EP0843757B1 - Method for reprocessing an ozone-containing gas present in a pulp bleaching plant - Google Patents
Method for reprocessing an ozone-containing gas present in a pulp bleaching plant Download PDFInfo
- Publication number
- EP0843757B1 EP0843757B1 EP96927319A EP96927319A EP0843757B1 EP 0843757 B1 EP0843757 B1 EP 0843757B1 EP 96927319 A EP96927319 A EP 96927319A EP 96927319 A EP96927319 A EP 96927319A EP 0843757 B1 EP0843757 B1 EP 0843757B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- ozone
- gas
- scrubber
- pulp
- alkaline medium
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/06—Treatment of pulp gases; Recovery of the heat content of the gases; Treatment of gases arising from various sources in pulp and paper mills; Regeneration of gaseous SO2, e.g. arising from liquors containing sulfur compounds
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
- D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone
Definitions
- the present invention relates generally to the bleaching of lignocellulosic materials for use in the pulp and paper industry, and more particularly to a method for reprocessing an ozone-containing gas present in a pulp bleaching plant.
- Ozone gas has been used in a wide variety of industrial applications which has included water and waste treatment and certain semiconductor manufacturing processes. Additionally, there have been many recent efforts to utilize ozone as a bleaching reagent for wood pulp and other lignocellulosic materials, so as to avoid the use of chlorine (and the attendant environmental problems) in such bleaching processes. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and the relatively high cost associated with its use have limited the development of satisfactory devices and processes for ozone bleaching of lignocellulosic materials.
- Ozone gas is very unstable, corrosive and is very toxic even in small concentrations.
- Environmental Protection Agency (EPA) air quality guidelines indicate that the ozone concentration in the atmosphere should be less than 0.12 ppm by volume.
- Occupational Safety and Health Administration (OSHA) guidelines stipulate that the allowable exposure of an individual to ozone for an eight hour period corresponds to a maximum ozone concentration in air of 0.1 ppm by volume.
- OSHA Occupational Safety and Health Administration
- One known method for destroying residual ozone present in a wood pulp bleaching off-gas is thermal destruction.
- the ozone-containing off-gas is supplied to a thermal destruct unit where the ozone gas is heated, typically to a temperature greater than 350° C, so as to dissociate the ozone into diatomic oxygen.
- Another known method for the destruction of residual ozone has been the use of a solid catalyst which causes ozone to decompose. While these methods may be advantageously utilized in industrial applications using relatively small volumes of ozone-containing gas, they are subject to the following limitation when applied in pulp bleaching applications.
- thermal destruct unit may be required to process both the off-gas from the pulp bleaching process and additionally a volume of fresh bleaching gas, at a significantly higher concentration of ozone, from an ozone generator. Since the decomposition of ozone is exothermal, the sudden input of fresh bleaching gas can cause the internal temperature of the thermal destruct unit to increase rapidly. This in turn may result in severe thermal stresses on the thermal destruct unit which may result in component failure.
- VOC's volatile organic compounds
- any residual pulp fibers present in the off-gas supplied to the solid catalyst bed may accumulate in the catalyst bed thereby restricting the gas flow and also reducing the gas-solids contact area.
- WO-A-9318226 teaches reprocessing the off-gases of an ozone bleaching plant by passing the off-gases through a scrubber using an alkaline solution to reduce the ozone concentration.
- Various alkaline solution sources are taught, and EPO 526 383 teaches the use of oxidized white liquor and extraction filtrate.
- this is accomplished by providing a method for reproducing an ozone-containing gas present in a pulp bleaching plant comprising the steps of:
- the Figure illustrates a portion of a pulp bleaching plant incorporating the method of the present invention for reprocessing an ozone-containing gas.
- the Figure illustrates a portion of a pulp bleaching plant 10 for bleaching lignocellulosic materials, such as medium or high-consistency wood pulp, with a gaseous bleaching reagent comprising ozone according to the present invention.
- the pulp bleaching plant 10 is included in a pulp mill which also includes a brownstock washing plant (not shown) and an oxygen delignification plant (not shown), each of which precedes plant 10 within the pulp mill.
- wood pulp is obtained from a digestion of wood chips, from repulping of recycled paper or from other sources, and is commonly processed in pulp and paper mills, such as plant 10, in slurry form in water.
- the term "consistency” is used to express the measured ratio of dry pulp fibers to water, or more specifically, the weight of dry pulp fibers in a given weight of pulp slurry or "pulp stock” as a percentage.
