US3807479A

US3807479A - Process for the evaporation of volatile alcohols and sulphur in the course of black liquor evaporation

Info

Publication number: US3807479A
Application number: US00306120A
Authority: US
Inventors: B Hubert; R Brannland; S Ekman
Original assignee: Mo och Domsjo AB
Current assignee: Mo och Domsjo AB
Priority date: 1971-11-19
Filing date: 1972-11-13
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30
Also published as: JPS4861701A; FI53230C; FI53230B; NO139568C; NO139568B; SE365008B; JPS528881B2; CA975114A

Abstract

A process is provided for reducing the emission of volatile alcohols and sulphur compounds during the evaporation of black liquor in several effects in the sulphate cellulose process. The liquor in those effects which are working with the most diluted liquor is evaporated in two or more steps on the same pressure level. The liquor is carried through the steps in series, the vapours obtained from these steps are condensed and at least a part of the condensates obtained are reintroduced into the process or purified by means of distillation.

Description

United States Patent [191 Brannland et al.

[ Apr. 30, 1974 Assignee: Mo Och Domsjo Aktiebolag,

Ornskoldsvik, Sweden Filed: Nov. 13, 1972 Appl. No.: 306,120

Foreign Application Priority Data Nov. 19, 1971 -Sweden 14791/ 71 U.S. Cl 159/47 R, 159/17 R, 159/17 P,

159/20, 159/17 VS, 202/174, 203/21 Int. Cl Bold l/26, B01d 3/00 Field of Search. 159/17 R, 17?, 47 R, 47 WL, 159/20 R, 17 VS; 202/174; 203/21 References Cited UNlTED STATES PATENTS 3,469,616 9/1969 Laquilharre 159/17 R X 3,580,818 5/1971 Villiers et al. i 203/11 X 3,627,646 12/1971 Osdor 159/18 X Primary Examiner.lack Sofer [5 7] ABSTRACT A process is provided for reducing the emission of volatile alcohols and sulphur compounds during the evaporation of black liquor in several effects in the sulphate cellulose process. The liquor in those effects which are working with the most diluted liquor is evaporated in two or more steps on the same pressure level. The liquor is carried through the steps in series, the vapours obtained from these steps are condensed and at least a part of the condensates obtained are reintroduced into the process or purified by means of distillation.

7 Claims, 3 Drawing Figures I STEAM FINAL EVAPORATED i uouon I D7 17 2o ii iilieusla [C NC Yc TO vacuum 33 l 1 l 1 i J c 15 [YB YB m/ ONDENSATE H OVAPOR Miiihizs 1 3 ,33

- VA /3 ,9 CONDENSATE i SEPARATOR 200 2 BLACK 1 LIQUOR 'CONDENSATE coFlENsATE PATENTED APR 3 0 i974 SHEET 2 OF 3 wEmzmm oQ w N QB NN E NH H a NN 9 w um 1 v PROCESS FOR THE EVAPORATION UlF VOLATILE ALCOHOLS AND suLPnun IN THE COURSE or BLACK Lrouon EVAPORATION The present invention relates to a process for the effective and economical evaporation of volatile alcohols and sulphur compounds, which are transformed into the condensate phase in the course of the evaporation of black liquor from the Kraft pulping process, and which cause water pollution by their high oxygen consumption in the course of biological degradation, when they are discharged to the surrounding water system through the outlet of the plant.

'3-5 atm, whereas the last apparatus works under vacuum on a pressure level of 0.1-0.4 atm.

Black liquor is a water solution containing partly organic material-mainly lignin, hemicellulose and alcohols-and partly inorganic material-mainly carbonate-, sulphide-, sulphate-and chlorideas well as sodiumand potassium ions'which solution usually has a dry content of 15-20 percent. In the course of the evaporation of black liquor, not only water vapour escapes but also volatile sulphurv compounds such as hydrogen sulphide, methyl mercaptan, dimethylsulphide and dimethyl disulphide and volatile alcohols, such as methanol and ethanol. Table l exemplifies the quantities of such compounds that can beevaporated in the course of a normal black liquor evaporation in five stages with about 15 percent initial dry content and about 58 percent final dry content, and a fresh steam pressure of 4 atm and 0.4 atm in the end condenser.

