CA1277530C - Heat recovery with differentiated multi-flash during production of high yield pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In the production of mechanical pulp, the pulp and process steam that are discharged from the refiner (defibrator) are passed through a series of pulp/steam separation devices (such as centrifugal separators, separator conveyors, and the like). The steam separated in the first separator is valuable process steam, which is passed to a reboiler to produce fresh steam. Turpentine, methanol, and like products may be reclaimed from the condensate of the reboiler. The steam separated in succeeding separator stages is used for other purposes suited to steam of the pressure and character produced in the successive separation stages. For instance the steam separated by the second separator is passed to a pre-steaming device for steaming of the chips (or like comminuted cellulosic fibrous material) being fed to the refiner. A cyclotube is provided after the last steam separation stage, and produces moisture laden hot air, which is passed through a heat exchanger to produce hot process water. The cyclotube is directly connected up to a latency tank, or like pulp storage tank.

Description

'7530 ~EAT RECOVERY WIT~ DI~FE~ENTIATED MUTLI-FLAS~
_ _ _ _ _ DURING PRODUCTION OF HIGH ~IELD PULP
BACKGROUND AND SUMMARY OF THE INVENTION

The production of mechanical pulps is of increasing interest since a higher yield can be obtained from a given amount of raw material utilizing mechanical pulping processes as compared to chemical pulping processes. Mechanical pulping, in general, refers to refiner mechanical pulping (RPM), thermomechanical pulping (TMP), chemi-mechanical pulping (CMP), chemithermomechanical pulping (CTMP), and board pulping. In each case a refiner (defibrator) is utilized as one of the basic components for breaking down the chips (or like comminuted cellulosic fibrous material) into progressively smaller bundles in a fibrillation process. During the refining action, frictional heat is developed, and that produces steam.
Despite the high yields available for mechanical pulps, in many areas of the world the production of such pulps is not economical because of the energy intensive nature of the fibrillation process. In such situations, it is necessary that the process steam generated during the refining process be effectively utilized, and in that way the mechanical pulping process can be made more economical. Typically, the pulp and process steam in the discharge from the defibrator are separated in a centrifu~al separator ~cyclone), or like device, and a significant portion of the separated steam is passed directly to a chips presteaming vessel for steaming the chips prior to their introduction into the defibrator. While such a - . :

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, --~;~77530 procedure does make the mechanical pulping operation more economical, it is much less efficient than is desired, and can result in the eventual build up of contaminants in the pulp to too high a level.
S The pulp discharged from the cyclone in the conventional mechanical pulping processes is in actuality "over qualified" for use in chips steaming. That is, it has such a high pressure and heat content that there are potentially many other more valuable uses for it than presteaming the chips. Additionally, about 50-90 percent of the terpenes (as well as other contaminants such as methanol), from the pulp pass into the stream of steam being discharged from the cyclone. When the steam is recirculated to the chip presteaming vessel to contact chips therein, these terpenes thus are returned to the pulp stream, and the level of terpenes in the pulp eventually builds up. For the production of some paper pulp products, this is extremely detrimental. For instance in the production of cardboard designed to hold liquids (such as for the packaging of milk products), the smell and taste of the product being packaged can be affected by the turpentine within the pulp.
According to the present invention, a method and apparatus are provided which utilize the process steam inherently produced during the defibrillation action in the most efficient manner, and in a manner such that build up terpenes, and like contaminants, does not occur. The basic feature according to the present invention is the flashing of the pulp stream discharged from the defibrator in a number of different stages, with a different "quality" of steam produced in each stage. The steam from each stage is then used in a - . , : ~ , 5;~

