FR2669947A1 - PROCESS FOR THE DISSOLUTION OF CELLULOSE - Google Patents

Abstract

a) Process for dissolving cellulose, b) process characterized in that in the black liquor, before the heat treatment, a gas containing at least 50% by volume of oxygen is introduced and in that, as gas containing oxygen, technically pure oxygen is used. c) The invention relates to methods for dissolving cellulose.

Description

The invention relates to a process for the dissolution of cellulose.

  process for dissolving the cellulose by means of a white liquor which consists essentially of Na and Na OH, resulting in a black liquor containing lignin and unreacted Na 2 S, from which a white solution is recovered

by heat treatment.

  Operations for the production of cellulose pulp typically work either according to the so-called sulphite process or

  what is called the sulphate process.

  In the case of the sulphite process, sulphites (for example Ca (HSO 3) 2 or Na HSO 3) are used to obtain the white liquor necessary for the dissolution of the cellulose As few sulfur compounds are released during the recovery white liquor from the black liquor by heat treatment, this process has hitherto been preferred in countries where there are strict obligations for the environment. However, this process has the disadvantage that the mechanical properties of

  cellulose fibers are worse.

  The sulphate process, in which sulphates, in particular Na 2 SO 4, are used to obtain the white liquor, allows on the other hand a much more efficient dissolution of the cellulose, so that a high quality paper can be produced when the recovery of white liquor from the black liquor by heat treatment, however, large quantities of sulfur compounds are released For this reason, this process is not used so far in countries where there is

  strict regulations for flue gas.

  The two dissolution methods are described, for example, in "Ullmanns Enzyklopadie der technischen Chemie, Band 17, 4, Auflage, Verlag Chemie Weinheim-York, York, Papier, Faserrohstoffe, pages 535 to 557" and in "Winnacker, Kuchler, Chemische

  Technology, Band 5, 4 Auflage, Carl Hanser Verlag -

  M nchen-Wien (1981), pages 605-626.

  The object of the present invention is to improve the sulphate process so that the emission of sulfur compounds during the recovery of the white liquor from the black liquor

  deleted to a large extent.

  This object is achieved, according to the invention, in that in the black liquor is introduced, before the heat treatment, a gas

  containing at least 50% by volume of oxygen.

  To allow an easier understanding of the invention, it is first necessary to go into more detail with regard to the principle of the dissolution of the cellulose according to the sulphate method. The starting materials for the white liquor necessary for the dissolution of the cellulose, are usually Na 2 O, Na 2 SO 4 and Ca OA from these compounds, the white liquor consisting essentially of Na 2 S and NaOH is obtained according to the following reaction equilibria: Na 2 O + C 02> Na 2 CO 3 Na 2 SO 4+ 4 CO> Na 2 S + 4 Ca 2 | CaO + H 20 _> Ca (OH) 2 Na 2 CO 3+ Ca (OH) 2> 2 Na OH + Ca CO 3 Cellulose is usually fired at about 160 ° C with white liquor, lignin acting as support frame in the

  cellulose then passing into solution and coloring this

  The solution thus colored black, which is called black liquor, contains, besides the dissolved lignin, also large quantities of unconverted Na 2 S. For chemical recovery, the black liquor is heat-treated, so typical in a recovery oven The heat resulting from the burning of the lignin content

  can then be used for steam production.

  From Na 2 S is born Na 2 SO 4 which is

  reused for the manufacture of white liquor.

  Up to now, about 3 to 5% of chemically dissolved Na 2 S was lost in the form of off-gas

  chemical recovery in recovery furnaces.

  It then escapes large amounts of H 25 and 502 through the chimney, which translates into a

  significant pollution of the environment.

  Surprisingly, this pollution of the environment can be avoided when, according to the invention, a gas containing at least 50% by volume of oxygen is introduced into the black liquor before the heat treatment. then based on the fact that thanks to the introduction of oxygen, the Na 2 S can already be oxidized to a large extent in Na 25203 before the heat treatment, so that in the recovery oven, it can no longer be used. escape from volatile sulfur compounds Volatile sulfur compounds originate in the recovery furnace on a boundary surface at which the conditions of an oxidizing and reducing reaction are contiguous In the upper zone of the recovery furnace, oxidizing reaction conditions, to burn the lignin content of the black liquor In the lower part, reducing reaction conditions are introduced to reduce the Na 2 S O 4 in Na 2 S with CO At the limiting surface, the Na 2 Sne can not be completely oxidized, so that volatile sulfur compounds are born This is prevented in that the Na 2 S is already oxidized to a large extent before the furnace of recovery by introduction of oxygen

technically pure.

