FI122534B - Arrangement for evaporation of black liquor - Google Patents

Description

ORGANIZATION FOR EVAPORATIVE BLACK LIQUID

The present invention relates to an arrangement for a pulp mill black liquor evaporation plant in which the initial liquor black liquor is carried out in a novel manner using steam compression evaporation.

For combustion in a recovery boiler, black liquor is most often concentrated in a multi-stage evaporator. The numbering of the phases is based on the order in which they are used. Most often, the desalination plant operates by supplying, for example, fresh steam to the evaporator at the highest pressure 10, which boils the liquid on the liquor side of the evaporator, producing steam having a lower pressure than the steam introduced on the evaporator side. The resulting steam is used to next heat the evaporator at lower pressure as the liquor flows through the evaporator upstream of the heating steam. Similarly, in the third stage, the fourth stage, etc., the same process occurs until the pressure and temperature of the vapor leaving the final stage of the steam flow are so low that it is technically and economically unviable to continue. The lowest pressure steam is usually condensed in the condenser with water or air. Typically, the evaporator has 5-7 steps. One conventional type of evaporation unit is the so-called. a falling film evaporator, in which liquor is fed to run down the surface of the heating element as a thin film to achieve a high heating power.

Vapor concentration (VCE), also called mechanical vapor re-compression (MVR), is used to concentrate black liquor for incineration, in which the vapor pressure of the vapor from the phase heating steam. Compression is necessary to raise the saturation temperature of the steam to provide a sufficient temperature difference over the evaporation step. The rise in saturation temperature must be greater than the rise in boiling point 30. Since the power requirement of the compressor or fan is proportional to the increase in pressure, steam compression evaporation is most frequently used when the boiling point rise is low. A limiting factor in the mechanical vapor compression of black liquor evaporation is the increase in the black liquor boiling point. As the solids content of black liquor rises, the boiling point increase rapidly and reduce the available 35-degree ceiling. In practice, the use of fan evaporators in the siphate pulp industry is limited to 2 pre-evaporators. wherein the lye is evaporated to a dry solids content of about 20-25%. The pre-evaporation is typically followed by the multi-stage evaporation described above to raise the black liquor solids to a desiccant dry content for combustion in a recovery boiler, typically 75-85%, up to 90%. Compressor evaporators are typically suitable for use as a drop film evaporator because they work efficiently even at small temperature differences.

The economic feasibility of steam compression evaporation depends on the fact that the price of electricity is sufficiently low compared to the price of the heat energy obtained. The advantage of this shark-10 method is that no external steam source is needed.

Fl patent publication 51835 discloses an evaporator where, for example, black liquor is first evaporated in a steam press evaporator, where the vapor removed from the evaporator is compressed by a compressor and returned to the same stage as a heat source. In the second evaporation step, the lye is further concentrated using steam removed from the condensate removed from the steam-compression evaporator as the heating medium.

Fl patent publication 118131 discloses an arrangement for increasing the capacity of a black liquor multi-stage evaporator by increasing the pressure of the vapor at least one of the evaporation stages by a compressor or blower. The higher pressure steam is passed to the next evaporation stage as a heating medium, so it is not a so-called. VOC rynpuristushaihdutus ..

As prices for additional fuel and green electricity - generally referred to as electricity produced from non-fossil fuels - continue to rise, various plant solutions that reduce plant steam consumption will become increasingly economically viable. By reducing steam consumption, the plant balance either saves extra fuel or has more steam in the condensate section of the turbine to generate electricity. The pulp mills are therefore seeking to sell more electricity out of the 30 economic gains.

The energy consumption of a mechanical vapor compression evaporator, which can also be referred to as a fan evaporator when using a fan, is often up to 70% lower when compared to, for example, a 7-stage serial evaporator. In the past, pu-35 halline vaporization plants have been used mainly in the sulphate pulp mill to obtain additional evaporation capacity. Only part of the required total evaporation can be done at the fan evaporator, since too much solids in the liquor to be evaporated raises the boiling point and requires a large pressure increase from the fan or compressor, which increases energy consumption.

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It is an object of the present invention to provide a more economically advantageous way, both technically and economically, to combine steam compression evaporation with a known black liquor multistage evaporator.

