US2216649A - Process of and apparatus for digesting fibrous materials - Google Patents

Description

Oct 1, 1940- A. D. NERRILL PROCESS OF AND APPARATUS FUR DIGESTING FIBROUS MATERIALS Filed June 27, 1955 amen Mot! AlberDMemdL Patented Oct. 1, 1949 STATES PATENT OFFICE Albert D. Merrill, Watertown, N. Y.

Application June 27, 1935, Serial No. 28,719

7 Claims.

This invention relates to improvements in a process of and apparatus for digesting fibrous material.

It has been recognized heretofore that the 6 quality and yield of pulp, produced for example by the sulphite process, is dependent to no inconsiderable degree upon the extent to which the chips are penetrated with acid prior to the actual cooking. If the chips are not thoroughly per- 10 meated with acid before the entire mass is raised to reaction temperature, differential cooking obtains with the result that the centers or cores of the chips may be undigested. This conditionof variation in the cooking is likewise accentuated when there is a marked difference in temperature in different parts of the digester.

To overcome these objections it has heretofore been proposed to secure a thorough penetration of the chips with acid by compacting the chips with hot acid, maintained below reaction temperatures, and at a high hydrostatic pressure. Such prior proposals have also included the suggestion of circulating the acid to and from the digester both during the penetrating or soaking period and the cooking period so as to establish substantially uniform thermal conditions through the digester and thus avoid differential delignification and produce a uniform pulp.

Such prior proposals, while very effective, re-

quired considerable apparatus, among other things a relatively large high pressure accumulator for storing the body of hot acid for the soaking period. I

A major object of the present invention is to devise a simplified process for "uniformly digesting a mass of fibrous material. I

A further object is to provide a process of delignifying fibrous materials in which the material is uniformly saturated with hot acid prior to cooking.

Yet another object is to provide an improved digester-storage tank system in which optimum heat recuperation is secured.

With these and other equally important objects in view the invention comprehends the concept of achieving the advantages of a thorough penetration with high temperature-high pressure acid, coupled with the advantages incident to uniform thermal conditions during treatment and to se- 50 cure the enumerated advantages in a greatly simplified apparatus.

As indicated hereinbefore, it has been proposed to pre-circulate hot high pressure acid through a mass of chips in a digester for the purpose of insuring a thorough penetration of the acid in the chips. Such prior processes were advantageously operated by employing a high pressure accumulator in the system, this accumulator functioning to recover the thermal and chemical values of the liquor and in addition providing a readily available supply of hot preconditioned acid for the soaking and cooking steps.

It has now been found that such major advantages can largely be secured in a special design simple unit comprising essentially a digester or battery of digesters and a low pressure storage tank or tanks.

,The present invention is an improvement on the process and apparatus disclosed in the United States patent to T. L. Dunbar, Number 1,974,420.

Considered more specifically the present process involves a treatment of fibrous materials with a suitable digesting agent, such as a typical bisulphite liquor, in such a manner as to secure a number of advantages with a minimum of apparatus. Such process thus involves charging the digester with chips, forcing a quantity of acid from a storage tank to the digester while heating the acid in transit with steam and/or directly or indirectly with hot liquor and gas obtained from another digester of the unit. The entering preheated and preconditioned acid is utilized to displace air from the digester, thus substantially precluding subsequent oxidation reactions. After the digester has been substantially or completely filled with hot acid an additional quantity of acid is forced into the digester so as to establish a high hydrostatic pressure on the liquor. In these circumstances, therefore, the hot acid readily penetrates the chips. To equalize the temperature throughout the digester and further to facilitate penetration of the acid in the chips, the acid is recirculated to and from the digester. During this recirculation the acid may be further heated, if desired or necessary, to bring it up to the optimum penetrating temperature.

After such preliminary soaking and precirculation period the temperature and pressure conditions within the digester are adjustedto initiate cooking. Duringthe cooking period hot gases-and liquorare relieved from the digester and the thermal and chemical values are recovered in the system, in part in a digester undergoing filling orsoaking and in part in the acid storage tank.

In order more clearly'to explain the underlying principles of the invention, a typical physical embodiment is shown in the single figure of the ac'-' companying drawing.

