US1837309A - Method of and apparatus for producing paper pulp - Google Patents

Description

R. B. WOLF Dec. 22, 1931.

METHOD AND APPARATUS FR PRODUCING PAPER PULP'.

Filed June 5, 1930 INVENTOR if".

V 84m-4r ATTORNEYS Patented Dec. 2.2, 1931 UNI-TED STATES PATENT OFFICE ROBERT R. WOLF,

OF NEW YoRR, N.'Y..Ass1eNoR 'ro LIGNO-CEL'LLOSE CORPORATION, OE NEW YORK, N. Y., A CORPORATION oF DELAWARE METHOD 0F AND APPARATUS FOR PRODUCING PAPER PULP Application filed .Tune 5,

This invention relates to the production of paper pulp and particularly to improvements in the sulphite process and in the recovery of the acid employed therein. i

To secure a proper Yunderstandino` of the invention, it is necessary to review briefly the development of the sulphite process for treating paper pulp which was described originally in the patent to Tilghman No. 70,485, November 5, 1867. In this process as originally applied, it was impossible to recover the blow-off or relief gases. These gases, consisting principally of sulphur dioxide, were discharged to the atmosphere and lost. The first attempt to recover the gases involved the absorption thereof in the cooking liquor as described in the Drewsen Patent No. 492,196 dated February 21, 189.3. The latter method was subject to certain disadvantages, however, and particularly the impossibility of retaining the sulphur dioxide gas in the acid at the high temperature resulting from the introduction of the hot gas thereto.

'The next improvement involved the introduction of so-called relief coolers between the digesters and the recovery tanks so that the temperature of the acid could be kept rela-` tively low, usually between 70 and 90 F. With such improvementsit was possible to recover the acid at a temperature of approximately 75 F. at atmospheric pressure, containing approximately 7% of sulphur dioxide, although in the ordinary mill the total sulphur dioxide content of the acid averages between 4 and 5%. v Y

The advantage of using strong acid was pointed out in the Wolf Patent No. 1,699,556, June 22, 1929. That patent also called attention to the fact that strong acid has not been used because the excess ofsulphur dioxide gas remaining in the digester Yat the end of the cook ordinarily could not be forced over into theA recovery tanks without advancing the l temperatures in the lcooking operation and thereby losing most of the benefit resulting from the use of strong acid. A method of recovering strong acid is described in the Wolf Patent No. 1,097,781, May 26, 1914, and

a simplified and more effective method is de` 1930. Serial No. 459,365.

crlibed in the Wolf Patent No. 1,499,898, July To get the best results, strong acid should be used at low temperature. This principle is recognized in the patent to Moore and Wolf, 1,110,454, September 15, 1914, which relates to a method of blowing the digesters into a blow pit and collecting the vomit stack gases and passing'them through a surface condioxide gases in the presence of moisture and because of the large relief lines necessitated by the volume of gases.

Another step qin the industry was in the direction' of making it possiblel to get the effect of stronger free acid by another method, namely, the method of 'relieving the digesters into an accumulator without cooling the relief gases in liquids. The first description of this method appears in the Clemm Patent No. 1,429,128, September 12, 1922, which is followed by the hot acid recovery system of Richter described in Patent No. 1,653,416,

patents. l

' In the hot acid recovery system, the emphasis is upon the recovery of heat from the relief gases. These gases are conducted, therefore, without cooling to a hot acid accumulator. In the ordinary mill a lnumber ofdigesters are connected to a common relief lme wh1ch carries the. gases from the digesters to the acld accumulators. This introduces seriousjdifculties, including a wide varia-A tion in the acid strength and in the temperature of the acid which isintroduced from the accumulators to thedigesters, but more particularly in the constant variation of the pres'- December 20, 1927, and in ther and later4 'sure in the relief line which seriously affects ,of the respective digesters. It is a matter o f common knowledge in the opera-tio of d1- gesters that the pressure therein should remain substantially constant. Th operator usually sets the relief valve a tie desired opening to maintain, so far as i-s possible, a constant pressure, but this control becomes wholly ineffective under the system described, in which the hot relief gases are conducted through a common relief line to the hot acid accumulators.