To achieve these objects, the present invention relates to an arrangement for evaporating black liquor, comprising at least a vapor compression evaporator comprising at least one heat transfer surface unit and a steam pressure boosting device, and a multi-batch evaporator having at least one and connections for supplying steam to each unit and for removing steam formed therefrom by evaporation of the black liquor. Characteristic of the arrangement according to the invention is that the vapor pressure evaporator unit and one multi-stage evaporator unit are connected so that the vapors formed in the black liquor evaporation are led to a common space connected to at least a . Preferably, the common space is further coupled to one of the vapors to conduct a subsequent evaporation step of the multi-stage evaporator.

The invention is carried out in such a way that the vapor compression evaporator unit and one multistage evaporator evaporator unit are arranged in the same shell vessel, i.e. in the same evaporator. Then, the same device has two heat transfer surface units, one of which is the steam generated in the previous stage of the multi-stage evaporator and the other from the steam boosting device. When the lye is evaporated on the outer surface of the heat transfer units 30, the vapor formed during evaporation enters the same space defined by the jacket and is typically discharged into one or more of the jacket, typically at the top. Part of this steam is used in the next lower pressure evaporation step and part of the steam is raised and returned to the heating steam for the steam compression evaporator unit.

Preferably, a heat transfer evaporator is used as the heat transfer surface unit, in which the evaporation surface is formed by a plurality of lamellae or tubes. Preferably, the liquor to be evaporated flows on the outer surface of the lamellae or on the inner surface of the tubes, while the heating steam is conducted inside or outside the lamellae. For example, in sheet-type lower film lamellar evaporators, the evaporation surface is almost all or a portion of the vertical surface in contact with the surrounding vapor space, leaving a space between the heat transfer elements to release the steam generated on their surface. In the evaporator operating on the principle of a free-flowing falling film, the steam flows freely into the 10 spaces defined by the jacket vessel, within which the heat transfer element units are located. The resulting steam flows substantially vertically upward in the free space between the heat transfer element units and the shell vessel wall to the top of the shell vessel. The steam is normally collected in the Pisa separator and allowed to flow out of the top of the pan. Each evaporation stage of a multistage batch evaporator has a single jacketed heat transfer surface unit 15 or more parallel jacketed heat transfer surface units.

The steam boosting device is a fan or a compressor.

According to the invention, the units of the steam compaction evaporator one multi-stage evaporator-20 plant are arranged in the same jacket, i.e. in one evaporator. There are then two heat transfer surface units in the same device in which one of the heating vapors is the vapor generated in the previous stage of the multi-stage evaporator from the second steam boosting device. In the units, the secondary vapors formed during the evaporation of the black liquor are conveyed to the same common space, which is the to-25 vapor space of the evaporator and from which, according to one embodiment, they are removed together through the same one. This outlet is connected in the downstream direction of the steam to the next evaporation step, in which part of the steam is fed directly to the heating steam. Said outlet connection is further connected to a pressure boosting device for increasing the pressure of the steam and returning some of the vapors after the pressure increase to the steam compression evaporator unit 30 for heating steam.

According to another embodiment of the invention, the vapor compression evaporator unit and one multistage batch evaporator unit are arranged within the same jacket such that the common secondary vapor space is partially divided by a septum so that the septum has an opening or 35 apertures to equalize the pressure between The purpose of dividing the common steam space into a partially combined steam space is to enhance the separation of the dirty condensate. It is known that:

When the feed or leach liquor is first evaporated, it first boils a large amount of substances (including methanol) with a lower boiling point than water. After five these vapors, together with water vapor is passed through a compressor or blower lämmitysväiiaineeksi the lower part of the step of the in-phase haihdutuslämpöpinnayksikön or sarjahaihdutta papilla heating-steam side, so ylösvirratessaan from a mixture of condensed water and the first mainly in the upper part of the approach and the steam partial pressure decreases more and more other vapors. The separately collected condensate from the remainder of the heat transfer surface (the so-called segregating portion of condensate 10) is enriched in volatile substances (such as methanol) and is referred to as a dirty condensate.

