Information
-
Patent Grant
-
6176971
-
Patent Number
6,176,971
-
Date Filed
Wednesday, November 18, 199826 years ago
-
Date Issued
Tuesday, January 23, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 162 23
- 162 29
- 162 46
- 162 47
- 162 261
- 162 250
-
International Classifications
-
Abstract
Useable steam, e.g. substantially clean useable steam, is produced from a chemical cellulose pulping system hot spent treatment liquor (e.g. black liquor) with optimum energy efficiency by passing the spent liquor to a reboiler, and then pressurizing (e.g. with an eductor, fan, or compressor) the clean steam discharged from the reboiler. The clean steam can be used to steam incoming chips (e.g. in a chip bin) without significantly increasing the TRS load on the pulp mill NCG system.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The processing of comminuted cellulosic fibrous material, for example, wood chips, to produce cellulose pulp is an energy-intensive process. Treatment temperatures greater than 100° C. and superatmospheric pressures are typical of most commercially-significant chemical treatments of cellulose, for example, the kraft process (also known as the sulfate process). What has made the kraft process so commercially successful is its inherent regeneration of energy from the combustion of waste cooking liquors and the regeneration of cooking chemical from the products of combustion through recausticization. However, due to the cost of energy in the late twentieth century, any further improvement to the energy efficiency of the chemical pulping process is welcome.
In published International application WO 96/32531 (the disclosure of which is hereby incorporated by reference herein) a method of recovering energy from spent cooking liquors by passing the liquors in heat-exchange relationship with cooler liquors, for example, in a steam generating device referred to as a “reboiler”, to produce fresh, uncontaminated steam, is disclosed. This method contrasted with earlier methods of recovering energy from spent liquor by using flash evaporation. Such earlier methods inherently entrained undesirable sulfur-bearing compounds, such as hydrogen sulfide [H
2
S], methyl mercaptan [CH
3
SH], dimethyl sulfide [CH
3
SCH
3
], and dimethyl disulfide [CH
3
SSCH
3
] as well as other gases, referred to collectively as Total Reduced Sulfur gases or TRS gases. When such steam generated by the flashing of spent cooking liquors was used to pretreat wood chips, the undesirable TRS gases introduced an additional treatment load on the Non-Condensable Gas (NCG) collection system, if present, or introduced the risk of releasing such undesirable, malodorous compounds to the environment.
Published PCT application WO 97/29236 discloses various methods for recovering and utilizing the heat of spent cooking liquor. These include the use of liquor-liquor heat exchangers, reflux boilers and evaporators. However, though this PCT application discloses a process that is similar to the process disclosed in WO 96/32531 in which a source of “clean” steam is provided, this PCT application does not disclose nor suggest the use of a process according to the present invention.
The present invention represents an improvement to the invention in WO 96/32531 by decreasing the temperature of the steam that can be produced by the reboiler or other steam generating device so that more energy in the form of heat can be recovered from the spent cooking liquor and re-used in and around the pulp mill.
In its broadest embodiment the present invention comprises or consists of a method and apparatus for increasing the energy efficiency of a cellulose digester heat recovery system by: (a) introducing a slurry of comminuted cellulosic fibrous material and liquid to a treatment vessel to produce cellulose pulp; (b) treating the material in the vessel with treatment chemical at a temperature greater than 140° C. and a pressure greater than 5 bar gage to produce a treated material and a spent treatment liquor, including sulphur compounds; (c) removing the spent treatment liquid from the vessel at a first temperature; (d) passing the spent liquor in indirect heat exchange relationship with a cooler liquid having a second temperature lower than the first temperature to produce steam (e.g. substantially clean steam) at a first pressure, and at a third temperature higher than the second temperature; (e) increasing the pressure of the steam produced to a second pressure higher (i.e. at least 10% higher and/or by at least about 0.2 bar gage and/or to at least 0.2 bar gage) than the first pressure; and (f) using the steam at the second pressure as needed in the digester system.
In the practice of the method set forth above, (e) may be practiced using a fan or compressor, or an eductor (also known as an ejector) wherein steam at a pressure of between about 3 and 13 bar gage is added to the steam from (d) in the eductor to increase the pressure of the steam from (d). When (e) is practiced so that the second pressure is at least 0.2 bar gage (e.g. 0.4 bar gage or more), (e) is also preferably practiced so that the temperature of the steam is at least 105° C. (e.g. 105-109° C. or more), and then (f) may be practiced to steam comminuted cellulose material being introduced into a feed system for the treatment vessel. In another aspect, (d) may be practiced by introducing as the cooler liquid an essentially sulfur-free liquid, or “clean” liquid, for example, essentially clean water, such that the steam produced is essentially clean steam, for example, containing little or no sulfur compounds. In a further embodiment of this invention, the above invention includes an additional step (g), during step (d), of producing a cooler spent cooking liquor from the spent liquor and using at least some of this cooler spent cooking liquor as the source of cooler liquid in step (d).
