the drum B. A doctor 14 removes the pulp layer from the drum B and delivers it to a repulper 16 whence it goes to the vat C and is drawn onto the drum C. A doctor 13 removes the pulp layer'from the drum C and delivers it to a repulper 2t) whence it goes to the vat D Where it is drawn onto the drum D. A doctor 22 removes the pulp from the drum D and delivers it to a repulper 24 whence it goes to other processing through a line 26.

A fresh water line 28 has one branch 30 feeding the final repulper 24 and another branch 32 feeding a shower pipe 34 directed against the face of the final drum D. A pump 36 removes liquor from the tank D" and delivers it to therepulper 20 and to a shower 38 directed against the face of the drum C. A pump 40 removes liquor from the tank C" and delivers it to the repulper 16 and to a shower 42 directed against the face of the drum B. A pump 44 removes liquor from the tank B" and delivers it to the repulper 12 and also to a shower 46 directed against the face of the drum A. A line 48 delivers the pulp, in its original strength liquor, to the vat A where it is drawn upon the surface of the. drum A. A pump 50 removes liquor from tank A" and delivers it to the recovery system.

It is to be noted that each of the tanks A", B, C" and D" is directly below its respective washing unit so that in effect the Walls of the tank form the vertical foundation of the washing unit and the washing unit acts substantially as a lid for the tank. This extremely compact relationship is impossible of accomplishment with conventional high vacuum washing. It is clear that the head against which the pumps 36, 40 and 44 must operate is greatly reduced over conventional practice and their power consumption is accordingly equally reduced. Y

In Figure 2 the tanks A", B", C and D" have about the same vertical relationship to the drums A, B, C and D as is shown in Figure 1. Instead of being placed under the drums, however, the tanks are offset to one side. Occasionally this is of advantage Where an installation has to be fitted into an existing mill.

Figure 3 is a schematic representation of a special case. Here an initial drum E corresponding to drum A of Figliquor.

ure rotates ina' vat'E' and'delivers extracted liquor to a tank E whence theliquoris" pumped bymeans (not shown) to the recovery system. A second drum F rotates in a vat F and delivers extracted liquor to a tank P" which is defined in part by an upwardly --extending-baffie 60. Liquor entering the tank-F" 'overflows-the'baffie60 to an auxiliary-tank 64 which is dividedinto two parts; by a downwardly extending bfie 62. A pulp matte 70 is doctored-oifthedrum E andfall'sinto-the left=handcompartment' of the tank 64- where it is 'defibered by an agitator-=58: The dilute defiberedstock' is picked tip-by apump 71- and-is delivered to the-vat F.

playing on the face-of the drum-E5 The-arrangement just described'o'bviously can-herepeatedas' often asrequired to carry out additional stages of washing.

drumrotatesin the direction-of the arrow 112, successive compartments -102 encounter a doctor 114 which delivers pulp-to a repulper 116. The compartments thenenter a vat 118- to which a pulp slurry is fed from a preceding.

repulper 120- or other source'of'supply. Liquor immediately begins to 110W from the vat 118'into each of the compartments 102 thence through the openings 106 to the channels l08 and then through the openings 110 into the interior of the drum. A certain layer of pulp is thereby built up on the-exterior of the drum 100 bythe hydrostatic'pressure of the slurry in the vat 118. Because the effective head is limited to the'rather small differential between the interior andexterior of the drum the initial web-formed onthe drum is low density uncompacted stock. I

As each compartment continues to rotate, there comes a=time when its orifice 106 arises above the level of stock in-the vat118, but since, in this region, the exit orifice1110 of theehannelf108 is stillsuhmerged, a'vacuumis created tending to suck shower Water'from the pipes 104 through the pulp matte. When the channel exit 110. finally rises above the'li'quor' level inside the drum the vacuum'is.

broken and the pulp thereupon is ready to be removed' by the doctor 114.

From the foregoing it will be clear that the only vacuum action or sucking action is occasioned internally by rota tion of the'drum and not by the application of any external source of sub atmospheric pressure such as a vacuum pump or barometric leg This-is the basis of the two above identified patents.

.It'is-most difiicult to-explain precisely 'why'a piece of equipment should remain available to the industry for more thanZO years and yet be put'only to the limited use of dewatering'pulp whereas when the same equipment is applied to countercurrent washing it produces'enormous advantages over the equipmenttheretofore used for such purposes. Probably it was the concensus of opinion in the industry that only a high" vacuum would serve-both to draw. the-initial liquor out of the pulp'and' also'to draw through the pulp the requisite shower-Water, though this must forever remain a-matter of conjecture. What actually appears to be the fact *however is that the low vacuumofthe valveless washer produces'a freer, lessdense formation of pulp on the drum and throughsuch asformation the' low vacuum is entirely adequate to drawtthezw'ash It is true that, due'to the'freer formationon the drum, more air may be drawn through thefvalveless I A pump72-througha I line 74 delivers liquor from the tank F to-ashower 76 washer than is drawn through the conventional high vacuum washer but; sincethe; travelof the commingled' air and liquor is for a shorter distance, and with less intimate mixing than necessarily must be the case with the high vacuum washer, liquor drawn through the valveless washer suffers far less from volumetric increase due to foam and is far easier to deaerate than liquor drawn through the high vacuumzwasher.

