WO1995023256A1

WO1995023256A1 - Process for generation and use of additional pulping chemicals in kraft-type process

Info

Publication number: WO1995023256A1
Application number: PCT/FI1995/000089
Authority: WO
Inventors: Paterson Mckeough
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 1994-02-28
Filing date: 1995-02-22
Publication date: 1995-08-31
Anticipated expiration: 1996-08-28
Also published as: FI102976B; FI940933L; FI102976B1; FI940933A0

Abstract

Disclosed is a method for improving the kraft pulping and chemical recovery processes. In the method non-traditional pulping chemicals are generated on site and used in addition to the traditional pulping chemicals. The additional active chemicals are certain organic sulphide compounds, in particular methyl mercaptide ion, which are derived from volatile compounds produced in significant quantities by heat-treating the spent pulping liquor at a temperature higher than the pulping temperature. The method has two main advantages. Firstly, pulping is enhanced by the additional pulping chemicals. Secondly, the recovery and reuse of pulping chemicals is considerably more flexible than in the conventional kraft process.

Description

PROCESS FOR GENERATION AND USE OF ADDITIONAL PULPING CHEMICALS IN KRAFT-TYPE PROCESS

Description

Technical field

The present invention concerns a favourable modification of the kraft pulping and chemical recovery processes. In the new method non-traditional pulping chemicals are generated on site and used in addition to the traditional kraft pulping chemicals.

The present invention has two main objects. Firstly, the said additional pulping chemicals enhance the pulping process itself. Secondly, the process for recovering and reusing the spent pulping chemicals is significantly more flexible than the conventional kraft chemical recovery process.

Prior art

In the kraft pulping process, lignocellulosic material, such as wood, is digested in a liquor containing, as main components, the active pulping chemicals sodium hydroxide (NaOH) and sodium hydrosulphide (NaHS). After digestion the spent pulping liquor, known as black liquor, is washed from the pulp, concentrated by evaporation and combusted in the chemical recovery boiler. The heat released through combustion of the black liquor organic material is recovered as steam while the inorganic material is converted into salts, mainly sodium carbonate (Na₂C0₃) and sodium sulphide (Na₂S) . The so-called white liquor, containing the above mentioned active pulping agents as main components, is prepared from these salts.

As described for example in Finnish Patent 85515 and in Finnish patent application 916097, high yields of sulphur gases can be obtained by thermal treatment of black liquor in the liquid phase at temperatures higher than the pulping temperature. The main gaseous products are methyl mercaptan (MM, CH₃SH) and dimethyl sulphide (DMS, (CH₃)₂S). Small amounts of hydrogen sulphide (H₂S) and dimethyl disulphide (DMDS, (CH₃)₂S₂) are also formed. The main reactions involved are the lignin demethylation reactions:

Lignin-0CH₃ + HS" → Lignin-0" + CH₃SH (1) Lignin-0CH₃ + CH₃S^" → Lignin-0^" + (CH₃)₂S (2)

The higher the severity of the thermal treatment the greater is the extent of demethylation. Black liquor heat treatment at low severity is used commercially as a method to lower the viscosity of black liquor thus enabling evaporation of black liquor to high solids concentration (Nikkanen, S., Bioresource Technology 46(1993)173-176). Note that demethylation by the above reactions also occurs to a slight extent during kraft pulping itself.

Two types of application of high-severity black liquor heat treatment have been suggested (Finnish patent application 916097). In the first type, the sulphur removed by heat treatment is converted into compounds, such as inorganic polysulphides, which can enhance pulping. In the second type, the sulphur is ultimately converted into the conventional pulping chemical, NaHS, but, because this sulphur recycle is independent of the conventional recovery cycle, cooking liquors of different sulphidities can be prepared and utilized in a single digestion. A third incentive for application of high- severity heat treatment would be to reduce the amount of sulphur in the conventional recycle loop (the rest being recycled in a separate loop) . This could be particularly advantageous when applying alternative black liquor conversion processes, such as black liquor gasification.