Also in the practice of the above method, the spent liquor removed in (c) is typically black liquor at about 140-160° C., and the method further comprises (h) passing the black liquor into indirect heat exchange relationship with a third liquid to recover some of the heat from the black liquor in the third liquid prior to the practice of (d). Typically this is achieved by passing the hot black liquor in heat exchange relationship with one or more liquids in one or more heat exchangers. Typically (d) is practiced using a steam generating device, for example a reboiler; and may further comprise positively cooling the spent liquor after it passes through the steam generating device. Again, this positive cooling may be effected by passing the liquor in heat exchange relationship with one or more liquids in one or more heat exchangers after the practice of (d).
In a preferred embodiment, the positive cooling of the spent liquor after the steam generating device is effected by passing the spent liquor in indirect heat exchange relationship with a fourth liquid to reduce the spent liquor temperature below the prevailing boiling temperature of the liquor (typically around 100° C.) so that the spent liquid may be stored in an unpressurized state, for example, in an unpressurized storage tank, prior to further treatment.
According to another aspect of the invention a method of producing steam having less noncondensible gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam, in a digester system, is provided. The method comprises: (a) Passing hot black liquor, including sulphur compounds, at a temperature of about 120-165° C. from a digester through a heat exchanger. (b) Passing an evaporable liquid to be evaporated through the heat exchanger into heat exchange contact with the hot black liquor so that the evaporable liquid is heated so that it is ultimately evaporated to produce steam having less condensable gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam. (c) Increasing the pressure of steam from (b) by at least 0.2 bar gage. And, (d) using the steam from (c) in the digester system.
In this method, preferably the evaporable liquid used in (b) is substantially free of sulphur compounds; and (b) is practiced to produce substantially clean steam. Step (c) may be practiced using an eductor, fan, or compressor, as described above, and to produce substantially clean steam at a pressure of at least 0.2 bar gage and a temperature of at least 105° C. The method may also comprise (e) passing the hot black liquor into indirect heat exchange relationship with another liquid to recover some of the heat from the black liquor in the other liquid prior to the practice of (b). In a preferred embodiment the noncondensible gases that are reduced are preferably one or more Total Reduced Sulfur (TRS) gases, as described above.
According to another aspect of the present invention there is provided a digester system comprising: A digester having a hot spent treatment liquor outlet. A feed system for feeding comminuted cellulose material to the digester. A steam generating device operatively connected to the spent treatment liquor outlet to use the heat of the spent treatment liquor to produce steam from another liquid, the steam generating device having a steam outlet and a spent treatment liquor outlet. A pressure increasing device, connected to the steam outlet, which increases the pressure of the steam from the steam generating device. And, a utilization device operatively connected to the pressure utilization device to utilize the steam therefrom.
In the digester system the pressure increasing device may be a fan, compressor, or eductor. The system may also comprise an indirect heat exchanger connected between the spent treatment liquor outlet and the steam generating device for recovery of some of the heat from the hot spent treatment liquor before that liquor is passed to the steam generating device; and a flow control valve between the pressure increasing device and the utilization device controlled by feedback from the utilization device, in which case preferably the utilization device comprises the feed system (e.g. a chip bin therein).
It is the primary object of the present invention to provide enhanced recovery and effective use of energy from spent cooking liquor during chemical pulping. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
schematically illustrates a typical prior art cellulose pulp treatment system over which the present invention is an improvement;
FIG. 2
schematically illustrates a system in which the present invention can be utilized for the digester shown in
FIG. 1
; and
FIGS. 3 and 4
schematically illustrate two preferred embodiments of the pressurizing device and procedure part of the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1
illustrates a prior art system
10
for treating comminuted cellulosic fibrous material, for example wood chips,
11
over which the present invention is an improvement and into which the present invention can be incorporated. System
10
includes a feed system
12
for introducing wood chips
11
and initiating the treatment, and a digester system
13
in which the chips are treated to produce a cellulose pulp
14
. Though the system
13
shown is a continuous digester system, it is understood that the present invention can also be implemented in a discontinuous, or batch, digester system.