`To appreciate this difficulty more fully, it is desirable to consider the normal operation of a digester. The digester is first filled with chips and with acid, the acid being as nearly as possible of uniform temperature and uniform chemical composition.` yThe cover of the digester is secured in place and steam is admitted into the bottom of the digester. The pressure is brought up to approXimately 7 5 pounds per square inch at the top of the digester` in a period ranging from 2 to 3 hours. After the pressure hasreached 75 pounds, which is the usual working pressure, no more steam can be admitted unless gas is relieved from the digester. This is accomplished by mea-ns of a relief valve on the cover. The speed at which the'temperature in the digester is raised depends upon the amount of opening of the relief valve. A

wide opening, permitting the` escape of a large amount of gas, lets in a large amount of steam, and therefore causes a rapid advance in temperature. The reverse is true if the relief valve is only slightly, opened; namely, the temperature will advance slowly. It is possible to control the speed of cooking yso that the variation vin practice is between hours and 20 hours, the purpose of the longer cooking time being to complete the disintegration of the wood chips into fibres at as low temperatures as is possible. If the pressure in the relief line varies constantly, the control of the cooking time is lost substantially, and the temperature of 'the digester will fluctuate widely because of the variations in the amount of steam introduced thereto.

` The rate of acceleration of temperature in the cooking operation is very important. In

\ other words. it makes quite a difference in the character of the pulp whether the temperature is advanced rapidly during the first part of the cook and slowly during the latter part or advanced slowly during the first part and rapidly during the latterpart ofthe cook.

The rate of advance of temperature is entircly a matter of control of the relief val-ve on the cover of the digester; hence it is very important for the pressure in the relief linesto be obsolutely constant so that when the cook sets his valve in a certain position he knows he will get a constant amount of relief.

delicate and requires the constant attention of a'highly skilled operator. The operator must gauge the condition of the digester by reference to the temperature of the digester as shown by the recording thermometer which is usually connected at one side of each digester. temperature are, because of the large mass of material involved, transmitted slowly, and the temperature record does not indicate absolutely the temperature condition throughout the digester. The record tends to lag behind the actual variations in temperature as they occur. Consequently the most skillful operators find it difficult to maintain a uniform and satisfactory operation under the most favorable conditions which may be met in digester operation as at present conducted.' It is the object ofthe present invention to provide a method and apparatus whereby the difliculties experienced heretofore in the operation of digesters and the recovery of acid therefrom are avoided, and whereby stronger acid canbe recovered and utilized more effectively and without losses heretofore incident to the operation of the sulphite process.

Another object of the invention is the-provision of a method andapparatus whereby the control of the cooking operation is facilitated and simplified so that a larger number of digesters can be controlled by a single operator to secure more uniform and satisfactory results. I Other objects and advantages of the invention will be apparent as it is better understood by reference tothe following specification and the accompanying drawing, which illustrates diagrammatically the preferred form of the invention.

In carrying out the invention, I employl one or more and preferably a plurality of digesters, and one or more acid recoveryuvessels, towhich the di gesters may be connected lines connected from theirtops to permit discharge of the relief gases. I provide, however, a steam main from which steam is supplied to the digesters at constant pressure. This may be accomplished readily by the introduction offan ordinary pressure regulat ing valve, for example of the diaphragm controlled type, between the source of steam. i. e., a boiler or a reducing turbine, and the steam main. Thus the pressure may be reduced uniformly from, Afor example, 150 pounds at the source to 125 pounds per square inch in the steam main. The steam at constant pressure is then deliveredthrough recording steam flow meters connected to each `of the digesters. Such meters are available needle of the steam flow meter shall follow` the predetermined chart. 'Ihe amount of steam entering the digester at constant pressure will thus correspond to the amount predetermined and necessary to accomplish the intended purpose.

' At the outlet from each of the digesters to the relief line, I provide a regulating valve with an auto-matic control, such as a Locke or Mason regulator, which permits the gas to escape from the digesters only when the pressure therein reaches a predetermined maximum. Thus, constant pressure caribe maintained in each of the digesters, regardless of the' pressure in the'relief line or lines. The gas escaping from the digesters passes through the relief line or lines to the acid recovery vessel, in which it mingles with the acid maintained therein and is dissolved, suitable pressure being maintained in the vessel to ensure anacid of the desired strength. The recovery vessel may be provided with a suitable valve to permit the escape of gas should the pressurerise above a predetermined maximum.

It is desirable, though not essential, to cool the gas before it enters the acid recovery vessel. Thus,'where a high free acid is desired, cooling of the relief gases may be effected by introducing a suitable cooling de- Vice in the relief line. The cooling device may be supplied with cooling water,-andvtuhe amount of water may be regulated by means of suitable valves to effect the desired cooling of the gases. Alternatively, the acid inthe recovery vessely may be cooledY by the introduction of a coil through which water is permitted to flow at the desired rate to remove the surplus heat, or the acid may be Withdrawn from the vessel and circulated through a cooling device and thence returned to the tank. The purpose of coolingthe acid or the relief gases is to maintain a relatively low temperature, thus permitting the absorption and retention of a larger proportion of sulphur dioxide gas at the pressure maintained in the recovery vessel. If dual recovery vessels arel employed, the relief gases rwill be directed alternately into one or the other of the vessels, depending upon the condition of the acid therein.

