US2870009A - Method and apparatus for the separation of liquids from cellulosic pulp - Google Patents

Description

RR, 9 mph NT w W 0 2 Jr 00 C 2 w C n m M o A J P E s E m m X R Jan. 20, v c c METHOD AND APPARATUS FOR THE OF LIQUIDS FROM CELLULOSIC PULP Filed June 8, 1956 ATTORNEYS METHOD AND APPARATUS FOR THE SEPARA- TION F LIQUIDS FRQM CELLULOSIC PULP Johan C. F. C. Richter, Karlstad, Sweden, assignor to Aktiebolaget Kamyr, Karlsta'd, Sweden, a Swedish company 7 Application June 8, 1956, Serial No. 590,218

Claims priority, application Sweden June 15, 1955 1 Claim. (Cl. 927) a separating tank, where a lower pressure is maintained than in the digester. As a consequence of this pressure drop, some of the water and liquid chemicals in the pulp are evaporated in the separating tank and pass out of the upper part thereof to be used as low pressure steam and gas for some heating purpose. The separating tank becomes filled with pulp and a pulp level responsive device controls the discharge of the pulp out of the lower part of the tank. Diluting water is usually added at the outlet of the tank in order to facilitate the dischargeof the pulp and also to lower itsvtemperature, so that the pulp can be fed out into atmospheric pressure without giving off any further considerable amount of steam. This diluting of the pulp makes recovery of the chemicals therein unduly expensive.

An object of the present invention is to improve the above method of withdrawing gases from the pulp by conveying the main part of the gas or steam set free from the pulp in the separating tank to a steaming vessel connected before or upstream from the digestenand feeding a smaller quantity of gas or steam out of the tank together with the pulp and preferably into a secondary separating tank.

Another object of this invention is to provide an apparatus or plant for the continuous digestion of cellulose, comprising a steaming vessel, a-digester and a steam separating tank, which plant is adapted for performing the above-mentioned method and is essentially characterized in that the steam separating tank, which by means of a conduit is connected to the steaming vessel in order to transfer the main part of the steam to said vessel, is pro vided at its bottom with a metering valve construction for transferring the pulp and gases preferably to a secondary separating tank. This valve constructionis designed to discharge the pulp from the main separating tank at a rate faster than it is fed thereinto, also serving to prevent open fluid pressure communication between the two separating tanks, and preferably comprising a rotary valve having individual cells or pockets therein driven at such a speed that the main separating tank always is practically emptied of pulp. By reason of the novel method of operation and design of the outlet valve of the separating tank, the structure of the plant and particularly of the separating tank may be considerably simplified. This is possible inasmuch as devices for automatically maintaining a pulp level in the separating tank can be omitted. Since the pulp in the tank never reaches an appreciable level, plugs or bridges will not be formed at the outlet, and therefore it is not necessary to I United States Patent 0 2,870,009 Patented Jan. 20, 1959 "ice In the drawing, 11 designates a steaming vessel provided with a feeding screw 13 orthe like also operating as a stirrer. Chips of a cellulosic material drop from a feeding conveyor 15,- to be mixed with a suitable digesting liquor from a conduit 17 and acted into one end of the steaming vessel .11 by means of a low-pressure feeding device .19 comprising a rotary cell wheel or are maintained-by suitable means (not shown).

valve 20. The chips and liquor are then steamed in vessel 11 at a'temperature preferably in the range of 110-120" C. by means of low-pressure steam supplied from a source to be later described. From the other end of the steaming vessel, the stuff is charged by means of a high-pressure feeding device 21, which may also be a rotary cell wheel or valve, into the upper end of an upright digester 23 in which a temperature of ISO-200 C. and a pressure sufiiciently high, for example 8 to 10 atmospheres gauge, to prevent boiling of the liquids At the bottom of the digester 23, the digested pulp is discharged into. a conduit 25 in which there is arranged a discharging device 27 comprising two valves 26' and 27' adapted to be opened and closed alternately.

The operation of said discharging device 27 is controlled by a suitable level responsive means 29 disposed,

in the digester whereby a certain pulp level is maintained therein. A rotary scraper 31 may be mounted in the bottom of the digester and serves to facilitate the discharge of the pulp into the conduit 25.