Though any feeding system can be used to introduce chips to system
10
, the system shown, and the preferred system when implementing the present invention, is the system described in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,736,006; 5,753,075; 5,766,418; and 5,795,438. This feed system is marketed under the trademark Lo-Level® by Ahlstrom Machinery Inc. of Glens Falls, N.Y.
Wood chips
11
are introduced via a conventional conveyor (not shown) for example, to an isolation device
16
. The isolation device
16
isolates the feed system from the atmosphere, and may be any type of conventional isolation device, for example, a Chip Gate as sold by Ahlstrom Machinery, but it is preferably a screw conveyor with a weighted hinged gate at its discharge as described in U.S. Pat. No. 5,766,416. The isolation device
16
introduces chips to a retention and steaming vessel
17
. Again, though any type of steaming or retention vessel may be used, the preferred vessel
17
is the one described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; 4,958,741; and 5,700,355 and marketed under the trademark DIAMONDBACK® by Ahlstrom Machinery. The DIAMONDBACK® steaming vessel is characterized as providing uniform steaming of wood chips while not requiring any form of vibration or agitation at its discharge. This uniform discharge without agitation is effected by using a unique single convergence geometry. Steam is provided to the bin
17
via one or more nozzles
18
. As is typical, bin
17
includes one or more conventional gamma-radiation level detectors (not shown), an exhaust vent
20
for collecting gases which can accumulate in the bin
17
, and a pressure controlling vent
21
.
Bin
17
discharges to a metering device
22
, which may be a Chip Meter sold by Ahlstrom Machinery, as shown, or one or more metering screws. The metering device optionally discharges steamed chips to a conventional pressure isolation device such as a star-type pressure isolation device, for example, a Low Pressure Feeder sold by Ahlstrom Machinery, but preferably discharges steamed chips directly to a conduit
24
, for example, a Chip Tube or Chip Chute as sold by Ahlstrom Machinery. The fully-steamed and slightly pressurized chips first encounter cooking liquor in conduit
24
. The cooking liquor is introduced via one or more conduits
27
. This cooking liquor may be any form of sulfate (e.g. white liquor) or sulfite cooking liquor and may contain yield or strength enhancing additives. Steam may also be introduced to conduit
24
via one or more conduits
25
. Conduit
24
may also have a gamma-radiation level detection system (not shown) for controlling the level of chips in the conduit
24
.
As is characteristic of the LO-LEVEL® feed system
12
, as described in the above U.S. patents, conduit
24
introduces a slurry of chips and liquor to the inlet of a slurry pump
28
. The slurry pump
28
may typically be a HIDROSTAL pump as sold by Wemco of Salt Lake City, Utah, though other equivalent pumps may be used. Additional liquid may be supplied to the inlet of the pump
28
from liquor tank
29
and conduit
30
. Pump
28
passes the chip slurry to the low pressure inlet of high-pressure transfer device
31
via conduit
32
. Device
31
is typically a high-pressure feeder as sold by Ahlstrom Machinery, though any type of transfer device can be used. The high-pressure feeder
31
includes a pocketed rotor which receives a slurry of chips from conduit
32
and via rotation exposes the slurry to a flow of high pressure liquid provided by pump
33
, known in the art as a Top Circulation Pump. The slurry is pressurized to 5-10 bar and transported to the top of vessel
34
, the digester, via conduit
35
. Excess liquor introduced to digester
34
via conduit
35
is separated using a separating device
36
, for example, a Top Separator, and returned to the feed system
12
by conduit
37
to supply the liquid pressurized by pump
33
.
Feed system
12
also utilizes conventional equipment typically included in such systems, including a Sand Separator
38
, an In-line Drainer
39
, a Level Tank
40
, a Feed Circulation Liquor Cooler
26
, and a Make-up Liquor Pump
41
, all sold by Ahlstrom Machinery. Cooking chemical, for example, kraft white, green, or black liquor, is typically introduced to the feed system
12
by conduits
42
,
43
and
27
.
Digester
34
may consist of a single vessel, as shown, or it may comprise multiple a vessels, for example, an impregnation vessel and a digester. Digester
34
typically includes at least two or more annular screen assemblies
44
,
45
,
46
and
92
. Any type of treatment may be preformed in vessel
34
, but preferably the treatment comprises or consists of one or more of the processes disclosed in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775, and marketed under the trademark LO-SOLIDS® by Ahlstrom Machinery.
