METHOD AND SYSTEM FOR TREATING SPENT PULPING LIQUOR

Abstract

A system (10) for treating spent pulping liquor (14) comprising lignin to provide green liquor (47) is disclosed. The system (10) comprises an evaporator (22), a recovery boiler (34), at least one filter (58, 74), and a green liquor plant (46). The evaporator (22) concentrates a first stream (18) of the spent pulping liquor (14) to provide a concentrated pulping liquor (30). The recovery boiler (34) incinerates the concentrated pulping liquor (30) to provide a smelt (42). The at least one filter (58, 74) filters a second stream (54) of the spent pulping liquor (14) to remove lignin therefrom to provide a permeate (66, 82). The green liquor plant (46) is for dissolving the smelt (42) from the recovery boiler (34) in the permeate (66, 82) from the at least one filter (58, 74) to provide green liquor (47). Also disclosed is a method for treating spent pulping liquor (14) to provide green liquor (47).

Claims

1. A system for treating spent pulping liquor comprising lignin to provide green liquor, the system comprising: an evaporator for concentrating a first stream of the spent pulping liquor to provide a concentrated pulping liquor; a recovery boiler for incinerating the concentrated pulping liquor to provide a smelt; at least one filter for filtering a second stream of the spent pulping liquor to remove lignin therefrom to provide a permeate; and a green liquor plant for dissolving the smelt from the recovery boiler in at least a part of the permeate from the at least one filter to provide green liquor.

2. The system of claim 1, wherein the at least one filter comprises: a first filter for filtering the second stream of the spent pulping liquor to provide a first retentate and a first permeate; and a second filter for filtering the first permeate to provide a second retentate and a second permeate; wherein the second permeate is used to dissolve the smelt in the green liquor plant.

3. The system of claim 2, wherein the first filter is a membrane filter with a molecular weight cut-off between 0.5 and 20 kDa and the second filter is a membrane filter with a molecular weight cut-off lower than first filter.

4. The system of claim 2, wherein the first filter is a membrane filter with a molecular weight cut-off of about 1 kDa.

5. The system of claim 1, wherein the second stream is between about 10 to 50% by volume of the spent pulping liquor.

6. The system of claim 1, wherein the second stream is about 25% by volume and the first stream is about 75% by volume of the spent pulping liquor.

7. The system of claim 1, further comprising a de-polymerization reactor for treating the first retentate to provide a de-polymerized lignin stream.

8. A method for treating spent pulping liquor comprising lignin to provide green liquor, the method comprising: (a) dividing the spent pulping liquor into a first stream and a second stream; (b) evaporating the first stream to provide a concentrated pulping liquor; (c) incinerating the concentrated pulping liquor to provide a smelt; (d) filtering the second stream to remove lignin, thereby obtaining a permeate with reduced content of lignin; (e) dissolving the smelt in at least part of the permeate to provide green liquor.

9. The method of claim 8, wherein step (d) filtering the second stream to obtain a permeate comprises: (d1) filtering the second stream of the spent pulping liquor to remove a first portion of lignin to provide a first retentate and a first permeate; and (d2) filtering the first permeate to remove a second portion of lignin to provide a second retentate and a second permeate; wherein at least part of the second permeate is used to dissolve the smelt in the green liquor plant in step (e).

10. The method of claim 8, wherein the second stream is between about 10 to 50% by volume of the spent pulping liquor.

11. The method of claim 8, wherein the pH of the permeate used to dilute the smelt is at least 11.

12. The method of claim 8, wherein part of the permeate is sent to the evaporator.

13. The system of claim 3, wherein the molecular weight cut-off of the second filter is between 0.1 and 0.8 kDa.

14. The method of claim 10, wherein the second stream is about 25% by volume of the spent pulping liquor.

15. The method of claim 11, wherein the pH of the permeate used to dilute the smelt is at least 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of an embodiment of the present invention, reference being made to the accompanying drawings, in which:

[0028] FIG. 1 is a system for treating spent pulping liquor comprising lignin according to an embodiment.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0029] The following description focuses on an embodiment of the present invention applicable to a system and method for treating spent pulping liquor to provide green liquor. However, it will be appreciated that the invention is not limited to this application but may be applied to other systems and methods which involve treating spent liquor e.g. soda liquor from a soda cooking process.

[0030] The table below lists features of the system 10 in FIG. 1.

TABLE-US-00001 Feature Reference numeral System 10 Spent pulping liquor 14 First stream of pulping liquor 18 Evaporator 22 Condensate 26 Concentrated pulping liquor 30 Recovery boiler 34 Flue gases 38 Smelt 42 Green liquor plant 46 Green liquor 47 White liquor plant 48 White liquor 50 Second stream of pulping liquor 54 First filter 58 First retentate 62 First permeate 66 De-polymerization reactor 70 De-polymerized lignin stream 71 Second filter 74 Second retentate 78 Second permeate 82 Combined lignin stream 86 Brown stock wash 90

[0031] As used herein the terms spent pulping liquor and black liquor are used interchangeably.