Numeral 33 designates a steam separating tank or blow tank, in a side of which the conduit 25 is substantially tangentially arranged so that a cyclone or swirling.

motion of the pulp is obtained in the tank. The pressure in tank 33 is preferably not very high and may be of the order of 1 atmosphere gauge, and by virtue of the pressure drop of the pulp coming from the digester 23 where the pressure is considerably greater, gases, mostly steam,

are given off from the pulp when it enters the tank 33.-

' It will be noted that the alternately opened and closed valves 26, 27 prevent the tank 33 from being exposed to the pressure in the digester. Connected to the tank 33 inside a screen 35 at the upper end thereof is a conduit 37, by means of which the gas or steam developed in tank 33 is returned to the steaming vessel 11. An additional amount of steam can be supplied to the steaming vessel throughza conduit 39. The steam condenses on the chips in the steaming vessel 11, heating the chips to' pressure is automatically held at a value corresponding to the desired steaming temperature.

The separating tank 33 has a conical or funnel-like bottom which is connected to a discharging apparatus 43.

Said apparatus consists of a stationary shell or casing 45 having an upper inlet opening 47, a lower outlet opening 49, and a rotary metering valve or. plug 51 having cells 3 or pockets 52 therein defined by walls 54. The plug 51 is rotated by means of a suitable prime mover and driving arrangement 56. When the plug or valve 51 rotates, the cells 52 are filled with pulp and steam as they move into registration and alignment with the inlet 47 disposed above said cells. As the cells 52 are respectively filled, rotation of the plug 51 moves them away from the opening 47, and the outer edges of the cell walls 54 engage the inner wall surface of thecasing 45 thereby completely closing oflf or sealing the cells until the latter reach a position aligned with the outlet opening 49 disposed below the respective cells. At this point, the cells empty into the outlet 49 leading to a secondary separating tank 53. The discharging apparatus 43 is of the metering or volumetric type and therefore never enables the inlet 47 and outlet 49 to be in direct fluid pressure communication with each other. Consequently, the steam pressure in the tank 33 can be maintained, even when the tank is empty and there is no pulp being fed therein, be-

cause the valve 51 isolates the pressuresin the tanks 33,

53 from each other.

The metering valve or plug 51 of the discharging apparatus 43 is driven at such a speed that its capacity, i. e., the cell volume times the cell frequency of rotation, preferably exceeds the normal feeding capacity of the discharging device 27 by a suitable value, e. g., about percent. Due to the faster discharge through valve 51, the tank 33 will always be maintained practically empty of pulp. When a small pulp lump is let out of the digester 23 by means of the discharging device 27, the pressure differential between digester 23 and tank 33 drives or forces the pulp through the conduit 25 into the tank 33 where it will travel in a spiral path downwardly along the inner wall thereof while giving off gas and steam due to the lower pressure in tank 33. The pulp thus moves tangentially and in a gradually closer spiral down along the funnel-shaped bottom of the tank 33 and finally flows through the opening 47 into the pockets of the pulp as a consequence of the pressure drop in passing from tank 33 to tank 53. The steam separated in tank 53 may be led oif through a conduit 55' to be used for heating water or any similar heating purpose.

The tank 53 preferably has the general shape of an inverted cone, the downwardly directed point of which is connected to a conventional pulp press 57. The press 57 preferably is of the so-called Davenport type and consists of two fiat conical perforated members 59 attached for rotation to shafts (not shown) which form a small angle with each other and one of which is driven. These members 59 are arranged with their apexes opposite and adjacent to each other, forming between them an annular vertical passage 61 of a varying triangular or trapezoidal transverse cross section. From the tank 53 the pulp drops into said passage, where it is grasped and carried forth by the rotating members 59 moving in the direction indithe valve 51 of the discharging apparatus 43. However,

the pockets will not be completely filled with pulp when they rotate past opening 47 tobe closed or sealed by the casing 45, but they will partly be filled with steam given off in the tank. This occurs because the pulp swirling down the conical bottom of tank 33 is discharged therefrom so rapidly by operation of the valve 51 that the pulp will flow along the inner peripheral wall of opening 47 and never block or completely fill said opening, whereby part of the steam liberated from the pulp in the tank 33 will flow through said opening into the pockets 52 of the valve 51 along with the pulp. When the discharging device 27 operates to alternately open and close valves 26', 27 at high frequencies, the successively released pulp lumps or masses will combine in the conduit 25 downstream from the valve 27' to form a substantially continuous stream or flow of pulp which is conveyed to tank 33. However, under virtually all conditions, only a small quantity of pulp will be present in the tank 33 and this pulp will have a swirling or spiral movement therein and no pulp will accumulate in the region of the inlet opening 47 of the discharging apparatus 43. Therefore, no scrapers or similar means need be provided at the inlet 47, nor is it necessary to supply diluting water to the pulp to facilitate its egress from tank 33. Additionally, as no pulp level is formed in the separating tank, there is no need for a level responsive device to maintain a desired level in the tank. Hence, the separating tank 33 can be very simple in construction and need not be equipped with any of these additional and expensive parts which are commonly used in prior art devices.