Table 1 Hydrogen sulphide 2400 grams per ton of pulp The amount of stripped volatile compounds in the different apparatus varies depending on the coupling scheme of the evaporation plant. Because of the fact 4 that the viscosity of the liquor increases with increasing concentration and decreasing temperature and that the risk for incrusts-that is the risk for solid precipitations on the heating surfacesincreases with increasing concentration and increasing temperature the normal coupling scheme lll-lV-V-l-ll has become the most usual at a five-effect plant. This means that the liquor is introduced in effect 111, for example at about 90C, and is then transported with self pressure via effect W at for example 80C to effect V, which is working at the lowest temperature, for example 65C, and is then preheated stepwise and introduced in effect I, which is working at the highest temperature, for example 125C, and is finally drawn off as thick liquor with 56-65 percent dry content from effect ll at for example 105C. Steam is supplied from the outside with for example 3.5 atm pressure to effect I, whereas vapour of the liquor leaving effect I is transferred with slightly .lower pressure, for instance 2.5 atm, to effect ll and so on, so that finally the liquor vapour from effect V has to be drawn off under vacuum on a pressure level of for instance 0.2 atm. With this coupling scheme the amounts of evaporated volatile compounds may be distributed according to Table ll.

Table ll Evaporated volatile compounds per ton of pulp Alcohols it is known that the greater amount of the volatile sulphur compounds can be separated and made useful by means of absorption with alkali (Swedish patent specification No. 226 789). By this, however, only the acid components in the evaporation vapours, that is H S and to a certain extent mercaptans, are collected and the biological oxygen consumption of the condensates is still high, mainly depending on their high methanol content. Methanol and ethanolcan, as is well known, easily be separated practically completely from water by distillation in a column provided with a dephlegmator. Since the alcohol content in the totally collected condensates is low, according to Table ll only about 0.1 g. per litre, the steam consumption of such a distillation will be very high, presumably about 1.5 tons per ton of pulp. This makes it desirable to separate the condensates from the different effects, whereby for instance the steam consumption may be brought down to about 40 percent, if only thetwo strongest condensates are treated, at which the coefficient of utilization will be about 75 percent. The cost of steam for distillation will, however, still be very high, especially since the heat content of the steam supplied at normal distillation only can be recovered in the. form of hot water from the dephlegmator of the distillation column, which cannot be further utilized in the process.

The present invention relates to a process for reducing the emission of volatile alcohols and sulphur compounds in the course of the evaporation of black liquor in several effects in the sulphate cellulose process, which does not show the above disadvantages. According to the invention the liquor is evaporated in those effects, which work with the most diluted liquor in two or more steps on the same pressure level, through which the liquor is conducted in series/vapours obtained from these steps are condensed and the condensates obtained are returned to the process or purified by distillation.

Surprisingly enough it has been found that the evaporation of alcohol in the first effects that the liquor encounters in its route can be considerably increased, if the evaporation of the liquor is carried out in two or more steps in each effect. This can be provided either by conducting the liquor in series through a number of evaporation apparatuses, which work on the same pressure level, or by providing an evaporation apparatus with shields (baffles) on the liquor side in the upper and lower part of the boiler compartment, so that two or more evaporation spaces, separated on the liquor side, are formed in the boiler compartment through which the liquor is conducted in series. The favourable effect of such division is shown in the following table, which states the total amount of evaporated methanol after evaporation in the first effect in the route of the liquor in the following number of steps:

one step 2.00 grams per ton of pulp two steps 2.21 do. three steps 2.29 do. four steps 2.33 do.

This technique can be used with the same good result in subsequent effects as long as the amount of methanol remaining in the liquor is of any technical importance from environment control point of view. The amount of condensate that shall be further treated or returned to the process can thus be decreased with about percent by a two-step evaporation, about 13 percent by three-step evaporation etc.

Further advantages may according to the invention also be obtained, if the vapour obtained at the evaporation of the liquor is condensed in several steps, whereby the earliest condensed fraction becomes poorer in methanol than the last condensed. The following table shows how the methanol distributes itself among two condensate fractions of mutually varying percental magnitude at an average content in the total condensate of 1 gram per litre of methanol:

First fraction of condensate Second fraction of condensate The table shows that it is possible to adjust the different condensate fractions at partial condensation so that a suitably high content of methanol will be obtained in the fraction, which shall be subjected to distillation. By stepwise condensation one can thus concentrate the amount of methanol into a smaller volume of condensate than the volume which is obtained at condensation in only one step. If one for example can allow oneself to discharge 8 percent of the amount of methanol without any treatment to the outlet, the amount of condensate, which has to be further treated or returned to the process, can according to the table be reduced to 60 percent of the amount of condensate obtained at condensation'in only one step.