According to one embodiment of the invention, said vapor compression evaporator unit and one multistage series evaporator unit are arranged inside different diaper vessels.

Removal of the repetitive vapors formed therein from the diapers is arranged in a common space connected to the steam pressure boosting device for returning a portion of the vapors to the steam compression evaporator unit for heating steam. Preferably, the space is also coupled to one of the vapors to conduct a subsequent evaporation step of the multi-stage evaporator. According to one embodiment, each diaper has a discharge connection which, outside the diaper 20 ends, joins into a single space which is connected to both the steam boosting device and the next evaporation stage of the multi-stage evaporator. Connecting the discharge ports may also be accomplished by connecting the vapor discharge conduit of the second jacket to the vapor space of the second jacket, serving as a common space and from where the repeat vapors are partially fed to the as heating.

According to a preferred embodiment of the invention, the removal, i.e. gasification, of the non-condensable gases produced in the steam compaction evaporator unit is carried out by the corresponding equipment of a multi-stage evaporator.

According to a preferred embodiment of the invention, the removal and treatment of the condensates generated in the steam compaction evaporator unit is carried out by means of devices for this purpose in the multistage evaporator.

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The invention will be described in more detail with reference to the accompanying drawings, in which Figure 1 schematically illustrates in detail the arrangement of a vapor compression evaporator unit with a multistage evaporator unit according to a preferred embodiment of the invention;

Figure 1 shows an evaporator having a vapor purifier-10 evaporator heat transfer surface unit 4 and a multi-stage evaporator heat transfer surface unit 3 operating within the mantle 2, which operate on the principle of a counting film. In this embodiment, the heat transfer surface units consist of a plurality of lamellae, the interior of which is supplied with heating steam and the liquor to be evaporated flows on their outer surface. The lye is thus heated by indirect contact with the steam inside the heat transfer elements. Instead of the lamellae 15, tubes can also be used, whereby the liquor can flow on their inner surface.

The black liquor to be evaporated, the feed liquor or the slurry from the previous evaporation step, is fed via a tube 5 to the lower part of the evaporator, from which the liquor is pumped by pump 6 to the distributor 19 above the heat transfer units The liquor to be evaporated is removed to the next evaporation step along line 13. Typically, steam is supplied to the heat transfer unit of the multi-stage evaporator through a conduit 8 from a previous evaporation stage operating at a higher pressure as the heating medium. Steam from the compressor 9 is introduced into the heat transfer surface unit 25 4 of the steam compression evaporator via line 12.

Since the heat transfer surface unit 4 of the steam compression evaporator and the heat transfer surface unit 3 of the multi-stage evaporator are located in the same jacket 2, the vapors evaporated from the black liquor 9, which is typically a compressor or fan. In the booster device 9, the pressure of the steam is increased so that it can be returned to the heating steam in the same step, its vapor compression heat transfer surface unit 4. The pipe 10 also has a branch pipe 11 through which part of the vaporizer 7! the steam removed from the nozzle 1 is introduced as a heating medium into the second evaporation step.

In the heat transfer surface units 3 and 4, the heating vapors form condensate which is discharged along the pipes 15 and 16 to the expansion tank and thence along the line 17 for treatment of the condensate in the multi-stage evaporator. The non-condensable gases formed in said units are also removed along a common line for further treatment.

An advantage of the present invention is that it is possible to make better use of known couplings with multi-stage evaporator equipment such as a liquor and steam transfer pipeline, a condensate system and a vacuum system.

Figure 2 shows a 7-stage multi-stage evaporator for black liquor. In this case, the evaporator comprises successive steps 1 to 7 which operate successively at decreasing pressures and temperatures in the direction of the steam. In the final liquor evaporation step 1, there are steps 1A and 1B. The evaporators shown in Fig. 2 are drop film lamellar evaporators, but other evaporators suitable for liquor evaporation can also be used in this case. Figure 2, where applicable, has the same reference numerals as Figure 1.

