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, in which a suspension containing lignin and chemical pulp is subjected to at least one process step for oxygen-assisted bleaching in a reactor, wherein the oxygen required for the oxygen-assisted bleaching is supplied to the reactor at least partly in the form of oxygen-containing nanobubbles.

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 the reactor for the oxygen-assisted bleaching, a feed for process water and/or a feed for a chemical used in the oxygen-assisted bleaching.

3. The process as claimed in claim 1, wherein the process step of oxygen-assisted bleaching is preceded by a wash stage in which wash water is supplied to the chemical pulp-containing suspension, and the oxygen-containing nanobubbles are supplied at least partly by generation of nanobubbles in the wash water for the wash stage.

4. The process as claimed in claim 1, wherein the oxygen-containing nanobubbles are supplied at least partly by generating nanobubbles in the suspension in a transport conduit for the chemical pulp-containing suspension that opens into the reactor for the oxygen-assisted bleaching or in the reactor for the oxygen-assisted bleaching itself.

5. The process as claimed in claim 1, wherein the process step of oxygen-assisted bleaching proceeds in multiple stages each conducted in a separate reactor, and the oxygen required is supplied to one of the reactors or multiple reactors at least partly in the form of oxygen-containing nanobubbles.

6. The process as claimed in claim 1, wherein the oxygen-assisted bleaching comprises a process step of oxygen delignification and/or a process step for oxygen-enhanced extraction and/or a process step for oxygen-enhanced peroxide bleaching, and wherein the oxygen required for at least one of these process steps is supplied at least partly in the form of oxygen-containing nanobubbles.

7. The process as claimed in claim 1, wherein the chemical pulp-containing suspension into which the oxygen in the form of nanobubbles is introduced in the process step of oxygen-assisted bleaching has a consistency between 8% and 35%, preferably between 10% and 14%.

8. An apparatus for production of bleached chemical pulp, having at least one reactor in which a lignin- and chemical pulp-containing suspension is subjected to at least one process step for alkaline oxygen-assisted bleaching, wherein the reactor and/or a feed for the chemical pulp suspension and/or for an aqueous fluid to be supplied to the reactor that has flow connection to the reactor has an assigned introduction apparatus for introducing oxygen in the form of oxygen-containing nanobubbles.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0025] 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

[0026] 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.

[0027] 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 or—as is customary in modern bleaching processes—in multiple stages in multiple reactors connected in series.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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 9—likewise in a manner known per se—is 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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

[0040] 1. Process [0041] 2. Chemical pulp suspension [0042] 3. Alkaline oxygen delignification [0043] 4. Alkaline oxygen-enhanced extraction [0044] 5. Oxygen-enhanced peroxide bleaching [0045] 6. Wash stage [0046] 7. Wash stage [0047] 8. Wash stage [0048] 9. Wash stage [0049] 10. Filtrate and wash water conduit [0050] 11. Suspension comprising bleached chemical pulp [0051] 12. - [0052] 13. Feed (for dilution water, NaOH) [0053] 14. Oxygen feed [0054] 15. Feed for wash water [0055] 16. Oxygen feed [0056] 17. Oxygen feed [0057] 18. Transport conduit [0058] 19. Oxygen feed [0059] 20. Oxygen feed [0060] 21. Oxygen feed [0061] 22. Transport conduit [0062] 23. Transport conduit [0063] 24. Introduction device