In aqueous solutions of sufficiently high pH, MM dissociates to form methyl mercaptide ion, CH₃S^". This ion is considerably more nucleophilic than hydrosulphide ion, HS^", and, in principle. would be more effective than hydrosulphide in increasing the rate of delignification during pulping. According to US Patent 3 451 889, kraft pulping is, in fact, enhanced by the addition of organic sulphide compounds, including MM. More specifically, higher pulp yields, at the same residual lignin content, can be achieved without detrimental effects on the pulp physical properties. To achieve these results at least 10 % of the total effective sulphidity should be due to the added organic sulphides. In another study (Ohara, S. , Meshitsuka, G. & Nakano, J. , J Japan Wood Res. Soc. 29(1983)9, 611-616), a liquor containing both NaOH and methyl mercaptide ion delignified wood at a significantly higher rate than the corresponding NaOH liquor. The reactions between lignin and methyl mercaptide ion were shown to be analogous to those between lignin and hydrosulphide ion in conventional kraft pulping.

In US Patent 3 451 889, only one way of obtaining the required organic sulphides on site was presented; namely, recycling of the organic sulphides formed during the pulping process itself. However, from published data on the kinetics of Reactions 1 and 2 (Douglass, I. & Price, L. , Tappi 49(1966)8, 335-342), it can be estimated that the steady-state concentration of methyl mercaptide ion, achievable by simply recycling MM formed during pulping, is quite small. In fact, it is much lower than the concentration level of organic sulphide additives recommended in US Patent 3 451 889. As stated above, the recommended minimum level is equivalent to 10 % of the total sulphidity.

Description of the invention

The present invention is a novel and elegant method of improving the kraft pulping and chemical recovery processes. The method eliminates the drawbacks of the prior art concerning the use of organic sulphides in kraft pulping. The method encompasses:

i) on-site generation of volatile organic sulphur compounds by heat treatment of the spent pulping liquor at elevated temperatur ii) absorption or condensation of some of these compounds, in particular MM and possibly DMDS, into part or all of the white liquor or cooking liquor, and

iii) utilization of the liquor so produced in one or more stages of the pulping process.

Further characteristics of the invention are evident from the accompanying claims.

The key feature of the novel concept is that organic sulphur compounds which can enhance pulping are generated in considerable quantities on site. Examples of the amounts of methyl mercaptide ion which can be fed to the digester in a steady-state situation will be given below. The new concept represents a significant improvement to the prior art where the only method proposed for obtaining organic sulphur compounds on site was the recycling of compounds formed during the pulping process itself. The amounts of active compounds, such as MM, which can be generated on a steady-state basis during pulping, are much lower lower than those required to significantly benefit pulping.

The novel concept has two main advantages:

- pulping is enhanced through the addition of organic sulphur compounds which are generated in sufficient quantity on site

- recovery and reuse of pulping chemicals is significantly more flexible than the conventional kraft recovery process.

The flexibility of the new chemical recovery concept is associated with the fact that sulphur is recycled in two distinct ways:

- by converting and recycling inorganic compounds produced during the black liquor conversion process, the conventional conversion process being combustion in the recovery furnace

- by recycling volatile organic sulphur compounds released during black liquor heat treatment prior to black liquor conversion.

Because only the former of these two recycle loops involves sodium, the recycle of sulphur and the recycle of sodium are not rigidly tied to each other as they are in the conventional process which encompasses only the former recycle loop. Thus, for example, in the new process the level of sulphur in the cooking liquor can be conveniently adjusted without affecting the level of sodium. Furthermore, cooking liquors of different effective sulphidities can be very conveniently prepared for use in a single digestion. In this respect the present invention offers considerable advantages compared to the method outlined in Finnish patent application 916097. The latter method requires complicated processing sequences for converting the organic sulphur compounds into the conventional pulping agent, NaHS. In the new method the effective sulphidities of cooking liquors can be conveniently adjusted by directly absorbing different amounts of MM into the liquors. In addition, the level of methyl mercaptide ion in the cooking liquor can be reduced during the pulping sequence by a temporary reduction in pressure.