- Various definitions are used, such as air-dry consistency (a.d.%) or oven-dry consistency (o.d.%), or moisture-free consistency (m.f.%).
- air-dry consistency a.d.% or oven-dry consistency (o.d.%)
- moisture-free consistency m.f.%
- Pulp bleaching plant 10 includes a conventional ozone generator 12 which is effective for producing ozone in a carrier gas, such as oxygen or air.
- a feed gas 14 is supplied to the ozone generator and may comprise either air or an oxygen-enriched gas.
- ozone is typically available at a relatively low concentration within the carrier gas.
- ozone typically has a concentration ranging from about 6% to about 14%, by weight, when using oxygen as the carrier gas.
- off-gas will refer to the mixture of ozone in an oxygen carrier gas, as well as other gases and vapors, such as gases which are produced as a by-product of the reaction of the ozone with the pulp, which are present at equilibrium conditions.
- fresh bleaching gas will be used to denote a mixture of ozone in an oxygen carrier gas, with an ozone concentration ranging from about 6% by weight to about 14% by weight, supplied from the ozone generator 12 which has not been reacted with the pulp and accordingly does not include reaction by-product gases.
- the ozone-containing gas, comprising fresh bleaching gas, produced by ozone generator 12 is supplied from generator 12 to an inlet 16 of a valve indicated at 18, via a conduit 20.
- the fresh bleaching gas discharges valve 18 through a first outlet 22 and is supplied to a pulp bleaching stage, indicated at 24, via a conduit 26.
- a wood pulp 28, which may comprise either medium or high consistency wood pulp, is also supplied to the pulp bleaching stage 24.
- the pulp bleaching stage comprises at least one, and may comprise a plurality of reactors (not shown), with at least a portion of the ozone in the fresh bleaching gas being reacted with the pulp within each of the reactors.
- the fresh bleaching gas supplied to the pulp bleaching stage 24 is converted into an off-gas which includes residual ozone which has not reacted with the pulp and by-product of reaction gases which includes carbon dioxide.
- the off-gas discharging from the pulp bleaching stage 24 is supplied to a first inlet 30 of a valve 32, via conduit 34.
- the off-gas then discharges from the valve 32 through an outlet 34 of valve 32 and is supplied to a gas inlet 36 of a scrubber 38 via conduit 40, with the residual ozone being substantially completely destroyed within scrubber 38 as subsequently discussed in greater detail.
- the method for destroying the residual ozone within scrubber 38 constitutes a central feature of the present invention.
- the fresh bleaching gas supplied from ozone generator 12 is allowed to purge through the pulp bleaching stage 24 where it is converted to off-gas having a relatively low ozone concentration and is then supplied to the inlet 36 of scrubber 38 via conduits 34 and 40 and valve 32.
- the first outlet 22 of valve 18 is closed causing the fresh bleaching gas supplied from ozone generator 12 to flow through inlet 16 of valve 18 and discharge from valve 18 through a second outlet 42.
- the fresh bleaching gas then flows through a conduit 44 to a second inlet 46 of valve 32 so that the fresh bleaching gas by-passes the pulp bleaching stage 24.
- the fresh bleaching gas is then supplied to the gas inlet 36 of scrubber 38 via outlet 34 of valve 32 and conduit 40. Accordingly, in these situations, the concentration of ozone within the ozone-containing gas supplied to scrubber 38 may be significantly higher than the ozone concentration present in the off-gas supplied to scrubber 38 during normal operation and normal shut-down of plant 10.
- Scrubber 38 is preferably substantially vertically oriented and includes a generally cylindrical housing 48 which forms an intemal chamber 50 therewithin. As shown in The Figure, the gas inlet 36 of scrubber 38 is in fluid communication with a lower portion 52 of the internal chamber 50. Scrubber 38 further includes a gas outlet 54 and a liquid inlet 56, each of which are in fluid communication with an upper portion 58 of chamber 50, and a liquid outlet 60 which is in fluid communication with the lower portion 52 of chamber 50. In the illustrative embodiment shown in The Figure, scrubber 38 further includes a plurality of vertically spaced trays 62, with each of the trays 62 being attached to an inner wall of housing 48 and extending radially inwardly therefrom.
- the trays 62 may be omitted from scrubber 38 and replaced with a packing medium (not shown) which preferrably comprises a plurality of rings, saddles, beads or balls made of materials resistant to ozone as well as the alkaline medium 66.
- a packing medium (not shown) which preferrably comprises a plurality of rings, saddles, beads or balls made of materials resistant to ozone as well as the alkaline medium 66.
- other suitable packing media may be used as long as sufficient contact surface area is provided.
