CA2098062A1

CA2098062A1 - Production of cellulose pulp bleaching chemicals

Info

Publication number: CA2098062A1
Application number: CA002098062A
Authority: CA
Inventors: Kaj Henricson; Jaana Suokas
Original assignee: Ahlstrom Corp
Current assignee: Andritz Oy
Priority date: 1992-06-10
Filing date: 1993-06-09
Publication date: 1993-12-11
Also published as: FI932620A0; FI932620A; SE9301940D0; SE9301940L; JPH06108391A

Abstract

ABSTRACT

Hydrogen peroxide for cellulose pulp bleaching is pro-duced directly at a pulp mill without electrolysis. Hy-drogen is produced from black liquor from the pulp mill pulping process, and that hydrogen is then reacted (e.g.
with oxygen and anthraquinone) to produce hydrogen perox-ide, which is in turn used to bleach pulp in a P stage directly on site. The hydrogen is produced from the black liquor by heat treatment to produce an off-gas containing DMS, and other hydrocarbons. Hydrogen can be produced from the off gas in a number of ways, including by react-ing it with a melt in a recovery boiler, or ash from a recovery boiler.

Description

fi~
...^, P~ODUCTIO~ OF CELLULOSE PULP BLEACHI~G CHEMICALS

Backgrou~d and sunmary o~ the invention In conventional bleach plants for bleaching of cellulose pulp, bleaching has typically been done with chlorine and chlorine dioxide, often associated with oxygen and hydro-gen peroxide steps or sequences. In a typical bleach plant which uses oxygen and hydrogen peroxide, the con-sumption of oxygen has been about 15-25 kg per ton of pulp produced, while the peroxide consumption has been about 0-5 kg per ton pulp produced. The rest of the bleaching chemicals consumption has been chlorine and chlorine dioxide. Conventionally, it has been considered uneconomical to manufacture oxygen and peroxide on site at a pulp mill~ Therefore these chemicals have typically been purchased~and brought to the mill. Typically perox-ide, after manufacture at a remote location, is evapor-ated to a strength of about 50~80 % in order to reduce transportation costs, the evaporation procedure and transportation costs obviously contributing to the rela-tively high price of purchased peroxide.

Recently, various pulping and bleaching techniques and combinations have been provided which allow for the chlorine free production of bleached cellulose pulp. For example, Canadian patent application No. 2,089,516 describes procedures for effectively producing bleached kraft pulp without employing chlorine containing chemi-cals. While such techniques are highly desirable from theenvironmental, and other, standpoints, the costs of the pulp production can be relatively high.

Conventionally, hydrogen peroxide is manu~actured by auto-oxidation of anthraquinone by air or oxygen. The solution of anthraquinone is hydrogenated according to .....
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the reaction (1) in the presence of a catalyst (e.g.
palladium):
f (1) :.
O OH
~ C2~5 ~ 3,C2~12 O O

2-ethylanthraquinone 2-ethylanthrahydroquinone ! In the next step the catalyst is separated and returned to the hydrogenation step. The resulting anthraquinone is then oxidized with either air or oxygen at approximately atmospheric pressure. The reaction (2) is exothermic, temperature being controlled by water cooled heat exchangers.

(2) OH
C ~C2l15 Cx~ C2~15 0~1 The resulting hydrogen peroxide is extracted into demineralized water. The concentration of hydrogen perox-ide/water solution is now 20-30 % W/W. Usually it is concentrated to 50-60 % w/w. Hydrogen for the reaction ~1) is usually produced by disassociating water into hydrogen and oxygen by electrolysis. The investment costs for a normal H202 plant (10000 ton/yr) presently are USD
20 million clivided about as follows: peroxide plant 50 %, electrolysis plant 30-40 %, and evaporation plant 10-20 %. The operating costs (USD 0.6/kg H2O2) divided in the same way. Accordingly, it is possible to decrease the price (USD 1.20/kg) of the hydrogen peroxide substan-. .~: . , ::: : ',. . . .

tially, if the electrolysis plant can be eliminated. Ifhydrogen peroxide is manufactured on site at a mill, further savings are obtained, because the costs of evap-orating, transporting, and diluting the hydrogen peroxide in the prior art are eliminated.