For example, in
FIG. 1
, one form of a LO-SOLIDS process is provide by removing liquor from vessel
34
via screen
44
and conduits
47
and
48
and forwarding the liquid in conduit
47
to treatment outside the digester
34
, for example, to flash evaporation in flash tank
49
via conduit
60
. The liquid removed from screen
44
via conduit
48
may be recirculated via pump
51
, heater
52
, conduit
53
, and a central distribution conduit (a “center-pipe”)
54
and reintroduced to the digester
34
in the vicinity of screen
44
. Heater
52
may also be bypassed via conduit
53
. Cooking liquor or dilution liquor may be introduced to conduit
53
via conduit
55
.
Digester
34
also includes a circulation associated with screen
45
. Liquor removed via screen
45
is preferably augmented in conduit
59
with cooking liquor and dilution liquor via conduits
15
and
56
. Cooking liquor is typically supplied via conduits
42
,
23
and
15
and pressurized via white liquor pump
19
or its spare
19
′. The dilution liquid
56
is typically supplied via conduits
25
and
56
and pressurized by filtrate pump
93
. Though wash filtrate is shown as the source of dilution in
FIG. 1
, it is understood that any source of low-dissolved-organic-material liquid may be used as diluent.
The augmented liquid in conduit
59
is then pressurized by pump
57
and passed through heat exchanger
58
prior to being reintroduced to the digester
34
in the vicinity of screen
45
via center-pipe
54
. Typically, the cooking liquor introduced via conduit
59
and center-pipe
54
passes counter-currently to the flow of wood chips due to the removal of liquid via screen
44
and conduit
47
. This counter-current flow of heated cooking liquor also raises the temperature of the slurry between screens
44
and
45
to cooking temperature, that is, to a temperature between about 140 and 180° C. Cooking typically proceeds between screens
45
and
46
such that the liquor removed via screen
46
is approximately at cooking temperature and contains spent cooking chemical and dissolved products of the cooking reaction. In the prior art system shown in
FIG. 1
, the cooking process progresses below screen
46
in co- or counter-current fashion. For example, in a circulation associated with the lowermost annular screen assembly
93
, liquor is removed from the digester
34
via conduit
94
, augmented with cooking liquor via conduit
95
and dilution liquor
96
, pressurized via pump
97
, heated by heater
98
and returned to the vicinity of screen
92
via center-pipe
54
.
The cook is terminated with the introduction of dilution liquid, for example, washer filtrate, at one or more locations via conduits
99
and
100
in the lower part of the digester
34
. The diluent may be cooled by a cooling heat exchanger
101
, for example, a Cold Blow Cooler as provided by Ahlstrom Machinery. The essentially fully-cooked pulp is discharged from the bottom of the digester
34
at a temperature of about 80 to 120° C., typically with the aid of a rotating discharge device
68
, into conduit
14
and forwarded to further treatment, for example, brown stock washing.
In typical prior art cooking systems, the hot spent liquor removed via screen
46
, typically at between 140-180° C. and 5 to 10 bar gage, is expanded under controlled conditions, that is, “flashed”, to remove water vapor and other volatile compounds in the liquor as steam, and to lower the temperature and pressure thereof. In
FIG. 1
, hot liquor removed by screens
44
and
46
is carried by conduits
60
and
61
, respectively to flash tank
49
.
The hot liquor in conduit
61
, typically at about 140-160° C. and 5-10 bar gage, may be passed through a heat exchanger
102
to heat other liquids. For example, heat exchanger
102
may be a Digester Steam Economizer, as sold by Ahlstrom Machinery, through which the diluent in conduit
56
is passed to heat the diluent prior to introducing it to cooking circulation conduit
59
. The liquor in conduit
60
, for example, at about 115-125° C., is typically cooler than the liquor in conduit
61
because the liquor in conduit
60
is removed earlier or prior to the chip slurry being heated to cooking temperature. Thus, the combined liquors introduced to flash tank
49
is only about 120 to 140° C.
In flash tank
49
the hot, pressurized liquor is exposed to a lower pressure, for example, a pressure between 0.8 and 1.5 bar gage, so that some of the water in the liquor rapidly evaporates, or “flashes”, to steam. This steam, typically at a temperature of about 103 to 113° C. and a pressure of about 0.1 to 0.6 bar gage, is then carried by conduit
64
for use in other parts of the mill. One preferred use of this steam is as the steaming medium in chip bin
17
, introduced via conduit
18
.