As shown in the drawing, the unit may comprise one or preferably a plurality of digesters l associated with an acid storage tank or tanks 2 of sufficient capacity to fulfill the requirements for the unit. The storage tank is supplied from the acid make-up unit, not shown.

The digesters may be of the type common in the art constructed of a suitable metal and lined with an acid resistant ceramic or equivalent material. It is provided with a removable manhole cover 3 and the usual blowoff elbow 4. Connected to the upper top of the digester is the top relief line 5 having a control valve 5. The portion of the line 5 terminating within the digester is provided with a suitable strainer means, as well understood in the art. The other end of the relief line may be connected to a header 8. Passage of relief fluids into the header may be controlled by the valve 6'. An air vent 1, provided with the control valve I, may be suitably connected with the upper portion of the digester, as for example through the line 5.

The acid storage tank 2 may be placed in communication with the digester through the line 8. Fluids passing from the header 6 through line 8 may be passed directly to the storage tank through the drop leg 9, controlled by the valve 8', or optionally, through the cooler l0, controlled by valve l, and thence through the drop leg H into the storage tank to be mixed with the acid liquor therein. Lines 9 and II preferably-terminate at a low point in the tank.

The digester is likewise provided with a'side relief line l2 so positioned on the digester as to 'establishthe desired liquid level during the cooking operation. This line is provided with a control valve l2 and the line terminates, as shown, at a low point within the separator l3. By-pass line ll connects the side relief line l2 with the top relief so that such side relief, in a manner more fully to be described, may be passed to the storage tank 3; by way of conduit 8.

Separator l3 may be of any suitable size and construction and is made of or lined with a suitable acid resistant material. The separator is provided with a knock-back or reflux coil I5 provided at its efllux'end with a pressure control valve l5. This valve is utilized to. maintain any desired pressure in the separator and thus control the extent or degree of separation of free gas from the side relief liquor. This knock-back coil communicates with the gas discharge line l6. Line l6 connects with the drop leg 9 through an injector fitting H. A valve I6 is provided in the line ii to control the passage of gas to the drop leg. It will be understood that with such a connection liquor passing downwardly through the line 9 imposes a suction on the line l6 andthus serves to withdraw and condense or absorb the gas from such line. It will be understood also that the line l8 may be connected through an injector type of fitting to the drop leg II from which the cooler liquids flow, thus increasing the absorption of as.

A by-pass line I8, controlled by valve l8, may be connected between line l8 and line 8 so that overhead gases from the separator may be admixed with fluids passing through line 8 to the cooler III. The connection [9 between lines i 8 and 8 may be of an educator type so as to establish a suction on line l8 and thus facilitate the withdrawal of gas from this line.

Acid is fed from the storage tank to the digester through the line 28 provided with the control valve 20'. Line 28 communicates with the header 2| and thence through the branch line 22, controlled by valve 22', to the blowoff elbow or any other suitable low point on the digester.

As indicated hereinbefore, the acid is heated in transit from the storage tank to the digester for the purpose of raising it to the optimum soaking or penetrating temperature. As has been explained heretofore, a desideratum in the present invention is to contact the chips in the digester with hot high pressure liquor while maintaining the temperature of the liquor below reaction temperatures for the purpose of securing maxi- 'mum penetration with substantially no reaction.

heat of the system.

As shown, the separator I3 is provided with a bottom drawofjf line 23, the passage of hot side relief liquor through which is controlled by valve 23'. This liquor is used to directly and/or indirectly heat the relatively cool acid passing from the storage tank to the digester. Thus the line 23 is connected by a branch, controlled by valve 23", to the indirect heat exchange 24 and this liquor constitutes the heating medium in such heat exchange. Acid liquor from the storage tank passing through line 20 enters the indirect heating coil 25 and is thus indirectly heated by the hot relief liquor.

The present system may also be employed not only to preheat the soaking liquor but also to precondition it. For this purpose the branch line 26 is provided. This line is connectedat one end into line 20 between control valve 20' and pump 21 or, if desired, directly to the digester. The flow of hot side relief liquor into line 28 at this point is controlled by valve 26. The branch line 28, controlled by valve 28', provides for the passage of hot side relief liquor into the acid feeding line 2!! beyond the eductor 28. In this manner hot relief liquor may be commingled or admixed with the relatively cooler. acid at the high and/or low pressure side of feed pump 21. Fresh acid from the storage tank therefore may be preheated and preconditioned by direct admixture with hot side relief liquor from the separator l2.