According to the present invention, a method is provided which greatly reduces the chemical costs for bleaching pulp, in particular at non-chlorine pulp mills. According to the present invention hydrogen peroxide is produced at a mill from black liquor produced from a sulfate pulping process, or the like, rather than by electrolysis. Ac-cording to the method of the invention, the price of the hydrogen peroxide is reduced by about 40-60 %, if it is used on site at the mill.

In producing hydrogen peroxide from black liquor it is also possible to generate suIphur and hydrocarbon con-taining gases from the cooking or pulp washing plants.
These gases can be converted to hydrogen gas. Especially during the cooking process, gases that can be converted to hydrogen can form and be released. The gases either directly from reactions of cooking liquor with wood, or from dissolved organic material in the cooking plant, can be utilized for this purpose. Thus, the term "black liquor" should be understood as a broad term covering both gaseous and dissolved reaction products formed dur-ing cooking. The cooking process is normally a sulphate process, but also may be sulfide cook, organosolv process (solvent pulping), or the like. Therefore the term "black liquor" as used in the present specification and claims is not restricted to liquor produced form the sulfate process nor to merely the waste liquor from the cooking plant taken to the evaporation plant, but relates to any 3~ gaseous and dis601ved reaction products formed during cellulose pulp cooking in general.

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It is known that the lignin in black liquor is split by a heat treatment, that is by heating the black liquor under pressure for a predetermined time, whereby gases contain-ing organic sulfur compounds, mainly dimethyl sulfide and methyl mercaptan, are generated, as shown in U.S. patent 4,929,307. The point at which the heat treatment is carried out between the digester and recovery boiler of a typical pulp mill is not limited. Preferably the heat treatment is carried out at a temperature of 170-350C. It is appreciated that even as much as 30 to 70 % of the sulfur in the black liquor may be removed. One purpose of the heat treatment is to decrease the sulfur content of the black liquor entering a recovery boiler or like re-covery process in order to minimize corrosion problems.
Alternatively partial gasification, partial combustion, or plasma burning can be used to produce suitable gas from the black liquor. In the method according to the present invention this sulfur containing gas is utilized in an advantageously different way.
The dimethyl sulfide and methyl mercaptan in the gas produced from black liquor heat treatment are converted to hydrogen sulfide and methane and/or other hydrocar-bons. At least the following known methods can be used:
partial oxidation, steam cracking, catalytic conversion, and injection to recovery boiler recycled ash or recovery boiler melt.

In the partial oxidation procedure, the gas is reacted with substoichiometric amounts of oxygen as follows:
(3) CXHy ~ x/2 2 -> x CO + y/2 Hz Any ~ulphur containing hydrocarbons are converted to hydrogen sulphide, carbon monoxide, and hydrogen. For example, Canadian patent application 2,060,819-6 describes a method of treating sulphur containing gases according to this principle.

, Using steam cracking, the organic sulfur gas is mixed with steam to a steam/hydrocarbon ratio of about 0.3-0.4/1.0, and the resulting mixture is subjected to heat-ing. The reaction products should include H2S, CH4, C2H4, H2 and other components.

The catalytic conversion method involves reaction of the organic sulfur gas with hydrogen over a suitable hydrode-sulfurization catalyst to produce hydrogen sulfide and methane by the reactions:
(4) ~CH3)2S + 2 H2 -> 2 CH4 ~ H2S

(5) (CH3SH + H2 -> CH4 + H2S

This reduction of the organic sulfur compounds is carried out at a temperature o~ about 200-300C. This method is described in detail in Canadian patent applica-tion No. 2,060,~19-6 and international application No.
PCT/US92/00924.