The cooler, lower pressure liquor, typically at a temperature of about 117 to 127° C. and a pressure of about 0.8 to 1.5 bar gage, is discharged from flash tank
49
into conduit
63
and pumped via pump
66
to further processing, for example, to spent liquor filtering device and chemical recovery. The steam in conduit
18
may also be supplemented by fresh steam or steam from other sources via conduit
67
.
Since the steam that is discharged in conduit
64
is produced from the direct evaporation of spent (sulphur compound containing) cooking liquor, it contains various amounts of TRS gases, such as hydrogen sulfide [H
2
S] or methyl mercaptan [CH
3
SH]. Introducing these sulfur-containing gases to the feed system
12
is undesirable since some accommodation must be made to prevent leakage of these malodorous gases to the environment, for example, by using a dedicated NCG collection system. It is difficult to prevent the release of the TRS gases via isolation devices and air-locks while introducing comminuted cellulosic fibrous material to the feed system
12
. For this reason, it is desirable to not produce sulfur-bearing steam streams for re-use in the feed system or anywhere else in the pulp mill.
WO 96/32531 introduced the concept of not flashing the spent cooking liquor,
60
,
61
, but instead using this hot liquor as a heating medium for liquids containing little or no sulfur compounds to produce a substantially sulfur-free steam. That earlier invention used a device referred to as a “reboiler” to pass the reduced-sulfur-containing liquid in heat-exchange relationship with the hot spent cooking liquor. However, that invention is thermodynamically limited by the “clean” steam pressure required. That is, a certain minimum pressure is required in the prior art system that is sufficient for using the steam produced as needed. This minimum pressure thermodynamically limits the temperature of the steam that can be produced and as a result limits the amount of cooling that can be done in the steam generating device, that is, reboiler. The present invention overcomes this disadvantage to provide not only a more efficient way of producing a substantially clean source of steam, but also to permit a more efficient recovery of energy from the hot spent cooking liquor.
FIG. 2
illustrates one embodiment of a system
110
according to the present invention. System
110
includes a digester
34
and related equipment that is essentially identical to the digester
34
and related equipment shown in FIG.
1
and is labeled with the same reference numerals. In addition
FIG. 2
also includes a steam generating device or reboiler
71
that replaces the flash tank
49
of FIG.
1
.
Similar to the system disclosed in WO 96/32531, the hot spent liquor in conduit
61
,
61
′, at about 110-170° C., preferably at about 120-150° C., is passed through a heat exchanger
71
, specifically a kettle-type reboiler, to heat a liquid
72
containing little or no sulfur compounds, to produce a source of steam
73
for use as needed, preferably to steam chips in the feed system
12
of FIG.
1
. The liquid in conduit
72
is typically mill water at a temperature of about 60-90° C., preferably about 70-80° C. At the same time, the hot liquor in
61
,
61
′ is cooled and discharged to conduit
74
, at about 80-120° C., preferably about 90-110° C., and forwarded to further treatment, for example, to a conventional fiber filtering device and then to a heat and chemical recovery system. The liquor in conduit
74
can be forwarded to one or more additional heat exchangers, for example, a Black Liquor Cooler
74
′ as sold by Ahlstrom Machinery, to further cool the liquor, to about 80-100° C., and recover heat from it. However, if the reboiler
71
is sufficiently large enough, substantially all cooling for heat recovery can be effected in the reboiler
71
and no further cooling is necessary. This can be particularly effective when a reduced volume of steam, or no steam, is required in the feed system
12
, for example, no steaming is done in chip bin
17
. Heated excess hot non-sulfur-containing liquid is discharged into conduit
75
and used as needed in the pulp mill.
In a preferred embodiment, the hot liquor
61
may also be passed in heat exchange relationship with other liquids in one or more heat exchangers
102
before the liquor is passed to reboiler
71
. The number of heat exchangers used depends upon the desired pressure and volume of the steam produced by the reboiler
71
, the capacity of the one or more heat exchangers
102
, and the temperature of the hot spent liquor
61
, among other things. The preferred medium heated in heat exchanger
102
is dilution filtrate introduced to one or more Lo-Solids® cooking circulation associated with digester
34
. For example, dilution liquor, such a washer filtrate, evaporator or heat exchanger condensate, or clean water, is preferably passed in conduit
76
, heated in heat exchanger
102
, for example, a Digester Steam Economizer as sold by Ahlstrom Machinery, and introduced to the Lo-Solids cooking circulation
59
via conduit
56
. In a preferred embodiment, only the hotter liquor removed from screens
46
, at about 140-160° C., is passed through heat exchanger
70
. The cooler spent liquor in conduit
60
, at a temperature of about 100-130° C., preferably about 110-120° C., bypasses the heat exchanger
102
and is introduced to conduit
61
′ downstream of heat exchanger
102
. In this way, only the hottest spent cooking liquor is used as a heating medium in heat exchanger
102
. The present invention is not limited to mixing the liquor streams
60
and
61
, but can also be applied to either stream alone. For example, only the hotter liquor in conduit
61
, with or without passing through heat exchanger
102
, may be passed to reboiler
71
to perfect the present invention.