manner to best take advantage oE its pressure and heat content.
For instance the steam separated in a first centrifugal separator is passed through a reboiler to produce fresh steam, and then the fresh steam is used to drive the turbine for powering the refiner, or for a like function suited for steam having that pressure and heat content. The steam from the second centrifugal separator (which has very little terpenes since most have been separated out in the steam stream from the first centrifugal separator) has a pressure and heat content ideally suited for chip presteaming. If additional centrifugal separators are used, the separated steam therefrom is utilized for other purposes particularly suited to the pressure and heat content of the separated steam.
By the practice of the present invention, the quantity of the primary process stream -- the highest quality steam -- can be increased 20-30 percent compared to conventional single flash procedures, while the quantity of moisture laden hot air produced in a cyclotube downstream of the centrifugal separator (which air is used for hot water production) is reduced. Further, according to the present invention turpentine from the pulp can be readily recovered so that it is a worthwhile by-product, instead of being a contaminant that may adversely affect the quality of the pulp produced.
It is the primary object of the present invention to provide a method and apparatus for the energy and cost-effective production of mechanical pulp. This and other objects of the invention will ', : ' ~'~7753() become apparent from the detailed description of the invention, and from the appended claims.

B~IEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a block diagram showing a conventional prior art system for separation and utlization of the process steam produced during mechanical pulping;

~ IGURE 2 is a block diagram schematically illustrating the practice of an exemplary method according to the present invention; and FIGURES 3 and 4 are schematic diagrams illustrating other exemplary procedures, and exemplary apparatus, according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Prior Art In a conventional single flash heat recovery system in the production of mechanical pulp, as illustrated in FIGURE 1, chips ln line 10 are optionally pretreated ~as with certain chemicals in the production of CMP, CTMP, and the like) in station 11, and then passed to a chip presteaming vessel 12 wherein the chips are steamed to make them more malleable, to increase the effectiveness of the defibrillation proces~. After presteaming in vessel 12, the chips are fed to a defibrator 13 wherein the refining action takes place, frictional heat being produced during the refining action changing a significant amount of the liquid in the chips-liquid , .
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7753~3 slurry within the defibrator 13 into process steam. The mechanical pulp produced by the defibrator 13~ with steam intimately mixed therewith, is discharged from the defibrator 13 in line 14 and passes to a cyclone 15 or like separating means for separating the pulp from the steam. The steam is discharged from the cyclone 15 in line 16, while the pulp is discharged in line 17. The pulp discharged from the cyclone 15 typically would have a pressure of about 4.5 bar (ABS). For a typical operation wherein 1.387 tons per bone dry metric ton (BDMT) of steam passes in line 16, about 0.3 tons would be recirculated in line 18 to the chip steaming vessel 12, while the remaining 1.087 tons would pass Lo a reboiler 19, where it would pass in heat exchange relationship with feed water to produce fresh steam in line 20, with the dirty condensate from the reboiler 19 sewered. The fresh steam in line 20 typically would be at a pressure of 4.0 bar, produced at a rate of 1.0 tons per BDMT.
The pulp in line 17 passes through a cyclotube 21 which is disposed in operative association with the pulp tank 22. A cyclotube is a large diameter (compared to that of line 17) tube (e.g. a vertical tube one meter in diameter and 7 meters high) in which pulp and gas (moist, saturated hot air) are separated. The moisture laden hot air that is produced by the flashing that takes place in the cyclotube passes in line 24 to a heat exchanger 25 and is used in the production of process hot water, which is discharged from the heat exchanger 25 in line 26. The feed fluid in line 24 is sewered after it passes through heat exchanger 25.