Increased heating economies and additional preconditioning of the soaking liquor may be secured by admixing gas evolved during the cooking operation with the fresh acid. As shown, this may be accomplished by inserting the eductor 28 in the feed line and connecting the suction side of the eductor with the line 30. Line 38, controlled by valve II', is connected, as shown, with the overhead gas line l8. When valve 28' is opened hot gases are aspirated from line It, under the action of the high velocity acid passing through the eductor, and such gases are intimately admixed and-absorbed by the acid. By controlling the valves 28, 28 and 25', the degree of preheat imparted to the cooler acid from the storage tank may be accurately controlled. Similarly, by controlling the quantity of the hot relief liquor passing through lines 28 and 28, the degree of heat imparted to the acid may be accurately controlled so as to secure the maximum absorption of gas passing into the stream from line 88.

The relatively cool acid flowing from the acid storage tank through line 28, preheated by indirect and direct exchange with hot relief gases and liquids, is further preheated in transit by steam. As shown, the steam line header 48 is provided with a branch line H, controlled by valve ll.

This branch line discharges into the blowofi elbow. If desired, additional branch lines 4i may be connected into line 26 at any desired point between the storage tank and the digester to achieve the maximum degree of preheat with optimum concentration of free gas. Preferably, as shown, the final heating with steam is achieved after the preliminary preheating and preconditioning by the hot digestion fluids. Steam line Q!) is also provided with branches 7.72, controlled by valves 42, which connect into the lower conical portion of the digester in the manner well known to those skilled the art.

The hot side relief liquor, after passing through the heat exchanger 24, may be picked up by pump 50 and forced through line 5! either directly into the acid storage tank to branch 52 or to line 53 through the cooler in and thence to the storage tank. If desired, the connection 54 between line 53 and the header 8 may be of the injector type of fitting so that the liquid forced through line 53 may be utilized to condense and absorb free gases in the relief fluids flowing through line 8'. The control of passage of the side relief liquor through the line 52 or 53 is controlled by valves 52' and 53 respectively.

As already indicated, according to the present invention relatively cool acid from the acid storage tank is first preheated and preconditioned for employment as a high temperature soaking liquor in the digester. In order to secure optimum penetration the liquor is preferably recirculated to and from the digester. Such recirculation tends to equalize the temperature throughout the digester and this uniformity of temperature, coupled with the fact that the acid is at high temperature and pressure, insures complete and uniform saturation of the chips with hot acid.

To secure this result the digester, as shown, is provided with a recirculation circuit. This may comprise the recirculation line 60, controlled by valve one end of which terminates in a perforated section 62 within the digester and the other end of which connects with the suction side of the pump 63. The high pressure side of the pump is connected through line 64 to the bottom portion of the digester. Interposed in this line is the heat exchange means 65. This may be an indirect heater, the heating medium of whichmay comprise steam, hot spent liquor and the like. If

desired, in lieu of and in addition to the indirect 7 be maintained or increased during the soaking heat the acid in another digester undergoing.

period by adding S02 gas or its'equivalent at this .time. For this purpose SO: gas may be withdrawn from the container and passed through line 61 and control valve 61 to the suction side of recirculating pump. By cdntrolsoaking. One method of accomplishing this, namely by admixing side relief liquor from separrator it with the stream of acid flowing from storage tank 2, has been described.

soalnng liquor during the precirculation period, that is to say during the period when the pump 83 is operating. To accomplish this the line I0 is connected to the suction side of pump 53. This line may be provided with a control valve l8 and connected at its other end to the header 6 or to a'separate high pressure header where such is employed. In these circumstances when, during the precirculation period, the digester comprises a closed cycle, that is to say shut oif from feed line 213, additional heat units may be imparted to the soaking liquor by withdrawing hot side relief liquor and forcing it through line if! into the recycle stream.