After the conversion of the organic sulfur compounds to hydrogen sulfide and hydrocarbons the H2S is separated from the gas mixture and utilized in the cooking cycle of the pulp mill. Separation can be carried out by adsorption, absorption, membrane technology, or by com-pression and partial condensation. Scrubbing with a se-lective solvent, such a methyl amine, can remove essen-tially all of the hydrogen sulfide from the gas mixture.
The produced H2S can be used in the pulping process and considerably improve the pulping yield by pre-impregnat-ing wood chips with H2S before cooking. The productionincrease based on fiber production is at least 10-12 %.
Hydrogen sulfide can also be separated by absorbing it into a caustic solution, such as white liquor, or NaOH
solution. Preferably it is absorbed into only a part of a whike liquor stream, whereby it is possible to produce different cooking liquor streams having different sulfi-dities. It is advantageous to use a high sulfidity liquor ' ' , ,.

at the beginning of cooking and low sulfidity liquors in the later stages. The produced H2S can also be used in other ways for modification of cooking liquors than by absorbing H2S into white liquor.

The sulfur gas can be treated by injecting it into recov-ery boiler recycled ash or recovery boiler melt, as described in Finnish patent applications 914585 and 914586, respectively. When DMS gas is injected into ash or melt, a gas containing ethene and/or other hydrocar-bons is obtained. The sulfur of the gas reacts with Na2CO3 of the ash or melt. Thus it is not necessary to separate H2S as would otherw1se be required.

The hydrocarbons containing gas which has been obtained as described above and from which H2S has been separated is used for production of hydrogen. Hydrogen can be gen-erated for instance by steam r0forming or sub-stoichiometric combustion. The gas mixture thus formed is introduced for instance into a PSA (pressure swing adsorption) unit, in which conventional adsorption methods are practiced to recover pure hydrogen. Also ;~; membrane technology can be utilized in order to yield pure hydrogen. The hydrogen thus obtained is used for manufacture of hydrogen peroxide by conventional methods, ; such as by autooxidation, as described above, and the remainder of the gas mixture can be used as a fuel in the pulp mill. Alternatively, or additionally, the reaction (1) can be used to separate hydrogen from the gases di-rectly, thus eliminating the need to produce pure hydro-gen.

The hydracarbons containing gas may also contain hydrogen to some degree, which may be recovered before separation of hydrogen sul~ide or before conversion of hydrocarbons to hydrogen. Also the sulfur containing gas may contain hydrogen, which may be recovered.

.. . .

2 ~ d It is the primary object of the present invention to pro-vide a process for the production of hydrogen peroxide directly on site at a pulp mill without utilizing elec-trolysis equipment. This and other objects of the inven-tion will become clear from an inspeckion of the detaileddescription of the invention and from the appended claims.
:
Brie desc~ip~ion of tha dr~wings FIGURE 1 is a schematic representation of one exemplary system for practicing exemplary methods acco~ding to the present invention; and FIGURE 2 is a schematic representation of another exemp-lary system.

D~ailed descriptio~ oi the dr~wings FIGURE 1 schematically illustrates a system utilizable for the practice of the process of the present invention.

Black liquor in line 10 from a sulfate digester or the like (not shown) is ~ed to a reactor 11, where sulfur is separated thererom in form of sulfurous gases by a heat treatment, such as described in U.S. patent 4,929,307.
The black liquor having a reduced sulfur content in line 12 that is produced is subjected to further treatment, such as to a recovery boiler (not shown) with conven-tional whitP liquor manufacture. The gas containingorganic sulfur compounds, mainly DMS and MM, in line 13 is fed into a hydroconverter 14, where the reactions (4) and (5), above, are carried out. The gas in line 13 may also include similar gases from other sources, such as from a digester and evaporation plant. The gas mixture containing CH4, H2S, and extra H2 in line 15 is introduced for instance into white liquor scrubber 16, in which H2S

:, , .. . . .
', ' ' , . ' ,, ~, ' , is separated by absorption and a substantially sulfur-free CH4 gas in line 17 is produced.

