TECHNICAL FIELD:

The present invention relates to the evaporation of spent liquor or other waste liquids, such as black liq¬ uor, sulphite liquor or bleaching department filtrate, from cellulose cooking, and the process relates to an evaporation process which yields a purer condensate of the evaporated liquid and a second condensate of the evaporated liquid which is more concentrated with re¬ gard to impurities.

STATE OF THE ART:

Spent liquor from cellulose cooking, so-called black liquor, has a dry substance content of 14-17%. These substances are cooking chemicals which are to be recov¬ ered and organic substances, principally lignin resi¬ dues, which are to be combusted. Direct combustion of black liquor is not possible, due to this low concen¬ tration of solid substances; for this reason, the black liquor has to be concentrated by evaporating off water. This evaporation takes place in various devices and re¬ sults m a concentrated black liquor whose concentra¬ tion is of the order of 80%. This concentrated black liquor is then combusted in a recovery boiler, with chemicals and heat energy being recovered.

The evaporation normally takes place in a series of ap¬ pliances which are in principle constructed as heat ex¬ changers. They consist of a cylindrical container of substantial height, for example 9-13 metres, and are provided internally with heat-exchanger tubes or heat- exchanger plates. In a preferred embodiment, the heat exchanger consists of a tube assembly which is placed vertically between two tube plates through which holes

are bored directly opposite the tube ends so that liq¬ uid, i.e. black liquor, can be caused to flow verti¬ cally down inside the tubes, with steam, which is to bring about the evaporation, being allowed to flow and condense on the outside of the tubes. The tube ends are joined to the tube plates such that there is no connec¬ tion between the inside and outside, respectively, of the tubes. The black liquor consequently forms a fal¬ ling film inside the tubes and releases a portion of its water content in the form of steam, so-called liq¬ uor steam, which is used as a heating medium in other appliances. Such an appliance is termed a falling film evaporator. Normally, several such appliances are cou¬ pled in series. While they can be arranged in such a manner that the liquor flows in countercurrent to the steam, co-current devices are also known. In these lat¬ ter devices, it is customary to have heat exchangers between the evaporation appliances in order to increase the temperature of the liquor.

The water which is evaporated in an evaporation appli¬ ance, a so-called effect, is released in steam form and is used for evaporation in another, subsequent, effect containing weaker liquor. Most of the liquor steam con- denses in this latter effect and forms _ condensate which is more or less impure. The liquor from the cook¬ ing department contains volatile compounds, such as methanol, terpenes and sulphur compounds, which are re¬ leased from the liquor in each evaporation effect. The impurity which occurs to the greatest extent in the condensate from the black liquor evaporation is metha¬ nol. Since methanol is more volatile than water, the liquor steam which is released from the weakest liquor will have the highest content of methanol. Condensates which are formed from liquor steam from stronger liq¬ uors contain proportionately much lower contents of methanol. If a liquor steam is allowed to condense out in two stages, a purer condensate and a more contami¬ nated condensate are then formed. This is because the

steam which does not condense out in the first stage contains a higher content of methanol and other vola¬ tile impurities.

According to the current technique, small constituent flows of the most contaminated evaporation condensates are collected together and conveyed, together with con¬ densate from the cooking department, to a stripper col¬ umn for a purifying treatment. Evaporation condensate which is purer is collected together separately and ei¬ ther reused in the mill as process water or else dis¬ charged. Since the condensates from the different ef¬ fects possess different degrees of purity, it is also customary, according to the current technique, to mix dirty condensates from one effect which yields rela¬ tively pure condensates with a pure condensate from an effect which yields condensates which are more contami¬ nated. This mixed condensate of intermediate purity can also be used in the process or be purified in different ways.

TECHNICAL PROBLEM:

The demands for a cleaner environment are nowadays be- coming gradually more stringent in most countries. The water which is to be released into rivers and the like must be pure, at least sufficiently pure to ensure that no environmental problems are associated with it. It is no longer acceptable to discharge semi-purified conden- sate. One way of decreasing the contaminants might be to reuse the condensate in the mill as process water instead of discharging it. However, with such a proce¬ dure, the problem then arises that various contaminants accumulate, thereby causing interference in the process and giving rise to problems in the working environment.