Figures SthroughGA, inclusive, deal-with arrangements for still further minimizing from conditions in Washers of the'type the subjectofthis application. Inthis type machine, since the matte formed on the drum is less dense than that formed on the high vacuum washer, considerable air is drawn through. with the liquor to the. interior: of. the drum hence into the receiving tank. In some cases it may not be enough merely to vent the tank since there would have to he at least some above-atmospheric pressure in the system to cause the air to leave the vent and this might cause flow-back at the takeoif point on the drum and'thus re-wet the washed sheet with liquor.

Referringnow to Figure 5 there is shown a drum 200 in a-vat 202. Liquor flows through one end 204 of'the drum into a receiving'tank 206. A pump 208 removes liquor from the tank 206 for re-circulation as'above described. The end of the drum 210 'oppositethe end 204 is completely closed. At theliquid delivery end 204 the drum itself is-completely open but the vat 202;is sealed to the drum by means of a deckle strap 212 and an end closure 214. The vat and tank are ventedby a pipe. 216 feeding a header 218. As shown in Figure 5A the header 218may serve any number of'washers-and communicate through a duct 220 either with a foam tank' 222' surmounted by a suction fan 224 or for the foam tank 222 and a suction -fan224 there may be substituted a'combination suction fan and foam breaker. The result is to establish within each unit a slightlysubatmospheric pressure, in' the region of a few'inches of water, which completely prevents any possible blowback-and serves, moreover,as an excellent means for the extraction of' foam from the system thereby permitting the receiving tank 206' could bemoved by a vacuum into the-foam-breaker but creation: andmaintenance of suflicient vacuum for this purposewonld be uneconomical. There is need for a very slight. vacuum Within the drum in order to evacuate the entrapped air which comes through the pulp matte. 'Ihere-isalso need for 'aslight vacuurnto bias the -flow offoamfrom the tank :toward .the foam-breaker and since: the foamseems to create a pressureof its own, only a bias ing vacuum is essential to direct the flow. To meet-this specialized condition, we propose to apply substantially the same vacuum to the line running :to the foam-breaker and also directlyto thedrum, thedifierential whichserves to bias thefoam'being supplied by thefoams own-independent pressure.

avoided.

The effects described in the above; paragraphilare.

achievedfintheform illustrated in Fig. 5 by the-provision of avacuumlineilS which .penetratesthe end. closure-214- of the drum 200. The line 215 mayrconnecttoan indeependent. vacuum source, or itmay-connect to-the foam tank 222. In either. case, however, hlow-backwithinthe drum .200 is:completely avoided.

vIn Figure 6 the drum.200.and vat 202'with deckle strap 212 and tank end closure 214 inall-respectsidenticalzwiththearrangementtof Figure 5.- Here, however, aductt226 By this means the foam is directed away from. the drum and. toward: thezfoam-breaker whileanyv blow-hack tendency within the drumv is completely delivers the efliuent from the drum 200 horizontally to a tank 228 placed adjacent the washer. A vent 230 is provided in the top of the tank 228 and communicates with a fan and foam breaker 232 as shown in Figure 6A. When the foam breaker is used the liquor recovered from the foam is returned to the system and therefore represents neither a loss nor a threat to the efficiency of the system.

In Figures 6 and 6A a line 231 is shown connecting the end closure 214 to the fan and foam breaker 228. The line 231 serves the same purpose as the line 215 of Figure 5.

What is claimed is:

l. A process of washing residual, highly forming, cooking liquor out of pulp comprising: operating on countercurrent principles a series of horizontal, forarninous cylindrical filters of the type wherein liquor is sucked inwardly through the foraminous surface of the cylinder wall under vacuum generated primarily as a result of rotation of the cylinder; separately collecting the liquor extracted by each filter; returning from each succeeding filter a portion of the liquor collected thereby as a shower on the face of the preceding filter; and using a portion of the remainder to dilute pulp concentrated on the preceding filter, the collection of liquor extracted by each cylinder occurring at a level only a little below the center line of such cylinder.

2. A process as set forth in claim 1 in which the liquor is stored in a reservoir at a' level only a little below the center-line of the cylinder.

3. A process as set forth in claim 1 including the steps of delivering extracted liquor to the interior of the cylinder; draining said liquor from the interior of the cylinder to a collection vat and enforcing the maintenance of a slight subatmospheric pressure within the interior of said cylinder and said vat.

4. A process as set forth in claim 1 in which the interior of each filter and its liquor collecting means are maintained at slight subatmospheric pressure.

References Cited in the file of this patent UNITED STATES PATENTS 1,042,724 Spangenberg Oct. 29, 1912 1,683,782 Johnson Sept. 11, 1928 1,816,132 Strindlund July 28, 1931 1,933,609 Wagner Nov. 7, 1933 2,077,059 Snyder Apr. 13, 1937 FOREIGN PATENTS 233,316 Germany of 1911

Claims (1)

1. A PROCESS OF WASHING RESIDUAL, HIGHLY FORMING, COOKING LIQUOR OUT OF PULP COMPRISING: OPERATING ON COUNTERCURRENT PRINCIPLES A SERIES OF HORIZONTAL, FORAMINOUS CYLIN-

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