The operation of the process will have been appreciated from the foregoing description. In treating a given mass of material, the manhole In addition side relief liquor may be admixed with the cover 3 is removed and the digester filled with chips from a suitable hopper or conveyor with or without the employment of a chip packing device. After the digester has been filled to the desired level with chips the cover is bolted on, Valves 5 and I are opened, valve 6, i2 and 60' being closed. Valve 20' is then opened and pump 21 operated to force relatively cool acid from the storage tank to the digester. In transit to the digester the acid is heated so as to raise the temperature to between '70 to or 0. As explained, this may be done, for example, by opening valve 26' and valve 23. In these circumstances pump 2'! takes suction. on line 26 and hot side relief liquor from the separator I3 is admixed with the relatively cool acid flowing through line 20. This preheats and preconditions the cool acid. Coincidentally valve 30' may be opened so that the high velocity stream of acid flowing through eductor 29 establishes a suction in line 30 and thus withdraws gas from the separator. This hot gas passing down through drop leg 30 is intimately mixed with and absorbed in the acid charge, additionally heating the latter and increasing its free gas content. Likewise, if necessary to attain the desired soaking temperature, direct steam may be admitted to the acid stream by opening valve 4 l and/or valves 42'. The preconditioned and partially preheated acid may then further be heated by hot relief liquor entering the acid stream from pipe 28 and indirectly'by the hot relief liquor in the heat exchanger 24. y

In some circumstances it is not desirable to impart the maximum degree of preheat tothe fresh acid until the digester is substantially filled so as to or the evolution of SO: gas during the filling operation. As the hot acid enters the digester at the bottom and flows upward, contained air is displaced and is discharged through the air vent 'I. This venting is continued until gas begins to discharge through 7 the vent, at which time valve 1' is closed and valve 6' opened. This overhead gas may be absorbed in the acid within the storage tank by preheated, in the manner described, that is to say by direct and indirect contact with hot relief liquid and by direct contact with hot gases and steam.

In the preferred operation the pump 21 is continued in operation until a high hydrostatic pressure is built up on the hot acid in the digester and in the latter operation the pump is operated until a pressure of the order of from ninety pounds or more is attained. During this period heat units are imparted to the acid so as to raise it to the optimum soaking temperature which, as noted hereinbefore, is just below reaction temperatures and may be approximately from 100 to 110 C.

-- It will be understood that while number I digester is on the soaking period, the material in other digesters of the battery are undergoing cooking and hot relief fluids are being discharged therefrom to the storage tank and/or the separator II in a manner more fully to be described. After the desired penetrating pressure has been established on the volume of acid in the digester, valves 20, 28' and 23 are closed'and pump, 21 stopped. It will be understood of course that in some installations having a large number of digesters the pump 21 and the preheating assemblage, including the heat exchanger 24, may be operated substantially continuously and the digester which is filled may be cut off from the system by operating valve 22' interposed between the blowdown elbow 4 and header 2 I. After such filling of the digester, valve 60' is opened and pump. 63 operated so as to circulate the acid to and from the digester for the purpose, as explained, of equalizing the temperature throughout. If, during the filling operation, the acid temperature has not been raised to the desired degree, additional heat unitsmay be imparted to the mass by heating the recirculating stream in its passage through the heat exchanger 65 and/or by introducing direct steam. Likewise,

during this recirculation period the acid concentration may be increased to any desired advantageous point by admitting S02 .gas from the container 56.

Similarly, as described hereinbefore, the temperature of the soaking liquor may be increased by directlyadmixing hot relief liquor withdrawn from another digester. This may be done by opening the valve 70 and withdrawing such liquor from manifold 6 through the line 10. It will be understood that a line may be provided from the bottom of separator l3 to the suction side of pump 63 so as to withdraw hot side relief liquor from the separator and admix it with the recirculating stream of soaking liquor.

Thus acid at a temperature of from approxi-.

mately 80 to 110 C. and under high hydrostatic pressure may be recirculated through the mass of the chips in the digester until such chips are thoroughly penetrated with the hot acid and are uniformly heated throughout. In ordinary circumstances, using a typical size digester, such precirculation preferably is continued. for a period of from one to four hours. In most circumstances, particularly where a high initial temperature is utilized, a soaking period of two hours achieves the advantages herein described.

After the precirculation-soaking period, the conditions within the digester are adjusted to cause reaction of the bisulphite liquor with the lignin for the purpose of delignifiying the fibers.