A further reduction in steam consumption for the stripping of methanol and other oxygen-consuming compounds can according to the invention be obtained, if one utilizes the heat content of the vapour used for evaporation in a more qualified way than to prepare hot water in the dephlegmator (reflux apparatus) of the distillation column. The economy of the evaporation process will thus be substantially improved, if the evaporation (stripping) column is connected to the evaporation system, in such a way that the dephlegmation is carried out in some of the heat exchangers of the evaporation plant.

A suitable way of carrying out the invention is to let vapour from a distillation column without any separate dephlegmator condense in the evaporator of the first effect, which will serve as a dephlegmator. At this the liquor is brought to boiling in the same way as when fresh steam is supplied to this evaporation apparatus.

According to another embodiment of the present invention the distillation column is placed on a lower pressure level at which liquor vapour from the first or the second effect, poor in methanol, is supplied to the column and methanol-containing vapour leaves the column and is transferred to the evaporators of the second respectively the third effect. The condensate obtained from these evaporators is returned to the column, whereas the methanol-rich vapour leaving the evaporators is led via a pressure-regulating valve to combustion or to a rectifier column for the preparation of pure methanol. Depending of the place of the column in the evaporation system the working pressure in the top of the column may vary between 0.5 and 4.0 atm. A closer description of the way of connecting the column is given in the working examples. As mentioned above deposits may form in those effects, which work with the most'concentrated liquor and these therefore must be washed at regular intervals. Such washing is usually carried out with non-evaporated liquor whereby condensates having a high methanol content are obtained in such effects that normally give condensates that are poor in methanol.

In order to reduce the water pollution by such washcouplings it has, according to the present invention, proved to be especially suitable to change the normal liquor-circulation described above, so that the most di-' luted liquor is introduced in an effect having a pressure level lower than according to the normal coupling scheme (supra), for instance in effect 1V in a five-stage plant, after which the liquor having been evaporated in two effects is taken out from the effect V as so called intermediate liquor optionally having a dry content of 18-25 percent. The liquor vapours from these effects are condensed in the way described above and transferred wholly or partly to an evaporation column. The intermediate liquor is preheated successively with evaporation vapours drawn off from the heat exchangers of the effects, in separate heat exchangers which work at lower temperature than the corresponding effects, and is introduced in the liquor compartment of the first effect. Those two effects which work with the highest liquor concentration, usually effect I and effect ll, are optionally provided with divided liquor compartments or two evaporators working parallelly on the steam side. The intermediate liquor is introduced in one of two halves of the divided liquor compartments of the first effect, or in one of the two parallelly working evaporators in the first effect and is then transported with self pressure to the corresponding parts of the second effect. Thereafter the liquor is transported, also with self pressure, to effect ill in the plant, after which the liquor is taken out as a thickened intermediate liquor having a dry content, which is suitable at this temperature, namely 22-30 percent. lf effect lll comprises several evaporators, the liquor may be conducted either in parallel or in series through these. The thickened intermediate liquor is after preheating brought to pass the second half of the divided liquor compartment respectively the second evaporator in effect I and thereafter the corresponding parts of II, after which the liquor is taken out'as thick liquor having a dry content of 55-65 percent at a temperature of l1l0C.

By letting the thinner intermediate liquor and the more thickened intermediate liquor regularly change route through the effects I and II one can obtain an effective washing of the thick liquor effects without strongly methanol-containing condensates being formed in these effects. The washing according to the invention is especially effective because of the fact that the intermediate liquor used has a lower dry content than is normal. Because of the fact that the intermediate liquor used for the washing of effect I is evaporated at a high temperature in said effect its liquor vapour can after condensation in a subsequent step be subjected to a number of expansions to the lowest pressure level, for instance vacuum, and thereby be freed from the main part of its methanol content.

The liquor circulation scheme described above requires greater amounts of stripping vapour from the heat exchangers of the effects for the liquor preheating than the normal liquor circulation scheme III-IV-V-I-II. Because of this fact more methanol is transferred to the secondary condensates of these vapours and the methanol content of this secondary condensates can optionally be recovered by stripping in a distillation column, .whereas the primary condensates from the effects II, III and IV may be left untreated. Through this one obtains the advantage that a smaller'amount of impure condensate has to be distilled than is the case at normal liquor circulation.