The methane is converted to hydrogen by steam reforming converter 18, which comprises or consists of catalyti-cally reacting a mixture of steam and methane at an elev-ated temperature to form a mixture of H2 and oxides of carbon. The net reaction is as follows:

(6) 1/2 CH4 + H2O -> 2 ~2 ~ 1/2 CO2 In the converter 18 the amount of methane that is con-verted to hydrogen is only about as much as will be con-sumed in the manufacture of hydrogen peroxide and in other processes. The hydrogen is separated from the prod-uct gas in line 19 containing H2, CO2 and unconverted CH4 by means of a PSA purification unit 20. The fuel gas thus obtained in line 21 can be used in a lime kiln as a fuel, in a power plant boiler or gas turbine process, or in other processes requiring heat in the pulp mill. The less the amount of hydrogen that is produced (in line 22), thegreater the heat value of the fuel gas that ensues (in line 21).

The hydrogen in line 22 produced on site at the pulp mill is discharged from the apparatus 20 and i6 split into two streams. One of these, line 22a, goes to a conventional hydrogen peroxide production apparatus 23, located at the pulp mill. The other stream, in line 22b, is fed to the hydroconverter 14, where the hydrogen is used to reduce the DMS and MM to methane and hydrogen sulfide.

The apparatus 23 may be any desired conventional appar-atus for producing ~22/ such as an apparatus capable of ; practicing an AO-process (reactions 1 and 2~. Air or substantially pure oxygen, or other oxygen-containing gas, in line 22 is used as an oxidant. A possible source of oxygen for peroxide manufacture is the oxygen contain-ing o~f gas from an ozone blea~hing stage~ By uji~g P~A
or VSA (~acuum ~ing ad~orption) technolo~ it is poss-ibl~ to aeparate 90-95 % oxygen ~rom the ~ir on ~ite. At t,he same ti~e the need ~or ox~gen ~or othGr purpos~, ~uch au ~l~aahiny, m~y be ~at~ d.

~ydrogen peroxide produced by the apparatu~ 23 i~ ~ed in line ~5, on ~ite in the pulp mill, directly to a P bleach 6tag~ to e~ect bleach~ ng o~ the cellul~s~ p~lp therein, or to a~other ~t~ge ~h~ch u~ei peroxid The hydro~en peroxide in line ~5 has ~ concentration of about 15-35 ~
whiC~ is a de~ira~le con~ent~atio~ for use directly i~ a peroxide ~agea Therefore the coet~ o~ ev~porating, transportingr and dlluti~g the hydr~ge~ pero~ide i~ ~h~
comme~cial prior ar~ are eliminated.

~wo al~erna~ive bleaching ~e~uence~, OAZEZPZ and ODED, are mo~t preferably employed. The ozo~e (Z) aan be pro-duced on site tOO, as ca~ the o~er ~leachin~ ahbmic~l~
as described in the int~rnational application PcT/FI93/ool48.
Hydrogen is generated in a suff icient amount ~or bleaching pulp and for catalytic conversion of DMS and MM.
In a typlcal example o~ tho pr~c~iae o the proeout in-v~ntion, a~ illus~rated in FIGURE 1, 1700 ton d.~./d of black li~uor is treated by pre6~ure heati~g ~11) in order to recover 82.5 kg/~on p~lp o~ ~MS, which is ca~alyti-cally converted to a g~G ~ixture cont~ining ~2.5 kg/ton pulp o~ C~4, 45.2 H2S ~nd 2.7 extra ~2 The hydrogen ~ul-fide i~ separated, and metha~e i~ con~erted to H2 (9.2 k~/ton) and CO2, I~ pulp i~ bleached according to the e~uence OAZE~PZ, 1.2 kg~ton p~lp hyd~ogen is required, while the amount o~ 0,6 kg H2/ton pulp i~ required for the se~uen~e OD~ y ~p~rating ~z a ~el gas ha~ing a ~ig-n~icantl~ high heat value (24.8-26.6 ~/~3) i~ obtainsd ~line 21). Most o the produced hydroge~l ~8 kg~to~ pulp) in line 22b i~ u~ed in th~ catalytic conv~ion oX DMS

