Process and apparatus for producing bleached cellulose

Abstract

In a process/an apparatus for producing bleached cellulose in which a lignin- and cellulose-containing suspension is subjected to at least one process step for oxygen-assisted bleaching in a reactor, such as alkaline oxygen delignification, oxygen-enhanced extraction or oxygen-enhanced peroxide bleaching, the oxygen required for the oxygen-assisted bleaching is supplied to the reactor at least partially in the form of oxygen-containing nanobubbles. The small size and high stability of the nanobubbles allow uniform distribution of the oxygen in the suspension and a comparatively long exposure time. The efficiency of the bleaching is thus substantially increased.

Claims

1. A process for producing bleached chemical pulp comprising: subjecting a suspension containing lignin and chemical pulp to a first process step for oxygen-assisted bleaching in a first reactor, wherein oxygen required for the first process step is supplied to the first reactor at least partly in the form of oxygen-containing nanobubbles via a first introduction device in a first transport conduit supplying the suspension to the first reactor; subjecting the suspension to a second process step for oxygen-assisted bleaching in a second reactor downstream from the first reactor, wherein oxygen required for the second process step is supplied to the second reactor at least partly in the form of oxygen-containing nanobubbles via a second introduction device in a second transport conduit supplying the suspension to the second reactor from the first reactor; and subjecting the suspension to a third process step for oxygen-assisted bleaching in a third reactor downstream from the second reactor, wherein oxygen required for the third process step is supplied to the third reactor at least partly in the form of oxygen-containing nanobubbles via a third introduction device in a third transport conduit supplying the suspension to the third reactor from the second reactor; wherein the second process step is different than the first process step, and wherein the third process step is different than the first process step and the second process step; and wherein the first process step for oxygen-assisted bleaching is preceded by a wash stage in which wash water is supplied to the suspension, wherein the oxygen-containing nanobubbles are supplied to the first reactor at least partly by a wash introduction device that is located in a feedline supplying the wash water to the wash stage and that generates the oxygen-containing nanobubbles in the wash water for the wash stage.

2. The process as claimed in claim 1, wherein the oxygen-containing nanobubbles are supplied at least partly by generating nanobubbles in a feed for fresh water having flow connection to at least one of the first reactor, the second reactor or the third reactor, in a feed for process water, and/or in a feed for a chemical used in the oxygen-assisted bleaching.

3. The process as claimed in claim 1, wherein the oxygen-containing nanobubbles are supplied to the first reactor at least partly by generating nanobubbles in the first reactor via a reactor introduction device.

4. The process as claimed in claim 1, wherein the first process step performed in the first reactor comprises a process step of oxygen delignification; wherein the second process step performed in the second reactor comprises a process step for oxygen-enhanced extraction; and wherein the third process step performed in the third reactor comprises a process step for oxygen-enhanced peroxide bleaching.

5. The process as claimed in claim 1, wherein the suspension into which the oxygen-containing nanobubbles are introduced has a consistency between 8% and 35%.

6. The process as claimed in claim 5, wherein the suspension into which the oxygen-containing nanobubbles are introduced has a consistency between 10% and 14%.

7. The process as claimed in claim 1, wherein the first process step performed in the first reactor comprises a process step of oxygen delignification.

8. The process as claimed in claim 7, wherein the oxygen required for the process step of oxygen delignification is supplied to the first reactor at least partly in the form of oxygen-containing nanobubbles via the first introduction device and via a reactor introduction device located in the first reactor.

9. The process as claimed in claim 7, wherein the oxygen required for the process step of oxygen delignification is supplied to the first reactor at least partly in the form of oxygen-containing nanobubbles via the first introduction device and via a fourth introduction device located in a chemical transport conduit supplying chemicals necessary for the process step of oxygen delignification.

10. The process as claimed in claim 1, wherein the second process step performed in the second reactor comprises a process step for oxygen-enhanced alkaline extraction.

11. The process as claimed in claim 1, wherein the third process step performed in the third reactor comprises a process step for oxygen-enhanced peroxide bleaching.