[0032] FIG. 1 illustrates a system 10 for treating spent pulping liquor 14 from a kraft pulp mill comprising lignin. A method for treating spent pulping liquor 14 will also be evident to the skilled person from the description of system 10.

[0033] Spent pulping liquor 14 is separated into a first stream 18 and a second stream 54. As already mentioned, spent pulping liquor 14, apart from lignin, comprises spent inorganic cooking chemicals, such as Na.sub.2CO.sub.3, Na.sub.2SO.sub.4, NaS.sub.2O.sub.3, and NaSO.sub.3. Typically, the pH of the spent pulping liquor 14 is 12 or higher, such as about 13.

[0034] The first stream 18 enters evaporator 22 wherein it is concentrated to form condensate 26 and concentrated pulping liquor 30. The concentrated liquor 30 then enters recovery boiler 34 for incineration. Flue gases 38 evolve from recovery boiler 34 and a smelt 42 is produced. Smelt 42 is then dissolved in an aqueous second permeate 82 obtained from treatment of the second stream 54 as will be described below. As already mentioned, the smelt comprises molten salts that consist mainly of Na.sub.2CO.sub.3 and Na.sub.2S. Additional aqueous liquids or process water available in the kraft mill (not shown) may be used to dissolve the smelt 42 if the quantity of second permeate 82 is insufficient. Dissolution of smelt 42 occurs in green liquor plant 46 to form green liquor 47 which is re-causticized using calcium hydroxide in white liquor plant 48. The regenerated white liquor 50, comprising NaOH and Na.sub.2S, from the white liquor plant 48 is recycled back to the wood cooking plant of the kraft pulping process (not shown).

[0035] The second stream 54 of spent pulping liquor enters first filter 58 wherein stream 54 is separated into a first retentate 62 and a first permeate 66. The first retentate 62 is rich in organic substances, including lignin, while the first permeate 66 is depleted of organic substances. The first retentate 62 is sent to de-polymerization reactor 70 wherein it is treated to produce a de-polymerized lignin stream 71. The first permeate 66 enters second filter 74 to yield a second retentate 78 and a second permeate 82 further depleted of organic substances. Optionally, some of the first permeate 66 is sent to evaporator 22 (cf. dashed line in FIG. 1). This may be advantageous, as the proportion of high-molecular compounds in the stream resulting from diluting the spent pulping liquor to be evaporated with the first permeate 66 will be lowered. Lowering the proportion of high-molecular compounds in the stream of spent pulping liquor to be evaporated will lower the viscosity of the spent pulping liquor to be evaporated, though the content of low molecular weight compounds, e.g. inorganic salts, remains the same. Thereby, the spent pulping liquor to be evaporated may be concentrated to a higher dry content before being incinerated, thereby improving the efficiency in the recovery boiler. Alternatively, some of the first permeate 66 may be used in the wood chip impregnation liquor or reintroduced elsewhere in the pulping process (not shown). The second retentate 78 has a high concentration of substances with a molecular weight distribution determined by the porosity cut-off of the first filter 58 (upper limit) and the second filter 74 (lower limit). Second retentate 78 is combined with de-polymerized lignin stream 71 to produce a combined lignin stream 86 suitable for manufacture of lignin bio-oil or other useful products. By de-polymerizing the first retentate 62, the molecular weight distribution of the combined lignin stream 86 becomes more narrow. Moreover, the combined lignin stream 86 is homogenous. A homogenous lignin stream with a lower and narrower molecular weight distribution is useful as a precursor for the production of liquid fuels. The second permeate 82 has a lower concentration of organic substances than the first permeate 66. At least a portion of the aqueous second permeate 82 is sent to the green liquor plant 46 for dissolving the smelt 42 as described above. The pH of the permeate 82 used to dissolve the smelt 42 is at least 11, typically at least 12, such as about 13. Optionally, some of the second permeate 82 is used to replace evaporation condensate in brown stock wash 90. It may further be discharged to other positions in the kraft mill including wood impregnation steps, oxygen delignification steps or to a bleach plant unit.

[0036] The first filter 58 and the second filter 74 are membrane filters operating in cross-flow mode. The membrane of the first filter 58 is an ultrafiltration membrane or an open nanofiltration membrane with a molecular weight cut-off in the range of 0.5 to 20 kDa, preferably about 1 kDa. The molecular weight cut-off of the membrane in the second filter 74 is lower than of the first filter 58 and is preferably between 0.1 and 0.8 kDa, such as about 0.2 kDa.