To initiate the cook valves 5', 6', l2 and i are opened and gas and liquor are relieved until the pressure and liquid level are reduced to the desired extent. In these circumstances hot fluids pass through the header 8 and cooler l0 and are admixed with the cooler acid in the storage tank, thus preheating and preconditioning this acid. If desired, during this period valve l2 may be opened and valve I4 in line Il may be closed, or only partially opened, so that some hot liquor is discharged to separator I3 to be employed, as described, as a direct or indirect heating medium for the acid employed for soaking in another digester.

It has been found that a rather decided drop in pressure and liquid level is advantageous after a soaking period of the character described, as this drop materially increases the speed with which the mass may be brought up to temperature. In the preferred operation, therefore, the pressure is dropped from the soaking pressure, of the order of 90 pounds more or less, down to from 30 to 50 pounds more or less. This, it will be appreciated, enables the employment of low pressure direct steam for cooking. The gases evolved as a result of this drop in pressure pass overhead through the header 8 and cooler Ill and are absorbed in the acid within the storage tank.

Upon such diminution in pressure and drop in liquid level, valves 42' are opened and valves 6' and I2 are closed. The admission of direct steam soon builds up the pressure within the digester and the temperature is rapidly increased. When the pressure has been built up to the normal desired operating level, of the order of 70 pounds gauge more or less, this pressure may be maintained at substantially this value by opening valves l2, l4 and 6' and passing fluids to. the acid storage tank either by way of drop leg 9 or drop leg II or such pressure may be maintained by closing valve 6' and I4 and relieving through valve l2 and line l2 into the separator I3.

During the cooking operation, in order to insure uniform thermal conditions throughout the mass, liquor may be recycled to and from the digester through the recycle line 60 by operating the pump 62 in the manner described. In these circumstances, particularly when the indirect heat exchanger 65 is utilized, additional heat units may be imparted and the extent of condensation and dilution of the acid commensurately reduced.

As is known, during the progress of the cook the liquid level tends to rise due to the increased amount of condensate. The desired liquid'level may be maintained after cooking is started by operating the side relief line in the manner de-' scribed. It will be understood that by proper manipulation of valves 5' and I2 the desired liquid level may be maintained.

As is known as the reaction proceeds a considerable quantity of free gas is evolved and this tends to build up an excessive pressure in the digester. In'the present operation such pressure is relieved by opening valve 5' and passing gas through header 8 and valve ll through the tion of the cooking period is maintained at approximately 75 pounds and temperatures at about 135 C.

Steam is shut ofi some time prior to the completion of the cook in the manner well known to those skilled in the art.

The pressure in the digester is then greatly reduced by relieving through header 8 down to approximately 20 or 30 pounds, after which the material is either discharged into the blow pit or is given a preliminary water wash in the digester and then discharged.

During the cooking operation, as noted here-' inbefore, optimum utilization of heat and chemical values is secured. The hot side relief liquor passing to separator I3 is utilized as a preheating medium and the gas evolved in the separator, to the extent determined by the setting of pressure control valve i5, is utilized to preheat and precondition the acid charge. The relief liquor passing through heat exchanger gives up a considerable quantity of-it's heat to the cooler acid and then is passed through line 5| either directly into the acid storage tank through line 52 or by way of line 53 through the cooler ID to the storage tank. In some circumstances, as where a relatively high pressure is carried on the separator l3, such side relief may flow under its own pressure into the storage tank. In other circumstances, and especially where a small size separator is employed, it is advisable to withdraw such hot liquor from the separator and through the heat exchanger at a relatively rapid rate. This may readily be done'by utilizing the pump 50 in line 5|. As noted hereinbefore, the owing streams of liquid in the system may be utilized by employing a suitable type of eductor fitting to more efiectively withdraw and absorb gases evolved in the system. This result can be attained in the manner described by employing ejector types of fitting at 54 and Hi.