EXAMPLE 1 In a five-effect plant with the normal liquor circulation IlI-IV-V-I-II (cf. FIG. l) 200 m of black liquor I were evaporated. The black liquor had a concentration corresponding to a dry solids content of 15 percent by weight at entry and the final concentration after the evaporation was 60 percent by weight, which corresponded to a total evaporation of 150 tons of water per hour. The plant was so constructed that the liquor was evaporated in each of the effects III, IV and V in two steps, at which the liquor vapours from .both the steps in one effect went to the common vapour compartment of the next effect.

In FIG. 1 the Roman FIGS. I-V represent different apparatus units, within which evaporation is carried out at a certain pressure level, i.e. effects. The pressure level decreases in order from effect I to effect V. In the effects I and II the evaporation is carried out in one step, whereas it is done in two steps in the effects III, IV and V, at which the liquor vapours from the two steps in one effect are led to the common vapour compartment of the boiler part of the next effect. The liquor, which for instance shall be evaporated in effect I enters from below into the boiler compartment 1 of the effect and is carried in boiler tubes 2 to a separating compartment 3 provided with separating devices 4. The evaporated liquor is separated in the separating compartment 3 from the stripped vapours and leaves through the tube 6 to the boiler compartment of the next effect. The vapours are collected in the vapour dome 5 and leave through the pipe 7 to the vapour compartment of the boiler part in the next effect. From effect II finally evaporated liquor (thick liquor) leaves through the pipe 8.

The effects III, IV and V have been provided with baffles 9 on the liquor side in the upper and lower part of the boiler part so that two separate compartments are formed through which the liquor is conducted in series. Black liquor from the pulp washing (thin liquor), which shall be evaporated, enters through the pipe 10 in the first boiler part of effect III, is evaporated in this, and is taken out from the separating compartment through the pipe 11 and is conducted through this to the second boiler compartment of effect III. The evaporated liquor is then carried via the pipe 12 from the separator compartmentto the first boiler compartment of effect IV, which works in the same way as effect III, and from effect IV further on to effect V in an analogous manner. The liquor from the second boiler compartments of effect V (intermediate liquor) is transferred via the pipe 13 to a buffer container 14 from which it via the pipe 15 can be introduced in the boiler compartment of effect I. The methanol-poor condensates from the vapour rooms of the boiler parts of the effects II and III are led via the condensate separators 33 and the pipes 16 to the main pipe 17, which conveys the condensates to another suitable place in the production, where its latent heat can be used or the condensate utilized for washing. The vapours from the vapour dome of effect V are condensed in series in the

condensers

18, 19 and 20. The condensates from the

condensers

18 and 19 are poor in methanol and are led via the pipe 21 to the main pipe 17 together with the condensates from the boiler compartment of effect IV and effect V obtained in the separators 33. The condensate from'the condenser 20, which is enriched in methanol, is led through the pipe 22 to the distillation column 23 to which fresh steam is supplied through the pipe 24 for the stripping of methanol and other volatile substances. Purified condensate is drawn ofi through the pipe 25 from the bottom of the distillation column, whereas the vapours leaving the top and containing water vapour, methanol and other volatile substances via the pipe 26 are supplied to the vapour room of the boiler part of effect I. The condensate from the vapour room of the boiler compartment in effect I is led together with uncondensed vapours via the pipe 27 to the condensate separator 28 and from there via the pipe 29 to the top of the distillation column 23. Uncondensed vapours leave the top of the condensate separator through the pipe 30. These vapours contain the main part of the volatile compounds contained in the black liquor, which can then be rendered innocuous by combustion. Gases and uncondensed vapours leaving the condenser 20 are led through the pipe 31 to the apparatus evacuating the evaporation plant.

The concentration of methanol in the liquor entering the plant was 0.37 gram/litre. The vapours leaving the three first effects contained the following amounts of methanol in kgs per hour:

Step I Step 2 effect Ill 32 1) effect IV 10 6 effect V 3 2 The condensates from these vapours contained 1.7 respectively 0.02 gram/litre of methanol and the liquor leaving effect V contained 0.03 gram/litre of methanol. The sulphur content of the condensates was 0.04, 0.04 and 0.05 gram/litre, the main part of which was hydrogen sulphide and the rest organic sulphides. The escape vapours from the condensate separators 33 belonging to the effects II-V were freed of the main part of their hydrogen sulphide content by washing with alkali.