From the vrecovery vessel or vessels, the

1 acid may be withdrawn from time to time asmay be required and delivered to the 'several digesters. The strong acid at a predetermined temperature can be introduced to the digesters at that temperature, thus permitting a further control of the operation of the digesters. Cooling of the gases or of the acid involves. a loss of heat. For example,

`the acid may be cooled from an average of 185 F. to a temperature of approximately 115 F., involving a loss of approximately 1,000,000' B. t. u. per ton of pulp. The loss represents a cost of only 40e per ton of pulp. The advantage of operating, however, with strong acid as described in this application, amounts to from $2.00 to $7.00 per ton, depending upon the price of wood, which offsets the loss of heat with a very wide margin.

The special advantage of the present invention resides, however, more particularly in facilitating the control of the cooking operation through the introduction lof steam atconstant pressure and at a predetermined rate. with automatic maintenance of the pressure in the digester after the pressure.V attains the predetermined maximum. The method as described avoids the uncertainty of attempting to control the operation through manipulation of the relief valve withl the consequent fluctuations both of the pressure in the4 digester and of the rate at which the steam is fed thereto. 'With the operation as described, it is possible to conduct `the cooking at a predetermined and' uniform rate and to control the operation by manipulating a valve in accordance with a visible-index which indicates with certainty the exact conditions which are effected when the valve is adjusted.

Referring to the drawing, 5 indicates the digesters of a group, each of which is connected to a relief line 6. AThe relief line is in turn connected with the bottom of a pressure acid recovery vessel 7. While cooling of the relief gases is not essential, I prefer to provide a cooler 8 which may be supplied with cooling water through a pipe 9 controlled by a valve 10, the water escaping through an outlet pipe 11. `Alternatively other suitable cooling means may be employed, as for example a coil 12 in the vessel 7 which is supplied 'with cooling water through a pipe 18 controlledV by a valve 14, the water escaping through ra pipe 15. It is to be understood that either or both of the cooling means may beemployed or omitted, depending upon the objects to be accomplish'ed. A pressure release valve 1-6 is provided preferably at the top of the acid recovery vessel, to avoid excessive pressures in the vessel.

desired point. This valve may be, for example, of the diaphragm-operated type, which is commonly employed for the purpose set forth. The. structure of the valve need not be described in detail, as it Will be understood by all those skilled in the art.

From the steam main 17 steam lines 20 are connected to each of the digesters 5 and a steam iow meter 21 is provided in each of the steam lines to indicate the amount of steam at constant pressure which is permitted to fiow through the lines under the control of valves 22. The latter valves are adapted to be manipulated by the operator to permit steam to iow to the digesters at a predetermined rate. The steam iow meters are Well known devices which need not be described in detail. One suitable type is provided with discs or paper dials adapted to receive an ink line indicating vthe predetermined rate of steam flow. These devices have also a pen on a movable arm, the movement of which corresponds to the iiow of steam through the device so that the meter will record accurately the amount of steam flowing at any given time. The operator may thus adjust the controlling valve so that the amount of steam flowing corresponds to the amount indicated by the line on the dial. l

At the top of each digester a manually operable valve 23 is provided, this valve being -normally open so that control of the relief is effected by an automatically controlled relief valve 24. The control is effected by a suitable regulator of the Lockey or Mason type, consisting for example of a diaphragm 25 connected by a pipe 26 to the top of the digester. The diaphragm is counterbalanced by a Weight 27 to permit predetermination of pressure to be maintained. The diaphragm is also connected to a Apilot valve 28 which controls the water pressure in a 'cylinder 29. A piston 30' in the cylinder is connected to the operating lever 31 of the valve- 24. Thus as the pressure tends to vary at the top of the digester, the valve 24 is opened or closed and a uniform pressure is maintained.

The gas which escapes from each of the several digesters 5, depending upon the operation of the valves 24, is delivered through the relief line 6 to the pressure acid recovery vessel 7 and is there absorbed. The strong acid thus produced may be utilized as hereinbefore indicated in the several digesters. If tWo pressure acid recovery vessels are employed, the gases may be directed into the vessels alternately.

With the method and apparatus as described, ressure variation in the relief line has no e ect Whatever upon the operation of the digester. Hence the diiiiculties which have been experienced heretofore because of such pressure variations inthe relief line are avoided entirely. The operation is simplified, furthermore, by controlling the rate atwhich steam is supplied to the digesters instead of by attempting to regulate manually the flow of relief gases from the digesters.

Various changes may made in the details of procedure and in the structure employed therefor Without departing from the invention or sacrificing any of its advantages.