According to the present invention the relatively clean steam
73
produced by reboiler
71
is pressured by a pressurization device
77
prior to being transferred via conduit
78
to the consumer or utilization device
79
of the steam. The pressure of the steam sent to device
79
is controlled by a pressure control valve
90
which receives a pressure control signal via loop
91
from the device
79
utilizing the steam. Utilization device
79
may be a digester feed system
12
(e.g. chip bin
17
) as shown in
FIG. 1
, or may be any other device in which a pressurized source of clean steam can be used. By pressurizing the steam in conduit
73
using device
77
, the steam produced by reboiler
71
may be produced at a lower pressure. Allowing the reboiler
71
to produce steam
73
at a lower pressure thermodynamically allows the lower pressure steam to be produced at a lower temperature. Allowing the steam
73
to be produced at a lower temperature, allows the temperature of the cooled spent liquor
74
to be lower and the temperature of the hot excess substantially sulfur-free liquid
75
to be hotter.
For example, in the prior art system of WO 96/32531, without the pressurization of steam
73
via device
77
, the pressure of the steam produced by reboiler
71
has to be great enough to supply the consumer
79
of the steam. For example, if the steam is used in the feed system
12
of the digester
34
, the steam must have a pressure of at least approximately 0.2 to 0.4 bar gage. This thermodynamically requires that the temperature of steam
73
be at least about 105-109° C. This minimum temperature of the steam limits the temperature of the cooled hot spent liquor
74
to about 106-114° C., depending upon the economics of the installation and the size of the reboiler
71
, among other things. The temperature of the hot sulfur-free liquid
75
is also cooled to approximately the same temperature of the steam
73
, that is, 105-109° C. Liquid
75
may not be removed continuously, but may be discharged intermittently as needed (this is referred to as “blow down”) to control the concentration of dissolved material present in the liquid in the reboiler. However, by introducing a pressurization step/device
77
, the minimum pressure of the steam
73
is substantially unlimited and may even be a vacuum. Thus, this lower pressure thermodynamically allows the temperature of steam
73
to have a substantially unlimited lower temperature. Furthermore, this permits the temperature of the liquid in conduit
74
to be lower and the temperature of the liquid in conduit
75
to be lower than in the system
10
of FIG.
1
.
FIGS. 3 and 4
schematically illustrate two specific embodiments of the pressurization step of the invention.
FIG. 3
illustrates the use of an eductor
80
as the pressurization device
77
of FIG.
2
. In this embodiment, the steam in conduit
73
from reboiler
71
of
FIG. 2
is introduced to the eductor
80
and mixed with the gas from a gas supply
81
at a higher pressure than the steam in conduit
73
(e.g. at least 10% higher). The resultant steam, at a higher pressure, is discharged from the eductor
80
to conduit
78
and passes to the utilization device
79
(for example, chip bin
17
of FIG.
1
). Gas supply
81
is preferably another steam supply, for example, a steam supply having a pressure between about 9 and 13 bar gage. One preferred source of steam
81
is medium pressure steam readily available in the pulp mill.
FIG. 4
illustrates another embodiment of the invention in which the pressurization device
77
of
FIG. 2
is a compressor or fan
82
. Compressor or fan
82
pressurizes the steam in conduit
73
so that it can be used in, for example, Chip Tube
24
of
FIG. 1
, or—as shown—chip bin
17
.
Other conventional pressurization devices aside from eductors, fans, and compressors may be used as the device
77
.
Data exemplary of the energy and temperature levels that can be expected in the digester system
110
of
FIG. 2
when utilizing the eductor
80
embodiment of
FIG. 3
are shown in Table 1. In contrast, the energy and temperature levels of a typical prior art system without an eductor
80
are shown in Table 2.