Effect I in the plant served as a dephlegmator for a distillation column, which was fed with the methanoland sulphur-containing condensates from effect IV and V and partly from the surface condenser after effect V, totally 70 m per hour with an average content of 1.0 gram per litre of methanol and 0.04 gram per litre of sulphur in the form of hydrogen sulfide, mercaptan and sulphides. The vapour from effect V was partially condensed in three steps with similar amounts of condensate from each step at which the condensates from the first two steps obtained such a low methanol content (less than 0.1 gram per litre) that they could be discharged to the outlet without any further treatment.

The methanol-enriched gases were drawn off from the condensate separator 28 in effect I via a pressure regulator 32, which kept the pressure in the distillation column at 3.6 atm. The extracted methanol, 63 kgs. per hour, was transferred together with the volatile sulphur compounds, 3 kgs. of sulphur per hour, to a destruction oven, whereas the condensate from the bottom of the distillation column, which contained 0.1 gram of methanol per litre was conducted to the outlet. 37 tons of steam per hour with the pressure 3.7 atm were supplied to the distillation column. The column was working with a pressure drop of 0.1 atm from the bottom of the column, where fresh steam was added, to the vapour compartment of the boiler part in effect I, where the condensation, necessary for the dephlegmation, took place. In series with the evaporation plant a back pressure turbine was working for generation of electricity. The pressure drop over the column, and the reduction of the condensation temperature with 1.5C due to the methanol content of the vapour supplied to effect ltotally 0.2 atm-brought about that the back pressure turbine had to be adjusted to a counter pressure of 3.8 atm instead of those 3.6 atm, which were ob- .tained without any distillation column. At an amount of EXAMPLE 2 A fit p.la ...w .th the .n aU quQ sirsi ti n IlI-IV-V-I-II (cf. FIG. 2) was working with three apparatuses in each effect except for in effect II, where one of the three evaporation apparatus had been replaced with a distillation column. The plant was so dimensioned that 80 tons of water were evaporated per hour in that-line, which contained the distillation column, and 100 tons per hour in the two other lines. The amount of liquor supplied was 375 m per hour. The initial content of dry substance was percent by weight and the final content of dry substance was 60 percent by weight.

In FIG. 2 the designations IA, 1B and" IC refer to the different apparatus units, which work in effect I. In effect II the evaporation apparatus IIA has been replaced with the distillation column 23. For the other effects the corresponding designations are valid as for effect I. The evaporation is carried out principally in the same way as in Example 1, but in each effect the evaporation takes place in several steps, at which the liquor is led in series through the plant. Black liquor from the pulp washing (thin liquor) enters through the pipe 10 to the boiler part of the apparatus IIIA, is evaporated in this and is withdrawn from the separator part through the pipe 11 and transferred to the boiler part of the apparatus I118. The liquor evaporated in this place is then transferred from the separator part via the pipe 12 to the apparatus IIIC and from there to effect IV, where the liquor circulation is similar. Effect V is working analogously at the liquor side. The liquor vapours from effect IV are led to the apparatus VA for partial condensation, after which a remaining vapour is taken out through the condensate separator 33 and supplied in parallel to the two other apparatus in effect V. The 1iquor from the apparatus VC (intermediate liquor) is carried via the pipe 13 to a buffer container 14, from which it via the pipe 15 is supplied to the boiler part of the apparatus IA. After having passed the apparatus IA, 18, IC, IIB and 11C, the finally evaporated liquor (thick liquor) leaves through the pipe 8.

Primary steam with a super atmospheric pressure of 3.5 atmospheres was added to all three apparatus in effect I through the pipes 19.

The methanol-poor condensates from the vapour rooms of the boiler compartments in effect 11 and from the apparatus [118 and IIIC are carried off to the washing department of the factory (not shown on the Figure).

The vapours from the vapour domes of effect V are condensed in the surface condenser 20. The condensate from the surface condenser 20 is poor in methanol and is led together with the methanol-poor condensate from the boiler compartment of the apparatus VA to the outlet through the pipes 20a. The condensates from the boiler compartments of effect IV and the apparatus VB and VC, which are methanol-enriched, are led via the separators 33 and the

pipes

21 and 22 to the distillation column 23, which is supplied with steam from the vapour dome of the apparatus IA through the pipe 24. Purified condensate is drawn off via the pipe 25 from the bottom of the distillation column, whereas vapours leaving the top, and containing water vapour, methanol and other volatile substances via the pipe 26 are supplied to the vapour room of the boiler compartment of the apparatus IIIA. The condensate from the vapour room of the boiler compartment of the apparatus 111A is led together with uncondensed vapours via the pipe 27 to the condensate separator 28 and from there via the pipe 29 to the top of the distillation column 23. Uncondensed vapours leave the top of the condensate separator via the pipe 30, which is provided with a pressure regulator 32. These vapours contain the main part of the volatile compounds occurring in the black liquor, which may be rendered harmless by combustion. Gases and uncondensed vapours leaving the surface condenser 20 are carried through the pipe 31 to the vacuum equipment of the evaporation plant.