For evaporation, steps 1A and 1B of evaporation step 1 are typically fed with factory steam through conduit 30 so as to heat the black liquor while condensing. In the evaporation step 1, steam is separated from the black liquor, which is introduced into the heating vapor in step 2 along the duct 31. In the evaporation step 2, in turn, steam at a temperature lower than the evaporation step 1 is discharged, which is further led to the next evaporation step 3 via channel 32. Similarly, in the evaporation stages 3, 4, 5 and 6, the secondary steam separated from the black liquor is always taken to the next evaporation step 4, 5, 6 and 7, respectively, to heat and evaporate the black liquor.

The leach liquor (feed liquor) is introduced into step 4, from where it flows to step 7. The liquor from step 30 is introduced to evaporate to step 6 through line 40 and further to step 5 through line 41 to form an intermediate liquor 42. The intermediate liquor is further passed to step 3, from which the liquor is passed through evaporation step 2 to final evaporation 1, which has two stages IA and IB connected in series on the liquor side. The liquor first evaporates on stage IB, from where it is led to stage IA to evaporate the liquor to a high solids content, i.e. about 75-90%. 35 Concentrated fuel liquor exits through line 43 for incineration.

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The vapor produced in the final step 7 is fed through line 33 to the evaporator vacuum system where it is e.g. is condensed by cooling water in a surface condenser (vacuum condenser) (not shown).

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In accordance with the present invention, a steam-press evaporator is coupled with respect to the flow direction of the black liquor of the multistage series evaporator. The connection is similar to that illustrated in more detail in Figure 1. Step 4 comprises a heat transfer surface unit 4 'of a vapor compression evaporator and a heat transfer surface unit 3 of a multi-stage evaporator 10 which operate on the principle of a diaphragm and are contained in the same sheath. The feed liquor 5 is introduced into the bottom of the evaporator of the evaporation stage 4, from which liquor is pumped through the distributor to the outer surfaces of the heat transfer surface units 3 and 4 ', which, when flowing downwards, evaporates steam. The secondary steam generated during evaporation is removed from the top of the evaporator to one through 10. Part of the steam is led in the downstream direction of the steam-15 to the next evaporation step 5. Part of the steam is again supplied to the pressure boosting device 9 provided in connection 10, where the steam pressure is increased so that this vapor can be returned as heating medium to the steam extraction evaporator 4. The condensates 17 and 23 generated in the heat transfer units can be treated in the condensate treatment system of the multi-stage evaporator.

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Figure 3 shows an alternative structure to the embodiment of Figure 1. In Fig. 3 - where applicable the same reference numerals as in Figs. sides. This design enhances the separation of dirty condensate.

The lean feed liquor is introduced along line 5 to the evaporator unit at the bottom of the jacket vessel 2, from where it is pumped via pump 7 'to line distributor above the steam compression heat transfer surface unit 4 30. The liquor flows down the outer surface of said heat transfer unit 4, and the concentrated liquor is removed through line 25 to a suitable evaporation stage. The secondary vapor formed is fed through a single 10 "at the top of the jacket to a booster device 9. The vapor at elevated pressure is introduced along line 12 to the lower portion of the inner portion of the heat transfer unit 4 where it flows upwardly. From the remainder of the heat transfer surface separated by a short partition 22 (the so-called condensation segregation part), the separately collected condensate is enriched in volatile substances (such as methanol), which is called the dirty condensate 23.

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Liquid from the second evaporation step via line 26 is introduced into the heat transfer unit 3 of the multi-stage evaporator in the jacket vessel 2. The liquor is pumped 6 "through line 7" to the top of the unit and made to drain down its outer surface. The evaporated secondary vapor is removed from the jacket 2 for a period of 10 'and transported in the steam flow direction 10 to the next evaporation step. The pure condensate is removed from the lower part of the inner part of the heat transfer units along lines 15 and 16 and further along line 17. The invention has the advantage that the treatment of condensates and the treatment of non-condensable gases can be performed by the same, i.e. multistage evaporator units.

Connecting the black liquor vapor compression evaporator to the multistage evaporator in accordance with the invention provides at least the following advantages: - the amount of equipment, piping, etc. required by the vapor compression evaporator is reduced, since common systems such as liquefaction and vapor extraction systems can be utilized.