- a flow of an alkaline medium 66 is supplied to the liquid inlet 56 of scrubber 38.
- the alkaline medium 66 preferably comprises one of the following: a sodium hydroxide solution; white liquor which is comprised mainly of sodium hydroxide, sodium sulfide and sodium carbonate; filtrate from an Eo stage (oxidative extraction stage not shown) of plant 10; post oxygen stage filtrate from the oxygen delignification plant which precedes plant 10; or brownstock washing filtrate supplied from the brownstock washing plant which precedes both the oxygen delignification plant and plant 10, with the brownstock washing filtrate alternatively referred to in the art as weak black liquor.
- each of the alkaline media has an alkali concentration ranging from about 1 of alkali/liter of solution to about 100 g of alkali/liter of solution.
- the ozone-containing gas supplied to the gas inlet 36 of scrubber 38 flows upward through scrubber 38 while the alkaline medium 66 supplied to the liquid inlet 56 of scrubber 38 flows downward through scrubber 38 under the action of gravity. Accordingly, the ozone-containing gas and alkaline medium 66 flow in countercurrent relationship with one another within scrubber 38 so as to provide intimate contact between the ozone-containing gas and the alkaline medium 66 within scrubber 38.
- the flowpath of the ozone-containing gas and the alkaline medium 66 through scrubber 38 is established by the vertically spaced trays 62.
- the flowpath of the ozone-containing gas and the alkaline medium 66 through scrubber 38 is determined by the packing medium.
- the use of the rings, saddles, beads or balls for the packing medium provides a very high surface area which facilitates intimate contact between the ozone-containing gas and the alkaline medium 66.
- the ozone present in the gas is substantially completely destroyed due to dissociation of the ozone into diatomic oxygen, thereby converting the ozone-containing gas into a substantially ozone-free gas.
- the substantially ozone-free gas discharging through gas outlet 54 of scrubber 38 has a reduced ozone concentration which meets allowable limits.
- the term "substantially ozone-free gas" is intended to also encompass a gas which is free of ozone, at least within detectable limits as measured with conventional equipment.
- the intimate contact between the ozone-containing gas and the alkaline medium 66 within scrubber 38 results in any carbon dioxide present in the ozone-containing gas being substantially completely absorbed by the alkaline medium 66.
- the gas discharging scrubber 38 through outlet 54 may be supplied to a pulp bleaching stage 68 of plant 10 via conduits 70 and 72 or alternatively may be supplied, or recirculated to the ozone generator 12 via conduit 70 and a conduit 74 (shown in dashed line).
- the pulp bleaching stage 68 may be disposed either upstream or downstream of the pulp bleaching stage 24 of plant 10. It is important to absorb the carbon dioxide if the off-gas is recirculated to ozone generator 12 to avoid an adverse impact on the efficiency of ozone generator 12 and it may also be advantageous to absorb any carbon dioxide present when the off-gas is supplied to pulp bleaching stage 68.
- stage 68 may comprise an Eo stage and the presence of any carbon dioxide may adversely affect the pulp processing in this stage.
- the alkaline medium discharges scrubber 38 through the liquid outlet 60 and may be supplied to other portions of the associated pulp mill for further processing as indicated by flow arrow 76.
- the present invention provides a cost-efficient method for destroying residual ozone in an ozone-containing gas used in a pulp bleaching plant.
- the relatively high electrical cost associated with operation of known ozone thermal destruct units is avoided.
- the method of the present invention also results in substantially complete absorption (by the alkaline medium) of any carbon dioxide which is present in the gas, which may not be achieved with the use of known thermal or catalytic destruct units.
- each of the alkaline media are commonly available from a variety of sources in the pulp mill which includes the pulp bleaching plant 10, which further enhances the cost-efficiency of the method of the present invention.