It will be appreciated that the present invention provides many advantages. The system comprises essentially, as the major units, only the digesters and storage tanks. The storage tank is made to perform a plurality of functions. In the first place it provides means of accumulating a large volume of acid and recovering some of the chemical and heat values, of the system. Since the storage tank, however, is under atmospheric pressure or at only slightly elevated pressure, the quantity of gas which may be absorbed and the degree of temperature rise is rather limited. In the present unit, however, additional absorption and increased recuperation of heat units is achieved by utilizing the separator l3 in conjunction with the acid preheating and preconditioning system 20-22-26-29 and 24. The present unit furthermore provides for optimum soaking and accelerated reactions in a simple type of unit. By utilizing the system as described the chips may be thoroughly penetrated with hot high pressure acid maintained at a substantially uniform temperature. In the present system although a relatively low pressure acid storage tank is employed, high hydrostatic pressure soaking with precirculation is nevertheless achieved by circulating directly to and from the digester and cutting out the acid storage tank from the system during the high pressure soak.

By subjecting the fibrous material to the preliminary high pressure high temperature soak the quantity of heat units required in the system is materially diminished. The system likewise advantageously and directly utilizes the hot liquor withdrawn from one digester undergoing cooking and utilizes this in another digester undergoing soaking. By this special correlation of contemporaneous soaking and cooking periods, in conjunction. with the other novel features of the unit, a large size accumulator is rendered unnecessary, while nevertheless obtaining substantially the full benefit of such accumulator. In other words, the soaking digester is made to act in part as a high pressure accumulator. This, in conjunction with the acid storage tank and the acid preheating system, effectively con-= serves the heat and chemical values.

It will be understood that suitable control means, understood by those skilled in the art, will be provided for the unit. Thus, for example, although it is not shown, the digester may be provided with the usual temperature and pressure indicating and/or recording devices. Similarly a thermostatic control may be associated with the digester and the heater 65 and operated to control the degree of heat imparted, by the heat exchange, during the soaking or cooking period. Likewise valve 61', associated with the source of S0 gas, may be controlled by a suitable electrolytic element positioned within the digester so that the amount of S02 gas admitted to the recycle stream is controlled according to the concentration of the liquor.

Therefore, while a preferred method of operation and type of apparatus has been described, it is to be understood that this is given merely as exemplifying the underlying principles of the invention and not as the exclusive method or means of eifectuating these principles.

I claim:

1. An apparatus of the character described comprising a digester and a low pressure acid storage tank, a valved conduit connecting the lower portion of the digester, a pump in said conduit for pumping a stream of acid from the tank directly to the digester, a valved side relief branch for the digester, a valved top relief branch for the digester, means connecting the top relief to the storage tank and means including a separator connecting the side relief to said valved conduit for directly admixing hot side relief with the stream of acid charged to the digester.

2. An apparatus of the character described comprising an enlarged digester and acid storage tank; an acid supply line extending from the tank to the digester; a side relief line connected with the digester and communicating with the storage tank, a separator and pressure reduction valve in such line and means, including an eductor, connecting the separator with the'said acid supply line.

3. In a process of the character described, utilizing a digester and acid-storage tank, that improvement which comprises withdrawing relatively cool acid from the storage tank and forcing it directly into the digester, separating side relief fluids into liquid and gaseous components 'and utilizing the flowing stream of acid to withdraw and iurtherpreheating' the acid flowing to the digester by introducing steam directly therein.

5. In a process 01! the characterxdescribed,

utilizing a digester and low pressure storage tank, that improvement which comprises pumping a stream or acid from the tank to the digester, withdrawing reliei fluids from the digester in which cooking is taking place, passing low presacid from a storage tank and forcing it into the digester, then sealing the digester and forcing in additional acid to build up a high hydrostatic pressure in the digester; heating a stream of acid in transit from the storage tank to the digester to raise the acid temperature to the order of 70 C. and then recirculating the acid directly to and from the digester under the said a high hydrostatic pressure; then reducing the pressure and cooking the material within the digester.

7. A process of digesting fibrous materials which comprises charging a digester with the material, withdrawing relatively cool acid from a storage tank and forcing it into the digester while displacing air from the digester with such entering acid; heating the stream or acid in transit to the digester by direct contact with hot fluids from another digester, then sealing the digester and. continuing the introduction of acid until a high hydrostatic pressure is established in the digester; then circulating the acid directly to and from the digester at such high hydrostatic pressure for a period of time sumciently prolonged to substantially completely saturate the material with acid and then raising the mass to reaction temperatures and cooking the material in the digester under a lower hydrostatic pressure.

' ALBERT D. MERRIIL.

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