The new method also opens the way for close coupling of the pulping and black liquor heat treatment processes. For example, heat, as well as active pulping chemicals, can be conveniently transferred from the black liquor heat treatment stage to the pulping stage. This convenient transfer is possible because the sulphur gases released in heat treatment are directly contacted with white liquor or cooking liquor. Contacting with cooking liquor can even be performed within the digester itself. Prior conversion of sulphur gases into other compounds, as outlined, for example, in Finnish patent application 916097, is not required. In some embodiments of the present invention conversion of volatile sulphur compounds into other compounds, such as NaHS or inorganic polysulphides, is necessary and/or desirable. In particular, DMS, which is not significantly absorbed into alkaline solution, is available for this purpose. In fact, when the amount of sulphur contained in the DMS component represents a significant fraction of the total amount of recycled sulphur, then at least part of the DMS must be converted into NaHS. The most economic way of converting DMS and other unabsorbed sulphur gases into, ultimately, NaHS is to co-process them with black liquor in the recovery furnace or other black liquor conversion reactor. Other options for conversion of the unabsorbed sulphur gases are those outlined in Finnish patent application 916097; namely:

- conversion into H₂S and subsequent absorption of H₂S into green liquor, white liquor or cooking liquor

- conversion into H₂S, conversion of H₂S into elemental sulphur and, by addition to white liquor or cooking liquor, conversion of elemental sulphur into inorganic polysulphides.

When the sulphur in the unabsorbed gases represents only a small fraction of the total amount of recycled sulphur, possible additional methods of using the unabsorbed gases are:

- combustion in on-site furnaces other than the recovery furnace, provided that such furnaces are equipped with a scrubber or other device for removing sulphur dioxide (S0₂) from the flue gases. For example, the lime kiln and, if installed, the odour gas incinerator could be used for this purpose.

- conversion into sulphur-containing chemicals, such as sulphuric acid, which are required in mill operations other than pulping. One possibility is to first combust the gases in an on- site furnace, such as the lime kiln or the odour gas incinerator, and then to convert the S0₂ so formed into the required chemical. This option would help some mills to reduce sulphidity levels without increasing emissions of sulphur compounds to the environment.

Inorganic polysulphides, which can be produced from sulphur gases as outlined in Finnish patent application 916097, enhance kraft pulping. Because of several striking analogies between the behaviour of inorganic polysulphides and that of organic polysulphides, it is probable that DMDS, an organic polysulphide, enhances kraft pulping in a similar fashion to inorganic polysulphides. DMDS is more easily absorbed or condensed into white liquor or cooking liquor than is DMS. However, during black liquor heat treatment, DMDS is formed to a much lesser extent than either MM or DMS. On the other hand, if desired, methyl mercaptide ion can be readily oxidized into

DMDS. In summary, additional active pulping agents which fall within the realm of the present invention include methyl mercaptide ion and DMDS.

The main advantages of the invention, enhancement of pulping and flexibility of chemical recovery, are both so significant that the realization of one, alone, would make the new method economically attractive.

The invention will be described in more detail by reference to the accompanying drawing which presents a block diagram of one embodiment of the invention. Block 1 encompasses digestion, digester blow and/or relief, and black liquor separation from the pulp (pulp washing). From block l the weak black liquor stream 2 is led to the evaporator 3. At an intermediate point in the evaporation sequence the black liquor stream 4 is diverted to the black liquor heat treatment process 5 where it is heated in the liquid phase to a temperature higher than the pulping temperature, for example 200 - 300°C, and held at that temperature for a time of 5 - 60 min. Elevated pressure is used to prevent excessive vapourization of water during the treatment. Volatile sulphur compounds, in particular MM and DMS, are formed and exit the process as stream 7. The ratio of MM to DMS in stream 7 is dependent, in part, on the process conditions applied in the heat treatment step. For example, the MM/DMS ratio will tend to increase when the operating pressure is decreased. The heat-treated liquor, stream 6, is returned to the evaporator 3 for further concentration by evaporation.

The concentrated black liquor stream 8 exiting the evaporator is led to block 9 which encompasses the recovery furnace, or alternative black liquor conversion reactor, and affiliated processes. Stream 10 comprises the inorganic compounds formed in the conversion process. In block 11 white liquor is prepared in the conventional way from the components of stream 10. The white liquor stream 12 contains, as main components, the active pulping agents NaOH and NaHS.