I claim:

l. The method of cooking sulphite pulp which comprises supplying steam under manual control to the digester containing the pulp at a predeterminedand regulated rate of iow While maintaining a substantially constant pressure in the digester.

2. The method of cooking sulphite pulp which comprises supplying steam under manual .control at constant pressure to the digester containing the pulp at a predetermined and regulated rate of flow While maintaining a substantially constant pressure in the digester.

3. The method of cooking sulphite pulp which comprises supplying steam under manual control to the digester containing the pulp at a predetermined and regulated rate of flow, and automatically controlling the delivery of relief gases from the digester.

4. The method of cooking sulphite pulp which comprises supplying steam under manual control to the digester containing the pulp at a predetermined and regulated rate of flow, and permitting relief gases to .escape from the digester only when the pressure therein attains a predetermined maximum.

5. The method of cooking sulphite pulp Which comprises supplying steam under manual control at constant pressure to the digester containing the pulp at a predetermined and regulated'rate of iow, and autoj matically controlling the delivery of relief gases from the digester.

6. The method of cooking sulphite pulp which comprises supplying steam under manual control at constant pressure to the digester. containing the pulp at a predetermined and regulated rate of flow, and permitting relief gases to escape from the digester only when the pressure therein attains a predetermined maximum.

7. The method of cooking sulphite pulp which comprises supplying steam under manual control to the digester containing the pulp at a predetermined and regulated rate of flow, automatically controlling the delivery of relief gases from the digester, and dischargin the relief gases into an acid absorbing me ium.

8. The method of cooking sulphite pulp which comprises supplying steam under manual control to the digester containing the pulp at a predetermined and regulated rate of flow, permitting -relief gases to escape from the digester when the pressure therein attains a predetermined maximum, maintaining the pressure automatically, and discharging the relief gases into an acid absorbing medlum.

9. The method of cooking sulphite pulp which comprises supplyingsteam under manual control at constant pressure tothe digester containing the pulp at a predetermined and regulated rate of flow, automatically controlling the delivery of relief gases from the digester,.and discharging the relief gases into an acid absorbing medium.I

10. The method of cooking sulphite pulp Which comprises supplying steam under manual control at constant pressure to the digester containing the pulp at a predetermined and regulated rate of flow, permitting relief gases to escape from the digester When the pressuretherein attains a predetermined maximum, maintaining this pressure automatically, and discharging the relief gases into an acid absorbin medlum.

11. The method o cooking sulphite pulp which comprises supplying steam under manual control tothe digester containing the pulp at a predetermined and regulated rate offlow While maintaininga substantially constant pressure in the digester and Withdrawing and discharging the reliefgases into an acid absorbing medium maintained under pressure.

ing the ulp at predetermined and regulated' flow and automatically controlling -rates o the delivery of relief gases from each of the digesters. f

14. The method of cooking sulphite pulp which comprises supplying steam under man- 'ual contro lated rates of ovv, automatically controlling 'the delivery of relief gases from each of the digesters, and dischargin the relief ases through a common relief n sorbing` medium.

15. he method of cooking'sulphite pulp which com rises su plying steam under manual contro toA a p urality of di esters" containing' the pul at pre'determine and regulated rates of ow, the delivery of relief gases from each of the digesters, discharging the relief gasesthrough -a common relief line tol an acid absorbin A medium in amtalned under fessure, andrcoo 05 mg the acid absorbing me um.

12. The method of cooking sulphite pulpv to a plurality of digesters cons taining thepup at predetermined and reguautomatically controllingl taining the pulp at predetermined and` regulated rates of How automatically controlling .the delivery of relief gases from each of the digesters, discharging the relief ases through a common relief line to an 4aci absorblng medium maintained under pressure,

and cooling the gases before they enter the- 16. The methodof cooking sulphite pulp which comprises supplying steam under manual control to a plurality of digesters conpulp, adigester manually operable means for l supplying steam at a constant rate of flow to the digester, and means for controlling automatically thedischarge of relief gases from the digester.

. 19. In anapparat-us for cookin sulphite pulp, a digester, manually opera le means ,for supplying steam at a constant rate of How to the` digester, means for controlling automatically the discharge of relief ases ,from the digester, and means for absor ing .the relief gases.

20. In an apparatus for cooking sulphite pulp, a` digester, manually operable means or supplying steam at a constant rate of flow to the digester, means for controlling automatically the discharge of relief gases -from the digester, means for absorbing the e to` an aci abrelief gases, and means for reducing the temperature of the gas-absorbing means.

In testimony whereof I air'ix my si ature.

ROBERT B. OLF.

` los l mi f. ist

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