For this reason, there has long been a great need within the cellulose industry to be able, on the one hand, to purify the condensates from the black-liquor

evaporation aggregates so that they become acceptable for discharge or reuse and, on the other hand, to carry out this purification economically without using sev¬ eral, or very large, so-called stripper columns.

SOLUTION:

According to the present invention, a process for evaporating spent liquor or other waste liquids from cellulose cooking, so-called black liquor, sulphite liquor or bleaching department filtrate, has therefore been developed in which the evaporation is performed in several evaporators, so-called effects, where aqueous steam which is driven off from one effect is conveyed onwards nto the next effect at a low level, and there, by means of condensation, in its turn drives off fur¬ ther steam from the liquor, and in which the liquor can be conveyed from effect to effect in an arbitrary man¬ ner, and in which the aqueous steam contains a variety of relatively low boiling point contaminants, which contaminants are enriched in the steam phase when the steam condenses so that a purer fraction of condensate and a dirtier fraction of condensate, which is more en¬ riched in contaminants, can be taken out separately from each other, which process is characterized in that the purer fraction from one effect is conveyed at a high level into another effect on the steam side where the steam which is arriving is purer than was the steam which was supplied to the effect from which the conden- sate came.

According to the invention, it is expedient for the dirtier fraction to be supplied, at a high level or at the very top, to another effect on the steam side where the steam which is arriving is more contaminated than was the steam which was supplied to the effect from which the condensate came.

According to the invention, it is expedient for the last effect in the direction of movement of the liquor to be operated using fresh steam.

According to the invention, it is desirable for the process to be carried out until the pure fraction can either be discharged or returned to the process.

According to the invention, it is expedient for the evaporation process to be performed in a so-called fal¬ ling film evaporator having one or more vertical parti¬ tions between the evaporation tubes.

According to the invention, it is expedient for two or more appliances to be coupled together in series.

DESCRIPTION OF THE FIGURES:

The invention will be deεcribed in more detail below with reference to the attached drawings, m which

Fig. 1 shows diagrammatically how the condensate streams are recov¬ ered using the current tech- nique,

Fig. 2 shows diagrammatically how the condensates are recovered m accordance with the present invention, and

Fig. 3 shows diagrammatically, and in section, an evaporation appli¬ ance which can be used in the process according to the pres¬ ent invention.

DETAILED DESCRIPTION:

Figure 1 shows a diagram of three effects which are in accordance with current technique and each of which yields one pure condensate and one dirty condensate. These condensates are neither completely pure nor fully dirty, but instead contain a certain quantity of con¬ taminants which is different for the three effects. These effects, 1, 2 and 3, are assumed to be of the so- called falling film type having vertical tubes, and they are provided with a vertical partition 4 so that pure condensate 5 and dirty condensate 6 can be removed from each of the effects. For the sake of simplicity, the paths taken by the liquor and the steam are not shown in this diagrammatic drawing.

The purest, or least contaminated, constituent streams are those which emanate from the effects which are far¬ thest to the left in the Figure. As can be seen in the Figure, the pure condensates from the two effects which are furthest to the left in the Figure, that is effects 1 and 2, are collected, for possible reuse, in a stream 7, while the dirtiest constituent streams from the right-hand effects, that is 2 and 3, are collected, for forwarding to a stripper, in a constituent stream 8, and the pure constituent stream 5 from effect 3, which is the least pure of the three constituent streams 5, is combined with the least dirty constituent stream 6 from effect 1 in a stream 9, which may be termed medium pure and which must be purified or reused m a process stage which has lower purity requirements.

Figure 2, which diagrammatically depicts the process according to the present invention, also shows three effects, with it being assumed that the installation consists of effects both to the right and left of those shown in the Figure. This is known technology. The ref¬ erence numbers which are given apply to the same ele¬ ments as those in Figure 1.

