US3441475A - Continuous pulping apparatus - Google Patents

Description

April 1969 c. 1.. TOMLINSON 3,441,475

CONTINUOUS PULPING APPARATUS Filed July 9, 1965 INVENTOR Charles Lester TOMLINSON am! 3; i

United States Patent US. Cl. 162-237 1 Claim ABSTRACT OF THE DISCLOSURE The present invention relates to an improved continuous pulping apparatus in which an impregnation zone is created by providing three circulating paths to said zone, the first of said paths taking liquor from the top of the zone and circulating same through an injection valve and back to the top of the zone, the second path taking liquor from the bottom of said zone and returning it at the top of said zone, and the third path taking liquor peripherally from the bottom of said zone and returning it axially at the bottom of said zone to flow substantially radially across said apparatus.

This invention relates to the making of pulp from cellulosic raw material, such as wood chips or the like. It relates more particularly to pulping in a continuous digester wherein the wood chips are made to flow continuously from an inlet point in the digester to a discharge point wherefrom a pulp is discharged.

Such continuous digesters are now well known in the art. However, their use has not hitherto been adapted to the most modern pulping techniques, eg those involving impregnation of the cellulosic raw material with the requisite amount of cooking chemical in an impregnation zone which is separate and distinct from the cooking zone. The present invention provides a system whereby a separate impregnation zone is created in a conventional digester said system permitting impregnation with a liquor of low concentration (less than about 50 grams per litre eflective Na O) at a high temperature (up to about 150 C.) and using a high liquor recirculation rate such that 1 lb. of wood is contacted with at least lbs. of liquor.

Thus the main object of the present invention is to provide a digester having means to circulate liquor through an impregnation zone and to curtain off said zone from the remainder of the digester thereby to permit operation using modern pulping techniques in modified conventional continuous pulping equipment.

Further features, objects and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.

The figure is a schematic illustration of the present pulping system.

In the illustrated embodiment chips are fed to a presteamer 10 through a rotary valve 8 and then from the pre-steamer via rotary valve 12 to the injector valve 14. These chips are contacted in the valve 14 with liquor at relatively high temperatures, say about 100l50 C. from line 16 and forced into the digester 18 through line 20. Liquor is circulated from the top of the digester via line 22, pump 24, line 16 and valve 14 to carry the chips along line 20 into the top of the digester. Circulated liquor in line 22 consists of liquor drawn from the top of the digester plus liquor from line 26 as will be explained hereinbelow. White liquor is generally added to the liquor in line 22 by pipe 33.

The valve 14 is an injector valve of the type used with digesters known in the art. As shown, valve 14 is separated from the pre-steamer by additional valve 2, and steam is added above valve 14, through line 13. In this 3,441,475 Patented Apr. 29, 1969 ice manner, the chips entering through valve 14 may be contacted with high temperature liquor without the usual bumping or blow-backs caused by the flashing of the high temperature liquor; moreover, the chips would be heated partially by the steam entering at 13.

Chips or other cellulosic raw material enter the presteamer where air is replaced with steam and then the chips pass via valve 12 into injector valve 14. As above described the valve 12 permits contacting of the chips in the valve 14 with high temperature liquor (i.e. about -150 C.) without incurring the problem of bumping or blow back. Thus the chips are contacted with impregnation liquor at about 100'150 C. in the valve 14 and the chips and liquor flow in cocurrent through line 20. A portion of the liquor in line 20 flows through the impregnation zone and the remainder is drawn ofi via line 22. That portion of the liquor flowing through the impregnation zone that is not sorbed by the chips is drawn off at the bottom of the impregnation zone through strainer 28.

The liquor drawn from the impregnation zone via strainer 28 is pumped through heat exchanger 30 or 50 depending on the moisture content of the chips as will be described herein below back to the impregnation zone via lines 26 and 34, as will be described in detail below.

The line 26 may be connected directly with the top of the digester or preferably as shown to the line 22. The amount of liquor fiow in line 26 determines the liquor recirculation rate in the main part of the impregnation zone since an amount of liquor equal to the flow in line 26 must flow through the impregnation zone in the diges'ter in cocurrent with the chips. Preferably at least about 10 lbs. of liquor per lb. of bone dry wood flow through the impregnation zone. This use of a high liquor recirculation during impregnation permits contacting of the wood with a low concentration liquor (less than about 50 grams per litre effective Na O) at a high temperature C.) with a minimum change in concentration as the liquor flows through the impregnation zone.