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z~

(in 14~, the rest (1.2 kg/ton) is used in line 22a to produce H2O2. The required amounts f ~22 are 20 kg ton pulp for the OAZ~ZPZ sequence, and 10 kg/ton pulp for the ODED sequence.
According to the present invention a method has been provided for the production of hydrogen peroxide on site at a mill for the bleaching of cellulosic material in an efficient and cost effective manner. When hydrogen perox-ide can be produced on site at a pulp mill from hydrogencoming from black liquor of the mill, it is possible to reduce the peroxide price considerably. The present in-vestment costs for the system of FIGURE 1 to practice the process of the invention are about USD 13 million (USD 10 million for the peroxide plant and USD 3 million for the H2 plant), which is about 50 % of the investment costs of the conventional process. The operating costs of the process of the invention are about USD 0.3/kg ff22, which are also about 50 % of those of the conventional process.
An alternative way to produce hydrogen peroxide also includes as the first step heat treating black liquor to produce a gas iIl FIGURE 1, e.g. using the apparatus 11 in FIGURE 2. Thi~ gas, in line 13, is injected into melt 30 in a recovery boiler 31, or is contacted with ash 32 from the boiler 31. When the temperature is high enough the sulfur in the gas in line 13 reacts with Na2CO3 to produce Na2S. An off-gas, in line 3~, is produced which contains hydrocarbons.
The hydrocarbons containing gas in line 34 can be con-verted to a gas containing hydrogen, as described above, and shown schematically at 35 in FIGURE 2. Hydrogen is separated from this gas mixture and used in manufacture of hydrogen peroxide. Alternatively, as shown in FIGURE
2, the hydrogen containing gas can be directly taken to the hydro~en peroxide process where hydrogen reacts with , , . '.. ;','''' ':
.,: . :::, ,: :
,:, , , , ''' :,' ~ ' ~Q~

anthraquinone according to formula (1). This reaction can be achieved by taking the anthraquinone solution in line 36 and the hydrogen containing gas in line 37 into an absorption column 38, where the reaction takes place. The gas in line 40 from absorption column 38 that is not consumed to produce hydrogen peroxide can be burnt in a boiler or lime kiln, shown schematically at 39 in FIGURE
2, to produce heat.

In FIGURE 2, hydrogen production takes place at 35. The production of hydrogen at 35 is not necessary, however.
Sometimes the gas in line 34 can contain enough hydrogen without conversion that it can be taken directly to col-umn 38. Alternatively, the reduction of anthraquinone according to reaction (1) may take place through some other reducing agent in line 37 other than hydrogen.

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 embodi-ment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

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Claims

1. A method of producing hydrogen peroxide at a pulp mill having a pulping process which produces black liquor, comprising the steps of:
(a) producing hydrogen from black liquor at the pulp mill; and (b) producing hydrogen peroxide at the pulp mill from the hydrogen from step (a).

2. A method as recited in claim 1, wherein step (a) is practiced by: (a1) pressure heating black liquor at a temperature and for a time period such that gases con-taining organic sulfur compounds are produced; and (a2) treating the gases to produce gases containing hydrogen.

3. A method as recited in claim 2, wherein step (a2) is practiced by separating hydrogen sulfide from a hydrocar-bon containing gas before conversion to hydrogen, by converting methane and/or other hydrocarbon containing gas to hydrogen, which is separated and used in step (b).

4. A method as recited in claim 3, wherein after separ-ation of hydrogen a gas mixture containing H2, CO2 and unconverted hydrocarbons is obtained and used as a fuel gas at the pulp mill.