  This is based on the following equality of reaction: Na 2 S + O 2 + 1/2 H 2 O> 1/2 Na 25203 + Na OH The starting Na 25203 is reduced by means of CO in Na 2 S in a similar way to the reduction of Na 2 SO 4, this Na 2 S being reused for the

manufacture of white liquor.

  As oxygen-containing gas, it is preferable to use technically pure oxygen, which is for example available as a liquid gas tank or which can be manufactured on site by means of an air decomposer. It is possible to use an oxygen-enriched gas, for example oxygen-enriched air. This oxygen-enriched gas can, for example, also be manufactured by means of an adsorption plant.

pressure alternation (PSA).

  The quantity of oxygen to be introduced into the black liquor is advantageously chosen so that the Na 2 S is completely oxidized to Na 2 O 5. Thus, the birth of volatile sulfur compounds in the recovery oven is totally prevented, so that even the strictest obligations with regard to

  concerning the environment, can be satisfied.

  The technically pure oxygen or the oxygen-enriched gas is advantageously injected into a discharge pipe containing the black liquor. Usually, the black liquor leaves under pressure the cooker in which the dissolution of the cellulose takes place and it is brought by discharge pipes. to a concentrator by evaporation and thence through a thickener to the recovery furnace Preferably, oxygen oxygen rich gas is injected into the discharge pipe where the pressure is at its maximum, so to obtain that the oxygen dissolves to a large extent in the black liquor Such a location is, for example immediately to the connection with the cooker, or directly in

downstream of discharge pumps.

  Apart from the fact that the emissions of H 25 and 502 are prevented, other important advantages are related to the invention. As no sulfur compounds are escaping from the chimney and the Na 2 S content of the black liquor can be totally reused for the manufacture of white liquor, the white liquor does not need to be completed as is necessary in the state of the art In addition, the energy of the steam that is put into In many locations in the production of cellulose pulp, this can be saved because, because of the exothermic reaction that converts Na 2 S to Na 25203, additional steam is produced. The sulphate process is environmentally compatible, so that it can now be implemented even in countries where there are strict environmental requirements. The process is then economical despite the costs. additional energy for the technically pure oxygen or the oxygen-enriched gas, because both the energy and the chemicals can be saved. In addition, it is not necessary to purify the gaseous effluents thanks to

  to what, profitability is also increased.

  The invention will be explained in more detail below with the aid of an exemplary embodiment.

  schematically represented in the single figure

  attached. This figure shows a flow diagram of an installation for the preparation of liquor

  black in a cellulose pulp mill.

  In the present embodiment, it is a cellulose pulp mill which comprises a continuous cooker for the production of eucalyptus cellulose pulp and four batch cookers for the production of cellulose pulp. For simplicity, only one cooker 1 is shown in the drawing In the black liquor flowing from the cookers,

  8 to 10 grams of Na 2 S per liter are still contained.

  The black liquor is fed through a discharge pipe 4 to an intermediate vessel 2 which must compensate for the fluctuations in the black liquor stream. The black liquor in the intermediate vessel 2 has a solids content of about 16 to about 18. From here, the black liquor is sent by delivery pumps 5 and another discharge line 6 to six evaporative concentrators 3 which are connected in series. In these concentrators by evaporation 3, hot steam is supplied against

current of black liquor.

  The vaporized fractions of the black liquor, the so-called steam, are withdrawn via a pipe 7 and brought to a vapor container 8 From there, the steam reaches a stripping installation 9 The waters exhausted are withdrawn via a pipe 10, while the gaseous fractions are introduced, via the pipe 11, into the recovery furnace 12 (or

recovery boiler).

  The non-vaporized black liquor fractions in the evaporative concentrators 3 are withdrawn via the pipe 13 and fed to a buffer tank 14. In this buffer tank 14, the black liquor already has a solids content of About 40% of this solids content is then fed, by means of a thickener to about 56%. By means of another buffer vessel 16, the black liquor finally arrives with a solids content of about 75%. In this recovery furnace 12, it is obtained from the black liquor of the slag which is discharged via line 17. From these slags, white liquor is again manufactured. which is again implemented for dissolution

cellulose in the cooker 1.