The methanol content of the entering thin liquor was 0.50 gram per litre. After evaporation in the effects III,

' Effect IV :IV and V, the methanol content of the liquor sank to 0.01 gram per litre. The same content was obtained in the evaporated thick liquor.

-The condensates-had .the following methanol contents:

Partial condensate End condenser 2.0 .g./l. Effect V 0.2 g./l. 0.9 do. Surface condenser 9.2 do.

gen sulphide and partially to organic sulphides. The

vent vapours from the boiler compartments of the effects II-V were freed of the main parts of their hydrogen sulphite content by means of washing with an alkaline water solution. After condensation in heat exchangers (not shown) the condensate was carried through the pipe 22 to the distillation column 23.

The total amount of condensate which via the pipe 22 went to. the top of the distillation column 23 was 90 m per hour and had a content of 1.7 grams per litre of methanol. 22 tons of steam per hour were supplied to the column through the pipe 24 at a super-atmospheric pressure of 2.4 atmospheres. The methanol recovered, 156 kgs. per hour, together with volatile sulphur impurities corresponding to an amount of sulphur of 5 kgs per hour were transferred to a destruction oven through the pipe 30. The purified condensate from the bottom of the column, which contained 0.15 gram per litre of methanol, was after heat exchanging transferred to outlet via the pipe 25. The pressure drop throughout the column was 0.1 atmosphere- The primary steam of effect I came from a counter pressure turbine, whose counter pressure and energy production was not influenced by the fact that the apparatus IlA was replaced with the distillation column 23. This is a consequence of the fact that the loss in temperature, which was caused by the pressure drop through the column. and by the content of methanol and other volatile compounds in the vapours from the top of the column to the boiler compartment of the apparatus IIIA, was considerably lower than the temperature drop in the apparatus IIA, replaced by the column, should have been.

The total steam consumption increased with 5 tons per hour as a consequence of the fact that the apparatus IIA was replaced with the distillation column 23. A separate distillation column for the same purpose should have required 22 tons of steam per hour.

By dividing the effects III and IV in three steps, each connected in series on the liquor side, it has been possible to carry about 83 percent of the methanol in the thin liquor to destruction. If the effects III and IV, as is usual, each had had only one liquor step, the amounts of methanol for destruction would have been reduced to 75 percent, which means that a50 percent greater amount of methanol would have accompanied the outflow to the recipient.

The partial condensation of the vapours from effect IV in the apparatus VA gives a condensate, which is poor in methanol. It can therefore be discharged to the outlet without any noticeable increase in the working load of the recipient with biochemical oxygenconsuming substances (BS). If the condensation of the liquor vapours from effect IV had been carried out in parallel in the three apparatus in effect V, as in the known manner, the BS-load on the recipient would have increased, since the condensate from the apparatus VA, which is discharged to the recipient, would have had a higher methanol content. Alternatively, this condensate may be supplied to the top of the distillation column 23, which, however, then would lose considerably more methanol to the recipient through the pipe '25. The total BS-charge on the recipient then would have been considerably greater than when using the arrangements withpartial condensation, which is shown in this example.

EXAMPLE 3 In a S-effect plant with the liquor circulation III-IV- V-I-II (cf. FIG. 3) 460 m of black liquor were evaporated per hour, the black liquor having an initial concentration of 15 percent by weight of dry substance and the evaporation being carried out to a final concentration of 60 percent by weight, which corresponded to a total evaporation of 350 tons of water per hour. The plant consisted of two lines of evaporation apparatus working in parallel on the vapour side and having the designations IA, IB, IIA, IIB, etc. Between the apparatus IB and IIB a distillation column 23 was arranged, the connection being carried out so, that the vapour from effect IB wassupplied to the bottom of the column via the pipe 7, and the vapour from the top of the column being supplied to the vapour room of the boiler part of the apparatus IIB. via the pipe 26.