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In the practice of the prior art procedure schematically illustrated in FIGURE 1, since the steam in line 16 has about 50-90% of the terpene~
from the pulp, a significant amount of turpentine is recirculated to the chips in line 18, ultimtely producing a build up of turpentine in the pulp so that the final mechanical pulp produced in line 28 contains significant traces of turpentine.
Additionally, the quantity of high quality fresh steam produced in line 20 is less than desirable.
The Invention One exemplary manner in which the invention may be practiced is illustrated schematically in FIGURE 2. In FIGURE 2 structures corresponding to those in the prior art of FIGURE 1 are illustrated by the same reference numeral only preceded by a "1" .
In the e~emplary form of the present invention illustrated in FIGURE 2, essentially all of the steam in line 116 passes to the reboiler 119. Thus where the amount of steam produced in line 116 is 1.387 tons per BDMT (same figures as for the prior art of FIGURE 1), the amount of fresh steam in line 120, at 4.0 bar, will be about 1.253 tons per BDMT. Also, according to the invention it is desirable to treat the dirty condensate from reboiler 119 at station 30 to recover the terpenes, methanol, or like substances therefrom, and after recovery of those materials the condensate is sewered.
In the practice of the invention as illustrated in FIGU~E 2, the pulp passing in line 117 passes to a second centrifugal separator 32. In this second centrifugal separator 32 the steam is discharged in line 33 while the pulp continues in .' ~ ' ~ .
- - . : ' 1.'~.77530 the same path 117 to the cyclotube 121. The steam in path 33 typically would have a pressure of 2.8 bar, with 0.3 tons per BDMT produced. This "quality" of steam is perfect for presteaming in the vessel 112, and contains very little turpentine since most of it has already been separated out in the first centrifugal separator 115. Should the amount of steam in line 33 be insufficient to properly effect presteaming in vessel 112, a small amount (e.g. up to about 0.05 tons per BDMT) of the steam from line 116 can be diverted into line 35, and be introduced into the steam in line 33 utilizing a Venturi injector, or the like.
Since more of the heat content of the discharge in line 114 is recovered prior to the cyclotube 121 in the FIGURE 2 embodiment, as compared to the FIGURE 1 embodiment, the amount of moisture laden hot air in the cyclotube discharge 124 will be reduced, and thus the amount of hot water discharge in line 126 reduced. Since turpenes have not been significantly recirculated, the final mechanical pulp produced, in line 128, will have significantly less turpentine than the pulp in line 28 in FIGURE 1.
In an exemplary process for the production of CTMP utilizing the procedures of FIGURES 1 and 2, the illustrative data in the following table shows that an increase in the amount of high quality process steam on the order of about 25 percent can be expected by practicing the invention:

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TABLE

Data:

Capacity 14 ADMT/h Installed electrical motor 15 MW
Motor, efficiency 96%
Reboiler, efficiency92%
1 ton steam/h x 0.8141 MW

Theoretical Process steam 0.96 x 15 = 17.69 T/h 0.814 Clean steam after reboiler 17.69 x 0.92 = 16.28 T/h Practical Single flash system - Fig. 1 Rate of clean steam pro-duction 1.000 T/BDMT = O.900T/ADMT
Rate x quality 14 x 0.900 = 12.60 T/h Recovery of energy 12.6 x_100, _ 77,4 (practical ~ theoretical) 16.28 Multi-flash system - Fi~. 2 Rate of clean steam pro-duction 1.253 T/BDMT = 1.128 T/ADMT
Rate x quality 14 x 1.128 =15.79 T/h Recovery of energy 15.79 x 100 _ 97 0%
(practical ~ theoretical) 16.28 Difference in recoverv (Fig. 2 vs. Fig. 1) -Steam 15.79 - 12.60 = 3.19 T/h Efficiency increase 3.19 x 100 - 2 (Fig. 2 over Fig. 1) 12.-60 ~ 5.3 :
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' -: - . . ~ ~, -7'753~:) While only a single additional separator has been illustrated in the embodiment in FIGU~E 2, according to the present invention it is possible to utilize additional flashing stages between the defibrator and the cyclotube, depending upon the initial pressure of the process steam in the pulp discharged from the refiner. In the FIGURE 3 embodiment, structures comparable to those in the FIGURES 1 and 2 embodiment have the same reference lQ numeral only with a "2" as a first digit, and in the FIGURE 4 embodiment a "3" appears as the ~irst digit.
In the PIGURE 3 embodiment for the production of TMP and CTMP, the initial chip lS treatment apparatus is slightly different than in the FIGURES 1 and 2 embodiment. Chips can be initially heated in chips bin 40, washed in washing vessel 41, impregnated with chemical in vessel 42, and then passed to steaming tube 43 wherein presteaming takes place. In this embodiment, an additional steam/pulp separating structure tsuch as a centrifugal separator, steam separating conveyor, or the like) 45 is disposed between the separator 232 a~d the cyclotube 221. Thus there are primary (from separator 215), secondary ~from separator 232), and tertiary (from separator 45) steam streams. As in the FIGURE 2 embodiment, the primary steam passes through reboiler 219 to produce clean steam in line 220, and the secondary steam in line 233 is utilized for chip presteaming in steam tube 43. The tertiary steam discharged in line 46 from third separator 45 typically would have a pressure of about 1. 5 bar (as compared to about 4. 5 bar in line 216, about 2.5 bar in line 233, and about 0.3 bar for the gas in line 22~), and would be combined , : ' ' ' . , .