TABLE 1
|
|
Temperatures and pressures for a system producing pulp at kappa
|
number of 25 according to the present invention. Assumes that the
|
amount steam 81 introduced to the eductor 80, equals the amount of
|
steam generated by the reboiler 73 and a 98% eductor efficiency.
|
Item Ref.
Temperature
Pressure
|
Description
No.
[° C.]
[Bars, gage]
|
|
Spent Liquor
61
162.6
|
Extracted from
|
Digester
|
Spent Liquor
61′
131.0
|
Introduced to
|
Reboiler After Heat
|
Exchanger
|
Hot Water Introduced
72
75.0
|
to Reboiler
|
Hot Water Removed
75
93.5
|
from Reboiler
|
Cooled Black Liquor
74
95.0
|
from Reboiler
|
Steam Generated by
73
93.5
−.20 to −.10
|
Reboiler
|
Steam Introduced to
81
200.0
13
|
Eductor
|
Steam to Chip Bin
78/17
105.0
0.1-0.2
|
|
TABLE 2
|
|
Temperatures and pressures for a system producing pulp
|
at kappa number of 25 according to the prior art.
|
Item Ref.
Temperature
Pressure
|
Description
No.
[° C.]
[Bars, gage]
|
|
Spent Liquor
61
162.6
|
Extracted from
|
Digester
|
Spent Liquor
61′
131.0
|
Introduced to
|
Reboiler After Heat
|
Exchanger
|
Hot Water Introduced
72
75.0
|
to Reboiler
|
Hot Water Removed
75
112.0
|
from Reboiler
|
Cooled Black Liquor
74
112.0
|
from Reboiler
|
Steam Generated by
73
110.0
0.2-0.4
|
Reboiler and
|
Introduced to Chip
|
Bin
|
|
Comparison of the data in Tables 1 and 2 clearly shows that the temperature of the steam produced in the reboiler
71
of the present invention (that is, 93.5° C.) is 16.5° C. degrees cooler than the temperature of steam the steam produced by the prior art reboiler (that is, 110° C.). The pressure of the steam produced by the reboiler
71
of the present invention (that is, about −0.2 bar gage) is about 0.5 bars less than the pressure produced by the prior art reboiler (that is, about 0.3 bars gage). This permits the temperature of the cooled black liquor from the reboiler
71
of the present invention (that is, about 95° C.) to be about 17° C. cooler than the prior art cooled spent liquor temperature (that is, about 112° C.). The temperature of the hot water produced in the reboiler
71
according to the present invention (that is, about 93.5° C.) is about 18.5° C. cooler than the hot water produced by the prior art system
10
(that is, about 112° C.). Thus, according to the present invention much more energy can be recovered from hot spent cooking liquor, and re-used, than can be recovered from the prior art system
10
.
It is to be understood that the data shown in Table 1 is representative of one set of conditions under which the present invention can be operated. These conditions will vary depending upon the economics, the related equipment and its operating conditions, etc. of the mill. For example, the reboiler
71
may also be operated at higher pressures (e.g., at 0.2 bar gage or higher) and higher temperatures (e.g., at 105° C. or higher) and still be used to effect the present invention. Also, the temperatures of the liquids flowing to and from the reboiler
71
may vary. For example, depending upon the size and type of the reboiler
71
or other heat exchanger used, the temperature of the cooler black liquor
74
may approach the temperature of the steam
73
produced. Other operating conditions will be apparent to those of skill in the art.
Thus, according to the present invention a method and apparatus for minimizing the escape of malodorous, TRS-laden, gases from the feed system of a cellulose material treatment system while recovering as much energy as possible from spent treatment liquor used in the system, is provided. The present invention requires less energy than prior art systems since the only energy leaving the system leaves with the pulp
14
, the spent cooking liquor
74
, and the steam (including recoverable turpentine)
20
(in FIG.
1
). Furthermore, all of these heat-containing streams leave this system at the temperature desired for the next downstream operation, that is, no heating or cooling of these streams is necessary for further treatment according to the present invention.
In the above description, the invention also encompasses all possible narrower ranges within any broad range.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
Claims
- 1. A method of producing chemical cellulose pulp in an energy efficient manner using a treatment vessel in a digester system, comprising:(a) introducing a slurry of comminuted cellulosic fibrous material and liquid to a treatment vessel to produce cellulose pulp; (b) treating the material in the vessel with treatment chemical at a temperature greater than 140° C. and a pressure greater than 5 bar gage to produce a treated material and a spent treatment liquor, including sulphur compounds; (c) removing the spent treatment liquid from the vessel at a first temperature; (d) passing the spent liquor in indirect heat exchange relationship with a cooler liquor having a second temperature lower than the first temperature to produce steam at a first pressure, and at a third temperature higher than the second temperature; (e) increasing the pressure of the steam to a second pressure by at least about 0.2 bar more than the first pressure; and (f) using the steam at the second pressure in the digester system.