12. The process as claimed in claim 1, wherein the wash stage is a first wash stage upstream from the first reactor, and the process further comprises: supplying wash water to the suspension in a second wash stage downstream from the first reactor and upstream from the second reactor; and supplying wash water to the suspension in a third wash stage downstream from the second reactor and upstream from the third reactor.

13. The process as claimed in claim 12, wherein the first wash stage is performed upstream from the first introduction device.

14. The process as claimed in claim 12, wherein the second wash stage is performed downstream from the first reactor and upstream from the second introduction device.

15. The process as claimed in claim 12, wherein the third wash stage is performed downstream from the second reactor and upstream from the third introduction device.

16. The process as claimed in claim 12, wherein the feedline supplies the wash water from the second wash stage to the first wash stage.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The drawing is intended to elucidate a working example of the invention in detail. The sole drawing (FIG. 1) shows a flow diagram for a working example of the process of the invention.

DETAILED DESCRIPTION

(2) FIG. 1 shows a process 1 for bleaching of chemical pulp which is used, for example, as precursor for the production of paper or other products. In this process, an aqueous chemical pulp suspension 2 comprising not only chemical pulp but also fractions of lignin passes through the successive oxygen-assisted bleaching stages of an alkaline oxygen delignification 3, an oxygen-enhanced alkaline extraction 4 and an oxygen-enhanced peroxide bleaching 5.

(3) In the oxygen delignification 3, the chemical pulp suspension 2 is treated with oxygen in an alkaline environment in a pressure-resistant reactor at high temperatures. This removes significant proportions of the lignin still present in the suspension by reaction with oxygen. For reasons of clarity, just one process step for oxygen delignification 3 is shown here; the oxygen delignification 3 may be effected here in a single reactor oras is customary in modern bleaching processesin multiple stages in multiple reactors connected in series.

(4) The oxygen delignification 3 requires an alkaline medium having a pH of about pH=11 at a temperature between 80 C. and 105 C., which, in the working example shown here, is achieved by the supply of NaOH and of hot steam to the reactor. The suspension here has an average consistency of, for example, 10% to 14%. Oxygen or an oxygenous gas is introduced into the reactor(s). In the comparatively unusual case nowadays of a one-stage oxygen delignification, the treatment is effected at a pressure of, for example, 7 to 8 bar in the feed and 4.5 to 5.5 bar in the output from the (single) reactor. The treatment time (retention time) here is, for example, 50 to 60 min. In the case of a two-stage oxygen delignification, there is generally a difference in pressure and reaction time in the two reactors. In the first stage, for example, a customary pressure is a pressure of 7 to 10 bar and a customary retention time is 10 to 15 minutes, and in the second stage a pressure of 3 to 5 bar with a retention time of about 1 h.

(5) In the subsequent alkaline extraction 4, the lignin remaining after the delignification is rendered largely soluble by means of NaOH. The addition of oxygen here enhances the bleaching action (EO, oxygen-enhanced extraction). The treatment is effected in a reactor at a temperature of, for example, 55 C.-80 C. and a pressure of, for example, between atmospheric pressure and 3-4 bar, with a residence time of, for example, 60 to 120 min.

(6) In the peroxide bleaching 5, the suspension is supplied, as a further bleaching agent, with a peroxide, especially hydrogen peroxide (H.sub.2O.sub.2). The efficiency of this process step can also be significantly improved by addition of oxygen (PO, oxygen-enhanced peroxide bleaching). The treatment is effected in a reactor, for example at atmospheric pressure and a temperature of, for example, between 85 C. and 90 C. or under an elevated pressure at temperatures of, for example, between 100 C. and 110 C.

(7) It will be apparent that process steps 3, 4, 5 shown here need not necessarily all be conducted, and all in the manner described here; in the context of the invention, it is possible for individual or multiple steps among these to be present, optionally in combination with further bleaching stages that are not described here.

(8) Connected upstream or downstream of the bleaching stages 3, 4, 5, in a manner known per se, are in each case wash stages 6, 7, 8, 9. In the final wash stage 9, an aqueous medium is supplied, for example fresh water or condensate. The filtrate obtained in the wash stage 9likewise in a manner known per seis fed via a filtrate and wash water conduit 10, in each case in countercurrent to the running of the chemical pulp suspension, to the respective prior wash stage 8, 7, 6. At the end of last wash stage 9, a suspension formed as intermediate with at least largely bleached chemical pulp 11 is fed to downstream processing steps that are of no further interest here.