During digester blow and/or relief and during black liquor evaporation, volatile sulphur compounds are released. In the conventional process most of these compounds are collected into so-called concentrated non-condensable gas (NCG) streams. In addition to the sulphur components, these streams typically contain other organic compounds such as methanol (CH₃0H) and turpentine, as well as significant quantities of air-derived oxygen (0₂) and nitrogen (N₂) . When methyl mercaptide ion is used in measurable quantities as an additional pulping agent, the amounts of sulphur gases released after pulping are greater than those released in the conventional kraft process. In particular, the amounts of MM and DMS are greater. MM originates from unreacted methyl mercaptide ion while DMS is formed during pulping according to Reaction 2. In the process of the accompanying figure, sulphur gases that would normally be collected into the concentrated NCG system are collected into two separate streams: stream 13, containing significant amounts of air-derived oxygen (0₂) and nitrogen (N₂), and stream 14, containing only minor amounts of such components.

The air-lean sulphur gas stream 14 is combined to the sulphur gas stream 7 exiting the black liquor heat treatment process, and the combined stream 15 is led to the absorber 16 where it is contacted with the white liquor stream 12. Stream 15 contains MM and DMS as major components and H₂S, DMDS, 0₂, N₂, and possibly CH₃0H and turpentine as minor components. In the absorber 16 most of the MM and H₂S in stream 15 is absorbed into the white liquor leading to the formation of methyl mercaptide ion and hydrosulphide ion, respectively. Depending on the absorber conditions some of the DMDS is also possibly absorbed or condensed into the white liquor. DMS is the main constituent of the unabsorbed gases, stream 17. The white liquor exiting the absorber, stream 18, is used in one or more stages of the digestion process. Pulping is enhanced as a consequence of the significant concentration of methyl mercaptide ion in the cooking liquor.

The primary option for utilization of the unabsorbed gases, stream 17, is co-conversion with black liquor in the black liquor conversion reactor 9 (in the conventional process, co- combustion in the recovery furnace). In this way the sulphur contained in these gases is conveniently converted into, ultimately, the active pulping agent NaHS. Alternatively, if stream 17 contains only a small proportion of the total amount of recycled sulphur, it can be combusted in block 19 which represents the lime kiln, the odour gas incinerator, or other such on-site furnace equipped with a device for removing S0₂ from the flue gases. Similarly, the air-rich sulphur gas stream 13 can be either processed in block 9 or, if its sulphur content is low enough, combusted in block 19.

Variations of the process configuration presented in the accompanying figure include:

- a configuration wherein some or all of the unabsorbed sulphur gases in stream 17 are converted into H₂S. The H₂S is either absorbed into the white liquor stream 18 or further converted into elemental sulphur which is added to the white liquor stream 18 .

- a configuration wherein the air-rich sulphur gas stream 13 is handled in a similar fashion to the air-lean stream 14. One way to achieve this is to collect most of the volatile sulphur compounds released after pulping into a single concentrated NCG stream and to either direct this stream, together with stream 7, to absorber 16 or to contact this stream with white liquor in a separate absorber.

- a configuration wherein the sulphur gas stream 15 is contacted with only part of the white liquor. The resultant liquor has a higher effective sulphidity than the white liquor that is not contacted with the sulphur gas stream. Cooking liquors of different effective sulphidities are then available or can be prepared for use in different stages of the digestion process.

During digestion methyl mercaptide ion is consumed in two main ways:

- in reactions with lignin leading to delignification of the lignocellulosic feed material. These delignification reactions are analogous to those occurring between lignin and hydrosulphide ion in conventional kraft pulping

- in the lignin demethylation reaction, Reaction 2 above.

The amounts of methyl mercaptide ion that can be inputted to the digester in a steady-state situation have been calculated for various extents of these two types of reaction. The other variables in the calculations were:

- the overall extent of lignin demethylation occurring during pulping and black liquor heat treatment

- the extent of recycle of MM released after pulping (including that released during black liquor evaporation) - the MM/DMS ratio in the product gas of the black liquor heat treatment step. When the other variables are fixed, the input of methyl mercaptide ion to the digester increases with this ratio.