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with heat recovered from the condensate for the reboiler in heat exchan~er 48, connected to a scrubber 50 for utilizing the heat and then passed in line 51 under the influence of fan 52 to chip bin 40. The fluid in line 51 cannot actually be considered as steam but is moisture saturated hot air, and performs a valuable function when i.e.
contacting frozen chips in bin 40.
In the FIGURE 4 embodiment, the defibrator and related structures can be the same as in the FIGURES 2 or 3 embodiments. The pulp and process steam in line 314, when the FIGURE 4 structure is utilized, would typically have process steam at a pressure of about 6-10 bar. If even higher pressure steam is produced in refiners in the future, additional flashin~ stations can be utilized. In the FIGURE 4 embodiment, three additional separators 332, 345, and 58 are utilized between the first separator 315 and the cyclotube 321. The steam in each o~ the lines 333, 346, and 59 is passed to a user station 60, 61, 62, respectively. The user station 60, 61, 62 employed is chosen depending upon the pressure and heat content of the steam in each of the lines 333, 346, and 59. Each of the lines 333, 346, 59 is preferably optionally connected to the preceding steam discharge line so that make up steam can be provided where one of the user statlons 60, 61, 62 requires additional steam. For instance the line 335 can provide some additional steam to line 333, which steam would be introduced by Venturi injector 336. Similarly Venturi injector 65 can introd~ce additional steam in line 346 from line 66, Venturi injector 67 can introduce additional steam in line 59 from line 68, and where it is desirable a Venturi iniector (69) may even be provided in the .

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discharge line 324 containing moisture saturated hot air from cycltube 321, which introduces steam from line 70.

It will thus be seen that according to the present invention an effective method and apparatus have been provided for increasing the production of high quality steam during mechanical pulping processes, and reducing the amount of turpenes, and the like, in the final mechanical pulp produced.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof lS within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and structures.

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Claims (7)

1. A method of producing mechanical pulp from comminuted cellulosic fibrous material, utilizing a defibrator, comprising steps of:
(a) steaming the material;
(b) introducing the steamed material, in a liquid slurry, into the defibrator;
(c) refining the material in the defibrator to produce pulp, frictional heat simultaneously being generated during refining to produce process steam;
(d) discharging the pulp and steam from the defibrator;
(e) effecting separation of the pulp and some of the steam discharged in step (d) at a first separation stage, and passing the separated steam in a first path and the separated pulp in a second path;
(f) separating additional steam from the pulp in the second path, at a second separation station, and passing the separated steam in a third path;
(g) utilizing essentially all the steam in the third path to steam the material in step (a); and (h) reboiling substantially all the steam passing in the first path, to produce fresh steam and without return of the steam in the first path for use in steaming the material in step (a).

2. A method as recited in claim 1 comprising the further step (i) of, after step (f), flashing the pulp and any additional steam passing in the second path to produce moisture saturated warm air, and passing the moisture saturated warm air through a heat exchanger to heat process water.

3. A method as recited in claim 1 comprising the further step of separating contaminants including at least one of terpenes and methanol, from the steam in the first path.

4. A method as recited in claim 3 wherein the separating of contaminants including at least one of terpenes and methanol from the steam is effected after the steam is passed through a reboiler.

5. A method as recited in claim 2 comprising the further step of, immediately after flashing of the pulp and other residual steam, passing the pulp to a latency tank.

6. A method as recited in claim 2 comprising the further step (j) of, between step (f) and step (h), separating additional steam from the pulp and passing the separated steam in a fourth path.

7. A method as recited in claim 1 wherein separation in steps (e) and (f) is accomplished utilizing first and second centrifugal separators, respectively.