US1784849A - Pulpmaking and apparatus therefor - Google Patents

Description

Dec. 16, 1930. G. A. RICHTER PULP MAKING AND AP ARATUS THEREFOR Filed Sept. 18, 1928 Patented Dec. 16, 1930 UNITED STATES GEORGE A. RICHTER, OF BERLIN,

NEW HAMPSHIRE, ASSIGINOR TO BROWN COMPANY,

PATENT OFFICE PULPMAKTNG AND APPARATUS THEREFOR Application filed September 18, 1928. Serial No. 306,739.

This invention in its broadest aspects relates to the heating of a confined mlxture of solid and liquid material to high temperature and pressure conditions, and has for its ob ject' to make possible a material decrease in the time necessary to reach the desired condition over that required by methods heretofore commonly practised. In accordance with the present invention, this object is attained by independently bringing the liquid material to or above the desired conditions of heat and pressure, and then adding it to the confined solid material and maintaining the mixed batch under such conditions for the desired period of time.

The process of the present invention may be applied to advantage in the chemical pulpmaking industry, where two processes of cooking the raw cellulosic material, e. g., chipped wood, have heretofore been generally employed. One process, which is known as direct cooking, involves introducing steam directly into the digester contents until pulping of the chips has been effected. The other process, which is known as indirect cooking and requires the use of a digester provided with a screen at its bottom, C0111- prises removing screened liquor from the digester, passing it through a tubular heater outside of the digester, and returning it in heated condition to the top of the digester. This latter process permits cooking of the chips under substantially uniform condi tions, since dilution of the liquor is avoided and substantially uniform temperature conditions are produced throughout the digester, owing to a positive rather than a natural circulation of the liquor from the bottom to the top of the digester. Pulpmakers therefore employ the latter process when a uniformly reproducible pulp is desired as the final product.

In both these cooking processes, a considerable fraction of the, time necessary to effect pulping is consumed in raising the bottom to the top of the digester, and should 5 steam be introduced too rapidly, violent ebullition in the charge and severe rocking ofthe digester are likely to take place. In the indirect cooking process, the rate at which liquor may be digester and then circulated through a heater is limited by the rate at which the screen and chips permit such withdrawal, and when using a screen of the usual type at the bottom of a large commercial digester, it is usually impossible to withdraw and heat more than about 600 gallons of liquor per minute, so that it may take about two hours to bring the digester contents to the temperature and pressure conditions desired for cookingfor instance, a temperature of about 335 F. and a pressure of about 90 pounds gage when kraft pulp is being produced.

Withdrawn from the c In accordance with the present invention,

the liquor employed for pulping the raw cellulosic material is independently heated to or above fiber-liberating temperature and pressure, and is then added in such condition to a confined charge of chips. may be rapidly heated to the desired conditions While unassociated with chips or pulp, for it may be withdrawn from a confined pool and circulated through a heater at high velocity and/or steam may be into at a high rate without causing violent disturbance in the liquor.

With these and other objects and features in View, the present invention may best be The liquor introduced thereunderstood from the tollowing more complete description thereof when taken in conjunction with the accompanying drawing, which illustrates more or less diagrammatically and conventionally a battery of digesters equipped with apparatus for conditionin the cooking liquor.

T he process of the present invention will now be described as applied specifically to an indirect krat't cooking process, but it is obvious that it may be applied to a direct kraft cooking process or to a direct or indirect sulphite cooking process.

Referring to the drawing, represents a storage tank for so-called white liquor, and 2 a similar tank for so-called black liquor. The white liquor is prepared by causticizing the smelted chemicals recovered from the black liquor associated with the pulped cellulosic material, and then filtering. As known to those skilled in the art, a mixture of both liquors (e. g., containing 20% to 50% black liquor) is usually employed in kraft cooking, the drawing accordingly showing a pump 3 removing white liquor from the tank 1 and black liquor from the tank 2, and the mixture being forced through a pipe 4 into a closed kettle 5. This kettle may be of a capacity suflicient to handle the amount of liquor necessary for pulping a digester charged with chipped wood. For instance, it may be designed to handle a batch of about 1600 cubic feet of liquor, this quantity of liquor being sufliicient to serve a digester of a capacity of about 2500 cubic feet. The mixture of liquors may initially be at a temperature of about 160 F., since the white liquor immediately after causticization and the black liquor as separated from pulp are at about this temperature. The batch of liquor in the kettle may be indirectly heated, a pump 6 being shown in conm'iunication with the bottom of the kettle and forcing the liquor through an indirect heater 7 and then returning the liquor through a pipe 8 to the top of the kettle. A valved pipe 9 is shown in communication with the bottom of the kettle, so that steam at suitable temperature and pressure may also be introduced directly into the liquor. The batch of liquor in the kettle may be rapidly heated to a temperature of 350 F. and a pressure of 100 pounds, this being somewhat above the temperature and pressure conditions usually employed for kraft cooking, so that when subsequently mixed with the relatively cold wood, the mixture will be substantially at fiber-liberating conditions. As indicated on the drawing, the liquor may be discharged from the bottom of the kettle selectively into any one of a battery of closed digesters 10, which may be already filled with chips, the air in the digester being allowed to escape through a vent pipe 16 at the top of the di ester and communicating through a pipe 1 with the top of the closed kettle 5. The air in the digester is thus gradually displaced by liquor, and since the system is closed, material lowering of the temperature and pressure of the liquor is avoided. The digester is closed after it has been supplied with cooking liquor, so that cooking may be effected therein while other batches of liquor are being heated to the desired condition in the kettle 5. Each of the digesters may be provided, as usual, at its bottom with a conical screen 11, the liquor being strained free of chips and pulp as it passes through the screen into an outlet pipe 12 communicating with the bottom of the digester. A pump '13 continuously forces screened liquor through an indirect heater 14 and thence through a pipe 15 to the top of the digester. The digester contents initially being substantially at fiberliberating conditions, liquor is removed from the digester and heated at a rate to maintain such conditions until the chipped wood has been ulped.

A batch of liquor sufiicient to supply a digester of about 2500 cubic feet capacity may be readily heated to the desired conditions in comparatively inexpensive apparatus, such as described, in a period in the order of magnitude of about 15 to 20 minutes, so that the same apparatus may be used to serve a whole battery of digesters, one digester being charged with chips, and liquor being heated and then added to the chips, while the other digesters are in operation. The decrease in time necessary for pulping the raw material thus efi'ected is of greater significance than ofi'hand appears, for pulp digesters are usually built in large units and a comparatively small increase in production in one digester results in a considerable saving over long periods when a battery of digesters are in continuous operation.

Having thus described certain embodiments of this invention, it is evident to those skilled in the art that various changes and modifications might be made therein without departing from the spirit or scope of invention as set forth in the appended claims.

What I claim is:

1. A process of pulping raw cellulosic material, which comprises heating a confined pool of cooking liquor to high temperature and pressure conditions, adding such liquor to confined raw cellulosic material while displacing the air in said latter and causing it to escape into and displace liquor in said pool, and cooking the confined charge of liquor and cellnlosic material until pulping has been effected.

2. Apparatus of the class described, comprising in combination a pulp digester, a v

kettle for heating cooking liquor to be used in said digester, means for discharging liquor from said kettle into said digester, and means for venting into said kettle above the In testimony whereof I have afiixed my signature.

GEORGE A. RICHTER.

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