Examples of the results of the calculations are given in Table 1.

TABLE 1 Examples of the amounts of methyl mercaptide that can be inputted to the digester (MM/DMS molar ratio in product gas of black liquor heat treatment = 1.0).

Overall fractional extent of 0.5 0.5 0.5 1.0 demethy1ation

Fraction of input mercaptide 0.25 0.25 0.25 0.25 consumed in delignification

Fraction of input mercaptide 0.25 0.75 0.25 0.25 consumed in demethylation during pulping (Reaction 2)

Fractional extent of recycle 0.5 0 1.0 0.5 of MM released after pulping

Amount of methyl mercaptide to 0.20 0.13 0.29 0.4 digester, mol/mol of methoxyl input (in lignocellulosic feed) From the results it is evident that, through application of the present invention, significant quantities of methyl mercaptide ion can be inputted to the digester in a steady-state situation. In a typical conventional kraft digestion about 0.8 moles of hydrosulphide ion are inputted to the digester for 1 mole of methoxyl (in the lignocellulosic feed material). If, in order to enhance pulping, at least 10 % of the effective sulphidity should be provided by the methyl mercaptide ion, the required minimum input of methyl mercaptide ion is about 0.08 mol/mol of methoxyl input. From the results of Table 1 it is evident that this level of input of methyl mercaptide ion can be readily achieved through application of the present invention. Furthermore, at high extents of MM recycle, the minimum input can be achieved at moderate extents of demethylation.

Claims

1. A method for generation and use of organic sulphide compounds, in particular methyl mercaptide ion, as additional pulping agents in a kraft-type pulping process, comprising: (i) heat treatment of black liquor in the liquid phase at a temperature higher than the pulping temperature, (ii) absorption of some or all of the methyl mercaptan so formed, and possibly absorption or condensation of other volatile compounds so formed, into part or all of the white liquor or cooking liquor, and (iii) utilization of the liquor so formed in one or more stages of the pulping process.

2. A method as in claim 1, wherein the said black liquor heat treatment is performed at a temperature in the range of 200 - 300°C and for a time of 5 - 60 min.

3. A method as in claim 1, wherein some or all of any methyl mercaptan released after pulping, and possibly other volatile compounds so released, are also absorbed or condensed into part or all of the white liquor or cooking liquor, thereby being recycled to the pulping stage.

4. A method as in claim 1 or claim 3, wherein some or all of the volatile compounds, which are contacted with white liquor or cooking liquor but which are not absorbed or condensed therein, are co-converted with black liquor in the recovery furnace or other black liquor conversion reactor.

5. A method as in claim l or claim 3, wherein some or all of the volatile compounds, which are contacted with white liquor or cooking liquor but which are not absorbed or condensed therein, are combusted in on-site furnaces, such as the lime kiln or the odour gas incinerator, with a further option to convert the sulphur dioxide so formed into a chemical, such as sulphuric acid, for utilization on site. ₆. _A method as in claim 1 or claim 3, wherein some or all of the sulp_hur-containing volatile compounds, which are contacted with white liquor or cooking liquor but which are not absorbed or condensed therein, are converted into hydrogen sulphide, and possibly further into elemental sulphur, followed by recycling of the said hydrogen sulphide or said elemental sulphur to the pulping stage.

7. A method as in claim 1 or claim 3, wherein cooking liquors of different effective sulphidities are prepared by dividing the whole amount of white liquor or cooking liquor into two or more parts and by absorbing methyl mercaptan to different extents into these parts, including the possibility that methyl mercaptan is not added at all to one part.

8. A method as in claim 1, wherein, at some point in the pulping sequence, a temporary pressure reduction is used to decrease the concentration of methyl mercaptide ion in the cooking liquor.

9. A method as in claim 1, wherein the said black liquor heat treatment process is closely coupled to the said pulping process enabling organic sulphide compounds and possibly heat to be transferred from the former process to the latter process with few or no intermediate steps.

10. A method as in claim 1 or claim 3, wherein organic polysulphide compounds, such as dimethyl disulphide, are included in the said other volatile compounds which are possibly used as additional pulping agents.