- the steam consumption compared to a multi-stage evaporator is reduced, whereupon steam can be used more for power generation, for example - the switch-steam and 7-stage serial evaporator circuit shown in Fig. 2 corresponds to a 9-11 stage serial evaporator, fuel cost and fuel dependent. Thus, I have considerable saving is also obtained, as required by the invention, the device is less.

While the present invention has been described herein in the most practical and preferred embodiment of the present state, it will be apparent to those skilled in the art that many modifications may be made within the scope of the invention.

Claims (10)

An arrangement for black liquor evaporation comprising at least one evaporation plant with meadow compression comprising at least one heat transfer surface unit (4) and a pressure elevation device (9) for meadow, and a multi-stage evaporation plant, each evaporation stage having at least one heat transfer unit (3). conduits (13) for conducting black liquor from one evaporative heat transfer unit to another, and conduits (8, 10) for feeding meadow into each unit and for diverting the mud formed therefrom during evaporation of black liquor, characterized in that the unit 10 (4, 4) ') at the evaporator evaporation plant and one (3) of the multi-stage evaporation plant units are coupled so that the vapors formed there during the evaporation of black liquor are led to a common space (18) connected at least to the pressure elevation device (9) for conducting part of the meadow there and feed it e The pressure rise to the unit (4, 4 ') of the evaporation plant with meadow compression is to be used as a thermal steam. 2. Arrangement according to claim 1, characterized in that the common space is additionally connected to a conduit (11) to guide a portion of the meadow to the next evaporation stage of the multi-stage evaporation plant. 20 Arrangement according to claim 1 or 2, characterized in that the unit (4) of the meadow evaporation plant and one (3) of the multi-stage evaporation plant are arranged within the same jacket (2), i.e. within the same evaporation device. 25 4. Arrangement according to claim 1, 2 or 3, characterized in that the secondary vapors formed during the evaporation of black liquor in the units of the meadow evaporation plant and the multi-stage evaporation plant are diverted from the common space through the same conduit, which outlet conduit is connected to the inlet current of the steam stream. to which part of the meadow is passed directly to be used as a heat meadow and which outlet line is connected to the meadow pressure rise device to raise the meadow pressure and return part of the meadows after the pressure rise to the unit at the evaporative evaporator plant to be used as a heat meadow. 35 Arrangement according to claim 1, 2 or 3, characterized in that the unit (4) of the meadow evaporation plant and one (3) of the multi-stage evaporation plant are arranged within the same jacket (2) so that the common space for the the secondary mud is partially divided by a partition (20) so that the partition is provided with an opening (21) or openings for equalizing the pressure between the sections that the partition forms within the manifold, and the secondary mantles are diverted through separate drainage pipes so that an outlet pipe ( 10 ') is coupled in the direction of flow of the meadow to the next evaporation stage, to which a portion of the meadow is passed directly to be used as a heat meadow and an outlet line (10') is connected to the meadow pressure raising device (9) to raise the meadow pressure and return part of it. of the meadows after the pressure rise to the unit at the evaporation plant with meadow compression for use as a heat meadow. Arrangement according to claim 1 or 2, characterized in that the unit of the meadow evaporation plant and one of the multi-stage evaporation plant 15 is arranged in separate sheaths and that the secondary steam evacuation formed therein from the sheaths is provided in a common space which is provided in a common space provided for meadow to pass part of the meadow there and return it to the unit of the meadow evaporation plant for use as a heat meadow and also connected to a line to direct part of the meadows to the next stage of the multi-stage evaporation plant. Arrangement according to any of the preceding claims, characterized in that the unit of the meadow evaporation plant and one of the multi-stage evaporation plant constitute the evaporation stage of the multi-stage evaporation plant, in which pre-evaporation of the liquor is carried out. Arrangement according to any one of the preceding claims, characterized in that the pressure raising device (9) for the meadow is a fan or compressor. Arrangement according to any of the preceding claims, characterized in that the drainage and treatment of condensate formed in the unit of the meadow evaporation plant is carried out with the multi-stage evaporation plant intended for this purpose. Arrangement according to any of the preceding claims, characterized in that the discharge of uncondensed gases formed in the unit at the evaporation plant with meadow compression, i.e. The degassing is carried out with the devices of the multi-stage evaporation plant. 5