The outlet opening 49 of the discharging apparatus 43 is-connected to the upper end of a secondary separating tank 53 which is held under substantially atmospheric pressure. The low-pressure steam discharged by the apparatus 43 will further expand in tank 53, an additional amount of steam will be accordingly liberated from cated by the arrow on the drawing. The passage 61 gradually narrows due to the angular inclination of the shafts of members 59 and it reaches a minimum size at the lower portion of the press, after which the passage again widens as it approaches an outlet 63 located on the opposite side of the press from the inlet thereto. The pulp is squeezed between the members 59, as the passage 61 gradually decreases, by a pressure directed at right angles to the direction of feed, without any rubbing or relative longitudinal sliding movement between the pulp and the rotating members 59. The liquor and what little water remains in the pulp is pressed or squeezed out of the pulp at the bottom of passage 61 in the press 57, passing through the perforations (not shown) in the members 59. These liquids are collected in the lower part of the press 57 to be conveyed through a conduit 64 to a recovery plant (not shown). When the pulp leaves the press 57, it may have a dry content of close to 40 percent, which indicates that a large portion of the liquor content of the digested pulp has been pressed off. Inasmuch as substantially all of the water in the pulp is removed in the separating tanks 33, 53 and no diluting water is added, the expense of adding heat or utilizing other procedures. during the recovery process to remove the water from the liquor is greatly reduced. Consequently, a more economical recovery of the chemicals involved may be effected. I

On their upwardly moving side, the passage between press members 59 widens, as stated hereinabove, whereby the pulp discharges generally tangentially through the outlet 63. Pulp that may tend to adhere or stick to the discs is removed by means of a scraper or the like (not shown) arranged in the upper part of the press.

From the press 57, the pulp is transferred to a suction filter 65 where the remaining digesting liquor is Washed off. For this purpose the pulp from the outlet 63 is first diluted to a consistency suitable for the operation of the filter. This may be done by pumping a liquid from the filter 65 through the conduit 67 to a mixer 69 located downstream from the outlet 63 of the press. The diluted pulp then flows into a bin 71, in which the consistency is made uniform before a pump 73 feeds the pulp to the filter 65 where it will be washed in the usual manner and then removed by any suitable means. The last filtrate formed in the filtering process preferably is collected in a vessel 75 and used as spray water in the first spray tubes indicated generally at 66 of the filter 65, whereas the first filtrate, which contains digesting liquor, is collected in the vessel 77. A part thereof is pumped through the conduit 67 and is used for dilution purposes, as described above, while the remainder is pumped through the conduit 79 to a recovery plant.

The present invention will thus be seen to efiectively and completely carry out the objects enumerated hereinabove. However, the apparatus or plant described herein A or any dewatering press or by a refiner. In certain in- 'stallations, it may be desirable but is not preferable for reasons explained above to supply diluting water in the secondary separating tank 53 in order to get a lower consistency and sometimes also a lower temperature. This applies particularly where there is no press but the separation of the digesting liquor from the cellulosic material takes place exclusively by washing on a filter. Additionally, the secondary tank 53 may be completely omitted, although this is not preferred, and the pulp discharged from the tank 33 will then be directly-conveyed from the outlet 49 through a suitable conventional conduit not shown, to the press 57. If the latter press is not employed, the conduit can be directly connected to the bin 71 for subsequent introduction into filter 65 as described. Therefore, this invention includes all modifications and embodiments encompassed within the scope of the following claim.

I claim:

when the latter is in the separating tank; conveying sub- A method of separating liquids from pulp formed out 20 of cellulosic material which is continuously transferred from a preheating vessel to a pressure digester under a relstantially all of the gases produced by said evaporation back to the preheating vessel; mechanically discharging the pulp from the separating tank to a receiving zone so as to maintain no appreciable pulp level within the separating tank; and the mechanical discharge of pulp from said separating tank being eifected independently of the pressure in the separating tank and the pressure in the receiving zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,991,244 De La Roza Feb. 12, 1935 2,011,799 De La Roza Aug. 20, 1935 2,534,324 Hildebrandt Dec. 19, 1950 FOREIGN PATENTS I 137,071 Sweden Aug. 26, 1952 175,783 Austria Aug. 10, 1953

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