- 2. A method as recited in claim 1 wherein (e) is practiced using an eductor.
- 3. A method as recited in claim 2 wherein steam at a pressure of between about 9 and 13 bar gage is added to the steam from (d) in the eductor to increase the pressure of the steam from (d).
- 4. A method as recited in claim 1 wherein (e) is practiced using a fan or compressor.
- 5. A method as recited in claim 1 wherein (e) is practiced so that the second pressure is at least 0.2 bar gage, and so that the steam is at a temperature of at least 105° C.
- 6. A method as recited in claim 5 wherein (d) is practiced to produce substantially clean steam, and wherein (f) is practiced to steam comminuted cellulose material being introduced into a feed system for the treatment vessel.
- 7. A method as recited in claim 1 wherein the liquor removed in (c) is black liquor at a temperature of between about 140-160° C.; and further comprising, after (c), (g) passing the black liquor into indirect heat exchange relationship with a third liquid to recover some of the heat from the black liquor in the third liquid prior to the practice of (d).
- 8. A method as recited in claim 1 wherein (d) is practiced using a steam generator; and further comprising positively cooling the spent liquor after it passes through the steam generator.
- 9. A method of producing steam having less noncondensible gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam, in a digester system, comprising:(a) passing hot black liquor, including sulphur compounds, at a temperature of about 120-165° C. from a digester through a heat exchanger; (b) passing an evaporable liquid to be evaporated through the heat exchanger into heat exchange contact with the hot black liquor so that the evaporable liquid is heated so that it is ultimately evaporated to produce steam having less noncondensible gases than steam produced by flashing black liquor, including sulphur compounds, directly into steam; (c) increasing the pressure of steam from (b) by at least 0.2 bar gage; and (d) using the steam from (c) in the digester system.
- 10. A method as recited in claim 9 wherein the evaporable liquid used is (b) is substantially free of sulphur compounds; and wherein (b) is practiced to produce substantially clean steam.
- 11. A method as recited in claim 10 wherein (c) is practiced to produce substantially clean steam at a pressure of at least 0.2 bar gage and a temperature of at least 105° C.
- 12. A method as recited in claim 11 where (d) is practiced to steam comminuted cellulose material prior to introduction thereof into the digester.
- 13. A method as recited in claim 10 further comprising (e) passing the hot black liquor into indirect heat exchange relationship with another liquid to recover some of the heat from the black liquor in the other liquid prior to the practice of (b).
- 14. A method as recited in claim 9 wherein (c) is practiced using an eductor, and introducing steam at a pressure of between about 3 and 13 bar gage with the steam from (b) into the eductor.
- 15. A digester system comprising:a digester having a hot spent treatment liquor outlet; a feed system for feeding comminuted cellulose material to said digester; a steam generator operatively connected to said spent treatment liquor outlet to use the heat of the spent treatment liquor to produce steam from another liquid, said steam generator having a steam outlet and a spent treatment liquor outlet; a pressure increasing device, connected to said steam generator steaming outlet, which increases the pressure of the steam from said steam generator; and a utilization device operatively connected to said pressure increasing device to utilize the steam therefrom.
- 16. A digester system as recited in claim 15 wherein said pressure increasing device comprises a fan or compressor.
- 17. A digester system as recited in claim 15 wherein said pressure increasing device comprises an eductor.
- 18. A digester system as recited in claim 15 further comprising an indirect heat exchanger connected between said spent treatment liquor outlet and said steam generator for recovery of some of the heat from the hot spent treatment liquor before that liquor is passed to said steam generator.
- 19. A digester system as recited in claim 15 further comprising a flow control valve between said pressure increasing device and said utilization device controlled by feedback from said utilization device.
- 20. A digester system as recited in claim 19 wherein said utilization device comprises said feed system.
US Referenced Citations (1)
Number |
Name |
Date |
Kind |
4668341 |
Nilsson |
May 1987 |
|
Foreign Referenced Citations (2)
Number |
Date |
Country |
WO9632531 |
Oct 1996 |
WO |
WO9729236 |
Aug 1997 |
WO |