The liquor circulated from strainer 28 via line 34 and pipe 36 forms a radial curtain of impregnation liquor between the end of pipe 36 and the strainer 28. This radial flow of liquors provides a curtain of suflicient flow to substantially abruptly end the impregnation zone whereby substantially the only liquor passing down through the curtain with the chips is the liquor absorbed on the chips. The amount of liquor flowing through the pipe 36 thus depends among other things on the total material flowing through the impregnation zone.

The flow through the digester, and in particular through the impregnation zone, is co-ordinated so that the chips leaving the impregnation zone carry substantially only the impregnating liquor sorbed thereon. Slight amounts of excess liquor may be allowed to pass with the chips if desired; however, it is preferred to keep this flow of excess liquor to a minimum since it will be wasted. Thus, the chips leaving the impregnation zone will usually carry liquid in a ratio of about 1.6-2.4 parts to l, of liquor to wood, based on the bone dry weight of the wood.

In the above-described system, the proper ratio of chemical to wood is maintained by varying the make up chemical added, in accordance with the total amount of liquid entering the system where the moisture content of the raw material is low, generally, however, the make up liquor flow is constant and liquor from strainer 28 bypasses heat exchange 30 and is conducted to heat exchanger 50 and subsequently flashed in tank 52 which is maintained at a constant pressure by a valve in line 54. Process steam is then available from line 54. The heat exchanger 50 is controlled in accordance with the liquid entering the system (i.e. with the raw material and the cooking chemical) to cause a variable amount of steam to flash in heat exchanger 52 and thereby maintain the average liquor concentration in the impregnation zone substantially constant. The liquor from the tank 52 is pumped via line 56 to the line 32 where it proceeds as above-described.

After the chips have been impregnated, they pass into the next zone which extends below the level of strainer 28 and down to, approximately, the level of strainer 41. The temperature in this zone is maintained by circulation of liquor, radially of the digester from pipe 40 to strainer 41. This liquor is drawn ofi through strainer 41 and circulated through line 42, pump 44, heat exchanger 46 and line 48 back to pipe 40. The chips passing through this zone at a speed of about 4"10" per minute, are heated to the required digestion temperature, the height of the zone, the temperature of the liquor and other variables being so co-ordinated that the chips reach the required temperature in the time necessary for them to travel from the upper to the lower boundary of this zone defined respectively by approximately the level of strainer 28 and approximately the level of strainer 41.

With the above-described arrangement, chips may be impregnated with the desired amount of chemical at a high temperature and then rapidly brought to digestion temperature.

The present invention has been described in relation to a down flow digestion, but it is evident that such a system may be used equally well with suitable modification in an up-fiow digester.

I claim:

1. An apparatus for continuous digestion of cellulosic material comprising a vertical digester having an impregnation zone and a digestion zone, means for supplying said cellulosic material to said digester including an injector valve, means defining a pair of circulating paths connected to said digester in said impregnation zone and means defining a recirculation path to said digester, one

4 of said pair of circulating paths including means for conducting liquor from said impregnation zone to said injector valve, said one of said pair of circulating paths feeding said liquor together with said cellulosic material from said injector valve to said digester adjacent one end of said impregnation zone, the other of said pair of circulating paths including means for withdrawing liquor from said digester adjacent the other end of said impregnation zone and means for conducting the withdrawn liquor to said one end of said impregnation zone whereby an amount of liquor equal to the amount of said withdrawn liquor is provided to flow with said cellulosic material through said impregnation zone, said recirculation path having a peripheral strainer in said digester between said other end of said impregnation zone and said digestion zone, means defining an axial outlet radially spaced from said strainer and approximately at the same level as said strainer and means for conducting liquor through said strainer and returning same to said digester via said axial outlet whereby a radial flow of liquor forms a radial curtain between said axial outlet and said peripheral strainer.

References Cited UNITED STATES PATENTS 2,695,232 11/1954 Richter l62-237 3,180,789 4/ 1965 Fuchiwaki 162237 3,200,032 8/1965 Richter et al 162-237 X 3,298,899 1/1967 Laakso 162237 X FOREIGN PATENTS 632,314 12/1961 Canada.

S. LEON BASHORE, Primary Examiner.

US. Cl. X.R. 162l9

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