The evaporation was. carried out principally in the same way as has been described in Example 1. The black liquor from the pulp washing (thin liquor) to be evaporated is supplied through the pipe 10 to the boiler compartment of the apparatus IVA, is evaporated there, and is drawn off from the separator part through the pipe 11, and flows through this to the boiler compartment of the apparatus IVB. The evaporated liquor then flows from the separator compartment through the pipe 12 to the boiler compartment of the apparatus VA in effect V, which works in the same way as effect IV.

The liquor from the apparatus VB (intermediate liquor l) flows through the pipe I3 to a buffer container 14 from which it via the pipe 15 is introduced in the boiler compartment of the apparatus IA in order to be further evaporated in the effects IIA, "IA and IIIB and is collected in a buffer container 35. The liquor from said container is carried through the pipe 36 into the boiler compartment of the apparatus IE to be finally evaporated in the apparatus IIB and drawn off as thick liquor through the pipe 8.

The circulation of the liquor can be changed in the effects I-III so that the liquor is circulated in the order IB-lIB-III-IIIB-IA-IIA through the apparatus, at which the apparatus IB and IIB are washed clean from incrustments, since they are brought to work with a thinner liquor. Because the wash liquor has passed four apparatus its content of methanol is low, and the condensates from the washed effects can be discharged to the outlet, since they contain only 0.2 respectively 0.l gram perlitre of methanol from effect I, respectively effect II.

The methanol-poor condensates from the vapour rooms of the boiler compartments in the effects III and IV as well as from the apparatus A are transferred to the outlet (not shown in the figure), since their heat contents have been utilized by cooling in heat exchangers. The vapours from the vapour domes in effect V first are partially condensed in the two parallel coupled condensers l8 and 19, thereafter to be finally condensed in the condenser 20. The condensates from the condensers l8 and 19 are poor in methanol and are led to the outlet through the pipe 21. The condensate from the condenser 20, which is methanol-enriched is together with the condensates from the boiler compartments in effect V, obtained in the separators 33, led through the pipe 22 to the distillation column 23, to which liquor vapour from the apparatus IB is supplied through the pipe 7 for evaporation of methanol and other volatile substances. Purified condensate is drawn off through the pipe 25 from the bottom of the distillation column whereas vapours leaving the top and containing steam, methanol and other volatile substances are supplied through the pipe 26 to the vapour room of the boiler compartment in the apparatus lIB. The condensate from the vapour room of the boiler compartment in the apparatus H8 is carried together with uncondensed vapours through the pipe 27 to the condensate separator 28 and from there through the pipe 29 to the top of the distillation column 23. Uncondensed vapours leave through the pipe 30 from the top of the condensate separator. These vapours contain the main part of the volatile compounds contained in the black liquor, which may be rendered harmless by combustion. Gases and uncondensed vapours from the condenser 20 are conducted through the pipe 31 to the vacuum equipment of the evaporation plant.

The methanol content of the supplied thin liquor in the plant was 0.45 gram per litre. The vapours leaving the two first effects contained the following amounts of methanol in kgs. per hour:

IV A W B 85 54 apparatus The condensates from these vapours contained the following methanol content in grams per litre:

of 2.0 grams per litre and an average sulphur content of 0.045 gram per litre distributed on partially hydro- I gen sulphide and partially organic sulphides.

The methanol-enriched gases were drawn off from the condensate separator 28 via a pressure regulator 32, which kept the pressure in the distillation column at 2.2 atm. The methanol recovered, 160 kgs. per hour, was carried in the state of vapour together with the volatile sulphur compounds, 4 kgs. of S per hour, to another distillation column (not shown on the figure) for purification and condensation of the methanol and destruction of the sulphur compounds. The condensate from the bottom of the distillation column, which contained 0.1 gram of methanol per litre, was discharged to the outlet through the pipe 25.

The practically total demethanolized liquor from effect V (intermediate liquor 1) had a methanol content of 0.05 gram per litre and a dry content of 22 percent. This liquor was used as a wash liquor in the apparatus IA and HA, after which it was further evaporated in the apparatus [HA and [HE and finally evaporated in the apparatus 1B and IIB. From there the liquor was drawn off with a content of dry substance of 60 percent (thick liquor).