  The black liquor already arrives at the outlet of the cooker 1 with a temperature of about 800 ° C. and an increased pressure. Consequently, immediately after the cooker 1, ideal conditions are created for the dissolution of the oxygen in the black liquor. , the oxygen that may be available in a liquefied gas tank, or that can be manufactured on site by means of an air decomposer or a PSA plant, is directly injected after the cooker 1 via In addition, downstream of the discharge pumps 5 and directly at the connections thereto, technically pure oxygen is injected into the discharge pipe 6 by means of nozzles 19. the discharge line 6 is about 100 meters since the

  nozzles 19 to the concentrator by evaporation 3.

  This length is sufficient to dissolve the oxygen in the black liquor to a large extent and to oxidize the Na 2 S to Na 25203 before the arrival at the concentrator by evaporation. The quantity of oxygen to be injected in total is chosen so that the fraction In the recovery furnace 12, only the sulfur-containing compounds with a high oxidation state are discharged into the black liquor stream, so that in this recovery furnace 12 he can not

  release sulfur compounds with low oxidation levels.

  The lignin fraction of the black liquor is burned in the upper part of the recovery oven 12 in which reaction conditions prevail.

  oxidizing (however often lack of oxygen).

  The energy thus released is used for the production of steam. This steam can be used at many locations in the cellulose pulp mill. In the lower part of the recovery furnace 12, Na 25203 is again reduced by CO in Na 2 S The resulting slag is

  reused for the manufacture of white liquor.

  There is thus no loss of Na 2 S, so that the Na 2 SO 4 must not be constantly completed for the manufacture of the white liquor, as is the case for the state of the art. Furthermore, there is no environmental pollution by H 2 S and 502 emissions, as is the case in conventional sulphate processes. As the oxidation of Na 25 to Na 25203 is exothermic, the energy additional form of steam is released which can be

  used in the cellulose pulp mill.

  The operating parameters for an exemplary embodiment are summarized below: Operating data: Volume flow rate in black liquor (Dry substance = 16 18%) 192 m 3 / h Delivery pipe length approximately 100 m Diameter of the pipe delivery pressure 200 mm Flow rate 1.70 m / s Line pressure 3 x 105 Pa Black liquor temperature 80 C Concentration in Na 2 S 8 g Na 25/1 02-Injection pressure 7.5 x 105 Pa f 02-flow meter indication (PE = 3 bar absolute) 44 m 3 02 / h 02-quantity injected + 1 m 0/44 x 7 r + 1 3 02 / h = 74 m 3 02 / h Measured values : Black liquor temperature rise approximately i C Reduction of Na 2 S in black liquor approximately 1.0 g / l Theoretical values: molar number in Na 2 S per hour 192 x 8000 mol / h = 78 mol / h = 19692 mol / h Oxygen requirement for all of Na 255 19692 x 32 g 02 / h = 630 kg O2 / h 471 m 3 02 / h Maximum possible oxidation of Na 25 471 8 g Na 25/11 1, 25 g Na 25 / l Maximum temperature rise: T = 1.25 Q 0.125 x 53 x 19692 T = x = l "c I m x cp 192 000 x 1 09 x 1

= 0.6 C

  When all 1.97 x 10 4 mol Na 2 S are oxidized by the wet chemical route, the addition

  50 t daily Na 2 SO 4 can be suppressed.

he

Claims (5)

  1. Process for the dissolution of cellulose by means of a white liquor which consists essentially of Na 2 S and NaOH, which results in a black liquor containing lignin and unprocessed Na 2 S, from of which a white solution is recovered by heat treatment, characterized in that in the black liquor, before the heat treatment, is introduced a gas containing at least 50% by volume of oxygen. 2. Method according to claim 1, characterized in that, as a gas containing   oxygen, technically pure oxygen is used.   Process according to Claim 1, characterized in that, as a gas containing   oxygen, oxygen enriched air is used.   4. Process according to any one of   Claims 1 to 3, characterized in that the gas   containing oxygen is introduced into the black liquor in an amount sufficient to completely oxidize before the heat treatment, the portion to Na 2 S to give Na 25203.   5. Process according to any one of   Claims 1 to 4, characterized in that the gas   containing oxygen is injected into a delivery line containing the black liquor.