(9) As mentioned, in bleaching stages 3, 4, 5, oxygen is supplied, directly or indirectly into the reactors that accommodate the respective bleaching stages 3, 4, 5. According to the invention, this introduces at least a portion of the oxygen in the form of nanobubbles having an average diameter between 20 nm and 1000 nm. The working example disclosed here, by way of example, shows various options for sites where oxygen can be introduced in the form of nanobubbles.

(10) For example, oxygen can be supplied to the alkaline oxygen delignification 3 by introduction of oxygen in the form of nanobubbles in a feed 13 for reflux water, into which the sodium hydroxide solution also required for the alkaline oxygen delignification 3 is also fed, as shown by the oxygen supply 14. However, the introduction of oxygen in the form of nanobubbles may also, additionally or alternatively, be effected in a feed 15 for wash water into the wash stage 6 upstream of the oxygen delignification 3 (oxygen feed 16), in an oxygen feed 17 that opens directly into the reactor (or one or more of the reactors) of the oxygen delignification 3 and/or in a transport conduit 18 that feeds the chemical pulp-containing suspension to the reactor (or one of the reactors) of the oxygen delignification 3, as indicated by oxygen feed 19.

(11) In the same way, there exist various options for the supply of oxygen in the alkaline extraction 4 and the peroxide bleaching 5 as well; at the same time, the drawing, for reasons of clarity, shows only oxygen feeds 20, 21 that open into a transport conduit 22, 23 arranged upstream of the respective reactor for the chemical pulp suspension.

(12) The introduction of the oxygen in the form of nanobubbles, incidentally, is not limited to the point of entry shown here; instead, the introduction can also be effected at other points that are not shown here.

(13) Incidentally, it is no way obligatory in the context of the invention for the oxygen to be introduced exclusively in the form of nanobubbles; instead, it is also possible that the oxygen is introduced in the form of nanobubbles in addition to other modes of introduction for the oxygen.

(14) Nanobubbles are produced in each case at the opening of the oxygen feeds 14, 16, 17, 19, 20, 21 into the respective fluid-conducting conduit 13, 15, 18, 22, 23 and/or the respective reactor in suitable introduction apparatuses 24. All that is required here is that, in operation of the introduction apparatuses 24, this at least with one apparatus that produces the nanobubbles, for example a nozzle, is surrounded by water or an aqueous fluid or a suspension, such that the nanobubbles can form in the aqueous phase. The nanobubbles are then entrained by the flow of the respective fluid and hence arrive in the respective reactor for the reaction 3, 4, 5. Incidentally, such an introduction device 24 that permits the production of oxygen-containing nanobubbles in the respective fluid may also be provided solely at one or some of the openings mentioned in the oxygen feeds 14, 16, 17, 19, 20, 21.

(15) The process of the invention makes it possible to use the oxygen introduced into the chemical pulp suspensions over the course of the various bleaching stages with significantly higher efficiency than is the case in prior art processes. The small size of the nanobubbles enables uniform distribution of the oxygen in the suspension and facilitates the transport of the oxygen directly to the lignin to be oxidized. In addition, the nanobubbles, in regions where there is a high oxygen demand, constitute a readily available reservoir for oxygen.

LIST OF REFERENCE NUMERALS

(16) 1. Process 2. Chemical pulp suspension 3. Alkaline oxygen delignification 4. Alkaline oxygen-enhanced extraction 5. Oxygen-enhanced peroxide bleaching 6. Wash stage 7. Wash stage 8. Wash stage 9. Wash stage 10. Filtrate and wash water conduit 11. Suspension comprising bleached chemical pulp 12. - 13. Feed (for dilution water, NaOH) 14. Oxygen feed 15. Feed for wash water 16. Oxygen feed 17. Oxygen feed 18. Transport conduit 19. Oxygen feed 20. Oxygen feed 21. Oxygen feed 22. Transport conduit 23. Transport conduit 24. Introduction device