42 tons of vapour per hour were supplied to the distillation column from the apparatus 1B. The column worked with a pressure drop of 0.08 atm, which caused a reduction of the temperature difference in the B- apparatus with 1C. On account of the content of methanol in the vapours supplied to the apparatus B one obtained there a temperature, which was 2C lower than if the aqueous vapour had been pure at the same pressure. In order to retain the total evaporation capacity, the fresh steam, which was taken from a counter pressure turbine, had to have l.5C higher temperature than if the distillation column had not existed, i.e. the vapour pressure had to be 0.15 atmosphere higher than the 3.5 atmospheres for which the plant was projected for without column.

In the back pressure turbine Megacalories per hour less energy as electric power were obtained out of the steam amount of 88 tons per hour. This loss must be compared with the cost of running a column with pure counter pressure steam and recovering the heat from the dephlegmator in the form of hot water. Such a column would need 22 tons per hour of steam with an energy content of 14,000 Megacalories per hour which means that the saving in energy obtained by the process shown in the example is 91 percent.

We claim:

1. A process for reducing the emission of volatile alcohols and sulphur compounds in the evaporation of black liquor in multiple effects in the sulphate cellulose process and in which at least those effects working with the most diluted liquor themselves comprise several serially connected steps, comprising pre-evaporating the liquor in those effects which are working with the most diluted liquor in two or more steps at the same pressure level, carrying the liquor through the steps in series, condensing the vapours obtained from these steps and reintroducing at least a part of the condensates thus obtained into the process.

2. The process of claim 1, in which theliquor of at least two subsequent evaporation effects of higher concentration is evaporated in two or more steps in each effect.

3. The process of claim 1, in which at least-30 percent, of the evaporation vapour from those effects which work with the most diluted liquor is condensed in a first methanol-poor fraction and the rest in a second methanol-enriched fraction, which fractions are reintroduced into the process or purified by distillation.

4. The process of claim 1, in which the final evaporathan the pressure prevailing in the distillation column, and in which evaporator uncondensed vapor resulting from the incomplete condensation and containing volatile alcohols and sulphur compounds after pressurereduction is discharged to destruction or recovery and from which evaporator the condensed liquid phase obtained in said incomplete condensation in the liquor evaporator is returned to the distillation column.

UNITED S'IAIES PATENT @FFlCE CERTIFICATE OF CQRRECTWN Patent NOBLSO'ZA'YQ Da e April 3 9 19/4 lnventofls) Rolf Karl August Brannland. et a1.

It is certified that error appears in the above-identified patent 21d that said Letters Patent are hereby corrected as shown below:

Column 2, Table II, line 19 "1000" should be listed under Column 2 of Table Q Column 11, line 5 e "parallel" should be in parallel Signed and ficaled this third ay of February 1976 [SEAL] Attest:

RUTH c. MASON c. MARSHALL DANN Altesting Officer (ommissinner oj'Patents and Trademarks

Claims

1. A process for reducing the emission of volatile alcohols and sulphur compounds in the evaporation of black liquor in multiple effects in the sulphate cellulose process and in which at least those effects working with the most diluted liquor themselves comprise several serially connected steps, comprising preevaporating the liquor in those effects which are working with the most diluted liquor in two or more steps at the same pressure level, carrying the liquor through the steps in series, condensing the vapours obtained from these steps and reintroducing at least a part of the condensates thus obtained into the process.

2. The process of claim 1, in which the liquor of at least two subsequent evaporation effects of higher concentration is evaporated in two or more steps in each effect.

3. The process of claim 1, in which at least 30 percent, of the evaporation vapour from those effects which work with the most diluted liquor is condensed in a first methanol-poor fraction and the rest in a second methanol-enriched fraction, which fractions are reintroduced into the process or purified by distillation.

4. The process of claim 1, in which the final evaporation of the most concentrated liquor is carried out in parallel with liquor received from an intermediate evaporation effect following pre-evaporation.

5. The process of claim 1, in which the vapours after condensation are supplied to a distillation column which is working with reflux and that the heat content of the vapours leaving the distillation column is recovered in the process.

6. The process of claim 5, in which the pressure in the top of the distillation column is kept at from 0.5 to 4 atm.

7. The process of claim 5, in which the reflux to the distillation column is obtained by bringing the vapour leaving the column to condense partially in indirect heat exchange with liquor in a liquor evaporator, in which the evaporation is carried out at a lower pressure than the pressure prevailing in the distillation column, and in which evaporator uncondensed vapor resulting from the incomplete condensation and containing volatile alcohols and sulphur compounds after pressure-reduction is discharged to destruction or recovery and from which evaporator the condensed liquid phase obtained in said incomplete condensation in the liquor evaporator is returned to the distillation column.