[0037] The second stream 54 that is fed to the first filter 58 is typically a weak spent pulping liquor (i.e. black liquor containing about 15% solids) from the wood digesters of the kraft pulping process as described above. Alternatively, the second stream 54 may be an intermediate black liquor (containing about 20-30% solids) from the black liquor evaporator 22 or a combination of a weak black liquor and an intermediate black liquor. An advantage of using an intermediate black liquor as the second stream 54 is that rosin soaps have usually been removed from the liquor. The rosin soaps may negatively affect the filtering process. Further are the rosin soaps a valuable by-product.

[0038] The second stream 54 is between about 10 to 50% by volume of the spent pulping liquor 14. Commonly, the first stream 18 is the major stream and the second stream 54 is the minor stream. The second stream 54 may be about 25% by volume and the first stream 18 may be about 75% by volume of the spent pulping liquor 14.

[0039] The present applicant has surprisingly found that the method and system of the invention significantly reduces the load on both the evaporation plant and the recovery boiler in a pulp mill whilst simultaneously recovering substantially all of the cooking chemicals for re-use in the pulping process. Avoiding a trade-off between reducing the load of the evaporation plant and the recovery boiler on the one hand, and the recovery of the cooking chemicals on the other is a major advantage. Thus, the present process allows for pulp production capacity of a kraft pulping mill to be increased whilst dispensing with the need to upgrade or replace the recovery boiler and/or evaporation plant. Further, the method and system of the invention allows for valorizing part of the lignin in the wood into useful products, contributing to the overall process economy.

EXAMPLE

[0040] The system 10 was operated as follows. Weak black liquor 14 from the digestion of softwood was used. The membrane of the first filter 58 had a molecular weight cut-off of 1 kDa while the membrane of the second filter 74 had a molecular weight cut-off of 0.2 kDa. The weak black liquor 14 was divided into a first stream 18 that was sent to the evaporator 22, recovery boiler 34 etc. as described above and a second stream 54 that was filtered through the first filter 58 operated with cross-flow velocities between 3 and 5 m/s, transmembrane pressures of 5 to 35 bar and at a temperature between 70 C. and 120 C. The first filter 58 separated between 75 and 90 wt. % of the lignin from the second stream 54. The first retentate 62 had a lignin concentration between 190 and 280 g/l (compared with 55 to 70 g/l in the initial weak black liquor 14 and second stream 54). The first retentate 62 was subjected to base catalysis in de-polymerization reactor 70 at a temperature between 250 and 290 C. for between 10 minutes and 4 hours to de-polymerize the lignin. The de-polymerization reactor 70 may contain one or more catalysts. The catalysts may be homogenous or heterogeneous. The catalyst may be selected from the group consisting of nickel, copper, molybdenum, and zirconium catalysts on a support, calcium compounds, sodium hydroxide, and potassium carbonate. Hydrogen gas, a hydrogen donor solvent, or a solvent such as phenol, cresol, or methanol may also be added to the de-polymerization reactor 70. The molecular weight distribution of the lignin was reduced from between 1 and 10 kg/mole in first retentate 62 to between 0.2 and 1.4 kg/mole in de-polymerized lignin stream 71. The first permeate 66 was filtered through second filter 74 at similar conditions to the first filter 58. The obtained second permeate 82 contained lignin and hemicellulose in concentrations of only about 2 wt. % of their concentrations in the second stream of black liquor 54. The second permeate 82 has low organic content and a high content of cooking chemicals including Na.sub.2CO.sub.3. Further, the organic compounds in this stream have a low molecular weight (less than 0.2 kDa). Aqueous streams with low content of organic material may be charged to green liquor plants which are not able to tolerate particulate organics in any quantities, though being equipped with efficient green liquor filters, but rather to tolerate salts and water. The second permeate 82 was sent to the green liquor plant 46. The obtained second retentate 78 contained lignin with a molecular weight distribution between that of the membrane filters i.e. between 0.2 and 1 kDa. This distribution was similar to the lignin in the de-polymerized lignin stream 71. The second retentate 78 was combined with the de-polymerized lignin stream 71 to produce a combined lignin stream 86 suitable for manufacture of lignin bio-oil or other useful products.

[0041] Although the present invention has been described above with reference to specific illustrative embodiments, it is not intended to be limited to the specific form set forth herein. Any combination of the above mentioned embodiments should be appreciated as being within the scope of the invention. Rather, the invention is limited only by the accompanying claims and other embodiments than the specific above are equally possible within the scope of these appended claims.

[0042] In the claims, the term comprises/comprising does not exclude the presence of other species or steps. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms a, an, first, second etc. do not preclude a plurality.