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- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Paper (AREA)
- Detergent Compositions (AREA)
- Treating Waste Gases (AREA)
Abstract
Description
- (e) flowing the ozone-containing gas into the bypass conduit;
- (f) flowing the ozone-containing gas from the bypass conduit to the scrubber;
- (g) supplying the alkaline medium to the scrubber;
- (h) flowing the ozone-containing gas in a countercurrent relationship with respect to the alkaline medium within the scrubber so as to provide intimate contact between the ozone-containing gas and the alkaline medium within the scrubber thereby substantially converting the ozone-containing gas into a substantially ozone-free gas;
Claims (13)
- A method for reducing an ozone concentration in an ozone-containing gas present in a pulp bleaching plant, the method comprising the steps of:(1) supplying the ozone-containing gas to a first valve, the first valve being in communication with a pulp bleaching vessel and a bypass conduit, the first valve being open to the pulp bleaching vessel and closed to the bypass conduit when pulp is being supplied to the pulp bleaching vessel, the first valve being closed to the pulp bleaching vessel and open to the bypass conduit when pulp is not being supplied to the pulp bleaching vessel;in the event pulp is being supplied to the pulp bleaching vessel, performing steps (a) through (d):(a) flowing the ozone-containing gas into the pulp bleaching vessel to bleach the pulp and produce an ozone-containing off gas;(b) flowing the ozone-containing off gas from the bleaching vessel to a scrubber;(c) supplying an alkaline medium to the scrubber;(d) flowing the ozone-containing off gas in a countercurrent relationship with respect to the alkaline medium within the scrubber so as to provide intimate contact between the ozone-containing off gas and the alkaline medium within the scrubber thereby substantially converting the ozone-containing off gas into a substantially ozone-free gas;in the event pulp is not being supplied to the pulp bleaching vessel, performing steps (e) through (h);(e) flowing the ozone-containing gas into the bypass conduit;(f) flowing the ozone-containing gas from the bypass conduit to the scrubber;(g) supplying the alkaline medium to the scrubber;(h) flowing the ozone-containing gas in a countercurrent relationship with respect to the alkaline medium within the scrubber so as to provide intimate contact between the ozone-containing gas and the alkaline medium within the scrubber thereby substantially converting the ozone-containing gas into a substantially ozone-free gas;(2) separately discharging the substantially ozone-free gas and the alkaline medium from the scrubber.
- The method as recited in claim 1, wherein the alkaline medium comprises a sodium hydroxide solution.
- The method as recited in claim 1, wherein the alkaline medium comprises white liquor.
- The method as recited in claim 1, wherein the alkaline medium comprises a post oxygen stage filtrate from an oxygen delignification process.
- The method as recited in claim 1, wherein the alkaline medium comprises weak black liquor.
- The method as recited in claim 1, wherein the alkaline medium comprises a filtrate from an Eo stage of the pulD bleaching plant.
- The method as recited in claim 1, wherein the alkaline medium has an alkali concentration ranging from about 1 g of alkali/liter of solution to about 100 g of alkali/liter of solution.
- The method as recited in claim 1, further comprising the step of:
reducing the carbon dioxide concentration of the ozone containing off-gas as the off-gas flows through the scrubber. - The method as recited in claim 1, wherein said step of separately discharging comprises the steps of:discharging the alkaline medium from a liquid outlet of the scrubber;discharging the substantially ozone-free gas from a gas outlet of the scrubber to an ozone generator.
- The method as recited in claim 1, wherein said step of separately discharging comprises the steps of:discharging the alkaline medium from a liquid outlet of the scrubber;discharging the substantially ozone-free gas from a gas outlet of the scrubber to a pulp bleaching stage of the pulp bleaching plant.
- The method as recited in claim 1, wherein:the scrubber is substantially vertically oriented and includes a generally cylindrical housing forming an internal chamber therein, the internal chamber having an upper portion and a lower portion, and the scrubber further includes a gas inlet and a liquid outlet in fluid communication with the lower portion of the internal chamber, and a gas outlet and a liquid inlet in fluid communication with the upper portion of the internal chamber;said step (b) further comprises flowing the ozone-containing off gas to the gas inlet of the scrubber;said step (f) further comprises flowing the ozone-containing gas to the gas inlet of the scrubber;said steps (c) and (g) further comprising supplying the alkaline medium to the liquid inlet of the scrubber;said step of separately discharging comprises the steps of:discharging the substantially ozone-free gas from the gas outlet of the scrubber; anddischarging the alkaline medium from the liquid outlet of the scrubber.
- The method of claim 1 wherein step (b) further comprises flowing the ozone-containing off gas to a second valve disposed opposite the bleaching vessel from the first valve, the second valve providing communication between the bleaching vessel and the scrubber, and, the bypass conduit and the scrubber, the second valve providing communication between the bleaching vessel and the scrubber when pulp is being supplied to the bleaching vessel, the second valve providing communication between the bypass line and the scrubber when pulp is not being supplied to the bleaching vessel.
- The method of claim 12 wherein step (b) further comprising flowing the ozone-containing of gas to the second valve and step (f) further comprises flowing the ozone-containing gas to the second valve.