Steam 10, which consists of liquor steam from the pre¬ ceding effect, is fed into effect 1 in the lower part of the section which yields the purer fraction 5 of the condensates. This steam 10 rises upwards in the effect and meets pure condensate 5 from effect 2, which con¬ densate is fed into effect 1 either at a higher level or at the very top. The steam 10 from the preceding ef¬ fect contains less volatile compounds than do the con- densates 5 from effect 2 and an exchange of volatile compounds will then take place by means of the volatile substances volatilizing and accompanying the steam which passes over into the section for the dirty con¬ densation fraction. The steam which condenses in this section for pure condensate is removed at the bottom and can be conveyed to the preceding effect or to dis¬ charge, via heat exchangers or flashing, or back to the process without any purifying treatment. The portion of the steam which passes over the upper edge of the par- tition 4 condenses in the dirty part. This condensation is virtually complete. An outlet, which is not shown, for non-condensable gases or the like is located in the lower part of this section. This outlet is connected to a vacuum pump.

For the sake of simplicity, the path taken by the liq¬ uor into effect 1 is not shown. The steam 11 which the liquor emits is removed from the so-called liquor steam space of the effect via droplet separators (not shown) and fed into the lower part of effect 2, as shown in the Figure. The dirty condensate stream 6 from effect 1 is fed into the top of effect 2 while the pure conden¬ sate stream 5 from effect 3 is fed into effect 2 at a lower level. This pure stream 5 can also be fed into the top of effect 2 or can be combined with the dirty constituent stream 6 from effect 1. The course of events in effect 2 and in effect 3 is the same as in effect 1.

Since the pure fractions 5 from each effect meet an even purer steam from the preceding effect, each con¬ stituent stream 5 becomes purer the further to the left in the Figure and each constituent stream 6 becomes ever dirtier the further to the right in the Figure.

Figure 3 shows diagrammatically, and in section, a part of a falling film evaporator in which the present in¬ vention can be performed. The evaporator includes a jacket 30 which stands vertically and is normally cy¬ lindrical and in which vertical tubes 31 are arranged. The spent liquor which is to be evaporated flows down through these tubes 31, and the tubes are heated by steam between the tubes. According to the invention, this steam is fed in at the lower part 32 of the jacket 30. According to the invention, this steam 11 comes from the preceding effect. The partition 4 is present in the cvlmdrical jacket 30. On that side of the par¬ tition 4 where the steam 11 enters, the latter rises upwards and meets the pure condensate 5 from the next effect and the condensate 6 from the preceding effect, which runs downwards on the outside of the tubes. An exchange of substances takes place during the condensa¬ tion and the lighter components accompany the steam over into the dirty part of the cylinder on the right- hand side, in the Figure, of the partition 4. Here, the remainder of the steam, including the contaminants, condenses and is removed, as dirty condensate, at 6, while the pure condensate, on the left-hand side of the partition 4, is removed at 5. Gases which cannot be condensed are vented via the valve 33.

Only one partition 4 has been shown in the Figure, which partition 4 divides the space into two parts, a clean part and a dirty part. Several partitions can be arranged in different patterns and can divide up the space into several parts having spaces of identical or different size. This does not imply any departure from the invention but only that the path taken by the steam

in the evaporation appliance is extended. This division into several parts can be regarded as several evapora¬ tors being assembled together.

In order to provide good contact between the downwardly flowing liquid or condensate and the ascending steam and thereby increase the efficacy of the fractional distillation, the condensates which are arriving can be dispersed uniformly and flow steadily down in step lad- ders between baffles which hold the tubes. Parts of the heating jacket can be provided with packing material, and the contact between steam and condensate can also be increased with the aid of invaginations or bulges on parts of the tubes. The steam/liquid equilibria are such that the segregation of contaminants is more effi¬ cient at relatively low pressure and temperature, for which reason a countercurrent process is to be pre¬ ferred (yields a high concentration at low pressure) .

Thus, the process according to the present invention makes it possible to achieve a much purer fraction than was previously the case and also to achieve a much more contaminated fraction, whose volume is consequently much smaller than previously. This dirty fraction can contain a relatively large quantity of methanol in ad¬ dition to other contaminants. Apart from the abovemen¬ tioned advantages of an environmental nature, the ad¬ vantage is also gained that it is economically much more favourable to handle a quantity of dirty conden- sate which is smaller as regards its volume.

The process according to the invention also applies to the surface condenser which is used together with ef¬ fects .

The invention is not limited to the embodiments shown, but can be varied in different ways within the scope of the patent claims.