5. A method as recited in claim 3, wherein hydrogen sul-fide is separated in the form of gas or by absorbing it into white liquor and used in pulp mill pulping process.

6. A method as recited in claim 1, wherein step (b) is practiced by utilizing a gas containing oxygen and an anthraquinone.

7. A method as recited in claim 2, wherein step (b) is practiced by utilizing a gas containing oxygen and an anthraquinone, and the gases from step (a2) are treated to form a gas mixture containing H2S, H2 and unconverted hydrocarbons, which mixture is contacted with the anthra-quinone to hydrogenate anthraquinone.

8. A method as recited in claim 7, wherein having removed the sulfur therefrom the rest of the gases are used as a fuel gas, and the obtained sulfur is used in the pulp mill pulping process.

9. A method as recited in claim 1, wherein step (a) is practiced by (a1) pressure heating black liquor at a temperature and for a time period such that gases con-taining organic sulfur compounds are produced; (a2) in-jecting the gases into melt or ash containing sodium carbonate to produce gases, which are used to produce hydrogen peroxide.

10. A method as recited in claim 9 comprising the further step of producing and separating hydrogen from the gases from step (a2), and using the separated out hydrogen to produce hydrogen peroxide in step (b).

11. A method as recited in claim 9, wherein step (b) is practiced by utilizing a gas containing oxygen and an anthraquinone, and by contacting the gases from step (a2) with the anthraquinone in order to hydrogenate it, and after hydrogenation, using the remaining gases as a fuel gas.

12. A method as recited in claim 11 comprising the fur-ther step of producing hydrogen from the gases from step (a2) before contacting the gases with anthraquinone.

13. A method as recited in claim 1 comprising the further step of using hydrogen peroxide produced in step (b) for bleaching cellulose pulp on site at the pulp mill.

14 14. A method as recited in claim 1, wherein step (b) is practiced to produce hydrogen peroxide having a concen-tration of about 15-35 %, and then comprising the further step of utilizing the hydrogen peroxide, without concen-tration or dilution, to bleach cellulose pulp.

15. A method of treating cellulose pulp at a pulp mill, comprising the steps of:
(a) producing hydrogen at the pulp mill without electrolysis;
(b) producing hydrogen peroxide at the pulp mill using the hydrogen from step (a); and (c) bleaching the cellulose pulp at the pulp mill with the hydrogen peroxide produced in step (b).

16. A method as recited in claim 15, wherein step (a) is practiced in part by (a1) injecting gases containing organic sulfur compounds produced at the pulp mill into melt or ash containing sodium carbonate, to produce in-termediate gases, and (a2) treating the gases from step (a1) to produce hydrogen.

17. A method as recited in claim 15, wherein step (b) is practiced by contacting the hydrogen from step (a) with a gas containing oxygen and an anthraquinone.

18. A method as recited in claim 15, wherein step (a) is practiced by treating organic sulfur compound containing gases produced at the pulp mill to produce primarily hydrogen sulfide and hydrocarbons, and then (a2) treating the hydrocarbons from step (a) to produce hydrogen.

19. A method as recited in claim 18, wherein step (a2) is practiced by separating hydrogen sulfide from hydrocarbon containing gas before conversion to hydrogen, and by converting methane and/or other hydrocarbon containing gas to hydrogen, which is separated and used in step (b).

20. A method as recited in claim 15, wherein step (c) further includes treating the pulp in a bleaching sequence including ozone produced on site.

21. A method as recited in claim 15, wherein step (b) is practiced to produce hydrogen peroxide having a consist-ency of about 15-35 %, and wherein that hydrogen peroxide is utilized, without dilution or concentration thereof, to practice step (c).

22. A method of treating cellulose pulp at a pulp mill in which black liquor is produced, comprising the steps of:
(a) reducing anthraquinone by using a reducing gas from the black liquor produced by the pulp mill; and (b) producing hydrogen peroxide by using the reduced anthraquinone from step (a).