Applications Claiming Priority (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US194595P | 1995-08-07 | 1995-08-07 | |
US19454 | 1995-08-07 | ||
US08/693,919 US5695604A (en) | 1996-08-05 | 1996-08-05 | Method for reprocessing an ozone-containing gas present in a pulp bleaching plant |
US693919 | 1996-08-05 | ||
PCT/US1996/012776 WO1997006305A1 (en) | 1995-08-07 | 1996-08-06 | Method for reprocessing an ozone-containing gas present in a pulp bleaching plant |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0843757A1 EP0843757A1 (en) | 1998-05-27 |
EP0843757B1 true EP0843757B1 (en) | 1999-10-20 |
Family
ID=26669715
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP96927319A Expired - Lifetime EP0843757B1 (en) | 1995-08-07 | 1996-08-06 | Method for reprocessing an ozone-containing gas present in a pulp bleaching plant |
Country Status (10)
Country | Link |
---|---|
EP (1) | EP0843757B1 (en) |
JP (1) | JPH10510469A (en) |
CN (1) | CN1198786A (en) |
AT (1) | ATE185857T1 (en) |
BR (1) | BR9609907A (en) |
CA (1) | CA2229046A1 (en) |
DE (2) | DE69604809T2 (en) |
ES (1) | ES2116952T3 (en) |
PT (1) | PT843757E (en) |
WO (1) | WO1997006305A1 (en) |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2008105108A1 (en) | 2007-02-27 | 2008-09-04 | Shigenori Aono | Process for production of bleached textile materials, equipment to be used in the process, and bleached textile materials obtained by the process |
WO2008105132A1 (en) * | 2007-02-27 | 2008-09-04 | Shigenori Aono | Process for producing bleached fiber product, apparatus for use therein, and thus obtained bleached fiber product |
WO2008108506A1 (en) | 2007-03-07 | 2008-09-12 | Nihon Cellulose Co., Ltd. | Method for treatment of black liquor containing no sulfide |
FI20080008L (en) * | 2008-01-08 | 2009-07-09 | Andritz Oy | Treatment of ozone-containing gas in connection with pulp bleaching |
Family Cites Families (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6126781A (en) * | 1991-08-01 | 2000-10-03 | Union Camp Patent Holding, Inc. | Process for conditioning ozone gas recycle stream in ozone pulp bleaching |
CA2107764A1 (en) * | 1992-03-02 | 1993-09-16 | Lawrence A. Hinkey | Combined ozone destruction and fiber scrubbing in paper pulp mills |
CA2078276C (en) * | 1992-09-15 | 2004-05-25 | Derek Hornsey | Recovery of oxygen-rich gas from ozone bleaching processes |
US5403441A (en) * | 1992-11-13 | 1995-04-04 | Union Camp Patent Holding, Inc. | Method for controlling an ozone bleaching process |
-
1996
- 1996-08-06 ES ES96927319T patent/ES2116952T3/en not_active Expired - Lifetime
- 1996-08-06 CA CA002229046A patent/CA2229046A1/en not_active Abandoned
- 1996-08-06 JP JP9508601A patent/JPH10510469A/en active Pending
- 1996-08-06 DE DE69604809T patent/DE69604809T2/en not_active Expired - Fee Related
- 1996-08-06 WO PCT/US1996/012776 patent/WO1997006305A1/en active IP Right Grant
- 1996-08-06 CN CN96197410.9A patent/CN1198786A/en active Pending
- 1996-08-06 EP EP96927319A patent/EP0843757B1/en not_active Expired - Lifetime
- 1996-08-06 PT PT96927319T patent/PT843757E/en unknown
- 1996-08-06 DE DE0843757T patent/DE843757T1/en active Pending
- 1996-08-06 BR BR9609907A patent/BR9609907A/en not_active Application Discontinuation
- 1996-08-06 AT AT96927319T patent/ATE185857T1/en not_active IP Right Cessation
Also Published As
Publication number | Publication date |
---|---|
DE69604809D1 (en) | 1999-11-25 |
EP0843757A1 (en) | 1998-05-27 |
ES2116952T3 (en) | 2000-02-01 |
ATE185857T1 (en) | 1999-11-15 |
WO1997006305A1 (en) | 1997-02-20 |
JPH10510469A (en) | 1998-10-13 |
DE843757T1 (en) | 1998-11-12 |
BR9609907A (en) | 1999-03-30 |
CN1198786A (en) | 1998-11-11 |
DE69604809T2 (en) | 2000-05-11 |
CA2229046A1 (en) | 1997-02-20 |
ES2116952T1 (en) | 1998-08-01 |
PT843757E (en) | 2000-04-28 |
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